Abstract:
A laser device includes a substrate and a multi-layer semiconductor structure formed on the substrate. The structure includes one or more active layers, which are adapted to amplify optical radiation at a plurality of different wavelengths, and at least two reflective regions, arranged to define at least one micro-cavity resonator containing the active layers and having an optical axis substantially perpendicular to the substrate. An electrode is coupled to apply an electrical current to the multi-layer semiconductor structure, causing the structure to emit laser radiation along the optical axis at the plurality of different wavelengths.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to opto-electronic semiconductor devices, and specifically to vertical-cavity surface-emitting laser (VCSEL) arrays that are capable of operating at multiple wavelengths simultaneously.  
         BACKGROUND OF THE INVENTION  
         [0002]    Opto-electronic semiconductor devices, such as light-emitting diodes (LEDs) and semiconductor diode lasers, typically emit radiation in a relatively narrow wavelength band. It is known in the art, however, to combine multiple opto-electronic elements, emitting light at different wavelengths, on a single semiconductor chip. For example, U.S. Pat. Nos. 5,386,428, 5,930,278, 5,982,799 and 6,411,642, whose disclosures are incorporated herein by reference, describe semiconductor laser arrays in which multiple lasers, operating at different wavelengths, are arranged side by side on a chip.  
           [0003]    Semiconductor LEDs and diode laser devices emit light due to recombination of electron-hole pairs in an active layer of the device. The wavelength of the radiation is determined by the bandgap in the active layer. It is known in the art to stack two or more active layers, one above the other, on a single substrate in order to generate multi-wavelength radiation. For example, U.S. Pat. No. 5,684,309, whose disclosure is incorporated herein by reference, describes LEDs based on stacked quantum wells, which are designed to emit different wavelengths that combine to produce white light. Stacked laser diode structures are described in U.S. Pat. Nos. 5,138,624, 5,708,674, 5,802,088, and 5,920,766, whose disclosures are also incorporated herein by reference. All these patents describe side-emitting structures, i.e., lasers (or LEDs) that emit radiation along an optical axis parallel to the plane of the substrate.  
           [0004]    Unlike conventional side-emitting semiconductor lasers, vertical-cavity surface-emitting laser (VCSELs) emit radiation along an optical axis that is generally perpendicular to the substrate. U.S. Pat. No. 6,174,749, whose disclosure is incorporated herein by reference, describes a multiple-wavelength VCSEL array. The described device uses a wavelength-shifting layer of variable thickness overlying the substrate. The thickness variation defines different VCSEL regions, side-by-side on the substrate, that emit radiation at different wavelengths.  
         SUMMARY OF THE INVENTION  
         [0005]    Embodiments of the present invention provide novel semiconductor laser devices, which emit laser radiation at multiple wavelengths along a common optical axis that is substantially perpendicular to the device substrate. Such devices typically comprise a VCSEL structure having multiple active layers, which amplify optical radiation at different, respective wavelengths. The VCSEL structure comprises at least two reflective regions, typically in the form of distributed Bragg reflectors, which define at least one micro-cavity resonator, oriented along the optical axis and containing the active layers. One or more electrodes are used to apply an electrical current to the VCSEL structure, causing the structure to emit laser radiation at the multiple different wavelengths simultaneously.  
           [0006]    The device may be configured to emit the laser radiation either through the substrate or through one of the electrodes (if a suitable transparent electrode structure is used). In either case, the laser radiation at all the different wavelengths is emitted coaxially, along the same optical axis, as defined by the micro-cavity resonator, unlike the side-by-side beams generated by devices known in the art. The wavelengths of the active layers in the VCSEL structure may be selected to give substantially any output light color, including white light. Furthermore, an array of multi-wavelength VCSEL structures of this sort may be formed together on a common substrate in order to create an intense, high-efficiency light source, which may be used in a wide range of applications.  
           [0007]    There is therefore provided, in accordance with an embodiment of the present invention, a laser device, including:  
           [0008]    a substrate;  
           [0009]    a multi-layer semiconductor structure formed on the substrate, the structure including:  
           [0010]    one or more active layers, which are adapted to amplify optical radiation at a plurality of different wavelengths; and  
           [0011]    at least two reflective regions, arranged to define at least one micro-cavity resonator containing the active layers and having an optical axis substantially perpendicular to the substrate; and  
           [0012]    an electrode, which is coupled to apply an electrical current to the multi-layer semiconductor structure, causing the structure to emit laser radiation along the optical axis at the plurality of different wavelengths.  
           [0013]    Typically, the one or more active layers include a plurality of active layers, each of which is adapted to amplify the optical radiation at a respective one of the different wavelengths.  
           [0014]    In some embodiments, the active layers include quantum wells having recombination energies that correspond to the different wavelengths. The different wavelengths are determined by selecting at least one of a composition of the quantum wells and a thickness of the quantum wells so as to provide electron and hole energy levels having the recombination energies that correspond to the different wavelengths.  
           [0015]    In a disclosed embodiment, the plurality of different wavelengths includes at least first and second wavelengths, and the at least one micro-cavity resonator includes a single resonator containing the active layers and having resonances at the first and second wavelengths. The at least two reflective regions may include first and second distributed Bragg reflectors (DBRs) containing the active layers therebetween, and the DBRs are adapted to reflect the radiation at both the first and second wavelengths. Typically, each of the DBRs includes a stack of alternating DBR layers, each having a respective dielectric index n and a respective thickness t that is chosen so as to satisfy  
       t   =         (       4   ×   m     +   1     )     ×   λ       4   ×   n                             
 
           [0016]    for λ equal to both of the first and second wavelengths, wherein m is an integer.  
           [0017]    In another embodiment, the one or more active layers include first and second active layers, which are adapted to amplify the optical radiation at respective first and second wavelengths, and the at least one micro-cavity resonator includes first and second resonators, coaxially aligned along the optical axis and containing the first and second active layers, respectively. Typically, the device has first and second sides and is arranged so that the laser radiation is emitted through the first side of the device, and the first resonator is located between the second resonator and the first side of the device, and the at least two reflective regions include first and second reflectors, containing the first active layer therebetween, wherein the first and second reflectors are substantially reflective at the first wavelength and substantially transparent at the second wavelength, and third and fourth reflectors, containing the second active layer therebetween, wherein the third and fourth reflectors are substantially reflective at the second wavelength. Alternatively, the at least two reflective regions include first, second and third reflective regions, such that the first and second reflective regions contain the first active layer therebetween and define the first resonator, while the second and third reflective regions contain the second active layer therebetween and define the second resonator.  
           [0018]    In an aspect of the invention, the different wavelengths are selected and an intensity of the radiation emitted at each of the different wavelengths is controlled so that the laser radiation is perceived as white light.  
           [0019]    There is also provided, in accordance with an embodiment of the present invention, a light source, including:  
           [0020]    a substrate;  
           [0021]    an array of vertical-cavity surface-emitting laser (VCSEL) structures formed on the substrate, each such VCSEL structure including one or more active layers, which are adapted to amplify optical radiation at a plurality of different wavelengths, the array further including at least two reflective regions, which are arranged to define, in each of the VCSEL structures, at least one micro-cavity resonator containing the active layer and having a respective optical axis passing through the VCSEL structure in a direction substantially perpendicular to the substrate; and  
           [0022]    electrodes, which are coupled to apply an electrical current to the VCSEL structures, causing each of the VCSEL structures to emit laser radiation along the respective optical axis at the plurality of different wavelengths.  
           [0023]    Typically, the light source includes an integrated circuit chip having pads, to which the electrodes are fixed so as to mount the array of VCSEL structures on the chip and to supply the electrical current through the pads to the electrodes.  
           [0024]    There is additionally provided, in accordance with an embodiment of the present invention, a method for producing a light source, including:  
           [0025]    forming a multi-layer semiconductor structure on a substrate, the structure including one or more active layers for amplifying optical radiation at a plurality of different wavelengths, and at least two reflective regions, arranged to define at least one micro-cavity resonator containing the active layers and having an optical axis substantially perpendicular to the substrate; and  
           [0026]    coupling an electrode to apply an electrical current to the multi-layer semiconductor structure, so as to cause the structure to emit laser radiation along the optical axis at the plurality of different wavelengths.  
           [0027]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a schematic, pictorial illustration of a multi-wavelength VCSEL structure, in accordance with an embodiment of the present invention;  
         [0029]    [0029]FIG. 2 is a schematic, sectional illustration showing details of a layer structure in a multi-wavelength VCSEL, in accordance with an embodiment of the present invention;  
         [0030]    [0030]FIG. 3 is a schematic, sectional illustration showing details of a distributed Bragg reflector (DBR), in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIGS. 4A and 4B are schematic plots of reflectivity as a function of wavelength for a micro-cavity used in a multi-wavelength VCSEL, in accordance with an embodiment of the present invention;  
         [0032]    [0032]FIGS. 5-7 are schematic energy level diagrams of quantum well structures in a multi-wavelength VCSEL, in accordance with embodiments of the present invention;  
         [0033]    [0033]FIGS. 8 and 9 are chromaticity diagrams that schematically illustrate the use of two or three laser wavelengths to create white light;  
         [0034]    [0034]FIGS. 10 and 11 are schematic, sectional illustrations of multi-wavelength VCSEL structures, in accordance with alternative embodiments of the present invention; and  
         [0035]    [0035]FIG. 12 is a schematic, sectional illustration of a multi-wavelength VCSEL array, in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0036]    [0036]FIG. 1 is a schematic, pictorial illustration of a multi-wavelength VCSEL structure  20 , in accordance with an embodiment of the present invention. Structure  20  is typically fabricated on a transparent sapphire substrate  22 , using methods of thin film deposition known in the art. The sapphire substrate provides structure  20  with the desired crystalline characteristics, mechanical structure and heat dispersion properties. An n-type GaN substrate layer  24  is deposited on the sapphire substrate. A gain region  26  is contained between a lower distributed Bragg reflector (DBR)  28  and an upper DBR  30 , which are formed over the substrate layers. Gain region  26  comprises multiple active layers, each designed to amplify optical radiation at a different, respective wavelength, as described in further detail hereinbelow. Typically, the active layers comprise intrinsic material with multiple quantum wells. Alternatively, gain region  26  may contain a junction between n- and p-type materials.  
         [0037]    Lower DBR  28 , upper DBR  30  and active layer  26  define an optical microcavity, having resonances at the gain wavelengths of region  26 . Lower DBR  28  typically comprises alternating layers of n-type materials, and thus serves both as the lower cavity reflector and as the n-type side of the semiconductor laser junction. In similar fashion, upper DBR  30  comprises alternating layers of p-type materials. The DBRs may comprise compliance layers, as are known in the art, in order to reduce the density of defects therein. Further details of the DBRs are also described below.  
         [0038]    Electrodes  32  and  34  contact the p-type upper DER  30  and n-type lower DER  28 , respectively. Electrodes  32  and  34  are positively and negatively biased, respectively, via conductors  36  and  38 , causing a current to flow through gain region  26 . As a result, the VCSEL structure emits a beam of multi-wavelength coherent radiation, represented by an arrow  40 , through substrate  22 . Alternatively, if electrode  32  is made of a transparent material, the beam may be emitted through the electrode.  
         [0039]    [0039]FIG. 2 is a schematic, sectional illustration of structure  20 , showing additional details of the layers making up the structure. Reference is also made to Table I, below, which describes quantitatively the composition and thickness of the layers, listed in order from the bottom up (in the frame of reference of FIG. 2).  
                                                                                                                                                           TABLE I                           LAYER COMPOSITION                        Thickness   Doping           Layer   Material   [nm]   [cm −3 ]                            Sapphire   Sapphire   500,000   —           substrate   (1000)           GaN substrate   GaN   738   5 * 10 18  (Si)           Buffer   In 0.02 Ga 0.98 N   183   5 * 10 18  (Si)           DBR   GaN   970   5 * 10 18  (Si)                21   DBR   In 0.2 Ga 0.8 N   907   5 * 10 18  (Si)           times   DBR   GaN   970   5 * 10 18  (Si)                Waveguide   Al 0.2 Ga 0.8 N   765   5 * 10 18  (Si)           Barrier   In 0.02 Ga 0.98 N   321   3 * 10 16  (Si)           Quantum well   In 0.15 Ga 0.85 N   5   5 * 10 18  (Si)           Barrier   In 0.02 Ga 0.98 N   10   3 * 10 16  (Si)           Quantum well   In 0.15 Ga 0.85 N   5   5 * 10 18  (Si)           Barrier   In 0.02 Ga 0.98 N   5   3 * 10 16  (Si)           Barrier   In 0.02 Ga 0.98 N   5   3 * 10 16  (Si)           Quantum well   In 0.32 Ga 0.68 N   4   5 * 10 18  (Si)           Barrier   In 0.02 Ga 0.98 N   10   3 * 10 16  (Si)           Quantum well   In 0.32 Ga 0.68 N   4   5 * 10 18  (Si)           Barrier   In 0.02 Ga 0.98 N   69   3 * 10 16  (Si)           Electron   Al 0.2 Ga 0.8 N   20   10 20  (Mg)           stopper           Waveguide   Al 0.2 Ga 0.8 N   765   5 * 10 19  (Mg)           DBR   GaN   970   5 * 10 19  (Mg)                22   DBR   In 0.2 Ga 0.8 N   907   5 * 10 19  (Mg)           times   DBR   GaN   970   5 * 10 19  (Mg)                DBR   In 0.2 Ga 0.8 N   907   5 * 10 19  (Mg)           DBR   GaN   970   5 * 10 19  (Mg)           Contact   GaN   907   10 20  (Mg)           Metal   Ni/Au   3/10   —                      
 
         [0040]    GaN substrate  24  is grown on sapphire substrate  22  in order to serve as the base for the structure above it. A compliance layer  50 , comprising In 0.02 Ga 0.98 N, is used to reduce the stresses caused by the lattice constant mismatch between the sapphire and GaN substrates. Layers  24  and  50  may also referred to as buffer layers.  
         [0041]    Reference is now made to FIG. 3, which shows details of lower DBR  28 , in accordance with an embodiment of the present invention. In the present example, DBR  28  comprises 21.5 cycles of alternating high-index layers  70  and low-index layers  72 . The high-index layers comprise In 0.2 Ga 0.8 N, while the low-index layers comprise GaN. For both of the laser emission wavelengths, λ 1  and λ 2 , Of structure  20 , each layer in DBR  28  satisfies the equation:  
             t   =         (       4   ×   m     +   1     )     ×   λ       4   ×   n               (   1   )                               
 
         [0042]    Here t is the thickness of the layer, n is the refractive index of the layer, λ is the wavelength and m is an integer. The characteristics of layers  70  and  72  in the present case are chosen for λ 1 =463 nm and λ 2 =572 nm.  
         [0043]    [0043]FIGS. 4A and 4B are schematic plots of the reflectivity of the microcavity structure in VCSEL structure  20 , as a function of wavelength in the vicinity of the two laser emission wavelengths of structure  20 . (The microcavity structure comprises DBR  28 , DBR  30 , and layers  52 - 60  shown in FIG. 2.) The reflectivity of each of the DBRs near the Bragg wavelength (with air at both sides) is approximately 0.99. The minimal microcavity reflectivity is achieved at the Bragg frequency, at which each DBR has its maximal reflectivity. Although in the present example, DBRs  28  and  30  are designed to operate at two particular wavelengths, the same approach can be used to design DBRs that support three or more laser wavelengths simultaneously (by satisfying equation (1) at all the wavelengths). As the number of wavelengths that must be supported increases, however, the thickness t of the DBR layers tends to increase, as well.  
         [0044]    Returning now to FIG. 2 and Table I, an n-type waveguide layer  52 , comprising Al 0.2 Ga 0.8 N, is formed over DBR  28 . Layer  52  serves as the n-side of the PIN junction containing gain region  26 . The gain region comprises two active layers  54  and  56 , which comprise quantum wells that are tuned to emit at the laser wavelengths λ 1  and λ 2 . Active layers  54  and  56  each comprise alternating layers of intrinsic-type (or lightly doped) material. In the present example, layer  54 , which emits at 463 nm, comprises quantum well layers of In 0.15 Ga 0.85 N alternating with barrier layers of In 0.02 Ga 0.98 N. Layer  56 , which emits at 572 nm, comprises quantum well layers of In 0.32 Ga 0.68 N alternating with barrier layers of In 0.02 Ga 0.98 N.  
         [0045]    In order to stop electrons at the upper end of the intrinsic (or lightly doped) region, a heavily-doped, p-type electron stop layer  58  is grown over gain region  26 . This layer stops electrons, but allows holes to pass through, so that recombination takes place in the gain region. Layer  58  is overlaid by a p-type waveguide layer  60 . Layers  58  and  60  typically comprise Al 0.2 Ga 0.8 N. Upper DBR  30  then comprises 23.5 cycles of alternating layers of high-index p-type In 0.2 Ga 0.8 N and low-index GaN. The alternating layers here are of the same thicknesses as layers  70  and  72  in lower DBR  28 . The use of a higher number (23.5) of cycles in upper DBR  30  causes most of the laser radiation from the microcavity to be emitted through lower DBR  28 . The last (upper) cycle of DBR  30 , comprising p-type GaN, serves as a contact layer  62 . This layer may be doped (more heavily than the layers below it) in order to achieve good ohmic contact with electrode  32 .  
         [0046]    [0046]FIG. 5 is a schematic energy level diagram, which shows details of the bandgap structure of active layers  54  and  56 , in accordance with an embodiment of the present invention. The horizontal axis in the figure corresponds to the Z-axis (i.e., the vertical axis) in FIG. 2. As noted above, active layers  54  and  56  comprise quantum well layers  80  and  82 , respectively, alternating with barrier layers  84 . The vertical axis in the figure represents energy levels within the active layers, wherein the upper solid line in the figure represents a conduction band  86 , and the lower solid line represents a valence band  88 .  
         [0047]    Electrons populate energy levels  92  and  98  within conduction band  86  in quantum well layers  80  and  82 , while holes populate energy levels  94  and  100  within valence band  88 . Recombination of an electron-hole pair in one of the quantum wells causes emission of a photon at a wavelength determined by the difference between the respective electron and hole energy levels, as indicated by arrows  90  and  96  in the figure. In the present example, the energy levels are determined by selecting the appropriate concentrations of In and Ga (or of other suitable quantum well materials) in layers  80  and  82 . Thus, the transition indicated by arrow  90  may correspond to emission at 463 nm, while the transition indicated by arrow  96  corresponds to emission at 572 nm. Alternatively, other wavelengths may be generated in like fashion, and further, different quantum well layers may be added so that structure  20  emits laser radiation at three or more wavelengths.  
         [0048]    [0048]FIG. 6 is a schematic energy level diagram, which shows details of the bandgap structure of two active layers in a multi-wavelength quantum well laser, in accordance with another embodiment of the present invention. This embodiment illustrates an alternative technique for adjusting the wavelength of laser emission, in this case by controlling the thickness of quantum well layers  110  and  112 . In thin layer  110 , allowed energy levels  116  and  118  are relatively far apart, so that photons emitted due to recombination in this layer have higher energy (shorter wavelength), as indicated by an arrow  114 . In thick layer  112 , allowed energy levels  122  and  124  are closer together, so that the emitted photons have lower energy (longer wavelength), as indicated by an arrow  120 .  
         [0049]    [0049]FIG. 7 is a schematic energy level diagram, which shows details of the bandgap structure of an active layer in a multi-wavelength quantum well laser, in accordance with yet another embodiment of the present invention. In this embodiment, a quantum well layer  130  is made sufficiently thick to support multiple electron energy levels  134  and  144  in conduction band  86 , and multiple hole energy levels  136  and  140  in valence band  88 . Judicious selection of the composition and the thickness of layer  130  allows three specific wavelengths to be generated due to recombination between different pairs of conduction band energy levels  134  and  144  with valence band energy levels  136  and  140 , as indicated by arrows  132 ,  138  and  142 .  
         [0050]    The methods of wavelength selection exemplified by FIGS. 5, 6 and  7  may be used individually or in combination to generate the desired combination of emission wavelengths. The relative intensities of emission at the different wavelengths may also be controlled. Various methods may be used for this purpose, including varying the distances of the quantum wells from the P- and N-sides of the junction, varying the spacings between the quantum wells, and using different numbers of quantum wells for different wavelengths. Usually, the closer the well is to the P-side the stronger its emission is. By adding more quantum wells to emit light of a certain color, the intensity of that color will be increased, while that of other colors will be reduced. Methods for creating chirped quantum well layers, with different emission frequencies and intensities, are described further, for example, in U.S. Pat. No. 6,504,171, whose disclosure is incorporated herein by reference. The emission wavelengths and intensities may be chosen so that the combined beam is perceived by the human eye as white light, or as substantially any other desired color.  
         [0051]    [0051]FIG. 8 is a chromaticity diagram, which schematically illustrates the combination of laser radiation from VCSEL structure  20  at two emission wavelengths, λ 1  and λ 2 , to generate white light, in accordance with an embodiment of the present invention. Each color in FIG. 8 is represented by corresponding x- and y-chromaticity components, in accordance with the well-known CIE model of human color vision. White light has coordinates x=y=0.33.  
         [0052]    As shown in FIG. 8, the CIE diagram can be used to visualize the addition of colors. When multiple wavelengths of light are combined, the resulting color can be found by calculating a weighed average of the original colors using their relative intensities. Thus, two light sources with specific colors can produce a range of colors defined by a line connecting them in the CIE diagram. In the present example, in which active layers  54  and  56  emit light at 463 nm and 572 nm, respectively, it can be seen that an intensity ratio of 1:1.6 between these wavelengths will give a white light output. In other words, driving active layer  54  to emit 1 W of radiation at 463 nm and active layer  56  to emit 1.6 W at  572  will give, in effect, 2.6 W of white light, with an optical efficiency of 412 lumen/W.  
         [0053]    [0053]FIG. 9 is a similar chromaticity diagram, which schematically illustrates the combination of laser radiation from a multi-wavelength VCSEL at three simultaneous wavelengths, in accordance with another embodiment of the present invention. Three light sources with specific wavelengths can achieve a range of colors defined by the triangle connecting them in the CIE diagram. In the present example, active layers emitting at 463 nm, 551 nm and 609 nm, respectively, are controlled so as to emit light with an intensity ratio of 1:1.16:0.82, thus generating white light with x=y=0.33. Alternatively, other combinations of two, three or more wavelengths may be used to achieve a similar effect. Using three or more wavelengths simultaneously in this manner allows a large range of colors to be generated by the VCSEL, by varying the relative intensities of emission at the different wavelengths.  
         [0054]    [0054]FIG. 10 is a schematic, sectional view of a multi-wavelength VCSEL structure  160 , in accordance with another embodiment of the present invention. Details of structure  160  are omitted here for the sake of brevity, but will be apparent to those skilled in the art based on the description of FIGS. 1-7 given above. Structure  160  comprises two separate active regions  162  and  164 , each designed to emit radiation at a different, respective wavelength. A lower DBR  166  and a middle DBR  168  (along with active region  162 ) define the optical microcavity for active region  162 , while middle DBR  168  and an upper DBR  170  (along with active region  164 ) define the optical cavity for active region  164 . Typically, active region  162  emits radiation at a longer wavelength than active region  164 , and upper DBR  170  is transparent to the longer wavelength. Alternatively, upper DBR  170  may be designed to form a part of the microcavity for the longer-wavelength radiation emitted by active region  162 . In either case, the laser radiation from both active regions  162  and  164  is emitted from VCSEL structure  160  in the upward direction (in the frame of reference of the figure) along a common optical axis.  
         [0055]    As in the embodiments described above, the emission wavelengths are chosen so as to give output light of a desired color, typically white light. A third active region, with an additional DBR, may similarly be added to generate a third wavelength. Additionally or alternatively, one of the active regions in structure  160 , along with its microcavity, may be designed for dual-wavelength operation, as in the embodiment described above.  
         [0056]    In order to provide carrier injection into both active regions  162  and  164 , structure  160  typically has a PNPN arrangement. Thus, regions  172  and  176  may comprise n-type material, while regions  174  and  178  comprise p-type material. Active regions  162  and  164  (and the adjoining waveguide layers) may comprise intrinsic material. Further details of the structure of the layers in FIGS. 10 and 11 may be derived, mutatis mutandis, from the order and composition of the layers listed above in Table I.  
         [0057]    [0057]FIG. 11 is a schematic, sectional view of a multi-wavelength VCSEL structure  180 , in accordance with yet another embodiment of the present invention. In this case, each of active layers  182  and  184  has its own, independent microcavity, wherein the two microcavities are coaxially aligned. The microcavity of active layer  182  comprises DBRs  186  and  188  (together with layer  182 ), while that of active layer  184  comprises DBRs  190  and  192  (together with layer  184 ). As in the preceding embodiment, active layer  184  is typically designed to emit radiation at a shorter wavelength than active layer  182 , and DBRs  190  and  192  are substantially transparent to the longer-wavelength radiation. Regions  194  and  198  typically comprise n-type material, while regions  196  and  200  comprise p-type material. The multi-wavelength laser beam is emitted from structure  180  in the upward direction.  
         [0058]    Although the embodiments described above use certain particular materials and layer structures, and are designed to emit radiation at particular wavelengths, the principles of the present invention may similarly be applied using structures and materials of other types, with different emission wavelengths. For example, the active layers in the laser structures may be based on direct recombination of electrons and holes in a direct band semiconductor, so that the energy bandgap determines the wavelength. As another example, quantum dots, as are known in the art, may be used in the active layer instead of quantum wells. All such alternative implementations are considered to be within the scope of the present invention.  
         [0059]    [0059]FIG. 12 is a schematic, sectional view of a multi-wavelength VCSEL array  210 , in accordance with an embodiment of the present invention. This array comprises a matrix of VCSEL structures, similar to structure  20 , shown in FIG. 1, and sharing sapphire substrate  22 , GaN substrate  24  and lower DBR  28 . To produce active region  26  and upper DBR  30 , the required layers are deposited over lower DBR  28 , and then these layers are separated, typically using an etching process (or selective growth), to define the individual VCSEL structures in the array. Optionally, lower DBR  28  may be separated among the individual VCSEL structures in this manner, as well, rather than shared among the structures as shown in FIG. 12.  
         [0060]    Metal contacts  32  and  34  are formed so as to contact the appropriate points on the VCSEL structure (as illustrated in FIG. 1, for example). An external passivation layer  212  protects the optical and electrical elements of the individual VCSEL structures. Metal contacts  32  are fastened with a conducting glue  214  or a soft metal, such as indium, to thickened pads  216  on a silicon chip  218 . External conductors  220  supply electrical current to chip  218  in order to drive the VCSEL structures in array  210 . Each of the VCSEL structures emits a multi-wavelength beam through substrate  22 , as indicated by arrows  222 .  
         [0061]    In this manner, array  210  serves as a high-intensity, extended light source, typically a white light source as described above. The light emitted by array  210  is relatively well-collimated, and can be further collimated (or alternatively, diffused) by means of additional, external optics, as are known in the art. Array  210  is capable of running on relatively low DC voltage, which may be supplied by a battery or by an AC/DC or DC/DC converter. The array may be driven by a DC current to emit continuous wave (CW) radiation, or it may alternatively be driven using short, high-current pulses to emit pulsed radiation. The current of the pulses may be chosen so as to optimize the power efficiency of the VCSEL structures, while the duty cycle of the pulses is controlled in order to give the desired output light intensity. If the repetition frequency of the pulses is sufficiently rapid, the pulses will be imperceptible to the human eye. Individual light sources based on a single VCSEL structure, as described above, can also be operated in this manner (i.e., low voltage, CW or pulsed). Another advantage of array  210 , however, is that even if certain individual VCSEL structures in the array are inoperative due to defects in the structure, the array as a whole is still capable of generating light of the desired intensity.  
         [0062]    Laser light sources produced in accordance with the present invention may be used in a wide range of lighting applications, for example, lighting of homes, industrial spaces, offices, streets and parking lots, as well as automotive headlights and taillights, stage lights, traffic lights, illuminated traffic signs, emergency lights, flashlights and magnifier lights. Such light sources are particularly useful in medical applications, such as overhead lamps and headlamps for surgery and dentistry, as well as miniature light sources for endoscopy and laparoscopy. In these and other applications, the individual multi-wavelength VCSEL structure or array may be combined with beam manipulation optics and other optical elements, as are known in the art, to achieve the desired illumination effects.  
         [0063]    It will be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.