Abstract:
A method and apparatus for pulse frequency modulation for analog optical communication. A train of optical pulses is generated. The spectrum of the optical pulses in the train of optical pulses can be broadened to provide a train of broad spectrum optical pulses. The broadening can be provided by self-phase modulation. Alternatively, broad spectrum optical pulses can be provided by merely having the optical pulses be less than 1 ps duration. A desired optical frequency slice from the train of spectrum broadened optical pulses is selected by a tunable Fabry-Perot filter. A desired optical frequency slice from the broad spectrum optical pulses is selected by a tunable Fabry-Perot filter. The tunable Fabry-Perot filter has a pair of Distributed Bragg Reflectors separated by an electro-refractive section. The electro-refractive section has tuning electrodes for applying transverse electric fields to the electro-refractive section, corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of communications, and, in particular, to optical frequency modulation devices useable in satellite communications systems. 
     BACKGROUND 
     Orbiting satellites are an important aspect of modern communication systems. Originally used for “single-bounce” communication, with a signal going up from one place on the surface of the earth and coming down in another, communication satellites are now being used to form complex networks in space, with each satellite in the network being able to communicate with many of the other satellites. Optical intersatellite links, with their high directionality, high energy efficiency, and tremendous information bandwidth, allow satellites to talk to one another, and to transmit a much larger amount of information. 
     Many satellite and terrestrial optical communication systems require transmission of analog optical signals. One known way to meet this transmission need is by employing amplitude modulation (AM) of an optical carrier. This approach, however, suffers from poor signal-to-noise ratio (SNR). 
     It is also well known that broadband modulation schemes, which utilize higher bandwidth than that of the transmitted information, may improve the SNR over that achieved with AM. One such technique is frequency modulation (FM). It is well known that the SNR of an FM system may be improved dramatically by using a higher frequency swing F FM  than the bandwidth of the transmitted information Δf, as described by in H. S. Black&#39;s “Modulation Theory”, published by D. Van Nostrand (1953), wherein: 
     
       
           SNR   FM   ∝SNR   AM ( f   FM   /Δf ) 2 ,  (1) 
       
     
     where SNR AM  is the signal-to-noise ratio of an AM communication system with identical optical power. 
     It is also known that FM optical signals can be obtained by modulating the current of a semiconductor laser. This technique, however, suffers from simultaneous amplitude modulation, and it provides a very limited frequency swing f AM &lt;20 GigaHertz (GHz). 
     In order to realize SNR advantages of optical FM, the FM receiver must include a limiter that eliminates amplitude noise of the received optical signal without affecting its frequency contents. Such all-optical limiters can be easily made for pulsed signals, e.g., based upon non-linear optical loop mirrors (NOLM), such as described in Wong et al. in Optics Letters, Vol. 22, 1997, p. 1150, or on SPM as described in the Mamyshev article, “All-optical Data Regeneration Based on Self-Phase Modulation Effect”, ECOC98, p. 475. These techniques, however, are hard to implement for continuous optical signals. 
     Therefore, to better help realize practical optical inter-satellite links, there exists a need for an effective FM system and method for analog optical communication. The present invention provides a solution to meet such need. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention an apparatus and method of optical frequency modulation is provided based upon spectral broadening of optical pulses via self phase modulation (SPM) followed by frequency selection by a fast tunable Fabry-Perot filter formed by two Distributed Bragg Reflectors (DBRs). 
     For realizing the SNR advantages of FM, an optical communication system needs an optical limiter in its receiver that equalizes the amplitude of a signal without changing its frequency. In a high bandwidth communication system, it is highly desirable to do limiting in the optical domain because of electronic speed limitations. While optical pulse reshaping and limiting based on SPM effect in optical fibers followed by spectral filtering has been described in the Mamyshev article, in accordance with the present invention this technique is combined with a tunable DBR filter to achieve FM pulses for optical communication. 
     The proposed system benefits from its pulsed format that allows the use of NOLM, such as described in the aforementioned Wong et al. article, or other optical regeneration techniques, such as delineated in the Mamyshev article, for optical limiting, which is not possible in a continuous wave FM system. 
     The advantages of the inventive approach described hereinbelow include: (1) a large FM swing of several hundred GHz, which is important for improving SNR, and (2) compatibility with optical limiting. 
     Therefore, in accordance with a preferred embodiment of the present invention a method and apparatus for pulse frequency modulation for analog optical communication is provided. A train of optical pulses is generated. The spectrum of the optical pulses in the train of optical pulses can be broadened to provide a train of broad spectrum optical pulses. The broadening can be provided by self-phase modulation. Alternatively, broad spectrum optical pulses can be provided by merely having the optical pulse duration be shorter than 1 ps. A desired optical frequency slice from the broad spectrum optical pulses is selected by a tunable Fabry-Perot filter. The tunable Fabry-Perot filter has a pair of Distributed Bragg Reflectors separated by an electro-refractive section. The electro-refractive section has tuning electrodes for applying transverse electric fields to the electro-refractive section, corresponding to an analog waveform being applied to the tuning electrodes, to provide a pulse-frequency modulated train of optical pulses. 
     Recovery of the analog waveform from the pulse-frequency modulated train of optical pulses is also provided. The pulse-frequency modulated train of optical pulses is spilt into a first optical beam and a second optical beam. A first photodetector is provided, the first photodetector providing a first current responsive to the first optical beam input thereon. The first photodetector has a first photodetector spectral response and is biased such that the first current is in a first direction. A second photodetector is also provided, the second photodetector providing a second current responsive to the second optical beam input thereon. The second photodetector has a second photodetector spectral response and is biased such that the second current is in the first direction. An input of a transimpedance amplifier is coupled to an output of the first photodetector and to an input of the second photodetector to provide an output of the transimpedance amplifier proportional to the difference between the first current and the second current. A first optical filter is provided to receive the first optical beam prior to incidence upon the first photodetector and a second optical filter is provided to receive the second optical beam prior to incidence upon the second photodetector. The first photodetector spectral response and the second photodetector spectral response are each broader than respective passbands of the first optical filter and the second optical filter to provide photocurrent vs. optical frequency characteristics determined by the respective first optical filter and the second optical filter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in schematic form an embodiment of a pulsed frequency modulator in accordance with the present invention. 
     FIG. 2 shows the spectra of a representative incident pulse and a representative pulse after self-phase modulation in accordance with the present invention. 
     FIG. 3 shows the spectra of a representative output pulse after filtering in accordance with the present invention. 
     FIG. 4 shows in schematic form an optical discriminator in accordance with the present invention. 
     FIGS. 5 a  and  5   b  show passbands of filters of the optical discriminator of FIG.  4 . 
     FIG. 6 shows in graph form a difference current of the optical discriminator of FIG. 4 as a function of frequency deviation of incident light. 
     FIG. 7 shows in schematic form an alternative embodiment of a pulsed frequency modulator in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, pulsed FM transmitter  10  in accordance with the present invention is shown. Train of equally spaced picosecond optical pulses  12   a  is generated by a mode-locked laser  14 . This is a mature technology capable of producing 40 GHz trains of pulses at =1.55 μm with sub-picosecond pulse duration. Similar to the optical regeneration scheme proposed by Mamyshev in the hereinabove referenced article, the train of pulses enters a length of fiber  16  that has a small normal dispersion (e.g., true wave fiber from Lucent). In fiber  16 , the pulses undergo self-phase modulation (SPM) that broadens their spectrum, resulting in output pulses 18a. The spectra of a representative incident pulse  12   b  and representative output pulse  18   b  are shown schematically in the FIG.  2 . It is important to emphasize that the spectrum of output pulses is almost rectangular-shaped after a normal dispersion fiber if its dispersion length, L D =τ 2 /β 2 , is larger than the non-linear length, L NL =cA eff /n 2 ω 0 I p , where c is the speed of light, τ, ω 0  and I p  are pulse duration, optical frequency and intensity, β 2 , A eff  and n 2 =2.6×10 −16  cm 2 /W are the fiber dispersion, effective core area and non-linear refractive index, respectively. 
     The bandwidth of SPM-broadened pulses is estimated as 
     
       
         Δω SPM ≈Δω 0 (2π/λ) n   2   I   p   L,   (2) 
       
     
     where Δω 0 ˜1/τ is the bandwidth of the input pulses and L is the fiber length. For example, a train of pulses with 20% duty cycle (e.g., 5 ps duration and 40 GHz PRF) and an average power of ˜40 mW experiences broadening of Δω SPM /Δω 0 ≈2.4 in 15 km of fiber. Shorter pulses, i.e., those having pulse duration shorter than 1 ps, of coming at 40 GHz require only one fifth of that length for the same amount of broadening. 
     The desired optical frequency slice is selected from SPM-broadened pulses by tunable Fabry-Perot filter  20  comprised of two DBR structures  22 ,  24  separated by electro-refractive (ER) section  26  supplied with electrodes. ER section  26  may be a passive Franz-Keldysh semiconductor waveguide, such as that described in the article by Delorme, et al. in IEEE Photonics Technical Letters, Vol. 7, (1995), p. 269. Alternatively, it can be made of LiNbo 3 . The filter has a narrow transmission bandwidth centered at 
     
       
         λ=2 nh /m,   (3) 
       
     
     where h is the spacing between the DBR reflectors, n is the semiconductor index of refraction and m is an integer. The spacing is chosen to provide sufficient free spectral range Δλ free , h&lt;λ 2 /(2n Δλ free ). For Δλ free =5 nm and n=3, this gives h&lt;75 μm. The spectral resolution of the filter, δλ, is determined by the reflectivity of the DBR structures, R,                δλ   λ     =         1   -   R2       4      R            λ     4      π                 nh                 (   4   )                                
     which gives δλ≈0.05 nm for R=0.9 and h=75 μm. Broader spectral bandwidth is easily achieved by reducing the reflectivity of DBR, R, or the spacing h. The smallest spacing h≈0.26 μm is determined by (3) with m=1, which gives δλ=15 nm for the same R. 
     The filter is tuned by applying transverse electric field to the ER section. A forward or reverse voltage may be applied to the semiconductor ER section, thus changing the refractive index by changing the carrier density or via Stark effect, respectively. The achieved wavelength shift is proportional to the change in refraction index, Δλ shift /λ=Δn/n. More than 2 nm wavelength shifts in both directions (4 nm total) have been demonstrated by applying this technique to active DBR lasers, as set forth in Delorme, et al. referenced above. The wavelength change of λ shift =2 nm corresponds to the optical frequency shift of Δv shift =−Δλ shift c/λ 2 =250 GHz. Such frequency swings will offer large SNR improvements in an FM communication system transmitting tens of GHz of information bandwidth, as evident from Equation (1). Resultant filtered pulse train  28   a  has a representative pulse spectra  28   b  shown schematically in the FIG.  3 . 
     In order to recover the modulation information, the frequency shift associated with each pulse must be converted into an electrical signal that is proportional to this shift. One way of doing this is shown in FIG.  4 . wherein optical discriminator  110  includes optical splitter  112 , which can be either a conventional 50:50 fiber optical splitter, (such as, for example, a Gould 22-1-0355-50-1120), or, for an unguided free-space beam, a bulk-optic 50:50 beam splitting cube (such as, for example, a Newport 05BC16NP.11), which receives the frequency-modulated optical beam  114 , either guided by conventional single-mode  116 , or propagated as an expanded free-space beam. Optical beam  114  is amplified by amplifier  113  and clipped by limiter  115 . Optical splitter  112  divides optical beam  114  into two equal-intensity beams  118 ,  120 . Optical filters  132 ,  134  are inserted in front of respective semiconductor photodetectors  131 ,  133 . Semiconductor photodetectors  131 ,  133  in this embodiment have a spectral response that is much broader than the passbands of filters  132   1 , 34 , depicted in FIGS. 5 a  and  5   b  respectively, so that the photocurrent vs. optical frequency characteristic of the filter-detector combination is determined by the filters alone. 
     Each of beams  118 ,  120  which pass through optical filters  132 ,  133  impinges upon respective photodetectors,  131 ,  133 . Photodetectors  131 ,  133  are biased (not shown) in such a way that the current flow is in direction  126 . If the frequency of light beams  118 ,  120  coincides with the passband of filter  132 , then the current I 1  through photodetector  131  will be large, and the current I 2  through photodetector  133  will be almost zero. Because almost no current can flow through the photodetector  133  ( i.e., it has been “optically” turned off), the entire current must flow into transimpedance amplifier  128 , (such as, for example, an Avantek ITA-02070 for applications below 1 GHz), coupled to the junction of photodetector  31  and photodetector  133 . Signal  130 , which is generally described as G (I 1 −I 2 ) where G is the gain of transimpedance amplifier  128 , out of transimpedance amplifier  128  will thus be proportional to I 1 , and will be positive. If the frequency of light beams  118 ,  120  is shifted so that it now coincides with the passband of filter  134 , the reverse will occur, namely current flow is now out of transimpedance amplifier  128 , not into it, so that output signal  130  will be proportional to current I 2  through photodetector  33 , and will be negative. The optical discriminator embodiment of FIG. 4 thus amplifies only the difference in detector currents. Electrically speaking, it has high common-mode rejection. 
     Optical filters  132 ,  134  are fabricated to have a Lorentzian line shape. In fact, commercially available Fabry-Perot filters, (such as, for example, the fiber-optic Micron Optics FFP-TF series), in addition to having the desired line shape, can also be mechanically tuned for whatever wavelength one desires. One can thus adjust the wavelength separation between the two filters so that ζ=Ω, insuring that one achieves the highest (maximum) linearity. An added advantage is that one can then readjust the filters to operate at other wavelengths. One device, therefore, can be manufactured that will satisfy a broad range of operating wavelengths, so that one could, for example, tune the discriminator to operate anywhere within the entire bandwidth of an Erbium doped fiber amplifier (1530-1560 nm wavelength). For very high-speed operation, the photodetectors of choice would, today, be InGaAs pin semiconductor devices. In particular, the use of fiber-optic filters together with dual-balanced, fiber-coupled detectors, such as the newly-developed NTT Electronics Corp NEL model KEPD2552KYG, would allow one to achieve greater than 20 GHz response using commercial off-the-shelf devices. 
     In essence, received light pulses are amplified, clipped, and then split into two branches. The pulses pass through filters that are shifted relative to a center frequency. The transmitted light is then converted into two electrical currents, the difference of which is fed into an electrical amplifier. In FIG. 6, there is shown a difference current as a function of the frequency deviation of the incident light. 
     As each pulse is processed by discriminator  110 , it generates a current pulse having an amplitude that is proportional wavelength offset of the pulse. One thus has a stream of pulses with each pulse having a height proportional to the analog signal used to generate the frequency shift of that pulse. By passing this pulse stream through a low-pass electrical filter, that pulse. By passing this pulse stream through a low-pass electrical filter, this pulse stream is converted into a continuous analog signal. 
     The criteria that must be satisfied in order to realize the processing gain associated with the desired FM system appear to have been met for the particular implementation desribed hereinabove: 
     (1) the system uses a bandwidth that is much larger than the modulation bandwidth of the analog signal; 
     (2) it is affected only by the noise in the bandwidth of the analog signal; and 
     (3) all amplitude noise associated with the utilized bandwidth has been removed by the limiter (pulse clipper). 
     There is, however, a point that should be made about the clipping process. A limiter is usually used to limit the absolute amplitude of the incoming signal. However, in accordance with the present invention, it serves two functions. As in other FM systems, it limits the pulse amplitude to some fixed value. However, unlike other applications, it also sets a lower threshold, below which no pulse will be passed. If it does not do this, then the optical amplifier noise that exists between pulses will generate noise by noise fluctuations at the photodetectors. These fluctuations have a large low-frequency component that will limit the amount of FM advantage that be passed (which one can easily do because of the very large CNR in the pulse), quiescent noise between pulses can be eleiminated. This double function has been demonstrated by a variety of nonlinear optical loop mirror limiter, as described in the hereinbefore mentioned Wong et al. article and in the SPM pulse regeneration scheme described in the Mamyshev article. 
     Referring to FIG. 7, there is shown in schematic form an alternative embodiment of a pulsed frequency modulator in accordance with the present invention. The pulsed frequency modulation shown in FIG. 7 would be similar in operation to that depicted in FIG. 1, with the exception that the pulse train  112   a  from mode-locked laser  114  would have a pulse duration shorter than 1 ps. As such, the short duration pulses have a broad spectrum and, therefore, a length of fibre for self-phase modulation to broaden the pulse spectrum is not needed. The desired optical frequency slice is selected from SPM-broadened pulses by tunable Fabry-Perot filter  120  comprised of two DBR structures  122 ,  124  separated by electro-refractive (ER) section  126  supplied with electrodes. As with the embodiment depicted in FIG. 1, the filter is tuned by applying transverse electric field to the ER section to correspond to the applied analog waveform. A resultant filtered pulse train  128   a  is thereby provided.