U.S. patent application number 09/206165 was filed with the patent office on 2001-12-06 for optical module and manufacturing method of optical module.
Invention is credited to NAKAYA, SUSUMU, TANAKA, HIDEYUKI.
Application Number | 20010048794 09/206165 |
Document ID | / |
Family ID | 26552713 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010048794 |
Kind Code |
A1 |
NAKAYA, SUSUMU ; et
al. |
December 6, 2001 |
OPTICAL MODULE AND MANUFACTURING METHOD OF OPTICAL MODULE
Abstract
An optical module according to the present invention has a stem
having an interconnection terminal; an optical device positioned on
the stem and electrically connected to the interconnection
terminal; a cap fixed onto the stem so as to enclose the optical
device within the cap; and a lens for optically coupling the
optical device with an optical member positioned outside the cap.
As a feature of this optical module, the cap is manufactured with a
high degree of precision so that the height of the cap with respect
to the stem can be used as a positional reference in the direction
of the optical axis of the optical system, which consists of the
optical device, the lens, and the optical member extending outside
the cap. In addition, the shape of the side wall of the cap is
designed so that it can function as a reference for maintaining the
coaxiality between the photo detector and the lens. The stem is
fixed to the cap at the proper position a high precision.
Inventors: |
NAKAYA, SUSUMU; (TOKYO,
JP) ; TANAKA, HIDEYUKI; (TOKYO, JP) |
Correspondence
Address: |
WENDEROTH LIND & PONACK
2
2033 K STREET N W
SUITE 800
WASHINGTON
DC
20006
|
Family ID: |
26552713 |
Appl. No.: |
09/206165 |
Filed: |
December 7, 1998 |
Current U.S.
Class: |
385/93 |
Current CPC
Class: |
G02B 6/4204 20130101;
G02B 6/424 20130101; G02B 6/4237 20130101; G02B 6/4263 20130101;
G02B 6/4206 20130101 |
Class at
Publication: |
385/93 |
International
Class: |
G02B 006/36 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 1997 |
JP |
341777/1997 |
Sep 30, 1998 |
JP |
278087/1998 |
Claims
What is claimed is:
1. An optical module comprising: a stem having an interconnection
terminal; an optical device positioned on the stem and electrically
connected to the interconnection terminal; a cap fixed onto the
stem so as to enclose the optical device within the cap; and a lens
for optically coupling the optical device with an optical member
positioned outside the cap, wherein, a height of the cap with
respect to the stem can be used as a positional reference in a
direction of an optical axis of an optical system.
2. An optical module according to claim 1, wherein the stem is
fixed to the cap and shape of a sidewall of the cap is designed so
that it can function as a reference for maintaining a coaxiality
between the optical device and the lens.
3. An optical module according to claim 1, wherein a top surface of
the cap is positioned a predetermined distance from the lens.
4. An optical module according to claim 3, wherein a focal point of
the lens is positioned below the top surface of the cap.
5. An optical module according to claim 3, the optical member
comprising: an optical fiber having an end surface which is
optically coupled to the optical device; a fiber holder for holding
the optical fiber and has an end surface which is fixed to the top
surface of the cap; and the end surface of the optical fiber is
aligned from the end surface of the fiber holder by a predetermined
length along the optical axis.
6. An optical module according to claim 3, wherein the stem is a
metallic stem, the cap is a metallic cap, and the metallic cap is
fixed to the stem by resistance welding.
7. An optical module according to claim 1, wherein the optical
device is a photo detector.
8. An optical module according to claim 1, wherein the optical
system consists of the optical device, the lens, and the optical
member.
9. A method of manufacturing an optical module comprising the steps
of: positioning an optical device on a stem which has an
interconnection terminal and electrically connecting the optical
device to the interconnection terminal; positioning an lens in a
predetermined position of a cap which has a predetermined height;
and fixing the cap onto the stem so as to enclose the optical
device within the cap, so that the height of the cap with respect
to the stem can be used as a positional reference in a direction of
an optical axis of an optical system.
10. A method of manufacturing an optical module according to claim
9, wherein the stem is fixed to the cap and shape of a side wall of
the cap is designed so that it can function as a reference for
maintaining a coaxiality between the optical device and the
lens.
11. A method of manufacturing an optical module according to claim
9, further comprising the steps of: holding an optical fiber which
has an end surface by a holder, so that the end surface of the
optical fiber is located a predetermined position from an end
surface of the holder; and fixing the end surface of the holder to
the top surface of the cap.
12. A method of manufacturing an optical module according to claim
11, wherein a focal point of the lens is positioned below the top
surface of the cap.
13. A method of manufacturing an optical module according to claim
9, wherein the optical device is set on a base and the base is
bonded to the stem, and the step of positioning the optical device
on the stem comprises fitting the base and the stem into a
predetermined guide hole of a coupling device and heating up the
coupling device for coupling the base and the stem.
14. A method of manufacturing an optical module according to claim
9, wherein an inner corner of the cap is chamfered.
15. A method of manufacturing an optical module according to claim
11, wherein the stem is a metallic stem, the cap is a metallic cap,
and the metallic cap is fixed to the stem by resistance
welding.
16. A method of manufacturing an optical module according to claim
15, wherein the stem has a first flange for the resistance welding,
the cap has a second flange for the resistance welding, and a
diameter of the second flange is set larger than a diameter of the
first flange.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of Invention
[0002] This invention relates to an optical module used to transmit
and receive optical signals in optical communication systems.
[0003] 2. Description of Related Art
[0004] In general, optical signals which have been transmitted
through optical fibers are received by the input ports of
receivers, and converted into electric signals by photo detector
modules using, for example, photo diodes (PD).
[0005] The size of the detection area of the photo detector used in
such a photo detector module is very small having a diameter of
about 80 .mu.m. In order to efficiently receive the optical beam
emitted from the optical fiber into the photo detector, a
collecting lens is typically used. For example, a technique of
attaching a ball lens directly to the cap of the photo detector is
known as a low-cost light-collecting method.
[0006] FIG. 5 illustrates an example of this type of photo detector
module. This photo detector module comprises a commercially
available stem 2, which is generally referred to as a TO-46 type
stem, and a photo detector 1 mounted on the stem 2. The stem 2 is
fixed to a cap 3 so that the photo detector 1 is accommodated in
the cap 3. A ball lens 4 made of low-temperature-melting glass is
attached to the middle of the cap. The cap 3 is fixed to a holder 5
by a bond 5, such as adhesive or solder.
[0007] An optical fiber 7 is connected to the holder 5 via a sleeve
9. To be more precise, the optical fiber 7 is inserted in a ferrule
8, and the ferrule 8 is inserted in and fixed to the sleeve 9 by
YAG welding. As a result of the YAG welding, nuggets 10 are formed
at the boundaries between the holder 5 and the ferrule 8, and at
the boundaries between the ferrule 8 and the sleeve 9.
[0008] In assembling the conventional photo detector, first, the
photo detector 1 is bonded to the stem 2. Then, the stem 2 is
fitted into the cap 3, to which the ball lens 4 has already been
fixed, and the stem 2 is fixed to the cap 3 by
resistance-welding.
[0009] Then, the holder 5 is fixed to the cap 3 at the end opposite
the stem 2 by a bond 6. The cap 3 can not be directly YAG welded
because the cap 3 is press-processed.
[0010] Finally, the optical fiber 7 is positioned in the optimal
position by an optical alignment technique, and then fixed to the
holder 5, via the sleeve 9, by YAG welding.
[0011] When this photo detector module is used for high-speed
transmission in a STM system, the optical signal which has been
propagated through the optical fiber 7 must be prevented from being
reflected by the surface of the photo detector 1 back to the
transmission path. For this reason, the end surfaces of the ferrule
8 and the optical fiber 7, which surface the ball lens 4, are
slanted by grinding or polishing for the purpose of inclining the
incident angle of the beam onto the photo detector 1, and of
reducing the return light to the optical fiber 7.
[0012] In particular, if the inclination of the end surface of the
optical fiber 7 is set to about 12 degrees, the incident angle of
the beam on the photo detector 1 becomes about 6 degrees, which can
reduce the reflected component of the signal light to less than -40
dB.
[0013] However, this arrangement causes a large variation in the
quantity of the reflected light because the coaxiality between the
photo detector 1 and the ball lens 4 is lost.
[0014] FIGS. 6(1) through 6(3) show the optical coupling and the
coaxiality between the photo detector 1 and the ball lens 4.
[0015] FIG. 6(1) illustrates the optical connection at the optimal
position with little separation at the ball lens 4. In this
arrangement, the light emitted from the optical fiber 7 strikes the
photo detector 1 at an optimal incident angle, and the amount of
the reflected light back to the optical fiber 7 via the ball lens 4
is greatly reduced. In this arrangement, the orientation of the end
surface of the optical fiber 7 does not affect the physical
properties of the optical connection between the ball lens 4 and
the photo detector 1.
[0016] However, if the optical axes of the photo detector 1 and the
ball lens 4 are offset from each other, as shown in FIGS. 6(2) and
6(3), the quantity of reflected light increases, while the coupling
efficiency decreases.
[0017] For example, if the separation between the photo detector 1
and the ball lens 4 and the orientation of the slanted end surface
of the optical fiber 7 satisfy particular conditions (that is, if
the orientation of the end surface of the optical fiber 7 is 12
degrees, and if the separation is about 0.11 mm, as shown in FIG.
6(2)), then a problem arises. In the example of FIG. 6(2), if the
optical fiber 7 is aligned to the optimal position, the light which
exits obliquely from the optical fiber 7 strikes the surface of the
photo detector 1 at a normal angle. The normal incident light is
reflected by the photo detector, and returns along the same path as
the incident light to the optical fiber 7. The amount of reflected
light may reach a maximum in this arrangement.
[0018] On the other hand, as shown in FIG. 6(3), if the orientation
of the inclined end surface of the optical fiber 6 is opposite the
orientation shown in FIG. 6(2), the quantity of reflected light is
reduced. However, in this arrangement, the light emitted from the
optical fiber 7 passes through the ball lens 4 at a position offset
from the center axis of the ball lens 4. As a result, the spherical
aberration of the ball lens 4 adversely affects the system, which
reduces the optical coupling efficiency and causes the
light-collecting ability of the photo detector to deteriorate.
[0019] In addition to these problems, the coaxiability between the
photo detector 1 and the ball lens 4 of the conventional photo
detector module may be offset by 0.25 mm at most due to the
following factors:
[0020] (1) Because there is no target or alignment mark on the stem
2, the photo detector 1 can not be precisely bonded;
[0021] (2) Because the cap 3 is press-processed, its dimensional
stability is insufficient and, accordingly, a large clearance is
required at the portion for receiving the stem 2, which causes the
optical axes of the stem 2 and the cap 3 to be offset from each
other during the resistance welding step; and
[0022] (3) It is mechanically difficult to accurately fix the ball
lens 4 in the center of the cap 3.
[0023] If the separation between the photo detector 1 and the ball
lens 4 is large, the amount of reflected light and the coupling
efficiency vary greatly with the slanting orientation of the end
surface of the optical fiber 7. In order to achieve a predetermined
desired performance, employing an optical alignment technique for
setting the orientation of the slanting end surface of the optical
fiber 7 at the optimal angle, while monitoring the reflected light
and the coupling efficiency, is indispensable. However, such an
optical alignment technique requires a number of steps, and the
production yield is lowered as a result.
[0024] In addition, it is also difficult in the conventional photo
detector module to precisely position the holder 5 with respect to
the cap 3 along the optical axis (i.e., the z-axis) because of the
existence of adhesive or solder. Since the optical fiber 7 must be
precisely positioned both in the direction perpendicular to the
optical axis and in the direcxtion parallel to the optical axis,
the ferrule 8 and the holder 5 need to be secured at two positions
via the sleeve 9 by YAG welding.
[0025] Furthermore, an adhesive or solder is also used to fix the
holder 5 to the cap 3, which is not preferable for maintaining the
reliability of the optical module for a long period of time.
SUMMARY OF THE INVENTION
[0026] Therefore, it is an object of the invention to overcome
these problems in the prior art, and to provide an optical module
which comprises a stem having an interconnection terminal; an
optical device positioned on the stem and electrically connected to
the interconnection terminal; a cap fixed onto the stem so as to
enclose the optical device within the cap; and a lens for optically
coupling the optical device with an optical member positioned
outside the cap. As a feature of this optical module, the cap is
manufactured with a high degree of precision so that the height of
the cap with respect to the stem can be used as a positional
reference in the direction of the optical axis of the optical
system, which consists of the optical device, the lens, and the
optical member extending outside the cap.
[0027] In addition, the shape of the side wall of the cap is
designed so that it can function as a reference for maintaining the
coaxiality between the photo detector and the lens. The stem is
fixed to the cap at the proper position a high precision.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] The above and other objects and features of the invention
will become more apparent from the following detailed description
of the preferred embodiments with reference to the attached
drawings, wherein:
[0029] FIG. 1 is a cross-sectional view of the optical module
according to an embodiment of the invention;
[0030] FIG. 2 is an exploded perspective view of the optical
module, which is used to explain the process of assembling the
optical module of the invention;
[0031] FIG. 3 illustrates how the stem is fixed to a base;
[0032] FIG. 4 illustrates how the stem is fixed to the cap;
[0033] FIG. 5 illustrates a conventional photo detector module;
and
[0034] FIG. 6 illustrates the relationship between the optical
coupling state and the coaxiability between the photo detector and
the ball lens.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] The preferred embodiments of the invention will now be
described in detail with reference to the attached drawings. FIG. 1
is a cross-sectional view of the optical module according to a
preferred embodiment of the invention, and FIG. 2 is an exploded
perspective view of the optical module of this embodiment, which
shows how this optical module is assembled.
[0036] In this embodiment, a photo detector 11 consisting of a
photo diode (PD) is used as the optical device. The photo detector
11 is mounted on a pad provided to the surface of a base 13 made of
ceramic. The base 13 is fixed to a metallic stem 12 which is
commercially available under the trade name of "TO-46 type". The
base 13 is capable of receiving a simple circuit including, for
example, a pre-amplifier 14 and a capacitor 15, in addition to the
photo detector 11. In this embodiment, these circuitry elements are
electrically connected to the interconnection terminal of the stem
12, whereby a photo detector module having a built-in pre-amplifier
is formed.
[0037] A metallic cap 16 is fixed to the stem 12 by resistance
welding, so that the photo detector 11 on the stem 12 is enclosed
within the cap 16. A ball lens 17 is fixed in the middle of the cap
16 by low-temperature-melting glass. The ball lens 17 is used to
optically couple the photo detector 11 with an optical member
positioned outside the cap 16.
[0038] The cap 16 is formed with a high degree of precision by
cutting processes, unlike the conventional cap which is formed by
press processes. The height of the cap 16 with respect to the stem
12 is used as a positional reference in the z-direction along the
optical axis of the system extending from the ball lens 17 to the
external optical member. In particular, the top wall of the cap 16
is shaped so that the top surface of the cap 16 is positioned a
predetermined distance from the ball lens 17. In other words, as
shown in FIG. 1, the thickness of the top wall of the cap 16 is
adjusted so that the focal point of the ball lens 17 is positioned
below the top surface of the cap 16.
[0039] The external optical member is an optical fiber 18 in this
embodiment. The end surface of the optical fiber 18 is optically
coupled to the cap 16. The optical fiber 18 is held by a ferrule 19
which functions as a fiber holder. The end surface of the ferrule
19 is fixed to the top surface of the cap 16 by YAG welding,
whereby the optical fiber 18 is also secured to the cap 16. The end
surface of the optical fiber 18 projects from the end surface of
the ferrule 19 by a predetermined length along the optical axis,
such that the focal point of the ball lens 16 is located within the
plane of the end surface of the optical fiber 18. As a result of
the YAG welding of the ferrule 19 to the cap 16, a nugget 20 is
formed at the boundary between the ferrule 19 and the cap 16.
[0040] Next, the process of assembling this photo detector module
will be described. Many techniques for achieving highly precise
alignment between the photo detector 11 and the ball lens 17 are
employed in this invention.
[0041] First, the base 13, on which a pad for receiving the photo
detector 11 is formed, is mounted on the stem 12. Then, the photo
detector 11 is bonded onto the pad. The stem 12 is fitted into the
cap 16, to which the ball lens 17 has already been secured, and
then the stem 12 is fixed to the cap 16 by resistance welding.
Finally, after the optical fiber 18 held by the ferrule 19 is
optically aligned with the photo detector 11, the ferrule 19 is
secured to the cap 16 by YAG welding.
[0042] The factors affecting the coaxiability between the photo
detector 11 and the ball lens 17 are listed below.
[0043] (1) Precision of the patterning of the pad on the base
13
[0044] (2) Positional accuracy of mounting the base 13 on the stem
12
[0045] (3) Accuracy of bonding the photo detector 11 onto the base
13
[0046] (4) Accuracy in resistance-welding the cap 16 to the stem
12
[0047] (5) Coaxiality between the cap 16 and the ball lens 17
[0048] The separations due to these factors are cumulated, which
results in a total separation between the photo detector 11 and the
ball lens 17.
[0049] Each of the above-listed factors will now be explained in
more detail.
[0050] (1) Precision of the patterning pad on the base 13
[0051] The base 13 is made of ceramic, and a pad for receiving the
photo detector 11 is formed on the base 13 by etching using a
photomask. The precision of the patterning of the pad is determined
by the precision of the alignment of the photomask.
[0052] (2) Positional accuracy of mounting the base 13 on the stem
12
[0053] In order to mount the base 13 on the stem 12 at the proper
position with a high degree of precision, the base 13 and the stem
12 are coupled using a coupling device 21 shown in FIG. 3, which
illustrates the coupling steps. The upper half of FIG. 3 shows
cross-sectional views, while the lower half shows the corresponding
perspective views.
[0054] The coupling device 21 has a guide hole 21-1 whose diameter
is slightly larger than the outer diameter of the base 13, and a
guide hole 21-2 whose diameter is slightly larger than the diameter
of the component-mounting area of the stem 12. The guide holes 21-1
and 21-2 are concentric.
[0055] In coupling the base 13 to the stem 12, the base 13 is set
into the guide hole 21-1 of the coupling device 21, as shown in
FIG. 3(1), and a bond (not shown), such as solder, is put on the
top surface of the base 13.
[0056] Then, the stem 12 is fitted into the guide hole 21-2 so that
the component-mounting area of the stem 12 comes into contact with
the top surface of the base 13 with the bond therebetween, as shown
in FIG. 3(2).
[0057] The coupling device 21, in which the base 13, the bond, and
the stem 12 are set, is heated up, as shown in FIG. 3(3), whereby
the base 13 and the stem 12 are bonded to each other.
[0058] Finally, the base 13 and the stem 12, which are now bonded
into one unit, are removed from the coupling device 21, as shown in
FIG. 3(4).
[0059] In this process, the coaxiality between the base 13 and the
stem 12 depends on the gap between the guide hole 21-1 and the base
13, and the gap between the guide hole 21-2 and the stem 12.
[0060] In this embodiment, a commercially available TO-46-type stem
is used. This stem 12 is press-processed, and the outer diameter of
the component-mounting part, including the systematic error (or
tolerance), is .phi.4.2.+-.0.025 mm. The base 13 is made of
ceramic, and its outer diameter tolerance depends on the amount of
contraction during the baking process. The actually measured
tolerance of the base 13 is .+-.0.03 mm. The diameters of the guide
holes 21-1 and 21-2 of the coupling device 21 are set slightly
larger than the maximum tolerances of the base 13 and the stem 12,
respectively. Accordingly, the maximum separation caused in the
worst case is about 0.065 mm. Since the circumference of the base
13 must not stick out beyond the circumference of the stem 12 even
in the worst case, the outer diameter of the base 13 is set
slightly smaller than the outer diameter of the stem 12.
[0061] (3) Accuracy of bonding of the photo detector 11 onto the
base 13
[0062] The photo detector 11 is mounted on the pad formed on the
base 13 by an ordinary bonding process. The uncertainty of the
positioning of the photo detector 11 with respect to the pad is
about 0.05 mm.
[0063] (4)Accuracy in resistance-welding the cap 16 to the stem
12
[0064] FIG. 4 illustrates how the cap 16 is fixed to the stem 12.
The stem 12 is fitted into the cap 16 so that the side wall of the
component-mounting part of the stem 12 comes into contact with the
inner surface of the edge of the cap 16. The coaxiality between the
cap 16 and the stem 12 depends on the gap between the outer
diameter of the component-mounting part of the stem 12 and the
inner diameter of the cap 16.
[0065] As has been explained above, the outer diameter of the
component-mounting part of the stem 12 is .phi.4.2.+-.0.025 mm. The
cap 16 is manufactured by cutting processes, and the uncertainty in
its inner diameter is .+-.0.025 mm or less. Accordingly, if the
inner diameter of the cap 16 is set to .phi.4.25.+-.0.025 mm, the
maximum separation between the cap 16 and the stem 12 is only 0.050
mm in the worst case.
[0066] The flange 23a of the stem 12 and the flange 23b of the cap
16 are fixed to each other by resistance welding. The inner corner
of the flange 23b of the cap 16 is chamfered, as indicated by the
numerical reference 22 in FIG. 4. In general, the boundary between
the flange 23a and the side wall of the press-processed stem 12
becomes distort during the resistance welding. However, the chamfer
22 can prevent such distortion from adversely affecting the bonding
of the stem 12 and the cap 16. The shape of the chamfer 22 is not
limited to the example shown in FIG. 4, and any shapes can be
selected as long as the flange 23b of the cap 16 does not come into
contact directly with the distortion of the stem 12.
[0067] In the example shown in FIG. 4, the diameter of the flange
23b of the cap 16 is set larger than the diameter of the flange 23a
of the stem 12. The flange 23b of the cap 16 is also thicker than
the flange 23a of the stem 12 because the cap 16 is formed by
cutting processes. During the resistance welding, the flange 23b of
the cap 16 and the flange 23a of the stem 12 are pressed against
each other at an appropriate pressure, and the flange 23b of the
cap 16, which is thicker and has a greater diameter than the flange
23a of the stem 12, can reliably receive and hold the stem 12
during the resistance welding.
[0068] (5) Coaxiality between the cap 16 and the ball lens 17
[0069] The ball lens 17 is secured to the cap 16 by
low-temperature-melding glass using an ordinary alignment device. A
conventional alignment device is sufficient to reduce errors in the
coaxiality and the position along the optical axis.
[0070] As has been described, each component is positioned and
aligned with a high degree of precision, and the overall coaxiality
between the photo detector 11 and the ball lens 17 is greatly
improved as compared with the conventional art.
[0071] In this embodiment, the coaxiality between the photo
detector 11 and the ball lens 17 is within 0.1 mm. It was confirmed
by experiment that, with this arrangement, the amount of reflected
light and the coupling efficiency do not greatly change with
variation of the slanting angles and the orientations of the end
surfaces of the optical fiber 18 and the ferrule 19. The
troublesome steps performed in the prior art of adjusting the
orientation of the ferrule 19 to the optimal angle, while
monitoring the physical properties of the optical system, can be
eliminated, and the work efficiency can thereby be improved.
[0072] The accuracy of positioning along the optical axis is also
improved because the cap 16 is manufactured by cutting processes,
and the height of the cap 16 can be set to the most appropriate
value so as to satisfy the predetermined optical-coupling
conditions. In other words, the cap 16 processed by cutting and
having a desired height in accordance with the predetermined
conditions is bonded directly to the stem 12 by resistance welding,
without requiring further positioning or adjustment along the
optical axis. This is a great advantage over the conventional
press-processed cap.
[0073] Thus, the cap 16 functions as a positional reference along
the optical axis of the optical system, which consists of the
optical device formed on the stem, the ball lens, and the external
optical member (i.e., the optical fiber). This facilitates the
optical alignment of the optical fiber, and the reflected light
from the top surface of the photo detector can stably be
reduced.
[0074] In addition, adhesive or solder used in the prior art is
eliminated, and the reliability and mechanical accuracy of the
entire module can be improved.
[0075] Although a photo detector module and its manufacturing
method have been described, the present invention can also be
applied to a light-emitting module. In this case, a ball lens is
incorporated into a cap processed by cutting, and the cap is
resistance-welded to a stem using the cap's height as a positioning
reference along the optical axis. Accordingly, a light-emitting
module using a laser diode (LD) is also included in the scope of
the invention.
* * * * *