WO2010146590A1 - Liquid crystal wavelength selective router - Google Patents

Liquid crystal wavelength selective router Download PDF

Info

Publication number
WO2010146590A1
WO2010146590A1 PCT/IL2010/000480 IL2010000480W WO2010146590A1 WO 2010146590 A1 WO2010146590 A1 WO 2010146590A1 IL 2010000480 W IL2010000480 W IL 2010000480W WO 2010146590 A1 WO2010146590 A1 WO 2010146590A1
Authority
WO
WIPO (PCT)
Prior art keywords
polarization
light
pixel
wavelength selective
wavelength
Prior art date
Application number
PCT/IL2010/000480
Other languages
French (fr)
Inventor
Yossi Corem
Eli Weinberg
Amos Eitan
Boris Frenkel
Seong Woo Suh
Original Assignee
Xtellus Ltd.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Xtellus Ltd. filed Critical Xtellus Ltd.
Priority to EP10789108A priority Critical patent/EP2443510A4/en
Priority to JP2012515628A priority patent/JP2012530930A/en
Priority to CN2010800365636A priority patent/CN102804051A/en
Publication of WO2010146590A1 publication Critical patent/WO2010146590A1/en

Links

Classifications

    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/29—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the position or the direction of light beams, i.e. deflection
    • G02F1/31—Digital deflection, i.e. optical switching
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/27—Optical coupling means with polarisation selective and adjusting means
    • G02B6/2706—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters
    • G02B6/2713—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations
    • G02B6/272—Optical coupling means with polarisation selective and adjusting means as bulk elements, i.e. free space arrangements external to a light guide, e.g. polarising beam splitters cascade of polarisation selective or adjusting operations comprising polarisation means for beam splitting and combining
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29304—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating
    • G02B6/29305—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means operating by diffraction, e.g. grating as bulk element, i.e. free space arrangement external to a light guide
    • G02B6/29311—Diffractive element operating in transmission
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/24—Coupling light guides
    • G02B6/26—Optical coupling means
    • G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
    • G02B6/293—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means
    • G02B6/29379—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device
    • G02B6/29395—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals with wavelength selective means characterised by the function or use of the complete device configurable, e.g. tunable or reconfigurable
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/1326—Liquid crystal optical waveguides or liquid crystal cells specially adapted for gating or modulating between optical waveguides
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2201/00—Constructional arrangements not provided for in groups G02F1/00 - G02F7/00
    • G02F2201/17—Multi-pass arrangements, i.e. arrangements to pass light a plurality of times through the same element, e.g. by using an enhancement cavity
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/58—Multi-wavelength, e.g. operation of the device at a plurality of wavelengths
    • G02F2203/585—Add/drop devices

Definitions

  • the present invention relates to the field of fast optical switches, whose operation is wavelength dependent, especially for use in reconfigurable optical add-drop multiplexer (ROADM) applications.
  • ROADM reconfigurable optical add-drop multiplexer
  • WDM wavelength division multiplexing
  • the polarization diversity may be performed by a polarized beam splitter, or a birefringent walk-off crystal, and the wavelength dispersion may be performed by a diffraction grating.
  • a polarization rotation device such as a liquid crystal polarization modulator, pixelated along the wavelength dispersive direction such that each pixel operates on a separate wavelength channel, is operative to rotate the polarization of the light signal passing through each pixel, according to the control voltage applied to the pixel.
  • the polarization modulated signals are then wavelength-recombined and polarization-recombined by means of similar dispersion and polarization combining components to those that were used to respectively disperse and split the input signals.
  • the direction in which the resulting output signal is directed is determined by whether the polarization of the particular wavelength channel was rotated by the polarization modulator pixel, or not.
  • the present invention seeks to provide new fiber-optical, wavelength selective switch structures, such as may be used for channel routing or blocking applications in optical communication and information transmission systems.
  • the devices are designed as a 2 x 1 switch, and use liquid crystal elements for switching.
  • the addition of a multiplexer and a demultiplexer enables such a basic 2 x 1 structure to be advantageously used as the core of a reconfigurable optical add-drop multiplexer (ROADM), with add and drop functionality, from and to a number of ports.
  • ROADM reconfigurable optical add-drop multiplexer
  • the switch uses a minimum of components, and can thus be economically constructed for large scale use in such systems.
  • the switch structure can also incorporate either a phase mode or a polarization mode LC attenuator cell, as a variable optical attenuator for any of the transfer paths through the device.
  • a phase mode or a polarization mode LC attenuator cell as a variable optical attenuator for any of the transfer paths through the device.
  • the switch uses polarization diversity for converting the input beams to a single defined polarization direction, followed by wavelength dispersion to spread the individual wavelength channels over the pixilated switching device.
  • This may be a polarization rotation element whose setting can be controlled by means of an applied electronic signal. This may either leave the polarization direction unchanged, or it may rotate it such that it is essentially orthogonal to the polarization of the input beam exiting the polarization diversity components.
  • the beam then proceeds to a birefractive wedge element which refracts light having the two orthogonal polarizations to different extents, thus separating the beams according to the control signal applied to the polarization rotation element through which each wavelength component of the beam passed.
  • the beams may thus be directed to different ports according to the control signal setting.
  • the beams can be directed through a pixelated phase changing element, which controls the transmission of each component of the beam by affecting the mode structure of the beam across the pixel thereby controlling its ability to couple readily into the output fiber.
  • a wavelength selective switch comprising: (i) a first optical port,
  • a pixilated polarization rotation element optically coupled to receive the dispersed light, and having pixels aligned generally in the dispersion plane and adapted to rotate the polarization of light passing through each pixel according to a control signal applied to the pixel, such that the polarization of a wavelength component of the dispersed light is rotated according to the control signal applied to the pixel through which the wavelength component passes, and
  • a reflective birefringent element in optical communication with light from the polarization rotation element, and disposed and oriented such that light which traverses a pixel of the polarization rotation element is reflected in a first or second direction according to the polarization of the received light, the elements being further orientated such that light from the reflective birefringent element reenters the polarization conversion element which reconstitutes the light to its original polarization.
  • the light may be reflected in a first direction when the control signal applied to the pixel is such as to generate no change in polarization of the optical beam, and in a second direction when the control signal applied to the pixel is such as to generate an essentially 90 degree rotation in the polarization of the optical beam.
  • the first direction may lead to a second optical port
  • the second direction may lead to a third optical port, such that the first optical port is connected optically to either of the second and third ports in accordance with the control signal.
  • implementations may further involve a switch as described above, in which the reflective birefringent element is aligned such that the light reflected in one of the directions returns collinearly with the optical path from the first optical port, the switch further comprising a circulator to separate light reflected in the one direction from light incident from the direction of the first optical port.
  • the switch should further comprise an isolator disposed in the optical path in the second direction, the isolator being aligned such that light directed in the second direction cannot enter an optical port disposed in the second direction.
  • the pixelated polarization rotation element may be a pixelated liquid crystal cell, in which case the control signal may be a voltage applied to electrodes across pixels of the liquid crystal cell.
  • Additional implementations may involve a wavelength selective switch according to any of the previously described designs, in which the reflective birefringent element is in the form of a wedge, such that the directions are angularly distinguished directions, or in the form of a block, such that the directions are laterally distinguished directions.
  • any such switches may be such as to direct light input to the first port, to either of the second and third ports or to direct light input to either of the second and third ports to the first port.
  • any of the above-described switches may further comprise a pixelated phase changing element, which controls the transmission of light passing through a pixel of the polarization rotation element by spatially varying the phase across light traversing the pixel, thereby controlling its ability to, couple into an output port.
  • the mode structure of the beam of the light is degraded by the voltage applied to the pixel of the phase changing element.
  • the pixelated phase changing element may comprise a comb-like electrode structure which applies a spatially undulating electric field across the pixel of the phase changing element, such that the phase of light passing therethrough undergoes corresponding spatially alternating changes. In either case, the degrading of the mode structure of the beam of the light controls its ability to couple into an output port fiber.
  • any of the wavelength selective switches described may further comprise a polarization mode attenuator, which controls the transmission of light passing therethrough, the attenuator comprising:
  • a pixelated birefringent polarization rotating element which rotates the polarization direction of light passing through a pixel thereof in accordance with the electric field applied across that pixel, and (ii) a serial linear polarizer, such that the attenuation of light passing through a pixel of the polarization mode attenuator is dependent on the degree to which the polarization of light traversing the birefringent polarization rotating element is parallel to the polarization direction of the linear polarizer.
  • a further exemplary implementation may comprise (i) a first optical port, (ii) a wavelength dispersive element in optical communication with the first optical port, such that wavelength components of light received from the first optical port are dispersed, (iii) a pixilated polarization rotation element, having pixels aligned generally to receive the dispersed wavelength components and adapted to rotate the polarization of light passing through each pixel in response to a control signal applied to the pixel, such that the polarization of a wavelength component of the dispersed light is rotated according to the control signal applied to the pixel through which the wavelength component passes, and (iv) a birefringent element disposed and oriented such that light which traverses a pixel of the polarization rotation element is directed in a first or second direction according to its polarization as determined by the control signal applied to the pixel.
  • Fig.1A illustrates schematically a block diagram of the functionality of a fixed Add/Drop ROADM, using a 2 x 1 WSS according to a first preferred embodiment of the present invention
  • Fig. 1 B is a schematic view of a reflective wavelength selective router as used in Fig. 1A, showing the component parts in more detail;
  • Figs. 2A-2C illustrate schematically a method of generating beam deflection using an LC cell in series with a birefringent crystal wedge
  • Figs. 3A-3C illustrate schematically a method of generating beam deviation using an LC cell in series with a .birefringent crystal block
  • Fig. 4 illustrates schematically a method of constructing the LC cell to implement the polarization switching required for the embodiments of Figs 2-3;
  • Fig. 5 illustrates schematically a method of constructing the LC cell to implement the beam attenuation required for the embodiments of Figs 2-3;
  • Fig. 6A-6C illustrate the effect of switching the LC cell ON or OFF on the transmission of a single segment of the embodiment of Fig. 5;
  • Figs 7A-7E illustrate schematically various reflective polarization mode switching embodiments
  • Figs. 8A -8D illustrate schematically more detailed drawings of the circulator configuration of Fig. 7E, showing the polarization changes in the beams as they traverse the switch assembly for four alternative transmission options;
  • Fig. 8E is a truth table showing the transmission paths for the four alternate switch positions shown in Figs. 8A to 8D.
  • Figs 9A-9C illustrate schematically a reflective polarization mode switching embodiment with phase attenuation incorporated.
  • Figs 10A-10E illustrate schematically a reflective polarization mode switching embodiment with polarization attenuation incorporated.
  • Fig. 1 illustrates schematically a block diagram of the functionality of a fixed Add/Drop ROADM, using a 2 x 1 WSS according to a first exemplary implementation of the devices of the present disclosure.
  • the ROADM inputs multi-wavelength light, made up of wavelengths ⁇ i, ⁇ 2 , ⁇ 3 , ... at its input port 10, and is designed to drop predetermined wavelengths at the local drop port 11 , or to add predetermined wavelengths at the local add port 12, and to output the resulting light signal at the output port 13.
  • the core of the device is the optically switched wavelength selective switch 15, which operates as a 2 x 1 switching router.
  • the optically switched wavelength selective switch 15 which operates as a 2 x 1 switching router.
  • the WSS has a through pass for wavelength ⁇ 3 , while it blocks other wavelengths ⁇ 1 ( ⁇ 2 with a high extinction ratio.
  • all of the pass paths, whether the through pass or the add pass, should have variable attenuation capability, to compensate for the different signal intensities arising from the different channels.
  • Fig. 1 B is schematic plan view of a reflective wavelength selective switch, as could be used in Fig. 1A, showing the component parts in more detail.
  • Fig. 1 B shows the plan view layout of a single channel path of the switch
  • the input (or output) beam of each port is input (or output) at the fiber interface block, which preferably comprises a fiber collimator 29 per port, followed by a birefringent walk-off crystal 21 , such as a YVO 4 crystal, preferably having a half wave plate 19 over part of its output face.
  • each channel thus comprises a pair of beams having the same polarization direction, as indicated by the vertical line on each of the beam outputs, and disposed in a predetermined plane, which, in the example shown in Fig. 1 B, is in the plane of the drawing.
  • these beams may then advantageously undergo lateral expansion in that same predetermined plane, in the preferred example shown in Fig. 1 B, by an anamorphic prism pair 23.
  • These laterally expanded beams are passed to a grating 24 for wavelength dispersion, again in the same predetermined plane, which, in the example shown in Fig. 1B, is in the plane of the drawing.
  • the dispersed wavelength components are then directed to the lens 25 for focusing on the beam switching and steering module 26, which comprises a pixelated polarization rotation element 27, and a beam steering device 28, shown in Fig. 1 B as a reflective element, operative to reflect each switched and steered beam back down the switch to the output positions of the birefringent crystal, and from there, after recombination, to the respective output collimator port.
  • This steering is performed in the direction perpendicular to the plane of the drawing and according to one preferred embodiment.
  • the beam steering device may be a MEMS array of mirrors.
  • a similar transmissive embodiment can equally be implemented, in which case the reflective elements 28 are replaced by a transmissive steering element embodiment, with the above mentioned input elements of the device repeated to the right of the beam steering device to deal with the outputting of the transmitted beams.
  • the WSS operates by means of polarization dependent beam deflection to switch the incoming signals.
  • Figs. 2A to 2C illustrate schematically a first advantageous method of achieving this.
  • the drawings show the switching function of a single pixel element of the pixels disposed along the wavelength dispersion direction of the device. It is to be understood that preceding this switching element, each input beam issuing from a fiber collimator is converted into a pair of closely disposed beams having the same predefined polarization direction for transmission through the switch, and through a beam expander, if used, and through the wavelength dispersion element, all as already shown in Fig. 1 B.
  • the switching assembly of Figs. 2A to 2C comprises a Liquid Crystal (LC) cell 20 in series with a birefringent crystal wedge 22.
  • the LC cell changes the polarization direction of light passing through, in accordance with the voltage applied to the LC cell electrodes, and the birefringent crystal wedge deflects the beam in accordance with the polarization of the incident light.
  • the beam can thus be directed according to the voltage applied to the LC pixel through which it is passing.
  • the incident light has a polarization perpendicular to the plane of the drawing.
  • Figs. 3A-3C illustrate schematically another exemplary method of deflecting the beam according to its polarization.
  • This embodiment is similar to that of Figs. 2A-2C, having an input LC cell 30 for switching, except that instead of the wedge 22, a block of birefringent crystal 32 is used.
  • This has the advantage that the beam is displaced laterally, rather than being deflected angularly, such that the beams remain parallel.
  • Fig. 4 in its 3 parts, which together illustrate schematically a method of constructing the LC switching cell, 20 or 30, used to implement the polarization switching shown in Figs. 2A-2C or 3A-3C.
  • the cell should be pixellated along the dispersion direction ⁇ of the WSS, such that different wavelengths (generated by means of a dispersive element, understood to be part of the device, but not shown in the drawings) fall on different pixels of the LC cell, as shown on the right hand side of Fig. 4.
  • the exemplary cell shown in Fig. 4 has a common back electrode 40, marked COM, while the front electrode, marked SEG is divided into pixels or segments 41 along the wavelength dispersion direction, as shown on the left hand side of Fig.
  • each wavelength channel can be separately switched.
  • the LC material lies between these two electrodes.
  • the rubbing axis of the LC is shown aligned at 45 deg. to the direction of input polarization, such that without application of an activating voltage between the electrodes of a particular segment, the input polarization will be rotated by 90 deg., while application of an activating voltage will leave the polarization unaffected.
  • the beam will thus be directed in the direction of either of the output ports of the device, according to whether or not, a voltage is applied to that particular wavelength pixel or segment.
  • Fig. 5 illustrates schematically the structure of an LC cell for implementing a method of attenuating the input beam of each wavelength channel, by use of phase mode manipulation, rather than the polarization mode switching shown in the previous drawings.
  • the common electrode 50 in this embodiment is constructed of a number of separate strips, in the form of a comb.
  • the front electrode, marked SEG is divided into pixels or segments 51 along the wavelength dispersion direction, as shown on the left hand side of Fig. 5, such that each wavelength channel can be separately switched.
  • the LC material lies between these two electrodes. Because of the comb structure of the COM electrode, the height of each segment 51 is divided into a number of separate parts, some of these parts being under the field influence of the common electrode and some without.
  • the attenuation can also be achieved by dividing the common electrode such that each segment 51 has a number of narrow strips, aligned vertically in the drawing direction of Fig. 5. In such an implementation, it is the phase changes occurring across the width of each segment that cause the destruction of the character of the mode of the beam passing through the segment, thus attenuating the beam passing through that segment.
  • phase perturbation generated by use of the embodiment of Fig. 5 there may also be an additional diffraction grating effect which can assist in attenuating the beam passing through a segment. Since the comb spacing is small, being typically from 100 microns down to 20 microns, and since at least the narrower of these spacings approaches the wavelength of the light used, when the grating is activated by application of voltage to the LC segment, this grating effect diffracts the light from its path, thus increasing the attenuation in addition to that due to the phase scrambling effect previously described.
  • Figs. 6A and 6B illustrate the effect of switching the LC cell ON or OFF, on the transmission of a single segment of the phase mode manipulation switch of Fig. 5.
  • Fig. 6A When the cell is OFF, there is no phase difference generated within the beam segment, and hence no attenuation, as shown in Fig. 6A.
  • Fig. 6B When the cell is ON, phase differences are generated across the height of the beam segment, and hence the beam is attenuated, as shown in Fig. 6B.
  • Fig. 6C shows a close up section of Fig. 6B, showing the periodic variation between two refractive indices ni and n 2 across the height of the segment. In neither of these situations is there any polarization change in the traversing light.
  • Figs. 7A to 7E illustrate schematically according to a further example of the implementation of the present invention, a switch mechanism similar to the transmissive polarization mode switching shown in Figs. 2A-2C, but using a reflective arrangement instead.
  • the birefringent wedge 70 has a reflective surface 71 at its face opposite to the direction of impingement of the incident light. Both before impingement on the wedge, and after reflection from the wedge, the light passes through the LC cell 74.
  • the light traversing the birefringent wedge 70 undergoes a deviation such that after reflection from the wedge, it returns at an angle ⁇ i to its incident direction. It then traverses the LC cell again, where its polarization is again rotated 90 deg., back to s-polarization, such that it is output with the same polarization as that with which it was input, but in a different propagation direction.
  • Fig. 7B there is shown the effect on the incident light when the LC cell is ON. In such a case, there is no polarization rotation, and the beam that traverses the birefringent wedge maintains the incident s-polarization. On passage through the wedge, it is diverted a different angle ⁇ 2 after reflection from the rear mirror 71. This difference in deflection angle is operative in enabling the switching of the beam to different output ports.
  • Fig. 7C there is shown the situation when the LC cell is partly switched, resulting in circularly polarized output light. This will result in a higher Insertion Loss than for the ON or OFF situations, because of the higher PDL (polarization dependent losses).
  • a birefringent wedge 70 in order to provide the change in direction imparted to the beam according to its polarization.
  • a wedge has been illustrated in Figs. 2A to 2C.
  • the implementation of the switch shown in Figs. 7A-7C requires three collimators at the input/output ports of the switch, as shown in Fig. 7D - one 75 to input the beam, and one each to output the beam at the two output ports 76, 77, corresponding respectively to the switched diversion angles ⁇ 2 and ⁇ 1 shown in Figs. 7B and 7A respectively.
  • the reflective birefringent wedge can be aligned at such an angle that the p-polarization beam is returned along its incident path, as shown in the configuration of Fig. 7E.
  • the . s-polarized beam will be returned along a path having a different angle of reflection.
  • only two collimators are required 77, 79, but a circulator 78 is necessary to separate the reflected output light from the input light.
  • Figs. 8A to 8 E schematically illustrate these features.
  • Figs. 8A to 8D show more detailed drawings of the circulator configuration of Fig. 7E, showing the polarization changes in the beams as they traverse the switch assembly for the four alternative transmission options.
  • Fig. 7E is explained in terms of a 1 x 2 switch
  • Figs. 8A to 8D illustrate a 2 x 1 configuration.
  • Fig. 8E is a truth table showing the routing of the various switch situations as a function of the LC cell status.
  • Fig. 8A shows the switching geometry of the circulator implementation of the switch.
  • IN 1 and IN 2 input ports
  • OUT output port
  • the input to port IN 2 is shown dashed as it is not active in the case of Fig. 8A.
  • a signal input via IN 1 traverses the input collimator COL 1 , followed by polarization separation, optional beam expansion and wavelength dispersion (none of which are shown in the figures), and is incident on the birefringent switching element 81 , which covers all of the input and output beam paths.
  • the switching element may advantageously be an LC cell.
  • the transmitted beam acquires an s-polarization, which is refracted by the birefringent wedge 82 in such a direction that it is incident on the reflecting surface 83 of the wedge normally and is returned back along its input path, for the circulator 85 and exits the switch at the OUT port.
  • Figs. 8A to 8D the retardation, ⁇ /2 or 0, generated by the birefringent switching element 81 is noted at the bottom edge of the element in the drawing, and the resultant polarization, p- or s-, is noted next to the beam transmitted through the birefringent switching element 81.
  • the inclination angles of the birefringent wedge 82 and the birefringent switching element 81 are shown in an exaggerated manner in the drawings of Figs. 8A to 8D, in order to clearly illustrate the way in which the different polarization beams are deflected in the birefringent wedge. In practice, the angle of inclination should be much smaller, typically of the order of 15° for a wedge having an apex angle of the order of 8°.
  • the switch status is switched over by setting the LC cell voltage such that no retardation is applied to the beam, and the beam input to port IN 1 is transmitted to the wedge with p-polarization.
  • the birefringence of the wedge is such as to refract this p-polarization at a different angle to that at which the s- polarization was refracted, and the relative alignment of the wedge is such that the beam is reflected back and exits the switch assembly towards the port IN 2.
  • this 2 x 1 switch configuration it is desired that input signals be directed from either input port only to the OUT port, it is necessary to prevent this signal from being output at port IN 2, and this is achieved by use of an isolator 86 in the beam path to port IN 2.
  • a signal input at port IN 2 passes though the isolator 86 in its forward, low insertion loss direction, is converted to p- polarization by the LC cell 81 , and then retraces the path of the beam of Fig. 8B in the opposite direction, until it is directed by the circulator 85 to the desired output port OUT.
  • LC cell setting is switched over to provide a retardation of ⁇ /2, such that the beam input from port IN 2 now enters the wedge with s- polarization, and is refracted in such a direction that it does not enter the beam path towards the circulator and port IN 1 (shown dashed in Fig. 8D), but is absorbed and lost somewhere in the walls or floor of the switch structure.
  • the switch assembly thus behaves as a 2 x 1 switch, as shown in the situations described in Figs. 8A and 8C, with the input at either of ports IN 1 and IN 2 being alternately switchable to the port OUT, depending on the switching state of the LC cell pixel through which those signals pass.
  • the LC cell is driven such that the retardation is somewhere between 0 and ⁇ /2, the circular or elliptical polarization resulting will be such that an input signal input at one port will be directed to both of the other two ports.
  • the need for the isolator is limited to the 2 x 1 switching situation. When the same switch structure is used in a 1 x 2 configuration, the additional isolator is not required, as explained in Fig. 8B above.
  • Figs. 9A to 9C illustrate schematically, according to a further preferred embodiment of the present invention, a polarization mode LC switch like that of Figs. 7A to 7E, but including a phase mode attenuator LC cell, like either of those of Figs. 5 and 6. It is generally necessary to be able to vary the attenuation passing through each port in order to equalize the optical throughput in the various ports.
  • Fig. 9A there is shown the birefringent reflective wedge 90, 91, with a switching LC cell 92.
  • a phase mode LC attenuator cell 94 is disposed in the beam paths, such that the switch is provided with attenuation for each port.
  • LC switching cell 92 and the LC attenuation cell 94 cannot be spatially coincident, there is a small bandwidth penalty in this kind of embodiment, since the pixel width subtended at the attenuator LC cell 94 by the focused beam on the surface of the mirror 91 , is larger than the pixel size of the switching LC 92, as the attenuating LC is further from the mirror than the switching LC.
  • the switch situations shown in Figs. 9A to 9C are essentially identical to those already shown in Figs. 7A to 7C, with the addition of the phase mode LC attenuator cell 94 disposed in the beam path, in order to be able to control the attenuation of signals passing through the switch.
  • the identity of the input and output polarization for a polarization-mode LC switch is important in this embodiment, since for the phase mode attenuator to operate correctly, this polarization must be the same. Furthermore, since the input and output polarizations are the same, a high PDL, high efficiency grating can be used in the system.
  • Figs. 10A to 10E illustrate schematically, according to a further preferred embodiment of the present invention, a polarization mode LC switch like that of Figs. 7A to 7E, but including a polarization mode attenuator LC cell, which comprises an LC cell 102 in combination with a polarizer plate 104.
  • Figs. 10D and 10E The operation of this embodiment is shown in Figs. 10D and 10E, where the various polarization changes through the device are shown.
  • the beam Moving from the input port at the left hand side of the drawings towards the birefringent wedge 108, the beam encounters the attenuator LC cell 102, which has its rubbing direction at 45 deg. to the direction of the input polarization, such that without application of an activating voltage between the electrodes of a particular segment, marked OFF as shown in Fig. 1OE, the input polarization will be rotated by 90 deg.
  • Application of an activating voltage, marked ON as shown in Fig. 1OD will leave the polarization unaffected.
  • the polarizer plate 104 has its direction of polarization parallel to the input polarization of the beam, so that when the LC cell is ON, as in Fig. 10D, the polarized beam, having an unchanged polarization direction, passes through attenuated only by the insertion loss. After this stage, the beam passes to the switching LC cell 106, and on to the reflective birefringent wedge 108 from which it is reflected at its characteristic angle, at least for the P-polarization shown in the drawing. If, on the other hand, the LC cell is OFF, as in Fig.
  • the polarization is rotated by 90 deg., such that it is now crossed with the polarizer plate polarization direction 104, and is thus attenuated by the polarizer extinction ratio, and is marked in the drawing as "o".
  • the signal is attenuated accordingly in its passage through the switch.
  • the polarization of the output beam is the same as that of the input beam, regardless of the ON/OFF switching status of the LC, such that the switch is polarization independent. This feature is desirable in optical switch technology, as it simplifies the design of the system in which the switches are used.

Abstract

A polarization independent switch, using polarization diversity for converting the input beams to a single defined polarization direction, followed by wavelength dispersion to spread the individual wavelength channels over a pixilated switching device. This may be a polarization rotation element whose setting can be controlled by means of an applied electronic signal. This may either leave the polarization direction unchanged, or it may rotate it such that it is essentially orthogonal to the polarization of the input beam exiting the polarization diversity components. The beam then proceeds to a birefractive wedge element which refracts light having the two orthogonal polarizations to different extents, thus separating the beams according to the control signal applied to the polarization rotation element through which each wavelength component of the beam passed. The beams may thus be directed to different ports according to the control signal setting. 2 x 1 switch configurations are shown.

Description

LIQUID CRYSTAL WAVELENGTH SELECTIVE ROUTER
FIELD OF THE INVENTION
The present invention relates to the field of fast optical switches, whose operation is wavelength dependent, especially for use in reconfigurable optical add-drop multiplexer (ROADM) applications.
BACKGROUND OF THE INVENTION
It is known in the field of optical communications to use optical wavelengths as optical carriers for carrying digital or analog information. Also, the different wavelengths may be used to discriminate one set or channel of information from another. When a plurality of wavelengths are coupled or multiplexed onto a single fiber, this is called wavelength division multiplexing (WDM). Use of such WDM increases the overall bandwidth of the system.
There is a need in such systems to switch packets of optical information passing along one fiber to any of a number of other fibers, according to the wavelength of the optical signal. Such a switch is known as an optical router. A number of wavelength dependent switches and routers exist in the prior art. In co- pending US Patent Applications Serial Nos. 10/492,484, 10/580,832, 11/911 ,047 and 12/066,249, all hereby incorporated by reference, each in its entirety, there are disclosed various wavelength selective switches and routers, wherein an input optical signal is polarization-split into two preferably perpendicular planes, and one of the polarization components rotated such that both polarization components are aligned in one direction, followed by spatial wavelength dispersal. The polarization diversity may be performed by a polarized beam splitter, or a birefringent walk-off crystal, and the wavelength dispersion may be performed by a diffraction grating. A polarization rotation device, such as a liquid crystal polarization modulator, pixelated along the wavelength dispersive direction such that each pixel operates on a separate wavelength channel, is operative to rotate the polarization of the light signal passing through each pixel, according to the control voltage applied to the pixel. The polarization modulated signals are then wavelength-recombined and polarization-recombined by means of similar dispersion and polarization combining components to those that were used to respectively disperse and split the input signals. At the output polarization recombiner, the direction in which the resulting output signal is directed is determined by whether the polarization of the particular wavelength channel was rotated by the polarization modulator pixel, or not.
The disclosures of each of the publications mentioned in this section and in other sections of the specification, are hereby incorporated by reference, each in its entirety.
SUMMARY OF THE INVENTION
The present invention seeks to provide new fiber-optical, wavelength selective switch structures, such as may be used for channel routing or blocking applications in optical communication and information transmission systems. The devices are designed as a 2 x 1 switch, and use liquid crystal elements for switching. The addition of a multiplexer and a demultiplexer enables such a basic 2 x 1 structure to be advantageously used as the core of a reconfigurable optical add-drop multiplexer (ROADM), with add and drop functionality, from and to a number of ports. The switch uses a minimum of components, and can thus be economically constructed for large scale use in such systems. The switch structure can also incorporate either a phase mode or a polarization mode LC attenuator cell, as a variable optical attenuator for any of the transfer paths through the device. Although the switch structure described in this application relates to a 2 x 1 structure, it is to be understood that according to the optical reciprocity principle, the described switches can be operated equally well as 1 x 2 switches.
The switch uses polarization diversity for converting the input beams to a single defined polarization direction, followed by wavelength dispersion to spread the individual wavelength channels over the pixilated switching device. This may be a polarization rotation element whose setting can be controlled by means of an applied electronic signal. This may either leave the polarization direction unchanged, or it may rotate it such that it is essentially orthogonal to the polarization of the input beam exiting the polarization diversity components. The beam then proceeds to a birefractive wedge element which refracts light having the two orthogonal polarizations to different extents, thus separating the beams according to the control signal applied to the polarization rotation element through which each wavelength component of the beam passed. The beams may thus be directed to different ports according to the control signal setting.
According to another implementation, the beams can be directed through a pixelated phase changing element, which controls the transmission of each component of the beam by affecting the mode structure of the beam across the pixel thereby controlling its ability to couple readily into the output fiber.
There is thus provided in accordance with an exemplary implementation of the devices described in this disclosure, a wavelength selective switch comprising: (i) a first optical port,
(ii) a polarization conversion element for endowing light input through the first optical port with a predetermined polarization direction,
(ii) a wavelength dispersive element in optical communication with the polarization conversion element, such that wavelength components of light received from the first optical port are dispersed in a dispersion plane,
(iv) a pixilated polarization rotation element, optically coupled to receive the dispersed light, and having pixels aligned generally in the dispersion plane and adapted to rotate the polarization of light passing through each pixel according to a control signal applied to the pixel, such that the polarization of a wavelength component of the dispersed light is rotated according to the control signal applied to the pixel through which the wavelength component passes, and (v) a reflective birefringent element in optical communication with light from the polarization rotation element, and disposed and oriented such that light which traverses a pixel of the polarization rotation element is reflected in a first or second direction according to the polarization of the received light, the elements being further orientated such that light from the reflective birefringent element reenters the polarization conversion element which reconstitutes the light to its original polarization.
In such a switch, the light may be reflected in a first direction when the control signal applied to the pixel is such as to generate no change in polarization of the optical beam, and in a second direction when the control signal applied to the pixel is such as to generate an essentially 90 degree rotation in the polarization of the optical beam. In this arrangement, the first direction may lead to a second optical port, and the second direction may lead to a third optical port, such that the first optical port is connected optically to either of the second and third ports in accordance with the control signal.
Other implementations may further involve a switch as described above, in which the reflective birefringent element is aligned such that the light reflected in one of the directions returns collinearly with the optical path from the first optical port, the switch further comprising a circulator to separate light reflected in the one direction from light incident from the direction of the first optical port. In such an implementation, the switch should further comprise an isolator disposed in the optical path in the second direction, the isolator being aligned such that light directed in the second direction cannot enter an optical port disposed in the second direction.
In any of the above described implementations, the pixelated polarization rotation element may be a pixelated liquid crystal cell, in which case the control signal may be a voltage applied to electrodes across pixels of the liquid crystal cell.
Additional implementations may involve a wavelength selective switch according to any of the previously described designs, in which the reflective birefringent element is in the form of a wedge, such that the directions are angularly distinguished directions, or in the form of a block, such that the directions are laterally distinguished directions.
Furthermore, any such switches may be such as to direct light input to the first port, to either of the second and third ports or to direct light input to either of the second and third ports to the first port.
Additionally, alternative implementations of any of the above-described switches may further comprise a pixelated phase changing element, which controls the transmission of light passing through a pixel of the polarization rotation element by spatially varying the phase across light traversing the pixel, thereby controlling its ability to, couple into an output port. In such a switch, the mode structure of the beam of the light is degraded by the voltage applied to the pixel of the phase changing element. The pixelated phase changing element may comprise a comb-like electrode structure which applies a spatially undulating electric field across the pixel of the phase changing element, such that the phase of light passing therethrough undergoes corresponding spatially alternating changes. In either case, the degrading of the mode structure of the beam of the light controls its ability to couple into an output port fiber.
Any of the wavelength selective switches described may further comprise a polarization mode attenuator, which controls the transmission of light passing therethrough, the attenuator comprising:
(i) a pixelated birefringent polarization rotating element which rotates the polarization direction of light passing through a pixel thereof in accordance with the electric field applied across that pixel, and (ii) a serial linear polarizer, such that the attenuation of light passing through a pixel of the polarization mode attenuator is dependent on the degree to which the polarization of light traversing the birefringent polarization rotating element is parallel to the polarization direction of the linear polarizer.
Finally, a further exemplary implementation may comprise (i) a first optical port, (ii) a wavelength dispersive element in optical communication with the first optical port, such that wavelength components of light received from the first optical port are dispersed, (iii) a pixilated polarization rotation element, having pixels aligned generally to receive the dispersed wavelength components and adapted to rotate the polarization of light passing through each pixel in response to a control signal applied to the pixel, such that the polarization of a wavelength component of the dispersed light is rotated according to the control signal applied to the pixel through which the wavelength component passes, and (iv) a birefringent element disposed and oriented such that light which traverses a pixel of the polarization rotation element is directed in a first or second direction according to its polarization as determined by the control signal applied to the pixel.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:
Fig.1A illustrates schematically a block diagram of the functionality of a fixed Add/Drop ROADM, using a 2 x 1 WSS according to a first preferred embodiment of the present invention; Fig. 1 B is a schematic view of a reflective wavelength selective router as used in Fig. 1A, showing the component parts in more detail;
Figs. 2A-2C illustrate schematically a method of generating beam deflection using an LC cell in series with a birefringent crystal wedge;
Figs. 3A-3C illustrate schematically a method of generating beam deviation using an LC cell in series with a .birefringent crystal block;
Fig. 4 illustrates schematically a method of constructing the LC cell to implement the polarization switching required for the embodiments of Figs 2-3;
Fig. 5 illustrates schematically a method of constructing the LC cell to implement the beam attenuation required for the embodiments of Figs 2-3;
Fig. 6A-6C illustrate the effect of switching the LC cell ON or OFF on the transmission of a single segment of the embodiment of Fig. 5;
Figs 7A-7E illustrate schematically various reflective polarization mode switching embodiments;
Figs. 8A -8D illustrate schematically more detailed drawings of the circulator configuration of Fig. 7E, showing the polarization changes in the beams as they traverse the switch assembly for four alternative transmission options;
Fig. 8E is a truth table showing the transmission paths for the four alternate switch positions shown in Figs. 8A to 8D.
Figs 9A-9C illustrate schematically a reflective polarization mode switching embodiment with phase attenuation incorporated; and
Figs 10A-10E illustrate schematically a reflective polarization mode switching embodiment with polarization attenuation incorporated.
DETAILED DESCRIPTION
Reference is now made to Fig. 1 , which illustrates schematically a block diagram of the functionality of a fixed Add/Drop ROADM, using a 2 x 1 WSS according to a first exemplary implementation of the devices of the present disclosure. The ROADM inputs multi-wavelength light, made up of wavelengths λi, λ2, λ3, ... at its input port 10, and is designed to drop predetermined wavelengths at the local drop port 11 , or to add predetermined wavelengths at the local add port 12, and to output the resulting light signal at the output port 13. The core of the device is the optically switched wavelength selective switch 15, which operates as a 2 x 1 switching router. In the embodiment shown in Fig. 1 , the WSS has a through pass for wavelength λ3, while it blocks other wavelengths λ1 ( λ2 with a high extinction ratio. In addition, all of the pass paths, whether the through pass or the add pass, should have variable attenuation capability, to compensate for the different signal intensities arising from the different channels.
Reference is now made to Fig. 1 B, which is schematic plan view of a reflective wavelength selective switch, as could be used in Fig. 1A, showing the component parts in more detail. Fig. 1 B shows the plan view layout of a single channel path of the switch The input (or output) beam of each port is input (or output) at the fiber interface block, which preferably comprises a fiber collimator 29 per port, followed by a birefringent walk-off crystal 21 , such as a YVO4 crystal, preferably having a half wave plate 19 over part of its output face. The output of each channel thus comprises a pair of beams having the same polarization direction, as indicated by the vertical line on each of the beam outputs, and disposed in a predetermined plane, which, in the example shown in Fig. 1 B, is in the plane of the drawing. After this polarization decomposition and conversion, these beams may then advantageously undergo lateral expansion in that same predetermined plane, in the preferred example shown in Fig. 1 B, by an anamorphic prism pair 23. These laterally expanded beams are passed to a grating 24 for wavelength dispersion, again in the same predetermined plane, which, in the example shown in Fig. 1B, is in the plane of the drawing. The dispersed wavelength components are then directed to the lens 25 for focusing on the beam switching and steering module 26, which comprises a pixelated polarization rotation element 27, and a beam steering device 28, shown in Fig. 1 B as a reflective element, operative to reflect each switched and steered beam back down the switch to the output positions of the birefringent crystal, and from there, after recombination, to the respective output collimator port. This steering is performed in the direction perpendicular to the plane of the drawing and according to one preferred embodiment. The beam steering device may be a MEMS array of mirrors.
A similar transmissive embodiment can equally be implemented, in which case the reflective elements 28 are replaced by a transmissive steering element embodiment, with the above mentioned input elements of the device repeated to the right of the beam steering device to deal with the outputting of the transmitted beams.
The WSS operates by means of polarization dependent beam deflection to switch the incoming signals. Reference is now made to Figs. 2A to 2C which illustrate schematically a first advantageous method of achieving this. The drawings show the switching function of a single pixel element of the pixels disposed along the wavelength dispersion direction of the device. It is to be understood that preceding this switching element, each input beam issuing from a fiber collimator is converted into a pair of closely disposed beams having the same predefined polarization direction for transmission through the switch, and through a beam expander, if used, and through the wavelength dispersion element, all as already shown in Fig. 1 B.
The switching assembly of Figs. 2A to 2C comprises a Liquid Crystal (LC) cell 20 in series with a birefringent crystal wedge 22. The LC cell changes the polarization direction of light passing through, in accordance with the voltage applied to the LC cell electrodes, and the birefringent crystal wedge deflects the beam in accordance with the polarization of the incident light. The beam can thus be directed according to the voltage applied to the LC pixel through which it is passing. In the exemplary device shown in Figs. 2A-2C, the incident light has a polarization perpendicular to the plane of the drawing. In Fig. 2A, with the LC voltage applied to fully activate the cell ON, there is no change in the polarization direction of the traversing light, and the beam passes through the wedge undeviated. In Fig. 2B, the LC is turned OFF, and the polarization is rotated through essentially 90 degrees, such that it is now in the plane of the drawing, and is thus deflected by the birefringence of the wedge, as shown. In Fig. 2C, an intermediate voltage is applied to the LC pixel, which rotates the polarization by 45 degrees, thus generating circular polarization, such that each perpendicular component of this circular polarization is directed by the wedge in a direction depending on the polarization of that component. Since the two components diverge in free space, the next optical component in the signal processing chain of the switch can be positioned at such a distance from the wedge that the angular deviation provides spatial separation of the components sufficient that each can be handled separately. This implementation has an advantage that the wedge can be thin, thus saving material costs. Reference is now made to Figs. 3A-3C which illustrate schematically another exemplary method of deflecting the beam according to its polarization. This embodiment is similar to that of Figs. 2A-2C, having an input LC cell 30 for switching, except that instead of the wedge 22, a block of birefringent crystal 32 is used. This has the advantage that the beam is displaced laterally, rather than being deflected angularly, such that the beams remain parallel. However, it has the disadvantage that in order to provide sufficient spatial separation of the two polarization components, it requires a longer block of material than the wedge embodiments of Figs. 2A-2C. The configurations of the three states shown in Figs. 3A to 3C are equivalent to those of Figs. 2A to 2C.
Reference is now made to Fig. 4 in its 3 parts, which together illustrate schematically a method of constructing the LC switching cell, 20 or 30, used to implement the polarization switching shown in Figs. 2A-2C or 3A-3C. The cell should be pixellated along the dispersion direction λ of the WSS, such that different wavelengths (generated by means of a dispersive element, understood to be part of the device, but not shown in the drawings) fall on different pixels of the LC cell, as shown on the right hand side of Fig. 4. The exemplary cell shown in Fig. 4 has a common back electrode 40, marked COM, while the front electrode, marked SEG is divided into pixels or segments 41 along the wavelength dispersion direction, as shown on the left hand side of Fig. 4, such that each wavelength channel can be separately switched. The LC material lies between these two electrodes. The rubbing axis of the LC is shown aligned at 45 deg. to the direction of input polarization, such that without application of an activating voltage between the electrodes of a particular segment, the input polarization will be rotated by 90 deg., while application of an activating voltage will leave the polarization unaffected. According to either of the switching assembly embodiments of Figs. 2A-2C and 3A-3C, the beam will thus be directed in the direction of either of the output ports of the device, according to whether or not, a voltage is applied to that particular wavelength pixel or segment.
Reference is now made to Fig. 5 in all its parts, which illustrates schematically the structure of an LC cell for implementing a method of attenuating the input beam of each wavelength channel, by use of phase mode manipulation, rather than the polarization mode switching shown in the previous drawings. The common electrode 50 in this embodiment is constructed of a number of separate strips, in the form of a comb. The front electrode, marked SEG is divided into pixels or segments 51 along the wavelength dispersion direction, as shown on the left hand side of Fig. 5, such that each wavelength channel can be separately switched. The LC material lies between these two electrodes. Because of the comb structure of the COM electrode, the height of each segment 51 is divided into a number of separate parts, some of these parts being under the field influence of the common electrode and some without. As a result, on application of a voltage between the COM and the SEG electrodes, different parts of the height of each pixel are subject to a different applied electric field across the thickness of the pixel. Since the rubbing direction of the cell for this implementation, is parallel to the input polarization direction, the polarization of the light is unaffected by the applied field in passage through the LC. On the other hand, application of a voltage across the cell will change the phase shift of the light passing through. The common comb electrode COM is held at ground potential. When no voltage is applied to a particular segment electrode, there will be no variation in the field between the common electrode and that segment electrode, resulting in uniform transmission of the light through that segment. On the other hand, if a voltage is applied to the segment electrode, then the field across the height of the segment varies periodically between successive "teeth" of the comb, thus periodically changing the refractive index of the liquid crystal material in the corresponding regions between the two electrodes, COM and SEG. This periodic variation in refractive index generates a periodic change across the height of the segment, in the phase of the light transmitted through the segment. This destroys the character of the mode of the beam, such that it cannot couple out in the same way as a uniform mode beam. The beam traversing that segment is therefore attenuated. Therefore, the beam traversing a segment is transmitted or blocked according to whether or not a voltage is applied to the electrode of that segment.
As an alternative to the horizontal comb structure applied to the common electrode 50, according to other implementations of these switches, the attenuation can also be achieved by dividing the common electrode such that each segment 51 has a number of narrow strips, aligned vertically in the drawing direction of Fig. 5. In such an implementation, it is the phase changes occurring across the width of each segment that cause the destruction of the character of the mode of the beam passing through the segment, thus attenuating the beam passing through that segment.
In addition to the phase perturbation generated by use of the embodiment of Fig. 5, there may also be an additional diffraction grating effect which can assist in attenuating the beam passing through a segment. Since the comb spacing is small, being typically from 100 microns down to 20 microns, and since at least the narrower of these spacings approaches the wavelength of the light used, when the grating is activated by application of voltage to the LC segment, this grating effect diffracts the light from its path, thus increasing the attenuation in addition to that due to the phase scrambling effect previously described.
Reference is now made to Figs. 6A and 6B which illustrate the effect of switching the LC cell ON or OFF, on the transmission of a single segment of the phase mode manipulation switch of Fig. 5. When the cell is OFF, there is no phase difference generated within the beam segment, and hence no attenuation, as shown in Fig. 6A. When the cell is ON, phase differences are generated across the height of the beam segment, and hence the beam is attenuated, as shown in Fig. 6B. Fig. 6C shows a close up section of Fig. 6B, showing the periodic variation between two refractive indices ni and n2 across the height of the segment. In neither of these situations is there any polarization change in the traversing light.
Reference is now made to Figs. 7A to 7E which illustrate schematically according to a further example of the implementation of the present invention, a switch mechanism similar to the transmissive polarization mode switching shown in Figs. 2A-2C, but using a reflective arrangement instead. The birefringent wedge 70 has a reflective surface 71 at its face opposite to the direction of impingement of the incident light. Both before impingement on the wedge, and after reflection from the wedge, the light passes through the LC cell 74. Although in Figs. 7A to 7E, for the sake of clarity, the LC cell 74 is shown at a distance from the birefringent reflective wedge 70, 71 , in practice these two components should be close together to ensure that the reflected beam passes through the same pixel as for its incident path. This same comment is also valid for Figs. 9A to 9C and 10A to 10E. For the exemplary implementation shown in Figs. 7A to 7E, when the cell is OFF, as in Fig. 7A, the polarization of the incident light is rotated 90 deg., from s-polarization to p-polarization. For the p-polarization, the light traversing the birefringent wedge 70 undergoes a deviation such that after reflection from the wedge, it returns at an angle θi to its incident direction. It then traverses the LC cell again, where its polarization is again rotated 90 deg., back to s-polarization, such that it is output with the same polarization as that with which it was input, but in a different propagation direction. In Fig. 7B, there is shown the effect on the incident light when the LC cell is ON. In such a case, there is no polarization rotation, and the beam that traverses the birefringent wedge maintains the incident s-polarization. On passage through the wedge, it is diverted a different angle Θ2 after reflection from the rear mirror 71. This difference in deflection angle is operative in enabling the switching of the beam to different output ports.
In Fig. 7C, there is shown the situation when the LC cell is partly switched, resulting in circularly polarized output light. This will result in a higher Insertion Loss than for the ON or OFF situations, because of the higher PDL (polarization dependent losses).
The implementations shown in Figs. 7A to 7C (and in 7D to 7E below) utilize a birefringent wedge 70 in order to provide the change in direction imparted to the beam according to its polarization. Such a wedge has been illustrated in Figs. 2A to 2C. However, it is to be understood that it is also feasible to generate the beam direction change according to its polarization using a birefringent block, as in the embodiments of Figs. 3A to 3C. In this case, instead of a polarization dependent angular deflection, the beam is given a polarization dependent lateral displacement.
The implementation of the switch shown in Figs. 7A-7C requires three collimators at the input/output ports of the switch, as shown in Fig. 7D - one 75 to input the beam, and one each to output the beam at the two output ports 76, 77, corresponding respectively to the switched diversion angles Θ2 and Θ1 shown in Figs. 7B and 7A respectively.
According to another example of these reflective polarization mode switches, the reflective birefringent wedge can be aligned at such an angle that the p-polarization beam is returned along its incident path, as shown in the configuration of Fig. 7E. The . s-polarized beam will be returned along a path having a different angle of reflection. In such an arrangement, only two collimators are required 77, 79, but a circulator 78 is necessary to separate the reflected output light from the input light.
However, the use of such a circulator in a 2 x 1 switch configuration, although it saves the need for one collimator, leads to other consequences in the construction of the switch, in the form of the need for an isolator in the port without the circulator. Figs. 8A to 8 E schematically illustrate these features. Figs. 8A to 8D show more detailed drawings of the circulator configuration of Fig. 7E, showing the polarization changes in the beams as they traverse the switch assembly for the four alternative transmission options. However, while Fig. 7E is explained in terms of a 1 x 2 switch, Figs. 8A to 8D illustrate a 2 x 1 configuration. Fig. 8E is a truth table showing the routing of the various switch situations as a function of the LC cell status.
Reference is now made to Fig. 8A, which shows the switching geometry of the circulator implementation of the switch. There are two input ports, labeled IN 1 and IN 2 and a single output port, labeled OUT. The input to port IN 2 is shown dashed as it is not active in the case of Fig. 8A. A signal input via IN 1 , traverses the input collimator COL 1 , followed by polarization separation, optional beam expansion and wavelength dispersion (none of which are shown in the figures), and is incident on the birefringent switching element 81 , which covers all of the input and output beam paths. The switching element may advantageously be an LC cell. When the cell voltage is set such that a retardation of λ/2 is applied to the beam, the transmitted beam acquires an s-polarization, which is refracted by the birefringent wedge 82 in such a direction that it is incident on the reflecting surface 83 of the wedge normally and is returned back along its input path, for the circulator 85 and exits the switch at the OUT port.
In all of Figs. 8A to 8D, the retardation, λ/2 or 0, generated by the birefringent switching element 81 is noted at the bottom edge of the element in the drawing, and the resultant polarization, p- or s-, is noted next to the beam transmitted through the birefringent switching element 81. It should also be noted that the inclination angles of the birefringent wedge 82 and the birefringent switching element 81 are shown in an exaggerated manner in the drawings of Figs. 8A to 8D, in order to clearly illustrate the way in which the different polarization beams are deflected in the birefringent wedge. In practice, the angle of inclination should be much smaller, typically of the order of 15° for a wedge having an apex angle of the order of 8°.
In Fig. 8B, the switch status is switched over by setting the LC cell voltage such that no retardation is applied to the beam, and the beam input to port IN 1 is transmitted to the wedge with p-polarization. The birefringence of the wedge is such as to refract this p-polarization at a different angle to that at which the s- polarization was refracted, and the relative alignment of the wedge is such that the beam is reflected back and exits the switch assembly towards the port IN 2. However, since in this 2 x 1 switch configuration, it is desired that input signals be directed from either input port only to the OUT port, it is necessary to prevent this signal from being output at port IN 2, and this is achieved by use of an isolator 86 in the beam path to port IN 2.
In Fig. 8C, for the same switch setting, a signal input at port IN 2 passes though the isolator 86 in its forward, low insertion loss direction, is converted to p- polarization by the LC cell 81 , and then retraces the path of the beam of Fig. 8B in the opposite direction, until it is directed by the circulator 85 to the desired output port OUT.
In Fig. 8D, the LC cell setting is switched over to provide a retardation of λ/2, such that the beam input from port IN 2 now enters the wedge with s- polarization, and is refracted in such a direction that it does not enter the beam path towards the circulator and port IN 1 (shown dashed in Fig. 8D), but is absorbed and lost somewhere in the walls or floor of the switch structure.
The above four alternate switch positions are summarized by the truth table shown in Fig. 8E. As is seen, the switch assembly thus behaves as a 2 x 1 switch, as shown in the situations described in Figs. 8A and 8C, with the input at either of ports IN 1 and IN 2 being alternately switchable to the port OUT, depending on the switching state of the LC cell pixel through which those signals pass..
It should also be understood that if the LC cell is driven such that the retardation is somewhere between 0 and λ/2, the circular or elliptical polarization resulting will be such that an input signal input at one port will be directed to both of the other two ports. Furthermore, it should be noted that the need for the isolator is limited to the 2 x 1 switching situation. When the same switch structure is used in a 1 x 2 configuration, the additional isolator is not required, as explained in Fig. 8B above.
Reference is now made to Figs. 9A to 9C which illustrate schematically, according to a further preferred embodiment of the present invention, a polarization mode LC switch like that of Figs. 7A to 7E, but including a phase mode attenuator LC cell, like either of those of Figs. 5 and 6. It is generally necessary to be able to vary the attenuation passing through each port in order to equalize the optical throughput in the various ports. In Fig. 9A, there is shown the birefringent reflective wedge 90, 91, with a switching LC cell 92. In addition, a phase mode LC attenuator cell 94 is disposed in the beam paths, such that the switch is provided with attenuation for each port. Since the LC switching cell 92 and the LC attenuation cell 94 cannot be spatially coincident, there is a small bandwidth penalty in this kind of embodiment, since the pixel width subtended at the attenuator LC cell 94 by the focused beam on the surface of the mirror 91 , is larger than the pixel size of the switching LC 92, as the attenuating LC is further from the mirror than the switching LC. The switch situations shown in Figs. 9A to 9C are essentially identical to those already shown in Figs. 7A to 7C, with the addition of the phase mode LC attenuator cell 94 disposed in the beam path, in order to be able to control the attenuation of signals passing through the switch.
It should be noted that the identity of the input and output polarization for a polarization-mode LC switch is important in this embodiment, since for the phase mode attenuator to operate correctly, this polarization must be the same. Furthermore, since the input and output polarizations are the same, a high PDL, high efficiency grating can be used in the system.
Reference is now made to Figs. 10A to 10E which illustrate schematically, according to a further preferred embodiment of the present invention, a polarization mode LC switch like that of Figs. 7A to 7E, but including a polarization mode attenuator LC cell, which comprises an LC cell 102 in combination with a polarizer plate 104.
The operation of this embodiment is shown in Figs. 10D and 10E, where the various polarization changes through the device are shown. Moving from the input port at the left hand side of the drawings towards the birefringent wedge 108, the beam encounters the attenuator LC cell 102, which has its rubbing direction at 45 deg. to the direction of the input polarization, such that without application of an activating voltage between the electrodes of a particular segment, marked OFF as shown in Fig. 1OE, the input polarization will be rotated by 90 deg. Application of an activating voltage, marked ON as shown in Fig. 1OD, will leave the polarization unaffected. The polarizer plate 104 has its direction of polarization parallel to the input polarization of the beam, so that when the LC cell is ON, as in Fig. 10D, the polarized beam, having an unchanged polarization direction, passes through attenuated only by the insertion loss. After this stage, the beam passes to the switching LC cell 106, and on to the reflective birefringent wedge 108 from which it is reflected at its characteristic angle, at least for the P-polarization shown in the drawing. If, on the other hand, the LC cell is OFF, as in Fig. 10E, the polarization is rotated by 90 deg., such that it is now crossed with the polarizer plate polarization direction 104, and is thus attenuated by the polarizer extinction ratio, and is marked in the drawing as "o". At any voltage in-between the fully OFF and fully ON levels, the signal is attenuated accordingly in its passage through the switch.
For all of the examples of the switches described hereinabove, it is noted that the polarization of the output beam is the same as that of the input beam, regardless of the ON/OFF switching status of the LC, such that the switch is polarization independent. This feature is desirable in optical switch technology, as it simplifies the design of the system in which the switches are used.
It is to be understood that although the embodiments, described hereinabove show a 1 x 2 switching configuration, because of the reciprocity principle in optics, the switch can also be used in a 2 x 1 configuration, with the exceptions stated with regard to the circulator embodiments of Figs. 8A to 8E.. Furthermore, such switches can be cascaded, leading to multiple switching configurations having compact dimensions and many more ports, input or output.
It is appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of various features described hereinabove as well as variations and modifications thereto which would occur to a person of skill in the art upon reading the above description and which are not in the prior art.

Claims

1. A wavelength selective switch comprising: a first optical port; a polarization conversion element for endowing light input through said first optical port with a predetermined polarization direction; a wavelength dispersive element in optical communication with said polarization conversion element, such that wavelength components of light received from said first optical port are dispersed in a dispersion plane; a pixilated polarization rotation element, optically coupled to receive said dispersed light, and having pixels aligned generally in said dispersion plane and adapted to rotate the polarization of light passing through each pixel according to a control signal applied to said pixel, such that the polarization of a wavelength component of said dispersed light is rotated according to said control signal applied to the pixel through which said wavelength component passes; and a reflective birefringent element in optical communication with light from said polarization rotation element, and disposed and oriented such that light which traverses a pixel of said polarization rotation element is reflected in a first or second direction according to the polarization of said received light, the elements being further orientated such that light from said reflective birefringent element reenters said polarization conversion element, which reconstitutes the light to its original polarization.
2. A wavelength selective switch according to claim 1 , wherein said light is reflected in a first direction when the control signal applied to said pixel is such as to generate no change in polarization of said optical beam, and in a second direction when the control signal applied to said pixel is such as to generate an essentially 90 degree rotation in the polarization of said optical beam.
3. A wavelength selective switch according to claim 1 , wherein said first direction leads to a second optical port, and said second direction leads to a third optical port, such that said first optical port is connected optically to either of said second and third ports in accordance with said control signal.
4. A wavelength selective switch according to claim 1 , wherein said reflective birefringent element is aligned such that said light reflected in one of said directions returns collinearly with the optical path from said first optical port, said switch further comprising a circulator to separate light reflected in said one direction from light incident from the direction of said first optical port.
5. A wavelength selective switch according to claim 4, further comprising an isolator disposed in the optical path in said second direction, said isolator being aligned such that light directed in said second direction cannot enter an optical port disposed in said second direction.
6. A wavelength selective switch according to any of the previous claims, wherein said pixelated polarization rotation element is a pixelated liquid crystal cell.
7. A wavelength selective switch according to claim 6, wherein said control signal is a voltage applied to electrodes across pixels of said liquid crystal cell.
8. A wavelength selective switch according to any of the previous claims, wherein said reflective birefringent element is in the form of a wedge, such that said directions are angularly distinguished directions.
9. A wavelength selective switch according to any of the previous claims, wherein said reflective birefringent element is in the form of a block, such that said directions are laterally distinguished directions.
10. A wavelength selective switch according to claim 3, wherein said switch directs light input to said first port to either of said second and third ports.
11. A wavelength selective switch according to claim 3, wherein said switch directs light input to either of said second and third ports to said first port.
12. A wavelength selective switch according to any of the previous claims, further comprising a pixelated phase changing element, which controls the transmission of light passing through a pixel of said polarization rotation element by spatially varying the phase across light traversing said pixel, thereby controlling its ability to couple into an output port.
13. A wavelength selective switch according to claim 12, wherein the mode structure of the beam of said light is degraded by the voltage applied to the pixel of said phase changing element.
14. A wavelength selective switch according to claim 12, wherein said pixelated phase changing element comprises a comb-like electrode structure which applies a spatially undulating electric field across said pixel of said phase changing element, such that the phase of light passing therethrough undergoes corresponding spatially alternating changes.
15. A wavelength selective switch according to claim 13, wherein said degrading of the mode structure of the beam of said light controls its ability to couple into an output port fiber.
16. A wavelength selective switch according to any of the previous claims, further comprising a polarization mode attenuator, which controls the transmission of light passing therethrough, said attenuator comprising: a pixelated birefringent polarization rotating element which rotates the polarization direction of light passing through a pixel thereof in accordance with the electric field applied across that pixel; and a serial linear polarizer, wherein the attenuation of light passing through a pixel of said polarization mode attenuator is dependent on the degree to which the polarization of light traversing said birefringent polarization rotating element is parallel to the polarization direction of said linear polarizer.
17. A wavelength selective switch according to any of the previous claims, wherein said arrangement of elements renders said switch polarization independent.
18. A wavelength selective switch comprising: a first optical port; a wavelength dispersive element in optical communication with said first optical port, such that wavelength components of light received from said first optical port are dispersed; a pixilated polarization rotation element, having pixels aligned generally to receive the dispersed wavelength components and adapted to rotate the polarization of light passing through each pixel in response to a control signal applied to said pixel, such that the polarization of a wavelength component of said dispersed light is rotated according to said control signal applied to the pixel through which said wavelength component passes; and a birefringent element disposed and oriented such that light which traverses a pixel of said polarization rotation element is directed in a first or second direction according to its polarization as determined by said control signal applied to said pixel.
PCT/IL2010/000480 2009-06-18 2010-06-17 Liquid crystal wavelength selective router WO2010146590A1 (en)

Priority Applications (3)

Application Number Priority Date Filing Date Title
EP10789108A EP2443510A4 (en) 2009-06-18 2010-06-17 Liquid crystal wavelength selective router
JP2012515628A JP2012530930A (en) 2009-06-18 2010-06-17 LCD wavelength selection router
CN2010800365636A CN102804051A (en) 2009-06-18 2010-06-17 Liquid crystal wavelength selective router

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
US21354409P 2009-06-18 2009-06-18
US61/213,544 2009-06-18

Publications (1)

Publication Number Publication Date
WO2010146590A1 true WO2010146590A1 (en) 2010-12-23

Family

ID=43355956

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/IL2010/000480 WO2010146590A1 (en) 2009-06-18 2010-06-17 Liquid crystal wavelength selective router

Country Status (4)

Country Link
EP (1) EP2443510A4 (en)
JP (1) JP2012530930A (en)
CN (1) CN102804051A (en)
WO (1) WO2010146590A1 (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103069320A (en) * 2012-06-12 2013-04-24 华为技术有限公司 Wavelength selection switch
CN116009327A (en) * 2023-01-16 2023-04-25 安徽共芯光子科技有限公司 Beam deflection device group and wavelength selective switch

Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2005041423A2 (en) 2002-10-17 2005-05-06 Stockeryale Inc. Wavefront polarization and phase scrambler
US20080226216A1 (en) * 2007-03-15 2008-09-18 Xtellus, Inc. Optical devices with integral isolators
US20080316585A1 (en) 2005-09-08 2008-12-25 Xtellus, Inc. Optical Wavelength Selective Router
US7492986B1 (en) * 2004-12-23 2009-02-17 Coadna Photonics, Inc. Apparatus and method for optical switching with liquid crystals and birefringent wedges
US20100183310A1 (en) * 2007-06-25 2010-07-22 Nippon Telegraph And Telephone Corporation Dispersion compensator

Family Cites Families (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5414540A (en) * 1993-06-01 1995-05-09 Bell Communications Research, Inc. Frequency-selective optical switch employing a frequency dispersive element, polarization dispersive element and polarization modulating elements
JP3447895B2 (en) * 1996-02-09 2003-09-16 株式会社東芝 Parameter setting method for projection display device
JP4069104B2 (en) * 2004-08-25 2008-04-02 日本電信電話株式会社 Optical frequency filter
JP4368286B2 (en) * 2004-10-08 2009-11-18 富士通株式会社 Optical switch device
WO2006123344A2 (en) * 2005-05-19 2006-11-23 Xtellus Inc. Single-pole optical wavelength selector
JP4842226B2 (en) * 2007-08-09 2011-12-21 日本電信電話株式会社 Wavelength selective switch

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2005041423A2 (en) 2002-10-17 2005-05-06 Stockeryale Inc. Wavefront polarization and phase scrambler
US7492986B1 (en) * 2004-12-23 2009-02-17 Coadna Photonics, Inc. Apparatus and method for optical switching with liquid crystals and birefringent wedges
US20080316585A1 (en) 2005-09-08 2008-12-25 Xtellus, Inc. Optical Wavelength Selective Router
US20080226216A1 (en) * 2007-03-15 2008-09-18 Xtellus, Inc. Optical devices with integral isolators
US20100183310A1 (en) * 2007-06-25 2010-07-22 Nippon Telegraph And Telephone Corporation Dispersion compensator

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See also references of EP2443510A4 *

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN103069320A (en) * 2012-06-12 2013-04-24 华为技术有限公司 Wavelength selection switch
WO2013185287A1 (en) * 2012-06-12 2013-12-19 华为技术有限公司 Wavelength selection switch
CN116009327A (en) * 2023-01-16 2023-04-25 安徽共芯光子科技有限公司 Beam deflection device group and wavelength selective switch

Also Published As

Publication number Publication date
CN102804051A (en) 2012-11-28
JP2012530930A (en) 2012-12-06
EP2443510A4 (en) 2012-12-12
EP2443510A1 (en) 2012-04-25

Similar Documents

Publication Publication Date Title
US7822303B2 (en) Optical wavelength selective router
JP6027628B2 (en) Method and apparatus for selectively switching wavelengths
US7014326B2 (en) Wavelength blocker
US7231110B2 (en) Wavelength selector switch
US9103991B2 (en) Multi-pole optical signal switch
EP2639611B1 (en) Wavelength selective switch
EP3266132B1 (en) Optical arrangement for managing diversity and isolation between ports in a wavelength selective switch
US20030175030A1 (en) Re-configurable optical add/drop multiplexer module and method
US9864148B1 (en) Optical arrangement for suppressing outerband crosstalk in a wavelength selective switch
WO2010146589A1 (en) Multiple port wavelength selectable router
US9521474B2 (en) Wavelength selective switch having multi-layer reflector
JP2009042558A (en) Wavelength selection switch
US20230061448A1 (en) Integrated Module Having Multiple Optical Channel Monitors With Shared Liquid Crystal Based Switching Assembly
EP2443510A1 (en) Liquid crystal wavelength selective router
US20110109869A1 (en) Reconfigurable wavelength selective cross-connect switch using liquid crystal cells
WO2010083507A1 (en) Single control liquid crystal optical switch and attenuator

Legal Events

Date Code Title Description
WWE Wipo information: entry into national phase

Ref document number: 201080036563.6

Country of ref document: CN

121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 10789108

Country of ref document: EP

Kind code of ref document: A1

WWE Wipo information: entry into national phase

Ref document number: 2012515628

Country of ref document: JP

NENP Non-entry into the national phase

Ref country code: DE

WWE Wipo information: entry into national phase

Ref document number: 2010789108

Country of ref document: EP