Manufacturable vertical extended cavity surface emitting laser arrays

Abstract

Arrays of vertical extended cavity surface emitting lasers (VECSELs) are disclosed. The functionality of two or more conventional optical components are combined into an optical unit to reduce the number of components that must be aligned during packaging.

1 . A semiconductor, vertical, extended-cavity, surface-emitting laser array with intra-cavity nonlinear frequency doubling, comprising: 
 an electrically pumped, surface-emitting semiconductor gain chip having an array of emitters formed in a common semiconductor substrate, each emitter generating light about a fundamental wavelength;    an end reflector spaced apart from said surface-emitting semiconductor gain chip to define an extended cavity for each of said array of emitters, said end reflector having a high reflectivity at the fundamental laser wavelength;    a nonlinear crystal disposed within the extended cavity to provide frequency doubling of light for the entire array of emitters;    a wavelength control element disposed within the extended cavity to provide wavelength control for the entire array of emitters; and    a polarization control element disposed within the extended cavity to provide polarization control for the entire array of emitters;    said surface-emitting laser array having an optical unit having the functionality of at least two of said gain chip, said end reflector, said nonlinear crystal, said wavelength control element, and said polarization control element to reduce the number of components that must be aligned during packaging.    
     
     
         2 . The laser array of  claim 1  wherein said end reflector is a volume Bragg grating, said volume Bragg grating serving said end reflector and said wavelength control element.  
     
     
         3 . The laser array of  claim 2  wherein the volume Bragg grating includes a high reflectivity optical coating deposited at an outer face of said volume Bragg grating to increase the circulating power intensity in the laser cavity at the fundamental wavelength and increase nonlinear conversion efficiency.  
     
     
         4 . The laser array of  claim 2 , wherein said nonlinear crystal is attached to said volume Bragg grating as an unit serving as said end reflector, wavelength control element, and nonlinear crystal.  
     
     
         5 . The laser array of  claim 2 , wherein said unit further includes a polarization control element attached to said nonlinear crystal such that said unit serves as said end reflector, wavelength control element, nonlinear crystal, and polarization control element.  
     
     
         6 . The laser array of  claim 5 , wherein said polarization control element is a waveplate.  
     
     
         7 . The laser array of  claim 6 , wherein said polarization control element is a wire grid polarizer.  
     
     
         8 . The laser array of  claim 1  wherein the wavelength control element is a thin-film coating interference filter  
     
     
         9 . The laser array of  claim 1  wherein the polarization control element is a coated beamsplitter providing loss for an undesirable polarization and substantially no loss for a desirable polarization of light at said fundamental frequency.  
     
     
         10 . The laser array of  claim 1  wherein the polarization control element is a planar element attached to another planar component to form a planar sub-unit.  
     
     
         11 . The laser array of  claim 10 , wherein said polarization control element is a wire-grid polarizer.  
     
     
         12 . The laser array of  claim 1  wherein the polarization control element is a birefringent crystal, which may be separate or the same as the nonlinear crystal, providing walk-off and misalignment for the undesirable polarization  
     
     
         13 . The laser array of  claim 1 , wherein a planar array of lenses is disposed in said extended cavity to provide transverse mode control for said array of lasers.  
     
     
         14 . The laser array of  claim 13  wherein the lenses are thermal lenses associated with each emitter of the surface-emitting array  
     
     
         15 . The laser array of  claim 1  wherein a planar array of apertures is disposed in the extended cavity.  
     
     
         16 . The laser array of  claim 15  wherein the planar array of apertures are lithographically integrated with the gain chip.  
     
     
         17 . The laser array of  claim 1 , wherein said polarization control element comprises a dichroically coated beamsplitter, said beamsplitter coupling light at a second harmonic frequency out of said extended cavity.  
     
     
         18 . The laser array of  claim 1 , wherein said cavity has a forward direction for beams traversing towards said end reflector and a backwards direction for beams traversing towards said gain chip, said beamsplitter receiving and re-directing second-harmonic beams traversing in the backward direction outside of the laser cavity.  
     
     
         19 . The laser array of  claim 19  further comprising a corner-turning mirror disposed outside of said extended cavity to direct an array of backward-propagating second-harmonic beams into parallel paths.  
     
     
         20 . The laser array of  claim 19  wherein said backward propagating second harmonic are reflected and in the same direction as second harmonic beams traveling in a forward direction coupled out of said extended cavity via said end reflector.  
     
     
         21 . The laser array of  claim 20  wherein the forward-propagating generated second-harmonic beams are reflected back into the cavity by a coating on the nonlinear crystal of the volume Bragg grating either along the original path or at an angle using a tilt or wedge and then re-collected by the dichroic beamsplitter together with the backward-propagating generated second-harmonic beams.  
     
     
         22 . The laser array of  claim 1  wherein a waveplate is disposed within the extended cavity to rotate the polarization of the backward-propagating generated second-harmonic beams which are then back-reflected onto their original path by a coating on the waveplate or the surface-emitting laser chip and then re-combined with the forward-propagating generated second-harmonic beams of the orthogonal polarization.  
     
     
         23 . The laser array of  claim 23  wherein the waveplate, the nonlinear crystal, and the volume Bragg grating are monolithically bonded to form a low-cost subunit defining the extended cavity.  
     
     
         24 . The laser array of  claim 1  wherein a waveplate is disposed within the extended cavity after the nonlinear crystal to rotate the polarization of the forward-propagating generated second-harmonic beams which are then back-reflected onto their original path by a coating on the waveplate or the end reflector such as a volume Bragg grating and then re-combined with the backward-propagating generated second-harmonic beams of the orthogonal polarization.  
     
     
         25 . The laser array of  claim 1  wherein all the optical elements in the extended cavity are mechanically aligned using passive alignment to a high-precision fiducial marks of the laser package.  
     
     
         26 . The laser array of  claim 1  wherein one of the optical elements is wedged or tilted and coated to provide a reflection and angular separation for the forward- and backward-propagating second-harmonic beams.  
     
     
         27 . The laser array of  claim 1  wherein an array of dome lenses is formed on the semiconductor surface-emitting chip to help optimize the spatial mode of the laser cavity and to focus the fundamental beams into the nonlinear crystal.  
     
     
         28 . The laser array of  claim 27  wherein the array of domes is coated with an anti-reflective coating at the fundamental wavelength and with a high-reflective coating at the second-harmonic wavelength to provide reflection of the backward-propagating second-harmonic beam which is also spatially expanded and thereby has a reduced spatial overlap with the forward-propagating second-harmonic beam.  
     
     
         29 . The laser array of  claim 1  in which the output array of second-harmonic beams is re-shaped by a diffractive optical element to achieve the desired illumination distribution.  
     
     
         30 . The laser array of  claim 1  in which speckle is reduced by the selection of the number of emitters and by promoting multi-longitudinal mode operation to decrease the overall spatial and spectral coherence of the laser light source.  
     
     
         31 . The laser array of  claim 1  in which the forward- and backward-propagating second-harmonic beams are recombined through an optical paths separating them by more than the coherence length of the laser to reduce interference between these beams.  
     
     
         32 . A laser array as in  claim 1  in which one or more of the elements are combined, and the remaining elements are packaged in such a fashion that the total volume of the package is less than one cubic inch.  
     
     
         33 . The laser array of  claim 32  in which the elements are combined and packaged in such a way that the total volume of the package is less than two cubic inches.  
     
     
         34 . A semiconductor, vertical, extended-cavity, surface-emitting laser array with intra-cavity nonlinear frequency doubling comprising 
 an electrically pumped, surface-emitting semiconductor gain chip having an array of emitters formed in a single semiconductor substrate, each emitter generating light about a fundamental wavelength;    a volume Bragg grating spaced apart from said surface-emitting semiconductor gain chip to form an end reflector defining an extended cavity for each of said array of emitters, said volume Bragg grating having a high reflectivity at the fundamental laser wavelength and providing spectral filtering to control the frequency of each emitter of said laser array; and    a nonlinear crystal to provide intra-cavity frequency conversion of light for the entire array of emitters.    
     
     
         35 . The laser array of  claim 34 , wherein said laser array is configured to require only one critical optical alignment in which said volume Bragg grating is packaged in alignment with said gain chip and other components do not require a critical optical alignment.  
     
     
         36 . The surface-emitting laser array of  claim 34 , further comprising a waveplate attached to said nonlinear crystal.  
     
     
         37 . The surface emitting laser of  claim 34 , wherein said nonlinear crystal is attached to said volume Bragg grating.  
     
     
         38 . The surface emitting laser of  claim 34 , wherein lenses for controlling the spatial mode of each emitter are formed in said gain chip.  
     
     
         39 . The surface emitting laser of  claim 38 , wherein said lenses comprise thermal lenses.  
     
     
         40 . The surface emitting laser of  claim 38 , wherein said lenses comprise an array of lenses bonded to said gain chip.  
     
     
         41 . The surface emitting laser of  claim 34 , further comprising an array of apertures formed on said gain chip for controlling a spatial mode.  
     
     
         42 . The surface emitting laser of  claim 34 , further comprising a polarizing element formed on another wavelength control element disposed within the extended cavity.