Abstract:
The invention is a laser assembly for reducing distortion ripple. The assembly includes a spherical lens which has a portion of its surface made optically asymmetric to prevent multiple reflections of scattered light within the lens.

Description:
FIELD OF THE INVENTION 
     This invention relates to semiconductor laser assemblies. 
     BACKGROUND OF THE INVENTION 
     Analog semiconductor lasers, Such as InP Distributed Feedback (DFB) lasers, are finding increasing use in cable TV transmission systems. In one design, the laser is mounted on the surface of a silicon substrate, and the light from the laser is focused onto an optical fiber by means of a spherical lens which is mounted in a cavity in the silicon substrate. (See, e.g., U.S. Patent Application of Anigbo, Ser. No. 60/00916, filed Dec. 22, 1995, which is incorporated by reference herein.) 
     One of the problems associated with fabricating such devices is to keep distortion ripple at a minimum. The &#34;distortion ripple&#34; is defined as the variation of the values of the composite second order distortions and the composite triple beat distortions due to reflection feedback as the temperature is changed. Generally, it is desired to keep the distortion ripple at or better than 4 dB over a laser temperature variation of 2 deg C, which is the typical temperature variation of the laser in the package. 
     SUMMARY OF THE INVENTION 
     The invention is a laser assembly which includes a semiconductor laser and a spherical lens bonded on a first portion of its surface to a supporting member and positioned with respect to the laser to receive light emitted therefrom at a second portion of the surface of the lens and transmit said light out of the lens at a third portion. The lens includes means on a fourth portion of its surface which is distinct from the first, second, and third portions for producing optical asymmetry at said fourth portion of the lens surface. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     These and other features of the invention are delineated in detail in the following description. In the drawing: 
     FIG. 1 is a cross sectional view of a portion of a laser assembly in accordance with the prior art illustrating a possible cause of distortion ripple; 
     FIG. 2 is a cross sectional view of a portion of a laser assembly in accordance with an embodiment of the invention; and 
     FIGS. 3-4 are graphs of distortion ripple as a function of laser current illustrating advantages of the invention in accordance with the same embodiment. 
    
    
     It will be appreciated that, for purposes of illustration, these figures are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     FIG. 1 illustrates a portion of an optical assembly, 10, in accordance with the prior art. The assembly includes a substrate, 11, which is typically silicon. A bonding pad, 12, is formed on a major surface of the substrate. The pad is typically Ti/Pt/Au. Mounted on top of the pad, 12, is a semiconductor laser, 13, which in this example is a 1.3 μm InP DFB laser, but could be any semiconductor analog laser. A spherical lens, 14, is mounted in a cavity, 15, etched in the surface of the substrate. The lens is typically made of M g  Al 2  O 3  or YAG or glass, and is positioned with respect to the laser, 13, to collimate the light from the facing edge of the laser as indicated schematically by the rays 16-18. (For a more detailed discussion of an optical assembly formed on a silicon substrate, see, for example, U.S. Patent Application of Anigbo, cited above.) The lens is bonded to a layer, 20, of aluminum formed on the surfaces of the cavity, 15, according to known techniques to form a three point mechanical contact, one of which is shown as 19, the other two being formed with the side walls of the cavity, 15, which are not shown in this view. (See, for example, U.S. Pat. No. 5,178,319 issued to Coucoulas, which is incorporated by reference herein.) 
     Applicants have discovered that a source of distortion ripple in such assemblies is scattered light within the lens, 14, as illustrated, for example by the dotted line, 20. Applicants theorize that since the lens is optically symmetrical, the scattered light is multiply reflected within the lens, 14, and gains in intensity due to constructive interference. The scattered light may then exit the lens at a point which interferes with the main beam, 16-18. Alternatively, the scattered light may exit at any point on the lens, 14, but the intensity of such light may be frequency dependent and thereby cause a nonlinearity in the intensity of the main beam at certain optical frequencies. 
     In accordance with a key feature of the invention, the scattered light, 20, is prevented from being multiply reflected by making the lens optically asymmetrical at some portion of the lens, 14, outside any portion, e.g. 19, which is bonded to the substrate, 11. 
     One technique for producing the asymmetry is illustrated in FIG. 2. Here, a top portion of the lens, 14, was coated with a material, 21, which in this example was either a silica-loaded epoxy or black wax. Other materials may be employed, e.g., organic or inorganic adhesives which may be filled or unfilled. The main requirements for the material are that it be not be highly reflective (i.e., has a reflectivity no greater than 60 percent) and that it have an index of refraction greater than the ambient. Preferably, the index of refraction of the material is equal to or greater than that of the lens, 14. Thus, for example non-metallic solders could also be used. The material, 21, was deposited by mechanical transfer, but other techniques could be used. The material had a thickness of approximately 0.01 inches but thicknesses in the range 0.001 to 0.03 inches would be useful. 
     As illustrated in FIG. 2, the presence of the material, 21, causes the scattered light, 20, to either be refracted out of the lens, 14, at the point of first incidence on the lens surface (ray 22), or, if the scattered light happens to be at less than the critical angle, reflected back into the lens (ray 23). In the latter case, however, the angle of reflection may be such that the scattered light will exit the lens, 14, at the next incidence on the lens surface due to the optical asymmetry produced by the material, 21. In either case, the scattered light, 20, does not have a chance to increase in intensity inside the lens, 14. 
     The dramatic improvement in distortion ripple between the prior art structure of FIG. 1 and the embodiment of FIG. 2 is illustrated in FIG. 3, which is a graph of distortion as a function of dc current supplied to the laser, 13. The dotted line curves, 30 and 31, show the second and third order distortion for different frequencies of modulation of the structure of FIG. 1, and the solid line curves, 32 and 33, show the distortion ripple for the same frequencies of modulation of the device of FIG. 2. Reduction of the distortion ripple from more than 15 dB to approximately 2 dB is achieved. (It will be noted that the distortion minimum at approximately 46 mA is not a ripple feature, but a typical null observed in most DFB lasers.) 
     FIG. 4 illustrates distortion ripple improvement for another embodiment of the invention. Here, the prior art device second and third order distortion is represented by curves 40 and 41. Curves 42 and 43 show distortion ripple for an assembly similar to FIG. 2, but with the black wax applied to the bottom of the lens, 14, so that the black wax covered the bottom portion of the lens which was not bonded to the aluminum metallization, 20, i.e., the portion not including site 19 and the two other sites (not shown) where the lens was attached to the sidewalls of the cavity, 15. The black wax was deposited on the bottom surface of the cavity, 15, and heated so that the black wax melted and surface tension drew the black wax under the lens, 14. As shown in FIG. 4, distortion ripple was reduced from approximately 6-15 dB to 1-4 dB. 
     Various additional modifications will become apparent to those skilled in the art. For example, the optical asymmetry of the lens, 14, can be achieved by mechanically altering the spherical lens by cleaving or grinding a flat on a portion of the surface of the lens outside the path of the main beam.