Abstract:
A polarized light beam having a coherence length L can be depolarized by splitting the beam into orthogonally polarized sub-beams and delaying one of the sub-beams relative to the other by a length larger than L. This spatial delay is created by splitting the beam in a walk-off crystal and disposing in the optical path of one of the sub-beams a slab of an optically dense material, while allowing the other sub-beam to propagate outside and near the slab. The sub-beams remain parallel to each other, allowing another walk-off crystal to be used to recombine the sub-beams. A dual-core fiber ferrule and a microlens array can be used to combine fiber-coupled output beams of two laser diodes in a single compact walk-off crystal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention claims priority from U.S. patent application Ser. No. 61/613,871 filed Mar. 21, 2012, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to optical components and modules, and in particular to optical components and modules for depolarizing light. 
       BACKGROUND OF THE INVENTION 
       [0003]    Most laser light sources emit polarized light during operation. While a polarized state of light can be advantageously used in some applications, in other applications it is detrimental. For example, for Raman amplification of an optical signal in a non-polarization maintaining optical fiber of a fiberoptic communication link, a depolarized Raman pump light source is needed. This is because a Raman amplification process is sensitive to mutual polarizations of the signal and the pump, which tend to be randomly fluctuating in non-polarization maintaining fibers. 
         [0004]    A common approach to depolarizing a polarized light beam having a coherence length L is to split the beam into two orthogonally polarized sub-beams, delay one of the sub-beams by a length larger than L relative to the other sub-beam, and recombine the sub-beams into an output optical beam. This causes the correlation of phase between the sub-beams to be lost, which effectively scrambles the output polarization. A polarization beam combiner is sometimes used in combination with the depolarizer, to combine and depolarize optical beams of two laser diodes at the same time. Using two laser diodes instead of one allows one to increase the output power of the depolarized optical beam, and to improve reliability via redundancy. 
         [0005]    Referring to  FIGS. 1A and 1B , a prior-art depolarized laser light source  100  includes first and second laser diodes  71  and  72  coupled via polarization-maintaining optical fibers  73  and  74  to input ports  81  and  82 , respectively, of a polarization beam combiner  8 . A birefringent crystal  10  is coupled to an exit port  83  of the polarization beam combiner  8 . In operation, lightwaves from each of the laser diodes  71  and  72  impinge onto the birefringent crystal  10 , as shown in  FIG. 1B . The crystal principal axis of the birefringent crystal  10  is disposed at 45 degrees with respect to the polarization directions of the combined lightwaves. Each of the combined lightwaves is split within the birefringent crystal  10  into two waves having orthogonal linear polarizations, one of which is delayed by a length L with respect to the other. When the delay L is larger than each coherence length of the lightwaves generated by the laser diodes  71  and  72 , the combined lightwaves in an output fiber  75  are depolarized. The light source  100  has been disclosed by Matsushita et al. in US Patent Application Publication 2002/0141698. 
         [0006]    One drawback of the light source  100  of Matsushita is that it usually requires a very long birefringent crystal  10 . By way of example, a Raman pump laser diode manufactured by JDS Uniphase Corporation of Milpitas, Calif., USA, has a coherence length of 60 mm. When using YVO 4  crystal  10  having Δn=n e −n o ≈0.2, one would require the YVO 4  crystal  10  to be at least 60 mm/0.2=300 mm long to depolarize the light emitted by this Raman pump diode. Such a long crystal is impractical to grow. 
         [0007]    Ziari et al. in U.S. Pat. No. 6,522,796 disclose a light source similar to the light source  100  shown in  FIG. 1 . The Ziari device uses a polarization beamsplitter cube in place of the polarization beam combiner  8 , and a length of polarization-maintaining (PM) optical fiber in place of the birefringent crystal  10 . A polarization maintaining fiber is also used by Fukushima in U.S. Pat. No. 5,692,082 to depolarize laser diode light. The length of the PM fiber must be large enough, so that the optical path difference (OPD) between the orthogonal polarization modes in the PM fiber is greater than the coherence length of the light source. By way of example, for the above mentioned Raman pump laser diode, a required length of a typical PM fiber with the birefringence Δn=n e −n o ≈3.7×10 −4  should be at least 60 mm/3.7×10 −4 =160 m. A 160 m long PM fiber is lossy, expensive, and bulky. 
         [0008]    Fidric et al. in U.S. Pat. No. 6,870,973 disclose a method allowing one to reduce the required length of the PM fiber. In a depolarized light source of Fidric et al., polarizations of multiple longitudinal modes of a Raman pump laser diode are overlapped, by converting half of the longitudinal modes to an orthogonal polarization state. As a result, a significantly shorter PM fiber length is required. The coherence length of this laser is only 9 mm, thus requiring only 24 m of PM fiber, or only 44 mm long YVO 4  crystals. However, these length values are still too long for constructing a compact and inexpensive depolarized light source. 
         [0009]    Another approach, taken by Yao et al. in US Patent Application Publication 2009/0225420, is to create the required optical path difference in a bulk-optic delay line or in a Michelson interferometer based on a polarization beamsplitter cube. The beams of orthogonal polarizations propagate along different directions in different optical paths, and one of the beams is delayed with respect to the other in a dedicated delay line. Optical path differences of tens of millimeters can easily be created in a bulk-optic delay line. Detrimentally, Michelson interferometers require complex optomechanical packaging to ensure stable operation. 
         [0010]    Tselikov et al. in U.S. Pat. No. 6,574,015 disclose a depolarizer based on a pair of polarization beam splitters and a fiberoptic delay line. One of the two orthogonally polarized sub-beams propagates in free space, and the other is coupled to a length of optical fiber. However, a fiberoptic delay line can create an unwanted temperature dependent variation of optical loss in one of the two optical paths for polarized sub-beams. 
         [0011]    Most of the above described depolarizers and beam combiners use optical polarizing beamsplitter cubes. In a polarizing beamsplitter cube, the orthogonally polarized incoming and/or outgoing optical beams are disposed at 90 degrees to each other. Since the inputs and outputs of the beam combiners and depolarizers are usually coupled to an optical fiber, the overall size of the device is increased due to a requirement to route all optical fibers on one end of the package, while observing a minimum bending radius of an optical fiber. 
         [0012]    Walk-off crystals can be used for combining or splitting orthogonally polarized beams. For example, Ziari et al. in U.S. Pat. No. 6,522,796 disclose, as an alternative, a polarization beam combiner having parallel input optical fibers coupled to a walk-off crystal through a couple of adjacently disposed lenses, thus not requiring the optical fibers to be bent within the package. This polarization beam combiner must use a walk-off crystal of sufficient length to create enough lateral displacement to accommodate two adjacent collimating lenses for coupling light into parallel fibers. For example, ˜20 m YVO 4  crystal would be required to combine two orthogonal polarized beams spaced 2.0 mm apart. It is desirable to further reduce size of a polarization beam combiner. 
         [0013]    Therefore, the prior art is lacking a compact, stable, reliable, and inexpensive depolarizer, especially a polarization beam combining depolarizer. 
       SUMMARY OF THE INVENTION 
       [0014]    A polarized light beam having a coherence length L can be depolarized by splitting the beam into orthogonally polarized sub-beams and delaying one of the sub-beams relative to the other by a length larger than L. According to the invention, this spatial delay can be created by splitting the beam in a walk-off crystal and disposing in the optical path of one of the sub-beams a slab of an optically dense material, such as glass or silicon, for example, while allowing the other sub-beam to propagate in air or inert gas near the slab. In this way, quite large optical path differences can be created in a very compact package, especially if a high-index slab material, such as silicon, is used. The sub-beams remain parallel to each other, allowing another walk-off crystal, preferably identical to the first one, to be used to recombine the sub-beams. Since the optical path difference can be generated mostly in the slab, the birefringent walk-off crystals can be made small. In accordance with another aspect of the invention, a dual-core fiber ferrule and a microlens array is used to combine fiber-coupled output beams of two laser diodes in a single compact walk-off crystal, resulting in a very compact polarization beam combining depolarizer. 
         [0015]    In accordance with the invention, there is provided an optical depolarizer comprising an in-coupling polarizer for defining a linear polarization of an input optical beam at 45 degrees with respect to a first axis, and a first walk-off crystal having first and second opposed ends. The first end of the first walk-off crystal is coupled to the in-coupling polarizer. The first walk-off crystal is sized and oriented to split the input optical beam launched at its first end into first and second parallel laterally offset sub-beams exiting from its second end. The first and second sub-beams at the second end are linearly polarized parallel and perpendicular, respectively, to the first axis. 
         [0016]    A slab of a transparent solid material, having a refractive index and a length between its first and second opposed ends, is coupled at its first end to the second end of the first walk-off crystal, and disposed in an optical path of the first sub-beam and not in an optical path of the second sub-beam. The slab length multiplied by the slab refractive index is at least 1 mm. A second walk-off crystal having first and second opposed ends is coupled at its first end to the slab&#39;s second end and sized and oriented to recombine the first and second sub-beams at the second end of the second walk-off crystal into an output optical beam. 
         [0017]    In operation, the input optical beam is launched into the in-coupling polarizer. The first walk-off crystal splits the input optical beam into the first and second sub-beams. The first sub-beam propagates in the slab from its first to its second end, and the second sub-beam propagates proximate the slab in air, neutral gas, or vacuum, whereby the depolarizing optical path difference is generated. The second walk-off crystal combines the sub-beams into a single depolarized output beam. 
         [0018]    The in-coupling polarizer can include a collimator lens and a polarization maintaining optical fiber having a first end for inputting the input optical beam, and a second end coupled to the first end of the walk-off crystal through the collimator lens. A stress direction at the second end of the polarization maintaining fiber is at 45 degrees with respect to the first axis, thereby defining the linear polarization of the input optical beam at 45 degrees with respect to the first axis. 
         [0019]    In one embodiment, the depolarizer can operate with not one but two laser diodes. In this beam-combining depolarizer, the in-coupling polarizer can include a third walk-off crystal having opposed first and second ends; first and second adjacently disposed collimator microlenses; and first and second polarization maintaining fibers each having a first end for inputting first and second input optical beams, respectively, and a second end coupled to the first end of the third walk-off crystal through the first and second collimator microlens, respectively. A stress direction at the second ends of the first and second polarization maintaining fiber is preferably oriented at +45 degrees and −45 degrees, respectively, with respect to the first axis. The second end of the third walk-off crystal is coupled to the first end of the first walk-off crystal. The third walk-off crystal is preferably oriented to define the linear polarization of the first and second input optical beams at +45 degrees and −45 degrees with respect to the first axis, and is sized to combine the first and second optical beams at the first end of the first walk-off crystal. 
         [0020]    The above disclosed depolarizers can also include a Faraday element-waveplate isolator stack for suppression of reverse-propagating light. 
         [0021]    In accordance with another aspect of the invention, there is further provided a laser source including an above described depolarizer coupled to a laser diode. The coherence length of the laser beam is smaller than the slab length multiplied by the slab refractive index, whereby the output optical beam is substantially depolarized. 
         [0022]    In accordance with another aspect of the invention, there is further provided a laser source including an above described beam-combining depolarizer coupled to a pair of laser diodes emitting first and second optical beams having first and second coherence lengths, respectively. The first and second coherence lengths are smaller than the slab length multiplied by the slab refractive index, whereby the output optical beam is substantially depolarized. 
         [0023]    In accordance with another aspect of the invention, there is further provided a method for depolarizing a linearly polarized optical beam, the method comprising: 
         [0024]    (a) providing an optical depolarizer described above; 
         [0025]    (b) coupling a first input optical beam having a first coherence length to the first end of the first walk-off crystal, wherein the first input optical beam is linearly polarized at 45 degrees with respect to the first axis, and wherein the slab length multiplied by the slab refractive index is selected to be larger than the first coherence length; 
         [0026]    (c) allowing the first input optical beam to propagate through the first walk-off crystal and split into first and second parallel laterally offset sub-beams at the second end of the first walk-off crystal, wherein the first and second sub-beams at the second end of the first walk-off crystal are linearly polarized parallel and perpendicular, respectively, to the first axis; 
         [0027]    (d) allowing the first sub-beam to propagate in the slab from the first to the second end thereof, and the second sub-beam to propagate proximate the slab in the air or the neutral gas; and 
         [0028]    (e) allowing the first and second sub-beams to propagate through the second walk-off crystal to recombine at the second end of the second walk-off crystal into the output optical beam. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    Exemplary embodiments will now be described in conjunction with the drawings, in which: 
           [0030]      FIG. 1A  is a block diagram of a prior-art depolarized light source; 
           [0031]      FIG. 1B  is a three-dimensional view of a birefringent crystal used to depolarize light in the depolarized light source of  FIG. 1A ; 
           [0032]      FIG. 2A  is a side view of a depolarizer of the invention; 
           [0033]      FIG. 2B  is a schematic view of an input polarization defined by an in-coupling polarizer of the depolarizer of  FIG. 2A ; 
           [0034]      FIG. 2C  is a schematic view of orientation of birefringent axes of walk-off crystals of the depolarizer of  FIG. 2A ; 
           [0035]      FIG. 3A  is a side view of an embodiment of the depolarizer of  FIG. 2A , wherein the in-coupling polarizer includes a PM fiber; 
           [0036]      FIG. 3B  is a frontal view of the PM fiber of  FIG. 3A ; 
           [0037]      FIG. 4A  is a side view of an embodiment of the depolarizer of  FIG. 3A  including an isolating stack, showing light propagating in a forward direction; 
           [0038]      FIG. 4B  is a side view of the depolarizer of  FIG. 4A , showing light propagating in a backward direction; 
           [0039]      FIG. 5A  is a side view of a polarization beam combiner according to the invention; 
           [0040]      FIG. 5B  is a front view of PM fibers of the polarization beam combiner of  FIG. 5A ; 
           [0041]      FIG. 5C  is a schematic view of orientation of a birefringent axis of walk-off crystals of the polarization beam combiner of  FIG. 5A ; 
           [0042]      FIG. 6A  is a schematic view of a beam combining isolating depolarizer of the invention, wherein the in-coupling polarizer includes a walk-off crystal, and the isolating depolarizer is similar to that of  FIGS. 4A and 4B ; 
           [0043]      FIG. 6B  is a frontal view of PM fibers of the beam combining isolating depolarizer of  FIG. 6A ; 
           [0044]      FIG. 6C  is a side view taken along the direction of view C denoted in  FIG. 6B , wherein the side view of  FIG. 6C  shows the birefringent axis orientation of a walk-off crystal of the in-coupling polarizer of the beam combining isolating depolarizer of  FIG. 6A ; 
           [0045]      FIG. 6D  is a polarization diagram showing relative polarizations of incoming optical beams in the beam combining isolating depolarizer of  FIG. 6A ; 
           [0046]      FIG. 7A  is a schematic view of a beam combining isolating depolarizer of the invention, having a Wollaston prism beam combiner; 
           [0047]      FIG. 7B  is a frontal view of PM fibers of the beam combining isolating depolarizer of  FIG. 7A ; 
           [0048]      FIGS. 8A to 8C  are schematic views of depolarized light sources of the invention, using: one free-space coupled laser diode ( FIG. 8A ); one fiber coupled laser diode ( FIG. 8B ); and two fiber coupled laser diodes ( FIG. 8C ); and 
           [0049]      FIG. 9  is a block diagram of a method for depolarizing light according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0050]    While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
         [0051]    Referring to  FIGS. 2A to 2C , an optical depolarizer  200  includes an in-coupling polarizer  202  for defining a linear polarization  204  of an input optical beam  206  at 45 degrees with respect to a first axis  208 . A first walk-off crystal  210  has first  211  and second  212  opposed ends. The first end  211  of the first walk-off crystal  210  is coupled to the in-coupling polarizer  202 . The first walk-off crystal  210  is sized and oriented to split the input optical beam  206  launched at its first end  211  into first  241  and second  242  parallel laterally offset sub-beams exiting from its second end  212 . The first and second sub-beams  241  and  242  at the second end  212  are linearly polarized parallel and perpendicular, respectively, to the first axis  208 . 
         [0052]    A slab  244  of a transparent solid material has a refractive index n. The slab  244  has a length L 2  between its first  247  and second  248  opposed ends. The slab length L 2  multiplied by the slab refractive index n is at least 1 mm, more preferably at least 3 mm, and most preferably 5 mm to 10 mm. The slab  244  is coupled at its first end  247  to the second end  212  of the first walk-off crystal  210  and disposed in an optical path of the first sub-beam  241  and not in an optical path of the second sub-beam  242 . When the input optical beam  206  is launched into the in-coupling polarizer  202 , the first sub-beam  241  propagates in the slab  244  from its first end to its second end, and the second sub-beam  242  propagates proximate the slab  244  in air, neutral gas, vacuum, or another medium having a refractive index close to  1 , for example between 1 and 1.1. 
         [0053]    A second walk-off crystal  220  has first  221  and second  222  opposed ends. The second walk-off crystal  220  is coupled at its first end  221  to the second end  248  of the slab  244 , and sized and oriented to recombine the first  241  and second  242  sub-beams at the second end  222  of the second walk-off crystal  220  into an output optical beam  207 . 
         [0054]    In  FIG. 2A , the input  206  and output  307  optical beams, and the sub-beams  241  and  242  are shown as propagating along the Z axis. In  FIG. 2B , the first axis  208  is parallel to the Y axis. In  FIGS. 2A and 2C , optical axes  213  and  223  of the first and second walk-off crystals  210  and  220 , respectively, are disposed in the YZ plane. The first and second walk-off crystals  210  and  220  can be made of YVO 4  or another suitable birefringent material, such as rutile or calcite, for example. For YVO 4  crystals  210  and  220 , the optical axes  213  and  223  makes an angle of 22.5 degrees with respect to the Y axis. When both the first  210  and the second  220  walk-off crystals are made of a same material, their lengths L 1  are equal to each other, because the walk-off created by the first walk-off crystal  210  needs to be exactly compensated by the second walk-off crystal  220 . Various walk-off crystals  210  and  220 , sized and shaped for creating lateral displacement along the Y-axis between the polarized sub-beams  241  and  242 , can be used. Various permutations of crystal orientations can also be used to achieve the same purpose of splitting and subsequent recombining the polarized sub-beams  241  and  242 . It is well known to the skilled person how to select axis orientation and the length of birefringent crystals to create a pre-defined amount of lateral displacement. The first walk-off crystal  210 , the slab  244 , and the second walk-off crystal  220  can be placed into mechanical holders ensuring their mutual disposition and orientation, or simply epoxied to each other into a single solid stack. 
         [0055]    A magnitude of the lateral displacement should be made sufficient for the polarized sub-beams  241  and  242  to be separated enough to propagate substantially without a clipping loss within and outside of the slab  244 , respectively, as shown in  FIG. 2A . By way of a non-limiting example, 20 mm long YVO 4  crystals  210  and  220  can be used, giving a displacement along the Y axis of approximately 2 mm. 
         [0056]    Advantageously, the depolarizer  200  of  FIG. 2  can be made very compact. By way of a non-limiting example, to generate an optical path difference of 9 mm, a 15 mm long slab of glass having the refractive index of 1.6 can be used. Thus, the total length of the depolarizer  200  becomes 20 mm (L 1 )+15 mm (L 2 )+20 mm (L 2 )=55 mm. This length can be further reduced if a crystalline silicon is used in place of glass. The refractive index of silicon at a typical Raman wavelength of 1.48 um is very high, approx. 3.5. Only L 2 =3.6 mm of silicon slab length will be required. To avoid disturbing the polarization state of the first sub-beam  241 , the crystal axis of the crystalline silicon should be preferably directed along the direction of propagation, that is, Z axis. 
         [0057]    Referring now to  FIGS. 3A and 3B  with further reference to  FIGS. 2A and 2B , an optical depolarizer  300  is an embodiment of the optical depolarizer  200  of  FIG. 2A . The optical depolarizer  300  of  FIG. 3A  includes a variant of the in-coupling polarizer  202 , including a collimator lens  302  and a PM optical fiber  310  having a first end  311  for inputting the input optical beam  206  and a second end  312  coupled to the first end  211  of the first walk-off crystal  210  through the collimator lens  302 . 
         [0058]    Referring specifically to  FIG. 3B , a stress direction  314  at the second end  312  of the PM fiber  310  is at 45 degrees with respect to the first axis  208 , thereby defining the linear polarization of the input optical beam  206  at 45 degrees with respect to the first axis  208 . The stress direction  314  is defined by a line passing through centers of stress rods  315 . It is to be noted, however, that the input optical beam  206  can be launched into the first end  311  of the PM fiber  310  with polarization either parallel to the local stress direction at the first end  311 , not shown, or perpendicular to the local stress direction at the first end  311 . In both cases, the launched polarization will be maintained in the PM fiber  310 , so that the input optical beam  206  at the first end  211  of the first walk-off optical crystal  210  will be polarized at 45 degrees away from the first axis  208 . Thus, 45 degrees with respect to the first axis  208  can include both directions, +45 degrees and −45 degrees, so that both launch conditions, along the “o-axis” and along the “e-axis” of the PM fiber  310  as they are sometimes called, are equally possible. 
         [0059]    In operation, the input optical beam  206  is coupled into the first end  311  of the PM fiber  310 , exits the second end  312 , gets collimated by the lens  302 ; and splits into the first  241  and second  242  sub-beams in the first walk-off crystal  210 . The first sub-beam  241  propagates through the slab  244 , and the second sub-beam  242  propagates in free space, e.g. air or inert gas, outside and near the slab  244 . Thus, an optical path difference is created between the first  241  and second  242  sub-beams. The sub-beams  241  and  242  are re-combined by the second walk-off crystal  220  to form the output optical beam  207 . If output fiber coupling of the output optical beam  207  is required, another, focusing lens  302  can be used to focus the output optical beam  207  into an output fiber  307 . 
         [0060]    Turning to  FIG. 4A  with further reference to  FIG. 3A , an isolating depolarizer  400  of  FIG. 4A  is a variant of the depolarizer  300  of  FIG. 3A . To attain the isolating property, the isolating depolarizer  400  of  FIG. 4A  includes a stack  411  of a Faraday element  401  and a half-wave waveplate  402 . The stack  411  is coupled between the second end  212  of the first walk-off crystal  210  and the first end  247  of the slab  244 . In operation, the Faraday element  401  rotates the polarization of the sub-beams  241  and  242  by 45 degrees, and the waveplate  402  rotates the polarization back by 45 degrees. As a result, the polarization of light propagating from the first walk-off crystal  410  to the second walk-off crystal  420  is substantially unaffected, so that the second walk-off crystal  420  can recombine the sub-beams  241  and  242  into the output optical beam  207 , which is then focused by the lens  302  into the output optical fiber  307 . Therefore, the performance of the isolating depolarizer  400  is substantially unaffected in the forward direction, except perhaps for a slight increase of insertion loss due to propagation of the sub-beams  411  and  412  through the Faraday element  401  of the stack  411 . 
         [0061]    Referring now to  FIG. 4B , the isolating property of the depolarizer  400  will be explained. Any residual light  407  emitted from the output optical fiber  307  will be collimated by the focusing right-side lens  302  and coupled to the second end  222  of the second walk-off crystal  220 . The second walk-off crystal  220  splits the residual light  407  into orthogonally polarized reverse sub-beams  441  and  442 , which then propagate within and outside, respectively, of the slab  244 . The waveplate  402  rotates the polarization of the reverse sub-beams  441  and  442  by 45 degrees, and the Faraday element  401  rotates the polarization by extra 45 degrees. As a result, the polarization of light propagating from the first walk-off crystal  410  to the second walk-off crystal  420  is rotated by 90 degrees. When the reverse sub-beams  441  and  442  enter the first walk-off crystal  410 , the first reverse sub-beam  441  will not be displaced, but the second reverse sub-beam  442  will be displaced downwards as shown in  FIG. 4B . This happens because the reverse sub-beams  441  and  442  had had their polarization rotated by the stack  411  by 90 degrees. As a result, the reverse sub-beams  441  and  442  will not be coupled into the second end  312  of the input optical fiber  310 , thereby achieving the isolating function. 
         [0062]    The Faraday element  401  and the half-wave waveplate  402  are mechanically coupled to each other to form the stack  411 , and oriented for rotating by 90 degrees polarization of light propagating from the second walk-off crystal  220  to the first  210 , while substantially not rotating the polarization of light propagating from the first walk-off crystal  210  to the second  220 , as explained above. Of course, the stack  411  can also be disposed proximate the second walk-off crystal  220 , or anywhere in the optical path of both reverse sub-beams  441  and  442  between the second end  212  of the first walk-off crystal  210  and the first end  421  of the second walk-off crystal  220 . If desired, the Faraday element  401  and the half-wave waveplate  402  can even be disposed separately from each other on opposite sides of the slab  244 . Furthermore, embodiments are possible where the forward propagating light has its polarization rotated by 90 degrees, and backward propagating light has its polarization not rotated. In the latter case, the second walk-off crystal  220  will have to be rotated about the Z axis by 180 degrees. 
         [0063]    According to one aspect of the invention, the overall size of any polarization beam combining depolarizer can be further reduced by using a compact polarization beam combiner based on a walk-off crystal. Turning now to  FIG. 5A , a polarization beam combiner  500  of the invention is shown. The polarization beam combiner  500  includes a walk-off crystal  560  having opposed first  561  and second  562  ends, first  501  and second  502  adjacently disposed collimator microlenses, and first  510  and second  520  PM fibers. A first input optical beam  531  is input into a first end  511  of the first PM fiber  510 , and a second input optical beam  532  is input into a first end  521  of the second PM fiber  520 . Second ends  512  and  522  of the first  510  and second  520  PM fibers, respectively, are coupled to the first end  561  of the walk-off crystal  560  through the first  501  and second  502  collimator microlenses, respectively. The walk-off crystal  560  is oriented and sized to combine the first  531  and second  532  input optical beams at its second end  562 , for coupling into an optional output optical fiber  570  through an optional focusing lens  571 . 
         [0064]    The polarization beam combiner  500  includes a first substrate  541  having a pair of parallel through openings  551 ,  552  for supporting therein the second ends  512 ,  522  of the first and second PM fibers  510  and  520 , respectively. The first  501  and second  502  collimator microlenses are disposed on a common second substrate  542 . The first  541  and second  542  substrates are mechanically affixed, for example epoxied or glass-soldered, to each other. Various types of microlenses  501  and  502  can be used, including, for example, gradient-index microlenses formed within the second substrate  542 . 
         [0065]    Turning to  FIG. 5B  with further reference to  FIG. 5A , stress directions  514  and  524  at the second ends  512 ,  522  of the first  510  and second  520  PM fibers, respectively, are oriented perpendicular to each other. The stress directions  514  and  524  can also be parallel to each other. What is important is to couple the input optical beams  531  and  532  into the PM fibers  510  and  520  such that their polarizations are perpendicular to each other at the first end  561  of the walk-off crystal  560 , to enable the walk-off crystal  560  to combine the input optical beams  531  and  532  at the second end  562  of the walk-off crystal  560 . 
         [0066]    Referring to  FIG. 5C  with further reference to  FIG. 5A , the orientation of a crystal axis  563  of the walk-off crystal  560  is shown. The crystal axis  563  is disposed in YZ plane. For YVO 4  crystal  560 , the crystal axis  563  makes an angle of 22.5 degrees with respect to the Y axis, as shown. 
         [0067]    Referring now to  FIG. 6A  with further reference to  FIGS. 4A ,  4 B, and  5 A, an isolating, beam-combining depolarizer  600  of  FIG. 6A  is similar to the isolating depolarizer  400  of  FIGS. 4A and 4B . In the isolating beam-combining depolarizer  600  of  FIG. 6A , the in-coupling polarizer  202  includes a third walk-off crystal  230  having opposed first  231  and second  232  ends; the first  510  and second  520  PM fibers for inputting the first  531  and second  532  input optical beams, respectively; and the first  501  and second  502  adjacently disposed collimator microlenses. The second ends  512  and  522  of the first  510  and second  520  PM fibers, respectively, are coupled to the first end  231  of the third walk-off crystal  230  through the first  501  and second  502  collimator microlenses, respectively. The second end  232  of the third walk-off crystal  230  is coupled to the first end  211  of the first walk-off crystal  210 . The in-coupling polarizer  202  of the isolating beam-combining depolarizer  600  is similar to the polarization beam combiner  500  of  FIG. 5A . One difference is that the orientation of a crystal axis  233  of the third birefringent crystal  230 , and the orientation of the second ends  512  and  522  of the first and second PM fibers  510  and  520 , respectively, are different. 
         [0068]    Referring to  FIGS. 6B and 6C  with further reference to  FIGS. 6A and 5B , stress directions  514  and  524  at the second ends  512  and  522  of the first and second PM fibers  510  and  520 , respectively, are oriented at +45 degrees and −45 degrees, respectively, with respect to the first axis  208 . The second ends  512  and  522  of the first  510  and second  520  PM fibers, respectively, are disposed on a line  602  at 45 degrees with respect to the first axis  208 . In comparison with  FIG. 5B , the second fiber ends  512  and  522  of  FIG. 6B  are rotated about Z axis by 45 degrees. 
         [0069]    Referring now to  FIGS. 6C and 6D  with further reference to  FIG. 5C , the crystal axis  233  of the third walk-off crystal  230  is disposed in a plane including the second fiber ends  512  and  522 , which is the plane of  FIG. 6C . For YVO 4  crystal  230 , the crystal axis  233  makes an angle of 22.5 degrees with respect to the line  602 , as shown. This orientation of the crystal axis  233  allows one to define linear polarizations  631  and  632  of the first  531  and second  532  input optical beams at +45 degrees and −45 degrees with respect to the first axis  208 . The orientations of the linear polarizations of the first  531  and second  532  input optical beams are shown in  FIG. 6D . 
         [0070]    Referring back to  FIG. 6A , the third walk-off crystal  530  is sized to combine the first  531  and second  532  optical beams at the first end  211  of the first walk-off crystal  210 . In comparison with  FIG. 5C , the crystal axis  233  of the third walk-off crystal  230  in  FIG. 6C  is rotated by 45 degrees about the Z axis. Referring specifically to  FIG. 6D , this rotation ensures that the polarization directions  631  and  632  of the first and second input optical beams  531  and  532 , respectively, are at 45 degrees with respect to the first optical axis  208 . 
         [0071]    The first  531  and second  532  optical beams are each split into the sub-beams  241  and  242 , which propagate in the first  210  and second  220  walk-off crystals in the same way as in the previously described depolarizers  200  of  FIG. 2A ,  300  of  FIG. 3A , and  400  of  FIG. 4A . The sub-beams  241  and  242  are focused by the optional right-side lens  302  into the optional output optical fiber  307 . 
         [0072]    Many variations of the depolarizer  600  are possible. As is known to a person skilled in the art, the input polarizations can be rotated by 90 degrees substantially without impacting the device performance. Furthermore, the polarization beam combiner  500  of  FIG. 5A  may be used instead of the in-coupling polarizer  202  in  FIG. 6A . In this case, a half-wave waveplate, not shown, will have to be added into the optical path between the polarization beam combiner  500  and the first walk-off crystal  210 , for rotating polarizations of the incoming optical beams  531  and  532  by 45 degrees. The construction shown in  FIG. 6A  is advantageous, however, because no such half-wave waveplate is required. If free space optical beam delivery is desired, the right-side lens  302  and the output optical fiber  307  may be omitted. The isolating stack  411  is also optional, although its inclusion allows the depolarizer  600  to act as an optical isolator. The relative position of the Faraday element  401  and the waveplate  402  may be varied as explained above. 
         [0073]    Turning to  FIGS. 7A and 7B  with further reference to  FIGS. 6A to 6D , an isolating, beam-combining depolarizer  700  of  FIG. 7A  is similar to the isolating, beam-combining depolarizer  600  of  FIG. 6A , the difference being that the in-coupling polarizer  202  is based on a Wollaston prism  710  and not on the walk-off crystal  230 . The Wollaston prism  710  has opposed first  711  and second  712  ends. In the isolating beam-combining depolarizer  700  of  FIG. 7A , the in-coupling polarizer  202  further includes a lens  702  and the first  510  and second  520  PM fibers. The second ends  512  and  522  of the first and second PM fibers  510  and  520 , respectively, are coupled to the first end  711  of the Wollaston prism  710  through the common lens  702 . 
         [0074]    Referring specifically to  FIG. 7B , the stress directions  514  and  524  at the second ends  512  and  522  of the first and second PM fibers  210  and  520 , respectively, are oriented at +45 degrees and −45 degrees, respectively, with respect to the first axis  208 . The second end  712  of the Wollaston prism  710  is coupled to the first end  211  of the first walk-off crystal  210 . The Wollaston prism  710  is oriented to define the linear polarizations  631  and  632  of the first and second input optical beams  531  and  532 , respectively, at +45 degrees and −45 degrees, respectively, with respect to the first axis  208 , as shown in  FIG. 6D . The Wollaston prism  710  is sized to combine the first and second optical beams  531  and  532  at the first end  211  of the first walk-off crystal  210 . 
         [0075]    Many variations of the depolarizer  700  are possible. As is known to a person skilled in the art, the input polarizations can be rotated by 90 degrees substantially without impacting the device performance. If free space optical beam delivery is desired, the right-side lens  302  and the output optical fiber  307  may be omitted. The isolating stack  411  is also optional, although its inclusion allows the depolarizer  700  to act as an optical isolator,—a quality desirable when laser diodes are used as a light source. 
         [0076]    The depolarizers  200 ,  300 ,  400 ,  600 , and  700  of  FIGS. 2A ,  3 A,  4 A,  6 A, and  7 A, respectively, can be used to construct depolarized light sources. Turning now to  FIG. 8A  with further reference to  FIG. 2A , a light source  800 A includes the optical depolarizer  200  of  FIG. 2A  and a free-space emitting laser diode  811  for emitting the input optical beam  206 . The laser diode  811  is coupled to the in-coupling polarizer  202  for defining polarization at 45 degrees as explained above. The coherence length of the laser diode  811  is smaller than the slab  244  length L 2  multiplied by the slab  244  refractive index n, whereby the output optical beam  207  is substantially depolarized. A half-wave waveplate, or another suitable polarization defining means can be used in place of the in-coupling polarizer  202 ; alternatively, the laser diode  811  or the depolarizer  200  can be simply rotated about the Z-axis, to define the proper incoming polarization direction. 
         [0077]    Referring to  FIG. 8B  with further reference to  FIG. 3A , a light source  800 B includes the optical depolarizer  300  of  FIG. 3A  and a laser diode  801  for emitting the input optical beam  206 . The laser diode  801  is coupled to the first end  311  of the polarization maintaining optical fiber  310 . The coherence length of the laser diode  801  is smaller than the slab  244  length L 2  multiplied by the slab  244  refractive index n, whereby the output optical beam  207  is substantially depolarized. The optical depolarizer  400  of  FIG. 4A  can be used in place of the optical depolarizer  300  of  FIG. 3A . 
         [0078]    Turning to  FIG. 8C  with further reference to  FIG. 6A , a light source  800 C includes the optical depolarizer  600  of  FIG. 6A , the laser diode  801  for emitting the first input optical beam  531 , and a second laser diode  802  for emitting the second input optical beam  532 . The laser diodes  801  and  802  are coupled to the first ends  511  and  521  of the first and second PM optical fibers  510  and  520 , respectively. The coherence length of the laser diodes  801  and  802  is smaller than the slab length L 2  multiplied by the slab refractive index n, whereby the output combined optical beam  207  is substantially depolarized. The optical depolarizer  700  of  FIG. 7A  can be used in place of the optical depolarizer  600  of  FIG. 6A . 
         [0079]    The depolarized light sources  800 A,  800 B, and  800 C can be used in a variety of applications, including Raman pumping of singlemode non-PM fibers, spectroscopy, illumination, etc. 
         [0080]    Referring to  FIG. 9  with further reference to  FIG. 2A , a method  900  for depolarizing the linearly polarized optical beam  206  includes a step  902  of providing an optical depolarizer of the invention, for example the optical depolarizer  200  of  FIG. 2A . The slab  244  length L 2  multiplied by the slab  244  refractive index n is selected to be larger than the coherence length of the input optical beam  206 . In a step  904 , the optical beam  206  is coupled to the first end  211  of the first walk-off crystal  210  at 45 degrees with respect to the first axis  208 . 
         [0081]    In a step  906 , the input optical beam  206  is propagated through the first walk-off crystal  210  and split thereby into the first  241  and second  242  parallel laterally offset sub-beams at the second end  212  of the first walk-off crystal  210 . The first  241  and second  242  sub-beams at the second end  212  of the first walk-off crystal  210  are linearly polarized parallel and perpendicular, respectively, to the first axis  208 . 
         [0082]    In a step  908 , the first sub-beam  241  is propagated in the slab  244  from the first  247  to the second  248  end thereof, and the second sub-beam  242  is propagated proximate the slab  244  in air or another low-index medium, as explained above. 
         [0083]    Finally, in a step  910 , the first  241  and second  242  sub-beams are propagated through the second walk-off crystal  220 , recombining at the second end  222  of the second walk-off crystal  220  into the output optical beam  207 . 
         [0084]    Referring again to  FIG. 9  with further reference to  FIGS. 6A and 7A , the method  900  can be used to combine and depolarize the first and second input optical beams  531  and  532 . To that end, the step  904  includes coupling the first optical beam  531  to the first walk-off crystal  210  at the polarization direction of 45 degrees with respect to the first axis  208 . In a step  905 , the second optical beam  532  is coupled to the first end  211  of the first walk-off crystal  210  at the polarization direction of −45 degrees with respect to the first axis  208 . In other words, the polarization directions of the first and second optical beams  531  and  532  are perpendicular to each other. 
         [0085]    In a step  907 , the second optical beam  532  is propagated through the first walk-off crystal  210  and split thereby into third and fourth parallel laterally offset sub-beams, polarized in the same way as the first  241  and second  242  parallel laterally offset sub-beams, and propagating along the same paths as the first  241  and second  242  sub-beams. 
         [0086]    In a step  909 , the third sub-beam is propagated in the slab  244  from the first  247  to the second  248  end thereof, and the fourth sub-beam is propagated proximate the slab  244  in the air or another low-index medium, as explained above. 
         [0087]    Finally, in a step  911 , the third and fourth sub-beams are propagated through the second walk-off crystal  220 , recombining at its second end into the output optical beam  207 . 
         [0088]    The foregoing description of one or more embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.