Abstract:
An optical pulse position modulation receiver relying on the gain dynamics in a semiconductor optical amplifier (SOA). Optical PPM signal pulses and periodic optical clock pulses at a different frequency and/or polarization than the signal pulses are coupled into an SOA. Due to the high optical power of the clock pulses, the SOA gain will drop to a small value after each clock pulse. The SOA will then amplify each signal pulse that follows the clock pulse, and the gain will depend on the delay between the signal pulse and the preceding optical clock pulse. The optical output of the SOA can then be converted to an electrical signal by a photodetector.

Description:
BACKGROUND 
   1. Field 
   The present disclosure relates to coherent detection of pulse position modulated signals. More particularly, the present disclosure describes a method and apparatus using a semiconductor optical amplifier (SOA) for demodulating optical pulse position modulated signals. 
   2. Description of Related Art 
   Many satellite and terrestrial optical communication systems require transmission of analog optical signals. The straightforward way to address this need is to modulate the amplitude of an optical carrier. This approach, however, suffers from poor signal-to-noise ratio (SNR). It is well known that broadband modulation techniques, which utilize higher bandwidth than that of the transmitted waveform, may improve SNR over that achieved with amplitude modulation. Pulse Position Modulation (PPM) is one of these techniques. In PPM, a temporal shift in the pulse position represents a sample of the transmitted waveform. The improvement in SNR near the Nyquist sampling frequency of a pulse position modulated signal over an amplitude modulated signal is shown below:
 
SNR ppm ∝SNR am (t p /τ) 2  
 
where t p  is the temporal spacing between unmodulated pulses and τ is the pulse duration.
 
   Conventional detection or demodulation of analog PPM optical signals, though, suffers from poor SNR at low frequencies. PPM signals are usually demodulated from the optical to electronic domain by a photodiode followed by a lowpass filter (LPF) that converts pulse position modulation to amplitude modulation. Such a demodulation technique is not capable of recovering the DC component, since the DC component is represented by a constant temporal shift of all pulses from their unmodulated positions. Moreover, the demodulated signals after the lowpass filter have very low amplitude at low frequencies. The amplitude increases linearly with frequency up to the Nyquist limit. Such frequency-dependent distortion is corrected by an integration circuit, which amplifies low-frequency noise accordingly, resulting in decreased SNR performance. 
   An apparatus and method for detecting an optical PPM signal are described in U.S. Pat. No. 6,462,860, issued to Ionov on Oct. 8, 2002. This patent application describes coherent wavelength converters that are used to generate preferably top hat shaped optical pulses. Top hat shaped optical pulses are preferred in the apparatus and method described in U.S. Pat. No. 6,462,860, since such pulses provide for better linearity of the detected optical PPM signal. Top hat shaped optical pulses may also be used in other optical systems known in the art. U.S. Pat. No. 6,462,860 describes the generation of top hat shaped pulses with non-linear optical mirrors (NOLMs). 
   Another embodiment of a top hat pulse generator suitable for use in optical pulse position modulation detection is described in U.S. patent application Ser. No. 10/341,689, filed on Jan. 13, 2003 and entitled “An Optical Top Hat Pulse Generator.” The apparatus described in U.S. Ser. No. 10/341,689 uses NOLMs that are controlled by first order solitons. Still another embodiment of a top hat pulse generator suitable for use in optical pulse position modulation detection is described in U.S. Provisional Patent Application Ser. No. 60/488,540 filed on Jul. 18, 2003 and entitled “Method and Apparatus for Optical Top-Hat Pulse Generation.” The embodiment described in U.S. Ser. No. 60/488,540 also uses an NOLM controlled by first order solitons, but the NOLM also comprises one or more sections of polarization maintaining fiber coupled to each other at 90 degree angles. 
   While optical PPM receivers based on optical top-hat pulse generation provide outputs with the desired linearity, such NOLM-based receivers have drawbacks due to their complexity. The NOLMs may require careful balancing and adjustments to achieve the desired linearity. Further, the NOLM-based receivers may also require a number of erbium-doped fiber amplifiers (EDFAs) and optical filters with flat dispersion. These components increase the complexity and cost of such NOLM-based receivers. Therefore, there is a need in the art for a PPM receiver with less complexity than those PPM receivers known in the art. 
   SUMMARY 
   An optical pulse position modulation receiver according to an embodiment of the present invention relies on the gain dynamics in a semiconductor optical amplifier (SOA). Optical PPM signal pulses and periodic optical clock pulses at a different frequency and/or polarization than the signal pulses are coupled into an SOA. Due to the high optical power of the clock pulses, the SOA gain will drop to a small value after each clock pulse. The SOA will then amplify each signal pulse that follows the clock pulse, and the gain will depend on the delay between the signal pulse and the preceding optical clock pulse. The optical output of the SOA can then be converted to an electrical signal by a photodetector. 
   A first embodiment according to the present invention provides an optical pulse position modulation receiver comprising: a semiconductor optical amplifier; a filter apparatus receiving an output of the semiconductor optical amplifier, said filter apparatus suppressing optical clock pulses output by the semiconductor optical amplifier and passing an amplified optical pulse position modulated signal output by the semiconductor optical amplifier; and an optical to electric converter receiving the amplified optical pulse position modulated signal, wherein the optical clock pulses input to the semiconductor optical amplifier have an optical power at a level to reduce or saturate the stimulated emission transition of the semiconductor optical amplifier. 
   Another embodiment according to the present invention provides an optical pulse position modulation detector comprising: optical amplification means receiving an input comprising optical pulse position modulated pulses and receiving optical clock pulses, said optical amplification means providing a gain for each pulse of said optical pulse position modulated pulses proportional to a delay from a preceding optical clock pulse of said optical clock pulses; filtering means receiving an output of said optical amplification means, said filtering means suppressing optical clock pulses output by said optical amplification means and passing amplified optical pulse position modulated pulses output by said optical amplification means; and optical to electric conversion means receiving said amplified optical pulse position modulated pulses and providing an electrical output corresponding to said amplified optical pulse position modulated pulses. 
   Still another embodiment according to the present invention provides a method for optical pulse position modulation detection comprising: optically amplifying an input comprising optical pulse position modulated pulses and optical clock pulses to provide an optical output having amplified optical pulses wherein each amplified optical pulse of the amplified optical pulses has a gain proportional to a delay between each optical pulse position modulated pulse and a preceding optical clock pulse; filtering the optical output to suppress optical clock pulses and to pass said amplified optical pulses. The filtered optical output may then be converted to an electrical signal by optical to electrical conversion. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an embodiment of an optical pulse position modulation receiver according to the present invention. 
       FIG. 2A  shows a timing diagram illustrating the gain dynamics of the SOA used in embodiments of the present invention. 
       FIG. 2B  shows a timing diagram for optical output power from the SOA for the optical signal pulses depicted in  FIG. 2A . 
       FIG. 3  shows the calculated gain dynamics of a SOA between two clock pulses for different driving currents. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some embodiments of the invention are shown. This invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. 
   A schematic diagram of a PPM demodulator  100  according to an embodiment of the present invention is shown in  FIG. 1 . The PPM demodulator  100  may also be referred to as a PPM receiver. Optical PPM signal pulses  110  and optical clock pulses  120  may have different wavelengths and/or different polarizations. The optical PPM signal pulses  110  and optical clock pulses  120  are combined and coupled into a semiconductor optical amplifier (SOA)  135 . In  FIG. 1 , the optical clock pulses  120  have a polarization that is orthogonal to that of the optical PPM signal pulses  110 . Therefore, in  FIG. 1 , the optical PPM signal pulses  110  and optical clock pulses  120  are combined in a polarization combiner  131  and directed to the SOA  135 . However, other means known in the art may be used to combine the optical PPM signal pulses  110  and optical clock pulses  120  for coupling into the SOA  135 , such as, for example, a wavelength multiplexer/demultiplexer, if the optical PPM signal pulses  110  and optical clock pulses  120  have different wavelengths. Still other wavelength independent and/or polarization independent coupling means may be used for coupling the optical PPM signal pulses  110  and the optical clock pulses  120  into the SOA  135 , such as a power combiner, a 3 dB optical coupler, a 10 dB optical coupler or any other such coupler known in the art. 
   The orthogonally polarized optical clock pulses  120  are filtered out from the optical PPM signal pulses  110  after the SOA  135  using another polarization coupler  135  (and/or a bandpass filter  137  if the signal  110  and clock pulses  120  have different wavelengths). The polarization coupler  131  is shown in  FIG. 1  for illustrative purposes. Alternatively, as noted above, a wavelength mux/demux, a 3 dB coupler, or other such devices may also be used for combining the signal  110  and clock pulses  120  before the SOA  135  and extracting the clock pulses  120  after the SOA  135 . 
   A timing diagram demonstrating the operational principle of the embodiment of the optical PPM demodulator according to the present invention based on gain dynamics in a semiconductor optical amplifier (SOA) is shown in  FIG. 2A . In a preferred configuration of the SOA  135 , the clock pulses  120  are sufficiently energetic to saturate the stimulated emission transition of the SOA  135 , so that the SOA gain drops to small value after each clock pulse  120 . In other configurations of the SOA  135 , the optical clock pulses  120  may have sufficient energy to merely reduce the stimulated emission transition of the SOA  135  to still provide that the SOA gain drops after each clock pulse  120 . Preferably, the signal pulses  110  are considerably weaker, so that they do not affect the SOA dynamics significantly. The SOA gain recovers between the clock pulses  120  and a quasi-equilibrium is maintained. As seen from  FIG. 2A , the gain experienced by a weak signal pulse  110  depends on its delay from the preceding clock pulse  120 .  FIG. 2B  illustrates the amplitude of the amplified signal pulse  130  after amplification by the SOA  135  according to the gain dynamics described above. 
     FIG. 1  illustrates an embodiment in which a reference signal channel  150  and a balanced detector  140  is used to extract the signal gain. The optical PPM signal pulses  110  in the reference channel  150  may be directed to the balanced detector  140  by mirrors  151  or other such optical means known in the art. At the balanced detector  140 , the amplified signal pulses  130  (which contain the optical PPM signal pulses  110  plus the gain) are subtracted from the optical PPM signal pulses  110  to obtain the gain, which, as described above, is proportional to the delay of each optical PPM signal pulse  110  from its corresponding clock pulse  120 . Preferably, the two arms of the receiver  100  have identical optical length and the balanced detector  140  is adjusted to give zero output when the optical PPM signal pulses  110  have minimal delays with respect to the clock pulses  120 . 
   Alternatively, the amplified signal pulses  130  may be coupled directly to a single-channel photodetector for conversion of the optical signals to electrical signals. In this case, the DC bias present in the output from the single-channel photodetector is preferably removed electronically. 
   As discussed above, embodiments of the PPM demodulator according to the present invention rely upon obtaining certain gain characteristics from the SOA or other such optical amplification devices. An analysis of the requisite gain dynamics is presented below. 
   It is assumed that the carrier density of the SOA does not change much in quasi-steady-state conditions, and, therefore, a linearized equation describing the dynamics of carrier density n may be used (see also Petermann, “Laser Diode Modulation and Noise,” Chapter 2, Kluwer Academic Publishers, Dordrecht/Boston/London, 1988): 
                     ⅆ     n   ⁡     (   z   )           ⅆ   t       =       I     e   ⁢           ⁢   V       -       n   ⁡     (   z   )       τ     -       g   ⁡     (   z   )       ⁢     v   g     ⁢   S               Eq   .           ⁢   1               
In Eq. 1, g is gain, τ is the carrier lifetime, v g  is the group velocity of light, S is the photon density, I is the current, and V is the SOA volume.
 
   The optical power changes along the SOA according to the following equation: 
   
     
       
         
           
             
               
                 
                   
                     ⅆ 
                     P 
                   
                   
                     ⅆ 
                     z 
                   
                 
                 = 
                 
                   
                     g 
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                       ( 
                       z 
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                   ⁢ 
                   P 
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
         
       
     
   
   Note that: 
                     v   g     ⁢   hvS     =         A     -   1       ⁢       ⅆ   P       ⅆ   z         =       A     -   1       ⁢     g   ⁡     (   z   )       ⁢   P               Eq   .           ⁢   3               
where A is the SOA cross section.
 
   Integrating Eq. 1 over the SOA length and using Eq. 3 and Eq. 4, the following is obtained: 
   
     
       
         
           
             
               
                 
                   A 
                   ⁢ 
                   
                     ⅆ 
                     
                       ⅆ 
                       t 
                     
                   
                   ⁢ 
                   
                     
                       ∫ 
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                       L 
                     
                     ⁢ 
                     
                       
                         n 
                         ⁡ 
                         
                           ( 
                           z 
                           ) 
                         
                       
                       ⁢ 
                       
                         ⅆ 
                         z 
                       
                     
                   
                 
                 = 
                 
                   
                     I 
                     e 
                   
                   - 
                   
                     
                       A 
                       τ 
                     
                     ⁢ 
                     
                       
                         ∫ 
                         0 
                         L 
                       
                       ⁢ 
                       
                         
                           n 
                           ⁡ 
                           
                             ( 
                             z 
                             ) 
                           
                         
                         ⁢ 
                         
                           ⅆ 
                           z 
                         
                       
                     
                   
                   - 
                   
                     
                       1 
                       hv 
                     
                     ⁢ 
                     
                       
                         ∫ 
                         0 
                         L 
                       
                       ⁢ 
                       
                         
                           
                             ⅆ 
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                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 4 
               
             
           
         
       
     
   
   Introducing the total number of carriers 
             N   =     A   ⁢       ∫   0   L     ⁢       n   ⁡     (   z   )       ⁢     ⅆ   z             ,         
Eq. 4 can be rearranged as follows:
 
                     ⅆ   N       ⅆ   t       =       I     e   ⁢               -     N   τ     -       Δ   ⁢           ⁢     P   ⁡     (   t   )         hv               Eq   .           ⁢   5               
where ΔP is the optical power gain.
 
   Assuming very short optical pulses, Eq. 5 can be integrated over the pulse duration to obtain:
 
 ΔN=ΔE/hv   Eq. 6
 
   Between the optical pulses ΔP=0, so Eq. 5 provides the following: 
                     ⅆ   N       ⅆ   t       =       I     e   ⁢               -     N   τ               Eq   .           ⁢   7               
which results in the following equation for N(t):
 
                   N   ⁡     (   t   )       =       τ   ⁢     I   e       +       (       N   ⁡     (   0   )       -     τ   ⁢     I   e         )     ⁢     exp   ⁡     (       -   t     /   τ     )                   Eq   .           ⁢   8               
where N(0) is the number of carriers left after each clock pulse.
 
   Eq. 8 may be linearized for t&lt;&lt;τ to obtain:
 
 N ( t )= N (0)+(τ I/e−N (0)) t/τ   Eq. 9
 
   In the steady-state condition, the drop in the total number of carriers caused by a clock pulse (and equal to the number of photons created via stimulated emission) is compensated by the driving current between the pulses:
 
Δ E/hv=−ΔN=TI/e−N (0) T/τ   Eq. 10
 
   Therefore, the saturated optical power may be calculated as follows:
 
Δ P/hv=I/e−N (0)/τ  Eq. 11
 
   For an SOA from JDS-Uniphase of San Jose, Calif. (Model No. CQF873), the saturated optical clock power at a current of I=0.3 A was found to be ΔP=20 mW, including a ˜3 dB loss from the SOA to an optical fiber. From the equations above, N(0)=1.7×10 9  (τ=1 ns) and ΔN=1.7×10 7  for T=100 ps. Therefore, the assumption that the small carrier density changes ΔN&lt;&lt;N is satisfied. Note, however, that embodiments of the present invention are not limited to SOAs available from JDS-Uniphase. 
   The gain of an SOA may be found from the following equation: 
   
     
       
         
           
             
               
                 G 
                 = 
                 
                   
                     
                       P 
                       out 
                     
                     / 
                     
                       P 
                       in 
                     
                   
                   = 
                   
                     exp 
                     ⁡ 
                     
                       ( 
                       
                         
                           ∫ 
                           0 
                           L 
                         
                         ⁢ 
                         
                           
                             g 
                             ⁡ 
                             
                               ( 
                               z 
                               ) 
                             
                           
                           ⁢ 
                           
                             ⅆ 
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                       ) 
                     
                   
                 
               
             
             
               
                 Eq 
                 . 
                 
                     
                 
                 ⁢ 
                 12 
               
             
           
         
       
     
   
   To calculate the gain dynamics, the following linearized relationship with the carrier density may be used (see also Petermann, “Laser Diode Modulation and Noise,” Chapter 2):
 
 g=a ( n−n   0 )  Eq. 13
 
Typically, n 0 ˜1.1×10 18  cm −3  and a ˜3×10 −16  cm 2 . However, in embodiments according to the present invention, the SOA preferably operates in heavily saturated conditions, where the emission cross section a is lower. Therefore, in these embodiments, a=10 −16  cm 2  may more accurately reflect the operating condition of the SOA. Combining Eq. 12 and Eq. 13, the SOA gain may be found as follows:
 
 G =exp( a ( N/A−n   0   L ))  Eq. 14
 
     FIG. 3  shows the calculated gain dynamics of a SOA between two clock pulses for different driving currents. It is noted that 0.6 A is typically the maximum current for SOAs from JDS-Uniphase. In the results presented in  FIG. 3 , it is assumed that A=10 −8  cm 2  and L=0.15 cm. From  FIG. 3 , it can be seen that at high currents, the SOA builds a significant gain (almost 9 dB) at the end of the clock window. However, at these higher currents, the gain is not linear. At smaller currents, the gain rises almost linearly, but the gain is significantly less. In preferred embodiments according to the present invention, the SOA current should be adjusted for suitable linearity and gain values. 
   As indicated above, the SOA should be operated so that the clock pulses saturate the stimulated emission transition. To saturate stimulated emission of an SOA by a single clock pulse, the energy of the clock pulse should exceed: 
                   E   hv     &gt;     A   a             Eq   .           ⁢   15               
For example, E&gt;12 pJ and the total power P&gt;120 mW for a 10 G/s pulse rate (T=100 ps).
 
   From the foregoing description, it will be apparent that the present invention has a number of advantages, some of which have been described herein, and others of which are inherent in the embodiments of the invention described herein. Also, it will be understood that modifications can be made to the apparatus and method described herein without departing from the teachings of the subject matter described herein. As such, the invention is not to be limited to the described embodiments except as required by the appended claims.