Abstract:
One embodiment of the present invention sets forth a system for compensating for the detrimental effects of all-order polarization mode dispersion. The system includes a broadband polarization correction module cascaded with a broadband phase correction module. Each of the modules includes an AWG chip as a wavelength dispersing element, as opposed to a bulk optic grating. Thus, aligning the optical components used to separate light beams of different wavelengths within the system is simpler, and the size of the overall system is reduced. Further, the AWG chip may be more easily aligned with the other optical components within the system, with the alignment being more robust, both mechanically and thermally, relative to prior art systems that include bulk optics. Since AWG chips may be fabricated using well-known fabrication techniques, overall manufacturability is also improved and costs are reduced.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to high-speed optical communications and, more specifically, to polarization mode dispersion compensation using an arrayed waveguide grating. 
         [0003]    2. Description of the Related Art 
         [0004]    As optical communication systems become faster and transmit signals to farther distances, certain physics-related phenomena begin limiting performance. Polarization mode dispersion (PMD) is the main limiting factor in long-haul optical communication systems beyond 10 Gb/s. PMD refers to a phenomenon where two different polarizations of light in a waveguide, which should travel at the same speed, actually travel at different speeds due to asymmetries and imperfections in the core of the fiber. Unless properly compensated for, the phenomenon may cause random spreading of optical pulses, which may render the bit stream being transmitted inaccurate and may ultimately limit the rate at which data can be transmitted through the fiber. 
         [0005]    The majority of research to date has focused on first-order and second-order PMD effects. However, as pulse durations continue to decrease and the bandwidth per channel continues to increase, first-order and second-order solutions to PMD issues have become inadequate. Consequently, an all-order broadband PMD compensation method has evolved that utilizes a two-stage compensation scheme. The scheme implements a broadband polarization correction setup in series with a broadband phase correction setup to independently equalize polarization spectra and phase spectra, respectively. 
         [0006]    One problem with broadband all-order PMD compensation systems is that they typically are implemented in free-space optics configurations, specifically using a bulk grating as a wavelength dispersing element. Using free-space optics has several drawbacks. First, aligning free-space optics components, especially gratings, is tedious, time-consuming and oftentimes error-prone. Second, the resulting alignments of free-space optical components are typically unstable due to, among other things, the mechanical properties of the materials used to make gratings. Finally, manufacturing gratings in large volumes using standard industry equipment is difficult, making free-space configurations expensive. 
         [0007]    As the foregoing illustrates, what is needed in the art is an all-order PMD compensation system that addresses one or more of the problems set forth above. 
       SUMMARY OF THE INVENTION 
       [0008]    One embodiment of the present invention sets forth a polarization mode dispersion compensation system. The system comprises a polarization correction module configured to produce a polarization-corrected signal that includes a first circulator configured to receive an optical input signal comprised of multiple wavelengths and route the optical input signal to one or more optical components, a first arrayed waveguide grating chip configured to receive the optical input signal from the first circulator and separate the optical input signal into a plurality of light beams, each light beam having a different wavelength, and a first lens configured to collimate light beams received from the first arrayed waveguide chip. 
         [0009]    One advantage of the disclosed system is that the polarization correction module includes an AWG chip as a wavelength dispersing element, as opposed to a bulk optic grating. Thus, aligning the optical components used to separate light beams of different wavelengths within the system is simpler, and the size of the overall system is reduced. Further, the AWG chip may be more easily aligned with the other optical components within the system, with the alignment being more robust, both mechanically and thermally, relative to prior art systems that include bulk optics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0011]      FIG. 1  is a conceptual diagram of an all-order PMD compensation system, according to prior art; 
           [0012]      FIG. 2  is a conceptual diagram of an all-order PMD compensation system, according to one embodiment of the invention; and 
           [0013]      FIG. 3  is a conceptual diagram of the AWG chip of  FIG. 2 , according to one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  is a conceptual diagram of an all-order PMD compensation system  100 , according to prior art. As shown, the PMD compensation system  100  includes, without limitation, a polarization correction module  110  connected to a phase correction module  130  to create a two-stage PMD compensation scheme. In the first stage, the polarization correction module  110  receives an input signal  150  consisting of multiple wavelengths having different states of polarization (SOP) and different phases and produces a polarization-corrected signal  160 . In the second stage, the phase correction module  130  receives the polarization-corrected signal  160  and produces a polarization- and phase-corrected output signal  170 . 
         [0015]    The polarization correction module  110  includes, without limitation, a circulator  112 , a grating  114 , a lens  116 , a liquid crystal modulator (LCM) array  118 , a 90% reflective mirror  120 , a polarimeter  122  and an SOP measurement system  124 . The input signal  150  is first received by the circulator  112 , which is configured to receive multiple input signals and route them in different directions. This functionality is used to route the uncompensated input signal  150  towards the grating  114  and the polarization-corrected signal  160  towards the phase-correction module  130 . Upon receiving the input signal  150 , the grating  114  acts as a wavelength-dispersing element, spatially separating the input signal  150  into beams of different individual wavelengths as a result of diffraction. The beams of different individual wavelengths are collimated by the lens  116  and received by the LCM array  118 , which corrects the wavelength-dependent polarization state of each beam to a fixed wavelength-independent polarization state by rotating the polarization states of the beams in a wavelength-by-wavelength fashion. Next, the 90% reflective mirror  120  allows 10% of the polarization-corrected light to reach the polarimeter  122 , which measures and records the polarization rotations of the different beams of light passed through the LCM array  118 . The other 90% of the polarization-corrected light is reflected back to the lens  116 , which focuses the beams of different individual wavelengths onto the grating  114 , which then multiplexes the individual beams into one signal consisting of multiple wavelengths. This signal is received by the circulator  112  and is transmitted as the polarization-corrected signal  160 , through the SOP measurement system  124 , to the phase correction module  130 . 
         [0016]    The phase correction module  130  includes, without limitation, a circulator  132 , a grating  134 , a lens  136 , a half-wave plate  138 , a phase-only LCM array  140  and a 100% reflective mirror  142 . The polarization-corrected signal  160 , which again consists of multiple wavelengths with different phases, is received by the circulator  132  and passed to the grating  134 , which, again acts as a wavelength-dispersing element by spatially separating the polarization-corrected signal  160  into beams of different individual wavelengths. The beams of different individual wavelengths are collimated by the lens  136  and are incident on the half-wave plate  138 , which rotates the fixed horizontal polarization of each beam transmitted from the grating  134  to a fixed vertical polarization, as required by the phase-only LCM array  140 . The phase-only LCM array  140  corrects the wavelength-dependent phase of each beam received from the half-wave plate  138  to a fixed phase in a wavelength-by-wavelength fashion. Next, the 100% reflective mirror  142  reflects all of the phase-corrected light back to the lens  136 , which focuses the beams of different individual wavelengths back onto the grating  134 , which multiplexes the individual beams into one signal consisting of multiple wavelengths. Finally, the one signal of multiple wavelengths is received by the circulator  134 , which produces the polarization- and phase-corrected output signal  170 . 
         [0017]      FIG. 2  is a conceptual diagram of an all-order PMD compensation system  200 , according to one embodiment of the invention. As shown, the PMD compensation system  200  includes, without limitation, a polarization correction module  210  connected to a phase correction module  230  to create a two-stage PMD compensation scheme. In the first stage, the polarization correction module  210  receives an input signal  250  consisting of multiple wavelengths having different SOP and different phases and produces a polarization-corrected signal  260 . In the second stage, the phase correction module  230  receives the polarization-corrected signal  260  and produces a polarization- and phase-corrected output signal  270 . 
         [0018]    The polarization correction module  210  includes, without limitation, a circulator  212 , an arrayed waveguide grating (AWG) chip  214 , a lens  216 , an LCM array  218 , a 90% reflective mirror  220 , a polarimeter  222  and an SOP measurement system  224 . The input signal  250  is first received by the circulator  212 , which is configured to receive multiple input signals and route them in different directions, similar to the circulator  112  of  FIG. 1 . This functionality is used to route the uncompensated input signal  250  towards the AWG chip  214  and the polarization-corrected signal  260  towards the phase-correction module  230 . Upon receiving the input signal  250 , the AWG chip  214  acts as a wavelength-dispersing element, spatially separating the input signal  250  into beams of different individual wavelengths as a result of each of the wavelengths undergoing a different phase shift while traveling through the AWG chip  214 . The beams of different individual wavelengths are collimated by the lens  216  and received by the LCM array  218 , which corrects the wavelength-dependent polarization state of each beam to a fixed wavelength-independent polarization state (for example, to a fixed horizontal polarization state) by rotating the polarization states of the different beams in a wavelength-by-wavelength fashion. Next, the 90% reflective mirror  220  allows 10% of the polarization-corrected light to reach the polarimeter  222 , which measures and records the polarization rotations of the different beams of light passed through the LCM array  218 . The other 90% of the polarization-corrected light is reflected back to the lens  216 , which focuses the beams of individual wavelengths onto the AWG chip  214 , which then multiplexes the individual beams into one signal consisting of multiple wavelengths. This signal is received by the circulator  212  and is transmitted as the polarization-corrected signal  260 , through the SOP measurement system  224  (where the polarization of the signal  260  can be measured), to the phase correction module  230 . 
         [0019]    The phase correction module  230  includes, without limitation, a circulator  232 , an AWG chip  234 , a lens  236 , a half-wave plate  238 , a phase-only LCM array  240  and a 100% reflective mirror  242 . The polarization-corrected signal  260 , which again consists of multiple wavelengths with different phases, is received by the circulator  232  and passed to the AWG chip  234 , which, again acts as a wavelength-dispersing element by spatially separating the polarization-corrected signal  260  into beams of different wavelengths. The beams of different individual wavelengths are collimated by the lens  236  and are incident on the half-wave plate  238 , which rotates the fixed polarization of each beam transmitted from the AWG chip  234  by ninety degrees (for example, a fixed horizontal polarization is rotated to a fixed vertical polarization), as required by the phase-only LCM array  240 . The phase-only LCM array  240  corrects the wavelength-dependent phase of each beam received from the half-wave plate  238  to a fixed phase in a wavelength-by-wavelength fashion. Next, the 100% reflective mirror  242  reflects all of the phase-corrected light back to the lens  236 , which focuses the beams of individual wavelengths back onto the AWG chip  234 , which multiplexes the individual beams into one signal consisting of multiple wavelengths. Finally, the one signal of multiple wavelengths is received by the circulator  234 , which produces the polarization- and phase-corrected output signal  270 . 
         [0020]    There are several differences between using a grating and an AWG chip in a PMD compensation system. The first difference relates to the physical phenomena used to separate the beams of different individual wavelengths. Since the grating relies on a phenomenon of diffraction, the overall system requires the presence of extended free space and additional components to collimate the light beams dispersed while traveling through that space. On the other hand, the functionality of the AWG chip is not based on diffraction. Rather, the beams of different individual wavelengths undergo different phase shifts as they travel through an array of waveguides of different lengths within the AWG chip, thereby separating the beams according to the individual wavelengths. Since the latter is not a free-space phenomenon, the AWG chip does not require the presence of extended free space in the overall system or any additional collimating components. Importantly, as described in greater detail below in  FIG. 3 , aligning the optical components in the overall system is far easier compared to the complicated alignment issues that exist in a system using bulk optics because the arrayed waveguides are already aligned within the AWG chip. The only alignment necessary is between the input and output of the AWG chip, which substantially simplifies the assembly of the overall system. 
         [0021]    The second difference between using an AWG chip and a grating is that the AWG chip may be implemented on silicon, which improves the mechanical and thermal stability of the final alignment within the overall system relative to a system using bulk optics because the mechanical and thermal properties of silicon are superior to the mechanical and thermal properties of the materials used to make gratings. In addition, AWG chips may be reproduced in volume using a mask, which enables the alignment of the optical components (e.g., the arrayed waveguides) within each AWG chip to be repeated consistently from chip to chip. The AWG chips are not only easier to manufacture than gratings, but the repeatability of the fabrication process reduces variations in the optical alignments between the different components in a PMD compensation system from system to system. Finally, an AWG chip requires much less space than a grating in the overall system since the AWG chip is small and thinner than the grating and, again, the AWG chip does not need any extended free space in the system for diffraction. 
         [0022]      FIG. 3  is a conceptual diagram of the AWG chip  214  or  234  of  FIG. 2 , according to one embodiment of the invention. As shown, the AWG chip  214  or  234  includes multiple arrayed waveguides  318 . On one side of the AWG chip  214  or  234 , the waveguides  318  are aligned with an input axis  350  of an external system, such as the circulators  212 ,  232  of  FIG. 2 , and, on the other side, the waveguides  318  are aligned with an output axis  370  of an external system, such as the lenses  216 ,  236  of  FIG. 2 . Importantly, and as previously described herein, each of the waveguides  318  has a different length. Thus, as an input signal of multiple wavelengths enters the AWG chip  214  or  234  along the input axis  350  and travels through the waveguides  318 , beams of different individual wavelengths undergo different phase shifts. As a result of this phenomenon, a beam of only one wavelength is output from each of the waveguides  318 . The number of arrayed waveguides  318  on the AWG chip  214  or  234  may be modified to ensure that a beam of each wavelength included in the input signal is able to travel through an individual channel. An accurate and precise alignment between an input signal of multiple wavelengths and multiple output signals, each consisting of a spatially separated beam of a different individual wavelength can be easily achieved by aligning the input axis  360  and the output axis  380  of the AWG chip  214  or  234  with the input axis  350  and the output axis  370 , respectively, of the external systems. 
         [0023]    In an alternative embodiment, the lens  216  or  236  may be included within the AWG chip  214  or  234 , respectively, as also shown on  FIG. 3 , to collimate the beams of individual wavelengths exiting the waveguides  318 . 
         [0024]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.