Optical module

Details Optical module Optical module

1998-09-22

2002-11-12

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Integrated optical circuit

C385S049000, C385S088000, C385S089000, C385S092000, C257S544000, C257S550000, C372S050121

Reexamination Certificate

active

06480639

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical module comprising an optical waveguide and a plurality of semiconductor devices integrated on a substrate, more specifically to an optical module which is able to reduce optical noise caused by reflections of leakage lights (or stray lights) in various paths within such a module, thereby reducing crosstalk between semiconductor devices.
2. Description of the Prior Art
Recently, towards construction of an optical subscriber system, necessity for development of low-cost optical modules has been widely recognized. Especially, cost effectivity is important for WDM optical transmitter and receiver modules for multiplexing and demultiplexing 1.3 &mgr;m /1.55 &mgr;m optical signals and performing bidirectional transmission and reception at 1.3 &mgr;m.
With the aim of cost reduction of optical modules, as described in the document (I) below, development is conducted for a hybrid integrated type optical module in which a laser diode (hereinafter in some cases referred simply to as LD), a photodiode (hereinafter in some cases referred to as PD) and the like are disposed directly on a silica optical waveguide substrate. (I) Yamada et al., Preprint of Proceedings for 1996 Spring Conference of the Society of Electronic Communications
FIGS. 27A and 27B
are diagrams showing the structure of a prior art optical module, including a perspective view and a sectional diagram showing an important part of the structure of optical waveguide. The optical module shown in
FIG. 27A
is the one which is described in the above document (I) and has been developed by the inventors.
In the optical module shown in
FIG. 27A
, a silica optical waveguide
2
is formed on a silicon (hereinafter abbreviated to as Si) substrate
1
provided with irregularities as a substrate, which is referred to a platform. On a Si recess
1
a
of the platform, an embedded type silica optical waveguide
2
is formed in such a configuration that a core
2
a
is embedded with a cladding layer
2
b
of a sufficient thickness. Using the optical waveguide
2
, a wavelength multiplexing/demultiplexing circuit (WDM circuit)
101
for multiplexing and demultiplexing 1.3 &mgr;m/1.55 &mgr;m and a Y-split circuit
102
for 1.3 &mgr;m light are formed.
As the wavelength multiplexing/demultiplexing circuit (WDM circuit)
101
, a wave multiplexing/demultiplexing function is achieved by a wavelength selection filter
10
inserted in a groove formed in the optical waveguide. Further, on a Si protrusion
1
b
provided in the vicinity of the end portions of two input/output waveguides of the Y-split circuit
102
, a recessed optical device mounting portion
15
is provided which is formed by recessing the optical waveguide substrate
2
, and on the thus formed recessed optical device mounting portion
15
, a semiconductor chip of LD
30
, a semiconductor chip for monitoring PD
32
and a semiconductor chip of a receiver PD
31
are directly mounted.
With this construction, the number of parts constituting the optical module can be substantially reduced. In
FIG. 27A
, reference numeral
4
indicates an optical fiber connection part, whereas
4
a
and
4
b
are optical fibers.
In this optical module, as shown in
FIG. 27B
, an embedded type optical waveguide
2
is used in which the core
2
a
is embedded with the cladding layer
2
b
of a sufficient thickness. Therefore, of the output lights from the LD
30
, the components which are not coupled to an optical transmission mode of the optical waveguide
2
are transmitted as leakage lights in the cladding layer
2
b
, which leak into the optical fiber
4
b
causing a noise of 1.55 &mgr;m port, so that a countermeasure thereto has been required. That is, it has been required to reduce crosstalk lights generated by leakage of 1.3 &mgr;m output lights from LD
30
into the optical fiber
4
b
of 1.55 &mgr;m output lights.
As a countermeasure thereto, it is effective to provide a light blocking area which is formed by removing an unnecessary part of the cladding layer
2
b
while remaining the vicinity of the core
2
a
(Terui et al., “Optical Waveguide Circuit”; Japanese Patent Application Laid-open No. No. 9-5548).
FIG. 28
is a plane diagram showing the structure of an example of an optical module provided with such a light blocking area, wherein the light blocking area
20
is formed by removing an unnecessary area of the cladding layer
2
b
(which may be referred to just as “cladding” or “cladding part”) in front of the recessed optical device mounting portion
15
, except the nearby area of the core
2
a
. With this construction, leakage lights from LD
30
can be prevented from reaching the optical fiber
4
b
for 1.55 &mgr;m output lights. Since the present invention is not directed to a wavelength multiplexing/demultiplexing circuit itself, detailed description thereof is omitted.
The optical module shown in
FIG. 28
is provided with a semiconductor chip LD
30
and a semiconductor chip for receiver PD
31
on the same substrate, however, since in an ordinary operation method, the LD
30
and the receiver PD
31
will never be driven simultaneously, turning round of the lights from the LD
30
to the receiver PD
31
is not a problem.
However, when the LD
30
and the receiver PD
31
are to be driven simultaneously, an important problem occurs in the optical module using the embedded type optical waveguide
2
. Specifically, because the LD
30
and the receiver PD
31
are disposed in the vicinity of each other on the substrate, the lights outputted from the LD
30
leak into the receiver PD
31
, which becomes a noise to the received optical signal. In the ordinary method of use, the LD
30
itself outputs lights of an intensity of +10 to +20 dBm. On the other hand, the receiver PD
31
is required to receive a weak optical signal of less than −30 dBm. Therefore, when receiving such a weak optical signal, the presence of leakage light from the LD
30
has been a critical problem.
From the past, the light leakage path from the LD
30
to the receiver PD
31
, as shown by the broken (First Path) line in
FIG. 29
, of forward and backward output lights from the LD
30
, is considered to be mainly a radiation component which is not coupled to the optical transmission mode of the optical waveguide
2
and inputted directly to the receiver PD
31
, and the leakage light component has been expected to be prevented, as shown in
FIG. 28
, by improving the relative positions of the LD
30
and the receiver PD
31
so that the receiver PD
31
is not positioned within the radiation angle of the output lights from the LD
30
, thereby preventing the radiation component from the LD
30
from being applied directly to the receiver PD
31
.
In addition to the above, the inventors have found that there exist second and third leakage light generation paths as shown by the dotted lines in FIG.
29
.
A second leakage light generation position is reflection from a backside wall of the recessed optical device mounting portion
15
. That is, some of the backward output lights from the LD
30
are reflected by a backside wall
150
of the recessed optical device mounting portion
15
and an optical waveguide substrate end portion
151
, and are incident to the receiver PD
32
.
A third leakage light generation path is caused by the light blocking area
20
itself. That is, the output lights from the LD
30
are reflected by a side wall
201
at a side closer to the LD
30
of the light blocking area
20
, and incident thereafter to the receiver PD
31
. This path can seemingly be prevented by filling the light blocking area
20
with a light absorber, however, in practice, even if it is filled with a light absorber, the third path is inevitably generated so far as there is a refractive index difference between the optical waveguide cladding layer
2
b
and the absorber.
As described above, the second and third leakage light generation paths are formed by reflection of leakage lights at a refracti

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