Abstract:
An optical polarization modulator (OPM) for use in an optical communication system includes two polarizing beam splitters (PBSs) and an adjusting stage coupled between the PBSs. One PBS receives an input optical signal with an arbitrary state of polarization (SOP) and splits it into its TE and TM components. The adjusting stage can change the amplitude and/or relative phase between the TE and TM components to help achieve a desired state of polarization (SOP). The ACS and PCS may include MZIs to both adjust the amplitude and the relative phase difference between the TE and TM components. Alternatively, the OPM may include an amplifier and a phase shifter for each component. The second PBS combines the adjusted components to form the output signal with a desired SOP. Another embodiment of the OPM includes an Y-junction coupler, two plasma optical-effect silicon phase shifter stages, a 2×2 3-dB coupler and a PBS. The Y-junction coupler splits the incoming optical signal into two equal portions (containing TM and TE components). The first stage adjusts the relative phase between the portions. The 2×2 3-dB coupler allows the phase adjusted portions to interact to adjust the amplitude. The second stage adjusts the phase difference between the two portions for TE and TM components and outputs the adjusted portions to the PBS via polarization maintaining fibers. The PBS then combines the TE component of one portion with the TM component of the other portion to form an output signal with the desired SOP.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present invention is related to U.S. patent application Ser. No. 09/811,171 entitled “Method and Apparatus For Steering An Optical Beam In A Semiconductor Substrate” filed Mar. 16, 2001 by A. Liu et al. and to U.S. patent application Ser. No. 10/004,030 entitled “Method and Apparatus Of A Semiconductor-Based Tunable Optical Dispersion Compensation System With Multiple Channels” filed Oct. 19, 2001 by S. Ovadia et al. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The field of invention relates to optical communication devices in general; and, more specifically, to optical polarization modulators and compensators.  
         BACKGROUND  
         [0003]    There are various methods to transmit information in fiber-optic communication systems. Some optical communication systems use state of polarization (SOP) modulation to transfer information. In a typical optical SOP system, a polarization modulator is used to control the SOP of an optical signal (i.e., a laser beam) by changing the phase and amplitudes of the optical signal&#39;s TE (transverse electrical) and TM (transverse magnetic) components (these components also referred to herein as the TE and TM components). The SOP can be used to achieve multi-level transmission (i.e., where each SOP can represent the value of multiple bits). SOP-based optical communication systems can be substantially insensitive to some nonlinear fiber effects, such as self-phase modulation and polarization dependent gain in some EDFAs (Erbium doped fiber amplifiers).  
           [0004]    Some polarization modulators use the thermo-optic effect to modulate the SOP. However, the speed of these thermo-optic based polarization modulators and compensators is relatively slow with typical symbol rates in the kHz range.  
           [0005]    Other polarization modulators use Lithium Niobate (LiNbO 3 ) devices, which provide greater symbol rate, but are relatively high in cost, form factor, and difficulty in implementing in an integrated circuit device.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified.  
         [0007]    [0007]FIG. 1 is a simplified schematic diagram illustrating an optical polarization modulator with controller and other electronic components omitted, according to a first embodiment of the present invention.  
         [0008]    [0008]FIG. 2 is a illustrating a cross-section of a plasma optical effect-based silicon phase shifter, according to one embodiment of the present invention.  
         [0009]    [0009]FIG. 3 is a diagram illustrating a perspective view of ridge-waveguide plasma optical-effect silicon phase shifter implementation of the phase shifter depicted in FIG. 2, according to one embodiment of the present invention.  
         [0010]    [0010]FIG. 4 is a simplified schematic diagram illustrating an optical polarization modulator with controller and other electronic components omitted, according to a second embodiment of the present invention.  
         [0011]    [0011]FIG. 5 is a simplified schematic diagram illustrating an optical polarization modulator with controller and other electronic components omitted, according to a third embodiment of the present invention.  
         [0012]    [0012]FIG. 6 is a simplified block diagram of an optical communication system using an optical polarization modulator according to one of the embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]    [0013]FIG. 1 is a simplified schematic diagram illustrating an optical polarization modulator  100 , according to a first embodiment of the present invention. In this embodiment, optical polarization modulator  100  includes optical phase control elements  101 - 104 , polarizing beam splitter (PBS)  106 , and Y-junction coupler  108 , 2×2 3-dB coupler  110  and polarization maintaining fibers (PMF)  114 ,  116  and  118 . In this embodiment, phase control elements  101 - 104  are silicon based optical phase control elements, implemented as described below in conjunction with FIGS. 2 and 3.  
         [0014]    In this embodiment, 2×2 3-dB coupler  110  is used to vary the light intensity of the TE or TM components at the output ports of phase control elements  101  and  102 . For example, 2×2 3-dB coupler  110  can be implemented as an evanescent coupler or as a 2×2 multi-mode interference (MMI) device.  
         [0015]    The elements of optical polarization modulator  100  are interconnected as follows. One port of Y-junction coupler  108  is connected to PMF  108  to receive a linearly polarized input optical signal. The other two ports of Y-junction coupler  114  are respectively connected to input ports of phase control elements  101  and  102 .  
         [0016]    Phase control element  101  has an output port connected to one input port of 2×2 3-dB coupler  110  and, similarly, phase control element  102  has an output port connected to the other input port of 2×2 3-dB coupler  110 . Phase control elements  101  and  102  are connected to receive a control signal V a1  and a control signal V a2 , respectively. Control signals V a1  and V a2  controls the induced phase difference between the two arms of the polarization modulator for the TE and TM components of the propagating optical signals after the phase control elements  101  and  102 . In one embodiment, control signals V a1  and V a2  are generated by a radio frequency (RF) signal generators (not shown). In another embodiment, one of phase control elements  101  and  102  does not receive a control signal.  
         [0017]    One output port of 2×2 3-dB coupler  110  is connected to an input port of phase control element  103 , whereas the other output port of 2×2 3-dB coupler  110  is connected to phase control element  104 . Phase control elements  103  and  104  are connected to receive a control signal V b1  and V b2 , respectively. Control signals V b1  and V b2  control the induced phase shift of the TE and TM components at each arm of the polarization modulator after phase control elements  103  and  104 . In one embodiment, control signals V b1  and V b2  are generated by other RF signal generators (not shown). Thus, the voltage difference V b1  and V b2  controls the relative phase difference between TE/TM and TM/TE components for the two arms of the polarization modulator.  
         [0018]    The output ports of phase control elements  103  and  104  are connected to the input ports of PBS  106  via PMFs  116  and  118 , respectively. PBS  106  has an output port connected to a single mode fiber (SMF)  120 . As will be described below, phase control elements  103  and  104  and PMFs  116  and  118  are used to control the relative phase difference between the TE and TM components of the output optical signal between the two arms of the polarization modulator.  
         [0019]    In operation, Y-junction coupler  108  receives a linearly polarized input optical signal (e.g., a laser beam of about 1550 nanometers) via PMF  114 . Y-junction coupler  108  then splits essentially one-half of the input signal power to phase control element  101  and the other half to phase control element  102 . Phase control elements  101  and  102  introduce a phase difference between their output signals, depending on control signals V a1  and V a2  as described below in conjunction with FIG. 2 (with control signals V a1  and V a2 ) corresponding to the control signal V G  in FIG. 2).  
         [0020]    2×2 3-dB coupler  110  then causes the phase shifted output signals of phase control elements  101  and  102  to interact. Depending on the phase difference introduced by phase control elements  101  and  102 , the amplitudes of the signals outputted by 2×2 3-dB coupler  110  are controlled. For example, by introducing a phase difference of π/2, −π/2, or 3π/2 radians, allowing the light intensity ratio to be maximized at the output of the 2×2 3-dB coupler. In contrast, by introducing no phase difference, the output signals of 2×2 3-dB coupler  110  would in an ideal system have the same amplitudes as the input optical signals received from phase control elements  101  and  102 .  
         [0021]    The intensities of the optical signals outputted by 2×2 3-dB coupler  110  and directed to phase control elements  103  and  104  (i.e., I 1  and I 2 , respectively) can be represented by the equations:  
           I   1   (TE,TM)   =E   0   2 ·cos 2 [Δφ 0   (TE,TM) +θ (TE,TM) ]  (1)  
           I   2   (TE,TM)   =E   0   2 ·sin 2 [Δφ 0   (TE,TM) +θ′ (TE,TM) ]  (2)  
         [0022]    where E 0  is the initial electrical amplitude, Δφ 0   (TE,TM)  is the induced phase shift for the TE and TM modes, θ (TE,TM)  and θ′ (TE,TM)  are the initial phases of the TE and TM modes received by phase control elements  103  and  104 , respectively.  
         [0023]    Phase control elements  103  and  104  then introduce a phase difference between their output signals, depending on control signals V b1  and V b2  as described below in conjunction with FIG. 2 (with control signal V b1  and V b2 ) corresponding to the control signal V G  in FIG. 2. The amplitudes of output electric field signals after the phase control elements  103  and  104  (i.e., E 1  and E 2 ) can be represented, respectively, by the equations:  
           E   1   =E   1,TE   +E   1,TM   =E   0   [e   i(Δφ′   0   +Δφ   1   +θ)   +e   i(Δφ′   0   +θ) ]  (3)  
           E   2   =E   2,TM   +E   2,TE   =E   0   [e   i(Δφ″   0   +Δφ   2   +θ″)   +e   i(Δφ″   0   +θ) ]  (4)  
         [0024]    where Δφ 1  and Δφ 2  are the induced phase shift for the TE and TM components at each arm of the polarization modulator after the phase control elements  103  and  104 , respectively; Δφ′ 0  and Δφ″ 0  are the initial phases of the TE and TM components when received by phase control elements  103  and  104 , respectively. By appropriate control of the control signals V b1  and V b2 , the desired induced phase shift difference, namely Δφ 1 −Δφ 2 ≠0 can be achieved.  
         [0025]    PMFs  116  and  118  then propagate the output signals from phase control elements  103  and  104  to PBS  106 . PBS  106  is configured so that when it receives the output signal from phase control element  103  via PMF  116 , it passes the TE mode to SMF  120  while reflecting the TM mode. In this way, the TM mode from PMF  116  does not contribute to the polarization modulator output signal. Similarly, PBS  106  is configured so that when it receives the output signal from phase control element  104  via PMF  118 , it passes the TM mode to SMF  120  while reflecting the TE mode. In this way, the TE mode from PMF  118  does not contribute to the polarization modulator output signal. Thus, PBS  106  combines the TE mode of the output signal of phase control element  103  with the TM mode of the output signal of phase control element  104 .  
         [0026]    Thus, by appropriate control of control signals V a1 , V a2 , V b1  and V b2 , polarization modulator  100  can generate an output signal with any SOP in the SMF  120 . In addition, because silicon-based phase control elements are used, polarization modulator  100  has a relatively high symbol rate (in the several GHz range), is more easily implemented in an integrated circuit device, and has a smaller cost and form factor compared to the previously described thermo-optic and LiNbO 3  polarization modulators  
         [0027]    [0027]FIG. 2 illustrates a cross-section of optical phase control element  101 , according to one embodiment of the present invention. In this embodiment, optical phase control elements  102 - 104  are essentially identical to optical phase control element  101 . In one embodiment, several trench capacitors are formed with polysilicon regions  229  disposed in semiconductor substrate  221 . In one embodiment, insulating regions  231  are disposed between polysilicon regions  229  and semiconductor substrate to form trench capacitors.  
         [0028]    In one embodiment, the wafer on which phase control element is disposed is a silicon-on-insulator (SOI) wafer. Accordingly, a buried insulating layer  225  is disposed between semiconductor substrate  221  and semiconductor substrate  227  of the SOI wafer. In addition, semiconductor substrate  221  is disposed between buried insulating layer  225  and insulating layer  237 . In one embodiment, insulating layer  237  is an interlayer dielectric layer of the wafer on which phase control element  101  is disposed.  
         [0029]    In one embodiment, an optical waveguide, such as for example a rib waveguide, is disposed in semiconductor substrate  221  between insulating layers  237  and  225 . As such, optical beam  223  is illustrated in FIG. 2 propagating from left to right. In one embodiment, optical beam  223  includes infrared or near infrared laser light. As mentioned, in one embodiment, semiconductor substrate  221  includes silicon. As known to those skilled in the art, silicon is partially transparent to infrared or near infrared light. For instance, in one embodiment in which phase control element  101  is utilized in telecommunications, optical beam  223  has an infrared wavelength in the range of approximately 1300 to 1550 nanometers. In one embodiment, insulating layers  225  and  237  include an oxide material. The oxide material has a smaller index of refraction that silicon and polysilicon; therefore, optical beam  223  is confined within the waveguide between insulating layers  225  and  237  as a result of total internal reflection.  
         [0030]    As shown in the embodiment of FIG. 2, polysilicon regions  229  are coupled to receive a control signal V G  through conductors  233  routed through insulating layer  237 . In the depicted embodiment, the trench capacitors formed by polysilicon regions  229  in semiconductor substrate  221  are biased in response the control signal V G  such that the concentration of free charge carriers in charged regions  235  is modulated. For instance, in one embodiment, when control signal V G  is varied, injected free electrons and holes included in charge regions  235  accumulate at the interfaces between the polysilicon regions  229  and insulating regions  231  and at the interfaces between semiconductor substrate  221  and insulating regions  231 . Accordingly, as optical beam  223  propagates through the waveguide between insulating layers  225  and  237 , optical beam  223  propagates through the modulated charged regions  235 .  
         [0031]    In one embodiment, the phase of optical beam  223  that passes through the charged regions  235  is modulated in response to control signal V G . In one embodiment, the phase of optical beam  223  passing through free charge carriers in charged regions  235  is modulated due to the plasma optical effect. The plasma optical effect arises due to an interaction between the optical electric field vector and free charge carriers that may be present along the propagation path of the optical beam  223 . The electric field of optical beam  223  induces a change in the velocity of the free charge carriers and this effectively perturbs the local dielectric constant of the medium. This in turn leads to a perturbation of the propagation velocity of the optical wave and hence the refractive index for the light, since the refractive index is simply the ratio of the speed of the light in vacuum to that in the medium. The free charge carriers are accelerated by the field and also lead to absorption of the optical field as optical energy is used up. Generally the refractive index perturbation is a complex number with the real part being that part, which causes the velocity change and the imaginary part being related to the free charge carrier absorption. In this embodiment, the amount of phase shift φ is determined using the equation:  
         φ=(2π/λ)Δ n·L   (5)  
         [0032]    where λ is the optical wavelength in vacuum and L is the interaction length.  
         [0033]    In the case of the plasma optical effect in silicon, the refractive index change Δn due to the electron (ΔN e ) and hole (ΔN h ) concentration change (which depends on control signal V G  as described above) is determined in this embodiment using the equation:  
               Δ                 n     =       -         e   2          λ   2         8                   π   2          c   2          ɛ   0          n   0                (         Δ                   N   e         m   e   *       +       Δ                   N   h         m   h   *         )               (   6   )                               
 
         [0034]    where n 0  is the nominal index of refraction for silicon, e is the electronic charge, c is the speed of light ε 0  is the permittivity of free space, m e * and m h * are the electron and hole effective masses, respectively.  
         [0035]    It is noted that four trench capacitors have been illustrated in FIG. 2 for explanation purposes with polysilicon regions  229  disposed in semiconductor substrate  221 . Other embodiments may have a greater or fewer number of trench capacitors in accordance with the teachings of the present invention, with the number of trench capacitors chosen to achieve the required phase shift. In particular, the interaction length L discussed in connection with Equation (5) above may be varied by increasing or decreasing the total number of trench capacitors of phase control element  101 .  
         [0036]    [0036]FIG. 3 is a diagram illustrating a perspective view of a portion of silicon-based phase control element that can be used to implement phase control element  101  (FIG. 2), according to one embodiment of the present invention. In this embodiment, the phase control element is implemented using a rib waveguide  325 . Rib waveguide  325  is disposed between insulating regions (not shown), similar to insulating regions  225  and  227  in FIG. 2. Conductors  233  (FIG. 2) are also omitted from FIG. 3 for clarity in describing the rib waveguide.  
         [0037]    Rib waveguide  325  is disposed in a semiconductor material  303  and includes regions of polysilicon  305 . In one embodiment, the semiconductor material  303  has a different index of refraction than polysilicon  305  such that periodic or quasi-periodic perturbations in an effective index of refraction are provided along an optical path through rib waveguide  325 .  
         [0038]    As shown, rib waveguide  325  includes a rib region  327  and a slab region  329 . In the embodiment illustrated in FIG. 3, the intensity distribution  319  of a single mode optical beam is shown propagating through the rib waveguide  325 . As shown, the intensity distribution  319  of the optical beam is such that of the majority of the optical beam propagates through a portion of rib region  327  towards the interior of the rib waveguide  325 . In addition, a portion of the optical beam propagates through a portion of slab region  329  towards the interior of the rib waveguide  325 . As also shown with the intensity distribution  319  of the optical beam, the intensity of the propagating optical mode of the optical beam is vanishingly small at the “upper corners” of rib region  327  as well as the “sides” of slab region  329 . This ridge waveguide shape allows rib waveguide  325  to support single mode propagation.  
         [0039]    [0039]FIG. 4 illustrates an optical polarization modulator  400 , according to a second embodiment of the present invention. In this embodiment, optical polarization modulator  400  includes the induced phase control elements  401 - 404 , PBSs  406  and  407 , 3-dB couplers  410 - 414 , input SMF  416 , PMFs  418 ,  419 ,  421  and  422 , and output SMF  425 . In this embodiment, the induced phase control elements  401 - 404  are plasma optical effect-based silicon phase shifters as described above in conjunction with FIGS. 2 and 3. However, in other embodiments of optical polarization modulator  400 , any suitable type of phase control element can be used. For example, other embodiments can use LiNbO 3  devices or, alternatively, devices based on the thermo-optic effect. In one embodiment, single-mode waveguides can be used instead of PMFs  418 ,  419 ,  421  and  422 .  
         [0040]    Also, in another embodiment, all or some of the elements that are part of device  400  could be fabricated on a single monolithic or hybrid chip. As an example, PBSs  406  and  407 , phase modulators  401 - 404 , Y-junction couplers  410 ,  411 ,  413 ,  414  and the connecting PMFs  418 ,  419 ,  421 ,  422  can be fabricated using the same silicon substrate. In addition, other substrates can be used; e.g., group Ill-V compound semiconductors.  
         [0041]    The elements of optical polarization modulator  400  are interconnected as follows. PBS  406  has an input port connected to input SMF  416 , and has one output port connected to an input port  410 A of 3-dB coupler  410  via PMF  418 . The other output port of PBS  406  is connected to an input port  411 A of 3-dB coupler  411  via PMF  419 .  
         [0042]    An output port  410 B of 3-dB coupler  410  is connected to an input port of phase control element  401  via PMF. 3-dB coupler  410  has another output port  410 C connected to an input port of phase control element  402  via PMF. The output port of phase control element  401  is connected to an input port  413 C of 3-dB coupler  413  via PMF. The output port of phase control element  402  is connected to another input port  413 B of 3-dB coupler  413  via PMF. Thus, the induced phase control elements  401  and  402  and 3-dB couplers  410  and  413  form a Mach-Zehnder Interferometer (MZI). Induced phase elements  401  and  402  are connected to receive control signals V A  and V B , respectively. In one embodiment, RF generators (not shown) generate control signals V A  and V B  under processor control (e.g., by a microprocessor or microcontroller executing a software or firmware program). This MZI is also referred to herein as the first MZI.  
         [0043]    Similarly, an output port  411 B of 3-dB coupler  411  is connected to an input port of phase control element  403  via PMF. 3-dB coupler  411  has another output port  411 C connected to an input port of phase control element  403  via PMF. The output port of phase control element  403  is connected to an input port  414 C of 3-dB coupler  414  via PMF. The output port of phase control element  404  is connected to another input port  414 B of 3-dB coupler  413  via PMF. Thus, the induced phase control elements  403  and  404  and 3-dB couplers  411  and  414  form a second MZI. Induced phase elements  403  and  404  are connected to receive control signals V C  and V D , respectively. In one embodiment, RF generators (not shown) generate control signals V C  and V D  under processor control.  
         [0044]    The output port  413 A of 3-dB coupler  413  is connected to one input port of PBS  407  via PMF  421 , whereas the output port of 3-dB coupler  414 A is connected to another input port of PBS  407  via PMF  422 . An output port of PBS  407  is connected SMF  425 .  
         [0045]    In operation, PBS  406  receives an input optical signal via SMF  416 , which PBS  406  splits into a TE component and a TM component. In one embodiment, PBS  406  directs the TE component to the first MZI (i.e., the MZI that includes phase control elements  401  and  402  in this example embodiment) via PMF  418 . In addition, PBS  406  directs the TM component to the second MZI (that includes phase control elements  403  and  404  in this example embodiment) via PMF  419 . In other embodiments, PBS  406  can be configured to direct the TE and TM components to the second and first MZIs, respectively.  
         [0046]    In this embodiment, the first MZI then adjusts the intensity or amplitude of the TE component as a function of the difference between control signals V A  and V B , as in a standard MZI. Similarly, the second MZI adjusts the intensity of the TM component as a function of the difference between control signals V C  and V D . The output signals of the first and second MZIs will have a relative phase difference that is a function of the difference between the above differences; i.e., (V A −V B ]−(V C −V D ). Thus, the two MZIs can adjust both the intensity of and the relative phase difference between the TE and TM components to achieve any desired SOP when combined by PBS  407  as described below.  
         [0047]    The output signal of the first MZI (i.e., the TE component in the above example embodiment) is directed to PBS  407  via PMF  421 . Similarly, the output signal of the second MZI (i.e., the TM component in the above example embodiment) is directed to PBS  407  via PMF  422 . PBS  407  then combines the TE and TM components and outputs the combined signal via output SMF  425 .  
         [0048]    [0048]FIG. 5 illustrates an optical polarization modulator  500 , according to a third embodiment of the present invention. This embodiment includes amplifiers  501  and  502 , phase control elements  504  and  505 . In one embodiment, amplifiers  501  and  502  are implemented with SOAs (semiconductor optical amplifiers). In addition, this embodiment includes PBSs  406  and  407 , input SMF  416 , PMFs  418 ,  419 ,  421  and  422 , and output SMF  425  as described above for optical polarization modulator  400  (FIG. 4).  
         [0049]    In this embodiment, phase control elements  504  and  505  are plasma optical effect-based silicon phase shifters as described above in conjunction with FIGS. 2 and 3. However, in other embodiments of optical polarization modulator  400 , any suitable type of phase control element can be used. For example, other embodiments can use LiNbO 3  devices or alternatively, devices based on the thermo-optic effect.  
         [0050]    Input SMF  416  is connected to an input port of PBS  406 . One output port of PBS  406  is connected to an input port of amplifier  501  via PMF  418 . Another output port of PBS  406  is connected to an input port of amplifier  502  via PMF  419 . The output ports of amplifiers  501  and  502  are respectively connected to input ports of phase control elements  504  and  505 . In this embodiment, phase control elements  504  and  505  are connected to receive control signals V E  and V F , respectively. In one embodiment, RF signal generators (not shown) generate control signals V E  and V F  under processor control. The output ports of phase control elements  504  and  505  are connected to PBS  407 . More specifically, in this example embodiment, the output port of phase control element  502  is connected via PMF  421  to an input port of PBS  407  that passes TE polarized light. Further, the output port of phase control element  505  is connected via PMF  422  to an input port of PBS  407  that passes TM polarized light. One of the output ports of PBS  407  is connected to SMF  425 .  
         [0051]    This example embodiment of optical polarization modulator  500  operates as follows. PBS  406  receives an input optical signal via SMF  416 , which PBS  406  splits into a TE component and a TM component. In this embodiment, PBS  406  directs the TE component to amplifier  501  via PMF  418 . In addition, PBS  406  directs the TM component to amplifier  502  via PMF  419 . In other embodiments, PBS  406  can be configured to direct the TE and TM components to amplifiers  502  and  501 , respectively.  
         [0052]    In this embodiment, amplifier  501  adjusts the intensity or amplitude of the TE component as a function of a control signal (not shown). Similarly, amplifier  502  adjusts the intensity of the TM component as a function of another control signal (not shown). The amplified TE and TM components outputted by amplifiers  501  and  502  are then received by phase control elements  504  and  505 , respectively. Phase control elements then adjust the phases of the amplified TE and TM components as a function of control signals V E  and V F . Control signals V E  and V F  are generated to achieve the desired relative phase difference. PBS  407  then combines the TE and TM components and outputs the combined signal via output SMF  425 .  
         [0053]    Also, in another embodiment, all or some of the elements that are part of device  500  could be fabricated on a single monolithic or hybrid chip. As an example PBSs  406  and  407 , phase modulators  504  and  505 , and the connecting waveguides  418 ,  419 ,  421 ,  422  can be fabricated using the same silicon substrate. Also, other substrates can be used; e.g., group III-V compound semiconductors.  
         [0054]    [0054]FIG. 6 illustrates an optical communication system  600 , according to one embodiment of the present invention. In this embodiment, optical system  600  includes an optical transmitter  601  connected to an optical receiver  602  via a SMF  603 . In addition, optical transmitter  601  includes an optical polarization modulator  610 , a controller  611  and an optical signal generator  612  (e.g., a laser). Controller  611  can include a processor (e.g., a microprocessor or microcontroller), along with memory (not shown) used to store instructions and data that are executed and operated by the processor (not shown). Optical transmitter  601  and optical receiver  602  can be part of optical transceivers or optical repeaters in some embodiments. Furthermore, optical transmitter  601  and optical receiver  602  can be part of wavelength-division-multiplexing (WDM) transmission system. In a WDM embodiment, for example, optical transmitter  601  and optical receiver  602  could include optical multiplexer(s) and/or demultiplexer(s) (not shown).  
         [0055]    In one embodiment, optical polarization modulator  610  is implemented in the same manner as optical polarization modulator  100  (FIG. 1). In other embodiments, optical polarization modulator  601  is implemented in the same manner as optical polarization modulator  400  (FIG. 4) or optical polarization modulator  500  (FIG. 5).  
         [0056]    In operation, optical transmitter  601  receives data to be modulated on an optical signal. In this embodiment, optical transmitter  601  uses optical polarization modulator  610  to transmit symbols that are defined by the SOP of an optical signal outputted by optical signal generator  612 . Each symbol can represent one or more bits, depending on the selected modulation format. As previously described, optical polarization modulator  601  can cause the output signal to have any desired SOP in response to received control signals. In this embodiment, controller  611  provides these control signals to optical polarization modulator  610 . The symbols are then transmitted to optical receiver  602  via SMF  603 .  
         [0057]    Embodiments of method and apparatus for implementing an optical polarization modulator are described herein. In the above description, numerous specific details are set forth to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.  
         [0058]    In addition, one skilled in the relevant art will recognize that the disclosed polarization modulator embodiments can be easily used as polarization compensators to compensate for polarization mode dispersion.  
         [0059]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable optical manner in one or more embodiments.  
         [0060]    In addition, embodiments of the present description may be implemented not only within a semiconductor chip but also within machine-readable media. For example, the designs described above may be stored upon and/or embedded within machine readable media associated with a design tool used for designing semiconductor devices. Machine-readable media also include media having layout information. Furthermore, machine-readable media for semiconductor chip design may be used in a simulation environment to perform the methods of the teachings described above.  
         [0061]    Thus, embodiments of this invention may be used as or to support software program executed upon some form of processing core (such as the CPU of a computer) or otherwise implemented or realized upon or within a machine-readable medium. A machine-readable medium includes any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium can include such as a read only memory (ROM); a random access memory (RAM); a magnetic disk storage media; an optical storage media; and a flash memory device, etc. In addition, a machine-readable medium can include propagated signals such as electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.).  
         [0062]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.