Abstract:
A green laser is made by constructing a laser cavity that includes within it a laser diode, a non-linear crystal such as KTP, and a lens. The boundaries of the cavity are not defined by the ends of the diode itself, but are instead defined by the ends of an apparatus that includes the lens and the KTP or similar crystal.

Description:
TECHNICAL FIELD 
     This invention relates to lasers, and more specifically, to an improved green laser for use in both commercial and military applications. 
     BACKGROUND OF THE INVENTION 
     Green lasers are advantageous over more widely used red lasers for a variety of reasons. Green lasers are easier on the human eye, more perceptible to a user, and more capable of providing clearer, projected images for users. 
     Most green lasers, a prior art example of which is shown in  FIG. 1 , suffer from a variety of drawbacks. First, power efficiency is relatively low, typically about 7% or less. Second, such lasers are only operable among very limited temperature ranges. Additionally, modulation speed is very limited. Other potential drawbacks of present green laser technologies include heat, cost and package size. 
       FIG. 2  shows a typical structure for a prior art green laser diode. A lens  202  is positioned in front of a laser diode  201  with so that the laser diode  201  excites a crystal to emit light at 1064 nanometers. The 1064 nanometer light is used to excite a nonlinear crystal such as KTP crystal, to generate second harmonic wave at 532 nanometers. This green laser light is then processed through beam expansion optics  205  as shown in  FIG. 2 . This prior arrangement suffers from several of the above described problems, including increased heat generation and package size. 
     An additional prior art green laser arrangement is shown in  FIG. 3 . A 1064 nanometer laser diode is arranged so that its output is processed through lenses  302  and focused on a PPMgLN crystal waveguides on a temperature controlled plate. The output from the nonlinear optical waveguide is processed through a collimating lens and color-filter  305 , resulting in a green laser output beam via single pass second harmonic generation. 
     The arrangement shown in  FIG. 3  is also subject to certain drawbacks. For example, there is a narrow temperature range over which such a device can operate, after which performance and power drastically degrade. Moreover, there are a large number of parts as shown in  FIG. 3 , some of which can be expensive. Further, in order to obtain sufficient power conversion efficiency, it is critical that all of the components be aligned properly, resulting in a complex and expensive manufacturing process, that utilizes many expensive parts. 
     There exists a need in the art for a more efficient and cost effective technology for producing green lasers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a prior art arrangement for generating green laser light; 
         FIG. 2  is a block diagram some further details of the prior art arrangement of  FIG. 1 ; 
         FIG. 3  depicts another green laser system of the prior art; 
         FIG. 4  shows a conceptual diagram of a green laser system in accordance with the present invention; 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In accordance with an embodiment of the present invention, a laser diode is positioned within a cavity that includes a non-linear optical crystal. The cavity that causes the lasing is positioned between rear side of the actual laser diode, and front side of a non-linear crystal. The arrangement permits the generation of a green light laser beam with an easier manufacturing technique. The method is extendable to other color lasers. 
     Shown at  FIG. 4  is a monitor photodiode  401  to the left of a laser diode  402 . An optional lens  403  is depicted to focus the light from the laser diode  402  onto a non-linear optical crystal  404 . The surfaces labeled A-F are properly selected to form a cavity that causes the green laser light. Specifically, surface A is high reflectivity at both the first and second harmonics of interest. A monitoring photodiode  401  is positioned to the left of the diode  402 , as shown. The monitoring photodiode detects a small amount of light from the diode  402 , and uses such detected light for diagnostic and control purposes such as automatic power control. 
     The surface at exhibit B is anti-reflection (AR) coated at both the fundamental and second harmonic wavelengths which simply permits the passage of light. Unlike a conventional laser, the surface of exhibit B is not set up to be partially reflective and cause the lasing because the space between surfaces A and B do not act as the laser cavity as in conventional products. The diode  402  is the gain media in this intra-cavity laser and second harmonic generation device. 
     A lens  403  is AR coated at both the fundamental and second harmonic wavelengths and focuses the output light on a non-linear optical crystal which produces the green light. The exact green laser wavelength is determined by laser gain media and corresponding cavity design. For the gain media with cavity that produce fundamental wavelength from 1040 nanometer to 1064 nanometer; the second harmonic wave produces green laser beam light from 520 nanometer to 532 nanometer. The crystal  404  may be a non-linear optical (NLO) crystal such as KTP (Potassium Titanyl Phosphate KTiOPO), although other such crystals may be used as well. 
     A surface E shown at  FIG. 4  is antireflective (“AR”) coated at both the fundamental and second harmonic wavelength. Light energy passes through that coating and through nonlinear optical crystal  404 , and encounters the surface  405  labeled F, which, together with surface  410  labeled A, form the laser cavity within which the light wave resonated before exiting. Surface F is high reflection (HR) coated at the fundamental wavelength and partially-transmission coated to output laser light energy at the second harmonic wavelength. 
     In operation, light emanates from the surface F at the second harmonic laser wavelength. Additionally, residual energy emanates at the fundamental laser wavelength because the high reflective coating at the fundamental wavelength is not one hundred percent reflective. Assuming the laser diode has its peak gain at 1040 nanometer to 1064 nanometer, the second harmonic output is in the visual green range, 520 nanometer and 532 nanometer. 
     Prior art arrangements typically include a either a pump laser to generate the fundamental wavelength at infra-red (IR), such as 1064 nanometer, or an IR pump laser such as a DBR laser 1064 nanometer to generate green in a single pass, second harmonic generation. However, various embodiments of the present invention actually encapsulate the non-linear optical crystal within the cavity, and also include an optional lens  403 , depending upon the balance of cost and electro-optical efficiency requirements. Also, the present invention includes a semiconductor diode  402  within the cavity in between surfaces A and F of  FIG. 4  and the diode provides optical gain. The optical oscillation wave within the cavity that sets up the laser beam has its endpoints at surfaces  410  and  405  (A and F). 
     The preferred laser diode  402  is in the near infra-red range, say 1040-1064 nanometer, in order to generate green although this is not critical to the present invention. If the gain of the laser diode and its corresponding HR coating are properly chosen to be near 920 nanometers, the invention can be applied to generate blue laser output. Other wavelengths may be used as well. By using such a laser diode  402 , electro-optical power conversion efficiency of about 20-25 percent can be achieved at the second harmonic wavelength, such percentage being measured as the fraction of input electrical power that appears as output light power at the second harmonic wavelength. 
     In addition to nonlinear crystal KTP, it has been found that other crystals such as GdCa4O(BO3)3, GdxY1-xCa4O(BO3)3 may also be used for wider temperature operation. 
     The length of the laser cavity, especially the length the nonlinear optical crystal is determined by the optimal conversion efficiency and power requirements. Additionally, the coatings for each of the surfaces indicated in  FIG. 4  as A, B, C, D E and F may be as follows, with each representing an example only: Surface A, HR coating for both fundamental (ω) and second harmonic waves (2ω), Surface B through E, AR coating for ω and 2ω waves and surface F, HR coating at ω) and partial transmission coating at 2ω waves. 
     The laser may be any type including of near IR wavelength. As is known in the art, near IR is above about 700 nm to below about 2000 nm. 
     Output filter  406  could be mounted inside a package housing and HR coated at surface G fundamental wavelength to rid of residual fundamental laser output from the laser cavity and AR coated the surface G at second harmonic wavelength. Surface H of the filter  406  can be AR coated for both fundamental and second harmonic wavelength. Alternatively, monitor photodiode  401  can be placed sideways so that reflections from filter can be used to monitor laser output to avoid significant tracking error due to a laser output front and back emission power differences. 
     While the above describes the preferred embodiment of the present invention, various other embodiments will be apparent to those of skill in the art. The scope of the invention therefore, is defined by the following claims.