Abstract:
The present application teaches systems and techniques for reducing the polarization dependency of an optical component by combining the actions of a circulator and a polarization beam combiner to separate an optical signal into a plurality of orthogonally oriented polarization components; rotate at least one of the components so that the polarization orientation of the components are parallel; propagate the components through respective input ports of an optical component at substantially the same time; rotate at least one of a plurality of output components from the optical component so that the polarization orientation of the output components are orthogonal; and recombine the plurality of output components into an output optical signal.

Description:
BACKGROUND  
         [0001]    The following description relates to systems and techniques for reducing the polarization mode dependency of an optical device. The techniques may be used on in conjunction with optical devices such as those used in optical communications systems.  
           [0002]    Different optical devices, including optical amplifiers, optical filters and combined optical, amplifying filters may be used in optical communication systems. These devices may be integrated in, for example, components such as switches, routers, multiplexers and other components for optical-signal-processing.  
           [0003]    Optical signals that are employed in communication systems may be polarized and may include more than one polarization mode. Two polarization modes are known as transverse-electrical (TE) and transverse-magnetic (TM). The optical signals may propagate through the communication system on optical fibers and may have an arbitrary polarization state. The TM component may be thought of as propagating perpendicular to an axis of the optical waveguide and the TE polarization mode may be thought of as propagating parallel to the axis of the optical waveguide.  
           [0004]    The performance of some optical components, including optical amplifiers and optical waveguide coupled components used in an optical communication system, may depend upon the polarization state of an incident optical signal received by the component. The optical devices may be polarization-dependent, meaning that the device has a different influence on the different modes of an incident signal.  
           [0005]    Incident signals with different polarization states may be affected by an optical device in different ways. In an optical amplifier, for example, the modal gain may be polarization mode dependent. The gain of the transverse-electrical and transverse-mechanical waves may be different. This variation in the response of an optical device to the different polarization states is known as polarization mode dispersion (PMD). Polarization mode dispersion is an inherent property of optical media. It can be caused by the difference in the propagation velocities of light in the orthogonal principal polarization states of the transmission medium. The net effect is that if an optical pulse contains both polarization components, then the different polarization components will travel at different speeds and arrive at different times, smearing the resultant optical signal. One result is that the gain may differ for TE-polarized and TM-polarized waves. The difference in gain between the differently polarized waves may result in an amplification of the TM wave that is different from the TE wave. Thus, the output optical signal from the amplifier may include TM and TE polarized waves that are in a different proportion than the input optical signal. The output signal of a polarization mode dependent device may have a different polarization state than the incident received signal.  
         SUMMARY OF THE DISCLOSURE  
         [0006]    The present application teaches systems and techniques for reducing the polarization dependency of an optical component.  
           [0007]    In one aspect, reducing the polarization dependency of an optical component is facilitated by separating an optical signal into a plurality of orthogonally oriented polarization components. At least one of the components is rotated so that the polarization orientation of the components are parallel and then the components are propagated through respective input ports of an optical component at substantially the same time. At least one of the output components of the optical component is rotated so that the polarization orientation of the output components are orthogonal. The orthogonal components are recombined into an output optical signal.  
           [0008]    In another implementation, the optical component is bi-directional.  
           [0009]    In another implementation, the optical component is an optical amplifier.  
           [0010]    In another aspect, a device for reducing the polarization dependency of an optical component is disclosed. The device includes an imaging element to receive an input optical signal from an input waveguide or provide an output optical signal to an output waveguide. A beam displacer/combiner is optically coupled to the first imaging element to split the input optical signal into a plurality of polarization component beams or to combine a plurality polarization component beams into the output optical signal. A first half-wave plate is optically coupled to the beam displacer to rotate a polarization orientation of the polarization component beams. A first nonreciprocal polarization rotator is optically coupled to the first half-wave plate to rotate the polarization orientation of the polarization component beams. A first beam angle turner is optically coupled to the first nonreciprocal polarization rotator to turn the polarization component beams through an angle, wherein a path of a polarized beam converges to or diverges from the longitudinal axis of the optical device depending upon the propagation direction of the polarization component beam. A second beam angle turner is optically coupled to the first beam angle turner to turn the polarization component beams through an angle, wherein the path of a polarized beam converges to or diverges from the longitudinal axis of the optical device depending upon the propagation direction of the polarization component beam. A second half-wave plate is optically coupled to the second beam angle turner to rotate the polarization orientation of the polarization component beams and a second nonreciprocal polarization rotator is optically coupled to the second half-wave plate to rotate the polarization orientation of the polarization component beams. A prism is optically coupled to the second half-wave plate o turn the polarization component beams through an angle toward the longitudinal axis and a second imaging element is optically coupled to the prism to couple the polarization component beams into respective inputs of a polarization dependent component or couple output polarization component beams from the polarization dependent component into the optical device.  
           [0011]    In some implementations, techniques described here may provide one or more of the following advantages. The technique may enable the use of a single optical to amplify an optical signal. The technique can reduce signal distortion of some optical devices. The modal components of the signal pass through the same optical loop and, therefore, may have substantially the same transfer function. The technique may be incorporated into the same package as an optical device, thereby reducing the assembly time and complexity.  
           [0012]    Details of one or more implementations are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description and drawings, and from the claims. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]    [0013]FIG. 1 is a cross-sectional view of an optical amplifier.  
         [0014]    [0014]FIG. 2 is a block diagram an implementation of circulator and polarization beam combiner coupled to an optical amplifier.  
         [0015]    [0015]FIG. 3 is an isometric view of the elements of an implementation of a circulator and polarization beam combiner.  
         [0016]    FIGS.  4 A-B illustrate a top and side view, respectively, of the circulator and polarization beam combiner of FIG. 3.  
         [0017]    FIGS.  5 A-B is a cross-sectional view showing the spatial location and polarization states of the component beams propagating through the circulator and polarization beam combiner of FIG. 3.  
         [0018]    [0018]FIG. 6 is an isometric view of the elements of an alternative implementation of a circulator and polarization beam combiner. 
     
    
       [0019]    Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0020]    The systems and techniques described here relate to reducing polarization mode dependency of optical components. The techniques may include passing an incident light beam signal through an optical circulator to direct the incident signal to a polarization beam combiner/splitter (PBC) that splits the beam into polarization components. The PBC output signals are provided to respective inputs of a bi-directional optical component. The resultant signals from the optical component are re-combined in the PBC. The combined signal is provided to the circulator, which directs the re-combined signal from the PBC to an output port. Thus, a reduced polarization mode dependent optical amplifier, for example, includes a circulator, a PBC and an optical amplifier. In an alternative implementation, an optical device is disclosed that performs both the function of a circulator and a PBC. The circulator and the PBC may be included in the same package as the optical component.  
         [0021]    [0021]FIG. 1 illustrates a semiconductor optical amplifier assembly  10 . Optical waveguides  12  and  14  are input and output single-mode waveguides, respectively. Single-mode optical waveguides are waveguides that propagate only one polarization mode of light. Imaging elements  16  and  18  may be disposed between the input and output waveguides  12  and  14 . A semiconductor optical amplifier (SOA)  20  is disposed between the imaging elements,  16  and  18 . Each waveguide,  12  and  14 , can provide incident optical signals to the SOA and receive optical signals from the SOA. SOA  20  is illustrated in a cross-sectional view. The amplifier may have an active layer  24  and a cladding layer  22  which, in cross-section, has an upper cladding layer  22   a  and lower cladding layer  22   b.    
         [0022]    A semiconductor optical amplifier is a bi-directional optical component. A bi-directional component can operate in both directions regardless of the direction a light signal passes through it. Incident light beam signals  26  and  28  may propagate through the SOA in forward and a reverse directions, respectively. Beams  26 ,  28  may be coupled to the amplifier  20  by imaging elements  16 ,  18 , respectively. The light intensity of each beam may increase as the beam propagates through the amplifier. Forward propagating beam  26  may then pass through imaging element  18  and may be provided to fiber  14 . Similarly, reverse propagating beam  28  then passes through imaging element  16  and is provided to fiber  12 .  
         [0023]    The modal gain of the amplifier may be different for different polarization modes of an incident beam. The transverse-magnetic TM and transverse-electric TE modes may have different gains due to intrinsic characteristics of the optical amplifier and may result in degradation of the incident signal.  
         [0024]    [0024]FIG. 2 is a block diagram  200  of an implementation of an optical system that includes a circulator and polarization beam combiner/splitter. An incident optical signal may be provided on an input waveguide  210  to a first port  224  of a circulator  212 . A single-mode waveguide  214  may optically couple a second port  226  of the circulator to a polarization beam combiner/splitter (PBC)  216 . Waveguides  218 ,  220  may optically couple PBC  216  outputs to respective inputs of a polarization dependent optical component  222 , such as an optical amplifier. Outputs from the optical component  222  may be coupled to the PBC by the waveguides  218 ,  220 . Output from the PBC may be coupled back to the circulator  212 . Circulator  212  may direct the PBC output to a circulator third port  228  which, in turn, may be coupled to an output optical waveguide  230 .  
         [0025]    In an implementation, an input optical signal may be received by the first port  224  of the circulator  212  on the input optical waveguide  210 . A feature of the circulator is to provide an input on the first port  224  as an output on the second port  226  and to provide an input on the second port as an output on the third port  228 . Thus, the input optical signal incident on the circulator first port  224  may be provided as an output signal on the circulator second port  226 . The waveguide  214  couples the optical signal on port  226  to the PBC  216 .  
         [0026]    The input optical signal may have more than one polarization component. The optical signal may have orthogonal transverse-magnetic (TM) and transverse-electrical (TE) polarization modes. The PBC  216  can split the optical signal provided on waveguide  214  into its TM and TE polarization components. The PBC also may be arranged to rotate one of the polarization components so that both modes propagate in the same orientation with respect to an optical wave guide. One polarized component beam may be provided to the optical component  222  on a first polarization-maintaining (PM) waveguide  220 . A second polarization component may be provided to the optical component  222  on a second PM waveguide  218 . Each polarization mode propagates through the optical component in a different direction. For example, the TE component may propagate in a forward direction through the optical component and the TM component in a reverse direction. The optical component may operate on both polarized components in substantially the same way because both components have the same orientation with respect to the axis of propagation.  
         [0027]    The output of the optical component resulting from the polarized component beam on first PM waveguide  220  may be provided to the PBC on the second PM waveguide  218 . Similarly, the output of the optical component resulting from the polarized component beam on second PM waveguide  218  may be provided to the PBC on the first PM waveguide  220 . The PBC may be arranged to rotate the previously rotated component back to the original orientation and subsequently re-combine the components into a single signal. The re-combined signal may be provided as an input to the circulator&#39;s second port  226  on waveguide  214 . As stated above, an input on the second port  226  may be provided by the circulator  222  as an output signal on the third port  228 . An output waveguide  230  may be coupled to the third port  228  to propagate the output optical signal.  
         [0028]    [0028]FIG. 3 is an isometric illustration of an implementation of the disclosed optical device that functions as a combined circulator and polarization beam combiner. The device has first and second optical ports at a proximal end that may be coupled to an input optical signal on a waveguide  210  and an output optical signal waveguide  230 , respectively. The device has third and fourth optical ports at a distal end that may be coupled to waveguides  218  and  220 , respectively, which, in turn, are coupled to a polarization dependent optical component (not shown). The elements of the device are arranged sequentially from the proximal end to the distal end and are optically coupled one to another along a longitudinal axis. The device may include a first imaging element  302 , a beam displacer  304 , a first half-wave plate  306   a - b  a first nonreciprocal beam rotator  308 , two beam angle turners  310 ,  312 , a second half-wave plate  314 , a second nonreciprocal beam rotator  316 , a prism  318  and a second imaging element  320 .  
         [0029]    First imaging element  302  is disposed along an optical path of the device and optically coupled to the input and output optical waveguides  210 ,  230 . Second imaging element  320  is disposed along the optical path and coupled to first and second PM waveguides  218 ,  220 . The imaging elements  302  and  320  include, for example, a collimating lens or graded index of refraction (GRIN) lenses. As shown in FIG. 3, the beam displacer/combiner  304  may be optically coupled to the first imaging element  302 . In one embodiment, the beam displacer/combiner includes a birefringent crystal. The crystal may be made, for example, of yttrium orthovanadate (YVO 4 ), rutile or barium borate (α-BBO). A first half-wave plate  306  is optically coupled to the beam displacer  304  and arranged to rotate the polarization state of a polarized beam component. The half-wave plate  306  is arranged so that segments  306   a  and  306   b  have different optical axes of rotation. The first nonreciprocal beam rotator  308  is optically coupled to the first half-wave plate  306 . In an embodiment, the first and second nonreciprocal rotators are nonreciprocal Faraday polarization rotators and may be made, for example, of yttrium-iron-garnet (YIG), or Bi-added thick film crystals. The Bi-added thick film crystals may include a combination of (YbTbBi) 3 Fe 5 O 12  and (GdBi) 3 (FeAlGa) 5 O 12 , or of YIG and Y 3x Bi x Fe 5 O 12 . Beam angle turners  310 ,  312  may be, for example, birefringent wedges and optically coupled distally from the first nonreciprocal beam rotator.  
         [0030]    The first and second beam angle turners  310 ,  312  change a beam propagation direction depending upon the beam polarization orientation and propagation direction. In an embodiment, the first and second beam angle turners may be Rochon prisms, Wollaston prisms, or modified Wollaston or Rochon prisms.  
         [0031]    The modified Rochon and Wollaston prisms differ from conventional Rochon and Wollaston prisms in the orientation of the optical axes of its wedges. In a modified Rochon prism, the optical axis of one of the wedges is oriented normal to the plane of normal incidence, which is the same as in a conventional Rochon prism. However, the optical axis in the other wedge is oriented forty-five degrees in the plane of normal incidence with respect to the optical axis orientation the wedge would possess in a conventional Rochon prism. Similarly, in a modified Wollaston prism, the optical axis of each of its birefringent wedges are oriented perpendicularly to each other and forty-five degrees in the plane of normal incidence with respect to the optical axis in a conventional Wollaston prism.  
         [0032]    The second half-waveplate  314  is optically coupled to the second beam angle turner  312 . Second nonreciprocal polarization rotator  316  is optically coupled distally to the second half-wave plate. Prism  318  is optically coupled to the second nonreciprocal polarization rotator  316  and is optically coupled to second imaging element  320 .  
         [0033]    [0033]FIGS. 4A and 4B illustrate a top and front view, respectively, of the implementation of the combined circulator and PBC of FIG. 3. FIG. 5A is a cross-sectional view showing the spatial location and polarization states of the component beams propagating in a forward direction through the combined device. The combined device performs both the function of the circulator  212  and PBC  216  of FIG. 2 by splitting an input signal on a waveguide  210  into polarized beam components in a forward direction, rotating the polarization direction of at least one of the component beams so that the components are polarized in the same orientation, and propagating the component beams to waveguides  218  and  220 , respectively. In the reverse direction, the device receives input component signals on waveguides  218  and  220  that have the same polarization orientation, rotates at least one of the components so that the polarization orientation of the components are orthogonal and combines both components into an output optical signal coupled to output waveguide  230 .  
         [0034]    Operation of the combined circulator and PBC in the forward direction will now be described with reference to FIGS.  4 A-B and  5 A. An input beam on waveguide  210  exits first imaging element  302 . FIG. 5A (section A-A) illustrates that the input beam includes two orthogonal light components. The input beam then enters beam displacer/combiner  304 , which acts as a polarization sensitive beam displacement plate. The input signal is decomposed into two orthogonal polarization components. Within the first beam displacer/combiner, the first polarized component beam may be referred to as an ordinary light ray (o-ray)  410  and the other component may be referred to as an extraordinary light ray (e-ray)  412 . The e-ray  410  walks off from the o-ray  414  through the beam displacer/combiner, with the result that there is a separation between the polarized beam components as illustrated in FIG. 5A (section B-B). Component beams  410  and  414  propagate through first half-wave plate  306   a  and  306   b , respectively. In this implementation, the half-wave plate rotates the polarization of the e-ray forty-five degrees counter-clockwise and the o-ray forty-five degrees clockwise. The component beams have the polarization orientation and spatial relationship illustrated in FIG. 5A (section C-C)  
         [0035]    The components  410 ,  414  then enter first nonreciprocal rotators  308 . The nonreciprocal rotator rotates the polarization orientation of both the e-ray and the o-ray forty-five degrees counter-clockwise and the polarization orientation of the components is illustrated in FIG. 5A (section D-D). Thus, the o-ray has the same polarization orientation as the input light beam and the polarization direction of the e-ray has been rotated so that both components now have the same polarization orientation but are spaced apart by the walk-off distance.  
         [0036]    Upon exiting the first nonreciprocal rotator  308 , both polarization components have the same polarization orientation before entering first beam angle turner  310 . First beam angle turner  310  turns both components towards the longitudinal axis of the optical device  416 . The components then exit the first beam angle turner and next propagate through second beam angle turner  312 , which is arranged to bend the components such that they are aligned with the longitudinal axis of second half-wave plate  314 . The polarization orientation remains the same as the components bend toward substantially the longitudinal axis of the device as illustrated in FIG. 5A (sections E-E and F-F).  
         [0037]    The component beams  410 ,  414  exit the second beam angle turner  312  and propagate through second half-wave plate  314 . Both the e-ray and the o-ray pass through the half-wave plate  314  that is arranged to rotate the polarization orientation forty-five degrees counterclockwise. as illustrated in FIG. 5A (section G-G). The waves then propagate through the second nonreciprocal rotator  316  and the polarization orientation id rotated another forty-five degrees in the counter-clockwise direction as illustrated in FIG. 5A (section H-H). Thus, the input beam on waveguide  210  as illustrated at FIG. 5A (section A-A), has been split into separate polarization components  410  and  414  having the same polarization orientation and separated by a walk off distance. The separated polarized beam components may bend toward the center after they propagate through a prism  318  and couple into two polarization maintaining fibers  218 ,  220  separately through the second imaging element  320 .  
         [0038]    The e-ray and the o-ray have the same polarization orientation and may be operated on by an optical component such as a semiconductor optical amplifier. A single amplifier may be used to amplify both polarization components by sending one component through the amplifier in a first direction and the other component in a second direction. Because the polarized components have the same polarization orientation, the amplifier will operate substantially the same on both components. After the polarization components exit respective sides of the amplifier, the polarized components will have the same polarization orientation.  
         [0039]    As described above, in the reverse direction the combined circulator and PBC can operate on polarization component beams having the same polarization orientation, rotate at least one of the components so that the components have orthogonal polarization orientation and combine the components into a single output optical signal.  
         [0040]    Operation of the combined circulator and PBC in the reverse direction will now be described with reference to FIGS.  4 A-B and  5 B. Each of two polarization component beams are received on PM fibers  218  and  220 , respectively. The polarization component beams are propagated through the second imaging element  320  and prism  318  into two parallel beams as illustrated in FIG. 5B (section J-J). The two polarization component beams are an o-ray  510  and an e-ray  514 .  
         [0041]    The components  510 ,  514  then propagate into second nonreciprocal rotator  308 . The nonreciprocal rotator rotates the polarization orientation of both the e-ray and the o-ray forty-five degrees counter-clockwise and the polarization orientation of the components is illustrated in FIG. 5B (section K-K). A feature of the nonreciprocal rotator is that the rotation of the polarization direction is the same for rays traveling in either direction through the nonreciprocal rotator.  
         [0042]    Component beams  510  and  514  propagate through second half-wave plate  314 . The second half-wave plate  314  rotates the polarization of both the e-ray and o-ray forty-five degrees clockwise, the direction of rotation being dependent on the direction of the component beam through the plate. The component beams have the polarization orientation and spatial relationship illustrated in FIG. 5B (section L-L). Both the e-ray and the o-ray have the same polarization orientation before propagating through the second beam angle turner  312 . Because the polarization orientation of the two beams are different than the forward propagating beams (compare FIG. 5B section L-L to FIG. 5A section C-C), the rays will be bent in a direction opposite to the forward propagating wave. In this implementation, the o-ray and the e-ray are bent away from the longitudinal axis. Component beams  510  and  514  propagate through the first beam angle turner  310  that is arranged to bend the beams substantially in the same direction as the longitudinal axis of the optical device. The component beams have the same polarization orientation but are offset from the longitudinal axis of the optical device as illustrated in FIG. 5B (sections M-M and N-N).  
         [0043]    The components  510 ,  514  then enter first nonreciprocal rotators  308 . The nonreciprocal rotator rotates the polarization orientation of both the e-ray and the o-ray forty-five degrees counter-clockwise and the polarization orientation of the components is illustrated in FIG. 5B (section P-P). Component beams  510  and  514  propagate through first half-wave plate  306   a  and  306   b , respectively. In this implementation, the half-wave plate rotates the polarization of the e-ray forty-five degrees clockwise and the o-ray forty-five degrees counter-clockwise. The component beams have polarization orientations that are orthogonal as illustrated in FIG. 5B (section R-R).  
         [0044]    The polarization components  510  and  514  then propagate through the beam displacer/combiner  304  where the components are recombined into the output optical and coupled to the output waveguide  230 . Thus, the separate polarization components having a parallel polarization orientation on waveguides  218  and  220 , respectively, have had the polarization direction of at least one of the components rotated so that the orientations are orthogonal and the beams combined and supplied as an output signal.  
         [0045]    [0045]FIG. 6 is an isometric illustration of an alternative implementation of the disclosed optical device that functions as a combined circulator and polarization beam combiner. The device has first and second optical ports at a first end that may be coupled to an input optical signal on a waveguide  210  and an output optical signal on waveguide  230 , respectively. The device has third and fourth optical ports at a second end that may be coupled to waveguides  218  and  220 , respectively, which, in turn, are coupled to a polarization-dependent optical component (not shown). The elements of the device may be arranged sequentially from the first end to the second end and are optically coupled one to another along a longitudinal axis. The device may include a first imaging element  302 , a beam displacer  304 , a first half-wave plate  306   a-b  a first nonreciprocal beam rotator  308 , a beam angle turners  310 , a second half-wave plate  314 , a second nonreciprocal beam rotator  316 , a prism  318  and a second imaging element  320 .  
         [0046]    The difference from the implementation disclosed in FIG. 3 is that in this alternative implementation of there is only one beam angle turner. The operation is substantially the same as described above in association with the description of FIG. 3. In this implementation, the beam angle turner  310  is arranged so that the two parallel polarization components  410 ,  414  (see FIG. 4A) are bent toward, and substantially parallel to, the longitudinal axis and after they propagate in a forward direction through one beam angle turner. In the reverse direction, the polarization components are bent away from, and essentially parallel to, the longitudinal axis of the device. Two beam angle turners may enable a smaller device than a single beam angle turner because of the focal length of the imaging devices  302 ,  320 .  
         [0047]    Other implementations are within the scope of the following claims.