Control of a launched polarization state of a frequency modulated beam coincident with an optical fiber principal axis

In an optical communication system for transmitting an optical fiber which may include an optical amplifier comprising an optical isolator, a polarization control system comprises at a send end a launch polarization controller for giving a launched state to the signal beam launched to the optical fiber. Received at a receive end as a received beam of a received state of polarization, the signal beam is split by polarization into first and second beams, each including a reception component of the modulating frequency and of a variable optical intensity. A detection signal is produced from the first and the second beams to have the modulating frequency and a variable electric intensity. Controlled by a launch controller supplied with the detection signal, the launch polarization controller keeps the launched state to minimize at the receive end the electric intensity. Preferably, the received beam is split into four beams, which are given four differents direction of polarization and, from which the detection signal is produced.

BACKGROUND OF THE INVENTION 
This invention relates to control of a launched polarization state of a 
signal beam in coincidence with a principal axis of polarization of an 
optical fiber. More particularly this invention relates to a method of 
controlling the launched state, a polarization control system, a 
polarization control device for controlling the launched state. 
As a prior-art polarization control system, a polarization control system 
for bidirectionally transmitted signal beams is disclosed in U.S. Pat. No. 
5,031,998 issued to Takashi Ono, the present inventor, and another 
hereinafter "th Ono patent". The specification of this United States 
patent is herein incorporated by reference. In such a polarization control 
system, polarization controllers are used. It is possible to use as 
polarization controllers the controller described in an article 
contributed to the Journal of Lightwave Technology, Volume 9, No. 10 
(October 1991), pages 1217 to 1224, entitled "Highly Practical Fiber 
Squeezer Polarization Controller" by Haruhito Shimizu et. al. The instant 
inventor is a coauthoer of this work. The, polarization control system of 
the United States patent is also described in a paper contributed by T. 
Ono, the present inventor, and three others to the Proceedings of ECOC 
'90, pages 419 to 422, under the title of "Novel Wideband Common 
Polarization Control Method for Coherent FDM Transmission System". 
Before the prior-art polarization control system was revealed, 
long-distance transmission of a signal beam through an optical fiber gave 
rise to accumulation of polarization dispersion in the optical fiber which 
resulted in an objectionable power penalty. Polarization dispersion 
results from a difference in a propagation time between two orthogonal 
principal states, or eigen modes, of an optical fiber. This difference 
distorts the shape of a received beam and degrades. Reception sensitivity 
depending upon a launched polarization state with which the signal beam is 
launched or supplied to the optical fiber. The principal states are 
described in an article contributed by C. D. Poole et. al. to the Journal 
of Lightwave Technology, Volume 6, No. 7 (July 1988), pages 1180 to 1190, 
entitled "Polarization Dispersion and Principal States in a 147 km 
Undersea Lightwave Cable". 
In prior-art polarization control systems, the signal beam and a pilot beam 
are bidirectionally transmitted through an optical fiber. It both ends of 
the optical fiber, polarization controllers are connected, and controlled. 
In this polarization control system, the signal beam is subjected to 
frequency division multiplexing (FDM) with its launched polarization state 
kept by the polarization controller to maintain coincidence with a 
principal axis of polarization defined by the principal states of the 
fiber. This suppresses adverse effects which would otherwise be imparted 
to the received beam. 
Although subjected to the frequency division multiplexing the signal beam 
was not frequency modulated in the prior-art polarization control system. 
Furthermore, the bidirectional transmission is not applicable to an 
optical fiber which includes an optical amplifier comprising an optical 
isolator. 
SUMMARY OF THE INVENTION 
Consequently it is an object of the present invention to provide a method 
of controlling a polarization state of an optical beam subjected to 
frequency modulation. 
It is another object of this invention to provide a polarization control 
method which is of the type described and which is applicable to an 
optical communication system comprising an optical fiber which may include 
an optical isolator. 
It is still another object of this invention to provide a polarization 
control method which is of the type described and which can suppress 
waveform distortion caused by polarization dispersion. 
It is yet another object of this invention to provide a polarization 
control method which is of the type described and which allows a high 
sensitivity to reception of the optical beam. 
It is a further object of this invention to provide a polarization control 
system and a polarization control device which is applicable to the 
polarization control method of the type described. 
Other objects of this invention will become clear as the description 
proceeds. 
According to an aspect of this invention, there is provided a method for 
controlling a polarization state of an optical beam subjected to frequency 
modulation by a modulating frequency for transmission as a signal beam 
through an optical fiber having a principal axis of polarization and which 
comprises the steps of: (A) launch controlling the polarization state into 
a launched state to supply a launched beam of the launched state to the 
optical fiber as the signal beam; (B) receiving the signal beam as a 
received beam which an intensity variable reception component of the 
modulating frequency to produce a detection signal representative of an 
intensity of the reception component, and (C) axis controlling, with the 
detection signal, the launch controlling step to keep the launched state 
in coincidence with the principal axis. 
According to a different aspect of this invention, there is provided a 
polarization control system for use in an optical communication system for 
transmitting as a signal beam, an optical beam modulated by a modulating 
frequency, through an optical fiber having a principal axis of 
polarization and which comprises: (A) a launch polarization controller for 
controlling the polarization state into a launched state to supply a 
launched beam of the launched state to the optical fiber as the signal 
beam; (B) receiving means for .receiving the signal beam as a received 
beam having a received state of polarization and including an intensity 
variable reception component of the modulating frequency to produce a 
detection signal representative of an intensity of the reception 
component; and (C) axis controlling means for controlling, which the 
detection signal, the launch polarization controller to keep the launched 
state in coincidence with the principal axis. 
According to another aspect of this invention, there is provided a 
polarization control device which is used in an optical communication 
system comprising a launch polarization controller controlling a 
polarization state of an optical beam modulated by a modulating frequency 
into a launched state to supply as a signal beam a launched beam of the 
launched state to an optical fiber having a principal axis of polarization 
and a control means for controlling the launch polarization controller, 
where the device comprises: (A) receiving means for receiving the signal 
beam as a received beam including an intensity variable reception 
component of the modulating frequency to produce a detection signal 
representative of an intensity of the reception components and (B) signal 
supply means for supplying the detection signal to the control means to 
make the control means transmit the detection signal to the launch 
polarization controller as a reception signal and control the launch 
polarization controller by the reception signal to keep the launched state 
in coincidence with the principal axis.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the description will begin with a polarization control 
system according to a first embodiment of the present invention. The 
polarization control system is for use in an optical communication system. 
In the example illustrated, the optical communication system is operable as 
a 10-Gb/s optical intensity modulation (IM) and direct-detection (DD) 
reception system, wherein a main signal beam is transmitted from a send 
end to a receive end through a main optical fiber 11, which is of a 
1.55-micrometer zero-dispersion type and is 100 km long. It will be 
presumed that the optical fiber 11 has a polarization dispersion of 50 ps. 
A main laser oscillator 13 comprises a semiconductor laser diode (not 
shown). A main bias current source 15 delivers a bias current of 100 mA to 
the laser diode to make the laser oscillator 13 produce a 1.5-micrometer 
main optical beam. A sinusoidal oscillator 17 superposes a sinusoidal 
signal of 100 kHz on the bias current to subject the optical beam to 
frequency modulation (FM). The sinusoidal signal is produced to have an 
amplitude for giving the optical beam a frequency shift of 10 GHz. In this 
manner, the optical beam is frequency modulated into the signal beam by a 
modulating frequency of 100 kHz. 
At the send end, a system input terminal 19 is supplied with a transmission 
data signal of 10 Gb/s. Controlled by the data signal, a main intensity 
modulator 21 subjects the signal beam to intensity modulation to make the 
signal beam transmit the data signal as a transmitted data signal to the 
receive end through the optical fiber 11. Before being launched or 
supplied to the optical fiber 11, the signal beam is passed through a 
launch polarization controller 23, which is a part of the polarization 
control system. 
Preferably in the manner described in the Shimizu et. al. article mentioned 
above, the polarization controller 23 is of a fiber squeezer type and is 
reliably and continuously operable to change a polarization state or plane 
of an input signal beam to a desired state of polarization. In any event, 
the polarization controller 23 gives a launched state of polarization to a 
launched beam for supply to the optical fiber 11 as the signal beam. 
At the receive end, the signal beam is branched by an optical coupler 25 
into two beam parts, both frequency modulated and carrying the transmitted 
data signal. One of the beam parts is delivered to a main optical receiver 
27 for detecting the transmitted data signal to deliver a reproduction of 
the transmission data signal to a system output terminal 29. 
The other beam part is delivered as a received beam to a reception 
polarization controller 31, which is preferably of the fiber squeezer 
type. During transmission through the optical fiber 11, the signal beam is 
subjected to the polarization dispersion. The beam reaches the 
polarization controller 31 with a received state of polarization and is 
processed in a manner which will presently be described. The polarization 
controller 31 controls the received state to a controlled state of 
polarization and produces a controlled beam with the controlled state. 
The controlled beam is delivered to a polarization splitter 35 and split 
into first and second split beams which are orthogonally polarized. Each 
of the first and the second split beams includes an optical reception 
component resulting has a variable optical intensity I.sub.o which depends 
on the received from the sinusoidal signal to have the modulating 
frequency and state of the received beam. 
In the illustrated example, the first and the second sit beams are 
delivered to two light receiving parts of a balanced optical receiver 
comprising dual pin photodiodes 35 and a signal amplifier 37. The balanced 
optical receiver (35, 37) produces an electric output signal including a 
demodulated signal as a detected component which results from the 
sinusoidal signal and has the modulating frequency and a variable electric 
intensity corresponding to the variable optical intensity. 
The output signal is delivered to a band-pass filter 39 which has a 
passband of a center frequency of 100 kHz and produces the detected 
component. Supplied with the detected component, an electric intensity 
detector 41 produces an electric detection signal representative of the 
variable electric intensity, namely, the variable optical intensity of the 
reception component. 
It may be mentioned here that an optical fiber has two orthogonal principal 
states, or eigen modes, for a polarized optical beam travelling 
therethrough and consequently a principal axis of polarization in the 
manner described in the Ono patent. Supplied with the detection signal 
from the receive end through a signal transmission line, a launch 
controller 43 controls at the send end the launch polarization controller 
23 to control the launched state into coincidence with the principal axis 
of the optical fiber 11. At the receive end, the detection signal is 
additionally delivered to a reception controller 45 to control the 
reception polarization controller 31 to keep the variable electric 
intensity at a maximum intensity. 
Turning to FIG. 2 in conjunction with FIG. 1, the optical fiber 11 is 
illustrated along a top or first row labelled (A). The send and the 
receive ends of the optical communication system are on the left and the 
right sides of the optical fiber 11 in the top row. The principal states 
are depicted by p(1) and p(2) on the left and the right sides of the 
optical fiber 11. 
In the manner described in above, the launched beam is frequency modulated 
to include a launched component having an optical frequency variable 
between first and second optical frequencies f(1) and f(2). The launched 
state of polarization does not vary throughout the launched component, 
namely, between the first and the second optical frequencies. 
It will first be assumed in the manner exemplified along a middle or second 
row labelled (B) that the launched beam is supplied to the optical fiber 
11 with the launched state incoincident with the principal axis of 
polarization of the optical fiber 11. In this event, the received state 
varies as a result of the polarization dispersion between first and second 
states as exemplified at the receive end, depending on the optical 
frequency of the optical reception component. When the received beam is 
caused to pass through the polarization splitter 33, the optical reception 
component in each of the first and the second split beams is given the 
optical intensity I.sub.o which varies with time t in accordance with the 
sinusoidal signal of the sinusoidal oscillator 17. As a consequence, it is 
possible for the balanced optical detector to produce the alemodulated 
component. 
It will now be assumed in the manner illustrated along a bottom or third 
row labelled (C) that the launched beam is supplied to the optical fiber 
11 with the launched state in coincidence with the principal axis of 
polarization in the optical fiber 11. In this event, the received state is 
in coincidence with the principal axis and is identical throughout the 
optical reception component. When the received beam passes through the 
polarization splitter 33, the optical reception component has a constant 
intensity that depends on angle of incidence of the received beam on the 
polarization splitter 33. The variable optical intensity I.sub.o is not 
variable with the time t. The batanoed optical receiver produces the 
demodulated signal with a zero intensity. 
It is therefore understood that the launched state is coincident with the 
principal axis of polarization when the variable electric intensity, and 
accordingly the electric detection signal, is minimized. The reception 
polarization controller 31 is used to keep the variable electric 
intensity, and consequently the detection signal, at the maximum 
intensity. This is done in order to make it possible for the balanced 
optical receiver detect the detection signal with a highest possible 
demodulating efficiency. 
Turning back to FIG. 1, each of the launch and the reception controllers 43 
and 45 includes a built-in microprocessor (not shown). In the manner 
described in the Ono et. al. patent, the launched and the controlled 
states are controlled by varying the launched and the controlled states 
within a narrow range, namely, by resorting to a peak search method. 
Inasmuch as the launch polarization controller 23 is used on the send end, 
the launched state is controlled by the detection signal with a 
propagation delay of about 1 ms. The launch controller 43 is therefore 
given a response speed of about 1.1 ms. The reception controller 45 is 
given a shorter response speed of 0.1 ms. 
If the signal beam were subjected to the polarization dispersion while 
transmitted through the optical fiber 11, the reception data signal of 10 
Gb/s would be obtained at the system output terminal 29 with an 
objectionably severe power penalty of 10 dB. With the polarization control 
system described above, it has been confirmed that the reception data 
signal is subjected to no power penalty. 
Reviewing FIG. 1, the launch controller 43 may be placed on the receive end 
as depicted. It is possible to understand a combination of the balanced 
optical receiver (35, 37), the band-pass filter 39, and the electric 
intensity detector 41 as a detection arrangement responsive to the first 
and the second split beams for detecting the optical reception component 
to produce the detected signal. Another combination of the reception 
polarization controller 31, the polarization splitter 33, and the 
detecting arrangement (35-41) serves as a receiving arrangement for 
receiving the signal beam to produce the detection signal. The launch 
controller 43 is alternatively called an axis control arrangement for 
controlling, with the detection signal, the launch polarization controller 
23 to keep the launched state in coincidence with the principal axis. The 
optical fiber 11 may include an optical comprised of an optical isolater 
amplifier which will be described in the following example. 
The polarization control system is referred to as a polarization control 
device when only its receive end is taken into consideration. In this 
event, a combination of the launch controller 43 and the signal 
transmission line is used as a control arrangement for controlling the 
launch polarization controller 23. A connection from the electric 
intensity detector 41 to the launch controller 43 serves as a signal 
supply arrangement for supplying the detection signal to the control 
arrangement to make the control arrangement transmit the detection signal 
to the launch polarization controller 23 as a reception signal and control 
the launch polarization controller 23 by the reception signal to keep the 
launched state in coincidence with the principal axis of polarization. 
Referring now to FIGS. 3(a) and 3(b), attention will be directed to a 
polarization control system according to a second embodiment of this 
invention. Similar parts are designated by like reference numerals and are 
similarly operable with optical and electric signals. The optical 
communication system serves as an optical amplifying and relaying system 
for relaying the signal beam with the signal beam amplified as it stands. 
In the example being illustrated, the main optical fiber 11 is used as a 
down propagation path with the send and the receive ends 1,000 km apart. 
Nineteen optical amplifiers 47(1), 47(2), . . . 47(19)are interposed at 
distances of 50 km. An auxiliary, or additional optical fiber 49 serves as 
the signal transmission line described in conjunction with FIG. 1 and is 
used as an up propagation path from the receive end to the send end. 
Ninteen up optical amplifiers 51(1), 51(2), 51(3), 51(4), ...51(19) are 
between the two ends interposed. Each of the optical amplifiers 47 
(suffixes omitted) and 51 (suffixes omitted) is an erbium doped fiber 
optical amplifier and includes an optical isolator (not shown). 
In each of the down and the up propagation paths including the optical 
amplifiers, there is an overall polarization dispersion of 150 ps. Lased 
at the send end in the main laser oscillator 13, the optical beam is 
frequency modulated by the sinusoidal signal and intensity modulated by 
the transmission data signal, which is now called a down data signal. 
At the receive end, the received beam is delivered to a beam branching unit 
53 and is branched into primary and secondary beams. Each of the primary 
and the secondary beams corresponds to the first or the second split beam 
described in connection with FIG. 1 and is likewise processed. More 
specifically, the primary beam is delivered to a first polarization 
splitter 33(1) and is split into first and second primary beams which are 
orthogonally polarized. The secondary beam is delivered to a second 
polarization splitter 33(2) and split into first and second secondary 
beams which are orthogonally polarized. The first and the second primary 
beams are collectively referred to as a first port signal. The first and 
the second secondary beams are collectively called a second port signal. 
It should be noted here that each of the polarization splitters 33 
(suffixes omitted) has an axis of polarization. The axis of the first 
polarization splitter 33(1) is directed parallel to the received state of 
polarization. The axis of the second polarization splitter 33(2) is 
directed to form an angle of 45.degree. with the received state. The first 
and the second polarization splitters 33 are therefore respectively called 
a parallel and an oblique polarization splitter. The first and the second 
primary beams are beams of 0.degree. and 90.degree. polarization relative 
to the received state. The first and the second secondary beams are beams 
of minus and plus 45.degree. polarization relative to the received state. 
Each of the first and the second primary beams includes a primary component 
of the modulating frequency. Each of the first and the second secondary 
beams includes a secondary component of the modulating frequency. Through 
first and second dual pin photodiodes 35(1) and 35(2), first and second 
signal amplifiers 37(1) and 37(2), and first and second band-pass filters 
39(1) and 39(2), the primary and the secondary components are delivered to 
first and second electric intensity detectors 41(1) and 41(2) and 
converted into first and second result signals representative of variable 
optical intensities of the primary and the secondary components. 
The first and the second result signals are added by an adder 55 to produce 
the detection signal described in conjunction with FIG. 1. It may be 
mentioned here that the principal axis of the optical fiber 11, is 
variable in accordance with external disturbances, such as the ambient 
temperature. The first and the second primary and secondary beams are, 
however, polarized with an angle difference of only 45.degree.. It is 
therefore possible to reliably detect the detection signal from at least 
one of the first and the second port signals. 
In the illustrated example, use is not made of the reception polarization 
controller 31 and of the reception controller 45 described in connection 
with FIG. 1. The detection signal is therefore delivered from the receive 
end to the send end through the up optical fiber 49 for delivery to the 
launch controller 43 alone. The up optical fiber 49 is additionally used 
in transmitting an up data signal in the manner described in the 
following. 
At the receive end, the detection signal is delivered to an 
analog-to-digital converter (A/D) 57 for producing a digital detection 
signal. The up data signal is supplied to an up terminal 59. A multiplexer 
(MX) 61 multiplexes the optical detection signal and the up data signal 
into a multiplexed signal. An auxiliary laser oscillator 63 generates an 
up optical beam when an auxiliary bias current source 65 is connected to 
the oscillator. Intensity modulated by the multiplexed signal at an 
auxiliary intensity modulator 67, the up optical beam is transmitted as an 
up signal beam through the auxiliary optical fiber 49. 
On the send end at which the up signal beam is received, a single optical 
receiver 69 receives the up signal beam as a received up beam. Supplied 
with the received up beam, a demultiplexer (DEMX) 71 produces an up data 
signal reproduction for delivery to an up output terminal 73 and an up 
digital detection signal reproduction. 
Supplied with the up digital detection signal reproduction, a 
digital-to-analog converter (D/A) 75 produces a reproduction of the 
detection signal, namely of the sinusoidal signal, as the reception signal 
mentioned above. Controlled by the reception signal, the launch controller 
43 controls the launch polarization controller 23 to keep at the receive 
end the detection signal minimum. The launch controller 43 has of a 
response speed of 11 ms which is shorter than a propagation time of the 
signal beam through the main optical fiber 11. 
When use is not made of the first and the second port signals, it has not 
always been always possible to produce the received data signal to the 
system output terminal 29 with the best possible reception sensitivity 
throughout a long time interval. It has, however, been confirmed that use 
of four polarization states for the first and the second primary and 
secondary beams enables reliable and long-continued reception of the 
received data signal with no adverse influences on the reception by the 
external disturbances. 
Reviewing FIGS. 3(a) and 3(b) , the polarization control system is again 
called a polarization control device when its receive end alone is taken 
into consideration. The control arrangement now comprises the launch 
controller 43 and the additional optical fiber 49. It is possible to 
transmit the detection signal in an analog type as it stands through the 
additional optical fiber 49 by superposing on the up optical beam either 
by amplitude or by frequently modulating a subcarrier signal. It is also 
possible to split by polarization the primary and the secondary beams into 
the first and the second primary and secondary beams using directions of 
polarization different from the four directions. exemplified above. 
Reviewing FIGS. 1, 5(a), and 3(b) , it is possible to use any one of known 
polarization controllers for the launch polarization controller 23 and for 
the reception polarization controller 31 if used. For example, equally 
well suited are a waveguide polarization controller made of lithium 
niobate, a polarization controller of a wavelength plate rotating type, 
and a liquid crystal polarization controller. 
While this invention has thus far been described specifically conjunction 
with only two preferred embodiments and several modifications, it will now 
be readily possible for one skilled in the art to put this invention into 
practice in various other manners. For example, it is possible to detect 
the detection signal either from at least one of the first and the second 
split beams or from at least one of the first and the second primary or 
secondary beams.