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Claims  |
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We claim:
1. An optical transmission or distribution system for broadband signals comprising:
an optical transmitter for providing transmitted optical power comprising:
a light source;
an electrical-to-light modulation means for modulating said light having a first non-linear modulation transfer characteristic having an associated shape, there being a bias point on said characteristic, said modulation means including an
information signal input for receiving an electrical modulation signal, an output for providing a modulated signal, and means for adjusting said modulation means;
one or more optical receivers;
a fiber optic distribution network for receiving said modulated light and distributing said modulated light to said optical receivers; and
a parametric feedback system comprising:
means coupled to said modulation means to superpose one or more pilot signals upon said electrical modulation signal;
preconditioning means coupled to said modulation means for providing electronic pre-conditioning of said electrical modulation signal, comprising:
means for providing non-linear predistortion to said modulation signal, having a second non-linear transfer characteristic; and
means to adjust said second non-linear transfer characteristic of said pre-conditioning means;
a monitoring optical receiver;
means to route a portion of modulated light to said monitoring optical receiver;
means coupled to said monitoring optical receiver to generate error signals indicative of the deviations of the system away from optimum points of minimal harmonic distortion and intermodulation associated with said system, by monitoring the
amounts of distortion generated by interaction of said pilot signals with the overall nonlinearity of said optical transmitter;
means to electronically process said error signals to provide processed error signals; and
means to feed back said processed error signals to said means for providing electronic pre-conditioning in order to adjust the overall transfer characteristic of said means for providing electronic pre-conditioning and said modulation means such
that the harmonic distortion and intermodulation components in the spectrum of the modulated signal at the output of said optical transmitter are reduced.
2. An optical transmission or distribution system as in claim 1 wherein said electrical-to-light modulation means comprises a continuous wave laser and an external integrated optic electro-optic modulator with one optical input and at least one
optical output.
3. An optical transmission or distribution system as in claim 2 wherein said external integrated optic modulator is selected from the group of devices consisting of an integrated optic Mach-Zehnder interferometer, a directional coupler, a
1.times.2 balanced bridge interferometer, and a 2.times.2 balanced bridge interferometer.
4. An optical transmission or distribution system as in claim 1 wherein said light source and said modulation means comprise a directly modulated laser.
5. An optical transmission or distribution system as in claims 2, 3, or 4 wherein said means for adjusting said modulation of said modulation means comprises an electronic circuit which additively superposes a varying biasing signal upon said
information signal.
6. An optical transmission or distribution system as in claims 2 or 3 wherein said means for adjusting said modulation of said modulator comprises an electrode structure on said modulator, electrically isolated from a broadband modulation
electrode structure and connected to an electrical biasing means separate from said information signal input of said modulation means.
7. An optical transmission or distribution system as in claims 1, 2, 3, or 4 wherein said pilot signals are chosen such that the spectrum of selected distortion components which are generated by said pilot signals in said system, is disjoint in
frequency from the spectrum of said electrical modulation signal, and is disjoint in frequency from the spectrum of the distortion components generated by said information signal.
8. An optical transmission or distribution system as in claim 7 wherein only one pilot signal is used, which has second and third harmonics before it is superposed on said electrical modulation signal which are sufficiently low such that they
cannot be detected by said means to generate error signals.
9. An optical transmission or distribution system as in claim 7 wherein at least two pilot signals are used such that at least one of the second order and third order intermodulation products generated by said pilot signals in the presence of
overall nonlinearity are disjoint in frequency from the spectrum of said electrical modulation signal or the spectrum of intermodulation and harmonic distortion components generated by said electrical modulation signal.
10. An optical transmission or distribution system as in claims 1, 2, 3, or 4 wherein said means for providing non-linear pre-distortion comprises an odd-wave nonlinear electrical network that nominally has an odd-wave transfer characteristic
and comprising a parametric adjusting input used to adjust the third order Taylor coefficient of the transfer characteristic of said means for providing non-linear pre-distortion.
11. An optical transmission or distribution system as in claim 10 wherein said odd-wave non-linear network comprises two nominally identical networks symmetrically or antisymmetrically inter-connected and connected to biasing networks.
12. An optical transmission or distribution system as in claim 11 wherein said means for providing non-linear pre-distortion comprises biasing means to modify the shape of the transfer characteristic or the magnitudes of at least one of the
first three Taylor series coefficients of said two nominally identical networks such that a set of biasing conditions may be established where the resulting overall transfer characteristic is odd-wave regardless of whether said two networks are precisely
matched to each other.
13. An optical transmission or distribution system as in claim 12 wherein said biasing means is used to adjust the even-wave components of the said means for providing non-linear pre-distortion by adjusting the operating points of said two
networks in order to reduce even-order distortion.
14. An optical transmission or distribution system as in claim 12 wherein said means for providing non-linear pre-distortion attains an odd-wave transfer characteristic by a second electronic parametric feedback system comprising:
means for detecting the presence of second order distortion, namely second harmonic or second order intermodulation products of said pilot signals at the electronic output of said preconditioning means; and
means to feed back the output of said means for detecting the presence of second order distortion to said biasing means in order to reduce or substantially null out the second order at the output of the pre-conditioning means, resulting in a
substantially odd wave characteristic.
15. An optical transmission or distribution system as in claim 11 wherein said odd-wave nonlinear network comprises two nominally identical one-port networks inverted with respect to each other and electrically coupled in series with each other.
16. An optical transmission or distribution system as in claim 15 wherein said one-port networks each comprise a parallel connection of a diode and a resistor and means for biasing said diodes in their active regions of operation.
17. An optical transmission or distribution system as in claim 16 wherein said means for biasing comprises means for providing a first current through said diodes to adjust the third order Taylor series coefficient and means for providing a
second current through one of said diodes to adjust the second order Taylor coefficient.
18. An optical transmission or distribution system as in claim 11 wherein said odd-wave nonlinear network comprises two nominally identical one-port networks inverted with respect to each other and coupled in parallel with each other.
19. An optical transmission or distribution system as in claim 18 wherein said one-port networks each comprise a parallel connection of a diode and a resistor and means for biasing said diodes in their active regions of operation.
20. An optical transmission or distribution system as in claim 19 wherein said means for biasing comprises means for providing a first current through said diodes to adjust the third order Taylor series coefficient and means for providing a
second current through one of said diodes to adjust the second order Taylor coefficient.
21. An optical transmission or distribution system as in claim 11 wherein said odd-wave nonlinear network comprises a differential driver having a first and a second output lead and two nominally identical two-port networks coupled in a
symmetric arrangement such that congruent inputs are connected to respective ones of said first and second output leads of said differential driver, and wherein the output signal of said odd-wave non-linear network is taken between congruent output ports
of said two-port networks.
22. An optical transmission or distribution system as in claim 21 wherein:
said two-port networks each comprise a bipolar junction transistor with a chain of N diodes connected to its base, a chain of M-1 diodes connected to its emitter, and a load resistor connected to its collector; and
wherein said odd-wave nonlinear network further comprises:
means to provide an output signal of said means for providing nonlinear pre-distortion as the differential voltage across the two collectors of said two transistors of said two-port networks; and
means to bias the transistor in its active region of operation and to bias the combination of two two-port networks to an odd-wave characteristic,
wherein said optical transmission or distribution system further comprises a pair of current sources driving, said chains of N diodes with a pair of currents having magnitudes centered about a selected current value.
23. An optical transmission or distribution system as in claim 22 wherein N=2 and M=3, nominally resulting in a transfer characteristic that provides a close approximation to the arc sine characteristic ideally required to cancel all orders of
distortion for an ideal sinusoidal transfer characteristic of the light vs. electrical drive of said modulation means.
24. An optical transmission or distribution system as in claim 1 wherein:
said means to generate error signals comprises a plurality of error circuits, each having an output; and
said means to electronically process comprises a plurality of circuits, each comprising a low-pass filter having an input coupled to an output of an associated one of said error circuits.
25. An optical transmission or distribution system as in claim 1 wherein said electrical modulation signal comprises a frequency division multiplexed multichannel signal combining a plurality of individual channel signals.
26. An optical transmission or distribution system as in claim 25 wherein said individual channel signals comprise video signals.
27. An optical transmission or distribution system as in claim 25 wherein said individual channels signals comprise digital signals.
28. An optical transmission or distribution system as in claim 1 wherein said modulation signal comprises a digital signal.
29. An optical transmission or distribution system as in claim 1 wherein said modulation signal comprises a spread spectrum signal.
30. An optical transmission or distribution system as in claim 1 wherein said preconditioning means has a transfer characteristic substantially identical to the inverse of the transfer characteristic of said modulation means.
31. An optical transmission or distribution system as in claim 1 wherein said preconditioning means further comprise means for providing gain.
32. An optical transmission or distribution system as in claim 31 wherein said means for adjusting the transfer characteristic of said pre-conditioning means comprises means for adjusting said means for providing gain.
33. An optical transmission or distribution system as in claim 1 wherein said electrical modulation signal comprises an analog broadband signal.
34. An optical transmission or distribution system as in claim 1 wherein said preconditioning means further comprises means for providing frequency equalization.
35. An optical transmission or distribution system as in claim 34 wherein said means for providing frequency equalization comprises an electrical network that provides an overall frequency response such that the variations over frequency of the
gain and of the group delay of the overall system are reduced.
36. An optical transmission or distribution system as in claim 35 wherein the frequency response of said modulation means and the frequency response of said means for providing electronic preconditioning provide substantially constant group
delay, while providing an amplitude response which diminishes a substantially small amount over frequency such that the roll-off of the frequency response of said modulation means and of said means for providing electronic preconditioning are
counteracted by said frequency equalizing electrical network and such that the group delay frequency dependence of said means for providing frequency equalization is substantially constant.
37. An optical transmission or distribution system as in claim 34 wherein said means to adjust said transfer characteristic of said pre-conditioning means comprises means for adjusting said frequency equalization.
38. An optical transmission or distribution system as in claim 1 wherein said means to adjust said transfer characteristic of said preconditioning means comprises means to modify the shape of the nonlinear transfer characteristic of said
preconditioning means.
39. An optical transmission or distribution system as in claim 1 wherein said means to adjust said transfer characteristic of said preconditioning means comprises means to adjust the magnitude of at least one of the Taylor series coefficients of
said transfer characteristic of said preconditioning means.
40. An optical transmission or distribution system as in claim 1 wherein said means for adjusting the modulation of said modulation means comprises means for adjusting said first non-linear modulation transfer characteristic.
41. An optical transmission or distribution system as in claim 1 wherein said means for adjusting the modulation of said modulation means comprises means for adjusting said bias point on said first non-linear modulation transfer characteristic.
42. An optical transmission or distribution system as in claim 2 wherein said continuous wave laser comprises a diode pumped solid state laser, or a distributed feedback semiconductor laser.
43. An optical transmission or distribution system as in claims 2 or 3 wherein said preconditioning means yields a substantially small second order Taylor coefficient of its transfer characteristic which said parametric feedback system acts to
cancel by providing feedback to an adjusting parameter of an opposing second order Taylor coefficient of said modulation means.
44. An optical transmission or distribution system as in claims 1, 2, 3, or 4 wherein said means to route a portion of transmitted optical power to said monitoring receiver comprises a device selected from the group of devices consisting of a
bulk optics beam-splitter, a fiber optic tap, and a fiber optic coupler.
45. An optical transmission or distribution system as in claims 1, 2, 3, or 4 wherein said means to route a portion of transmitted optical power to said monitoring receiver comprises an integrated optic tap or coupler contained on an integrated
optic substrate which also contains said modulation means.
46. An optical transmission or distribution system as in claims 1, 2, 3, or 4 wherein said means to extract error signals is selected from the group of harmonic distortion detectors consisting of synchronous, coherent, and lock-in electronic
detector circuits comprising a demodulation means performing a function selected from the group of functions consisting of multiplication, chopping, and switching, of the input signal with a reference signal harmonically related to said one or more pilot
signals.
47. An optical transmission or distribution system as in claim 46 wherein said harmonic distortion detectors comprise a second order distortion detector and a third order distortion detector, wherein the reference signal for the second order
distortion detector has a frequency twice the fundamental frequency of a selected pilot signal, and wherein the reference signal for said third order distortion detector has a frequency triple the fundamental frequency of said selected pilot signal.
48. An optical transmission or distribution system as in claim 46 wherein the reference signal for the second order distortion detector has a frequency equal to a sum or difference of the fundamental frequencies of a first and a second pilot
signal, and wherein the reference signal for the third order distortion detector has a reference frequency equal to .+-.2w.sub.1 .+-.w.sub.2 or .+-.w.sub.1 .+-.2w.sub.2 in the case of two pilot signals or .+-.w.sub.1 .+-.w.sub.2 .+-.w.sub.3 in the case
of three pilot signals, where w.sub.1, w.sub.2 and w.sub.3 are the fundamental frequencies of said first, second, and third pilot signals, respectively.
49. An optical transmission or distribution system as in claim 4 wherein said parametric feedback system acts to cancel or reduce a second order Taylor coefficient of the transfer characteristic of said directly modulated laser by providing
feedback to an adjusting parameter of the second order Taylor coefficient of the transfer characteristic of said preconditioning means.
50. An optical transmission or distribution system as in claim 4 wherein said directly modulated laser comprises a distributed feedback semiconductor laser, an external cavity laser, or a Fabri-Perot semiconductor laser. |
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Description |
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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,
"Feasibility of Multichannel VSB/AM Transmission on Fiber Optic Links," 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 R. Joseph, "System Characteristics of Direct Modulated and Externally Modulated RF Fiber-Optic
Links", Journal Lightwave Technol., Vol. LT-5, pp. 180-387. 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, "Optical Waves in Crystals", (Ch. 7: Electro-optics, Ch. 11: Guided waves and integrated optics), Wiley, 1984: R. Alferness, "Titanium Diffused
Lithium Niobate Waveguide Devices", in "Guided-Wave Optoelectronics", 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.
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. 4. 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 (absorption 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., over a limited
range around Q, 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.
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, "Comparison of Second and Third Order Distortion in Intensity
Modulated InGaAsP Lasers and an LiNbO.sub.3 External Modulator", Paper WK2, OFC89' Conference on Optical Fiber Communications, Houston, Texas, 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 .mu.m 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, "High Sensitivity optical analog link using an external modulator", CLEO Apr. 24-28, 1989, Baltimore, Md.
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.
Copending U.S. patent application Ser. No. 07/370,711, filed Jun. 23, 1989 on an invention entitled "Optical Distribution of Analog Signals Using Optical Modulators With Complementary Outputs" teaches 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.
Copending U.S. patent application Ser. No. 07/370,711 teaches structures which utilize modulators with pairs of complementary outputs, 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 output ports, such an optical modulator having multiple output ports is used to provide multiple output signals which are simultaneously routed to a plurality of
optical receivers, or simultaneously routed to complementary input ports of a single optical receiver.
Complementary modulation means include those described by R. Alferness, "Titanium Diffused Lithium Niobate Waveguide Devices", in "Guided-Wave Optoelectronics", to. Tamir ed., Springer-Verlag, 1988, and include:
a directional coupler (FIG. 5), including electrodes for receiving signals for modulation,
a Y-fed directional coupler (FIG. 6), including electrodes for receiving a modulating signal; and
balanced-bridge interferometers (also known as 1.times.2 and 2.times.2 switches): devices which consist of either two directional couplers or a y-junction and a directional coupler, with a two-arm interferometer in between (FIGS. 7a and 7b,
respectively).
Additional complementary modulation means are described in copending patent application Ser. No. 07/370,711.
Linearity of Electro-optic Modulation Means
Both Mach-Zehnder type modulators and certain types of complementary output modulators such as the balanced bridge interferometer are described by a voltage-intensity transfer characteristic which is a raised cosine as shown in FIG. 4. In the
case of directional coupler type devices, the complementary outputs are both described by raised cosines but the curves are shifted with respect to each other such that when one is at minimum the other is at maximum, and the sum of the two is constant,
corresponding to constant output power from the two complementary outputs, the best operating bias point for linear operation is quadrature point Q which is located half way between the on and off states.
In general, the bias point is determined by the geometry of the device, e.g. the imbalance in length between the two arms in a Mach-Zehnder modulator, the length of the interaction regions in directional coupler type devices, as well as by static
DC voltages applied to the electrodes of the device. Other factors such as optical damage in the presence of excess optical power, temperature variations, and the like may cause the bias point to drift.
Around quadrature point Q the transfer characteristic appears to be an odd function. At quadrature point Q, the second order harmonic distortion is nulled out, as are all even orders of distortion. Only odd orders of distortion remain present
at quadrature bias point Q. The most important source of non-linearity remaining is the third order harmonic distortion, since the 5th, 7th, and subsequent higher odd orders of distortion are usually smaller than the third order.
Mathematically, the characteristic around the quadrature bias point appears to be of the form sin(.theta.), which may be expanded into a power series as equation ##EQU1## where the modulation angle .theta. is proportional to the voltage v(t)
applied to the electrodes: ##EQU2##
The absence of a second order term .theta.2 is apparent in this case, however if the device deviates from quadrature point Q by a small angle .theta..sub.b its characteristic is given by ##EQU3##
The most obvious effect of non-quadrature biasing is the appearance of the second order harmonic distortion term: ##EQU4##
Another method to assess the presence or absence of second and third order distortion terms is by looking at the second and third order derivatives of the transfer characteristic ##EQU5##
To suppress the second order term one needs to have g"(0)=0. The characteristic sin(.theta.+.theta..sub.b) has a null second order derivative with respect to .theta. at the point .theta..sub.b =0.
The Problem
The problem of analog transmission via fiber optics has received strong impetus with the introduction of some AM fiber links based on novel distributed feedback (DFB) semiconductor lasers. Analog AM transmission systems and distribution networks
benefit from fiber-optic transport, since the distance between electrical repeaters can be significantly increased as compared with electronic distribution networks. However, the requirements on second-order and third-order linearity are very stringent
for directly modulated lasers to be used in the optical transmitters, since the presence of non-linearities causes intermodulation distortion which, in the case of CATV transmission for example, shows up as intolerable degradation of the television
picture. Because of non-linearity of the optical source, the modulating signal amplitude has to be limited to small values in order to maintain the composite triple beat and second order distortion specifications under a tolerable level. A reduction of
the optical source non-linearity would directly translate into the ability to increase the modulation signal amplitude while still maintaining the composite distortion specifications. An increased modulation amplitude (modulation index) is equivalent to
better signal to noise ratio, i.e., the ability to distribute the analog signal over larger distances, to split the signal to more receiving sites, or to transmit more channels on the same link. The main obstacle with AM fiber transmission is how to
overcome the non-linearity limitations of optical sources for analog transmission. Currently used approaches include:
1. Careful laser device selection, using distributed feedback lasers and other types of semiconductor low-noise lasers, and trying to carefully select individual devices or modify the fabrication process in order to produce a more linear
response, for example preventing leakage current around the active lasing area. The problem with this approach is that currently the yield for devices with sufficient linearity is quite low, and reliable fabrication techniques for more linear laser
devices have not yet been found.
2. Feedforward techniques, as described by J. Koscinski, "Feasibility of multichannel VSB/AM transmission on fiber optic links" NCTA Technical papers, Las Vegas 1987 p. 17. In these techniques, compensation of the nonlinearity is achieved by
isolating the distortion produced in a nonlinear circuit and subsequently injecting the processed error back into the circuit. The disadvantages of these methods are in the requirements to use matched sources and the cost of two optical sources and the
complexity of delay and gain balancing.
3. Negative feedback techniques, such as described by J. Koscinski, "Feasibility of multichannel VSB/AM transmission on fiber optic links," NCTA Technical papers, Las Vegas 1987 p. 17, rely on a photodiode to monitor the optical signal and
provide the necessary feedback signal. The amount of distortion compensation depends on the feedback gain. Although the application of negative feedback is straightforward, large bandwidth requirements may create problems at high frequencies rendering
this technique impractical.
An alternative to directly modulating lasers is using external modulators in conjunction with CW lasers. A directly modulated laser such as a DFB laser tends to be more sensitive to optical back reflection, and noisier than a CW laser which is
modulated externally. The back reflection into the laser yields a non-linear light vs. current response when coupled with chirping effects of the directly modulated laser. The advantages of using modulators vs. lasers in terms of the non-linear
intermodulation distortion have been discussed by G. E. Bodeep, T. E. Darcie, "Comparison of second and third order distortion in intensity modulated InGaAsP lasers and an LiNbO.sub.3 external modulator," Paper WK2, OFC89' Conference on Optical Fiber
Communications, Houston, Texas, February 1989, where it was concluded that external modulators tend to have lower second order distortion but higher third order distortion then directly modulated lasers. However, the ability to maintain the second order
distortion of a modulator at a sufficiently low level cannot be taken for granted, and depends on how close to the quadrature point the device is biased. Fabrication imperfections, temperature changes, optical damage, and the like may cause the bias
point to drift away from quadrature, in which case the second order distortion becomes more substantial. In order to use external modulators effectively for analog transmission applications, the drift in the bias point (which causes second harmonic
distortion) as well as the third order harmonic distortion of the device must be eliminated or greatly reduced, since prior art modulators provide adequate linearity only over a very limited range around the quadrature point.
It is generally known that external modulators have a transfer characteristic that is considerably more stable than that of lasers. Unlike the lasers' light vs. current curve, the shape of the modulators' transfer characteristic is generally
unaffected by optical power, temperature, aging, and the like, although the quiescent point of operation on the fixed transfer characteristic is affected by these factors.
If means were found to extend the linear range around the quadrature point and to reliably maintain the quadrature point, then the modulation index of the analog information signal could be increased and the performance of analog links improved,
with better signal to noise ratio, and the ability to provide longer links.
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 is a graph depicting a raised cosine transfer characteristic and the quadrature point of linear operation of an external modulator;
FIG. 5 depicts a typical prior art directional coupler;
FIG. 6 depicts Y-fed directional couplers;
FIGS. 7a and 7b depict balanced-bridge interferometers;
FIG. 8 depicts one embodiment of an optical transmission or distribution system constructed in accordance with the teachings of this invention;
FIG. 9 is one embodiment of a traveling wave balanced-bridge interferometer suitable for use in accordance with the teachings of this invention;
FIG. 10 is one embodiment of a synchronous harmonic detector suitable for use as one or both detectors 907 and 908 of FIG. 8;
FIG. 11 is a diagram of another embodiment of a transmission or distribution system constructed in accordance with the teachings of this invention;
FIG. 12 is one embodiment of an odd-wave linearizer circuit suitable for use in accordance with the teachings of this invention;
FIGS. 13a, 13b, and 13c describe three embodiments of this invention utilizing predistortion networks;
FIG. 14 is a model useful for analyzing the embodiment of FIG. 13c;
FIG. 15 is a schematic diagram of one embodiment of a circuit suitable for use as the embodiment of FIG. 13b;
FIG. 16 is an embodiment of the topology of FIG. 13c;
FIG. 17 is a model of the circuit of FIG. 16 useful for analysis;
FIGS. 18a, 18b, and 18c are graphs depicting the odd-wave transfer characteristic of equation 69; and
FIG. 19 is a diagram depicting one embodiment which utilizes a plurality of pilot tones.
SUMMARY
The solutions provided by this invention for linearizing the response of electro-optic modulators are of great interest to the cable television industry in its attempt to expand its networks and reduce prices by cutting down on maintenance
expenses and to bring high-definition television (HDTV) to customers. The solutions provided by this invention are also of great interest to the telephone industry in its attempt to bring broadband ISDN services over optical fiber to the homes of
residential customers.
The invention discloses a general technique of taking advantage of the stability and repeatability of the modulator transfer characteristic in order to correct for second and third order distortion. These objectives are achieved in a way
compatible with the nature of the video distribution frequency formats.
The method of this invention for linearizing the amplitude transfer characteristic of integrated optic devices for analog transmission is accomplished by injecting a pilot signal, monitoring the harmonic distortion content of the optical output,
and feeding back bias signals to parametrically tune the operating points of the integrated optic modulator and of a predistortion network. General principles for realizing suitable parametrically tunable predistortion networks are also presented.
DETAILED DESCRIPTION
FIG. 8 depicts one embodiment of an optical transmission or distribution system constructed in accordance with the teachings of this invention which is suitable for distribution of broadband analog electronic signals. Transmitter system 900
includes modulator means 902, a fiber optic distribution network 903 including one or more splitters 904, and at least one optical receiver 905.
The following discussion, by way of example, describes the teachings of this invention when used for the distribution of multiple video channels for cable television. It is known that the composite video signal in a cable TV distribution system
and in off the air TV broadcast systems is a highpass signal starting at 54 MHz, with the individual AM/VSB modulated video channels stacked in frequency at intervals of 6 MHz (except for some frequency gaps used for FM radio, mobile radio, etc.)
In accordance with the teachings of this invention, the low-pass vacant region (e.g. from 0 to 54 MHz in USA Cable Systems) of multichannel analog transmission (or other vacant spectral regions such as gaps between channel groups) is utilized for
transmitting one or more pilot tones generated by pilot tone oscillator 910, which are used to monitor the presence of second and third order distortion, via second order distortion detector 907 and third order distortion detector 908 respectively.
Signals from detectors 907 and 908 resulting from the monitoring of the pilot tones at the receivers are fed back, via loop processor 909, to modulator driver 906 in order to hold modulator 902 at the quadrature bias operating point, to the effect of
eliminating or reducing second order distortion, and to parametrically tune non-linear predistortion circuits within modulator driver 906 in order to eliminate or reduce its inherent second order distortion and eliminate or reduce third-order distortion
and second order distortion of the overall system by generating opposing distortions in modulator 902 and in the compensating non-linearity within modulator driver 906.
The unique form of feedback used in accordance with the teachings of this invention for improving the linearity of analog transmission signals is labelled here "parametric feedback," since the feedback is applied to parameters of the modulator
and its driver rather than to the modulating signal itself.
Comparing the parametric feedback method of this invention with prior art negative feedback methods described by Koscinski, supra, it is apparent that the difference lies in the fact that in accordance with the present invention, slowly varying
feedback is applied to the parameters of the predistortion network and modulator, which control the quiescent point, whereas in the prior art fast varying feedback is applied to the input signal itself. In the present invention, output diagnostic
signals of intermediate frequency are monitored and used to actuate the feedback, whereas in the prior art the high frequency output signal is used to actuate the feedback. The advantages of utilizing lower frequency signals in the feedback loop are
apparent.
While a predistortion network affecting the input signal itself may be an obvious first step to the challenge of extending the small-signal range of linear operation of a modulator, in the wake of device to device variations and device drifts
this solution may not be viable. The adaptive parametric feedback solution taught in this invention, and tailored to the particular characteristics of electro-optic modulators, provides a consistent solution to the problem of improving non-linearity of
electro-optic modulators for analog transmission as required for distribution of video signals via fiber optics and other emerging applications.
The combined effect of the anti-distortion measures of this invention is to reduce the overall non-linearity of the optical analog transmitter to the degree that cable television system specifications of composite second order and composite
triple beat are met for higher modulation indexes, i.e., for higher signal to noise ratios.
In one embodiment, transmitter 900 of this invention includes a continuous wave laser, such as a DBF semiconductor laser or a solid state diode pumped laser, or an external cavity or a semiconductor Fabri-Perot laser serving as optical source
901. The output of optical source 901 is fed into modulator means 902, which may be, for example, an electro-optic Mach-Zehnder modulator, directional coupler, balanced bridge interferometer, or more generally any suitable modulating means. Modulating
means 902 is electrically driven by modulation driver 906 which serves several functions:
1) Modulation driver 906 linearly amplifies the information signal and feeds the result to modulating means 902 conditioned at the proper level in amplitude and in impedance.
2) Modulation driver 906 provides a DC or slowly varying control voltage which establishes the modulator bias point. This slowly varying control voltage is used to steer modulating means 902 to the quadrature point where the second-order
derivative of the transfer characteristic is nulled out. A typical embodiment, as shown in FIG. 11, comprises a bias-tee circuit combining the AC and quasi-DC components while keeping the two paths mutually isolated. The bias-tee circuit combines the
quasi-DC control voltage with the broadband information voltage (i.e., the composite video signal in the case of CATV distribution).
3) In another novel embodiment biasing electrodes are provided in the integrated optic modulator separate from the high frequency modulating electrodes. Unlike prior art where DC and high frequency signals are combined in one set of electrodes,
the claimed embodiment results in separating the high-speed circuitry from the biasing electronics. The high frequency electrodes do not have to carry any DC currents. In this case the modulation driver simply routes the quasi-DC signal to the biasing
electrode. An embodiment for a travelling wave balanced bridge interferometer is shown in FIG. 9.
4) Modulation driver 906 provides a parametrically tunable non-linear predistortion to the useful information signal in order to compensate for the residual non-linearity of modulating means 902, after the modulating means is brought to the
vicinity of the quadrature point. Parametrically tunable means that the quiescent operating points of the circuit (the parameters) can be tuned to affect the amounts of various orders of non-linearity.
5) Modulation driver 906 superposes one or more pilot signals from pilot tone oscillator 910 upon the electrical information signal, such that the spectrum of the pilot signal(s) and the second and third harmonics of the pilot spectral components
fall outside the passband of the useful information signal to be distributed. The second and third harmonics of the spectral components of the pilot signal(s) are monitored at the output of optical receiver 905 by harmonic distortion detectors 907 and
908 and used to actuate the parameters of the modulation driver 906 in such a way that the overall second and third order distortion is nulled out. This parametric feedback is a form of negative feedback technique, however the feedback is not applied in
real-time to the fast varying information signal but rather it is slowly applied to the parameters controlling the overall nonlinearity of the system, in order to null the overall nonlinearity out.
To this end, an optical splitter is placed upon at least one optical output of modulating means 902 to route a portion of the transmitted optical power to a monitoring optical receiver (which can be thought of as part of transmitter 899).
Optical splitter 904 can, for example, be realized either as a fiber optic directional coupler or be integrated on the same integrated optic substrate with the modulating means 902, or be realized as a bulk optic beam splitter.
Monitoring optical receiver 905 feeds its output signal into two synchronous harmonic detectors 907 and 908, which detect the presence in the optical output of modulating means 902 of second and third harmonic spectral components of the pilot
tone, respectively. In the case when multiple tones are used, the detectors detect the presence of intermodulation tones corresponding to the pilot tones.
One possible embodiment of a synchronous harmonic detector suitable for use as one or both detectors 907 and 908 is shown in FIG. 10. Synchronous harmonic detector 1000 comprises lock-in detector 1001, formed by mixer 1002 followed by low-pass
filter 1003. Mixer 1002 multiplies the signals received on its two inputs, i.e. the electronic signal from monitoring optical receiver 905 (FIG. 8) and a reference signal at a harmonic or intermodulation tone of the pilot tone(s). The reference signal
is phased by phase-shifter 1004 such that it is in phase with the harmonic to be monitored. The use of ph | | |
