Abstract:
A laser comprising: a front mirror and a rear mirror which are disposed so as to establish a reflective cavity therebetween; a gain region disposed between the front mirror and the rear mirror, the gain region being constructed so that when the gain region is appropriately stimulated by light from a pump laser, the gain region will emit light; and one of the front mirror and the rear mirror being positioned to admit pump light into the reflective cavity, the one of the front mirror and the rear mirror having a low and substantially constant reflectance over a pumping wavelength range and having a high and substantially constant reflectance over a lasing wavelength range.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to photonic devices in general, and more particularly to lasers.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lasers are well known in the art. A laser typically comprises a front mirror and a rear mirror which are disposed so as to establish a reflective cavity therebetween. An active, or gain, region is disposed between the front mirror and the rear mirror. The gain region is constructed so that when the gain region is appropriately stimulated, it will emit light. The rear mirror is typically substantially fully reflective at the lasing wavelength, and the front mirror is typically partially reflective at the lasing wavelength so as to allow a beam of laser light to be emitted therefrom.  
           [0003]    As is well known in the art, the gain region may be stimulated by electrical current (“electrically pumped”) or it may be stimulated by light (“optically pumped”).  
           [0004]    The present invention is directed to optically pumped lasers and, more particularly, to an improved optically pumped laser having a substantially constant power output.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention comprises an improved optically pumped laser having increased efficiency.  
           [0006]    In one form of the invention, there is provided a laser comprising: a front mirror and a rear mirror which are disposed so as to establish a reflective cavity therebetween; a gain region disposed between the front mirror and the rear mirror, the gain region being constructed so that when the gain region is appropriately stimulated by light from a pump laser, the gain region will emit light; and one of the front mirror and the rear mirror being positioned to admit pump light into the reflective cavity, the one of the front mirror and the rear mirror having a low and substantially constant reflectance over a pumping wavelength range and having a high and substantially constant reflectance over a lasing wavelength range.  
           [0007]    In another form of the invention, there is provided a method of lasing, the method comprising: providing a laser comprising: a front mirror and a rear mirror which are disposed so as to establish a reflective cavity therebetween; a gain region disposed between the front mirror and the rear mirror, the gain region being constructed so that when the gain region is appropriately stimulated by light from a pump laser, the gain region will emit light; one of the front mirror and the rear mirror being positioned to admit pump light into the reflective cavity, the one of the front mirror and the rear mirror having a low and substantially constant reflectance over a pumping wavelength range and having a high and substantially constant reflectance over a lasing wavelength range; and providing pump light to the one of the front mirror and the rear mirror being positioned to admit pump light into the reflective cavity.  
           [0008]    In still another form of the invention, there is provided a method of constructing a mirror for an optically pumped laser, the method comprising: selecting a reflectance profile for the mirror; querying a database to generate a mirror configuration comprising materials, thicknesses and number of mirror pairs so as to construct the mirror having the reflectance profile; and constructing the mirror based on the mirror configuration. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features of the present invention will be more fully disclosed by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:  
         [0010]    [0010]FIG. 1 is a schematic side sectional view of a tunable VCSEL formed in accordance with the present invention;  
         [0011]    [0011]FIG. 2 is a graph of a first reflectivity curve for a preferred embodiment of the present invention in which the novel mirror is configured for low and constant reflectance of pumping wavelengths of light over a broad pumping wavelength; and  
         [0012]    [0012]FIG. 3 is a graph of a second reflectivity curve for another preferred embodiment of the present invention in which the novel mirror is configured for high and constant reflectance of lasing wavelengths of light in two windows of pumping wavelength ranges. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]    Looking first at FIG. 1, there is shown a schematic diagram of a novel laser  5  formed in accordance with the present invention. Optically pumped laser  5  is a tunable vertical-cavity surface-emitting laser (VCSEL) of the sort disclosed in pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER (Attorney&#39;s Docket No. CORE-33), and in pending prior U.S. patent application Ser. No. 09/543,318, filed Apr. 5, 2000 by Peidong Wang et al. for SINGLE MODE OPERATION OF MICROELECTROMECHANICALLY TUNABLE, HALF-SYMMETRIC, VERTICAL CAVITY SURFACE EMITTING LASERS (Attorney&#39;s Docket No. CORE-53), and in pending prior U.S. patent application Ser. No. 09/750,434, filed Dec. 28, 2000 by Peidong Wang et al. for TUNABLE FABRY-PEROT FILTER AND TUNABLE VERTICAL CAVITY SURFACE EMITTING LASER (Attorney&#39;s Docket No. CORE-67). The three aforementioned patent applications are hereby incorporated herein by reference.  
         [0014]    More particularly, and looking now at FIG. 1, there is shown a tunable VCSEL 5. VCSEL 5 generally comprises a substrate  10 , a bottom mirror  15  mounted to the top of substrate  10 , a bottom electrode  20  mounted to the top of bottom mirror  15 , a thin membrane support  25  atop bottom electrode  20 , a top electrode  30  fixed to the underside of thin membrane support  25 , a reinforcer  35  fixed to the outside perimeter of thin membrane support  25 , and a confocal top mirror  40  set atop thin membrane support  25 , with an air cavity  45  being formed between bottom mirror  15  and top mirror  40 .  
         [0015]    As a result of this construction, a Fabry-Perot cavity is effectively created between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the lasing Fabry-Perot cavity.  
         [0016]    A gain region (or “active region”)  55  is positioned between bottom mirror  15  and bottom electrode  20 . As a result, when gain region  55  is appropriately stimulated, e.g., by optical pumping, lasing can be established between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the laser&#39;s resonant cavity, and hence tune VCSEL 5.  
         [0017]    In accordance with the present invention, one of the mirrors, specifically the one positioned to admit pump light into the air cavity, has a very low reflectance over a pumping wavelength range and a very high reflectance over a lasing wavelength range. In addition, this mirror is configured such that the reflectance over the pumping wavelength range and the lasing wavelength range are each substantially constant or flat.  
         [0018]    Now looking at FIG. 2, in a preferred embodiment of the present invention, there is shown a first reflectivity curve  60  corresponding to one preferred embodiment of top mirror  40  (see FIG. 1). Top mirror  40  is configured to provide a low and substantially constant reflectance over a pumping wavelength range  65 . Top mirror  40  is also configured to provide a high and substantially constant reflectance over a lasing wavelength range  70 . Preferably, top mirror  40  is constructed to provide substantially no reflectance to light within pumping wavelength range  65  and a reflectance of about 99.9% to light within lasing wavelength range  70 .  
         [0019]    Now looking at FIG. 3, in another preferred embodiment of the present invention, there is shown a second reflectivity curve  75  corresponding to another preferred embodiment of top mirror  40  (see FIG. 1). Top mirror  40  is configured to provide two ranges, or “windows”  80 , of low and substantially constant reflectance over two portions of pump wavelength ranges. Top mirror  40  is also configured to provide a high and substantially constant reflectance over a lasing wavelength range  85 .  
         [0020]    In a preferred embodiment of the invention, top mirror  40  (see FIG. 1) is a distributed Bragg reflector formed out of mirror pairs. Preferably, the material and thickness of each layer of the mirror pair is selected to produce the desired reflectance profiles, such as curve  60  (see FIG. 2) and curve  70  (see FIG. 3), around a pumping wavelength, range, such as pumping wavelength range  65  (see FIG. 2) or pumping wavelength range  80  (see FIG. 3), and a lasing wavelength range, such as lasing wavelength range  70  (see FIG. 2) or lasing wavelength range  80  (see FIG. 3). A computer program may be used for generating possible configurations of mirror materials and layers. These configurations may then be analyzed to discard ones that are impossible to create. As an example of one preferred embodiment of the present invention, and referring to the reflectance profile of curve  60  (See FIG. 2), a pumping region of about 950 nm to about 1340 nm is created at pumping wavelength range  65  with a transmission rate of greater than about 99%, while a lasing region of about 1528 nm to about 1560 nm has a reflectance of greater than about 99.9% at lasing wavelength range  70 .  
         [0021]    Looking now at FIGS. 1 and 2, a method is disclosed for constructing top mirror  40  (see FIG. 1) with a tailored reflectance profile, such as curve  60  (see FIG. 2) or curve  75  (see FIG. 3). In addition, a method is disclosed for lasing with a VCSEL 5 having a top mirror  40  with a tailored reflectance profile, such as curve  60  (see FIG. 2) or curve  75  (FIG. 3).  
         [0022]    A tailored reflectance profile, such as that of curve  60  (see FIG. 2) or curve  75  (see FIG. 3), can be achieved by selecting and configuring mirror materials and layer thicknesses for low and substantially constant reflectance over pumping wavelength range  70 . These mirror materials may be selected and configured from either dielectric or semiconductor materials.  
         [0023]    It is desirable to provide a tailored reflectance to top mirror  40  to create a wideband window of low and substantially constant reflectance profile within the typical interference fringe spectrum of a distributed Bragg reflector. This wideband window of pumping wavelength range  65  provides improved stability in the coupling of tunable VCSEL 5 to an optical pump source, which counteracts several common effects of an irregular reflectance profile and provides low sensitivity to manufacturing variations.  
         [0024]    A first common effect of an irregular reflectance profile includes imprecise tuning of pump light due to a varying reflectivity profile over a pumping wavelength range. Such a varying reflectivity profile produces, in turn, varying amounts of coupling of reflectivity and changes to the power output of a tunable VCSEL.  
         [0025]    A second common effect of an irregular reflectance profile is “device to device” variation. This occurs when one tunable VCSEL has a differing reflectivity profile over its wavelength range than that of another tunable VCSEL. These differing profiles cause, in turn, variations in the power output profile from one VCSEL to another tunable VCSEL.  
         [0026]    A third common effect of an irregular reflectance profile is reduced power output due to heating, which causes a shift in the reflectivity profile of the pumping wavelength range due to changes in the optical index of the tunable VCSEL.  
         [0027]    A fourth common effect of an irregular reflectance profile is reduced power output due to heating noise, which causes a shift in the cavity wavelength and affects coupling of reflected light therein. Heating noise comprises physical motion induced by heating of thin membrane support  25 .  
         [0028]    These four effects reduce the effectiveness of tunable VCSEL 5 as the level of output power varies due to changes in the reflectivity profile as the pumping wavelength of the optical pump remains constant. The present invention provides a substantially constant portion of reflectivity curve  60  over the pumping wavelength range  65  (see FIG. 2) or substantially constant portions of reflectivity curve  75  over windows  80  in of the pumping wavelength range. These substantially constant portions eliminate variations in reflectivity over desired wideband sections of pumping wavelength ranges and, in turn, permit VCSEL 5 to provide a substantially constant power output.  
         [0029]    It is to be understood that the present invention is by no means limited to the particular constructions and method steps disclosed above and/or shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims.