Abstract:
An optical communication system uses a radio frequency (RF) signal for communicating an analog communication signal. It comprises an optical transmitter and receiver. The transmitter generates a reference light beam, generates a shifted light beam that is shifted in frequency by the RF frequency, and responds to the analog communication signal and produces a communication light beam having a phase modulation corresponding to the analog communication signal. The optical receiver comprises first means to interferometrically combine the reference light beam with the shifted light beam to provide a first heterodyne signal, second means responsive to the first heterodyne signal to produce a first electrical signal at the optical beat frequency corresponding to the RF, third means to interferometrically combine the communication light beam with the shifted light beam to provide a second heterodyne signal including information regarding the state of phase of the communication light beam, fourth means responsive to said second heterodyne signal to produce a second electrical signal at the optical beat frequency corresponding to the RF and with a phase corresponding to the state of phase of the communication light beam, and fifth means responsive to said first and said second electrical signals to provide an output signal that has a linear correspondence to the state of optical phase of said communication light beam and the analog communication signal. Also disclosed are the transmitter and the receiver.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following commonly assigned applications: “Heterodyne Wavefront Sensor”, Ser. No. 09/283,604, having inventors Stephen Brosnan, Donald Heflinger and Lee Heflinger; “A Dynamic Optical Micrometer”, Ser. No. 09/283,484, having inventors Donald Heflinger and Lee Heflinger; “A Dynamic Optical Phase State Detector”, Ser. No. 09/282,946, having inventors Donald Heflinger and Lee Heflinger; “Improved High Average Power Fiber Laser System With High-Speed, Parallel Wavefront Sensor”, Ser. No. 09/282,579, having inventors Stephen Brosnan, Donald Heflinger, and Lee Heflinger; “Linear Analog Optical Communication System With Two Transmitted Beams And Phase Modulation”, Ser. No. 09/285,207, having inventors Donald Heflinger and Lee Heflinger; and “Optical Communication System With A Single Polarized, Phase Modulated Transmitted Beam”, Ser. No. 09/283,053, having inventors Donald Heflinger and Lee Heflinger; filed concurrently with this patent application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to optical communication systems, and more particularly to an optical communication system utilizing phase modulation that provides substantially linear recovery of an analog communication signal. 
     2. Description of the Prior Art 
     Analog optical communication links are known in the prior art. Conventional optical analog links employ intensity modulation techniques to convey the analog information on an optical beam of light. Such analog optical links are utilized by the cable television industry to transmit video images using the conventional RF analog modulation format for television video. Intensity detection at the receiver using conventional photodetectors enables the light intensity to be linearly converted to an analog voltage corresponding to the signal that is to be transmitted by the link. However, inherent to these analog intensity modulation optical links is an acceptance of a non linearity associated with the intensity modulators used in the transmitter. Mach-Zehnder intensity modulators, which are commonly employed in optical intensity modulation analog links, have a non linear transfer function that yields a sinusoidal intensity variation with a linearly changing applied analog modulation voltage. Similarly, electro absorption modulators also yield a non linear intensity variation to a linearly applied analog modulation voltage. 
     This inherent non linearity associated with intensity modulators has led to a consideration of using optical phase modulation in the transmitter as an alternative to intensity modulation. Optical phase modulators that can achieve a linear change in the state of the optical phase with a linearly changing analog modulation voltage are known in the art. Modulators can be made from electro optic materials that change their refractive index linearly with applied electric field supplied by a linearly changing analog modulation voltage. The linearly changing refractive index causes the optical path length through the modulator to linearly change. This linearly changing optical path length causes a linearly changing state of optical phase corresponding with a linearly changing analog modulation voltage. Thus, an optical phase modulator can be used in the transmitter to deliver a linearly varying optical signal in contrast to the inherent non linearity associated with intensity modulators. 
     The utilization of a linear phase modulator in an analog optical communication link requires that the state of optical phase be detected at the receiver. Conventional approaches for this utilize optical interference techniques that cause the phase varying light to become detectable with photodetectors as intensity variations. A common approach used for optical phase state detection is to interfere the phase modulated communication light with an unmodulated reference beam of light that has been split from the initial light source prior to applying the phase modulation. The process of utilizing optical interference techniques to detect the state of optical phase leads to a non linear sinusoidal intensity variation that corresponds to the linearly varying state of optical phase. Thus, this conventional phase detection process leads to a non linearity in the detected analog signal. This non linearity inherent in the conventional phase detection process negates the linearity achieved by the phase modulator and results in an analog optical communication link that is as non linear as the conventional intensity modulation analog optical link. Thus, all analog optical communication links are degraded in performance by an inherent non linearity that distorts the original analog signal that is to be conveyed. 
     What is needed, therefore, is an analog optical communications system that is capable of detecting the state of optical phase of a phase modulated communication signal in a way so as to produce an analog voltage signal that is linearly related to the state of optical phase of the phase modulated optical signal. Such an analog optical communications system thus will be capable of conveying an analog signal without any non linear distortion. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally an optical communication system for communicating an analog communication signal. 
     Briefly, the optical communication system comprises an optical transmitter and an optical receiver separated by an analog optical link. The transmitter comprises means for generating a reference light beam, means for generating a shifted light beam that is shifted in frequency from the frequency of said reference light beam by a predetermined radio frequency (RF) and means responsive to the analog communication signal to generate a phase modulated communication beam. 
     The optical receiver comprises first means responsive to the reference light beam and the shifted light beam to interferometrically combine these two beams to provide a first heterodyne signal, second means responsive to the first heterodyne signal to produce a first electrical signal at the optical beat frequency corresponding to the RF, third means responsive to the communication light beam and the shifted light beam to interferometrically combine the communication light beam with the shifted light beam to provide a second heterodyne signal including information regarding the state of phase of the communication light beam, fourth means responsive to the second heterodyne signal to produce a second electrical signal at the optical beat frequency corresponding to the RF with a phase corresponding to the state of phase of the communication light beam, and fifth means responsive to the first and second heterodyne signals to provide an output signal that corresponds to the state of optical phase of the communication light beam. More particularly, the detection of the relative phase of the RF signals is performed by a digital dividing technique that provides a high resolution determination over several optical wavelengths. Both the RF beat frequency signal and the RF modulation signal are converted to digital waveforms that are digitally divided, preferably by an equal number of powers of two, so that the resulting outputs are square waves. The divided signals are applied to an exclusive OR gate that provides a pulse waveform having a duty cycle that reflects the relative phase of the two RF frequencies. The pulse waveform is filtered to create an average voltage that linearly corresponds to the particular state of phase in the communication light beam and the analog communication signal. 
     Other aspects of the invention separately describe the optical transmitter and the optical receiver. 
     The foregoing and additional features and advantages of this invention will become apparent from the detailed description and accompanying drawing figures below. In the figures and the written description, numerals indicate the various elements of the invention, like numerals referring to like elements throughout both the drawing figures and the written description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of the optical communication system in accordance with the present invention. 
     FIG. 2 is a timing diagram illustrating the waveforms developed at several elements in the receiver of the optical communication system illustrated in FIG.  1 . 
     FIG. 3 is a timing diagram illustrating the waveforms developed at several elements in the receiver of the optical communication system after the state of phase of the phase modulated signal has been changed. 
     FIG. 4 is a plot of voltage versus state of optical phase representing the output signal produced by the optical receiver. 
     FIG. 5 is a schematic block diagram of the optical communication system, according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the schematic block diagram of FIG. 1, the present invention provides an optical communication system, generally designated by the numeral  10 , including an optical transmitter  7  and an optical receiver  8  separated by an analog optical link  9 . As will be described the optical communication system  10  produces a voltage signal that linearly corresponds to the state of phase of the phase modulated communication light beam, which in turn corresponds to the voltage of an analog communication signal. This state of phase is also referred to as a particular wavefront state. The particular embodiment shown in FIG. 1 uses free-space optical interconnections; however, as will be described, an all fiber optic embodiment is also possible. 
     In the optical transmitter  7  an optical source  12  generates a beam of coherent light at an optical frequency v, which is applied on an optical fiber or a free-space beam  14  to an optical frequency shifter  16 . Preferably, the optical source  12  is a semiconductor laser diode such as a distributed feedback (DFB) laser, although any coherent source such as a helium neon (HeNe) laser can also be used. A portion of this light is split by a fiber optic coupler or mirror  15  which is directed by fiber or mirror  17  to an optical frequency shifter  16 . A radio frequency (RF) oscillator  18  generates an RF fixed frequency electrical signal which is applied through an amplifier or RF driver  20  to the optical frequency shifter  16 , and serves to create an upshifted beam of light. Preferably the RF signal is a sinusoidal signal but any fixed RF frequency may be utilized that is much higher than the highest frequencies of the analog communication signal that is applied at  46 . The optical frequency shifter  16  serves to shift the optical frequency of the light beam  14  by an amount of frequency corresponding to the RF. In the preferred embodiment, the optical frequency shifter  16  is an optical modulator such as a Mach-Zehnder modulator followed by a narrow pass band optical filter  19  to extract the shifted side band light. Alternatively, the optical frequency shifter can be an acousto-optic modulator. 
     In the case of using a Mach-Zehnder modulator as an optical frequency shifter, the modulator is biased at the minimum light transmission so that the delivered light will be directed into just the upper and lower side bands at an optical frequency that is shifted either up or down by the RF. By filtering this light with a narrow pass band optical filter  34 , such as a Fabry-Perot filter or a Bragg grating filter, it is possible to extract just the light that is either upshifted or down shifted in frequency. For the Mach-Zehnder modulator, the RF signal can be as high at 40 GHz. 
     In the case of the acousto-optic modulator an acoustic sound wave is generated in an optically transparent medium by use of a piezoelectric transducer and the applied RF. This sound wave provides a traveling Bragg grating with a period that corresponds to the RF and diffracts the incident light into an up shifted and/or down shifted light beam. The first order of diffraction is shifted in optical frequency by the RF, the second order of diffraction is shifted in optical frequency by twice the RF and so on. The acousto-optic modulator can use RF signal frequencies up to 2 GHz. 
     The output light from the optical frequency shifter is the first of the three transmitted beams. In the case of the Mach-Zehnder modulator frequency shifter, it is filtered by the narrow pass band optical filter  19  and delivered as the frequency shifted beam  24 . This beam is carried by optical analog link  9 . 
     A beam splitter or fiber optic coupler  26  sends a portion of the unshifted beam  22  into a first input port of a straight optical phase modulator  50  having an optical output  52 . An analog communication voltage signal  46 , such as a signal in the radio frequency (RF) range or a video signal, is amplified by analog signal driver  48  and applied to the electrical modulation input terminal of the straight optical phase modulator  50 . 
     The straight phase modulator  50  is a linear optical device that comprises an electro-optic medium that has a refractive index that depends linearly on the voltage applied across the medium. Thus, the refractive index of the optical medium is dynamically changed according to the applied voltage signal. When the refractive index of the optical medium is changed, the “optical path length” is changed and this causes a change in the relative optical phase of the light delivered by the straight phase modulator (relative refers to the state of the phase at the output when there is an applied voltage compared to the state of the phase at the output when there is no applied voltage). Accordingly, the straight phase modulator  50  effects the state of the optical phase of the light delivered at output  52  so as to have a linear dependence with the applied voltage. By changing the optical path length it produces a phase modulated communication beam  54  that corresponds linearly with the analog communication voltage signal that is applied at  46 . 
     In this patent the phrase “optical path length” is characterized and mathematically defined as the product of the physical propagation distance and the associated refractive index of the medium through which the light propagates. It should be recognized that adjusting the optical path length can be accomplished by adjusting the physical propagation distance or adjusting the index of refraction of the medium. 
     The remaining portion of the beam  22  is sent by beam splitter or fiber optic coupler  26  to reflector or mirror  28  which is preferably an approximately 45° mirror and reflects the beam, referred to as the reference light beam  30 , out of the transmitter  7  and through the analog optical link  9 . 
     In the preferred embodiment, the light beams  24 ,  30 , and  54  are transmitted by the transmitter  7  through the analog optical link  9  via separate optical fibers. Alternatively, the link may be free space but the three beams must remain separate. 
     The receiver  8  receives the light beams  24 ,  30 , and  54  after they pass through the analog optical link  9 . More particularly, the reference beam  30  is reflected off 45° beam combiner  32  or optical fiber coupler. Simultaneously, the upshifted light beam  24  is applied through beam splitter or fiber optic coupler  42  so that a portion is directed to beam combiner  32  where it interferometrically combines with the reference beam  30  and is applied to photodetector  60 . The beam combiner  34  combines the light from the frequency upshifted and the frequency unshifted beams so that the optical interference between the beams performs a heterodyne of these two optical frequencies which generates the beat frequency representative of the RF modulation frequency from RF source  18 . Preferably, the optical path length of the two beams are equal. The beam splitter achieves this optical interference and performs the heterodyne by making the two beams co-linear and superimposed on each other, although other techniques, including fiber optic combining techniques, also can be used. 
     It should be recognized that the degree of coherence provided by the particular source used dictates the optical path lengths and the particular beam recombination geometry. Thus sources at various wavelengths, optical paths of different lengths and different mirror geometries may be employed. It is important, however, that the light from the two beams be combined so that the optical interference that occurs between the two beams performs the heterodyne which generates the beat frequency corresponding to the RF modulation frequency. 
     The photodetector  60  responds to the intensity variations of the combined beams by optically heterodyning the two optical frequencies to create the reference RF beat frequency signal. This is applied to divide chain path  62  as shown in dashed lines. In this way, an RF drive signal reference is generated in the receiver using optical interference. An amplifier  64  amplifies the detected RF beat frequency signal to a sinusoid at a preselected amplitude that can trigger a digitizing circuit. More particularly, a Schmidt trigger  66  converts the sinusoid into a digital waveform signal at  68  which corresponds to the RF beat frequency. However, other components that are functionally equivalent to a Schmidt trigger and that yield a signal that can be sent to a digital divider also can be used. 
     The digital waveform signal at  68  is then provided to a digital divider  70  that creates at its output  72  a square wave that is lower in frequency by the particular integer divisor used in the divide chain. Simple digital flip-flop dividers make it convenient to divide by a particular power of two. Mathematically, the square wave frequency is: 
     
       
           f /2 m , where  m =1, 2, 3, 4 . . .  
       
     
     and f is the RF drive signal frequency. The power of two used for the division depends on the desired resolution of the phase. It will be apparent that division by integers, denoted as N, other than powers of two also will result in similar performance provided the output of the divider chain is a square wave and that both chains  62  and  76  divide identically. Division by powers of two is the preferred embodiment because of its simplicity and ease of implementation. 
     A portion of the upshifted light beam  24  is sent via the beamsplitter  42  to a beam combiner  74 . Similarly, the phase modulated communication beam  54  is delivered to the beam combiner  74 . The beam combiner  74  interferometrically combines the beams  24  and  54 , performs a heterodyne of the optical frequencies, and applies the results to photodetector  75 . The beam combiner  74  combines the light from the frequency upshifted and the phase modulated communication beams so that the optical interference between the beams performs an optical heterodyne that generates the beat frequency representative of the RF drive signal from oscillator  18 . Preferably, the optical path length of the two beams are equal. The beam combiner achieves the optical interference by making the two beams co-linear and superimposed on each other, but other techniques, including fiber-optic combining techniques, can be used. 
     The photodetector  75  responds to the intensity variations in the interference of the combined beams, and by optically heterodyning the two optical frequencies the RF beat frequency signal is generated. The photodetectors in this invention are preferably PIN photodiodes, but other photodetectors such as avalanche photodiodes or photomultiplier tubes can be used. This beat frequency signal is applied to path  76  as shown in dashed lines. The particular state of phase of the detected RF beat frequency relative to the original RF drive signal reference corresponds directly to the particular state of phase of the phase modulated communication light beam  54 . An amplifier  78  amplifies the detected RF beat frequency signal to a sinusoid at a preselected amplitude that can trigger a digitizing waveform. More particularly, a Schmidt trigger  80  converts the sinusoid into a digital waveform signal at  82  which corresponds to the RF beat frequency. However, other components that are functionally equivalent to a Schmidt trigger and yield a signal that can be sent to a digital divider also can be used. 
     The digital waveform signal at  82  is then provided to a digital divider  84  that creates at its output  86  a square wave that is lower in frequency by the particular integer divisor used in the divide chain. Just as was the case in divide chain  62 , simple digital flip-flop dividers make it convenient to divide by a particular power of two. Mathematically, the square wave frequency is: 
     
       
           f /2 m , where  m =1, 2, 3, 4 . . .  
       
     
     and f is the RF driver signal frequency. The power of two used for the division must be identical to that used in divider  70 . It will be apparent that division by integers, denoted as N, other than powers of two also will result in similar performance provided the output of the divider chain is a square wave and that both chains  62  and  76  divide identically. 
     For analog communication signal frequencies less than 1 MHz, the digital dividers  70  and  84  can be FAST TTL flip-flops manufactured by Fairchild Corporation and designated as Model 74F74. For higher analog communication signal frequencies, the digital dividers  70  and  84  are made from ECL logic or digital GaAs or InP high speed integrated circuit logic which will enable analog communication signal frequencies up to 2 GHz. The two resulting square waves at  72  and  86  have an offset in their relative phase dependent on the original offset in phase that was created by the straight optical phase modulator  52 . 
     With reference now to FIG. 2 timing diagrams are shown for the signals appearing at the outputs  72  and  86  as they are processed through successive stages of the optical receiver. More particularly, the offset is shown by the displacement in the transitions in the timing diagram between the signal  72  at FIG. 2A, which is the divided square wave signal derived from the RF drive signal, and the signal  86  at FIG. 2B, which is the divided square wave signal from the phase modulated communication beam beat frequency. As illustrated one period of the square waves shown in FIGS. 2A and 2B is 2 m /f. When these square wave signals are combined by an exclusive OR gate  90  it produces a pulse wave form shown by the signal at FIG. 2C, with a duty cycle dependent on the changed phase due to the straight optical phase modulator  50 . This duty cycle is then sent through a low pass filter  92  that develops a DC voltage on its output as shown in FIG. 2D, having a magnitude that is dependent on the duty cycle. Preferably, the filter  92  is a conventional resistor capacitor integrator circuit that has a time constant that is less than the time transitions in the analog communication voltage signal applied at  46 . 
     As shown in FIG. 3, new waveforms are created as a new interference pattern is developed by the optical receiver. This results in the square wave signal at FIG. 3B being developed by the digital divider  70 . As shown the signal is displaced relative to the signal at FIG. 3A (and also to the signal shown in. FIG. 2B as illustrated by the dashed lines) and corresponds to the phase change. This leads to a new pulse wave form at the output of the exclusive OR gate  90  with an increased duty cycle as shown by the signal at FIG.  3 C. This increased duty cycle creates a larger DC voltage at the output  94  of the low pass filter  92  as shown in FIG. 3D that has a linear dependence on the actual phase change. Plotting the voltage at the output  94  as a function of the state of optical phase in radians of the phase modulated communication beam leads to a repeating linear triangular wave form that spans several optical wavelengths of phase change as shown in FIG.  4 . Note that in FIG. 4 the independent variable is the state of optical phase in radians (not time as shown in FIGS. 2 and 3) and that the voltage increases linearly from a minimum to a maximum when the state of optical phase is changed by πN radians. For the special case of division by powers of two, this is also equal to π2 m  radians, where m is the integer power of two used in the divide chains  62  and  76 . 
     Also note that the linear triangular pattern repeats after the phase has changed by several wavelengths of optical phase. In normal use the division ratio will be chosen to that the multiple wavelengths of phase change all takes place on a single slope or segment of the response curve of FIG. 4, thus giving a linear response without passing over the peaks or valleys of the triangle wave, thereby avoiding ambiguity. Since the state of optical phase in the phase modulated communication beam has a direct linear correspondence to the analog communication signal, the output voltage at  94  will have a direct linear correspondence to the analog communication voltage signal applied at input  46 . 
     Referring now to FIG. 5, an alternative embodiment of the optical communication system  10  is illustrated. many of the parts of the system  10  are identical in construction to like parts in the system illustrated in FIG. 1 described above, and accordingly, there have been applied to each part of the system in FIG. 5 a reference numeral corresponding to the reference numeral that was applied to the like part of the system described above and shown in FIG.  1 . 
     The fundamental difference between the system  10  of FIGS. 1 and 5 is that this embodiment shows the application of an acousto optic modulator as a frequency shifter. This type of frequency shifter does not require a narrow band optical filter. 
     The acousto-optic modulator  16  creates an acoustic sound wave that forms a traveling Bragg grating and generates two optical beams  22  and  24  from the modulated beam of light. The optical beam  22  comprises the unshifted zeroth order beam of transmitted coherent light at frequency v that passes directly through the modulator  16  and the optical beam  24  is a first order Bragg diffracted beam that is up shifted in optical frequency by the RF modulation frequency (v+40 MHz) and is directed at the Bragg diffraction angle. Alternatively, both beams can be shifted. In any event the beam  24  is separated from the beam  22 . 
     In addition, the acousto-optic modulator  16  can generate other beams that can be used besides the unshifted beam  22  and the upshifted beam  24  shown in FIG.  1 . In particular, it is possible to use a down-shifted beam (not shown) that can be generated by the acousto-optic modulator in place of the upshifted beam, or to shift both beams. 
     The upshifted beam  24  is reflected off mirror  36  via a grazing incident reflection to deliver the transmitted shifted beam  37 . Simultaneously, the reference light beam  22  is reflected off beam splitter  26  to the mirror  28  where it delivers the transmitted reference beam  36 . In addition, the reference light that is transmitted through the beam splitter  26  is sent to the straight phase modulator  50  to create the transmitted phase modulated beam  54 . These three beams are conveyed by link  9  to the optical receiver. There the light is processed exactly as it was in FIG.  1 . 
     Accordingly the present invention provides a voltage signal that varies linearly according to the optical state of phase of the communication light beams, and hence corresponds to the analog communication signal. 
     Obviously, many modifications and variations of the present invention are possible in view of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.