WO2003005621A1 - Diffractive fourier optics for optical communications - Google Patents
Diffractive fourier optics for optical communications Download PDFInfo
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- WO2003005621A1 WO2003005621A1 PCT/US2002/019797 US0219797W WO03005621A1 WO 2003005621 A1 WO2003005621 A1 WO 2003005621A1 US 0219797 W US0219797 W US 0219797W WO 03005621 A1 WO03005621 A1 WO 03005621A1
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- 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
- G02B6/2931—Diffractive element operating in reflection
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- 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
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- 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
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- 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
- G02B6/29313—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 characterised by means for controlling the position or direction of light incident to or leaving the diffractive element, e.g. for varying the wavelength response
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- 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
- G02B6/29391—Power equalisation of different channels, e.g. power flattening
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- 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
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- 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
- G02B6/29398—Temperature insensitivity
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- 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
- G02B6/32—Optical coupling means having lens focusing means positioned between opposed fibre ends
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/02—Wavelength-division multiplex systems
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- 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
- G02B6/2753—Optical coupling means with polarisation selective and adjusting means characterised by their function or use, i.e. of the complete device
- G02B6/2793—Controlling polarisation dependent loss, e.g. polarisation insensitivity, reducing the change in polarisation degree of the output light even if the input polarisation state fluctuates
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J14/00—Optical multiplex systems
- H04J14/02—Wavelength-division multiplex systems
- H04J14/0221—Power control, e.g. to keep the total optical power constant
Definitions
- This invention relates to systems and methods for separating, modifying or combining wavelength multiplexed signals in optical communications, and more particularly to such systems and methods for controlling, modulating, switching or otherwise manipulating individual wavelength signals in dense wavelength division multiplexed beams.
- the invention described herein discloses numerous design features and fabrication approaches specifically tailored for and necessitated by the demanding optical and mechanical requirements of fiberoptic devices.
- These devices comprise a family of components including dynamic channel equalizers, gain equalizers, band splitters, interleavers, and dynamic add/drop multiplexers.
- a multiport, wavelength selective, multi-channel variable attenuator and blocker component is the basis of this family of devices.
- the key features of this component include a unique three dimensional folded optical design, an imaging configuration which improves on the 4-f configuration, optimized anamo ⁇ hic optics for polarization management, compactness and stability, high performance liquid crystal spatial light modulators (LC-SLMs), and precision alignment processes.
- LC-SLMs liquid crystal spatial light modulators
- a compact, high optical efficiency system and method for manipulating multi- wavelength optical wavelength signals is based upon three-dimensional refolding, high resolution imaging and modification of wavelength dispersed beams within a compact volume with high optical efficiency.
- the DWDM optical signal is first transformed from the fiber mode into an anamo ⁇ hically shaped, free space propagating beam by a novel micro-optic system.
- the beams are reflected at slightly different elevations off opposing and spaced apart reflecting and diffractive surfaces so as to converge and focus to asymmetrically shaped, sagittally dispersed individual wavelength components which are separated and projected at the plane of an array of control elements.
- control elements may be either dynamic or static, may reflect or transmit, or both, wherein the modified individual wavelength signals are subsequently rediffracted and reimaged into the symmetrical fiber modes for re-launch into the same or another fiber(s).
- the beam refolding paths may be reversely directed through the same reflecting and diffractive surfaces or through a set of adjacent elements.
- the system provides a very compact, optically efficient and versatile approach which is adaptable to many different applications, and which are optimized to achieve high performance optical specifications.
- a combination of high wavelength resolution, compactness and low loss can be attained by employing a diffraction grating hi the Littrow configuration, with the grating rulings transverse to the long axis of the anamo ⁇ hic beam, and a Mangin reflector system for collimating and refolding the beams, and later converging and imaging the wavelength beams onto spatially separate locations.
- the diffraction grating is angled at the nomhial Littrow angle to reflect and disperse individual wavelengtli components within a sagittal plane, symmetrically distributed about an axis substantially normal to the Mangin mirror system.
- 50 GHz optical devices demand a highly refined optical system.
- channel spacings are greater, e.g. 100 GHz, more design options are feasible, and a polarization independent grating of 600 1pm and a single Mangin reflector may be suitable.
- more than one input beam can be launched into the optical system, sharing the reflecting and focusing surfaces while impinging the surfaces at slightly different levels.
- the polarization components are also separated and refolded in parallel paths to accept more than one beam.
- the signal modulators or modifiers for the individual spatially dispersed wavelength components can be used in different system configurations to provide channel equalization, channel blocking, channel add/drop, band splitting and interleaving, or channel switching.
- a particular example of a channel equalizer/blocker in accordance with the invention uses the reciprocal path configuration for refolding the beams before and after individual control, and an array of reflective liquid crystal elements at the image plane. After phase retardation or variable rotation by the reflective liquid crystal cells, as determined by control signals applied to the cells, the polarization sensitive elements in the signal path block or divert that portion of the signal to be rejected.
- the wavelength signal components are variably attenuated for channel equalization pu ⁇ oses, or fully extinguished (>40 dB) for block pu ⁇ oses.
- the polarization splitter comprises a Wollaston prism combination close to the input anamo ⁇ hic collimator device, to separate the orthogonal s and p polarization components into separate beams diverging at a small angle, such that the two beams coincide and overlap at the plane of the modulator.
- This overlap at the LC-SLM ensures that both polarization components experience the same amount of retardation, which is necessary to give zero PDL and PDF by design.
- Both polarization components are also polarized parallel to one another and parallel to the grating rulings. In their return paths from the reflective liquid crystal elements, the upper beam is directed along the lower beam path and visa versa. Following recombination upon the second pass of the grating, the wavelength components are collapsed back into two orthogonally polarized beams, which are then reunified by the polarization splitter into a single beam for return to the optical fiber mode.
- a specifically patterned template may be disposed at the image plane, to reflect or transmit individual channel signals.
- the system may be single sided, reflecting signals reciprocally back along the same optics to a common input/output port, or to a separate output port slightly displaced from the input.
- the system can also be double sided, transmitting channel signals through the template and into a second complementary diffractive Fourier optics assembly. Both optical assemblies can be used at the same time, increasing the combinations that are possible with a static system.
- two different level gratings and/or two different ports can be used in each beam refolding combination, so that either port can be used as either input or output.
- the static multichannel template can be used to provide selective channel blocking, drop and throughput patterns, 1x2 channel splitting, and interleaver operation.
- the wavelength signal modifier array may comprise static or dynamic elements which shift the elevation level of one or more channel input signals or block of signals, for direction to one or more different level output ports.
- DWDM signals from one input port can be divided into odd and even channels combined at separate output ports. This takes advantage of the fact that multiple low profile beam patterns at different levels can be refolded concurrently within the same compact volume.
- zero twist nematic liquid crystal cells which function as half wave retardation elements for full extinction, and as variable retardation elements for analog control of attenuation.
- the dispersion of the cells in the sagittal plane may be chi ⁇ ed to precisely map the dispersion plane to the ITU wavelength standard.
- Other types of spatial light modulators, such as MEMs and twisted nematic liquid crystal cells may be used for dynamic signal control depending upon the application.
- High density microlithographic patterns may be used to define the multi-channel controls for static blocking, attenuation, reflection or transmission of signals in different channels.
- Various tuning and alignment techniques can be used to reduce insertion loss and reduce optical aberrations.
- the ambient pressure and/or environment within the enclosed housing may be used to achieve period and phase tuning, and the positions and angles of various optical elements are maintained within close limits to maintain beam uniformity, low PDL and low PDF.
- Associated optical circuits and devices can be employed to provide other capabilities for meeting multi-channel requirements of DWDM systems, based on the use of diffractive Fourier optics modules, which may employ one or more fiberoptic ports. Combinations of fiberoptic splitters, circulators and thin film filters may be used to enable unique system combinations for achieving add/drop multiplexing, demultiplexing, and equalization.
- Fig. 1 is a schematic diagram of a system displaying diffractive and Fourier optics
- Fig. 2 is a schematic diagram of beam paths between a diffractive grating and mirror, showing typical beam angles;
- Fig. 3 is a graph of transmission versus frequency showing stopband characteristics
- Fig. 4 is another schematic diagram of beam paths between a diffraction grating and concave mirror when polarization components are split and propagated separately;
- Fig. 5 is a simplified perspective view of a diffractive Fourier optics system in accordance with the invention for DWDM optical beams, configured to function as a gain equalizer, and including reciprocal incident and return optical paths and an array of reflective liquid crystal cells for controlled the diffracted beams;
- Fig. 6 is a simplified plan view of some principal optical elements in the system of Fig. 5, showing the refolding paths of incident beams directed to liquid crystal cells at the object plane and illustrating the sagittal dispersion variations;
- Fig. 7 is a simplified perspective view of the principal optical elements in the system of Figs. 5 and 6, showing the separation of polarization components in elevation in the beam;
- Fig. 8 is a perspective view, without beam paths, of the configuration of a practical example of a system corresponding to Figs. 5-7, with the cover removed from the housing of the system;
- Fig. 9 is a side view of the elements of collimator anamo ⁇ hic converter input optics and a Wollaston prism beamsplitter that may be used in the system of Figs. 5-8;
- Fig. 10 is an enlarged, perspective view of one example of polarization management and liquid crystal cell optics showing an example of the beam paths of the two polarization components of a single diffracted wavelength optical signal at a liquid ciystal cell;
- Fig. 11 is a simplified diagram of multiple low profile anamo ⁇ hic beam patterns incident on the reflective surface of the Mangin mirror during operation of the system of Figs. 5-9;
- Fig. 12 is a combination side view and block diagram, generalized in some respects, of a different compact system in accordance with the invention for concurrently diffractively refolding beams from two different DWDM signals, and also employing a high numerical aperture Wollaston beamsplitter for two incident beams;
- Fig. 13 is a perspective view of an alternate system using diffractive Fourier optics with separate input and return paths using three dimensional refolding and a patterned template for selective control of multiple channels;
- Fig. 14 is a different perspective of the system of Fig. 13, showing beam paths therein;
- Fig. 15 is a perspective view of the arrangement of waveplates in relation to the patterned template in the system of Figs. 13 and 14;
- Fig. 16 is a perspective view of a microlithographic template that may be employed in the system of Figs. 13 to 15;
- Fig. 17 is a block diagram of a programmable add/drop multiplexer employing a power splitter and a pair of channel equalizers in accordance with the invention;
- Fig. 18 is a block diagram of a system using three channel equalizers and two power splitters in a four port configuration for providing add, drop and throughput outputs; and Fig. 19 is a graph of the dependence of frequency period on pressure of the surrounding atmosphere. 5 Detailed Description of the Invention
- New systems and methods for manipulating multiwavelength optical signals to accomplish a wide range of functions are based on the combined use of diffractive and Fourier optics together with a fixed or dynamic template. By angularly dispersing the
- the optical system described herein utilizes a fiber coupled, diffraction grating based, double pass spectrometer which has been designed to achieve high passband flatness and low chromatic dispersion.
- any combination of output channels can be configured or modified in time delay and/or optical power.
- a spatially varying transmission pattern is directly mapped to a wavelength varying transmission pattern by the optical system.
- a MEMS mirror or liquid crystal array could be used to permit dynamic wavelength routing, switching or attenuation.
- a compact low profile multiwavelength system with multiple ports is configured with reflection gratings, anamo ⁇ hic beams and reflective optics, which disperse wavelength components through close parallel planes angled to fold into reciprocal paths within the unit.
- the optical design utilizes Fourier optics principles to image the wavelength dispersed input fiber mode anamo ⁇ hically onto a liquid crystal spatial light modulator (LC-SLM).
- LC-SLM liquid crystal spatial light modulator
- the diverging beam waist of the input expands and is collimated by the first focusing element of focal length f ls to achieve full illumination of the grating rulings, and the second focusing element of focal length f 2 transforms the angular dispersion at the grating into a continuum of beam waists spatially separated according to wavelength.
- the focal lengths i and f 2 can be different, in which case a magification factor from the object to the image planes is introduced.
- Mangin mirror assemblies provides advantages in high resolution spectrometers which demand focusing optics of diffraction limited optical resolution.
- spherical mirrors have served as focusing lenses. Spherical mirrors produce aberrations which lead to crosstalk degradation. Parabolic mirrors eliminate aberrations for on-axis rays, but off-axis rays still experience significant aberrations. These aberrations become problematic in compact implementations of high resolution 50 GHz filtering applications.
- Spherical abberation causes rays entering near the edges of the optic to focus at different longitudinal distances from the focusing optic.
- Coma causes rays entering near the edges of the optic to focus at different transverse distances along the focal plane. Both these effects produce a loss of optical resolution and, as a result, degrade the adjacent channel isolation and passband flatness.
- a Mangin type reflector with refractive front surface and a reflective back surface, each surface spherical with a different radius of curvature is inco ⁇ orated.
- a shared single collimator for both input and output beams reduces the complexity and stability requirements of the fiber coupled spectrometer. Only a single collimator must be precisely aligned and welded into the optical system. The return beam tracks the input beam, so that slight collimator misalignments do not compromise optical performance. The output beam can than be extracted from the input fiber by using mature circulator technologies.
- the 4-f configuration places/practical lhnitations on the size of the device, which can be overcome by inco ⁇ orating an alternate "imaging" type configuration.
- An advantage of the imaging type configuration is the ability to use a tight vertical fold angle.
- a single reflective lens assembly which is double passed serves as both focusing or Fourier transforming optics.
- the well-known but restrictive 4-f configuration can be adopted.
- both the object plane (i.e., the input optical fiber) and the image plane (i.e., the location of the modulator array) are located at the focal planes of the shared lens. This requirement does not need to be met, however, in compact, high resolution optical system. In fact, in the most compact designs, it is often advantageous to move away from the 4-f configuration.
- the modulator array and input beam waist are not placed at the focal planes, but rather at the image/object planes, respectively.
- the key design factor to maintain high optical resolution, wide flat pass bands, and low chromatic dispersion is that the Gaussian beam waists lie precisely at the LC-SLM plane. Maintaining this "imaging" condition is more important than maintaining precise f spacing between each optic. For example, the requirements on the positions of the input collimator and LC- SLM relative to the focusing element(s) are much looser than the tolerances on the position of the LC-SLM relative to the beam waist locations.
- the beam waist location depends of several interrelated factors, including the input collimator location and the focusing element locations.
- the optical design described herein is not limited by the 4-f condition.
- the only condition which must be satisfied is that the input beam waist is mapped to an output beam waist coinciding with the modulator array or fixed template. That is, the distances between optical elements must be selected such that the anamo ⁇ hic beam waist produced at the input collimator is re-imaged at the plane of the modulator array. This ensures that the advantages of wide, flat passbands and zero chromatic dispersion are achieved. Specifically, if the collimator object plane is greater than a distance f from the focusing optic, then the image plane is less than the distance f from the focusing optic.
- this flexibility reduces mechanical interference issues between the input lens assembly and the diffraction grating, translating into a tighter fold and a low profile package. It is desirable to place the input anamo ⁇ hic beam waist behind the plane of the diffraction grating to avoid mechanical interference issues which would lead to increased fold angle, potential off-axis beam aberrations, and package height.
- the more general imaging type (non-4-f) spectrometer configuration is also described.
- anamo ⁇ hic beams within the fiber coupled spectrometer provides several advantages.
- Anamorphic beam shaping optics reduce the height of the device without compromising optical performance and also increase the power handling rating by enlarging the optical beam at the modulator array.
- the collimator assembly utilizes anamo ⁇ hic optics to shape the beam in the transverse and sagital dimensions before launching the free space beam into the folded optical system.
- the resolution is related to the total number of rulings illuminated on the grating. Therefore, the optics expand the beam from the fiber to the diffraction grating plane.
- the fill factor of the optical beam on the diffraction grating normal to the grating vector does not impact optical resolution.
- the spot size at the LC-SLM pe ⁇ endicular to the array direction, should be sufficiently large to maintain average power levels below the power threshold for degradation in optical response. It is advantageous that the spot size parallel to the array direction for any particular wavelength should be minimized such that the spot is several times narrower that the required pixel width for 50 GHz spacing.
- the typical spot size at the LC-SLM array is 8 to 10 um horizontal by 100 to 250 um vertical. Note, however, that the sense of beam asymmetry at the grating is inverted by the focusing element.
- the spot size in the vertical direction at the grating and the Mangin mirror is relatively small ( ⁇ 1mm) compared to the spot size in the horizontal direction ( ⁇ 25 mm). This allows tight folding in the vertical direction, while maintaining near horizontal incidence angle on the grating and the Mangin.
- the beam is normally incident to the LC-SLM reflective plane.
- the three dimensional fold is a superior approach to the common two dimensional fold used in grating spectrometers, providing numerous performance advantages.
- First, the three dimensional fold reduces optical aberrations, achieves high wavelength resolution, and reduces insertion loss is a most compact package.
- Second, the three dimensional fold reduces the number of optical elements by enabling multiple passes through shared optics.
- the use of anamo ⁇ hic beams of reduced dimension normal to the dispersion plane allows the optical system to be tightly folded in this transverse dimension.
- the dispersion of a diffraction grating is limited to one dimension and oriented in a sagital plane.
- Fiber coupling may be accomplished through holes drilled in the center of the diffraction gratings, which enables the on-axis beam paths and keeps size to a minimum. It also permits the use of common coUimating and focusing optics to further reduce package size and minimize component count.
- the beam paths are folded to substantially shorten the length of the package. Mmimization of the fold angles is desirable to reduce aberration effects which compromise resolution. This becomes increasingly important for channel spacings below 50 GHz.
- the 2D folded design places fundamental geometrical constraints on the design which prevent these angles from being minimized.
- the 3D folded design described herein takes advantage of the third dimension, in which the beam is collimated and relatively small, to dramatically reduce the angles of incidence.
- the beams at the Mangin mirror are two cigar shaped spots slightly displaced from one another in the vertical direction (y).
- the beams at the diffraction grating are elongated in the dispersion direction (x) and overlap at a single location, hi the 2D fold design, the fold direction (x) is parallel to the dispersion direction (x).
- the fold direction (y) is orthogonal to the dispersion direction (x), represented schematically in Fig. 2.
- Additional input and output ports to the ones shown in Fig. 1 can be provided by spatially multiplexing additional diffraction gratings and optical fibers in planes above and below the plane of Fig. 1.
- the anamo ⁇ hic profile of the optical beams permits independent beam paths to be stacked or multiplexed in the vertical dimension of the device. These new ports can be accessed, for example, by redirecting the incident light on the template by reflection to the desired input or output fibers.
- Both polarization independent and polarization dependent diffraction gratings are commercially available. Each type has its own advantages and disadvantages in optical system applications.
- 600 lines/mm gratings are available in low PDL versions, and 1100 1200 lines/mm gratings are generally polarization dependent when operated in the 1550 nm range.
- Polarization independent gratings simplify the polarization management optics, but have limited angular dispersion because of the upper limit on the ruling density.
- Wollaston Prism The use of a Wollaston prism at the input to a grating spectrometer provides an improved method of polarization management.
- the Wollaston beam splitter provides overlapping beams at the LC-SLM plane, which eliminates the dependences of PDL on the level of variable attenuation due to LC nonuniformities.
- the Wollaston transforms the beams such that their polarizations are suitably aligned to the grating ruling direction.
- a Wollaston prism separates an unpolarized beam into two linear and orthogonally polarized beams angularly displaced relative to one another. Once the beams propagate a distance such that they are adequately separated, their polarizations can be rotated to be parallel to one another.
- the Wollaston ensures that the two polarization components precisely overlap in a single spot at the modulator plane. If an LC-SLM is coincident with the modulator plane, then both polarization components will experience identical amounts of retardation. This overlap eliminates any mechanisms for PDL, which can become problematic at high attenuation states (e.g., 10 to 20 dB).
- the Wollaston creates a ring-like optical path, in which one polarization propagates clockwise through the ring, and the orthogonal polarization propagates counterclockwise through the ring.
- the benefit of such approach is that both polarizations experience identical propagation effects even though they are spatially separated outside of the image and object planes.
- Dynamic optical filtering devices require active elements to achieve a programmable optical response, such as MEMs or liquid crystal modulators.
- MEMs or liquid crystal modulators To achieve a combination of high blocking efficiency and precisely controlled variable attenuation within the same modulator, it is advantageous to utilize zero twist nematic LC-SLM's.
- a first advantage of zero twist nematic LC-SLM's is that they provided improved stability and attenuation resolution in the variable attenuation mode.
- a second advantage is that they provide in excess of 40 dB extinction.
- a third advantage is that LC-SLM's have no moving parts, in contrast to MEMs type devices, which consequently raise reliability concerns.
- Fiberoptic components typically require polarization management optics to ensure polarization independent operation.
- polarization dependent elements such as gratings and liquid crystal type modulators
- the input states of polarization must be fixed and well defined.
- the input beam is split into its two orthogonal and linear polarization components before reaching these elements.
- both polarization components it is desirable for both polarization components to travel over substantially identical beam paths.
- the input beam is typically separated into orthogonal polarization components at the input/output collimators, these components propagating in parallel through the entire optical system. It is desirable to minimize this beam separation, as imperfections in coatings and optical surfaces would cause differential loss between two spatially separate beams, which can create an elevated level of PDL.
- the two polarization components overlap throughout the optical system until they reach the LC- SLM subassembly, wherein they are spatially separated by greater than 100 um for processing by the liquid crystal, and immediately combined back into a single beam for completion of the return path to the collimator.
- PDF 0.5 GHz polarization dependent frequency
- This angular misalignment budget includes errors resulting from alignment irregularities when rubbing and aligning the patterned LC backplate, the ITO front plate, and the birefringent compensating plate.
- the PBD subassembly is the deteraiining factor in achieving this specification.
- the PBD subassembly utilizes two PBD's of different optical axis orientations. To ensure that the displaced beams exit from the subassembly with the correct spatial orientation and state of polarization (necessary to achieve high extinction and low PDF), precise control of the optical thickness is necessary.
- the optical thickness depends on the index of refraction and physical thickness. Typically, it is adequate to ensure that the physical thicknesses are precisely matched (to within 1 um).
- the LC-SLM pixel widths map directly to channel spacing.
- the LC-SLM subassembly may be rotated slightly relative to the optical beam propagation direction to change the channel spacing. For example, a 25 MHz channel spacing change corresponds to 0.0286 degree rotation.
- the next step is measure PDL and insert an appropriate PDL compensation waveplate.
- This waveplate has a single pass retardation between 0 and ⁇ /2 radians.
- the PBD assembly introduces some amount of retardation because PBD 1 and PBD 2 are not exactly matched in optical path length.
- the added waveplate ensures that the total round trip retardation of the pbd and waveplate assembly, across all wavelengths, is ⁇ radians. This ensures that the polarization of the beam is rotated by 90 degrees between the first pass and the second pass.
- the waveplate is tilted along the LC-SLM pixel array direction to translate the beams as heeded along the LC- SLM.
- a +/- 10 degree rotation corresponds to a physical shift at the plane of the LC of +/- 5 um, which corresponds to an absolute frequency shift of +/- 10 GHz.
- the thicker the waveplate the greater the amount of frequency tuning for a given angular change.
- the Littrow configuration is the preferred approach.
- the light at the target wavelength (1547 mn, for example) is diffracted back along the incident beam direction.
- Littrow operation also ensures that the maximum resolution for a given grating pitch is achieved.
- smaller incidence angles are used, allowing smaller diffraction gratings and a more compact mechanical design.
- Fig. 2 illustrates the relative orientation of the optical beams relative to the median plane.
- Angle ⁇ lx is the declination angle of the input beam (O)
- angle 0 2x is the angle of the focusing reflector (MM)
- ⁇ 3x is the angle of the diffraction grating (DG)
- 0 4x is the angle of the pickoff mirror (PM)
- ⁇ 3x is equal to 1.1 degrees
- ⁇ 3y the tilt of the grating lines
- ⁇ 3y the tilt of the grating lines
- diffraction occurs in the x-z plane, which is orthogonal to the fold direction.
- the angles ⁇ lx and ⁇ 3x can be minimized, which improves the optical performance characteristics of the system.
- a grating with a finer ruling is preferred.
- the Littrow angle in first order (1100 lines/mm) is 58.2 degrees.
- the reflective diffraction gratings are generally of the replicated type, and can be holographic or ruled, sinusoidal or blazed. In the Littrow configuration the light at the target wavelength (1547 nm, for example) is diffracted back along the incident beam direction.
- the theoretical frequency resolution of a diffraction grating based optical filter is determined by the number of illuminated rulings.
- the resolution is further determined by the relative size of the wavelength impulse response at the LC-SLM plane and the pixel format of the LC-SLM. For example, a device with pixels chhped in width from 70 to 95 um, 2.5 um inteipixel gaps, and 43 dB maximum extinction is described herein. Fringing fields effectively increase the inte ⁇ ixel gaps to an effective width equal to the liquid crystal cell gap. For the reflective half wave retarder, the liquid crystal cell gap is approximately 4 to 5 um wide.
- the optical beam at LC-SLM plane is transformed to have an 8 um diameter in the horizontal direction and 200 to 250 um in the vertical direction.
- the theoretical stopband over which 40 dB extinction is maintained is equal to 37 GHz, corresponding to the more rapid roll-off illustrated in Fig. 3.
- the more gradual roll-off illustrated in Fig. 3 corresponds to the case of un-optimized optics which do not meet the performance requirements of 50 GHz systems.
- the Wollaston beamsplitter is located at the input plane after the anamo ⁇ hic beam shaping optics, and separates the input beam into two beams separated by an angle 2 ⁇ w , as illustrated in Fig. 4.
- Angle ⁇ lx is the declination angle of the input beam (O)
- angle ⁇ 2x is the angle of the focusing reflector (MM)
- ⁇ 3x is the angle of the diffraction grating (DG)
- ⁇ x is the angle of the pickoff mirror (PM)
- ⁇ 3x is equal to 2.37 degrees
- ⁇ 3y the tilt of the grating lines
- the grating is oriented such that it reflects all wavelengths into the median plane.
- the preferred grating has 1100 rulings per mm. The wavelengths are dispersed such that the center of the operating band, 1547 nm for the C band, reflects back in the Littrow geometry at 58.2 degrees.
- WP is a waveplate array, and consists of two half waveplates, the top oriented at 45 degrees relative to the p polarized beam, to transform it into an s polarized beam, and the bottom oriented at 0 degrees relative to the s polarized beam, to maintain it as an s polarized beam and precisely equalize the two path lengths.
- Equalization of path lengths is necessary to image both beam waists at precisely the same location in the LC-SLM image plane. This ensures that the waists of the optical beams lie in the Fourier plane to provide optimal resolution characteristics. This optical system also achieves both low excess loss and low polarization dependent loss.
- Dynamic functionality is provided by the introduction of an electronically addressable, pixelated spatial light modulator into the wavelength dispersed image plane of the input optical fiber.
- a reflective type LC-SLM transforms incident beams of linear polarization into reflected beams of transformed polarization in a wavelength channel independent fashion.
- the use of proven LC technology is attractive because of the widely available LC-SLM manufacturing services and the extensive long term reliability data available for this technology.
- These cells serve as variable attenuator arrays, for example.
- the ellipticity and azimuth angle of the transformed beams are determined by those control voltages applied directly to the liquid crystal pixels.
- Polarization management optics are designed to transform this polarization modulation into amplitude modulation for an arbitrary input state of polarization.
- the liquid crystal pixel elements are tall normal to the grating dispersion direction (transverse plane) and narrow parallel to the dispersion direction (sagital plane), and the impinging wavelength dispersed beams are spatially separated and distributed in a linear fashion within the sagital plane.
- the individual polarization orientations of the dispersed beams are voltage controllable upon propagating through the active pixel elements.
- Each cell operates on only a single wavelength channel, and by utilizing the design features of this invention, achieve an extinction ratio in excess of 40 dB.
- the cell widths are defined lithographically such that they precisely map to a 50 GHz wide spectral splice.
- the widths of the cells in the array may be chi ⁇ ed to achieve accuracy in the center f equency of each passband relative to the ITU grid.
- the voltages may advantageously be varied across each pixel to provide uniformly high extinction or an arbitrary grey scale attenuation across the operating wavelength range.
- a high extinction polarizer placed directly in front of the LC-SLM transforms the variable state of polarization rotation of the LC-SLM into variable attenuation.
- the optical axis of the LC-SLM is oriented at 45 degrees to the pixel array direction.
- the polarizer is therefore oriented at 0 degrees to provide maximum contrast.
- the LC-SLM produces nominally zero polarization rotation under zero applied voltage; this is the transmissive state. At some larger voltage, the LC-SLM produces 90 degrees of polarization rotation; this is the high rejection state.
- Intermediate voltage values provide intermediate attenuation values.
- static and dynamic optical switching and filtering components based on modulator arrays and spectrometers offer superior performance for applications to problems in wavelength management, such as channel equalization, gain equalization, interleaving, band splitting and reconfigurable add/drop multiplexing.
- the combinations of Fourier optic principles and diffraction gratings are simplified and reduced in size to provide components of exceptional compactness, stability, and optical performance.
- a reflective design shares a single input/output port, and a circulator is used to extract the reflected output signal present at the input port.
- An input/output terminal receives the input WDM signal and directs an anamo ⁇ hic, low profile beam toward a concave reflector of the Mangin type.
- the focused and reflected anamo ⁇ hic beam is angled slightly downward to a diffraction grating in a Littrow configuration, in a direction normal to the wavelength dispersion direction and at a declination angle which has been minimized by anamo ⁇ hically shaping the beams.
- the beam Upon Littrow-type reflection from the grating, the beam is refolded towards the Mangin mirror.
- the mirror reflects and redirects the converging beam, toward an angled pick-off mirror at the level of a modulator array subassembly (an LC-SLM in the case of active control, a static reflective mask in the case of a static device), wherein the individual wavelength components of the diffracted beam impinge and are spatially localized on the elements of the modulator array.
- an LC-SLM in the case of active control, a static reflective mask in the case of a static device
- each cell introduces precise polarization control to affect optical attenuation on a cell-by-cell basis, such that the beams are reflected back through the mirrors and diffractive optics, restoring the processed WDM beam at the input/output terminal with some programmable transmission spectra characteristic.
- a dynamic gain equalizer 10 in accordance with the invention is designed to equalize the power of the individual channels in a DWDM communication system while meeting the demanding requirements of 25 and 50 GHz channel spacings with high optical efficiency.
- the optical system is fully contained within a hermetic housing 12, in which the pressure and/or gas composition may be adjusted for period tuning, as described below.
- Input signals on an optical waveguide (not shown) carrying the DWDM signals are provided via an optical circulator 14 to a collimator and anamo ⁇ hic converter device 16 which inco ⁇ orates lens elements (see Fig. 5), for first collimating the input beam and then shaping it into an anamo ⁇ hic beam pair.
- the anamo ⁇ hic pattern here has a major axis and a minor axis, the major:minor ratio being in excess of about 20:1, and the beam height (transverse direction) at its most fully divergent position being of the order of 1 mm, and the beam width (sagital plane) at its most fully divergent position being of the order of 25 mm.
- the beam is adjacently directed through a beam polarization splitter 18, to be described in more detail hereafter relative to Figs. 9 (and 12), which separates the input beam, which has an arbitrary state of polarization, into its s and p polarization components.
- the sagittally diverging patterns of the separated polarization beams (only one of which is shown in Fig. 5) are incident on the first element of a Mangin mirror system 20.
- a concave reflector alone does not provide the beam shaping and corrective beam shaping that is needed for 50 and 100 GHz channel spacings.
- the Mangin mirror system which provides refractive power from two refractive surfaces and a selectable index of refraction, is preferred. The requirements of systems with less than 50 GHz spacing are more demanding, however.
- the Mangin mirror system 20 includes a first transmissive lens 22 of selected front and back spherical curvatures, and a second transmissive lens 24 also having spherical front and back curvatures, and having a high reflection coating on its back spherical surface.
- the multiple corrections introduced by these spherical surfaces in relation to the refractive indices of the lenses provide precise angular adjustments in both the sagittal and transverse directions while also controlling beam tilt, beam convergence and beam divergence at successive beam reflections.
- Fig. 5 which illustrates forward directed beam folds by solid lines and reverse folds by dotted lines, only the outer margins of the beams are indicated, since the closely adjacent polarization components cannot clearly be depicted at this scale. However, the outlines of the four reflected patterns on the back surface of the second lens 24 are shown in dotted lines.
- the divergent anamo ⁇ hic input beam is collimated on its first reflection off the back surface of the second lens 24 in both the sagittal and transverse directions, so as to project an anamo ⁇ hic pattern of constant height and sagittal width toward a grating 26 at the opposite limit.
- the correction provided by the double Mangin mirror system 20 tilts the reflected beam downwardly, as shown by the dotted marginal parallel lines.
- the grating 26 shown is a diffraction grating, capable of high diffraction efficiency.
- the diffraction grating 26 is set at the Littrow angle, here 58.2° to the central sagittal axis of the beam and bisects the image plane at a level below the anamo ⁇ hic/collimator device 16.
- the diffraction grating 26 itself has a slight tilt, to reflect sagittally dispersed beams upwardly, again in the forward direction, to a second level (lower than the first) on the Mangin mirror system 20.
- the diffraction grating 26 has a dense grating line pattern, of the order of 1100-1200 lines/mm (in this example 1100 lines/mm).
- the impinging collimated but anamo ⁇ hic beam pattern is sagittally dispersed across the surface of the grating 26, thus engaging a high proportion (>90%) of the grating lines on the surface.
- the grating lines also vary from precisely transverse in the central region to tilt angles of up to 2° towards the end regions, since the grating is polarization dependent. With both polarization components being in parallel orientation to the grating lines, and in s polarization direction, there is minimal loss of diffraction efficiency.
- the sagittally dispersed beams that diffractively reflect from the diffraction grating 26 are collimated in their return to the double Mangin system 20 where they are now tilted slightly upwards on reflection and caused to converge to define beam waists at a image plane which is at an elevation between the hiput/output optics 16 and the grating 26 within the housing 12.
- the modulator image plane in this instance is laterally displaced in the housing 12 from the principal beam paths, so the converging beams are reflected sideways off an angled pick off mirror 30 which is supported at the proper level on a mount 32 that extends upwardly from the bottom of the housing 12.
- the pair of linearly polarized beams constituting the individual wavelength signals are of asymmetrical shape, but their major dimension is now in the transverse direction to the sagittal plane, having relatively small separation as they approach coincidence at the image plane at an inclination angle of 3° and 2°.
- the polarization components pass without attenuation through a polarizer element 34 with its optical axis disposed for full transmission of vertically polarized light, the state of both polarized beam components at this point.
- the waists of the beam spots for each individual wavelength overlap and are approximately 10-11 microns wide by 150-250 microns tall, distributing the beam energy across the cell surface and keeping local power intensity in an acceptable range ( ⁇ 100 mW per wavelength).
- the polarized beam components then impinge on the individual cells 37 in the liquid crystal spatial light modulator (LC-SLM) array 38).
- the liquid crystal cells 37 are of the zero twist nematic type, with a preferred "director" (rubbing axis) aligned at 45° to the s polarization direction.
- the narrow sagittal dimensions of the beam spots and the sagittal separation between spots enables use of a compact LC-SLM array.
- the pixels of the array can be mapped to beam spot locations in the sagittal direction.
- the individual cells of the LC-SLM array 38 are separately amplitude controlled by voltage control electronics in conventional fashion, driven by a square wave signal of 4 to 20 kHz modulation frequency and 0 to 10 volts in amplitude.
- Control voltage levels for the cells may be derived in conventional fashion by power monitor and processing circuits 42 which receive an input DWDM signal, measure the power in each channel and provide corrective output signals to the voltage control electronics 40 to equalize gain.
- Input DWDM signals after monitoring are fed as shown to the input to the optical circulator 14, and after equalization may be supplied to other circuits or an EDFA (not shown).
- the zero twist nematic cells 37 function as a variable phase retarder.
- the LC cell plus the embedded retardation plate provide precisely a half wave of retardation in round trip. In this event the two polarization components are reflected back toward the system but are redirected or absorbed by the polarizer element 34.
- the polarizer element 34 is a polarization beam displacer (PBD)
- the rejected energy is diverted out of the field of view of the output fiber.
- the zero twist nematic crystal serves as a variable phase retarder.
- the elipticity and azimuth of the polarized beam components determine the proportion of energy that is blocked or diverted by the polarization sensitive device 34.
- the two polarization components per wavelength signal that are reflected from the cell 37 are again at 3° and 2° angles, but each moves along the incident path of the opposite beam.
- the liquid crystal 37 is tilted slightly or inco ⁇ orates a thin wedge element to control the angle of reflections, and to minimize unwanted back reflections. Consequently the modified signals, whether extinguished or attenuated, are returned back through the optical system for refolding along paths reciprocal to the input paths.
- the separate polarization components maintain the small divergence angle until reaching the Mangin mirror system 20, where they are shifted into parallelism, for rediffraction upon reflection at the diffraction grating 26.
- the rediffracted beams after the diffraction grating 26 are converted to the collimated anamo ⁇ hic beam and, after again reflecting off the Mangin mirror system 20, the convergent anamo ⁇ hic beam is directed slightly upwardly through the beam polarization splitter 18.
- the two returning polarization components are returned to orthogonal polarization relationships and coincidence, and the wavelengtli components are recombined.
- the anamo ⁇ hic lenses and collimator lens 16 thus form a DWDM output beam with equalized channels, which is transferred via the optical circulator 14 to an output waveguide (fiber) for propagation outwardly.
- a 1 mm (O.D.) collimator 16a element generates a downstream beam waist of 200 ⁇ m spot size at a forward distance of 10 to 15 mm.
- An anamo ⁇ hic mode converter comprising three high index cylindrical lenses 16b, 16c, 16d, each having optical power in one axis, then introduce the geometric asymmetry or anamo ⁇ hic shape in the beam cross-section.
- the output of this mode converter provides an elliptical beam waist, 10 microns by 200 microns, several millimeters after the last cylindrical lens 16d.
- the beam polarization splitter device 18 in this example is an air-gap type of Wollaston prism positioned such that the elliptical spot occurs within the interior air gap of the beamsplitter.
- the beamsplitter 18 comprises two air spaced yitrium vanadate (YVO 4 ) wedges 18a, 18b at 3° divergences which, together with the mode converter in the anamo ⁇ hic/collimator device 16 are tilted downward at 2.5° relative to the sagittal or horizontal (as seen in the Figures) plane of the optical system.
- the beamsplitter 18 provides as output two polarization component beams diverging at ⁇ 0.6° and of orthogonal polarization relative to each other.
- the beamsplitter 18 also includes a half waveplate 19a in the path of the horizontally polarized beam to rotate that beam to the vertical orientation.
- a glass plate 19b of equal optical path length is in the other beam path, to equalize optical path length and minimize PDL.
- the polarization of the two elliptically shaped and diverging beams are thus parallel to the vertical orientation, or s polarization.
- the Mangin mirror assembly 20 introduces cumulative corrections which reflect the diverging input pair of polarized beams as collimated anamo ⁇ hic beams, deflected downwardly at 2.5° relative to the nominal horizontal plane.
- the beam patterns for each polarization component are only about 1 mm high, but about 20 mm or more in the sagittal direction.
- Fig. 11 demonstrates approximately (the drawing is not to scale) how the four low profile beam patterns project onto the reflective back surface of the Mangin niirror 24. With each anamo ⁇ hic pattern only about 1 mm high, and with only 1 mm between the patterns, it can be seen that a system less than 2 cm high can support multiple refolded beams with dense patterns on the reflective surface.
- the two elements 22, 24 of the Mangin mirror assembly 20 are configured for low optical aberrations such that the theoretical optical resolution is approached.
- the design is optimized to image Gaussian beams with a ⁇ 2 degree field into diffraction limited spots, and to accommodate field of view requirements imposed by the need to image four beams (in this example).
- the highly dispersive diffraction grating (1100 lines/mm) imposes added requirements because of large angular deviations of the edge rays.
- the specifications of the elements of the Mangin mirror combination are as follows:
- the parallel collimated beams from the Mangin niirror combination 20 after the first reflection are incident on the grating 26, which is oriented at 58.2 degrees such that the Littrow angle occurs at 1547 nm.
- the reflected central wavelength of 1545 nm is horizontal to the nominal plane, but in order to dispose all wavelengths from 1530-1565 nm in a horizontal plane at the image plane, the grating lines on each side of the incident beam are rotated up to 2° relative to the vertical. Other wavelengths are diffracted in the horizontal plane with ⁇ 2.5° of the Littrow wavelength.
- the diffracted beams that subsequently reflect off the Mangin mirror assembly 20 angle slightly upwardly but converge, with the separate s and p polarization components being at their slightly different (3.0° and 2.0°) angles, as they angle the pickoff niirror 34 toward the array 38. These angles are maintained until the two beam waists are supeiposed at the cell surfaces 38a at to the image plane at the LC-SLM array 38 (Fig. 10) after passing the polarization dependent element 34.
- the elements of the system may be disposed in precisely adjusted locations, and angled in particular ways.
- the LC-SLM assembly 38 may inco ⁇ orate a silica wedge (1.5°) which is tilted, with the assembly 38, so that the incident beams at 2.0° and at 3.0° each reflect back on the path of the other.
- the Wollaston beamsplitter is preferably aligned so that the optical path lengths of both polarized beams are equal as they exit the last surface.
- an offset is used to assure that the mirrored surface of the Mangin mirror assembly 20 is at equal distances from the centerline of the mode converter.
- a polarization dependent device 36 such as a polarized plate (e.g. "Polarcor") has its optical axis oriented at 0° to the pixel array direction, while the optical axis of each LC cell is oriented at 45° to the pixel array direction.
- the cell is arranged to produce nominally zero polarization rotation under zero applied voltage, providing the transmissive state.
- the cell introduces 90° of polarization rotation, for the high extinction or rejection state at the device 34.
- Intermediate voltage values provide a responsive range of attenuation values, and the control voltage may be adjusted by known expedients to compensate for nonlinearities in the response curves.
- rejected beam power is displaced at an angle in the PBD 34, and out of the field of view of the useful signal bema.
- a diffuse surface may be provided on the element to scatter the rejected beam power sufficiently and avoid intense heating of a supporting structure.
- the rejected beam power may be displaced from the input beam, to be directed back into the optical system to retrace the optical path to a location separate from the anamo ⁇ hic/collimator 16 for dissipation or even as a diverted signal.
- Each channel can be selectively attenuated as required for gain equalization in accordance with a known or measured gain wavelength profile. This profile can also inco ⁇ orate the precompensation needed for both a pre-equalization EDFA and a serially disposed post-equalization EDFA, to provide a fully adjusted DWDM signal at an adequate power level for transmission.
- Fiberoptic gain equalizers compensate for continuous gain and loss variation with wavelength across the transmission band.
- EDFA amplifiers exhibit variations in gain with wavelength, temperature, aging and the number of channels. Therefore, an optical component placed before, after, or within an optical amplifier can dynamically alter the gain characteristics to achieve any desired response.
- a wavelength independent response is desired.
- built-in optical channel monitoring functionality may be part of the gain equalizer.
- the gain equalizer of this invention may advantageously utilize an identical optical design as the channel equalizer; the only difference is the pixel format of the LC- SLM.
- a 4096 1-D pixel array with 1.8 um pixel pitch, fully reflective back mirror plane, and nominally half wave round-trip retardation has been utilized.
- This element consists of zero twist nematic liquid crystal cells in which the rubbing or alignment layer is oriented such that the optical axis of the cell is precisely parallel to the long axis of the LC array.
- the backplane is fabricated using standard silicon processing and lithographic techniques.
- the pixel width is selected to be less than the optical impulse response, the width of the optical beam at the LC-SLM image plane corresponding to a single wavelength of mfinitesimal linewidth.
- the 1/e 2 width of optical impulse response is nominally 10 um
- the pixel pitch is 1.8 um.
- the inte ⁇ ixel gaps are sufficiently narrow (about 0.25 um) such that fringing fields impart a negligible effect on the optical response. These gaps are also highly reflective to maintain low insertion loss.
- This device utilizes a silicon back plane to economically achieve conductor routing for a large number of densely spaced pixels.
- This same high pixel count LC-SLM can serve as a channel equalizer. Not only the attenuation, but also the attenuation slope within the passbands can be independently tailored.
- this device is channel spacing independent, as the widths of the passbands and stopbands can be arbitrarily controlled to within 3.5 GHz, which is the optical frequency spacing corresponding to the 1.8 um pixel pitch for a 600 1pm grating.
- the reflective backplane exhibits continuous high reflectivity even between pixels, so that no light is lost in the inte ⁇ ixel gaps. This is important in order to maintain low insertion loss.
- the identical LC-SLM used in the channel equalizer may be used herein for the gain equalizer.
- the impulse response of the optical beam at the LC-SLM plane should be wider in the sagital plane, so that transmission ripple arising from the inte ⁇ ixel gaps is sufficiently small ( ⁇ 0.1 dB).
- Each pixel is nominally 28 um wide for 50 GHz channels, with a 3 um gap.
- the sagital optical beam width is preferably 30 to 35 um to maintain the highest level of resolution with the lowest passband ripple. The response rolls off 10 dB within about 100 GHz.
- the optical beam is made wider by utilizing a different collimating lens design.
- An anamoiphic lens may be used to achieve a spot size of 35 um wide by 70 um tall at the SLM.
- This approach further has the advantage that the electronics and optics (with the exception of a single input lens) are common among both the channel and gain equalizer components. This allows the technology platform to be leveraged across different product types.
- FIG. 13 An example of a multi-wavelength diffractive Fourier filter for static or unchanging patterns of separation of predetermined wavelengths is shown in the perspective views of Figs. 13 and 14.
- the major optical components of the assembly are chosen for the less stringent operating requirements of 50 GHz or greater channel spacing.
- Individual wavelength signals are modified by a template pattern rather than dynamic elements, to demonstrate the versatility of the concept, although it will be apparent to those skilled in the art that many implementations are feasible.
- the diffractive Fourier optics filter 110 is again within the compact volume of a temperature stable housing 112 (the top of which is not shown) having four input/output ports (114, 115, 116, 117 mounted in pairs of different sides of one end wall 119.
- the two upper ports 114, 115 are also on different sides and define complementary output/input optics.
- the upper ports 114, 115 and lower ports 116, 117 each lie in upper and lower planes respectively relative to a median horizontal plane within the housing 112.
- Each of the ports 114-117 includes interior collimating and anamo ⁇ hic lens elements, as described in the prior example, between an exterior optical fiber and an internal access point leading into the housing 112.
- the ports 114-117 are each mounted to communicate with the internal beam path volume via separate microapertures or pinholes 124 in central regions of different reflective grating elements 120, 121, 122, 123 respectively, paired as spaced apart upper and lower gratings on each side of the housing 112. These are gratings at the Littrow angle, which have 600 lines/mm and are at 27.7° to the incident beam direction.
- the reflective grating faces of these elements 114-117 are slightly tilted to reflect images to a different plane, since the upper gratings 120, 121 line in an upper plane and the lower gratings 122, 123 lie in a lower plane, leaving a median plane in which a pickoff mount 125 is disposed.
- the mount 125 supports angled pickoff mirrors 126, 127 which are on opposite sides of a central template structure 130 (shown in more detail in the enlarged view of Figs. 16 and 17) that is aligned with the vertical midplane of the housing 112.
- the template 130 support comprises a lower base mount 132, the sides of which are parallel to the central longitudinal axis of the housing 112, and below the median plane.
- the base mount 132 sides support waveplate carriers 134, 135 each of which retains a waveplate 137 or 138 that lies in a vertical plane along the longitudinal axis.
- the waveplates 137, 138 are on opposite sides of the static (i.e. fixed pattern) template 130 which is configured to transmit or block individual beams or beam sectors in the spatially dispersed array of diffracted beams.
- the template 130 has reflective surfaces for drop wavelengths and, by way of illustration, spaced apart apertures or transmitting areas for throughput wavelengths.
- the array 141 of parallel reflectors each define an elongated rectangular outline, which may be filled with a highly reflecting material such as gold, and which covers a selected individual channel or group of adjacent wavelength channels.
- the reflective array 141 is sandwiched between transparent substrates 142, 143, so that throughputs go through the reflectors and drop channels (in this instance) are reflected back. Referring again to Figs.
- the refolding optical paths are formed between the diffraction grating pairs 120, 122 and 121, 123 at one path end and different ones of a pair of spaced apart Mangin mirrors 144, 145 at the other path end.
- the Mangin mirrors 144, 145 again have back surfaces with reflective coatings and will typically include anti- reflection coatings on their front surfaces. With wider channel spacings, the beam shaping and correction of a single Mangin mirror is typically sufficient for optical performance requirements.
- the radii of curvatures and the thickness of the glass body of the Mangin mirror assembly 144, 145 are selected to provide the refractive as well as deflective correction and directional control of impinging beams. This system also folds and refolds an input beam to provide all of the functions essential to the diffractive Fourier optics concept in a compact and versatile manner by repeated diffractive beam dispersion and combination to Fourier transform and restore images after wavelength manipulation.
- Fig. 14 schematically shows the outlines of the beam paths, with multiple folding of beams from two input fibers through two separate halves of the diffractive Fourier optic system to two output fibers on opposite sides of the system.
- the lower left 111 (as seen from the input/output side) of the input fibers feeding a diverging anamo ⁇ hic beam through the microaperture 124 in the center of the lower left grating.
- the diverging beam strikes the first Mangin mirror 144 at the mirror's center plane, which coincides with the horizontal median plane of the optical system.
- the Mangin mirror 144 functions as a first collimating element and reflects an anamo ⁇ hically shaped but collimated beam onto the first upper grating 120.
- the grating 120 since it receives the collimated beam at Littrow angle, then reflects it back, diffracted but collimated and parallel to its focal axis, to the first Mangin mirror 144 .
- the diffracted beam reflects again off the Mangin niirror 144, but is now convergent and angled downwardly toward the pickoff mirror 126, which is at the median plane of the template 130.
- the template 130 lies along the image plane of the converging beams at the median level of the optical system, and the wavelength varying diffracted components of the beam are spatially distributed across the template 130, so that each beam can be separately processed.
- the template 130 divides the focused beams into drop and throughput channels by selective reflection of some channels back to the first pickoff 126, and transmission of the others through to the second pickoff 122.
- the paths of the reflected and throughput beams are symmetrically opposite relative to the vertical median plane of the optical system. Considering the reflected side of this symmetry, the reflected beam returns divergently to the pickoff mirror 26 on the input side of the system 110, then reflects back to the first Mangin mirror 144. At this point the pattern is at a lower plane, and the reflected or drop beams are refolded and directed reversely to span the lower grating 122.
- the reflection of re-diffracted beams goes back to the median level of the first Mangin mirror 144, from which it is refolded and angled slightly upwardly, converging toward the upper grating 120 on the first side.
- the beams pass through the micro-aperture or pinhole 124 in the mid-region of the upper grating 120 and pass through the drop output 114.
- the system 110 functions as an interleaver, dividing alternate channels into the different ones of two output fibers.
- channels can be disposed in a predetermined pattern or divided into two adjacent groups, thus providing a 1x2 splitter for the DWDM input signals.
- control elements at the local plane can be dynamically controllable elements, such as an array of transmissive liquid crystal elements.
- Add/drop multiplexers typically consist of arrangements of three port optical devices with an input, drop (or add) and throughput. This can be achieved by using a two port channel equalizer, combined with power splitters. Alternately, a three-port device can be fabricated using a double collimator and power redirecting polarization diversity optics in front of the LC-SLM. This approach has the advantage of utilizing shared cylindrical collimator optics to provide the anamo ⁇ hic beam.
- FIG. 12 The versatility of the concept inherent in the compact three dimensional refolding scheme with low profile anamo ⁇ hic beams is illustrated by the system of Fig. 12, which uses the capability for concurrent transmission of more than one beam at different levels in the beam path volume. Only unique portions of the system are shown in Fig. 12, and it is to be understood that the general features of systems (eg Figs 5-11 or Figs 13-14 )/one of the examples of diffractive Fourier optics may be used.
- a pair of 1 mm O.D. collimators 70, 71, precisely mounted in a base 72 at different elevations are in-line with the group of three shared lenses in the set of anamo ⁇ hic converter lenses 75 (as in Fig. 9).
- the input signal from the first collimator 70 is shaped into an anamo ⁇ hic beam elongated in the sagittal plane (transverse to the plane of the paper). Shaped output from the anamo ⁇ hic converter 75 is again split into two precisely diverging polarized beam components (s and p polarization), but in a single or double Wollaston beamsplitter 80.
- the double Wollaston beamsplitter 80 shown in Fig. 12 may be advantageous in providing a high numerical aperture device, one independent of beam incidence location on the beamsplitter.
- the device 80 comprises two beam pairs of birefringent (YVO ) or wedges 81, 82 and 83, 84 respectively, the wedges of each pair being oppositely tapered and having taper angles of 2.5°.
- the two pairs of optical wedges 80, 81 and 83, 84 respectively are separated by a quartz half waveplate 86. Consequently, an incident DWDM beam may enter the beamsplitter 80 in either direction at a lower or higher elevation, but in either event encounters nominally equal path lengths between the two wedge pairs independent of where the beam enters.
- the quartz half waveplate 86 rotates the two polarization components by 90° between the two wedge pairs, and in the forward direction the s and p polarization components exit at a less than 2.0° separation angle.
- the separate polarization components propagate first through a waveplate 102, and thereafter pass successively through a pair of PBDs 104, 105 with optical axes orthogonally disposed relative to the beam polarization direction.
- the first PBD 104 has its optical axis oriented to transfer the beams straight through without angular deviation, but the second PBD 105 angles the beams downward to the level of an LC-SLM cell 108.
- the same paths are used at all other sagittally dispersed individual wavelength signals and their respective cells in the LC-SLM array.
- the LC cells 108 each transform the polarization of the incident beams by that amount needed for signals to be dropped.
- the drop signals pass through the second PBD without deflection, but when they reach the first PBD 104, they angle to the lower elevation to a level corresponding to that of the lower collimator 71.
- the precise 1 mm spacing between the collimators is equaled by using two PBDs 104, 105 of 7.07 mm thickness and diagonal displacement of one optical axis relative to the other.
- the throughput signals reflected from the cell 108 are not rotated or transformed in polarization so they retrace the incident path through the second PBD 105 and then the first PBD 104.
- the parallel sets of drop and throughput beams then retrace the refolding paths through the diffractive Fourier optics system 80, the beamsplitter 86, and anamo ⁇ hic beam shaper 75 to the separate collimator 71.
- channel equalizers 150, 152 are arranged to provide programmable add/drop functionality as well as channel by channel attenuation adjustment. This is achieved by joining two dynamic channel equalizers 150, 152 in parallel with the outputs of a 50-50 power splitter 154.
- This arrangement includes control electronics 156 for governing the individual channel operations in each of the two equalizers 150, 152.
- the channel equalizers 150, 152 may operate in a complementary fashion with one equalizer extinguishing channels, while the other equalizer attenuates channels as needed. Any combination of wavelength channels can then be dropped or added at a particular location.
- Gain equalizers may also be used in tandem with one or more fiber amplifiers, such as EDFAs, to equalize the gain in the channels, transmitted through the amplifier system.
- the equalizer will be in series with both an input EDFA and an output EDFA so as to compensate the characteristics of the first and precompensate the characteristics of the second. This insures that adequate balance of power at each channel is maintained in the channels as they propagate throughout the network and as the signals are subjected to wavelength dependent loss arising from the fibers attenuation profile, wavelength dependent amplifier gain, or wavelength dependent component losses.
- dynamic channel equalizers/blockers and other channel controls in accordance with the invention When dynamic channel equalizers/blockers and other channel controls in accordance with the invention are employed with optical circulators, they can provide add/drop wavelength capabilities at some signal loss (typically 1.0 to 1.5 dB loss per round trip through the component. If two closely separated collimators are employed, together with beam splitting polarization diversity optics, then circulators can be added to provide the desired add, drop and throughput functionalities. Drop signals reflected back after signal blocking in the optics to a first circulator go to the designated output. The throughput wavelength signals go to a second circulator and to the throughput terminal.
- signal loss typically 1.0 to 1.5 dB loss per round trip through the component.
- One, two or three reflective equalizer/blockers may be used with wavelength independent power splitters to provide add/drop and throughput capabilities with limited or full equalization.
- Fig. 18 depicts the full equalization capability, using three equalizer blockers 170, 172, 174 and two wavelength independent power splitters 176, 178.
- the splitters can be fused couplers with a splitting ratio of 38:62, giving 4.2 dB and 2.1 dB loss respectively.
- the splitting ratio is selected to balance the losses from serial components in transmission to the express, drop and add ports.
- the input signals are fed to the first splitter 176, which provides 38% input to the first equalizer/blocker 170, the blocked wavelengths being the drop output.
- the 62% brand output from the splitter 176 is applied to the second splitter/combiner 178.
- the wavelength components in channels output from the second equalizer/blocker 172 have been attenuated 4.2 dB, equal to the attenuation of add signals at the 38% arm of the second splitter 178.
- the equalizer/blockers provide independent blocking or attenuation control of each channel in the drop, express and add ports.
- Figs. 17 and 18 demonstrate the feasibility of systems that employ the functionalities of circulators or splitter together with the dynamic channel controls of equalizer/blockers to provide add/drop and throughput functions alone, or with added equalization.
- Those skilled in the art will recognize that the same capability of these subunits can also be employed in other combinations, with more or fewer input and output ports, to provide a variety of routing and switching functions.
- the superior optical performance of these various fiber coupled grating spectrometers enables dramatic improvements in system performance and wavelength management capabilities.
- the low loss (3- 4 dB) results in less need for reamplification.
- the low inherent chromatic dispersion of the device ( ⁇ 10 ps/nm) does not compromise the dispersion budget, which results in less frequent electronic regeneration.
- the flat passbands (0.1 dB) allow the add/drop to be cascaded in a ring type network where passband narrowing effects must be avoided.
- high channel extinction > 40 dB) eliminates any system penalty arising from coherent crosstalk.
- the low profile design ( ⁇ 20 mm tall) enables this enhanced functionality within a limited volume. This device offers independent control of attenuation for each channel using simple control algorithms. Unlike programmable Fourier synthesis type filters, this design does not suffer from potential liquid crystal reset issues (discontinuities in phase for transitions through ⁇ retardation) and is optically transparent in the power-off state.
- the wavelength dependent diffraction angle is also a function of the index of refraction of the surrounding atmosphere.
- the index exhibits a relatively strong dependency on air pressure. Note that air exhibits ⁇ 10 "5 index dispersion across the 1500 to 1600 nm wavelength range. As the atmospheric pressure is reduced, the index decreases linearly from the 1 atm value to the vacuum index.
- the mapping of each particular wavelength onto a location on the LC-SLM plane depends of several factors dependent on the optical design.
- the index of refraction of the atmosphere surrounding the spectrometer can shift the location of each wavelength. Air at 20 °C and 1 atm has an index of refraction of 1.0002926 (D line). If air is replaced with 1 atm He, with an index of refraction of 1.000036 (D line), the absolute frequency shifts approximately -50 GHz. Alternately, if air is replaced with CO 2 , with index of refraction of 1.000451 (D line), the absolute frequency shifts approximately +30 GHz. Therefore, by altering the composition of the atmosphere, the absolute frequency of the channel equalizer can be shifted during final packaging.
- Moisture corrections to dry air index values are made by Lorentz's formula: +0.000041 (Pmo/760), where P H2O is the vapor pressure in mm Hg.
- Humidity variation can therefore produce up to 8 GHz of absolute frequency shift. Note that this also produces about 2 GHz of center frequency walk-off from channel one to channel hundred.
- FIG. 19 illustrates the effect of a reduction in pressure from 1 atm to vacuum within the optical package. Note that channel 1 and channel 100 (for 50 GHz channel spacing) walk-off in frequency by different amounts. Therefore, in addition to the phase, the period also changes. Practical pressure tuning is therefore limited to about +/- 0.1 atm, to maintain frequency walk-off below +/- 2 GHz while phase tuning over a +/- 10 GHz range.
- the DWDM control systems for individual wavelength signals described herein address several needs of next generation optical networks not met by present components and systems.
- the diffraction grating approach achieves wide passbands and precise stop bands necessary for 25, 50 and 100 GHz operation, in contrast to component technology which generally does not scale well at channel spacings of 50 GHz and below.
- the compact, three dimensional refolding concept provides great versatility, in enabling systems to be realized which have different numbers of inputs and outputs.
- the approach is a relatively low cost answer for large scale deployment of new components technologies. It has a relatively simple optical system with a low parts count and enables a wide variety of low cost manufacturable solutions of problems to be made available.
- any of a number of suitable spatial light modulators including MEMs and micro-mechanical light diverters may be used.
- devices and methods in accordance with the invention open wide possibilities for analog and digital control employing signal attenuation, equalization, switching and routing of signals.
Abstract
Description
Claims
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EP02747944A EP1413078A4 (en) | 2001-07-06 | 2002-07-03 | Diffractive fourier optics for optical communications |
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US30375901P | 2001-07-06 | 2001-07-06 | |
US60/303,759 | 2001-07-06 | ||
US34796401P | 2001-11-07 | 2001-11-07 | |
US60/347,964 | 2001-11-07 |
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Cited By (3)
Publication number | Priority date | Publication date | Assignee | Title |
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EP1932033A2 (en) * | 2005-09-08 | 2008-06-18 | Xtellus Inc. | Optical wavelength selective router |
US7484102B2 (en) | 2004-09-07 | 2009-01-27 | Microsoft Corporation | Securing audio-based access to application data |
KR101923821B1 (en) | 2017-06-05 | 2018-11-29 | 국방과학연구소 | Design method of dielectric multilayer thin film diffraction grating |
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Also Published As
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EP1413078A4 (en) | 2005-11-09 |
EP1413078A1 (en) | 2004-04-28 |
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