APPARATUS FOR COUPLING A MULTIPLE EMITTER LASER DIODE TO A MULTIMODE OPTICAL FIBER

Abstract: A laser system includes a laser diode source (10) with a plurality of output facets (12, 14, 16). The laser diode source (10) produces a divergent output along an emission path. A microlens (26) is optically coupled to the diode source (10). The microlens (26)receives the divergent output along the emission path and modifies the divergence of the output in a high numerical direction of the microlens (26). A power source is coupled to the laser diode source (10).

Description: APPARATUS FOR COUPLING A MULTIPLE EMITTER LASER DTOPE TO A MULTTMODE OPTICAL FIBER BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to solid-state lasers. More specifically, the present invention pertains to laser diodes and coupling of laser diodes to multimode optical fibers. Description of Related Art It is often necessary to couple a laser diode to a multimode optical fiber. The standard procedure is to butt couple the laser diode to the optical fiber, placing the optical fiber as close as possible to the emitting face of the laser diode. The output from a typical laser diode has very high angular divergence in the direction perpendicular to the diode junction, having a typical numerical aperture (NA) of 0.4 to 0.6. The numerical aperture is the sine of the half angle of the diode emission. The NA in the direction parallel to the junction is much smaller, typically 3 times less. In order to couple efficiently to a laser diode, a multimode optical fiber must have an acceptance angle or NA comparable to the NA of the laser diode. A multimode optical fiber ideal for coupling to a laser diode would have an NA of 0.3 or above. Such a multimode optical fiber is somewhat difficult to design since it requires a cladding having a very low index of refraction. Moreover, since the energy emitted from the laser diode diverges rapidly, the end of the optical fiber must be placed very close to the diode facet to which it is to be coupled. Accurately achieving such placement requires very tight mechanical tolerances and can lead to practical fabrication problems due to optical feedback to the laser diode. An object of the present invention is to provide a convenient means of coupling a low NA multimode optical fiber to a laser diode. The resulting low A fiber output can ' have high brightness, where brightness is defined as W/cm 2 /unit solid angle. SUMMARY OF THE INVENTION A microlens, such as a small diameter multimode optical fiber is used to collimate the output emissions of a laser diode before butt coupling the output of the laser diode to an optical fiber. The optical fiber used as the microlens lens is chosen such that its diameter roughly equals the diameter of the fiber to be coupled to the laser diode. The collimation is performed in the high NA direction of the output of the laser diode. For optimal coupling, the diameter of the butt coupled fiber diameter is preferably chosen to be 20 to 50% bigger than the lateral dimension of the laser diode of diode array. The diameter of the collimating microlens is then chosen to be roughly equal to this diameter. The NA of the butt coupled fiber is chosen to be roughly equal to the low NA direction of the diode, typically 0.1 to 0.2. The microlens and the butt coupled end of the fiber may be anti-reflection coated to reduce reflections off these surfaces. The microlens is oriented so as to collimate the high numerical aperture direction of the laser diode output. In a presently preferred embodiment, the butt coupled fiber may be rectangular in shape so as to reduce the total amount of glass in the fiber. The width of the fiber is chosen to be slightly larger (about 20-50% larger) than the diode emitting area. The height is chosen to be as small as possible, typically around 30 to 50 microns. The diameter of the microlens should then be chosen to be approximately equal to the height of rectangle. The arrangement of the present invention for precollimation of a diode emitter can be easily extended to a laser diode bar by extending the fiber precollimating lens along the length of the bar. An array of fibers, with spacing matching the diode separation on the bar, may be butt coupled to the diode bar resulting in very high efficiency coupling of diode bars to fiber arrays. BRTEF DESCRIPTION OF THE FIGURES FIG. 1 is a perspective view of an arrangement according to the present invention for coupling the output from a laser diode bar into an optical fiber bundle. FIG. 2 is graph showing the output power from a fiber bundle as a function of the input power to the fiber bundle in a system using the present invention. FIG. 3 is a block diagram of an embodiment of the present invention used as a pump source for a laser system. FIG. 4 is a perspective view of a presently preferred embodiment of an arrangement according to the present invention for coupling the output from a laser diode bar into an optical fiber bundle. DETAILED DESCRIPTION It has been discovered that by using a microlens to collimate the laser diode before butt coupling to an optical fiber significantly improves coupling efficiently and permits the use of fibers with a low NA (0.1). Moreover, it has been discovered that a piece of multimode optical fiber can serve as an inexpensive microlens. The optical fiber should be chosen such that its diameter roughly equals the diameter of the optical fiber to be coupled to the laser diode. By coUimating the high NA direction of the emission from a laser diode, the butt- coupled fiber may be located further away form the diode laser (it can now be spaced several hundred microns from the laser diode emitting surface) thus reducing the mechanical tolerances of the assembly. In a presently preferred embodiment of the invention, the microlens is cylindrical in cross section. Those of ordinary skill in the art will recognize, however, that other cross sectional shapes, such as elliptical and hyperbolic, could prove to be useful in the present invention for correction of particular spherical aberrations as is known in the are, i.e. Kingsdale, Lens Design Fundamentals, Academic Press 1978. The prior art arrangement for coupling the output of laser diode bar having a plurality of emitters into a plurality of optical fibers which may be part of a fiber bundle includes placing the end of each optical fiber very close to the diode facet to which it is to be coupled because the energy emitted from the laser diode diverges rapidly. Accurate placement of the fiber end relative to the laser diode requires very tight mechanical tolerances and thus leads to practical fabrication problems. In addition, the output from a typical laser diode has very high angular divergence in the direction perpendicular to the diode junction, having a typical NA of 0.5 to 0.6. The NA in the direction parallel to the junction is much smaller, typically 3 times less. A multimode optical fiber having an NA to ideally match the laser diode output would require a cladding having a very low index of refraction. It is difficult to produce such a fiber. The present invention solves these practical problems and provides a solution which avoids the critical placement problems of the prior art arrangement, and allows utilization of readily available multimode optical fibers. Referring first to FIG. 1, an arrangement according to the present invention for coupling the radiation emitted from laser diode bar 10, having a plurality of emitters 12, 14, and 16, into multimode optical fibers 18, 20, and 22, which may be part of an optical fiber bundle 24 includes placing a microlens 26 in between the emitting facets of emitters 12, 14, and 16, and the ends of multimode optical fibers 18, 20 and 22. The optimal spacing of the microlens 26 from the emitting surfaces of the emitter facets may be determined by the following equation: d=R(2-n)/2(n-l), where d is the distance from the laser diode to the lens, n is the index of refraction of the optical fiber lens, and R is the radius of the optical fiber. A distance of approximately 60 microns from the near edge of the microlens to the diode facet is satisfactory for a 250 micron diameter fiber having an index of refraction of 1.5. The optimal spacing of the optical fiber end from the microlens should be as small as possible. Imagings other than collimation may sometimes be desirable and are intended to fall within the scope of the present invention. Fore example, it may be advantageous to refocus the highly divergent emission from a laser diode emitter in order to stay within the aperture of a small diameter or rectangular optical fiber to which the laser diode radiation is to be coupled. The various diode-to-lens spacing distances d and other setup parameters for such applications may readily be determined by those of ordinary skill in the art by reference to widely available works, such as F. Jenkins and H. White, Fundamentals of Optics, McGraw-Hill, 4th ed. 1976, Chapter 3, pp. 44-57. For optimal coupling, the diameter of the optical fiber to be coupled is typically chosen to be 20 to 50% bigger than the lateral dimension of the laser diode of diode array. For example given a laser diode array having a 200 micron broad area emitter, the optical fiber is chosen to be approximately 250 microns in diameter. In a presently preferred embodiment, the diameter of microlens 26 is chosen to be roughly equal to the diameter of the optical fiber to be coupled. The diameter of microlens 26 may be chosen to be less than the diameter of the optical fiber to be coupled without loss in coupling efficiency. However, if such smaller microlens diameters are used, the alignment of the microlens will be made more difficult. Microlens 26 must be carefully placed with respect to the output facets of the laser diode bar in order to properly collimate them. This may be accomplished by carefully aligning the microlens and securing it in place with a suitable epoxy such as Tra-Bond BB-2151 available from Tra-Con in Medford Mass. The NA of the optical fiber to be coupled is chosen to be roughly equal to the low NA direction of the diode, typically 0.1 to 0.15. This combination of microlens and optical fiber results in greater than 80% coupling of the laser diode emitted energy into the multimode optical fiber. The microlens and the butt coupled end of the fiber may be anti-reflection coated to reduce reflections from these surfaces. As is shown in FIG. 1, the optical fibers 18, 20, and 22 may be rectangular in cross section so as to reduce the total amount of glass in the fiber. The width of such a rectangular fiber is preferably chosen to be slightly bigger than the diode emitting area. The height is chosen to be as small as possible, typically around 30 to 50 microns. Heights smaller than 30 to 50 microns may be used but make alignment difficult, the diameter of the microlens 26 is then preferably chosen to equal the height of the rectangle, 30 to 50 microns in the example given. The resulting fiber output is brighter since the total emission area is smaller. As illustrated in FIG. 1, the arrangement of the present invention for precollimation of a single laser diode emitter can be easily extended to a laser diode bar containing a plurality of laser diode emitters by extending the microlens 26 along the length of the bar to intercept the output radiation from the multiple emitters along the length of the bar. An array of optical fibers, with spacing matching the diode separation on the bar, can be efficiently butt coupled to the laser diode bar 10, resulting in very high coupling of diode bars to fiber arrays. In the example given above, ten 1 watt laser diode emitters contained on a 1 cm diode bar, each emitter having a 200 micron wide emission region, available from Spectra Diode Labs of San Jose, California, were coupled to a fiber array including 10 optical fibers, each having a 250 micron diameter and an output NA of 0.11, by use of a 250 micron diameter optical fiber lens spaced approximately 50 microns from the emitting surfaces of the diodes and about 300 microns from the ends of the optical fibers. The overall coupling efficiency of this arrangement was measured at about 90% as shown by the graph of FIG. 2. The output diameter of the resulting fiber bundle is about 1 mm and the NA is 0.11. If smaller diameter emission regions are needed (for example, 0.33 mm, to pump a solid state laser) then the emitting face of the fiber optic bundle can be demagnified by using a simple high NA lens, as will be appreciated by those of ordinary skill in the art, to suitably reduce the diameter of the image of the output of the fiber bundle. The present invention is useful for a variety of applications. One such application is as a pump source for a laser system. As an example, FIG. 3 is block diagram illustrating how the present invention may be used as a pump source for a solid state laser, although those of ordinary skill in the art will realize that the present invention is adaptable for pumping other kinds of laser systems in other manners without departing from the concept of the invention. Referring now to FIG. 3, a bar 30 of doped solid state laser material and an output coupler 32 form a laser cavity 34. As will be appreciated by those of ordinary skill in the art, the end of laser cavity 34 may be a highly reflective coating on the face of the bar 30. An optical fiber bundle 36, into which laser diode radiation has been coupled, terminates in an output coupler 38, as is well known in the art. An imaging lens 40, having a focal length f, is placed in the optical path of radiation from the optical fiber bundle 36, at a distance 4f from the surface of the output coupler 38 and at a distance 4f/3 from the face of the bar 30 forming one end of the laser cavity 34. Those of ordinary skill in the art will recognize that other distances may be employed depending on the optical coupling desired. The particular distances employed in particular embodiments depend entirely on the optical coupling desired and selection of particular distances to achieve particular ends is well within the level of ordinary skill in the art. Those of ordinary skill in the art will recognize the geometry disclosed in FIG. 3 as an end-pumped cavity geometry, but such persons will immediately recognize that the arrangement disclosed with respect to FIG. 3 could easily be used for side pumping a laser cavity with minimal and trivial rearrangement An important consideration in the design of high-energy output optical systems of the nature described herein is the positional stability of the optical components over the temperature ranges expected to be encountered during normal system operation. The thermal characteristics of optical fibers such as those suitable for use as fiber lenses in the present invention are such that the fiber lens may tend to bow during thermal cycling if it is mounted with both of its ends fixed. Given the small spacing between the optical elements in a typical system in which the present invention may be utilized, even small amounts of bow due to thermal excursions can be intolerable. According to another aspect of the present invention, an optical fiber lens according to the present invention may be mounted in a manner which avoids the problems caused by thermal expansion during operation. Referring now to FIG. 4, a perspective view of a presently preferred embodiment of a optical fiber mounting arrangement according to the present invention for coupling the output from a laser diode bar into an optical fiber bundle is shown. In the embodiment of FIG. 4, an assembly configured according to the present invention includes a laser diode bar 10 disposed on a mounting member 40. As is known in the art, mounting member 40 may comprise a material suitable for providing heat transfer from laser diode bar 10, and is preferably formed from a material having a thermal expansion coefficient matching that of laser diode bar 10. Laser diode bar 10 is clamped down to mounting member 40 by a contact plate 42 held in place by fastening means such as the three screws 44 shown in FIG. 4. A spacer 46 is provided at the rear of clamp member 42. The rear portion of clamp member 44 is shown shaped as a spade terminal 48, providing a convenient way to supply a power supply potential to the laser diode bar 10. A first end of optical fiber lens 26 may be held in a fixed position at a selected distance from the emitting faces of laser diode bar 10 by one of any number of well known fastening means, including, but not limited to, a pressure clamp (not shown), a dab of epoxy resin 50, such as Tra-Con 2151, available from Tra-Con, Incorporated, of Medford, Massachusetts. The other end of optical fiber lens 26 is slidably mounted along its axis. This may be accomplished in a number of ways. As illustrated in FIG. 4, a cylindrical clamp 52, is furnished with an inner sleeve 54, which may be formed from a length of hollow precision bore tubing. The tubing may be made of quartz, sapphire, polyimide, etc. Suitable tubing may be purchased from Polymicro Technologies of Phoenix, Arizona, the outside diameter of the optical fiber should match the inside diameter of the tubing. Cylindrical clamp 52 is affixed to mounting member 40 in a position such that the axis of the cylindrical aperture of cylindrical clamp 52 and inner sleeve 54 lies along the axis of optical fiber lens 26. The second end of optical fiber lens 26 is thus slidably positioned in the cylindrical aperture of cylindrical clamp 52. Alternatively, a first end of optical fiber 26 may be affixed to mounting member 40 with a dab of non-wetting or non-adhering adhesive. When hardened, the adhesive will restrict the motion of the fiber to a direction along its axis. By mounting the optical fiber lens 26 as taught herein, a thermally stable configuration is achieved, this allows reliable operation of the optical system over a greater temperature range. A presently preferred embodiment of the invention has been disclosed. From the disclosure in conjunction with the drawings, those of ordinary skill in the art will be able to make changes and modifications to the specifically described embodiments without departing form the scope of the invention which is intended to be limited only by the scope of the appended claims. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents. What is claimed is:

Claims: CLAIMS 1. A laser system, comprising: a laser diode bar, said diode bar with a plurality of emitter regions, each emitting region with an output facet, said diode bar having output in a high numerical aperture direction and a low numerical aperture direction, a microlens having a linear axis oriented substantially perpendicular to the emission path of radiation from said emitter regions, positioned in the emission path of said emitter regions, spaced from said diode bar by a distance selected to achieve a desired modification of the divergence of the diode bar output in its high numerical aperture direction, a power source coupled to the laser diode bar. 2. The laser system of claim 1, wherein the divergence of the output of the laser diode bar in its high numerical aperture direction is reduced by the microlens. 3. The laser system of claim 1 or 2, wherein the output of the laser diode bar in its high numerical aperture direction is focussed by the microlens. 4. The laser system of claim 1 or 2, wherein the output facets of the emitting regions are imaged in the high numerical aperture direction to a selected plane by the microlens. 5. The laser system of claim 1 or 2, wherein the output facets of the emitting regions are relayed in the high numerical aperture direction to a selected plane by the microlens. 6. The laser system of claim 1 or 2, wherein the microlens has a substantially round cross-sectional geometry. 7. The laser system of claim 1 or 2, wherein the microlens has a non-round cross-section, with the cross-section chosen to provide a desired optical modification to the output of the diode bar in its high numerical aperture direction. 8. The laser system of claim 1 or 2, further including a slip-fit sleeve on at least one end of said microlens. 9. A laser system, comprising: a laser diode bar, said diode bar with a plurality of emitter regions, each emitting region with an output facet, said diode bar having output in a high numerical aperture direction and a low numerical aperture direction, a microlens having a linear axis oriented substantially perpendicular to the emission path of radiation from said emitter regions, positioned in the emission path of said emitter regions, spaced from said diode bar by a distance selected to achieve a desired modification of the divergence of the diode bar output in its high numerical aperture direction, one or more optical fibers positioned to receive light form the emitting regions of said diode bar and microlens combination, and a power source coupled to the laser diode bar. 10. The laser system of claim 9, wherein the divergence of the output of the laser diode bar in its high numerical aperture direction is reduced by the microlens. 11. The laser system of claim 9, wherein the output of the laser diode bar in its high numerical aperture direction is focussed by the microlens. 12. The laser system of claim 9, wherein the output facets of the emitting regions are imaged in the high numerical aperture direction to a selected plane by the microlens, with one or more of said optical fibers positioned near the selected plane. 13. The laser system of claim 9, wherein the output facets of the emitting regions are relayed in the high numerical aperture direction to a selected plane by the microlens, with one or more of said optical fibers positioned near the selected plane. 14. The laser system of claim 9, wherein the microlens has a substantially round cross-sectional geometry. 15. The laser system of claim 9, wherein the microlens has a non- round cross-section, with the cross-section chosen to provide a desired optical modification to the output of the diode bar in its high numerical aperture direction. 16. The laser system of claim 9, further including a slip-fit sleeve on at least one end of said microlens.

Inventors: BAER THOMAS M, HEAD DAVID F, BAER, THOMAS, M., HEAD, DAVID, F.
Applicants: SPECTRA PHYSICS LASERS INC, SPECTRA-PHYSICS LASERS, INC.
Application year: 1996
Main classification: G02B6
IPC codes: G02B6