Optical transmitter, and method of controlling bias voltage...

Details Optical transmitter, and method of controlling bias voltage... Optical transmitter, and method of controlling bias voltage...

2001-04-24

2004-12-28

Negash, Kinfe-Michael (Department: 2633)

Optical communications

Transmitter

Including compensation

C398S182000, C398S195000, C359S239000

Reexamination Certificate

active

06836622

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an optical transmitter of an external modulation method employed in an optical communication system and a control method of a bias voltage to an optical modulator employed therein. More particularly, this invention relates to an optical transmitter using an optical modulator of Mach-Zehnder type and a control method of a bias voltage to an optical modulator employed therein.
BACKGROUND OF THE INVENTION
Conventionally, the optical communication system uses the direct modulation method. In this method, light intensity signal proportional to an electric signal serving as a driving current is obtained by generating an optical modulation signal with a driving current for a laser diode. However, in an ultra-fast broadband optical communication system having a transmission rate exceeding several Gbit/s, the wavelength of light changes at the direct modulation, which is known as chirping and limits a transmission capacity.
On the other hand, the chirping occurs less frequently in the external modulation method. Furthermore, in the external modulation method, modulation is relatively easy in an operating band of 10 GHz or higher, and therefore, has been applied to an ultra-fast broadband optical communication system with a large capacity. The most popular optical modulator as the external modulator is a Mach-Zehnder optical modulator using lithium niobate (LiNbO
3
).
An output optical signal I(t) modulated by a modulation signal S(t) by using the Mach-Zehnder optical modulator is expressed by Equation (1):
I
(
t
)=
k
{1+cos (&bgr;·
S
(
t
)+&dgr;)}  (1)
where k represents a proportion coefficient, &bgr; represents a degree of modulation, and &dgr; represents a phase at the operating point of the Mach-Zehnder optical modulator.
Given that the modulation signal S(t) is a binary digital signal, the degree of modulation &bgr; is &bgr;=&pgr;, and the initial phase &dgr; is &dgr;=&pgr;/2 by applying an adequate DC voltage (bias voltage) to the Mach-Zehnder optical modulator, then the Mach-Zehnder optical modulator outputs the output optical signal I(t) that switches ON/OFF in proportionate to the modulation signal S(t).
Given that the degree of modulation &bgr; is &bgr;=2&pgr;, the initial phase &dgr; is &dgr;=0 by applying an adequate bias voltage to the Mach-Zehnder optical modulator, and the modulation signal S(t) is used, then when a sine wave having a repeating frequency R is input, the output optical signal I(t) is expressed by equation (2).
I
(
t
)=
k
{1+cos (2&pgr;·sin (2
&pgr;R
(
t
)))  (2)
Hence, the output optical signal I (t) expressed by equation (2) is output as an optical signal that switches ON/OFF at a repeating frequency 2R that is double the repeating frequency R.
There would be no problem if the phase &dgr; is constant. However, a typical optical modulator using lithium niobate has a problem that the operating point undesirably drifts. Two types of drift are known. That is, a thermal drift induced by the pyroelectric effect caused by a temperature change; and a DC drift induced by a charge distribution over the surface of the element of the optical modulator produced by the bias voltage applied to the electrode of the optical modulator. In order to compensate variance of the operating point caused by these types of drift, it is necessary to apply a bias voltage to the optical modulator in such a manner so as to attain an optimal operating point.
FIG. 8
is a block diagram depicting an arrangement of a conventional optical transmitter capable of stabilizing a bias voltage applied to the optical modulator using lithium niobate (see Japanese Patent Application Laid-Open No. 5-142504). Continuous optical signals emitted from a light source
101
are input into a Mach-Zehnder optical modulator
103
using lithium niobate. A terminator
114
is connected to the Mach-Zehnder optical modulator
103
and a driving signal for driving the Mach-Zehnder optical modulator
103
and a bias voltage are applied to the Mach-Zehnder optical modulator
103
through a node TT.
Output optical signal modulated by the Mach-Zehnder optical modulator
103
is output to an output terminal
120
through a branching filter
104
, and a part of the output optical signal is input into a photo diode
105
. The photo diode
105
converts the input part of the output optical signal into an electric signal, amplifies the electric signal by means of a pre-amplifier
106
, and outputs the same to a synchronous detector circuit
107
.
The synchronous detector circuit
107
conducts synchronous detection between the electric signal input from the pre-amplifier
106
and a low frequency signal output from a dither signal generator
112
. The synchronous detector circuit
107
includes a mixer
117
, which mixes the electric signal input from the pre-amplifier
106
with the low frequency signal output from the dither signal generator
112
. The resulting mixed signal is input into a low pass filter
109
through an operational amplifier
108
, and the signal having passed through the low pass filter
109
is output to a bias voltage control circuit
110
.
The bias voltage control circuit
110
includes a DC voltage
118
and an adder
119
. The adder
119
adds an output signal from the synchronous detector circuit
107
and a bias voltage output from a DC power source
118
, and outputs the sum as a bias voltage to the Mach-Zehnder optical modulator
103
from the node TT through an inductor
111
. On the other hand, a driving signal is input into an input terminal
121
and output to a low frequency superimposing circuit
113
through a driving circuit
124
. The low frequency superimposing circuit
113
superimposes the input driving signal and a low frequency signal output from the dither signal generator
112
, and applies the resulting signal as a driving signal to the Mach-Zehnder optical modulator
103
from the node TT through a capacitor. Hence, both the driving signal superimposed with the low frequency signal and the bias voltage under the bias voltage control are applied to the Mach-Zehnder optical modulator
103
from the node TT.
How the bias voltage to the Mach-Zehnder optical modulator is controlled in the conventional optical transmitter will now be explained with reference to
FIG. 9
to FIG.
11
.
FIG. 9
is a view explaining a modulation operation of the Mach-Zehnder optical modulator
103
when a bias voltage (phase &dgr;) is at an adequate value. Operating characteristic curve
130
of the Mach-Zehnder optical modulator
103
represents the operating characteristic curve expressed by the equation (1), and indicates a state where the bias voltage (phase &dgr;) is adequately set. In this case, upon input into the Mach-Zehnder optical modulator
103
, a driving signal (input signal)
131
, which has been superimposed with the low frequency signal, is modulated by the operating characteristic curve
130
and output as an output optical signal
132
. The output optical signal
132
does not include a low frequency component (f[Hz]) of the low frequency signal superimposed on the driving signal, and a low frequency component (2f[Hz]) double the low frequency component (f[Hz]) is generated. Thus, after a part of the output optical signal
132
is received by the photo diode
105
, amplified by the pre-amplifier
106
, and let undergo the synchronous detection by the synchronous detector circuit
107
, the resulting signal outputs “0”. In this case, because no signal component is added by the adder
119
of the bias voltage control circuit
110
, the current bias voltage is maintained and applied intact to the Mach-Zehnder optical modulator
103
.
On the other hand,
FIG. 10
is a view explaining a modulation operation by the Mach-Zehnder optical modulator
103
when the bias voltage is at a relatively high value compared with an adequate value. Operating characteristic curve
140
of the Mach-Zehnder optical modulator

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