Patent Publication Number: US-8526829-B1

Title: System, method and apparatus for clockless PPM optical communications

Description:
RELATED APPLICATIONS 
     This application is a Continuation in Part of “System, Method and Apparatus for Clockless PPM Optical Communications” filed Feb. 24, 2009, application Ser. No. 12/391,798 by S. I. Ionov, which is a divisional of application Ser. No. 10/973,696 filed Oct. 25, 2004, now U.S. Pat. No. 7,515,835. The disclosure of each of the previously filed applications identified above is incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to clockless pulse position modulation (PPM) communication technology and particularly to optical clockless pulse position modulation communication technology. 
     BACKGROUND INFORMATION 
     Many satellite and terrestrial optical communication systems require transmission of analog signals. A straightforward way to address this need is to modulate the amplitude (AM) of an optical carrier. This approach, however, suffers from a poor Signal to Noise Ratio (SNR) at a distant receiver. It is well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted waveform, may improve the SNR over that achieved with AM. Pulse position modulation (PPM) is one of such techniques. In PPM, a shift in the pulse position represents a sample of the transmitted waveform, as shown in  FIG. 1 . It can be shown that for a given power, SNR PPM ∝SNR AM (t p /t) 2 , where t p  is the spacing between un-modulated pulses and t is the pulse duration, respectively. See H. S. Black, “Modulation Theory”, D. Van Nostrand Co. (1953). 
     The implementations of PPM for optical communications require new techniques for generating trains of optical pulses whose positions are shifted in proportion to the amplitude of a transmitted waveform. A bandwidth of Δf=1-10 GHz and higher is of interest for future communications. Since pulse repetition frequencies (PRF) of 1/t p &gt;2Δf are required for sampling a signal of bandwidth Δf, GHz trains of picosecond (ps) pulses may be required for realizing the advantages of PPM. For example, a free space optical link designed to transmit waveforms with Δf=10 GHz bandwidth requires sampling rates of PRF=1/t p ≧2Δf=20 GHz. By employing 1-2 ps-long optical pulses, a 30 dB gain is realized over an AM system with equal optical power. 
     Optical PPM offers large SNR improvements in power-starved optical links. This technology, however, requires development of new types of optical PPM receivers. The simplest and most basic PPM decoder, which is based on an integrating circuit, suffers from poor performance at low frequencies. See H. S. Black, “Modulation Theory”, noted above. Though newly invented PPM decoders overcome the low-frequency shortcomings of the simplest decoder, these improvements come at the cost of higher complexity. See, for example, S. I. Ionov, “Detection of optical analog PPM streams based on coherent optical correlation”, U.S. Pat. No. 6,462,860; S. I. Ionov, “A practical design of a PPM receiver with optical top hat pulse generator controlled by solitons”, U.S. patent application Ser. No. 10/341,689 filed Jan. 13, 2003 which is based upon U.S. Ser. No. 60/383,343 filed May 23, 2002; I. Ionov “Method and Apparatus for PPM Demodulation Using A Semiconductor Optical Amplifier”, U.S. Pat. No. 7,330,304 filed Nov. 3, 2003; I. Ionov, “Method and apparatus for optical top-hat pulse generation”, U.S. patent application Ser. No. 10/735,071 filed Dec. 12, 2003 which is based upon U.S. Ser. No. 60/488,540 filed Jul. 18, 2003; and S. I. Ionov, “Interferometric PPM Demodulators based on Semiconductor Optical Amplifiers”, U.S. Pat. No. 7,149,029 filed Jan. 11, 2005. 
     In the past, ElectroOptic delay generators shift the temporal position of an optical pulse in proportion to the applied voltage. Such a PPM modulator provides seamless means for a PPM encoding scheme wherein a temporal displacement of an optical pulse from its unmodulated position represents a sample of the transmitted waveform. Such an ElectroOptic delay generator has been described in: Method and apparatus for Electro-optic Delay Generation of Optical Signals U.S. Pat. No. 6,466,703 filed Apr. 7, 2000; 
     More recently, U.S. Pat. No. 7,330,304 demonstrated a PPM decoder based on the gain dynamics of a semiconductor optical amplifier. When fed by two optical streams—a PPM signal and clock, the decoder produces an electric output that is proportional to the delays between the corresponding signal and clock pulses and changes on the pulse-by-pulse scale. 
     A PPM communication system based on such an encoder and decoder requires optical clock pulses, which must be either transmitted alongside with the PPM signal or regenerated at the receiver side. This requirement puts an unnecessary burden on the communication system, which requirement is eliminated according to the present disclosure. 
     SUMMARY OF THE INVENTION 
     Referring to  FIG. 1 , a PPM Transmitter  100  includes an Optical Clock Generator  10  for generating equally-spaced optical pulses with a sampling period T; an Encoder  50  for transforming an incoming waveform U(t) into a linear combination V(t) of U(t) and a delayed output V(t−kT) according to a rule V(t)=U(t)+aV(t−kT), where k is a positive integer, V(t) is a voltage generated by the Encoder  50  and a is a coefficient. An Optical Delay Generator  20  delays optical pulses generated by the Optical Clock Generator  10  in proportion to the voltage V(t), such that Δt n =bV(t), where b is another coefficient and where Δt is the amount of delay imposed by the Optical Delay Generator  20 . The PPM Transmitter  100  generates a stream of optical pulses Q(t) from a clock signal internal to the Transmitter  100 . However, the PPM Transmitter  100  functions with a PPM Receiver  200  for communicating data without the need to separately transmit a clock signal to the Receiver  200  or otherwise provide a separate clock signal at the Receiver  200 . 
     The PPM Receiver  200  drives a PPM Decoder  40  with a received stream of optical pulses Q(t) and a version of Q(t) delayed by ckT in a Scaled Delay Module  30 . The output of the PPM Decoder  40  is a voltage proportional to the input signal U(t). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows the optical encoding/decoding scheme and required apparatus. 
         FIGS. 2A through 2E  illustrate the operation of the invention. 
         FIG. 3  shows an embodiment of one PPM demodulator suitable for the system described in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows the optical encoding/decoding scheme and required apparatus. 
     The scheme uses a well-known PPM format, wherein the input analog signal is encoded by a PPM Transmitter  100  in the temporal delay of samples of a periodic optical clock. The Receiver  200  decodes the received series of optical pulses. 
     The PPM Transmitter  100  consists of three major parts:
         1. An Optical Clock Generator  10  that generates equally-spaced optical pulses with the sampling period T.   2. An Encoder  50  that transforms the incoming waveform U in (t) into a linear combination V(t) of U in (t) and a delayed output V(t−kT) according to the rule
 
 V ( t )= U   in ( t )+ aV ( t−kT ),  [Eq. 1]
 
where k is a positive integer, T is the sampling (i.e., optical clock) period and a is a coefficient.
   3. A Optical Delay Generator  20  that delays the optical pulses supplied by the Optical Clock Generator  10  in proportion to the applied voltage V(t), i.e.,
 
Δ t   n   =bV ( t )= bU   in ( t )+ abV ( t−kT )),  [Eq. 2]
 
where b is another coefficient.
       

     The PPM Receiver  200  consists of:
         1. An Optical Splitter Circuit that splits the incoming signal into two channels (signal and clock) and delays the latter by ck sampling periods in a Scaled Delay  30  module where c is another scale factor applied to Δt n-k ;   2. A PPM Decoder  40  that converts the delay between the two channels into an electrical waveform
 
 U   out ( t )=Δ t   n   −cAt   n-k   =bU   in ( t )+ abV ( t−kT ))− cbV ( t−kT )
 
 U   out ( t )= bU   in ( t );  c=a,   [Eq. 3]
 
i.e., recovers the original waveform U in (t).
       

     The Transmitter  100  in  FIG. 1  comprises the Encoder  50 , a Optical Clock Generator  10  and a Optical Delay Generator  20 . 
     The Optical Clock Generator  10  in the PPM Transmitter  100  may be any commercially available source, e.g., an optical clock produced by Pritel Inc. or an ERGO pulse generating laser produced by GigaTerra. 
     An Electro-Optic delay generator has been described in: Method and apparatus for Electro-Optic Delay Generation of Optical Signals U.S. Pat. No. 6,466,703 filed Apr. 7, 2000; and Optical Pulse Delay Method and Apparatus U.S. patent application Ser. No. 12/009,569 filed Jan. 17, 2008 both of which are incorporated by reference in this application as though fully set forth. 
     In an embodiment of the Encoder circuit  50 , the incoming signal U in (t) is directed to a device shown in block diagram form in  FIG. 1 . One embodiment for the Encoder circuit  50  may use a Digital Signal Processor. In an alternative embodiment, the Encoder circuit  50  may comprise a programmable digital computer. Alternatively, the Encoder circuit  50  may comprise discrete components as shown in System, Method and Apparatus for Clockless PPM Optical Communication by Ionov, U.S. Pat. No. 7,515,835 filed Oct. 25, 2004 and incorporated by reference. The round trip time of the signal in the Encoder&#39;s loop is chosen equal to kT, where T is the sampling (i.e., optical clock) period and k is an integer. The gain a must be less than one for a bounded V(t). 
       FIGS. 2A through 2E  explain the operation of this invention.  FIG. 2A  shows the output pulses  11  of an optical clock  10  with a period T. These pulses  11  are delayed in Optical Delay Generator  20  according to equation 1. The delayed clock pulses  12  are shown in  FIG. 2B  as the solid arrows and the delay Δt is relative to the clock pulse  11  shown as a dashed arrow for reference only. The set of pulses  12  in  FIG. 2B  are transmitted to the Receiver  200  and are shown in  FIG. 2C  as received signal  13 .  FIG. 2C  is essentially a copy of the pulses in  FIG. 2B . The received signal  13  of  FIG. 2C  is split and delayed in Optical Splitter Circuit  45  by kT periods and a bit less as set by the value c, as shown in  FIG. 2D  for the received signal delayed  14 . In  FIG. 2D  k is two and c is less than one. The two signals, Received Signal  13  and Received Signal Delayed  14 , go into a PPM Decoder  40  where the Received Signal  13  is connected to the signal input of the PPM Decoder  40  and the Received Signal Delayed  14  is connected to the clock input of the PPM Decoder  40 . The operation of the PPM Decoder  40  is illustrated in the Decoder Output  15  of  FIG. 2E . The decoder output is an electrical current or voltage that is proportional to the time between the arrival of the signal input and the arrival of the clock input. One consequence of this design is that the initial modulation Δt must be less than T/2 to preserve the order of comparison between the signal input and the clock input. 
     The PPM Receiver  200  is shown in  FIG. 1  and comprises an Optical Splitter Circuit  45  and a PPM Decoder  40 . The Optical Splitter Circuit  45  comprises an optical splitter and a Scaled Delay  30 . 
     The Scaled Delay  30  in the PPM Receiver  200  can be assembled with standard Commercial Off the Shelf (COTS) components such as optical fiber or waveguide splitters and optical delay lines (adjustable or fixed). 
       FIG. 3  shows a basic PPM Decoder  40  based on a Semiconductor Optical Amplifier  135 . The clock input and signal input are combined in the Combiner  131  and then directed to the Semiconductor Optical Amplifier (SOA)  135 . The Semiconductor Optical Amplifier (SOA)  135  acts as a pulse-position to pulse-intensity converter as described in U.S. Pat. No. 7,330,304, filed Nov. 3, 2003 and incorporated by reference. One way for the SOA  135  to do this conversion is for each clock pulse to deplete the carrier-population and thus the gain of the SOA  135 . The carrier population then recovers at a known rate after that clock pulse has ended because a continuous flow of carriers are supplied to the SOA  135  by means of the applied bias current. The gain experienced by the following position-modulated signal pulse provides a measure of the amount of gain recovery. Thus, the intensity of the amplified signal pulse is related to the time delay between that signal pulse and the preceding clock pulse. For the intensity of the amplified signal pulse to be proportional to the delay, the signal pulse input has to be stable (or at least its drift known) and have a well known amplitude. The output of the SOA  135  is in turn directed to a Photodetector  142  and Amplifier  140 . Since the output of the SOA  135  has an optical amplitude proportional to the time delay between pulses and the Photodetector  142  converts the input optical signal into an electrical signal, the output of the Amplifier  144  is an electrical signal proportional to the time delay between pulses, ie, to the input analog signal. 
     In an alternative arrangement, the PPM Decoder  40  may further comprise a fixed output Repeater  133  that outputs a copy pulse at a known, stable amplitude when the input optical signal arrives such that the gain applied by the SOA can be calculated from the amplitude of the output of the SOA  135 . Since the output of the SOA  135  depends solely on the gain and the gain depends only on the difference in arrival times of a clock pulse and a signal pulse, driving the Photodetector  142  with the output of the SOA  135  will produce an output proportional to the scaled value of the input U in (t). 
     From the foregoing description, it will be apparent to those skilled in the art that the present invention has a number of advantages, some of which have been described above, 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 disclosed apparatus described herein without departing from the teachings described herein. As such, the invention is not to be limited to the described embodiments except as required by the appended claims.