U.S. patent application number 09/773660 was filed with the patent office on 2001-07-12 for optical modulation apparatus and method of controlling optical modulator.
Invention is credited to Ishikawa, George, Nakamoto, Hiroshi, Nishizawa, Yoshinori, Ooi, Hiroki, Yamamoto, Takuji.
Application Number | 20010007508 09/773660 |
Document ID | / |
Family ID | 18271898 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007508 |
Kind Code |
A1 |
Ooi, Hiroki ; et
al. |
July 12, 2001 |
Optical modulation apparatus and method of controlling optical
modulator
Abstract
An optical modulator having a voltage--optical output
characteristic in which optical output varies periodically with
respect to a voltage value of an electrical drive signal is driven
by a modulator driving voltage signal, which has an amplitude of
2.multidot.V.pi. between two light-emission culminations or two
light extinction culminations of the voltage--optical output
characteristic. A low-frequency superimposing unit superimposes a
prescribed low-frequency signal on the modulator driving voltage
signal, and an operating-point controller controls the operating
point of the optical modulator by detecting operating-point drift
of the optical modulator based upon the low-frequency signal
component contained in an optical signal output from the optical
modulator and controlling the bias voltage of the optical modulator
in dependence upon the drift of the operating point of the optical
modulator.
Inventors: |
Ooi, Hiroki; (Kawasaki-shi,
JP) ; Nakamoto, Hiroshi; (Kawasaki-shi, JP) ;
Ishikawa, George; (Kawasaki-shi, JP) ; Yamamoto,
Takuji; (Kawasaki-shi, JP) ; Nishizawa,
Yoshinori; (Kawasaki-shi, JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
700 11TH STREET, NW
SUITE 500
WASHINGTON
DC
20001
US
|
Family ID: |
18271898 |
Appl. No.: |
09/773660 |
Filed: |
February 2, 2001 |
Current U.S.
Class: |
359/245 ;
359/238; 359/246 |
Current CPC
Class: |
G02F 1/0123 20130101;
G02F 2203/21 20130101 |
Class at
Publication: |
359/245 ;
359/246; 359/238 |
International
Class: |
G02F 001/01; G02B
026/00; G02F 001/03; G02F 001/07 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 25, 1998 |
JP |
10-333958 |
Claims
What is claimed is:
1. An optical modulation apparatus including an optical modulator
having a voltage--optical output characteristic in which optical
output varies periodically with respect to a voltage value of an
electrical drive signal, and a drive signal generator for
generating an electrical drive signal which drives the optical
modulator by an amplitude between two light-emission culminations
or two light extinction culminations of the voltage--optical output
characteristic, said apparatus comprising: a low-frequency
oscillator for generating a prescribed low-frequency signal;
low-frequency superimposing means for superimposing the prescribed
low-frequency signal on the drive signal; low-frequency signal
detection means for detecting operating-point drift of the optical
modulator based upon the low-frequency signal component contained
in an optical signal output from said optical modulator; and
operating-point control means for controlling the operating point
of the optical modulator in dependence upon the drift of the
operating point of the optical modulator.
2. The apparatus according to claim 1, wherein said optical
modulator includes: optical waveguides that branch on a light input
side and merge on a light output side; two signal electrodes for
applying phase modulation to optical signals in the branched
optical waveguides on both sides; and two drive-signal input
terminals for inputting complimentary drive signals to respective
ones of said signal electrodes.
3. The apparatus according to claim 2, wherein optical duobinary
modulation, in which a binary data signal is converted to a 3-value
electrical signal and the 3-value electrical signal is converted to
an optical signal, is performed.
4. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal by varying a center level of the drive signal by said
low-frequency signal.
5. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal by
controlling gain of the drive signal.
6. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that phases of upper and lower
envelopes of said drive signal coincide.
7. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that only an upper or lower envelope
of said drive signal varies.
8. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that amplitudes of upper and lower
envelopes of said drive signal differ.
9. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that frequencies of upper and lower
envelopes of said drive signal differ.
10. The apparatus according to claim 1, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that phases of upper and lower
envelopes of said drive signal differ.
11. An optical modulation apparatus including an optical modulator
having optical waveguides that branch on a light input side and
merge on a light output side, two signal electrodes for applying
phase modulation to optical signals in the branched optical
waveguides on both sides and two drive-signal input terminals for
inputting complimentary drive signals to respective ones of said
signal electrodes, and possessing a voltage--optical output
characteristic in which optical output varies periodically with
respect to a voltage value of an electrical drive signal; and a
drive signal generator for generating complimentary drive signals
having an amplitude between a light-emission culmination and a
neighboring light-extinction culmination of the voltage--optical
output characteristic of said optical modulator, said apparatus
comprising: a low-frequency oscillator for generating a prescribed
low-frequency signal; low-frequency superimposing means for
superimposing the prescribed low-frequency signal on the drive
signal; low-frequency signal detection means for detecting
operating-point drift of the optical modulator based upon the
low-frequency signal component contained in an optical signal
output from said optical modulator; and operating-point control
means for controlling the operating point of the optical modulator
in dependence upon the drift of the operating point of the optical
modulator.
12. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency on the drive
signal applied to at least one of the signal electrodes; said
operating-point control means controls the operating point of the
optical modulator by controlling bias voltage of said one of the
signal electrodes based upon the operating-point drift of the
optical modulator; and means is provided for fixing the center of
the voltage of the drive signal, which is applied to the other
signal electrode, at ground voltage.
13. An optical modulation apparatus for modulating an optical
signal by inputting a drive signal to an optical modulator having
optical waveguides that branch on a light input side and merge on a
light output side, two signal electrodes for applying phase
modulation to optical signals in the branched optical waveguides on
both sides and two drive-signal input terminals for inputting
complimentary drive signals to respective ones of said signal
electrodes, and possessing a voltage--optical output characteristic
in which optical output varies periodically with respect to a
voltage value of an electrical drive signal, said drive signal
having an amplitude between a light-emission culmination and a
neighboring light-extinction culmination of the voltage--optical
output characteristic, said apparatus comprising: a drive signal
generator for generating complementary drive signals having an
amplitude that is one-half said amplitude and for inputting these
drive signals to respective ones of said signal electrodes; a
low-frequency oscillator for generating a prescribed low-frequency
signal; low-frequency superimposing means for superimposing the
low-frequency signal on the drive signal; low-frequency signal
detection means for detecting operating-point drift of the optical
modulator based upon the low-frequency signal component contained
in an optical signal output from said optical modulator; and
operating-point control means for controlling the operating point
of the optical modulator in dependence upon the drift of the
operating point of the optical modulator.
14. The apparatus according to claim 13, wherein said low-frequency
superimposing means superimposes the low-frequency on the drive
signal applied to at least one of the signal electrodes; said
operating-point control means controls the operating point of the
optical modulator by controlling bias voltage of said one of the
signal electrodes based upon the operating-point drift of the
optical modulator; and means is provided for fixing the center of
the voltage of the drive signal, which is applied to the other
signal electrode, at ground voltage.
15. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal by varying a center level of the drive signal by said
low-frequency signal.
16. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal by
controlling gain of the drive signal.
17. The apparatus according to claim 13, wherein said low-frequency
superimposing means superimposes the low-frequency signal by
controlling gain of the drive signal.
18. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that phases of upper and lower
envelopes of said drive signal coincide.
19. The apparatus according to claim 13, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that phases of upper and lower
envelopes of said drive signal is the opposite of each other.
20. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that only an upper or lower envelope
of said drive signal varies.
21. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that amplitudes of upper and lower
envelopes of said drive signal differ.
22. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that frequencies of upper and lower
envelopes of said drive signal differ.
23. The apparatus according to claim 11, wherein said low-frequency
superimposing means superimposes the low-frequency signal on the
drive signal in such a manner that phases of upper and lower
envelopes of said drive signal differ.
24. The apparatus according to claim 11, further comprising
operating-point changeover means for shifting a voltage range used
in modulation in the voltage--optical output characteristic of said
optical modulator.
25. The apparatus according to claim 11, further comprising a reset
switch for resetting, to a prescribed initial state, an operating
point on the voltage--optical output characteristic of said optical
modulator.
26. The apparatus according to claim 11, wherein said low-frequency
detecting means detects light that leaks from an optical waveguide
of the optical modulator and detects a low-frequency component from
the detected light.
27. The apparatus according to claim 11, wherein the optical
modulator has a half-wave plate inserted in the middle of the
branched optical waveguide on each side of the optical modulator
and is capable of modulating light of any polarization.
28. An optical modulation apparatus which includes an optical
modulator having a voltage--optical output characteristic in which
optical output varies periodically with respect to a voltage value
of an electrical drive signal, and a drive circuit for generating
complimentary drive signals having an amplitude between a
light-emission culmination and a neighboring light-extinction
culmination of the voltage--optical output characteristic of said
optical modulator, or an amplitude that is one-half of said
amplitude, said apparatus comprising: a low-frequency oscillator
for generating a prescribed low-frequency signal; low-frequency
superimposing means for superimposing the prescribed low-frequency
signal on the drive signal; low-frequency signal detection means
for detecting operating-point drift of the optical modulator by
detecting a frequency signal component, whose frequency is twice
that of said low-frequency signal, contained in an optical signal
output from said optical modulator; and operating-point control
means for controlling the operating point of the optical modulator
in dependence upon the drift of the operating point of the optical
modulator.
29. A method of controlling an optical modulator, which has a
voltage--optical output characteristic in which optical output
varies periodically with respect to a voltage value of an
electrical drive signal, by the electrical drive signal, which has
an amplitude between two light-emission culminations or two light
extinction culminations of the voltage--optical output
characteristic, said method comprising the steps of: superimposing
a prescribed low-frequency signal on the drive signal; detecting
operating-point drift of the optical modulator based upon the
low-frequency signal component contained in an optical signal
output from said optical modulator; and controlling the operating
point of the optical modulator in dependence upon the drift of the
operating point of the optical modulator.
30. The method according to claim 29, further comprising the steps
of: using, as said optical modulator, an optical modulator which
includes optical waveguides that branch on a light input side and
merge on a light output side two signal electrodes for applying
phase modulation to optical signals in the branched optical
waveguides on both sides, and two drive-signal input terminals for
inputting complimentary drive signals to respective ones of said
signal electrodes; and generating two complimentary drive signals
having an amplitude between a light-emission culmination and a
neighboring light-extinction culmination of the voltage--optical
output characteristic of the optical modulator, and inputting the
complimentary drive signals to respective ones of said signal
electrodes.
31. A method of controlling an optical modulator for modulating an
optical signal by inputting a drive signal to the optical
modulator, which includes two signal electrodes for applying phase
modulation to optical signals on both sides of the optical
modulator and two drive-signal input terminals for inputting
complimentary drive signals to respective ones of said signal
electrodes, and which possesses a voltage--optical output
characteristic in which optical output varies periodically with
respect to a voltage value of an electrical drive signal, said
drive signal having an amplitude between a light-emission
culmination and a neighboring light-extinction culmination of the
voltage--optical output characteristic, said method comprising the
steps of: generating complementary drive signals having an
amplitude that is one-half said amplitude; superimposing a
prescribed low-frequency on one complimentary drive signal;
detecting operating-point drift of the optical modulator based upon
the low-frequency signal component contained in an optical signal
output from said optical modulator; and controlling the operating
point of the optical modulator in dependence upon the drift of the
operating point of the optical modulator.
32. The method according to claim 30, wherein the low-frequency
signal is superimposed on the drive signal in such a manner that
phases of upper and lower envelopes of said drive signal
coincide.
33. The method according to claim 31, wherein the low-frequency
signal is superimposed on the drive signal in such a manner that
phases of upper and lower envelopes of said drive signal is the
opposite of each other.
34. The method according to claim 30, wherein the low-frequency
signal is superimposed on the drive signal in such a manner that
only an upper or lower envelope of said drive signal varies.
35. The method according to claim 30, wherein the low-frequency
signal is super imposed on the drive signal in such a manner that
amplitudes of upper and lower envelopes of said drive signal
differ.
36. The method according to claim 30, wherein the low-frequency
signal is superimposed on the drive signal in such a manner that
frequencies of upper and lower envelopes of said drive signal
differ.
37. The method according to claim 30, wherein the low-frequency
signal is superimposed on the drive signal in such a manner that
phases of upper and lower envelopes of said drive signal differ.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to an optical modulation apparatus
and to a method of controlling an optical modulator. More
particularly, the invention relates to an optical modulation
apparatus and to a method of controlling an optical modulator,
wherein even if the operating point of an optical modulator the
optical output of which varies periodically with respect to a
driving voltage fluctuates owing to a change in ambient temperature
or aging, the fluctuation in operating point can be compensated for
in stable fashion. More specifically, the present invention relates
to a control method for stabilizing the operating point of a
Mach-Zehnder optical modulator (referred to as an "MZ-type optical
modulator) in an optical transmitter used in a time-division
multiplexing (TDM) or wavelength-division multiplexing (WDM)
optical transmission system.
[0002] The explosive increase in the quantity of available
information in recent years has made it desirable to enlarge the
capacity and lengthen the distance of optical communications
systems. In-line optical amplifier systems which accommodate a
transmission speed of 10 Gbps are now being put to practical use.
Even greater capacity will be required in the future, and research
and development is proceeding in both the TDM and WDM aspects of
optical transmission.
[0003] Direct modulation
[0004] Intensity modulation and direct detection (so-called "direct
modulation") is the simplest technique to use for an electro-optic
conversion circuit employed in an optical communications system.
According to this technique, a current that activates a
semiconductor laser is turned on and off directly by the "0"s and
"1"s of a data signal to control the emission and extinction of the
laser beam. When a laser per se is turned on and off directly,
however, the light signal experiences a fluctuation in wavelength
(so-called "chirping") owing to the properties of the
semiconductor. The higher the transmission speed (bit rate) of the
data, the greater the influence of chirping. The reason for this is
that an optical fiber exhibits a chromatic dispersion property
wherein propagation velocity varies for different wavelengths. When
chirping is caused by direct modulation, propagation velocity
fluctuates, waveforms are distorted during propagation through
optical fiber and it becomes difficult to perform long-distance
transmission and transmission at high speed.
[0005] External modulation
[0006] For the reasons mentioned above, external modulation is used
for high transmission speeds of 2.5 to 10 Gbps. According to
external modulation, a laser diode emits light continuously and the
emitted light is turned on and off by the "1"s and "0"s of data
using an external modulator. The above-mentioned MZ-type modulator
primarily is used as the external modulator. FIGS. 32A and 32B are
diagrams useful in describing the MZ-type modulator, in which FIG.
32A is a schematic view of the construction of the modulator and
FIG. 32B is for describing the modulating operation.
[0007] Shown in FIG. 32A are a distributed-feedback semiconductor
laser diode (DFB-LD) 1 used in long-distance transmission at a
speed of greater than 1 Gbps, an MZ-type modulator 2 and optical
fibers 3a, 3b. The MZ-type modulator 2 includes on an LiNbO.sub.3
substrate, (1) an input optical waveguide 2a formed on the
substrate for introducing light from the laser diode 1, (2)
branching optical waveguides 2b, 2c and (3) an output optical
waveguide 2d formed on the substrate for outputting modulated
light; (4) two signal electrodes 2e, 2f formed on the substrate for
applying phase modulation to the optical signals in the branching
optical waveguides 2b, 2c, and (5) a signal input terminal 2g
formed on the substrate for inputting an NRZ electrical drive
signal to one of the signal electrodes, namely the electrode
2e.
[0008] If a voltage applied to the signal electrodes 2e, 2f is
controlled by the "1"s and "0"s of data, the branching optical
waveguides 2b, 2c develop a difference in refractive index and the
light waves of the optical signals in the optical waveguides
develop a difference in phase between them. For example, if the
data is a "0", control is performed in such a manner that the phase
difference between the light waves of the optical signals in the
two optical waveguides 2b, 2c becomes 180.degree.; if the data is a
"1", control is performed in such a manner that the phase
difference between the light waves of the optical signals in the
two optical waveguides 2b, 2c becomes 0.degree.. If this
arrangement is adopted, superimposing the optical signals of the
two optical waveguides 2b, 2c will make it possible to output the
input light upon modulating it (turning it on and off) by the "1"s
and "0"s of the data.
[0009] As shown in FIG. 32B, the optical output characteristic of
the MZ-type optical modulator, which has a voltage difference
between the two electrodes thereof, varies periodically in
dependence upon the applied voltage. Point A represents the
culmination of the light emission and point B the culmination of
extinction. The range of the voltage over one period is 2V.pi..
When data is a "1", therefore, voltage having an amplitude of V.pi.
is applied between the signal electrodes 2e, 2f, whereby light is
emitted. When data is a "0", a voltage of zero is applied between
the signal electrodes 2e, 2f, whereby light is extinguished.
[0010] The MZ-type optical modulator described above is
advantageous in that transmitted light exhibits little chirping.
However, a change in the temperature of the LiNbO.sub.3
constituting the substrate, prolonged application of an electric
field thereto and aging thereof are accompanied by polarization of
the substrate per se, electric charge remains on the surface of the
substrate and the bias voltage across the signal electrodes
fluctuates. Consequently, the voltage--optical output
characteristic of the MZ-type optical modulator fluctuates to the
left and right from the ideal curve a in FIG. 33 to the curves b
and c. In other words, the operating point of the MZ-type optical
modulator drifts with the passage of time, thereby the on/off light
level changes and causes inter symbol interference between codes
(refer to output eye patter in FIG. 33).
[0011] Bias control method in NRZ modulation
[0012] Accordingly, in order to stabilize the operating point, the
conventional practice is to perform control in such a manner that
the bias voltage is increased correspondingly if the curve shifts
to the right and decreased correspondingly if the curve shifts to
the left. More specifically, there has been proposed a compensation
method (referred to as "automatic bias-voltage control" (ABC)
below) which includes superimposing a low-frequency signal on an
electrical drive signal, detecting the amount of drift of the
operating point and the direction of this drift, and controlling
the bias voltage by feedback (see the specification of Japanese
Patent Application Laid-Open No. 3-251815). FIG. 34 is a diagram
showing the construction of a circuit for stabilizing the operating
point of an optical modulator that implements the currently
available method of compensating the modulator operating point, and
FIG. 35 is a diagram useful in describing the principle of
operating-point stabilization.
[0013] Shown in FIG. 34 are the semiconductor laser diode (DFB-LD)
1, the MZ-type optical modulator (LN optical modulator) 2, the
optical fibers 3a, 3b and a drive circuit 4. An NRZ electric signal
(the data signal) is input to the drive circuit 4, which proceeds
to generate an electrical drive signal SD having an amplitude
(=V.pi.) between the culmination A of light emission and the
culmination B of light extinction in the voltage--optical output
characteristic (see FIG. 32B) of the MZ-type optical modulator 2. A
low-frequency oscillator 5 generates a low-frequency signal SLF
having a low frequency f.sub.0 (e.g., 1 KHz), a low-frequency
superimposing circuit 6 for superimposes a low-frequency signal on
the drive signal SD, an optical branching unit 7 branches the
optical signal from the optical modulator 2, and a light receiver
(PD) 8 such as a photodiode converts the optical signal output by
the optical modulator 2 to an electrical signal. Numeral 9 denotes
an amplifier. A phase comparator 10 detects and outputs a phase
difference .theta. between the low-frequency signal component of
the frequency f.sub.0 contained in the optical signal output by the
optical modulator 2 and the low-frequency signal output by the
low-frequency oscillator 5. A low-pass filter (LPF) 11 rectifies
the output signal of the phase comparator 10, and a bias supply
circuit 12 controls the bias voltage, which is applied to a signal
electrode, in such a manner that the phase difference .theta. will
become zero.
[0014] The low-frequency superimposing circuit 6 subjects the drive
signal of the MZ-type optical modulator 2 to amplitude modulation
by the signal having the low frequency of fo, the photodiode 8
converts the output light of the optical modulator 2 to an
electrical signal, the phase comparator 10 performs a phase
comparison between the low-frequency signal impressed upon the
drive signal and the low-frequency signal component contained in
the optical signal, and the bias supply circuit 12 controls the
bias voltage applied to the signal electrode in such a manner that
the phase difference .theta. will become zero.
[0015] The optimum operating points of the MZ-type optical
modulator are points A and B (see FIG. 35) at which the two levels
of the waveform of the drive signal SD give the maximum and minimum
output optical powers. In the case that there is no fluctuation in
the voltage--optical output characteristic of the MZ-type optical
modulator 2. Even if the signal SLF having the low frequency
f.sub.0 is impressed upon the drive signal SD, upper and lower
envelopes ELU, ELL of the output light do not contain the f.sub.0
component and a frequency component that is twice f.sub.0 appears
in the ideal state (curve a).
[0016] On the other hand, if the characteristic curve shifts to the
left or right from a to b or from a to c (if the operating point
shifts to the left or right) in the manner illustrated, the upper
and lower envelopes ELU, ELL of the output light both become
signals modulated by the same phase. These signals contain the fo
component. In addition, the phases of the upper and lower envelopes
ELU, ELL of the output light in characteristic curve b are the
opposite of the phases of the upper and lower envelopes ELU, ELL of
the output light in characteristic curve c.
[0017] By virtue of the foregoing, the direction in which the
operating point drifts can be detected by comparing the phase of
low-frequency signal SLF superimposed on the drive signal and the
phase of the low-frequency signal component contained in the
optical signal. The bias voltage can be controlled in such a manner
that this phase difference will become zero.
[0018] Optical duobinary modulation
[0019] In a case where an increase in capacity is intended by TDM,
a factor is that chromatic dispersion (GVD) governs transmission
distance. Dispersion tolerance is inversely proportional to the
square of the data transmission speed (the bit rate). A dispersion
tolerance that is about 800 ps/nm in a 10-Gbps system, therefore,
deteriorates to about {fraction (1/16)}of this figure, namely to
about 50 ps/nm, in a 40-Gbps system. One method of reducing
waveform degradation due to chromatic dispersion is optical
duobinary modulation. (For example, see A. J. Price et al.,
"Reduced bandwidth optical digital intensity modulation with
improved chromatic dispersion tolerance", Electron. Lett., vol. 31,
No. 1, pp. 58-59, 1995.)
[0020] In comparison with the NRZ modulation scheme, optical
duobinary modulation reduces the bandwidth of the optical signal
spectrum to about half thereby it reduces the effects of chromatic
dispersion. For example, whereas the bandwidth of the optical
signal spectrum of a 10-Gbps NRZ signal is 10 GHz in terms of
frequency and 0.2 nm in terms of wavelength, the bandwidth of the
optical signal spectrum of a 10-Gbps duobinary signal is 5 GHz in
terms of frequency and 0.1 nm in terms of wavelength. Because the
velocity of light differs depending upon wavelength, the larger the
bandwidth of the spectrum of the optical signal, the greater the
amount of change in the velocity at which light propagates and,
hence, the greater the distortion of the waveform caused by
long-distance transmission. Accordingly, if the bandwidth of the
spectrum of the optical signal can be made small by optical
duobinary modulation, the amount of fluctuation in velocity can be
reduced and the dispersion tolerance can be increased.
[0021] FIG. 36 is a diagram showing the construction of a
modulation apparatus that relies upon optical duobinary modulation,
FIGS. 37A, 37B are diagrams useful in describing the principle of
optical duobinary modulation, and FIGS. 39A, 39B are waveform
diagrams of the associated signals.
[0022] Shown in FIG. 36 are the semiconductor laser diode (DFB-LD)
1 and the MZ-type optical modulator 2 having two signal electrodes
for applying phase modulation to the optical signals in the optical
waveguides on both sides, and drive-signal input terminals for
inputting complimentary drive signals to the signal electrodes.
[0023] A precoder 21 encodes a 40-Gbps binary NRZ electrical input
signal. A D-type flip-flop (D-FF) 22 extracts and stores the output
of the precoder 21 at a 40-GHz clock and outputs a non-inverted
signal D and an inverted signal *D. Phase shifters 23a, 23b adjust
the output phases of the flip-flop 22 and apply there outputs to
amplitude adjusters 24a, 24b, respectively. The outputs thereof are
applied to electrical low-pass filters 25a, 25b, respectively,
having a bandwidth that is one-fourth the bit rate (=40 Gbps). Bias
adjustment circuits (bias tees) are shown at 26a, 26b and
terminators at 27a, 27b. The binary NRZ electrical input signal
encoded by the precoder 21 is made 3-value electrical signals S1
and S2 having inverted signs by passage through the low-pass
filters 25a, 25b, and these signals are in turn passed through the
bias tees 26a, 26b, thereby generating complimentary 3-value
electrical drive signals (push-pull signals) S1', S2' that are
applied to the respective ones of the two symmetrical signal
electrodes of the MZ-type optical modulator 2.
[0024] In the MZ-type optical modulator 2, the driving amplitude
necessary to turn the CW light on and off generally is V.pi. (see
FIG. 37B) based upon the voltage--optical output characteristic. In
optical duobinary modulation, however, each of the two signal
electrodes is subjected to push-pull modulation by the amplitude
V.pi.. (This is modulation in which voltages that are always
opposite in sign are applied to the two electrodes). The voltage
applied to the optical modulator 2 is the voltage difference
(=S1'-S2') between the input signals S1' and S2'. In optical
duobinary modulation, in other words, the MZ-type optical modulator
2 is modulated by a driving amplitude 2V.pi., namely an amplitude
that is twice V.pi.. Further, the bias voltage (the center voltage
of the electrical signal) is set in such a manner that the optical
modulator is driven between two light-emission culminations A, A on
the voltage--optical output characteristic curve.
[0025] The details of optical duobinary modulation will now be
described.
[0026] As shown in FIG. 38, the precoder 21 includes a NOT gate 21a
for inverting an input signal an, a 1-bit (25 ps) delay gate 21b,
and an EX-OR gate 21c for outputting a signal c.sub.n obtained by
taking the exclusive-OR between the preceding output c.sub.n-1 and
the present inverted input b.sub.n. If reference is had to a truth
table of the inverted signal b.sub.n, the preceding output signal
c.sub.n-1 of the EX-OR gate and the present output signal c.sub.n
of the EX-OR gate, we have the following:
[0027] (1) c.sub.n=c.sub.n-1 (no change in sign) if b.sub.n="0"
holds; and
[0028] (2) c.sub.n=1-c.sub.n-1 (sign inverted) if b.sub.n="1"
holds.
[0029] A low-pass filter 25a has a bandwidth which is only
one-fourth of the bit rate, namely 10 GHz. Consider two successive
bits of the input signal c.sub.n. If the input data varies at high
speed in the manner "0, 1" or "1, 0", the low-pass filter 25a
cannot follow up this change and outputs 0.5, which is the level
intermediate the 0 and 1 levels. If the input data is two
successive "1"s , namely "1, 1", the low-pass filter 25a outputs
the level 1.0; if the input data is two successive "0"s, namely "0,
0", the low-pass filter 25a outputs the level 0.0. More
specifically, the low-pass filter 25a:
[0030] (3) outputs the 0.0 level in a case where the output c.sub.n
of the precoder is successive "0"s ("00": no change in sign);
[0031] (4) outputs the 1.0 level in a case where the output c.sub.n
of the precoder is successive "1"s ("11": no change in sign);
and
[0032] (5) outputs the 0.5 level in a case where the sign of the
output c.sub.n of the precoder reverses ("01" or "10").
[0033] From (1) to (5) above, the output of the low-pass filter 25a
changes if the sign of the precoder output changes. That is, the
low-pass filter 25a outputs the 0.0 or +1.0 level as the output
d.sub.n if the input data an is "1", and outputs the +0.5 level as
the output d.sub.n if the input data a.sub.n is "0". Similarly, the
low-pass filter 25b outputs the 0.0 or -1.0 level as the output *dn
if the input data a.sub.n is "1", and outputs the 0.5 level as the
output *d.sub.n if the input data a.sub.n is "0". Accordingly, if
the level .+-.1.0 is .+-.V.pi. and the level .+-.0.5 is
.+-.V.pi./2, then 2V.pi. or 0 is input across the signal electrodes
of the MZ-type optical modulator 2 when the input data an is "1"
and V.pi. is input across the signal electrodes of the MZ-type
optical modulator 2 when the input data an is "0". As a result,
with reference to FIG. 37B,
[0034] (1) "1" is output (light is emitted) if the input data an is
"1", at which value 2V.pi. or 0 is input across the signal
electrodes of the MZ-type optical modulator 2; and
[0035] (2) "0" is output (light is extinguished) if the input data
an is "0", at which value V.pi. is input across the signal
electrodes of the MZ-type optical modulator 2.
[0036] Thus, the waveforms of the output signals S1, S2 from the
low-pass filters 25a, 25b are as shown in FIG. 39A, and the optical
signal output S3 from the MZ-type optical modulator 2 becomes as
shown in FIG. 39B.
[0037] The characterizing feature of the optical duobinary
modulation method is that the bandwidth of the optical signal
spectrum is approximately half that obtained with the conventional
NRZ modulation method described above. This makes it possible to
reduce the effects of chromatic dispersion.
[0038] Further, in accordance with optical duobinary modulation,
channels can be disposed at higher density in the WDM scheme. In a
case where the intent is to enlarge capacity by the WDM technique,
bandwidth of wavelength at which a optical amplifier can amplify
are limiting factors. However, if optical duobinary modulation is
used, the fact that this method provides a narrow bandwidth for the
optical signal spectrum can be utilized and channels can be
disposed at a higher density within the amplification bandwidth of
the light amplifier.
[0039] Further, in optical duobinary modulation, chirping can be
reduced because of push-pull drive. Chirping occurs and the
direction thereof reverses when the applied voltage of an optical
modulator increases and decreases. With optical duobinary
modulation, however, the electrodes are driven by mutually
complimentary electrical signals. Consequently, when the applied
voltage increases at one electrode, it decreases at the other, and
when the applied voltage decreases at one electrode, it increases
at the other. Since the optical phase of the output optical signal
is the sum of the optical phases produced at the two electrodes,
chirping is reduced by cancellation.
[0040] An advantage of the MZ-type optical modulator is the fact
that transmitted light experiences little chirping, as mentioned
above. However, a change in the temperature of the LiNbO.sub.3
constituting the substrate and the aging thereof are accompanied by
temporal drift of the operating point of the voltage--optical
output characteristic.
[0041] For this reason, it is necessary to control the bias voltage
in dependence upon drift of the operating point, just is in the NRZ
modulation scheme, in optical duobinary modulation as well.
However, the problems set forth below arise when the
operating-point compensation technique of NRZ modulation is applied
directly to optical duobinary modulation. FIG. 40 is a diagram
useful in describing a case where the operating-point compensation
technique of NRZ modulation is applied directly to optical
duobinary modulation.
[0042] With optical duobinary modulation, the driving voltage is
made twice that used in NRZ modulation. Consequently, if the
voltage--optical output characteristic shifts to the left or right
from the ideal characteristic a to b or c, the envelopes ELU, ELL
of the optical signal corresponding to the ON-side and OFF-side
portions EU and EL of the electrical driving signal of the
modulator subjected to low-frequency modulation take on mutually
opposite phases and cancel each other out, making it impossible to
detect the signal component of the low frequency f.sub.0. The
problem that arises, therefore, is that the ABC control method
employed in the conventional NRZ modulation method cannot be
applied to a modulation scheme, which includes optical duobinary
modulation, wherein an optical modulator is driven between two
light-emission culminations or between two light-extinction
culminations of the voltage--optical output characteristic.
[0043] Another problem is that the conventional ABC control method
only assumes use of an MZ-type optical modulator configured for
electrode drive on one side. This means that it is necessary to
also consider setting of an operating point in a case where an
optical modulator configured for driving electrodes on both sides
is used in optical duobinary modulation, NRZ modulation and RZ
modulation.
SUMMARY OF THE INVENTION
[0044] Accordingly, an object of the present invention is to make
it possible to compensate for drift of the operating point that
accompanies a variation in the voltage--optical output
characteristic of an optical modulation apparatus in which an
optical modulator is driven by the amplitude between two
light-emission culminations or two light-extinction culminations of
the voltage--optical output characteristic.
[0045] Another object of the present invention is to so arrange it
that the operating point can be controlled to assume the proper
position even if the voltage--optical output characteristic of the
optical modulator varies in a case where the optical modulator,
which is configured for driving electrodes on both sides, is used
in optical duobinary modulation, NRZ modulation and RZ
modulation.
[0046] According to a first aspect of the present invention, when
an optical modulator having a voltage--optical output
characteristic in which optical output varies periodically with
respect to a voltage value of an electrical drive signal is driven
by the electrical drive signal, which has an amplitude (=2V.pi.)
between two light-emission culminations or two light extinction
culminations of the voltage--optical output characteristic, (1) a
prescribed low-frequency signal is superimposed on the drive
signal, (2) operating-point drift of the optical modulator is
detected based upon the low-frequency signal component contained in
an optical signal output by the optical modulator, and (3) the
operating point of the optical modulator is controlled in
dependence upon the operating-point drift (NRZ modulation, RZ
modulation).
[0047] According to a second aspect of the present invention, two
mutually complimentary drive signals having an amplitude between a
light-emission culmination and a neighboring light-extinction
culmination of a voltage--optical output characteristic of an
optical modulator are generated, a low-frequency signal is
superimposed on at least one of these complimentary drive signals,
and the drive signals are input to the signal electrodes to drive
electrodes on both sides of the optical modulator (optical
duobinary modulation).
[0048] In the first and second aspects of the present invention,
the optical modulator is an optical modulator, e.g., an MZ-type
optical modulator, having optical waveguides that branch on a light
input side and merge on a light output side, two signal electrodes
for applying phase modulation to optical signals in the branched
optical waveguides on both sides, and two drive-signal input
terminals for inputting complimentary drive signals to respective
ones of the signal electrodes.
[0049] Further, in the first and second aspects of the present
invention, examples of methods of superimposing a low-frequency
signal on a drive signal are:
[0050] (1) superimposing the low-frequency signal on the drive
signal in such a manner that phases of upper and lower envelopes of
the drive signal coincide;
[0051] (2) superimposing the low-frequency signal on the drive
signal in such a manner that only an upper or a lower envelope of
the drive signal varies;
[0052] (3) superimposing the low-frequency signal on the drive
signal in such a manner that amplitudes of upper and lower
envelopes of the drive signal differ;
[0053] (4) superimposing the low-frequency signal on the drive
signal in such a manner that frequencies of upper and lower
envelopes of the drive signal differ; and
[0054] (5) superimposing the low-frequency signal on the drive
signal in such a manner that phases of upper and lower envelopes of
the drive signal differ.
[0055] In accordance with the first and second aspects of the
present invention as described above, a low-frequency signal
component can be detected from an optical signal output by an
optical modulator, and operating-point drift that accompanies
fluctuation of the voltage--optical output characteristic of the
optical modulator can be compensated for by a simple arrangement.
Further, in accordance with optical duobinary modulation of the
second aspect of the present invention, the influence of chromatic
dispersion can be reduced and chirping can be diminished by
push-pull drive.
[0056] According to a third aspect of the present invention, an
optical modulator having optical waveguides that branch on a light
input side and merge on a light output side, two signal electrodes
for applying phase modulation to optical signals in the optical
waveguides on both sides, two drive-signal input terminals for
inputting complimentary drive signals to respective ones of the
signal electrodes, and a voltage--optical output characteristic
that varies periodically, is driven by a drive signal that has an
amplitude (=V.pi.) between a light-emission culmination and a
neighboring light extinction culmination of the voltage--optical
output characteristic. At this time, (1) complimentary drive
signals whose amplitude is one-half of the amplitude (=V.pi.) are
generated, (2) a prescribed low-frequency signal is superimposed on
one of the complimentary drive signals, and (3) operating-point
drift of the optical modulator is detected based upon the
low-frequency signal component contained in an optical signal
output by the optical modulator, and the operating point of the
optical modulator is controlled in dependence upon the
operating-point drift.
[0057] The third aspect of the present invention is such that when
the optical modulator is driven by the drive signal that has an
amplitude V.pi. between the light-emission culmination and the
neighboring light extinction culmination of the voltage--optical
output characteristic, two complimentary drive signals of amplitude
V.pi./2 are generated and the optical modulator is subjected to
push-pull drive by these complimentary drive signals. As a result,
chirping can be reduced. Moreover, the low-frequency signal
component can be detected reliably from the optical signal output
by the optical modulator, thereby making it possible to compensate
for drift of the operating point.
[0058] In accordance with the first through third inventions, as
described above, the low-frequency signal component can be detected
reliably from the optical signal output of the optical modulator by
way of a simple arrangement, thereby making it possible to
compensate for operating-point drift that accompanies variation of
the voltage--optical output characteristic of the optical
modulator, even in a case where an optical modulator configured for
drive on both sides is used in optical duobinary modulation, NRZ
modulation and RZ modulation.
[0059] Other features and advantages of the present invention will
be apparent from the following description taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1 is a diagram showing the principle underlying an
optical modulation apparatus according to the present
invention;
[0061] FIG. 2 is a diagram useful in describing the principle
underlying a first method according to the present invention
(same-phase low-frequency modulation applied to ON and OFF sides of
an electrical drive signal);
[0062] FIG. 3 is a diagram useful in describing the principle
underlying a second method according to the present invention
(low-frequency modulation applied only to the ON side of an
electrical drive signal);
[0063] FIG. 4 is a diagram useful in describing the principle
underlying a third method according to the present invention
(low-frequency modulation of different amplitudes applied to ON and
OFF sides of an electrical drive signal);
[0064] FIG. 5 is a diagram useful in describing the principle
underlying a fourth method according to the present invention
(low-frequency modulation of different frequencies applied to ON
and OFF sides of an electrical drive signal);
[0065] FIG. 6 is a diagram useful in describing the principle
underlying a fifth method according to the present invention
(low-frequency modulation of different phases applied to ON and OFF
sides of an electrical drive signal);
[0066] FIG. 7 is a diagram showing the construction of an optical
modulation apparatus according to a first embodiment;
[0067] FIG. 8 is a waveform diagram of signals associated with FIG.
7;
[0068] FIG. 9 is a first modification of the optical modulation
apparatus according to the first embodiment;
[0069] FIG. 10 is a waveform diagram of signals associated with
FIG. 9;
[0070] FIG. 11 is a second modification of the optical modulation
apparatus according to the first embodiment;
[0071] FIG. 12 is a waveform diagram of signals associated with
FIG. 11;
[0072] FIG. 13 is a diagram showing the construction of an optical
modulation apparatus according to a second embodiment;
[0073] FIG. 14 is a waveform diagram of signals associated with
FIG. 13;
[0074] FIG. 15 is a modification of the optical modulation
apparatus according to the second embodiment;
[0075] FIG. 16 is a waveform diagram of signals associated with
FIG. 15;
[0076] FIG. 17 is a diagram showing the construction of an optical
modulation apparatus according to a third embodiment;
[0077] FIG. 18 is a waveform diagram of signals associated with
FIG. 17;
[0078] FIG. 19 is a diagram showing the construction of an optical
modulation apparatus according to a fourth embodiment;
[0079] FIG. 20 is a waveform diagram of signals associated with
FIG. 19;
[0080] FIG. 21 is a diagram showing the construction of an optical
modulation apparatus according to a fifth embodiment;
[0081] FIG. 22 is a waveform diagram of signals associated with
FIG. 21;
[0082] FIG. 23 is a diagram showing the construction of an optical
modulation apparatus according to a sixth embodiment;
[0083] FIG. 24 is a waveform diagram of signals associated with
FIG. 23;
[0084] FIG. 25 is a diagram showing the construction of an optical
modulation apparatus according to a seventh embodiment;
[0085] FIG. 26 is a diagram showing the construction of an optical
modulation apparatus according to an eighth embodiment;
[0086] FIG. 27 is a waveform diagram of signals associated with
FIG. 26;
[0087] FIGS. 28A and 28B are diagrams useful in describing
switching of a bias point of a modulator;
[0088] FIG. 29 is a diagram showing the construction of an optical
modulation apparatus according to a ninth embodiment;
[0089] FIGS. 30A and 30B are diagrams useful in describing a case
where a light receiver is incorporated in a substrate;
[0090] FIG. 31 is a diagram showing the construction of an optical
modulator capable of modulating any polarized light wave;
[0091] FIGS. 32A and 32B are diagrams for describing a Mach-Zehnder
optical modulator;
[0092] FIG. 33 is a diagram useful in describing the problems
caused by drift of the operating point of an optical modulator;
[0093] FIG. 34 is a diagram showing the construction of a circuit
for stabilizing the operating point of an optical modulator in NRZ
modulation;
[0094] FIG. 35 is a diagram showing the principle underlying the
circuit for stabilizing the operating point of an optical modulator
in NRZ modulation;
[0095] FIG. 36 is a diagram showing an example of the construction
of a modulator using optical duobinary modulation;
[0096] FIGS. 37A and 37B are diagrams for describing the principle
of optical duobinary modulation;
[0097] FIG. 38 is another diagram for describing the principle of
optical duobinary modulation;
[0098] FIGS. 39A and 39B are waveform diagrams showing signals
associated with the optical duobinary modulator; and
[0099] FIG. 40 is a diagram useful in describing a case where a
technique similar to that of NRZ modulation is applied to optical
duobinary modulation.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0100] (A) Overview of the invention
[0101] (a) Basic construction
[0102] FIG. 1 is a diagram showing the basic construction of a
first optical modulation apparatus according to the present
invention. Shown in FIG. 1 are a semiconductor laser (DFB-LD) 51,
an optical modulator (e.g., an MZ-type optical modulator) 52 the
voltage--optical output characteristic whereof varies periodically,
a drive signal generator 53 for generating electrical drive signals
SD, SD' that drive the optical modulator by an amplitude
2.multidot.V.pi. between two light-emission culminations A, A or
two light extinction culminations B, B of the voltage--optical
output characteristic, a low-frequency oscillator 54 for generating
a prescribed low-frequency signal, a low-frequency superimposing
unit 55 for superimposing the low-frequency signal on the drive
signal SD, an optical branching unit 56 for branching an optical
signal output by the optical modulator 52, a low-frequency signal
detector 57 for detecting the low-frequency signal component
contained in an optical signal output by the optical modulator and
detecting operating-point drift of the optical modulator based upon
the low-frequency signal component, and an operating-point control
unit 58 for controlling the position of the operating point by
controlling the bias voltage of the optical modulator in dependence
upon the direction of drift of the operating point of the optical
modulator.
[0103] When the optical modulator 52 is driven by the electrical
signal having the amplitude 2.multidot.V.pi., the low-frequency
superimposing unit 55 superimposes a low-frequency signal SLF on
the electrical drive signal SD output by the drive signal generator
53. The low-frequency signal detector 57 detects the low-frequency
signal component contained in the optical signal output by the
optical modulator 52, and the operating-point control unit 58
discriminates the direction of operating-point drift based upon
this detected low-frequency signal component and controls the bias
voltage of the optical modulator 52. More specifically, the
operating-point control unit 58 controls the operating point in
such a manner that the center level of the electrical drive signal
(the modulator driving voltage signal) applied to the modulator
will coincide with the level of the extinction culmination B of the
characteristic curve and the levels on both sides of the electrical
drive signal will coincide with the light-emission culminations A,
A of the characteristic curve.
[0104] (b) Method of superimposing low-frequency signal
[0105] Methods of superimposing the low-frequency signal on the
drive signal are as follows:
[0106] (1) a first method (FIG. 2) of superimposing the
low-frequency signal SLF on the drive signal SD in such a manner
that the phases of upper and lower envelopes EU, EL of the
modulator driving voltage signal (the input electrical drive signal
of the modulator) will coincide;
[0107] (2) a second method (FIG. 3) of superimposing the
low-frequency signal SLF on the drive signal SD in such a manner
that only the upper envelope EU or lower envelope EL of the
modulator driving voltage signal will vary;
[0108] (3) a third method (FIG. 4) of superimposing the
low-frequency signal SLF on the drive signal SD in such a manner
that the amplitudes of the upper and lower envelopes EU and EL of
the modulator driving voltage signal will differ; and
[0109] (4) a fourth method (FIG. 5) of superimposing the
low-frequency signal SLF on the drive signal SD in such a manner
that the frequencies of the upper and lower envelopes EU and EL of
the modulator driving voltage signal will differ; and
[0110] (5) a fifth method (FIG. 6) superimposing the low-frequency
signal SLF on the drive signal SD in such a manner that the phases
of the upper and lower envelopes EU and EL of the modulator driving
voltage signal will differ.
[0111] As shown in FIG. 2, the first method is a method of
performing low-frequency modulation in such a manner that the
envelopes EU and EL corresponding to the ON and OFF sides,
respectively, of the modulator driving voltage signal take on the
same phase. The optimum operating points of the MZ-type optical
modulator 52 are the points A, A at which the two levels of the
waveform of the modulator driving voltage signal give the maximum
output optical power and the point B at which the intermediate
level gives the minimum output optical power. In the case that
there is no fluctuation in the voltage--optical output
characteristic of the MZ-type optical modulator 52. Even if the
signal SLF having the low frequency f.sub.0 is impressed upon the
modulator driving voltage signal, the upper and lower envelopes
ELU, ELL of the output light do not contain the f.sub.0 component
and a frequency component that is twice f.sub.0 appears in the
ideal state (curve a).
[0112] On the other hand, if the characteristic curve shifts from a
to the left or right, as indicated by b, c (if the operating point
shifts to the left or right), the upper and lower envelopes ELU,
ELL of the output light will contain the f.sub.0 component. In this
case, the envelopes EU and EL on the ON and OFF sides,
respectively, of the modulator driving voltage signal are made
identical in phase, whereby the envelopes ELU, ELL of the optical
signal are made identical in phase, unlike the situation
illustrated in FIG. 40. As a consequence, the f.sub.0 component is
not canceled out and can be detected reliably. Moreover, the phase
of the envelopes ELU, ELL of the output light is inverted depending
upon the direction in which the characteristic curve shifts. This
means that signal component of the superimposed low frequency
f.sub.0 can be detected even if the voltage--optical output
characteristic of the modulator shifts to the left or right from
the ideal curve a to the curve b or c, i.e., even if the operating
point varies from the optimum point. Further, since the phase of
the signal of the f.sub.0 component differs by 180.degree.
depending upon the direction in which the operating point drifts,
the direction in which the operating point drifts can be detected
by comparing this phase with the phase of the low-frequency signal
SLF superimposed upon the electrical drive signal SD.
[0113] As shown in FIG. 3, the second method is a method of
performing low-frequency modulation only on the ON side (or on the
OFF side). The second method is such that if the characteristic
curve shifts from a to the left or right, as indicated by b, c,
only the envelope ELU on the upper side of the output light will
contain the f.sub.0 component. As a result, the low-frequency
signal component can be detected reliably. Moreover, the phase of
the envelope ELU of the output light is inverted depending upon the
direction of shift. This means that signal component of the
superimposed low frequency fo can be detected even if the
voltage--optical output characteristic of the modulator shifts to
the left or right from the ideal curve a to the curve b or c, i.e.,
even if the operating point varies from the optimum point. Further,
since the phase of the signal of the fo component differs by
180.degree.depending upon the direction in which the operating
point drifts, the direction in which the operating point drifts can
be detected by comparing this phase with the phase of the
low-frequency signal SLF superimposed upon the electrical drive
signal SD.
[0114] As shown in FIG. 4, the third method is a method of
performing amplitude modulation in such a manner that the envelopes
EU and EL corresponding to the ON and OFF sides of the modulator
driving voltage signal take on different amplitudes. If the
voltage--optical output characteristic curve shifts from a to the
left or right, as indicated by b, c, the upper and lower envelopes
ELU, ELL of the output light will contain the f.sub.0 component. In
this case, the envelopes EU and EL on the ON and OFF sides,
respectively, of the modulator driving voltage signal are opposite
in phase, and therefore the phases of the envelopes ELU, ELL of the
optical signal also are opposite in phase. However, since the
amplitudes of the envelopes EU, EL are different, the signal
obtained by combining the envelopes ELU, ELL of the optical signal
does not become zero and the f.sub.0 component can be detected
reliably. Moreover, the phase of the signal obtained by combining
the envelopes ELU, ELL of the output light is inverted depending
upon the direction of shift. This means that the signal component
of the superimposed low frequency f.sub.0 can be detected even if
the voltage--optical output characteristic of the modulator shifts
to the left or right from the ideal curve a to the curve b or c,
i.e., even if the operating point varies from the optimum point.
Further, since the phase of the signal of the f.sub.0 component
differs by 180.degree.depending upon the direction in which the
operating point drifts, the direction in which the operating point
drifts can be detected by comparing this phase with the phase of
the low-frequency signal SLF superimposed upon the electrical drive
signal SD.
[0115] As shown in FIG. 5, the fourth method is a method of
obtaining different frequencies f.sub.1, f.sub.2 of low-frequency
modulation of the envelopes EU and EL corresponding to the ON and
OFF sides of the modulator driving voltage signal. If the
voltage--optical output characteristic curve shifts from a to the
left or right, as indicated by b, c, the upper and lower envelopes
ELU, ELL of the output light will contain the f.sub.1, f.sub.2
components, respectively. As a consequence, these signal components
can be detected reliably. Moreover, the phases of the envelopes
ELU, ELL of the output light are inverted depending upon the
direction in which the operating point shifts. This means that
signal components of the superimposed frequencies f.sub.1, f.sub.2
can be detected from the output light of the modulator even if the
voltage--optical output characteristic of the modulator shifts to
the left or right from the ideal curve a to the curve b or c.
Further, since the phases of the signal components of the
frequencies f.sub.1, f.sub.2 differ by 180.degree.depending upon
the direction in which the operating point drifts, this direction
can be detected.
[0116] As shown in FIG. 6, the fifth method is a method of
obtaining different phases of low-frequency modulation of the
envelopes EU and EL corresponding to the ON and OFF sides of the
modulator driving voltage signal. If the voltage--optical output
characteristic curve shifts from a to the left or right, as
indicated by b, c, the upper and lower envelopes ELU, ELL of the
output light will contain the f.sub.0 component. In this case, the
phases of the envelopes EU, EL on the ON side and OFF side of the
modulator driving voltage signal are offset by .theta., and
therefore the signal obtained by combining the envelopes ELU, ELL
of the optical signal does not become zero and the f.sub.0
component can be detected reliably. Moreover, the phase of the
signal obtained by combining the envelopes ELU, ELL of the output
light is inverted depending upon the direction of shift. This means
that the signal component of the superimposed low frequency fo can
be detected even if the voltage--optical output characteristic of
the modulator shifts to the left or right from the ideal curve a to
the curve b or c. Further, since the phase of the signal of the
f.sub.0 component is inverted depending upon the direction in which
the operating point drifts, this direction can be detected.
[0117] (c) Optical modulator configured for electrode drive on both
sides
[0118] The optical modulator is not defined above. Used as the
optical modulator 52 is an LN optical modulator (MZ-type optical
modulator) configured for electrode drive on both sides, having (1)
optical waveguides 52a, 52b that are formed on the LiNbO.sub.3
substrate and branch on a light input side and merge on a light
output side; (2) two signal electrodes 52c, 52d for applying phase
modulation to optical signals in the branched optical waveguides on
both sides; (3) two drive-signal input terminals 52e, 52f for
inputting complimentary drive signals to respective ones of the
signal electrodes; and (4) a bias-voltage input terminal 52g.
[0119] In a case where use is made of such an optical modulator
having driven electrodes on both sides thereof, the drive signal
generator 53 generates two mutually complimentary drive signals
(push-pull drive signals) SD, SD' having an amplitude V.pi. between
a light-emission culmination A and a neighboring light-extinction
culmination B of the voltage--optical output characteristic of the
optical modulator are generated, and the low-frequency
superimposing unit 55 superimposes the low-frequency signal SLF on
at least one of the electrical drive signals, i.e., the drive
signal SD, and inputs the resulting signal to the signal electrode
52c. The other drive signal SD' is input to the signal electrode
52d. Thus, both sides of the modulator are driven. It should be
noted that the above-described low-frequency signal superimposing
method can be applied also in a case where the optical modulator
driven on both its sides is used in NRZ modulation and RZ
modulation, etc.
[0120] (B) Embodiment
[0121] (a) First embodiment
[0122] FIG. 7 is a diagram showing the construction of an optical
modulation apparatus according to a first embodiment. This is an
example in which an LN optical modulator (MZ-type optical
modulator) configured for drive on both sides (i.e., the modulator
has driven electrodes on both sides) is used as the optical
modulator and low-frequency modulation is carried out in such a
manner that envelopes on the ON and OFF sides, respectively, of the
modulator driving voltage signal applied to the optical modulator
will take on the same phase (see FIG. 2 showing the principle
underlying the first method of the present invention). FIG. 8 is a
waveform diagram of signals associated with FIG. 7.
[0123] Shown in FIG. 7 are the semiconductor laser (DFB-LD) 51, and
the MZ-type optical modulator 52 the voltage--optical output
characteristic whereof varies periodically. The optical modulator
52 includes the optical waveguides 52a, 52b that are formed on the
LiNbO.sub.3 substrate and branch on the light input side and merge
on the light output side, the two signal electrodes 52c, 52d for
applying phase modulation to optical signals in the branched
optical waveguides on both sides, and two drive-signal input
terminals 52e, 52f for inputting complimentary drive signals to
respective ones of the signal electrodes, and bias-voltage input
terminals 52g, 52h for inputting bias voltages to the signal
electrodes.
[0124] The arrangement further includes the drive signal generator
53 which generates the two mutually complimentary drive signals
(push-pull drive signals) SD, SD' having the amplitude V.pi.
between the light-emission culmination A and the neighboring
light-extinction culmination B of the voltage--optical output
characteristic (see FIG. 1) of the optical modulator 52. The drive
signal generator 53, which corresponds to the circuitry from the
precoder 21 to the low-pass filters 25a, 25b of FIG. 36, converts
the binary input data to the 3-value push-pull drive signals SD,
SD' and outputs these signals. The drive signal SD is a 3-value
signal, namely a signal having a V.pi. or 0 level if the input data
is "1" and a V.pi./2 level if the input data is "0". The drive
signal SD' is a 3-value signal having a -V.pi. or 0 level if the
input data is "1" and a -V.pi./2 level if the input data is
"0".
[0125] The arrangement further includes the low-frequency
oscillator 54 for generating the prescribed low-frequency signal
SLF, e.g., a frequency f.sub.0 of 1 KHz, and the low-frequency
superimposing unit 55, which is constituted by a coil L for passing
a low-frequency signal and a capacitor C1 for cutting direct
current, for superimposing the low-frequency signal on one drive
signal, namely the drive signal SD. The low-frequency superimposing
unit 55 uses a bias tee to vary the center voltage of the drive
signal SD by the low-frequency signal of frequency f.sub.0 at the
input side of the optical modulator 52. The optical branching unit
56 branches the optical signal output by the MZ-type optical
modulator 52. A light receiver 57a such as a photodiode converts
the branched light to an electrical signal, an amplifying circuit
57b amplifies the output of the photodiode 57a, a phase comparator
57c to which the low-frequency signal SLF output by the
low-frequency oscillator 54 and an electrical signal conforming to
the photodiode output are input detects the low-frequency signal
component contained in the photodiode output by a phase comparison
and outputs the detected low-frequency signal component as a signal
indicative of the drift of the operating point of the modulator,
and a low-pass filter 57d smoothens the output of the phase
comparator. The photodiode 57a, amplifying circuit 57b, phase
comparator 57c and low-pass filter 57d construct the low-frequency
signal detector 57 of FIG. 1 that detects drift of the operating
point of the optical modulator. In order to raise the precision of
the phase comparator, a bandpass filter for the frequency f.sub.0
can be inserted on the output side of the amplifying circuit
57b.
[0126] The bias supply circuit (operating-point control unit) 58,
which is constituted by a bias tee 58a and a 50-.OMEGA. terminator
58b, controls the position of the operating point by controlling
the bias voltage Vb1 applied to the signal electrode 52a in
dependence upon the direction of drift of the operating point of
the optical modulator. The bias tee 58a has a coil L, which is for
supplying the signal electrode 52a of the optical modulator with
the bias voltage Vb1, and a capacitor C for inputting a
high-frequency signal from the modulator to the terminator 58b. A
bias tee 59, which consists of a coil L and capacitor C, supplies
the other signal electrode 52b of the modulator with a bias voltage
Vb2. A terminator 60 is connected to the bias tee 59. Drive
circuits 61, 62 input the drive signals SD, SD', which are output
by the drive signal generator 53, to the respective signal
electrodes of the optical modulator 52, thereby driving the
modulator.
[0127] In the first embodiment, the amplitude of the modulator
driving voltage signal applied to the optical modulator 52 is
2.multidot.V.pi. (the voltage between the two light-emission
culminations A, A of the voltage--optical output characteristic).
As a result, the modulator performs push-pull modulation, in which
drive signals [see (a) and (d) in FIG. 8] of mutually inverted
amplitude V.pi. output by the drive circuits 61, 62 are input to
the optical modulator 52. Chirping of the optical modulated signal
is made zero by this push-pull modulation and degradation of the
transmitted waveform can be reduced.
[0128] The voltage plotted along the horizontal axis of the
voltage--optical output characteristic (FIG. 1) of the optical
modulator 52 is not the absolute value of the potentials of both
electrodes, but is the potential difference between these
electrodes. The bias voltage Vb2 corresponding to the drive circuit
62 therefore is fixed at zero (or at another constant voltage)
using the bias tee 59. Only the bias voltage Vb1 corresponding to
the drive circuit 61 is controlled based upon operating-point
drift. Further, low-frequency amplitude modulation is performed
using the low-frequency signal SLF, which is output by the
low-frequency oscillator 54, superimposed solely on the drive
signal from the drive circuit 61 by the low-frequency superimposing
unit 55.
[0129] Capacitances C1, C2 and C3 interrupt the bias voltages
applied to the signal electrodes of the optical modulator at the
indicated positions. It is required that the capacitance C3 be made
a sufficiently large value so as to be capable of passing the
low-frequency signal SLF.
[0130] The centers of the output signals from the low-frequency
superimposing unit 55 and drive circuit 62 agree with the bias
voltages Vb1, Vb2 (=0 V) of the optical modulators 52a, 52b, and
therefore the signal waveforms become as shown in (c) and (e) of
FIG. 8. As a result, the modulator driving voltage signal applied
to the optical modulator 52 has the amplitude 2.multidot.V.pi.
[indicated by (c)-(e) in FIG. 8], which corresponds to the
potential difference across both electrodes, as well as the
envelopes EU, EL, on the ON and OFF sides, modulated by the low
frequency f.sub.0 and at the same phase.
[0131] If the operating point of the optical modulator 52 drifts
from the optimum value, a low-frequency signal component having a
phase conforming to the direction of drift is produced in the
optical signal output by the optical modulator 52. From this point
onward, therefore, the bias voltage Vb1 of the optical modulator is
controlled in a direction that will cancel out this low-frequency
component. More specifically, the optical branching unit 56
branches part of the optical signal output from the optical
modulator 52, the photodiode 57a converts this optical signal to an
electrical signal, and the amplifying circuit 57b inputs the
electrical signal to the phase comparator 57c upon amplifying the
signal to the required amplitude. The phase comparator 57c, to
which the low-frequency signal SLF output by the low-frequency
oscillator 54 and the electrical signal conforming to the
photodiode output are input, extracts the low-frequency signal from
the photodiode output by a phase comparison and inputs the
extracted low-frequency signal to the bias supply circuit 58. The
latter controls the bias voltage Vb1 in such a direction that the
low-frequency signal component in the photodiode output will be
come zero.
[0132] FIG. 7 illustrates a method in which low frequency is
superimposed solely upon the drive circuit 61. However, it is also
possible to subject the drive signals from both drive circuits 61,
62 to similar low-frequency amplitude modulation simultaneously at
phases that are the opposite of each other. In such case the
low-frequency modulated amplitude of the modulator driving voltage
signal indicated at (c)-(e) in FIG. 8 would double.
[0133] FIG. 9 illustrates a first modification of the optical
modulation apparatus according to the first embodiment. Components
identical with those shown in FIG. 7 are designated by like
reference characters. In the first embodiment, the center voltage
of the drive signal SD is varied by the low-frequency signal of
frequency f.sub.0 at the input side of the optical modulator 52. In
this modification, however, the bias voltage input to the signal
electrode 52c can be varied by the low-frequency signal of
frequency f.sub.0. The modification of FIG. 9 differs from the
first embodiment in the following respects:
[0134] (1) The output terminal of the low-frequency oscillator 54
and the output terminal of the bias supply circuit 58 are connected
to vary the bias voltage Vb1 by the low-frequency signal of
frequency f.sub.0.
[0135] (2) The bias voltage Vb1, the amplitude of which is varied
by the low-frequency, is input to the signal electrode 52c of the
optical modulator 52 via the low-frequency superimposing unit 55.
The capacitances C1, C2 and C3 interrupt the bias voltages, which
are applied to the signal electrodes of the optical modulator, at
the indicated positions, thereby preventing input of these voltages
to the drive circuits and low-frequency oscillator.
[0136] The signal waveforms associated with the arrangement of FIG.
9 are identical with those of the first embodiment, as illustrated
in FIG. 10. That is, the modulator driving voltage signal [see
(c)-(e) in FIG. 10] applied to the optical modulator 52 has the
amplitude 2.multidot.V.pi. as well as the envelopes EU, EL, on the
ON and OFF sides, both modulated by the low frequency f.sub.0 and
at the same phase. Subsequent control of the operating point is the
same as in the first embodiment.
[0137] FIG. 11 illustrates a second modification of the optical
modulation apparatus according to the first embodiment. Components
identical with those shown in FIG. 7 are designated by like
reference characters. In both the first embodiment and first
modification, the signal electrodes of the optical modulator 52
that apply the modulating signals are common with the bias
electrodes that apply the center voltage. However, these electrodes
can be provided as separate electrodes for drive signals and for
the bias voltages. Providing these electrodes as separate
electrodes makes it possible to eliminate the capacitances C1, C2
for interrupting the bias voltages.
[0138] The modification of FIG. 11 differs from the first
embodiment in the following respects:
[0139] (1) The output terminal of the low-frequency oscillator 54
and the output terminal of the bias supply circuit 58 are connected
to vary the bias voltage Vb1 by the low-frequency signal of
frequency f.sub.0.
[0140] (2) The electrode 52c is separated into electrodes
52c.sub.1, 52c.sub.2 for the drive signal and bias voltage,
respectively, and the electrode 52d is separated into electrodes
52d.sub.1, 52d.sub.2 for the drive signal and bias voltage,
respectively.
[0141] (3) The drive signals output by the drive circuits 61, 62
are input to the signal electrodes 52c.sub.1, 52d.sub.1,
respectively.
[0142] (4) The bias voltage Vb1 whose amplitude is varied by the
low-frequency signal is input to the bias voltage electrode
52c.sub.2 of the optical modulator 52 via the low-frequency
superimposing unit 55, and the bias voltage Vb2 (=0) is input to
the bias voltage electrode 52d.sub.2.
[0143] (5) The capacitances C1 and C2 are deleted.
[0144] The bias voltage output by the low-frequency superimposing
unit 55 has a waveform upon which the low-frequency signal is
superimposed, as shown at (c) in FIG. 8. As a result of separately
providing the electrodes for the drive signals and the electrodes
for the bias voltages, a modulator driving voltage signal indicated
at (a)+(c)-(d) in FIG. 12 enters the optical modulator 52. The
modulator driving voltage signal possesses a waveform having the
amplitude 2.multidot.V.pi. as well as the envelopes EU, EL, on the
ON and OFF sides, modulated by the low frequency f.sub.0 and at the
same phase.
[0145] In FIG. 11, the electrodes in the arrangement of the first
modification (FIG. 9) are separated into electrodes for drive
signals and electrodes for bias voltages. However, the electrodes
in the arrangement of the first embodiment shown in FIG. 7 can also
be separated into electrodes for drive signals and electrodes for
bias voltages.
[0146] (b) Second embodiment
[0147] FIG. 13 is a diagram showing the construction of an optical
modulation apparatus according to a second embodiment, and FIG. 14
shows the associated signal waveforms. The second embodiment
differs from the first embodiment in the method of superimposing
the low-frequency signal. Components identical with those shown in
FIG. 7 are designated by like reference characters.
[0148] According to the first embodiment, the arrangement is such
that the input side of the optical modulator 52 is provided with
the low-frequency superimposing unit 55 to vary the center voltage
of the drive signal SD by the low-frequency signal. According to
the second embodiment, however, the gains of the drive circuits 61,
62 are varied by a low-frequency signal, thereby amplitude
modulating the drive signals by the low-frequency signal.
[0149] Shown in FIG. 13 are the semiconductor laser (DFB-LD) 51,
and the MZ-type optical modulator 52, the drive signal generator 53
which generates the two mutually complimentary drive signals
(push-pull drive signals) SD, SD' [(a), (e) in FIG. 14] having the
amplitude V.pi., the low-frequency oscillator 54 for generating the
low-frequency signal SLF of frequency f.sub.0, and an amplitude
modulating signal generator 55', to which the low-frequency SLF is
input, for generating two amplitude modulating signals SAM.sub.1,
SAM.sub.2 [(c), (f) in FIG. 14] whose phases are displaced from
each other by 180.degree.. The amplitude modulating signal
generator 55' functions as low-frequency superimposing means for
superimposing a low-frequency signal upon the drive signals SD,
SD'.
[0150] Also shown in FIG. 13 are the optical branching unit 56 for
branching the optical signal output by the MZ-type optical
modulator 52, the photodiode 57a, the amplifying circuit 57b for
amplifying the output of the photodiode 57a, the phase comparator
57c for detecting the low-frequency signal component contained in
the photodiode output and outputting the low-frequency signal
component as a signal indicative of the drift of the operating
point of the modulator, and the low-pass filter 57d for smoothing
the output of the phase comparator. Also shown are the bias supply
circuit (operating-point controller) 58 for controlling the
position of the operating point by controlling the bias voltage
Vb1, which is applied to the signal electrode 52c, based upon the
low-frequency signal component contained in the photodiode output,
namely the drift of the operating point of the optical modulator
52. The drive circuits 61, 62 input the drive signals SD, SD',
which are output by the drive signal generator 53, to the
respective signal electrodes 52c, 52d of the optical modulator 52,
thereby driving the modulator. The drive circuits 61, 62 have gain
control terminals to which the amplitude modulating signals
SAM.sub.1, SAM.sub.2, respectively, from the amplitude modulating
signal generator 55' are applied. The capacitances C1, C2 interrupt
the bias voltages, which are applied to the respective signal
electrodes of the modulator, at the positions they occupy.
[0151] By mutually inverting the amplitude modulating signals
SAM.sub.1, SAM.sub.2, which are applied to the drive circuits 61,
62, respectively, in the manner shown at (c) and (f) in FIG. 14,
the drive circuits 61 and 62 are made to output drive signals
indicated at (d) and (g), respectively, in FIG. 14. As a result,
the modulator driving voltage signal applied to the optical
modulator 52 becomes a potential difference [indicated at (d)-(g)
in FIG. 14] that is applied between the signal electrodes 52c, 52d.
This is the same as the waveform of the first embodiment shown in
FIG. 8. Subsequent control of the operating point is the same as in
the first embodiment.
[0152] In FIG. 14, "1", "0" correspond to the logic of the input
electrical signal. Because of push-pull drive, the drive signal (g)
takes on the "1" level at the instant the drive signal (d) assumes
the "1" level, and therefore the envelope EU indicated at (d)-(g)
becomes as shown at d1-g1. Similarly, the drive signal (g) takes on
the "0" level at the instant the drive signal (d) assumes the "0"
level, and therefore the envelope EL indicated at (d)-(g) becomes
as shown at d0-g0. In FIG. 14, (e) is a signal that is the inverse
of (a).
[0153] FIG. 15 illustrates a modification of the optical modulation
apparatus according to the second embodiment. Components identical
with those shown in FIG. 13 are designated by like reference
characters. In the second embodiment, the electrodes for entering
the drive signals are common with the electrodes for entering the
bias voltages. However, these electrodes are provided as separate
electrodes for drive signals and for the bias voltages in this
modification. Providing these electrodes as separate electrodes
makes it possible to eliminate capacitances for interrupting the
bias voltages.
[0154] The modification of FIG. 15 differs from the second
embodiment in the following aspects:
[0155] (1) The electrode 52c is separated into electrodes
52c.sub.1, 52c.sub.2 for the drive signal and bias voltage,
respectively, and the electrode 52d is separated into electrodes
52d.sub.1, 52d.sub.2 for the drive signal and bias voltage,
respectively.
[0156] (2) The drive signals output by the drive circuits 61, 62
are input to the signal electrodes 52c.sub.1, 52d.sub.1,
respectively.
[0157] (3) The bias voltage Vb1 (=Vb) is input to the bias voltage
electrode 52c.sub.2 of the optical modulator 52, and the bias
voltage Vb2 (=0) is input to the bias voltage electrode
52d.sub.2.
[0158] (4) The capacitances C1 and C2 are deleted.
[0159] By mutually inverting the amplitude modulating signals
SAM.sub.1, SAM.sub.2, which are applied to the drive circuits 61,
62, respectively, in the manner shown at (c) and (f) in FIG. 16,
the drive circuits 61 and 62 are made to output drive signals
indicated at (d) and (g), respectively, in FIG. 16. As a result of
separately providing electrodes for the drive signals and bias
voltages in the modification of FIG. 15, the modulator driving
voltage signal applied to the optical modulator 52 takes on a value
obtained by adding the bias voltage Vb1 (=Vb) of the bias electrode
52c.sub.2 to the potential difference input between the two signal
electrodes 52c.sub.1, 52d.sub.1. The waveform of this modulator
driving voltage signal is as indicated at (d)+(h)-(g) in FIG. 16.
This is a waveform similar to that of the second embodiment.
[0160] (c) Third embodiment
[0161] FIG. 17 is a diagram showing the construction of an optical
modulation apparatus according to a third embodiment. Components
identical with those of the second embodiment shown in FIG. 13 are
designated by like reference characters. In the second embodiment,
the drive signals SD, SD' are both amplitude modulated by
low-frequency signals, whereby the ON and OFF sides of the
modulator driving voltage signal are modulated by low-frequency
signals having the same phase. In the third embodiment, only one of
the drive signals SD, SD' is amplitude modulated by a low-frequency
signal, whereby only the ON side or OFF side of the modulator
driving voltage signal is modulated by the low-frequency
signal.
[0162] The third embodiment shown in FIG. 17 differs from the
second embodiment of FIG. 13 in that the low-frequency signal SLF
of frequency f.sub.0 is input to the gain control terminal of the
drive circuit 61 as the amplitude modulating signal SAM.sub.1; the
gain of the drive circuit 62 is not controlled. If the amplitude
modulating signal SAM.sub.1 is input to the gain control terminal
of the drive circuit 61, the gain of this drive circuit changes. As
a result, the drive circuit 61 outputs a drive signal of the kind
shown at (c) in FIG. 18. Since the gain of the other drive circuit
62 remains constant, however, this drive circuit outputs a drive
signal of the kind indicated at (e) in FIG. 18, the center of this
signal being the bias voltage Vb2 (=0). As a result, the modulator
driving voltage signal applied to the optical modulator 52 develops
a potential difference [indicated at (c)-(e) in FIG. 18] that is
applied between the signal electrodes 52c, 52d. This is the
waveform shown in FIG. 3. Accordingly, from this point onward the
operating point is controlled in such a manner that the
low-frequency signal component of frequency f.sub.0 contained in
the optical signal output by the optical modulator 52 will become
zero.
[0163] (d) Fourth embodiment
[0164] FIG. 19 is a diagram showing the construction of an optical
modulation apparatus according to a fourth embodiment. Components
identical with those of the second embodiment shown in FIG. 13 are
designated by like reference characters. In the second embodiment,
the drive signals SD, SD' are modulated respectively by the
low-frequency signal SAM.sub.1 and the low-frequency signal
SAM.sub.2 obtained by inverting the signal SAM.sub.1, whereby the
ON and OFF sides of the modulator driving voltage signal are
modulated by low-frequency signals having the same phase. In the
fourth embodiment, the drive signals SD, SD' are amplitude
modulated by the low-frequency signals SAM.sub.1, SAM.sub.2 of
identical phase but different amplitude, whereby the ON and OFF
sides of the modulator driving voltage signal are modulated by
low-frequency signals of identical phase but different
amplitudes.
[0165] The fourth embodiment of FIG. 19 differs from the second
embodiment of FIG. 13 in the following respects:
[0166] (1) First and second amplitude modulating signal generators
55a, 55b constituted by amplifiers having different gains are
provided instead of the amplitude modulating signal generator 55',
and the low-frequency signal SLF is input to each of these signal
generators.
[0167] (2) The amplitude modulating signal SAM.sub.1 output by the
first amplitude modulating signal generator 55a is input to the
gain control terminal of the drive circuit 61, and the amplitude
modulating signal SAM.sub.2 output by the second amplitude
modulating signal generator 55b is input to the gain control
terminal of the drive circuit 62.
[0168] By varying the amplitudes of the amplitude modulating
signals SAM.sub.1, SAM.sub.2, which are applied to the drive
circuits 61, 62, respectively, as shown at (c) and (f) in FIG. 20,
the drive circuits 61, 62 output the drive signals indicated at (d)
and (g), respectively, in FIG. 20. As a result, the modulator
driving voltage signal applied to the optical modulator 52 develops
a potential difference [indicated at (d)-(g) in FIG. 20] that is
applied between the signal electrodes 52c, 52d. This is the
waveform shown in FIG. 4. Accordingly, from this point onward the
operating point is controlled in such a manner that the
low-frequency signal component of frequency f.sub.0 contained in
the optical signal output by the optical modulator 52 will become
zero.
[0169] (e) Fifth embodiment
[0170] FIG. 21 is a diagram showing the construction of an optical
modulation apparatus according to a fifth embodiment. Components
identical with those of the second embodiment shown in FIG. 13 are
designated by like reference characters. In the fifth embodiment,
the drive signals SD, SD' are modulated respectively by the
low-frequency signal SAM.sub.1 and the low-frequency signal
SAM.sub.2 obtained by inverting the signal SAM.sub.1, whereby the
ON and OFF sides of the modulator driving voltage signal are
modulated by low-frequency signals having the same phase. In the
fifth embodiment, the drive signals SD, SD' are amplitude modulated
by the amplitude modulating signals SAM.sub.1, SAM.sub.2 having
different frequencies, whereby the ON and OFF sides of the
modulator driving voltage signal are modulated by signals of
different frequencies.
[0171] The fifth embodiment of FIG. 21 differs from the second
embodiment of FIG. 13 in the following respects:
[0172] (1) First and second low-frequency signal oscillators 54a,
54b for generating low-frequency signals of frequencies f.sub.1 and
f.sub.2, respectively, are provided.
[0173] (2) A low-frequency signal SLF.sub.1 of frequency f.sub.1 is
input to the gain control terminal of the drive circuit 61 as the
amplitude modulating signals SAM.sub.1, and a low-frequency signal
SLF.sub.2 of frequency f.sub.2 is input to the gain control
terminal of the drive circuit 62 as the amplitude modulating
signals SAM.sub.2.
[0174] (3) First and second phase comparators 57c.sub.1, 57c.sub.2
are provided. The inputs to the phase comparators 57c.sub.1,
57c.sub.2 are the low-frequency signals SLF.sub.1, SLF.sub.2 output
by the low-frequency signal oscillators 54a, 54b, respectively, as
well as the electrical signal conforming to the photodiode output
signal. The first and second phase comparators 57c.sub.1, 57c.sub.2
detect and output the low-frequency signal components of
frequencies f.sub.1, f.sub.2, respectively, contained in the
photodetector output.
[0175] (4) Low-pass filters 57d.sub.1, 57d.sub.2 for smoothing the
signals output by the first and second phase comparators 57c.sub.1,
57c.sub.2 are provided.
[0176] (5) An averaging circuit 57e is provided. This circuit
calculates the average value of the low-frequency components of
frequencies f.sub.1, f.sub.2, which are contained in the
photodetector output signal, and inputs the average value to the
bias supply circuit 58.
[0177] (6) The bias supply circuit 58 controls the bias voltage in
such a manner that the average value becomes zero.
[0178] When the amplitude modulating signal SAM.sub.1 of frequency
f.sub.1 is input to the gain control terminal of the drive circuit
61, the gain of the drive circuit 61 changes and the latter outputs
a drive signal of the kind indicated at (d) in FIG. 22. When the
amplitude modulating signal SAM.sub.2 of frequency f.sub.2 is input
to the gain control terminal of the drive circuit 62, the gain of
the drive circuit 62 changes and the latter outputs a drive signal
of the kind indicated at (g) in FIG. 22. As a result, the modulator
driving voltage signal applied to the optical modulator 52 becomes
a potential difference [indicated at (d)-(g) in FIG. 22] that is
applied between the signal electrodes 52c, 52d. This is the same as
the waveform shown in FIG. 5. From this point onward, therefore,
the operating point is controlled in such a manner that the
low-frequency signal components of frequencies f.sub.1, f.sub.2
contained in the optical signal output by the optical modulator 52
will become zero. For example, if the bias point of optical
modulator 52 shifts from the optimum value, both signal components
of the low-frequencies f.sub.1, f.sub.2 appear in the optical
signal and the phase of each signal gives a direction of control
for changing the bias point to the optimum position. Accordingly,
the averaging circuit 57e calculates the average value of both
signal components, and bias control is performed in such a manner
that this average value will become zero. This makes it possible to
improve the precision of control.
[0179] (f) Sixth embodiment
[0180] FIG. 23 is a diagram showing the construction of an optical
modulation apparatus according to a sixth embodiment. Components
identical with those of the second embodiment shown in FIG. 13 are
designated by like reference characters. In the second embodiment,
the drive signals SD, SD' are modulated respectively by the
low-frequency signal SAM.sub.1 and the low-frequency signal
SAM.sub.2 obtained by inverting the signal SAM.sub.1, whereby the
ON and OFF sides of the modulator driving voltage signal are
modulated by low-frequency signals having the same phase. In the
sixth embodiment, the drive signals SD, SD' are modulated
respectively by the low-frequency signal SAM.sub.1 and
low-frequency inverted signal SAM.sub.2 of different phases,
whereby the ON and OFF sides of the modulator driving voltage
signal are modulated by low-frequency signals of different
phases.
[0181] The sixth embodiment of FIG. 23 differs from the second
embodiment of FIG. 13 in the following respects:
[0182] (1) A first delay circuit 71 for delaying the low-frequency
signal SLF of frequency f.sub.0 by a prescribed time T is provided,
and the delayed signal output by the delay circuit 71 is input to
the gain control terminal of the drive circuit 62 as the amplitude
modulating signals SAM.sub.2.
[0183] (2) A second delay circuit 72 for delaying the low-frequency
signal SLF by half the delay time of the first delay circuit 71
(i.e., by T/2) and inputting the delayed signal to the phase
comparator 57c is provided.
[0184] (3) The phase comparator 57c senses the direction of a shift
in the bias point of the optical modulator by comparing the phase
of the low-frequency signal output by the delay circuit 72, whose
delay is T/2, and the phase of the low-frequency signal component
in the optical signal.
[0185] When the amplitude modulating signal SAM.sub.1, which is
obtained by inverting the low-frequency signal SLF, is input to the
gain control terminal of the drive circuit 61, the gain of the
drive circuit changes. As a result, the drive circuit 61 outputs a
drive signal of the kind indicated at (c) in FIG. 24. When the
amplitude modulating signal SAM.sub.2, which is obtained by
delaying the phase of the low-frequency signal SLF by T, is input
to the gain control terminal of the drive circuit 62, the gain of
the drive circuit 62 changes and the drive circuit 62 outputs a
drive signal of the kind indicated at (g) in FIG. 24. As a result,
the modulator driving voltage signal applied to the optical
modulator 52 becomes a potential difference [indicated at (d)-(g)
in FIG. 24] that is applied between the signal electrodes 52c, 52d.
This is the same as the waveform of the first embodiment shown in
FIG. 6. Accordingly, from this point onward the operating point is
controlled in such a manner that the low-frequency signal component
of frequency f.sub.0 contained in the optical signal output by the
optical modulator 52 will become zero.
[0186] For example, if the bias point of the optical modulator 52
drifts, a low-frequency signal component delayed by T/2, which is a
phase delay that conforms to the direction of the shift, appears in
the optical signal. This makes it possible to sense the direction
of the shift in the bias point of the optical modulator by
comparing the phase of the low-frequency signal, which enters via
the delay circuit 72 that applies the delay T/2, and the phase of
the low-frequency signal contained in the optical signal. (
[0187] g) Seventh embodiment
[0188] FIG. 25 is a diagram showing the construction of an optical
modulation apparatus according to a seventh embodiment. Components
identical with those of the first embodiment shown in FIG. 7 are
designated by like reference characters. The associated signal
waveforms are identical with those of the first embodiment shown in
FIG. 8.
[0189] In the first embodiment, a frequency component identical
with the frequency f.sub.0 generated by the low-frequency
oscillator 54 is detected from the optical signal to control the
operating point. However, as will be understood from FIGS. 2 to 6,
when the operating point of the optical modulator is at the optimum
value, a low-frequency signal component whose frequency is twice
the frequency f.sub.0 (i.e., 2.multidot.f.sub.0) appears in the
optical signal and this signal component takes on a maximum value.
According to the seventh embodiment, therefore, the low-frequency
signal component of frequency 2.multidot.f.sub.0 contained in the
optical signal is detected and the operating point is controlled so
as to maximize this signal component.
[0190] As shown in FIG. 25, the arrangement of the first embodiment
is additionally provided with a frequency multiplier 73 for
doubling the frequency f.sub.0 of the low-frequency signal SLF
output by the low-frequency oscillator 54. The low-frequency signal
of frequency 2.multidot.f.sub.0 output by the frequency multiplier
73 and the electrical signal conforming to the optical signal
output by the optical modulator are input to the phase comparator
57c, which proceeds to detect the low-frequency of frequency
2.multidot.f.sub.0 in the optical signal by phase comparison. The
bias supply circuit 58 controls the bias voltage, which is applied
to the signal electrode 52c of the optical modulator, so as to
maximize the low-frequency signal component.
[0191] (h) Eighth embodiment
[0192] According to the foregoing embodiments, the modulation
apparatus generates the two mutually complimentary drive signals
(push-pull drive signals) SD, SD' each having the amplitude V.pi.
between the light-emission culmination A and the neighboring
light-extinction culmination B of the voltage--optical output
characteristic of the optical modulator 52, and inputs these drive
signals to the respective signal electrodes of the optical
modulator, which is of the type driven on both sides thereof,
whereby a modulator driving voltage signal of 2.multidot.V.pi. is
applied to the optical modulator. However, if the objective is to
eliminate chirping of the optical modulated signal by push-pull
modulation and reduce transmission waveform degradation, it is not
always necessary to apply a modulator driving voltage signal of
2.multidot.V.pi. to the optical modulator. According to an eighth
embodiment, therefore, two complimentary drive signals SP, SP' each
of amplitude V.pi./2 are generated and the signals SP, SP' are
input to the respective signal electrodes of the optical modulator,
which is of the type driven on both sides thereof, whereby a
modulator driving voltage signal of V.pi. is applied to the optical
modulator to generate an NRZ optical signal or an RZ optical
signal.
[0193] FIG. 26 is a diagram showing the construction of an optical
modulation apparatus according to an eighth embodiment, and FIG. 27
is a waveform diagram of the associated signal waveforms.
Components identical with those of the first embodiment shown in
FIG. 7 are designated by like reference characters. The eighth
embodiment of FIG. 26 differs from the first embodiment of FIG. 7
in the following respects:
[0194] (1) A push-pull drive signal generator 74 for generating two
complimentary drive signals SP, SP' each of amplitude V.pi./2 is
provided.
[0195] (2) The low-frequency superimposing unit 55 superimposes the
low-frequency signal of frequency f.sub.0 upon the drive signal SP
in such a manner that the phases of the envelopes EU, EL on the ON
and OFF sides, respectively, of the modulator driving voltage
signal will be displaced from each other by 180.degree.[see (c) of
FIG. 27].
[0196] (3) The amplitude of the modulator driving voltage signal is
made V.pi. and the phases of the envelopes EU, EL on the ON and OFF
sides, respectively, of the modulator driving voltage signal are
displaced from each other by 180.degree.[see (d)-(f) of FIG.
27].
[0197] The center of the output signal from the low-frequency
superimposing unit 55 and the center of the output signal from the
drive circuit 62 are made to coincide with the bias voltages Vb1
and Vb2 (=0V), respectively, of the signal electrodes 52c, 52d,
respectively. These output signal waveforms, therefore, are as
shown at (d) and (f) of FIG. 27. As a result, the modulator driving
voltage signal applied to the optical modulator 52 has the
amplitude V.pi. [indicated by (d)-(f) in FIG. 27], which
corresponds to the potential difference across both electrodes, as
well as the envelopes EU, EL, on the ON and OFF sides, modulated by
the low frequency f.sub.0 and at the 180.degree.phase
difference.
[0198] If the operating point of the optical modulator 52 drifts
from the optimum value, a low-frequency signal component having a
phase conforming to the direction of drift is produced in the
optical signal output by the optical modulator 52. From this point
onward, therefore, the bias voltage Vb1 of the optical modulator is
controlled in a direction that will cancel out this low-frequency
component.
[0199] Thus, in accordance with the eighth embodiment, chirping of
the optical modulated signal is made zero by push-pull modulation,
degradation of a transmission waveform can be reduced and drift of
the operating point can be compensated for by control of bias
voltage.
[0200] According to the eighth embodiment, the operating point is
controlled upon detecting, from the optical signal, a frequency
component identical with the frequency f.sub.0 generated by the
low-frequency oscillator 54. However, it is also possible to adopt
an arrangement similar to that of the seventh embodiment (see FIG.
25), in which the low-frequency signal component of frequency
2.multidot.f.sub.0 contained in the optical signal is detected and
the operating point is controlled in such a manner that this signal
component takes on the maximum value.
[0201] Further, as described above, the low-frequency signal of
frequency f.sub.0 is superimposed upon the drive signal SP in such
a manner that the phases of the envelopes EU, EL on the ON and OFF
sides, respectively, of the modulator driving voltage signal will
be displaced from each other by 180.degree.. However, it is also
possible to adopt the following arrangements:
[0202] (1) a low-frequency signal is superimposed upon the drive
signal SP or SP' in such a manner that only one envelope of the
envelopes EU, EL on the ON and OFF sides, respectively, of the
modulator driving voltage signal will vary, or
[0203] (2) a low-frequency signal is superimposed upon the drive
signals SP or SP' in such a manner that the amplitudes of the
envelopes EU, EL on the ON and OFF sides, respectively, of the
modulator driving voltage signal will differ, or
[0204] (3) a low-frequency signal is superimposed upon the drive
signals SP or SP' in such a manner that the frequencies of the
envelopes EU, EL on the ON and OFF sides, respectively, of the
modulator driving voltage signal will differ, or
[0205] (4) a low-frequency signal is superimposed upon the drive
signals SP or SP' in such a manner that the phases of the envelopes
EU, EL on the ON and OFF sides, respectively, of the modulator
driving voltage signal will differ, and the operating point is
controlled upon detecting a frequency component, which is identical
with the frequency f.sub.0, from the optical signal. When the
operating point is controlled by any of these methods, the
arrangements of the second through sixth embodiments can be
applied.
[0206] (i) Ninth embodiment
[0207] The periodicity of the voltage--optical output
characteristic of the optical modulator is such that
2.multidot.V.pi. is equal to one period of voltage. Accordingly,
the optical modulation apparatus can be provided with a function
for changing over the range of drive in terms of the
voltage--optical output characteristic of the optical
modulator.
[0208] For example, in a case where modulation is carried out by a
driving amplitude of V.pi. in NRZ modulation, the optical modulator
is provided with a function for shifting the bias voltage by V.pi.
between VbA and VbB, as shown in FIG. 28A, and the range of drive
voltages is made to change from A to B by this shifting of the bias
voltage. This shifting of the operating point can be applied
directly to the optical modulation apparatus (see FIG. 26) of the
eighth embodiment, which generates the NRZ, RZ signals using the
optical modulator that is driven on both sides.
[0209] Further, in a case where modulation, such as optical
duobinary modulation, is performed by a driving amplitude of
2.multidot.V.pi., the optical modulation apparatus is provided with
a function for shifting the bias voltage by 2.multidot.V.pi.
between VbA and VbB, as shown in FIG. 28B, and the range of drive
voltages is made to change from A to B by this shifting of the bias
voltage.
[0210] The above-described changeover of the operating point can be
applied to a case where chirping is set to a direction advantageous
for transmission and to a case where it is necessary to select a
range in which the voltage--optical output characteristic has the
proper shape. The changeover can be realized by intentionally
shifting the bias voltage by a fixed amount in response to an
externally applied changeover signal.
[0211] FIG. 29 is a diagram showing the construction of an optical
modulation apparatus according to a ninth embodiment having an
operating-point shifting function. Components identical with those
of the first embodiment shown in FIG. 7 are designated by like
reference characters.
[0212] An operating-point changeover circuit 81 shifts the bias
voltage by a fixed amount of voltage in response to an externally
applied changeover signal CS, thereby changing over the range of
drive of the voltage--optical output characteristic.
[0213] An operating-point reset circuit 82 forcibly resets the bias
point to zero in response to an operating-point reset signal RS. It
is necessary to reset the bias point (1) when the system is
initially put into operation and (2) when bias-point drift becomes
so large during system operation that the bias voltage of the
modulator, which is controlled to achieve stability, exceeds an
allowable range. In such cases the operating-point reset circuit 82
forcibly resets the bias point to zero by the operating-point reset
signal RS, which enters from the outside.
[0214] As shown in FIG. 29, the operating-point changeover circuit
81 is constituted by a fixed-voltage power supply (although a
variable-voltage power supply is possible) and a switching
arrangement for switching the bias supply line, and the
operating-point reset circuit 82 is constituted by a point of
ground potential (GND) and a switching arrangement for switching
the bias supply line. However, other methods are acceptable as long
as the same operating-point changeover function and operating-point
reset function can be implemented. Further, the operating-point
changeover signal CS or operating-point reset signal RS is entered
as necessary and the changeover of the operating point by the
operating-point changeover circuit or reset of the operating point
by the operating-point reset circuit is performed in accordance
with the entered signal.
[0215] It should be noted that the components of FIG. 29 for
shifting the operating point and for resetting the operating point
can be applied to the eighth embodiment of FIG. 26 as is.
[0216] (j) Position of photodetector
[0217] In each of the foregoing embodiments, the optical modulator
is externally provided with the optical branching unit 56 and
photodiode 57a. However, the same function can be achieved by
incorporating a photodiode 57a in the LiNbO.sub.3 substrate 52 of
the optical modulator 52 and detecting the light intensity of the
light emission produced within the optical modulator, as shown in
FIG. 30A. (See ECOC'97 vol. 2, pp. 167-170, Y. Kubota et al.,
"10Gb/s Ti; LiNbO3 Mach-Zehnder modulator with Built-in Monitor
Photodiode Chip".)
[0218] Specifically, at extinction of the MZ-type optical modulator
52, light energy is not actually extinguished even though light
waves, displaced in phase by 180.degree., that propagate through
branched optical waveguides 52a, 52b are combined. Rather, a
combining of modes takes place owing to the width of the optical
waveguides, and radiated light ascribable to surplus modes radiates
to the exterior of the optical waveguides from interference points.
If viewed from directly above the substrate, the radiated light
radiates along an extension line of a branched optical waveguide
52a', as shown in FIG. 30B. Accordingly, a hole HL is cut into the
substrate at a prescribed position along the extension line, the
photodiode 57a is imbedded within the hole HL and is electrically
wired. If this arrangement is adopted, an optical branching unit
and external photodiode can be dispensed with, thereby making it
possible to simplify structure.
[0219] (k) Arrangement for dealing with input light of any
polarization
[0220] If the fiber routing between the light source and optical
modulator is of great length or the arrangement is not one in which
polarization is fixed, then it is necessary to adopt an arrangement
in which the optical modulator modulates light of any polarization.
FIG. 31 shows an example of the construction of an MZ-type optical
modulator capable of dealing with such a situation. Shown in FIG.
31 are two branched optical waveguides 52a, 52b within the optical
modulator, electrodes 52c, 52d to which electrical signals for
modulating the optical signals in the respective optical waveguides
are input, and half-wave plates 91, 92 inserted in the middles of
the respective optical waveguides. The half-wave plates are
obtained by cutting holes into the substrate at the optical
waveguides and filling the holes with a material exhibiting
birefringence. The width of the half-wave plates is decided in such
a manner that the optical path difference between the polarization
modes caused by birefringence will be half the signal
wavelength.
[0221] The efficiency of phase modulation in the optical waveguides
of the optical modulator is better in the TM mode than in the TE
mode. If light of any polarization having mixed TE-mode and TM-mode
components enters the modulator, the TM-mode components undergo
phase modulation along the first half of the optical waveguides
52a, 52b (in front of the half-wave plates), after which they are
converted to TE-mode components by the half-wave plates 91, 92. The
TE-mode components then undergo phase modulation along the second
half of the optical waveguides (in back of the half-wave plates).
Conversely, the TE-mode components do not undergo phase modulation
in front of the half-wave plates, are converted to TM-mode
components by the half-wave plates 91, 92 and then are phase
modulated in the second half of the optical waveguides.
[0222] Accordingly, by giving consideration to design, such as the
electrode length for obtaining the amount of phase modulation
necessary in the first and second halves of the optical waveguides,
modulation can be performed even in cases where light of any
polarization impinges upon the modulator.
[0223] Thus, in accordance with the present invention as described
above, when an optical modulator is driven by an electrical drive
signal, which has an amplitude of 2.multidot.V.pi. between two
light-emission culminations or two light extinction culminations of
a voltage--optical output characteristic, a low-frequency signal
component can be detected reliably from the optical signal output
of the optical modulator by way of a simple arrangement, and
fluctuation of the voltage-optical output characteristic of the
optical modulator, namely drift of the operating point, can be
compensated for using this low-frequency component. By applying the
operating-point control method of the present invention to optical
duobinary modulation, the effects of waveform dispersion can be
reduced. Moreover, push-pull drive makes it possible to reduce
chirping.
[0224] In accordance with the present invention, when an optical
modulator is driven by a driving signal having an amplitude between
a light-emission culmination and a neighboring light-extinction
culmination of a voltage--optical output characteristic, two
complimentary drive signals each having an amplitude of V.pi./2 are
generated and the optical modulator is subjected to push-pull drive
by these drive signals. As a result, chirping can be reduced.
Moreover, low-frequency signal components can be detected reliably
from the optical signal output by the optical modulator, and it is
possible to compensate for drift of the operating point.
[0225] In accordance with the present invention, a range used in
modulation can be shifted in the voltage--optical output
characteristic of an optical modulator. As a result, chirping can
be set to a direction advantageous for transmission, or a range
having the proper shape can be selected from the shape of the
voltage--optical output characteristic, and the modulator can be
driven using this range.
[0226] In accordance with the present invention, an operating point
on the voltage--optical output characteristic of an optical
modulator can be set to a prescribed initial point. Accordingly, in
the event that bias-point drift becomes so large at the start of
system operation or during system operation that the bias voltage
exceeds an allowable range, the bias point can be forcibly reset to
zero and the system restarted.
[0227] In accordance with the present invention, an arrangement is
adopted in which a photodiode is imbedded in the substrate of an
optical modulator, light leaking from an optical waveguide is
detected and low-frequency components are extracted from the
detected light. This makes it possible to dispense with an optical
branching unit, thereby simplifying overall structure.
[0228] In accordance with the present invention, a half-wave plate
is inserted in the middle of a branched optical waveguide on each
side of the modulator. This makes it possible to modulate light of
any polarization.
[0229] In accordance with the present invention, control is
performed so as to maximize a signal component of frequency
2.multidot.f.sub.0 contained in the output of a photodetector,
thereby making it possible to compensate for operating-point drift
that accompanies fluctuation of the voltage--optical output
characteristic of the optical modulator.
[0230] The present invention is such that when an optical modulator
configured to be driven on both its sides is used in optical
duobinary modulation, NRZ modulation or RZ modulation, a
low-frequency signal component can be detected reliably from the
optical signal output of the optical modulator by way of a simple
arrangement, and it is possible to compensate for operating-point
drift that accompanies fluctuation of the voltage--optical output
characteristic of the optical modulator.
[0231] As many apparently widely different embodiments of the
present invention can be made without departing from the spirit and
scope thereof, it is to be understood that the invention is not
limited to the specific embodiments thereof except as defined in
the appended claims.
* * * * *