Abstract:
An apparatus having a topology that allows building complicated optical programmable arrays useful for manipulating the phase and/or amplitude of an optical signal. Sophisticated filtering and other optical signal processing functionality can be programmed into the array after a chip containing the array has been fabricated. This programming capability is analogous to that of electronic field programmable gate arrays (FPGA&#39;s). Apparatus described herein will provide a powerful tool for processing optical signals or very broadband electrical signals.

Description:
TECHNICAL FIELD 
   This disclosure relates to optical signal processing. More particularly, this disclosure relates to programmable arrays of optical signal processing elements that can be used to implement a variety of optical signal processing functions, such as filtering. 
   BACKGROUND 
   Field programmable gate arrays (FPGA&#39;s) have been widely used in electronic world. FPGA&#39;s are arrays of gates and logic elements that can be programmed to perform desired functions after the FPGA has been manufactured. Typically, an FPGA consists of an array of logic elements, for example, gates, lookup table RAM&#39;s, and flip-flops, interconnected together by programmable interconnect wiring. After the circuit chip is made, it can be programmed by users to perform different electronic functions by changing the interconnections and the functions of the individual blocks in the array. This type of circuit has been proven very powerful in new system prototyping and in situations where circuit system functions need be defined in the field. The needs of electronic circuit design met by FPGA&#39;s are also present in optical circuit design and it would be desirable to have an FPGA-like structure available to designers of optical systems. 
   SUMMARY 
   There are single individual optical filters built on monolithic platforms which are used for optical signal processing, such as band pass or notch filtering. Described herein is an apparatus having a topology that allows building complicated optical programmable arrays useful for manipulating the phase and/or amplitude of optical signals. Sophisticated filtering and other optical signal processing functionality can be programmed into the array after a chip containing the array has been fabricated similar to the way electronic FPGA&#39;s are programmed. Apparatus in accordance with the invention will provide a powerful tool for processing optical signals or very broadband electrical signals. 
   The basic construction of a generalized FPGA-like programmable optical array is described below. In one illustrative embodiment, the programmable array comprises a plurality of optical waveguides forming a rectangular grid of cells. Optical rings are located in the cells of the grid. A plurality of programmable coupling elements controllably couple selected ones of the waveguides together. Another plurality of programmable coupling elements controllably couple selected ones of the optical rings to selected ones of the waveguides. By selectively controlling the amount of coupling provided by the coupling elements, a variety of different optical signal processing functionalities can be implemented by the programmable array. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  shows an illustrative programmable optical array in accordance with this invention. 
       FIG. 2(   a ) shows an illustrative 4 th  order pole-zero filter that can be implemented in the array of  FIG. 1 . 
       FIG. 2(   b ) shows how the filter of  FIG. 2(   a ) is implemented in the array of  FIG. 1 . 
       FIG. 3(   a ) shows an illustrative notch filter that can be implemented in the array of  FIG. 1 . 
       FIG. 3(   b ) shows how the filter of  FIG. 3(   a ) is implemented in the array of  FIG. 1 . 
       FIG. 4(   a ) shows an illustrative filter cascade implemented in an array like the one shown in  FIG. 1  by using an optical reroute block. 
       FIG. 4(   b ) shows the frequency response of the filter cascade shown in  FIG. 4(   a ). 
       FIG. 5  shows an illustrative programmable optical array in accordance with the invention having a rerouting block and a gain block. 
       FIG. 6  shows a more detailed depiction of a four cell array in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   Programmable filter shapes and characteristics are critical in processing broadband optical signals. A programmable optical filter matrix architecture can be achieved in an optical programmable array in accordance with this invention. In an array in accordance with this invention, identical tunable unit cells are arranged in a geometric matrix that can be programmed to implement many different optical filtering characteristics and other signal processing functionality such as gain blocks and routing blocks. Multiple elementary cells can be cascaded or connected in parallel to form a more complex filter system of prescribed performance. An array in accordance with this invention can contain a variety of programmable ring-waveguide and waveguide-waveguide couplings and a variety of programmable waveguide routes through the array so that the array can function as an FPGA-like array for programmable optical signal processing. 
   Recent breakthroughs in semiconductor lithography have allowed people to build optical components by using a standard CMOS processing. See, for example, M. S. Rasras, D. M. Gill, S. S. Patel, A. E. White, K. Y. Tu and Y. K. Chen and etc., “Tunable Narrowband Optical Filter in CMOS,” OFC2006, paper OFC-PD13, 2006. One of the successful examples is a fourth order filter with programmable center frequency and bandwidth. Interestingly, the same filter arrangement can be reprogrammed to function as a notch filter. Apparatus in accordance with this invention allows the programming of the connection, the amplitude coupling, the phase adjustment, and the gain of amplifier elements, to deliver various filter types such as Butterworth, Elliptic, and Chebychev filters, or various filter functions such as low pass, high pass, and notch filters. Moreover, the filters can be cascaded or connected in parallel to tailor the filtering spectrum to meet individualized needs. Elements on the circuit can include digital or analog optical modulators, waveform generators, or optoelectronic mixed signal components as its subcomponents. Electrical FPGA&#39;s coexisting with optical FPGA-like circuits on the same chip may be used to perform more complicated signal processing. 
   A) BASIC STRUCTURE OF AN ILLUSTRATIVE OPTICAL FIELD PROGRAMMABLE ARRAY 
   An optical filter can be constructed by cascading various numbers of coupled optical ring structures, each coupled ring structure forming a pole-zero pair in the frequency domain. By properly positioning pole-zero pairs and adjusting the power coupling into the ring, a box-like filter can be formed with impressive filter shape. See, for example, the Rasras et al. article cited above. Similarly, one can implement other types of filters, such as notch filters, by using a small set of programmable optical components. 
     FIG. 1  shows a generalized array  10  of optical components that can be programmed to implement a variety of filters and other optical signal processing functions. For example, one or more of pole-zero filters, all-zero filters, all-pole filters, notch filters, band-pass filters, low pass filters, and high pass filters may be implemented in the array of  FIG. 1 . In some embodiments of the invention, notch, band-pass, low pass, and high pass filters may be formed by properly tuning pole-zero, all-zero, or all pole filters. The structure of  FIG. 1  also permits the implementation of other optical signal processing elements, such as routing elements and amplification elements. 
   The array  10  of  FIG. 1  comprises a plurality of generally parallel waveguides, numbered  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 , and  28  at each end of a respective waveguide. Each waveguide extends in a stair-steep fashion at a 45 degree angle from lower left to upper right in  FIG. 1  thereby forming a rectangular grid bounding a plurality of generally rectangular cells  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52 . A plurality of optical rings  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 , and  76  are located in the cells  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 ,  46 ,  48 ,  50 , and  52  formed by the waveguides  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 , and  28 . 
   The waveguides  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 , and  28  are aligned such that adjacent waveguides get close to one another at the corners of the stair steps as illustratively shown at reference numeral  29  at the junction between waveguides  12  and  14  in the lower right hand corner of  FIG. 1 . The amount of coupling between adjacent waveguides can be controlled by way of a programmable coupling element located at one or more of the junctions between waveguides. The example of the invention shown in  FIG. 1  comprises programmable coupling elements  78  and  80  that selectively couple waveguides  14  and  16 . Programmable coupling elements  82 ,  84 , and  86  selectively couple waveguides  16  and  18  together. Programmable coupling elements  88 ,  90 ,  92 , and  94  selectively couple waveguides  18  and  20  together. Programmable coupling elements  96 ,  98 ,  100 , and  102  selectively couple waveguides  20  and  22  together. Programmable coupling elements  104 ,  106 , and  108  selectively couple waveguides  22  and  24  together. Programmable coupling elements  110  and  112  selectively couple waveguides  24  and  26  together. Finally, programmable coupling element  114  selectively couples waveguides  26  and  28  together. 
   Each of the optical rings  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 , and  76  is coupled to each of the four waveguide segments that form the boundaries of the respective cell  30 ,  32 ,  34 ,  36 ,  38 ,  40 ,  42 ,  44 ,  46 ,  48 ,  50 , or  52  in which the ring is located. This programmable coupling is achieved by way of programmable coupling elements like those programmable coupling elements just described that selectively couple the waveguides together. The programmable coupling elements that couple the optical rings to the waveguide segments that define the cells are given the same reference numerals as their respective optical rings followed by the letters a, b, c, or d. For example, programmable coupling elements  54   a ,  54   b ,  54   c , and  54   d  selectively couple ring  54  in cell  30  to segments of waveguides  18  and  20  in  FIG. 1 . 
   One or more of the rings  54 ,  56 ,  58 ,  60 ,  62 ,  64 ,  66 ,  68 ,  70 ,  72 ,  74 , and  76  in the Example of the invention shown in  FIG. 1  contains an optical phase shifter that controls the phase of optical signals flowing in each respective ring. An illustrative phase shifter in one of the rings  58  is numbered  58   e  in  FIG. 1 . One or more of the waveguide segments may also contain a phase shifter that controls the phase of optical signals flowing through one or more of the waveguides  12 ,  14 ,  16 ,  18 ,  20 ,  22 ,  24 ,  26 , and  28 . An illustrative one of those phase shifters in one of the waveguides  14  is numbered  14   a  in  FIG. 1 . 
   The waveguides, rings, and coupling elements in the example of the invention shown in  FIG. 1  may be built using standard silicon based complementary metal oxide semiconductor (CMOS) technology. The programmable couplers shown in  FIG. 1  may be any coupler that can be controlled to provide a predetermined amount of coupling between optical structures such as waveguides and rings. Examples of such coupling elements include, for example, Mach-Zehnder (MZ) interferometers. MZs are composed of two 2×2 directional couplers separated by waveguides at both ends. By tuning the differential phase across the MZ arms, the coupling ratios can be tuned to any desired level. Full coupling and zero coupling (i.e. switching) also are possible using this structure. 
   B) LOW PASS FILTERS EMBEDDED IN THE FILTER ARRAY 
     FIGS. 2(   a ) and  2 ( b ) show an illustrative filter that may be embedded in the optical filter array shown in  FIG. 1 .  FIG. 2   a  illustrates a basic 4 th  order pole-zero filter having a filter spectrum as shown on the filter output ports. The 4 th  order pole-zero filter design of  FIG. 2(   a ) may be created, for example, by a silicon CMOS process.  FIG. 2(   b ) illustrates the same basic filter embedded in an optical array structure similar to that shown in  FIG. 1 . 
   Input light  116  is split by a coupler  118 . One component of the input light  116  is coupled to optical rings  120  and  122  by means of couplers  130  and  132  in the waveguide forming the upper arm of the filter. The other component of the input light  116  split by coupler  118  is coupled to optical rings  124  and  126  by means of couplers  134  and  136  in the waveguide forming the lower arm of the filter. The light components flowing in the upper and lower arms of the filter are recombined in a coupler  128  to form a 4 th  order elliptical filter function. The coupling ratio between waveguides at the input and output are adjustable and the coupling between ring and the waveguides are also adjustable. There are also phase shifting elements  119 ,  121 ,  123 ,  125 ,  127 , and  129  on the waveguides and the rings as shown in  FIG. 2(   a ). Such a structure can be implemented as a sub set of the filter array elements shown in  FIG. 1 . 
   The structure of  FIG. 2(   a ) can be achieved in the array of  FIG. 1  by turning on coupling elements  96  and  100 , corresponding to couplers  118  and  128  in  FIG. 2(   a ), to couple waveguides  20  and  22 . Coupling element  54   a  is turned on to couple optical ring  54  to one of the horizontal segments of waveguide  20 . Coupling element  62   a  is turned on to couple optical ring  62  to another one of the horizontal segments of waveguide  20 . Coupling element  60   d  is turned on to couple optical ring  60  to one of the vertical segments of waveguide  22 . Coupling element  68   d  is turned on to couple optical ring  68  to one of the vertical segments of waveguide  22 . Phase shifting elements like phase shifting elements  119 ,  121 ,  123 ,  125 ,  127 , and  129  shown in  FIG. 2(   a ) may be fabricated into any appropriate place in the array, including throughout the entire array, to control the phase of light flowing through the device. If a filter like the one shown in  FIG. 2(   a ) is to be implemented, then a phase shifting device may be fabricated (a) in waveguide  20  between coupling element  96  and coupling element  54   a ; (b) in each of the optical rings  54 ,  60 ,  62 , and  68 ; and (c) in waveguide  22  between coupling element  96  and  60   d.    
   A phase adjuster can be placed at any place on any of the waveguides and rings in the array of  FIG. 1  to provide the ability to tune the performance of any filter or other component embedded in the array. All coupling and phase adjustments can be achieved, for example, with a heating element closely built next to a waveguide or optical ring. 
   Although there are many coupling elements in the array of  FIG. 1 , construction of a 4 th  order filter only requires six of those coupling elements. Allowing the rings to couple to a waveguide on any of its four sides permits versatile filter construction, convenient input/output placements, and a higher density of devices built on the same area. 
   C) NOTCH FILTERS EMBEDDED IN THE OPTICAL PROGRAMMABLE ARRAY 
   Another useful filter example that could be implemented in the array of  FIG. 1  is a notch filter which is commonly used to remove an unwanted frequency from an optical signal.  FIG. 3(   a ) is a diagram showing an illustrative notch filter. The filter of  FIG. 3(   a ) has only one ring  138  with a Mach-Zehnder (MZ) structure. The output of the filter is represented by the curve  140  in  FIG. 3(   b ). A detailed discussion of such notch filters is found, for example, in Madsen, C. K.; Cappuzzo, M.; Chen, E.; Gomez, L.; Griffin, A.; Laskowski, E. J.; Stulz, L.; Wong-Foy, “A tunable ultra-narrowband filter for subcarrier processing and optical monitoring,” Optical Fiber Communication Conference, 2004. OFC 2004, TUL5, 2004. 
     FIG. 3(   b ) shows a notch filter like the one shown in  FIG. 3(   a ) implemented in the filter array of  FIG. 1  by turning on one ring  64  and three couplers  64   c ,  82 , and  86 . The phase shifters labeled in  FIG. 3(   b ) may be used to tune the filter. 
   D) COMPACT CONSTRUCTION OF A CASCADED FILTER IN AN OPTICAL PROGRAMMABLE ARRAY 
   In the above sections B and C, a low pass filter and a notch filter constructed in small compact areas of the filter array have been demonstrated.  FIG. 4(   a ) shows cascaded band-pass and notch filters in an optical programmable array with an optical reroute element.  FIG. 4(   b ) shows the output spectrum of cascaded band-pass and notch filters. 
   By using optical reroute cell, like the optical reroute cells  142  and  154  shown in  FIG. 4(   a ), the system would allow the optical path to be rerouted in opposite directions such that two filters can be programmed using nearby waveguides and rings. The optical reroute cell  142  is constructed of two optical couplers  144  and  146 . Coupler  144  controllably couples waveguides  148  and  150  together. Coupler  146  controllably couples waveguides  150  and  152  together. The couplers  144  and  146  and the waveguides  148 ,  150 , and  152  allow the optical path through the array to be redirected by switching either one of the couplers depending on the transmission direction. One possible application of reroute cells  142  and  154  in  FIG. 4(   a ) is to cascade a band pass filter like the one shown in  FIG. 2(   b ) with a notch filter like the one shown in  FIG. 3(   b ), such that the band-pass spectrum of the  FIG. 2(   b ) filter can be provided with a sharp notch produced by the filter of  FIG. 3(   b ). The band pass filter center frequency and bandwidth are tunable and so is the notch filter center frequency. With this tunability and field programmable capability, optical arrays in accordance with this invention can become very powerful in that more complicated systems can be hierarchically constructed. 
   E) OPTICAL PROGRAMMABLE ARRAY WITH GAIN CELLS 
   As the complexity of the system described above increases, optical losses begin to add up and the system eventually will render itself a useless device at a certain level of complexity. Therefore placing gain elements in the programmable array would be an important improvement. In one embodiment of the invention, an optical programmable array will have a dedicated area where gain elements  156  are embedded as shown in  FIG. 5 . Because of the flexibility of rerouting the optical paths, some of the middle stage connections will be routed to a gain cell to compensate for losses in the elements, as also shown in  FIG. 5 . In the example of the invention shown in  FIG. 5 , the optical programmable array has filter block  158 , an optical gain block  160 , and an optical rerouting block  162 . The operations of those blocks are programmable. One of the typical gain cells available would be the semiconductor optical amplifier (SOA) which can be turned on to provide gain and turned off to shut off the transmission. A reroute element can provide the path going through the SOA&#39;s. 
   G) IMPLEMENTATION OF AN INTEGRATED RING RESONATOR CELL FOR FILTER ARRAY 
     FIG. 6  shows another embodiment of a programmable optical array in accordance with the invention comprising four representative cells of a potentially larger array.  FIG. 6  depicts four rectangular rings  162 ,  164 ,  166 , and  168 . The rings  162 ,  164 ,  166 , and  168  contain Mach-Zehnder (MZ) couplers on four sides. MZ couplers are also located in each of waveguides  159 ,  161 ,  163 ,  165 ,  167 , and  169  that surround each ring and they are adjacent to a corresponding MZ coupler contained one of the rings. Two representative pairs of MZ couplers in the rings and waveguides are given reference numbers  170  and  172  in  FIG. 6 . MZ coupler pairs are also formed in the waveguides to selectively couple pairs of waveguides together. One of those MZ coupler pairs is labeled with reference numeral  174  in  FIG. 6 . Each MZ coupler pair has a heating element to adjust the coupling ratio. Two representative ones of those heaters are labeled with reference numerals  176  and  178  in  FIG. 6 . 
   H) CONCLUSION 
   In accordance with this invention, an optical array can be programmed to implement various optical filter characteristics with center frequency and bandwidth tuned as desired. The platform is such that basic elements are on a single semiconductor chip and they can be programmed dynamically like an electronic FPGA circuit. Optical rerouting and gain blocks were included to redirect and amplify the optical signal in the array which allows optimization of the system performance. It is also envisioned that other elements such as digital or analog optical modulators, optical detectors, and electronic drivers and receivers can also be part of the element library eligible for inclusion into embodiments of programmable arrays in accordance with this invention. 
   The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited by the claims set forth below.