WO2003054595A1

WO2003054595A1 - Arrangement and method for characterizing fiber gratings

Info

Publication number: WO2003054595A1
Application number: PCT/SE2002/002127
Authority: WO
Inventors: Leif Nyholm; Bengt Sahlgren
Original assignee: Proximion Fiber Optics Ab
Priority date: 2001-12-13
Filing date: 2002-11-22
Publication date: 2003-07-03
Also published as: AU2002366934A1; SE0104191D0

Abstract

A method and an arrangement for characterizing in-fiber gratings are disclosed. According to the invention, the characteristics of a fiber grating are analyzed by reflecting light in a measurement branch of an interferometer from the grating, and subsequently interfering the reflected light with light from a reference branch of the interferometer. A first measurement value is recorded. Then, the polarization state of the light in the reference branch is switched to an orthogonal state, whereupon a second measurement value is recorded. A polarization insensitive interference value is obtained by taking the quadratic sum of the first and the second measurement values.

Description

ARRANGEMENT AND METHOD FOR CHARACTERIZING FIBER GRATINGS

Field of the invention

The present invention relates to an arrangement and a method for characterizing fiber gratings. More particularly, the invention relates to white light interferometry characterization of in-fiber Bragg gratings .

Background of the invention A prior art technique for white light interferometry for the purpose of characterization of in-fiber gratings is disclosed in, for example, "Characterization of Fiber Bragg Gratings by Use of Optical Coherence-Domain Reflectometry" , Petermann et al . , J. of Lightwave Tech., vol. 17, no. 11, p. 2371-2378. White light interferometry for said purpose is sometimes also known as Optical Coherence-Domain Reflectometry (OCDR) or Optical Low Coherence Reflectometry (OLCR) .

In general, white light interferometry is based on the utilization of a light source having low, or limited, temporal coherence in an interferometer set-up, where the grating under test (GUT) or device under test (DUT) is placed in one of the interferometer branches. The low temporal coherence (short coherence length) of the light source provides a narrow spatial region in which interference is obtained. Hence, by varying the optical length of a reference branch, the measurement (or characterization) region may be moved along the GUT. Interference is only obtained in a region in the GUT for which the length of the two interferometer branches are substantially equal. In this way, it is possible to move the measurement region along the GUT. The term "white light" is utilized in order to express the fact that a broad band light source is employed in the set-up. However, it is to be understood that the light source is not truly "white" . The light source may, for example, be a light emitting diode having a wavelength width of about 50 ran, giving a spatial dimension of the measurement region of slightly less than 100 μm. It is appreciated that a more broad band light source provides a higher spatial resolution, and vice versa.

However, the fibers used in the aforementioned arrangements are typically not polarization maintaining, which may cause polarization fade-out of the detected interference signal. For this reason, it is not possible to distinguish between a decrease in grating strength on the one hand, and polarization fade-out on the other hand.

Hence, there is a need for improved methods and arrangements for characterization of fiber gratings, which address the problem of polarization fade-out.

Summary of the invention

Therefore, a main object of the present invention is to provide an arrangement and a method for characterizing fiber gratings, for which polarization fade-out in the interference signal is eliminated, or at least drastically reduced. In other words, characterization of gratings according to the invention is independent of (immune to) polarization twisting occurring in fibers. This object is achieved by the method and the arrangement according to the appended claims.

Furthermore, the arrangement and method according to the present invention provides additional advantages, as will be elucidated in the detailed description below. According to the invention, grating characterization that is independent of (immune to) polarization fade-out in the interference signal is obtained by controlling the polarization state in at least one of the branches of the interferometer. The characterization is performed by making two measurements, wherein the polarization state in one of the interferometer branches is switched between two orthogonal states after the first measurement and before the second measurement. A polarization insensitive interference value is then derived from the first and the second measurement values .

To this end, an arrangement according to the present invention comprises means for switching between orthogonal polarization states in one of the interferometer branches. In a preferred embodiment, said means for switching includes a phase modulator. The action of the phase modulator is to delay one polarization component with respect to the other such that the total polarization state is mirrored in an optical axis of the phase modulator. Preferably, the orientation of the optical axis of the phase modulator is such that the polarization state is rotated by 90 degrees. It is to be noted, however, that for

"unpolarized" light, rotation of the polarization state is not meaningful unless the rotated polarization state is compared with a portion of the light for which no rotation has been effected. It is^" preferred to work with pure polarization states within the arrangement according to the invention. For this reason, a polarizer is preferably provided downstream from the phase modulator, in order to filter out one pure polarization state. In most cases, the selected pure polarization state will be linear. However, a circular polarization state is also conceivable.

Furthermore, according to the invention, it is preferred to employ two measurement regions within the grating under test (GUT) . In this way, perturbations in the set-up may be cancelled out by comparing the measurement values obtained within each of said measurement regions. Only perturbations occurring between the two measurement regions will affect the result of the measurement. In most cases, any perturbations occurring between the measurement regions, which are typically about a centimeter apart, are negligible. In order to provide for two different and longitudinally spaced measurement regions, the reference branch of the interferometer arrangement comprises two different paths, and means for selectively switching between said two paths. The optical lengths of said paths are different, such that interference is obtained in two different regions in the GUT. Since a comparatively broad-band light source is employed, the coherence length of the light is limited. For this reason, and as mentioned above, interference is only obtained when the optical path lengths in the measurement branch and the reference branch differ by at most one coherence length. The means for switching between the two paths is preferably a polarizing beam splitter, and a means for switching the polarization between a first state that is reflected by said beam splitter and a second state that is passed by said beam splitter. The means for switching the polarization state may be a phase modulator together with an optional polarizer.

Another preferred feature of the invention is a possibility of scanning the measurement region (s) along the GUT. This feature is preferably implemented by means of a moveable corner reflector in a delay line of the reference branch of the arrangement . By moving the corner reflector, and thus alter the optical length of the reference branch, the region in the GUT for which interference is obtained (i.e. the measurement region) is scanned along the GUT.

Furthermore, by employing a number of phase modulators, which each may be in either of two possible states, the arrangement according to the invention can be switched between eight different states. Advantageously, cycling between these eight different states is employed during operation, to record measurement values corresponding to different states in a time multiplexed fashion.

Brief description of the drawings

The. features and advantages of the present invention will be more fully appreciated when the following detailed description is read in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic diagram of a first preferred arrangement according to the invention;

Fig. 2 is a schematic diagram of a second preferred arrangement according to the invention;

Fig. 3a and 3b illustrate the operation of a polarization selective feature of the arrangement according to the invention;

Fig. 4a and 4b illustrate the operation of a feature for selecting between two alternative light paths in the arrangement according to the invention; Fig. 5 is a flow chart schematically showing the steps performed in a first embodiment of the method according to the invention; and

Fig. 6 is a flow chart schematically showing the steps performed in a second embodiment of the method according ^"to the invention.

Throughout the drawings, like parts are designated like references.

Detailed description of preferred embodiments An arrangement for characterizing a fiber grating according to the present invention is schematically shown in figure 1. An arrangement of this general kind is also known as a White Light Interferometer (WLI) .

In general, the arrangement is an interferometer with two branches. At an input side, there is provided a power splitter for dividing incident light into two portions and for sending each of said portions into a respective branch of the interferometer. More specifically, with reference to figure 1, the arrangement 10 comprises a measurement branch (a first branch) 101 and a reference branch (a second branch) 102. At an input side of the arrangement, the two branches 101 and 102 are interconnected by a first 50/50 optical coupler 12. At an output side, the two branches are again interconnected by a second 50/50 optical coupler 14. The second optical coupler is arranged to bring light from the two branches back together, and thus cause interference between light from the two branches. In the measurement branch, the device under test (DUT) or grating under test (GUT) 15 is introduced. In general, the interference caused by bringing the two portions of light back together at the second coupler 14 depends on the characteristics of the GUT 15.

More specifically, the GUT 15 is coupled to the measurement branch by means of a first optical circulator 16. The circulator 16 operates to pass light incident from a first terminal 16.1 to a second terminal 16.2, and to pass light incident from the second terminal 16.2 to a third terminal 16.3. The GUT 15 is connected to the second terminal 16.2, and acts as a retro reflector for light output from the second terminal to direct said light back towards said second terminal. In this way, any light having a wavelength within a reflectance band of the GUT 15, and being directed into the measurement branch of the arrangement, will eventually arrive at the second coupler 14. It is to be noted, however, that interference between light from the measurement branch and light from the reference branch is only obtained for light reflected by the GUT 15 at a location that makes the optical distances in the two branches substantially equal. In this way, although light is reflected from every portion of the GUT 15, measurement is only performed in (interference is only obtained for) a selected region in the GUT 15 that fulfills the requirement that the optical distances traveled by light in each respective branch are substantially equal.

Turning now to the reference branch 102 of the arrangement 10, a length of fiber on the input side is terminated by a first lens 18. This lens is preferably a gradient index lens (GRIN-lens) , but could also be of some other type. Beyond the lens 18, light in the reference branch is in free propagation without any waveguiding means such as a fiber. It is to be understood, however, that any of the optical components (to be described in more detail below) in the reference branch may alternatively be carried out as in-fiber components .

After the lens 18, the light passes through a first phase modulator PM1 and a first polarizer PI. The polarizer PI is aligned such that vertically polarized light (with reference to the drawing) is passed. Hence, beyond the polarizer PI, the light is linearly polarized in the vertical direction. Continuing in the light path, there is arranged a first polarizing beam splitter PBSl, which is arranged to reflect vertically polarized light and to pass horizontally polarized light. Light coming from the first polarizer PI is vertically polarized, and hence reflected by the polarizing beam splitter PBSl (downwards in figure

1) •

After the polarizing beam splitter PBSl, the light encounters a series of components arranged to rotate the polarization direction from vertical to horizontal. First, there is arranged a Faraday Rotator FR, which is operative to rotate the polarization direction 45 degrees in a predetermined sense of rotation. Now, having a polarization direction of 45 degrees from the vertical (and, hence, from the horizontal) , the light passes an optional second polarizer P2. The second polarizer P2 is merely for extra precaution, in case the rotation provided by the Faraday Rotator is not perfectly 45 degrees. Next, there is arranged a half-waveplate (λ/2 -plate) having its optical axis at 22.5 degrees with respect to the horizontal. The action of the waveplate is to mirror the polarization direction of linearly polarized light about its optical axis. In effect, the ^■ polarization direction of the light becomes horizontal. The waveplate is optional and may be left out from the arrangement. However, the waveplate is preferred because it is more convenient to work with horizontal polarizations than, for example, a polarization at 45 degrees. Furthermore, the second polarizer P2 may be left out, since its only purpose is to filter out any residual light having an undesired polarization.

Next, the light passes a second phase modulator PM2 , which is used to selectively switch the polarization of the light between vertical and horizontal . More specifically, if the phase modulator is switched off, no effect will be caused upon the polarization of the light. On the other hand, if the phase modulator is switched on, the polarization state of the light will be rotated 90 degrees. After the second phase modulator PM2 , the light continues to a second polarizing beam splitter PBS2. Depending on the polarization state of the light after the passage of the second phase modulator PM2 , light will either be ^'reflected or transmitted at the second polarizing beam splitter PBS2. By controlling the polarization state of the light by means of the second phase modulator PM2 , light can be selectively directed towards a first retro reflecting mirror Ml or a second retro reflecting mirror M2. In this way, the length of the reference branch of the interferometer can be switched between a first length LI corresponding to reflection from the first mirror Ml, and a second length L2 corresponding to reflection from the second mirror M2. In either case, light is reflected by one of the mirrors Ml and M2 back through the polarizing beam splitter PBS2 , the phase modulator PM2 , the waveplate, the polarizer P2 and the Faraday rotator FR. The action of the Faraday rotator, which will be described below, will cause the polarization state of the light to be such that light is now passed through the first polarizing beam splitter PBSl. After passage of the first polarizing beam splitter PBSl, the light passes a third phase modulator PM3. As for the first and second phase modulators, the third phase modulator PM3 is used for selective switching of the light between two polarization states, either horizontal or vertical. Again, if the phase modulator is switched off, no effect will be caused upon the polarization of the light. On the other hand, if the phase modulator is switched on, the polarization state of the light will be rotated 90 degrees. With the setup shown in the figures, light passing the first polarizing beam splitter PBSl is always polarized horizontally. Therefore, if the third phase modulator is switched off, light will be horizontally polarized after the passage of the same. If the third phase modulator is switched on, light will be vertically polarized after the passage .

Hence, the third phase modulator is used for selectively sending horizontally or vertically polarized light into a length of fiber 103 on the output side of the reference branch. Preferably, and as schematically shown in the figure, light is launched into said length of fiber 103 by means of a second lens 20. The second lens 20 is preferably a GRIN-lens.

The length of fiber 103 is coupled to a second optical circulator 17. The second circulator 17 operates to pass light incident from a first terminal 17.1 to a second terminal 17.2, and to pass light incident from the second terminal 17.2 to a third terminal 17.3.

The third terminal 17.3 of the second optical circulator 17 is coupled to the second 50/50 optical coupler 14, by means of which light from the measurement branch 101 and light from the reference branch 102 are brought back together to interfere. The thus produced interference is detected by a differential photo detector 22 to provide an output signal indicative of the interference, and hence ^'the measurement performed on the GUT 15.

In order to scan the measurement region (s) through the GUT 15, a delay line 30 is preferably coupled to the second terminal 17.2 of the second optical circulator 17. For different lengths of the delay line 30, interference will be obtained for light reflected at different positions in the GUT 15 when light from the measurement branch and the reference branch is brought back together at the second 50/50 optical coupler.

In the shown embodiment, the delay line 30 comprises a translatable corner reflector 31 and a mirror 32. The translatable corner reflector 31 can be translated parallel to a light path in order to adjust the length of the delay line 30. For convenience, a third lens 33 is provided at an end face of a length of fiber, which in turn is coupled to the second optical circulator 17.

Furthermore, it may be preferred to utilize a fourth lens 34 in front of the mirror 32 in order to compensate for divergence of the light beam, and hence to minimize optical losses in the delay line. In order to accurately control the position of the translatable corner reflector (and hence the length of the delay line) , interferometric means is preferably employed. For example, a mirror (not shown) could be mounted on a sledge that carries the corner reflector, said mirror acting as a mirror in a Michelson type interferometer.

As explained above, the interferometer according to the present invention is based on a broad band, low temporal coherence light source. The coherence length of the light from the light source should preferably be shorter that the grating under test. In the most preferred embodiment of the invention, the coherence length of the light from the light source is about 1 mm. In this way, interference is only obtained for light that has traveled substantially the same optical distances in the measurement branch and in the reference branch, respectively. It is to be noted that the optical distance generally equals the geometrical distance times the effective refractive index. Although the light source should be broad band, the center wavelength thereof still needs to be matched to the local grating period of the GUT 15 in the measurement region. For this reason, it is preferred to use wavelength control in order to tune the center wavelength of the light source in accordance with the current position of the measurement region in the GUT 15.

Therefore, the light launched into the interferometer is preferably obtained as follows. Light is originally obtained from a white light source WLS, emitting light in a broad wavelength band. The WLS may be an erbium-doped fiber amplifier (EDFA) . Next, the emitted "white" light is filtered by a tunable spectral filter 24 (such as a tunable Fabry-Perot etalon) , and provided to the interferometer. Preferably, the filtered light is amplified, for example by means of a second Erbium-doped fiber amplifier EDFA, before being launched into the interferometer. In the preferred embodiment of the invention, a light source having a wavelength width Δλ (after the filter) of about 1 nm is used, giving a coherence length l_c of about 1 mm. Consequently, the measurement region in the grating under test 15 has a longitudinal dimension of about 1 mm. The phase modulators in the arrangement are preferably comprised of a pair of lithium niobate crystals, which are mounted at 45 degrees with respect to the horizontal plane and orthogonal with respect to each other. The crystals are electrically controlled to jointly act as a half-wave plate, and when horizontally or vertically polarized light is sent through the crystals, the polarization direction can either be rotated 90 degrees (mirrored in the optical axis of the crystal) if the phase modulator is switched on, or unaffected if the modulator is switched off.

In the following, the function and operation of the arrangement according to the invention will be described in more detail .

Each of the phase modulators PM1 , PM2 and PM3 introduces a possibility to switch between two different polarization states. Therefore, in an arrangement having three phase modulators, as shown in figure 1, eight (2x2x2=8) different states are possible for the arrangement .

The first phase modulator PM1 , together with the first polarizer PI, selects one linear polarization component of the light in the reference branch 102. The polarization component selected by the first phase modulator PM1 will be parallel to the passing direction of the polarizer PI, but could correspond to either of the polarization components of the light in the measurement branch 101 upstream from said phase modulator

PM1 by virtue of the polarization rotation introduced by said first phase modulator PM1. This is schematically illustrated in figures 3a and 3b. At any rate, the selected polarization component will only interfere with the corresponding, parallel component of the light in the measurement branch 101, since polarization components that are originally orthogonal in terms of polarization can be regarded as incoherent and cannot interfere . However, it is to be noted that the polarization direction of the light in the measurement branch 101 may rotate during its propagation through the fiber. Therefore, interference may be obtained even if the polarization component selected by the first phase modulator PM1 is rotated with respect to the light in the measurement branch 101 before light from the measurement branch 101 and the reference branch 102 are brought back together. Of course, it is also possible that the polarization state of the light in the reference branch 102 may rotate during its propagation through the fiber. As will be described below, this situation is handled by means of the third phase modulator PM3 , which is employed for switching between different polarization directions to be launched into the fiber 103.

After the first optical coupler 12, the light in each of the branches 101 and 102 can be regarded as being composed of two orthogonal polarization components. This is true regardless of the actual polarization state of the light before the optical coupler 12. For convenience, let these two polarization components be horizontal and vertical (with reference to figure 1) . After the optical coupler 12, the horizontal components will always be coherent to each other, regardless of whether they are twisted or rotated at a later stage. Below, the horizontal polarization component will be referred to as the first component, and the vertical polarization component as the second component . In the reference branch, the first and/or the second polarization component will be altered depending on the state of the arrangement (i.e. of the phase modulators) . In effect, the originally horizontal polarization component, the first component, may be either horizontally or vertically polarized at later stages in the reference branch.

In the following, the above-mentioned eight different states of the arrangement will be described in more detail with reference to figure 1. In the first state of the arrangement, the first phase modulator, the second phase modulator and the third phase modulator are all switched off. In this state, which could be called the (000) -state, a vertical polarization direction is selected by the first polarizer PI. Since the first phase modulator is switched off, this vertical direction corresponds to the second component (the originally vertical component) . The situation when the first phase modulator is switched off is schematically shown in figure 3a.

Hence, the second component, still vertically polarized, is incident upon the first polarizing beam splitter PBSl and reflected towards the Faraday rotator FR (i.e. downwards in the figure) .

The Faraday rotator FR is arranged to rotate this vertically polarized second component 45 degrees. To ensure a 45 degree polarization, light passes a second polarizer P2 after the Faraday rotator FR. Next, the light passes a half-wave plate λ/2, which has its optical axis oriented at 22.5 degrees with respect to the horizontal, such that the second component (now polarized at 45 degrees) is mirrored in the optical axis. In effect, the second component (the originally vertical component) is horizontally polarized after the half-wave plate .

Light now encounters the second phase modulator PM2 , which is also switched off in this (000) -state. Therefore, the polarization state is not affected by the second phase modulator PM2 , and the light is still horizontally polarized.

Beyond the second phase modulator PM2 , light is incident upon the second polarizing beam splitter PBS2, which is operative to pass light having a horizontal polarization, and to reflect light having a vertical polarization. Hence, the light (which is horizontally polarized) is passed through the second polarizing beam splitter PBS2 towards the first mirror Ml. The effect on the polarization of the light is shown in more detail in figures 4a and 4b, figure 4a referring to light propagating downwards and figure 4b referring to light propagating upwards.

When the light has been reflected off the first mirror Ml, it propagates upwards and is again passed through the second polarizing beam splitter PBS2. Since the second phase modulator PM2 is switched off, the polarization remains unaltered and in the horizontal direction. The half-wave plate mirrors the polarization direction in its optical axis, making the light polarized at 45 degrees, allowing it to pass the second polarizer P2. Next, the light enters the Faraday rotator FR on its way upwards. The Faraday rotator is operative to rotate the polarization direction in an absolute predetermined sense, i.e. in the same absolute sense as during the propagation downwards. Consequently, the light is horizontally polarized after the second passage of the Faraday rotator FR.

The light now being horizontally polarized, it is passed through the first beam splitter PBSl and continues upwards towards the third phase modulator PM3. Since, in this (000) -state, the third phase modulator PM3 is switched off, the polarization remains horizontal also after PM3.

The light is then input into the fiber 103, and passes the second optical circulator 17 and the delay line 30, and is mixed with the light from the measurement branch by the second optical coupler 14 to interfere. It is to be understood that the actual polarization direction of the light from the reference branch may twist during the propagation through the fiber 103, such that it is no longer a pure polarization state when it is caused to interfere with light from the measurement branch at the coupler 14. Nevertheless, it will always correspond to the same polarization component selected by the first phase modulator PM1 and the first polarizer PI (which is the originally vertical polarization component, since PM1 is switched off in this state of the arrangement) . Consequently, light from the reference branch can only interfere with that component of the light from the measurement branch that was originally vertically polarized. Moreover, there must be parallel components at the second optical coupler 14 for interference to occur. Having now described the (000) -state in which all of the phase modulators are switched off, it is straightforward to describe the other states of the arrangement. The following eight states are possible:

In the (100) -state, the first phase modulator PM1 is switched on. Therefore, the polarization state is switched and the originally horizontal polarization component is now vertical, and hence passed by the first polarizer PI. After the first polarizer, the light is vertically polarized, and the functional description is equivalent to above. At the second optical coupler 14, the horizontal component of the light from the measurement branch is now analyzed.

In the (010) -state, the second phase modulator PM2 is switched on. The second phase modulator PM2 , together with the second polarizing beam splitter PBS2, selects one of the two different paths in the reference branch (defined by the mirrors Ml and M2) . Consequently, the second phase modulator PM2 is used for selecting the location of the characterization region in the grating and for rapidly switching the location by switching between the first and the second mirror Ml and M2. It should be recalled that the light source employed in the arrangement has a limited temporal coherence, giving a confined characterization region at the location in the grating for which the total optical lengths of the measurement branch and the reference branch are substantially equal. In the (010) -state with the second phase modulator PM2 switched on, the polarization of light is rotated to vertical, such that the path defined by the second mirror M2 is selected, since vertically polarized light is reflected by the second polarizing beam splitter PBS2.

In the (001) -state, the third phase modulator PM3 is switched on. The effect is that the horizontal polarization coming from the first beam splitter PBSl is rotated to vertical. Hence, the third phase modulator PM3 selects between vertical and horizontal polarization. Since light in the measurement branch may rotate during the propagation through the fiber, the polarization state of the light from the measurement branch may be mixed (i.e. not pure) at the second optical coupler 14. Furthermore, interference is only obtained between polarization components that are parallel to each other. For this reason, even if, for example, a vertical polarization component was originally selected by the first phase modulator PM1 and the first polarizer PI, not all light from the measurement branch may still be vertically polarized. In order to achieve interference, and thus a measurement, of all light corresponding to the original vertical polarization, the third phase modulator is used for selecting either of two polarization directions to be launched into the fiber 103. By making two measurements, one with PM3 switched off and one with PM3 switched on, interference is obtained also for light that has rotated its polarization during the propagation in the measurement branch.

In other words, the third phase modulator PM3 is used for selecting if vertically polarized light or horizontally polarized light is to be mixed with light from the measurement branch. For example, if a vertical polarization component was selected by the first phase modulator, then the light in the reference branch always correspond to the vertical polarization component of the light in the measurement branch, which makes interference possible when light from the measurement branch is brought back together with light from the reference branch. However, the polarization state of the light in the reference branch 'may (and will.), be rotated by the third phase modulator PM3 such that, when it interferes with light from the measurement branch, it is either horizontally polarized or vertically polarized (or mixed due to polarization twisting in the fiber 103) . The third phase modulator determines whether this selected polarization component should be allowed to interfere with the vertical or the horizontal polarization component of the light in the measurement branch. If the polarization of the light in the measurement branch was not rotated during the propagation in the fiber, interference will only be obtained if the selected vertical component is mixed with the vertical component of the light in the measurement branch. On the other hand, if the polarization of the light in the measurement branch was actually rotated somewhat during the propagation in the fiber, some interference will also be obtained when the selected vertical component is mixed with the horizontal component of the light in the measurement branch. It is to be noted that, regardless of whether the polarization direction is rotated by the third phase modulator, it will always correspond to the polarization originally selected by the first phase modulator PM1 and the first polarizer PI. Having described the action of each of the phase modulators PM1 , PM2 and PM3 , it should be apparent what the function of the other states are.

In conclusion, the first phase modulator PM1 selects one of two original polarization components to be analyzed. PM2 selects one of two different paths in the reference branch, thereby selecting one of the two possible measurements regions in the GUT 15. PM3 selects the polarization direction to be launched into the fiber 103 and thus to be interfered with light from the measurement branch at the coupler 14. It can be said that the first phase modulator PM1 determines for which polarization direction the grating should be characterized; the second phase modulator PM2 determines which of two possible characterization regions should be used; and the third phase modulator PM3 determines whether the selected polarization component in the reference branch should interfere with a first or a second polarization component in the measurement branch.

Figure 5 shows a schematic flow chart of the measurement procedure .

To characterize the grating 15 in a selected polarization direction (say, vertical) , this selected polarization direction is filtered out by the first phase modulator PM1 and the first polarizer PI. Then the interference is detected to obtain a first measurement value 501. Once the first measurement value has been obtained, the polarization direction to interfere with light from the measurement branch is rotated, or switched, 90 degrees 502 by use of the third phase modulator PM3. Then, the interference is detected again to obtain a second measurement value 503. Finally, a polarization insensitive interference value is derived from the first and second measurement values 504, said polarization insensitive interference value being indicative of the grating characteristics.

The first measurement value corresponds to light remaining in the original polarization direction, and the second measurement value corresponds to light that has been rotated (causing interference fade-out) during the propagation in the fiber. The quadratic sum of the first and the second measurement values is therefore constant for a selected characterization region in the grating, and thus indicative of the grating characteristics of the fiber grating under test (GUT) in that characterization region. Hence, a polarization insensitive interference value is obtained by taking the quadratic sum of the first and the second measurement values .

During operation, the arrangement according to the invention is cycled through the eight different states

(as determined by the phase modulators PM1, PM2 and PM3) . For each state of the arrangement, a measurement value is preferably recorded in a time multiplexed fashion. When one full cycle, wherein the eight states have been cycled through, the length of the delay line is adjusted in order to move the measurement regions along the GUT 15.

The cycling scheme for one position of the delay line is schematically shown in figure 6.

First, one polarization component of the light sent into the reference branch is selected 601 by means of the first phase modulator PM1. The selected polarization component is made to have a vertical polarization direction 602 (regardless of its original polarization direction) , such that it is reflected by the first polarizing beam splitter PBSl towards the second beam splitter PBS2. The polarization of the light is then rotated 90 degrees 603 by means of the Faraday rotator FR and the half-wave plate λ/2 before it enters the second phase modulator PM2. Then, by means of said second phase modulator ^"PM2 , either the first or the second path is selected 604, by switching (or not switching) the polarization direction such that light is either passed or reflected by the second beam splitter PBS2. After reflection in either of the mirrors Ml or M2 , light continues towards the third phase modulator PM3 , which selects 605 whether vertically or horizontally polarized light is to be launched into the fiber 103. Following this, a first measurement value is recorded 606.

The cycling then continues to the next state of the arrangement by changing the third phase modulator PM3 such that the other polarization direction 607 is launched into the fiber 103 and a measurement value is again recorded 606.

Then, the other path in the reference branch is selected 608, and two more measurement values are recorded according to above.

Following this, the other polarization component is selected 609 by the first phase modulator PM1 , and the process continues to obtain four additional measurement values . Consequently, in one cycle of measurements, eight measurement values are recorded, which form the basis for the actual characterization of the grating under test.

After a full cycle, the length of the delay line is adjusted 610, and a new cycle starts. In the following, the measurement by means of the differential detector 22 will be described in more detail .

Let the electromagnetic field from the measurement branch be described by

and the electromagnetic field from the reference branch by

E₂-e^"iωt. (2)

Then, the field to the upper input ("+" -input) of the detector 22 is given by

+ (l/Λ/2)E₂-e-^i(ωt+φ) (3)

and the field to the lower input ("-"-input) of the detector 22 by

+ (l/V2)E_re-^i(ωt+,|)) (4) where φ is the phase shift introduced to the electromagnetic field that is coupled to the "other" side .

The quantity actually detected by the detector 22 is the irradiance of the electromagnetic fields, i.e. the time average of the energy. If the time average is taken over one single period, the temporal phase term (cot) can be left out. The energy is proportional to the field times its complex conjugate, according to

A-A^* = ( 1 / 2 ) ( E_x ² + E₂ ² + E E₂ ^*-e^+iψ + Eι^*E₂-e^{_ φ}) ( 5 )

for the upper field, and

B-B^* = (1/2) (E_x ² + E₂ ² + E₁E₂ ^*-e^"iφ + E_x ^*E₂-e^+iφ) (6)

for the lower field.

Taking into account the requirement that energy must be conserved in the optical coupler 14, the total input energy must equal the total output energy, such that

EiEi^* + E₂E₂ ^* = A-A^* + B-B^* (7)

Substitution for equations (5) and (6) then gives the solution that cp=+π/2. Without any loss of generality, it is then possible to choose cp=+π/2.

Consider now the differential detector 22. The detector output signal is proportional to the difference input signal, namely

A-A - B-B^* = i(EχE₂ i E ) (8)

In practice, there is of course a phase difference between the two electromagnetic fields that has notyet been taken into account. To take the phase difference between the fields into account, let the fields be described by Ei = CrE₀-e^{" iφ} ( 9 )

E₂ = C₂-E₀ ( 10 )

where Ci and C are real valued constants describing the amplitude and φ is the phase difference between the two fields when they arrive at the detector. Taking, for simplicity, Ci = C₂ = C, the detected difference signal as described by equation (8) can now be written as

A-A^* - B-B^* = -iC²(e^iφ - e^"iφ) E₀ ² (11)

or, using Euler formulas

A-A^* - B-B^* = 2C²-sin(φ)E₀ ². (12)

It is now clear that φ corresponds to the interference between the electromagnetic fields, and will vary as the measurement regions are scanned along the GUT 15. Hence, the interference is detected by the differential detector 22, as explained mathematically above .

By using a first and a second measurement region, as is suggested in accordance with the present invention, any drift in the system can be cancelled out by subtracting the measurement value associated with the first region from the measurement value associated with the second region, and hence eliminate any perturbation that is common to the two regions. What is actually obtained is a phase difference between the measurement regions, and the overall phase of the grating under test is obtained by simply integrating the phase difference along the grating. Another embodiment of the arrangement according to the present invention is schematically shown in figure 2. The phase modulator PM1 and the polarizer PI are both shown, in this embodiment, as in-fiber components. However, it is to be understood that these components could also be of standard type, optically connected to the fiber by means of, for example, GRIN lenses. In this case, the dual path feature has been omitted. Hence, for each setup of the delay line 30, only one characterization region is obtained. The arrangement comprises a phase modulator PM, corresponding to the first phase modulator of the arrangement shown in figure 1, and an optional polarizer P for filtering out the selected polarization component. Also shown in the figure is the delay line 30. It is preferred to include the delay line in the arrangement in order to provide for scanning of the characterization region along the grating 15. However, if only one region of the grating is to be characterized, it may be possible to omit (or lock at a fixed position) the delay line.

The phase modulator PM and the polarizer P is used for selecting one of two orthogonal polarization components from the light in the reference branch 102.

First, the polarizer P selects one polarization component from the light sent into the reference branch 102. The polarization component selected by the polarizer is the component for which the grating under test 15 is to be characterized. For example, if a vertical polarization component is selected by the polarizer P, the GUT 15 is analyzed regarding its characteristics for vertically polarized light. In order to take any possible polarization twisting in the fibers into account, the phase modulator PM is used for switching the polarization beyond the polarizer between two orthogonal states. As mentioned above, the polarization component selected in the reference branch can only interfere with the corresponding component in the measurement branch. For example, if the vertical polarization component is selected by the polarizer P, interference is only obtained at the second optical coupler 14 between light from each respective branch having this original polarization. However, and as mentioned above, the polarization direction may rotate or twist during the propagation of the light along the fibers. Therefore, some light might be polarized in another direction when it arrives at the coupler 14. This situatiδon is handled according to the invention by making two measurements, between which the polarization state of light in the reference branch is switched between two orthogonal states after the first measurement and before the second measurement .

If it is desired to analyze the GUT 15 also for another polarization component, the polarizer P is simply turned or replaced, such that the desired polarization component is passed.

Hence, in the arrangement shown in figure 2, two measurement values are taken, wherein the total polarization state of the light is switched to an orthogonal state by the phase modulator PM between the taking of said two measurement values.

For example, in a case where the polarizer P selects vertically polarized light, the vertical component in the reference branch 102 is first interfered with the vertical component in the measurement branch 101 (phase modulator ^'PM switched off - no rotation) to obtain a first measurement value. Then, the horizontal component in the reference branch 102 is interfered with the vertical component in the measurement branch 101 (phase modulator PM switched on - polarization state rotated) to obtain a second measurement value. Finally, a polarization insensitive interference value is derived from said first and second measurement values by taking the quadratic sum. Since interference is always obtained with respect to the vertical component in the measurement branch (as selected by the polarizer) , the characterization of the grating is only performed for substantially vertically polarized light. The effect of the grating on horizontally polarized light is not detected in this case. In many cases, however, it is sufficient to characterize the grating for a selected polarization direction. If the polarizer P is omitted, the operation of the inventive arrangement is still adequate, provided that there is no birefringence in the grating under test. If there is birefringence in the GUT, it will, in fact, act as two individual gratings (one for each refractive index) . In this latter case, multiple measurement need to be carried out in order to distinguish between the two effective gratings.

In conclusion, a method and an arrangement for characterizing in-fiber gratings are disclosed. According to the invention, the characteristics of a fiber grating are analyzed by reflecting light in a measurement branch of an interferometer from the grating, and subsequently interfering the reflected light with light from a reference branch of the interferometer. A first measurement value is recorded. Then, the polarization state of the light in the reference branch is switched to an orthogonal state, whereupon a second measurement value is recorded. A polarization insensitive interference value is obtained by taking the quadratic sum of the first and ^'the second measurement values.

Claims

«. *• ii __m_L27 CLAIMS

1. A method of characterizing a fiber grating, said method being performed in an interferometer having a first and a second branch, and said fiber grating being optically coupled to the first branch, the method comprising the steps of: sending light into the interferometer, said light having a coherence length that is shorter than said fiber grating, thereby defining a characterization region in said fiber grating; dividing said light into a first portion and a second portion, and directing the first portion into the first branch and the second portion into the second branch; detecting the interference to obtain a first measurement value ; switching the polarization state of a selected one of said first and second portions of the light from a first polarization state to a second polarization state, said first and second polarization states being orthogonal with respect to each other; and, following upon said switching, detecting the thus produced interference to obtain a second measurement value; and deriving a polarization insensitive interference value from said first and second measurement values, said polarization insensitive interference value being indicative of the grating characteristics.

2. The method as claimed in claim 1, wherein the polarization state of the second portion of the light in the second branch is switched.

3. The method as claimed in claim 1 or 2 , wherein the polarization insensitive interference value is derived by taking the quadratic sum of the first and second measurement values .

4. The method as claimed in any one of the preceding claims, wherein the coherence length of the light is shorter than 1 cm.

5. The method as claimed in any one of the preceding claims, wherein the coherence length of the light is about 1 mm.

6. The method as claimed in any one of the preceding claims, further comprising the step of adjusting the length of the second branch of the interferometer, thereby adjusting the location of the characterization region in the grating.

7. The method as claimed in any one of the preceding claims, further comprising the step of filtering out one linear polarization component of the light in the second branch to interfere with the light in the first branch.

8. The method as claimed in any one of the preceding claims, further comprising the step of selectively sending trie light in the second branch along either of at least two alternative paths, said at least two paths having different optical lengths, such that a first and a second characterization region in the grating are defined.

9. The method as claimed in claim 8, wherein a first path measurement value and a second path measurement value are recorded, and a difference value between said first and second path measurement values is calculated, said difference value indicating a phase difference between light reflected at the first and the second characterization region in the grating.

10. The method as claimed in claim 9, wherein a number of difference values are calculated for different positions of the characterization regions in the grating, and further comprising the step of integrating said difference values along the grating to obtain a phase profile of the grating.

11. The method as claimed in any one of the preceding claims, further comprising the step of selecting one of two orthogonal polarization components of the light in the second branch to interfere with light from the first branch, such that the effect of the grating on said selected polarization component of light is characterized.

12. An arrangement for characterizing fiber gratings, comprising interferometer means having a first and a second branch, a grating to be characterized, which is optically connected to said first branch, a broad band light source, which is arranged to send light into said interferometer means, said light source being arranged to emit light having a coherence length that is shorter than said grating to be characterized; a first coupler for sending a first portion of the light from the light source into the first branch and a second portion of said light into the second branch, means for switching the polarization state of the light in a selected one of the first and the second branch, a second coupler for bringing light from the first and the second branch back together to interfere, and a detector for detecting the thus produced interference .

13. The arrangement as claimed in claim 12, wherein the means for switching the polarization state is arranged in the second branch of the interferometer means .

14. The arrangement as claimed in claim 12 or 13, further comprising an adjustable optical delay line, which is arranged in the light path of the second branch, and which is operative to adjust the optical path length of said second branch.

15. The arrangement as claimed in any one of the claims 12 to 14, wherein the second branch comprises at least two alternative paths of different path lengths, and means for switching between said at least two alternative paths.

16. The arrangement as claimed in claim 15, wherein the means for switching between the alternative paths comprises a polarizing beam splitter that is operative to reflect light of a first polarization state and to pass light of a second polarization state; and means for switching between said first and said second polarization states .

17. The arrangement as claimed in claim 16, wherein the means for switching between the first and the second polarization states is a phase modulator.

18. The arrangement as claimed in any one of the claims 12 to 17, further comprising means for selecting one of two orthogonal polarization components of the light in the second branch to interfere with light from the first branch, such that the effect of the grating on said selected polarization component of light may be characterized.