Optical coupling circuit element

Abstract: An optical coupling circuit element providing one transparent substrate (7), a first micro Fresnel lens (11) formed on one side surface (31) of said substrate, and a second micro Fresnel lens (21) formed on the other side surface (31b) of said substrate, so that coherent light incident into said first micro Fresnel lens is projected, through said transparent substrate, on said second micro Fresnel lens to be left therefrom as a collimating beams, which is useful for directing light emitted from coherent source such as semi-conductor laser (5) to optical communication means such as optical fiber (6) for condensation.

Description: The invention relates to an optical coupling circuit element and more particularly to an optical circuit coupling element using a micro Fresnel lens for collimating light emitted from a coherent source such as a semiconductor laser to optical communication means such as an optical fiber for condensation. Optical fiber communication for a long distance and high capacity uses semiconductor laser having higher coherency as a source, since light emitted from the semiconductor laser is required to be effectively directed to optical communication means such as an optical fiber. A far field pattern of light emitted from semiconductor laser usually has an elliptical cross section. Such light when collimated by use of a customary axially symmetric optical lens will be gathered elliptically, so that when the light is input to a core of an optical fiber whose aperture angle is in right circular conical shape, it will be poor in coupling efficiency in comparison with a circular collimation. To solve the problem, various constructions have been proposed previously. Use of a micro Fresnel lens with a hologram on a transparent substrate for an optical circuit coupling element has been hitherto proposed (Japanese Patent Publication No. SHO 47(1972)-43670). Use of the micro Fresnel lens realized reduction of spherical aberration that is difficult for the conventional lens to achieve, and lowering of ellipticity of elliptically emitted light from semiconductor laser used as a source. In recent years, a computer generated hologram has been used for the fabrication of micro Fresnel lens, wherein in a similar manner for production of IC, a hologram pattern is formed by exposure on a transparent substrate which is then diced similarly to an IC chip. The computer designed hologram can be formed by writing on EB resist with electron beam. By changing of dosage of electron beam, a saw-tooth shaped diffraction grating can be achieved and a micro Fresnel lens having diffraction efficiency of almost 100% can be fabricated. Fig. 10 shows a conventional example using a micro Fresnel lens to direct the light emitted from semiconductor laser to an optical fiber. A micro Fresnel lens 13 formed on a transparent substrate 7 can realize outgoing of elliptic light (spherical wave) 5a from a semiconductor 5 in a manner of having a round shape on the section of an optical fiber 6. In detail, the light passing the micro Fresnel lens can be converted into a spherical wave 5b collimating circularly to a core 6a at the optical fiber 6. In other words, the spherical wave converges into a shape of cone S in which a right circular cone having as its top the center A of the core 6a of the optical fiber 6 is cut with the micro Fresnel lens [see Fig. 10(b)]. Thus, the micro Fresnel lens 13 is arranged in such a manner that the mother line m of the core 6a of optical fiber 6 is placed on an extension of a line 1 connecting a center of circle defined by the bottom of that right circular cone with a top thereof (the line 1 will be called hereunder "axis of right circular cone"). When using the micro Fresnel lens 13 shown in Fig. 10, the optical axis at the outgoing side, i.e., the axis of right circular cone does not correspond to the optical axis at the incidence, i.e., the axis of elliptic incident ray. When such an off-axial type construction is used, the following problems will occur. First, since the arrangement is poor in symmetry, the optical axis is likely to be deflected due to thermal expansion of mount material, etc. Also, locations or angles in setting modules are hard to be decided. Furthermore, the optical system cannot be made compact as a whole. To solve the abovesaid problems, two micro Fresnel lenses may be used to enable an arrangement having a favorable symmetry and such a coaxial type construction that the optical axis at the incidence side are coaxially disposed with the optical axis at the outgoing side, i.e., the axis of the right circular cone. However, the coaxial construction with two micro Fresnel lenses causes an additional problem. Generally, as an optical system increases the number of constituent optical elements, accuracy of the entire system will lower due to cumulation of specific positional deflections of the optical elements. That is, in the case of using two micro Fresnel lens, there causes the problem of positional deflection between the two lenses. The present invention has been designed to overcome the abovesaid problems. The use of Fresnel lenses in arrangements which avoid the effects of chromatic and wave-length change aberration in a collimated beam is observed additionally in GB 2,021,807 and EP 0268357. The latter publication also discloses the use of holograms as gratings disposed on opposite faces of a transparent body. The present invention is aimed to provide an optical coupling circuit element which can be arranged with a favorable symmetry to enable an incident ray from a coherent source having an elliptic far field pattern to be converted into a spherical wave converging into a shape of cone forming a part of right circular cone. Thus, the invention provides an optical coupling element for converting a beam of coherent light diverging from a light source to a converging beam of light and comprises first and second microhologram Fresnel lenses each provided on a surface of a transparent substrate whereby the first Fresnel lens receives the diverging beam and transmits the respective light via the substrate to the second Fresnel lens, characterised in that the first Fresnel lens is constructed to transform an incident beam of light of elliptical cross-section into a beam projecting a far field pattern of substantially circular shape and the second Fresnel lens has an elliptical shape and projects the beam of circular cross-section as a converging spherical-wave beam having a circular cone shape. In such a construction, the optical axis of the coherent light from the light source and the optical axis of a beam from the second micro Fresnel lens can be set on the same line, whereby the diverging spherical wave beam from the coherent source can be focused as a converging spherical wave beam having the shape of a right circular cone or slanted circular cone so as to focus on a core of an optical fiber, thereby enhancing the coupling efficiency of source to optical fiber. Also, since two micro Fresnel lenses are formed on one transparent substrate, the positional misalignment between the two lenses can be eliminated. In another aspect, the present invention provides an optical coupling circuit element, for collecting a coherent light beam emitted by a light source and directing the said light beam to an optical communication means, comprising first and second micro Fresnel lenses each provided on a surface of a transparent substrate characterised in that the first Fresnel lens is formed on one side surface of the substrate, a reflective mirror is formed on an opposed side surface of the substrate and the second Fresnel lens is formed parallel to said first Fresnel lens on said one side surface of said substrate, so that light from the said source incident on said first Fresnel lens propagating through said transparent substrate to said reflective mirror is reflected by said mirror to pass through said substrate again to said second Fresnel lens and leaves the second lens as a converging beam. In such a construction, since two micro Fresnel lenses are formed on the one side face of the transparent substrate, and the reflecting mirror is formed on the other side face, the projected light to pass the first micro Fresnel lens and to be changed in direction by the reflecting mirror is enabled to be condensed to the optical fiber in the shape of right circular cone, thereby preventing coupling efficiency of source and optical fiber from lowering. Also, since the two micro Fresnel lens are formed on the transparent substrate, the positional deflection between the lens can be eliminated. Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings wherein: Fig. 1 is a perspective view of an optical coupling circuit element of one embodiment of the present invention, illustrating the state that laser beam is directed to an optical fiber. Figs. 2 (a) and (b) are explanatory views showing the state of converging of light on horizontal and vertical planes with respect to the optical coupling circuit element of Fig. 1. Figs. 3 (a), (b) and (c) are explanatory views for explaining of optical elements providing operation of micro Fresnel lens, using three types optical lens. Figs. 4 - 6 are explanatory views showing constructions for the 2nd to 4th embodiments of the invention. Fig. 7 is an explanatory view showing construction of an optical coupling circuit element of the 5th embodiment of the invention, illustrating the state that laser beam is directed to optical fiber. Fig. 8 is an explanatory view of correcting the slanting of light with respect to the 2nd to 4th embodiments. Figs. 9 (a), (b) and (c) are explanatory views of function of an optical coupling circuit element of the invention. Figs. 10 (a), (b) and (c) are explanatory views for showing a conventional prior art optical coupling circuit element. Generally, a wave optical consideration will be required to effectively feed light into a single mode fiber. Also, for effectively coupling incident light to an optical fiber, the wave front of the incident light should correspond to a waveguide front of the optical fiber. Referring to Fig. 9, a single mode fiber 30 is formed in axial symmetry, and a core 31 as a waveguide front of the fiber 30 is naturally round. For effectively converging light to a round end face 30a of the fiber 30, it is enough that a waist of converging beam comes to the end face 30a and a shape of the beam shape is round. To form such a shape of beam, the wave front P of the beam is naturally a spherical wave 32, so that a plane wave 33 is transmitted to the inside of the core 31. As shown in Fig. 9 (b) and (c), it is necessary to form a micro Fresnel lens in such a manner that light is to leave as a spherical wave converging to a top A of a right circular cone T. In the present invention, as shown in Fig. 1, two micro Fresnel lenses 11, 21 are arranged on one transparent substrate 7, so that the micro Fresnel lens 21 at the fiber side requires to have a shape for formation of a spherical wave, i.e., to be a hologram of elliptic shape in the case of a coaxial system, while the micro Fresnel lens 11 at a semiconductor laser 5 side requires, in the case of coaxially system, to be able to convert an elliptic beam of the semiconductor laser 5 to a round beam for spherical wave. A hologram having a magnitude covering the ellipse of beam, a thick pitch in the major axis direction of the ellipse, and a thin pitch in the minor axis direction of the ellipse can be preferably used as the micro Fresnel lens 11 (see Fig. 1). In the present invention, an adjustment method for a coupled circuit comprising two holograms is as follows in brief. 1. First, to fix a stem mounting a chip of the semiconductor laser to decide the optical axis, 2. to set holograms perpendicularly to the optical axis, and simultaneously, to generally adjust an interval between the semiconductor laser and the hologram, 3. to set an end of an optical fiber on the optical fiber to generally adjust an interval between the hologram and the optical fiber, 4. to switch on the semiconductor laser, a monitor beam coming out of output of the optical fiber and to move the holograms vertically and fixes them at a point where the output beam becomes maximum, 5. to move the input end of the optical fiber along the optical axis and fix the fiber at a point where the output beam becomes maximum. The coherent source to be used in the invention may be a semiconductor laser. The light in the invention may be a spherical wave H having a shape of right circular cone T and converging its top with respect to a center of the core 6a of the optical fiber 6 (see Figs. 1, 5, 6, or 9) and a spherical wave having a shape of a slanting circular cone J and converging accordingly. Reference A is a apex of the cone J. The first micro Fresnel lens of the invention may preferably has a thick pitch at a position corresponding to a major axis direction of an elliptic far field pattern of coherent light and a thin pitch at a position corresponding to a minor axis direction. The second micro Fresnel lens of the invention may preferably be an elliptic shape. The first and second micro Fresnel lens can be fabricated by a conventional technique, e.g., using a computer designed hologram. In the same way as for production of IC, a hologram pattern is formed by exposure on a transparent substrate and is put into dicing as the IC chip. The computer generated hologram can be formed by writing on EB resist with electron beam. Change of dosage of electron beam may provide a saw-tooth shaped diffraction grating to provide a micro Fresnel lens having a diffraction efficiency of almost 100%. Fig. 1 shows one embodiment of the optical coupling circuit element, showing the state that light from a semiconductor laser 5 is taken into an optical fiber 6 through the optical coupling circuit element. a Divergent spherical wave issued from the semiconductor laser 5 goes into a micro Fresnel lens 11 formed on one face 31a of a transparent substrate 7 of a rectangle parallel-linked. The other side face 31b of the substrate 7 has a micro Fresnel lens 21. Since the far field pattern of outgoing light of semiconductor is elliptic usually, incident light is projected elliptically on the micro Fresnel lens 11 on which a hologram is formed to project circularly that incident light on the micro Fresnel lens 21 through the transparent substrate 7. Also, on the micro Fresnel lens 21 is provided a hologram which transmits the circularly projected light as a spherical wave converging to an apex A of a right circular cone having a round bottom. A center of core 6a of an optical fiber 6 is located at the apex A of the right circular cone, so that the converging spherical wave can be effectively taken into the optical fiber 6. In the case that the micro Fresnel lens 21 at the fiber side is perpendicular to an extended line of an axis of the core 6a in the present example, the micro Fresnel lens 11 at the semiconductor side requires to circularly transmit a circularly projected incident light on the micro Fresnel lens 21 at the fiber side. Figs. 2 (a) and (b) show the state on a parallel plane (called hereinafter horizontal plane) to a junction of the semiconductor laser in Fig. 1 and a plane perpendicular to the junction and having an optical axis (called hereinafter vertical plane). On the horizontal plane, width of light spread on the micro Fresnel lens 11 is equal to that on the lens 21, while on the vertical plane, width of widely spread light on the lens 11 is corrected to be equal to that on the horizontal plane. Figs. 3 (b) and (c) replace the micro Fresnel lens 11, 21 in Figs. 2 (a) and (b) with optical lens. Fig. 3 (a) further replaces the lens of Figs. 3 (b) and (c) with another two optical lens. In Fig. 3 (a), solid line shows the state on the vertical plane and dotted line shows that on the horizontal plane. Incident light is first changed to parallel rays by a collimating lens 1 that is a customary optical lens in axial symmetry. A lens 3 is a cylindrical lens, and functions as a convex lens only in a direction perpendicular to an interface of the semiconductor (called hereinafter vertical direction) but does not function in a direction parallel to the semiconductor interface and perpendicular to the optical axis (called hereinafter horizontal direction). Rays are contracted only in vertical direction and corrected to be circular at lens 4 spaced at an interval l from lens 3. The lens 4 is a cylindrical lens operating a concave lens only in vertical direction but not in the horizontal direction. Light corrected circularly by the lens 3 is changed by the lens 4 to be parallel rays keeping circularity and then reaches an objective lens 2. The objective lens is an optical lens in usual axial symmentry, so that the parallel rays changed to be circular are converted, with keeping the circularity, into a spherical wave collimating in a right circular cone shape so as to be collected to the core at the end face of the optical fiber 6. Use of four lenses as in Fig. 3 (a) can correct the elliptic light to be circular. The number of lenses can be reduced. For instance, a first lens 10 shown in Figs. 3 (b) and (c) are composed of lens 1 and lens 3, and a second lens 20 of lens 2 and lens 4. As shown in Fig. 3 (b), on the horizontal plane, there is no function of two cylindrical lenses 3 and 4, and focal length f 1H of first lens 10 in the horizontal direction is equal to that f₁ of the collecting lens 1. f 1H = f₁ (1) Also, focal length f 2H of second lens 20 in the horizontal direction is equal to that f₂ of the objective lens 2. f 2H = f₂ (2) As shown in Fig. 3 (c), function of the two cylindrical lenses 3 and 4 appear on the vertical plane, so that focal length of first lens 10 in the vertical direction f 1V is represented by the following formula when focal length of lens 3 in the vertical direction is f 3V . And, length of first lens 20 in the vertical direction f 2V is represented by the following formula when focal length of lens 4 in the vertical direction is f 4V . Focal length f 3v and f 4v of the cylindrical lens can be obtained as follows. In an ellipse of far field pattern of the emitting light from semiconductor laser, when a ratio of major axis to minor axis is e, f 3v = -e f 4v (e > 1, f 4v < 0) (5), and an interval between lens 3 and 4 is 1 = f 3v + f 4v (6) From these relationship Then, f 1v and f 2v can be obtained from the formula (3), (4) and (7). Also, diameters in horizontal and vertical directions of holograms formed on the micro Fresnel lens 11, 21 can be obtained by inputting f 1v , f 2v , f 1H , f 2H into f of the following formula. γ n = √ nfλ + n²λ²/4 (8) wherein n is degree of zones, and λ is wavelength of laser beam. The optical circuit coupling element 31 in Fig. 1 uses the micro Fresnel lens 11, 21 obtained by the abovesaid calculation. The embodiments shown in Figs. 4 - 6 can prevent the generation of noises by restraining a part of light emitted from the semiconductor laser from returning thereto due to reflection by the micro Fresnel lens nearer the laser. In these cases, the micro Fresnel lenses 11, 21 with hologram are formed on the both side of the transparent substrate, and the face of the micro Fresnel lens nearer the semiconductor laser 5 in the optical coupling circuit element and wave normals extending at an intersection of the lens face with the optical axis of the emitting beam from laser are adapted not to be parallel with the optical axis of outgoing light from the laser. An optical coupling circuit element 32 in Fig. 4 provides micro Fresnel lenses 11, 21 on both surfaces of a transparent substrate, each surface being parallel. Fig. 5 shows an example wherein one micro Fresnel lens 11 at an optical coupling circuit element 33 and nearer a semiconductor laser 5 is adapted to be slanted not perpendicular with respect to the optical axis of the laser beam. An optical coupling circuit element 34 in Fig. 6 is made to be convex at its face nearer the semiconductor laser 5 and provides on the convex surface with the micro Fresnel lens 11, and an optical axis of the laser beam (indicated by solid line) is off-set by h from an axis of the convex portion (by dotted line). Surfaces of the micro Fresnel lens 11 in the abovesaid coupling elements 32 - 34 are not perpendicular to the optical axis of the laser beam, so that the hologram for the micro Fresnel lens 21 nearer the optical fiber 6 at each of the coupling elements is formed in a manner of capable of correcting the slanting of light passing through the lens 11. The construction shown in Fig. 8 can change pitches of the diffraction grating of the hologram corresponding a direction of laser beam to a diffract beam at any desirable angles. To change light l₁ , l₂ by angle ϑ₁, pitch d₁ is set as d₁sinϑ₁ = λ [d₁sinϑ₁ = nλ (n = ±1, ±2, ......)] and to change light m₁, m₂ by angle ϑ₂, pitch d₂ is set as d₂sinϑ₂ = λ [d₂sinϑ₂ = nλ (n = ±1, ±2, ......)] By gradually changing the pitches in the surface of hologram accordingly, slanting of the surfaces can be corrected. The optical coupling circuit elements 33 and 34 in Figs. 5 and 6 have the surfaces at the fiber side perpendicular to an optical axis, so that the laser beam emitted elliptically is projected circulaly on the micro Fresnel lens 21 at the fiber side through the micro Fresnel lens 11 at the laser side, whereby enabling the outgoing light to be a spherical wave converging in a right circular cone shape. In the coupling element 32 of Fig. 4, an existing surface at the micro Fresnel lens 21 at the optical fiber side is not perpendicular to an optical axis of the outgoing light, so that in order to make the outgoing light to be spherical wave converging in a slanted circular cone shape, micro Fresnel lens 11 projects incident light on the micro Fresnel lens 21 in such a shape of cross section that the right circular cone is transected by the surface of the micro Fresnel lens 21. Fig. 7 shows an embodiment wherein two micro Fresnel lenses 11, 21 are disposed on the same surface of a transparent substrate 7. The other side opposite to that surface thereof is formed a reflective mirror 14. The laser beam elliptically outgoing from a semiconductor laser 5 is directed to the reflective mirror 14 by the micro Fresnel lens 11 and reflected by the mirror 14 so as to be projected on the micro Fresnel lens 21. In this instance, the light is projected circularly on the lens 21 as a spherical wave converging in a right circular cone shape having the circularity as its bottom, and then focused on a top A of the right circular cone corresponding to the center C of core portion of optical fiber 6. By using the optical coupling circuit element as disclosed in the embodiment, the semiconductor laser 5 and the optical fiber 6 can be mounted integrally to enable to constitute a compact module.

Claims: An optical coupling circuit element for converting a beam of coherent light diverging from a light source to a converging beam of light (H) and comprising first (11) and second (21) micro-hologram Fresnel lenses each provided on a surface of a transparent substrate (7) whereby the first Fresnel lens (11) receives the diverging beam and transmits the respective light via the substrate to the second Fresnel lens (21), characterised in that the first Fresnel lens is constructed to transform an incident beam of light of elliptical cross-section into a beam projecting a far field pattern of substantially circular shape and the second Fresnel lens has an elliptical shape and projects the beam of circular cross-section as a converging spherical-wave beam having a circular cone shape. An optical coupling circuit element of Claim 1 including a coherent light source in the form of a semiconductor laser (5). An optical coupling circuit element of Claim 2 wherein said coherent light source transmits a diverging spherical wave beam to the said first Fresnel lens (11). An optical coupling circuit element of any preceding claim wherein the pitch of the grating rings of said first Fresnel lens is smaller on one lens diameter in comparison to the pitch on a diameter orthogonal thereto. An optical coupling circuit element of Claim 2 or Claim 3 wherein said converging beam is in the form of a right circular cone (H). An optical coupling circuit element of any of Claims 2 to 4 wherein said first and second Fresnel lenses (11, 21) have flat lens surfaces that are arranged parallel to each other, and an optical axis of said diverging light beam incident on said first Fresnel lens is parallel to a line perpendicular to the flat lens surfaces thereof, and wherein the optical axis of light propagating from said first Fresnel lens onto said second Fresnel lens through said transparent substrate and the optical axis of the converging beam leaving said second Fresnel lens are colinear. An optical coupling circuit element according to any preceding claim wherein the said first and second micro-hologram Fresnel lenses are disposed on opposite surfaces of the transparent substrate. An optical coupling circuit element according to any of Claims 1 to 4 wherein said converging beam has a slanted cone shape. An optical coupling circuit element according to any of Claims 1 to 7 wherein said first and second Fresnel lenses have flat lens surfaces and the lenses are arranged non-parallel to each other with the optical axis of coherent light being obliquely incident on said first Fresnel lens relative to a line perpendicular to the first lens surfaces thereof, and wherein the optical axis of light passing through said transparent substrate from said first Fresnel lens to said second Fresnel lens and the optical axis of the converging beam leaving said second Fresnel lens are colinear. An optical coupling circuit element of Claim 9 wherein said first Fresnel lens (11) has a lens surface which taken as a whole is generally convex and has a central axis displaced a fixed interval from the optical axis of incident coherent light and said second Fresnel lens (21) taken as a whole has a flat surface, and wherein the optical axis of coherent light incident on said first Fresnel lens oriented obliquely to the lens surfaces thereof, the optical axis of light propagated from said first Fresnel lens to said second Fresnel lens through said transparent substrate (7) and the optical axis of a converging bundle of rays leaving said second Fresnel lens are colinear. An optical coupling circuit element for collecting a coherent light beam emitted by a light source (5) and directing the said light beam to an optical communication means (30), comprising first (11) and second (21) micro Fresnel lenses each provided on a surface of a transparent substrate (7) characterised in that the first Fresnel lens (11) is formed on one side surface of the substrate, a reflective mirror (14) is formed on an opposed side surface of the substrate and the second Fresnel lens (21) is formed parallel to said first Fresnel lens on said one side surface of said substrate, so that light from the said source incident on said first Fresnel lens propagating through said transparent substrate to said reflective mirror is reflected by said mirror to pass through said substrate again to said second Fresnel lens and leaves the second lens as a converging beam. An optical coupling circuit element of Claim 11 comprising a light source in the form of a semiconductor laser (5). An optical coupling circuit element set forth in Claim 12 wherein said light source transmits a diverging spherical wave beam to the said first Fresnel lens (11). An optical coupling circuit element set forth in Claim 12 or Claim 13 wherein said converging beam is a spherical wave beam having a right circular cone shape. An optical coupling circuit element set forth in Claim 13 or Claim 14 when appendant to Claim 13 wherein said first Fresnel lens (11) has a lens surface perpendicular to the optical axis of said diverging light beam and said second Fresnel lens (21) has a lens surface perpendicular to the optical axis of said converging beam. An optical coupling circuit element of Claim 11 or any claim appendant thereto wherein the first and second lenses (11, 21) are hologram elements and the first Fresnel lens is constructed to project a far field pattern of a substantially circular cross-section on said second Fresnel lens and wherein the pitches of the grating rings of said first Fresnel lens are made smaller across a central diameter relative to those on a central diameter orthogonal thereto.

Inventors: INOGUCHI KAZUHIKO, NAKATSU HIROSHI, OKUMURA TOSHIYUKI, TAKIGUCHI HARUHISA, INOGUCHI, KAZUHIKO, NAKATSU, HIROSHI, OKUMURA, TOSHIYUKI, TAKIGUCHI, HARUHISA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1989
Main classification: G02B6
IPC codes: G02B6, G02B3