CA2146758A1 - Postdistortion circuit for reducing distortion in an optical communications system - Google Patents

Abstract

A postdistortion network (36) for a receiver at a receiver location (12) in an optical transmission system is disclosed. The receiver converts an optical signal carried over an optical transmission link (11) to an RF signal. The postdistortion network (36) includes an optical terminal for receiving the RF signal and a distortion generator for generating a distortion signal from the RF
signal. An adjusting circuit adjusts the phase and amplitude of the distortion signal based on distortion at said receiver location (12). The adjusted distortion signal is combined with the RF signal in order to cancel or substantially suppress distortion in the RF input signal. The present invention permits individual receivers in an optical transmission system to be compensated for dis-tortion which may be unique to that receiver location. A communication system including a predistortion network and a postdis-tortion network is also disclosed.

Description

wo 94/09574 2 1 4 6 7 5 8 PCr/USs3/09615 POSTDISTORTION CIRCUIT FOR REDUCING
DISTORTION IN AN OPTICAL COMMU~CATIONS SYSTEM

BACKGROUND OF THE INVENTION

Technical Field The present invention generally relates to the linearization of nonlinear optical tr~n~miscion systems by the generation of a compensation signal and, more particularly, to a method and apparatus for compensating the nonlinearities of an optical transmission system including at least one of a semiconductor laser, an optical amplifier, and an optical fiber communications link by the generation of a postdistortion signal.
Des~ Jlion Of The Prior Art Optical communications system which include a semiconductor laser, an optical fiber communication link, and an optical receiver are well known in the art.
These communications systems are adapted to carry a wide range of information including voice, video and data.
The typical optical communic~tions system includes a laser transmitter which transduces an electrical information signal into an optical signal. The optical signal is then carried over an optical fiber communications link where it is converted back to an electrical signal by a photodet~tor of an optical receiver.
The tr~nsnllission scheme may be analog or digital and the modulation scheme amplitude, phase, or frequency, or any combination of the above.

Wo 94/09574 2 2 1 4 6 7 5 8 Pcr/US93/0961S

One of the most advantageous optical communication systems from the viewpoint of simplicity and bandwidth considerations is an analog scheme where the optical intensity of the semiconductor laser is amplitude modulated. The optical tr~ncmiccion system, including the semiconductor laser, optionally an optical amplifier, and optical fiber comm1~niC~tions link is required to convert the electrical information signal linearly into an optical signal and to transmit the optical signal linearly over the communications link. In general, distortions caused by the semiconductor laser, the optical amplifier, and the fiber optic communications link cause the system to operate in less than an optimum manner.
Increasingly, this type of optical communication system is playing an important role in the delivery of high quality signals in all types of CATV architectures.
Distortion in optical tr~ncmiccion systems can origin~te from several different sources. One of the primary sources is the electrical to optical transducer, a laser diode in most systems. Another contributor is the optical communications link and, more recently, any optical amplifier in the optical link.
Some of these sources produce similar distortion signals which may even cancel others, but usually each distortion has its own unique char~ctenctirs and should be compensated for independently.
A laser diode generally exhibits distortion from several identifi~ble causes.
The first is generally that caused by the non-linearities of its LI (light intensity as a function of e1~t~ic~1 current) transfer functioll. The nonlinearity may be Wo 94/09~74 2 1 ~ 6 7 5 8 Pcr/US93/09615 superlinear (L increases at increasing rates for increases in I), or it may be sublinear (L increases at decreasing rates for increases in I). This type of laser diode distortion causes mainly second order and higher intermodulation products that are in~ep~ndent of frequency. This type of distortion can be corrected by generating a distortion signal which is generally similar (sum and difference beats of the carrier frequencies) with a similar amplitude but opposite phase. Another characteristic distortion of the laser diode is where the amplitude and phase of the modulating signal is distorted as a function of frequency changes. To correct for such a distortion, a co~-~pensation signal which varies in amplitude and phase as a function of frequency is advantageous.
The distortion generated by an optical link is generally caused by phase and amplitude dispersion. Generally, phase dispersion causes the different modulating frequencies of an optical signal to be phase shifted differ~ nt amounts. Phase dispersion is plu~ ional to the length of the optical link and causes second order distortion with amplitude prùpolLional to distortion frequency. Present optical communi~ ons systems use a 1330 nm. optical wavelength signal to minimi7e phase dispersion. However, newer systems that operate at a 1550 nm. optical wavelength are capable of minimi7in~ amplitude signal losses but with effect of increasing phase dispersion. It would be advantageous to compensate for the increased phase dispersion of the 1550 nm. system while maintaining its lower optical loss. Optical amplifiers, particularly Erbium doped fiber ampLifiers wo 94/0g574 ~ 1 9 S 7 5 8 pcr/l,ss3/o96ls (EDFA), allow greater lengths of the optical fibers to be used but produce their own distortion in the form of an amplitude versus frequency characteristic.
One technique of compencating for distortion from the above-described sources utilizes a predistortion signal to compensate the RF signal modulating the laser transminer for the distortion in the optical communications system. Such predistortion is discussed in commonly ~ccigned, copending Application Serial No. 805,251 entitled "Method and Apparatus for Predistortion" and Application Serial No. 805,259 entitled "Method and Apparatus for PredistortionH, which are incorporated herein by reference. However, predistortion networks cannot generally compensate for distortion which may be unique to a particular receiver or receiver location. For example, distortion caused by fiber dispersion is dependent upon fiber length and thus may be different at different receiver locations serviced by the same tr~n~miner. Additionally, the receiver itself may introduce distortion which may vary from receiver to receiver.
SUMl~RY OF THE INVENTION
It is an object of the present invendon to provide co",pensadon for distordon at a receiver location in an optical communications system.
It is another object of the present invendon to provide compensadon at a receiver location for distortion caused by modulation of a laser diode.
It is another object of the present invendon to provide co,l,pcnsation at a receiver locadon for distordon generated by an opdcal amplifier.

wo 94/09574 2 1 9 6 7 5 8 PCr/USs3/0961~

It is another object of the present invention to provide compensation at a receiver location for distortion due to chromatic dispersion in an optical fiber.
It is another object of the present invention to provide an improved optical communication system for compenc~ing distortion.
The present invention provides a postdistortion method and apparatus for the compensation of a nonlinear optical transmission system. In one preferred embodiment, the tr~nClr icsion system can be used for the carriage of a broadband television signal for a CATV system. The optical tr~ncmiccion system includes a semiconductor laser which acts as an electrical signal to optical signal transducer, optionally a fiber amplifier which increases the optical signal strength, and a fiber optic communications link which carries the optical signal to an optical receiver including a photodPtector. The postdistortion method and apparatus includes a distortion generator which accepts the photodete~tor output and generates a distortion signal of the same general type as the distortion inherent in the optical tr~ncmicsion system.
In one plefe.l~,d embodim~pnt~ the invention includes a direct path and a distortion path. An RF output of the photodete~tor, which may be a broadband multich~nn~PI CATV signal, is split between the direct path and the distortion path.
The distortion path has a distortion generator fed by a portion of the output of the photodetPctor to produce a distortion signal essPnti~lly equivalent to that which is produced by the optical communir~tions system. The distortion generated in the 21q6758 W094/09s74 - 6- Pcr/uss3/o96l~

distortion path is then recombined with the signal in the direct path to produce a cancellation or substantial suppression of the distortion.
The present invention is adapted to compensate for the distortion caused by the modulation of a laser diode with an RF input signal with a multiplicity of carriers. In addition, it will compensate for distortion generate~ in an optical amplifier due to its variation in gain with respect to optical wavele~gth.
Furthermore, the postdistortion network of the invention is particularly well-suited to co,l,pensate for distortion due to chromatic dispersion in the optical fiber. Since this distortion is dependent on the length of the fiber path from the transmitter to a receiver, its characteristics can vary from one receiver location to another.
Since postdistortion is implemented at receiver locations, the postdistortion compensation signal may be adjusted to compensate for distortion conditions unique to that location.
According to one aspect of this embo~irnent, a square-law device is used to generate the distortion signal in the distortion generator. A square-law device generates composite second order (CS0) distortion without generating other unwanted distortion products. CS0 distortion in a particular cll~nnel is the total power of the separate sum and difference beats of the other ch~nn~ls falling within that ch~nn~l. In one implementation of the invention, the square law device sele~ted is a field effect transistor, such as a GaAsFET (Gallium Arsenide Field Effect Transistor), operated in the non-linear region near pinch off. A GaAsFET

WO 94~0g574 2 1 ~ 6 7 5 8 PCr/USs3/09615 is chosen because of its good high frequency characteristics across the bandwidth of interest. In addition, because it is a volsge controlled device, the operating point of the device can be varied easily and precisely to generate the distortion characteristic desired. A device to invert of the phase of the distortion signal is further provided to produce the proper phase for cancellation of the system distortion.
In another embodiment of the invention, a distortion generator is utilized in a configuration which permits the cancellation of the fundamental frequency and the composite triple beat (CTB) component of the distortion. A triple beat is a third-order intermodulation product of two or three fundamental carriers which combine such as (fl + f~ + f3), (fl + f2 - f3), etc. and CTB is the composite cf all the beat frequencies falling in a particular cll~nnel for a particular range of frequencies.
Another embodiment includes a plurality of distortion generators, at least one of which has provisions for generating and cancelling CTB distortion, and at least one of which has provisions for generating and cancelling CSO distortion.
The present invention also provldes an optical communications system including a tr~ncmit~er having a predistortion network and a receiver having a postdistortion network, thereby affording increased ability to compensate for distortion in the system.

Wo 94/09574 8 2 1 4 6 7 5 8 Pcr/US93/09615 BRIEF DESCRlPrION OF THE DRAWINGS
These and other objects, features and advantages of the present invention will be better understood from a reading of the following detailed description in conjunction with the accompanying drawings in which:
Figure 1 is a system block diagram of an optical communications system including a postdistortion circuit in accordance with the present invention;
Figure 2 is a block diagram of a first embodiment of the postdistortion circuit illustrated in Figure 1;
Figure 3A is a block diagram of a first embodiment of a distortion generator for the postdistortion circuit of Figure 2;
Figure 3B is a sch~m~ic diagram of the distortion generator of Figure 3A;
Figure 4A is a block diagram of another embodiment of a distortion generator for the postdistortion circuit of Figure 2;
Figure 4B is a schem~tic diagram of the distortion generator of Figure 4A;
Figure 5A is a block diagram of another embodiment of a distortion generator for the postdistortion circuit of Figure 2;
Figure 5B is a sch~ tic diagram of the distortion generator of Figure 5A;
Figure 6 is a block diagram of an embodiment of the I-ch~nnel variable-gain network of Figure 2;
Figure 7 is a sche~n~tic diagram of the I-channel variable-gain network of Figure 6;

2 1 g 6 7 5 8 Pcr/US93/09615 Figure 8 is a schematic diagram of one embodiment of the Q-channel variable-gain network of Figure 2;
Figure 9 is a block diagram of another embodiment of the postdistor~on circuit illustrated in Figure 1 used to cancel multiple orders of distortion;
Figure 10 is a block diagram of an embodiment of the distortion generator of Figure 9;
Figure 11 is a schematic diagram of the distortion generator of Figure 10;
and Figure 12 is a system block diagram of an optical communications system in accordance with another embodiment of the present invention.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMEN~S
Figure 1 is a system block diagram of an optical communication system in accordance with the present invention. The system includes an optical transmitter 10, an optical communic~tions link 11, and one or more optical nodes 12. For clarity in the Figures, the details of one only optical node 12 are shown. The other nodes are similarly configured. The RF input to optical tr~nsmitt~r 10 may, for example, be a broadb~nd CATV signal consicting of a plurality of frequency-division-multiplexed video carriers, although the invention is not limited in this respect. The RF input signal is amplified by RF ~mplifi~r 14, the output of which modul~tes the bias current in laser diode 15. The bias current is provided by a current source 16. Modulation of the laser current causes the intensity of the wo 94/09574 2 1 4 6 7 5 8 PCr/USs3/0961s optical output to vary in a nearly linear fashion with respect to the input signal.
The current source and amplitude of the modulation current are adjusted for optimum link carrier-to-noise and distortion performance.
The optical output from optical transmitter 10 is coupled to optical communications link 11 for transmission to one or more optical nodes. Optical communication link 11 includes optical fibers 20 and 21 and may optionally include an optical amplifier 22 and an optical coupler 23 for feeding a plurality of optical nodes.
Optical node 12 includes a photodetector 30, for example, a PIN
photodiode, which transduces the intensity modulated signal into an amplitude modulated electric current signal. Photodetector 30 is reverse biased from a voltage source 26 through resistors 28 and 32. The photodetector output is amplified by photodetector amplifier 34, which may be a push-pull transimpedance amplifier or other suitable type designed for CATY applications. The RF output of photodet~tor amplifier 34 is processed by postdistortion compensation network 36 to reduce distortions generated from causes described above. The output of the postdistortion co,J-pensation network is amplified by buffer amplifier 38 for distribution to the CATY network.
Figure 2 is a block diagram of one embodiment of the postdistortion co",pensation network 36 illustrated in Figure 1. The RF input to postdistortion network 36 is the output from photodetector amplifier 34 of Figure 1. The RF

wo 94/0957~ 2 1 ~ 6 7 5 8 Pcr/us93/o96ls signal with distortion is split into two paths by directional coupler 110 with most of the output connected to a plug-in pad 112 in a first main path, and a smaller amount connected to a plug-in pad 115 in a second postdistortion path. The main path includes plug-in pad 112 and a delay network 114. Plug-in pad 112 is used to set the RF output level from the postdistortion network. Delay networl~ l 14, which may be a coaxial tr~nsmi~sion line, distributed delay line, or other suitable delay network, provides a main path signal delay which is substantially e~ual to the time for a signal to pass through the postdistortion path.
The distortion path includes a plug-in pad 115, a distortion generator 120, directional couplers 122, 132, and 134, and a distortion signal adjustment network 123. Plug-in pad 115 is used to set the signal level to distortion generator 120 to optimize the output of the distortion generator with respect to the amplitude of the desired distortion, supp.ession of high-order distortion, and suppression of the broadb~nd RF signal. The broadb~nd RF signal with distortion passes from plug-in pad 115 to distortion generator 120. The purpose of distortion generator 120 is to generate intermodulation distortion, primarily second order (CSO), third order (CTB), or both, while subst~nti~lly suppressing the undistorted broa~lb~nd signal.
The output of distortion generator 120 is provided to adjustment network 123 for adjusting the phase and amplitude of the distortion signal. Specifically, the output of distortion generator 120 is split by directional coupler 122 into two paths: an in-phase path which will be referred to as the I ch~nnel, and a 90-degree-shifted Wo 94/09574 1 2 21 4 6 7 5 8 pcI/uss3/os6ls quadrature path which will be referred to as the Q channel. The I channel incudes an I channel variable gain network 124 and the Q channel includes differentiator 126, Q channel variable gain networlc 128, and delay 130. The gain and phase of the signals in the I and Q channels are adjusted as described in greater detail below. The outputs of the I and Q channels are combined into a postdistortion compensation signal by directional coupler 132. Directional coupler 134 provides an output from the postdistortion path for test purposes. The postdistortion compensation signal from directional coupler 132 is combined with the RF input signal with distortion in the main path by directional coupler 116. With appropriate adjustments to the gain and phase of the signals in the I and Q
channels, the combination of the RF input signal with distortion and the postdistortion co-~-pensation signal cancels or substantially supplesses the distortion from the RF input signal.
Figure 3A is a block diagram of one embodiment of distortion generator 120 illustrated in Figure 2. In particular, the distortion generator illustrated in Figure 3A generates second-order (CS0) distortion and inc~udes push-pull transformer 200 and first and second non-linear devices 202 and 204. First and second ports C and D of transformer 200 ~es~;Li~ely provide out of phase signals to non-linear devices 202 and 204, which are preferably FET arnplifiers. The outputs of non-linear devices 202 and 204 are combined such that the even ordered distortion components add and the odd ordered distortion c~lllponents cancel.

-21~6758 w094/09574 - 1 3- Pcr/US93/09615 Figure 3B is a schematic diagram of distortion generator 120 illustra~ed in Figure 3A and the non-linear devices include respective field effect transistors 220 and 220'. The push-pull transformer 200 illustrated in Figure 3A includes a balun 210 and a transformer 212 which provide a balanced output from the unbalanced RF input. A first output of transformer 212 is coupled to non-linear device 202 and a second output of transformer 212 is coupled to non-linear device 204. Non-linear device 202 includes a capacitor 214 coupled between the first output of transformer 212 and the control gate of FET 220. Resistor 233 and capacitor 236 are connected in series between a reference potential and a node between capacitor 214 and the control gate of FET 220. A gate bias is connected to a node between resistor 233 and capacitor 236. Capacitor 224 and resistor 240 are connected in series between output node 225 and a voltage source of a voltage +V. Capacitor 244 is connected between ground and a node between resistor 240 and the voltage source. The current terminals of transistor 220 are connected between ground and a point between capacitor 224 and resistor 240.
Non-linear device 204 is similarly arranged and will not be discusse~ in detail.
The b~l~nced output from transformer 212 is coupled through capacitors 214 and 214' to the gates of field effect transistors 220 and 220'.
Resistors 233 and 233' are terrninations for transformer 212. Field effect transistors 220 and 220' are biased from gate bias sources 230 and 230' to near pinch-off. In this region, the transistor drain current is a nonline~r function of -Wo 94/09574 21~ 6 7 5 8 Pcr~uS93/o96l5 gate voltage and can be described by the Curtice Cubic Model for field effect transistors. Accordingly, the drain current in transistor 220 can be expressed as Il d. = aO + a,V" + a2V2~, + a3V3"
where Il ~" is the drain current, V" is the gate-source voltage, and aO, a" a2, and a3 are constants in the nonlinear FET model. The gate-source voltage for FET 220' is the negative of the gate-source voltage of FET 220 since the field effect transistors are fed from a b~l~nced source. Thus, the drain current in 220' can be exp,~ssed as I2 d, = aO + a,(-V") + a2(-VGs)2 + a3(-V3")3 The transistor outputs are connecte~ in parallel to output terminal 225 through c~pacitors 224 and 224'. Summing the output currents gives = 2a2(V")2 Thus, the linear broadband signal and odd-order CTB distortion is cancelled at the output of the transistors. Drain voltage for the transistors is supplied through resistors 240 and 240'. Capacitors 236, 236', 244, and 244' are RF bypass c~pacitors. The gate bias sources 230 and 230' may be adjusted for best balance to minimi7c the fund~mental and odd-order-distortion col-,ponents at the output of the distortion generator.
TPncistors 220 and 220' are preferably GaAsFET devices because of their highly non-linear pinch-off region, common in many FETs, and its excellent high frequency characteristics across the CATV Sp~llUI~I. The device is also precisely wo 94/09574 21 4 6 7 5 8 Pcr/~s93/o96l5 controllable to tailor the distortion generated by the gate voltage. Most FETs have a nonlinear region of their operating drive which is of use.
Figure 4A is a block diagram of a distortion generator 120' which may also be utilized in the present invention. In particular, the distortion generator illustrated in Figure 4A generates second-order (CSO) distortion and includes hybrid transforrner 305 and diodes 307 and 309, which diodes are respectively coupled to ports B and D of hybrid transformer 305. Distortion generated in the diodes is reflected to port C of hybrid transformer 305, which port constitutes the output of the distortion generator.
Figure 4B is a schematic diagram of the distortion generator 120' illustrated in the block diagram of Figure 4A. Transformers 302 and 304 form a 4-port hybrid with ports labeled A, B, C, and D. Port A receives the RF input from plug-in pad 115. Port C constitutes the output of distortion generator 120' . Port D is coupled to ground through capacitor 312 and diode 309. Port B is coupled to ground through car~ritor 310 and diode 307. A first terminal of potentiometer 313 is conne~ted to the anode of diode 309 through resistor 314. A second terminal of potentiometer 313 is conne~ted to the anode of diode 307 through resistor 316. The first terminal of potentiometer 313 is coupled to ground through c~r~itor 318. The second terminal of potentiome~er 313 is coupled to ground through c~r~ tor 320.
The RF input is fed into port A of hybrid 305. The output of hybrid 305 -Wo 94/09574 214 6 7 5 8 Pcr/us93/o96l5 is split equally between ports B and D with a 180-degree phase difference between the signals at ports B and D. Port C is isolated from port A, i.e., with ports B
and D terminated, no power is coupled from port A to port C. Signals reflected from ports B and D add in phase at port C and out of phase at port A. Thus, the broadband input signal appears across diodes 30~ and 309 but with a 180-degree phase difference. The current through the diodes is an exponential function of the voltage across the diodes. This diode nonlinearity causes harmonic and intermodulation currents in the diodes which are coupled to ~orts A and C of the hybrid. Re~ause the diodes are driven anti-phase, the even-ordered distortion components reinforce at port C of the hybrid and cancel at port A. Odd-order distortion co,.,ponents cancel at port C and add at port A. Potentiometer 313 may be adjusted to minimize the amount of odd-order distortion, principally composite triple beat (CTB). Accordingly, the output of distortion generator 120' is second order (CSO) distortion.
Figure 5A is a block diagrarn of a distortion generator 120" which may also be utilized in the present invention. In particular, the distortion generator illustrated in Figure SA generates third-order (CTB) distortion and includes directional coupler 406, diodes 408 and 410 coupled to port D of directional coupler 406, and resistor 415. Diodes 408 and 410 are connected in an opposed, parallel relationship between port D and a reference potential such as ground.
Distortion is generated in the diodes 408 and 410 and reflec~ed by dire~tion~l Wo 94/09574 21 ~ 6 7 ~ 8 Pcr/us93/o96l5 coupler 406 to pon C thereof, which port constitutes the output o~ the postdistonion generator. Resistor 415 provides a termination for port B of directional coupler 406.
Figure SB is a schematic diagrarn of the distonion generator 120"
illustrated in the block diagram of Figure SA. Directional coupler 406 includes pons A, B, C, and D. Port A receives the RF input from plug-in pad 115. Pon B is coupled to ground through resistor 415. Port D of directional coupler 406 is coupled to the cathode of diode 408 and to the anode of diode 410. Resistor 412, capacitors 416 and 418 and resistor 414 are connected in series between voltages +V and -V. A point between capacitors 416 and 418 is coupled to ground. The anode of diode 408 is connected to a node between resistor 412 and capacitor 416 and the cathode of diode 410 is connected to a node between capacitor 418 and resistor 414.
Directional coupler 406, which may be a 12-dB coupler, couples the broadb~nd RF input at port A to port B with a loss of approximately O.S dB and to the diodes 408 and 410 at pon D with a loss of approximately 12 dB. Port C
is isolated from input port A. The current through the diodes 408 and 410 is an eA~onential function of the voltage across the diodes. This diode nonline~rity causes harmonic and intermodulation currents in the diodes which are coupled to port A of the hybrid with a loss of 12 db and to pon C with a loss of appro~im~t~ly O.S dB. Rec~se the diodes are connected back-to-back, the diode wo 94/09574 2 I ~ 6 7 5 8 p,~/US93/096l~

current is an odd function of the diode voltage. Thus, even-order distortion components are cancelled and only odd-order distortion (CTB) components are refiected to the output of the distortion generator at port C of directional coupler 406. Diodes 408 and 410 are biased from voltage sources through resistors 412 and 414. The voltage sources may be adjusted to balance the diode characteristics and minimize the refection of the fun~mPn~l components and second-order-distortion to the output of the distortion generator. Capacitors 416 and 418 are RF
bypass capacitors.
Figure 6 is a block diagram of I-channel variable-gain network 124 illustrated in Figure 2. I-ch~nnel variable-gain network 124 includes phase selector 300, directional coupler 302, amplifiers 306 and 308 with corresponding gain adjustments 314 and 316, diplex filter 318 including high pass filter 310 and low pass filter 312, and an amplitude and delay equalizer circuit 320. Distortion from the distortion generator is coupled through directional coupler 122 of Figure 2 to the 0-180 degree phase selector network 300. Phase selector network 300 is used to select a phase app,opliate for compenc~ting the distortion in the RF input signal. The output of phase selector network 300 is suppLied to a 3-dB coupler 302. The output of coupler 302 feeds in antiphase the high-pass and low-pass s~ions of the I~h~nnPI network. ReC~Ilse of the complex nature of the distor~ion mech~nicmc described previously, the co~,posite distortion will in general vary in rn~gnitude and phase as a function of the channel frequency.

w0 94t09574 1 9 2 l ~ 6 7 5 8 Pcr/usg3/o96l~

Accordingly, independent gain adjustments 314 and 316 for amplifiers 306 and 308, respectively, allow for independent adjustment of distortion at high channels and low channels.
The output of the high-pass and low-pass sections are combined at the output of the diplex filter 318 which is comprised of high-pass filter 310 and low-pass filter 312. Diplex filter 318 is of a Butterworth design. The Butterworth filter has the characteristic that the phase versus frequency characteristic of the high-pass and low pass sections is identical except for an offset of 90 degrees times the filter order. The diplex filter as shown in Figure 6 is of order 2. Thus, the antiphase inputs from directional coupler 302 add in phase at the output of the diplex filter. As a result of the Butterworth characteristic, the gain of the high-and low-channels can be changed without c~-~nging the phase at the output. In this implementation, a linear phase characteristic is obtained for the I-channel with independent high- and low-channel gain adjustments. Amplitude and delay equaliær circuit 320 enables a flat arnplitude and linear phase les~onse to be obtained. The delay equalizer equalizes primarily for the characteristic delay of the Butterworth filter. The amplitude equaliær equalizes for the 3-dB increase in gain at the cross-over frequency of the diplex filter.
Figure 7 is a det~ile.d schematic of the I~hannel variable gain network illustrated in Figure 6. The I-channel input from the distortion generator via directional coupler 122 is connected to a jumper or switch assembly which allows wo 94/09574 - 2 o 21 ~ 6 75 8 PCr/~s93/o96l5 the I-channel output to be switched between 0 and 180 degrees as necessary for cancellation of the optical-link distortions. When terminal 1 is jumpered to terminal 3 and terrninal 2 is jumpered to terminal 4, the phase of the I channel input is shifted 180 degrees. When terminal 1 is jumpered to terminal 2 and terminal 3 is jumpered to terrninal 4, the phase of the I channel input is not shifted. Transformer 302 includes a balun 702 and a 4: 1 impedan~e transformer 704 which provide a balanced input to the FET amplifiers. A first output of transformer 302 is coupled to the control gate of field effect transistor 720 through c~pacitor 706. One current terminal of transistor 720 is coupled to ground and the second terminal is coupled to a power supply +Vd through resistors 724 and 728. Capacitor 780 is coupled between ground and a point between resistors 724 and 728. The output of amplifier 798 is coupled to the control gate of transistor 720 through resistors 778 and 710. Capacitor 711 is coupled between ground and a point between resistors 778 and 710. The output of amplifier 798 is coupled to the inverting input terminal of the amplifier through c~p~ or 776. The non-inverting input of amplifier 798 is col~n~d to a point between resistors 724 and 728. A contact of gain adjusting potentiometer 314 is coupled to the inverting terminal of anplifier 798 through resistor 774.
Potentiometer 314 and resistor 772 are connected in series between the power source +Vd and ground. Amplifier 798 is conne~ted to power supply +Vd and the ground potential. FET ~mplifier 308 is similarly configured and not discussed W094/09574 - 21 - Pcr/uss3/o96l5 in detail here.
The gain of the field effect transistors is controlled by current-feedback loops. The gains are controlled by the setting of the gain adjustment potentiometers 314 and 314'. The voltage across resistors 728 and 728' is compared with the voltage across potentiometers 314 and 314', and the error voltage is amplified by amplifiers 798 and 798'. This error voltage is fed through resistors 778, 710 and 778' and 710' to the gates of transistors 720 and 720' to regulate the transistor drain currents and gains. Resistors 710 and 710' provide the resistive termination for transforrner 704. Capacitors 706 and 706' are coupling c~r~citors. ~r~citors 776 and 776' set the time constant of the current-fe~dbacl~ loops. C~I-acitors 711, 711', 780, and 780' are by-pass capacitors.
The output of the amplifier stages 306 and 308 add in-phase at the output of the diplex filter 318. Diplex filter 318 includes a low-pass section 312 with a capacitor 740 and an inductor 744 and a high-pass section 310 with an inductor 736 and a c~p~citor 738.
The amplitude equalizer of circuit 320 is a bridge-T equalizer including resistors 742, 748, and 750, inductors 751 and 752, and c~racitors 745 and 754.
Inductor 751 and c~pacitQr 745 are ,espee~ely conne~ted in parallel across the terminals of resistor 742. The first terminal of resistor 742 is connected to ground through resistor 748, inductor 752 and c~pacitor 754. The second terminal of resistor 742 is conn~t~ to ground through resistor 750, inductor 752 and WO 94/09574 - 2 2 2 1 ~ 6 7 S 8 Pcr/US93,096,s capacitor 754. The amplitude equalizer equalizes for the 3-dB increase in gain at the cross-over frequency of diplex filter 318.
The delay equalizer of circuit 320 equalizes for the characteristic delay of the Butterworth filter and amplitude equalizer and includes capacitors 760 and 766 and inductors 762 and 764. Capacitor 760 is connected bet-veen the output of the amplitude equalizer and the I channel output. A first terminal of capacitor 760 is connected to ground through inductor 762 and capacitor 766 and a second terminal of c~pacitor 760 is connected to ground through inductor 764 and capacitor 766.
Figure 8 is a detailed schem~tic of Q channel 136 illustrated in Figure 2.
The Q-channel input from the distortion generator is supplied to balun 801 and then to a selector network 802 comprising a jumper or switch assembly to enable the Q-ch~nn~l output to be switched between 90 and 270 degrees. Adding the output of the variable-gain Q channel of phase 90 or 270 degrees with the output of the variable-gain I-ch~nnPI of phase 0 or 180 degrees enables the output distortion to be adjusted in magnitude and phase over a 360-degree range as necesc~ry for cancellation of the optical-link distortions.
The output of selector network 802 is differentiated by differentiator 126 which causes a 90-degree phase lead in the distortion co-,-ponents. Resistor 804 and inductor 808 differentiate the signal. Resistor 803 and capacitor 806 provide a complementary input ~dmitt~nce so that the input imped~nce to the differentiator is a constant re~ist~rlc~ The output of the differentiator 126 is amplified by WO 94/09574 - 2 3 ~ 2 1 4 6 7 5 8 PCr/US93/096ls FET 820 in the variable gain network 128. The arrangement of components in variable gain network 128 is substantially the same as the arrangement of components of amplifier 304 and will not be discussed in detail here. In short, the gain of FET 820 is controlled by a conventional current-feedback loop in manner similar to that in the I-channel. The gain is controlled by the setting of the gain adjustment potentiometer 814. The voltage across resistor 828 is compared with the voltage across potentiometers 814, and the error voltage is amplified by operational amplifier 829. This error voltage is fed through resistors 810 and 812 to the gate of the FET 820 to regulate the FET drain current and gain. Delay 130, which might be provided by a coaxial line, or distributed or printed delay line, adds a signal delay to the Q c~nnel to match the total signal transit time through the I-channel.
Figure 9 is a block diagram of an embodiment of the postdistortion network 36 illustrated in Figure 1 for c~nce~ling multiple-orders of distonion, primarily CSO and ~ . The post~1ictortion network of Figure 9 includes a distortion gene~ator 820 for generating second and third order distonion. The distonion path includes a first distortion path for the second order distortion col.lponents and a second distortion path for the third order distortion CGI.,pOnentS.
The l~sp~ e outputs of the adjusting networks in each of these paths is combined by directional coupler 850. The post~ictortion compensation signal from direction~l coupler 850 is combined with the RF input signal with distonion at -Wo 94/09574 2 1 4 6 7 5 8 Pcr/US93/09615 directional coupler 116'. The details of this arrangement are apparent from the discussion above and will not be repeated here.
Figure 10 is a block diagram of one embodiment of distortion generator 820 illustrated in Figure 9 for generating second-order (CSO) and third-order (CTB) distortion. It is apparent that distortion generator 820 consists of connecting port A of the distortion generator illustrated in Figure 4A to the port B
of the distortion generator of Figure 5A. The RF input at port A of directional coupler 406' is coupled to port B with a loss of approximately 0.5dB. The theory of operation of this circuit is evident from the above discussions of Figures 4B
and 5B.
Figure 11 is a schematic diagram of the distortion generator 820 illustrated in the block diagram of Figure 10. Again, it is apparent that distortion generator 820 is obtained by connecting port A of the distortion generator of Figure 4B to port B of the distortion generator illustrated in Figure 5B. The theory of operation of this circuit is evident from the above discussiQns of Figures 4B and SB.
Figure 12 is a system block diagram of an optical communication system in accordance with another embodiment of the present invention. The system includes an optical tr~nsmitter 910, an optical communiC~ions link 911, and one or more optical nodes 912. For clarity in the Figures the details of only one optical node 912 are shown. The other nodes are similarly configured. The RF
input to optical tr~n~mittPr 910 may, for example, be a broadb~nd CATV signal wo 94/09574 - 2 5 2 1 4 6 7 5 8 PCr/Us93/09615 consisting of a plurality of frequency-division-multiplexed video carriers, although the invention is not limited in this respect. The RF signal is amplified by RF
arnplifier 914 and provided to predistortion network 950. Predistortion network 950 generates a predistortion signal to compensate the RF signal modulating the laser diode for distortion which will be generated in ~he communication system. Predistortion network 950 may be constructed, for example, as shown in commonly ~csign~d Application Serial Nos. 805,251 and 805,259 or as shown in Figures 9 and 10 above. The output of predistortion network 950 modulates the bias current in laser diode 915. The bias current is provided by a current source 916. Modulation of the laser current causes the intensity of the optical output to vary in a nearly linear fashion with respect to the input signal. The current source and arnplitude of the modulation current are adjusted for optimum link carrier-to-noise and distortion performance.
The optical output from optical tr~ncmitt~r 910 is coupled to optical communirations link 911 for tr~ncmiccion to one or more optical nodes. Optical communication link 911 includes optical fibers 920 and 921 and may optically include an optical amplifier 922 and an optical coupler 923 for fe~ding a plurality of optical nodes.
Optical nodes 912 includes a photodetector 930, for example, a PIN
photodiode, which transduces the intensity modul~ted signal into an amplitude modulated electric current signal. Photodet~ctor 930 is reverse biased from a -Wo 94/09574 2 1 4 6 7 5 8 PC~/US93/09615 voltage source 926 through resistor 928 and 932. The photodetector output is amplified by photodetector amplifier 934, which may be a push-pull transimpedance amplifier or other suitable type designed for CATV applications.
The RF output of photodetector amplifier 934 is processed by postdistortion compensation network 936 to reduce distortiolls, which may, for example, be unique to a receiver location. The output of the postdistortion compensation network is amplified by buffer amplifier 938 for distribution to the CATV
network. It will be apparent that an optical communications system constructed in accordance with the above te~chings offers increased capabilities for compensating for distortion.
While there has been shown and described the preferred embodiments of the invention, it will be evident to those skilled in the art that various modifications may be made thereto without departing from the spirit and scope of the invention which is set forth in the appended claims.

Claims (10)

WE CLAIM:

1. A postdistortion network for a receiver at a receiver location in an optical communications system, said receiver converting an optical signal carried over the optical communications system to an RF signal, said postdistortion network comprising:
an input terminal for receiving the RF signal;
a distortion generator for generating a distortion signal from the RF
signal;
adjusting means for adjusting the phase and amplitude of the distortion signal based on distortion at said receiver location; and combining means for combining the adjusted distortion signal with the RF signal to cancel or substantially suppress distortion in said RF signal.

2. The postdistortion network according to claim 1, wherein said distortion generator comprises a distortion generator for generating second order distortion.

3. The postdistortion generator according to claim 2, wherein said distortion generator for generating second order distortion comprises:
a transformer having an input coupled to said input terminal and first and second outputs for respectively outputting a first transformer output and a second transformer output out-of-phase with the first transformer output;
a first non-linear device receiving said first transformer output;

a second non-linear device receiving said second transformer output;
and combining means for combining outputs of said first and second non-linear devices.

4. The postdistortion network according to claim 3, wherein said first and second non-linear devices comprise field effect transistor amplifiers.

5. The postdistortion network according to claim 1, wherein said distortion generator comprises a distortion generator for generating third order distortion.

6. The postdistortion network according to claim 5, wherein said distortion generator for generating third order distortion comprises:
a directional coupler having a first port for receiving said RF signal and coupling a portion of said RF signal to a third port;
a diode circuit including a first diode having an anode coupled to said third port and a cathode coupled to a reference potential and a second diode having a cathode coupled to said third port and an anode coupled to the reference potential,

7. The postdistortion network according to claim 1, wherein said distortion generator comprises a distortion generator for generating second and third order distortion.

8. An optical communications system, comprising:
a transmitter for converting an electrical input signal to an optical signal, said transmitter including a laser diode based by a bias current modulated by the electrical input signal to generate said optical signal and a predistortion network for generating a predistortion signal to compensate the electrical input signal for distortions in said optical communications system; and a receiver for converting the optical signal of said transmitter to an electrical signal, said receiver including a photodetector for generating the electrical signal and a postdistortion network for generating a postdistortion signal to compensate the electrical signal for distortions in said optical communications system.

9. The optical communications system according to claim 8, wherein said predistortion network comprises a distortion generator for generating second and third order distortion.

10. The optical communications system according to claim 8, wherein said predistortion network comprises a distortion generator for generating second and third order distortion.