U.S. patent application number 11/481925 was filed with the patent office on 2006-11-09 for subassembly and optical module.
Invention is credited to Masahiro Uekawa.
Application Number | 20060251362 11/481925 |
Document ID | / |
Family ID | 32992990 |
Filed Date | 2006-11-09 |
United States Patent
Application |
20060251362 |
Kind Code |
A1 |
Uekawa; Masahiro |
November 9, 2006 |
Subassembly and optical module
Abstract
A subassembly includes a supporting substrate having a V-shaped
groove at which a member is disposed, a laser diode mounted at the
supporting substrate, and a lens element. The lens element includes
a lens portion formed at a surface of an optical substrate, a
projection portion having a contour which places the projection
portion in contact with the V-shaped groove at the supporting
substrate when the lens element is mounted, and a rectangular
handling portion having a groove on an upper surface thereof for
identifying the lens formation surface, and the lens element is
positioned relative to the laser diode. An optical module includes
the subassembly, a package used to package the subassembly and an
interface. The interface includes an optical fiber to be optically
coupled with the laser diode via the lens element and is positioned
as it comes in contact with the package.
Inventors: |
Uekawa; Masahiro; (Kanagawa,
JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W.
SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
32992990 |
Appl. No.: |
11/481925 |
Filed: |
July 7, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10786024 |
Feb 26, 2004 |
|
|
|
11481925 |
Jul 7, 2006 |
|
|
|
Current U.S.
Class: |
385/93 ; 385/88;
385/92 |
Current CPC
Class: |
G02B 6/4292 20130101;
G02B 6/3652 20130101; G02B 6/2746 20130101; G02B 6/3636 20130101;
G02B 6/3692 20130101; G02B 6/4206 20130101 |
Class at
Publication: |
385/093 ;
385/088; 385/092 |
International
Class: |
G02B 6/36 20060101
G02B006/36 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 19, 2003 |
JP |
JP2003-075630 |
Nov 21, 2003 |
JP |
JP2003-391958 |
Claims
1. An optical module comprising: a supporting substrate having a
first groove at which a member is disposed; an optical element
mounted at said supporting substrate; a lens element positioned
relative to said optical element, said lens element including a
lens portion formed at a surface of an optical substrate, a
projection portion that comes in contact with said first groove at
said supporting substrate when said lens element is mounted, and a
rectangular handling portion extending in a direction orthogonal to
a direction in which said first groove extends and having a second
groove on an upper surface of said rectangular handling portion for
identifying a surface of said lens element on which said lens
portion is formed; one or more package components used to package
said supporting substrate mounted with said optical element and
said lens element; and an interface that includes an optical fiber
to be optically coupled with said optical element via said lens
element and is positioned as said interface comes in contact with
said one or more package components.
2. An optical module according to claim 1, wherein said lens
portion is constituted of a diffractive optical element.
3. An optical module according to claim 1, wherein the optical
substrate is a silicon crystal substrate.
4. An optical module according to claim 1, wherein said optical
element is either a light emitting element or a light receiving
element.
5. An optical module according to claim 1, wherein said supporting
substrate is held in an airtight space formed by said one or more
package components.
6. An optical module according to claim 1, wherein said one or more
package components are coaxial package components.
7. An optical module comprising; a supporting substrate having a
first groove at which a member is disposed; an optical element
mounted at said supporting substrate; a lens element positioned
relative to said optical element, said lens element including a
lens portion formed at a surface of an optical substrate to cause a
light flux to exit in a direction different from the direction of
an incident light flux, a projection portion that comes in contact
with said first groove at said supporting substrate when said lens
element is mounted, and a rectangular handling portion extending in
a direction orthogonal to a direction in which said first groove
extends and having a second groove on an upper surface of said
rectangular handling portion for identifying a surface of said lens
element on which said lens portion is formed; one or more package
components used to package said supporting substrate mounted with
said optical element and said lens element; and an interface that
includes an optical fiber, which is optically coupled with said
optical element via said lens element and has a diagonal end
surface, and is positioned as said interface comes in contact with
said one or more package components.
8. An optical module according to claim 7, wherein said lens
portion is constituted of a diffractive optical element.
9. An optical module according to claim 7, wherein the optical
substrate is a silicon crystal substrate.
10. An optical module according to claim 7, wherein said optical
element is either a light emitting element or a light receiving
element.
11. An optical module according to claim 7, wherein said supporting
substrate is held in an airtight space formed by said one or more
package components.
12. An optical module according to claim 7, wherein said one or
more package components are coaxial package components.
13. A subassembly comprising: a supporting substrate having a first
groove at which a member is disposed; a lens element that is
mounted at said supporting substrate, said lens element including a
lens portion formed at a surface of an optical substrate, a
projection portion which comes in contact with said first groove at
said supporting substrate when said lens element is mounted, and a
rectangular handling portion extending in a direction orthogonal to
a direction in which said first groove extends and having a second
groove on an upper surface of said rectangular handling portion for
identifying a surface of said lens element on which said lens
portion is formed; and an isolator element that is mounted at said
supporting substrate and has an isolator function.
14. A subassembly according to claim 13, wherein said lens portion
is constituted of a diffractive optical element.
15. A subassembly according to claim 13, wherein the optical
substrate is a silicon crystal substrate.
16. A subassembly according to claim 13, wherein the section of
said first groove at said supporting substrate is any of
substantially V-shaped, substantially trapezoidal, substantially
semicircular and substantially rectangular.
17. A subassembly according to claim 13, wherein said projection
portion has a circular arc shape.
18. An optical module comprising: a supporting substrate having a
first groove at which a member is disposed; a lens element that is
mounted at said supporting substrate and includes a lens portion
formed at a surface of an optical substrate, a projection portion
which comes in contact with said first groove at said supporting
substrate when said lens element is mounted, and a rectangular
handling portion extending in a direction orthogonal to a direction
in which said first groove extends and having a second groove on an
upper surface of said rectangular handling portion for identifying
a surface of said lens element on which said lens portion is
formed; an isolator element that is mounted at said supporting
substrate and has an isolator function; and a package component
having a pedestal portion, wherein said supporting substrate having
said lens element and said isolator element mounted thereat is set
at said pedestal portion.
19. An optical module according to claim 18, further comprising a
means for magnetic field application that is disposed at said
pedestal portion and applies a magnetic field to said isolator
element.
20. An optical module according to claim 18, wherein said package
component is a coaxial package component.
21. An optical module comprising: a supporting substrate having a
first groove at which a member is disposed; an optical element
mounted at said supporting substrate; a lens element positioned
relative to said optical element, said lens element including a
lens portion formed at a surface of an optical substrate, a
projection portion that comes in contact with said first groove at
said supporting substrate when said lens element is mounted, and a
rectangular handling portion extending in a direction orthogonal to
a direction in which said first groove extends and having a second
groove on an upper surface of said rectangular handling portion for
identifying a surface of said lens element on which said lens
portion is formed; an isolator element mounted at said supporting
substrate and having an isolator function; one or more package
components used to package said supporting substrate mounted with
said optical element, said lens element and said isolator element;
and an interface that includes an optical fiber to be optically
coupled with said optical element via said lens element and is
positioned as said interface comes in contact with said one or more
package components.
22. An optical module according to claim 21, further comprising a
means for magnetic field application that applies a magnetic field
to said isolator element.
23. An optical module according to claim 21, wherein said optical
element is either a light emitting element or a light receiving
element.
24. An optical module according to claim 21, wherein said
supporting substrate is held in an airtight space formed by said
one or more package components.
25. An optical module according to claim 21, wherein said one or
more package components are coaxial package components.
Description
[0001] This application is a divisional of U.S. application Ser.
No. 10/786,024, filed Feb. 26, 2004.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a subassembly and an
optical module that are ideal in applications in optical
communication systems and the like.
[0004] 2. Description of the Related Art
[0005] An optical module in the related art is normally achieved by
packaging a subassembly substrate at which a laser diode is
mounted, a lens provided to achieve optical coupling and an optical
fiber. The lens is constituted of a ball lens or an aspherical lens
in most cases. Coaxial optical modules known in the related art
include can-type optical modules, pigtail-type optical modules
(see, for instance, Japanese Patent Laid-open Publication No.
5-343709) and receptacle-type optical modules. The optical axes are
adjusted for alignment by utilizing an aligning device while
monitoring the light output.
[0006] The need for optical modules achieving a higher degree of
coupling efficiency that can be, at the same time, offered at lower
prices have risen in recent years. The semiconductor laser module
disclosed in Japanese Patent Laid-open Publication No. 8-23138, for
instance, is manufactured by using fewer parts and through a
simpler alignment process in order to achieve a cost reduction.
More specifically, a holder having a ball lens attached thereto is
utilized, and the holder and the laser package are fixed. After a
ferrule in which an optical fiber is housed is set against the
holder, the ferrule is aligned along directions perpendicular to
the optical axis.
[0007] In addition, an optical module equipped with an isolator has
been often used in recent years to prevent return light from
entering the laser diode. The isolator may be constituted mainly
with a Faraday rotator and polarizers mounted on the two sides of
the Faraday rotator, for instance. In some cases, magnets that
apply magnetic fields to the Faraday rotator are disposed around
the Faraday rotator and the polarizers. An optical module equipped
with an isolator usually adopts a structure in which the isolator
is located toward the optical fiber end with a small light flux
diameter so as to acheive miniaturization of the isolator (see, for
instance, U.S. Pat. No. 5,841,922). The miniaturization of the
isolator is a crucial priority since it greatly contributes to
lowering the cost of the optical module and also helps to achieve
miniaturization of the optical module itself.
SUMMARY OF THE INVENTION
[0008] However, it is still difficult to achieve miniaturization of
the optical module in the related art described above which
normally includes a lens such as a ball lens with a significant
external diameter that is mounted by using a holding member such as
a cap. In addition, the coupling efficiency of such an optical
module is greatly compromised unless the ball lens is set at a
predetermined position relative to the laser diode with a high
degree of accuracy. Accordingly, an optical module that allows
highly accurate positioning has been eagerly sought.
[0009] An object of the present invention, which has been achieved
by addressing the problems discussed above, is to provide a new and
improved subassembly and a new and improved optical module that
enable highly accurate positioning and miniaturization.
[0010] In order to achieve the object described above, a first
aspect of the present invention provides an optical module
comprising: a supporting substrate having a groove at which a
member is disposed; an optical element mounted at the supporting
substrate; a lens element positioned relative to the optical
element, where the lens element includes a lens portion formed at a
surface of an optical substrate and a projection portion that comes
in contact with the groove at the supporting substrate when the
lens element is mounted, and a rectangular handling portion
extending in a direction orthogonal to a direction in which the
groove at the optical substrate extends and having a groove on an
upper surface thereof for identifying a surface of the lens element
on which the lens portion is formed; one or more package components
used to package the supporting substrate mounted with the optical
element and the lens element; and an interface that includes an
optical fiber to be optically coupled with the optical element via
the lens element and that is positioned as it comes in contact with
the package component.
[0011] A second aspect of the present invention provides an optical
module comprising: a supporting substrate having a groove at which
a member is disposed; an optical element mounted at the supporting
substrate; a lens element positioned relative to the optical
element, where the lens element includes a lens portion formed at a
surface of an optical substrate to cause a light flux to exit in a
direction which is different from the direction of the incident
light flux, a projection portion that comes in contact with the
groove at the supporting substrate when the lens element is
mounted, and a rectangular handling portion extending in a
direction orthogonal to a direction in which the groove at the
optical substrate extends and having a groove on an upper surface
thereof for identifying a surface of the lens element on which the
lens portion is formed; one or more package components used to
package the supporting substrate mounted with the optical element
and the lens element; and an interface that includes an optical
fiber to be optically coupled with the optical element via the lens
element, which has a diagonal end surface, and that is positioned
as it comes in contact with the package component.
[0012] A third aspect of the present invention provides a
subassembly comprising: a supporting substrate having a groove at
which a member is disposed; a lens element that is mounted at the
supporting substrate and that includes a lens portion formed at a
surface of an optical substrate, a projection portion which comes
in contact with the groove at the supporting substrate when the
lens element is mounted, and a rectangular handling portion
extending in a direction orthogonal to a direction in which the
groove at the optical substrate extends and having a groove on an
upper surface thereof for identifying a surface of the lens element
on which the lens portion is formed; and an isolator element that
is mounted at the supporting substrate and has an isolator
function.
[0013] A fourth aspect of the present invention provides an optical
module comprising: a supporting substrate having a groove at which
a member is disposed; a lens element that is mounted at the
supporting substrate and that includes a lens portion formed at a
surface of an optical substrate, a projection portion which comes
in contact with the groove at the supporting substrate when the
lens element is mounted, and a rectangular handling portion
extending in a direction orthogonal to a direction in which the
groove at the optical substrate extends and having a groove on an
upper surface thereof for identifying a surface of the lens element
on which the lens portion is formed; an isolator element that is
mounted at the supporting substrate and has an isolator function;
and a package component having a pedestal portion. This optical
module is characterized in that the supporting substrate having the
lens element and the isolator element mounted thereat is set at the
pedestal portion.
[0014] A fifth aspect of the present invention provides an optical
module comprising: a supporting substrate having a groove at which
a member is disposed; an optical element mounted at the supporting
substrate; a lens element positioned relative to the optical
element, where the lens element includes a lens portion formed at a
surface of an optical substrate, a projection portion that comes in
contact with the groove at the supporting substrate when the lens
element is mounted, and a rectangular handling portion extending in
a direction orthogonal to a direction in which the groove at the
optical substrate extends and having a groove on an upper surface
thereof for identifying a surface of the lens element on which the
lens portion is formed; an isolator element that is mounted at the
supporting substrate and has an isolator function; one or more
package components used to package the supporting substrate mounted
with the optical element, the lens element and the isolator
element; and an interface that includes an optical fiber to be
optically coupled with the optical element via the lens element and
that is positioned as it comes in contact with the package
component.
[0015] A sixth aspect of the present invention provides a
subassembly comprising: a supporting substrate having a groove at
which a member is disposed; a light emitting element that is
mounted at the supporting substrate and emits light with a first
wavelength; a lens element which includes a lens portion formed at
a surface of an optical substrate and a projection portion that
comes in contact with the groove at the supporting substrate when
the lens element is mounted, which is positioned relative to the
light emitting element and which converts divergent light emitted
from the light emitting element to substantially parallel light; a
wavelength dividing filter that is mounted at the supporting
substrate and has a function of dividing light into different
wavelengths; and a light receiving element at which light with a
second wavelength having been divided through the wavelength
dividing filter enters.
[0016] A seventh aspect of the present invention provides an
optical module comprising: a supporting substrate having a groove
at which a member is disposed; a light emitting element that is
mounted at the supporting substrate and emits light with a first
wavelength; a first lens element which includes a lens portion
formed at a surface of an optical substrate and a projection
portion that comes in contact with the groove at the supporting
substrate when the lens element is mounted, which is positioned
relative to the light emitting element and which converts divergent
light emitted from the light emitting element to substantially
parallel light; a wavelength dividing filter that is mounted at the
supporting substrate and has a function of dividing light into
different wavelengths; a light receiving element at which light
with a second wavelength having been divided through the wavelength
dividing filter enters; one or more package components used to
package the supporting substrate having the light emitting element,
the first lens element and the wavelength dividing filter mounted
thereat and the light receiving element; a second lens element that
converts the substantially parallel light to convergent light; and
an interface which includes an optical fiber at which the light
with the first wavelength having been converted to the convergent
light enters and the light with the second wavelength exits toward
the second lens element and which is positioned as it comes in
contact with the package component.
[0017] An eighth aspect of the present invention provides a
subassembly comprising: a supporting substrate having a first
groove and a second groove adopting a first structure and a groove
adopting a second structure which is positioned between the first
groove and the second groove adopting the first structure; a light
emitting element that is mounted at the supporting substrate and
emits light with a first wavelength; a first lens element which
includes a lens portion formed at a surface of an optical substrate
and a projection portion that comes in contact with the first
groove adopting the first structure when the first lens element is
mounted, which is positioned relative to the light emitting element
and which converts divergent light emitted from the light emitting
element to substantially parallel light; a second lens element
which includes a lens portion formed at a surface of an optical
substrate and a projection portion that comes in contact with the
second groove adopting the first structure when the second lens
element is mounted and which converts the substantially parallel
light to convergent light; a wavelength dividing filter that is
disposed at the groove adopting the second structure and has a
function of the dividing light into different wavelengths; and a
light receiving element at which light with a second wavelength
having been divided through the wavelength dividing filter
enters.
[0018] A ninth aspect of the present invention provides an optical
module comprising: a supporting substrate having a first groove and
a second groove adopting a first structure and a groove adopting a
second structure which is positioned between the first groove and
the second groove adopting the first structure; a light emitting
element that is mounted at the supporting substrate and emits light
with a first wavelength; a first lens element which includes a lens
portion formed at a surface of an optical substrate and a
projection portion that comes in contact with the first groove
adopting the first structure when the first lens element is
mounted, which is positioned relative to the light emitting element
and which converts divergent light emitted from the light emitting
element to substantially parallel light; a second lens element
which includes a lens portion formed at a surface of an optical
substrate and a projection portion that comes in contact with the
second groove adopting the first structure when the second lens
element is mounted and which converts the substantially parallel
light to convergent light; a wavelength dividing filter that is
disposed at the groove adopting the second structure and has a
function of dividing light into different wavelengths; a light
receiving element at which light with a second wavelength having
been divided through the wavelength dividing filter enters; one or
more package components used to package the supporting substrate
having the light emitting element, the first lens element, the
second lens element and the wavelength dividing filter mounted
thereat and the light receiving element; and an interface that
includes an optical fiber at which the light with the first
wavelength having been converted to the convergent light enters and
the light with the second wavelength exits toward the second lens
element, and is positioned as it comes in contact with the package
component.
[0019] In the structures described above, the lens portions may be
constituted with a diffractive optical element. The optical
substrate may be a silicon crystal substrate. The optical element
may be a light emitting element such as a laser diode or a light
receiving element such as a photodiode. It is desirable that the
supporting substrate having the various members mounted thereat be
held in an airtight space. The package component may be a coaxial
package component.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective showing the structure of the
subassembly achieved in a first embodiment of the present
invention;
[0021] FIG. 2 is a sectional view of the structure adopted in the
optical module in the first embodiment of the present
invention;
[0022] FIG. 3 shows the relationship between the position along the
Z direction and the coupling efficiency;
[0023] FIG. 4 is a sectional view of the structure adopted in the
optical module in a second embodiment of the present invention;
[0024] FIG. 5A is a perspective showing the structure of the
subassembly achieved in a third embodiment of the present
invention;
[0025] FIG. 5B is an exploded perspective showing the structure of
the subassembly achieved in the third embodiment of the present
invention;
[0026] FIG. 6 is a plan view of the structure adopted in an optical
module in the third embodiment of the present invention;
[0027] FIG. 7A is an exploded perspective showing the structure
adopted in the optical module in the third embodiment of the
present invention;
[0028] FIG. 7B is a perspective showing the structure adopted in
the optical module in the third embodiment of the present
invention;
[0029] FIG. 8 is a partial sectional view showing the structure
adopted in an optical module in the third embodiment of the present
invention;
[0030] FIG. 9 is a perspective showing the structure of the
subassembly achieved in a fourth embodiment of the present
invention;
[0031] FIG. 10 is a sectional view of the structure adopted in the
optical module in the fourth embodiment of the present
invention;
[0032] FIG. 11 is a perspective showing the structure of the
subassembly achieved in a fifth embodiment of the present
invention;
[0033] FIG. 12 is a sectional view of the structure adopted in the
optical module in the fifth embodiment of the present
invention;
[0034] FIG. 13 is a partial sectional view showing the structure of
an optical module in the related art; and
[0035] FIG. 14 is a partial sectional view showing the structure of
an optical module in the related art.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The following is a detailed explanation of the embodiments
of the present invention with reference to the drawings. It is to
be noted that in the following explanation and attached drawings,
the same reference numerals are assigned to components having
substantially identical functions and structural features to
preclude the necessity for a repeated explanation thereof.
[0037] In a typical mode of the present invention, the subassembly
includes a supporting substrate, an optical element and a lens
element. The optical element and the lens element are mounted onto
the supporting substrate. A groove at which a member is to be
disposed is formed at the supporting substrate. The optical element
may be, for instance, a light emitting element or a light receiving
element. The lens element includes a lens portion formed at a
surface of an optical substrate, and a projection portion having a
shape that allows the projection portion to come into contact with
the groove at the supporting substrate when the lens element is
mounted. By setting the projection portion at the groove, the lens
element is positioned relative to the optical element with a high
degree of accuracy along a direction perpendicular to the optical
axis.
[0038] In addition, in a typical mode of the present invention, the
optical module comprises mainly the subassembly described above, a
package component and an interface. The package component is used
to package the subassembly in an airtight state. The interface is
fixed onto the package component and functions as a connector
terminal. The interface includes an optical fiber that is optically
coupled with the optical element via the lens element. The
interface is positioned as it comes in contact with the package
component.
First Embodiment
[0039] The structures of the subassembly and the optical module
achieved in the first embodiment of the present invention are now
explained with reference to FIGS. 1 and 2. FIG. 1 is a perspective
of a subassembly 100 achieved in the first embodiment of the
present invention. FIG. 2 is a sectional view of an optical module
102 achieved in the first embodiment of the present invention by
using the subassembly 100. The subassembly 100 includes a
supporting substrate 110, a laser diode 120 and a lens element
130.
[0040] The supporting substrate 110 may be constituted of, for
instance, a silicon crystal substrate. The size of the supporting
substrate 110 may be 1 mm.times.1 mm.times.2 mm.times.2 mm. As
shown in FIG. 1, a V-shaped groove 112 having a V-shaped section is
formed at the supporting substrate 110. The V-shaped groove 112, at
which a member is disposed, is formed from one end of the
supporting substrate 110 to extend to a middle point of the
supporting substrate 110. The V-shaped groove 112 may be formed so
as to achieve dimensions that allow a single mode optical fiber
with a 125 .mu.m diameter to be placed therein. The V-shaped groove
112 can be formed through, for instance, anisotropic etching. In
addition, an indented groove 113 having a rectangular section is
formed at the supporting substrate 110 along a direction that is
perpendicular to the direction along which the V-shaped groove 112
extends. The indented groove 113 prevents light emitted from the
laser diode 120 from becoming blocked.
[0041] The laser diode 120, which is a light emitting element, is
disposed at an area on the supporting substrate 110 where the
V-shaped groove 112 is not present, on a line extending from the
V-shaped groove 112.
[0042] The lens element 130 is constituted of an optical substrate
which is a silicon crystal substrate in this example. The lens
element 130 is constituted mainly of a lens portion 132 formed at a
surface of the optical substrate, a projection portion 136 that
comes in contact with the V-shaped groove 112 when the lens element
130 is mounted, and a handling portion 134 that allows the lens
element 130 to be held with ease for handling.
[0043] The lens portion 132 is constituted of a diffractive optical
element formed at one surface of the optical substrate. In this
embodiment, the lens portion 132 is formed in a circular shape with
a diameter of, for instance, 50 to 125 .mu.m. The lens portion 132
may be formed by using the photolithography technology and etching
technology adopted in semiconductor manufacturing processes. In the
following explanation, the surface of the lens element located on
the side where the lens portion 132 is formed is referred to as a
lens formation surface. In this example, the direction extending
perpendicular to the lens formation surface matches the direction
along which the optical axis extends.
[0044] An edge portion 133 located along the bottom side of the
lens portion 132 constitutes part of the periphery of the lens
portion 132 and assumes the shape of an arc ranging along the
contour of the circumference of the lens portion 132. The contour
of the arc-shaped edge portion 133 is continuously present from the
lens formation surface to the surface facing opposite the lens
formation surface, as the semi-cylinder constituting part of the
substantially cylindrical shape, the central axis of which is the
optical axis of the lens portion 132. This semi-cylindrical portion
projecting downward from the middle position of the handling
portion 134 is referred to as the projection portion 136. The shape
of the projection portion 136 allows the projection portion 136 to
fit in the V-shaped groove 112 so that the lens element 130 can be
mounted at the supporting substrate 110 while the projection
portion 136 is set in contact with the V-shaped groove 112. In
other words, the projection portion 136 assumes a shape that allows
the lens element 130 to be positioned along two directions (the X
direction and the Y direction) that are perpendicular to the
optical axis simply by mounting the lens element 130 with the
projection portion 136 in contact with the V-shaped groove 112. It
is to be noted that while the edge portion 133 is formed around the
lens element 130 in this example, the edge portion 133 may instead
be constituted with the external circumference of the lens portion
132.
[0045] The handling portion 134 ranging so as to surround the top
side of the lens portion 132 has a greater width than the lens
portion 132 within a plane substantially parallel to the surface of
the lens portion 132, and assumes the shape of a bar extending to
the left and the right in FIG. 1. The handling portion 134 is
formed so as to constitute an integrated unit with the lens portion
132, the edge portion 133 and the projection portion 136. The upper
surface of the handling portion 134 is flat and, as a result, the
lens element 130 can be held from above by a means for holding with
ease. The means for holding may be, for instance, a means for
negative pressure holding such as a vacuum suction cup capable of
vacuum holding the lens element 130.
[0046] If the wavelength of light originating from the light source
of the optical system in which the lens element 130 is utilized is
1.3 .mu.m or 1.55 .mu.m, the ideal material to constitute the lens
element 130 is a silicon crystal substrate. The lens element 130
can be manufactured through the photolithography technology and
etching technology adopted in semiconductor manufacturing
processes. A photolithography step and an etching step may be
executed repeatedly on the silicon substrate to prepare the lens
portion 132 constituted of a diffractive optical element. The
substrate may then be etched until a desired depth is achieved
through deep etching or the like by forming a pattern with a shape
corresponding to that of the lens element 130 as a photo mask
pattern to form the lens element 130. By adopting such a method,
the lens element 130 can be mass produced with a high degree of
accuracy at low cost.
[0047] At the subassembly 100, the laser diode 120 and the lens
portion 132 of the lens element 130 are positioned and disposed so
that they share a common optical axis. The lens element 130 can be
positioned with ease along the two directions perpendicular to the
optical axis by mounting the lens element 130 with the projection
portion 136 in contact with the V-shaped groove 112. The lens
portion 130 is positioned along the optical axis (the Z direction)
by using image recognition technology, as explained later.
[0048] The thickness of the lens element 130 along the optical axis
may be set to, for instance, 100 .mu.m. The distance between the
lens element 130 and the laser diode 120 may be, for instance, 80
.mu.m. In addition, the length of the handling portion 134 of the
lens element 130 along the major axis may be within a range of 250
to 500 .mu.m. The lens element 130 having a lens portion
constituted of a diffractive optical element, as described above,
is smaller in size than lenses used in optical modules in the
related art. In addition, since the distance between the optical
element and the lens element, which are optically coupled, along
the optical axis is greatly reduced, the light flux propagated
between these elements is allowed to maintain a small diameter as a
result. Thus, the subassembly 100, which includes the laser diode
120 and the lens element 130 to be optically coupled with the laser
diode 120, is also extremely small in size.
[0049] Next, the optical module 102, which includes the subassembly
100, is explained with reference to FIG. 2. The optical module 102
comprises the subassembly 100, a package 150 and an interface 170.
In FIG. 2, the subassembly 100 is packaged by using package parts
constituting the package 150, and the interface 170 is fixed in
contact with the package 150.
[0050] The package 150 is a coaxial package that includes a cap 152
having a substantially cylindrical external shape, a header 154
constituting a pedestal portion, a stem 156 having a substantially
disk shape, and electrode terminals 158. One end of the header 154
is fixed onto one surface of the stem 156, whereas a stage against
which the subassembly 100 is to be abutted is formed at the other
end of the header 154. It is to be noted that this contact stage is
not a crucial structural feature and the header may instead adopt a
structure that does not include any stage. The subassembly 100 is
fixed onto the header 154. A photodiode 160, which is a light
receiving element, is fixed on the surface of the stem 156 located
toward the header 154. The photodiode 160 is used to monitor the
light traveling from the rear end surface of the laser diode 120.
The laser diode 120 and the photodiode 160 are each electrically
connected with an electrode terminal 158.
[0051] The cap 152 is constituted of a metal such as iron. One end
of the cap 152 is fixed to the stem 156, whereas the interface 170
is fixed to the other end of the cap 152. A barrier wall 153 is
formed inside the cap 152, with a flat window 151 mounted over a
portion of the barrier wall 153. The flat window 151 is constituted
of a material that allows the light emitted from the laser diode
120 to be transmitted. A sealed space S1 formed by the stem 156,
the cap 152, the barrier wall 153 and the flat window 151 is
maintained in an airtight state. The subassembly 100 is held inside
the airtight space S1. The front end of the interface 170 is
inserted in the space on the other side of the barrier wall 153 and
the flat window 151, opposite from the side where the airtight
space S1 is located. The length of the cap 152 along the optical
axis is set in advance during the design stage so that when a large
diameter portion of the interface 170 and the end of the cap 152
are abutted to each other, the point at which light is condensed
through the lens element 130 is positioned at the end surface of an
optical fiber 171.
[0052] The interface 170 is a receptacle type connector terminal
having the optical fiber 171, a ferrule 172 and a sleeve 173. The
end surface of the optical fiber 171 located toward the lens
element 130 is formed as a diagonal surface. As a result, the light
emitted from the laser diode 120 is prevented from re-entering the
laser diode 120 after being reflected at this end surface and
external light having traveled through the optical fiber 171 is
also prevented from going back to the outside after being reflected
at the end surface. The sleeve 173 constituting an external frame
includes a larger diameter portion which is abutted against the end
of the cap 152. The optical fiber 171 and the ferrule 172
surrounding the optical fiber 171 are inserted and fixed inside the
sleeve 173, ranging from one end of the sleeve 173 to a middle
point of the sleeve 173. A hollow portion 174 is formed inside the
sleeve 173 to range from the middle point to the other end, and a
connector (not shown) is inserted in the hollow portion.
[0053] In the optical module 102, the laser diode 120, the lens
portion 132 and the optical fiber 171 are set so that they share a
common optical axis. The divergent light emitted from the laser
diode 120 is converted to convergent light at the lens element 130,
travels through the flat window 151 and then enters the optical
fiber 171 so that the light is condensed at a point at the end
surface of the optical fiber 171. Thus, the laser diode 120 is
optically coupled with the optical fiber 171 via the lens element
130.
[0054] The following is an explanation of an example of a
manufacturing method that may be adopted to manufacture the
subassembly 100 and the optical module 102. First, the supporting
substrate 110 having the V-shaped groove 112 formed thereat is
prepared. The laser diode 120 is then set on the supporting
substrate 110 by positioning the laser diode 120 with a high degree
of accuracy from above the supporting substrate 110 with a marker
(not shown), and the laser diode 120 is bonded onto the supporting
substrate 110 with solder or the like. Next, the lens element 130
is set with the projection portion 136 in contact with the V-shaped
groove 112. With this, the lens element 130 is positioned along
directions (the X direction and the Y direction) perpendicular to
the optical axis. The position of the lens element 130 along the
optical axis (the Z direction) is set by using a marker (not shown)
provided in advance at the supporting substrate 110. Once it is
verified that the lens element 130 is set at the correct position,
the lens element 130 is bonded onto the V-shaped groove 112. The
adhesive that may be used to bond the lens element 130 may be a
thermosetting resin, a UV (ultraviolet)-setting resin or solder.
Through the process described above, the subassembly 100 is
manufactured.
[0055] While the accuracy with which the laser diode 120 and the
lens element 130 are mounted depends upon the accuracy of the
bonder, they can be mounted at an accuracy level of .+-.3 .mu.m
with ease. While the mounting accuracy with which the lens element
130 is mounted along the directions that are perpendicular to the
optical axis is dependent upon the accuracy with which the
projection portion 136 is manufactured and also the accuracy with
which the V-shaped groove 112 is manufactured, the lens element 130
can be mounted at an accuracy level as high as .+-.1 .mu.m.
[0056] Next, the subassembly 100 is set on the header 154, the
subassembly 100 is positioned along the optical axis by abutting
the subassembly 100 against the stage formed at the header 154 and
the subassembly 100 is then fixed by using a thermosetting resin or
solder. It is to be noted that even when no contact stage is
provided, the subassembly 100 can still be mounted at the header
154 with a mounting accuracy as high as .+-.10 .mu.m. Since the
laser diode 120 and the lens element 130 are already aligned with
each other and are positioned relative to each other, the mounting
accuracy of the subassembly 100 only affects the efficiency with
which the subassembly 100 and the optical fiber 171 are optically
coupled with each other. When the subassembly 100 is mounted, the
wiring of the laser diode 120 is electrically connected through
wire bonding. Then, the cap 152 is attached and one end of the cap
152 is welded and fixed onto the stem 156. The subassembly 100 is
held in the airtight space S1 in this state.
[0057] Next, the interface 170 is inserted at the other end of the
cap 152 and the large diameter portion of the interface 170 is
abutted against the end of the cap 152. As explained earlier, the
length of the cap 152 along the optical axis is set in advance at
the design stage so that the light is condensed at a point at the
end surface of the optical fiber 171 in this state. Thus, by
abutting the interface 170 against the end of the cap 152 in this
manner, the position along the optical axis can be determined. In
the abutted state, the laser diode 120 is made to emit light and
the interface 170 is aligned along directions that are
perpendicular to the optical axis while monitoring the light
exiting the optical fiber 171. When the alignment is completed, the
contact area where the large diameter portion and the cap 152 are
in contact with each other is fixed through welding or the like.
Through the processing described above, the optical module 102 is
manufactured.
[0058] As described above, the lens element 130 is positioned with
a high degree of accuracy along three directions, i.e., the X
direction, the Y direction and the Z direction, relative to the
laser diode 120. In particular, very precise positioning is
achieved along the X direction and the Y direction with ease simply
by placing the projection portion 136 at the V-shaped groove 112.
As the positioning accuracy along the X direction and the Y
direction greatly affects the optical coupling efficiency, an
improvement in the optical coupling efficiency can be achieved in
the optical module 102 adopting the structure described above. In
addition, no aligning step needs to be performed along the Z
direction.
[0059] The distance between the lens element 130 and the optical
fiber 171 is several times the distance between the lens element
130 and the laser diode 120 in the optical module 102. When the
magnifying power of the optical system is taken into consideration,
the position at which the light is condensed on the side toward the
optical fiber 171 greatly changes if the position of the lens
element 130 along the Z direction changes even slightly relative to
the laser diode 120. For instance, if the lens element 130 is not
positioned relative to the laser diode 120 with accuracy and there
is an error that cannot be disregarded in individual optical
modules, the extent of inconsistency of the position at which the
light is condensed on the side toward the optical fiber 171 becomes
significant and the alignment of the optical fiber 171 along the Z
direction becomes difficult. In contrast, the lens element 130 is
positioned with a high degree of accuracy relative to the laser
diode 120, and thus, the extent of inconsistency in the light
condensing position can be minimized in the optical module 102 in
the embodiment.
[0060] In addition, a greater alignment tolerance is afforded with
regard to the alignment of the lens element 130 and the optical
fiber 171 as compared to the tolerance for the alignment of the
laser diode 120 and the lens element 130. Let us now consider an
example in which the distance between the laser diode 120 and the
lens element 130 is approximately 80 .mu.m and the distance between
the lens element 130 and the optical fiber 171 is approximately 500
.mu.m. FIG. 3 presents a graph of the coupling efficiency which is
measured relative to the misalignment occurring along the Z
direction in this situation. The horizontal axis in FIG. 3
represents the position along the Z direction, whereas the vertical
axis in FIG. 3 represents the coupling efficiency. In FIG. 3, the
curve L relates to the misalignment of the laser diode 120 and the
lens element 130, whereas the curve F relates to the misalignment
of the optical fiber 171 and the lens element 130. Both curves in
FIG. 3 are normalized by setting the coupling loss at zero in
correspondence to the Z position 0 indicating no misalignment. FIG.
3 indicates that even a slight extent of misalignment of the laser
diode 120 and the lens element 130 results in a great reduction in
the coupling efficiency. In contrast, even a large extent of
misalignment between the lens element 130 and the optical fiber 171
does not significantly affect the coupling efficiency. In other
words, while it is necessary to mount the laser diode 120 and the
lens element 130 with a high degree of accuracy, the optical fiber
171 does not need to be mounted with such rigorous precision.
[0061] By taking these points into consideration, the optical fiber
171 is positioned along the Z direction simply by abutting the
large diameter portion of the interface 170 against the end of the
cap 152 and no further alignment is performed along the Z direction
in the optical module 102. In addition, the lens element 130 set at
the correct position along the X direction and the Y direction
relative to the laser diode 120 through a very simple method. Thus,
since the positioning process is simplified and only a small number
of aligning steps need to be performed, the optical module 102 can
be manufactured with greater ease. Ultimately, a coupling
efficiency of approximately 50% is achieved between the laser diode
120 and the optical fiber 171.
[0062] As explained above, the optical module 102 in the first
embodiment is provided as a highly efficient and low-cost optical
module that also achieves a high level of mass productivity. In
addition, since a smaller lens element constituted of a diffractive
optical element is used and thus the lens size is reduced as
compared to that of a ball lens, the focal length can be reduced as
well in the optical module 102 in the first embodiment. As a
result, it is possible to miniaturize the optical module by
adopting the first embodiment.
Second Embodiment
[0063] Next, the subassembly and the optical module achieved in the
second embodiment of the present invention are explained with
reference to FIG. 4. FIG. 4 is a sectional view of an optical
module 202 achieved in the second embodiment of the present
invention. The optical module 202 differs from the optical module
102 in the first embodiment in that the optical system is
constituted as an axial shift-type optical system. The following
explanation focuses on this feature, and a repeated explanation of
the structural features of the second embodiment which are
identical to those of the optical module 102 is omitted.
[0064] A lens element 230 is included in a subassembly 200
constituting the optical module 202. The lens portion of the lens
element 230 is constituted of a diffractive optical element that
emits a light flux along a direction that is different from the
direction in which the incident light flux enters. In this context,
this structure is referred to as an "axial shift". The lens element
230 in this second embodiment only differs from the lens element
130 in the first embodiment in the optical performance of its lens
portion, and otherwise assumes a structure which is similar to that
of the lens element 130. In addition, the components other than the
lens element 230 are identical to those used in the first
embodiment.
[0065] In FIG. 4, the optical axis M of the laser diode 120 is
indicated with a one-point chain line. The optical axis M matches
the central axis of the package 150. Divergent light is emitted
from the laser diode 120 as a light flux with a central axis
matching the optical axis M. This divergent light is converted to
convergent light at the lens portion of the lens element 230 and is
also deflected so that the central axis of the light flux extends
along a direction at an angle relative to the optical axis M. As
described above, the direction along which the light is emitted
from the lens element 230 in the second embodiment is different
from the direction along which the light is emitted from the lens
element 130 in the first embodiment. For this reason, the light
entering the optical fiber 171 achieves an even larger angle
relative to the normal line of the entry end surface of the optical
fiber 171 than in the first embodiment and, as a result, the
insertion loss at the optical fiber 171 can be reduced. For
instance, if the entry end surface of the optical fiber 171 has an
angle of 9.degree. relative to the direction that is perpendicular
to the optical axis M, optical coupling is achieved with the
central axis of the light flux that has exited the lens portion,
achieving an angle of 4.degree. on the downward side relative to
the optical axis M. Such an axial shift-type lens element can be
manufactured with ease by designing the diffractive optical element
constituting the lens portion so as to deflect light at a desired
angle and forming the diffractive optical element in conformance to
the design, as in the first embodiment.
[0066] The optical module 202 in the second embodiment can also be
manufactured as described above for the first embodiment. After the
subassembly 200 is mounted at the header 154, the interface 170 is
abutted against the cap 152 to achieve positioning along the Z
direction, and then alignment along the X direction and the Y
direction is performed by monitoring the light emitted from the
optical fiber 171. In this case, the center of the optical fiber
171 is offset from the center of the package 150. When the
alignment is completed, the abutted area is fixed through welding
or the like. The subassembly 200 is packaged by using package parts
constituting the package 150 and is then held in the air tight
space S1, as in the case of the subassembly 100 of the first
embodiment.
[0067] If a ball lens or the like in the related art is used to
direct light toward the end surface of the optical fiber so that
the light enters the optical fiber along a direction that is
different from the direction in which the optical axis M extends, a
special alignment step needs to be performed to offset the lens
from the central position, which is bound to complicate the
manufacturing process. However, the lens element in the second
embodiment only needs to be mounted by taking advantage of the
external contour of the lens element, as in the first embodiment,
and it is not necessary to perform any alignment step to offset the
lens element from the central position. In the second embodiment, a
higher level of coupling efficiency is achieved than in the first
embodiment. In addition, the second embodiment provides a low-cost
and compact optical module that enables highly accurate
positioning, as does the first embodiment.
Third Embodiment
[0068] Next, the subassembly and optical modules achieved in the
third embodiment of the present invention are explained with
reference to FIGS. 5A, 5B, 6, 7A, 7B and 8. FIG. 5A is a
perspective showing the structure of a subassembly 300 achieved in
the third embodiment of the present invention, and FIG. 5B is an
exploded perspective of the structure of the subassembly 300. This
third embodiment is characterized in that the subassembly 300
includes an isolator element. The following explanation focuses on
this feature, and some structural features similar to those in the
first embodiment are not explained again.
[0069] The subassembly 300 includes a supporting substrate 310, a
laser diode 120, a lens element 330 and an isolator element 340.
The laser diode 120, the lens element 330 and the isolator element
340 are mounted in this order over specific intervals on the
supporting substrate 310.
[0070] The supporting substrate 310 is constituted of a silicon
crystal substrate. As shown in FIGS. 5A and 5B, the supporting
substrate 310 adopts a two-level structure that includes a stage
and has a V-shaped groove 312 at which a member is to be disposed
and formed thereat. In the following explanation, the upper surface
of the upper level portion of the supporting substrate 310 is
referred to as an upper level surface 314, whereas the upper
surface of the lower level portion of the supporting substrate 310
is referred to as a lower level surface 316. The V-shaped groove
312 is formed to extend from one end of the upper level surface 314
located toward the staged side to a middle point. The stage can be
formed through, for instance, dicing, and the V-shaped groove 312
can be formed through, for instance, anisotropic etching.
[0071] As shown in FIG. 5A, the lens element 330 is positioned at
the V-shaped groove 312. The laser diode 120 is set at a position
where the V-shaped groove 312 is not present on a line extending
from the V-shaped groove 312 at the upper level surface 314. The
isolator element 340 is set on the lower level surface 316. The
individual components are positioned and mounted as described
above.
[0072] The lens element 330 is constituted of an optical substrate
which is a silicon crystal substrate in this example. As is the
lens element 130 in the first embodiment, the lens element 330 is
constituted mainly of a lens portion 332 formed at a surface of an
optical substrate, a projection portion 336 which comes in contact
with the V-shaped groove 312 when the lens element 330 is mounted
and a handling portion 334 which is used to hold the lens element
330 for easy handling.
[0073] While the lens portion 332 of the lens element 330 is
constituted of a diffractive optical element, its optical
performance, e.g., the focal length and the target wavelength, is
not necessarily the same as that of the lens portion 132 of the
lens element 130 in the first embodiment. In addition, the handling
portion 334 of the lens element 330 has a different shape from that
of the handling portion 134 of the lens element 130. Other
structural features and the method adopted to manufacture the lens
element 330 are identical to those of the lens element 130.
[0074] A groove 337 that is used to identify the lens formation
surface is formed at the upper surface of the handling portion 334.
The groove 337, which has a section with a substantially
rectangular shape and extends along a direction substantially
perpendicular to the lens formation surface, is positioned on one
side of the handling portion 334 instead of at the center of the
handling portion 334. Thus, the handling portion 334 adopts an
asymmetrical structure to the left and to the right relative to a
virtual plane that contains the center of the lens portion 332 and
is perpendicular to the upper surface. This asymmetry makes it
possible to distinguish the lens formation surface of the lens
element 330 from the opposite surface. It is to be noted that while
a single groove 337 having a section with a substantially
rectangular shape is provided to facilitate identification of the
lens formation surface, the number of grooves and the shape of the
grooves are not limited to those adopted in this example as long as
asymmetry is achieved.
[0075] In addition, positioning grooves 338a and 338b are formed at
the lower surface of the handling portion 334 facing opposite the
upper surface, on the two sides of the projection portion 336. The
grooves 338a and 338b having a section with a substantially
rectangular shape and extending along a direction that is
substantially perpendicular to the lens formation surface are each
set on either side of the lens portion 332. The lower surface is
placed in close proximity to the supporting substrate 310 when the
lens element 330 is mounted at the supporting substrate 310. The
grooves 338a and 338b are used for positioning along a direction
that is parallel to the direction in which the V-shaped groove 312
extends when mounting the lens element 330 at the supporting
substrate 310. It is to be noted that the number of positioning
grooves and their shape are not limited to those adopted in the
example.
[0076] The thickness of the lens element 330 along the optical axis
may be set to, for instance, 100 .mu.m. The distance between the
lens element 330 and the laser diode 120 may be, for instance, 80
.mu.m. In addition, the length of the handling portion 334 of the
lens element 330 along the major axis may be within a range of 250
to 500 .mu.m. The lens element 330 having a lens portion
constituted of a diffractive optical element, as described above,
is smaller in size than lenses used in optical modules in the
related art. In addition, since the distances between the laser
diode and the lens element, and between the laser diode and the
optical fiber, which are optically coupled along the optical axis,
are greatly reduced, the light flux propagated between these
elements is allowed to maintain a small diameter as a result.
[0077] The laser diode 120 is a light emitting element which is set
at a position so that the light emitted from the laser diode 120
enters the lens portion 332 of the lens element 330.
[0078] The isolator element 340 is constituted of polarizers 342a
and 342b and a Faraday rotator 344 disposed between the polarizers
342a and 342b. The isolator element 340 has a function of an
isolator whereby light advancing along a specific direction is
transmitted and light advancing along the opposite direction is
blocked. With this isolator function, any return light is prevented
from entering the laser diode 120. In addition, the isolator
element 340 is set at a slight angle of inclination relative to the
optical axis to prevent return light having exited the laser diode
120 and having been reflected at the surface of the isolator
element 340 from entering the laser diode 120.
[0079] As shown in FIG. 5A, the subassembly 300 adopting the
structure described above is an extremely compact subassembly which
still includes an isolator element 340. The overall length of the
subassembly 300 along the optical axis is less than approximately 1
mm in the third embodiment.
[0080] Next, an optical module constituted by using the subassembly
300 is explained. FIG. 6 is a plan view showing the structure of an
optical module 301 constituted by using the subassembly 300. FIG.
7A is an exploded perspective of the optical module 301 and FIG. 7B
is a perspective of the optical module 301. It is to be noted that
in FIGS. 6, 7A and 7B, details such as the grooves which are formed
at the lens element 330 are not included in the illustrations.
[0081] The optical module 301 includes the subassembly 300, two
magnets 346a and 346b, a package component and a photodiode 160.
The package component used in conjunction with the optical module
301 is a coaxial package component which includes a header 354
constituting a pedestal upon which the subassembly 300 is placed, a
stem 356 assuming a substantially disk shape, and electrode
terminals 358. The header 354 is fixed onto one surface of the stem
356. The electrode terminals 358, which pass through the stem 356
and are fixed, extend to the other surface of the stem 356. It is
to be noted that other than FIGS. 7A and 7B, the drawings do not
include an illustration of the electrode terminals 358 extending to
the other surface.
[0082] The subassembly 300 and the magnets 346a and 346b are
positioned and fixed onto the header 354. The magnets 346a and 346b
constitute a means for magnetic field application that applies a
magnetic field to the Faraday rotator 344 of the isolator element
340 and are disposed on the two sides of the isolator element
340.
[0083] The photodiode 160 is a light receiving element and is fixed
onto the surface of the stem 356 at a position above the header 354
so as to monitor the light originating from the rear end surface of
the laser diode 120. It is to be noted that the laser diode 120 and
the photodiode 160 are each electrically connected with an
electrode terminal 358.
[0084] As described above, in the optical module 301, the
supporting substrate 310 having the laser diode 120, the lens
element 330 and the isolator element 340 mounted thereupon and the
magnets 346a and 346b are disposed on the header 354 in an
extremely compact configuration.
[0085] FIG. 8 is a partial sectional view showing the structure of
an optical module 302 constituted by using the subassembly 300. A
cap 352 and an interface 170 are shown in sectional form. It is to
be noted that FIG. 8 does not include an illustration of the groove
337 at the lens element 330. The optical module 302 adopts a
structure achieved by adding the substantially cylindrical cap 352
and the interface 170 to the structure of the optical module 301.
The interface 170 is identical to that in the first embodiment.
Some of the structural features identical to those having been
referred to earlier are not explained.
[0086] The cap 352 is a coaxial package part constituted of a metal
such as iron. One end of the cap 352 is fixed to the stem 356,
whereas the interface 170 is fixed to the other end of the cap 352.
A barrier wall 353 is formed inside the cap 352, with a flat window
351 mounted over a portion of the barrier wall 353. The flat window
351 is constituted of a material that allows the light emitted from
the laser diode 120 to be transmitted. A sealed space S3 formed by
the stem 356, the cap 352, the barrier wall 353 and the flat window
351 is maintained in an airtight state. The subassembly 300 is held
inside the airtight space S3. The front end of the interface 170 is
inserted in the space on the other side of the barrier wall 353 and
the flat window 351 opposite from the side where the airtight space
S3 is located. The length of the cap 352 along the optical axis is
set in advance during the design stage so that when the large
diameter portion of the interface 170 and the end of the cap 352
are abutted to each other, the point at which light is condensed
through the lens element 330 is positioned at the end surface of
the optical fiber 171, as in the first embodiment.
[0087] The divergent light emitted from the laser diode 120 is
converted to convergent light at the lens element 330, is
transmitted through the isolator 340, travels through the flat
window 351 and then enters the optical fiber 171 so that the light
is condensed at a point at the end surface of the optical fiber
171. Thus, the laser diode 120 is optically coupled with the
optical fiber 171 via the lens element 330.
[0088] The following is an explanation of an example of a
manufacturing method that may be adopted to manufacture the
subassembly 300 and the optical module 301 and 302. First, the
supporting substrate 310 having the V-shaped groove 312 and the
stage formed thereat is prepared, the laser diode 120 is set at the
upper level surface 314 by positioning the laser diode 120 with a
high degree of accuracy from above the supporting substrate 110
with a marker (not shown), and then the laser diode 120 is bonded
onto the supporting substrate 110 with solder or the like. Next,
after identifying the lens formation surface of the lens element
330 by using the grooves 337, the lens element 330 is disposed with
the projection portion 336 in contact with the V-shaped groove 312.
With this, the lens element 330 is positioned along the direction
that is perpendicular to the upper level surface 314. The position
of the lens element 330 along the optical axis is set by using the
positioning grooves 338a and 338b and a marker (not shown) provided
in advance at the supporting substrate 310. The lens element 330
can be set by holding its upper surface or side surface which is
flat with an appropriate means for holding. Once it is verified
that the lens element 330 is set at the correct position, the lens
element 330 is bonded onto the V-shaped groove 312. The adhesive
that may be used to bond the lens element 330 may be a
thermosetting resin, a UV (ultraviolet)-setting resin or solder.
Next, the isolator element 340 is positioned at the lower level
surface 316 and then is bonded at the specific position by using a
resin or the like. Through the processing described above, the
subassembly 330 is manufactured.
[0089] The optical module 302 can then be manufactured by setting
the subassembly 300 at the header 354 and bonding the subassembly
300 onto the header 354 with a thermosetting resin, solder or the
like. After the subassembly 300 is bonded onto the header 354, the
magnets 346a and 346b are set and fixed onto the header 354. Then,
the wiring of the laser diode 120 is electrically connected through
wire bonding. Through the steps described above, the optical module
301 is manufactured.
[0090] Then, the cap 352 is mounted and one end of the cap 352 is
fixed to the stem 356 through welding. In this state, the
subassembly 300, which is packaged by using the package parts
constituting the package 350, is held in the airtight space S3.
Next, the interface 170 is inserted through the other end of the
cap 352, the large diameter portion of the interface 170 and the
end of the cap 352 are abutted against each other, alignment is
performed as in the first embodiment, and the interface 170 is
fixed. Through the steps described above, the optical module 302 is
manufactured.
[0091] A compact isolator element used in the related art has to be
disposed near the end of the optical fiber where the light flux
diameter is small. In addition, while the polarizers and the
Faraday rotator can be miniaturized to a certain extent in an
optical module having an isolator adopting the structure in the
related art, the presence of the magnets disposed around the
isolator hinders further miniaturization.
[0092] However, since the lens element is disposed in close
proximity to the laser diode, the light flux diameter is reduced in
the third embodiment. For this reason, the isolator element can be
mounted on the package header, and furthermore, the magnets can
also be mounted at the header 354 in the third embodiment. Thus, a
highly compact structure is realized by disposing numerous parts on
the header 354 in the third embodiment. The length of the
subassembly 300 along the optical axis is less than approximately 1
mm. Accordingly, an extremely small distance L1 of approximately 1
mm is achieved between the surface of the stem 356 and the end of
the optical fiber 171. Furthermore, the external diameter D1 of the
stem 356 does not exceed approximately 3 mm.
[0093] As described above, the third embodiment provides, as does
the first embodiment, a highly compact optical module that
facilitates high precision positioning and does not require any
complicated alignment process. In addition, since the third
embodiment allows the use of a small isolator element and does not
require a member which holds the lens such as a cap, a cost
reduction is achieved. The results of a measurement executed on the
optical module 302 manufactured as described above confirm that
light is output from the optical fiber 171 with a 50% coupling
efficiency.
Fourth Embodiment
[0094] Next, the subassembly and the optical module achieved in the
fourth embodiment of the present invention are explained with
reference to FIGS. 9 and 10. FIG. 9 is a perspective of a
subassembly 400 achieved in the fourth embodiment of the present
invention. FIG. 10 is a sectional view of an optical module 402
achieved in the fourth embodiment of the present invention by using
the subassembly 400. The fourth embodiment is characterized by the
optical module 402 being constituted as a single fiber
bidirectional optical module that bidirectionally propagates two
types of light signals with different wavelengths through a single
optical fiber. The following explanation focuses on this feature,
and some of the structural features which are identical to those in
the first embodiment are not explained repeatedly.
[0095] The subassembly 400 includes a supporting substrate 410, a
laser diode 420, a lens element 430, a wavelength dividing filter
440 and a photodiode 442. The laser diode 420, the lens element 430
and the wavelength dividing filter 440 are mounted in this order on
the supporting substrate 410 over specific intervals.
[0096] The supporting substrate 410 may be constituted of, for
instance, a silicon crystal substrate. The supporting substrate 410
adopts a two-level structure which includes a stage and has a
V-shaped groove 412 at which a member is to be disposed and formed
thereat, as does the supporting substrate 310 shown in FIG. 5B. In
the following explanation, the upper surface of the upper level
portion of the supporting substrate 410 is referred to as an upper
level surface 414, whereas the upper surface of the lower level
portion of the supporting substrate 410 is referred to as a lower
level surface 416. The V-shaped groove 412 is formed to extend from
one end of the upper level surface 414 located on the staged side
to a middle point.
[0097] The laser diode 420 is a light emitting element that emits
light with a wavelength .lamda.1 for transmission. The laser diode
420 is disposed at a position where the V-shaped groove 412 is not
present, on a line extending from the V-shaped groove 412 on the
upper level surface 414.
[0098] The lens element 430 is disposed at the V-shaped groove 412.
The lens element 430 is constituted of an optical substrate which
may be a quartz substrate or a silicon substrate. As does the lens
element 130 in the first embodiment, the lens element 430 includes
a lens portion 432 constituted of a diffractive optical element
which is formed at one surface of the optical substrate, a
projection portion having a contour that allows the projection
portion to come into contact with the V-shaped groove 412 when the
lens element 430 is mounted and a handling portion that allows the
lens element to be held with ease for handling. The optical
performance of the lens portion 432 differs from that of the lens
portion 132 of the lens element 130. Other structural features and
a manufacturing method that may be adopted to manufacture the lens
element 130 are substantially similar to those of the lens element
430.
[0099] The wavelength dividing filter 440 is disposed at the lower
level surface 416. The wavelength dividing filter 440, which
achieves wavelength selectivity, has a function of dividing light
into different wavelengths. For instance, if two types of light
with different wavelengths .lamda.1 and .lamda.2 enter the
wavelength dividing filter 440, the light with the wavelength
.lamda.1 is transmitted and the light with the wavelength .lamda.2
is reflected. The wavelength dividing filter 440 may be constituted
by using, for instance, a multilayer film mirror. In this example,
the wavelength dividing filter 440 adopts a structure which is
achieved by enclosing a dielectric multilayer film with two glass
blocks. The dielectric multilayer film has a function of allowing
the light with the wavelength .lamda.1 to be transmitted and
reflecting the light with the wavelength .lamda.2. The wavelength
dividing filter 440 is set so that the dielectric multilayer film
intersects the optical axis of the light emitted from the laser
diode 420 with a 45.degree. angle.
[0100] At the upper surface of the wavelength dividing filter 440,
a photodiode 442 which receives light is set and fixed with solder.
The photodiode 442, which is constituted of a plane entry-type
light receiving element, is directly mounted at the wavelength
dividing filter 440 so that the light receiving portion of the
photodiode 442 and the wavelength dividing filter 440 face opposite
each other with no lens or spacer present between them in this
example.
[0101] At the subassembly 400, the laser diode 420 and the lens
element 430 are positioned and set so that the laser diode 420 and
the lens portion 432 of the lens element 430 share the same optical
axis. The lens element 430 is positioned along the two directions
that are perpendicular to the optical axis with ease by placing the
lens element with the projection portion of the lens element 430 in
contact with the V-shaped groove 412.
[0102] Next, an optical module 402 constituted by using the
subassembly 400 is explained with reference to FIG. 10. The optical
module 402 includes the subassembly 400, a lens element 451, a
package 450 and an interface 170. The lens element 451 is
constituted of a diffractive optical element formed at a surface of
an optical substrate in this embodiment. However, the lens element
451 may be constituted of a ball lens or an aspherical lens instead
of a diffractive optical element. As shown in FIG. 10, the
subassembly 400 is packaged by using package parts (components)
constituting the package 450, and the interface 170 is fixed in
contact with the package 450.
[0103] The package 450 is a coaxial package that includes a cap 452
having a substantially cylindrical external shape, a header 454
constituting a pedestal portion, a stem 456 having a substantially
disk shape, and electrode terminals 458. One end of the header 454
is fixed onto one surface of the stem 456, whereas a stage against
which the subassembly 400 is to be abutted is formed at the other
end of the header 454. It is to be noted that this contact stage is
not a crucial structural feature and the header 454 may instead
adopt a structure that does not include any stage. The subassembly
400 is fixed onto the header 454. A photodiode 160, which is a
light receiving element, is fixed on the surface of the stem 456
located toward the header 454. The photodiode 160 is used to
monitor the light traveling from the rear end surface of the laser
diode 420. The laser diode 420 and the photodiodes 160 and 442 are
each electrically connected with an electrode terminal 458.
[0104] The cap 452 is constituted of a metal such as iron. One end
of the cap 452 is fixed to the stem 456, whereas the interface 170
is fixed to the other end. A barrier wall 453 is formed inside the
cap 452, with the lens element 451 mounted over a portion of the
barrier wall 453. A sealed space S4 formed by the stem 456, the cap
452, the barrier wall 453 and the lens element 451 is maintained in
an airtight state. The subassembly 400 is held inside the airtight
space S4. The front end of the interface 170 is inserted in the
space on the other side of the barrier wall 453 and the lens
element 451, opposite from the side where the airtight space S4 is
located. The length of the cap 452 along the optical axis is set in
advance during the design stage so that when the large diameter
portion of the interface 170 and the end of the cap 452 are abutted
to each other, the point at which light is condensed through the
lens element 451 is positioned at the end surface of the optical
fiber 171.
[0105] The operation of the optical module 402 adopting the
structure as described above will now be explained. The divergent
light with the wavelength .lamda.1 emitted from the laser diode 420
is converted to substantially parallel light at the lens element
430 and is then transmitted through the wavelength dividing filter
440. Subsequently, the light is converted to convergent light at
the lens element 451, is condensed toward the end surface of the
optical fiber 171 and is transmitted to the outside from the
optical fiber 171. In addition, the light signal with the
wavelength .lamda.2 input from the outside to the optical module
402 is propagated through the optical fiber 171 and is emitted as
divergent light from the diagonal end portion of the optical fiber
171 toward the lens element 451. This emitted light is converted to
substantially parallel light at the lens element 451 and enters the
wavelength dividing filter 440. Then, the light is reflected from
the dielectric multilayer film constituting the wavelength dividing
filter 440 so as to advance along a direction that is perpendicular
to the upper surface of the supporting substrate 410 and enters the
photodiode 442. Since the light entering the photodiode 442 is
substantially parallel light, the photodiode 442 can be positioned
with ease. As described above, the optical module 402 functions as
a bidirectional transmission/reception module.
[0106] As explained above, the fourth embodiment achieves an
advantage in that a single fiber bidirectional optical module, in
which two types of light signals with different wavelengths are
propagated bidirectionally through a single optical fiber, is
provided in a compact configuration, in addition to the advantages
of the first embodiment.
[0107] It is to be noted that while an explanation is given above
in reference to the fourth embodiment on an example in which the
light traveling between the lens element 430 and the lens element
451 is a substantially parallel light beam, the present invention
is not limited to this example. The light beam diameter may change,
e.g., the light beam diameter may gradually increase, while
traveling between the lens element 430 and the lens element 451.
Alternatively, instead of the lens element 430 and the lens element
451, the lens element 130 and the flat window 151 respectively may
be disposed to allow non-parallel light to be transmitted through
the wavelength dividing filter 440.
Fifth Embodiment
[0108] Next, the optical module achieved in the fifth embodiment of
the present invention is explained with reference to FIGS. 11 and
12. FIG. 11 is a perspective of a subassembly 500 achieved in the
fifth embodiment of the present invention. FIG. 12 is a sectional
view of an optical module 502 achieved in the fifth embodiment of
the present invention by using the subassembly 500. The structure
adopted in the optical module in the fifth embodiment is achieved
basically by replacing the lens element 451 with a flat window and
adding a second lens element in the subassembly 400 in the optical
module 402 in the fourth embodiment. The following explanation
focuses on these structural features, and some of the structural
features which are similar to those of the fourth embodiment are
not explained again.
[0109] The subassembly 500 includes a supporting substrate 510, a
laser diode 420, two lens elements 430 and 530, a wavelength
dividing filter 440 and a photodiode 442. The supporting substrate
510 may be constituted of, for instance, a silicon crystal
substrate. Two V-shaped grooves 512a and 512b and an indented
groove 516 are formed at the upper surface of the supporting
substrate 510. These grooves are formed so that the two V-shaped
grooves 512a and 512b run on a single straight line communicating
with the indented groove 516 located between them. The V-shaped
groove 512a is formed to extend to a middle point of the supporting
substrate 510 from the indented grooves 516, whereas the V-shaped
groove 512b is formed to extend from the indented grooves 516 to
one end of the supporting substrate 510.
[0110] The V-shaped grooves 512a and 512b both have a section with
a V shape. The V-shaped grooves 512a and 512b may be formed so as
to allow a single mode optical fiber with a diameter of 125 .mu.m
to be placed therein. The V-shaped grooves 512a and 512b can be
formed through, for instance, anisotropic etching. The indented
groove 516, at which the wavelength dividing filter 440 is
disposed, has a flat area at its bottom surface. While the section
of the indented groove 516 is substantially rectangular in this
example, the present invention is not limited to this example. The
indented groove 516 can be formed through dicing or the like.
[0111] The laser diode 420 is a light emitting element that emits
light with a wavelength .lamda.1 for transmission. The laser diode
420 is disposed on the supporting substrate 510 at a position where
the V-shaped groove 512a is not present, on a line extending from
the V-shaped groove 512a.
[0112] The lens element 430 is set at the V-shaped groove 512a
located closer to the laser diode 420, whereas the lens element 530
is disposed at the V-shaped groove 512b located further away from
the laser diode 420. The lens elements 430 and 530 are each
constituted of an optical substrate such as a quartz substrate or a
silicon substrate and respectively include lens portions 432 and
532, each constituted of a diffractive optical element formed at
one surface of the optical substrate. The lens portions 432
converts the light emitted from the laser diode 420 to
substantially parallel light, whereas the lens portions 532
converts the substantially parallel light to convergent light. The
lens elements 430 and 530 differ from each other only in the
optical performance of the respective lens portions 432 and 532,
and their other structural features such as their shapes are
identical. As does the lens element 130 in the first embodiment,
the lens elements 430 and 530 include projection portions having a
shape that allows the projection portions to come into contact with
the V-shaped grooves 512a and 512b when the lens elements are
mounted, and handling portions that are used to hold the lens
elements for easy handling. It is to be noted that while the shape
of the front ends of the handling portions of the lens elements 430
and 530 is slightly different from that of the front end of the
handling portion of the lens element 130, this difference does not
affect the mounting process.
[0113] The wavelength dividing filter 440 is disposed at the
indented groove 516. At the upper surface of the wavelength
dividing filter 440, a photodiode 442, which receives light, is set
and fixed with solder.
[0114] At the subassembly 500, the laser diode 420 and the lens
elements 430 and 530 are positioned and set so that the laser diode
420 and the lens portions 432 and 532 of the lens elements 430 and
530 share a common optical axis. The lens elements 430 and 530 can
be positioned along the two directions that are perpendicular to
the optical axis simply by placing the projection portions of the
individual lens elements in contact with the V-shaped grooves 512a
and 512b, respectively.
[0115] Next, the optical module 502 achieved by using the
subassembly 500 is explained with reference to FIG. 12. The optical
module 502 includes the subassembly 500, a package 550 and an
interface 170. The package 550 is a coaxial package that includes a
cap 552 having a substantially cylindrical external shape, a header
554 constituting a pedestal portion, a stem 556 having a
substantially disk shape, and electrode terminals 558. One end of
the header 554 is fixed onto one surface of the stem 556, whereas a
stage against which the subassembly 500 is to be abutted is formed
at the other end of the header 554. It is to be noted that this
contact stage is not a crucial structural feature and the header
554 may instead adopt a structure that does not include any stage.
The subassembly 500 is fixed onto the header 554. A photodiode 160,
which is a light receiving element, is fixed on the surface of the
stem 556 located toward the header 554. The photodiode 160 is used
to monitor the light traveling from the rear end surface of the
laser diode 420. The laser diode 420 and the photodiodes 160 and
442 are each electrically connected with an electrode terminal
558.
[0116] The cap 552 is constituted of a metal such as iron. One end
of the cap 552 is fixed to the stem 556, whereas the interface 170
is fixed to the other end. A barrier wall 553 is formed inside the
cap 552, with a flat window 551 mounted over a portion of the
barrier wall 553. The flat window 551 is constituted of a material
that allows the light emitted from the laser diode 420 and the
light emitted from the optical fiber 171 to be transmitted. A
sealed space S5 formed by the stem 556, the cap 552, the barrier
wall 553 and the flat window 551 is maintained in an airtight
state. The subassembly 500, which is packaged by using the package
parts (components) constituting the package 550, is held inside the
airtight space S5. The front end of the interface 170 is inserted
in the space on the other side of the barrier wall 553 and the flat
window 551 opposite from the side where the airtight space S5 is
located. The length of the cap 552 along the optical axis is set in
advance during the design stage so that when the large diameter
portion of the interface 170 and the end of the cap 552 are abutted
to each other, the point at which light is condensed through the
lens element 530 is positioned at the end surface of the optical
fiber 171.
[0117] The operation of the optical module 502 adopting the
structure as described above is explained. The divergent light with
the wavelength .lamda.1 emitted from the laser diode 420 is
converted to substantially parallel light at the lens element 430
and is then transmitted through the wavelength dividing filter 440.
Subsequently, the light is converted to convergent light at the
lens element 530, is condensed toward the end surface of the
optical fiber 171 and is transmitted to the outside from the
optical fiber 171. In addition, the light signal with the
wavelength .lamda.2 input from the outside to the optical module
402 is propagated through the optical fiber 171 and is emitted as
divergent light from the diagonal end portion of the optical fiber
171 toward the lens element 530. This emitted light is converted to
substantially parallel light at the lens element 530 and enters the
wavelength dividing filter 440. Then, the light is reflected from
the dielectric multilayer film constituting the wavelength dividing
filter 440 so as to advance along a direction that is perpendicular
to the upper surface of the supporting substrate 510 and enters the
photodiode 442. As described above, the optical module 502
functions as a bidirectional transmission/reception module.
[0118] As explained above, the optical module in the fifth
embodiment achieves functions and advantages similar to those of
the optical module in the fourth embodiment. In addition, since the
two lens elements are both included in the subassembly, the lens
elements can be positioned and mounted with greater ease in the
embodiment.
[0119] FIGS. 13 and 14 show optical modules in the related art,
presented as examples for comparison. FIG. 13 is a partial
sectional view of an optical module 802 in the related art, in
which a laser diode and an optical fiber are optically coupled via
a ball lens. FIG. 14 is a partial sectional view of an optical
module 902 in the related art that includes an isolator.
[0120] The optical module 802 shown in FIG. 13, which does not
include an isolator, couples a laser diode 820 and an optical fiber
871 by using a ball lens 830 having a large diameter. In the
optical module 802, the laser diode 820 is mounted on a header 854
which, in turn, is fixed onto a surface of a stem 856. A cap 810
used to hold the ball lens 830 is disposed around the laser diode
820 so as to enclose the header 854. One end of a cylindrical
portion 852 constituting a cylindrical package member is bonded to
the cap 810. The optical fiber 871 and a ferrule 872 are inserted
and fixed in the cylindrical portion 852.
[0121] As FIG. 13 illustrates, the distance between laser diode 820
and the ball lens 830 is significant in the optical module 802
which utilizes a large diameter ball lens 830. In addition, the cap
810 holding the ball lens 830 is also bound to be large, and for
these reasons, the optical module 802 cannot be provided as a
compact unit. The distance L2 from the surface of the stem 856 to
the end of the optical fiber 871 is at least 5 mm and the external
diameter D2 of the stem 856 is approximately 5 mm in the optical
module 802.
[0122] An optical module 902 shown in FIG. 14 adopts a structure
achieved by disposing an isolator 940 on a side near an end of an
optical fiber 971 with magnets 946a and 946b set on the two sides
of the isolator 940. In the optical module 902, a laser diode 920
is located on a header 954 fixed onto one surface of a stem 956. A
cap 910 used to hold a lens 930 is provided around the laser diode
920 so as to enclose the header 954. One end of a cylindrical
portion 952 constituting a cylindrical package member is bonded to
the cap 910. The optical fiber 971 and a ferrule 972 are inserted
and fixed in the cylindrical portion 952.
[0123] As FIG. 14 clearly indicates, the optical module 902
includes the lens 930 which is larger than the lens element 330
used in the third embodiment of the present invention with the cap
910 disposed around the lens 930. For this reason, the distance
between the laser diode 920 and the optical fiber 971 is much
greater than that in the optical module 302 in the third embodiment
of the present invention. In the optical module 902, the distance
L3 from the surface of the stem 956 to the end of optical fiber 971
is approximately 5 mm and the external diameter D3 of the stem 956
is approximately 5 mm.
[0124] While the present invention has been particularly shown and
described with respect to preferred embodiments thereof by
referring to the attached drawings, the present invention is not
limited to these examples, and it will be understood by those
skilled in the art that various changes in form and detail may be
made therein without departing from the spirit, scope and teaching
of the invention.
[0125] While an explanation is given in reference to the first
through fifth embodiments on examples in which the present
invention is adopted in an optical module achieved by using a
coaxial package, the present invention is not limited to this
application and it may instead be adopted in conjunction with a
flat package that is substantially rectangular parallelepiped in
shape. In addition, a lens element, a lens portion, a handling
portion, a projection portion and the like taking on shapes other
than those in the examples above may be used. While the lens
portion is formed at one surface of an optical substrate in the
examples explained above, a lens portion may instead be formed at
each of the two surfaces of an optical substrate. Furthermore, the
lens portion may be formed at a surface other than the surface at
which the lens portion is formed in the examples explained above.
Moreover, the quantity of lens elements included in the subassembly
is not limited to those in the examples explained above. A lens
element may be formed by using an optical substrate constituted of
any of GaAs, InP, GaP, SiC, Ge and the like as well as the
materials mentioned earlier. While the supporting substrate is a
silicon crystal substrate in the embodiments explained above, the
supporting substrate may instead be a ceramic substrate, an
aluminum nitride substrate, an alumina substrate, a silicon carbide
substrate or the like. The shape of the section of the groove
formed at the supporting substrate to dispose a lens element is not
limited to that adopted in the examples, and instead, the groove
may be formed to have a section that is substantially V-shaped,
substantially trapezoidal, substantially semicircular or
substantially rectangular.
[0126] While the optical fiber and the laser diode are coupled in
the first through third embodiments, a light receiving element such
as a photodiode may be utilized in place of the laser diode.
[0127] The axial shift-type lens element achieved in the second
embodiment may also be used in the third through fifth
embodiments.
[0128] In the fourth and fifth embodiments, elements having other
optical functions, e.g., an isolator, a deflector, a wave plate and
a filter, may be used instead of the wavelength dividing filter 440
and the photodiode 442. In addition, depending upon the specific
structure of the wavelength dividing filter that is used, the
photodiode 442 may be disposed at a side of the wavelength dividing
filter instead of on the wavelength dividing filter.
* * * * *