Abstract:
The invention is a Demodulator for an Optical Analog Pulse Position Modulated signal suitable for inclusion in receivers for Free Space Optical communication systems. In one embodiment the Demodulator may use the pulse position modulated optical information signal and the clock signal with different wavelengths. By proper biasing of a Semiconductor Optical Amplifier and selection of wavelengths for the information signal and the clock signal, the performance of the Demodulator is made insensitive to noise in the received signals.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No.-05-C-0044 Optical Analog Pulse Position Modulation System. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to the structure and fabrication of optical analog demodulators for pulse position modulated signals. 
     BACKGROUND OF THE INVENTION 
     Most Free Space Optical (FSO) links make use of intensity modulation or amplitude modulation of the laser light. However, the signal-to-noise ratio (SNR) of an intensity modulated optical link is limited by the shot noise in the number of photons collected at the receiver. Pulse-position modulation (PPM) can provide a means to achieve a link in which the signal-to-noise ratio can exceed the limits of shot noise. Thus, the transmitted power can be reduced, the distance of the link can be increased, the sizes of the transmitter and receiver apertures can be reduced, and the link performance can be more tolerant to the variable attenuation arising from atmospheric turbulence. Optical PPM links have been developed for transmission of digital information in long-distance or high-loss non-waveguided optical links such as deep-space links. These optical links have been demonstrated with discrete-level modulation using pulse-position modulation transmitters and photon-counting receivers that have as many as 256 levels of position modulation or 256 time slots [S. S. Muhammad, P. Brandl, E. Leitgeb, O. Koudelka and I. Jelovcan, “VHDL based FPGA Implementation of 256-ary PPM for free space optical links,” Proc. 9 th  Intl. Conf. Transparent Optical Networks, 2007, pp. 174-177]. Decoding the received signal typically involves detection and discernment of the presence of optical energy in the various time slots. The SNR demands for a receiver for continuous-level, analog optical PPM systems are much more stringent. 
     An alternative modulation scheme for an optical link uses optical analog pulse-position modulation (OAPPM) that transmit pairs of short optical pulses through free space, with the relative time-positions of, or the time-delay between, those two pulses in a pair being proportional to the time-sampled analog value of an input RF signal. As noted before, conventional optical receivers of optical links that carry intensity modulated light with direct detection of that light by a photo-detector have an output SNR that is no greater than the intensity to noise ratio of their input light. A Demodulator in an OAPPM system converts the relative time-positions of pairs of input optical pulses into an analog output voltage or current. One pulse of the pair, generally the signal pulse, has its time-position modulated and the other pulse of the pair, generally the clock reference pulse, has a fixed time position. In those cases when it is desired for the FSO link to have a lower transmit power or to cover a longer distance, the optical intensity of the received pulses can be low. Hence, it is important that the Demodulator be tolerant of intensity noise in the input pulses that are received via the free-space optical (FSO) link. Such low-intensity optical pulses may have substantial shot noise, or noise in the number of photons that comprise a pulse. An optical amplifier at the front-end of the OAPPM receiver can increase the optical power of those pulses in a pair, but that amplification process adds even more intensity noise, which could further degrade the performance of optical links that carry analog information. 
     A prior art OAPPM Demodulator is described in S. I. Ionov, “Method and apparatus for PPM demodulation using a semiconductor optical amplifier,” U.S. Pat. Nos. 7,605,974 and 7,330,304B2. The OAPPM Demodulator and functionality in the U.S. Pat. No. 7,330,304 is reproduced in  FIGS. 1 ,  2 A and  2 B of this application. This OAPPM Demodulator accepts pairs of input optical signal and clock pulses  110  and  120 , respectively, for which the variable position of one pulse in a pair (the signal pulse  110 ) is modulated relative to the fixed position of the other pulse in that pair (the clock pulse  120 ). The OAPPM Demodulator  100  needs to be able to distinguish between, and separate, the signal and clock pulses  110  and  120  of a pair. One of the prior ways to enable this separation is for the two pulses to have different optical wavelengths [see the &#39;304 patent]. The primary component in the OAPPM Demodulator  100  is a semiconductor optical amplifier (SOA)  135  that acts as a pulse-position to pulse-intensity converter [see the &#39;304 patent]. As shown in  FIGS. 2A and 2B , one way for the SOA  135  to do this conversion is for each clock pulse  120  to deplete the carrier-population and thus the gain of the SOA  135 . The carrier population then recovers gradually after that clock pulse  120  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  110  provides a measure of the amount of gain recovery. Thus, the intensity of the amplified signal pulse  130  is related to the time delay between that signal pulse  110  and the preceding clock pulse  120 . Returning to  FIG. 1 , the amplified clock pulses on the output of the SOA (if any) and the amplified signal pulses  130  that are output from the SOA are separated from each other by an optical filter or demultiplexer  131  in  FIG. 1 . If necessary, an optical bandpass filter  137  may be used to further block the clock pulse  120 . These intensity modulated signal pulses  130  are then coupled to a photodetector  140  that has a low-pass frequency response, for producing the output RF waveform while removing the aliasing spurs that are caused by the discrete-time sampling process associated with analog pulse-position modulation. 
     Continuing with  FIG. 1 , the prior OAPPM Demodulator  100  makes use of an SOA  135  as a pulse-position to pulse-intensity converter, as does the SOA of the present disclosure. However, the prior OAPPM Demodulator  100  assumes that there is a near-perfect optical limiter before the SOA  135  that removes any intensity noise in the signal and clock pulses coupled into the SOA  135 . Such a near-perfect optical limiter has not been developed. Moreover, the prior OAPPM Demodulator  100  in  FIG. 1  only requires that the wavelength of the signal pulse  110  and of the clock pulse  120  be different so that those two pulses can be separated. There is no requirement of any relationship between the wavelengths of the signal pulse  110  and clock pulses  120  to the gain characteristics of the SOA  135 . There also is no requirement on the temporal shape of the signal pulse  110  and clock pulse  120  coupled into the SOA  135 . The prior OAPPM Demodulator  100  could have a balanced pair of photodiodes comprise the photodetector  140  instead of a single photodiode. The prior art photodetector  140  with balanced photodiodes have the clock pulses  120  and the signal pulses  110  coupled, respectively, into their two optical inputs. 
     In the prior OAPPM Demodulator  100 , the optical pulse that samples the recovering gain of the SOA  135  (the signal pulse  110 ) would have an intensity that depends both on the intensity of the input sampling signal pulse  110  as well as on the gain of the SOA  135  at the time of that signal pulse  110 . However, because a near-perfect optical limiter is not available, the intensity of the input sampling signal pulses  110  would have substantial variations. Also, since the wavelength of the gain depleting clock pulses  120  could be longer than the wavelength of the gain sampling signal pulses  110 , it actually would not be possible for those gain depleting clock pulses  120  to deplete the gain of the SOA  135  to a fixed point. Instead the depleted gain level of the SOA  135  would depend on the varying intensity of the gain-depleting clock pulse  120 . As a result, the starting point for the gain recovery would fluctuate because of the noisy intensity of the gain-depleting clock pulse  120 . This would transfer the intensity noise of the gain-depleting clock pulse  120  onto the modulated intensity of the gain-sampling signal pulse  110 . But to achieve a desired high SNR performance, it is important the demodulation process be independent of fluctuations in the intensity of the gain-depleting clock pulses  120 . 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, the Demodulator has features that make it tolerant of the intensity noise of both its input signal pulses and its input clock-reference pulses. In another aspect of the invention the Demodulator also has features that may make it tolerant of absolute timing noise or jitter in those signal pulses and clock pulses. The Demodulator contains and uses a semiconductor optical amplifier (SOA) as a pulse-position to pulse-intensity converter. The Demodulator is part of an OAPPM receiver that also contains an optical amplifier that amplifies the intensity of the pairs of signal and clock-reference pulses that it receives. These amplified signal pulses are coupled to the SOA and they act to briefly deplete the gain of the SOA, by stimulating emission of light by the SOA. Contrary to the prior art, the signal pulses are used to reset the SOA gain while the clock pulses are used to sample the gain. 
     The Demodulator may also contain an optical injection-locked short-pulse laser that generates sampling pulses, which may be called the clock pulses. These clock pulses have low intensity-noise and are synchronized to the received clock-reference pulses. In effect, the optical injection-locked short-pulse laser normalizes the received clock-reference pulses. The clock pulses from the short-pulse laser are coupled to the SOA, and act to sample the gain of the SOA because the amplification of the clock pulse depends on the gain of the SOA at the time the clock pulse is passing through the SOA. Thus, the intensity of these amplified clock pulses is modulated according to the sampled gain of the SOA. The sampled gain is dependent on the time between the clock pulse relative to a prior gain depleting pulse. 
     The level of the bias current supplied to the SOA, the wavelengths of both the clock pulses and the signal pulses and the gain characteristics of the SOA are selected to achieve a resetting of the gain of the SOA by each received signal pulse. This resetting is accomplished by having the SOA reach the transparency condition when the signal pulse is coupled into the SOA. The Demodulator may also contain another optical amplifier for increasing the intensity of the signal pulses to ensure the SOA achieves transparency. 
     The Demodulator may contain an optical pulse replicator of the received signal pulses that converts the received signal pulses into a wide effective pulse with steep rising and falling edges of a single signal pulse. A wide effective pulse is realized as a plurality of multiple narrow signal pulses. An alternative embodiment may comprise a variation of the optical pulse replicator that has an optical wavelength-selecting splitter and an optical wavelength-selective combiner. 
     The Demodulator may also contain a photodetector that converts the series of amplified and intensity-modulated clock pulses to an output electrical voltage or current. Another embodiment of the invention contains a balanced photodetector pair instead of a single photodetector. The amplified clock pulses, obtained from the output of the SOA, are coupled into one of the two optical inputs of this balanced photodetector pair and the original clock pulses produced by the short-pulse laser are coupled into the other input of this balanced photodetector pair. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, together with the specification, illustrate exemplary embodiments of the invention, and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates the prior art demodulator. 
         FIGS. 2A and 2B  show the operation of the prior art demodulator shown in  FIG. 1 . 
         FIG. 3  shows one embodiment of an optical analog pulse position modulation Demodulator of the invention. 
         FIG. 4A  illustrates an alternative embodiment of an optical analog pulse position modulation Demodulator. 
         FIG. 4B  illustrates another alternative embodiment of an optical analog pulse position modulation Demodulator. 
         FIG. 5  illustrates an optical pulse replicator. 
         FIG. 6  illustrates a wavelength splitter combined with an optical pulse replicator. 
         FIG. 7  illustrates a typical SOA gain spectra versus wavelength as a function of the bias current applied to the SOA. 
         FIG. 8  illustrates the gain as a function of wavelength of an SOA measured for several different values of its bias current and for two different values of the intensity of the probe light. 
         FIG. 9   a  illustrates one aspect of the invention as a short pulse laser clock. 
         FIG. 9   b  illustrates one aspect of the invention as a short pulse laser clock. 
         FIG. 10  illustrates another aspect of the invention as another short pulse laser clock. 
         FIG. 11  shows an aspect of the invention as a short pulse laser clock. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the invention are shown and described, by way of illustration. As those skilled in the art would recognize, the described exemplary embodiments may be modified in various ways, all without departing from the spirit or scope of the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. 
     Embodiments of the invention are described herein with reference to block diagram illustrations that are schematic illustrations of idealized embodiments of the invention. It is understood that many of the blocks will have different appearances compared to those shown. Further, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques, tolerances, or particular vendor are expected. Embodiments of the invention should not be construed as limited to the particular embodiments of the components illustrated herein but are to include deviations in components that result, for example, from vendor selection. A component illustrated or described as having particular functionality or performance parameters may be replaced with another component of substantially the same functionality and performance. Thus, the Demodulators illustrated in the figures are schematic in nature and the blocks shown are not intended to illustrate the precise composition of a Demodulator and are not intended to limit the scope of the invention. 
     The problems previously described are at least partially solved by selecting a Semiconductor Optical Amplifier (SOA) and its operational characteristics such that the bias current and the optical wavelengths of the optical signal and optical clock pulses in a Demodulator minimize the effects of intensity and timing jitter in the signal and clock pulses. Furthermore, the effects of intensity variations and jitter may be attenuated by using a short pulse laser triggered by the clock pulse to sample the SOA gain. This short pulse laser may be considered as normalizing the optical clock pulse input in terms of amplitude, duration and spectral content. 
     In one aspect of the invention, the signal pulses are used to deplete the gain of the SOA while the received clock pulses are used to sample the gain of the SOA. 
     In another aspect of the invention the Demodulator preferably sets the position-modulated optical signal pulses to a wavelength that is shorter than the wavelength of the fixed-position optical clock pulses. In addition, the invention may specify the wavelength of those signal pulses to be selected such that the SOA can reach a transparency condition when the signal pulses are coupled into the SOA. In general, the wavelength of transparency is less than or equal to the wavelength of the signal pulse, which is less than or equal to the wavelength of the clock pulse. In an alternative embodiment of the invention, the wavelength of the clock pulses is selected to be on the short-wavelength side of the wavelength for which the gain of the SOA is maximal. In another aspect of the invention, the Demodulator may further include an optical pulse replicator. Furthermore, the Demodulator may include an optical-injection locked short-pulse laser source. In an alternative embodiment of the invention, the Demodulator includes a balanced photodetector pair with two optical inputs such that the clock pulse produced by the laser source may be coupled into one of those inputs and the optical-wavelength filtered, amplified clock pulse output from the SOA, may be coupled into the other of those inputs. 
     One embodiment of the invention of an optical analog pulse-position Demodulator  300  is illustrated in  FIG. 3 . This Demodulator  300  is part of an OAPPM receiver  303  of an OAPPM link (not shown) that receives pairs of short optical pulses  301 . One pulse of the pair  301 , the optical signal pulse  316 , has its time-position modulated and the other pulse of the pair, the optical clock pulse  312 , has a fixed time position. Without implying a limitation, the relative time-positions of, or the time-delay between, those two pulses in a pair  301  is proportional to the time-sampled analog value of an input microwave or RF signal supplied to the OAPPM transmitter (not shown) of that OAPPM link. The function of the Demodulator  300  in an OAPPM system is to convert the relative time-positions of pairs of its input optical pulses  301  into an analog  399  output voltage or current. The OAPPM receiver  303  also may include an optical amplifier  304  that amplifies the received optical signal and optical clock pulses as well as a light-capturing optical aperture  302 , such as a telescope. In a preferred embodiment the optical amplifier  304  may comprise a low noise fiber amplifier. 
     The Demodulator  300  may have less sensitivity to jitter than prior art because: 1) the received optical clock pulses  312  are regenerated by the short pulse generating laser  330 , 2) the received optical signal pulse  316  is used to drive the SOA  370  to transparency and finally 3) each optical signal pulse  316  is compared against its particular optical clock pulse  312  rather than comparing the optical signal pulse  316  to a clock derived from a series of clock reference pulses. 
     Optical clock pulse  312  and optical signal pulse  316  may be generated with different wavelengths by passing the optical clock pulse in the modulator (not shown) through a pulse compressor and then through optical filters to select the desired wavelengths. 
     The Demodulator  300  in  FIG. 3  includes a wavelength splitter  310  that separates the optical clock pulse  312  from the optical signal pulse  316 . This is possible because the optical clock pulse  312  and optical signal pulse  316  are at different wavelengths. The optical clock pulse  312  drives a short pulse generating laser  330  which produces a normalized clock pulse  314  at substantially the same wavelength as the optical clock pulse  312  but at a substantially consistent amplitude. The fixed amplitude, fixed duration normalized clock pulses  314  may be delayed by module  340  to allow for adjustment of the timing of the normalized clock pulses  314 , producing delayed normalized clock pulse  315 . The optical signal pulse  316  in the received pulse pair  301  is selected by the wavelength splitter  310  and drives the optical pulse replicator  320 . The optical pulse replicator  320  accepts as input a single optical signal pulse  316  and outputs a plurality of signal pulses  318 . The plurality of signal pulses  318  may be amplified by optical amplifier  350  to produce the amplified plurality of signal pulses  319 . The amplified plurality of signal pulses  319  are combined with the clock pulse  315  in the wavelength combiner  360 . The output  317  of the wavelength combiner  360  is the input to the semiconductor optical amplifier (SOA)  370 . The plurality of signal pulses  319  may be considered one pulse with a preferably short rise and fall time. The plurality of signal pulses  319  are energetic enough to ensure the gain of the SOA  370  is fully depleted irrespective of the energy of the optical signal pulse  316 . As described later, the input optical signal pulse  316  may be conditioned to maintain the SOA  370  in its transparent state. The delayed normalized clock pulses  315  are then amplified by a gain determined by the SOA  370  bias current (not shown in  FIG. 3 ) supplying carriers to the SOA  370 . The output of the SOA  370 , amplified clock pulse  313 , may be filtered by the optical filter  380  that substantially passes only the delayed normalized clock pulses  315  after amplification by the SOA  370 . Any optical energy at the wavelength of the plurality of signal pulses  319  may be blocked by the optical filter  380 . The optical filter  380  passes substantially only the wavelength of the normalized clock pulse  314 . The output  311  of the optical filter  380  is then the optical clock pulse  312 , normalized and amplified by the SOA  370  according to the time separation between the optical signal pulse  316  and optical clock pulse  312  in the received input pulse pair  301 . The output  311  is then passed to a photodetector  390  where the optical output  311  is converted to an electrical signal to generate the analog output  399 . This analog output  399  is a reproduction of the analog signal used in the transmitter (not shown) to modulate the position of the signal pulses relative to the clock pulses in each pair in input optical pulse,  301 . 
     An alternative embodiment using clock and signal pulses of different polarizations may be realized by replacing the wavelength splitter  310  with one that operates on polarization as in  131  in  FIG. 1  and replacing the wavelength filter  380  with one that filters based on polarization as in  131  in  FIG. 1 . The wavelength combiner  360  may be replaced with a polarization combiner. Components that operate on polarization are commercially available and known to practitioners in the art. The operation of the polarization coupler  131  is described in U.S. Pat. No. 7,605,974 to Ionov, issued Oct. 20, 2009 and incorporated by reference herein. 
       FIG. 4A  shows an alternative embodiment  400 A of the invention. The components, signals and modules in  FIG. 4A  that are the same in  FIG. 3  have the same reference numbers as in  FIG. 3 . In the embodiment  400 A in  FIG. 4A , the short pulse laser  330  is replaced by modules  415 ,  435  and  445 . Module  415  is an optical power splitter, module  435  is an optical variable delay line and module  445  is a variable optical attenuator. In addition, the optical clock pulses  312  drive a balanced photodetector  492 . The clock signal  421  from the variable optical attenuator  445  provides a reference level in the balanced photo-detector  492  for comparison against the amplified clock pulse  311  from the SOA  370 . This comparison of clock pulses  421  and  311  in the balanced photodetector  492  results in some cancellation of clock noise and spurs. While  FIG. 3  uses a wavelength combiner  360  that requires specific wavelengths,  FIG. 4A  shows an alternate embodiment to combine two light signals of different wavelengths using an optical coupler  425  instead of a wavelength combiner  360 . 
     The remaining components of the Demodulators  300  and  400 A are passive wavelength selective components. The optical wavelength splitter  310  separates the input PPM optical signal pulses  316  from the input optical clock pulses  312 . An optical filter  380  or optical wavelength selector passes the amplified clock pulses  313  on to the photo-detector  390  or  492 . A person of ordinary skill in the art may appreciate that these components can be realized using commercially available optical wavelength multiplexers/demultiplexers. An array waveguide grating (AWG) is a preferred way to realize these components because the AWGs fabricated from silica optical waveguides can handle very high levels of optical power without being damaged. 
     In an alternative embodiment  400 B shown in  FIG. 4B  the Demodulator uses a balanced photo-detector pair  492 . One photo-detector of that pair  492  is connected to the filtered optical output  311  of the SOA  370 . The other photo-detector of the pair  492  is connected to the output  421  of the variable optical attenuator  445 . The output  421  is split from the normalized clock pulse  314  of the short pulse generating laser  330 . The light in the paths to both photo-detectors in  492  originate from the same short pulse generating laser  330 , hence noise in the short pulse generating laser  330  may be cancelled out. The adjustable optical attenuator  445  may be used to balance the average photo-detected currents. In this way, only the intensity modulation of the amplified clock pulses  313  will be observed in the output  499  from the balanced photo-detector pair  492 . An adjustable delay  340  may also be added to equalize the overall lengths of the two paths from the normalized clock pulse  314  to the photo-detector inputs  311  and  421 . By also balancing the delay paths, any low-frequency noise in the intensity of the normalized clock pulse  314  may be cancelled. 
     The preceding embodiments are by way of example and not limitation. Other embodiments of the principles of the invention are possible and within the contemplation of the invention. The essential function of a Demodulator embodying the principles of the invention is the use of two pulses wherein the original analog value is encoded in the time spacing of the two pulses. One pulse, the depleting pulse, is used to force the SOA  370  to a transparent condition. A second pulse, the sampling pulse, is normalized then used to sample the gain of the SOA  370 . The amplified sampling pulse produced by the SOA  370  has an intensity proportional to the time spacing between the input pulse pair and hence the original analog value. Other labels may be used for the depleting and sampling pulses such as signal and clock without implying a limitation. 
     The various components of the Demodulators  300 ,  400 A and  400 B can be made to have optical-fiber pigtails that allow them to be connected together. In general, the gain of the SOA  370  and the injection locking function of the short-pulse laser  330  are sensitive to the polarization of the light. Thus, to more easily control the operation of the Demodulators  300 ,  400 A, and  400 B, it is preferable to use polarization maintaining fiber and polarization maintaining passive components where possible. Also, polarization controllers or adjusters as well as polarizers (polarization filters) may be added, as needed. 
       FIG. 5  shows one embodiment of a multi-wavelength optical pulse replicator  320  comprising power splitters  415 , variable delay modules  501  through  516  and combiners  425 . The combiner  425  output  522  may be amplified by optical amplifier  350  to generate the burst of signal pulses  524 . The amplifier  350  may be an Erbium Doped Fiber Amplifier (EDFA), a pump/pumped laser device or similar device known in the art. 
     The multi-wavelength optical pulse replicator circuit  320  illustrated in  FIG. 5  takes a single signal-pulse input  520  and produces a burst of signal pulses  522  having multiple copies of that signal pulse  520 .  FIG. 5  shows, without implying a limitation, 16 copies. Other multiples may be used and the number of copies is not limited to a multiple of two. The time-spacing between two successive pulses of the burst of signal pulses  522  should be sufficiently large that those signal pulses do not overlap. Overlap between pulses, which can have optical phase interference, could produce noise in the intensities of those pulses. However, the time-spacing also should be sufficiently small that the SOA  370  experiences minimal recovery from the gain depletion caused by one pulse of the burst of signal pulses  524  before the next pulse of the burst of signal pulses  524  arrives at the SOA  370 . In this way, each successive pulse of the burst of signal pulses  524  causes the SOA  370  gain to be depleted more and more. But because the final pulse of the burst of signal pulses  524  is narrow and thus has a sharp edge, the termination of the SOA  370  gain depletion caused by the burst of signal pulses  524  is abrupt. 
     The multi-wavelength optical pulse replicator  320  of  FIG. 5  comprises a branching network based on power splitters  415  and optical combiners  425 . As illustrated in  FIG. 5 , this branching network comprises a tree-arrangement of 1×2 combiners  425 . Ideally, since the pulses arriving at the junction of each 1×2 combiner  425  do not overlap, the combination is done without any optical interference occurring between those pulses. Instead, the combined output just suffers an insertion loss of 3 dB. As a result, the total insertion loss for an example 16-to-1 combiner, which has 4 cascaded stages of 1×2 combiners  425 , would be 12 dB. The multi-wavelength optical pulse replicator  320  may have additional optical losses arising, for example, from propagation of light in waveguides  501 - 516 , from mode-mismatches at the junctions, and from the coupling of the optical waveguides  501 - 516  to an optical fiber (which are at the input and output ends of the optical combiner  425  and power splitter  415 ). The output of the multi-wavelength optical pulse replicator  320  in the Demodulator  300  is connected to an optical amplifier  350  that increases the intensity of the multiple pulses in a burst  524 . In one embodiment, this intensity of the multiple pulses in burst  524  preferably has a value comparable to or even greater than the intensity of the single input pulse  520 . An erbium-doped fiber amplifier (EDFA) may be used for the optical amplifier  350 , but other kinds of optical amplifiers that have similarly long gain recovery times also could be used. 
     The input side of the multi-wavelength optical pulse replicator  320  of  FIG. 5  contains an optical-waveguide circuit that has a branching waveguide network comprised of power splitters  415  that makes multiple copies of the input pulse  520 . The multi-wavelength optical pulse replicator  320  then has a set of optical delay segments  501 - 516  of differing length that give each of those 16 copies a different amount of time delay. The length of each delay segment  501 - 516  is a successively increasing multiple of a pre-determined delay increment. The multi-wavelength optical pulse replicator  320  preferably uses a longer delay increment when the input pulse is wider. In one exemplary embodiment, if the delay increment is 3 psec, the first copy of the input pulse  520  would be given an added time delay of zero psec. The second copy would be given an added time delay of 3 psec. The third copy would be given an added time delay of 6 psec; the fourth copy would be given an added time delay of 9 psec, and so on. The 16 th  copy would be given an added time delay of (N-1)t, where t is the incremental time delay and N is the number of copies. For this example, that time delay of the 16 th  copy would be 45 psec. The length increment is determined by the refractive index of the optical waveguided mode in the circuit. If that refractive index has a value of 1.5, the length increment would be approximately 0.6 mm. The foregoing does not imply a limitation. Other embodiments with more or fewer optical delay elements and uniform or non-uniform delay increments may be used. 
       FIG. 5  shows one embodiment of the multi-wavelength optical pulse replicator  320 . However, the multi-wavelength optical pulse replicator  320  of  FIG. 5  may be improved to increase the signal to noise ratio. With analog systems, the dynamic range and the signal to noise ratio often must be greater than many tens of dB. For real optical pulses, the edges of the pulse are not abrupt on the order of tens of dB but instead there still is significant energy even at large time values away from −3 dB power level of the pulse. Optical interference occurs in the overlapping tails of the replicated pulses that have the same wavelength and that are coherent with each other. This interference results in optical intensity variations being added by the multi-wavelength optical pulse replicator  320  like the one shown in  FIG. 5 . An alternative multi-wavelength optical pulse replicator  320  is shown in  FIG. 6  for the Demodulator  300  that avoids this optical interference by exploiting the spectrum of wavelengths in the input to the multi-wavelength optical pulse replicator  320 . 
       FIG. 6  shows an alternative embodiment of the multi-wavelength optical pulse replicator  320 . The pulse-position modulated output from a modulator in the transmitter generally is compressed by a subsequent wavelength shifting stage. The optical signal pulse  316  output of the wavelength splitter  310 , is the input  620  to the multi-wavelength optical pulse replicator  320  in  FIG. 6 . The multi-wavelength optical pulse replicator  320  of  FIG. 6  uses an array waveguide grating (AWG)  610  with small channel spacing (in one embodiment 1.6 nm) to carve out pulses at several different wavelengths. Each of those pulses  603  and  604  is sent to a separate pulse replicator  606  comprising one or more power splitters  415 . Alternatively, the array waveguide grating  610  may be replaced with an optical wavelength splitter  310 . The output of the replicators  606  are guided through optical delay segments  611 - 618  that have delay values chosen to avoid any overlap between the replicated pulses. The delay segments  611 - 618  are functionally the same as  501 - 516 . The outputs of the optical delay segments  611 - 618  are then combined in combiners  425 . The outputs of the combiners  425  are combined using another AWG  660 . Thus, adjacent pulses in the final signal burst  624  have differing wavelengths and will not interfere with each other. Without implying a limitation,  FIG. 6  shows the input  620  being decomposed into two wavelengths  603  and  604 . A person skilled in the art will appreciate that the input  620  may be decomposed into a plurality of wavelengths according to the principals described herein. 
     Successful performance of the embodiments of the invention depend on operating the SOA  370  at preferred wavelengths and with a preferred bias current.  FIG. 7  illustrates the gain of a typical SOA  370  as a function of wavelength for a particular bias current as it experiences first a gain depleting signal pulse  319  then the clock pulse  315 .  FIG. 7  is illustrative only and not of a specific device. Curve  701  shows the SOA  370  gain when both the signal pulse  319  and the clock pulse  315  are not supplied to the SOA. This gain distribution is determined by the bias current level. Curve  702  shows the SOA  370  gain after depletion by the signal pulse  319 . As indicated by the arrow  704 , the SOA  370  gain shifts up and toward the shorter wavelengths as the bias current replenishes the carriers. The SOA  370  bias current and the plurality of signal pulses  319  wavelength are chosen such that the energy of the plurality of signal pulses  319  is sufficient to substantially bring the SOA  370  to a transparency condition for that wavelength, where transparency is defined as the output power equals the input power. In this way, any higher level of the plurality of signal pulses  319  energy will result in absorption of the signal light and thus generation of additional carriers to maintain that transparency condition. As a result, any fluctuations in the intensity of the plurality of signal pulses  319  will not cause a fluctuation in the SOA  370  gain if the SOA  370  is at its transparency point for that plurality of signal pulse  319  wavelength. When the plurality of signal pulses  319  ends, the SOA  370  gradually returns to the gain spectrum  701  shown in  FIG. 7 . Thus, having a more intense plurality of signal pulses  319  would allow a larger bias current for the SOA  370 . For the larger bias current, there would be a larger swing in the gain of the SOA  370  after the plurality of signal pulses  319  has ended. The larger swing in the gain of the SOA  370  results in a larger swing in the amplification of the subsequent clock signal. 
     As noted previously, to achieve full depletion of the gain, in one preferred embodiment, the wavelength of the plurality of signal pulses  319  is matched with the bias-current dependent gain spectrum of the SOA  370 , as illustrated in  FIG. 7 . The SOA  370  has a material transparency wavelength, λ transparency , that is set by the level of the bias current. Light at shorter wavelengths (higher photon energy) than λ transparency  experience net absorption and generate electrical carriers that supplement the carriers injected by the bias current. Light at longer wavelengths than λ transparency  experience net positive gain and deplete the electrical carrier population in the SOA  370 , because of the stimulated emission. The curve  702  represents the quasi-equilibrium gain spectrum resulting from gain-depletion by a long signal pulse  319  (or pulse burst) whose wavelength is longer than λ transparency . A sufficiently long and intense plurality of signal pulses  319  can establish a new transparency condition such that the SOA  370  now becomes transparent for the wavelength λ sig  of the plurality of signal pulse  319 . After the plurality of signal pulses  319  ends, the gain spectrum gradually shifts back toward the curve  701  as additional electrical carriers are supplied to the SOA  370  via the bias current. In addition, the transparency condition reverts back toward its original value with transparency occurring at the λ transparency  and no longer at λ sig . The curve  703  represents the gain spectrum of the SOA  370  after the SOA  370  is depleted by a strong and sufficiently long clock pulse  315  at the clock wavelength λ clk , which is longer than the wavelength λ sig  of the plurality of signal pulse  319 . The SOA  370  gain spectrum gradually shifts again back to the curve  701  after each clock pulse  315  has ended. 
     The typical SOA  370  gain spectrum in  FIG. 7  shows that the gain is larger for longer wavelengths of the light but the spectrum eventually has a downward curvature as the wavelength is increased. Thus, a clock pulse, or gain-probing pulse, at a longer wavelength would experience a larger time-average level of the SOA gain. However, the swing in the gain caused by the gain-depletion due to the signal burst and the gain-recovery due to the continuously supplied (DC) bias current becomes smaller and smaller as the clock wavelength is increased beyond a maximum. The amplitude of the RF output  399  (in  FIG. 3 ) from the photo-detector  390  of the OAPPM Demodulator  300  is proportional to the swing in the SOA  370  gain. The noise on the RF output  399  is proportional to the time-average gain level. Thus, it is preferable to have the clock wavelength only slightly greater than the wavelength of the plurality of signal pulses  319 , although alternative embodiments may use substantially the same wavelength for both clock pulses  315  and plurality of signal pulses  319 . Typical values for the clock pulse  315  wavelength and the plurality of signal pulse  319  wavelength depend on the SOA  370  selected. For maximum RF signal output  399  from the OAPPM Demodulator  300 , it is preferable to operate in the regime for which the SOA  370  gain is increasing with wavelength rather than decreasing with wavelength. These constraints guide the selection of the preferred types of SOA  370  devices to use in the OAPPM Demodulator  300 . Typical SOA  370  amplifiers, by example and not to imply a limitation, are the BOA1080, BOA1004 and SOA1117 by Covega Corporation of Jessup Md. 
     In one aspect of the invention, the plurality of signal pulse  319  wavelength λ sig  is chosen to be shorter than the clock pulse  315  wavelength λ clk . Furthermore, the signal pulse wavelength λ sig  and the SOA  370  bias current, are chosen to make the gain fully depleted by the plurality of signal pulses  319 . In another aspect of the invention, the effective duration of the plurality of signal pulses  319  to the SOA  370  is sized so that the total optical energy producing the SOA  370  gain depletion can be increased without increasing the peak intensity of the (gain depleting) plurality of signal pulses  319 . 
     If the plurality of signal pulses  319  has sufficient energy, it can act like a temporary “holding beam” to reset the carrier population to a new transparency wavelength that matches the wavelength λ sig  of the signal pulse  319 . Ideally, regardless of the gain spectrum the SOA  370  had before the plurality of plurality of signal pulses  319  arrives, that plurality of signal pulses  319  should reset the carrier population to the same level, thereby reducing the sensitivity to the plurality of signal pulses  319  variation. Also, as long as the energy of the plurality of signal pulses  319  is sufficiently large for each plurality of signal pulses  319  to temporarily establish a new transparency wavelength at λ sig , the carrier population at the end of each plurality of signal pulses  319  will be at the same value, thereby reducing the sensitivity to fluctuations in the intensity of the plurality of signal pulses  319 . 
       FIGS. 5 and 6  described previously illustrate optical pulse replicators  320  for stretching the signal pulse  316  without increasing the peak intensity of the plurality of signal pulses  319 . In one preferred embodiment, the optical pulse replicator  320  effectively supplies a long signal pulse  316  in the form of a plurality of signal pulses  319  to the SOA  370 , with the total energy sufficient to fully deplete the gain of the SOA  370 . The long time duration of the plurality of signal pulses  319  allows the total energy of the light depleting the SOA  370  gain to be increased without making its peak power so high that the SOA  370  or the optical combiners  360  and filters  380  in the Demodulator  300  become damaged. 
     Without implying a limitation,  FIG. 8  shows the typical operating characteristics of an exemplary semiconductor optical amplifier (SOA)  370  used in various embodiments of the invention. The SOA  370  is the model BOA1080 by Covega Corporation of Jessup Md.  FIG. 8  illustrates one aspect of the invention as selecting the wavelengths, intensity, and bias current for the SOA  370 . 
       FIG. 8  shows the photodetected output signals  802 ,  804 ,  806  and  808  that were measured by a photodetector placed to detect the light  313  at the output from the SOA  370 , for the particular SOA  370  bias current levels shown.  FIG. 8  also shows the photodetected input signals  809  and  810  that were measured by a photodetector placed to detect the light  317  at the input to the SOA  370 , for two different intensity levels of that input light  317 . The wavelength of the continuous-wave input light  317  was tuned between 1.52 micrometers and 1.58 micrometers and the input and output light of the SOA  370  was measured at each wavelength set-point. Input  809  has an intensity level of zero dBm and output  802  corresponds to that input. Output curve  802  results from the 250 ma bias current. Input  810  has an intensity level of −10 dBm and the corresponding output curves are  804 ,  806  and  808  at bias currents of 250 mA, 200 mA and 150 mA respectively. Curves  802 ,  804 ,  809  and  810  were extrapolated in  FIG. 8  to facilitate the following explanations. These extrapolations are shown as dotted lines at the shorter wavelengths for curves  802 ,  804 ,  809  and  810  in  FIG. 8 . 
     The gain applied to a given SOA  370  input signal  317  at a particular wavelength is the vertical distance (dB relative scale) on the plot of  FIG. 8  between an input curve  809 ,  810  and its associated output curve  802 ,  804 ,  806 ,  808  for a particular bias current to the SOA  370 . The gain  812  at a wavelength of 1.568 micrometers for input  810 , intensity −10 dBm, and bias current 150 mA is shown as an example. If the input curves  809 ,  810  are considered as indicating reference levels of zero gain, the output curves  802 ,  804 ,  806 ,  808  can be considered as being the gain curves of the SOA for their associated levels of input intensity and bias current. The effect of increasing the bias current is to shift the gain curves up (indicating higher gain) and to the shorter wavelengths, as shown by gain curves  804 ,  806  and  808 . Compare the input and output curves  809  and  802 ,  810  and  804  for a 250 mA bias current at a wavelength of 1.54 micrometers. The photodetected output level indicated by curve  802  for −0 dBm input  809  is approximately 6 dBm for a photodetected input level of −2.5 dBm. Thus, SOA  370  input signal  317  at 1.54 micrometers and intensity level 0 dBm will receive a gain of approximately 8.5 dB for an SOA  370  bias current of 250 mA. Also, as shown by curves  810  and  804 , SOA input signal  317  at 1.54 micrometers and intensity level −10 dBm will receive a gain of approximately 10 dB for an SOA bias current of 250 mA. This shows it is preferable to use lower input power signals to maximize the gain available and thereby the sensitivity of SOA  370  output to variations in input  317  timing. 
     Continuing with  FIG. 8 , the transparency point is a condition at which the input light experiences zero dB gain. This condition is established for a given SOA at particular combinations of SOA  370  bias current, input light wavelength and power level. For the −10 dBm input  810  and with a bias current of 150 mA the transparency point  814  for output curve  808  is at 1.547 micrometers. Since the Optical Analog PPM Demodulators ( 300 ,  400 A, and  400 B) use the SOA  370  to measure the time shift of the clock pulse  315 , in one preferred embodiment, it is preferable to use a signal pulse  319  whose wavelength is shorter than the wavelength of the clock pulse  315 . In this way, even if the gain for the signal pulse  319  is zero dB, the gain for the clock pulse  315  will have a value greater than one (&gt;0 dB) and the clock pulse  315  will be amplified by the SOA  370 . In one preferred embodiment, the wavelength of the signal pulse  319 , the wavelength of the clock pulse  315 , and the SOA  370  bias current are chosen so the wavelength of the signal pulse  319  is at or near the SOA  370  transparency point. Moreover, the wavelength of the clock pulse  315  is chosen to be longer than the signal pulse  319  wavelength yet shorter than the wavelength for the maximum gain. In addition, the power level of the signal pulse  319  may be chosen to ensure the signal pulse  319  experiences minimal gain. Likewise the clock pulse  315  power may be chosen to experience maximum amplification. Alternatively, the clock pulse  315  power may be chosen to maximally deplete the gain of the SOA. 
     Further continuing with  FIG. 8 , the transparency point for −10 dBm input level and 250 mA bias current is approximately at a wavelength of 1.514 micrometers, as indicated by an extrapolation of curves  804  and  810 . If the input level is increased to 0 dBm level, for the same 250 mA bias current, the transparency point would shift to a longer wavelength of 1.518 micrometers, as indicated by an extrapolation of curves  802  and  809 . Thus, if an input signal  317  at a wavelength of 1.514 micrometers has its intensity increase from a level of −10 dBm to a level of 0 dBm, that input signal  317  would experience negative gain (in dB) and the light would be absorbed instead by the SOA  370 , thereby augmenting the gain of the SOA  370 . The photo-generated carriers from the absorption of that light supplement the carriers supplied by the bias current and act to shift the gain curve up (higher gain) and toward shorter wavelengths (i.e., away from the gain curve associated with  804  and toward the gain curve associated with  802 ). Alternatively, if an input signal  317  at a wavelength of 1.518 micrometers has its intensity decrease from a level of 0 dBm to a level of −10 dBm, that input signal  317  would experience positive gain (in dB) and the light would be amplified by the SOA  370 . The amplification process depletes the carriers in the SOA  370  and acts to shift the gain curve downward (lower gain) and toward longer wavelengths (i.e., away from the gain curve associated with  802  and  804  and toward the gain curve associated with  804  and  810 ). These examples illustrate the selection of the SOA  370  bias current, wavelength of input signal  317 , and the power level or intensity of input signal  317  to be at or near a transparency point. Any increase of the input signal  317  power level or intensity would result in absorption of that excess signal power to restore the transparency condition and any decrease of the input signal  317  power level or intensity would result in amplification of the input signal  317 , accompanied by depletion of the SOA  370  gain, until the transparency condition is reached again. Considering the examples above, if the input signal  317  has a wavelength of 1.512 micrometers and a minimum intensity of −10 dBm, for an SOA bias current of 250 mA, any fluctuation of the intensity of the input signal  317  between the levels of −10 dBm and 0 dBm should not result in a net change of the SOA gain. Now consider an input signal pulse  317  that has a wavelength of 1.512 micrometers and an intensity of at least −10 dBm and an SOA  370  biased at a current of 250 mA, if the duration of that input signal  317  is sufficiently long, the SOA will reach a transparency condition such that any fluctuations in the intensity of that input signal  317  will not further change the gain of SOA  370  while that input signal  317  continues to be supplied to the SOA. A more intense input signal  317  can reach the transparency condition in a shorter time whereas a less intense input signal  317  will reach the transparency condition in a relatively longer time. 
     Now, consider an input signal  317  that has energy between the wavelengths of 1.530 and 1.534 micrometers (as illustrated by dotted line  811 ), still referring to  FIG. 8 . Suppose the intensity of that input signal  317  can fluctuate between −10 dBm and 0 dBm. The bias current of the SOA  370  would preferably be set at 200 mA to have that signal pulse always reach the transparency condition by depleting the gain with a sufficiently long input signal  317 . However, if the bias current of the SOA  370  were set at 250 mA or at 150 mA, the intensity of that input signal  317  may not always be sufficiently high to produce a transparency condition. Note that the transparency condition might be reached by absorbing or by amplifying that input signal  317  depending on its wavelength and input power. 
     The preceding paragraph discussed setting the characteristics of the input signal  317  and the SOA  370  bias current to facilitate depleting the gain of the SOA  370  and setting the characteristics of the clock pulse  315  so as to maximize the change in gain experienced by the clock pulse  315 . The same discussion applies to any signal intended to deplete the carriers of the SOA  370  and any signal intended to experience the gain of the SOA  370 . The labels “signal pulse” and “clock pulse” do not imply a limitation that only a pulse derived from the optical clock pulse  312  may experience the gain of the SOA  370 . In an alternate embodiment, the signal pulse  316  may be coupled to the short pulse laser  330  and optical clock pulse  312  may be coupled to the optical pulse replicator  320 . 
     The pulse intensity noise (or amplitude jitter) of the optical signal pulse  316  will affect the link SNR and is addressed in part by the operation of the SOA  370  described in connection to  FIG. 8 . Pulse timing jitter of the received optical signal pulse  316  and optical clock pulse  312  will also affect the link SNR. To mitigate the effects of the pulse-to-pulse timing jitter, the optical signal pulse  316  and the optical clock pulse  312  are preferably handled as matched pairs of those pulses. The time delay  340  acts to make the clock pulse  315  of a pair always be the one that immediately follows the plurality of signal pulse  319  of that same pair. To address the pulse intensity noise of optical clock pulses  312 , the Demodulators  300  and  400 B may include a short-pulse generating laser  330  that is synchronized to the optical clock pulses  312 . The short pulse generating laser  330  may be implemented in one or more configurations  900   a ,  900   b ,  1000  and  1100  shown in  FIGS. 9   a ,  9   b ,  10  and  11 . Functionally, the devices  900   a ,  900   b ,  1000  and  1100  normalize the input optical clock pulse  312  in intensity, time and spectrum. Where the reference numbers used in  FIGS. 9   a ,  9   b ,  1000  and  1100  are the same in the least two significant digits indicates the same signal or component. 
     The pulse-position modulator in the transmitter of an OAPPM link may contain a mode-locked laser source that generates a train of optical pulses. Each of those pulses samples the RF signal input to that transmitter and has their pulse position modulated according to the sampled voltage of that RF input signal. The OAPPM transmitter can be configured to transmit not only the pulse-position modulated pulses but also a set of timing-reference pulses as groups of two-pulse pairs. One pulse in a pair is the timing-reference pulse and the other pulse in that pair is the position-modulated pulse. If the timing reference pulse of a pair is derived from the same sampling pulse that was supplied to the modulator and that became the position-modulated pulse of the pair, there is no relative timing jitter between those two pulses of a pair. Pulses in different pairs can have relative timing jitter but not the two pulses in the same pair. The preferred Demodulator processes the two pulses in each pair of received pulses as a pair through the adjustment of time delay  340 . The preferred Demodulator uses the timing-reference pulse of a pair to control the timing of the normalized clock pulse  314  generated by the short pulse generating laser  330  in the Demodulator  300  or  400 B. The preferred Demodulator uses the position-modulated optical signal pulse  316  of the same pair to deplete the gain of the SOA prior to the arrival of the corresponding optical clock pulse  312 . The corresponding normalized clock pulse  314  is synchronized to the optical clock pulse  312  of that same pair. In this way, there is minimal timing jitter between the gain-depleting optical signal pulse  316  and the subsequent normalized clock pulse  314 . 
     The Demodulators  300  and  400 B in the OAPPM receiver preferably use a short-pulse generating laser  330  that is not actively mode locked by a periodic electrical RF waveform but instead is optical-injection locked by a train of optical pulses. A block diagram of an exemplary short-pulse generating laser  330  implemented as an Optical-Injection Locked Fiber Laser  900   a  is shown in  FIG. 9   a . The Optical-Injection Locked Fiber Laser  900   a  has a ring cavity that includes a Gain Element segment  904 , three Optical Couplers  902 ,  906  and  910  and an Optical Isolator  916 . A first Optical Coupler  910  is used to couple laser-generated light  999  out of the cavity, and to the output of the Short-Pulse Generating Laser  330 . A second Optical Coupler  902  is used to couple the Clock Reference Input Pulse  901  into the cavity. These Clock Reference Input Pulses  901  for the injection locking are obtained from the OAPPM transmitter and preferably are the optical clock pulses  312  supplied to the short-pulse generating Laser  330 . In the configuration illustrated, a third Optical Coupler  906 , which typically would be wavelength selective, is used to couple pump light from the Pump Laser  908  into the cavity for pumping the Gain Element  904 . Since the Gain Element  904  could generate light that travels in both directions around the ring cavity, the Optical Isolator  916  selects a preferred direction for the laser light to travel around the ring cavity. The pump light from the Pump Laser  908  could be supplied to travel in either or both directions through the Gain Element  904 . The Clock Reference Input Pulses  901  are coupled to travel in the same direction as the laser-generated light, clockwise in  FIG. 9   a . The ring cavity of the Optical-Injection Locked Fiber Laser  900   a  is gated by the RF Generator  930 . The timing of the RF Generator  930  is adjusted with the RF Delay Line  931 . 
     Continuing with  FIG. 9   a . The Gain Element  904  is pumped continuously but with the pumping level set such that the gain of the laser  900  is close to but not quite sufficient to sustain lasing. The received Clock Reference Input Pulse  901  is the input optical clock pulse  312  of the received pair  301  amplified by Low Noise Amplifier  304  and are then injected into the laser cavity through Optical Coupler  902 . The Clock Reference Input Pulse  901  is sufficiently energetic such that it can further pump the Gain Element  904  to bring it above the lasing threshold. In addition, some of the photons in that Clock Reference Input Pulse  901  can stimulate emission by the Gain Element  904 , with that emission being sustainable because the laser is now above the lasing threshold. The lasing light quickly depletes the available gain of the Gain Element  904  so that a short pulse  999  is obtained that is coupled out through the output coupler  910 . Since the lasing is initiated by the Clock Reference Input Pulse  901 , the timing of the laser output pulse  999  is synchronized with the Clock Reference Input Pulse  901 . However, the intensity of the output pulse  999  is determined primarily by the average gain of the Gain Element  904  and is much less sensitive to variations in the intensity of the Clock Reference Input Pulse  901 . 
     It may be beneficial to have a Optical-Injection Locked Fiber Laser  900   a  whose round trip gain is sufficient to lase only during the times the Clock Reference Input Pulse  901  are to be coupled into the Optical-Injection Locked Fiber Laser  900   a . The RF Generator  930 , RF Delay Line  931  and Optical Modulator  920  act to suppress lasing in the cavity except when the Clock Reference Input Pulses  901  arrive by synchronizing the Clock Reference Input Pulses  901  with the RF Generator  930 . The Optical Modulator  920  acts like a shutter or gate with a time-varying optical attenuation. The Optical Modulator  920  is preferably driven at the same frequency as the pulse-repetition frequency of the Clock Reference Input Pulses  901  by the RF Generator  930 . The RF Generator  930  generates a sinusoidal modulation waveform and the adjustable RF Delay Line  931  has its time delay adjusted to synchronize the Optical Modulator  920  with the arrival of the Clock Reference Input Pulses  901 . The result is the Optical-Injection Locked Fiber Laser  900   a  has an output largely insensitive to variations in the Clock Reference Input Pulse  901 . 
     The DC bias of this Optical Modulator  920  and the RF modulation voltage from the RF Generator  930  may be set such that the gain of the Optical-Injection Locked Fiber Laser  900   a  is low, because of attenuation by the Optical Modulator  920 , except near the time of the arrival of the Clock Reference Input Pulse  901 . This reduces the amplified spontaneous emission noise of the laser and hence the noise added to the output  999 . 
     The Optical Filter  912  in the Optical-Injection Locked Fiber Laser  900   a  may be used to select the wavelength of the lasing light to match the wavelength of the Clock Reference Input Pulse  901 . This Optical Filter  912  is helpful in reducing the amplified spontaneous emission coupled to the output  999  of the Optical-Injection Locked Fiber Laser  900   a . The Cavity Length Adjuster  914  can be an adjustable optical delay line. For an Optical-Injection Locked Fiber Laser  900   a , the output light  999  is not obtained solely by a build up of light from many passes through the laser cavity. However, in one preferred embodiment, the pulse-repetition interval of the Clock Reference Input Pulse  901  is substantially a multiple of the cavity round-trip time, which can be adjusted by the Cavity Length Adjuster  914 . The Optical-Injection Locked Fiber Laser  900   a  also may contain a Dispersion Compensator  940  that serves to narrow the pulse duration of the emitted light. The Optical Filter  912  and the Dispersion Compensator  940  are preferably placed close to the Output Coupler  910 , so that the output light  999  will have a low noise floor and a short pulse width. 
     A block diagram of an alternative Short-Pulse Generating Laser  330  implemented as an Optical-Injection Locked Fiber Laser is shown in  FIG. 9   b . The Optical-Injection Locked Fiber Laser  900   b  differs from the Optical-Injection Locked Fiber Laser  900   a  in that the Pump Input Coupler  906  is replaced with an Optical Circulator  917 . Another variation shown in  FIG. 9   b  uses the second Optical Coupler  902  to couple the Clock Reference Input Pulse  901  into the ring, and also could be used to couple a portion of both the laser-generated light as well and the Clock Reference Input Pulse  901  out of the ring as monitor pulses  998 . These two sets of pulses  901  and  908  that are coupled out by the second Optical Coupler  902  can be monitored with an optical cross-correlator or other fast pulse-displaying instrument as a means to set the initial time-delay  340  in  FIGS. 3 and 4B . 
     A variation of the embodiment  900   b  is shown in  FIG. 10 . The RF Generator  930  is replaced with an RF Bandpass Filter  1064 , a RF Driver Amplifier  1062 , a Dielectric Resonator RF Filter  1060 , and a Photo Detector  1050  that receives the Clock Reference Input Pulse  1001 . In operation the Clock Reference Input Pulses  1001  are converted into an electrical signal  1051  by the Photo Detector  1050 . The electrical signal  1051  has a frequency spectrum that decays with increasing frequency. The electrical signal  1051  passes through the Dielectric Resonator RF Filter  1060  to generate an electrical signal with a substantially uniform frequency response. The output of the Dielectric Resonator RF Filter  1060  overdrives the RF driver Amplifier  1062  to produce a squared off output in signal  1063 . The signal  1063  is cleaned up by the RF Bandpass Filter  1064  to produce a Control Signal  1065 . The Control Signal  1065  may be delayed by the RF Delay Line  1031  to produce the Control Signal  1052  for the Optical Modulator  1020 . In this way the gating of the ring cavity in the Optical-Injection Locked Fiber Laser  1000  is controlled with a substantially square wave instead of a sinusoid. 
       FIG. 11  shows another aspect of this invention in a further embodiment of short-pulse generating laser  300  as an Optical-Injection Locked Fiber Laser  1100 . This embodiment is substantially the same as the Optical-Injection Locked Fiber Laser  1000  except the second Optical Coupler  902  is omitted. With this embodiment, lasing of the Optical-Injection Locked Fiber Laser  1100  is further decoupled from the energy of the Clock Reference Input Pulse  1101 . The last two digits of the reference numbers in  FIGS. 10 and 11  are the same to identify the same components, functions and signals. 
     While the principles of the invention has been described in connection with certain exemplary embodiments, it is to be understood by those skilled in the art that application of the principles of the invention are not limited to the disclosed embodiments, but, on the contrary, are intended to cover various modifications included within the spirit and scope of the appended claims and equivalents thereof