Light coupling device to couple a light source to an optical fiber

Abstract: A light coupling device to couple a light source (14) to an optical fiber (4) through a GRIN lens (3) is disclosed. A focal length (f₁) between a light source unit (9) and a GRIN lens (3) and a focal length between the GRIN lens (3) and a optical fiber end-face can separately be adjusted. A light power receiving element assembly has the GRIN lens (3) which is received to an end of a cylindrical sleeve (1) of the assembly. To the other end of the sleeve (1) a ferrule (6) with an optical fiber (4) is inserted so that the ferrule (6) can move along the optical axis thereof. In a light source assembly, a light source unit (9) is secured to an end of a cylindrical sleeve (8) of the assembly. The cylindrical sleeve (1) of the light power receiving element assembly is placed at the position that the light powermeter connected to the end-face (15) of the optical fiber (4) indicates the maximum light power while the cylindrical sleeve (1) of the light power receiving element assembly is moved in the sleeve (8) of the light source assembly along the optical axis thereof. And the cylindrical sleeve (1) of the light power receiving element assembly is set to the cylindrical sleeve (8) of the light source assembly by a setscrew (13). The ferrule (6) with the optical fiber (4) is set to the cylindrical sleeve (1) of the light power receiving element assembly by a setscrew (12) after the adjustment is carried out in the aforementioned manner.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a light coupling device to couple a light source to an optical fiber, and more specifically a light coupling device to couple a light source to an optical fiber, which is constructed so that the focal length between the light source unit and a GRIN lens and the focal length between the GRIN lens and the optical fiber end-face can be adjusted. 2. PRIOR ART FIG. 4 shows a schematic diagram of a light coupling device to couple a light source to an optical fiber, which is constructed to feed light power radiated from the light source, i.e., a laser diode, to the optical fiber through a GRIN lens (see also EP-A-447 403 ). In FIG. 4, a light source unit 17 of can type wherein a light emitting element 18 is arranged in the central portion thereof. A GRIN lens 19 (a selfoc® lens of Nihon Ita Glass Co., Ltd.) is used to converge the light from the light emitting element 18. In a ferrule 20 there is an optical fiber consisting of a core 21 and a clad 22 which is inserted into a central hole 32 of the ferrule and fastened thereto. A diverging light beam 23 radiated from the emitting element 18 in the X and Y directions have different magnitudes, respectively. The average radiation angle 20 in each direction is approximately 50 degrees. The light beam is distributed in accordance with the Gaussian distribution in magnitude. The GRIN lens 19 is used to convert the light beam distributed over the wide angle into spot light beam 24 distributed within a small spot, so that the light power output from the GRIN lens 19 is gathered to the end-face of the optical fiber core 21. High light coupling efficiency is attained by aligning the optical axes of the light emitting element 18 in the optical source unit 17, the GRIN lens 19, and the optical fiber core 21, and by optimizing the focal length f 1 between the end-face of the light emitting element 18 in the light source unit 17 and the GRIN lens 19 and the focal length f 2 between the GRIN lens 19 and the optical fiber core 21. Since the core diameter of the single-mode optical fiber measures 9 to 10µm, the light source unit 17 cannot be precisely fabricated to satisfy this requirement. FIGS. 5A and 5B show the front and side views of light source unit 17, respectively. According to the specification of the light source unit 17, as shown in FIGS. 5A and 5B, the diameter of the circular mount of the light source unit 17 has a dimensional allowance of 50µm and deviation "e" of the light emitting element 18 to the circular mount measures 100µm between centers. The light emitting element 18 has a positioning error of 150 (=100 + 50)µm due to the above dimensional deviations. Positioning error Δ at the emitting end-face of the light emitting element 18 is 100µm referring to the center of the circular mount. The preferable dimensional allowances are as follows: The alignment error of the optical axes of the light emitting element 18 in the light source unit 17, the GRIN lens 19, and the optical fiber core 21 should be less than 2µm. The error of the focal length f 1 between the end-face of the light emitting element 18 in the light source unit 17 and the GRIN lens 19 should be less than ±100µm. The error of focal length f 2 between the GRIN lens 19 and the end-face of optical fiber core 21 should be less than ±100µm. FIG. 6 partially shows a cross-sectional view of a conventional light coupling device. A light source unit 26 of FIG. 6 is not secured to a frame 28 in the upper portion of FIG. 6 (referring to the center line of the device). A light source unit 26 shown in FIG. 6 is secured to the frame 28 in the lower portion of FIG. 6 referring to the center line of the device. The optical axis of the light source unit 26 is aligned to that of a GRIN lens 27, and then the light source unit 26 is mounted to the frame 28 by resistive welding. The frame 28 has conical projections 29. The conical projections 29 of the frame 28 are used for resistive welding (see the upper half portion of FIG. 6.). When resistive welding is carried out, one of the conical projection 29 is deformed as shown in the lower portion of FIG. 6 so that the conical projection 29 becomes flat. If the focal length f 1 between the light source unit 26 and the GRIN lens 27 shifts from the calculated value by Δf as shown in FIG. 6, no adjustment can be done after the welding. A sleeve 31 into which a ferrule 30 is inserted is fastened to the frame 28. The light source unit is mounted to the frame, after the optical axis alignment is accomplished. SUMMARY OF THE INVENTION The object of the present invention is to present a light coupling device to couple a light source to an optical fiber, wherein the focal length f 1 between the light source unit and a GRIN lens and the focal length f 2 between the GRIN lens and an optical fiber end-face can be adjusted during the assembling process of the device. A light coupling device to couple a light source to an optical fiber in accordance with the present invention wherein a light power output from the light source unit, i.e., a laser diode, is gathered by a GRIN lens and then fed to the optical fiber. The device consists of a light power receiving element assembly wherein the GRIN lens is mounted to one end of first through-hole of first cylindrical sleeve and a ferrule with an optical fiber is inserted into the first through-hole from the other end so that the ferrule can move along the axis of the first through-hole, and a light source assembly consisting of a second cylindrical sleeve which can concentrically couple with the first cylindrical sleeve of the light power receiving element assembly so that the first cylindrical sleeve can move along the optical axis thereof within second through-hole of the second cylindrical sleeve, wherein the light source unit is fastened to the second through-hole of second cylindrical sleeve at one end thereof so that the gap between the inner diameter of the second through-hole of the second cylindrical sleeve and the outer diameter of the first cylindrical sleeve may be within a predetermined allowance. The method of fabricating the light coupling device to couple the light source to the optical fiber in accordance with the present invention, consists of a first adjustment process to find a first focal length between the light source unit and the GRIN lens while the light power receiving element assembly is moved along the axis thereof on condition that the light power receiving element assembly is set in the second through-hole of the light source assembly and that the end-face of the optical fiber attached to the ferrule is connected to a light powermeter when the light source unit is kept put on, and a second adjustment process to find a second focal length between the GRIN lens and the ferrule with the optical fiber while the ferrule which is set in the first through-hole of the light power receiving element assembly is moved along the axis thereof. In the forementioned configuration of the present invention, the light power receiving element assembly and the ferrule with the optical fiber are placed in a position where the light powermeter indicates the maximum output during the first and second processes. The predetermined allowance is specified as 2µm. In the forementioned configuration of the present invention, a first setscrew which fastens the inner surface of the first through-hole in the first cylindrical sleeve of the light source receiving element assembly to the outer surface of the ferrule with the optical fiber by pressing the ferrule to the other side is provided in one side of the first cylindrical sleeve. And a second setscrew which fastens the inner surface of the second through-hole in the second cylindrical sleeve of the light source assembly to the outer surface of the first cylindrical sleeve of the light power receiving element assembly by pressing the first cylindrical sleeve to the same side as the ferrule is provided in the second cylindrical sleeve on the same side as the first cylindrical sleeve. In the forementioned configuration of the present invention, an adhesive agent is put on both the outer surface of the first cylindrical sleeve of the light power receiving element assembly and the outer surface of the ferrule with the optical fiber before the first and second lengths are adjusted. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B show an embodiment of a light coupling device to couple a light source to an optical fiber, wherein FIG. 1A shows a cross-sectional view of an embodiment of the light power receiving element assembly and FIG. 1B shows a cross-sectional view of a light source assembly; FIG. 2 shows a cross-sectional view of the embodiment of the light coupling device of the present invention. FIGS. 3A shows a cross-sectional view of the device, indicating how to adjust focal length f 1 between the light emitting element and a GRIN lens; FIG. 3B shows the cross-sectional view of the device, indicating how to adjust the focal length f 2 between the GRIN lens and the end-face of the ferrule with an optical fiber; FIG. 4 shows a layout of components of a conventional light coupling device to couple a light source to an optical fiber, wherein light power radiated from the light source, i.e., a laser diode, is fed to the optical fiber through the GRIN lens; FIGS. 5A and 5B show a configuration of a light source unit wherein FIG. 5A shows a cross-sectional front view of the light source and FIG. 5B shows a cross-sectional side view of the light source unit; FIG. 6 shows a cross-sectional view of a conventional light coupling device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is described hereafter referring to the drawings. In FIGS. 1A and 1B there is shown an embodiment of a light coupling device to couple a light source to an optical fiber in accordance with the present invention. FIG. 1A shows a cross-sectional view of a light power receiving element assembly and FIG. 1B shows a cross-sectional view of a light source assembly of the present invention. FIG. 2 shows a cross-sectional view of the light coupling device of the present invention. The light power receiving element assembly of FIG. 1A is set in the light source assembly of FIG. 1B. FIGS. 3A and 3B show the cross-sectional views of the device, respectively. FIG. 3A shows the cross-sectional view of the device, indicating how to find and adjust the focal length f 1 between the light source unit and the GRIN lens. FIG. 3B shows the cross-sectional view of the device, indicating how to find and adjust the focal length f 2 between the GRIN lens and the input end of the ferrule with an optical fiber. In FIG. 1A, a GRIN lens 3 is secured to at one end a through-hole 2 of a cylindrical sleeve 1. An optical fiber 4 is inserted into a central hole 5 of a ferrule 6. The ferrule 6 is received in the cylindrical sleeve 1 at the other end thereof. A thread 10 is arranged in part of a flange 1a of the cylindrical sleeve 1. A setscrew 12 mates with the thread 10. The light power receiving element assembly R shown in FIG. 1A consists of the cylindrical sleeve 1 and ferrule 6 with an optical fiber. As shown in FIG. 1B the light source unit 9 is concentrically installed in through-hole 7a of a cylindrical sleeve 8, wherein a through-hole 7 is provided to receive light power receiving element assembly R therein so that light power receiving element assembly R can move along the axis of the cylindrical sleeve 8 without any substantial play, and secured there after the alignment or adjustment is established. A light emitting element 14 is placed in the interior of the light source unit 9. A thread 11 is provided in the middle of the cylindrical sleeve 8 so that the thread 11 can mate with a setscrew 13. A light source assembly L consists of the cylindrical sleeve 8 and the light source unit 9. The light power receiving element assembly R is coupled to the light source assembly L, as shown in FIG. 2. The length between the end-face of the light emitting element 14 in the light source unit 9 and the GRIN lens 3 is adjusted to the focal length f 1 as shown in FIG. 3A. The other end-face 15 i.e., the output end of the optical fiber 4 is connected to an light powermeter (not shown), and then light power receiving element assembly R is moved forwards or backwards along the optical axis thereof while the light power is output from the light source unit 9. The light power receiving element assembly R is stopped at the position where the light power, measured by the light powermeter, is maximum. And the light power receiving element assembly R is fastened there by the setscrew 13 so as to keep focal length f 1 . The length between the GRIN lens 3 and the end-face of ferrule 6 with the optical fiber is adjusted to the focal length f 2 as shown in FIG. 3B. The ferrule 6 with the optical fiber 4, which is set in the through-hole 2 of light power receiving element assembly R, is moved forwards or backwards along the optical axis thereof. The ferrule 6 with the optical fiber 4 is stopped at the position where the light power displayed on the light powermeter is maximum, and the ferrule with the optical fiber 4 is secured there by the setscrew 13 so as to keep the focal length f 2 . A setscrew 12 is used to fasten the ferrule 6 with optical fiber 4 to the cylindrical sleeve 1, eliminating in the other side the gap between the ferrule 6 with the optical fiber 4 and the inner surface of cylindrical sleeve 1 of the light power receiving element assembly R. The setscrew 13 is used to secure the cylindrical sleeve 1 to the cylindrical sleeve 8, eliminating the gap in the other side between the inner surface of the through-hole 7 of the cylindrical sleeve 8 of the light source assembly L and the outer surface of the cylindrical sleeve 1 of the light power receiving element assembly R. The gap between the inner surface of the cylindrical sleeve 8 of the light source assembly L and the cylindrical sleeve 1 of the light power receiving element assembly R is specified as 1 to 2µm or less for the product precisely fabricated. The gap between the inner surface of the cylindrical sleeve 1 of the light power receiving element assembly R and the outer surface of the ferrule 6 with the optical fiber 4 is specified as 1 to 2µm or less for the product precisely fabricated. If the outer surface of the cylindrical sleeve 1 of the light power receiving element assembly R and the outer surface of the ferrule 6 with the optical fiber 4 are covered with an adhesive agent of epoxy resin. The inner space of the device is sealed from outer atmosphere completely. As described heretofore, the light coupling device to couple the light source to the optical fiber, in accordance with the present invention, consists of a light power receiving element assembly wherein the GRIN lens is fastened to one end of the first through-hole of the first cylindrical sleeve and the ferrule with the optical fiber is inserted into the other end of the first through-hole of the first cylindrical sleeve so that the ferrule can move along the optical axis thereof. In the light source assembly, the first cylindrical sleeve of the light power receiving element assembly can move along the optical axis thereof and the light source unit is concentrically aligned to the second through-hole of the second cylindrical sleeve, which can mate with the outer surface of the first cylindrical sleeve within a predetermined allowance, and secured there at one end of the second cylindrical sleeve. According to the above configuration, the focal length f 1 between the light emitting element of the light source and the GRIN lens, and the focal length f 2 between the GRIN lens and the end-face of the optical fiber can separately be adjusted. In the conventional device, if a length f 1 between a light emitting element of a light source is once secured, it can not be adjusted. On the contrary in the device according with the present invention the distance between the light emitting element and the GRIN lens can be re-adjusted even after the adjustment is completed in the light coupling device. The construction of the light coupling device in accordance with the present invention is more simplified than the conventional device. In accordance with the present invention, high light coupling efficiency can be accomplished by separately adjusting the distance between the light emitting element and the GRIN lens and the distance between the GRIN lens and the input end of the optical fiber to the focal lengths f 1 and f 2 . While the present invention has been described above only with respect to the embodiment thereof, it should of course be understood that the present invention should not be limited only to the embodiment but various changes or modifications may be made without departure from the scope of the present invention as defined by the claims.

Claims: A light coupling device to couple a light source (14) to an optical fiber (4) comprising: a light power receiving element assembly (R) having a GRIN lens (3) secured to one end of a first through-hole (2) of a first cylindrical sleeve (1), and a ferrule (6) with an optical fiber (4) being inserted into said first cylindrical sleeve (1) at the other end thereof so that said ferrule (6) can move along the axis of said first cylindrical sleeve; a light source assembly (L) consisting of a light source unit (9) and a second cylindrical sleeve (8) which coaxially receives said first cylindrical sleeve (1) of said light power receiving element assembly (R) so that said first cylindrical sleeve (1) can move along the axis of said second cylindrical sleeve (8) within a second through-hole (7) of said second cylindrical sleeve (8), said light source unit (9) being concentrically received in one end of said second through-hole (7) of said second cylindrical sleeve (8) wherein; the gap between the outer surface of said first cylindrical sleeve (1) and the inner surface of said second through-hole (7) of said second cylindrical sleeve (8) being less than a predetermined allowance; the assemblies being secured to one another after an adjustment of their relative position. A light coupling device to couple the light source (14) to the optical fiber (4), as set forth in claim 1, wherein the adjustment process comprising; a connecting process of a light powermeter to the output end-face (15) of said optical fiber (4) attached to said ferrule (6) on condition that said light power receiving element assembly (R) is installed in said second through-hole (7) of said light power assembly (L) while light power being radiated from said light source unit (9); a first adjustment process to find the focal length (f 1 ) between said light source unit (9) and said GRIN lens (3) while said light power receiving element assembly (R) is moved forwards or backwards along said optical axis thereof; and a second adjustment process to find the focal length (f 2 ) between said GRIN lens (3) and said ferrule (6) with said optical fiber (4) while said ferrule (6) with said optical fiber (4), which having been installed in said first through-hole (2) of said light power receiving element assembly (R) being moved forwards or backwards along the optical axis thereof. A light coupling device to couple said light source (14) to said optical fiber (4), as said forth in claim 2, wherein said first and second adjustment processes, said light power receiving element assembly (R) and said ferrule (6) with said optical fiber (4) being set at the position where the light powermeter indicating a maximum output power. A light coupling device to couple said light source (14) to said optical fiber (4) as said forth in claim 1, wherein said predetermined allowance is 2 µ m. A light coupling device to couple said light source (14) to said optical fiber (4) as said forth in claim 1 wherein; a first setscrew (12) being used to fasten said ferrule (6) with said optical fiber (4) to said first cylindrical sleeve (1), eliminating in the other side the gap between said ferrule (6) with said optical fiber (4) and said inner surface of said first through-hole (2) in said first cylindrical sleeve (1) of said light power receiving element assembly (R); and a second setscrew (13) being used to fasten said first cylindrical sleeve (1) to said second cylindrical sleeve (8), eliminating in the other side of said gap between said inner surface of said second through-hole (7) in said second cylindrical sleeve (8) of said light source assembly (L) and said outer surface of said first cylindrical sleeve (1) of said light power receiving element assembly (R). A light coupling device to couple said light source (14) to said optical fiber (4) as said forth in claim 1; wherein said outer surface of said first cylindrical sleeve (1) of said light power receiving element assembly (R) and said outer surface of said ferrule (6) with said optical fiber (4) being covered with layers of adhesive agent before said adjustment is carried out to set said focal lengths (f 1 , f 2 ) in place.

Inventors: TAKAHASHI MITSUO, TAKAHASHI, MITSUO
Applicants: SEIKO GIKEN KK, SEIKOH GIKEN CO., LTD.
Application year: 1995
Main classification: G02B6
IPC codes: G02B6, H01S5