The polarization-dependent distortion of an optical signal transmitted through an optical fiber is reduced by aligning the polarization of the optical signal to minimize the received signal distortion. A polarization controller (a device which can change the polarization of light in an optical fiber) may be located at either the input or output end of a long haul optical fiber system and is used to align the polarization of the signal to minimize the received signal distortion. Automatic operation of the polarization controller can be obtained by using a steepest-descent method based on a distortion measure of the received signal for the optical signal transmitted through the optical fiber to generate control signals which are used to control the polarization controller.

TECHNICAL FIELD 
This invention relates to lightwave transmission systems. More 
particularly, this invention relates to apparatus for minimizing 
polarization dependent distortion in high speed long haul terrestrial and 
undersea lightwave systems, as well as analog lightwave systems. 
BACKGROUND OF THE INVENTION 
Many factors in lightwave communication systems make the optical signal 
propagation polarization dependent. These factors include 
polarization-dependent loss and polarization mode dispersion in the fiber 
and the system components. 
For example, consider polarization mode dispersion (PMD). The core of an 
optical fiber is not truly symmetrical, and therefore the propagation of 
an optical signal at one polarization, for example, vertical, will be 
different than the propagation of the optical signal at another 
polarization, for example, horizontal. With no polarization-dependent 
loss, for each frequency there exist a pair of orthogonal input states of 
polarization for which the corresponding output states of polarization are 
orthogonal and are independent of wavelength to the first order. These 
states are referred to as the principle states of polarization (PSP) in 
the fiber. With sufficiently narrow bandwidth, such as with external 
modulation of a single-frequency laser, optical signals transmitted in 
either of these two states are undistorted at the receiver, but have, in 
general, different time delays. A signal with arbitrary polarization can 
be expressed in terms of a sum of signals in each PSP, and thus will be 
received as two signals with different delays. The received signal is 
therefore distorted unless it is transmitted with one of the two PSPs. 
If the PSPs of the fiber remained constant, then one-time corrective 
measures could be taken when a system is installed to avoid its adverse 
effects. However, the PSPs as well as the time delay changes with time. 
Factors that cause the PSPs and delay to change with time include changes 
in temperature caused by, e.g., sunlight and ocean currents. In addition, 
any change of position or movement of the optical fiber will cause a 
change of PSPs and also a change in the time delays. Thus, for a given 
input polarization the received signal distortion varies with time. 
With large PMD, or with wider bandwidth signals, such as with direct laser 
modulation, or with polarization-dependent loss, the signal propagation 
may no longer be adequately described by the PSP model. However, the 
distortion of the received signal is still polarization dependent. 
Furthermore, since, for a given input state of polarization, the state of 
polarization of the optical signal in the fiber changes with time, the 
distortion of the received signal also changes with time, even when the 
distortion is due to components whose polarization-dependent properties 
are constant. 
It is recognized that if the polarization-dependent distortion of an 
optical pulse signal traversing an optical fiber can be minimized, then 
the bit rate with which, and/or distance over which, information can be 
transmitted over a fiber optic transmission channel can be increased. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, it is recognized that the 
foregoing problem can be solved by utilizing, in the fiber optic 
transmission context, a technique for aligning the polarization of an 
optical signal being transmitted via an optical fiber to the input state 
of polarization with minimum received signal distortion or by receiving 
only the output state of polarization with minimum signal distortion. 
In preferred embodiments of the invention, more particularly, a 
polarization controller, a device which can change the polarization of 
light in an optical fiber, is coupled to correct the polarization of light 
as it either enters or leaves a long haul terrestrial or undersea optical 
fiber. Using a gradient search algorithm based on a measure of the 
distortion of the received signal that was transmitted over the optical 
fiber, control signals are generated for controlling the polarization 
controller to substantially reduce polarization dependent distortion.

DETAILED DESCRIPTION 
Polarization-dependent distortion is a significant factor which can limit 
the maximum bit rate-distance in high speed, long haul terrestrial and 
undersea lightwave systems. The source of this distortion includes 
polarization-dependent loss and polarization mode dispersion. It can cause 
significant signal degradation of the optical signal at data rates of 2.5 
Gbps and above in optical amplifier systems. 
Let us first consider polarization mode dispersion. The effect of 
polarization mode dispersion (PMD) in coherent and direct detection 
lightwave systems, including first order frequency effects along with 
higher order effects have been studied and the results were presented in 
various publications such as "Polarization-Dispersion Limitations in 
Lightwave Systems" in Technical Digest, Optical Fiber Communications 
Conference, New Orleans, La., Jan. 25-28, 1988, p. 93 by R. E. Wagner et 
al.; "Polarization-Dependent Pulse Compression and Broadening Due to 
Polarization Dispersion in Dispersion-Shifted Fiber", Optics Letters, Vol. 
13, February 1988, pp. 155-157 by C. D. Poole, et al.; and "Fading in 
Lightwave Systems Due to Polarization-Mode Dispersion", IEEE Photonics 
Technology Letters, Vol. 3, January 1991, pp. 68-70 by C. D. Poole et al. 
Experimental results have shown that the first order effects dominate with 
external modulation of the transmitting laser or with FSK modulation. 
First order effects of polarization mode dispersion arise because of a 
differential delay time of components of a wave aligned with two 
orthogonal principal states of polarization of a fiber. In this instance, 
PMD is a linear distortion in the received electrical signal. 
In a single mode optical fiber, it has been shown that for each frequency 
there exists a pair of orthogonal input and corresponding output states of 
polarization, referred to as principle states of polarization (PSPs). 
Signals transmitted in either of these two states have no first order PMD, 
but the signals in the two states have, in general, different time delays. 
Thus, a signal transmitted through an optical fiber with arbitrary 
polarization can be described mathematically using these PSPs as basic 
functions. From this mathematical relationship it was concluded by the 
inventors that the first order PMD can be avoided if the transmit (or 
receive) polarization is aligned to one of the PSPs of the fiber. With 
wider bandwidth signals, such as with direct laser modulation, or with 
polarization-dependent loss, the signal propagation may no longer be 
adequately described by the PSP model. However, the distortion of the 
received signal is still polarization dependent, and performance can be 
improved by aligning the input polarization to the polarization with 
minimum signal distortion. Thus, even with these polarization-dependent 
impairments, the received signal distortion can be reduced by proper 
adjustment of the transmit or receive polarization. Therefore, by 
adjusting the polarization, the maximum bit rate and/or distance of a 
given optical fiber system can be increased. Note that since the 
propagation characteristics of the fiber are continuously changing, this 
adjustment must be continuous or at least often repeated. 
Referring to FIG. 1, there is illustrated an embodiment of the invention 
for aligning the polarization of light into an optical fiber to one of the 
PSPs using an optical polarization controller at the input end of the 
optical fiber. Specifically, a laser transmitter 10 is coupled to transmit 
an optical signal from a first station 12 via a long optical fiber 14 to a 
second station 16. At the second station, an optical to electrical 
detector 18 detects the optical signal from the optical fiber 14 and 
converts it into an electrical signal. The signal from the detector is 
directed to a threshold detector and timing recovery circuit 20. The 
threshold detector samples each received symbol of the received signal to 
determine if it is a "1" or a "0", and the timing recovery circuit 
establishes timing signals which are used to determine the instant when 
the received signal is to be sampled. The output signal of the threshold 
detector is a stream of well defined "0s" and "1s", and this signal is the 
electrical equivalent of the transmitted optical signals. To determine how 
to adjust the polarization controller, a measure of the performance of the 
receiver is required. In the embodiment shown in FIG. 1, the bit error 
rate (BER) is used as the performance measure. Since it is usually 
desirable to have the receiver output error-free, though, a second 
detector with added noise is used to generate errored data, with the BER 
corresponding to the level of distortion. As shown in FIG. 1, the signal 
from the detector 18 is also combined with noise at 22, and the resultant 
signal is directed to a threshold detector 24. Thus, the signal from 
detector 24, because of the added noise, has relatively high error rate 
compared to the signal from the detector 20. The two signals are directed 
to an exclusive OR gate 26 where the relatively error free signal from 
detector 20 is compared with the relatively high error rate signal from 
detector 24. The output signal from the gate 26 is directed to a counter 
28 where the bit error rate (BER) is determined. 
The BER signal from the counter 28 is directed to a control circuit 30 
which performs a gradient search as described in detail in the book, 
"Digital Communications" by John C. Proakis, pp. 369 published by 
McGraw-Hill Book Company in 1983. 
Specifically, the control circuitry 30 generates a signal to rotate the 
polarization controller 32 output signal polarization in the direction 
that reduces the BER. The polarization controller 32 is located at the 
input of the optical fiber 14 and can be a Oshima Computer programmable 
polarization controller as described in the publication "Proposal for a 
Fiber-Optic Endlessly Rotatable Fractional Wave Device and Its Application 
to Lightwave Technologies" in Electronics and Communications in Japan, 
Part 2, Vol. 71, No. 6, 1988, pp. 36-47, by T. Matsumoto et al. This 
device consists of two fiber loops that are rotated to operate like 
quarter-wave and half-wave plates. The output polarization is determined 
by the settings of the two plates, i.e., the polarization can be described 
by two parameters. 
The gradient descent technique works as follows. The BER with the 
polarization controller at the initial setting is measured. The control 
circuitry then changes the setting of one plate of the controller by a 
small amount and notes the change in the BER. If the BER decreases, the 
setting was changed in the right direction. If, however, the BER 
increases, the setting is adjusted by twice that amount in the opposite 
direction so that the setting has now been changed in the right direction 
from the initial position. This process is repeated for the other plate, 
and the entire process is continuously repeated to find and track the 
polarization with minimum distortion. 
The gradient search method can be visualized by examining the BER surface. 
The plot of the BER versus the two settings that determine the 
polarization, where the first setting can be along the X axis, the second 
setting can be along the Y axis and the BER is along the Z axis is a three 
dimensional BER surface. With PMD and external modulation, this surface 
has only two local minima, one at each PSP, which for small PMD, have 
equal BER. Thus, the gradient search method locates and tracks one of the 
PSP's. With high PMD, wide bandwidth signals, or polarization-dependent 
loss, the local minima may no longer be equal, but in all cases the 
gradient search method will still locate and track a minima which reduces 
distortion. 
Although a polarization controller at the transmitter is adequate for most 
systems, consideration should be given to the feedback requirements. For 
example, information must be transmitted from the receiver back to the 
transmitter. In addition, for long transmission distance where 
polarization-dependent distortion is a problem, the delay in the feedback 
due to propagation delay may make tracking the PSPs difficult if the PSP's 
change too rapidly. 
These problems can be overcome by transmitting the optical signal from the 
laser transmitter to the long haul optical fiber with fixed polarization 
and locating the polarization controller at the receiving end of the 
optical fiber 14 which is located at the second station 16. 
Referring to FIG. 2, there is illustrated a block diagram of a fiber optic 
transmission system embodying the principles of the invention by changing 
the polarization of optical signal such that signals in the two PSP's are 
detected separately. 
In FIG. 2, a laser transmitter 40 is coupled to transmit an optical signal 
from a first station 42 via a long haul terrestrial or undersea optical 
fiber 44 to a second station 46. At the second station, the received 
optical signal has its polarization rotated such that signals in the PSP's 
of the fiber are separated by a polarization splitter 48 which divides the 
received optical signal into two signals polarized at 90.degree. to each 
other. The two orthogonally polarized signals are directed toward separate 
detectors 50, 52 which convert the received optical signals into 
electrical signals. The electrical signal from detector 50 is directed to 
a first receiver 54; and the electrical signal from detector 52 is 
directed to a second receiver 56. The outputs of the two receivers are 
directed to a combiner-selector 58, and to a control circuit 60. The 
output signals from the control circuit 60 are directed to polarization 
controller 62. It is to be noted that, in FIG. 2, the polarization 
controller is located at the output of the optical fiber 44 and, as 
described in FIG. 1, the polarization controller can be an Oshima Computer 
programmable polarization controller. 
Referring to receiver 54, the signal from the detector 50 is directed to 
threshold detector and timing recovery circuit 64 which samples each 
received electrical symbol to determine whether it is a "1" or a "0". The 
output signal of the threshold detector is a series of "1s" and "0s" which 
is the electrical equivalent of the optical symbols transmitted over the 
optical fiber 44. The electrical signal from the detector 50 is also 
combined with noise at 66, and the resultant signal is directed to a 
threshold detector 68. The signal at the output port of the threshold 
detector has a relatively small BER while the signal from detector 68, 
because the added noise has a relatively high BER. The two signals, one 
from threshold detector 64 and the other from threshold detector 68, are 
directed to the input ports of an exclusive OR gate 70 which compares the 
two signals. The output signal from the exclusive OR gate 70 is directed 
to a counter 72 which determines the actual BER. 
The receiver 56 is similar in all respects to the receiver 52 and, 
therefore, to avoid repetition, a recitation of the connections between 
the various components is not given again. However, as the various 
components and the functions of the various components of receiver 56 are 
similar to those of receiver 54, the reference numerals used to identify 
the components of receiver 54 are repeated for the similar components of 
receiver 56. 
The output signals from the counters 72 of receivers 54, 56 are directed to 
the control circuit 60 which, as noted previously, generates control 
signals that adjust the setting of polarization controller 62. 
In operation, a signal from the first station 42 via optical fiber 44 to 
second station 46 is split at polarization splitter 48 into two beams, one 
of which is directed toward detector 50 and the other of which is directed 
toward detector 52. Referring to receiver 54, the detector 50 detects the 
received optical signal from polarization splitter 48 and converts it into 
an electrical signal. This signal is directed to a threshold detector and 
timing recovery circuit 64 and, after being combined with noise at 66, to 
threshold detector 68. The threshold detectors sample each received symbol 
and determines if it is a "1" or a "0". In addition, threshold detector 64 
generates clock signals which are required for timing purposes and 
forwards these signals to the threshold detector 68. The signal from 
detector 64 which has a relatively low BER and the signal from detector 68 
which has a relatively high BER are directed to the two input ports of 
exclusive OR gate 70. It is noted that the relatively high BER of the 
signal from detector 68 is due primarily to the added noise. The output 
signal from the exclusive OR gate 70 is directed to counter 72 which 
generates a signal which represents the bit error rate. 
Referring to receiver 56, the signal from the polarization splitter 48 
which is received by detector 52 is converted into an electrical signal 
which is directed to the two threshold detectors 64, 68 of receiver 56. As 
noted previously, the operation of receiver 56 is similar to that of 
receiver 54. 
The BER signals from counters 72 of receivers 54, 56 are directed to 
control circuit 60. The control circuit 60 examines the BER signals from 
receivers 54, 56 and determines at some instant of time which signal has 
the lower BER. 
Note that with PMD, the two receivers will generally have different BER's 
because the power received in each polarization will differ. In addition, 
higher order effects of PMD can also make the BER of the two PSP's differ. 
Thus, control circuitry in using the gradient search method uses the lower 
BER in determining the adjustment of the polarization controllers as 
described before. In addition, the control circuitry also selects the 
output from the receiver with the lower BER as the output bits. For 
example, if the BER from counter 72 in receiver 56 is lower than that from 
counter 72 in receiver 54, control circuitry 60 would generate a select 
signal which causes combine/selector 58 to output the bit from detector 64 
in receiver 56. 
Although the above implementation is the preferred embodiment, there are 
many variations for the invention. For example, in FIG. 1, the second 
threshold detector can be avoided when forward error correction coding is 
used since the BER before correction can be used as the performance 
measure, with the corrected output bits nearly error-free. Other 
performance measures, such as signal-to-noise ratio, and received signal 
eye opening in either the vertical (amplitude) or horizontal (time) 
direction, can be used. Other techniques to determine the polarization 
controller adjustment can also be used. With the performance measures of 
signal-to-noise ratio or eye opening, gradient ascent, rather than 
descent, can be used to maximize these performance measures. Also, the 
performance measure (e.g., BER) surface can be periodically scanned and 
the controller setting located at that setting with the optimum 
performance. 
For the receive polarization controller of FIG. 2, the selector can be 
replaced by a combiner that weights, and/or delays, and then sums the two 
signals output from the photodetectors 50 in receiver 54 and 56 to 
minimize distortion, followed by threshold detection to determine the 
output bits. 
Finally, the invention is not limited to digital optical communications 
systems. Since analog cable TV systems using optical fibers with 
subcarrier multiplexing can also suffer polarization-dependent distortion, 
the invention is also useful in these systems. 
It will thus be appreciated that those skilled in the art will be able to 
devise numerous arrangements which, although not explicitly shown or 
described herein, embody the principles of the invention. Accordingly, all 
such alternatives, modifications and variations which fall within the 
spirit and broad scope of the appended claims will be embraced by the 
principles of the invention.