U.S. patent application number 12/494812 was filed with the patent office on 2010-04-01 for optical signal transmitter.
This patent application is currently assigned to FUJITSU LIMITED. Invention is credited to Yuichi Akiyama, Takeshi Hoshida, Hideyuki Miyata.
Application Number | 20100080571 12/494812 |
Document ID | / |
Family ID | 41490422 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100080571 |
Kind Code |
A1 |
Akiyama; Yuichi ; et
al. |
April 1, 2010 |
OPTICAL SIGNAL TRANSMITTER
Abstract
An optical signal transmitter includes first and second
modulation units, a combiner, and a control unit. The first and
second modulation units generate first and second modulated optical
signals, respectively. The combiner combines the first and second
modulated optical signals to generate a polarization multiplexed
optical signal. The control unit controls at least one of the first
and second modulation units so that the optical powers of the first
and second modulated optical signals become approximately equal to
each other.
Inventors: |
Akiyama; Yuichi; (Kawasaki,
JP) ; Miyata; Hideyuki; (Kawasaki, JP) ;
Hoshida; Takeshi; (Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700, 1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
FUJITSU LIMITED
Kawasaki
JP
|
Family ID: |
41490422 |
Appl. No.: |
12/494812 |
Filed: |
June 30, 2009 |
Current U.S.
Class: |
398/184 ;
398/185; 398/186; 398/188 |
Current CPC
Class: |
H04J 14/06 20130101;
H04B 10/532 20130101; G02F 1/0123 20130101; H04B 10/5561 20130101;
H04B 10/505 20130101; H04B 10/54 20130101; G02F 1/0136
20130101 |
Class at
Publication: |
398/184 ;
398/185; 398/188; 398/186 |
International
Class: |
H04B 10/04 20060101
H04B010/04; H04B 10/12 20060101 H04B010/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2008 |
JP |
2008-247210 |
Claims
1. An optical signal transmitter comprising: a first modulation
unit configured to generate a first modulated optical signal; a
second modulation unit configured to generate a second modulated
optical signal; a combiner configured to combine the first and
second modulated optical signals to generate a polarization
multiplexed optical signal; and a control unit configured to
control at least one of the first and second modulation units so
that optical powers of the first and second modulated optical
signals become approximately equal to each other.
2. The optical signal transmitter according to claim 1, wherein the
first and second modulation units respectively have a drive circuit
generating a drive signal for modulating an optical signal in
accordance with transmission data, and the control unit controls an
amplitude of the drive signal of at least one of the first and
second modulation units.
3. The optical signal transmitter according to claim 1, wherein the
first and second modulation units respectively have a modulator
using an electro-optic effect, and the control unit controls a bias
of the modulator of at least one of the first and second modulation
units.
4. The optical signal transmitter according to claim 1, wherein the
first and second modulation units respectively have an optical
attenuator, and the control unit controls the optical attenuator of
at least one of the first and second modulation units.
5. The optical signal transmitter according to claim 1, further
comprising: a first monitor unit configured to obtain a first
monitor value representing an optical power of the first modulated
optical signal that is output from the first modulation unit; and a
second monitor unit configured to obtain a second monitor value
representing an optical power of the second modulated optical
signal that is output from the second modulation unit, wherein the
control unit controls at least one of the first and second
modulation units so that the first and second monitor values become
approximately equal to each other.
6. The optical signal transmitter according to claim 5, wherein the
first and second modulation units respectively have a modulator
using an electro-optic effect and a bias control circuit
controlling an operating point of the modulator, and the first and
second monitor units are respectively realized using a
photodetector for the bias control circuit.
7. The optical signal transmitter according to claim 1, wherein the
first and second modulation units respectively have a phase
modulator and an intensity modulator connected in series with the
phase modulator; each intensity modulator has a intensity
modulation driver circuit generating an intensity modulation drive
signal for intensity modulation in accordance with a clock signal;
and the control unit controls an amplitude of an intensity
modulation drive signal of the intensity modulator of at least one
of the first and second modulation units.
8. An optical signal transmitter comprising: a first light source;
a first modulation unit configured to modulate an optical signal
generated by the first light source to generate a first modulated
optical signal; a second light source; a second modulation unit
configured to modulate an optical signal generated by the second
light source to generate a second modulated optical signal; a
combiner configured to combine the first and second modulated
optical signals to generate a polarization multiplexed optical
signal; and a control unit configured to control at least one of
the first and second light sources so that optical powers of the
first and second modulated optical signals become approximately
equal to each other.
9. An optical signal transmitter comprising: a first modulation
unit configured to generate a first modulated optical signal; a
second modulation unit configured to generate a second modulated
optical signal; a combiner configured to combine the first and
second modulated optical signals to generate a polarization
multiplexed optical signal; and a control unit configured to
control at least one of the first and second modulation units,
wherein the first and second modulated optical signals have a same
symbol rate and have timings shifted with respect to each other by
a predetermined time; and the control unit controls at least one of
the first and second modulation units in accordance with the symbol
rate component in the polarization multiplexed optical signal.
10. The optical signal transmitter according to claim 9, wherein
the control unit controls at least one of the first and second
modulation units so as to minimize the symbol rate component in the
polarization multiplexed optical signal.
11. An optical signal transmitter comprising: a first light source;
a first modulation unit configured to modulate an optical signal
generated by the first light source to generate a first modulated
optical signal; a second light source; a second modulation unit
configured to modulate an optical signal generated by the second
light source to generate a second modulated optical signal; a
combiner configured to combine the first and second modulated
optical signals to generate a polarization multiplexed optical
signal; and a control unit configured to control at least one of
the first and second light sources, wherein the first and second
modulated optical signals have a same symbol rate and have timings
shifted with respect to each other by a predetermined time; and the
control unit controls at least one of the first and second light
sources in accordance with the symbol rate component in the
polarization multiplexed optical signal.
12. An optical signal transmitter comprising: an intensity
modulation unit configured to adjust an intensity of first and
second wavelength components; a demultiplexer configured to extract
the first and second wavelength components; a first modulation unit
configured to generate a first modulated optical signal from the
first wavelength component obtained by the demultiplexer; a second
modulation unit configured to generate a second modulated optical
signal from the second wavelength component obtained by the
demultiplexer; a multiplexer configured to multiplex the first and
second modulated optical signals to generate a multiplexed optical
signal; and a control unit configured to control the intensity
modulation unit so that optical powers of the first and second
modulated optical signals become approximately equal to each
other.
13. The optical signal transmitter according to claim 12, wherein
the intensity modulation unit has an LN modulator; and the control
unit controls a bias of the LN modulator.
14. The optical signal transmitter according to claim 12, wherein
the multiplexer performs polarization multiplexing of the first and
second modulated optical signals.
15. An optical signal transmitter comprising a first modulation
unit configured to generate a first modulated optical signal; a
second modulation unit configured to generate a second modulated
optical signal; a combiner configured to combine the first and
second modulated optical signals to generate a polarization
multiplexed optical signal; and a control unit configured to
control at least one of the first and second modulation units in
accordance with a received signal quality of the polarization
multiplexed optical signal.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority of the prior Japanese Patent Application No. 2008-247210,
filed on Sep. 26, 2008, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The present invention relates to an optical signal
transmitter, and may be applied to, for example, an optical signal
transmitter used in a polarization multiplexing transmission
system.
BACKGROUND
[0003] The demand for realizing a super-high-speed (over 40 Gbit/s,
i.e., 100 Gbit/s, for example) optical transmission system has been
increasing rapidly. For this reason, the development has been
underway, for the practical realization of an optical transmission
system adopting a multi-value modulation system (for example an
RZ-DQPSK modulation system using quadrature phase modulation) that
has been applied to the radio system. However, as the transmission
signal speed increases, solving problems related to the viability
of the electric signal circuit, and problems related to the
degradation of optical transmission signals (such as the
transmission signal spectrum degradation due to an optical filter,
the signal degradation due to chromatic dispersion and accumulation
of optical noises) becomes harder.
[0004] As a method for solving these problems to realize a
large-capacity long-distance transmission system, an optical
transmission system adopting the polarization division multiplexing
and digital coherent detection has been attracting attention. The
research and development for the commercialization of these
techniques are in progress, and the techniques are disclosed in,
for example, a document 1 (G.Charlet et al., "Transmission of
16.4Tbit/s Capacity over 2,550 km using PDM QPSK Modulation Format
and Coherent Receiver" presented at the OFC'08 Paper PDP3.), a
document 2 (J. Renaudier, et al., "Linear Fiber Impairments
Mitigation of 40-Gbit/s Polarization-Multiplexed QPSK by Digital
Processing in a Coherent Receiver," J. Lightwave Technology., vol.
26, No. 1, pp. 36-42, January 2008.), and a document 3 (O.
Bertran-Pardo et al., "Nonlinearity Limitations When Mixing 40-Gb/s
Coherent PDM-QPSK Channels With Preexisting 10-Gb/s NRZ Channels"
IEEE Photonics Technology Letters, Vol. 20, No. 15, pp. 1314-1316,
August 2008.).
[0005] According to the polarization division multiplexing, two
data streams are transmitted using two polarized waves having the
same wavelength and being orthogonal to each other. For this
reason, the polarization division multiplexing contributes to the
improvement of the characteristics of the electric signal
generation circuit, cost reduction, size reduction and
power-consumption reduction, as the modulation speed is reduced to
half. In addition, effects due to the quality degradation factors
such as the dispersion in the optical transmission path are
reduced, improving the characteristics of the optical transmission
system as a whole. For example, patent document 1 (Japanese
Laid-open Patent Publication No. 62-024731) and patent document 2
(Japanese Laid-open Patent Publication No. 2002-344426) disclose
transmission systems using the polarization division
multiplexing.
[0006] In an optical signal transmitter that generates a
polarization multiplexed signal, a modulator is provided for each
polarization signal. For this reason, a difference in optical power
between polarized waves of the optical signal may occur due to the
variation of the characteristics (for example, optical loss)
between the modulators, or, the variation of the optical losses of
an optical splitter, an optical combiner and the like. The
difference in optical power of the polarized waves causes the
degradation of the transmission characteristics.
SUMMARY
[0007] An optical signal transmitter of one aspect of the invention
includes: a first modulation unit configured to generate a first
modulated optical signal; a second modulation unit configured to
generate a second modulated optical signal; a combiner configured
to combine the first and second modulated optical signals to
generate a polarization multiplexed optical signal; and a control
unit configured to control at least one of the first and second
modulation units so that optical powers of the first and second
modulated optical signals become approximately equal to each
other.
[0008] An optical signal transmitter of another one aspect of the
invention includes: a first modulation unit configured to generate
a first modulated optical signal; a second modulation unit
configured to generate a second modulated optical signal; a
combiner configured to combine the first and second modulated
optical signals to generate a polarization multiplexed optical
signal; and a control unit configured to control at least one of
the first and second modulation units. The first and second
modulated optical signals have a same symbol rate and have timings
shifted with respect to each other by a predetermined time. The
control unit controls at least one of the first and second
modulation units in accordance with the symbol rate component in
the polarization multiplexed optical signal.
[0009] An optical signal transmitter of another one aspect of the
invention includes: an intensity modulation unit configured to
adjust an intensity of first and second wavelength components; a
demultiplexer configured to extract the first and second wavelength
components; a first modulation unit configured to generate a first
modulated optical signal from the first wavelength component
obtained by the demultiplexer; a second modulation unit configured
to generate a second modulated optical signal from the second
wavelength component obtained by the demultiplexer; a multiplexer
configured to multiplex the first and second modulated optical
signals to generate a multiplexed optical signal; and a control
unit configured to control the intensity modulation unit so that
optical powers of the first and second modulated optical signals
become approximately equal to each other.
[0010] The object and advantages of the invention will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0011] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0012] FIGS. 1A and 1B are diagrams illustrating the configuration
of an optical signal transmitter according to the first aspect.
[0013] FIG. 2 is a diagram explaining polarization division
multiplexing.
[0014] FIG. 3 illustrates the first embodiment of an optical signal
transmitter.
[0015] FIG. 4 is a diagram illustrating the operation of an LN
modulator.
[0016] FIGS. 5A and 5B illustrate an embodiment of a computing
unit.
[0017] FIG. 6 illustrates the second embodiment of an optical
signal transmitter.
[0018] FIG. 7 illustrates the third embodiment of an optical signal
transmitter.
[0019] FIG. 8 is a diagram illustrating the operation of an LN
modulator used as an RZ modulator.
[0020] FIG. 9 is a diagram illustrating the bias of an LN
modulator.
[0021] FIG. 10 illustrates the fourth embodiment of an optical
signal transmitter.
[0022] FIG. 11 illustrates the fifth embodiment of an optical
signal transmitter.
[0023] FIGS. 12A and 12B are diagrams illustrating modified
configurations of the first aspect.
[0024] FIG. 13 illustrates the sixth embodiment of an optical
signal transmitter.
[0025] FIG. 14 is a diagram illustrating the configuration of an
optical signal transmitter according to the second aspect.
[0026] FIGS. 15A and 15B are diagrams explaining Time-Interleaved
polarization division multiplexing.
[0027] FIG. 16 is a diagram illustrating the spectrum of an output
signal of a photodetector.
[0028] FIG. 17 illustrates the seventh embodiment of an optical
signal transmitter.
[0029] FIG. 18 illustrates the eighth embodiment of an optical
signal transmitter.
[0030] FIG. 19 illustrates the ninth embodiment of an optical
signal transmitter.
[0031] FIG. 20 illustrates the tenth embodiment of an optical
signal transmitter.
[0032] FIG. 21 illustrates the eleventh embodiment of an optical
signal transmitter.
[0033] FIG. 22 is a diagram illustrating a modified configuration
of the second aspect.
[0034] FIG. 23 illustrates the twelfth embodiment of an optical
signal transmitter.
[0035] FIG. 24 is a diagram illustrating the configuration of an
optical signal transmitter according to the third aspect.
[0036] FIG. 25 illustrates the thirteenth embodiment of an optical
signal transmitter.
[0037] FIG. 26 is a diagram explaining the operation of an LN
modulator used as CS-RZ modulation.
[0038] FIG. 27 is a diagram illustrating the relationship between
the optical power of subcarriers and a bias.
[0039] FIG. 28 illustrates the fourteenth embodiment of an optical
signal transmitter.
[0040] FIG. 29 is a diagram (1) illustrating the configuration for
performing feedback control in accordance with the received signal
quality.
[0041] FIG. 30 is a diagram (2) illustrating the configuration for
performing feedback control in accordance with the received signal
quality.
[0042] FIG. 31 is a diagram (3) illustrating the configuration for
performing feedback control in accordance with the received signal
quality.
DESCRIPTION OF EMBODIMENTS
[0043] FIGS. 1A and 1B are diagrams illustrating the configuration
of an optical signal transmitter according to the first aspect. The
optical signal transmitter according to the first aspect transmits
a polarization multiplexed optical signal obtained by the
polarization division multiplexing of first and second modulated
optical signals. At this time, a difference between the optical
powers of the first and second modulated optical signals causes the
deterioration of the characteristics of the polarization
multiplexed optical signal. Therefore, in the first aspect, the
powers of the first and second modulated optical signals are
controlled to be equal or approximately equal.
[0044] A light source (LD) 1 is, for example, a laser diode, which
generates an optical signal having a frequency. The optical signal
is, for example, a continuous wave (CW) that is split by, for
example, an optical splitter and directed to modulation units 10,
20.
[0045] The modulation unit 10 modulates an input optical signal in
accordance with transmission data X to generate a modulated optical
signal X. In the same manner, the modulation unit 20 modulates an
input optical signal in accordance with transmission data Y to
generate a modulated optical signal Y. The modulation units 10, 20
are configured to respectively include a modulator (in this
example, a Mach-Zehnder LN modulator) with which the power of the
output light periodically changes with respect to the drive
voltage. Furthermore, ABC (Auto Bias Control) circuits 11, 21 are
provided, in order to control the operating points (i.e., the bias)
of the LN modulators of the modulators 10, 20. The ABC circuits 11,
21 apply a low-frequency voltage signal to the corresponding LN
modulator, and respectively adjust the operating point (i.e., the
DC bias voltage) of the LN modulator, in accordance with the
low-frequency component contained in the output lights of the
modulators 10, 20.
[0046] While an LN modulator is described herein as an example of
the optical modulator, this is not a limitation. In other words,
the optical modulator is not limited to the LN modulator, and may
be a modulator using an electro-optic material, i.e., for example,
a modulator configured with a semiconductor material such as
InP.
[0047] Optical attenuators 12, 22 respectively adjust the powers of
the modulated optical signals X, Y. The optical attenuators 12, 22
are not essential constituent elements. In addition, the optical
attenuators 12, 22 may be disposed either on the input side of the
modulators 10, 20, or within the modulators 10, 20, or on the
output side of the modulators 10, 20.
[0048] A polarization beam combiner (PBC) 31 performs polarization
multiplex for the modulated optical signal X and modulated optical
signal Y to generate polarization multiplexed optical signal. At
this time, according to the polarization multiplex (or DP: dual
polarization), as illustrated in FIG. 2, X polarized wave and Y
polarized wave being orthogonal to each other are used.
Specifically, the modulated optical signal X is propagated using
the X polarized wave, and the modulated optical signal Y is
propagated using the Y polarized wave.
[0049] A computing unit 41 calculates the difference between a
monitor signal X representing the characteristics of the modulated
optical signal X and a monitor signal Y representing the
characteristics of the modulated optical signal Y. The monitor
signals X and Y are, in the configuration illustrated in FIG. 1A,
obtained using the DC component of the monitor signals referred to
by the ABC circuits 11, 21. In the configuration illustrated in
FIG. 1B, the monitor signals X, Y are obtained from the spilt-off
portions of the modulated optical signals X, Y output from the
modulators 10, 20. A control unit 42 generates, in order to perform
feedback control, a control signal C for making the difference
between the monitor signals X, Y zero. The "zero" here does not
require being exactly zero, and includes a sufficiently small
value.
[0050] The control signal C generated by the control unit 42
controls, for example, the amplitude of drive signals of the
modulators 10, 20. Alternatively, the control signal C may be used
to control the bias of the LN modulators provided in the modulators
10, 20. Furthermore, in the configuration in which the optical
attenuators 12, 22 are provided, the control signal C may control
the attenuation amount of the optical attenuators 12, 22. In either
case, a feedback system for making the difference between the
monitor signals X, Y zero is formed.
[0051] The feedback control in the configuration described above
makes the powers of the modulated optical signals X, Y
approximately equal to each other. That is, the powers of the X
polarized wave and the Y polarized wave of the polarization
multiplexed optical signal become approximately equal to each
other. Therefore, the transmission characteristics of the
polarization multiplexed optical signal are improved.
[0052] FIG. 3 illustrates the first embodiment of the optical
signal transmitter. In this embodiment, it is assumed that
transmission data X and transmission data Y are transmitted by
means of a polarization multiplexed optical signal. In addition, in
the first embodiment, data are transmitted according to the
NRZ-DQPSK modulation. Meanwhile, the modulation method is not
limited to the DQPSK, DPSK, and other multi-value modulation
methods. For example, as disclosed in U.S. Patent Application
Publication No. 2006/0127102, the optical transmitter may be
equipped with an optical modulator that changes the optical phase
as a vector by filtering a data signal.
[0053] In FIG. 3, the modulation unit 10 illustrated in FIG. 1A or
FIG. 1B has a DQPSK optical modulator 13, driver circuits 14a, 14b,
and a photodetector (PD) 15. The DQPSK optical modulator 13 has, in
this embodiment, LN modulators 13a, 13b, and a .pi./2 phase shift
element 13c. The LN modulators 13a, 13b are, in this embodiment,
Mach-Zehnder interferometers. The LN modulator 13a is disposed in
one of an I arm or a Q arm, and the LN modulator 13b is disposed in
the other of the I arm or the Q arm, The .pi./2 phase shift element
13c gives a phase difference .pi./2 between the I arm and the Q
arm. The .pi./2 phase shift element 13c is realized with, for
example, a material of which optical path length changes in
accordance with the voltage or temperature.
[0054] The driver circuit 14a drives the LN modulator 13a using a
drive signal Data I. The driver circuit 14b drives the LN modulator
13b using a drive signal Data Q. Here, the drive signals Data I,
Data Q are generated, for example, by encoding the transmission
data X using a DQPSK encoder. The driver circuits 14a, 14b may
respectively have an amplifier and control the amplitudes of the
drive signals Data I, Data Q. While the output of the driver
circuits 14a, 14b is a differential output in FIG. 3, it may be a
single output.
[0055] FIG. 4 is a diagram explaining the operation of the LN
modulator. The power of the output light of the LN modulator
changes periodically with respect to the driving voltage. Here, the
drive amplitude is "2V.pi.". Meanwhile, "V.pi." is a
half-wavelength voltage, which is a voltage for the power of the
output light of the LN modulator to change from a local minimum
value to a local maximum value. Therefore, in FIG. 3, a decrease in
the amplitude of the drive signal Data I results in a decrease in
the amplitude of the output optical signal of the LN modulator 13a,
lowering the average power of the output light of the LN modulator
13a. In the same manner, a decrease in the amplitude of the drive
signal Data Q lowers the average power of the output light of the
LN modulator 13b. The amplitudes of the drive signals Data I, Data
Q are controlled, for example, by adjusting the gain of the
amplifier provided in the driver circuits 14a, 14b, respectively.
In addition, when adopting a fixed-gain amplifier, a similar effect
can be obtained by adjusting the amplitude of the input signal to
the amplifier. The powers of the output lights of the LN modulators
13a, 13b are controlled to be equal to each other, which is to be
described in detail later.
[0056] The photodetector 15 converts the output light of the DQPSK
optical modulator 13 into an electric signal. In this embodiment,
the DQPSK optical modulator 13 outputs a pair of complementary
optical signals. Then, one of the pair of optical signals is
directed to the polarization beam combiner 31, while the other
optical signal is directed to the photodetector 15. Therefore, the
electric signal obtained by the photodetector 15 represents the
output light of the DQPSK optical modulator 13. As another method
for inputting an optical signal to the photodetector 15, a leakage
light of the output combiner of the DQPSK optical modulator 13 may
be used.
[0057] The ABC circuit 11 controls, for example, the drift of the
LN modulators 13a, 13b in accordance with the dithering method. In
this case, the ABC circuit 11 generates a low-frequency voltage
signal. The frequency f.sub.0 of the low-frequency voltage signal
is sufficiently low with respect to the symbol rate of the
transmission data X, Y. The low-frequency voltage signal is given
to the LN modulators 13a, 13b. When the low-frequency voltage
signal is given to the LN modulator 13a, the output light of the LN
modulator 13a includes "f.sub.0 component" and/or "2f.sub.0
component", and the f.sub.0 component and/or the 2f.sub.0 component
are extracted from the output signal of the photodetector 15. Then,
the ABC circuit 11 adjusts the DC bias voltage to be applied to the
LN modulator 13a, using the extracted frequency components. The
same applies to the LN modulator 13b. In addition, the ABC circuit
11 is capable of adjusting the phase shift amount of the .pi./2
phase shift element 13c in accordance with the dithering
method.
[0058] Meanwhile, when the operating point of the modulator is
shifted by adjusting the DC bias voltage to be applied to the LN
modulator 13a, the average power of the output light of the LN
modulator 13a changes. Specifically, for example in FIG. 4, when
the DC voltage of the drive signal is adjusted, the corresponding
output optical signal changes, and thus the average power of the
output light also changes. Therefore, the power of the output light
of the LN modulator 13a can be controlled by adjusting the DC bias
voltage to be applied to the LN modulator 13a.
[0059] The configuration and the operation of the modulation unit
20 illustrated in FIGS. 1A and 1B are basically the same as those
of the modulation unit 10. That is, the modulation unit 20 has a
DQPSK optical modulator 23, driver circuits 24a, 24b, and a
photodetector 25. Then, the driver circuits 24a, 24b drive the
DQPSK optical modulator 23 in accordance with the transmission data
Y.
[0060] The optical signal transmitter configured as described above
transmits a pair of transmission signals X, Y using a polarization
multiplexed optical signal. Specifically, the DQPSK optical
modulator 13 is driven in accordance with the transmission data X
to generate the modulated optical signal X. In the same manner, the
DQPSK optical modulator 23 is driven in accordance with the
transmission data Y to generate the modulated optical signal Y. The
modulated optical signal X and the modulated optical signal Y are
directed to the polarization beam combiner 31. Then, the
polarization beam combiner generates the polarization multiplexed
optical signal by performing polarization multiplexing of the
modulated optical signals X, Y. The polarization multiplexed
optical signal is transmitted via an optical fiber transmission
path.
[0061] At this time, the output lights of the DQPSK optical
modulators 13, 23 are respectively converted into electric signals
by the photodetectors 15, 25, and given to the computing unit 41 as
monitor signals X, Y. The monitor signals X, Y may be the DC
components of the output signals of the photodetectors 15, 25.
Alternatively, when the computing unit is realized with a processor
such as a DSP, the DC components may be obtained by sampling the
output of the photodetectors 15, 25. In either case, the computing
unit 41 obtains signals representing the average powers of the
output lights of the DQPSK optical modulators 13, 23. Hereinafter,
it is assumed that the monitor signals X, Y represent the average
powers of the output lights of the DQPSK optical modulators 13,
23.
[0062] The computing unit 41 calculates the difference between the
monitor signals X, Y. Here, the computing unit 41 is realized with,
while it is not a particular limitation, for example, a subtractor
utilizing a differential amplifier circuit such as the one
illustrated in FIG. 5A. In addition, the computing unit 41 may be
configured using a comparator such as the one illustrated in FIG.
5B. In this configuration, an averaging circuit is disposed for the
output of the comparator. While the averaging circuit is not an
essential configuration, the averaging of the output signal of the
comparator makes the control by the control unit 42 easy. In
addition, when the computing unit 41 is realized with a processor
such as a DSP, the monitor signals X, Y are converted into digital
data, and digital calculation is performed.
[0063] The control unit 42 generates a control signal C for making
the difference obtained by the computing unit 41 zero. The control
signal C is given to, in this example, driver circuits 14a, 14b,
24a, 24b. In otherwords, the amplitudes of drive signals Data I,
Data Q that drive the DQPSK optical modulator 13, and/or the
amplitudes of drive signals Data I, Data Q that drive the DQPSK
optical modulator 23 are controlled by the control signal C. For
example, when the power of the output light of the DQPSK optical
modulator 13 is larger than the power of the output light of the
DQPSK optical modulator 23, the control unit 42 generates a control
signal C for decreasing the amplitudes of the drive signals Data I,
Data Q that drive the DQPSK optical modulator 13 (or, for
increasing the amplitudes of the drive signals Data I, Data Q that
drive the DQPSK optical modulator 23). This feedback control makes
the powers of the output lights of the DQPSK optical modulators 13,
23 approximately equal to each other. In other words, the optical
powers of the X polarized wave and the Y polarized wave of the
polarization multiplexed optical signal become approximately equal
to each other.
[0064] While monitor signals X, Y are generated using the
photodetectors 15, 25 in the configuration illustrated in FIG. 3,
the monitor signals X, Y may be generated in accordance with the
configuration illustrated in FIG. 1B. In other words, the modulated
optical signals X, Y directed to the polarization beam combiner 31
may be split off, and the monitor signals X, Y may be generated
from the split-off portions. However, in the configuration
illustrated in FIG. 3, the monitor signals X. Y can be obtained
using the photodetectors 15, 25 for the ABC circuits 11, 21,
contributing to cost reduction.
[0065] In addition, while the amplitude of the drive signals
driving the DQPSK optical modulators 13, 23 are controlled in the
configuration illustrated in FIG. 3, other elements may be
controlled. In other words, for example, the bias of the DQPSK
optical modulators 13, 23 may be controlled. In this case, the
control signal C is given to the ABC circuits 11, 21. Then, the ABC
circuits 11, 21 control, as explained with reference to FIG. 4, the
DC bias voltage in accordance with the control signal C.
Alternatively, in the configuration such as the one illustrated in
FIG. 1A or 1B in which optical attenuators 12, 22 are provided,
each optical attenuator may be controlled in accordance with the
control signal C. In this case, the monitor signals X, Y may be
generated using the optical signal split off on the output side of
the optical attenuator.
[0066] FIG. 6 illustrates the second embodiment of the optical
signal transmitter. The optical signal transmitter according to the
second embodiment has an RZ optical modulator on the input side or
on the output side of the DQPSK optical modulator. In the example
illustrated in FIG. 6, RZ optical modulators 51, 61 are disposed on
the output side of the DQPSK optical modulators 13, 23,
respectively. That is, in the second embodiment, data are
transmitted in accordance with the RZ-DQPSK modulation.
[0067] The RZ optical modulators 51, 61 are, for example,
Mach-Zehnder LN modulators, which perform RZ modulation in
accordance with drive signals generated by driver circuits 52, 62.
Here, the drive circuits 52, 62 generate drive signals synchronized
with a symbol clock. The drive signal is, while it is not a
particular limitation, a sine wave having the same frequency as the
symbol clock. In addition, the amplitude of the drive signal is,
for example, V.pi..
[0068] The ABC circuit 11 controls, not only the drift of the DQPSK
optical modulator 13 but also the drift of the RZ optical modulator
51. In the same manner, the ABC circuit 21 controls, not only the
drift of the DQPSK optical modulator 23, but also the drift of the
RZ optical modulator 61. Meanwhile, the configuration and the
operation of the computing unit 41 and the control unit 42 are
similar to those in the first embodiment.
[0069] In the optical signal transmitter configured as described
above, the control signal C generated by the control unit 42 is
given to driver circuits 14a, 14b, 24a, 24b. That is, feedback
control is performed for the amplitude of at lest on of the drive
signals driving the DQPSK optical modulators 13, 23.
[0070] FIG. 7 illustrates the third embodiment of the optical
signal transmitter. In the third embodiment, the control signal C
generated by the control unit 42 is given to driver circuits 52, 62
driving RZ optical modulators 51, 61. The driver circuits 52, 62
generate, as described above, a drive signal synchronized with a
symbol clock.
[0071] FIG. 8 is a diagram explaining the operation of an LN
modulator used as the RZ optical modulators 51, 61. In the LN
modulator, when it is used as the RZ optical modulators 51, 61, the
amplitude of the drive signal is, for example, V.pi.. Here, a
decrease in the amplitude of the drive signal results in a decrease
in the average power of the output light of the LN modulator. That
is, the control of the drivers 52, 62 using the control signal C to
control the amplitude of the drive signal of the RZ optical
modulators 51, 52 results in a change in the average power of the
output light of the RZ optical modulators 51, 61. Therefore, for
example, when the power of the output light of the RZ optical
modulator 51 is larger than the power of the output light of the RZ
optical modulator 61, the control unit 42 generates a control
signal C for decreasing the amplitude of the drive signal driving
the RZ optical modulator 51 (or, for increasing the amplitude of
the drive signal driving the RZ optical modulator 61). The feedback
control makes the powers of the output lights of the RZ optical
modulators 51, 61 approximately equal to each other. That is, the
optical powers of the X polarized wave and the Y polarized wave of
the polarization multiplexed optical signal become approximately
equal to each other.
[0072] FIG. 9 is a diagram explaining the bias of the LN modulator
used as the RZ optical modulators 51, 61. Here, the state in which
the operating point is adjusted to the center and the state in
which the operating point is shifted from the center are
illustrated. In this case, as illustrated in FIG. 9, if the
operating point is shifted from the center, the average power of
the output light of the LN modulator decreases. In other words, the
average power of the output light is controlled by adjusting the DC
bias voltage applied to the LN modulator. Therefore, the control
unit 42 is able to make the optical powers of the X polarized wave
and the Y polarized wave of the polarization multiplexed optical
signal approximately equal to each other by adjusting the DC bias
voltage, utilizing this characteristic of the LN optical
modulator.
[0073] FIG. 10 illustrates the fourth embodiment of the optical
signal transmitter. In the fourth embodiment, the control signal C
generated by control unit 42 is given to ABC circuits 11, 21. At
this time, the ABC circuits 11, 21 control the DC bias voltage of
the RZ optical modulators 51, 61 in accordance with the control
signal C. The relationship between the DC bias voltage and the
power of the output light of the LN modulator is as described with
reference to FIG. 9.
[0074] FIG. 11 illustrates the fifth embodiment of the optical
signal transmitter. In the fifth embodiment, optical attenuators
12, 22 are provided to adjust the power of each of the modulated
optical signals. The optical attenuators 12, 22 may be disposed
between the DQPSK optical modulators 13, 23 and the RZ optical
modulators 51, 61, or may be disposed on the output side of the RZ
optical modulators 51, 61. In addition, the optical attenuators may
be disposed respectively between the LD1 and the DQPSK optical
modulator 13, and between the LD1 and the DQPSK optical modulator
23.
[0075] The control signal C generated by the control unit 42 is
given to the optical attenuators 12, 22. The optical attenuators
12, 22 adjust the power of the modulated optical signal in
accordance with the control signal C. When the optical attenuators
12, 22 are disposed on the output side of the RZ optical modulators
51, 61, the monitor signals X, Y are generated from the optical
signal split off on the output side of the optical attenuators 12,
22.
[0076] FIGS. 12A and 12B are diagrams illustrating modification
examples of the first aspect. The configurations illustrated in
FIGS. 12A and FIG. 12B correspond to the optical signal
transmitters illustrated in FIG. 1A and FIG. 1B, respectively.
[0077] In the optical signal transmitters illustrated in FIG. 12A
and FIG. 12B, light sources 2, 3 are provided for the modulation
units 10, 20, respectively. The modulation unit 10 generates a
modulated optical signal X using the output light of the light
source 2, and the modulation unit 20 generates a modulated optical
signal Y using the output light of the light source 3.
[0078] In the optical transmitter configured as described above,
the control signal C generated by the control unit 42 is given to
the light sources 2, 3. Then, the light sources 2, 3 controls the
light-emitting power, making it possible to make the optical powers
of the X polarized wave and the Y polarized wave of the
polarization multiplexed optical signal approximately equal to each
other.
[0079] FIG. 13 illustrates the sixth embodiment of the optical
signal transmitter. In the sixth embodiment, the light-emitting
power of the light sources 2, 3 are adjusted in accordance with the
control signal C generated by the control unit 42. For example,
when the power of the output light of the RZ optical modulator 51
is larger than the power of the output light of the RZ optical
modulator 61, the control unit 42 generates a control signal C for
decreasing the light-emitting power of the light source 2 (or, for
increasing the light-emitting power of the light source 3). The
feedback control makes the output optical powers of the modulated
optical signals X, Y approximately equal to each other.
[0080] FIG. 14 is a diagram illustrating the configuration of the
optical signal transmitter according to the second aspect. In the
optical signal transmitter according to the second aspect, a signal
is transmitted using Time-Interleaved Polarization Multiplex.
[0081] FIGS. 15A and 15B are diagrams illustrating Time-Interleaved
Polarization Multiplex. In a general (aligned) polarization
multiplexing, as illustrated in FIG. 15A, the pulses of the X
polarized wave and the Y polarized wave are transmitted at the same
timing. On the other hard, according to the Time-Interleaved
Polarization Multiplex, as illustrated in FIG. 15B, the pulses of
the X polarized wave and the Y polarized wave are transmitted in
the state where they are shifted by time .DELTA.t with respect to
each other. The shift time .DELTA.t corresponds to, for example,
one fourth of the symbol period.
[0082] In an optical signal transmitter according to the
Time-Interleaved Polarization Multiplex system, the operation
timings of the modulators 10, 20 are shifted with respect to each
other by one-fourth period of the symbol clock, in order to realize
the shift time .DELTA.t. In the configuration illustrated in FIG.
14, the modulator 10 operates in synchronization with a clock
signal CLK1, and the modulator 20 operates in synchronization with
a clock signal CLK2. The frequency of the clock signals CLK1, CLK2
is the same, and corresponds to the symbol rate.
[0083] The Time-Interleaved Polarization Multiplex makes it
possible to suppress the degradation of transmission quality due to
non-linear noises in the optical fiber. The Time-Interleaved
Polarization Multiplex is described, for example, D.Van Den Borne,
et. al., "1.6-b/s/Hz Spectrally Efficient Transmission Over 1700 Km
of SSMF Using 40.times.85.6-Gb/s POLMUX-RZ-DQPSK", J. Lightwave
Technology., Vol. 25, No. 1, January 2007.
[0084] The optical signal transmitter according to the second
aspect has, as illustrated in FIG. 14, an optical splitter 71, a
photodetector (PD) 72, a mixer 73, and a control unit 74. The
optical splitter 71 splits off an interleaved polarization
multiplexed optical signal output from the polarization beam
combiner 31. The photodetector 72 is, for example, a photodiode,
and converts the split-off interleaved polarization multiplexed
optical signal into an electric signal. The mixer 73 multiplexes an
output signal of the photodetector 72 and the clock signal CLK2 to
generate a monitor signal M. The control unit 74 generates a
control signal D in accordance with the monitor signal M. In this
regard, the optical splitter 71 may be integrated with the
polarization beam combiner 31.
[0085] FIG. 16 is a diagram illustrating the spectrum of an output
signal of the photodetector 72. The spectrum is a result of the
simulation for changing the optical power difference between the
modulated optical signals X, Y. The symbol rate is 21.5G. The
modulated optical signals X, Y are RZ-DQPSK optical signals.
[0086] When the optical powers of the modulated optical signals X,
Y are different from each other, the peak of the optical power
appears at the frequency corresponding to the symbol rate. In the
example illustrated in FIG. 16, when the optical powers of the
modulated optical signals X, Y are different from each other only
by 0.1 dB, the peak of the optical power appears at 21.5 GHz. In
addition, as the difference between optical powers of the modulated
optical signals X, Y increase, the optical power at 21.5 GHz also
increases.
[0087] On the other hand, when the optical powers of the modulated
optical signals X, Y are the same, the peak of the optical power
does not appear at the frequency corresponding to the symbol rate.
Therefore, the optical power of the modulated optical signals X, Y
become equal to each other, when the optical power is monitored at
the frequency corresponding to the symbol rate and feedback control
is performed so as to make the monitored optical power minimum.
[0088] Therefore, in the second aspect, a frequency component fs
corresponding to the symbol rate is extracted from an output signal
of the photodetector 72. In the example illustrated in FIG. 14, the
frequency component fs is extracted from an output signal of the
photodetector 72 by multiplexing the output signal of the
photodetector 72 with the clock signal CLK2 using the mixer 73.
Then, the signal extracted by the mixer 73, or a signal
representing the power of the frequency component fs is given to
the control unit 74 as a monitor signal M. Meanwhile, the frequency
component fs may be extracted using a bandpass filter. In this
case, the frequency component fs can be extracted without using the
clock signal CLK2.
[0089] The control unit 74 generates a control signal D to minimize
the monitor signal M. The control signal D controls, for example,
the amplitude of at least one of the drive signals of the
modulation units 10, 20. Alternatively, the control signal D may
control the bias of the LN modulator provided in at least one of
the modulation units 10, 20. Furthermore, in a configuration in
which the optical attenuators 12, 22 are provided, the control
signal D may control the attenuation amount of at least one of the
optical attenuators 10, 20. In either case, a feedback system for
minimizing the monitor signal M (that is, for minimizing the
frequency component fs) is formed. Meanwhile, the optical
attenuators 12, 22 maybe disposed between the LD1 and the
modulators 10, 20, respectively.
[0090] The configuration described above makes it possible to make
the powers of the modulated optical signals X, Y approximately
equal to each other, in the second aspect as well. That is, the
powers of the X polarized wave and the Y polarized wave of the
polarization multiplexed optical signal can be approximately equal
to each other. Therefore, the transmission characteristics of the
polarization multiplexed optical signal are improved. In addition,
as illustrated in FIG. 16, since the spectrum changes significantly
with respect to a slight difference (in the embodiment, only by 0.1
dB) between the optical powers of the modulated optical signals X,
Y, an optical power adjustment with a high accuracy can be
performed.
[0091] FIG. 17 illustrates the seventh embodiment of the optical
signal transmitter. The configuration of the seventh embodiment is
basically the same as that of the first embodiment. However, the
feedback system of the seventh embodiment is different from that of
the first embodiment.
[0092] In the optical signal transmitter according to the seventh
embodiment, as explained with reference to FIG. 14, the amplitude
of the drive signal of the DQPSK optical modulators 13, 23 is
controlled so as to minimize the monitor signal M. However, in this
embodiment, which one of the output lights of the DQPSK optical
modulators 13, 23 has a larger power is not necessarily detected.
For this reason, in the feedback control using the control signal
D, the following procedures are performed, for example. Here, it is
assumed that a monitor signal M1 is detected.
[0093] When the monitor signal M1 is smaller than a threshold
level, it is determined that the power difference between the
modulated optical signals X, Y is sufficiently small. In this case,
the amplitude of the drive signals of the DQPSK optical modulators
13, 23 are maintained. When the monitor signal M1 is larger than
the threshold level, a control signal D for decreasing the
amplitude of the drive signal of the DQPSK optical modulator 13 is
generated, and a monitor signal M2 is detected. If the monitor
signal M2 is smaller than the monitor signal M1, it is determined
that the control direction is correct, and after that, the control
signal D for decreasing the amplitude of the drive signal of the
DQPSK optical modulator 13 is generated until the monitor signal
becomes smaller than the threshold level. On the other hand, if the
monitor signal M2 is larger than the monitor signal M1, it is
determined that the control direction is wrong, and a control
signal D for decreasing the amplitude of the drive signal of the
DQPSK optical modulator 23 is generated until the monitor signal
becomes smaller than the threshold level.
[0094] While the amplitude of the drive signal driving the DQPSK
optical modulators 13, 23 is controlled in the configuration
illustrated in FIG. 17, other elements may be controlled. In other
words, the bias of the DQPSK optical modulators 13, 23 may be
controlled. In this case, the control signal D is given to the ABC
circuits 11, 21. Then, the ABC circuits 11, 21 control, as
explained with reference to FIG. 4, the DC bias voltage in
accordance with the control signal D. Alternatively, in an
embodiment such as the one illustrated in FIG. 14 in which the
optical attenuators 12, 22 are provided, each attenuator may be
controlled in accordance with the control signal D.
[0095] FIG. 18 through FIG. 21 illustrates the eighth through
eleventh embodiments. The configuration of the eighth through
eleventh embodiments is similar to that of the second through fifth
embodiments. However, the feedback system of the eighth through
eleventh embodiments adopts the configuration described with
reference to FIG. 14.
[0096] FIG. 22 is a diagram illustrating a modification example of
the second aspect. In the optical signal transmitter illustrated in
FIG. 22, light sources 2, 3 are provided for the modulation units
10, 20, respectively, in the same manner as in the configuration
illustrated in FIG. 12A, FIG. 12B. The modulation unit 10 generates
a modulated optical signal X using output light of the light source
2, and the modulation unit 20 generates a modulated optical signal
Y using output light of the light source 3. Here, the optical
attenuators 12, 22 may be disposed either on the input side or on
the output side of the modulation units 10, 20.
[0097] In the optical signal transmitter configured as described
above, the control signal D generated by the control unit 74 is
given to at least one of the light sources 2, 3. Then, the light
sources 2, 3 control the light-emitting power in accordance with
the control signal D. This makes it possible to make the optical
powers of the X polarized wave and the Y polarized wave of the
polarization multiplexed optical signal approximately equal to each
other.
[0098] FIG. 23 illustrates the twelfth embodiment of the optical
signal transmitter. The configuration of the twelfth embodiment is
similar to that of the sixth embodiment. However, the feedback
system of the twelfth embodiment adopts the configuration described
with reference to FIG. 14.
[0099] In the third embodiment, a plurality of subcarriers with
different frequencies are generated, and a plurality of data sets
are transmitted with each subcarrier.
[0100] FIG. 24 is a diagram illustrating the configuration of the
optical signal transmitter according to the third aspect. In FIG.
24, alight source (LD) 1 outputs light having a wavelength A. The
output light of the light source 1 is, for example, a continuous
wave. A modulation unit 80 has an LN modulator, and generates a
plurality of optical subcarriers 1 through n from the output light
of the light source 1. The wavelengths .lamda.1 through .lamda.n
are different from each other by .DELTA..lamda..
[0101] A demultiplexer 91 separates the plurality of subcarriers 1
through n by each wavelength. The subcarrier 1 through n are
respectively directed to modulation units 10-1 through 10-n. The
configuration and operation of each of the modulation units 10-1
through 10-n are the same as those of the modulation units 10, 20
of the first or second aspect. Specifically, the modulation units
10-1 through 10-n respectively generate modulated optical signals 1
through n by modulating the subcarriers 1 through n with
corresponding transmission data. Then, the modulated optical
signals 1 through n are multiplexed by a multiplexer 30, and output
to an optical fiber transmission path. The multiplexer 30 is, for
example, a polarization beam combiner or a wavelength multiplexer.
Thus, a plurality of data streams are transmitted using a plurality
of wavelengths .lamda.1 through .lamda.n. In this case, the
plurality of data streams may be transmitted in accordance with the
OFDM system.
[0102] A computing unit 43 compares the powers of output lights of
the modulation units 10-1 through 10-n. Then, a control unit 44
generates a control signal E for making the powers of the output
lights of the modulation units 10-1 through 10-n approximately
equal to each other. The control signal E is given to, for example,
the modulation unit 80. In this case, the bias of the LN modulator
provided in the modulation unit 80 is controlled in accordance with
the control signal E.
[0103] FIG. 25 illustrates the thirteenth embodiment of the optical
signal transmitter. In the thirteenth embodiment, a CS-RZ optical
modulator 81 is provided as the modulation unit 80. The CS-RZ
optical modulator 81 is a Mach-Zehnder LN modulator, which operates
as an intensity modulator. A driver circuit 82 generates a drive
signal for driving the CS-RZ optical modulator 81. The frequency of
the drive signal is "fc/2".
[0104] The configuration in which a plurality of subcarriers are
generated using an optical modulator is described in, for example,
A.Sano, H.Masuda, et al., "30.times.100-Gb/s all-optical OFDM
transmission over 1300 km SMF with 10 ROADM nodes"
[0105] A continuous wave having a wavelength .lamda. output from
the light source 1 is input to the CS-RZ optical modulator 81. In
addition, the CS-RZ optical modulator 81 is driven, as described
above, by the drive signal having a frequency fc/2. Meanwhile, the
amplitude of the drive signal in the CS-RZ modulation is generally
2V.pi., as illustrated in FIG. 26. In this case, in the output
light of the CS-RZ optical modulator 81, a pair of subcarriers with
wavelength .lamda.1 and .lamda.2 are generated. The difference
between wavelengths .lamda.1 and .lamda.2 of the pair of
subcarriers corresponds to the frequency fc.
[0106] The output light of the CS-RZ optical modulator 81 is
directed to an interleaver 92. The interleaver 92 corresponds to
the demultiplexer 91 as illustrated in FIG. 24, and operates as an
optical switch. The interleaver 92 extracts the .lamda.1 and
.lamda.2 components, and directs the .lamda.1 component to a DQPSK
optical modulator 13, while directing the .lamda.2 component to a
DQPSK optical modulator 23. That is, the subcarriers .lamda.1,
.lamda.2 are directed to the DQPSK optical modulators 13, 23.
[0107] The configuration and operation of the DQPSK optical
modulators 13, 23 and the polarization beam combiner 31 are the
same as those in the first and second aspects. Therefore, the DQPSK
optical modulator 13 generates a modulated optical signal X by
modulating the subcarrier .lamda.1 using transmission data X. In
the same manner, the DQPSK optical modulator 23 generates a
modulated optical signal Y by modulating the subcarrier .lamda.2
using transmission data Y. The polarization beam coupler 31
performs polarization multiplexing of the modulated optical signals
X, Y.
[0108] The operations of the computing unit 43 and the control unit
44 are similar to those of the computing unit 41 and the control
unit 42 in the first aspect. Specifically, the computing unit 43
calculates the difference between the powers of output lights of
the DQPSK optical modulators 13, 23. At this time, the DC
components of signals detected for the ABC circuits 11, 21 may be
used as the powers of the output lights of the DQPSK optical
modulators 13, 23. Alternatively, the powers of the output lights
of the DQPSK optical modulators 13, 23 may be detected using the
split-off portions of the optical modulation signals X, Y directed
to the polarization beam coupler 31. Then, the control unit 44
generates a control signal E for making the difference obtained by
the computing unit 43 zero.
[0109] The control signal E is given to, for example, the CS-RZ
optical modulator 81. In this case, the control signal E controls
the DC bias voltage of the CS-RZ optical modulator 81. At this
time, the spectrum of the output light of the CS-RZ optical
modulator 81 changes in accordance with the DC bias voltage, as
illustrated in FIG. 27. In other words, the intensity of each
subcarrier can be adjusted by controlling the DC bias voltage of
the CS-RZ optical modulator 81. Therefore, in the third aspect, the
power balance of the subcarrier .lamda.1, .lamda.2 are adjusted by
controlling the DC bias voltage of the CS-RZ optical modulator 81,
so as to make the difference between the powers of the output
lights of the DQPSK optical modulator 13, 23 zero. This makes it
possible to make the optical power of the plurality of multiplexed
and transmitted subcarriers constant, improving the transmission
quality.
[0110] While the DC bias voltage of the CS-RZ optical modulator 81
is controlled in the configuration illustrated in FIG. 25, other
elements may be controlled using the control signal E. In other
words, the amplitude of the drive signal of the DQPSK optical
modulators 13, 23, the DC bias voltage of the DQPSK optical
modulators 13, 23, the attenuation amount of the optical
attenuators 12, 22 may be controlled using the control signal
E.
[0111] FIG. 28 illustrates the fourteenth embodiment of the optical
signal transmitter. The configuration of the optical signal
transmitter according to the fourteenth embodiment is similar to
that of the thirteenth embodiment illustrated in FIG. 25. However,
in the fourteenth embodiment, a multiplexer 32 is provided instead
of the polarization beam combiner 31 illustrated in FIG. 25. The
plurality of subcarrier signals (modulated optical signals X, Y)
are multiplexed and transmitted by the multiplexer 32.
[0112] FIG. 29 through FIG. 31 are diagrams illustrating the
configuration for performing feedback control in accordance with
the received signal quality. FIG. 29, FIG. 30, FIG. 31 illustrates
the configuration in which the feedback control in accordance with
the received signal quality is applied to the optical signal
transmitter illustrated in FIG. 1A-1B, FIG. 12A-12B, FIG. 24,
respectively.
[0113] As illustrated in FIG. 29 through FIG. 31, a receiver 100
has an optical receiver unit 101, a decision unit 102, and an FEC
error count unit 103. The optical receiver unit 101 receives an
optical signal (here, polarization multiplexed optical signal)
transmitted from a transmitter, and converts it into an electric
signal. The decision unit 102 decides each symbol of the received
signal, and recovers the transmission data stream. The FEC error
count unit 103 counts the FEC error number (or, error frequency) of
the recovered transmission data stream, thereby obtaining the bit
error rate (BER) information.
[0114] A control unit 111 controls the amplitude of the drive
signal of the modulation unit, the DC bias voltage of the
modulation unit, or the attenuation amount of the optical
attenuator according to the BER information. At this time, for
example, feedback control for minimizing the BER is performed,
thereby appropriately adjusting the power balance between the X
polarized wave and the Y polarized wave of the polarization
multiplexed optical signal. While the control unit 111 is provided
within the optical transmitter in the examples illustrated in FIG.
29 through FIG. 31, the configuration may also be made so as to
dispose the control unit 111 in the receiver 100.
[0115] While the first through third aspects described above
illustrates the configurations for transmitting a DQPSK signal, the
configuration is not limited to this, and modulated optical signals
in other formats may be transmitted by the optical signal
transmitters according to the first through third aspects.
[0116] In addition, in the first through third aspects, the
feedback control adjusting the optical powers of the modulated
optical signals X, Y are, for example, periodically repeated.
Alternatively, the feedback control described above may be
performed at the time of the initial setting and under a
predetermined condition (for example, when the temperature of the
optical signal transmitter changes).
[0117] According to the embodiments of the first aspect, even when,
for example the characteristics of the first and second modulation
units are not the same, the optical powers of the first and second
modulated optical signals transmitted by the polarization
multiplexed optical signal become approximately equal to each
other.
[0118] According to the embodiments of the second aspect, the
symbol rate component in the polarization multiplexed optical
signal depends on the power difference between the first and second
modulated optical signals. Therefore, the control of at least one
of the first and second modulation units in accordance with the
symbol rate component in the polarization multiplexed optical
signal makes the optical powers of the first and second modulated
optical signals approximately equal to each other.
[0119] According to the embodiments of the third aspect, the
intensity of the first and second wavelength components for
generating the first and second modulated optical signals is
adjusted by controlling the intensity modulation unit. Therefore,
even when, for example the characteristics of the first and second
modulation units are not the same, the optical powers of the first
and second modulated optical signals transmitted by the
polarization multiplexed optical signal become approximately equal
to each other.
[0120] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiment(s) of the
present inventions has (have) been described in detail, it should
be understood that the various changes, substitutions, and
alterations could be made hereto without departing from the spirit
and scope of the invention.
* * * * *