Optical modulator, bias control circuit therefor, and...

Details Optical modulator, bias control circuit therefor, and... Optical modulator, bias control circuit therefor, and...

1999-12-13

2001-12-25

Schuberg, Darren (Department: 2872)

Optical waveguides

Temporal optical modulation within an optical waveguide

Electro-optic

C385S008000, C385S009000, C385S016000, C385S039000, C385S040000, C359S237000

Reexamination Certificate

active

06334004

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical modulator, a bias control circuit therefor, and an optical transmitter including the optical modulator.
2. Description of the Related Art
In an optical fiber communication system, a modulation rate is increasing with an increase in capacity of the system. In direct intensity modulation of a laser diode, wavelength chirping is a problem. The chirping causes waveform distortion when an optical signal passes an optical fiber having chromatic dispersion. From a standpoint of fiber loss, the most desirable wavelength to be applied to a silica fiber is 1.55 &mgr;m. At this wavelength, a normal fiber has a chromatic dispersion of about 18 ps/km
m, which limits a transmission distance. To avoid this problem, an external modulator has increasingly been expected.
As a practical external modulator, a Mach-Zehnder type optical modulator (LN modulator) using LiNbO
3
(lithium niobate) as a substrate has been developed. Continuous-wave light (CW light) having a constant intensity from a light source is supplied to the LN modulator, in which a switching operation using interference of light is carried out to obtain an intensity-modulated optical signal.
The LN modulator has a frequently pointed-out defect that it causes operation point drift. To cope with the operation point drift, light output from the LN modulator is monitored, and control for operation point stabilization is carried out according to an electrical signal obtained as the result of this monitoring.
FIG. 1
is a plan view of a conventional modulator chip in an LN modulator. This modulator chip has an optical waveguide structure
4
provided by a dielectric chip
2
. The dielectric chip
2
is formed of lithium niobate. In this case, the optical waveguide structure
4
is obtained by thermal diffusion of Ti (titanium).
The optical waveguide structure
4
has an input port
6
for receiving an input beam from a light source (not shown) and an output port
8
for outputting a modulated optical signal. The optical waveguide structure
4
further has a first Y branch
10
and a second Y branch
12
respectively connected to the input port
6
and the output port
8
, and first and second paths
14
and
16
for connecting the Y branches
10
and
12
.
The input beam supplied to the input port
6
is branched into first and second beams substantially equal in optical power to each other by the first Y branch
10
. The first and second beams are guided by the paths
14
and
16
, respectively, and then interfere with each other at the second Y branch
12
. According to a phase difference between the first and second beams at the second Y branch
12
, switching is carried out between a coupling mode where an output beam is obtained at the output port
8
and a leaky mode where a leaky beam is radiated from the second Y branch
12
into the dielectric chip
2
, thereby outputting an intensity-modulated optical signal from the output port
8
.
To change the phase difference between the first and second beams, a grounding electrode
18
is provided on the first path
14
, and a signal electrode
20
is provided on the second path
16
. The signal electrode
20
is configured as a traveling wave type such that an input end
20
A is connected to an internal conductor of a connector
22
and an output end
20
B is connected to an internal conductor of a connector
24
. Shields of the connectors
22
and
24
and the grounding electrode
18
are grounded. The electrodes
18
and
20
are formed by vapor deposition of Au (gold), for example. Although not shown, a single or plural stabilizing buffer layers formed of Si and/or SiO
2
may be provided between the dielectric chip
2
and the electrodes
18
and
20
.
Operation point drift will now be described with reference to FIG.
2
. In an LN modulator, an operation characteristic curve is drifted by a temperature change or aged deterioration in general (which is referred to as operation point drift). In
FIG. 2
, reference numerals
26
and
28
denote an operation characteristic curve and an output optical signal waveform, respectively, in the case that no operation point drift occurs, and reference numerals
30
and
32
denote an operation characteristic curve and an output optical signal waveform, respectively, in the case that an operation point drift toward positive voltage occurs. Reference numeral
34
denotes a waveform of an input signal or modulating signal (drive voltage).
The operation characteristic curve is represented as a periodic change in output optical power with an increase in voltage. In the example shown, the periodic change is given by a sine curve. Accordingly, by using voltages V
0
and V
1
respectively providing a minimum value and a maximum value of the optical power, respectively corresponding to the two logical values (the high level and low level) of the input signal as a binary signal to thereby perform effective switching between the coupling mode and the leaky mode mentioned above, efficient binary modulation can be performed.
When the voltages V
0
and V
1
are constant upon occurrence of the operation point drift, the extinction ratio of the output optical signal is degraded as shown by reference numeral
32
by the periodicity of the operation characteristic curve. Accordingly, when the operation point drift occurs in an amount of dV, the voltages V
0
and V
1
must be changed to (V
0
+dV) and (V
1
+dV), respectively, thereby compensating for the operation point drift.
FIG. 3
is a block diagram of a conventional optical transmitter (optical modulator) designed so as to effect operation point stabilization. CW light as an input beam from a laser diode (LD)
36
is supplied to the input port
6
of the modulator chip
2
shown in
FIG. 2
, for example. An output beam from the output port
8
of the modulator chip
2
is divided into two branch beams by an optical coupler
38
. One of the two branch beams is launched into an optical fiber transmission line (not shown), and the other branch beam is supplied to a photodetector (PD)
40
. The photodetector
40
is provided by a photodiode, for example. In this case, the photodetector
40
outputs a current signal. Therefore, this current signal from the photodetector
40
is converted into a voltage signal by a current/voltage (I/V) converter
42
. Thereafter, the voltage signal output from the I/V converter
42
is supplied through a bandpass filter
44
to a phase comparator circuit
46
.
A low-frequency signal (pilot signal) output from an oscillator
48
is used for operation point stabilization. The pilot signal is supplied to the phase comparator circuit
46
and a drive circuit
50
. The drive circuit
50
may be composed of a variable-gain amplifier for amplifying a data input signal and a low-pass filter connected to the output of the variable-gain amplifier. In this case, the gain of the variable-gain amplifier is changed by the low-frequency signal, and as a result, the low-frequency signal is superimposed on the data input signal. By the use of the low-pass filter, the low-frequency signal is superimposed on both the low level and high level of the data input signal in opposite phases. A resultant signal is then supplied as a modulating signal to the connector
22
of the modulator chip
2
.
The phase comparator circuit
46
is provided by a synchronous detector circuit, for example. The phase comparator circuit
46
performs phase comparison between the low-frequency signal from the oscillator
48
and a low-frequency component from the photodetector
40
. The result of this phase comparison appears in a DC component of an output signal from the phase comparator circuit
46
. Then, the bias circuit
52
performs feedback control of a bias voltage to be supplied to the connector
24
of the modulator chip
2
, according to the DC component. In this feedback loop, the bias voltage is adjusted so that the low-frequency component from the photodetector
40
is minimized.
Referring to
FIG. 4

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