Abstract:
A system and method for quantizing a photonic signal involves passing the photonic signal through a photonic crystal. The photonic crystal has localized defects for splitting the photonic signal into a plurality of quantized photonic components and for directing the quantized photonic components to a set of optical detectors. A digital conversion of the photonic signal can occur by performing a threshold comparison of the quantized components, either in the electrical domain through comparators or in the optical domain through optical limiters.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates generally to systems and methods for quantizing photonic signals and, more specifically, to systems and methods for analog-to-digital conversion of photonic signals.  
       BACKGROUND  
       [0002]     An analog-to-digital converter is a ubiquitous component found in many different types of systems, such as but certainly not limited to computer, data, control, sensor, communication, and telecommunication systems. The analog-to-digital converters receive analog signals and provide a digital signal to another component, such as a processor. As the systems within which the analog-to-digital converters are used develop and become more sophisticated, the performance of the analog-to-digital converter is becoming more important.  
         [0003]     For instance, as the development of a software defined radio continues, the need for high-speed analog to digital conversion to directly digitize RF to microwave signals rather than to down-converting the signal to IF becomes necessary to improve performance, to simplify design, to reduce noise, to reduce interference, and to reduce cost. The current technology of analog to digital conversion is primarily done at the electronic signal. The high speed sampling of the electronic signal has been limited to the stability of the clock jitter, thermal noise of the electronics, electromagnetic interference of other electronic devices and circuits, cross-talk, and coupling noises of interconnect lines.  
         [0004]     The use of an optical signal has benefited many high speed communication applications due to the inherent inertness of the optical signals to the EMI noise and minimal cross-talk and coupling to close by devices. Optical signals can also travel relatively long distances without compromising severe signal distortion and attenuation at high modulation speed.  
         [0005]     To obtain even better performance, some work has been done in performing the analog-to-digital conversion in the optical domain. a photonic analog-to-digital converter can take advantage of the high speed analog signal of the optical domain and convert it to a high speed digital signal in the electronic domain for further signal processing. The photonic analog-to-digital converter can therefore achieve high speed analog to digital conversion beyond today&#39;s technology. The photonic analog-to-digital converter can provide a system with the low noise, low distortion, and high-speed characteristic of photonics while leveraging the more established high-speed digital electronics for low cost signal processing. The digital signal is also less sensitive to noise and can be processed using today&#39;s semiconductor technology at relatively high speed.  
         [0006]      FIG. 1  illustrates an example of a photonic analog-to-digital converter  10  using a conventional optical splitter  12 , saturable absorbers  14 , optical delay lines  16 , photo-diode detectors  17  and electronic comparators  19 . With this analog-to-digital converter, incoming photonic signals are split into a plurality of photonic components and directed toward the saturable absorbers  14 . The saturable absorbers  14  quantize the photonic signal by preventing any signal from reaching the photo-diode detectors  17  unless the photonic components exceed a certain threshold level. The comparators  19  then digitize the output by comparing the output of the photo-diode detectors  17  to a reference level for the digital signal. A digital encoder  18  combines the individual bits from the comparators to form a digital signal.  
         [0007]     A problem with this approach is that the use of conventional optical waveguides  16  and/or fiber splitter  12  makes it difficult to miniaturize the analog-to-digital converter  10 . This is due to the minimum-bending radius allowed in order to minimize the scattering losses at the bends of the waveguides  16  and splitter  12 . Secondly, the use of saturable absorbers  14  to quantize the optical signal creates a highly inefficient conversion. Most of the original optical signal will be wasted through absorption. As number of optical split channels required for the number of digital bit resolution is channels=2 n  bits, the device  10  quickly becomes impractical to implement.  
       SUMMARY  
       [0008]     The invention addresses the problems above by providing systems and methods for quantizing photonic signals. A system according to a preferred embodiment has a photonic crystal with a periodic structure that forms a plurality of optical splitters and waveguides. The photonic signal passes through the photonic crystal and is separated into a plurality of quantized photonic components through the optical splitters. The quantized photonic components are then routed to a plurality of optical detectors for generating a set of electrical signals. The periodic structure of the photonic crystal also forms waveguides for routing the photonic signal through the various optical splitters and also for routing the quantized photonic components to the optical detectors.  
         [0009]     In one embodiment, the system forms a photonic analog-to-digital converter. The splitters divide the photonic signal into successively smaller quantized photonic components each of which represents a different bit within a digital output. According to one aspect, the quantized photonic components are directed to optical limiters which pass the quantized photonic components to the optical detectors only if the quantized photonic components exceed a certain threshold intensity. According to another aspect, the quantized photonic components are routed to the optical detectors and a set of comparators forms individual bits of a digital signal by comparing outputs of the optical detectors to a set of intensity thresholds. An encoder forms the digital signals by combining the individual bits derived from each detector.  
         [0010]     Because the analog-to-digital conversion is performed optically, the analog-to-digital converter can operate at much higher speeds and is much less susceptible to EMI, cross-talk, and noise. Also, since the routing and splitting of the photonic signals is performed within the crystal and not with conventional fiber waveguides and/or splitters, the analog-to-digital converter can be manufactured much smaller and can more easily accommodate higher of bits. The analog-to-digital converters according to the invention therefore offer a more beneficial and practical solution to analog-to-digital conversion and with improved overall performance.  
         [0011]     Other advantages and features of the invention will be apparent from the description below, and from the accompanying papers forming this application. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention and, together with the description, disclose the principles of the invention. In the drawings:  
         [0013]      FIG. 1  is a diagram of a conventional photonic analog-to-digital converter using saturable absorbers;  
         [0014]      FIG. 2  is an example of a waveguide using a crystal according to an embodiment of the invention;  
         [0015]      FIG. 3  is an example of a splitter using a crystal according to an embodiment of the invention;  
         [0016]     FIGS.  4 (A), (B), and (C) are examples of different periodic pattern structures for photonic crystals;  
         [0017]      FIG. 5  is a diagram of a photonic analog-to-digital converter according to an embodiment of the invention;  
         [0018]      FIG. 6  is a diagram of an optical limiter structure using alternating dielectric and optical Kerr coefficients; and  
         [0019]      FIG. 7  is a diagram of a photonic analog-to-digital converter according to another embodiment of the invention using an optical limiter. 
     
    
     DETAILED DESCRIPTION  
       [0020]     In recent years, a photonic crystal has been studied for creating optical waveguides. Unlike conventional index confined waveguides, photonic crystals guide the optical waves by prohibiting certain energy states within certain frequency ranges to exist within the periodic structure. As a result, the electromagnetic waves can be guided through tight bends without any significant scattering losses.  FIG. 2  is a diagram that represents the path  26  of travel of a photonic signal  22  through a crystal  20 . The photonic crystal  20  enables smaller optical devices to be interconnected than can be achieved through conventional optical waveguides and/or fibers.  FIG. 3  is a diagram of a photonic crystal  30  that forms a splitter  32  for separating an incoming photonic signal  34  into two photonic components  36  and  38 . The photonic crystal  30  has the ability to create a highly efficient optical splitter  32  close to 100% transmission over certain frequency range.  
         [0021]     The photonic crystals according to the invention have periodic dielectric structures that contain materials with alternating different dielectric constants. As shown in FIGS.  4 (A) to  4 (C), the periodic structures can form square  42 , triangular  44 , honeycomb  46 , or other patterns for creating a photonic bandgap used in confining electromagnetic waves within the localized defect region. Additional information on photonic crystals may be found in M. Loncar et al., Applied Physics Letters, Vol. 77, No. 13, Sep. 25, 2000, and M. Bayindir et al., Applied Physics Letters, Vol. 77, No. 24, Dec. 11, 2000, both of which are incorporated herein by reference.  
         [0022]     An optical signal quantizer  50  will now be described with reference to  FIG. 5 . The quantizer  50  has a photonic crystal  52  which defines a highly efficient optical splitter  54  and optical waveguide  55  bends at sub-micron radii. The quantizer  50  includes a plurality of optical splitters  54   a  to  54   c  for successively splitting an incoming photonic signal  56  into smaller quantized photonic signals. In this example, each optical splitter  54  separates a photonic signal into two equal photonic signals. It should be understood that in other embodiment of the invention, the splitters  54  could separate a photonic signal into more than two photonic signals and/or may separate a photonic signal into unequal photonic signals. With reference to  FIG. 5 , the incoming photonic signal is separated into two photonic signals quantized at 50% for each signal. The second optical splitter  54   b  receives one of the 50% quantized photonic signal and divides the signal in half to form two 25% quantized photonic signals. Finally, the third optical splitter  54   c  receives one of the 25% quantized signals and forms two 12.5% quantized optical signals.  
         [0023]     The outputs from the optical splitters  54  are routed through waveguides  55  to optical detectors  51 . The waveguides  55  preferably are designed to provide a uniform delay for all quantized photonic signals. Thus, each quantized photonic signal of the incoming photonic signal will reach the optical detectors  51  at the same time. The waveguides  55  include turns, such as 90 degree turns  55   a,  which may be provided with practically no losses. The quantized photonic signals are then converted into electrical signals by the optical detectors  51 . A set of comparators  57  set threshold levels for the digital signals and provide outputs in digital form. The outputs from the comparators  57  forms individual bits of the digital signal which are combined together through digital encoder  59 . Thus, with the quantizer  50 , the photonic crystal  52  quantizes the incoming photonic signal  52  into quantized photonic signals by successively splitting the photonic signal. With the high-speed photodiode detectors  51 , comparators  57 , and the digital encoder  59 , the quantizer  50  can provide a high-speed optical analog to digital converter.  
         [0024]     In the embodiment shown in  FIG. 5 , the analog signals from the optical detectors  51  are input to the comparators  57  in order to form the digital bits of a digital signal. With this embodiment, the analog electrical signals are compared to threshold levels set by the comparators  57 . According to another aspect of the invention, the quantized photonic signals are compared to threshold intensity levels, thereby resulting in an even faster conversion speed. The comparison of the quantized photonic signals can be performed in a number of ways, such as through the use of an optical limiter. According to this aspect, a photonic crystal quantizes photonic signals and routes the quantized signals directly to the optical limiter. The output of the optical limiter is provided to photodiodes and then to a digital encoder. This approach eliminates any analog signal in the electrical domain. The use of a non-linear distributed feedback structure as an optical limiter can remove the use of the electronic comparators  57  in the photonic analog-to-digital system  50  of  FIG. 5 . Additional information on optical limiters may be found in L. Brzozowski et al., IEEE Journal of Quantum Electronics, Vol. 36, No. 5, May 2000, which is incorporated herein by reference.  
         [0025]     An example of an optical limiter  60  will now be described with reference to  FIG. 6 . The optical limiter  60  is comprised of a one-dimensional periodic structure  62  with alternating dielectric constants  64   a  and  64   b  which possesses alternating positive and negative non-linear Kerr coefficients This structure can provide an optical hard-limited effect that prevents light transmission up to certain intensity level and quickly switches to a fixed limit of output transmission after the certain intensity threshold is reached. The threshold can be determined by the selection of the alternating material dielectrics and their corresponding Kerr coefficients. The thresholds for the optical limiters may be set to the same value or to different values. The wavelength λ of operation can be tuned by changing the layer thickness and spacing. The shape quality of the optical signal response curve can be determined by the number of alternating layers.  
         [0026]      FIG. 7  is a diagram of a quantizer  70  according to another embodiment of the invention. The quantizer  70  includes the photonic crystal  52  which receives the incoming photonic signal  56  and forms quantized photonic signals. The quantized photonic signals are passed through optical limiters  72  which block the quantized photonic signal if it is less than a set threshold intensity level or passes the quantized photonic signal if it meets or exceeds the threshold intensity level. The set of optical detectors  51  receives the outputs from the optical limiters  72  and outputs from the detectors  51  form the individual bits of a digital signal. The digital encoder  59  forms the digital signal from the individual bits.  
         [0027]     The foregoing description of the preferred embodiments of the invention has been presented only for the purpose of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching.  
         [0028]     The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated.