Optical distribution of analog and digital signals using optical modulators with complementary outputs
INTRODUCTION 
     Although digital communication has been gaining in importance, analog signal transmission is still an important transmission method, particularly in regard to the distribution of video signals. The broadcast distribution of television signals and the distribution of television signals via cable are most obvious examples of analog communications. At the high bandwidths required for video signals, the attenuation of transmission lines such as coaxial cable and the bandwidth of repeater amplifiers are significant factors dictating the use of the transmission format which best preserves bandwidth, namely Amplitude Modulation (AM). In particular, AM-vestigial sideband (AM-VSB) modulation is used in television. The penalty to be paid for the reduced bandwidth of AM as compared to digital transmission is a higher signal to noise ratio required for AM transmission (about 30 dB more than for digital transmission). 
     In particular, fiber optical transmission has been considered for cable television (CATV) systems as a means to transmit AM/frequency division multiplexed (AM/FDM) signals over longer distances, without repeaters, as described by J. Koscinski, &#34;Feasibility of Multichannel VSB/AM Transmission on Fiber Optic Links,&#34; NTCA Technical papers, Las Vegas 1987, p. 17. Such fiber optic AM links apply optical intensity modulation upon a light source, sending the modulated light signal via fiber to a receiver which converts the optical intensity signal back to an electrical signal. The modulating electrical signal is a composite signal with the AM video channels multiplexed in frequency, hence the term AM/FDM. 
     The useful length of a fiber optic analog link is determined by factors such as the optical power available, the relative intensity noise (RIN) of the optical source (which is typically a laser), and the sensitivity of the optical receiver. Also of importance is the non-linearity of the modulation characteristics of the optical source. Second order and third order distortion cause cross-talk, intermodulation distortion, and interference among the transmitted channels. 
     With present technology, the limiting factors for optical AM links used for cable television transmission are the RIN of the laser source and the second order distortion resulting from the light-current modulation characteristics. 
     External modulation of the light source has been proposed and demonstrated for analog transmission via fiber, as described by W. E. Stephens and to. R. Joseph, &#34;System Characteristics of Direct Modulated and Externally Modulated RF Fiber-Optic Links&#34;, Journal Lightwave Technol., Vol. LT-5, pp. 180-387, 1987. When using an externally modulated light source, as shown in FIG. 1, the laser is run in the continuous wave (CW) mode, i.e. at constant output power into an optical modulator, which also has an electrical input port. The resulting optical output signal from the optical modulator is a modulated light beam, with an optical intensity envelope that is a replica of the modulating electrical signal. 
     Typical prior art electro-optic amplitude modulators are described by Amnon Yariv and Pochi Yeh, &#34;Optical Waves in Crystals&#34;, Ch. 7: Electro-optics, Ch 11: Guided waves and integrated optics), Wiley, 1984: R. Alferness, &#34;Titanium Diffused Lithium Niobate Waveguide Devices&#34;, in &#34;Guided-Wave Optoelectronics&#34;, to. Tamir ed., Springer-Verlag, 1988. Such a typical prior art amplitude modulator may be structured as either a phase modulator between crossed polarizers (FIG. 2), or an interferometric device superposing the outputs of two phase modulators (FIG. 3), i.e. a Mach-Zehnder interferometer. In turn, an electro-optic phase modulator is constructed by passing the optical beam through a material which is electro-optic, i.e. when an electric field is applied to it, the refractive index of the material and thus the speed of light and the time of flight delay, changes linearly with the applied voltage. 
     Of particular interest are integrated-optic modulators, where the electro-optic interactions occur in optical waveguides that are patterned into optical substrates via microlithographic techniques. Such integrated-optic devices tend to require lower drive voltages than bulk modulators, since the small dimensions of the optical guides are such that the electric fields across the optical guides are very intense. 
     Another type of modulation acts upon the state of polarization of the light beam, rather than its intensity, as shown in FIG. 4. Polarization modulation is achieved not by phase modulation, but by conversion between two orthogonal propagation modes of an integrated optical waveguide, i.e. TE-TM mode conversion, as described by R. Alferness, &#34;Titanium Diffused Lithium Niobate Waveguide Devices&#34;, in &#34;Guided-Wave Optoelectronics&#34;, to. Tamir ed., Springer-Verlag, 1988. One example of such a polarization modulator is shown in FIG. 4. Polarization modulator 400 includes optical waveguide 401, and electrode 404 placed in close proximty. Located on the opposite side of waveguide 401 is electrode 402, which receives a voltage V 1  which serves to phase shift the optical beam propagating within optical waveguide 401. Electrode 403 is located out-of-plane to electrode 404, and receives a voltage V 2  which serves to cause mode conversion of the optical beam propagating within waveguide 401. Such modulators have been used to modify an arbitrary state of polarization of an input light beam into a linearly polarized output beam. Such devices, when operated at low modulation rates, are called polarization controllers. 
     The advantage of externally modulating a laser as opposed to directly modulating the drive current of the laser is that the so called chirp effect--a parasitic FM modulation created when modulating the laser current--is avoided. Also, more significant for AM fiber optic transmission, the intensity noise of lasers which are run CW at constant power tends to be lower than that of lasers under broadband modulation. 
     The main disadvantage of external modulation is the need for a second optical device-the additional modulator, with increased cost, complexity, and insertion loss through the modulator. A large fraction of this loss is inherent or intrinsic in the physics of a modulator, which dictate a voltage-light characteristic that is typically a raised cosine curve, as shown in FIG. 5. The maximum points A of the curve correspond to maximum transmission (on-state) of the device, where ideally or intrinsically in the absence of excess losses (absorbtion of light in the guiding material) the transmission is 100%. The minimum points B represent the off-state of the device, where ideally all the light is blocked from reaching the output. For analog transmission, the device is biased at the 50% transmission point Q, the so called quadrature point. At the quadrature bias point Q, half the light is dissipated in the device, however, at this point the linearity is the best, i.e. modulation signal voltage deviations from this state translate into proportional intensity deviations with only slight distortion, caused by the deviation of the sine curve from a straight line. 
     Analog AM transmission systems and distribution networks benefit from fiber-optic transport, since the distance between electrical repeaters is greatly increased. However, the requirements on RIN and on second-order linearity are very stringent for directly modulated lasers to be used in the optical transmitters. An alternative to using directly modulated lasers is using external modulators. 
     Unfortunately, a major disadvantage of using external modulators is the intrinsic 50% loss upon biasing at the quadrature point of linear operation. Furthermore, it is important to find ways of further reducing the intensity noise, since this would result in improvement of link performance. 
     Furthermore, in the tree-and-branch architecture used by the cable industry for video distribution, which is described by K. to. Deschler, &#34;Cable Television Technology&#34;, McGraw Hill, 1986, one would typically like to distribute many optical fibers from the headend of a cable network, so it is advantageous to find methods of reducing the number of transmitters used to modulate the video signals onto the outgoing multiple fibers. 
     The advantage of using external modulators vs. directly modulated lasers in terms of the non-linear intermodulation distortion have been discussed by G. E. Bodeep and to. E. Darcie, &#34;Comparison of Second and Third Order Distortion in Intensity Modulated InGaAsP Lasers and an LiNbO 3  External Modulator&#34;, Paper WK2, OFC89&#39; Conference on Optical Fiber Communications, Houston, Tex., February 1989, where it was concluded that external modulators tend to have lower second order distortion but higher third order distortion than directly modulated lasers. However, the ability to keep down the second order distortion of an external modulator depends on how close to the quadrature point the device is biased. Various fabrication imperfections, temperature changes, optical damage, etc. cause the bias point to drift away from the quadrature point, in which case the second order distortion may become substantial. 
     A recent experimental demonstration of AM fiber transmission utilizing an external Mach-Zehnder modulator, and a high power low RIN 1.3 μm solid state Nd:YAG laser pumped by a high power GaAlAs/GaAs laser diode array, is described by G. E. Betts, L. M. Johnson, C. H. Cox III, S. E. Lowney, TuJ19, &#34;High Sensitivity optical analog link using an external modulator&#34;, CLEO &#39;89 Apr. 24-28, Baltimore, Md. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 depicts a typical prior art system including a laser and an external modulator; 
     FIG. 2 is an example of a prior art amplitude modulator utilizing a phase modulator located between cross polarizers; 
     FIG. 3 is an example of a typical prior art Mach-Zehnder interferometer; 
     FIG. 4 depicts a typical prior art polarization modulator; 
     FIG. 5 is a graph depicting a raised cosine transfer characteristic and the quadrature point of linear operation or an external modulator; 
     FIG. 6 depicts a typical prior art directional coupler; 
     FIG. 7 depicts a typical prior art Y-said directional couplers; 
     FIG. 8 depicts a typical balanced-bridge interferometer of a prior art; 
     FIG. 9 depicts another complementary output optical modulator; 
     FIG. 10 depicts yet another optical complementary output modulator; 
     FIG. 11 depicts a balanced optical receiver constructed in accordance with the teachings of this invention; 
     FIG. 12a depicts an optical system constructed in accordance with the teachings of this invention which utilizes a polarization modulator to complementary optical signals to be transmitted in different polarizations on a single optical path. 
     FIG. 12b depicts the polarization state of the structure of FIG. 12a; 
     FIG. 13 depicts an alternative embodiment as the structure of FIG. 12a which includes a polarization controller for insuring the polarization states are correct; 
     FIG. 14 is yet another embodiment of an optical system constructed in accordance with the teachings of this invention including a polarization control loop fed back from the receiving end of an optical link to the transmitter end of the optical link; 
     FIGS. 15a-15d depict a comparison between the use of prior art external modulators and the use of external modulators in accordance with the teachings of this invention; and 
     FIGS. 16a-16c depicts a comparison of power delivered to a plurality of receivers, using prior art techniques and in accordance with the teachings of this invention. 
    
    
     SUMMARY 
     In accordance with the teachings of this invention, a novel optical system suitable for the transmission of analog and/or digital signals is taught including an optical source, an optical modulator having at least one optical input port for receiving the optical signal from the optical source, a modulation port for receiving a modulating signal, and complementary output ports. The complementary modulated output signals are applied to one or more optical receivers at the other end of an optical transmission link. In one embodiment of this invention, the two complementary signals are provided to two separate optical receivers, thereby allowing distribution of optically modulated signals, for example, in a cable television or other type distribution system. In another embodiment of this invention, the two complementary modulated signals are applied to a balanced receiver, thereby providing information content to the complementary receiver at twice the power level available on either one channel alone. Furthermore, utilizing a balanced receiver, common mode error signals cancel, thereby increasing the signal to noise ratio of the system. 
     In one embodiment of this invention, a single optical transmission channel is used, and the two optically modulated signals are separated in polarization so as to be capable of being separately detected on the receiving end of the link. In one embodiment of this invention, feedback means is used to insure proper polarization states between the two complementary signals. 
     DETAILED DESCRIPTION 
     This specification discloses the application of integrated-optic devices, and the exploitation of the peculiar tree-and-branch architecture of video distribution, to effectively eliminate the intrinsic waste of 50% of optical power in external optical modulators that are biased in their linear region of operation. This specification also discloses a new architecture to reduce intensity noise, and to reduce the even orders of modulating signal distortion for imperfectly biased modulating devices. 
     Complementary Output Modulation 
     In prior art external modulation techniques for fiber-optic AM transmission, half the optical power is dissipated in the external modulating device because of the need to bias the modulator in quadrature, half-way between its on and off states, in order to attain the maximum degree of linearity. 
     One major advantage of this invention is the ability to utilize substantially all the optical power, including the 50% which is wasted in prior art modulators. The power previously wasted in prior art modulators is utilized in accordance with the teachings of this invention to accomplish transmission to either a different receiver, as required in situations involving distribution of signals to multiple points as in cable television, for example, or to the same receiver, in which case a novel signal processing technique is used to recombine the two signals. When the previously wasted power is routed to the same receiver, an important benefit results: partial cancellation of RIN and even orders of distortion. 
     Complementary Output Devices 
     In accordance with the teachings of this invention, modulators with pairs of complementary outputs are used, such that optical energy is not wasted but is rather transferred from one output to the other in accordance with the modulating signal. Unlike prior art integrated optic modulators having multiple ouput ports, in accordance with the teachings of this invention, such an optical modulator having multiple ouput ports is used to provide multiple output signals which are simultaneously routed to a plurality of optical receivers (FIG. 15c), or simultaneously routed to complementary input ports of a single optical receiver (FIG. 15d). One embodiment of a complementary output modulator constructed in accordance with the teachings of this invention uses a directional coupler which couples light energy from one waveguide to the other in response to the modulating signal. 
     In a single output modulator, half the optical power is dissipated in the device, on the average. The modulated optical signal rides on a bias CW component. When the modulating signal goes positive, the optical power rises with respect to the 50% bias level, and vice versa. 
     In a directional coupler or other modulating means with dual complementary outputs, whenever the light energy signal in one output is suppressed to less than its bias level, it reappears in the other arm as an equal signal above its bias level. Thus, the sum of the two output powers is constant with only constant absorbtion loss. 
     Complementary modulation means suitable for use in accordance with the teachings of this invention include those described by R. Alferness, &#34;Titanium Diffused Lithium Niobate Waveguide Devices&#34;, in &#34;Guided-Wave Optoelectronics&#34;, to. Tamir ed., Springer-Verlag, 1988, and include: 
     a directional coupler (FIG. 6), including electrodes for receiving signals for modulation, 
     a Y-fed directional coupler (FIG. 7), including electrodes for receiving a modulating signal; and 
     the balanced-bridge interferometer (also known 
     as a 2×2 switch): a device which consists of two directional couplers with a two-arm interferometer in between (FIG. 8). 
     An additional complementary modulation means is shown in FIG. 9, consisting of integrated optic device 1090 including Y-junction 1091 which receives light from optical source 1092. Y-junction 1091 feeds two-arm phase-shifting interferometer 1093, which in turn feeds directional coupler 1094. The input signal is split into two equal components. Each propagates over one arm of interferometer 1093. The interferometer arms are sufficiently separated to prohibit evanescent coupling between them. The two interferometer arms are connected to the two input ports of directional coupler 1094. The coupler may be equipped with control electrodes 1194 to adjust the splitting ratio as desired, for example to 50/50. A similar device has been used in another context as a one-by-two switch, having one input port and two complementary output ports. 
     Yet another complementary modulation means, shown in FIG. 10, consists of an integrated optic device 1060 comprising two-arm phase-shifting interferometer 1065 with arms sufficiently separated to prevent coupling therebetween followed by directional coupler 1066. External bulk-optic beam-splitter is used to inject light from optical source 1061 into the two input ports of two-arm interferometer 1065, thus performing off-chip the 50/50 splitting task. 
     Of importance, in all these embodiments the integrated optic couplers are used not in the conventional way as digital switches which serve to direct all light energy from one port to the other. Rather, the couplers are used in this invention in a novel manner to provide two complementary output signals for the purposes of analog modulation. 
     In particular, incorporating directional coupler type devices rather than interferometric modulators in the transmitter makes efficient use of the available power, by serving additional receivers in the broadcast distribution networks with power than was previously wasted in the substrates of prior art modulator devices. 
     Preferably, a CW laser source of high power and low RIN is used in conjunction with the integrated optic complementary means of modulation. One suitable source is a solid state diode pumped laser such as a Nd:YAG laser providing an output at approximately 1.3 μm wavelength, and pumped by a laser diode array. Another optical source suitable for this application is a distributed feedback laser (DFB) or an external cavity laser operating at either approximately 1.3 μm or approximately 1.5 μm wavelength, or an Er:YAG or Er doped glass laser operating at approximately 1.5 μm wavelength. 
     In one embodiment of this invention, an optical source and a complementary output modulator are both formed as a single integrated optic device, such as a InGaAsP/InP semiconductor laser monolithically integrated with a complementary output modulator. Alternatively, GaAs/GaAlAs can be used to form such a monolithic device. 
     A Single Balanced Optical Receiver 
     In one embodiment of this invention, as shown in FIG. 11, the two complementary outputs which carry modulation signals of opposite polarities are routed to the same optical receiver 900 rather than to two separate optical receivers. In one such embodiment, the receiver is a balanced receiver which uses two photodetectors 902-1;902-2 in order to detect the signals on the two complementary channels 901-1;901-2. The signal processing scheme required to combine the two signals consists of electronic subtraction of the two channels, for example, by means of differential amplifier 905. Since the modulated optical signals in channels 901-1 and 901-2 are derived with opposite polarities from the same electrical modulating input signal, this subtraction results in reinforcing the resultant signal to double the value of the modulation signals in each channel. 
     In addition to making use of power previously wasted in the integrated-optic device substrate of prior art devices, the advantage of this embodiment is substantial in terms of the reduction in spurious signals due to RIN. Since the two outputs of the modulating device are derived from the same optical laser source, the laser intensity excess fluctuations are common to the two signal paths, i.e. they appear with equal amplitudes and the same polarity. Subtraction of the two photo-detected signals cancels these common-mode signals. 
     Balancing of Gains and Matching of Delays 
     One requirement for the successful operation of the embodiment of FIG. 11 is that the two channels be balanced in gain and matched in delay, i.e. the complementary optical modulation signals which are presumably generated with opposite polarity and with equal amplitude at the transmitter maintain opposite polarity and equal amplitude at the receiver, whereas the common mode signals which are common to the two paths will arrive in phase at the receiver and therefore perfectly cancel. For this purpose, the gains in the two paths should be equal (or if the signals are launched with unequal amplitudes, the ratio of the gains should be equal to the inverse ratio of launch amplitudes) and the two optical paths should be equal in delay, such that the 180 degree phase relationship between the signals at the transmitter is maintained at the receiver. 
     In practice, the delay difference between the two arms should not exceed a small fraction of the RF wavelength corresponding to the highest frequency being transmitted, a condition that could be attained if the two fibers are strung in the fiber optic cable such that they stay in tight proximity (e.g. fibers from the same tube are chosen). 
     One way of adjusting the gain difference in the two arms for perfect cancellation of the noise and the even orders of harmonic distortion is to monitor the generation of the second-harmonic of a pilot tone signal at the output of the differential amplifier which subtracts the two channels. 
     Upon perfect balancing of the two paths this second-harmonic signal should disappear. In one embodiment, an automatic gain control signal is used for making the gain in the two channels equal, by monitoring the generation of the second harmonic signal. 
     In one embodiment, an electrical delay line is used to match the delay between the two paths, but in a preferred embodiment equalization is performed in the optical domain, by trimming the fiber lengths. Test equipment methods such as the use of OTDR (Optical Time Domain Reflectometry) and OFDR (Optical Frequency Domain Reflectometry) with high resolution are suitable for use in adjusting the two fibers to equal length. 
     Polarization Multiplexing of Complementary Channels 
     In another embodiment of this invention (FIG. 12a), the two complementary channels are multiplexed in polarization by polarization modulator 128, and transmitted over a single fiber 129, separated by polarization splitter 132, and the two complementary channels applied to balanced receiver 131. This is achieved by identifying each complementary channel with one of two orthogonal linear states of polarization in the fiber and modulating the polarization of the two states in a complementary fashion such that when the amplitude of one polarization increases, the amplitude of the other polarization decreases. For linear states of polarization, this corresponds to rotating the plane of polarization of the signal. For the generic elliptic polarization state, the push-pull complementary operation corresponds to changing the tilt of the polarization ellipse (this change of tilt may or may not be accompanied by a change of ellipticity, as depicted in FIG. 12b. At the receiver end, a means of demultiplexing the two polarizations is required. This means is provided by a polarization diversity receiver which separates the incoming optical signal into two orthogonal polarizations, and detects each polarization separately in a photo-detector. The outputs of the two photodetectors are then signal processed by subtraction as in the previous embodiment. This subtraction of the two polarization powers results in reinforcing the two useful transmitted signals, since the modulation signal affects the two polarizations with opposite polarities. Thus, subtracting two equal quantities with opposite polarities, results in doubling each of the original quantities. 
     One advantage of the embodiment of FIG. 12 is that the relative delay between the two polarization channels is assured to be maintained with good accuracy, since the two polarizations propagating in the same fiber see essentially the same optical path. 
     Polarization Control 
     A polarization modulator should be used to rotate the state of polarization of the launched light around the 45 degree position of the bias state. Maintaining balanced gains in this embodiment is more difficult. If polarization maintaining fiber is used, the state of polarization as launched at the transmitter will tend to be maintained at the receiver. With polarization maintaining fiber, the two principal axes of the fiber are preferably aligned with the axes of the polarization diversity receiver, and the bias state of polarization at the transmitter, at the output of the polarization modulator, is preferably at 45 degrees with respect to the axes of the fiber. 
     If regular fiber is used, the problem is encountered that polarization fluctuations may occur, wherein a given state of polarization launched at the transmitter evolves along the fiber due to residual birefringence of the regular fiber, into a different state of polarization at the receiver. A means is needed to actively modify the bias state of polarization (i.e. the DC component of the state of polarization) at the output of the transmission fiber such as it is oriented at 45 degrees with respect to the polarization detection axes of the polarization diversity receiver. 
     In the embodiment of FIG. 13, polarization controller 1301 is installed in the receiver, ahead of the polarization diversity receiver 131, to achieve this effect. Again the generation of a second harmonic signal would accompany a non-balanced state, wherein the modulated energies in the two polarizations are unequal. In one embodiment, this signal is used to feed back to polarization controller 1301, so as to bring the two polarizations into balance. 
     Polarization Controller in the Transmitter 
     Another embodiment, particularly well suited for cable television distribution signals, includes a polarization controller in the transmitter rather than in the receiver (FIG. 14). The particular utility of this embodiment with respect to cable television distribution has to do with the harsh environmental conditions that receivers are exposed to, whereas transmitters reside in a temperature-conditioned head-end. Another advantage of this invention is that the polarization controller may be integrated on the same integrated-optic chip as the polarization modulator in the transmitter. 
     In this embodiment, the control signals required to actuate polarization controller 1401 are transmitted from receiver 131 back to the transmitter. Fiber 129 itself could be used as a communication channel to achieve this telemetric transmission and close the polarization control loop. Alternatively, as shown in FIG. 14, an additional fiber link 1429 is used to feed back signals from receiver 131 to polarization controller 1401 located in transmitter 1489. In any event, feedback loop transmitter 1402 and feedback loop receiver 1403 are used to interface to the optical path used. The bandwidth of the system will be limited by the delay in the communication loop but, fortunately, the rate of change of polarization in regular single mode fiber is generally slow. 
     In one embodiment, the functions of polarization controller and polarization modulator are combined in the same integrated optics device consisting of a tandem combination of phase modulator and a TE-TM mode converter. Furthermore, in one embodiment a single electro-optic waveguide is used to carry superposed control fields from two different sets of electrodes in order to integrate the two functions of polarization control and polarization modulation. 
     Comparison with Prior Art 
     The following table shows a comparison in output power for a variety of prior art devices, as compared with a device constructed in accordance with the teachings of this invention. Assuming an optical source is used which is capable of producing peak output power of P peak , and the modulation is a sine wave, a directly modulated laser would provide an output beam having peak power P peak  and thus average optical power (0.5) P peak . As shown, a device constructed in accordance with the teachings of this invention provides two output signals, each at -3 dB from the CW optical source power level of P peak , with only approximately 2 dB loss. Thus the total output power of the device of this invention allows a total average power level of only -2 dB from the optical source, a significant improvement over the prior art. 
     
                       TABLE                                                       
______________________________________                                    
           Output Power                                                   
           When Modulated by                                              
                           Intrinsic                                      
                                    Total                                 
Device     a Sinewave      Loss     Loss                                  
______________________________________                                    
Directly   one output port 0        3 db                                  
modulated  @ (0.5) P.sub.peak (-3 db)                                     
laser                                                                     
Mach-Zehnder                                                              
           one output port @                                              
                           2 db     5 db                                  
           (0.5) P.sub.peak (-3 db)                                       
Directly   two output ports each                                          
                           0.1 db   3.1 db                                
modulated  @ one-half (0.5) P.sub.peak                                    
laser coupled                                                             
           (-6 db)                                                        
to a Y-splitter                                                           
Mach-Zehnder                                                              
           two output ports, each                                         
                           2 db     5 db                                  
Modulator  @ one-half (0.5) P.sub.peak                                    
coupled to (-6 db)                                                        
a Y-splitter                                                              
One embodiment                                                            
           two output ports,                                              
                           ≦2 db                                   
                                    ≦2 db                          
of FIG. 15c                                                               
           each @ (0.5) P.sub.peak                                        
           (-3 db)                                                        
______________________________________                                    
 
    
     An additional comparison of Mach-Zehnder external modulation vs. complementary output modulation vs. direct laser modulation is now described with reference to FIGS. 16a-16c. All three compared systems serve four receivers. Any number of multiple receivers can be served in such a fiber optic distribution system by repeatedly splitting the outputs using fiber optic splitters or couplers. For comparing these systems of FIGS. 16a-16c, assume that three laser sources 161 provide peak optical power P, and that the intrinsic losses of the modulators are represented by a fraction (1/L). Ignoring the attenuation in the optical channels (e.g. fibers), which are assumed equal in the three cases, and neglecting the small losses in the fiber-optic couplers, and assuming a sinusoidal modulating signal which causes the source (either the laser or the modulator) to make an excursion (1/M) of the full on-off optical power range, the amplitude at the sinusoidal intensity modulation received by each receiver in the three cases is given as follows: 
     Mach-Zehnder Modulator 
     The prior art Mach-Zehnder modulator system of FIG. 16a provides a total optical power applied to the four receivers 166 to be equal to P/(8LM). The factor of 8 is composed of three factors of 2, two caused by couplers 163, 164, and 165, and one caused by the intrinsic loss of Mach-Zehnder modulator 162. 
     Complementary Output Modulator 
     The embodiment of a complementary output modulator system constructed in accordance with the teachings of this invention shown in FIG. 16c provides a total optical power applied to the four receivers 166 to be equal to P/(4LM). The factor of 4 is composed of two factors of two, one caused by splitters 184 and 185, and the other caused by the fact that the optical power is used by complementary output modulator 183 to provide two complementary output signals. 
     Directly Modulated Laser (P/8M) 
     The prior art directly modulated laser system of FIG. 16b provides a total optical power applied to the four receivers 166 to be equal to P/(8LM). The factor of eight is composed of three factors of two: one factor of two because the average power is half the peak power P of optical source 161, and two factors of two caused by couplers 173, 174, and 175. In this case, the modulation losses (1/L) are avoided, however, a power output is reduced by a factor of 2 as compared to the complementary output modulator system of FIG. 16c. This is due to the fact that when the modulating signal goes fully negative, the output power of optical source 161 reaches a minimum, whereas complementary output modulator 183 of the system of FIG. 16c applies the energy extracted from the first arm when the power ouput of the first arm is decreased, and applies this power to the second arm to cause the second output signal to reach maximum power. 
     As has now been explained, the complementary output modulation technique of this invention is twice as efficient in optical power than the prior art Mach-Zehnder systems, and more efficient by a factor of 2/L prior art systems which use a directly modulated laser. A typical value of L is 1.5, so the present invention is more efficient than a system using a directly modulated optical source in terms of delivered power, namely by a factor of 1.33. Of course, additional advantages of external modulation are the reduced RIN of the unmodulated optical source and the ability to obtain significantly less second order harmonic distortion and intermodulation and well characterized third order harmonic distortion and intermodulation. 
     All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. 
     Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims.