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Description  |
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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 .mu.m /1.55 .mu.m
optical signals and performing bidirectional transmission and reception at
1.3 .mu.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 1a of the platform, an embedded type silica
optical waveguide 2 is formed in such a configuration that a core 2a is
embedded with a cladding layer 2b of a sufficient thickness. Using the
optical waveguide 2, a wavelength multiplexing/demultiplexing circuit (WDM
circuit) 101 for multiplexing and demultiplexing 1.3 .mu.m/1.55 .mu.m and
a Y-split circuit 102 for 1.3 .mu.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 1b 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 4a and 4b are optical fibers.
In this optical module, as shown in FIG. 27B, an embedded type optical
waveguide 2 is used in which the core 2a is embedded with the cladding
layer 2b 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 2b, which leak into the optical fiber 4b causing a noise of
1.55 .mu.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 .mu.m output lights from LD 30 into the optical fiber 4b of 1.55
.mu.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
2b while remaining the vicinity of the core 2a (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 2b (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 2a. With this construction, leakage lights from LD
30 can be prevented from reaching the optical fiber 4b for 1.55 .mu.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 2b and the
absorber.
As described above, the second and third leakage light generation paths are
formed by reflection of leakage lights at a refractive index discontinuity
portion, and the basic cause thereof is common.
Leakage lights due to the second and third paths become those transmitted
to optical devices other than the light emitting devices to generate a
noise and, at the same time, are incident again as the leakage lights to
the light emitting device itself. As a result, there is a problem in that
when the return lights are strong in intensity, it causes a return light
noise of the light emitting device itself.
Yet further, in effect, apart from the above described leakage light paths,
there is another path in which the leakage lights from these light
emitting devices are reflected by a bottom surface or side wall of the
optical waveguide substrate itself to enter the light receiving device.
FIGS. 30A and 30B schematically show the state. In this module, the core
part 2a and the cladding part 2b of the optical waveguide are formed on a
silica glass substrate 10, and the light emitting device 30 and the light
receiving device 31 are provided to couple with the core part 2a. However,
with such a simple construction, as shown by the arrows in FIG. 30B, stray
lights easily reach the light receiving device 31. As a measure for such a
problem, heretofore a method to block lights transmitting the above
described cladding part, a method of using a wavelength selective filter
or the like has been considered.
A construction example shown in FIGS. 31A and 31B is a simplified
construction which is applied with a method to block lights transmitting
the cladding part (e.g., above-described Japanese Patent Application
Laid-open No. 9-5548 "Optical Waveguide Circuit"). In this example, as
shown in FIG. 31A, a light blocking groove 20 is formed on the surface of
the cladding part 2b so that the transmission of stray lights is
suppressed by reflection or scattering by the side surface of the groove
20. In this case, the optical module is constructed to be provided with
the light emitting device 30 and the light receiving device 31 so that it
is connected to an external device by the same output port through the
Y-split optical waveguide 2a.
In general, in such a module, stray lights from the light emitting device
30 not coupled with the optical waveguide 2 enters the light emitting
device 31 resulting in the generation of a noise. Geometrical optical
paths of stray lights are, for example, as shown by the arrows in FIG.
31B. A greater part of the stray lights are reflected or scattered on the
side surface of the groove 20, and the amount of stray lights entering the
light emitting device 31 is reduced. As to formation of such a groove,
when a silica glass optical waveguide is used as an optical waveguide,
since fine processing of silica glass by machining is generally difficult,
formation of the groove is performed by a physicochemical method such as
plasma etching or the like, different from machining. For this reason, it
is very difficult to form a groove of large depth, and a shallow groove is
formed on the surface of the substrate. Therefore, stray lights
transmitting below the substrate are difficult to be blocked by this
groove, and the stray lights of this part reach the light receiving device
31 while repeating reflections.
Furthermore, a construction example shown in FIGS. 32A to 32C is the one
that is applied with a method of using a wavelength selective filter
(e.g., Inoue et al., Japanese Patent Application No. 9-151825
"Bidirectional WDM Optical Transmission and Reception Module"). In this
example, the optical module is constructed such that receive light and
transmit light have wavelengths .lambda.in and .lambda.out differing from
each other, these both light waves are respectively transmitted or
reflected by the wavelength selective filter 10 and connected through the
same port to an external device (FIGS. 32A, 32B). Since the wavelength
selective filter 10 has a wavelength selectivity, it can also reflect
stray lights from the light emitting device 30 as shown by an arrow in the
sectional diagram FIG. 32C.
However, in this method, the wavelength selective filter 10 is inserted in
a very narrow groove 12, and, for an insertion of the filter deep into the
substrate, it is required to form a groove of a very high aspect ratio,
which involves a technical difficulty. Therefore, since a groove is formed
with an appropriate depth, this method is not effective to the stray
lights transmitting below the substrate as with the above-described
example. Moreover, this method cannot be applied to an optical module
using the same wavelength.
Still further, it has been found through studies conducted thereafter,
noises are generated due to further leakage light paths apart from the
reflection by the light blocking groove formed by removing the optical
waveguide cladding or at recesses for optical device mounting or the like,
and from the stray lights transmitting below the substrate.
For example, in the module of the structure of FIGS. 32A to 32C, another
path has been found where a strong scattering of light is generated at the
part of the wavelength selective filter 10, and, after repeating multiple
scattering, lights reach the light receiving device 31 through a space
above the substrate. This path is generated when a structure largely
protruding from the substrate is formed as shown in the figures, and had
not been recognized as a problem in the past.
That is, in the above structure, considering the fact that optical devices
generally having a thickness of about 100 to 200 .mu.m protrude greater
than the cladding of optical waveguide generally having a thickness of
several tens of .mu.m, leakage lights transmitting through a space over
the optical waveguide is investigated, and, as a result, it has been found
that the leakage lights have a large influence on the generation of
crosstalks.
A first object of the present invention, in order to solve the
above-described prior art technical problems in an optical module in which
an optical waveguide and optical semiconductor devices are integrated on a
substrate, is to provide a technology as a first aspect thereof, which can
prevent reflection of the basically horizontal movement of the stray
lights from a light emitting device at a refractive index discontinuity
part, which reflection is incident thereafter to semiconductor devices.
A second object of the present invention, in order to solve problems with
such a prior art optical module, as a second aspect thereof, is to provide
a construction for effectively suppressing optical noises due to the
lights leaking below the substrate and reflected by the bottom surface or
side wall, resulting in a degradation of signals.
A further object of the present invention, in order to solve the problems
of leakage lights scattered on the substrate or in the vicinity of the
filter and transmitting a space above the substrate, as a third aspect
thereof, is to provide a structure of optical module which can efficiently
suppress the leakage lights to reduce crosstalk.
SUMMARY OF THE INVENTION
An optical module according to the present invention has a silicon
substrate, a plurality of optical semiconductor devices integrated on the
silicon substrate, and an optical waveguide for performing transmission of
optical signals by the optical semiconductor devices, wherein the silicon
substrate contains an impurity (dopant) for increasing the number of
carriers in the silicon substrate thereby suppressing optical crosstalk
between the plurality of optical semiconductor devices.
Further, in particular, to achieve the first object, the optical waveguide
comprises a core part for coupling the semiconductor devices with each
other on the substrate and a peripheral cladding layer of the core part,
or in a construction where each optical fiber is coupled to each
semiconductor device, an electrical resistivity of some part or all of the
silicon substrate is 0.1 .OMEGA.cm or less, or a lower part of a light
receiving device of the optical semiconductor is made high in resistance
and a lower part of a light emitting device of the optical semiconductor
is made low in resistance.
To achieve the second object of the present invention, the optical
waveguide is an embedded type optical waveguide in which the core part is
embedded with the cladding layer, a backside wall of a recess formed in
the cladding layer is formed not to be perpendicular to the optical axis
of the semiconductor device, and the cladding layer other than the
vicinity of the core part is removed to form a further light blocking area
in front or rear of the recess in such a manner that the optical waveguide
is not divided, wherein the light blocking area formed at the rear of the
recess is filled with a black light blocking substance, and the side wall
thereof is set obliquely.
Still further, a plurality of recesses are provided, of which between at
least those disposed side by side in a longitudinal direction of the
optical waveguide, a light blocking area is also formed by removing the
cladding layer other than the vicinity of the core of the optical
waveguide in such a manner that the optical guide is not divided, the rear
side wall is set not to be perpendicular to the optical axis of the
semiconductor optical device, or the side wall of the light blocking area
is formed not to cross at right angles with the optical axis of the
semiconductor optical device.
To attain the third object of the present invention, the optical module has
a further filter inserted in a groove formed in the optical waveguide,
each of the optical semiconductor devices is locally covered with a
transparent resin, the parts protruding upward from the optical waveguide
are all coated with a light absorber, and, in this case, either each of
them is covered with separate caps or all of them are covered with a
single cap.
According to the first aspect of the present invention, in an optical
module, all of the leakage light generation paths including generation of
leakage lights in the horizontal direction from the light emitting device
caused by the presence of a refractive index discontinuity in the optical
waveguide can be eliminated, thereby reducing crosstalk optical noises
generated due to the leakage lights.
Further, according to the same aspect of the present invention, leakage
lights from the light emitting device incident to other optical devices on
the same optical waveguide substrate and generate noises can be prevented,
and generation of return light noises in the light emitting device can
also be prevented.
Still further, according to the second aspect of the present invention, in
an optical module in which an optical waveguide and optical semiconductor
devices are integrated on a substrate, optical noises due to leakage
lights below the substrate degrading the signals can be efficiently
suppressed to obtain a high light reception sensitivity, thereby providing
an optical module construction of improved functions.
Yet further, according to the third aspect of the present invention, stray
lights transmitting above the optical integrated substrate, which have not
been taken into consideration in the past, can be efficiently suppressed,
thereby enabling an optical module with minimized optical crosstalks. In
particular, it is apparent that when the light emitting device and the
light receiving device are included in the optical module, the present
invention provides an optical module construction which is very effective
in achieving an optical module with superior reception characteristics.
The above and other objects, effects, features and advantages of the
present invention will become more apparent from the following description
of embodiments thereof taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are schematic diagrams showing the structure of the optical
module according to an embodiment 1 of the present invention;
FIG. 2 is a plane diagram showing the construction of an optical module
fabricated according to a prior art for comparing with the optical module
of embodiment 1 of the present invention;
FIGS. 3A to 3E are diagrams showing an example of fabrication method of the
optical module of embodiment 1 of the present invention;
FIG. 4 is a plane diagram showing the structure of the optical module
according to an embodiment 2 of the present invention;
FIG. 5 is a plane diagram showing the construction of an optical module
fabricated according to a prior art for comparing with the optical module
of embodiment 2 of the present invention;
FIG. 6 is a plane diagram showing the structure of the optical module
according to an embodiment 3 of the present invention;
FIGS. 7A and 7B are diagrams showing the construction of the optical module
of an embodiment 4 of the present invention;
FIG. 8 is a plane diagram showing the structure of the optical module
according to an embodiment 5 of the present invention;
FIG. 9 is a plane diagram showing the structure of the optical module
according to an embodiment 6 of the present invention;
FIG. 10 is a plane diagram showing the structure of the optical module
according to an embodiment 7 of the present invention;
FIG. 11 is a plane diagram showing the structure of the optical module
according to an embodiment 8 of the present invention;
FIGS. 12A and 12B are diagrams showing the construction of the optical
module of an embodiment 9 of the present invention;
FIGS. 13A and 13B are diagrams showing the construction of the optical
module of an embodiment 10 of the present invention;
FIGS. 14A and 14B are diagrams showing the construction of the optical
module of an embodiment 11 of the present invention;
FIG. 15 is a diagram showing changes in propagation loss at 1.3 to 1.5
.mu.m wavelength region against electrical resistivity;
FIGS. 16A and 16B illustrate a modified example of the embodiment of FIGS.
14A and 14B, showing an embodiment 12;
FIG. 17 is a further modified example of the embodiment of FIGS. 14A and
14B, showing an embodiment 13;
FIG. 18 is a still further modified example of the embodiment of FIGS. 14A
and 14B;
FIG. 19 is a yet further modified example of the embodiment of FIGS. 14A
and 14B, showing an embodiment 14;
FIG. 20 is a diagram showing the construction of the optical module
according to an embodiment 15 of the present invention;
FIG. 21 is a perspective view showing an embodiment 16 of the present
invention in the construction of FIG. 12A, the entire device being covered
with a light absorbent resin;
FIG. 22 is an exploded view showing parts of inside construction of FIG.
21;
FIG. 23 shows a configuration of the optical module before providing the
structure for suppressing leakage lights, wherein an optical waveguide
such as an optical fiber is omitted for simplicity;
FIG. 24 is a diagram showing an embodiment 17 in which a light absorbent
resin is coated including a filter;
FIG. 25 is a diagram showing an embodiment 18 provided with caps as a light
blocking body on respective optical devices and filter;
FIG. 26 is a diagram showing an embodiment 19 in which the entire
construction is covered with a single cap, rather than using discrete caps
shown in FIG. 25;
FIGS. 27A and 27B are diagrams showing the construction of a prior art
optical module;
FIG. 28 is a plane diagram showing the construction of an example of prior
art optical module provided with a light blocking area;
FIG. 29 is a plane diagram for explaining a leakage light generation
circuit in a prior art optical module;
FIGS. 30A and 30B are diagrams for explaining the state of leakage lights
entering the light receiving device in the optical module;
FIGS. 31A and 31B are diagrams for explaining a method for blocking lights
transmitting through a cladding part;
FIGS. 32A to 32C are diagrams for explaining a method using a wavelength
selective filter.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the following, embodiments according to the first aspect of the present
invention will be described in detail with reference to the drawings.
In all of the drawings for describing the embodiments, those having similar
functions have similar reference numerals, and detailed description
thereof is omitted.
Embodiment 1
FIGS. 1A and 1B are diagrams showing the construction of the optical module
according to an embodiment 1 of the present invention, in which FIG. 1A is
a perspective view and FIG. 1B is a plane view.
The optical module of the present embodiment is composed of an embedded
type silica optical waveguide 2 comprising a cladding layer 2b and a core
2a formed on a silicon (Si) substrate 1, a semiconductor chip of LD 30
disposed on the Si substrate 1, a semiconductor chip of a monitor-receiver
PD (which may be referred to simply as monitor PD) 32 and a semiconductor
chip of a receiver PD 31. The optical waveguide 2 is formed of two
straight waveguides, and connected to ends of which are two optical fibers
(4a, 4b) with an optical fiber connection part 4.
Here, the optical waveguide 2 is an embedded type optical waveguide 2 in
which the core 2a is embedded with the cladding layer 2b having a
sufficient thickness, and the "cladding layer of a sufficient thickness"
means that the cladding layer is set thicker than a depth for the lights
transmitting through the core 2a to come out to the cladding layer 2b,
which normally requires a thickness equal to or greater than the core
size.
The three semiconductor devices (30, 31 and 32) are all disposed on a
recessed optical device mounting portion 15 formed by removing the optical
waveguide 2 (core 2a and cladding layer 2b) into a recessed form. To
prevent output lights from the LD 30 from leaking directly to the receiver
PD 31, the receiver PD 31 is disposed in such a manner that it is not
positioned within the radiation angle of the backward output lights from
the LD 30.
A characteristic feature of the present embodiment is that the rear side
wall 150 of the recessed optical device mounting portion 15 is disposed
obliquely. In the present embodiment, the rear side wall has an angle of
about 80 degrees with respect to a perpendicular line of the optical axis
of the backward output light from the LD 30 (about 10 degrees to a
perpendicular line of the optical axis of backward output light from the
LD 30). As a result, the backward output light from the LD 30, after being
reflected by the rear side wall 150 of the recessed optical device
mounting portion 15, can be prevented from leaking into the receiver PD
31. In the optical module of the present embodiment, crosstalk light from
the LD 30 to the receiver PD 31 was about -30 dB.
For comparison, an optical module according to the prior art as shown in
FIG. 2 was fabricated and the crosstalk light therein was measured. In
this comparative example, the LD 30 and the receiver PD 31 are disposed on
the same recessed optical device mounting portion 15, and the rear side
wall 150 of the recessed optical device mounting portion 15 is almost
perpendicular to the optical axis. In the module according to the prior
art, since some of the backward output lights from the LD 30 leak into the
receiver PD 31 due to the reflection from the rear side wall 150 of the
recessed optical device mounting portion 15, crosstalk light from the LD
30 to the receiver PD 31 was about -25 to -27 dB. From the above
comparison, the effect of the optical circuit construction of the present
embodiment is apparent.
Next, an example of fabrication method of the optical module of the present
embodiment will be briefly described with reference to FIGS. 3A to 3E.
First, the flat Si substrate 1 is patterned to etch an area other than a
Si terrace (protruded portion) 50 to a depth of about 30 .mu.m where the
semiconductor chip of LD 30, the semiconductor chip of monitor receiver PD
32 and the semiconductor chip of receiver PD 31 are to be mounted. On the
etched portion, a glass layer to be a lower cladding layer 51 is formed by
a flame deposition method (FIG. 3A). After that, flat polishing is
performed until the surface of the Si terrace 50 is exposed (FIG. 3B).
This surface becomes a level reference surface for the optical waveguide
when the semiconductor chip of LD 30, the semiconductor chip of monitor PD
32 and the semiconductor chip of receiver PD 31 are packaged.
Subsequently, a height adjusting cladding layer (second lower cladding
layer) 52 to be a height adjusting layer is formed. Next, a core layer 53
is deposited to a thickness of about 7 .mu.m (FIG. 3C). After the core
layer is etched into an optical waveguide pattern, an upper cladding layer
54 is deposited (FIG. 3D). Here, deposition of all of the cladding layers
and the core layer is achieved by using the flame deposition method.
Thereafter, only the Si terrace 50 is etched until the Si terrace is again
exposed. Finally, electrode wirings for the LD 30, the monitor receiver PD
32 and the receiver PD 31 are deposited, together with a mounting solder
55 (FIG. 3E).
Embodiment 2
FIG. 4 is a plane view showing a construction of the optical module
according to an embodiment 2 of the present invention. In the present
embodiment, the structure in the vicinity of the recessed optical device
mounting portion 15 is same as embodiment 1. However, the module of the
present embodiment differs from the embodiment 1 in that a light blocking
area 20 is provided in front of the recessed optical device mounting
portion 15.
The light blocking area 20 is provided to prevent forward output lights
from the LD 30 from leaking into the optical fiber 4b. A characteristic
feature of the present embodiment resides in that a side wall 201 at the
LD 30 side of the light blocking area 20 is slanted by an angle of about
70 degrees relative to the optical axis of the forward output lights of
the LD 30. As a result, the path of leakage lights caused by the forward
output lights from the LD 30 reflected by the side wall 201 of the light
blocking area 20 and leaking into the receiver PD 31 could be cut out. In
the thus fabricated optical module of the present embodiment, crosstalk
light from the LD 30 to the receiver PD 31 was less than -30 dB.
For comparison, an optical module according to the prior art as shown in
FIG. 5 was fabricated and measured for crosstalk light. In this
comparative example, the rear side wall 150 of the recessed optical device
mounting portion 15 and the side wall 201 of the light blocking area 20
are both disposed almost perpendicular to the optical axis of the LD 30.
As a result, in the module according to the prior art, crosstalk light
from the LD 30 to the receiver PD 31 is decreased to about -20 to -24 dB,
thus an effect of crosstalk light degradation due to the reflected lights
is apparent. From the above comparison, the effect of the optical circuit
construction of the present embodiment is apparent.
Embodiment 3
FIG. 6 is a plane diagram showing the construction of an optical module
according to an embodiment 3 of the present invention. The basic structure
of the present embodiment is same as the above embodiment 1. However, the
optical module of the present embodiment differs from the above embodiment
1 in the points that (1) the recessed optical device mounting portion 15
is filled with a transparent resin 5 for potting sealing, (2) the rear
side wall 150 of the recessed optical device mounting portion 15 is set
nearly perpendicular to the optical axis of rear output light from the LD
30, (3) further, as a most characteristic structure, a light blocking area
21 is provided at the rear side of the recessed optical device mounting
portion 15 which is filled with a black light absorbent substance, and its
side wall 211 is set obliquely.
In the present embodiment, the reason why the rear side wall 150 of the
recessed optical device mounting portion 15 is not disposed obliquely is
that since the recessed optical device mounting portion 15 is filled with
the transparent resin 5, reflection due to refractive index discontinuity
at the side wall becomes negligibly small. However, in this case,
reflection at an end of the optical waveguide is a problem, and the
reflection therefrom generates a leakage light path. To prevent this, the
light blocking area 21 is disposed at the rear of the recessed optical
device mounting portion 15. As a result, as the crosstalk light from the
LD 30 to the receiver PD 31, a value of about -30 to -33 dB was obtained.
Embodiment 4
FIGS. 7A and 7B are diagrams showing the construction of an optical module
according to an embodiment 4 of the present invention, in which FIG. 7A is
a perspective diagram and FIG. 7B is a plane diagram. The optical module
of the present embodiment comprises an embedded type silica optical
waveguide 2 including a cladding layer 2b and a core 2a formed on a Si
substrate 1, and a semiconductor chip of LD 30, a semiconductor chip of
monitor receiver PD 32 and a semiconductor chip of receiver PD 31, which
are disposed on the Si substrate 1.
The optical waveguide 2 is formed of two straight waveguides, an end of
which is connected with optical fibers 4a and 4b. The LD 30 and monitor
receiver PD 32 are disposed on the recessed optical device mounting
portion 15 formed by recessing the optical waveguide 2 (cladding layer
2b), and the receiver PD 31 is disposed on a recessed optical mounting
portion 16 formed by recessing the optical waveguide 2 (cladding layer
2b). Further, a light blocking area 20 formed by removing the cladding
layer 2b other than the vicinity of the core 2a is provided between the
recessed optical device mounting portion 15 and the recessed optical
device mounting portion 16.
The light blocking area 20 may be filled with an absorbent material in the
inside, or an opaque metal film or the like may be formed on the side wall
of the light blocking area 20. In the present embodiment, a gold thin film
was formed on the light blocking area 20 side wall.
A characteristic of the present embodiment is that the LD 30 and the
receiver PD 31 are disposed respectively on the different recessed optical
device mounting portions (15, 16), the light blocking area 20 is provided
in between, and of the side walls, the side wall 201 closer to the output
end of the LD 30 is disposed obliquely to have an angle of 70 degrees with
respect to the optical axis of the forward output lights of the LD 30. In
addition, the rear side wall 150 of the recessed optical device mounting
portion 15 for mounting the LD 30 is disposed oblique | | |
