Rod lens fixing method

Abstract: A rod lens fixing method which can prevent the generation of cracks in a rod lens due to annealing to be carried out after welding a lens assembly. The rod lens fixing method includes the steps of providing a flanged split sleeve having an axial slit (25) extending continuously from one end of the flanged split sleeve to the other end thereof, the axial slit (25) being wide enough at at least a flange (24) of the flanged split sleeve to permit laser welding; press-fitting the rod lens (6) into the flanged split sleeve; and applying a solder into the axial slit (25) to solder the rod lens (6) to the flanged split sleeve. This method further includes the steps of 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 (24) and in the axial slit (25). The laser welding in the axial slit (25) of the flange (24) causes the acting of shrinkage forces in the flange to thereby prevent the generation of cracks in the rod lens (6) during the annealing.

Description: The present invention relates to a rod lens fixing method for fixing a rod lens in a sleeve to produce an assembly for use in an optical semiconductor module or the like and to such an optical semiconductor module 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 predetermined 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 µm 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 one 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. 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. According to 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 the 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 the flange including points within 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 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. 2) 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. 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, in which: 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 a perspective view of a further preferred embodiment of the present invention; Fig. 8 is an elevational view of the lens assembly shown in Fig. 7, illustrating a welding method according to the embodiment of Fig. 7; Fig. 9 is a perspective view of a further preferred embodiment of the present invention; and Fig. 10 is an elevational view of the lens assembly shown in Fig. 7, illustrating a welding method according to the embodiment of Fig. 9. 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 to Fig. 7, there is shown a lens assembly 38 according to a further 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. 8 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. 9 there is shown a lens assembly 46 according to a further 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 preferred embodiment of Fig. 7. That is, as shown in Fig. 10, 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.

Claims: A rod lens fixing method comprising the steps of: providing a flanged split sleeve (23) having an axial slit (25) extending continuously from one end of said flanged split sleeve to the other end thereof, said axial slit being wide enough at at least a flange (24) of said flanged split sleeve to permit laser welding; press-fitting a rod lens (6) into said flanged split sleeve (23); applying a solder (26) into said axial slit (25) to solder said rod lens (6) to said flanged split sleeve; inserting said flanged split sleeve into a bore formed at one end portion of a further sleeve (12), said bore having a diameter larger than an outer diameter of said flanged split sleeve (23) and smaller than an outer diameter of said flange (24); and laser-welding (P) said flange (24) to one end surface of said sleeve at a plurality of points (P) on said flange including points within said axial slit (25). A rod lens fixing method according to claim 1, wherein said flange of said flanged split sleeve (23) is gradually thinned as a circumferential distance from said axial slit increases; at least one hole (29) is formed through at thin-walled portion of said flange; and said flange is laser-welded through said hole to said one end surface of said further sleeve. A rod lens fixing method according to claim 1 or 2, wherein some of said weld points are one the outer circumference of said flange. A rod lens fixing method according to claim 1, wherein said flange is formed with a plurality of radial slits (53) arranged in circumferentially spaced relationship from one another; and said laser-welding points are in said radial slits of said flange, each of said points being set on only one side of the respective radial slit. A rod lens fixing method according to claim 4 further comprising providing said flanged sleeve with a plurality of first slits extending from said flange to an axially intermediate portion thereof, said first slits being arranged in circumferentially spaced relationship from one another, and a plurality of second slits extending from the other axial end of said flanged sleeve to said axially intermediate portion, said second slits being arranged in alternate relationship with respect to said first slits; applying said solder into said first slits and said second slits to solder said rod lens to said flanged sleeve; and laser-welding said flange to said one end surface of said sleeve at a plurality of points in said first slits of said flange. A method of producing an optical semiconductor module including performing the method of any preceding claim to produce a rod lens assembly and the further step of incorporating the rod lens assembly into an optical semiconductor module.

Inventors: ISHIZAKA TETSUO C O FUJITSU LI, MASUKO TAKAYUKI C O FUJITSU LI, OHYA TOSHIO C O FUJITSU LIMITE, SATOH SHUNICHI C O FUJITSU LIM, ISHIZAKA, TETSUO, C/O FUJITSU LIMITED, MASUKO, TAKAYUKI, C/O FUJITSU LIMITED, OHYA, TOSHIO, C/O FUJITSU LIMITED, SATOH, SHUNICHI, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1991
Main classification: G02B7
IPC codes: G02B6, G02B7