Abstract:
A highly compact laser scanner with no moving parts includes a vertical cavity surface emitting laser (VCSEL). A microlens is mounted directly to the light emitting surface of the VCSEL via flip chip bonding. The VCSEL array may be one or two dimensional. The laser array may be used in a variety of applications including free space optical communications.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates generally to the field of miniaturized optical scanners.  
           [0003]    2. Description of Related Art  
           [0004]    Optical communications systems which employ optical scanning and other optical systems often employ laser light sources. In recent years, a particular type of laser, a vertical cavity surface element laser, or VCSEL, has come into use as a laser light source for optical systems. One of many applications which use optical scanning in general, and VCSEL scanning in particular, is free space optical communications, which encompass telecommunications, cable and satellite television.  
           [0005]    Some optical beam scanning systems use mechanically driven scanning elements which require complex drive systems and relatively high power requirements to operate. Still other scanning systems include using deformable micro mirrors which operate in response to electric field changes. Other scanning systems include acousto-optical light beam deflection. Yet other scanning systems use electro-optical or magneto-optical beam deflectors. Many of these systems use optically-phased arrays which operate analogously to radio-frequency phased-array radar systems. However, the optical scanning systems described above are limited in terms of beam switching speeds and deflection angles.  
           [0006]    Miniaturized laser beam scanners also exist for use in various optical scanning applications, including image formation and free space optical communication systems. One prior art miniature laser scanning array, known as a monolithic integration of a VCSEL array with a microlens, is shown in FIG. 1. This monolithic integrated VCSEL comprises a VCSEL array and a built-in microlens formed in the GaAs substrate of the VCSEL. One problem with the device shown in FIG. 1 is that the VCSEL substrate material has to be transparent to the laser wavelength.  
           [0007]    Another example of a miniature solid state laser array is disclosed in U.S. Pat. No. 6,121,983, incorporated by reference herein in its entirety. The &#39;983 patent discloses a linear array of laser faces of a vertical cavity surface effect laser (VCSEL) array. The light output of this laser array is scanned using a mirror array. Each VCSEL element is provided with a microlens to reduce the divergence of the laser beam emitted by that VCSEL element. Each microlens is separated from its corresponding VCSEL element by a spacer to permit the focal point of the lens to coincide with the location of the corresponding VCSEL element. The scanning is performed using the mirror array. VCSEL elements having attached microlenses element to reduce laser light beam divergence are illustrated in FIG. 2.  
           [0008]    Another prior art VCSEL array is disclosed in U.S. Pat. No. 6,246,708, incorporated by reference herein in its entirety. The &#39;708 patent discloses a VCSEL array which integrated electronic components including matrix addressing circuitry, in the form of transistor elements, which allow addressing of the laser structure.  
         SUMMARY OF THE INVENTION  
         [0009]    A need exists for a micro laser scanner which overcomes the problems associated with the aforementioned prior art devices.  
           [0010]    This invention provides a micro laser beam scanner formed by integrating a refractive microlens on a VCSEL array.  
           [0011]    This invention separately provides methods for integrating a refractive microlens onto a VCSEL array.  
           [0012]    This invention separately provides an integrated refractive microlens and methods for integrating a refractive microlens onto a linear array of VCSEL elements.  
           [0013]    This invention separately provides an integrated refractive microlens and methods for integrating a refractive microlens onto a two-dimensional array of VCSEL elements.  
           [0014]    This invention separately provides an integrated refractive microlens and methods for integrating a refractive microlens onto a circular array of VCSEL elements.  
           [0015]    In various exemplary embodiments, integrating the microlens and the VCSEL array is achieved by flip-chip bonding a microlens chip on a VCSEL substrate. In various exemplary embodiments of the systems and methods of this invention, the microlens is used to direct the optical output from individual pixels of the VCSEL array in different directions. Such systems can be used not only in free-space communications, but also in other optical beam scanning applications, such as, for example, xerographic exposure systems, optical radars, etc.  
           [0016]    The micro laser scanner according to this invention is designed to work for any laser wavelength and with any optical materials. The proposed micro laser scanner involves no moving parts.  
           [0017]    These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Various exemplary embodiments of this invention will be described in detail with reference to the following figures, wherein:  
         [0019]    [0019]FIG. 1 shows a prior art monolithic integrated VCSEL array with a microlens for laser beam scanning;  
         [0020]    [0020]FIG. 2 shows a prior art VCSEL array with microlens for each VCSEL emitter and a mirror scanner;  
         [0021]    [0021]FIG. 3.is a cross sectional view of one exemplary embodiment of a micro laser scanner according to the invention;  
         [0022]    [0022]FIG. 4 is a plan view of a one-dimensional array embodiment of a micro laser scanner according to the invention;  
         [0023]    [0023]FIG. 5 is a plan view of a two-dimensional array embodiment of a micro laser scanner according to the invention; and  
         [0024]    [0024]FIG. 6 is a plan view of a circular two-dimensional array embodiment of a micro laser scanner according to the invention.  
         [0025]    [0025]FIG. 7 is a cross-sectional view of a vertical cavity surface emitting laser structure and integrated electronic circuit components usable with the invention.  
         [0026]    [0026]FIG. 8 is a plan view of the device shown in FIG. 7. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]    A more complete appreciation of the invention and many of the attendant advantages thereof will be readily appreciated and understood by referring to the following detailed description and the accompanying drawings in which like reference numerals denote like elements as between the various drawings.Various features and advantages of the invention will become more apparent upon study of the following description, taken in conjunction with the drawings, in which: FIG. 1 shows a cross-section of a prior art monolithic integrated vertical cavity surface effect laser (VCSEL) device  100 . The VCSEL device  100  has a plurality of pixels or elements  110  that are arranged in a one-dimensional array  105 . The VCSEL pixels or elements  110  are formed as part of a semiconductor laser structure  107 . A gallium arsenide (GaAs) layer  120  is formed over an emitting surface of the VCSEL pixels or elements  110 . A microlens  130  is formed on a surface  122  of the GaAs layer  120  opposite the surface of the GaAs layer that is adjacent to the VCSEL pixels or elements  110 . The VCSEL device  100  has a limited operational bandwidth because the GaAs substrate is highly absorptive of the laser light emitted by the VCSEL elements  110 , when the wavelength of the emitted laser light is shorter than 850 nm.  
         [0028]    [0028]FIG. 2 shows a second conventional VCSEL device  200  having microlenses  230 . As shown in FIG. 2, the VCSEL device  200  includes a substrate  220  on which an array  205  of the VCSEL pixels or elements  210  is formed. A spacer layer  240  is formed on or over the VCSEL pixels or elements  210 . A plurality of microlenses  230  is formed on or over the spacer layer  240 . Each of the microlenses  230  is associated with one of the VCSEL pixels or elements  210 . Each microlens  230  reduces the divergence of the laser beam emitted by the corresponding VCSEL element  210 . Each microlens element  230  is spaced from each corresponding VCSEL element  210  by the spacer layer  240  so that each VCSEL element  210  is located at the focus of the corresponding microlens  230 .  
         [0029]    [0029]FIG. 3 shows an exemplary embodiment of an integrated device  300  according to this invention. As shown in FIG. 3, the integrated device  300  includes a substrate layer  302 . A semiconductor structure  307  is formed on or over the substrate layer  302 . The semiconductor structure  307  includes an array  305  of two or more VCSEL elements  310 . A substrate or plate  350  is attached to the semiconductor structure  307  via solder bumps  340 . This type of bond arrangement is known as flip chip bonding. In various exemplary embodiments, the substrate or plate  350  may be made of, for example, quartz or any material with a suitable refractive index and dimensional stability.  
         [0030]    A microlens  330  is formed on or over the plate  350  such that the microlens  330  is positioned geometrically over the array  305  of the VCSEL elements  310 . Outputs from different VCSEL elements are refracted to different directions after passing through the microlens  330 . Scanning is thus obtained by sequentially turning on and off different ones of the VCSEL elements  310 .  
         [0031]    Accuracy of alignment between the VCSEL pixels or elements  310  and the microlens  330  is achieved by reflowing the solder bumps  340  located between the semiconductor structure  301  and the substrate  350  carrying the microlenses  330 . Solder wetting metal pads (not shown) are provided on both the surface of the semiconductor structure  307  and on the surface of the substrate  350  which faces the VCSEL pixels/elements  310 . The VCSEL device  300  can be fabricated using the techniques set forth in U.S. Pat. No. 6,208,681, incorporated herein by reference in its entirety.  
         [0032]    The &#39;681 patent allows fabrication of the VCSEL pixels or elements  310  in the arrays  305  at pitches or spacings between centers of the VCSEL pixels or elements  310  of less than about 7 μm. In various exemplary embodiments, where the VCSEL array pitch is about 6 μm, a suitable microlens focal length would be 500 μm. The center of the array  305  of the VCSEL pixels or elements  310  is generally located at the focal point of the microlens  330 . In such an arrangement, the scan angle which can be obtained from this exemplary embodiment of the integrated VCSEL according to this invention by turning on in sequence two adjacent VCSEL pixels or elements  310  in sequence is 0.68 degree. Thus, in an array  305  having 10 VCSEL pixels or elements  310 , where each pixel is turned on in sequence, a scan angle of 6 degrees can be obtained.  
         [0033]    [0033]FIG. 4 is a top plan view of a one-dimensional array  305  having 5 VCSEL pixels or elements  310 . The number of VCSEL pixels or elements  310  shown in FIG. 4 is illustrative only, and may vary depending on the application.  
         [0034]    [0034]FIGS. 5 and 6 show other exemplary embodiments of the integrated VBCSEL of this invention. Instead of providing a one-dimensional array  305  of the VCSEL pixels or elements  310 , as shown in FIG. 4, FIGS. 5 and 6 show two-dimensional arrays  305  of VCSEL pixels or elements  310 .  
         [0035]    [0035]FIG. 6 illustrates a two-dimensional array  305  of the VCSEL pixels or elements  310  set out in a circular pattern centered at the axial focus of the microlens  330 . This arrangement will result in a beam scan pattern in the form of a spherical cone.  
         [0036]    [0036]FIG. 7 illustrates a vertical cavity surface emitting laser structure and integrated electronic component structure usable with the invention. This structure is disclosed in greater detail in U.S. Pat. No. 6,246,708 incorporated herein by reference in its entirety. The integrated VCSELs according to this invention may incorporate integrated scanning electronic components, as disclosed in FIG. 7 or may use external addressing and scanning circuitry, which is also disclosed in the &#39;708 patent.  
         [0037]    [0037]FIG. 7 shows a vertical cavity surface emitting laser (VCSEL) pixel  310 . VCSEL pixel element  310  includes a substrate  354 , a lower mirror structure  358 , spacer layers  364  and  360  above and below an active layer  362 , and an upper mirror structure  368 , through which laser light beam B is emitted. A passivation layer  311  is provided over the VCSEL pixel element  310 . FIG. 7 also shows first and second transistor structures  308  and  309 , each having gate electrodes  112  and  114 , respectively, and insulating layer  116  and  118 , respectively. Channel materials  320  and  322 , respectively, are provided over the gate electrodes. Passivation material  324  and  326 , respectively is provided over the channel materials  320 ,  322 , respectively.  
         [0038]    A source  328  and drain  330  are provided for the first transistor  108 , and a source  328 ′ and drain  329 ′ are provided for second transistor  309 . The drain  329  is in electrical communication with the gate  314  through a conductive region  315 . Similarly, drain  329  is in electrical communication with electrode  370  of the laser structure through a conductive via  313 .  
         [0039]    [0039]FIG. 8 is a plan view of the device shown in FIG. 7, showing one embodiment of the integrated VCSEL and associated electronic components  310 . In a three-dimensional array, there may also be VCSELs  310  above and below the shown VCSEL  310 . A voltage from a voltage source (not shown) is applied to a drive line  380  to drive the VCSEL  310 . FIG. 8 also shows a capacitor  382  which connects the gate  314  of the transistor  308  and the drive line  380 . The capacitor  382  comprises an extension of the gate contact material of the transistor  309  which underlies part of the drive line  380 .  
         [0040]    Details of operation of the array are set forth in the incorporated &#39;708 patent. The flip-chip microlens array pixels  310  disclosed herein may be turned on and off using the circuitry shown in FIGS. 7 and 8.  
         [0041]    It should be understood that the array  305  of VCSEL pixels or elements  310  need not be centered along the axis of the microlens  330 , but may be shifted in any direction by any amount as long as beam deflection of at least one of the VCSEL pixels or elements  310  occurs.  
         [0042]    The microlenses  330  used in this invention may be fabricated on any suitable substrate  350  that results in transmission of a broad wavelength band or bands of light beams which can be generated by the VCSEL lasers  310  of the array  305  and also of any pump light should those lasers be able to be optically pumped. Suitable substrate materials include glass and quartz, among others.  
         [0043]    In one exemplary embodiment of a method for fabricating the microlens chip  330  according to this invention, the substrate  350 , formed of quartz, was flip chip bonded to the semiconductor structure  307 . Then, one or more thick dielectric films of SiO 2  (not shown) were deposited on or over the substrate  350 . Next, a microlens  330  was formed in or on the SiO 2  layers formed on or over substrate  350  by reflowing a photoresist pattern. Then, the microlens  330  was transferred to the substrate  350  by one-to-one plasma etching.  
         [0044]    In another exemplary embodiment, the refractive microlenses  330  may be obtained commercially, and deposited directly on or over the substrate  350 .  
         [0045]    In various exemplary embodiments, a microlens  330  may be made of quartz, which has a refractive index of 1.455, silicon., which has a refractive index of 3.5, or a nitride. The refractive indices of nitrides varies depending on the type of nitride used, but for the various exemplary embodiments of the microlens  330  according to this invention, a reasonable assumption for the refractive index of a nitride microlens  330  is about 2.03. In various exemplary embodiments of this invention, a microlens  330  diameter may be about 130 microns, the difference in thickness from the relatively thin edge to the relatively thick center of a microlens  330  may be about 5 microns, and the focal length of a microlens  330  is equal to the radius of curvature of the microlens  330  divided by the refractive index of the microlens minus 1, i.e., focal length=((radius of curvature)/(refractive index−1)).  
         [0046]    While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.