Abstract:
An apparatus and method for enhancing light using stimulated Raman scattering in a potassium gadolinium tungstate (KGW) crystal. The stimulated Raman scattering is utilized to add wavelength diversity for reduced speckle and to change the color of the light to a more desirable combination of wavelengths. Digital projection is one application that may benefit from lower speckle and shifted color. Color-based stereoscopic projection is enabled by the addition of stimulated Raman bands at specific wavelengths.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    There are many advantages of using laser light sources to illuminate digital projection systems, but the high coherence of laser light tends to produce undesirable speckle in the viewed image. Known despeckling methods generally fall into the categories of polarization diversity, angle diversion, and wavelength diversity. In the laser projection industry, there has been a long-felt need for more effective despeckling methods. 
       SUMMARY OF THE INVENTION 
       [0002]    In general, in one aspect, an optical apparatus that includes a potassium gadolinium tungstate (KGW) crystal, a green laser, and a digital projector. The green laser illuminates the KGW crystal, a stimulated Raman scattering in the KGW crystal enhances an aspect of the light output from the KGW crystal, and the light output of the KGW crystal illuminates the digital projector. 
         [0003]    Implementations may include one or more of the following features. The aspect of the light output from the KGW crystal may be speckle or color. The light output from the KGW crystal may include a band of green light and a band of red light. There may also be a multipass cell, and the green laser may illuminate the KGW crystal with multiple passes that are determined by the multipass cell. There may also be a KGW resonant cavity and the resonant optical condition for the KGW crystal may be determined by the KGW resonant cavity. The KGW crystal may be located inside the green laser resonant cavity. The light output from the digital projector may meet the Digital Cinema Initiative color requirements. The light output of the KGW crystal may be separated into two wavelength bands. The first wavelength band may be used to form the first-eye image of a stereoscopic image projected from the digital projector. The second wavelength band may be used to form the second-eye image of the stereoscopic image. The green laser may include a fiber laser. 
         [0004]    In general, in one aspect, a method of despeckling that includes generating a green laser beam from a green laser, focusing the laser beam into a potassium KGW crystal, generating stimulated Raman scattering light in the KGW crystal, using the stimulated Raman scattering light to enhance an aspect of the light output from the KGW crystal, and using the light output of the KGW crystal to illuminate a digital projector. 
         [0005]    Implementations may include one or more of the following features. The aspect of the light output from the KGW crystal may be speckle or color. The light output from the KGW crystal may include a band of green light and a band of red light. The green laser beam may illuminate the KGW crystal with multiple passes that are determined by a multipass cell. The green laser beam may illuminate the KGW crystal with a resonant optical condition that is determined by a KGW resonant cavity. The KGW crystal may be located inside the green laser resonant cavity. The light output from the digital projector may meet Digital Cinema Initiative color requirements. There may also be a step of separating the light output of the KGW crystal into a first wavelength band and a second wavelength band. The first wavelength band may be used to form the first-eye image of a stereoscopic image in the digital projector. There may also be a step of projecting the stereoscopic image from the digital projector. The green laser may include a fiber laser. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0006]      FIG. 1  is a top view of laser projection system with a potassium gadolinium tungstate (KGW) crystal in a multipass cell; 
           [0007]      FIG. 2  is a top view of a laser projection system with a KGW crystal in a KGW resonant cavity; 
           [0008]      FIG. 3  is a top view of a laser projection system with a KGW crystal in a green laser resonant cavity; 
           [0009]      FIG. 4  is a spectral graph of a a laser projection system with a KGW crystal in a multipass cell; 
           [0010]      FIG. 5  is a color chart of a laser projection system with a KGW crystal in a multipass cell; 
           [0011]      FIG. 6  is a spectral graph of a laser projection system with a KGW crystal in a resonant cavity; 
           [0012]      FIG. 7  is a color chart of a laser projection system with a KGW crystal in a resonant cavity; 
           [0013]      FIG. 8  is a spectral graph of a stereoscopic laser projection system with a KGW crystal in a multipass cell; 
           [0014]      FIG. 9  is a color chart of a stereoscopic laser projection system with a KGW crystal in a multipass cell; 
           [0015]      FIG. 10  is a computer-simulated time graph of stimulated Raman scattering in a KGW crystal; 
           [0016]      FIG. 11  is a computer-simulated spectral graph of stimulated Raman scattering in a KGW crystal; 
           [0017]      FIG. 12  is a flowchart of a method of laser projection with a KGW crystal; and 
           [0018]      FIG. 13  is a flowchart of a method of stereoscopic laser projection with a KGW crystal. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Conventional laser projection systems are typically constructed with narrow-band laser sources. The narrow bands of light tend to produce speckle patterns in the projected images. Spectral broadening of the laser sources may be used to add wavelength diversity that reduces the level of speckle. By using stimulated Raman scattering in a potassium gadolinium tungstate (KGW) crystal, additional Stokes-shifted peaks may be added to help reduce laser speckle with wavelength diversity. The Stokes-shifted peaks are individually broadened compared to the starting peaks, so the wavelength broadening effect is magnified when compared to adding additional narrow peaks. With appropriate balancing of wavelengths and amplitudes, Digital Cinema Initiative (DCI) color points can be achieved for red, green, and blue primary colors, along with the DCI white point. 
         [0020]      FIG. 1  shows a top view of a laser projection system with a potassium gadolinium tungstate (KGW) crystal in a multipass cell. Green laser  100  generates first light beam  102 . First light beam  102  illuminates first lens assembly  104 . First lens assembly  104  produces second light beam  106 . Second light beam  106  illuminates KGW crystal  108  and generates SRS light that is shown as included in second light beam  106 . Second light beam  106  is reflected by first reflector  110  to produce third light beam  112 . Third light beam  112  is reflected by first reflector  110  to produce fourth light beam  114 . Fourth light beam  114  illuminates KGW crystal  108  and generates SRS light that is shown as included in fourth light beam  114 . Fourth light beam  114  is reflected by second reflector  118  to produce fifth light beam  118 . Fifth light beam  118  is reflected by second reflector  118  to produce sixth light beam  120 . Sixth light beam  120  illuminates KGW crystal  108  and generates SRS light that is shown as included in sixth light beam  120  and seventh light beam  124 . Sixth light beam  120  illuminates second lens assembly  122 . Second lens assembly  122  produces seventh light beam  124 . Seventh light beam  124  illuminates digital projector  126 . KGW crystal  108 , first reflector  110 , and second reflector  118 , form a multipass cell. The number of passes through the KGW crystal may be modified to attain a longer or shorter path length as desired to convert more of less of the green laser light to SRS light. Three passes through the KGW crystal are shown in  FIG. 1 , but any number of passes may be utilized. The lens assemblies may be any combination of lens or other optical elements that are designed to collect and shape the beam for optimal effect in each part of the system. The reflectors may be corner cube prisms, flat mirrors, or other optical elements that reflect the beams appropriately. To construct a three-color projection system, red and blue lasers may be added, but are not shown in  FIG. 1 . Digital projector  126  may be a projector based on digital micromirror (DMD), liquid crystal device (LCD), liquid crystal on silicon (LCOS), or other digital light valves. Additional elements may be included to further guide or despeckle the light such as additional lenses, diffusers, vibrators, or optical fibers. 
         [0021]      FIG. 2  shows a top view of a laser projection system with a KGW crystal in a KGW resonant cavity. Green laser  200  generates first light beam  202 . First light beam  202  illuminates first lens assembly  204 . First lens assembly  204  produces second light beam  206 . Second light beam  206  illuminates first partial mirror  208 . First partial mirror  208  produces third light beam  210 . Third light beam  210  illuminates KGW crystal  212 . KGW crystal  212  produces fourth light beam  214 . Fourth light beam  214  illuminates second partial mirror  216 . Second partial mirror  216  produces fifth light beam  218 . Fifth light beam  218  illuminates second lens assembly  220 . Second lens assembly  220  produces sixth light beam  222 . Sixth light beam  222  illuminates digital projector  224 . First partial mirror  208 , KGW crystal  212 , and second partial mirror  216  from a KGW resonant cavity such that light is reflected multiple times between first partial mirror  208  and second partial mirror  216 . KGW crystal  212  generates SRS light that is shown as included in third light beam  210 , fourth light beam  214 , fifth light beam  218 , and sixth light beam  222 . First partial mirror  208  and second partial mirror  2167  have partial reflection designed to form a resonant cavity at desired wavelengths. First partial mirror  208  may be spherically shaped to help stabilize the beam in the resonant cavity. Partial mirrors may be constructed with dichroic coatings to transmit and reflect various wavelengths at desired levels and to convert the desired amount of the green laser light to SRS light. The lens assemblies may be any combination of lens or other optical elements that are designed to collect and shape the beam for optimal effect in each part of the system. To construct a three-color projection system, red and blue lasers may be added, but are not shown in  FIG. 2 . Digital projector  224  may be a projector based on digital micromirror (DMD), liquid crystal device (LCD), liquid crystal on silicon (LCOS), or other digital light valves. Additional elements may be included to further guide or despeckle the light such as additional lenses, diffusers, vibrators, or optical fibers. 
         [0022]      FIG. 3  shows a top view of a laser projection system with a KGW crystal in a green laser resonant cavity. Green laser  300  generates first light beam  320 . First light beam  320  illuminates first lens assembly  322 . First lens assembly  322  produces second light beam  324 . Second light beam  206  illuminates digital projector  326 . Green laser  300  includes mirror  302 , lasing crystal  306 , second harmonic generation (SHG) crystal  310 , KGW crystal  314 , and partial mirror  318 . Mirror  302  and partial mirror  318  from a green laser resonant cavity at desired wavelengths such that light is reflected multiple times between mirror  302  and partial mirror  318 . Third light beam  304  carries infrared (IR) light, unshifted green light, and SRS light between mirror  302  and lasing crystal  306 . Fourth light beam  308  carries IR light, unshifted green light, and SRS light between lasing crystal  306  and SHG crystal  310 . Fifth light beam  312  carries IR light, unshifted green light, and SRS light between SHG crystal  310  and KGW crystal  314 . Sixth light beam  316  carries IR light, unshifted green light, and SRS light between KGW crystal  314  and partial mirror  318 . Partial mirror  318  allows some light to pass through to generate first light beam  320 . Lasing crystal  306  generates coherent IR light and may be constructed from neodymium-doped yttrium aluminum garnet (Nd:YAG), neodymium-doped yttrium vanadate (Nd:YVO 4 ), neodymium-doped yttrium lithium fluoride (Nd:YLF), or another lasing material. SHG crystal  310  converts some of the coherent IR light to unshifted green light and may be constructed from lithium triborate (LBO) or other nonlinear optical material. KGW crystal  314  converts some of the unshifted green light to SRS light. Mirror  302  may be spherically shaped to help stabilize the beam in the resonant cavity. Partial mirrors may be constructed with dichroic coatings to transmit and reflect various wavelengths at desired levels and to convert the desired amount of the unshifted green laser light to SRS light. The lens assembly may be any combination of lens or other optical elements that are designed to collect and shape the beam for optimal effect in the digital projector. To construct a three-color projection system, red and blue lasers may be added, but are not shown in  FIG. 3 . Digital projector  326  may be a projector based on digital micromirror (DMD), liquid crystal device (LCD), liquid crystal on silicon (LCOS), or other digital light valves. Additional elements may be included to further guide or despeckle the light such as additional lenses, diffusers, vibrators, or optical fibers. 
         [0023]      FIG. 4  shows a spectral graph of a laser projection system with a KGW crystal in a multipass cell. The horizontal axis represents wavelength in nanometers and the vertical axis represents normalized light intensity. First peak  400  is generated by blue laser diodes at 465 nm. Second peak  402 , third peak  404 , fourth peak  406 , fifth peak  408 , and seventh peak  412  correspond to the spectral output of the laser projection system shown in  FIG. 1 . Second peak  402  is formed from unshifted green light at 520 nm. Third peak  404 , fourth peak  406 , fifth peak  408 , and seventh peak  412  are formed from SRS light at 542 nm, 565 nm, 619 nm, and 650 nm. Sixth peak  410  is generated by red laser diodes at 637 nm. Second peak  402 , third peak  404 , and fourth peak  406  represent a despeckled green primary color. Fifth peak  408 , sixth peak  410 , and seventh peak  412  represent a despeckled red primary color. An SRS peak at 591 nm is not shown because it is filtered out by low projector transmission between the green and red bands. The vertical axis is normalized to second peak  402 . Although not shown to scale in  FIG. 4 , second peak  402  is typically a very narrow peak that has a width of much less than one nanometer, whereas the other peaks typically have bandwidths in the range of 1 to 5 nm each. Third peak  404 , fourth peak  406 , fifth peak  408 , and seventh peak  412  are broadened by the SRS process. First peak  400  and sixth peak  410  are broadened by the properties of the laser diodes that are used to create those peaks. The amplitudes of third peak  404 , fourth peak  406 , fifth peak  408 , and seventh peak  412  are determined by the KGW conversion parameters such as the beam diameter and length in the KGW crystal and the green laser peak power and pulse shape. The amplitudes of first peak  400 , second peak  402 , and sixth peak  410  are determined by the output powers of the blue, green, and red lasers respectively. 
         [0024]      FIG. 5  shows a color chart of a laser projection system with a KGW crystal in a multipass cell. The x and y axes represent the x and y coordinates of the Commission Internationale de L&#39;Eclairage (CIE) 1931 color space. First triangle  500  (solid line) shows the color gamut that results from the laser spectrum of  FIG. 4 . The red primary color is shown by first point  506 . The green primary color is shown by second point  504 . The blue primary color is shown by third point  508 . The white color is shown by fourth point  502 . Second triangle  510  (dashed line) shows the DCI standard color gamut that is required for cinema applications. Because first triangle  500  includes the entire area of second triangle  510 , the laser system can meet the requirements of the DCI color space. Fourth point  502  meets the requirements of the DCI standard white point. 
         [0025]      FIG. 6  shows a spectral graph of a laser projection system with a KGW crystal in a resonant cavity. The horizontal axis represents wavelength in nanometers and the vertical axis represents normalized light intensity. First peak  600  is generated by blue laser diodes at 465 nm. Second peak  602 , third peak  604 , and fourth peak  606  correspond to the spectral output of the laser projection systems shown in  FIG. 2  and  FIG. 3 . Second peak  602  is formed from unshifted green light at 528 nm. Third peak  604  and fourth peak  606  are formed from SRS light at 550 nm and 554 nm. Fifth peak  608  is generated by red laser diodes at 637 nm. Second peak  602 , third peak  604 , and fourth peak  606  represent a despeckled green primary color. The vertical axis is normalized to second peak  602 . Although not shown to scale in  FIG. 6 , second peak  602  is typically a very narrow peak that has a width of much less than one nanometer, whereas the other peaks typically have bandwidths in the range of 1 to 5 nm each. Third peak  604  and fourth peak  606  are broadened by the SRS process. First peak  600  and fifth peak  608  are broadened by the properties of the laser diodes that are used to create those peaks. The amplitudes of third peak  604  and fourth peak  606  are determined by the KGW conversion parameters such as the beam diameter and length in the KGW crystal and the green laser peak power and pulse shape. The amplitudes of first peak  600 , second peak  602 , and fifth peak  608  are determined by the output powers of the blue, green, and red lasers respectively. 
         [0026]      FIG. 7  shows a color chart of a laser projection system with a KGW crystal in a resonant cavity. The x and y axes represent the x and y coordinates of the CIE 1931 color space. First triangle  700  (solid line) shows the color gamut that results from the laser spectrum of  FIG. 6 . The red primary color is shown by first point  706 . The green primary color is shown by second point  704 . The blue primary color is shown by third point  708 . The white color is shown by fourth point  702 . Second triangle  710  (dashed line) shows the DCI standard color gamut that is required for cinema applications. Because first triangle  700  includes the entire area of second triangle  710 , the laser system can meet the requirements of the DCI color space. Fourth point  702  meets the requirements of the DCI standard white point. 
         [0027]      FIG. 8  shows a spectral graph of a stereoscopic laser projection system with a KGW crystal in a multipass cell. The horizontal axis represents wavelength in nanometers and the vertical axis represents normalized light intensity. First peak  800  is generated by blue laser diodes at 445 nm. Second peak  802  is generated by blue laser diodes at 465 nm. Third peak  804 , fifth peak  808 , sixth peak  810 , and seventh peak  812  correspond to the spectral output of the laser projection system shown in  FIG. 1 . Third peak  804  is formed from unshifted green light at 520 nm. Fifth peak  808 , sixth peak  810 , and seventh peak  812  are formed from SRS light at 542 nm, 565 nm, and 619 nm. Fourth peak  806  at 540 nm is generated by a separate green laser. Eighth peak  814  is generated by red laser diodes at 637 nm. Ninth peak  816  is generated by red laser diodes at 657 nm. First peak  800 , third peak  804 , sixth peak  810 , seventh peak  812 , and eighth peak  814  (dotted line) represent primary colors for one eye of a stereoscopic image. Second peak  802 , fourth peak  806 , fifth peak  808 , and ninth peak  816  (solid line) represent primary colors for the other eye of the stereoscopic image. Third peak  804  and third peak  810  represent a despeckled green primary color for one eye of the stereoscopic image. Fourth peak  806  and fifth peak  808  represent a despeckled green primary color for the other eye of the stereoscopic image. Seventh peak  812  and eighth peak  814  represent a despeckled red primary color for one eye of the stereoscopic image. An SRS peak at 591 nm is not shown because it is filtered out by low projector transmission between the green and red bands. The vertical axis is normalized to third peak  804 . Although not shown to scale in  FIG. 8 , third peak  804  and fourth peak  806  are typically very narrow peaks that have a width of much less than one nanometer, whereas the other peaks typically have bandwidths in the range of 1 to 5 nm each. Fifth peak  808 , sixth peak  810 , and seventh peak  812  are broadened by the SRS process. First peak  800 , second peak  802 , eighth peak  814 , and ninth peak  816  are broadened by the properties of the laser diodes that are used to create those peaks. The amplitudes of fifth peak  808 , sixth peak  810 , and seventh peak  812  are determined by the KGW conversion parameters such as the beam diameter and length in the KGW crystal and the green laser peak power and pulse shape. The amplitudes of first peak  800 , second peak  802 , third peak  804 , fourth peak  806 , eighth peak  814 , and ninth peak  816  are determined by the output powers of the blue, green, and red lasers respectively. 
         [0028]      FIG. 9  is a color chart of a stereoscopic laser projection system with a KGW crystal in a multipass cell. The x and y axes represent the x and y coordinates of the CIE 1931 color space. First triangle  900  (solid line) shows the color gamut that results from second peak  802 , fourth peak  806 , fifth peak  808 , and ninth peak  816  (solid line) laser spectrum of  FIG. 8 . The red primary color is shown by first point  906 . The green primary color is shown by second point  904 . The blue primary color is shown by third point  908 . The white color for these peaks is shown by fourth point  902 . Second triangle  910  (dotted line) shows the color gamut that results from first peak  800 , third peak  804 , sixth peak  810 , seventh peak  812 , and eighth peak  814  (dotted line) laser spectrum of  FIG. 8 . The red primary color is shown by fifth point  914 . The green primary color is shown by sixth point  912 . The blue primary color is shown by seventh point  916 . The white color for these peaks is again shown by fourth point  902 . Third triangle  918  (dashed line) shows the DCI standard color gamut that is required for cinema applications. Because first triangle  900  and second triangle  910  almost include the entire area of third triangle  918 , the laser system is close to meeting the requirements of the DCI color space for each eye separately. When the two eyes are combined, the average will meet the DCI requirements better than each eye separately. Fourth point  902  meets the requirements of the DCI standard white point for each eye. 
         [0029]    A computer model was utilized to calculate the conversion properties of a KGW crystal with a pulsed laser beam pump that creates Raman gain in the crystal to produce Stokes-shifted peaks of SRS light. The model utilizes several parameters of the crystal and laser source to determine the Stokes-shifted peaks. For the example shown in  FIG. 10 , the Stokes shift was 768 cm −1 , the Raman gain cross section was 1.4×10 −9 , the average laser spot size in the crystal was 250 micrometers in diameter, the laser pulse energy was 2×10 −3  joules, the input pulse full-width half-maximum was 70 ns, the crystal physical length was 50 mm with 5 passes (total effective length of 250 mm), the spontaneous Raman seed power was 1×10 −9  J, the quantum defect level was 0.95, and the crystal transmission was 99% over the total effective length. The input pulse to the KGW crystal was based on the output pulse from a green fiber laser that has a pulse shape with exponential decay.  FIG. 10  shows a computer-simulated time graph of SRS in a KGW crystal. The x-axis represents time in nanosecond, and the y-axis represents intensity in arbitrary units. First curve  1000  shows the input pulse with exponential decay. Second curve  1002  shows the residual energy that is not Stokes shifted. Third curve  1004  shows the first Stokes-shifted peak. Fourth curve  1006  shows the second Stokes-shifted peak. Fifth curve  1008  shows the third Stokes-shifted peak. Sixth curve  1010  shows the fourth Stokes-shifted peak. Seventh curve  1012  shows the fifth Stokes-shifted peak. Overall,  FIG. 10  describes the evolution in time of SRS process. 
         [0030]    The model used to generate  FIG. 10  was used with the same parameters to generate  FIG. 11  which shows a spectral graph of SRS in a KGW crystal. First peak  1100  shows an unshifted peak used to pump the crystal at 520 nm. Second peak  1102  shows the first Stokes-shifted peak at 542 nm. Third peak  1104  shows the second Stokes-shifted peak at 565 nm. Fourth peak  1106  shows the third Stokes-shifted peak at 591 nm. Fifth peak  1108  shows the fourth Stokes-shifted peak at 619 nm. Sixth peak  1110  shows the fifth Stokes-shifted peak at 650 nm. 
         [0031]      FIG. 12  shows a flowchart of a method of laser projection with a KGW crystal. In step  1200 , a green laser light beam is generated. In step  1202 , the green laser light beam is focused into a KGW crystal. In step  1204 , SRS light is generated in the KGW crystal. In step  1206 , the SRS light is used to enhance the light output from the KGW crystal. In step  1208 , the light output of the KGW crystal is used to illuminate a digital projector. In step  1210 , the digital projector is used to project an image. 
         [0032]      FIG. 13  shows a flowchart of a method of stereoscopic laser projection with a KGW crystal. In step  1300 , a green laser light beam is generated. In step  1302 , the green laser light beam is focused into a KGW crystal. In step  1304 , SRS light is generated in the KGW crystal. In step  1306 , the SRS light is used to enhance the light output from the KGW crystal. In step  1308 , the light output from the KGW crystal is separated into first and second wavelength bands. In step  1310 , the first wavelength band is used to form a first-eye image in a digital projector. In step  1312 , the second wavelength band is used to form a second-eye image in a digital projector. In step  1314 , the first-eye image and the second-eye image are used to project a stereoscopic image. Both stereoscopic images may be projected from a single digital projector or, alternately, each stereoscopic image may be projected from a separate digital projector. 
         [0033]    In addition to the DCI color space shown in  FIG. 5 ,  FIG. 7 , and  FIG. 9 , other target color spaces can be achieved with SRS light generated in a KGW crystal. One such color space is The International Telecommunication Union Radiocommunication (ITU-R) Recommendation  709  also known as Rec.  709 . 
         [0034]    The computer model utilized to calculate the results shown in  FIG. 10  and  FIG. 11  can be used to optimize the Stimulated Raman conversion process and transfer of power in the series of cascaded Raman shifts to longer wavelengths. This enables design of a system that efficiently converts power to higher-order Stokes peaks. It also enables the calculation of the spectral output behavior of the system. This can be utilized to provide a spectrum that is controlled to meet the requirements specific applications such as the DCI specification. This model is a simplification of the general problem of nonlinear processes in crystals. It does not account for four wave mixing effects for example. However the results of the model are in general agreement with experimentally determined results. 
         [0035]    KGW is a biaxial crystal with Raman shifts that are dependent on polarization orientation. The Raman shift is either 768 cm −1  or 901 cm −1 . The 768 cm −1  shift is advantageous for despeckling because minimal peak spacing enables the maximum number of peaks to fit into the visible bands in order to achieve maximum despeckling. The crystal is typically cut to allow propagation along the b-axis. The output wavelength from the Raman crystal may be controlled by an optical waveplate that controls the polarization orientation of the pump laser beam. 
         [0036]    As shown in  FIG. 3 , a green laser source may be constructed with multiple wavelength output in the green and red bands by utilizing a solid-state laser that includes a neodymium or ytterbium-doped crystal (such YAG, YVO 4  or YLF) to provide an IR laser beam at a wavelength of approximately one micron with multiple nonlinear processes occurring in the laser cavity. By placing a nonlinear conversion crystal in the laser cavity it is possible to utilize the high finesse of the IR laser cavity to efficiently convert IR radiation to visible green light. The laser cavity may include several mirrors with high reflectivity at a wavelength near one micron. The high finesse of the laser cavity increases the IR flux in the cavity to provide the intense beam needed for the nonlinear conversion processes. The laser cavity may have curved mirrors or lensing elements that control the laser mode size in the cavity. The laser mode may be shaped to provide a beam size that meets the nonlinear conversion conditions at the location of the nonlinear crystal to create green light. The nonlinear crystal may be constructed from a material such as LBO or potassium titanyl phosphate (KTP). A second nonlinear crystal constructed from a material such as KGW may be placed in the laser cavity such that the visible green light interacts with the KGW to produce additional SRS wavelengths of light in the green and red bands. The laser cavity may have mirrors and/or polarizers that control the green laser light in order to enable the SRS process in the KGW crystal. Specific wavelengths may be extracted from the cavity by a partially-reflective mirror. This mirror may let some or none of the shortest-wavelength visible light escape out of the laser cavity. Stokes-shifted light may be output from the laser cavity by the partially-reflective mirror. A polarization control technique utilizing one or more waveplates and a polarizer in the cavity may also be used to extract the Stokes-shifted wavelengths. The laser may be pumped or energized by a laser-diode assembly that pumps the laser crystal to an excited state which then creates the one micron laser radiation 
         [0037]    Other implementations are also within the scope of the following claims.