U.S. patent application number 09/789510 was filed with the patent office on 2002-01-17 for optical transmitter and optical transmission system.
Invention is credited to Nakamoto, Hiroshi.
Application Number | 20020005975 09/789510 |
Document ID | / |
Family ID | 18706750 |
Filed Date | 2002-01-17 |
United States Patent
Application |
20020005975 |
Kind Code |
A1 |
Nakamoto, Hiroshi |
January 17, 2002 |
Optical transmitter and optical transmission system
Abstract
The present invention aims at providing an optical transmitter
utilizing a Mach-Zehnder type optical modulator, in which the
optical modulator is capable of readily optimizing an optical
wavelength chirp to be added to a transmission optical signal. To
this end, the optical transmitter of the present invention includes
a Mach-Zehnder type optical modulator having two arms driven by two
drive signals, respectively, and is constituted to comprise:
amplitude adjusting parts for adjusting the amplitudes of the drive
signals, respectively; phase adjusting parts for adjusting the
phases of the drive signals, respectively; an amplitude controlling
part for feedback controlling the amplitude adjusting parts so that
an amplitude ratio between the drive signals becomes a value
corresponding to an optimum optical wavelength chirp amount; and a
phase controlling part for feedback controlling the phase adjusting
parts so that the phases of the drive signals are brought into an
antiphase relation.
Inventors: |
Nakamoto, Hiroshi;
(Kawasaki, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
18706750 |
Appl. No.: |
09/789510 |
Filed: |
February 22, 2001 |
Current U.S.
Class: |
359/254 ;
359/237; 359/245 |
Current CPC
Class: |
G02F 1/225 20130101;
H04B 10/5051 20130101; G02F 1/212 20210101; H04B 10/50577 20130101;
G02F 2203/25 20130101; G02F 1/0121 20130101; G02F 2201/16 20130101;
H04B 10/541 20130101; H04B 10/505 20130101 |
Class at
Publication: |
359/254 ;
359/237; 359/245 |
International
Class: |
G02F 001/00; G02B
026/00; G02F 001/03; G02F 001/07 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 2000 |
JP |
2000-210489 |
Claims
What is claimed:
1. An optical transmitter utilizing a Mach-Zehnder type optical
modulator, said Mach-Zehnder type optical modulator including: a
light input end for receiving light; a first arm and a second arm
for branching the light from said light input end to propagate the
branched light, respectively; a light output end for synthesizing
the branched light propagated through said first and second arms to
output the resultant light; a first electrode for applying a first
drive signal to the first arm to thereby drive the first arm; and a
second electrode for applying a second drive signal to the second
arm to thereby drive the second arm, comprising: amplitude
adjusting parts for adjusting the respective amplitudes of the
first and second drive signals; phase adjusting parts for adjusting
the respective phases of the first and second drive signals; an
amplitude controlling part for detecting the respective amplitudes
of the first and second drive signals, to thereby feedback control
the amplitude adjusting parts; and a phase controlling part for
detecting the respective phases of the first and second drive
signals, to thereby feedback control the phase adjusting parts.
2. An optical transmitter according to claim 1, wherein said
amplitude controlling part detects the respective amplitudes of the
first and second drive signals after propagated through the first
and second electrodes, respectively, and said phase controlling
part detects the respective phases of the first and second drive
signals after propagated through the first and second electrodes,
respectively.
3. An optical transmitter according to claim 1, wherein said
amplitude controlling part detects the respective amplitudes of the
first and second drive signals before being applied to the first
and second electrodes, respectively, and said phase controlling
part detects the respective phases of the first and second drive
signals before being applied to the first and second electrodes,
respectively.
4. An optical transmitter according to claim 1, wherein when said
optical transmitter comprises: low frequency signal superimposing
parts, each of which superimposes a predetermined low frequency
signal symmetrically on a "1" side and a "0" side of each of the
first and second drive signals; and a drift controlling part for
detecting a low frequency signal component included in the optical
signal output from the Mach-Zehnder type optical modulator to
thereby judge an occurring state of an operating point drift, and
for controlling the operating point of the Mach-Zehnder type
optical modulator so that the operating point drift is compensated
for, the amplitudes of the low frequency signals superimposed on
the first and second drive signals, respectively, are varied
corresponding to an amplitude ratio corresponding to an optical
wavelength chirp amount.
5. An optical transmitter according to claim 1, wherein when said
optical transmitter comprises: low frequency signal superimposing
parts, each of which superimposes a predetermined low frequency
signal on either one of a "1" side and a "0" side of each of the
first and second drive signals; and a drift controlling part for
detecting a low frequency signal component included in the optical
signal output from the Mach-Zehnder type optical modulator to
thereby judge an occurring state of an operating point drift, and
for controlling the operating point of the Mach-Zehnder type
optical modulator so that the operating point drift is compensated
for, the amplitudes of the low frequency signals superimposed on
the first and second drive signals, respectively, are kept constant
independently of an amplitude ratio corresponding to an optical
wavelength chirp amount.
6. An optical transmitter according to claim 1, wherein said
Mach-Zehnder type optical modulator includes a light modulating
part, which is connected serially to a preceding stage of said
light input end or a latter stage of said light output end, so as
to modulate the light input into the optical transmitter in a two
staged manner.
7. An optical transmitter utilizing an external modulator made up
by serially connecting a Mach-Zehnder type optical modulator and an
optical phase modulator, comprising: an amplitude adjusting part
for adjusting an amplitude of a drive signal for driving said
optical phase modulator; a phase adjusting part for adjusting a
phase of said drive signal; an amplitude controlling part for
detecting the amplitude of said drive signal and for feedback
controlling said amplitude adjusting part so that said amplitude of
the drive signal becomes a value corresponding to an optical
wavelength chirp amount set to reduce transmittal degradation of an
optical signal; and a phase controlling part for detecting the
phase of said drive signal and for feedback controlling said phase
adjusting part so that said phase is matched with a phase of a
signal for driving the Mach-Zehnder type optical modulator.
8. An optical transmitter according to claim 7, wherein said
external modulator uses a polarization scrambler instead of the
optical phase modulator.
9. An optical transmission system comprising: a plurality of
optical transmitters for transmitting optical signals of different
wavelengths, an optical multiplexer for multiplexing the optical
signals from said optical transmitters to transmit the multiplexed
optical signal to a transmission path; and an optical demultiplexer
for demultiplexing the optical signal transmitted through said
transmission path into optical signals of respective wavelengths;
and a plurality of optical receivers for receiving and processing
the optical signals of respective wavelengths demultiplexed by said
optical demultiplexer, wherein the optical transmitter according to
claim 1 or claim 7 is adopted as each of said plurality of optical
transmitters, and in each of said plurality of optical
transmitters, the setting of the optical wavelength chirp amount is
adjusted based on receipt information transmitted from each of the
optical receivers corresponding to the applicable wavelength of the
applicable optical transmitter and corresponding to the wavelengths
adjacent to the applicable wavelength.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates to an optical transmitter
utilizing an external modulator such as a Mach-Zehnder type optical
modulator and an optical transmission system utilizing such an
optical transmitter, and particularly to an optical transmitter and
an optical transmission system for transmitting an optical signal
added with a required optical wavelength chirp.
[0003] (2) Related Art
[0004] In an optical communications system having a large capacity
over a long distance, it is required to reduce degradation of an
optical signal when transmitting the same through a transmission
path. It is known that a waveform distortion due to a self phase
modulation, which is one of the degradation causes of optical
signal, can be corrected at an optical transmitting terminal side
by adding an optical wavelength chirp (hereinafter simply called
"chirp") to the optical signal. It is also known that the optimum
value of such a chirp depends on power of optical signal to be
transmitted and on wavelength dispersion of a transmission path.
For example, a wavelength division multiplexing (WDM) transmission
system using 30 waves requires mutually different optimum chirp
amounts for optical signals in 30 channels, respectively.
[0005] Known as a conventional technique for adding a chirp to an
optical signal is to utilize a Mach-Zehnder type optical modulator
formed of lithium niobate (LiNbO.sub.3; hereinafter called "LN"),
for example. There has been also proposed a technique to render a
chirp amount to be variable, by driving the aforementioned type
Mach-Zehnder optical modulator by two drive signals corresponding
to bifurcated arms (optical waveguides), and by varying a ratio
between amplitudes of the drive signals (see Japanese Unexamined
Patent Publication Nos. 7-7206 and 9-80363, for example).
Concretely, the chirp amount becomes 1 when the modulator is driven
by setting a ratio between voltage amplitudes of the two drive
signals to be 1:0 (i.e., only one of the drive signals is input),
for example, and becomes 0 when the voltage amplitudes of the drive
signals are equivalent to each other.
[0006] In the aforementioned conventional optical transmitters
utilizing Mach-Zehnder type optical modulators, it is required to
adjust the amplitudes of respective drive signals so as to optimize
the chirp amount. In optimally adjusting the amplitudes of the
drive signals, possible variations of time delays of respective
signals require an adjustment of a phase difference between the two
drive signals. However, this phase adjustment has been extremely
laborious work, and difficult to practice.
SUMMARY OF THE INVENTION
[0007] The present invention has been carried out in view of the
conventional problems as described above, and it is therefore an
object of the present invention to provide an optical transmitter
and an optical transmission system capable of readily conducting an
adjustment of a chirp amount.
[0008] To achieve the object, with one aspect of an optical
transmitter according to the present invention, there is provided
an optical transmitter utilizing a Mach-Zehnder type optical
modulator, the Mach-Zehnder type optical modulator including: a
light input end for receiving light; a first arm and a second arm
for branching the light from the light input end to propagate the
branched light, respectively; a light output end for synthesizing
the branched light propagated through the first and second arms to
output the resultant light; a first electrode for applying a first
drive signal to the first arm to thereby drive the first arm; and a
second electrode for applying a second drive signal to the second
arm to thereby drive the second arm, comprising amplitude adjusting
parts for adjusting the respective amplitudes of the first and
second drive signals; phase adjusting parts for adjusting the
respective phases of the first and second drive signals; an
amplitude controlling part for detecting the respective amplitudes
of the first and second drive signals, to thereby feedback control
the amplitude adjusting parts; and a phase controlling part for
detecting the respective phases of the first and second drive
signals, to thereby feedback control the phase adjusting parts.
[0009] In the optical transmitter having such a constitution, the
light input into the light input end of the Mach-Zehnder type
optical modulator is bifurcated to be propagated through the first
and second arms. The respective lights propagated through the first
and second arms are synthesized into a resultant light and
thereafter, the resultant light is output from the light output
end. This Mach-Zehnder type optical modulator is applied with the
first and second drive signals to first and second electrodes,
respectively, to thereby cause changes in phases of the respective
lights propagated through the first and second arms, respectively,
so that intensity modulations of the respective lights are
conducted in accordance with the first and second drive signals,
and simultaneously a chirp is added corresponding to a ratio
between the amplitudes of the first and second drive signals. The
amplitudes of the first and second drive signals are monitored by
the amplitude controlling part, and feedback controlled by the
amplitude adjusting part so that an amplitude ratio between the
first and second drive signals becomes a value corresponding to the
optimum value of a chirp amount. Further, the phases of the first
and second drive signals are monitored by the phase controlling
part and feedback controlled by the phase adjusting part so that
these phases are brought into, for example, an antiphase relation
and, then, the first and second drive signals are applied to the
first and second electrodes, respectively. In this way, there can
be realized an optical transmitter capable of readily optimizing
the chirp to be added to an optical signal.
[0010] As a concrete constitution of the optical transmitter, the
amplitude controlling part may detect the respective amplitudes of
the first and second drive signals after propagated through the
first and second electrodes, respectively, and the phase
controlling part may detect the respective phases of the first and
second drive signals after propagated through the first and second
electrodes, respectively. Alternatively, the amplitude controlling
part may detect the respective amplitudes of the first and second
drive signals before being applied to the first and second
electrodes, respectively, and the phase controlling part may detect
the respective phases of the first and second drive signals before
being applied to the first and second electrodes, respectively.
[0011] Further, when the optical transmitter comprises: low
frequency signal superimposing parts, each of which superimposes a
predetermined low frequency signal symmetrically on a "1" side and
a "0" side of each of the first and second drive signals; and a
drift controlling part for detecting a low frequency signal
component included in the optical signal output from the
Mach-Zehnder type optical modulator to thereby judge an occurring
state of an operating point drift, and for controlling the
operating point of the Mach-Zehnder type optical modulator so that
the operating point drift is compensated for, it is preferable that
the amplitudes of the low frequency signals superimposed on the
first and second drive signals, respectively, are varied
corresponding to an amplitude ratio corresponding to an optical
wavelength chirp amount.
[0012] According to such a constitution, the amplitude of each low
frequency signal to be superimposed on both sides of each of the
first and second drive signals in order to detect the operating
point drift is adjusted in accordance with the amplitude ratio
corresponding to the optimum chirp amount, together with an
amplitude of a main signal. In this way, in the sum signal of the
first and second drive signals, a superimposition ratio of the low
frequency signals becomes constant. Thus, even if the amplitude
ratio between the first and second drive signals is changed when
the setting of the chirp amount is changed, the low frequency
signals to be detected at the drift controlling part becomes
constant. As a result, even when the chirp amount is controlled by
adjusting the amplitude ratio between the first and second drive
signals, no affection is imposed on the detection and control of
the operating point drift based on the superimposition of the low
frequency signals.
[0013] Further, when the optical transmitter comprises: low
frequency signal superimposing parts, each of which superimposes a
predetermined low frequency signal on either one of a "1" side and
a "0" side of each of the first and second drive signals; and a
drift controlling part for detecting a low frequency signal
component included in the optical signal output from the
Mach-Zehnder type optical modulator to thereby judge an occurring
state of an operating point drift, and for controlling the
operating point of the Mach-Zehnder type optical modulator so that
the operating point drift is compensated for; it is preferable that
the amplitudes of the low frequency signals superimposed on the
first and second drive signals, respectively, are kept constant
independently of an amplitude ratio corresponding to an optical
wavelength chirp amount.
[0014] According to such a constitution, the low frequency signals
having constant amplitudes independent of the chirp amount setting
are superimposed on one sides of the first and second drive
signals, respectively. In this way, in the sum signal of the first
and second drive signals, a superimposition ratio of the low
frequency signals becomes constant. Thus, even if the amplitude
ratio between the first and second drive signals is changed when
the setting of the chirp amount is changed, the low frequency
signals to be detected at the drift controlling part becomes
constant. As a result, no affection is imposed on the detection and
control of the operating point drift.
[0015] Further, in the optical transmitter, the Mach-Zehnder type
optical modulator may include a light modulating part, which is
connected serially to a preceding stage of the light input end or a
latter stage of the light output end, so as to modulate the light
input into the optical transmitter in a two staged manner.
[0016] In the optical transmitter having such a constitution, the
optical signal input into the Mach-Zehnder type optical modulator
is modulated by being propagated through the light input end, first
and second arms, and light output end, and further modulated at the
light modulating part. The optical signal as modulated in such a
two staged manner is added with a chirp controlled to the optimum
value at the time of the former light modulation, similarly to the
aforementioned case. In this way, it becomes possible to transmit
such as a high-speed optical signal in an RZ data format, and to
readily adjust the optimum chirp amount.
[0017] With another aspect of the present invention, there is
provided an optical transmitter utilizing an external modulator
made up by serially connecting a Mach-Zehnder type optical
modulator and an optical phase modulator, comprising: an amplitude
adjusting part for adjusting an amplitude of a drive signal for
driving the optical phase modulator; a phase adjusting part for
adjusting a phase of the drive signal; an amplitude controlling
part for detecting the amplitude of the drive signal and for
feedback controlling the amplitude adjusting part so that the
amplitude of the drive signal becomes a value corresponding to an
optical wavelength chirp amount set to reduce transmittal
degradation of an optical signal; and a phase controlling part for
detecting the phase of the drive signal and for feedback
controlling the phase adjusting part so that the phase is matched
with a phase of a signal for driving the Mach-Zehnder type optical
modulator. The external modulator may include a polarization
scrambler instead of the optical phase modulator.
[0018] In the optical transmitter of such a constitution, the light
input into the external modulator is intensity modulated at the
Mach-Zehnder type optical modulator and phase modulated at the
optical phase modulator, to thereby be added with the chirp. Since
the chirp amount to be added to the optical signal at this time is
varied corresponding to the amplitude of the drive signal for the
phase modulation, the amplitude adjusting part is feedback
controlled so that the amplitude of the drive signal monitored at
the amplitude controlling part becomes a value corresponding to the
optimum value of the chirp amount. Further, since the phase of the
amplitude-adjusted drive signal is required to be matched with the
phase of the drive signal for driving the Mach-Zehnder type optical
modulator, the phase adjusting part is feedback controlled in
accordance with the phase of the drive signal monitored by the
phase controlling part. In this way, it becomes possible to conduct
the adjustment of the optimum chirp amount, even in a constitution
utilizing an external modulator made up by combining a Mach-Zehnder
type optical modulator with an optical phase modulator.
[0019] The optical transmission system according to the present
invention comprises: a plurality of optical transmitters for
transmitting optical signals of different wavelengths, an optical
multiplexer for multiplexing the optical signals from the optical
transmitters to transmit the multiplexed optical signal to a
transmission path; and an optical demultiplexer for demultiplexing
the optical signal transmitted through the transmission path into
optical signals of respective wavelengths; and a plurality of
optical receivers for receiving and processing the optical signals
of respective wavelengths demultiplexed by the optical
demultiplexer; wherein the aforementioned optical transmitter
according to the present invention is adopted as each of the
plurality of optical transmitters; and in each of the plurality of
optical transmitters, the setting of the optical wavelength chirp
amount is adjusted based on receipt information transmitted from
each of the optical receivers corresponding to the applicable
wavelength of the applicable optical transmitter and corresponding
to the wavelengths adjacent to the applicable wavelength.
[0020] According to the optical transmission system having such a
constitution, the chirp amount to be added to the optical signal of
each wavelength is adjusted to become the optimum value at each
optical transmitter, while taking account of an influence on the
adjacent wavelengths. In this way, the optimization of the chirp
amount for the optical signal of each wavelength can be readily
conducted, to thereby allow acquisition of an excellent
transmission characteristic.
[0021] Other objects, features and advantages of the present
invention will become more apparent from the following description
of preferred embodiments when read in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a block diagram showing an essential constitution
of an optical transmitter according to a first embodiment of the
present invention;
[0023] FIG. 2 is an enlarged plan view of a substrate part utilized
in the first embodiment;
[0024] FIG. 3 is a block diagram showing an essential constitution
of an optical transmitter according to a modified example of the
first embodiment;
[0025] FIG. 4 is a block diagram showing an essential constitution
of an optical transmitter according to a second embodiment of the
present invention;
[0026] FIG. 5 is a view showing exemplary waveforms of drive
signals input into a Mach-Zehnder type optical modulator in the
second embodiment, in which 5A, 5B and 5C show waveforms of a first
drive signal, a second drive signal, and a sum of the first and
second drive signals, respectively;
[0027] FIG. 6 is a block diagram showing an essential constitution
of an optical transmitter according to a third embodiment of the
present invention;
[0028] FIG. 7 is a view showing exemplary waveforms of drive
signals input into a Mach-Zehnder type optical modulator in the
third embodiment, in which 7A, 7B and 7C show waveforms of a first
drive signal, a second drive signal, and a sum of the first and
second drive signals, respectively;
[0029] FIG. 8 is a block diagram showing an essential constitution
of an optical transmitter according to a fourth embodiment of the
present invention;
[0030] FIG. 9 is a diagram for explaining a phase relation among
first, second and third drive signals in the fourth embodiment;
[0031] FIG. 10 is a block diagram showing an essential constitution
of an optical transmitter according to a modified example of the
fourth embodiment;
[0032] FIG. 11 is a block diagram showing an essential constitution
according to a fifth embodiment of the present invention;
[0033] FIG. 12 is a block diagram showing a constitution of an
embodiment of an optical transmission system according to the
present invention; and
[0034] FIG. 13 is a graph for explaining an influence on adjacent
channels, accompanying to an increase in a chirp amount, in the
embodiment of the optical transmission system.
DETAILED DESCRIPTION OF THE INVENTION
[0035] There will be described hereinafter embodiments according
the present invention, with reference to the accompanying
drawings.
[0036] FIG. 1 is a block diagram showing an essential constitution
of an optical transmitter according to a first embodiment of the
present invention.
[0037] In FIG. 1, the present optical transmitter comprises a light
source (LD) 1, a Mach-Zehnder type optical modulator 2 for
externally modulating the light from the light source 1, and a
chirp controlling circuit 3 for controlling a chirp to be added to
an optical signal modulated by the Mach-Zehnder type optical
modulator 2.
[0038] The light source 1 is a typical one for generating light of
a required wavelength band such as by using a laser diode. The
light generated by the light source 1 is kept in a predetermined
polarized state such that a modulation efficiency at the
Mach-Zehnder type optical modulator 2 is maximized and transmitted
to a light input end 20A of the Mach-Zehnder type optical modulator
2.
[0039] The Mach-Zehnder type optical modulator 2 includes, for
example, a substrate part 20, a branch circuit 21, driving circuits
22.sub.1, 22.sub.2, variable attenuators (ATT) 23.sub.1, 23.sub.2
as amplitude adjusting parts; and variable delay circuits (DLY)
24.sub.1, 24.sub.2 as phase adjusting parts.
[0040] In the substrate part 20 as shown in an enlarged plan view
of FIG. 2, continuous-wave (CW) light from the light source 1 is
input into a light input end 20A, and, thereafter bifurcated to be
propagated through a first arm 20B.sub.1 and a second arm
20B.sub.2, respectively, and then multiplexed into a resultant
light. This resultant light is output to the exterior via a light
output end 20C. Formed on the first arm 20B.sub.1 and second arm
20B.sub.2 are a first electrode 20D.sub.1 and a second electrode
20D.sub.2 in approximately channel shapes, respectively, and these
electrodes 20D.sub.1, 20D.sub.2 are applied with a first drive
signal DS1 and a second drive signal DS2 which are usually in
opposite phase to each other, respectively, to be described later.
Here, the first drive signal DS1 is input into an input terminal
P1.sub.lN positioned at the light input end 20A side of the first
electrode 20D.sub.1, advances on the first arm 20B.sub.1, and is
output from an output terminal P1.sub.OUT positioned at the light
output end 20C side. Further, the second drive signal DS2 is input
into an input terminal P2.sup.IN positioned at the light input end
20A side of the second electrode 20D.sub.2, advances on the second
arm 20B.sub.2, and is output from an output terminal P.sub.OUT
positioned at the light output end 20C side. Although not shown,
there may be formed an earth electrode so as to enclose the first
and second electrodes 20D.sub.1, 20D.sub.2. Further, the electrodes
20D.sub.1, 20D.sub.2 shall be applied with required DC voltages,
respectively, in addition to the drive signals DS1, DS2.
[0041] The branch circuit 21 bifurcates a data signal DATA
transmitted at a required bit rate, and sends the bifurcated
signals to driving circuits 22.sub.1, 22.sub.2, respectively. The
bit rate of this data signal can be arbitrarily set, for example,
at a higher one exceeding 10 Gbit/s.
[0042] The driving circuits 22.sub.1, 22.sub.2 generate signals as
origins of the drive signals DS1, DS2, respectively, such as by
amplifying the bifurcated data signals from the branch circuit 21
to predetermined levels, respectively. Note, the signal to be
generated by the driving circuit 22.sub.1 and the signal to be
generated by the driving circuit 22.sub.2 are kept in a phase
relation opposite to each other, and the drive signals DS1, DS2
operating in a push-pull manner shall be input to the substrate
part 20.
[0043] The variable attenuators 23.sub.1, 23.sub.2 attenuate the
respective signals output from the driving circuits 22.sub.1,
22.sub.2, respectively, so that a ratio between the amplitudes of
respective signals output from the driving circuits 22.sub.1,
22.sub.2 becomes a value corresponding to a required chirp amount.
Amounts attenuated by the variable attenuators 23.sub.1, 23.sub.2
are controlled by control signals output from an electric power
comparison circuit 33 to be described later.
[0044] The variable delay circuits 24.sub.1, 24.sub.2 are provided
to delay the signals output from the variable attenuators 23.sub.1,
23.sub.2, respectively, to thereby adjust phases of the signals,
respectively. Delay amounts by the variable delay circuits
24.sub.1, 24.sub.2 are controlled in accordance with control
signals output by a phase comparison circuit 34 to be described
later, so that phases of the signals are brought into an antiphase
relation.
[0045] The chirp controlling circuit 3 includes, for example,
branch circuits 31.sub.1, 31.sub.2, electric power detectors
(DET's) 32.sub.1, 32.sub.2, the electric power comparison circuit
(POWER COMP) 33, and the phase comparison circuit (PHASE COMP) 34.
Here, the electric power detectors 32.sub.1, 32.sub.2 and electric
power comparison circuit 33 cooperatively function as an amplitude
controlling part, while the phase comparison circuit 34 functions
as a phase controlling part.
[0046] The branch circuits 31.sub.1, 31.sub.2 bifurcate the first
and second drive signals DS1, DS2 output from the output terminals
P1.sub.OUT, P.sub.OUT of the substrate part 20, respectively. One
of the first drive signals DS1 bifurcated by the branch circuit
31.sub.1 is transmitted to the electric power detector 32.sub.1,
and one of the second drive signals DS2 bifurcated by the branch
circuit 31.sub.2 is transmitted to the electric power detector
32.sub.2. Both of the others of the first and second drive signals
bifurcated by the branch circuits 31.sub.1, 31.sub.2, respectively,
are transmitted to the phase comparison circuit 34.
[0047] The electric power detectors 32.sub.1, 32.sub.2 detect
electric powers of the first and second drive signals DS1, DS2
bifurcated by the branch circuits 31.sub.1, 31.sub.2, respectively,
and output the respective detection results to the electric power
comparison circuit 33.
[0048] The electric power comparison circuit 33 compares values of
the electric powers detected by the electric power detectors
32.sub.1, 32.sub.2 so as to obtain a ratio between the amplitudes
of the first and second drive signals, and generates control
signals for feedback controlling the attenuation amounts of the
variable attenuators 23.sub.1, 23.sub.2, respectively, so that this
ratio becomes a value corresponding to the optimum value of a chirp
amount.
[0049] The phase comparison circuit 34 compares the phases the
first and second drive signals DS1, DS2 bifurcated by the branch
circuits 31.sub.1, 31.sub.2, respectively, with each other, and
generates control signals for feedback controlling the delay
amounts of the variable delay circuits 24.sub.1, 24.sub.2,
respectively, so that the phases of the drive signals are brought
into an antiphase relation.
[0050] There will be described an operation of the first
embodiment.
[0051] In the present optical transmitter, CW light generated by
the light source 1 is externally modulated by the Mach-Zehnder type
optical modulator 2. This Mach-Zehnder type optical modulator 2 is
applied with the first and second drive signals DS1, DS2 to the
electrodes 20D.sub.1, 20D.sub.2 to thereby cause changes in the
phases of respective lights propagated through the first and second
arms 20B.sub.1, 20B.sub.2, respectively. A phase difference between
the respective lights becomes 0 or .pi., resulted in an ON or OFF
state of the light to be output from the light output end 20C. In
this way, there is conducted an intensity modulation corresponding
to the first and second drive signals DS1, DS2.
[0052] In the light modulation utilizing the Mach-Zehnder type
optical modulator 2, there is essentially caused a wavelength
change. Concretely, in one optical pulse modulated by the
Mach-Zehnder type optical modulator 2, there is generated such a
phenomenon called a red shift in which the wavelength shifts from a
short wavelength (blue side) to a long wavelength (red side) with
time lapse, or a phenomenon called blue shift in which the
wavelength shifts from a long wavelength (red side) to a short
wavelength (blue side) with time lapse. In the present optical
transmitter, there is added a chirp to transmission light making
use of the aforementioned wavelength change.
[0053] To add a required amount of chirp to an optical signal at
the Mach-Zehnder type optical modulator 2, it is necessary to
suitably adjust an amplitude ratio and the phase relation between
the first and second drive signals. The amplitude ratio between the
first and second drive signals shall be firstly considered. For
example, when a required chirp amount is .alpha., if the optimum
driving voltage is set as V .pi. rassuming that the Mach-Zehnder
type optical modulator 2 is to be driven by only one of the driving
electrodes, a voltage V1 of the first drive signal and a voltage V2
of the second drive signal can be represented as follows:
V1=(1+.alpha.).multidot.V.pi./2, and
V2=(1-.alpha.).multidot.V.pi./2.
[0054] Thus, the amplitude ratio between the first and second drive
signals is determined corresponding to the optimum value of the
chirp amount .alpha. to be set depending on power of the optical
signal to be transmitted and on wavelength dispersion of a
transmission path. In this embodiment, the aforementioned amplitude
ratio corresponding to the optimum value of the chirp amount
.alpha. is previously set in the electric power comparison circuit
33, and the amplitudes (voltages) of the first and second drive
signals are feedback controlled by adjusting the attenuation
amounts of the variable attenuators 23.sub.1, 23.sub.2 so that the
optimum chirp amount is added to the optical signal. Note, in
feedback controlling these amplitudes, there shall be
simultaneously conducted at the electric power comparison circuit
33 such a control that a sum of electric powers detected by the
electric power detectors 32.sub.1, 32.sub.2 becomes a value
corresponding to the optimum driving voltage V.pi. in case of
driving by only one of the electrodes as described above.
[0055] Further, the phase relation between the first and second
drive signals is adjusted such that the phases of the first and
second drive signals DS1, DS2 advancing through the electrodes
20D.sub.1, 20D.sub.2, respectively, are brought into an antiphase
relation. Here, the phases of the first and second drive signals
DS1, DS2 output from the output terminals P1.sub.OUT, P2.sub.OUT of
the electrodes 20D.sub.1, 20D.sub.2 are compared with each other by
the phase comparison circuit 34, and the phases of the first and
second drive signals are feedback controlled by adjusting the delay
amounts of the variable delay circuits 24.sub.1, 24.sub.2 so as to
keep the antiphase relation.
[0056] Concretely, for example, the delay amounts of the variable
delay circuits 24.sub.1, 24.sub.2 may be adjusted so as to obtain a
computation result corresponding to 1 (one) time slot of a data
signal, by computing a logical product of both drive signals after
logically inverting one of the two drive signals to be input into
the phase comparison circuit 34. As a concrete setting condition of
this phase control, it is preferable to conduct feedback
controlling so that the phases of the first and second drive
signals DS1, DS2 are brought into an antiphase relation within a
range less than 10% for 1 time slot of data.
[0057] According to the first embodiment as described above, the
amplitudes and phases of the first and second drive signals DS1,
DS2 are monitored and feedback controlled, to thereby enable
realization of the optical transmitter capable of readily adjusting
the chirp amount to the optimum value.
[0058] There will be now described a modified example of the first
embodiment.
[0059] FIG. 3 is a block diagram showing an essential constitution
of an optical transmitter according to a modified example of the
first embodiment. Like reference numerals as used in FIG. 1 are
used to denote identical elements in FIG. 3, and the same rule
applies corresponding to the following.
[0060] In FIG. 3, the constitution of this optical transmitter is
different from that of the first embodiment shown in FIG. 1, in
that: branch circuits 35.sub.1, 35.sub.2 for monitoring the first
and second drive signals DS1, DS2 are inserted, for example,
between the variable attenuators 23.sub.1, 23.sub.2 and between the
variable delay circuits 24.sub.1, 24.sub.2, respectively; and
instead of the first and second drive signals DS1, DS2 output from
the output terminals P1.sub.OUT, P2.sub.OUT, respectively, of the
Mach-Zehnder type optical modulator 2, first and second drive
signals DS1, DS2 bifurcated by the branch circuits .sup.351,
35.sub.2 are input into the branch circuits .sup.311, 31.sub.2,
respectively. Note, terminators 41.sub.1, 41.sub.2 are connected to
the output terminals P1.sub.OUT, P.sub.OUT of the Mach-Zehnder type
optical modulator 2, respectively. The remaining constitution other
than the above is identical with that of the first embodiment.
[0061] In this way, the optical transmitter according to the
present invention is not limited to such a constitution that the
chirp is controlled by monitoring the first and second drive
signals DS1, DS2 having passed through the electrodes 20D.sub.1,
20D.sub.2 of the Mach-Zehnder type optical modulator 2,
respectively. It is also possible to control the chirp by
monitoring the first and second drive signals DS1, DS2 before input
into the electrodes 20D.sub.1, 20D.sub.2, respectively.
[0062] In this modified example, the branch circuits 35.sub.1,
35.sub.2 for monitoring the first and second drive signals DS1, DS2
have been inserted between the variable attenuators 23.sub.1,
23.sub.2 and between the variable delay circuits 24.sub.1, 24.sub.2
, respectively. However, it is also possible to insert these branch
circuits 35.sub.1, 35.sub.2 between the variable delay circuits
24.sub.1, 24.sub.2 and the input terminals P1.sub.IN, P2.sub.IN of
the Mach-Zehnder type optical modulator 2, respectively.
[0063] There will be described hereinafter a second embodiment of
the present invention.
[0064] In this second embodiment, there will be considered a
situation where the present invention is applied to an optical
transmitter having a function to compensate for an operating point
drift of a Mach-Zehnder type optical modulator. Note, the
"operating point drift" of a Mach-Zehnder type optical modulator
means a phenomenon in which a fluctuation is caused in input and
output characteristics of a Mach-Zehnder type optical modulator
such as due to a DC voltage to be applied to the modulator, a
temperature change, and a change with time lapse.
[0065] FIG. 4 is a block diagram showing an essential constitution
of an optical transmitter according to the second embodiment of the
present invention.
[0066] In FIG. 4, the present optical transmitter is provided with,
in addition to the constitution of the first embodiment shown in
FIG. 1: superimposing circuits 50.sub.1, 50.sub.2 as low frequency
signal superimposing parts for superimposing low frequency signals
of a frequency f.sub.0 on the first and second drive signals DS1,
DS2, respectively; a low frequency signal detecting part 51 for
detecting a low frequency signal component included in the optical
signal output from the Mach-Zehnder type optical modulator 2, and
for comparing a phase of the detected low frequency signal
component with a phase of the low frequency signal before
superimposition, to thereby detect an operating point drift
direction; and a drift controlling part 52 for controlling the
operating point of the Mach-Zehnder type optical modulator 2 into
the same direction as the operating point drift direction detected
by the low frequency signal detecting part 51, according to this
drift direction. The thus described constitution for compensating
for the operating point drift of the Mach-Zehnder type optical
modulator 2 is the same as a known constitution such as described
in Japanese Unexamined Patent Publication No. 3-251815. The
constitution of this embodiment is characterized by such a
countermeasure that the control of the amplitude ratio between the
first and second drive signals corresponding to the optimum chirp
amount never affects on the detection and control of the operating
point drift using a low frequency signal.
[0067] Each of the superimposing circuits 50.sub.1, 50.sub.2
superimposes a predetermined low frequency signal output from a low
frequency oscillator (not shown), symmetrically on a "1" side and a
"0" side of a high-speed main signal output from the pertinent one
of the driving circuits 22.sub.1, 22.sub.2 (to thereby amplitude
modulates the main signal), and outputs the main signal
superimposed with the low frequency signal to the pertinent one of
the variable attenuators 23.sub.1, 23.sub.2.
[0068] The low frequency signal detecting part 51 branches a
portion of the optical signal output from the Mach-Zehnder type
optical modulator 2, for example, converts the thus branched light
into an electrical signal, and extracts therefrom a frequency
f.sub.0 component signal. The low frequency signal detecting part
51 further compares a phase of the extracted frequency f.sub.0
component signal with that of the low frequency signal from the low
frequency oscillator, and outputs a signal corresponding to a
difference between those phases to the drift controlling part
52.
[0069] The drift controlling part 52 controls values of DC voltages
to be applied to the electrodes 20D.sub.1, 20D.sub.2 of the
Mach-Zehnder type optical modulator 2, respectively, according to
the signal of the low frequency signal detecting part 51.
[0070] Since the concrete circuit constitutions of the
aforementioned low frequency signal detecting part 51 and drift
controlling part 52 are disclosed in detail in the aforementioned
Japanese Unexamined Patent Publication No. 3-251815, the
explanation thereof shall be omitted herein.
[0071] In the optical transmitter having the aforementioned
constitution, the low frequency signal is symmetrically
superimposed on the "1" side and "0" side of each of the high-speed
main signals output from the driving circuits 22.sub.1, 22.sub.2 ,
respectively. Each of the main signals superimposed with the low
frequency signal is: attenuated by the pertinent one of the
variable attenuators 23.sub.1, 23.sub.2 so that an amplitude ratio
between the main signals corresponds to the optimum chirp amount;
delayed by the pertinent one of the variable delay circuits
24.sub.1, 24.sub.2 so that the phases of the main signals are
brought into an antiphase relation; and input into the pertinent
one of the electrodes 20D.sub.1, 20D.sub.2 of the Mach-Zehnder type
optical modulator 2.
[0072] FIG. 5 is a view showing exemplary waveforms of drive
signals input into the Mach-Zehnder type optical modulator 2, in
which 5A shows a waveform of the first drive signal DS1 to be input
into the input terminal P1.sub.IN, 5B shows a waveform of the
second drive signal DS2 to be input into the input terminal
P2.sub.IN, and 5C shows a waveform corresponding to a sum of the
first and second drive signals DS1, DS2.
[0073] As shown in FIG. 5A and FIG. 5B, each of the first and
second drive signals DS1, DS2 is superimposed with the low
frequency signal of the frequency f.sub.0 at the "1" side and "0"
side, and the overall amplitude of each of the first and second
drive signals DS1, DS2, including the superimposed component, is
controlled according to the amplitude ratio corresponding to the
optimum chirp amount. This means that the amplitude of the low
frequency signal to be superimposed on the drive signals so as to
detect the operating point drift is adjusted in accordance with the
amplitude ratio corresponding to the optimum chirp amount,
simultaneously with the adjustment of the amplitudes of the
high-speed main signals. When such first and second drive signals
DS1, DS2 are input into the electrodes 20D.sub.1, 20D.sub.2,
respectively, the light is modulated in accordance with a signal
corresponding to the sum of the first and second drive signals DS1,
DS2 as shown in FIG. 5C. In this sum signal of the first and second
drive signals DS1, DS2, a superimposition ratio of the summed low
frequency signals becomes constant (Vf.sub.0/V.sub.p=constant in
FIG. 5C). Thus, even when the amplitude ratio between the first and
second drive signals DS1, DS2 is varied by varying the setting of
the optimum chirp amount, the low frequency signal to be detected
by the low frequency signal detecting part 51 becomes constant. As
a result, even when the chirp amount is controlled by adjusting the
amplitude ratio between the first and second drive signals, no
affection is imposed on the detection and control of the operating
point drift based on the superimposition of the low frequency
signal.
[0074] In this way, according to the second embodiment, when the
low frequency signal is to be superimposed symmetrically on the "1"
side and "0" side of each of the first and second drive signals
DS1, DS2, the amplitude of the low frequency signal to be
superimposed on each of the drive signals is also adjusted
according to the amplitude ratio corresponding to the optimum chirp
amount. Thus, the superimposition ratio of the summed low frequency
signals in the sum signal of the first and second drive signals
DS1, DS2 becomes constant, to thereby assuredly enable the
detection and control of the operating point drift. This enables an
easy adjustment of the optimum chirp amount, and realization of an
optical transmitter capable of stably conducting an external
modulation of an optical signal while compensating for the
operating point drift of the optical modulator.
[0075] There will be now described a third embodiment of the
present invention.
[0076] In this third embodiment, there will be considered a
constitution different from the second embodiment in which the
present invention has been applied to the optical transmitter
provided with the function for compensating for the operating point
drift of the Mach-Zehnder type optical modulator.
[0077] FIG. 6 is a block diagram showing an essential constitution
of an optical transmitter according to the third embodiment of the
present invention.
[0078] In FIG. 6, the constitution of this optical transmitter is
different from that of the second embodiment shown in FIG. 4, in
that: instead of the superimposing circuits 50.sub.1, 50.sub.2 used
in the second embodiment, superimposing circuits 50.sub.1',
50.sub.2 ' for superimposing the low frequency signal on either one
of the "1" side and "0" side of the pertinent one of the first and
second drive signals are provided between the variable attenuators
23.sub.1, 23.sub.2 and the variable delay circuits 24.sub.1,
24.sub.2, respectively. The remaining constitution other than the
above is identical with that of the second embodiment.
[0079] In the optical transmitter having the aforementioned
constitution, the output signals (high-speed main signals) from the
driving circuits 22.sub.1, 22.sub.2 are transmitted to the
superimposing circuits 50.sub.1', 50.sub.2', respectively, after
attenuated by the variable attenuators .sup.231, 23.sub.2 ,
respectively, so as to attain the amplitude ratio corresponding to
the optimum chirp amount. At each of the superimposing circuits
50.sub.1', 50.sub.2', the low frequency signal of the frequency
f.sub.0 is superimposed on either one of the "1" side and "0" side
of the pertinent one of the output signals (high-speed main
signals) which have been adjusted to have the required amplitude
ratio therebetween. Further, the signals superimposed with the low
frequency signal are delayed by the variable delay circuits
24.sub.1, 24.sub.2, respectively, so that the phases of the signals
are brought into an antiphase relation, and then input into the
electrodes 20D.sub.1, 20D.sub.2 of the substrate part 20,
respectively.
[0080] FIG. 7 is a view showing exemplary waveforms of drive
signals input into the Mach-Zehnder type optical modulator 2, in
which 7A shows a waveform of the first drive signal DS1 to be input
into the input terminal P1.sub.IN, 7B shows a waveform of the
second drive signal DS2 to be input into the input terminal
P2.sub.IN, and 7C shows a waveform corresponding to a sum of the
first and second drive signals DS1, DS2.
[0081] As shown in FIG. 7A and FIG. 7B, in the first and second
drive signals DS1, DS2, low frequency signals having mutually
identical constant amplitudes independent of the amplitude ratio
between the high-speed main signal components are superimposed on
the "0" sides of the amplitude-adjusted high-speed main signal
components, respectively, in this situation. Input of such first
and second drive signals DS1, DS2 into the electrodes 20D.sub.1,
20D.sub.2, respectively, results in a modulation of the light in
accordance with the signal corresponding to the sum of the first
and second drive signals DS1, DS2 as shown in FIG. 7C. It is noted
in FIG. 7C that the part upper than OV corresponds to the first
drive signal DS1 (the polarity has been reversed) and the part
lower than OV corresponds to the second drive signal DS2. In this
sum signal of the first and second drive signals DS1, DS2, the
superimposition ratio of the low frequency signals becomes constant
(Vf.sub.0/V.sub.p=constant in FIG. 7C). Thus, even when the
amplitude ratio between the high-speed main signal components of
the first and second drive signals DS1, DS2 is varied by varying
the setting of the optimum chirp amount, the low frequency signal
to be detected by the low frequency signal detecting part 51
becomes constant. As a result, even when the chirp amount is
controlled by adjusting the amplitude ratio between the first and
second drive signals, no affection is imposed on the detection and
control of the operating point drift based on the superimposition
of the low frequency signal.
[0082] In this way, according to the third embodiment, when the low
frequency signal is to be superimposed on either one of the "1"
side and "0" side of each of the first and second drive signals
DS1, DS2, the amplitude of the low frequency signal to be
superimposed on the respective drive signals is adjusted to be
constant independently of the amplitude ratio corresponding to the
optimum chirp amount. Thus, the superimposition ratio of the summed
low frequency signals in the sum signal of the first and second
drive signals DS1, DS2 becomes constant, to thereby obtain the same
effect as in the second embodiment.
[0083] In the third embodiment, the superimposing circuits
50.sub.1', 50.sub.2' have been provided between the variable
attenuators 23.sub.1, 23.sub.2 and the variable delay circuits
24.sub.1, 24.sub.2, respectively. However, the superimposing
circuits 50.sub.1', 50.sub.2' may be provided between the variable
delay circuits 24.sub.1, 24.sub.2 and the input terminals
P1.sub.IN, P2.sub.IN of the Mach-Zehnder type optical modulator 2,
respectively.
[0084] Further, the second and third embodiments have been
constituted such that the chirp is controlled by monitoring the
first and second drive signals DS1, DS2 having passed through the
electrodes 20D.sub.1, 20D.sub.2 of the Mach-Zehnder type optical
modulator 2. However, similarly to the situation as explained in
the modified example of the first embodiment, it is also possible
to control the chirp by monitoring the first and second drive
signals DS1, DS2 before input into the electrodes 20D.sub.1,
20D.sub.2, respectively.
[0085] There will be described hereinafter a fourth embodiment of
the present invention.
[0086] In this fourth embodiment, there will be considered a
situation where the present invention is applied to an optical
transmitter capable of transmitting a high-speed optical signal in
an RZ data format, by connecting two Mach-Zehnder type optical
modulators in a serial two staged manner.
[0087] FIG. 8 is a block diagram showing an essential constitution
of an optical transmitter according to a fourth embodiment of the
present invention;
[0088] In FIG. 8, the present optical transmitter is constituted to
include: a light source (LD) 1; a Mach-Zehnder type optical
modulator 2' for externally modulating light from the light source
1 in a two staged manner; and a chirp controlling circuit 3' for
controlling the chirp to be added to the optical signal modulated
by the Mach-Zehnder type optical modulator 2'.
[0089] The constitution of the Mach-Zehnder type optical modulator
2' is different from that of the Mach-Zehnder type optical
modulator 2 used in the first embodiment, in that: instead of the
substrate part 20 for conducting the modulation in a single stage
manner, there is adopted a substrate part 60 for conducting a
modulation in a two staged manner by serially connecting a similar
Mach-Zehnder type optical modulator to the preceding stage of the
substrate part 60, and there are provided a driving circuit (ATT)
61 and a variable delay circuit (DLY) 62 at the preceding stage
side so as to provide a drive signal. The latter stage side of the
substrate part 60 is identical with the substrate part 20 of the
first embodiment, and the constitution for providing the first and
second drive signals DS1, DS2 to such a latter stage side is also
identical with that of the first embodiment.
[0090] At the preceding stage side of the substrate part 60, the CW
light from the light source 1 is input into a light input end 60A,
and thereafter bifurcated to be propagated through first arm
60B.sub.1 and second arm 60B.sub.2, respectively, and then
multiplexed into a resultant light to be output from a light output
end 60C to the light input end 20A of the latter stage side. Formed
on the first arm 60B.sub.1 is an electrode 60D to which a third
drive signal DS3 to be described later is applied from the light
input end 60A side. In such a substrate part 60 of a two-staged
constitution, the CW light from the light source 1 is intensity
modulated at the preceding stage side in accordance with the data
signal in an NRZ format, and then intensity modulated at the latter
stage side in accordance with a clock signal corresponding to the
modulation at the preceding stage side, to thereby finally generate
an optical signal in an RZ data format. Note, similarly to the
first embodiment, the chirp amount to be added to the optical
signal is controlled by adjusting the amplitude ratio between the
first and second drive signals DS1, DS2 to be applied to the latter
stage side.
[0091] The driving circuit 61 generates a signal as an origin of
the drive signal DS3 such as by amplifying a data signal (DATA) at
a required bit rate and in an NRZ format to a predetermined level,
and outputs the signal to the variable delay circuit 62.
[0092] The variable delay circuit 62 is to delay the signal output
from the driving circuit 61 to thereby adjust a phase of the
signal. A delay amount of this variable delay circuit 62 is
controlled by a control signal output from a phase comparison
circuit 34' to be described later.
[0093] The constitution of the chirp controlling circuit 3' is
different from that of the chirp controlling circuit 3 used in the
first embodiment, in that: instead of the phase comparison circuit
34, there is provided the phase comparison circuit (PHASE COMP) 34'
for comparing the phases of the first and second drive signals DS1,
DS2 bifurcated by the branch circuits 31.sub.1, 31.sub.2 with a
phase of the third drive signal having passed through an electrode
60D at the preceding stage side of the substrate part 60.
[0094] The phase comparison circuit 34' generates control signals
for feedback controlling the respective delay amounts of the
variable delay circuits 24.sub.1, 24.sub.2, 62, so that the phases
of the first through third drive signals DS1, DS2, DS3 have mutual
relations as shown in a waveform diagram of FIG. 9. Namely,
concerning the first and second drive signals DS1, DS2, the phase
comparison circuit 34' compares the phases of the first and second
drive signals DS1, DS2 bifurcated by the branch circuits 31.sub.1,
31.sub.2, respectively, and feedback controls the delay amounts of
the variable delay circuits 24.sub.1, 24.sub.2, respectively, so
that the phases of the first and second drive signals DS1, DS2 are
brought into an antiphase relation, similarly to the first
embodiment. Concerning the third drive signal DS3, the phase
comparison circuit 34' compares the phases of the first and second
drive signals DS1, DS2 with the phase of the third drive signal
having passed through the electrode 60D at the preceding stage side
of the substrate part 60, and generates the control signal for
feedback controlling the delay amount of the variable delay circuit
62 so that the timing, at which the first and second drive signals
DS1, DS2 become the maximum or the minimum, coincides with a
transitional point of the data or with a substantial center of 1
unit data length of the third drive signal DS3.
[0095] In the optical transmitter of the aforementioned
constitution, the CW light generated by the light source 1 is
NRZ-data modulated at the preceding stage side of the Mach-Zehnder
type optical modulator 2'. At this time, although the optical
signal is added with a chirp, this chirp is constant. Further, the
NRZ-data modulated optical signal is further modulated in
accordance with the clock signal at the latter stage side of the
Mach-Zehnder type optical modulator 2', and thus converted into an
RZ data format. At this time, since the electrodes 20D.sub.1,
20D.sub.2 at the latter stage side are applied with the first and
second drive signals DS1, DS2 feedback controlled so that these
first and second drive signals DS1, DS2 are brought into an
antiphase relation with the amplitude ratio corresponding to the
optimum chirp amount similarly to the first embodiment, the
adjustment of the chirp amount can be readily conducted.
[0096] In this way, according to the fourth embodiment, there can
be obtained the same effect as the first embodiment, by applying
the present invention, for the side which conducts the modulation
by simultaneously driving two arms, even in such a constitution
that the Mach-Zehnder type optical modulators connected in a serial
two staged manner so as to transmit a high-speed optical signal
such as in an RZ data format.
[0097] The fourth embodiment described above has been constituted
to control the chirp by monitoring the first through third drive
signals DS1, DS2, DS3 having passed through the electrodes
20D.sub.1, 20D.sub.2, 60D of the Mach-Zehnder type optical
modulator 2', respectively. However, it is also possible to control
the chirp by monitoring the first through third drive signals DS1,
DS2, DS3 before input into the electrodes 20D.sub.1, 20D.sub.2,
60D, respectively, similarly to the situation described in the
modified example of the first embodiment. FIG. 10 shows a block
diagram showing an essential constitution in such a situation. In
this situation, a branch circuit 63 for extracting the third drive
signal DS3 is provided between the driving circuit 61 and the
variable delay circuit 62, and the bifurcated third drive signal
DS3 is transmitted to the phase comparison circuit 34'. Connected
to an output terminal of the electrode 60D is a terminator 413.
[0098] There will be now described a fifth embodiment of the
present invention.
[0099] In the fifth embodiment, there will be considered a
situation where the present invention is applied to an optical
transmitter utilizing an external modulator made up by serially
connecting a Mach-Zehnder type optical modulator and an optical
phase modulator.
[0100] FIG. 11 is a block diagram showing an essential constitution
according to the fifth embodiment of the present invention.
[0101] In FIG. 11, this optical transmitter comprises: a light
source (LD) 1; an external modulator 7 for modulating light from
the light source 1 by a Mach-Zehnder type optical modulator and an
optical phase modulator serially connected to each other; and a
chirp controlling circuit 8 for controlling a chirp to be added to
the optical signal modulated by the external modulator 7.
[0102] The external modulator 7 includes, for example, a substrate
part 70, driving circuits 71, 73, variable delay circuits (DLY) 72,
75, and a variable attenuator (ATT) 74.
[0103] The substrate part 70 conducts an intensity modulation by
the Mach-Zehnder type optical modulator arranged at the preceding
stage side, and a phase modulation by the optical phase modulator
arranged at the latter stage side, to thereby add a chirp to the
optical signal. These Mach-Zehnder type optical modulator and
optical phase modulator are formed on a single LN substrate.
Further, the light, kept in a polarized state such that the
modulation efficiency is maximized, is input into one end of the
Mach-Zehnder type optical modulator from the light source 1.
[0104] Concretely, in the Mach-Zehnder type optical modulator, CW
light from the light source 1 is input into a light input end 70A.
This CW light is then bifurcated to be propagated through a first
arm 70B.sub.1 and a second arm 70B.sub.2, respectively, and
thereafter multiplexed into a resultant light which is output from
a light output end 70C to the optical phase modulator at the latter
stage side. Formed on the first arm 70B.sub.1 is an electrode 70D
to which a drive signal DSa as described later is applied from the
light input end 70A side.
[0105] In the optical phase modulator at the latter stage side, the
optical signal from the light output end 70C at the preceding stage
side is input into an optical waveguide 70E, and this optical
waveguide 70E is formed with an electrode 70F at a predetermined
portion thereof. This electrode 70F is applied with a drive signal
DSb to be described later from a light input side.
[0106] The drive signal DSa for driving the Mach-Zehnder type
optical modulator of the substrate part 70 is generated at the
driving circuit 71 and variable delay circuit 72. The driving
circuit 71 generates a signal as an origin of the drive signal DSa
such as by amplifying a data signal (DATA) at a required bit rate
to a predetermined level, and outputs the signal to the variable
delay circuit 72. This variable delay circuit 72 is to delay the
signal output from the driving circuit 71 to thereby adjust a phase
of the signal. A delay amount of this variable delay circuit 72 is
controlled in accordance with a control signal output from a phase
comparison circuit 83 to be described later.
[0107] The drive signal DSb for driving the optical phase modulator
of the substrate part 70 is generated by: the driving circuit 73;
the variable attenuator 74 as an amplitude adjusting part; and the
variable delay circuit 75 as a phase adjusting part. The driving
circuit 73 generates a signal as an origin of the drive signal DSb
such as by amplifying a clock signal (CLOCK) corresponding to the
data signal used in the intensity modulation at the preceding stage
side to a predetermined level, and outputs the signal to the
variable attenuator 74. The variable attenuator 74 is to attenuate
the signal output from the driving circuit 73 so that the amplitude
of this signal becomes a value corresponding to a required chirp
amount, and this attenuation amount is controlled in accordance
with the detection result of an electric power detector 82. The
variable delay circuit 75 is to delay the signal output from the
variable attenuator 74 to thereby adjust a phase of the signal. A
delay amount of this variable delay circuit 75 is controlled in
accordance with a signal output from a phase comparison circuit
83.
[0108] The chirp controlling circuit 8 includes, for example, a
branch circuit 81, the electric power detector (DET) 82 and the
phase comparison circuit (PHASE COMP) 83. Here, the electric power
detector 82 corresponds to an amplitude controlling part, and the
phase comparison circuit 83 corresponds to a phase controlling
part.
[0109] The branch circuit 81 bifurcates the drive signal DSb having
passed through the electrode 70F of the substrate part 70, and
sends the bifurcated signals to the electric power detector 82 and
phase comparison circuit 83, respectively. The electric power
detector 82 detects electric power of the drive signal DSb
bifurcated by the branch circuit 81, and notify the result to the
variable attenuator 74. The phase comparison circuit 83 compares
the phase of the drive signal DSb branched from the branch circuit
81 with the phase of the drive signal DSa having passed through the
electrode 70D of the substrate part 70, and generates control
signals for feedback controlling the delay amounts of the variable
delay circuits 72, 75 so that the timing, at which the drive signal
DSb becomes the maximum or the minimum, coincides with a
transitional point of the data or with a substantial center of 1
unit data length of the drive signal DSa.
[0110] In the optical transmitter having the aforementioned
constitution, the CW light generated by the light source 1 is
intensity modulated in the Mach-Zehnder type optical modulator at
the preceding stage side of the external modulator 7, according to
the drive signal DSa. Further, the intensity modulated optical
signal is phase modulated in the optical phase modulator at the
latter stage side according to the drive signal DSb, so that the
chirp is added to the optical signal. At this time, the chirp
amount to be added to the optical signal is varied according to the
amplitude of the drive signal DSb. Thus, the attenuation amount of
the variable attenuator 74 is feedback controlled making use of the
detection result of the electric power detector 82, so that the
amplitude of the signal output from the variable attenuator 74
becomes a value corresponding to the optimum value of the chirp
amount to be set according to: the power of the optical signal to
be transmitted; and the wavelength dispersion of the transmission
path. Further, since the phase of the amplitude-adjusted drive
signal DSb is required to be matched with the phase of the drive
signal DSa at the preceding stage side, there is conducted a phase
adjustment by controlling the delay amounts of the variable delay
circuits 72, 75 by the control signals generated by the phase
comparison circuit 83. In this way, the optical signal, which has
been intensity modulated at the preceding stage side, is added with
the optimum chirp at the latter stage side.
[0111] According to the fifth embodiment as described above, it
becomes possible to realize an optical transmitter capable of
readily adjusting the optimum chirp amount also in a constitution
adopting an external modulator made up by serially connecting a
Mach-Zehnder type optical modulator with an optical phase
modulator, by conducting a feedback control so that the amplitude
of the drive signal DSb of the optical phase modulator becomes a
value corresponding to the optimum value of the chirp amount, and
the phase of the drive signal DSb is matched with the phase of the
drive signal DSa of the Mach-Zehnder type optical modulator.
[0112] As an applied example of the fifth embodiment, it is
possible to adopt a polarization scrambler instead of an optical
phase modulator. Polarization scramblers have a function to change
a phase difference between two polarization components of light to
thereby change a polarized state, and are identical with a
Mach-Zehnder type optical modulator and an optical phase modulator,
for example, in that a wavelength change is essentially caused.
Concretely, such as by entering light into an optical phase
modulator made of LN while tilting the plane of polarization of the
light relative to an optical axis of the optical phase modulator by
45.degree., it is possible to change a phase difference between two
polarization components of the incident light by a birefringence of
LN. By adopting such a polarization scrambler, it becomes possible
to reduce the correlation of polarizations between channels, and to
readily conduct the adjustment of the optimum chirp amount by
applying the present invention similarly to the fifth
embodiment.
[0113] There will be described an optical transmission system
according to the present invention.
[0114] FIG. 12 is a block diagram showing a constitution of an
optical transmission system according to an embodiment of the
present invention.
[0115] In FIG. 12, the present optical transmission system
comprises: n units of optical transmitters TX1, TX2, . . . TXn for
transmitting optical signals of different wavelengths,
respectively; an optical multiplexer 90 for wavelength multiplexing
the optical signals output from the optical transmitters TX1 to TXn
and for transmitting the wavelength multiplexed optical signal to a
transmission path L; optical repeaters 91 inserted in the
transmission path L at predetermined intervals; an optical
demultiplexer 92 for demultiplexing the optical signal repeatedly
transmitted via the transmission path L and optical repeater 91,
into optical signals of respective wavelengths; and n units of
optical receivers RX1, RX2, . . . RXn for receiving and processing
the optical signals of respective wavelengths demultiplexed by the
optical demultiplexer 92.
[0116] Each of the optical transmitters TX1 to TXn is applied with
anyone of the optical transmitters shown in the aforementioned
first through fifth embodiments, and generates an optical signal
added with a chirp of a required amount set according to such as
the wavelength dispersion of the transmission path L. Here, the
setting of the chirp amount in each of the optical transmitters TX1
to TXn is adjusted corresponding to the receipt information from
the associated one of the optical receivers RX1 to RXn.
[0117] Each of the optical receivers RX1 to RXn receives the
demultiplexed optical signal of the associated wavelength from the
optical demultiplexer 92, and conducts a receive processing such as
data reproduction. Here, there is monitored information concerning
a coding error rate when correcting a coding error by an
error-correcting code processing in each of optical receivers RX1
to RXn, and the coding error rate is transmitted as receipt
information to the associated one of the optical transmitters TX1
to TXn.
[0118] Note, the optical multiplexer 90, optical repeaters 91 and
optical demultiplexer 92 are the same with those used in a typical
optical transmission system.
[0119] In the optical transmission system having the aforementioned
constitution, the chirp amount to be added to the optical signal is
adjusted to the optimum value at each of the optical transmitters
TX1 to TXn, identically with the first through fifth embodiments.
At this time, when a chirp amount to be added to an optical signal
of a certain wavelength (supposed to be a channel k [Ch.k]) is so
increased as shown in FIG. 13, the post-transmission spectrum of
the optical signal received by the associated optical receiver
spreads in a manner as indicated by a broken line in FIG. 13,
resulting in cross talk to neighboring channels, to cause a
possibility of degradation of transmission qualities of the
neighboring channels. As such, in this embodiment, there is
conducted the control of the chirp amount so as not to degrade the
coding error rates of the neighboring channels, by monitoring the
coding error rate of each of the optical receivers RX1 to RXn, and
by adjusting the optimum chirp amount of the pertinent channel
while considering an affection on the neighboring channels.
[0120] Concretely, in controlling a chirp amount for a channel k,
the chirp amount is adjusted by firstly using a coding error rate
sent from the associated optical receiver for the channel k, and by
setting an amplitude ratio of the drive signal, for example, so
that the coding error rate is minimized. Next, by using the coding
error rates sent from the optical receiver for the channel k-1 and
the optical receiver for the channel k+1 corresponding to both
neighboring channels, respectively, the chirp amount at the optical
transmitter for the channel k is finely adjusted so that the
respective coding error rates are decreased. By sequentially
conducting such a chirp-amount control for respective channels,
there is conducted optimization of a chirp amount taking account of
an influence on neighboring channels.
[0121] Note, it is difficult to conduct a chirp-amount control
using a coding error rate as described above, when an S/N ratio of
an optical transmission system is in such an excellent state that
no coding errors are caused in an optical receiver. Even in such a
situation, it is also possible to conduct a chirp-amount control,
such as by changing the setting of pre-emphasis to be conducted at
the transmitting side to thereby intentionally degrade the SIN
ratio to such an extent that the SIN ratio can be corrected by an
error-correcting code processing.
[0122] According to the present optical transmission system as
mentioned above, it becomes possible to realize an optical
transmission system capable of readily obtaining an excellent
transmission characteristic, by utilizing an optical transmitter
capable of readily conducting an adjustment of the optimum chirp
amount, and by conducting optimization of the chirp amount while
taking account of an influence on neighboring channels by using
receipt information such as a coding error rate obtained by an
optical receiver.
[0123] In the aforementioned embodiment, there has been considered
a coding error rate as the receipt information to be obtained at
each optical receiver. However, the receipt information to be used
in the present invention is not limited thereto, and it is possible
to utilize various information representing receipt
characteristics. Further, although there has been exemplified a
constitution in which optical repeaters are arranged in the
transmission path L, the present invention may be applied to a
system requiring no optical repeaters.
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