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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rod lens fixing method for fixing a rod
lens in a sleeve to be used for an optical semiconductor module or the
like by means of soldering and laser welding.
2. Description of the Prior Art
In an optical communication system, for example, employing an optical fiber
as an optical transmission line, an optical semiconductor module is used
for the purpose of introducing an outgoing light from an optical
semiconductor device such as a laser diode (LD) and a light emitting diode
(LED) into the optical fiber. In the optical semiconductor module, the
optical semiconductor device and an incident end surface of the optical
fiber are fixed in a predetrmined positional relation, and a condensing
lens is provided therebetween. In this kind of optical semiconductor
module, a relative positional relation between components has a direct
influence upon an optical coupling efficiency. Accordingly, it is required
to position each component with a high accuracy, e.g., 1 .mu.m or less.
Moreover, it is also required to maintain such a positioning accuracy for
a long period of time.
FIG. 1A is an elevational view of a lens assembly manufactured by a lens
fixing method in the prior art, and FIG. 1B is a vertical sectional view
of the lens assembly shown in FIG. 1A. Reference numeral 2 denotes a
flanged split sleeve formed at its own end with a flange 4. The flanged
split sleeve is formed of stainless steel or the like. A condensing rod
lens 6 is press-fitted in the flanged split sleeve 2, and is fixed by
solders 8 and 9 to the flanged split sleeve 2 at a portion of an axial
slit 5 and opposite ends of the flanged split sleeve 2. To enable the rod
lens 6 and the flanged split sleeve 2 to be welded together, an outer
circumference of the rod lens 6 and a predetermined portion of the flanged
split sleeve 2 are plated with gold. Thus, a lens assembly 10 is
constructed. As shown in FIG. 2B, the lens assembly 10 is inserted into a
stepped sleeve 12 from one end thereof, and as shown in FIG. 2A, the
flange 4 is laser-welded to one end surface of the stepped sleeve 12 at
four points P. On the other hand, a ferrule 14 connected to an optical
fiber 16 is inserted into the stepped sleeve 12 from the other end
thereof. After a relative positional relation between the rod lens 6 and
the ferrule 14 in the stepped sleeve 12 is adjusted, the ferrule 14 is
similarly laser-welded to the stepped sleeve 12. In this way, a fiber
collimator (virtual fiber assembly) 18 is formed.
After the lens assembly 10 and the ferrule 14 are laser-welded to the
stepped sleeve 12, annealing of the fiber collimator 18 is usually
repeated for a long period of time, so as to remove stresses due to the
laser welding. During this annealing, tensile forces act in the flange 4
of the flanged split sleeve 2 to expand the flange 4. As a result, the rod
lens 6 is drawn in radially outward directions, causing the generation of
cracks in the rod lens 6 as shown by arrows C in FIG. 2A from near the
slit 5 of the flange 4.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a rod lens
fixing method which can prevent the generation of cracks in a rod lens.
It is another object of the present invention to provide a fiber collimator
manufactured by a rod lens fixing method which can prevent the generation
of cracks in a rod lens.
In accordance with an aspect of the present invention, there is provided a
rod lens fixing method comprising the steps of providing a flanged split
sleeve having an axial slit extending continuously from one end of the
flanged split sleeve to other end thereof, the axial slit being wide
enough at at least a flange of the flanged split sleeve to permit laser
welding; press-fitting a rod lens into the flanged split sleeve; applying
a solder into the axial slit to solder the rod lens to the flanged split
sleeve; inserting the flanged split sleeve into a bore formed at one end
portion of a sleeve, the bore having a diameter larger than an outer
diameter of the flanged split sleeve and smaller than an outer diameter of
the flange; and laser-welding the flange to one end surface of the sleeve
at a plurality of points on an outer circumference of the flange and in
the axial slit.
In accordance with other aspects of the present invention, there are
provided rod lens fixing methods as mentioned below.
(1) A plurality of slits are formed in a flange of a flanged split sleeve
in such a manner that at least a part of the slits extends in a
circumferential direction of the flange. The flange is laser-welded to one
end surface of a sleeve at a plurality of points on an outer circumference
of the flange, the points lying on extensions of straight lines connecting
a center line of the rod lens to substantially central portions of the
circumferential slits formed in the flange.
(2) A plurality of radial slits are formed in a flange of a flanged split
sleeve in such a manner as to be arranged in circumferentially spaced
relationship from one another. The flange is laser-welded to one end
surface of a sleeve at a plurality of points in the radial slits of the
flange, each of the points being set on only one side of the respective
radial slit.
(3) A plurality of first slits are formed in a flanged sleeve in such a
manner as to extend from a flange formed at one axial end of the flanged
sleeve to an axially intermediate portion thereof and be arranged in
circumferentially spaced relationship from one another. Further, a
plurality of second slits are formed in the flanged sleeve in such a
manner as to extend from the other axial end of the flanged sleeve to the
axially intermediate portion and be arranged in alternate relationship
with respect to the first slits. The flange is laser-welded to one end
surface of a sleeve at a plurality of points in the first slits of the
flange, each of the points being set on only one side of the respective
first slit.
(4) An outer circumferential surface of a rod lens is metallized, and
thereafter, opposite ends of the rod lens are chamfered. The rod lens is
press-fitted into a flanged split sleeve, and is soldered thereto. The
flanged split sleeve is inserted into a bore of a sleeve, and a flange of
the flanged split sleeve is laser-welded to one end surface of the sleeve
at a plurality of points on an outer circumference of the flange.
(5) An axial groove is formed on an inner circumferential surface of a
flanged split sleeve in such a manner as to extend in opposed relationship
to an axial slit of the flanged split sleeve. A solder is applied into the
axial slit and the axial groove to solder a rod lens to the flanged split
sleeve. The flanged split sleeve is inserted into a bore of a sleeve, and
a flange of the flanged split sleeve is laser-welded to one end surface of
the sleeve at a plurality of points on an outer circumference of the
flange.
(6) An outer circumferential surface of a rod lens is partially metallized
at an axially central portion only thereof except at and near opposite
ends of the rod lens. The rod lens is press-fitted into a flanged split
sleeve, and is soldered thereto. The flanged split sleeve is inserted into
a bore of a sleeve, and a flange of the flanged split sleeve is
laser-welded to one end surface of the sleeve at a plurality of points on
an outer circumference of the flange.
In accordance with a further aspect of the present invention, there is
provided a fiber collimator comprising a stepped sleeve having opposite
large-diameter portions and an intermediate small-diameter portion formed
therebetween, the stepped sleeve having a first bore extending from one
end thereof and a second bore extending from the other end thereof so as
to communicate with the first bore; a lens assembly comprising a flanged
split sleeve and a rod lens press-fitted in the flanged split sleeve and
soldered thereto; and a fiber assembly comprising a ferrule and an optical
fiber fixedly inserted in the ferrule; the fiber collimator being
manufactured in accordance with the following steps of inserting the
ferrule into the second bore of the stepped sleeve; fixing an outer
circumference of the ferrule to the other end of the stepped sleeve at a
plurality of points by means of first laser welding; inserting the lens
assembly into the first bore of the stepped sleeve; fixing an outer
circumference of a flange of the flanged split sleeve to the one end of
the stepped sleeve at a plurality of points be means of second laser
welding; and penetrating the small-diameter portion of the stepped sleeve
at a plurality of points by means of third laser welding to fix the
small-diameter portion to the ferrule by means of the third laser welding.
The above and other objects, features and advantages of the present
invention and the manner of realizing them will become more apparent, and
the invention itself will best be understood from a study of the following
description and appended claims with reference to the attached drawings
showing some preferred embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is an elevational view of a lens assembly fixed by a conventional
method;
FIG. 1B is a vertical sectional view of the lens assembly shown in FIG. 1A;
FIG. 2A is an elevational view of a fiber collimator manufactured by a
conventional method;
FIG. 2B is a vertical sectional view of the fiber collimator shown in FIG.
2A;
FIG. 3 is a perspective view of a first preferred embodiment of the present
invention;
FIG. 4 is a perspective view of a modification of the first preferred
embodiment;
FIG. 5 is a schematic view illustrating a slit formed in a flange according
to the modification shown in FIG. 4;
FIG. 6 is a perspective view of another modification of the first preferred
embodiment;
FIG. 7 is an elevational view of a lens assembly according to a second
preferred embodiment of the present invention;
FIG. 8 is a vertical sectional view of the lens assembly shown in FIG. 7;
FIG. 9 is a vertical sectional view of a fiber collimator manufactured by
the method according to the second preferred embodiment;
FIG. 10 is an elevational view of the fiber collimator taken in the
direction of arrow X in FIG. 9;
FIG. 11 is a perspective view of a third preferred embodiment of the
present invention;
FIG. 12 is an elevational view of the lens assembly shown in FIG. 11,
illustrating a welding method according to the third preferred embodiment;
FIG. 13 is a perspective view of a fourth preferred embodiment of the
present invention;
FIG. 14 is an elevational view of the lens assembly shown in FIG. 11,
illustrating a welding method according to the fourth preferred
embodiment;
FIG. 15 is an elevational view of a fifth preferred embodiment of the
present invention;
FIG. 16 is a cross section taken along the line A--A in FIG. 15;
FIG. 17 is an elevational view of a sixth preferred embodiment of the
present invention;
FIG. 18 is a cross section taken along the line B--B in FIG. 17;
FIG. 19 is a vertical sectional view of a seventh preferred embodiment of
the present invention;
FIG. 20 is an exploded perspective view of a fiber collimator according to
an eighth preferred embodiment of the present invention; and
FIG. 21 is a vertical sectional view of the fiber collimator shown in FIG.
20.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring first to FIG. 3, there is shown a perspective view of a fiber
collimator (virtual fiber assembly) 20 adopting a lens fixing method
according to a first preferred embodiment of the present invention.
Reference numeral 22 denotes a lens assembly comprising a condensing rod
lens 6 and a flanged split sleeve 23 integrally formed at its one end with
a flange 24. The rod lens 6 is press-fitted in the flanged split sleeve
23, and is fixed thereto by a solder 26. In this lens assembly 22, a slit
25 of the flange 24 is so wide as to permit spot welding.
A sleeve portion of the lens assembly 22 is inserted into a stepped sleeve
12, and the flange 24 of the lens assembly 22 is laser-welded to one end
surface of the stepped sleeve 12 at two points P in the wide slit 25 and
the other two lower points P on the outer circumference of the flange 24.
Thus, the lens assembly 22 is fixed in the stepped sleeve 12 to form the
fiber collimator 20. As the flange 24 is laser-welded to the stepped
sleeve 12 at the two points P in the slit 25, tensile forces act in the
flange 24 in the directions depicted by arrows F during annealing to be
carried out after the welding step. As a result, shrinkage forces act in
the rod lens 6 near the slit 25 during annealing to prevent the generation
of cracks in the rod lens 6 from near the slit 25.
Referring to FIG. 4, there is shown a perspective view of a lens assembly
22' according to a modification of the first preferred embodiment
described above. In this modification, a wide slit 25' of the flange 24 is
tapered. Such a tapered shape of the wide slit 25' of the flange 24
improves the workability of laser welding at the points P as shown in FIG.
5.
Referring to FIG. 6, there is shown a perspective view of a lens assembly
20" according to another modification of the first preferred embodiment.
In this modification, a flange 28 is so tapered as to be gradually thinned
as the circumferential distance from the wide slit 25 increases. Further,
two through holes 29 are formed through a thin-walled portion of the
flange 28. The flange 28 is laser-welded to one end surface of the stepped
sleeve at two points P in the through holes 29 in addition to two points P
in the slit 25, thereby fixing the lens assembly 20" in the stepped
sleeve. The two points P in the through holes 29 where the laser welding
is carried out are set on the outside in the radial direction of the
flange 28, so as to prevent the generation of tensile forces acting in the
flange 28 in the radially outward directions thereof and thereby prevent
the generation of cracks in the rod lens 6.
Referring next to FIGS. 7 and 8, a lens assembly 30 according to a second
preferred embodiment of the present invention will be described. A flange
32 of a flanged split sleeve 31 is formed with a pair of slits 34
extending circumferentially from an axial slit 33 and a pair of slits 35
extending radially inwardly from the outer circumference of the flange 32
on the opposite side of the axial slit 33 and further extending
circumferentially from the radially inward ends of the radial slit
portions in the opposite directions. The rod lens 6 is press-fitted in the
flanged split sleeve 31, and is fixed thereto by the solder 26 to form the
lens assembly 30.
A sleeve portion of the lens assembly 30 thus formed is inserted into the
stepped sleeve 12 from one end thereof as shown in FIG. 9, and the flange
32 of the lens assembly 30 is laser-welded to one end surface of the
stepped sleeve 12 at four points P.sub.2 as shown in FIG. 10. On the other
hand, a ferrule 14 of a fiber assembly 15 is inserted into the stepped
sleeve 12 from the other end thereof. The fiber assembly 15 is constructed
of the ferrule 14 and an optical fiber 16 fixedly inserted in the ferrule
14. After a relative positional relation between the lens assembly 30 and
the fiber assembly 15 is adjusted, the fiber assembly 15 is laser-welded
to the other end surface of the stepped sleeve 12 at four points P.sub.1
circumferentially spaced at 90.degree. from one another.
In the various embodiment described herein, the order of the laser welding
to be carried out at the various points is to be noted. That is, the
ferrule 14 of the fiber assembly 15 is first laser-welded at the four
points P.sub.1. Secondly, the lens assembly 30 is laser-welded at the four
points P.sub.2. Finally, a thin-walled portion of the stepped sleeve 12 is
penetrated by a laser beam at two points P.sub.3 to be laser-welded to
ferrule 14. Thus, a fiber collimator (virtual fiber assembly) 36 is
completed.
As described above, the flange 32 is formed with the slits 34 and 35, and
the flange 32 is laser-welded to the one end surface of the stepped sleeve
12 at the four points P.sub.2. Accordingly, tensile forces acting in the
flange 32 in the radially outward directions during annealing after the
welding step is absorbed by the slits 34 and 35. As a result, tensile
forces acting in the rod lens 6 is relaxed, thereby preventing the
generation of cracks in the rod lens 6. Furthermore, in assembling the
fiber collimator 36, the laser welding at the points P.sub.3 is carried
out in the final stage. Accordingly, in annealing the fiber collimator 36,
shrinkage forces act in the stepped sleeve 12, thereby preventing the
generation of cracks in the rod lens 6.
Referring to FIG. 11, there is shown a lens assembly 38 according to a
third preferred embodiment of the present invention. A flanged sleeve 40
of the lens assembly 38 is formed with a plurality of slits 43 extending
radially of a flange 42 and extending axially of a sleeve portion of the
flanged sleeve 40 continuously from the radial slits of the flange 42 to
an intermediate position of the sleeve portion of the flanged sleeve 40.
The flanged sleeve 40 is further formed with a plurality of slits 45
axially extending from the other end on the opposite side of the flange 42
to the intermediate position of the sleeve portion in such a manner that
the slits 45 are arranged in alternate relationship with respect to the
slits 43.
In constructing the lens assembly 38, the rod lens 6 is press-fitted into
the flanged sleeve 40, and a solder is poured into the slits 43 and 45 to
fix the rod lens 6. During cooling of the solder, tensile stresses act in
the rod lens 6 between the adjacent slits 43 and between the adjacent
slits 45 due to a difference in coefficient of thermal expansion of
materials can be reduced. For example, assuming that each number of the
slits 43 and 45 is n, the tensile stresses can be reduced to 1/n times
those in the case of one slit.
As the tensile stresses acting in the rod lens 6 are reduced for the above
reason, the generation of cracks in the rod lens 6 can be prevented in
constructing the lens assembly 38.
The sleeve portion of the lens assembly 38 is inserted into the stepped
sleeve 12, and the flange 42 of the lens assembly 38 is laser-welded to
one end surface of the stepped sleeve 12. In the welding step, the flange
42 is laser-welded at four points P in some of the slits 43 of the flange
42 on the only one side of each slit 43 as shown in FIG. 12. In this
manner, each point P of the laser welding is set on the only one side of
each slit 43 of the flange 42. Therefore, stresses due to shrinkage of the
welded portions upon annealing can be made almost zero.
Referring to FIG. 13, there is shown a lens assembly 46 according to a
fourth preferred embodiment of the present invention. A flanged split
sleeve 48 is formed with an axially extending slit 52. A flange 50 of the
flanged split sleeve 48 is formed with a plurality of radially extending
slits 53 arranged in circumferentially spaced relationship from one
another. The rod lens 6 is press-fitted into the flanged split sleeve 48,
and a solder 54 is poured into the slit 52 to fix the rod lens 6, thus
forming the lens assembly 46. Then, a sleeve portion of the lens assembly
46 is inserted into the stepped sleeve 12, and the flange 50 of the lens
assembly 46 is laser-welded to one end surface of the stepped sleeve 12 in
the same manner as that in the third preferred embodiment. That is, as
shown in FIG. 14, the flange 50 is laser-welded to the stepped sleeve 12
at four points P in the slit 52 and some of the slits 53 of the flange 50
on the only one side of each slit. In this manner, each point P of the
laser welding is set on the only one side of each slit of the flange 50.
Therefore, stresses due to shrinkage of the welded portions upon annealing
can be made almost zero.
Referring to FIGS. 15 and 16, a lens fixing method according to a fifth
preferred embodiment of the present invention will now be described.
Reference numeral 58 denotes a flanged split sleeve integrally formed with
a flange 60. A condensing rod lens 6' is press-fitted in the flange split
sleeve 58, and is fixed thereto by soldering. The flanged split sleeve 58
is formed of stainless steel, and a gold plating for enabling the
soldering is therefore formed on the inner circumferential surface of the
sleeve 58. Similarly, the outer circumferential surface of the rod lens 6'
is also plated with gold so as to enable the soldering. Further, edge
portions 6a of the opposite ends of the rod lens 6' are chamfered after
plated with gold. As the edge portions 6a are chamfered, the rod lens 6'
is fixed to the flanged split sleeve 58 primarily by a solder 62 poured
into a slit 61 of the sleeve 58. Thus a lens assembly 56 is constructed.
Then, the lens assembly 56 is inserted into the stepped sleeve 12 as shown
in FIG. 9, and is fixed thereto by laser welding, thus constructing a
fiber collimator. In annealing the fiber collimator, tensil forces act in
the flange 60 in the radially outward directions thereof. However, as the
edge portions 6a of the rod lens 6' are chamfered, the tensil forces are
not applied directly to the rod lens 6' thereby preventing the generation
of cracks in the rod lens 6'.
Referring to FIGS. 17 and 18, a lens fixing method according to a sixth
preferred embodiment of the present invention will now be described. In
this preferred embodiment, an axially extending groove 65 is formed on the
inner circumferential surface of the flanged split sleeve 58 on the
opposite side of the slit 61. A gold plating is formed on the inner wall
of the groove 65 only. The rod lens 6 is press-fitted into the flanged
split sleeve 58, and the solder 62 is poured into the slit 61 and the
groove 65 to thereby fix the rod lens 6 in the flanged split sleeve 58.
Thus, a lens assembly 64 is constructed.
Then, the lens assembly 64 is inserted into the stepped sleeve 12 as shown
in FIG. 9, and is fixed thereto by laser welding, thus constructing a
fiber collimator. In annealing the fiber collimator, tensile forces act in
the flange 60 in the radially outward directions thereof. However, as the
rod lens 6 is fixed to the flanged split sleeve by the solder 62 in the
slit 61 and the groove 65 only, the tensile forces are not applied
directly to the rod lens 6, thereby preventing the generation of cracks in
the rod lens 6.
Referring to FIG. 19, a lens fixing method according to a seventh preferred
embodiment of the present invention will now be described. In this
preferred embodiment, a gold plating 7 is formed on the outer
circumferential surface of the rod lens 6 at its axially central portion
only. In other words, no gold plating is formed at and near the opposite
end portions of the outer circumferential surface of the rod lens 6. On
the other hand, the inner circumferential surface of the flanged split
sleeve 58 is entirely plated with gold. The rod lens 6 is press-fitted
into the flanged split sleeve 58, and is fixed thereto by the solder 62.
As the gold plating 7 is not formed at and near the opposite end portions
of the outer circumferential surface of the rod lens 6, but is formed at
the axially central portion only, the rod lens 6 is soldered to the
flanged split sleeve 58 at the axially central portion only of the rod
lens 6. Thus, a lens assembly 66 is constructed.
Then, the lens assembly 66 is inserted into the stepped sleeve 12 as shown
in FIG. 9, and is fixed thereto by laser welding, thus constructing a
fiber collimator. In annealing the fiber collimator, tensile forces act in
the flange 60 in the radially outward directions thereof. However, as the
rod lens 6 is fixed by soldering at its axially central portion only to
the flanged split sleeve 58, the tensile forces are not applied directly
to the rod lens 6, thereby preventing the generation of cracks in the rod
lens 6.
Referring to FIGS. 20 and 21, a fiber collimator manufactured by a fixing
method according to an eighth preferred embodiment of the present
invention will now be described.
Reference numeral 72 denotes a flanged split sleeve integrally formed with
a flange 72a. The flanged split sleeve 72 is formed with an axial slit 72c
extending from one end having the flange 72a over the full length of a
lens receiving portion 72b. The flange split sleeve 72 is formed of
stainless steel for permitting laser welding. A solderable metal film such
as a gold film is formed on the outer circumferential surface of the rod
lens 6 and the inner circumferential surface of the flanged split sleeve
72. The rod lens 6 is pressed-fitted into the flanged split sleeve 72, and
is fixed thereto by soldering. Thus, a lens assembly 76 is constructed.
Reference numeral 78 denotes a stepped sleeve for receiving the lens
assembly 76 and fixing the same therein. The stepped sleeve 78 has
opposite large-diameter portions 78a and 78b and an intermediate
small-diameter portion 78c formed therebetween. The stepped sleeve 78 is
formed with a large-diameter bore 79 at one end portion thereof and with a
small-diameter bore 81 at the other end portion so as to communicate with
the large-diameter bore 79. The stepped sleeve 78 is formed of stainless
steel for permitting laser welding. The large-diameter portion 78a of the
stepped sleeve 78 is formed with a plurality of axial slits 80 extending
from one end of the sleeve 78 in such a manner that the slits 80 are
arranged at circumferentially equal intervals.
Reference numeral 86 denotes a fiber assembly comprising a ferrule 84 and
an optical fiber 82 fixedly inserted in the ferrule 84. The fiber assembly
86 is fixedly inserted in the small-diameter bore 81 of the stepped sleeve
78. An end surface 84a of the ferrule 84 opposed to an inside end surface
of the rod lens 6 is inclined with respect to a plane vertical to a center
line of the optical fiber 82, so that a reflected light from the end
surface 84a is introduced in a direction different from that of an
incoming optical path.
The fiber collimator according to the preferred embodiment is manufactured
in accordance with the following procedure. First, the rod lens 6 is
fixedly inserted into the flanged split sleeve 72 form the lens assembly
76. On the other hand, the fiber assembly 86 having the ferrule 84 and the
optical fiber 82 integrated together is inserted into the small-diameter
bore 81 of the stepped sleeve 78, and the end surface of the
large-diameter diameter portion 78b of the stepped sleeve 78 is
laser-welded to the outer circumferential surface of the ferrule 84 at
four points. Then, the lens assembly 76 is inserted into the
large-diameter bore 79 of the stepped sleeve 78, and a relative position
between the end surface of the optical fiber 82 and the rod lens 6 is
adjusted. Thereafter, the flange 72a of the flanged split sleeve 72 of the
lens assembly 76 is laser-welded to the end surface of the large-diameter
portion 78a of the stepped sleeve 78 by irradiating a laser beam at our
points of the outer circumference of the flange 72a.
Then, the small-diameter portion 72c of the stepped sleeve 78 is penetrated
at four points by a laser beam to be welded to the ferrule 84. Finally,
the slits 80 of the stepped sleeve 78 are spot-welded at positions
preferably on the lens assembly 76 side with respect to the axially
central positions of the slits 80. The reason why the spot welding is
carried out at such positions is that shrinkage forces at the spot-welded
portions of the slits 80 are readily applied to the flange 72a.
In the fiber collimator manufactured by the above method, shrinkage forces
act in the stepped sleeve 78 by the laser welding carried out through the
small-diameter portion 78c of the stepped sleeve 78. Furthermore, the
shrinkage forces at the spot-welded portions of the slits 80 act in the
stepped sleeve 78 in such directions as to reduced the diameter of the
stepped sleeve 78 on the flange 72a side of the lens assembly 76.
Accordingly, in annealing the fiber collimator, tensile forces acting in
the flange 72a are relaxed by these shrinkage forces, thereby preventing
the generation of cracks in the rod lens 6.
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