MXPA97005846A - Optical system that uses nearly incoherent processing for the correction of distors - Google Patents

Abstract

The present invention relates to a system for providing compensation for optical non-linearities in an optical system that transmits data signals in a data transfer frequency band, said system comprising: an optical source for providing a coherent main optical beam; beam of light to receive said optical light beam and provide first and second beam of divided optical light, a frequency generator for generating a radio frequency (RF) modulation signal, whose frequency spectrum includes said frequency band of transmission frequency. data, RF signal bypass means for extracting a reference portion of said RF modulation signal, main modular means for receiving said RF modulation signal and modulating said first divided optical beam to output a beam of modulated main optical light that has modulated and distorted optical components; quasi-incoherent ompensation including: means for generating an optical beam synthesized incoherent with said main optical beam in said data frequency signal band, so that no optical interference product occurs between the main optical and synthesized light beams, within said frequency band of data transfer when said main optical and synthesized light beams are combined, means for generating an error signal indicative of the difference between said RF signal and said portion of distortion of the main optical beam, modulating means of compensation for receiving said error signal and modulating said second beam of optical light divided to output an optical beam of modulated compensation, and optical combining means for receiving said beams of main optical and compensation light and providing them with a beam of light Output compensates

Description

OPTICAL SYSTEM THAT USES NEARLY INCOHERENT PROCESSING FOR THE CORRECTION OF DISTORTION TECHNICAL FIELD The present invention relates generally to systems for correcting non-linear distortion in fiber optic communication systems and, more particularly to an optical network employing optical modulators and adding a correction signal on a fiber optic cable.

BACKGROUND OF THE INVENTION The use of electro-optical modulators for dynamic high-scale fiber optic link applications in the field of communications and other applications has been hampered by the non-linearity of these devices. The Mach-Zehnder modulator, which is the working horse of the analogous links, has an intrinsic non-linearity due to the interferometric nature of its operation. The linearization of these devices became necessary in most demand applications. The linearization of electroptimatic modulators for high dynamic scale applications has taken many forms. Basically, there are two linearization classes that have been explored. The first class contains those devices that have electronic predistortion, which electronically produces the correction for a third-order distortion before feeding the signal to a modulator. This aspect is limited in its ability to maintain the appropriate amount of correction on very large signal applications, where higher order non-linearities occur. In addition, the stability required in many applications is being reduced and devices that incorporate electronic predistortion are difficult to produce. The other aspect, which has been explored on a limited basis, is found in the classic feed-forward techniques used in the design of high-performance, high-frequency RF amplifiers for many years. In this aspect, the non-linear element is the modulator and is directly driven by the supplied RF signal. The output of this non-linear element is then electronically compared to the input, and an error signal is produced. The error signal is simply the difference between the input and the output of the non-linear element. This error signal is then amplified, fed forward and combined with the output of the first non-linear element. Care should be taken to match the amplitude and phase of the error signal and the output of the element in order to ensure adequate cancellation of the error signal. The illustrative prior art is shown in reference D 1, patent of E. U.A. No. 5, 166, 509 of Curran and shows a non-linearity reduction circuit of optical modulator. An optical modulator or laser source 10 has a detector 14 for detecting its output signal, and producing a corresponding output signal. A bypass device 12 is connected to derive a portion of the input modulation signal as a reference signal, which is compared to and subtracted from the detector output signal through a subtraction unit 18 to produce a signal output error proportional to the laser noise / distortion components. The error signal is amplified by the amplifier 34 and fed to an external modulator, to reduce or cancel the noise / distortion in the laser output signal. Reference D2, patent of E. U.A. No. 5,289,550 to Plastow shows a demodulated light source with a linear transfer function and method that uses it, including two modulatable optical sources. The output of the first modulator, with a low noise of high power, is sampled, compared with the input signal, and used to generate an error signal used to modulate the second optical source, with a moderate noise and low power. The output of the second modulator is combined with a delayed output of the first modulator to remove the linearity outputs. It may be desirable to have a system for correcting harmonic distortion in fiber optic networks, which is of independent wavelength and which allows the use of an individual light source. The present system is designed towards said invention.

COMPENDIUM OF THE INVENTION An object of the present invention is to provide an apparatus for correcting the non-linear distortion through a forward feed correction apparatus in a fiber optic system having an individual source of laser beam. Another object of the present invention is to provide an apparatus of the above type, which alters the coherence of an optical signal through an optical phase modifying device, so that a stable feed forward correction is obtained without false interference signals within a selected radio frequency band. A further object of the present invention is to provide an apparatus of the above type, in which the phase-modifying device uses a sine-wave variation dependent on the time (or combination of sine waves) of the differential optical phase of the optical signals. Still another object of the present invention is to provide an apparatus of the above type, in which the phase modifier device generates interference or noise terms outside the radio frequency band. Another object of the present invention is to provide an apparatus of the above type characterized by an optical delay of a source within a limited temporal coherence to mitigate the interference between the combination of the main and corrective optical signals.

Another object of the present invention is to provide an apparatus of the above type characterized by orthogonal polarization states for the main optical and corrective signals. A further object of the present invention is to provide an apparatus of the above type, in which the optimization of the feed-forward network is achieved via a parametric control of optical and electrical components. In accordance with the present invention, a system for providing compensation for non-linearities in an optical system that transmits data signals in a data transfer frequency band, includes an optical source for providing a coherent main optical light beam; a beam splitter for receiving the optical light beam and providing first and second divided optical light beams and a frequency generator for generating a radio frequency (RF) modulation signal, whose frequency spectrum includes the frequency signal band of data transfer. There is a derivation of RF signal to extract a reference portion of the RF modulation signal as well as a main modulator to receive the RF modulation signal and modulate the first divided optical light beam to ot an optical light beam Modulated main that has modulated and distorted optical components. A quasi-incoherent compensation apparatus includes a mechanism for generating a synthesized optical light beam incoherent with the main optical light beam in the data frequency signal band, such that no optical interference products occur between the optical light beams main and synthesized within the frequency band of data transfer, when the main or synthesized optical light beams are combined. There is also a mechanism for generating an error signal indicative of the difference between the RF signal and said distortion portion of the optical light beam. A compensation modulator receives the error signal and modulates the second split optical light beam to ot an optical light beam of modulated compensation. An optical combiner receives the main and synthesized light beams and provides it with an ot light beam.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified schematic representation of a known optical system that provides correction for non-linear distortion. Figure 2 is a simplified schematic representation of a system that provides optical correction for non-linear distortion as provided in accordance with the present invention. FIG. 3 is a diagrammatic representation of modulated optical signals, including a signal as compensated by the system of FIG. 2. FIG. 4 is a diagrammatic representation of modulated optical signals showing a composite triple pulse measurement.

Figure 5 is a simplified schematic representation of a first embodiment of the present invention using a phase delay. Figure 6 is a simplified schematic representation of a second embodiment of the present invention using orthogonal polarization states. Figure 7 is a simplified schematic representation of another embodiment of the present invention using small amplitude servo control of forward feed parameters. Figure 8 is a simplified schematic representation of a further embodiment of the present invention using small amplitude servo control with a pilot tone added to the main modulator. Figure 9 is a simplified schematic representation of another alternative embodiment of the present invention utilizing an active control of relative signal gains. Figure 10 is a simplified schematic representation of another alternative embodiment of the present invention generating two output light beams.

DESCRIPTION OF THE PREFERRED MODALITY In an optical fiber link correction of harmonic distortion can be obtained through the use of a known system similar to that shown in Figure 1,. In this system 10, a laser 12 generates an optical signal 1 to be presented to the modulator 16. A radio frequency signal generator 18 presents an RF signal on the line 20 to the modulator, while a portion thereof is derived on line 22 for the generation of an error signal. This portion derived from the RF signal is combined with a selective signal on line 24. The selective signal is initially produced by a detector 26 connected to an optical branch 28 on the output side of the modulator and then amplified through the amplifier. 29. A representation of the RF spectral content of the signal generated by the signal generator is shown in the phase graph 30, while the phase of the modulated optical signal is indicated in the phase graph 32. In these graphs, the phase is indicated by the arrows pointing upwards. This combination of optical signal is carried out in such a way that the result is the exact signal necessary to correct any error in the modulator. An error correction signal on line 34 is then used to activate a laser beam diode 36, which will produce an optical signal indicated at 38 which carries the error information. The output of the laser diode, whose relative phase is shown by the descending arrows of the graph 40, is then combined with the modulated optical signal presented by the original modulator through the use of an optical branch 42. The resulting output signal 44 presented on the fiber is the output copropagation of the modulator with the error signal. In the detector at the end of the optical fiber (not shown), these two signals are transformed into photocurrents, which are added, thus producing the desired sum of the original signal outside the modulator and the error signal. The resulting sum should ideally be a signal that resembles the original RF input. This implementation is full of several disadvantages. The strongest is the fact that the CW laser source used for the modulator and the laser diode are of different wavelengths. This difference in wavelengths is always large enough that the two signals do not propagate on the fiber at exactly the same speeds towards the dispersion of individual individual fibers currently used. This difference is the rate at which the phase shear stress between the error and the signal arises as they propagate down the fiber. After approximately 5-10Okm of propagation, the system is unable to correct the non-linear distortions of the modulator to the degree typically required (20dB). Since it is conceivable that these can be produced at exactly the same wavelength, this is not practical. One solution to this problem is to use the same laser beam source divided into two forms, one for the main modulator and the other used to generate the optical error signal. In this arrangement, the laser beam diode of the system of Figure 1 is simply replaced by a modulator similar to the main modulator. This could be acceptable if it were not for the fact that, in the thin optical coupler, the main optical signal and the error signal are coherently added in the optical domain, producing crossed terms, which are not present in the system 10. The crossed terms are extremely sensitive to the relative phase of two optical signals. A solution is achieved by controlling exactly the optical phase of the two light beams, but such control is extremely difficult to achieve to the necessary degree. Referring now to Figure 2, there is shown a system 45 provided in accordance with the present invention, having a forward wavelength topology of individual wavelength, which mitigates the coherent effects of combined light beams. The present invention provides, in essence, a system with a second optical light beam source by synthesizing it from a portion of the laser beam light of the system. The main requirement of the synthesized optical light beam is that, when combined with the laser beam of the system, the resulting light beam does not produce any interference at frequencies within the band of interest, that is, the light beams are quasi-incoherent. Various embodiments of the present invention are detailed, which manipulate the optical phases of the main "error" light beams and synthesized to produce the quasi-coherent combined light beams without any harmful false interference term within the frequency response. necessary of the entire optical system. The techniques used in these modalities include frequency change polarization rotation and time delay. The forward feed correction of non-linear distortion products is obtained with an individual laser beam source. The system 45 can be divided into main optical and correction circuits, 46, 47. The main optical circuit includes a laser 48 that provides an optical light beam 50 that functions as the optical carrier. There is a first modulator 52 that the "main" modulator, while a second modulator, "error correction" or forward feeding, is also provided. A radio frequency generator 56 presents an RF signal on the line 58 to the main modulator, while a portion of the RF signal is extracted on the branch 60 for the presentation of the delay and an equalization circuit 62, whose output is presented to a signal combiner 63. A portion of the laser beam is also extracted by an optical splitter 64, which presents the optical light beam removed at the input of the correction modulator. A portion of the main modulated optical light beam is also derived, detected by the photodetector 65 and amplified by the amplifier 66 and presented first to the delay and equalization circuit 68 and then to a second input port on the combiner. A difference signal between the output of the main modulator and the input signal thereto is amplified by the amplifier 69 and fed on the line 70 to the correction modulator. The two optical light beams of the main and correction modulators are then combined in the final optical coupler 72 to achieve cancellation of the non-linear distortion products. The problems arising from the interference between the optical light beams can be mitigated in the frequency band of interest by appropriately modifying the differential optical phase imposed by the main phase modulator. In the forward feeding mode of Figure 2, the error signal is generated in a known way and fed to the correction modulator, it generates the optical version of the error. As noted, the light beams arriving at the final optical coupler are coherent in the RF and optical domains. With the present invention, the optical coherence of these fields is altered by the phase modulator 74, which receives the optical light beam extracted before the presentation of the correction modulator. The phase of the extracted light beam is changed to move the light beam out of the band through the use of an individual tone constant amplitude signal, such as a 2 GHz tone provided by the phase modulation signal generator 76. Alteration of the coherence of the optical fields is done in order to produce no optical interference between the light beams in the final optical coupler at any frequency within the desired bandwidth.

The simplest, but not the only, way to view the present invention is to use a frequency change element instead of, for the purpose of the present, an equivalent of the phase modulator. Examples of frequency change elements may include acoustic devices or integrated optical circuits, complexes to achieve the same. If the optical signal (carrier) entering the error modulator has a frequency deviation relative to the carrier entering the main modulator, the resulting interference from the optical light beams in the output coupler could occur at a frequency equal to this deviation. The resulting photocurrent in a detector placed at the end of a fiber optic cable, for example, could contain the two signals, error and principal, and an interference term, which is around the frequency deviation. This can be expressed as: Isaiida = S2 Principal + S2 Error + F (Spr¡nciPai x Serror) Ec. 1 where l, a? da is the photocurrent in the detector, Sprmcipai and SErr? r are the optical field amplitudes of the principal and error modulators, respectively, and F is a function of the product represented in this argument. It is in this function, F, where interference crossing terms are retrieved. However, as noted above, the frequency content of the signals represented by the F function could be centered around the frequency deviation and outside the band of interest. In the embodiment shown in Figure 2, the phase modulator is used, effectively, to change the frequency of the optical carrier a frequency greater than twice the bandwidth. In addition, if the phase modulator is activated so that the peak-to-peak phase deviation equals 2,405 radians (corresponding to J0, the first zero of the Bessel function), the spectrum of the optical carrier signal that enters The error modulator will have power at frequencies deflected from the carrier at multiples of the drive frequency, but none at the original carrier frequency. In this special case, the bearer is said to be exhausted. Thus, any optical interference between the error light beam and the main beam will not produce any false signal within the bandwidth of the system, just as in the pure frequency change example set above. Since a simple phase modulation of the optical carrier light beam has been demonstrated, there are other waveforms, which can achieve the same desired result. The possible waveforms that can be used to minimize the coherent crossover terms in Eq. 1, which give rise to band interference effects, can be determined by observing the nature of the interference crossover term given by F and proportional to a cosine function, as shown below: F (SxS) - cos (0 (t)) Ec. 2 This implies that, in order to eliminate the effects of this term within the RF bandwidth of interest, 0 (t) must be chosen so that the average time of this term is approximately zero. cos (0 (t)) * 0 Eq. 3 where the average time is comparable with the inverse of the highest frequency in the RF band of interest. During practice, this time must not be greater than the inverse of twice the highest frequency. There are a number of solutions that satisfy this condition. Below is a partial list. • Sinewave / cosine wave with an amplitude of 2,405 radians or any other amplitude corresponding to zero in J0. • Modulated frequency signal with an amplitude of 2,405 radians or any other amplitude that corresponds to zero in J0. • A combination of harmonic signals. • Random noise of limited bandwidth with the appropriate characteristics in order to exhaust the vehicle.

Due to the one-to-one correspondence between the voltage applied to the phase modulator and the induced phase shift, any solution to Eq. can be made by applying an electrical signal in the form of the solution directly to the phase modulator electrodes. Those skilled in the art will then observe that the present invention corrects non-linear distortion in a fiber optic system, using interferometric modulators such as the Mach-Zehnder interferometer, although other apparatuses that provide optical modulation may be equivalently substituted. The present invention involves the addition of a corrosion signal on the optical transmission path, which contains error cancellation information. In addition, a light source can be used and the system can be made independent of the wavelength. This is possible due to the frequency change or phase mixing of the additive correction signal achieved through the use of an energy phase or modulator, which ensures the addition of correction light signal incoherently or quasi-incoherently within the width of interest band. The system of the present provides correction, which is also substantially independent of the length of the link in the fiber optic systems. Figure 3 illustrates graphically the performance characteristics of the system of Figure 2. Diagram 78 shows the amplitude against the frequency signal presented to the system with and without correction. Two frequency tones at 40 and 40.1 Mhz were used as the RF input signal to the system as a typical simulated input. A 400 Mhz tone was applied to the phase modulator to achieve the elimination of the coherent crossover term established in equation 1. The input R F signals were adjusted to produce an appreciable third order distortion as evidenced by the supports at 39.9 and 40.2 Mhz at the 80 uncorrected mark.

When the corrective network is enabled, there is a reduction of the distortion supports of more than 30 db, as in the mark 82, indicating a vastly improved linearity of the system. An additional perspective of the present invention can be seen referring to Figure 4. In this, a diagram 84 indicative of system performance is shown using cable simulated television (CATV) carriers at frequencies consistent with NTSC specifications of national telecommunications standard. . In this case, a channel system 60 with a modulation index (OMI) of about 6.4% per channel was used, the carrier centered at 289.25 MHz. The phase modulator signal has an approximate frequency of 1 GHz and a sufficient amplitude to eliminate the coherent cross terms between the main and error optical light beams. The mark 86 demonstrates the performance of the system with the bearer signal on. There is a component of system pulse signal (CTV), as evidenced by the large amount of signal present when the carrier signal was turned off (mark 88). This indicates that there is a significant and unacceptable distortion in the system. When the corrective network is turned on (mark 90), there is a significant reduction in the CTB signal, which corresponds to a significant improvement in the linearity of the system. The present invention encompasses modalities that use alternatives to externally modify coherence, in order to achieve a reduction in the coherent crossover term of Eq. 1.

One mode takes the advantage of a natural finite coherence length or temporal coherence length of any laser beam source. In this alternative embodiment, a portion of the laser beam can be delayed for a period much greater than the characteristic coherence time of the laser, tc. If this delayed light source is used as the light source for the forward power modulator, the lack of coherence between it and the light of the main phase modulator will ensure a reduction or elimination of the coherent crossing term. Figure 5 is a simplified schematic illustration of a first alternative embodiment of the present invention. The system 92 is substantially equal to that shown with respect to Figure 2, but also includes a delay element 94, which receives the optical light beam removed before presentation to the error correction modulator. The delay element delays the beam of light extracted for a period much greater than the coherence time, tc. This allows incoherent addition in the final coupler to form the optical output light beam. The delay element issues the need for a phase modulator and associated signal generator. The coherence time is commonly expressed as a characteristic length, Lc. This length is the distance over which the light could travel in the coherence time. The typical coherence lengths for laser sources vary from ten meters to several hundred meters for diode lasers, while diode pumped solid state lasers (DPSS) have a coherence length of many kilometers. The delay element is preferably a fiber optic coil of appropriate length. Note that from a practical point of view, it is currently not cost effective to include the fiber section in a fiber delay line necessary to achieve the required delay for DPSS laser beams. However, the fiber delay lines for diode lasers are made with moderate fiber lengths and are, therefore, economically viable. Another, simpler alternative aspect to modalize the present invention is to use orthogonal polarization states for the forward and forward feed error modulators. Interference between combined light beams is avoided in this mode, if the orthogonality of the two optical signals is maintained. Referring now to Figure 6, a second alternative system 96 is schematically shown. The system 96 is substantially the same as the system shown with respect to Figure 2 and is obtained by providing the modulators with a bias maintenance fiber 98, 100 in their respective outputs to receive the modulated optical light beams. There is also a 90 degree polarization rotator 102 that receives the optical light beam modulated by the correction modulator. Then, a rotated correction light beam 104 is combined with the main modulated optical light beam 100 coupling it to the orthogonal states of a bias maintenance coupler, 106.

This provides a feed forward correction signal, stable to the degree that the two signals are truly orthogonal and therefore does not interfere. Figure 7 is a simplified illustration of a third embodiment of the present invention. Shown in the Figure is a system 8 that is substantially equal to that shown with respect to Figure 2 with the inclusion of elements that allow active control. In general, the parameters that govern the amplitude of the correction signal and, therefore, the degree of cancellation of the distortion products in a system output light beam need to be precisely controlled in order to ensure optimum performance . Once the corrective RF signal phase is adjusted to maximize the cancellation of the distortion products, the only parameter is the relative RF gain between the main and correction optical circuits through the final coupler. This gain can easily be adjusted via an electrical control of the RF signal gain either from the 1 10 or 1 12 amplifier or by adjusting the amount of optical energy emanating from the output of either or both modulators by varying the amount of optical coupling provided by the couplers 1 14 or 1 16 or by adding an additional intensity modulator 1 18 in any of the correction (or main) optical circuits. Subsequently, this relative gain can be optimized by checking the cancellation of either the distortion products or a test signal using a small amplitude servo technique or another common optimization technique. In the embodiment shown in Figure 7, the system 108 also includes an output light beam coupler 120, which has a portion of the output light beam toward a detector 122, which signals are provided to the amplitude servo control circuit 124. small of a known type. The output control signals are presented on lines 126, 128, to amplifiers 1 10 and 1 12. Alternatively, the control signals can be represented to the optical dividers 1 14, 1 16, or to the modulator either main or of correction. Figure 8 is a simplified schematic illustration of a fourth embodiment of the present invention. The figure shows a system 130 which is also substantially the same as that shown with respect to that of Figure 2, with most of the aspects of the system 108 shown in Figure 7. However, the system 130 includes a generator of frequency 132 to provide a pilot signal in the frequency band of data for purposes of producing a small amplitude. This signal is added to the input signal of R F to the main modulator after the derived RF signal is divided for the error correction circuit. Consequently, the signal combiner receives the RF input signal, which is a reference, while the signal presented to the combiner by the amplifier 1 10 contains the modulator error plus the error terms introduced by the small amplitude signal. Also included in this mode is a filter 134 for filtering the signals received from the photodetector 122.

During operation, a pilot or test tone is continuously fed to the main modulator of the system 130 after the initial RF coupler. This pilot tone resembles the forward feed system as a distortion product, which must be canceled. Actually, there is no interference, in addition to its origin, between this tone and any distortion product created in the main modulator. The degree of cancellation of this tone will exactly mimic the cancellation of any distortion and is, therefore, an excellent measure of system performance. The RF gain (G2) of the amplifier 1 10 is controlled via an output signal of a controlled voltage gain element 136 or AGC. The AGC signal is continuously sent as a small amplitude oscillation around some level of gain error, which is generally slowly variable. This oscillation gain of small amplitude will be evident at the output of the detector that verifies the final output. Filtering is used to reduce the bandwidth of the signal entering the servo circuit, but the frequencies around the pilot tone frequency are retained. These frequencies can be placed anywhere in the bandwidth of the system. The amplitude of the detected pilot tone that emerges from the filtering network will vary in synchronization with the small amplitude oscillation gain. The phase, in relation to the small amplitude oscillation signal, and its amplitude can be used to determine the magnitude and direction of deviation of the optimal cancellation through the use of normal synchronous detection schemes used in servo feedback systems. The phase to the feedback loop is chosen so that it activates the detected pilot tone to a minimum, thus indicating a maximum cancellation. Since an individual pilot tone was used in the embodiment of Figure 8, any combination of tone or noise signals can be used to verify cancellation. In this way, several regions of the operating RF spectrum can be simultaneously verified. In many applications, a dual output light beam operation of the system is desired. An example of such a system is shown schematically in Figure 9, wherein two main modulator output light beams are corrected for distortion and are available for use. The system 138 provides the forward feed correction in basically the same way as it was used with the individual balance systems with the following modifications. The system 138 includes a dual output master modulator 140, 142 to generate main light beams 144 and 146, and a dual output correction modulator 142 generates forward feed light beams 158, 150. In this system, the phase The RF signals must be simultaneously set for maximum cancellation at each output. This is preferably achieved by manufacturing the system with closely controlled fiber sections, in order to ensure a small relative phase deviation between all the main and correction signals in each combiner 152, 154. The control of the interference cancellation can be achieved using the control circuit 156 by varying the relative gain of either the main modulator output beams or the correction modulator output light beams. For example, the gain of the amplifier 158 (G2) and the coupling provided by the coupler 160 (kO) can be manipulated in a simple manner to obtain optimal cancellation of each of the outputs of the system, simultaneously. Any combination of electrical or optical elements that vary the relative gain of the main or correction signals can be used in a similar way. In addition, closed-loop optimization can be achieved as shown * schematically in Figure 10, with system 162 using the same principles discussed previously with the system of the Figure. In system 162, two servo-links 164, 166 are formed, each checking the cancellation of one of the output light beams, with separate servo control circuits 168, 170 optimizing the content of each output light beam 172, 174. In an alternative embodiment, for example, the embodiment of the Figure 2 can be modified by inserting the phase modulator into the main optical light beam, thus creating the differential optical phase.

Claims (10)

1 .- A system (45) for providing compensation for optical non-linearities in an optical system that transmits data signals in a data transfer frequency band, said system comprising: an optical source (48) to provide a beam of light coherent main optical (50); light beam dividing means (64) for receiving said optical light beam (50) and providing first and second divided optical light beams; a frequency generator (56) for generating a radio frequency (RF) modulation signal, whose frequency spectrum includes said data transfer frequency signal band; RF signal bypass means (60) for extracting a reference portion of said RF modulation signal; main modular means (52) for receiving said RF modulation signal and modulating said first divided optical light beam to output a modulated main optical light beam having modulated and distorted optical components; a quasi-inconsistent compensation apparatus (47) that includes; means (74) for generating a synthesized optical light beam incoherent with said main optical light beam in said data frequency signal band, so that no optical interference product occurs between the main and synthesized optical light beams, within said frequency band of data transfer when said main and synthesized optical light beams are combined; means (65, 66, 68) for generating an error signal indicative of the difference between said RF signal and said distortion portion of the main optical light beam; compensating modulator means (54) for receiving said error signal and modulating said second divided optical light beam for outputting an optical beam of modulated compensation light; and optical combining means (72) for receiving said main and compensating optical light beams and providing therewith a compensated output light beam.

2. The system (45) according to claim 1, wherein the quasi-incoherent compensation apparatus further comprises; detector means (65) for receiving a first portion of said main optical light beam modulated to provide its electrical signal equivalents; signal combining means (63) for receiving said signals from the detector means and said reference RF signals and generating from the same signals corresponding to the difference between said received signals; a phase modulation frequency generator (76) for generating a phase modulation signal at a frequency outside said data frequency signal; phase modulation means for receiving said second divided optical light beam and the phase modulation signal for providing a second phase modulated optical light beam; and correction modulator means (54) for receiving said difference signal and said second phase-modulated optical light beam, as part of said synthesized light beam, a modulated correction optical beam having a substantially equal distorted optical component in magnitude a but opposite in phase to said modulated main optical light beam distortion component.

3. The system (45) according to claim 1, wherein the output optical light beam is described by, aiida = S2 Main + S2 Error + F (SPr / "c / Pa / x SErr? r) where lsa? da is the photocurrent in the detector, Sprí, c / Pa and S? rror are the optical field amplitudes of the main modulators (52) and compensation modulators (54), respectively, and F is a function of the product represented in this argument and wherein said compensation generating means generate a synthesized optical light beam such that: r ~ (SPrjnc¡pal X SError) ~ COS (0 (t)) * 0 within the frequency band of data transfer.

4. - The system (45) according to claim 3, wherein 0 (t) corresponds to a sine / cosine wave with an amplitude of 2,405 radians or an amplitude that corresponds to zero in a function, JO.

5. The system (45) according to claim 3, wherein 0 (t) corresponds to a modulated frequency signal with an amplitude of 2,405 radians or an amplitude that corresponds to zero in a Bessel function, JO.

6. The system (45) according to claim 3, wherein 0 (t) corresponds to a combination of harmonic signals.

7. The system (45) according to claim 3, wherein it has a random noise of limited bandwidth selected in order to exhaust a carrier frequency. The system (45) according to claim 1, wherein said quasi-incoherent compensation apparatus further comprises: detecting means (65) for receiving a first portion of said main optical light beam modulated to provide signal equivalents electric of the same; a signal combiner (63) for receiving said signals from the detector means and said reference RF signals and generating therefrom signals corresponding to the difference between said received signals; time delay means (94), which receive the second divided optical light beam, to provide a time delay, whose magnitude is selected to be greater than a coherence time for said main optical light beam; and correction modulator means (54) for receiving said difference signal and the second time-delayed optical light beam for output, as part of said synthesized light beam; a modulated correction optical beam of light having a distorted optical component substantially equal in magnitude a but opposite in phase to the distortion component of the modulated main optical light beam. 9. The system according to claim 1, wherein said coherent main optical light beam has an initial polymerization state and wherein the quasi-incoherent compensation apparatus further comprises: sensing means for receiving a first portion of the beam of main optical light modulated to provide its electrical signal equivalents; signal combining means for receiving the signals from the detector means and the reference RF signals and generating from the same signals corresponding to the difference between said received signals; correction modulator means for receiving said difference signal and said second optical light beam, as part of the synthesized light beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude a but opposite in phase to modulated main optical light beam distortion component; rotary biasing means (102) for receiving said synthesized, modulated optical light beam and for generating a synthesized optical light beam rotated to a 90 degree phase; and wherein the optical combining means (106) further comprises means for maintaining said polarization states of the synthesized, modulated, rotated optical light beam, and said modulated main optical light beam. 10. The system according to claim 1, wherein the quasi-incoherent compensation apparatus further comprises: sensor means for receiving a first portion of the main optical light beam modulated to provide its electrical signal equivalents; signal combining means for receiving the signals of the detector means and the reference RF signals and generating therefrom, signals corresponding to the difference between said received signals; correction modulator means for receiving the difference signal and the second optical light beam for outputting, as part of the synthesized light beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude a but opposite in phase to the modulated main optical light beam distortion component; wherein said system comprises: a pilot tone signal generator (132) for providing a pilot frequency signal for presentation to said main modulator in combination with the RF modulation signal sequence to said RF signal derivation; output signal detector means (122) for generating electrical signal equivalents of the optical output light beam; and small amplitude servo control circuit means (124) receiving said output light beam detector signals having a portion of pilot tone signal, said small amplitude servo control circuit means including automatic gain control means for varying the magnitude of gain control signals presented to the combining means at about a value thereof corresponding to a minimum of the pilot tone signal portion of detector signal of the output light beam. The system according to claim 1, wherein said quasi-incoherent compensation apparatus further comprises: sensor means (65) for receiving a first portion of the main optical light beam modulated to provide its electrical signal equivalents; signal combining means (63) for receiving said signals from the detector means and said reference RF signals and for generating from the same signals corresponding to the difference between the received signals; correction modulator means (54) for receiving the difference signal and said second optical light beam for outputting, as part of the synthesized light beam, a modulated correction optical beam having a distorted optical component substantially equal in magnitude a but opposite in phase to the modulated main optical light beam distortion component; wherein said main modulator means (52) further comprises: means (142) for generating two main output light beams and the correction modulator means further comprising means for generating two modulated correction optical light beams; and second combinator means (154) for receiving said second modulated correction light beams and for generating a second compensated output light beam therefrom. 12. The system according to claim 2, wherein further comprises: means for selecting the phase of said RF signal; means for generating signals indicative of optical energy detected in the compensated output light beam; and gain adjusting means (136) for receiving said compensated output light beam energy signals and for adjusting said relative RF signal gain between the main and synthesized optical beams; said RF signal phase and said relative RF signal gain selected to maximize the cancellation of the distorted optical components. 13. - The system according to claim 12, wherein the gain adjustment means further comprises means for adjusting the magnitude of the optical energy removed from the main optical light beam. 14. The system according to claim 12, wherein the gain adjustment means (156) further comprises means for adjusting the magnitude of the modulation through the correction modulating means. 15 - The system according to claim 12, wherein the pilot tone signal generator (132) further comprises means for providing a plurality of pilot frequency signals for presentation to the main modulator in combination with the RF modulation signal subsequent to the RF signal derivation; and wherein the automatic gain control means of the small amplitude servo control circuit means (170) further comprises means for varying the magnitude of the gain control signals presented to the combining means around the values corresponding to a minimum. corresponding to the pilot signal tone portions of the output light beam detector signal.