Optical fiber light coupling interface with an enlarged incident surface

Abstract: An optical fiber light coupling interface (100) with an enlarged input surface (7) has a light coupling portion consisting of an enlarged optical fiber input end-face portion in diameter and a pair of tapered portions (22, 23), and an optical reflection layer formed outside said light coupling portion so that light power incidents thereon from inside is totally reflected therefrom. The light coupling portion having the enlarged optical fiber input end-face which consists of an enlarged core (17) and an enlarged clad (18) which are fabricated by such processes that a stress of compression is applied to a part of the optical fiber (19) along the optical axis thereof so that the radius of the art of the optical fiber is enlarged while the part of the optical fiber is fused by heating, the optical fiber is cut along the plane perpendicular to the optical axis of the optical fiber at the point where the radius of the optical fiber is enlarged, and then the plane is polished. The pair of tapered portions (22, 23) are characterized in that the radii of the core (17) and clad (18) of the normal optical fiber portion near the enlarged optical fiber input end-face (7) are gradually decreased as the distance from said enlarged optical fiber input end-face increases and then increased again until it goes to the normal optical fiber radius after passing through the minimum point of radius.

Description: 1. Field of the Invention The present invention relates to an optical fiber light coupling interface with an enlarged incident surface to couple the light power from a light source to an optical fiber, and especially to a light coupling device of an optical fiber which is input the light power dispersed from such a light source i.e., a laser diode or a light emitting diode to an enlarge incident end-face of which emits diverging light, and outputs it to an optical fiber. 2. Prior Art A number of conventional light coupling devices, each of which couples light power from a light source to a single-mode optical fiber, have been proposed and used. A conventional light coupling device is constructed using a lens system consisting of one or more optical lenses, and the system is arranged in a space between a light source and an optical fiber end-face. In this type of light coupling device, spot radius of the light beam radiated from the light source is adjusted to be the mode radius of the optical fiber core in order to improve the efficiency of the light coupling between the light source and the optical fiber. A reflection light loss at a surface of an optical lens in an optical conventional coupling device is approximately 14% for each lens. Optical power (Pa) incidents on a core of an optical fiber can propagate effectively through the optical fiber core of the conventional light coupling device. Since a core of a single mode optical fiber is 9 to 10 µm in diameter, existing alignment errors of the optical axes among the light source, optical fiber, and optical lens system remarkably decrease the efficiency of light coupling . Efficiency η of light coupling is given in terms of the alignment error among the axes of these optical components by expression (1) η=exp(-2d 2 /ω 2 ) where d is an alignment error(µm), and ω is the mode radius of the optical fiber. Assume that efficiency η of light coupling at the alignment error of 0µm is 100%. Efficiency η of light coupling for d=2.5µm and ω=5µm is then given as approximately 60%. If an optical lens system is used to propagate a light beam in an optical coupling device, it is necessary alignment errors of the optical axes among the optical components to be corrected. Efficiency η of the light coupling system depends on the accuracy of correction. In addition, reflection light losses at the surfaces of the lenses are added each time the light beam passes through a lens surface. Due to these difficulties, efficiency η of the light coupling of the system was limited to at most approximately 40% in most cases. A light coupling device of direct coupling type or light coupling device of simple structure, wherein no lens system is arranged in the space between the light source, i.e., a laser diode or a light emitting diode, and the single-mode optical fiber end-face, as shown in Figure 4, has been recommended. Figure 4 shows the principle of operation of a light coupling between a light source and an optical fiber end-face wherein no lens system is arranged in the space therebetween. In Figure 4, 1 denotes a light source, i.e., a laser diode, 2 denotes the core of an optical fiber 6, 3 denotes the clad of the optical fiber 6, 4 denotes the total light beam radiated, and the light beam 5 incidents on the core 2 of the optical fiber 6. When a laser diode is used as the light source, the intensity of light 4 emitted from the laser diode 1 in radiation angle r is distributed in accordance with the Gaussian distribution, and the light beam is coherent. Due to the diffraction of coherent light beam with the Gaussian distribution, an elliptic radiation pattern is formed. The elliptic pattern has a spreading of 40 to 60 degrees along the XX' axis and a spreading of 20 degrees along the YY' axis. Light power Pa incident on the core 2 of the optical fiber is calculated by Pa=I 0 {1-exp(-2a 2 /ω z 2 )} where I 0 is the intensity of the light power emitted from the light source, "a" is the radius of the core of the single-mode optical fiber (5=µm), and "ω z " is the radius the light beam incident on the optical fiber end-face, at distance z measured from the light source. Average radiant angle  r of the total light flux 4 is assumed to be 25 degrees. The numerical aperture for the light power incidents on the core of the single-mode optical fiber is assumed to NA= 1 =5.3 degrees. Light power Pa incidents on the end-face of optical fiber core 2 is calculated to be approximately 8%. Remaining light power Pa, which is approximately 92% of the total light power, is incident on optical fiber clad 3 and other area. The light power incidents on optical fiber clad 3 is radiated to outer surface 6 of the optical fiber clad 3 becomes a radiation loss. Assume that optical fiber end-face 7 approaches light source 1 of a laser diode as far as possible, the light power incident on the optical fiber core 2 at the incident angle of NA= 1 ≒5.3 degree or more cannot propagate along the optical fiber, although the light power incidents on optical fiber core 2 at an incident angle of less than NA= 1 =5.3 degrees can propagate along the optical fiber. A light coupling device of simple structure, which is built in accordance with the direct coupling structure as shown in Figure 4, is easy to build, but not put into practical use, in most cases are due to its low efficiency of light coupling. However, since no optical lens is used to simplify the configuration of the assembly in the above method of direct coupling, a number of variations have been proposed to improve the efficiency of light coupling. For instance, a light coupling device shown in Figure 5 is one of them. And it is described in "Ideal Microlenses for Laser to Fiber Coupling" by Christopher A. Edwards, et.al., IEEE Journal of Lightwave Technology, Vol. 11, No.2, PP.252-257, (February 1993). Figure 5 shows an example of a cross-sectional view of the light coupling device constructed in accordance with the method of direct coupling. A tapered portion 9 wherein the radius of an optical fiber 8 is reduced toward the end-face of the optical fiber 8 is formed by fusing and drawing the optical fiber 8 so that the mode radius of a core 10 is extended, and a hemisphere microlens 11 is formed at the top of the tapered portion 9 due to surface tension caused by fusing the optical fiber end-face. In Figure 5, distance z between a light source of laser diode 1 and the optical fiber end-face is 8.5µm, and radius R of the surface curvature of the microlens 11 is 5.7µm. The efficiency η of the light coupling is reported to be approximately 50%. In this example, the numerical aperture is small because of the very small radius of microlens 11. Errors can occur in aligning the optical axes between the light source and the optical fiber. Both of them limit the efficiency of light coupling of the device. An optical connector employing field modification invented by Nolam, et.al., is disclosed in US patent No. 4,763,976, will be explained referring to Figure 6. Figure 6 shows a cross-sectional view of the optical connector. In Figure 6, a glass tube 15 having optical refractive index n 3 which is smaller than the optical refractive index n 1 of an optical fiber clad 14 is concentrically arranged in a unit structure around outer surface 13 of the clad 14 of an optical fiber 12, and the end-face of the optical fiber 12 is finished to be small by fusing and drawing them together. In the example shown in Figure 6, the mode radius of optical fiber core 16 is extended twice as large as the normal mode radius for the normal optical fiber so that the efficiency of light coupling might not be decreased even if an alignment error has occurred in between the optical axes. The proposed device shown in Figure 6 is aimed to improve the efficiency of light coupling. When the ratio of the end-face diameter to the normal optical fiber diameter is 1 to 4 in the tapered portion, the mode radius ω of the optical fiber core is reported to be 10µm. If the offset (d) of the optical axes between the light source and the optical fiber is 2.5 µm, the efficiency (η) of light coupling is calculated to be 88% because only the offset of the optical axes is considered to decrease the efficiency of light coupling. From the technical point of view, finishing of an optical fiber to make a tapered portion in the shape shown in Figure 6 is however not so easy, and the practical fabrication is considered to be difficult. Both alignment errors due to the offset of the optical axes between the light source and the optical fiber, and the numerical aperture (NA) of the optical fiber core reduce the light power which is effectively input to the optical fiber core. These limiting factors reduce the efficiency of light coupling to 50% or less in many conventional devices. SUMMARY OF THE INVENTION The object of the present invention is to provide an optical fiber light coupling interface with an enlarged input surface whereon light power is entered, in order to accept the light power at an incident angle in the wide angle range so that the offset between the optical axes of the light source and optical fiber can be disregarded, if any, to keep the efficiency of light coupling high, and that the incident light power can propagate at high efficiency. The optical fiber light coupling interface with an enlarged input surface built to fulfill the object of the present invention, is such a device that which feeds the light power from the light source to the optical fiber. The optical fiber light coupling device optical fiber in accordance with the present invention is constructed using an optical reflection layer and light coupling portion which consists of an extended optical fiber input end-face consisting of an enlarged core and an enlarged clad which are fabricated by such processes that a stress of compression applies to a part of the optical fiber along the optical axis thereof so that the radius of a part of the optical fiber is enlarged while the part of the optical fiber is fused by heating. The optical fiber is cut along the plane perpendicular to the optical axis of said optical fiber at the point where the radius of the optical fiber is enlarged, and then the plane is polished. A pair of tapered portions where the radii of the core and the clad of the normal optical fiber portion near the enlarged optical fiber input end-face are gradually decreased as the distance from the enlarged optical fiber input end-face increases, and then increased again until the normal optical fiber radius after passing through the minimum point of radius. The optical reflection layer is formed outside the coupling means. The optical reflection layer which reflects the total light incidents thereon can be a mirror coating layer of dielectric material or metal film layer with high reflection coefficient, i.e., an aluminum, copper, gold, or silver film. The light coupling portion is mounted to a ferrule which has been made to accept said extended optical fiber input end-face at one end of a hole bored at the center thereof. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a cross-sectional view of an embodiment of the optical fiber coupling interface with an enlarged input surface of the present invention. Figure 2 shows the steps in manufacturing the optical fiber light coupling interface with an enlarged input surface of the present invention. Figure 3 shows relationships of the mode radius (ω), radius(a) of an optical fiber core, and radius (b) of the optical fiber clad as functions of the ratio of the minimum radius of the ratio of the minimum radius of the tapered portion to that of the normal optical fiber. Figure 4 shows the principle of operation of the light coupling in a conventional light coupling device wherein no lens system is used to couple a light source to an optical fiber end- face. Figure 5 shows a cross-sectional view of a conventional light coupling device built to improve the efficiency of light coupling when an alignment error has occurred among the optical axes of the optical components. Figure 6 shows a cross-sectional view of another example of a conventional light coupling device built to improve the efficiency of light coupling when an alignment error of the optical axis has occurred. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT The optical fiber light coupling interface with an enlarged input surface of the present invention, will be described in detail referring to the drawings. Figure 1 shows the cross-sectional view of an embodiment of the optical fiber light coupling interface with an enlarged input surface of the present invention. Figure 2 shows the steps in manufacturing the optical fiber light coupling interface with an enlarged input surface of the present invention as shown in Figure 1. Figure 2(A) shows the first process of forming an elliptical-shaped enlarged portion 20. The portion 20 is formed by applying a stress of compression along the optical fiber axis while a single-mode optical fiber 19 having a cross-section consisting of concentric core 17 and clad 18 is partly fused by heating. Figure 2(B) shows the second process of fabricating the pair of tapered portions 22, 23 having a smallest diameter at midpoint 21. These portions 22, 23 are formed by fusing and drawing the optical fiber 19 near the elliptical-shaped enlarged portion 20. Figure 2(C) shows a process of forming an optical reflection layer 24 outside the optical fiber 19 so that the input light power (Pb) propagating in the clad is kept within the optical fiber 19 and that numerical aperture NA is extended. The optical reflection layer 24 is formed by the process of depositing a mirror coating layer of dielectric material having high reflection coefficient onto the outside surface of the optical fiber. The optical reflection layer 24 is also formed by a process of evaporating a metal film layer having high reflection coefficient, i.e., aluminum, copper, gold, or silver on the outside surface of the optical fiber. Thereafter, the enlarged input end-face can be formed by polishing the end-face of elliptically shaped enlarged portion 20. After the total light reflection layer shown in Figure 2(C) is formed, the optical fiber coupling interface with an enlarged input surface will be inserted into a ferrule 25 as shown in Figure 1. When single-mode optical fiber 19 is inserted into the ferrule 25 and mounted there, the enlarged elliptically shaped portion 20 is put into a beveled portion 27 at one end of a central hole 26 bored in the cylindrical ferrule 25. After the insertion process, ferrule 25 is polished at both ends 28, 29 thereof together with the end-faces of the optical fiber. Thereafter, the optical axis of light source 30, i.e., an LD or an LED, is aligned to that of the optical fiber light coupling device of having the enlarged input end-face, which is built in accordance with the present invention, and both the light source and light coupling device are built in a package to construct the assembly of the light source and light coupling device. As described heretofore, a mirror coating layer of dielectric material or metal film layer, having high reflection coefficient, is used in the light coupling portion of optical fiber type 100 which is built in accordance with the present invention. The mirror coating layer of dielectric material has a reflection coefficient of 98% or more when the light beam is incident on this mirror coating layer at an incident angle of 45 degree or less. Such a metal film layer as a copper, gold, or silver film has a reflection coefficient of 98% or more when the light beam is incident on this metal film layer at right angle. For further information, refer to "General Catalog" published by Newport Co., Ltd., page J-62 (1990), and to "Machine Design Handbook" published by Maruzen Co., Ltd., page 89 (1973). Light power I 0 radiated from light source 30, i.e., an LD or an LED, is coupled to the end-face 28 at high coupling efficiency as shown in Figure 1. The characteristics of the optical fiber light coupling interface with an enlarged input surface will be explained hereinafter. Firstly, when the core radius a of the optical fiber which receives the light beam is enlarged twice as large as the normal core radius, light power Pa at the optical fiber core end-face is increased and light coupling efficiency η tends to increase and be free from offset d between the optical axes of the light source 30 and the optical fiber. Secondly, when a pair of tapered portions are formed by fusing and drawing the optical fiber, mode radius ω is extended to the same size as clad radius b of the optical fiber. Since an apparent core area of the optical fiber is extended to the same size as clad radius b thereof, light power Pb which is input to the clad area of the optical fiber converges into the core thereof, wherein light power Pa is propagating along the core thereof, while light power Pb travels along a pair of tapered portions 21, 22. Thirdly, since in a conventional device such as shown in Figure 4, light power Pb incidents on the optical fiber clad end-face is normally lost because it is radiated outside the clad, no light power can propagate within the clad. In accordance to the present invention the optical reflection layer 23 is evaporated onto the outer surfaces of a pair of tapered portions 22, 23 so that the light power traveling the clad of the tapered portions can propagate through the clad while no radiation loss can occur in the clad portions. If a light beam is incident on the optical fiber at an angle which is equal to or less than the numerical aperture (NA) of the single mode optical fiber (= angle of the radius ≤ approximately 5.3 degrees), light power Pa input to optical fiber core 17 can propagate along the core. If light power Pa is input to optical fiber core 17 at the incident angle of numerical aperture NA or more, light power Pa generally goes out and enters into the clad. However, the optical fiber light coupling interface with an enlarged input surface of the present invention, is characterized in that the light power within the clad portions of the tapered portions converges into the core portions of the pair of tapered portions 21, 22. This implies that the numerical aperture (NA) can be disregarded. Light power Pb input to the clad end-face can propagate within the tapered portions 21, 22 without any restriction caused by incident angle since the clad portions of the pair of tapered portions 21, 22 are optically isolated from free space outside the clad portions by evaporated layer 23 with high reflection coefficient. The light power which can be incident on the optical fiber end-face at an incident angle which is equivalent to or less than the Brewster's angle converges into the core of the optical fiber. The Brewster's angle  b is given by eq. (3)  b = tan -1 n 1 where n 1 is the refractive index of the optical fiber. If n 1 is 1.47,  b becomes 55.8 degrees. Since the angle of radiation for a laser diode is normally 25 to 30 degrees, the light power from the laser diode can easily be input to the optical fiber. A change in mode radius ω due to the change of radius a in the optical fiber core has been cited in "Loss analysis of single-mode file splice", Bell System Technical Journal, Vol.56, No.5, pp.703 by Marcuse D. (1977). According to Marcuse, mode radius ω is given by eq. (4) when radius a of the optical fiber core changes. ω = a(0.65 + 1.619/V 1.5 + 2.879/V 6 ) where V is the normalized frequency given by eq. (5). V = (2πan 1 /λ)•{2(n 1 -n 2 )/n 1 } 1/2 where λ(µm) is the wavelength of the light emitted from the light source, a(µm) is the radius of the optical fiber core, n 1 is the refractive index of the optical fiber core, and n 2 is the refractive index of the optical fiber clad. Figure 3 shows the relationships of the mode radius(ω), radius(a) of the optical fiber core, and radius(b) of the optical fiber clad as functions of the ratio (D min /D n ) of the minimum radius of the core and clad of the tapered portions to those of the normal optical fiber. In Figure 3, λ=1.31µm, a=5µm n 1 =1.47, n 2 =1.46, and b=62.5µm are assumed. The optical fiber light coupling interface with an enlarged input surface of the present invention, is as described heretofore characterized in that a pair of tapered portions are formed by fusing and drawing a standard single-mode optical fiber near the incident end-face, radius a of optical fiber core 17 is reduced along the tapered portions, and that light power Pb propagating in the clad portions of the tapered portions converges into optical fiber core 17 wherethrough light power Pa is propagating. As radius a of optical fiber core 17 becomes small, mode radius ω of optical fiber core 17 is extended as shown in Figure 3. The area where mode radius ω is extended is optically regarded as the optical fiber core area. The optical fiber light coupling interface with an enlarged input surface of the present invention is operated based on this mode of optical properties. Light power Pa propagating within optical fiber core 17, which is input from the light source to optical fiber core 17, is partly going out to the mode radius area in the pair of tapered portions due to Evanescent effect, and is combined with light power Pb propagating within the optical fiber clad. This mode of propagation results in combined light power (Pa+Pb) propagating within the optical fiber clad. As mode radius ω becomes small as the light beam goes propagating in second tapered portion 22 whose radius becomes large as the distance from the incident end-face increases beyond the midpoint of the tapered portions, the propagating light power (Pa+Pb) is concentrated into the area within radius a of the optical fiber core. The combined light power P=Pa+Pb finally converges into optical fiber core 17 so that light power P can propagate within optical fiber core 17. When the ratio of the radius of the tapered portion at its minimum to that of the normal optical fiber is approximately 40% in Figure 3, radius b of the optical fiber clad and mode radius ω are respectively 27 to 28 µm, which are the same. When this ratio is 40%, the object of the present invention can be fulfilled. Angle t which defines the radius change with distance for the tapered portions of a single mode optical fiber in the optical fiber light coupling interface with an enlarged input surface of the present invention, needs not be strictly determined, but is preferred to be set at a value which is equal to or less than threshold angle c of the optical fiber used to fabricate the light coupling device of optical fiber type. Threshold angle c of the single mode optical fiber shown in Figure 3 is approximately 3.6 degrees. As described heretofore, the optical fiber light coupling interface with an enlarged input surface of the present invention feeds the light power from the light source to the optical fiber. The optical fiber light coupling device is constructed using an optical reflection layer and a light coupling portion. The light coupling portion consists of the enlarged optical fiber input end-face, consisting of the enlarged core and the enlarged clad, and the pair of tapered portions. These enlarged core and clad are fabricated by such processes that a stress of compression applies to a part of the optical fiber along the optical axis of the optical fiber so that the radius of the part of the optical fiber increases while the part of the optical fiber is fused by heating, the optical fiber is cut along the plane perpendicular to the optical axis of the optical fiber at the point where the radius of the optical fiber is extended, and that the cut plane is polished. The pair of tapered portions are characterized in that the radii of the core and clad of the normal optical fiber portion near the enlarged optical fiber input end-face portion are gradually decreased as the distance from the enlarged optical fiber input end-face portion increases, and then increased again until it goes to the normal optical fiber radius after passing through the minimum radius. The optical reflection layer is formed outside the coupling means. All the light power emitted from the light source is received by the enlarged optical fiber end-face containing both the core and clad becomes the effective light power which propagates along the core. An allowance of the offset of the optical axes between the light source and single mode optical fiber can be increased as compared with that in the prior art. Assume that radiation angle r of the light beam emitting from the light source is 25 degrees, and that distance z between the light source and the incident end-face of the optical fiber is 100µm. At that time, radius ωz of the light beam becomes approximately 47µm. If allowance d of the offset of the optical axes between the light source and the optical fiber is defined as ±5µm for the practical use, the light coupling device of the present invention can be assembled without any light axis adjustment. If allowance d is 5µm, the light coupling loss in the light coupling device of the present invention is 2.2%. Even if the light coupling device is built without any light axis adjustment, high coupling efficiency can easily be obtained.

Claims: A light coupling interface (100) for feeding light from a light source (30) into a transmission optical fiber (19) or any other light propagation medium comprising an optical fiber coupling portion having : an enlarged optical fiber input end-face (7) which consists of an enlarged core (17) and an enlarged clad (18) which are fabricated by processes such as applying compression forces to a part (20) of an optical fiber along the optical axis thereof so that the radius of said part of the optical fiber (19) is enlarged while said part of the optical fiber is softened by heating, cutting said optical fiber along a plane perpendicular to the optical axis of said optical fiber at the point where said radius of the optical fiber is enlarged, and polishing said plane so as to from said input end face ; a pair of tapered portions (22, 23) where the radii of the core (17) and clad (18) of a normal optical fiber portion near said extended optical fiber input end-face (7) gradually decrease with increasing distance form said enlarged optical fiber input end-face, and then increase again up to the normal optical fiber radius after having passed through a minimum ; the coupling interface further comprising an optical reflection layer (24) formed on the outer surface of said light coupling portion. The optical fiber light coupling interface with an enlarged input end face as claimed in Claim 1, wherein said optical reflection layer (24) which reflects the total incident light thereon is a mirror coating layer of dielectric material or metal film layer with high reflection coefficient. The optical fiber light coupling interface with an enlarged input end-face as claimed in Claim 1, wherein said light coupling portion is mounted in a ferrule (25) adapted to accept said enlarged optical fiber input end-face at one end of a hole (26) bored at the center thereof.

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