Optical modulation device having bias reset means

An optical modulation device comprises an optical modulator in which the operating point is varied according to a bias voltage. A phase deviation of an operating point from a selected optimal operating point is detected by a phase comparator based on an output light of the optical modulator. The bias voltage is generated within a predetermined voltage range by a DC amplifier so as to reduce the phase deviation. A reset circuit resets the bias voltage at the ground voltage associated with the selected optimal operating point when the optical modulator is powered up and passes the bias voltage to the optical modulator at all other times.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical modulation devices, and in 
particular to an optical modulation device comprising an optical modulator 
in which a bias voltage causes modulation characteristics to be changed. 
2. Description of the Related Art 
Some optical modulators use the electro-optic or magneto-optic effect to 
modulate an input light wave in amplitude according to an input signal. 
These optical modulators are designed to set an operating point at one of 
optimal operating points according to a predetermined bias voltage applied 
thereto. However, since the modulation characteristics is changed due to 
variations of temperature and deterioration over time, a current operating 
point frequently varies. In order to avoid such a variation, a feedback 
control circuit is generally provided with the optical modulator. More 
specifically, the feedback control circuit monitors a deviation of a 
current operating point from the optimal operating point by using the 
output light of the optical modulator and controls the bias voltage so as 
to operate the optical modulator at the optimal operating point. 
As an example of the feedback control circuit described above, an automatic 
control circuit of a bias voltage applied to a Mach-Zehnder optical 
modulator is disclosed in Japanese Patent Unexamined Publication No. 
4-294318. This feedback control circuit monitors the respective power 
averages of two complementary outputs having phases opposite to each other 
and generates a monitoring voltage corresponding to the difference between 
the two power averages. The bias voltage of the Mach-Zehnder optical 
modulator is adjusted based on the monitoring voltage which is generated 
at the time when the current operating point is optimal. Since the 
monitoring voltage is generated by calculating the difference between the 
respective power averages of the complementary outputs, the power 
difference for use in control is doubled in comparison to a feedback 
control based on a power average of one output, resulting in more reliable 
control of the bias voltage applied to the optical modulator. 
However, since it is not initially determined which operating point is 
selected from the optimal operating points, the following problem arises 
in the conventional control circuit. Generally, the available range of the 
bias voltage is restricted within the power supply voltage supplied to a 
DC amplifier (or a bias voltage generating circuit). Therefore, the bias 
control needs to be performed within the available output voltage range of 
the DC amplifier. In other words, in cases where the optical modulator is 
initially set at an optimal operating point corresponding to a position 
close to the upper or lower limit of the available output voltage range of 
the DC amplifier, there is a possibility that a bias voltage to be applied 
to the optical modulator is above the upper limit or below the lower limit 
of the DC amplifier when a drift of the modulation characteristics occurs 
as mentioned above. As a result, the bias voltage generated by the DC 
amplifier becomes fixed at the upper or lower limit, which causes the 
modulated light output from the optical modulator to be distorted. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an optical modulation 
device which performs optical modulation without distorting the modulated 
light signal even when a drift of the modulation characteristics occurs 
due to temperature variations and deterioration over time. 
Another object of the present invention is to provide a bias control 
circuit and method for setting a bias voltage for use in feedback control 
at an optimal operating point associated with the center of the available 
voltage range of a bias voltage supplying circuit. 
In accordance with an aspect of the present invention, an optical modulator 
is reset at an initial optimal operating point by a predetermined bias 
voltage when initialized and is controlled according to a bias voltage 
obtained based on an output light of the optical modulator at all other 
times. More specifically, the optical modulation device is comprised of an 
optical modulator which modulates an input light wave according to a 
modulating signal at an operating point which is determined by a bias 
voltage. The optical modulator exhibits a predetermined modulation 
characteristic having a plurality of optimal operating points. The optical 
modulation device is further comprised of a deviation detector for 
detecting a deviation of the operating point from a selected optimal 
operating point based on an output light of the optical modulator, a bias 
voltage generator for generating the bias voltage within a predetermined 
voltage range so as to reduce the deviation, and a reset circuit for 
resetting the bias voltage at a predetermined voltage associated with the 
selected optimal operating point when the optical modulator is 
initialized. 
Preferably, the reset circuit is comprised of a timer for timing a 
predetermined time period starting from power-up, and a switch for 
switching the bias voltage to the predetermined voltage during the 
predetermined time period starting from power-up. 
The predetermined voltage is preferably positioned at the center of the 
predetermined voltage range of the bias voltage. More specifically, the 
center voltage of the predetermined voltage range is preset at the ground 
voltage. 
The optical modulator necessitates a bias-controlled modulation 
characteristic such that an operating point is changed by a bias voltage. 
Preferably, a Lithium Niobate material is used in the optical modulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an optical modulation device is comprised of an 
optical modulation section and a bias control section. The optical 
modulation section includes a laser diode 101, an optical modulator 102, a 
drive amplifier 103 and an optical coupler 104. The bias control section 
is comprised of a photodetector 105, a band-pass filter 106, a phase 
comparator 107, a low-frequency oscillator 108, a reset circuit 109 and a 
DC amplifier 110. 
The laser diode 101 outputs a light wave of a predetermined wavelength to 
the optical modulator 102. Receiving the light wave from the laser diode 
101, the optical modulator 102 modulates the light wave in amplitude 
according to a modulating signal MS received from the drive amplifier 103. 
The optical modulator 102 uses the electro-optic effect of Lithium Niobate 
(LiNbO.sub.3) to perform the optical modulation. As another electro-optic 
material, Lithium Tatalate (LiTaO.sub.3) may be used. The optical 
modulator 102 exhibits the modulation characteristics as shown in FIG. 4 
where a plurality of optimal operating points appear repeatedly at 
predetermined intervals. An optimal operating point is initially selected 
and is thereafter adjusted according to a bias voltage Vb received from 
the DC amplifier 110. 
The drive amplifier 103 receives transmission data from an external source 
as well as a low-frequency signal LFS of a sine wave from the 
low-frequency oscillator 108. The low-frequency signal LFS has a frequency 
sufficiently lower than that of the transmission data. Superposing the 
low-frequency signal LFS on the transmission data, the drive amplifier 103 
outputs the modulating signal MS to the optical modulator 102. 
The modulated light wave ML is output from the optical modulator 102 to the 
optical coupler 104 through which the modulated light wave ML is output to 
an external device and to the photodetector 105 as a light wave CL for use 
in the bias control. The light wave CL is received by the photodetector 
105 such as a PIN photodiode which converts the light wave CL into an 
electrical signal CS and outputs it to the band-pass filter 106. The 
band-pass filter 106 allows a low-frequency component LCS to pass while 
stopping others. The low-frequency component LCS has the same frequency as 
the low-frequency signal LFS and is output to the phase comparator 107. 
The phase comparator 107 receives the low-frequency signal LFS from the 
low-frequency oscillator 108 and the low-frequency component LCS from the 
band-pass filter 106. Comparing the low-frequency signal LFS with the 
low-frequency component LCS, the phase comparator 107 outputs a phase 
difference signal Pd to the reset circuit 109. 
The reset circuit 109 selects one of the phase difference signal Pd and the 
ground voltage according to a timer incorporated therein or a control 
signal CTRL. The signal selected by the reset circuit 109 is output to the 
DC amplifier 110. More specifically, the reset circuit 109 outputs the 
ground voltage to the DC amplifier 110 during a preset time period 
starting from power-on. The timer is reset for the time period ranging 
from several hundred milliseconds to 1 second as described later. 
The DC amplifier 110 is designed to generate the bias voltage Vb ranging 
from the negative power supply voltage to the positive according to the 
signal selected by the reset circuit 109. More specifically, when 
receiving the ground voltage from the reset circuit 109, the DC amplifier 
110 fixes the bias voltage Vb at the ground voltage, and when receiving 
the phase difference signal Pd, the DC amplifier 110 adjusts the bias 
voltage Vb according to the phase difference signal Pd. Since the reset 
circuit 109 outputs the ground voltage to the DC amplifier 110 during the 
preset time interval starting from power-on, the ground voltage is output 
as the bias voltage Vb to the optical modulator 102 during that interval. 
In addition, the central voltage of the bias voltage range available in 
the ground voltage is preset at the ground voltage. Receiving the bias 
voltage Vb from the DC amplifier, the operating point of the optical 
modulator 102 is set at a phase position of the modulation characteristics 
corresponding to the bias voltage Vb, as shown in FIG. 4. 
As illustrated in FIG. 2, the reset circuit 109 is comprised of an analog 
switch 201 and a timer circuit 202. The analog switch 201 selects one of 
the phase difference signal Pd and the ground voltage according to the 
reset control signal RCS received from the timer circuit 202. More 
specifically, when receiving a reset signal, e.g. the value 1 of the reset 
control signal RCS, from the timer circuit 202, the analog switch 201 
selects the ground voltage, and when a set signal, e.g. the value 0 of the 
reset control signal RCS, the analog switch 201 selects the phase 
difference signal Pd. The selected one is output to the DC amplifier 110. 
The timer circuit 202 outputs the reset signal to the analog switch 201 
during the preset time period starting when powered up and outputs the set 
signal at all other times. Power-up may be detected by the timer circuit 
202 itself as described later. Alternatively, the timer circuit 202 may 
detect the power-up by receiving a control signal CTRL from a main 
controller (not shown). 
The DC amplifier 110 is typically comprised of one or more operational 
amplifiers driven by two power supply voltages +V and -V, and is designed 
to have a dynamic range between +V and -V. In other words, the output 
voltage, that is, the bias voltage Vb of the DC amplifier 110 is available 
within the range between +V and -V. Furthermore, the DC amplifier 110 is 
designed such that the ground voltage is output when the ground voltage is 
input and the ground voltage is at the center of the dynamic range between 
+V and -V. 
FIG. 3 shows an example of the timer circuit 202. In this example, a 
comparator 301 is employed to output the reset control signal RCS to the 
analog switch 201. A reference voltage Vref, or a threshold level, 
supplied to the non-inverting input of the comparator 301 is generated by 
a voltage divider comprising two resistors R1 and R2 connected in series. 
The comparison voltage Vi supplied to the inverting input of the 
comparator 301 is generated by an integrator circuit comprising a resistor 
R3 and a capacitor C connected in series. Since the comparison voltage Vi 
is a voltage across the capacitor C, when powered on, the comparison 
voltage Vi rises toward the power supply voltage Vcc with the time 
constant determined by the resistor R3 and the capacitor C. Therefore, the 
reset signal of the value 1 is output from the comparator 301 to the 
analog switch 201 until the comparison voltage Vi reaches the reference 
voltage Vref. When the comparison voltage Vi is greater than the reference 
voltage Vref, the set signal of the value 0 is output to the analog switch 
201. 
OPERATION 
FIG. 4 shows modulation characteristics of the optical modulator 102, where 
the horizontal axis indicates input signal voltages and the vertical axis 
indicates output light intensities. As indicated by a solid line 401, the 
optical modulator 102 exhibits a modulation characteristic similar to a 
sine wave. In such a modulation characteristic, an optimal operating point 
of the optical modulator 102 should be selected so as to achieve the 
broadest dynamic range with respect to the input signal MS. Since the 
modulation characteristic is similar to a sine wave, the optical modulator 
102 has a plurality of optimal operating points as indicated by reference 
numerals 403, 404 and 405. 
However, as mentioned above, the modulation characteristic is frequently 
changed due to temperature variations and deterioration over time of the 
electro-optic material. Such a drift of the modulation characteristic is 
shown by a broken line 402, where the respective optimal operating points 
403-405 are shifted as shown by the arrows, for example, the point 404 of 
the solid line 401 is shifted to the point 406 of the broken line 402. In 
cases where such a drift of the modulation characteristic occurs, the bias 
control is performed as follows. 
As mentioned above, the driver amplifier 103 outputs the modulating signal 
MS to the optical modulator 102 with superposing the low-frequency signal 
LFS of a sine wave on the transmission data. If the optical modulator 102 
has no change in modulation characteristic, the band-pass filter 106 
should output the same sine wave signal LCS as the low-frequency signal 
LFS. Since the phase of the sine wave signal LCS is identical to that of 
the low-frequency signal LFS, the phase comparator 107 outputs the phase 
difference signal Pd of the ground voltage to the DC amplifier 110 through 
the reset circuit 109 which is in the set state. Therefore, the optical 
modulator 102 holds its initial optimal operating point (404) which has 
been set by the reset circuit 109 on power-up. 
If the optical modulator 102 changes in modulation characteristic as shown 
by the broken line 402 and the optimal operating point is shifted with 
respect to the input signal voltage, then a current operating point 407 is 
deviated from the shifted optimal operating point 406 as shown in FIG. 4. 
Therefore, the extracted signal LCS by the band-pass filter 106 is 
deviated in phase from the low-frequency signal LFS originally generated 
by the low-frequency oscillator 108. The phase difference signal Pd 
representing the phase deviation is output to the DC amplifier 110 through 
the reset circuit 109 which is in the set state, and thereby the bias 
voltage Vb applied to the optical modulator 102 is varied such that the 
optical modulator 102 follows the shift of the optimal operating point. In 
this manner, the optical modulator 102 keeps operating at the optimal 
operating point which is initially set by the reset circuit 109 regardless 
of any drift of the modulation characteristic. 
The initial optimal operating point 404 is selected by the reset circuit 
109 outputting the ground voltage to the DC amplifier 110 on power-up. 
More specifically, when powered up, the reset circuit 109 is turned to the 
reset state in which the ground voltage is output to the DC amplifier 110 
for the preset time period after power-up. Therefore, the bias voltage Vb 
is fixed at the ground voltage (zero Volts) which is the center of the 
available voltage range as shown in FIG. 4. 
After the preset time period has lapsed, the reset circuit 109 is turned to 
the set state in which the phase difference signal Pd is transferred to 
the DC amplifier 110 passing through the reset circuit 109. Therefore, the 
bias voltage Vb is varied according to the phase difference signal Pd, 
causing the operating point of the optical modulator 102 to be shifted to 
the nearest optimal operating point 404. In this manner, the search for 
the initial bias voltage corresponding to an optimal operating point is 
started from the ground voltage, that is, the center of the available 
range of bias voltage. Therefore, the optical modulator 102 is set at an 
optimal operating point closest to the ground voltage. If the initial bias 
voltage corresponding to the optimal operating point is known, the reset 
circuit 109 may output an initial voltage other than the ground voltage to 
the DC amplifier 110 such that the initial bias voltage Vb is output to 
the optical modulator 102. 
FIG. 5 shows another embodiment of the present invention. In this 
embodiment, the DC amplifier 110 is connected to the optical modulator 102 
through a reset circuit 501 which is the same circuit arrangement as the 
reset circuit 109 as shown in FIG. 2. The reset circuit 501 is comprised 
of an analog switch SW and a timer. The analog switch SW selects one of 
the bias voltage and the ground voltage according to the reset control 
signal received from the timer. More specifically, when receiving a reset 
signal from the timer, the analog switch SW selects the ground voltage, 
and when receiving a set signal, the analog switch SW selects the bias 
voltage received from the DC amplifier 110. The selected one is output as 
the bias voltage Vb to the optical modulator 102. It is apparent that this 
arrangement can also obtain the same advantages as the above-mentioned 
arrangement as shown in FIG. 2. 
Although, in the above embodiments, the center of the available voltage 
range is set at the ground voltage, the center of the available voltage 
range may be set at 7.5 volts in cases where the available voltage range 
spans from 0 to 15 volts. 
In addition, instead of the optical modulator using the electro-optic 
material such as LiNbO.sub.3 and LiTaO.sub.3, another type of optical 
modulator may be employed, for example, a waveguide-type optical modulator 
using the magneto-optic effect such as a Mach-Zehnder optical modulator. 
The important thing is that an optical modulator employed in the present 
invention is capable of changing in operating point according to a bias 
voltage. 
Furthermore, the bias control section of the above modulation devices may 
employ another scheme such that the bias voltage is controlled based on 
the average power of the output light of the optical modulator.