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 optical transmitter comprises means for generating a first reference light beam, and means responsive to the analog communication signal to produce a communication light beam having phase modulation corresponding to the analog communication signal. The optical receiver comprises first means responsive to the reference light beam and the RF signal to develop a shifted light beam that is shifted in frequency by the RF frequency, second means to interferometrically combine the communication light beam with the shifted light beam to provide a heterodyne signal including information regarding the state of phase of the communication light beam, third means responsive to said heterodyne signal to produce an 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 fourth means responsive to the RF signal and the electrical signals to provide an output signal that has a linear correspondence to the state of optical phase of the communication light beam and the analog communication signal. The optical transmitter and optical receiver are also invented.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following commonly assigned applications: “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; “Optical Communication System With Phase Modulation”, Ser. No. 09/285,215, 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 two transmitted beams and 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 a linear optical communication system for communicating an analog communication signal. 
     Briefly, the linear analog 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 and means responsive to the analog communication signal to generate a phase modulated communication beam. 
     The receiver comprises means responsive to the transmitted reference light beam and an RF signal and operative to develop a shifted light beam that is shifted by the RF frequency, means responsive to the communication light beam and the shifted light beam and operative to interferometrically combine the communication light beam with the shifted light beam to provide a heterodyne signal including information regarding the state of phase of the communication light beam, means responsive to the heterodyne signal and operative to produce an 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 means responsive to the RF signal and the electrical signal and operative to provide an output signal that has a linear correspondence to the state of optical phase of the communication light beam and the analog communication signal. 
     More particularly, the detection of the relative phase of the electrical signal includes means for converting the RF signal into a first digital waveform, a first digital divider for dividing the first digital waveform by a predetermined integer to form a first square wave, means for converting the electrical signal into a second digital waveform, a second digital divider for dividing the second digital waveform by the same predetermined integer to form a second square wave, an exclusive OR circuit for processing the first and the second divided square waves to form a pulse waveform, and means for integrating the pulse waveform to provide the output signal having a magnitude that varies linearly relative to the state of optical phase of the communication light beam and the analog communication signal. 
     Other aspects of the invention separately describe the transmitter and the 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 pulse 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 an alternative embodiment of the optical communication system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As illustrated in the schematic block diagram of FIG. 1, the present invention provides a linear analog 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 linear analog optical communication system  10  utilizes two transmitted light beams and generates a voltage signal that linearly corresponds to the state of the phase of the phase modulated communication light beam, which in turn corresponds to the voltage of an analog communication signal. The dynamic state of the phase modulated light beam 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 a beam splitter or fiber optic coupler  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. 
     The beam splitter or fiber coupler  16  sends a portion of the optical source beam into a first input port of a straight optical phase modulator  20  having an optical output  22 . An analog communication voltage signal  26 , such as a signal in the radio frequency (RF) range or a video signal, is amplified by analog signal driver  28  and applied to the electrical modulation input terminal of the straight optical phase modulator  20 . 
     The straight phase modulator  20  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 optical phase 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  20  effects the state of the optical phase of the light delivered at output  22  so as to have a linear dependence with the applied voltage. By changing the optical path length it produces a phase modulated communication beam  24  that corresponds linearly with the analog communication voltage signal that is applied at  26 . 
     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 optical source beam is sent by beam splitter or fiber optic coupler  16  to optical path  30  by either fiber or reflector  29  which is preferably an approximately 45° mirror and reflects the optical signal. This light is now referred to as the reference light beam  30  and is delivered out of the transmitter  7  and through one of the two beams of the analog optical link  9 . The other transmitted beam is the phase modulated communication beam  24 . These two light beams must remain separated, either by having independent non overlapping free-space paths or by each being carried by a separate optical fiber. In the preferred embodiment, the light beams  30  and  24  transmitted by the transmitter  7  through the analog optical link  9  are carried by separate optical fibers. 
     The receiver  8  receives the light beams  55  and  56  after they pass through the analog optical link  9 . In the receiver  8 , a radio frequency (RF) oscillator  31  generates an RF fixed frequency electrical signal which is applied through an amplifier or RF driver  32  to an optical frequency shifter  33 . The optical frequency shifter  33  is positioned to receive the reference light beam  56  and serves to shift the optical frequency of the beam  56  by an amount corresponding to the RF. In the preferred embodiment, the optical frequency shifter  33  is an optical modulator such as a Mach-Zehnder modulator followed by a narrow pass band optical filter to extract the shifted side band light, but it can be an acousto-optic modulator. 
     In the case of using a Mach-Zehnder modulator as an optical frequency shifter  33 , 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  38 , 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. 
     In the case of an acousto-optic modulator, an acoustic sound wave is generated in an optically transparent medium by a piezoelectric transducer and the applied RF. This sound wave provides a traveling Bragg grating with a period that corresponds the RF and diffracts the incident light into an upshifted 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 RF drive signal from oscillator  31  must be well above the highest frequency component of the analog communication signal. In the case of an acousto-optic modulator, this RF signal is limited to roughly 2 GHz. For the Mach-Zehnder modulator this RF signal can be as high as 40 GHz. It should be recognized that any fixed RF frequency can be used as long as it is higher in frequency than the highest analog frequencies in the communication signal. 
     The RF drive signal form oscillator  31  is also sent to the divide chain  62 . An amplifier  64  amplifies the RF drive 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 at the RF drive signal 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. 
     The upshifted light  35  that has been frequency shifted by optical frequency shifter  33  and filtered by narrow pass band optical filter  38  is conveyed by optical fiber or mirror  42  to beam combiner  74 . Beam combiner  74  can be a 50—50 beam splitter or a fiber optic coupler. Similarly, the phase modulated communication beam  55  is applied to the beam combiner  74 . The beam combiner  74  interferometrically combines the beams 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  31 . Preferably, the optical path length of the two beams are equal. The beam splitter achieves this 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 variation in the interference of the combined beams by optically heterodyning the two optical frequencies to create the RF beat frequency signal. This 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 driver signal from oscillator  31  corresponds directly to the particular state of phase of the phase modulated communication light beam  24 . An amplifier  78  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  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 signal waveform 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 communications 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  20 . 
     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 signal 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  20 . 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  26 . 
     As shown in FIG. 3, new waveforms are created as a new interference pattern is developed by the optical interferometer. This results in the square wave signal at FIG. 3B being developed by the digital divider  70 . As shown its 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 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 equal to π2 m  radians, where m is the integer power of two used in the divide chains. 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 so that the multiple wavelength 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  26 . 
     Referring now to FIG. 5, an alternative embodiment of the linear analog 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 and an optical interference means for obtaining the RF drive signal reference. This type of frequency shifter  33  does not require a narrow band optical filter. 
     The acousto-optic modulator  33  creates an acoustic sound wave that forms a traveling Bragg grating and generates two optical beams  34  and  35  from the modulated beam of light. The optical beam  34  is denoted as the reference beam and comprises the unshifted zeroth order beam of transmitted coherent light at frequency v that passes directly through the modulator. The optical beam  35  is a first order Bragg diffracted beam that is up shifted in optical frequency by the RF modulation frequency (v+2 GHz for the upper frequency limit of an acousto optic modulator) and is directed at the Bragg diffraction angle. Alternatively, both beams can be shifted. In any event, the beam  35  is separated from the beam  34 . 
     The reference beam  34  is reflected off 45° mirror  36  to a beam combiner  37 . Simultaneously, the upshifted light beam  35  is applied through beam splitter  43  to the beam combiner  37  where it interferometrically combines with the reference beam  34  and is applied to photodetector  60 . The beam combiner  37  combines the light from the frequency upshifted and the frequency unshifted beams so that the optical interference between the beams performs a heterodyne of the two optical frequencies which generates the beat frequency representative of the RF drive signal generated by oscillator  31 . 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, but 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 generates the beat frequency corresponding to the RF drive signal frequency from oscillator  31 . 
     In addition, the acousto-optic modulator  33  can generate other beams that can be used besides the unshifted beam and the upshifted beam shown in FIG.  5 . 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 photodetector  60  responds to the intensity variations of the combined beams that are generated 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. The photodetectors in this invention are preferably PIN photodiodes, but other photodetectors such as avalanche photodiodes or photomultiplier tubes can be used. 
     In this way, the RF drive signal reference is generated using optical interference in this alternative embodiment instead of being provided by a direct connection as was done in the embodiment shown in FIG.  1 . The RF drive signal reference generated from optical interference is used exactly as it was in the embodiment shown in FIG. 1 once it is applied to divide chain path  62 . Beam splitter  43  provides a portion of the upshifted light  35  for use at beam combiner  74 , in a similar manner to mirror  42  in the embodiment of FIG.  1 . Accordingly, the present invention provides a linear voltage signal that represents the optical state of phase of the phase modulated communication light beam. 
     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.