Abstract:
An optical demodulator for use as a pulse position demodulator. The demodulator has one or more semiconductor optical amplifiers coupled to receive an optical signal to be demodulated at first end thereof and for receiving optical control pulses at a second end therefore, the optical signal to be demodulated and the optical control pulses counter-propagating in said one or more semiconductor optical amplifiers in order to determine a delay or phase shift there between; and a detector coupled to the one or more semiconductor optical amplifiers for recovering, as an electrical output signal, the delay or phase shift in the optical signal.

Description:
TECHNICAL FIELD 
   This disclosure relates to pulse position modulation demodulators. 
   BACKGROUND INFORMATION 
   Many satellite and terrestrial optical communication systems require transmission of analog optical 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). 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 /τ) 2 , where t p  is the spacing between un-modulated pulses and τ—the pulse duration, respectively. See H. S. Black, Modulation Theory, D. Van Nostrand (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. Typically a bandwidth of Δf=1–10 GHz and higher is of interest for inter-satellite communications. Since pulse repetition frequencies (PRF) of 1/t p &gt;2 Δf are required for sampling a signal of bandwidth Δf, GHz trains of picosecond (ps) pulses are required for realizing the advantages of PPM. For example, an optical inter-satellite 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. One optical PPM receiver based on top hat pulse generation (THPG) has been proposed. See S. I. Ionov, “Detection of optical analog PPM streams based on coherent optical correlation”, U.S. Pat. No. 6,462,860. See also S. I. Ionov, “Optical top hat pulse generator”, US Published Patent Application No. 2003/0219195, and “PPM demodulator based on PM NOLM with improved conversion efficiency”, U.S. patent application Ser. No. 10/735,071 filed Dec. 12, 2003 which is based upon 60/488,540 filed Jul. 18, 2003. 
   The present disclosure describes a significantly simpler approach to PPM decoding. Because of its simplicity, the proposed device is expected to be more robust. The technology alluded to above utilizes fiber-based designs. The major drawback of the fiber-based design is in its complexity. The previous receivers were based on non-linear optical loop mirrors (NOLM) that require careful balancing and adjustments. They also need a number of EDFAs and optical filters with flat-dispersion. 
   The reader is also directed to U.S. patent application Ser. No. 10/701,378 filed Nov. 3, 2003 which relates to a PPM demodulator based on the gain dynamics of a semiconductor optical amplifier (SOA). 
   This disclosure relates to a different implementation of a PPM demodulator based on interferometric schemes involving SOAs. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a schematic diagram of an interferometric all-optical PPM demodulator based on UNI (Ultrafast Non-linear Interferometer) architecture; 
       FIG. 2  is a timing diagram of clock (i.e., control) and signal pulses in the SOA of the UNI and MZI (Mach-Zehnder Interferometer) embodiments of a demodulator; 
       FIG. 3  is a schematic diagram of an interferometric all-optical PPM demodulator based on MZI architecture; 
       FIG. 4  is a schematic diagram of an interferometric all-optical PPM demodulator based on SLALOM (Semiconductor Laser Amplifier in a Loop Mirror) architecture; and 
       FIG. 5  is a timing diagram of clock (i.e., control) and signal pulses in SOA of the SLALOM demodulator. 
   

   DETAILED DESCRIPTION 
   Schematic diagrams of interferometric PPM demodulators based on SOA are shown in  FIGS. 1 ,  3  and  4 . The pictured architectures are somewhat similar to ultra-fast all-optical switches based on the UNI (Ultrafast Non-linear Interferometer), the MZI (Mach-Zehnder Interferometer), and the SLALOM (Semiconductor Laser Amplifier in a Loop Mirror), respectively. See C. Schubert, J. Berger, S. Diez, H. J. Ehrke, R. Ludwig, U. Feiste, C. Schmidt, H. G. Weber, G. Topchyski, S. Randel, and K. Petermann, “Comparison of Interferometric All-Optical Switches for Demultiplexing Applications in High-Speed OTDM Systems”,  J. Lightwave Tech ., vol. 20 (4), 2002, pp. 618–624. The difference between the architectures of the PPM demodulators shown in  FIGS. 1 and 3  and the corresponding ultra-fast all-optical switches is that the control pulse counter-propagates with respect to one of the interfering signal pulses. Also, operating conditions have been selected for achieving linear PPM (ramp-like) response. This contrasts with typical switching applications that seek to achieve a rectangular switching window with vertical edges and also attempt to maximize the contrast between the ON and OFF states. 
   Preferably, the SOAs in all these demodulator embodiments operate in the gain-transparent mode. See S. Diez, R. Ludwig, and H. G. Weber, “Gain-transparent SOA switch for high-bitrate OTDM add/drop multiplexing,”  IEEE Photon. Technol. Lett ., vol. 11 (1), 1999, pp. 60–62. In this mode of operation, the wavelength of control (or clock) pulses is in the spectral gain region of SOA (e.g., 1.3 mm), whereas the wavelength of the signal pulses is longer, corresponding to photon energy below the band-gap of the semiconductor material. Alternatively, both wavelengths may be in the gain region. In this case, the SOA should be current-biased to near-transparent conditions (i.e. with no significant gain or loss). 
   In the disclosed UNI demodulator  100 , the incoming signal pulses  102  are polarized at 45 degrees with respect to a first polarizing beam splitter (PBS)  106 , so that they are split into two orthogonally-polarized beams of equal power that pass along optical legs F and S from the output of splitter  106  towards a second PBS  116 . The optical lengths of legs F and S are preferably identical except for an intentional delay (Δτ) inserted by element  108  (which may be a thickness of glass) in the relatively slower leg S. The delay (Δτ) between the orthogonally-polarized pulses at the second PBS  116  is set close to the clock period. The polarization controllers  112 ,  114  in each leg F, S ensure that both beams are combined by the second PBS  116  and launched into the SOA loop  122  in the clockwise direction in the depicted embodiment. As a result, both signal beams enter the SOA from one end thereof and move in a common direction through the SOA, while the control or timing pulse on optical path  118  enters the SOA from its opposite end. The polarization controller  124  in the SOA loop  122  assures that the faster signal component, which arrived via the faster leg F, returns to beam splitter  106  via the slower leg S and the slower signal component, which arrived in the SOA loop  122  via the slower leg S, returns via the faster leg F. Thus, the delay between the two polarization components is cancelled at the output of the interferometer. 
   In an alternative embodiment, the first polarizing beam splitter  106  and the unequal arms F, S before the second beam splitter  116  may be replaced by a single section of a PM fiber oriented at 45 degrees with respect to the polarization of the incoming signal pulses. See C. Schubert, S. Diez, J. Berger, R. Ludwig, U. Feiste, H. G. Weber, G. Topchyski, K. Petermann, and V. Krajinovic, “160 Gbit/s all-opticaldemultiplexing using a gain-transparent Ultrafast non-linear interferometer (gt-UNI),”  IEEE Photon. Technol. Lett ., vol. 13, 2001, pp. 475–477. 
   The operation of the UNI PPM demodulator  100  is better understood by considering the timing diagram of signal and clock optical pulses in the SOA. See  FIG. 2 . In  FIG. 2  four different possible timing situations are depicted (top to bottom in the figure). In each situation the SOA is shown with two signal pulses, both traveling in the same direction through the SOA, with a clock or control signal (shown in black) propagating in an opposite direction through the SOA. The right most signal pulse or component is depicted upright and it is the signal pulse that arrives earlier since it traveled via the faster leg F. The left most signal pulse depicted for each timing situation arrives later since it traveled via the slower leg S. The control or timing pulse is depicted in black to differentiate it from the two signal pulses. In the four timing situations depicted by this figure the timing pulse goes from being relatively close to the slow leg signal pulse (S) to being relatively close to the fast leg signal pulse (F) to being very close to the slow leg signal pulse (S) when viewing the four situations from top to bottom in  FIG. 2 . Of course, those skilled in the art will appreciate that these four timing situations are merely exemplary of the possible timing situations that will occur in these optical circuits. 
   In  FIG. 2 , the delayed signals are depicted in a bent fashion which represents the fact that the slow component of the signal is orthogonal to that of the fast one in a UNI PPM demodulator. 
   The timing of clock versus signal pulses in the SOA is set in such a way that minimum and maximum delays between the slow signal pulse or component and the corresponding clock or control pulse (at the moment when the clock or control pulse enters the SOA  126 ) represent, respectively, the minimal and maximal values of the transmitted waveform. In this arrangement, the fast component or pulse (F) of the signal always exits the SOA  126  before the corresponding clock pulse enters into it. The crosshatched region in the SOA has a length that is equal to one half the distance between the slow leg component or pulse and the control pulse at the moment when the control pulse enters the SOA. The total length of the SOA  126  is preferably chosen to be sufficiently long that at a maximum delay between the signal and clock pulses, the latter collides with the slow component or pulse of the signal before exiting the SOA  126 . For example, for a 10 G/s pulse rate, L≧Tc/2n=4.5 mm, where T=100 ps is the pulse period and n=3.3 is the index of refraction. 
   The clock pulse changes the carrier density in the SOA (in the crosshatched region), which affects the phase shift of the slow signal component with respect to the fast one (the cross hatched lines in the SOA depict the length along the SOA that the control or clock pulse has traveled when it encounters (collides with) the slow signal component or pulse and thus depicts where the carrier density of the SOA has been changed by the clock or control pulse when the collision occurs). The length of the crosshatched region is proportional to the relative phase shift. As a result, when the two polarized signal components recombine at the output  107  of beam splitter  106 , they do so into a different polarization state (which is determined by their relative phase shift), and a portion of the signal pulse will pass through the output polarizer  130 . As seen from  FIG. 2 , the length of the SOA, which is affected by the clock pulse and sampled by the slow signal component or pulse (S), is proportional to the delay between the signal and the clock. Therefore, the corresponding phase shift between the two polarized components of the signal is also proportional to the delay, assuming that the clock pulse is not attenuated significantly in the SOA. If the latter condition is not satisfied, the SOA should be current-biased for near-transparency at the clock wavelength as previously mentioned. 
   The optical field amplitude after the polarizer is E=E o (cos(ωt+φ)cos(α)+cos(ωt)sin(α)), where φ is the relative phase shift acquired by the slow signal component in the SOA  126  and α is the tilt of the output polarizer  130  with respect to the principal polarization direction of the signal. The polarization controller  128  and the polarizer  130  at the output of the interferometer are adjusted to have zero signal transmission at minimal delay between the signal and the clock pulses. In this case, α=−π/4 and E/E o ∝sin(φ/2)≈φ/2. The electric amplitude of the signal is converted to electrical current in a homodyne detector  132 , which beats the output signal against a phase-locked local oscillator  136  on a photodiode  134 . The local oscillator  136  is preferably implemented by a DFB laser diode. Since the output current of the homodyne receiver  132  is proportional to the amplitude of the output signal (in the linear regime), it is also proportional to the phase shift φ, which is, in turn, proportional to the delay between the signal and clock optical pulses. Therefore, the electrical output of the UNI demodulator  100  depicted in  FIG. 1  is proportional to the delay between the signal and clock pulses. In an MZI PPM demodulator  300  as shown in  FIG. 3 , the signal is preferably split by a 3 dB coupler  301  into two components that travel in two separate legs  304 ,  306  (each leg has the same optical length in this embodiment, but for the existence of the intentional optical delay  312 ,  314 ) before they are recombined by a second 3 dB coupler  324 . A clock pulse  303  induces two non-linear index changes in the two legs  304 ,  306  of the interferometer that depend on the relative delays between the counter-propagating control and signal pulses in each leg. The control pulse in one leg  304  of the interferometer is delayed (by optical delay  305 ) by about one clock period with respect to that in the other leg  306 , and the length of each SOA  308 ,  310  is traveled by optical pulses in one half of the clock period. The clock (i.e., control) pulse is aligned similarly to the alignment in the UNI demodulator. In one leg, the control pulse collides with the corresponding signal pulse at the entrance of the SOA if maximum PPM delay is present. In this configuration, the phase shift experienced by the signal pulse in the first leg  304  is proportional to the delay between the signal and the clock. This operation is similar to that in the UNI demodulator (see again the timing diagram of  FIG. 2 ) although now two signal pulses in the two legs have the same polarization. Also, the signal pulses may be aligned in time whereas control pulses are split. However, the relative pulse positions of control and signal pulses are similar to that of the previously described UNI PPM demodulator embodiment. The control or timing pulse in the other leg  306  always enters the SOA after the corresponding signal pulse has already exited, thereby imprinting no phase shift on the signal pulse. Therefore, the differential phase shift between the two signal components is proportional to the PPM delay between the signal and control pulses. 
   Alternatively, the signal pulses in one leg  304  or  306  could be delayed instead of delaying the corresponding control pulses as described above. 
   The MZI demodulator is balanced in the absence of clock pulses either for a 50/50 split at the output, or for zero output from the detector leg (see  FIG. 3 ). In the first case (the 50/50 split), a simple photodiode  326  provides linear conversion of the PPM phase shift into the output current. In the second case (the zero output), a homodyne detector is utilized for linear conversion, similar to the UNI embodiment of  FIG. 1 . For illustrative purposes, only the first case (the 50/50 split embodiment with the simple photodiode) is shown in  FIG. 3  since an exemplary homodyne detector is already shown in connection with the embodiment of  FIG. 1 . 
   To balance the interferometer, an optical delay (OD)  312 ,  314 , an attenuator (ATN)  316 ,  318  and a polarization controller (PC)  320 ,  322  are preferably used in each leg  304 ,  306 . The optical delays  312 ,  314  are optional—they may be needed if bulk optics and fibers are used and/or if there exists a temperature imbalance between the two legs to assure correct timewise pulse alignment. In addition, active phase stabilization must be implemented for proper operation so it may be necessary to also include a phase shifter in at least one of the legs  304 ,  306 . One of the advantages of the SLALOM and UNI embodiments is that timewise pulse alignment is achieved inherently by the configurations of the light paths employed so that optical delay devices in each leg, comparable to ODs  312 ,  314 , and/or phase shifters are ordinarily not needed in those embodiments. 
     FIG. 4  depicts the SLALOM demodulator embodiment  400 . In this embodiment incoming signal pulses  402  are split by an adjustable 3 dB input-output coupler  405  into two components, one propagating in the clockwise direction in the loop  404  depicted in the figure (so that the signal initially enters leg  404 ) and the other in the counterclockwise direction in  FIG. 4  (so that the counterclockwise signal initially enters leg  406 ). In this embodiment the SOA  426  is placed timewise asymmetrically in loop  402  due to the presence of an optical delay element  408  in leg  406 , so that the counter-propagating pulses reach the SOA  426  with an intentional delay equal to the clock period (Δτ). Otherwise, without the intentional delay, both legs have the same amount of delay associated therewith. Without a control pulse, the loop is balanced for 100% reflection by a PC  420  and the adjustable 3 dB coupler  406 . The length of the SOA is chosen so as to be crossed by the counter-propagating optical pulses in one half of the clock period (0.5·Δτ). 
   A timing diagram explaining the operation of the SLALOM demodulator is shown in  FIG. 5 . The clock pulse always arrives at the SOA  426  prior to the corresponding co-propagating signal pulse. As a result, the co-propagating signal pulse in the clockwise (CW) direction in the depicted loop always acquires maximum phase shift. (This is opposite to the operation of the UNI or MZI demodulator embodiments shown in  FIG. 2 , where the fast component never acquires a phase shift in the disclosed embodiments.) On the other hand, the counter-propagating signal pulse—in the counterclockwise (CCW) direction in the depicted loop—acquires a phase shift that is proportional to the PPM delay between the signal and clock pulses (similar to the operation of the disclosed UNI and MZI demodulators). Therefore, the differential phase shift acquired by the two signal components is proportional to φ∝1−t PPM , i.e. proportional to the negative of the transmitted waveform. 
   The output optical field of the SLALOM demodulator  400  is proportional to sin(φ)≈φ. The electric amplitude of the signal is converted to electrical current in a homodyne detector  428 , which beats the output signal against a phase-locked local oscillator  432  on a photodiode  430 . Since the output current of the homodyne receiver is proportional to the amplitude of the output signal (in the linear regime), it is also proportional to the phase shift φ, which is, in turn, proportional to the negative of the delay between the signal and clock optical pulses. Therefore, the electrical output of the SLALOM PPM demodulator  400  is proportional to the negative of the delay between the signal and clock pulses. The optical paths in the previously described embodiments are assumed to be in free space. However, after accounting for path lengths, the paths could be in other media such as optical fibers. If optical fibers are utilized and the delays in the optical fibers are appropriately adjusted, then the optical delay devices, such as element  108  in the embodiment of  FIG. 1 , may be omitted. 
   Having described this technology in connection with certain embodiments thereof, modification will now doubtlessly suggest itself to those skilled in the art. As such, the present invention is not to be limited to the disclosed embodiments except as specifically required by the appended claims.