Abstract:
The present invention provides several novel diffraction limited microlens configurations which are especially valuable for use in conjunction with laser diodes, and optical fibers. Collimators, circularizers and focusers (couplers) are provided.

Description:
The United States Government has rights in this invention pursuant to Contract No. W-7405-Eng-48 between the United State Department of Energy and the University of California for the operation of Lawrence Livermore National Laboratory. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to copending application Ser. No. 07/591,462, titled Method for Fabrication of Cylindrical Microlenses of Selected Shape, filed Sep. 28, 1990 by James J. Snyder and Thomas M. Baer and also copending application Ser. No. 07/591,409, titled Laser Diode Assembly Including a Cylindrical Lens, filed Sep. 28, 1990 by James J. Snyder and Patrick Reichert, both of which are hereby incorporated by reference as if fully set forth herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to microlenses, and more particularly to microlenses for use with laser diodes and integrated optics. 
     2. Description of Related Art 
     The related art is described in the section of the same name in each of the above-referenced applications of which this application is a CIP. 
     SUMMARY OF THE INVENTION 
     The present invention provides several novel diffraction limited microlens configurations which are especially valuable for use in conjunction with laser diodes. 
     The first two configurations are Crossed Lens Collimators. This configuration converts the astigmatic elliptical output beam from a typical low-power laser diode into a small (e.g., around 100 microns diameter), circular, diffraction limited collimated beam. The laser diode with lens assembly would be very compact and would provide a small laser beam over propagation distances of several tens of centimeters. This might be particularly attractive to the manufacturers of bar code scanners, for example, since the beam would be round and would contain most of the light from the laser diode. 
     The third configuration is a Circularizer. This configuration converts the highly divergent, astigmatic, and elliptical cross section output beam from a typical laser diode into a moderately divergent, spherical wave with a circular cross section which appears to emanate from a virtual point source. This configuration would be of use to people who wish to collimate or focus the laser beam at some fairly large diameter compared to the 100 microns or so available with the Crossed Lens Collimator. Since the laser diode with the lens would mimic a conventional point source of a few microns diameter, it could be collimated or tightly focussed using an inexpensive, macroscopic, conventional lens of moderate speed. This could be applicable to compact disk recorders and players, laser printers, bar code scanners, and laser range finders. 
     The fourth configuration is a Crossed Lens Focuser. This configuration is similar to the Crossed Lens Collimator in appearance but the design provides for focussing to a circular spot at some finite distance from the optic. One possible application is for coupling laser diode light into a single mode optical fiber. This application requires that the light be focussed so that it matches the Numerical Aperture of a single mode optical fiber, typically about 0.1. If the optic is diffraction limited, the coupling from the diode into the fiber can be nearly 100%. 
     All of these configurations display the surprising result that the effect on the wavefront surface of any lens aberration, not just those due to errors in fabricating the preform, scale with the lens size. That is, if a particular lens design is reduced in all its dimensions by some factor, the wavefront distortion due to aberrations in the lens is reduced by the same factor. This includes, for example, aberrations due to the divergence of the laser diode beam in the direction parallel to the lens axis. This means that lens configurations which would be useless due to aberrations when using macroscopic lenses can provide diffraction limited performance when scaled to sizes of the order of a few hundred micrometers. In particular, a crossed cylinder lens configuration can provide a diffraction limited circular beam 100 microns in diameter in spite of the severe aberrations along the diagonals between the lens&#39; axes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B show orthogonal views of a first preferred embodiment of a crossed lens collimator. 
     FIGS. 2A and 2B show orthogonal views of another preferred embodiment of a crossed lens collimator. 
     FIGS. 3A and 3B show orthogonal views of the details of a single-element lens diode beam circularizer. 
     FIGS. 4A and 4B show orthogonal views of a preferred embodiment of a single mode diode fiber coupler. 
     FIGS. 5A and 5B show orthogonal views of a preferred embodiment of a fiber optic coupler. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of the following discussion, lens surfaces will be described in terms of the sag, or z-axis height of the surface relative to the vertex height, as a function of the radius. All of these lenses are cylindrical, so that the sag is given as a function of x, and is independent of y. A convenient equation for a lens surface is the general equation for a conic section, since many aspheric designs are in fact conic sections in first approximation. Quadratic (and higher order terms) can be added to the equation if desired. 
     The equation we use to describe the sag z of a cylindrical lens as a function of the distance x from the axis of the lens is ##EQU1## where c is the curvature of the lens surface, s is the lens shape, and a2 is the coefficient of the quadratic term used if needed to optimize the lens design. These three parameters plus the distance z0 from the diode to the vertex of the curved surface, and the width w and height h of the lens is given for each of the lens designs disclosed. For all designs, the lens material is SFL6 glass (available from Schott Glass). All of these lenses are preferably fabricated by the pulling technique disclosed in the two U.S. patent applications Ser. Nos. 07/591,462 and 07/591,409, referenced above. 
     Shown in FIGS. 1A and 1B is a first preferred embodiment of a crossed lens collimator. In this particular embodiment the lens system is optimized to collimate a Toshiba TOLD9211 visible laser diode, which has 13 microns astigmatism and 29 degrees half angle divergence (the &#34;fast&#34; axis) by 8.6 degrees divergence half angle (the &#34;slow&#34; axis). The diode is located at the origin of the axes. 
     The system is made up of two plano-convex cylindrical lenses. The two cylindrical lenses are oriented orthogonally to each other, with the cylinder axis of the first lens parallel to the x-axis and the cylinder axis of the second lens parallel to the y-axis. The first lens L1 is optimized for collimating light in the plane of the fast axis. The second lens L2, which is rotated 90 degrees about the optical axis (z-axis) from L1, is optimized for collimating along the slow axis. Although the figures show the two lenses as being juxtaposed, which is sometimes convenient, for some applications it may be desirable for them to be separated. The spacing between the curved surfaces of the lenses is chosen to achieve a desired ellipticity, (i.e. aspect ratio). The following table provides the details of the optical system with the ellipticity chosen to be unity (i.e. a circular output beam). 
     First lens L1 (nearest diode): 
     c1=0.010684/micron 
     s1=-2.1684 
     z01=118 microns 
     w1=200 microns 
     h1=220 microns 
     Second lens L2: 
     c2=0.002518/micron 
     s2=0.5745 
     z02=780 microns 
     w2=443 microns 
     h2=443 microns 
     This configuration converts the astigmatic, elliptical output beam from a typical low-power laser diode into a small (e.g., around 100 microns diameter), circular, diffraction limited collimated beam. The laser diode with lens assembly would be very compact and would provide a small laser beam over propagation distances of several tens of centimeters. This might be particularly attractive to the manufacturers of bar code scanners, for example, since the beam would be round and would contain most of the light from the laser diode. 
     FIGS. 2A and 2B illustrate another preferred embodiment of a crossed lens collimator. In this configuration lens L1&#39; is an immersion lens adjacent the laser diode (and preferably optically cemented thereto). Lens L2&#39; is spaced apart from L1&#39; to achieve ellipticity of unity; however other spacings will produce other ellipticities. The following table provides the details of this embodiment, which is designed to collimate a laser diode source with divergences of 30 degrees by 15 degrees (half-angle). Astigmatism is assumed to be zero, but minor modifications in the position of the second lens surface can be made to compensate for any astigmatism. 
     First lens L1&#39;(nearest diode): 
     c1&#39;=0.010373/micron 
     s1&#39;=0.684383 
     z01&#39;=220 microns 
     w1&#39;=230 microns 
     h1&#39;=220 microns 
     Second lens L2&#39;: 
     c2&#39;=0.0063/micron 
     s2&#39;=-1.9412 
     z02&#39;=300 microns 
     w2&#39;=230 microns 
     h2&#39;=200 microns 
     FIGS. 3A and 3B illustrate the details of a single-element lens diode beam circularizer. This convex-concave lens circularizes and symmetrizes the output from a SONY SLD151/V visible laser diode. The laser astigmatism is 35 microns, and the divergence is 22.5 degrees by 8.2 degrees (half angle). 
     First surface S1 (convex, nearest diode): 
     c=0.010481/micron 
     s=-2.222482 
     z01&#34;=120 microns 
     w=410 microns 
     h&#34;=410 microns (vertex to vertex) 
     Second surface S2 (concave): 
     c&#39;=0.00330/micron 
     s&#39;=-2.143688 
     z02&#34;=530 microns 
     The first surface S1 reduces the divergence of the output beam along the fast axis of the diode. The second surface S2 is located at a distance from S1 in order to provide a desired ellipticity, and is shaped to change the divergence of the output beam along the fast axis to coincide with the slow axis. Note that other surface shapes and spacings can be selected to provide differing amounts of divergence along the fast axis. 
     This configuration converts the highly divergent, astigmatic, and elliptical cross section output beam from a typical laser diode into a moderately divergent, spherical wave with a circular cross section which appears to emanate from a virtual point source. This configuration would be of use to people who wish to collimate or focus the laser beam at some fairly large diameter compared to the 100 microns or so available with the Crossed Lens Collimator. Since the laser diode with the lens would mimic a conventional point source of a few microns diameter, it could be collimated or tightly focussed using an inexpensive, macroscopic, conventional lens. This could be applicable to compact disk recorders and players, laser printers, bar code scanners, and laser range finders. 
     FIGS. 4A and 4B illustrate a preferred embodiment of a single mode diode fiber coupler (or focuser). This design focuses the light from a SONY SLD151/V visible laser diode into a single mode optical fiber with N.A.=0.1. In this embodiment, the numerical aperture of the focussed beam matches the numerical aperture of the optical fiber, thus providing optimal mode matching of the light to the single mode optical fiber, and thereby maximizing the coupling efficiency. The laser astigmatism is 35 microns, and the divergence is 22.5 degrees by 8.2 degrees (half angle). The focuser is similar to the collimator except that it focuses light at a finite distance from the lens while the collimator focuses at infinity. 
     First lens L&#39;&#34;(nearest diode): 
     c1&#39;&#34;=0.006547/micron 
     s1&#39;&#34;=-2.222965 
     z01&#39;&#34;=200 microns 
     a21&#39;&#34;=0.0005055/micron 
     w1&#39;&#34;=280microns 
     h1&#39;&#34;=280 microns 
     Second lens L2&#39;&#34;: 
     c2&#39;&#34;=0.013882/micron 
     s2&#39;&#34;=-0.002635 
     z02&#39;&#34;=700 microns 
     a22&#39;&#34;=-0.0048344/micron 
     w2&#39;&#34;=220 microns 
     h2&#39;&#34;=220 microns 
     In yet another embodiment shown in FIGS. 5A and 5B, a coupler can be constructed using a first crossed lens system to collimate the output from a single mode optical fiber, which is then directed into another crossed lens which refocuses the light into a second single mode optical fiber. The system is arranged so that the ellipticity introduced in the light beam by the first lens pair is removed by the second lens pair, resulting in very efficient coupling of light between the two fibers. The two lens pairs are typically identical for optimum coupling efficiency, but different pairs can also be used. In a preferred illustrative embodiment, each pair is a collimator pair as shown in FIGS. 1A and 1B with the same design parameters. 
     Those skilled in the art will understand that there are many variations of the above embodiments which fall within the purview of the invention. For example, the lens system shown in FIGS. 4A and 4B could be reconfigured in a manner similar to that shown in FIGS. 2A and 2B.