Abstract:
A system and method for demodulating an optical differential-phase-shift-keyed (DPSK) input signal using a Fabray-Perot etalon filter. In one embodiment the system receives a transmitted wavefront from the etalon filter and uses a detector to generate an electrical waveform from the transmitted wavefront. A comparator is used to receive an output from the detector and to generate a signal in accordance with each phase shift (i.e., bit transition) in the optical DPSK input signal. A latching flip-flop receives an output from the comparator and generates a digital signal representative of the bit pattern of the DPSK input signal. The system and method does not require the precisely matched dual optical paths of a Mach-Zehnder interferometer, and therefore is substantially less susceptible to thermal effects that could influence the operation of a conventional Mach-Zehnder interferometer in DPSK demodulation operations.

Description:
FIELD 
       [0001]    The present disclosure relates to demodulators used with optical differential-phase-shift-keyed (DPSK) communication, and more particularly to an interferometer of less complex construction that is able to demodulate an optical DPSK signal without the need for optical path length control schemes that need to be matched exactly to the data rate, and that might compromise performance of the interferometer. 
       BACKGROUND 
       [0002]    Optical Differential-Phase-Shift-Keyed (DPSK) demodulators have traditionally been formed by using the well known Mach-Zehnder interferometer. This device has essentially two “arms” that form distinct optical paths that are used to detect phase shifts in the optical DPSK signal. The Mach-Zehnder interferometer requires a physical length that is determined by the data rate of the communication link with which it is being used. The two arms of the Mach-Zehnder interferometer must maintain a differential optical path length equal to the distance that light travels in one bit period. As a result, it is highly sensitive to temperature and other environmental factors that cause variations in the physical dimensions of the optical components that it uses. 
         [0003]    To address the undesirable influence that thermal factors have on the performance of present day Mach-Zehnder interferometers that are used in DPSK optical demodulator applications, these devices have typically employed a control loop to regulate the path length of one of the optical paths. As will be appreciated, this adds significantly to the complexity and cost of the interferometer. 
         [0004]    Other apparatuses and methods for demodulating a DPSK optical signal have involved optical filter discriminator approaches that attempt to detect the phase shift changes in the bit pattern of the optical DPSK signal. However, such approaches typically only use the transmitted wavefront portion of the input optical signal, and not the larger reflected energy provided by the reflected wavefront component of the signal. Such approaches further typically do not attempt to make use of both the transmitted and reflected energy components of the optical DPSK signal. Using both the transmitted and reflected energy 1) increases the received signal, and 2) improves the signal-to-noise ratio, since common-mode noise of the dual detectors is cancelled out. Time alignment of the transmitted and received pulses is less difficult than alignment of the two arms of a Mach-Zehnder demodulator. 
       SUMMARY 
       [0005]    The present system and method is directed to a system for demodulating an optical DPSK signal that is more sensitive and less complex and less costly construction than other forms of optical demodulator systems. 
         [0006]    In one embodiment the system makes use of a Fabry-Perot etalon filter for receiving an optical DPSK input signal and generating a wavefront component therefrom each time a phase shift occurs in the optical DPSK input signal. A detector system detects the wavefront component output from the etalon filter to enable bit transitions in a data sequence of the optical DPSK input signal to be detected, and the optical DPSK input signal to be reconstructed as a digital output signal. A comparator is used to compare the electrical signal with a reference threshold signal, and an output of the comparator is used to signal a phase shift (i.e., bit transition) in the optical DPSK input signal. A logic component receives an output of the comparator and generates a digital output signal indicative of the bit pattern of the optical DPSK signal. 
         [0007]    In another embodiment the detector system makes use of a first detector for receiving a transmitted wavefront component of the optical DPSK input signal, and a second detector for receiving a reflected wavefront component of the DPSK input signal. A comparator receives the output from each detector and generates a signal indicative of a phase shift in the wavefronts of the signals output from the etalon filter. The signal path lengths relative to the comparator are matched during initial construction of the system. The comparator output is fed to an edge-triggered logic component, which may comprise a latching flip-flop that generates a digital output signal indicative of the bit pattern of the optical DPSK signal. The physically small size of the etalon filter serves to significantly reduce or substantially eliminate the susceptibility of the etalon filter to thermal changes that could significantly affect the performance of a conventional Mach-Zehnder interferometer. 
         [0008]    The various embodiments and methods of the present disclosure are substantially unaffected by thermal changes that typically adversely affect performance of other dual arm demodulator systems. The present system and method is expected to find utility in both free-space and optical fiber implementations. 
         [0009]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0011]      FIG. 1  is a simplified block diagram of one embodiment of an optical DPSK demodulator in accordance with the present disclosure; 
           [0012]      FIG. 2  shows a graph of two exemplary waveforms that illustrate the difference in the output of the etalon filter of  FIG. 1  for two different levels of finesse of the etalon filter (The desired width is a function of the data rate; the effect of one transition should be gone before the next transition can occur); 
           [0013]      FIG. 3  is a diagram illustrating the phase shifts in the reflected signals produced by the etalon filter of  FIG. 1 ; 
           [0014]      FIG. 4  is a graph that shows two curves representing the transmitted wavefront (dotted line) and the reflected wavefront (solid line) of an optical DPSK signal being filtered by an etalon filter, and illustrates how the energy of the reflected wavefront is released over a significantly shorter time period (in this example 10 picoseconds) than the time period corresponding to the switching frequency of the DPSK signal (in this example 100 picoseconds); 
           [0015]      FIG. 5  illustrates two exemplary waveforms, one produced by a conventional Mach-Zehnder interferometer and the other produced by a optical DPSK demodulator of the present system and method; and 
           [0016]      FIG. 6  is a block diagram of another embodiment of the system of the present disclosure that makes use of both the transmitted and reflected wavefront components of the optical DPSK input signal. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0018]    Referring to  FIG. 1 , an optical differential-phase-shift-keyed (DPSK) demodulator system  10  is illustrated in accordance with one embodiment of the present disclosure. The system  10  includes, in this example, a Fabry-Perot etalon filter  12 , an optical detector  14 , a comparator  16 , and a latching, edge triggered flip-flop  18 . Components  14 ,  16  and  18  may be viewed as collectively comprising a detector system  20 . The Fabry-Perot etalon filter  12  will be referred to as the “etalon filter  12 ” for discussion purposes. 
         [0019]    The etalon filter  12  receives an optical DPSK input signal  22  and generates a transmitted wavefront  24  and a reflected wavefront  26  therefrom. The transmitted wavefront  24  is directed to an input of the detector  14 , such as a high-speed InGaAs detector. The detector  14  produces an electrical output signal at an output signal, indicated by waveform  30 , at an output  28 . As long as no phase shifts (i.e., bit transitions) occur in the optical DPSK input signal  22  data sequence, the amplitude  30   a  of the transmitted wavefront  24  is constant. However, when a phase shift of 180 degrees (corresponding to a bit transition) occurs in the optical DPSK input signal  22  data sequence, a decreased-intensity pulse occurs on the transmitted wavefront  24 , while a corresponding increased-intensity pulse occurs on the reflected wavefront  26 . The width of pulse depends on the finesse (i.e., resolution) of the etalon filter  12 . 
         [0020]    With brief reference to  FIG. 2 , curves  32  and  34  illustrate two different waveforms produced by a carrier wave length at approximately 1550 nanometers for two different values of etalon filter  12  finesse. The filter finesse of a value of 150 produces the curve  32 , which represents an output signal from the etalon filter  12 . The signal represented by curve  32  forms a significantly narrower pulse with less energy than the output signal represented by curve  34 , which is produced with a finesse value of 300. However, the narrower pulse may have a better signal-to-noise ratio. The pulse width should be optimized for the best signal-to-noise ratio for the desired data rate, within the practical constraints of producing high finesse filters. 
         [0021]    With further reference to  FIG. 1 , the electrical waveform  30  at the output  28  of the detector  14  is fed into the inverting input  36  of the comparator  16 . A reference threshold level signal  38  is applied to the non-inverting input  40  of the comparator  16 . The output  42  of the comparator  16  may be a logic “0” level signal when the magnitude of the signal on the non-inverted input  36  is less than the value of the threshold level signal  38  on the inverting input  40 . For example, this might produce a logic “0” output signal on the output  42  of the comparator  16 . However, when the value of the signal  30  on the inverting input  36  increases beyond the threshold level signal  38  on the non-inverting input  40 , the output  42  of the comparator  16  may go to a logic level “1” value. The output waveform  44  present on the output  42  of the comparator  16  forms a digital saw-toothed waveform, as shown in  FIG. 1 . 
         [0022]    When the output waveform  44  on output  42  changes to a logic 1 level, the latching flip-flop  18  receives the output waveform  44  on its input  46  and generates a digital output waveform  50  on its output  48 . The latching flip-flop  18  is only triggered on the trailing edge of each pulse of the output waveform  44  (i.e., only when there is a phase shift in the optical DPSK input signal  22  data sequence. As a result, the digital output signal  50  from the latching flip-flop  18  represents a reconstruction of the bit sequence of the optical DPSK input signal  22 . 
         [0023]    The optical principle upon which use of the system  10  is predicated is based, in part, on an unconventional application of the transient shift in the transmission and/or reflection characteristics of the etalon filter  12  that occurs when the phase of the optical DPSK input signal  22  is shifted over a relatively short period of time. In general, the conventional application of a convention Fabry-Perot etalon filter is based on its steady state operation as depicted in  FIG. 3 . If δ is an integral multiple of 2π radians at the operational wavelength range of interest, the transmitted amplitude A t (t), to within the internal absorption limitations of the etalon filter  12 , equals the incident amplitude A i (t), while the reflected amplitude A r (t) equals zero. This situation is dependent upon constructive interference of the reflecting wavefront component  24  internal to the etalon filter  12 . Typically it is assumed that the phase of an electromagnetic wave varies continuously in space and time only due to propagation. However, if the phase φ(t) is shifted at the source, this additional phase term will affect the wavefront, or signal, transmitted (A t (t)) through (i.e., transmitted wavefront component  24 ) and reflected (A r (t)) from the etalon filter  12  (i.e., reflected wavefront component  26 ), as shown in Equations (1) and (2) below. The delayed phase terms in the summations occur as a result of the multiple reflections internal to the etalon filter  12 . 
         [0000]    
       
         
           
             
               
                 
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         [0024]    where:
       θ—angle of incidence of incoming wavefront;   A i (t); θ′—angle of refraction of wavefront inside of etalon filter;   n′—refractive index of etalon filter material;   n—refractive index of surrounding material;   r—reflection coefficient from outside of the etalon filter material;   r′=−r—reflection coefficient from inside of the etalon filter material;   t—transmission coefficient from outside into the etalon filter material;   t′=t—transmission coefficient from inside to outside of the etalon filter material;   h—thickness of the etalon filter;   λ o —free space wavelength of wave;   c—free space speed of light; and   v=c/λ o —frequency of the optical input wave signal (i.e., signal  22 ).       
 
         [0037]      FIG. 4  illustrates an exemplary optical input waveform  22  having a transmitted wavefront component, represented by waveform  52 , and a reflected wavefront component, represented by waveform  54 , where the phase of the optical input waveform  22  is reversed (shifted by π radians) every 100 picoseconds (pS). In this example waveform  52  corresponds to transmitted wavefront  24  in  FIG. 1 , and waveform  54  corresponds to reflected wavefront  26  in  FIG. 1 . Waveforms  52  and  54  also represent waveforms that are generated from the application of equations (1) and (2) given above. The phase transition occurs over a 10 pS interval, designated by reference numeral  56 . It appears that the magnitude of reflectivity of the etalon filter  12  is substantially greater than 1, which is a physical impossibility for a passive device such as an etalon filter. This is explained by the fact that the etalon filter  12  actually releases its stored energy over a substantially shorter time interval than the 100 pS time interval (in this example) that the phase shifting occurs in. Thus, the signal surge represented by waveform  54  provides a significant signal-to-noise advantage for bit detection purposes and enables phase shifts in the optical DPSK input signal (i.e., the bit pattern) to be directly detected without bit-to-bit comparison. Thus, the system  10  does not require the two arms of conventional Mach-Zehnder interferometers to detect the phase shift in the optical DPSK input signal  22 . Furthermore, the system  10  is much less susceptible to thermal conditions that affect the detectability of conventional Mach-Zehnder interferometers. In  FIG. 5 , the exemplary output waveform  44  produced by comparator  16  is presented in comparison to an exemplary output waveform  58  from a typical Mach-Zehnder interferometer. 
         [0038]    With brief reference to  FIG. 6 , a system  100  in accordance with other embodiment of the present disclosure is illustrated. The system  100  is similar in some respects to the system  10 , and common components will be designated by reference numerals increased by 100 over those used in connection with the system  10 . 
         [0039]    The system  100  differs from the system  10  by making use of both the transmitted wavefront  126  and the reflected wavefront  124  from an etalon filter  112 . The incoming optical DPSK-modulated beam  22  is linearly polarized so that it is reflected by a polarizing beam splitter  172 . It passes through the Faraday rotator  170  and into the etalon filter  112 . The transmitted wavefront  126  is received by one half of a differential detector  114   a  while the reflected wavefront  124  passes through the Faraday rotator  170  and the polarization beam splitter  172  to the other half of the differential detector  114   b . The Faraday rotator  170  is a non-reciprocal device that rotates the polarization of a transmitted beam in the same sense relative to the direction of propagation. The thickness of the Faraday rotator  170  determines the amount of polarization. In this application the polarization is rotated by 45 degrees on each pass, in order that the reflected beam  124  from the etalon filter  122  is transmitted by the polarization beam splitter  172  to the second detector  114   b.    
         [0040]    The waveform produced at the output  128   a  of the first detector  114   a  illustrates the decreased-intensity pulse that occurs each time the detector  114   a  detects a phase shift in the transmitted wavefront  126 . This waveform is indicated by waveform  130   a . The second detector  114   a  produces an increased-intensity waveform  130   b  at its output  115  from the reflected wavefront  124  each time a phase shift occurs in the optical DPSK input signal  22 . The output  128   a  from the first detector  114   a  and the output  115  from the second detector  114   b  are fed into the inputs of a comparator  116 . When a phase shift occurs in the optical DPSK input signal  22 , the signal on the non-inverting input  140  of the comparator  116  will be different than the magnitude of the signal on the inverting input  136 , thus causing the comparator  116  to produce a signal  144  at its output  142 . The signal  144  is used to drive a latching flip-flop  118  that produces a digital output signal  150  at its output  148  representative of the bit pattern of the optical DPSK input signal  22 . Thus, the system  100  makes use of both the transmitted wavefront  126  and the reflected wavefront  124 , and does not require a separate reference threshold input on the comparator  116 . It will be appreciated that the propagation path lengths between the etalon filter  112  and the first detector  114   a , and between the etalon filter  112  and the second detector  114   b , will need to be matched to a fraction of the bit period (typically within about 5%) during initial construction and/or tuning of the system  100 . Differential detectors with a variable optical delay on one input are commercially available. One such component is the “Balanced Photodiode Lab Buddy with Variable Optical Delay Line”, commercially available from Discovery Semiconductors, Inc. of Ewing, N.J. The system  100  is expected to provide an even higher signal-to-noise ratio for the output signal  150  of the system  100 . 
         [0041]    The various embodiments and methods described herein thus enable the bit pattern of an optical DPSK input signal to be detected and reconstructed without the use of a conventional dual arm interferometer such as a Mach-Zehnder interferometer. The various embodiments and methods do not require any control loops to be constructed to account for thermal changes in the environment in which the demodulator is being used. The use of a Fabry-Perot etalon filter, which is significantly smaller in physical size than a conventional Mach-Zehnder interferometer, helps to significantly reduce the susceptibility of the filter to thermal changes that could affect performance. The use of a Fabry-Perot etalon filter also significantly simplifies the construction of a DPSK demodulator system, as well as potentially enabling the system to be used in applications and/or environments where significant thermal changes would render conventional Mach-Zehnder DPSK demodulators unreliable. 
         [0042]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.