Abstract:
An integrated broadband polarization controller is based on planar waveguide technology. The integrated broadband polarization controller emulates the polarization control function of an existing single channel polarization controller in a device that can be built in planar waveguide technology, and is expanded from single channel operation to broadband channel operation by designing the control degrees of freedom to be wavelength selectively addressable.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of optoelectronics and, more specifically, to polarization controllers. 
     BACKGROUND OF THE INVENTION 
     Light is an electromagnetic wave composed of electric and magnetic fields. The orientation of these fields defines the light&#39;s polarization. As light propagates through an optical fiber, variables such as temperature and stress can cause random and arbitrary changes in the state of the light&#39;s polarization, reducing the distance that the light travels due to signal degradation. These changes in the state of polarization can cause problems in fiber optic applications such as optical communication systems and sensors. Polarization controllers can reduce signal degradation in optical systems by converting any incoming polarization state back to the intended polarization state during optical transmissions. The principle of the polarization controller is that a desired polarization state is obtained by using appropriate phase retarders or phase shifters which can transform a state of polarization (SOP) to another arbitrary SOP. For the purposes of achieving this result, two design aspects of polarization controllers are generally considered critical. First, the controller must be able to convert an arbitrary, time variant state of polarization into a specific, desired polarization state. Second, the polarization controller must be able to convert the state of polarization for a wide range of wavelengths for use in optical communication systems, such as those utilizing wavelength division multiplexing (WDM). 
     Previous work in the field has proposed several different concepts for polarization controllers possessing the two desired design aspects described above, based on mechanically tunable as well as electro-optic effects. For example, see R. Noe, H. Heidrich, D. Hoffman, “Endless Polarization Control Systems for Coherent Optics,” Journal of Lightwave Technology, pp. 1199-1207, 1988. It seems that presently, the straight-forward solution for achieving a polarization controller with the ability to convert the state of polarization for a wide range of wavelengths consists of an optical demultiplexer, single channel polarization controllers, and an optical multiplexer. This hybrid combination however, can be costly and physically occupies a large form factor. 
     SUMMARY OF THE INVENTION 
     The invention comprises a method and apparatus for providing integrated broadband polarization control. The invention enables an integrated polarization controller, suitable for realization on low-cost material in planar waveguide technology, and compact in size, which can transform a SOP to another arbitrary SOP for a wide range of wavelengths for use in optical communication systems, such as those utilizing wavelength division multiplexing (WDM). 
     A method for providing broadband polarization control according to an embodiment of the invention includes the steps of splitting an optical signal into a first polarization component and a second polarization component, the second polarization component orthogonal to the first polarization component, retarding the phase of the first polarization component, and recombining the first polarization component and the second polarization component. Alternatively, the method can further include the steps of converting the first polarization component into the orthogonal polarization prior to the retarding, such that the first polarization component and the second polarization component propagate with the same polarization, and converting the first polarization component back to its original polarization state prior to recombining it with the second polarization component. 
     An apparatus for providing integrated broadband polarization control according to another embodiment of the invention includes a splitting optic for splitting received optical signals into a first polarization component and a second polarization component, the second polarization component orthogonal to the first polarization component, and the first and second polarization components propagating through separate branches of the broadband polarization controller, at least one wavelength selectable phase shifter, for retarding the phase of the first polarization component, and a combining optic for combining the first polarization component and the second polarization component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
     FIG. 1 depicts an embodiment of a single channel polarization controller according to prior art; 
     FIG. 2 depicts an embodiment of a single channel polarization controller based on planar waveguide technology, according to the Jones matrix product; 
     FIG. 3 depicts a simplified design of the single channel polarization controller of FIG. 2; and 
     FIG. 4 depicts an embodiment of an integrated broadband polarization controller based on planar waveguide technology. 
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts an embodiment of a single channel polarization controller design. (See R. Noe, H. Heidrich, D. Hoffman, “Endless Polarization Control Systems for Coherent Optics,” Journal of Lightwave Technology, pp. 1199-1207, 1988). The polarization controller  100  of FIG. 1 provides polarization tuning of an input optical signal&#39;s state of polarization (SOP) to a desired output SOP. Briefly stated, an optical signal propagates through the waveguide  120  and subsequently reaches the polarization controller  100 . The polarization controller of FIG. 1 includes three fiber squeezers  110   1 ,  110   2 ,  110   3 . The SOP of the input optical signal is then manipulated to a desired SOP due to the squeezing influence that the fiber squeezers  110   1 ,  110   2 ,  110   3  have on the fiber birefringence. Unfortunately, fiber squeezers, as depicted in FIG. 1, occupy a large form factor and cannot be designed in silicon optical bench technology. Thus there exists a need to emulate the polarization control function of an existing single channel polarization controller in a device, suitable for realization on low cost material in planar waveguide technology. 
     The control function of the fiber squeezers in FIG. 1 is translated into planar waveguide technology. The three fiber squeezers  110   1 ,  110   2 , and  110   3 , which are implemented in the controller, are represented in Jones calculus by the two matrices A and B represented below:                  A        (   α   )       =     [           cos                 α             j   ·   sin                   α                 j   ·   sin                   α           cos                 α           ]       ;       B        (   β   )       =       [           exp        (     j                 β     )           0           0         exp        (       -   j                   β     )             ]     .               (   1   )                                
     In the above matrices, α and β represent variables that describe the squeezing influence in different planes, on the fiber birefringence. Thus the design of the polarization controller  100  of FIG. 1 is described by the Jones matrix product as follows:                T        (     α   ,   β              ,   γ     )       =       [           cos                 γ             j   ·   sin                   γ                 j   ·   sin                   γ           cos                 γ           ]                [                    exp                   (     j                 β     )           0           0         exp                   (       -   j                   β     )             ]                [                      cos                 α             j   ·              sin                   α                 j   ·   sin                   α           cos                 α           ]     .               (   2   )                                
     In planar waveguide technology, matrices of type B are more readily identifiable than matrices of type A. Subsequently, matrices of type A can be rewritten as a product of a type B matrix and matrices that describe 3 dB couplers as follows:                A   (              α   )     =       [           cos                 α             j   ·   sin                   α                 j   ·   sin                   α           cos                 α           ]     =       [           1   /     √   2               -   1     /     √   2                 1   /     √   2             1   /     √   2             ]                                       [                      exp                   (     j                 α     )           0           0         exp                   (       -   j                   α     )             ]                            [           1   /     √   2             1   /     √   2                   -   1     /     √   2             1   /     √   2             ]     .                       (   3   )                                
     Using the previous expression, the Jones matrix product can be rewritten as follows:                T        (     α   ,   β   ,   γ     )       =         [           1   /     √   2               -   1     /     √   2                 1   /     √   2             1   /     √   2             ]                [                      exp                   (     j                 γ     )           0           0         exp                   (       -   j                   γ     )             ]                                         [           1   /     √   2             1   /     √   2                   -   1     /     √   2             1   /     √   2             ]                [                      exp                   (     j                 β     )           0           0         exp                   (       -   j                   β     )             ]                [                                  1   /     √   2               -   1     /     √   2                 1   /     √   2             1   /     √   2             ]                                       [                      exp                   (     j                 α     )           0           0         exp                   (       -   j                   α     )             ]                [           1   /     √   2             1   /     √   2                   -   1     /     √   2             1   /     √   2             ]                         (   4   )                                
     3 dB couplers possessing the matrix form used in Equation (3) above can be represented by conventional couplers. A conventional coupler has to be manipulated by adding constant phase shifts at the input and the output ports so that the desired transfer matrix is achieved. The added phase shift is realized wavelength independent by changing the path-length in the order of a fraction of the wavelength. A constant phase amount can then be factored out and neglected in the calculation, as it represents a constant phase that is common to both arms. A device based on planar waveguide technology and possessing the same overall transfer matrix as for the above described single channel polarization controller can be constructed using these results. 
     By representing each matrix in the product of Equation 4 by a corresponding device (phase shifters and couplers), the result is the design for the single channel polarization controller shown in FIG.  2 . FIG. 2 depicts an embodiment of a single channel polarization controller  200  based on planar waveguide technology, according to the Jones matrix product. The single channel polarization controller  200  includes a waveguide  210 , four couplers  220   1 ,  220   2 ,  220   3 , and  220   4 , two mode converters  230   1 , and  230   2 , a polarization beam splitter  240 , a polarization beam combiner  250 , and six phase shifters  260   1 ,  260   2 ,  260   3 ,  260   4 ,  260   5 , and  260   6  separated into two groups. Briefly stated, an optical signal enters the waveguide  210  and gets separated into TE and TM polarized components by the polarization beam splitter  240 . After being separated into TE and TM polarized components, one of the polarizations is converted into the orthogonal one by the first mode converter  230   1 , and subsequently, both signal components propagate with the same polarization along two separate branches of the polarization controller. Examples of this kind of mode mapping are detailed in “PMD Emulator restricted to first and second order PMD Generation,” L. Moeller, H. Kogelnik, ECOC &#39;99. vol. II, pp. 64-65, 1999. This technique allows the exploitation of the interference effects between the original TE and TM polarized components in order to achieve mode conversion. 
     The converted polarized component of the optical signal then propagates along the first branch of the waveguide traversing the four couplers  220   1 ,  220   2 ,  320   3 , and  220   4  and the first group of three phase shifters  260   1 ,  260   2 , and  260   3  until finally being converted back to its original polarization state by the second of the mode converters  330   2 . The three phase shifters  260   1 ,  260   2 , and  260   3  retard the phase of the converted polarized component of the optical signal in the three variable planes, α(λ), β(λ), γ(λ). The second polarized component of the optical signal propagates along the second branch of the waveguide traversing the four couplers  220   1 ,  220   2 ,  220   3 , and  220   4  and the second group of three phase shifters  260   4 ,  260   5 , and  260   6 . The three phase shifters  260   4 ,  260   5 , and  260   6  retard the phase of the second polarized component of the optical signal in the three variable planes, α(λ), β(λ), γ(λ). The two polarized components are then recombined by the polarization beam combiner  250 . When recombined, the interference effects of the TE and TM components produce a desired polarization state. 
     The design of the single channel polarization controller of FIG. 2 is simplified as shown in FIG.  3 . FIG. 3 depicts a simplified design of the single channel polarization controller of FIG.  2 . The simplified single channel polarization controller  300  includes a waveguide  310 , four couplers  320   1 ,  320   2 ,  320   3 , and  320   4 , two mode converters  330   1 , and  330   2 , a polarization beam splitter  340 , a polarization beam combiner  350 , and three phase shifters  360   1 ,  360   2 , and  360   3 . Briefly stated, an optical signal enters the waveguide  310  and gets separated into TE and TM polarized components by the polarization beam splitter  340 . After being separated into TE and TM polarized components, one of the polarizations is converted into the orthogonal one by the first mode converter  330   1  so both signal components propagate with the same polarization along the two separate branches of the polarization controller. 
     The converted polarized component of the optical signal then propagates along the first branch of the waveguide traversing the four couplers  320   1 ,  320   2 ,  320   3 , and  320   4  and the three phase shifters  360   1 ,  360   2 , and  360   3  until finally being converted back to its original polarization state by the second of the mode converters  330   2 . The three phase shifters  360   1 ,  360   2 , and  360   3  retard the phase of the optical signal in the three variable planes, α(λ), β(λ), γ(λ). The second polarized component of the optical signal propagates along the second branch of the waveguide traversing only the four couplers  320   1 ,  320   2 ,  320   3 , and  320   4 . The two polarized components are then recombined by the polarization beam combiner  350 . When recombined, the interference effects of the TE and TM components produce a desired polarization state. 
     The configuration of FIG. 3 is used as a single channel polarization controller. In order for this set up to be suitable for use in a WDM system, the phase shifting has to be carried out independently for the various wavelengths. It would then be necessary to implement wavelength selectable phase shifters instead of the phase shifters in FIG.  3 . 
     Wavelength selectable phase shifters can be built using an optical demultiplexer (DeMux), an array of parallel phase shifters, and an optical multiplexer (Mux), as depicted in FIG.  4 . Thus the complete design, possessing broadband polarization control for all of the WDM channels of a WDM system and possessing the same transfer function for each channel depending on α(λ), β(λ), γ(λ), such as the fiber squeezer based device for single channel controllers, is given by the overall design shown in FIG.  4 . 
     FIG. 4 depicts an embodiment of an integrated broadband polarization controller  400  based on planar waveguide technology. The broadband polarization controller  400  includes a waveguide  310 , four couplers  320   1 ,  320   2 ,  320   3 , and  320   4 , two mode converters  330   1 , and  330   2 , a polarization beam splitter  340 , a polarization beam combiner  350 , and three wavelength selectable phase shifters  460   1 ,  460   2  and  460   3  (collectively  460 ). Briefly stated, an optical signal enters the waveguide  310  and gets separated into TE and TM polarized components by the polarization beam splitter  340 . After being separated into TE and TM polarized components of the incoming optical signal, one of the polarizations is converted into the orthogonal one by the first mode converter  330   1  so that both signal components propagate with the same polarization along the two separate branches of the broadband polarization controller. The converted polarized component of the optical signal then propagates along the first branch of the waveguide through the first coupler  320   1  until it reaches the first wavelength selectable phase shifter  460   1 . 
     The three wavelength selectable phase shifters  460  are comprised of three optical demultiplexer  462   1 ,  462   2 ,  462   3  (collectively  462 ), an array of parallel phase shifters  464   1 - 464   n  (collectively  464 ), and three optical multiplexers  466   1 ,  466   2 ,  466   3  (collectively  466 ). Each wavelength selectable phase shifter  460  is comprised of one demultiplexer  462 , an array of parallel phase shifters  464 , and a multiplexer  466 . In the embodiment shown in FIG. 4, the optical demultiplexers  462  and the optical multiplexers  466  are Arrayed Waveguide Grating (AWG) filters. 
     The converted polarized component of the optical signal reaches the first demultiplexer  462   1  and is separated into a plurality of wavelength components. The number of wavelengths, and subsequently, the number of phase shifters  464 , are determined by the number of wavelength components that the converted polarized component of the optical signal is divided into by the first demultiplexer  462   1 . The wavelength components of the converted polarized component of the optical signal then propagate through individual phase shifters  464 . Each phase shifter  464  is chosen for the particular wavelength region it will operate on. Each phase shifter  464  retards the phase of the signal in equal amounts to the other phase shifters  464  to produce a resultant signal with a different phase than the input signal. The various wavelength components of the optical signal pass through the respective phase shifters  464  and are then recombined by the first multiplexer  466   1 . The optical signal then propagates through the waveguide, passing through the second coupler  320   2 , until it reaches the second wavelength selectable phase shifter  460   2 . The optical signal is phase shifted in the second variable plane by the second wavelength selectable phase shifter  460   2  and propagates though the third coupler  320   3  to the third wavelength selectable phase shifter  460   3 . The converted polarized component of the optical signal is phase shifted in the third variable plane by the third wavelength selectable phase shifter  460   3  then propagates through the fourth coupler  320   4  until finally being converted back to its original polarization state by the second of the mode converters  330   2 . 
     The second polarized component of the optical signal propagates along the second branch of the waveguide traversing only the four couplers  320   1 ,  320   2 ,  320   3 , and  320   4 . The two polarized components are then recombined by the polarization beam combiner  350 . When recombined, the interference effects of the TE and TM components produce a desired polarization state. 
     The broadband polarization controller of FIG. 4 provides the ability to convert an arbitrary, time variant state of polarization into a specific, desired polarization state, and the ability to convert the state of polarization for a wide range of wavelengths for use in WDM systems, on low-cost material in planar waveguide technology, and in a compact size. Although the demultiplexers  462  and multiplexers  466  of FIG. 4 were illustrated to be Arrayed Waveguide Grating filters, it would be evident to those skilled in the art that other embodiments of the present invention would include other planar waveguide components to be used as the demultiplexers and the multiplexers in a broadband polarization controller. 
     In another embodiment of a broadband polarized controller, the mode converters at the input and output of the broadband polarization controller can be eliminated. In this embodiment, the amounts for α(λ), β(λ), γ(λ) phase shifting have to be adjusted to compensate for the fact that the two polarization components of the optical signal are not propagating through the two separate branches of the broadband polarization controller with the same polarization. 
     In other embodiments of the present invention, broadband polarization controllers, similar to the broadband polarization controller of FIG. 4, can be designed containing additional control elements to enhance the tolerance of the design. This is similar to single channel polarization controllers with four or more fiber squeezers. 
     In another embodiment of the present invention, a broadband polarization controller is built for endless transformation of an arbitrary input state of polarization into an arbitrary output state of polarization by combining two of the described broadband polarization controllers. The overall setup is then simplified by removing the polarization beam splitter and combiner and connecting the corresponding input and output ports directly to each other. The number of multiplexers and demultiplexers is then reduced to five. 
     While the forgoing is directed to some embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.