Module for optical transmitter

Details Module for optical transmitter Module for optical transmitter

2002-11-22

2004-05-11

Reichard, Dean A. (Department: 2831)

Optical waveguides

Integrated optical circuit

C385S088000, C385S092000

Reexamination Certificate

active

06735353

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a module for optical transmitter including an opto-electronic integrated circuit chip obtained by forming an opto-electronic optical modulator and a driver circuit for modulator as an integrated circuit, and an optical transmission system using the same.
2. Description of the Related Art
In a conventional optical transmitter, a multiplexer, an optical modulator, and a driver circuit for modulator are different modules, and the respective modules are provided on the base of the optical transmitter. Further, the connection between the respective modules is established by a characteristic impedance-matched cable or connector. The photograph of the external view thereof is disclosed in “Electronic Technology”, 2000, Nov. issue, pp. 7-8.
Further, the system block diagram thereof is disclosed in “FIFTH ASIA-PACIFIC CONFERENCE ON COMMUNICATIONS AND FOURTH OPTOELECTRONICS COMMUNICATIONS CONFERENCE, APCC/OECC'99, PROCEEDINGS”, pp. 12-13.
SUMMARY OF THE INVENTION
In the foregoing prior art, a multiplexer module, a module of a driver circuit for optical modulator, and a module of an optical modulator are connected by a cable or a connector. Accordingly, the connection loss of an electric signal occurs at the connection part, resulting in degradation in response characteristic from the electric signal to an optical signal.
Further, the area occupied by the modules is increased due to the cable or the connector, resulting in a large transmitter.
The present inventors prototyped a module for optical transmitter wherein an optical modulator and a driver circuit for modulator are formed as an OEIC for implementing a compact high-performance optical transmitter.
FIG. 2
is a top view of the interior of the prototyped module for optical transmitter seen by removing the upper part of the storage case thereof.
FIG. 3
is a side view seen from the cross section of the portion along line D-D″ shown in FIG.
2
.
FIG. 4
is a perspective view of the prototyped OEIC chip.
FIG. 5
is a cross sectional view of the portion along line E-E′ shown in FIG.
4
.
For the prototyped module for optical transmitter, as shown in
FIG. 2
, an OEIC chip
80
is connected to an Au (gold)-plated wiring pattern
82
formed on a ceramic substrate
81
by a bonding wire
83
. A thermister
84
for detecting the inside temperature of the module is also connected thereto similarly, and connected to an input/output (I/O) terminal
86
by a lead wire
85
.
A storage case
87
made of a metal serving as a housing is provided with a connector
88
for inputting a high-frequency signal to the OEIC chip
80
and a terminal
89
for applying dc voltage. Each terminal is connected to the Au-plated pattern
82
formed on the ceramic substrate
81
by each lead wire
85
. A single mode fiber
90
is connected to an isolator
91
, and inputs and outputs optical signals to and from the OEIC chip
80
through an aspherical lens
92
for fiber coupling.
Further, as shown in
FIG. 3
, the OEIC chip
80
is fixed on a carrier
93
made of CuW (copper tungsten). Further, the carrier
93
is connected to a peltier cooler
94
, and placed on the bottom
95
of the case
87
. Thus, the module is so configured that the OEIC chip
80
is cooled by passage of a prescribed current through the I/O terminal
96
of the peltier cooler
94
.
Further, for the prototyped OEIC chip
80
, as shown in
FIGS. 4 and 5
, an opto-electronic optical modulator
97
and a driver circuit
98
for modulator are formed on the same InP substrate
99
. Further, a characteristic impedance-matched transmission line (G-S-G line)
100
composed of ground wire G-signal wire S-ground wire G for connecting the optical modulator
97
and the driver circuit
98
, a pad
101
for input signal, and a pad
102
for dc bias are included therein. An interlayer insulating film
103
is formed under the respective pads
101
and
102
, and the transmission line
100
. To the opto-electronic optical modulator
97
, an optical waveguide
104
is connected. Further, the substrate
99
made of InP has been reduced in thickness to 100 &mgr;m, implementing such a configuration as to enhance the heat dissipation effect.
The present inventors mounted the foregoing prototyped module for optical transmitter in an optical transceiver system, and evaluated the characteristics. As a result, they found that the module for optical transmitter using the OEIC chip
80
is more degraded in terms of optical transmission characteristics than the one manufactured by using separate modules.
Further, the present inventors considered that this is caused by the thermal transmission from the driver circuit
98
for modulator with large power consumption to the optical modulator
97
, and performed the numerical analysis on the thermal resistance for defining the thermal transmission path.
FIG. 6
shows a simplified thermal path of the prototyped module for optical transmitter. This thermal path is composed of the thermal resistance Rth
1
of the semiconductor substrate
99
, the thermal resistance Rth
2
of the wiring metal formed on the surface of the OEIC chip
80
, the thermal resistance Rth
3
of the gas filled in the module for optical transmitter, the input power P
1
of the OEIC chip
80
, the input power P
2
of the peltier cooler
94
, the temperature T
1
of the driver circuit
98
for modulator, the temperature T
2
of the optical modulator
97
portion, and the ambient temperature Ta.
The thermal resistances Rth
1
to Rth
3
of the respective elements are expressed by the following equation (1).
Rthn=L/&lgr;A
  (1)
wherein n=1, 2, or 3; L denotes the distance between two points of the optical modulator and the driver circuit for modulator; &lgr;, the thermal concuctivity; and A, the cross-sectional area.
The analysis was performed assuming as follows: the distance L is 1 mm; the thickness of the wire, 5 &mgr;m; the width of the wire, 1 mm; and the width of the OEIC chip, 1.7 mm. As a result, the thermal resistance Rth
2
on the substrate surface was 250° C./W. In contrast, the thermal resistance Rth
1
of the substrate portion was 33.4° C./W, which is about 13% of the thermal resistance of the substrate surface.
The prototyped module for optical transmitter is an enclosed housing. Accordingly, the thermal transmission by a gas is considered to be smaller than the thermal transmission from the wiring metal. Thus, the thermal resistance Rth between the optical modulator
97
and the driver circuit
98
for modulator is determined two dimensionally in a simplified manner, and it can be expressed as the following equation (2):
Rth
=(Rth
1
×Rth
2
)/(Rth
1
+Rth
2
)  (2)
The thermal resistance between the two points can be considered as a simple combined resistance. Therefore, it has been shown that the heat generated at the driver circuit
98
for modulator was conducted mainly through the substrate
99
to the optical modulator
97
to degrade the extinction characteristic of the optical modulator
97
.
Further, although the peltier cooler was disposed in the prototyped module for optical transmitter, presumably, it was not capable of controlling the temperature rise and the temperature change in the optical modulator
97
due to the thermal path along the lateral direction of the substrate
99
from the driver circuit
98
for modulator.
Herein, simple provision of a large-capacity peltier cooler exhibiting a large cooling effect may also be mentioned as one of the countermeasures. This, however, leads to an increase in power consumption of the module for optical transmitter itself, and hence is not a preferred countermeasure from the viewpoint of implementing lower power consumption of the optical transmitter.
Therefore, it is essential to solve the thermal transfer problem for improving the performance than that of the conventional optical transmitter.
Under such circumstances, it is therefore an object of the present invention to provide

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