Abstract:
A monoblock laser cavity incorporates optical components for a short-pulse laser. These optical components are ‘locked’ into alignment forming an optical laser cavity for flash lamp or diode laser pumping. The optical laser cavity does not need optical alignment after it is fabricated, increasing the brightness of the monoblock laser.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a divisional patent application of copending application Ser. No. 12/848,272, filed Aug. 2, 2010, entitled “Beam Quality of the Monoblock Laser Through Use of a 1.5 Micron External Cavity Partial Reflector.” The aforementioned application is hereby incorporated herein by reference. 
    
    
     GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, sold, imported, and/or licensed by or for the Government of the United States of America. 
    
    
     FIELD OF THE DISCLOSURE 
     The disclosure relates to a monoblock laser cavity having optical components for a short-pulse laser. 
     BACKGROUND INFORMATION 
     Laser range finders are an increasingly vital component in high precision targeting engagements. The precise and accurate range to target information is an essential variable for fire control of weapons. This information is easily, and timely, provided by laser range finders. 
     Unfortunately, known laser range finders are bulky, heavy and expensive. These laser range finders were not developed with the individual field use in mind. 
     Monoblock laser makes the development/fabrication of a very low cost, compact laser range finder feasible. Unfortunately, the beam divergence of known monoblock lasers is rather large (typically between 8 and 14 mRad). Such a laser has a fairly low brightness, wherein a sizable optic is needed to collimate the monoblock laser output. 
     SUMMARY 
     A monoblock laser cavity having optical components is disclosed for a short-pulse laser. One exemplary embodiment of a monoblock laser cavity includes a gain medium having one coated end surface, a juncture in the medium, and another end surface; a passive Q-switch having one end surface optically facing said another end surface of the gain medium; and an optical parametric oscillator crystal having one end surface and an output face. The one end surface of the optical parametric oscillator crystal optically is configured to face another end surface of the Q-switch. An output coupler is placed on the output face of the optical parametric oscillator crystal. Such a monoblock laser can improve the brightness and decrease the beam divergence of the monoblock laser. 
     In one aspect, an exemplary embodiment of a monoblock laser cavity arrangement can be based on an external cavity partial reflector. Such an exemplary arrangement comprises an Nd:YAG gain medium having one coated end surface, a juncture in the medium having a Brewster&#39;s angle for polarization, and another end surface; a passive Q-switch having one end surface optically facing said another end surface of the gain medium; an optical parametric oscillator crystal having one end surface and an output face, said one end surface of the optical parametric oscillator crystal optically facing another end surface of the Q-switch, wherein an output coupler is placed on said output face of the optical parametric oscillator crystal; and an external cavity partial reflector having one end surface disposed to optically face said output coupler. 
     Yet, another exemplary embodiment of a monoblock laser cavity arrangement can be based on a curved-surface external cavity partial reflector. Such an exemplary arrangement comprises an Nd:YAG gain medium having one coated end surface, a juncture in the medium having a Brewster&#39;s angle for polarization, and another end surface; a passive Q-switch having one end surface optically facing said another end surface of the gain medium; an optical parametric oscillator crystal having one end surface and an output face, said one end surface of the optical parametric oscillator crystal optically facing another end surface of the Q-switch, wherein an output coupler is placed on said output face of the optical parametric oscillator crystal; and a curved-surface external cavity partial reflector having said curved-surface disposed to optically face said output coupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the disclosure will become readily apparent in light of the detailed description and the attached drawings, wherein: 
         FIG. 1  depicts an exemplary embodiment of a monoblock laser cavity as disclosed; 
         FIG. 2  shows an exemplary embodiment of a monoblock laser cavity arrangement having an external cavity partial reflector for improved beam quality; and 
         FIG. 3  shows another exemplary embodiment of a monoblock laser cavity arrangement having a curved external partial reflector for improved beam quality. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an exemplary monoblock laser cavity. It is shown as a flat-flat or stable resonator configuration. As configured in relation to a YAG optical bench  150 , an Nd:YAG gain medium  110  has one end surface  111  coated to have a surface optical property, e.g., High-Reflection Coating of HR@1064 nm; and a juncture  112  in the medium  110  having a Brewster&#39;s angle for polarization. A passive Q-switch  120  (e.g., Cr4+:YAG passive QSw) has one end surface optically facing another end surface  113  of the Nd:YAG gain medium  110 . An optical parametric oscillator (OPO) crystal  130  is configured in relation to another end of the YAG optical bench  150 , one end surface  131  of the OPO crystal  130  being optically facing another end surface of the Q-switch  120 . The one end surface  131  of the OPO crystal  130  can have surface coatings, e.g., Anti-Reflection Coating of AR@1064 nm, and High-Reflection Coating of HR@1570 nm. Such an exemplary configuration can be acutely sensitive to angular deviations of the mirrors from the optical axis. It can also allow high order modes of lasing to degrade the beam quality. 
     An ouput coupler  132  can be placed on the output face of the OPO crystal  130 . The output coupler  132  can consist of coatings for the OPO cavity  130  as exemplified in  FIG. 1 . Coatings on the output face of the OPO cavity  130  can therefore serve as the output coupler  132  of the 1064 nm pump cavity as shown in  FIG. 1 . For example, the output face of the OPO crystal  130  can have High-Reflection Coating of HR@1064 nm, and PR@1570 nm, which serve to function as an output coupler  132 . As  FIG. 1  also shows, the alignment of the OPO conversion cavity is solely due to the tolerance achieved in the fabrication process of the OPO crystal  130  (how well the face-face parallelism is) since the OPO cavity coatings are processed onto the crystal faces (e.g.,  131  and/or  132 ). This leads to a simple alignment of only the 1064 nm pump cavity of the monoblock  100 . The pump cavity can be aligned fairly well in order to produce an appreciable output. In contrast, a malalignment of the OPO cavity  130  can lead to a poor output beam quality. 
     An Improved Beam Quality of the Monoblock Laser 
       FIG. 2  shows an exemplary embodiment of a monoblock laser cavity arrangement  200  having an external cavity partial reflector  240  for improved beam quality. For example, use of an exemplary 1.5 micron external cavity reflector  240  is depicted in  FIG. 2 . It is comprised of all the same optical components except that a new component, an external cavity partial reflector  240 , is added. For example, as configured in relation to a YAG optical bench  250 , an Nd:YAG gain medium  210  has one end surface  211  coated to have a surface optical property, e.g., High-Reflection Coating of HR@1064 nm; and a juncture  212  in the medium  210  having a Brewster&#39;s angle for polarization. A passive Q-switch  220  (e.g., Cr4+:YAG passive QSw) has one end surface optically facing another end surface  213  of the Nd:YAG gain medium  210 . An optical parametric oscillator (OPO) crystal  230  is configured to define a 1 st  OPO cavity, one end surface  231  of the OPO crystal  230  optically facing another end surface of the Q-switch  220 . The one end surface  231  of the OPO crystal  230  can have surface coatings, e.g., Anti-Reflection Coating of AR@1064 nm, and High-Reflection Coating of HR@1570 nm. 
     As further exemplified in  FIG. 2 , an output coupler  232  can be placed on the output face of the OPO crystal  230 . The output coupler  232  can consist of coatings for the OPO cavity  230  as exemplified in  FIG. 2 . Coatings on the output face of the OPO cavity  230  can therefore serve as the output coupler  232  of the 1064 nm pump cavity as shown in  FIG. 2 . For example, the output face of the OPO crystal  230  can have High-Reflection Coating of HR@1064 nm, and PR@1570 nm, which serve to function as an output coupler  232  optically facing the external cavity partial reflector  240  to produce a 1.57 μm output. 
     One end surface of the external cavity partial reflector  240  facing the output coupler  232  can have a surface coating, e.g., Anti-Reflection Coating of AR@1570 nm. Another end surface of the external cavity partial reflector  240  can a surface coating, e.g., PR@1570 nm to produce 1.57 μm improved beam output. As exemplified, the external cavity partial reflector  240  serves two purposes. First, it lengthens the Optical Parametric Oscillator (OPO) cavity (e.g., from the 1 st  OPO cavity length to the 2 nd  OPO cavity length as exemplified in  FIG. 2 ) which lowers the total number of longitudinal modes capable of being supported within the laser resonator. The second, and more important purpose, is that the external cavity partial reflector  240  is aligned to ONLY the back reflector of the OPO resonator. 
     In contrast, the output coupler  132 , which is placed on the output face of the OPO crystal  130  as seen in  FIG. 1  (original monoblock laser), is optically aligned with the 1064 nm ‘pump’ cavity and alignment to the OPO conversion cavity is fixed by crystal fabrication! The output coupler is thus seldom ‘perfect’ (due to real world fabrication inaccuracies) for both cavities as configured in  FIG. 1 . 
     The external cavity partial reflector  240 , being optically aligned to only the OPO cavity&#39;s back reflector, can optimize the OPO resonator&#39;s performance. Optimal performance can be achieved by aligning only to the OPO back reflector and by creating a longer OPO cavity (as seen  FIG. 2 ). This greatly reduces the number of higher order lasing modes generated which leads to a much improved beam quality. 
     A Monoblock Laser Cavity Arrangement with a Curved External Partial Reflector for Improved Beam Quality 
     The monoblock laser cavity arrangements discussed above related to a flat-flat cavity. Alternatively, a curved surface  341  can be added to an exemplary external cavity partial reflector as shown in  FIG. 3 . This will make the cavity an unstable cavity which would make it less sensitive to angular deviations of the mirrors with respect to the optical axis. Such an alternative arrangement can also mitigate the number of modes allowed to propogate within the laser cavity (the higher order mode are subjected to more loss) so the output leaving the laser cavity consists of the lower order modes for a better beam quality. 
       FIG. 3  shows such an exemplary embodiment of a monoblock laser cavity arrangement  300  having a curved-surface external partial reflector  340  for improved beam quality. Specifically, use of such an exemplary 1.5 micron (or 1.57 μm output) external cavity reflector  340  having a curved surface  341  is depicted in  FIG. 3 . For example, as configured in relation to a YAG optical bench  350 , an Nd:YAG gain medium  310  has one end surface  311  coated to have a surface optical property, e.g., High-Reflection Coating of HR@1064 nm; and a juncture  312  in the medium  310  having a Brewster&#39;s angle for polarization. A passive Q-switch  320  (e.g., Cr4+:YAG passive QSw) has one end surface optically facing another end surface  313  of the Nd:YAG gain medium  310 . An optical parametric oscillator (OPO) crystal  330  has one end surface  331  of the OPO crystal  330  optically facing another end surface of the Q-switch  320 . The one end surface  331  of the OPO crystal  330  can have surface coatings, e.g., Anti-Reflection Coating of AR@1064 nm, and High-Reflection Coating of HR@1570 nm. 
     As further exemplified in  FIG. 3 , an ouput coupler  332  can be placed on the output face of the OPO crystal  330 . The output coupler  332  can consist of coatings for the OPO cavity  330  as exemplified in  FIG. 3 . For example, the output face of the OPO crystal  330  can have High-Reflection Coating of HR@1064 nm, and PR@1570 nm, which serve to function as an output coupler  332  optically facing the external cavity partial reflector  340  to produce a 1.57 μm output. The curved end surface  341  of the external cavity partial reflector  340  facing the output coupler  332  can have a surface coating, e.g., PR@1570 nm. Another end surface  342  of the external cavity partial reflector  340  can have a surface coating, e.g., Anti-Reflection Coating of AR@1570 nm to produce a 1.57 μm improved beam output. 
     The overall output energy of such an alternative exemplary embodiment of the monoblock laser cavity with an external cavity partial reflector may be slightly less than that of the other exemplary embodiments of monoblock laser cavity (how much depends on the amount of 1.5 micron reflection selected for the external cavity partial reflector (from 10% to 80% for typical monoblock laser cavities). But the achievable far field beam divergence can be significantly less to yield an overall increase in the laser&#39;s ‘brightness’. Accordingly, a smaller afocal can be used in a laser range finder system incorporating such embodiments. 
     The various embodiments as disclosed can improve the brightness of the monoblock laser (tighter beam divergence). The tighter beam divergence (improved brightness) of the improved monoblock laser allows for use of a smaller diameter optic to collimate the laser output for use in a laser range finder. 
     The various exemplary embodiments can be small with minimal impact to the monoblock. They can utilize known bonding techniques for monoblock construction. 
     The monoblock laser with improved beam quality through use of a 1.5 micron external cavity partial reflector is still a simple module that requires none of the labor extensive alignment procedures as alternative laser range finder solid state laser sources. No optical holders have to be fabricated, no complex engineering is required to design the optical cavity, and no precise laser cavity alignment(s) are required. Production labor and material costs are greatly reduced. 
     The improved monoblock laser cavity is a modular component. The modularity lends to ease of configuration for different pump sources. It can be incorporated in a flash lamp pumped or laser diode pumped system. 
     The various exemplary embodiments may be used as the laser source in very compact laser range finders. For example, they cant generate eye safe laser output for eye safe laser range finding. These laser range finders can have both military and commercial applications. The compact configuration of the improved monoblock laser cavity also lends itself to placement in other laser-based portable/hand-held devices. These may be medical devices, industrial tools or scientific equipment that would benefit from the size/weight reduction, dependable performance, and low cost. 
     It is obvious that many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as described.