Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-02-02
2002-03-26
Ben, Loha (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S238000, C359S276000, C359S279000, C359S199200
Reexamination Certificate
active
06362913
ABSTRACT:
BACKGROUND OF THE INVENTION
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.
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.
Direct modulation
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.
External modulation
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.
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
3
a
,
3
b
. The MZ-type modulator
2
includes on an LiNbO
3
substrate, (1) an input optical waveguide
2
a
formed on the substrate for introducing light from the laser diode
1
, (2) branching optical waveguides
2
b
,
2
c
and (3) an output optical waveguide
2
d
formed on the substrate for outputting modulated light; (4) two signal electrodes
2
e
,
2
f
formed on the substrate for applying phase modulation to the optical signals in the branching optical waveguides
2
b
,
2
c
, and (5) a signal input terminal
2
g
formed on the substrate for inputting an NRZ electrical drive signal to one of the signal electrodes, namely the electrode
2
e.
If a voltage applied to the signal electrodes
2
e
,
2
f
is controlled by the “1”s and “0”s of data, the branching optical waveguides
2
b
,
2
c
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
2
b
,
2
c
becomes 180°; 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
2
b
,
2
c
becomes 0°. If this arrangement is adopted, superimposing the optical signals of the two optical waveguides
2
b
,
2
c
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.
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&pgr;. When data is a “1”, therefore, voltage having an amplitude of V&pgr; is applied between the signal electrodes
2
e
,
2
f
, whereby light is emitted. When data is a “0”, a voltage of zero is applied between the signal electrodes
2
e
,
2
f
, whereby light is extinguished.
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
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
).
Bias control method in NRZ modulation
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.
Shown in
FIG. 34
are the semiconductor laser diode (DFB-LD)
1
, the MZ-type optical modulator (LN optical modulator)
2
, the optical fibers
3
a
,
3
b
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&pgr;) 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
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
Ishikawa George
Nakamoto Hiroshi
Nishizawa Yoshinori
Ooi Hiroki
Yamamoto Takuji
Ben Loha
Fujitsu Limited
Staas & Halsey , LLP
LandOfFree
Optical modulation apparatus and method of controlling... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Optical modulation apparatus and method of controlling..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Optical modulation apparatus and method of controlling... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2872221