Abstract:
A diode-pumped solid state pulsed laser includes an intracavity nonlinear crystal for wavelength conversion by difference frequency mixing and a secondary resonant cavity containing an additional nonlinear crystal for parametric amplification. Primary and secondary cavities are capable of injection seeding and wavelength stabilization resulting in a very narrow, stable, and well defined spectral output. The combination of pump diode pulsing, the implementation of the intracavity parametric oscillator and parametric amplifier results in very efficient operation. Optical fiber coupled parametric oscillator byproduct light allows simple and non-invasive wavelength diagnostics and monitoring upon connection to an optical spectrum analyzer.

Description:
FIELD OF INVENTION 
     The present invention, in general, is directed to a mid-wave infrared (MWIR) optical laser system. More specifically, the present invention is directed to a system and method of generating a narrow wavelength mid-wave infrared (MWIR) laser signal using a combination of an optical parametric oscillator (OPO) and an optical parametric amplifier (OPA) intracavity configuration. 
     BACKGROUND OF THE INVENTION 
     Mid-wave infrared (MWIR) lasers are increasingly utilized in many fields, including remote sensing. More specifically, numerous chemical species detection systems, including differential absorption lidar (DIAL) systems, employ MWIR lasers as an active source. 
     Typical conventional MWIR lasers utilized in DIAL systems include a diode-pumped, solid-state, pulsed laser cavity which feeds an independent, non-linear crystal based optical parametric oscillator (OPO) and an optical parametric amplifier (OPA) cavity for wavelength conversion by difference frequency mixing. If the application system requires a very narrow, stable and well defined spectral output, these cavities may be injection seeded and phase locked. The result is a complex laser system that suffers from many deficiencies including: (a) a large number of components which results in a laser system that has a large volume, weight and power consumption; (b) large component costs; (c) poor long term performance; and (d) maintenance difficulties. 
     The present invention, however, provides a system and method for improved efficiency, compactness and ease of maintenance. As will be explained, the present invention uses a diode-pumped solid state laser with an intracavity non-linear OPO and OPA to provide wavelength conversion by difference frequency mixing. 
     SUMMARY OF THE INVENTION 
     To meet this and other needs, and in view of its purposes, the present invention provides a laser system including a first region formed by a first optical cavity for generating a first laser light, and a second region formed by a second optical cavity for generating a second laser light. Also included is an overlap region formed by the first and second regions for mixing the first and second laser light and generating a third laser light, wherein the third laser light is a desired primary output signal. The first region is larger than the second region, and a portion of the second region is located within the first region. 
     The overlap region includes a first nonlinear optical crystal (which may include an optical parametric oscillator (OPO)) for mixing the first and second laser light and generating the third laser light. The second region includes a second nonlinear optical crystal (which may include an optical parametric amplifier (OPA)) for amplifying the third laser light, prior to outputting the third laser light. 
     The first region receives a pump laser signal. The first region includes a rod resonating at a frequency of the first laser light, and the rod converts the received pump laser signal into the first laser light. The first region includes a Q-switch, which converts the first laser light into a pulsed first laser light. The overlap region includes an output port for individually outputting the first, second and third laser light for diagnostic purposes. 
     The first region may receive a first CW seed laser signal for spectrally locking and narrowing the first laser light. The second region may receive a second CW seed laser signal for spectrally locking and narrowing the second laser light. The third laser light may correspond to a spectral feature of a gas or vapor. 
     Another embodiment of the present invention is a mid-wave infrared (MWIR) laser system. The system includes a primary cavity and a secondary cavity, in which a portion of the secondary cavity overlaps into the primary cavity to form an overlapping region. The primary cavity resonates at a first wavelength and the secondary cavity resonates at a second wavelength. The overlapping region mixes the primary wavelength and the secondary wavelength and generates a third wavelength as a primary output signal. The overlapping region includes an optical parametric oscillator (OPO) for mixing the first wavelength light and second wavelength light and producing the third wavelength light, which varies between 2.8 um and 4.8 um. The secondary cavity includes an optical parametric amplifier (OPA) for converting the second wavelength light, which varies between 1.2 um and 1.6 um, into an additional third wavelength light. The primary cavity receives a pump laser signal, which is equal to or less than a 1.0 um wavelength. A 1.0 um rod is located in the primary cavity for converting the received pump laser signal into the first wavelength light of approximately 1.0 um. 
     A Q-switch is located in the primary cavity for converting the third wavelength light into a pulsed third wavelength light. The Q-switch includes driver electronics for controlling pulse width of the pulsed third wavelength light. A 1.0 um seed laser signal is received by the primary cavity for spectrally locking and narrowing the first wavelength light. A 1.5 um seed laser signal is received by the secondary cavity for spectrally locking and narrowing the second wavelength light. The overlapping region includes an output port for directing the first, second and third wavelengths of light to diagnostic equipment. 
     The first cavity is bounded by a high reflectance (HR) device at each end. The second cavity is also bounded by a high reflectance (HR) device at each end. 
     It is understood that the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention may be understood from the following detailed description when read in connection with the accompanying figures: 
         FIG. 1  is a block diagram of a compact, efficient, seeded, mid-wave infrared OPO/OPA laser system, in accordance with an embodiment of the present invention; and 
         FIG. 2  is a mechanical schematic of an exemplary compact, efficient seeded, mid-wave infrared OPO/OPA laser system, in accordance with an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described by referring to elements shown in  FIGS. 1 and 2 . It will be understood that the elements shown or described may take other forms known to those skilled in the art. 
     The present invention provides a pulsed laser light output in the MWIR spectral region at a pulse repetition rate that may vary between 1 Hz and 100 kHz and a pulse width that may vary between 1 and 100 nanoseconds. The spectral line width of the light output is very narrow, measuring less than 1 GHz. 
       FIG. 1  is a block diagram of a laser system, in accordance with an embodiment of the present invention. As shown (and by way of example), laser system  500  generates a 3.4 um MWIR pulsed laser light  130 . The laser system  500  includes a base MWIR laser subsystem, generally designated as  180 , which is compact and efficient. The base MWIR laser subsystem  180  includes a 1.0 um cavity (also referred to herein as a primary cavity), designated as  30 , and a 1.5 um cavity (also referred to herein as a secondary cavity), designated as  110 . As shown, a portion of the 1.5 um cavity  110  is located inside the 1.0 um cavity  30 , resulting in a 1.0 um and 1.5 um cavities overlap region, designated as  140 . This overlap region  140  includes a first nonlinear optical crystal, such as an optical parametric oscillator (OPO) crystal, designated as  90 . As will be explained, it is in this overlap region  140 , also referred to herein as an intracavity, where the primary difference frequency mixing occurs. 
     Laser system  500  also includes a pulsed laser diode source, which provides a wavelength of 1.0 um or less (shown designated as  20 ). As an example,  FIG. 1  shows an 800 nm pulsed laser diode source and electronics module, generally designated as  10 , which produces an 800 nm pulsed pump laser light  20 . The 800 nm pulsed pump laser light  20  is directed into the 1.0 um cavity  30 . Within the 1.0 um cavity  30 , a 1.0 um rod  40  and a Q-switch  50  are connected in series to convert the 800 nm pulsed pump laser light  20  into a 1.0 um pulsed laser light  80  (also referred to as a pump laser light). The Q-switch  50  is controlled by a Q-switch driver signal  70  generated by Q-switch driver electronics module  60 . 
     The 1.0 um pulsed laser light  80  is pumped into the OPO crystal  90 , the latter converting the 1.0 um pulsed laser light  80  into a 1.5 um pulsed laser light  120  and a 3.4 um pulsed laser light  130 . The resulting 3.4 um pulsed laser light  130  is amplified by a second nonlinear optical crystal, for example an optical parametric amplifier (OPA) crystal  100 , which consumes the excess 1.5 um pulsed laser light  120  within the 1.5 um cavity  110 . The frequency mixing of the 1.0 um pulsed laser light  80  and the 1.5 um pulsed laser light  120  within the OPO crystal  90  results in the creation of an idler laser light and a 3.4 um pulsed laser light  130  (also referred to as an output laser light). This 3.4 um pulsed laser light  130  is the primary signal of the base MWIR laser subsystem  180 . It will be appreciated that the primary signal of 3.4 um is exemplary only and may include other wavelengths, as will be explained below. 
     The OPO and the OPA provide the following crystal difference frequency mixing:
 
OPO: 1/λ output =1/λ pump −1/λ idler  
 
OPA: 1/λ output =1/λ idler −1/λ dump  
         having the following wavelength definitions:       

     λ pump : This wavelength is determined by the composition of the laser rod. If the rod is Nd:YAG, the wavelength is 1064 nm. If the rod is Nd:YLF, the wavelength is 1053 nm or 1047 nm, depending on rod crystal axis orientation. 
     λ output : This wavelength may vary depending on the requirements of a specific application and may be anywhere in the range from 2.8 μm to 4.8 μm. In the example shown in  FIG. 1 , the wavelength output is 3.4 um. 
     λ idler : This wavelength may also vary in the range of 1.2 μm to 1.6 μm and is determined by the crystal phase matching conditions of the OPO crystal that control the ratio between λ idler  and λ output . Those conditions include crystal temperature, angle of incidence, and polling period (when the crystal used is periodically polled lithium niobate). 
     λ dump : This wavelength may also vary in the range of 1.6 μm to 3.8 μm and is determined by the crystal phase matching conditions of the OPA crystal that control the ratio between λ idler  and λ dump  Those conditions include crystal temperature, angle of incidence, and polling period (when the crystal used is periodically polled lithium niobate). 
     The phase matching conditions of the OPO and OPA crystals require that the difference frequency mixing equations, shown above, are satisfied simultaneously. The output wavelength (λ output ) of the laser may be tuned by simultaneous temperature and/or angle tuning of the OPO and OPA crystals (for example, using the OPO-OPA thermal controller  150  shown in  FIG. 1 ) 
     In the embodiment illustrated in  FIG. 1  (and  FIG. 2 ), the following wavelengths are used:
         λ pump =1.0 μm,   λ output =3.4 μm,   λ idler =1.5 μm, and   λ dump =2.7 μm.       

     If frequency locking and narrowing of the primary 3.4 um pulsed laser light  130  is desired, additional components may be easily incorporated into laser system  500 , as shown in  FIG. 1 . For example, the 1.0 um pulsed laser light  80  may be spectrally locked and narrowed using an injected 1.0 um continuous wave (CW) seed laser light, generally designated as  200 . This CW seed laser light may be produced and controlled by a 1.0 um seed laser source and electronics module  190 . 
     In addition, if desired, a similar locking and narrowing scheme may be used for the 1.5 um pulsed laser light  120  with a 1.5 um CW seed laser light  220 , the latter produced and controlled by a 1.5 um seed laser source and electronics module  210 . 
     Furthermore, the present invention offers a side benefit in that the 1.0 um and 1.5 um cavities overlap region  140  may produce diagnostic leakage light  230 , as shown in  FIG. 1 . All the different laser signals may be analyzed using test equipment  240 , such as an optical spectrum analyzer or other diagnostic equipment. It is unconventional to have all the different laser signals available at one location, but the present invention provides such an advantage. 
       FIG. 2  is a mechanical drawing of an exemplary compact, efficient seeded, mid-wave infrared OPO/OPA laser system, generally designated as  600 , in accordance with an embodiment of the present invention. As shown, the 800 nm pulsed pump laser light  20  is directed into an 800 nm pump fiber mount  250 . The output from the 800 nm pump fiber mount  250  is a diverging version of the 800 nm pulsed pump laser light  20 . The diverging light is then provided into an 800 nm lens  260 , which again converges the 800 nm pulsed pump laser light  20 . 
     This converging 800 nm pulsed pump laser light  20  now enters the 1.0 um cavity  30 . The physical ends of this 1.0 um cavity are defined by (a) a high reflectance (HR) 1.0 um optic, piezoelectric actuator (PZT) and mount assembly  270  at one end of the cavity, and (b) an HR 1.0 um/3.4 um/99% reflectance (R) 1.5 um optic and mount assembly  320  at the other end of the cavity. Within this 1.0 um cavity  30 , the 800 nm pulsed pump laser Light  20  is converted into a 1.0 um light by the 1.0 um rod  40  (which may, for example, include a Nd: YLF or Nd:YAG crystal). This 1.0 um light is then converted into a 1.0 um pulsed laser light  80  by using a Q-switch  50 , in combination with a 1.0 um ½ waveplate  280  and a 1.0 um ¼ waveplate  300 , as shown. 
     A 1.0 um thin film polarizer (TFP)  290  and an HR 1.0 um/anti-reflective (AR) 1.5 um/3.4 um optic and mount assembly  310  may also be included in this cavity, as shown and described later. 
     The 1.0 um pulsed laser light  80  becomes a new pump laser light which is directed into the OPO crystal  90  (which may, for example, include a periodically poled lithium niobate, or PPLN crystal). Within the OPO crystal  90 , the 1.0 um pulsed laser light  80  is converted into a 1.5 um pulsed laser light  120 , as shown. This 1.5 um pulsed laser light  120  resonates within the 1.5 um cavity  110 , between (a) the HR 1.0 um/3.4 um/99% reflectance 1.5 um optic and mount assembly  320  at one end of the 1.5 um cavity and (b) the HR 1.5 um/AR 3.4 um optic and PZT mount assembly  330  at the other end of the 1.5 um cavity. Within this 1.5 um cavity  110 , the 1.5 um light is converted into a 3.4 um light by the OPA crystal  100 . 
     As shown, the 1.0 um cavity  30  and the 1.5 um cavity  110  share a unique overlapping portion that is also referred to herein as the 1.0 um and 1.5 um cavities overlap  140 . It will be appreciated that the HR 1.0 um/anti-reflective (AR)1.5 um/3.4 um optic and mount assembly  310 , which is disposed in the overlapping region, allows frequency mixing to occur within the OPO crystal  90 . This is due to the fact that assembly  310  allows both the 1.0 um cavity  30  and the 1.5 um cavity  110  to resonate. The frequency mixing produces a 3.4 um pulsed laser light  130 , which through appropriate coatings in the 1.5 um cavity  110  is allowed to exit as a primary output source of the base MWIR laser subsystem  180 , shown in block diagram form in  FIG. 1  and in mechanical form in  FIG. 2 . 
     It will be appreciated that if frequency locking and narrowing of the primary 3.4 um pulsed laser light  130  is desired, a few components may be easily employed. In  FIG. 2 , for example, the 1.0 um pulsed laser light  80  may be locked and narrowed using an injected 1.0 um CW seed laser light  200  (output from module  190  shown in  FIG. 1 ). The 1.0 um CW seed laser light  200  is directed into a 1.0 um seed collimator  350 , which then directs the 1.0 um CW seed laser light  200  into a combination 1.0 um isolator and ½ waveplate  360 . This combination element  360  provides desired polarization and isolation functions. The 1.0 um CW seed laser light  200  is injected into the 1.0 um cavity  30  by way of a 1.0 um lens  380  followed by the 1.0 um TFP  290 . 
     Also included in system  600  of  FIG. 2  (if desired) is a 50% reflectance/1.0 um optic and mount assembly  370 , which allows both leakage levels of the 1.0 um pulsed laser light  80  and the 1.0 um CW seed laser light  200  to be routed into a 1.0 um phase detector  390 . The comparative phase of these two lights may then be measured. The electronics portion of the 1.0 um seed laser source and electronics module  190  ( FIG. 1 ) may then be used to lock the phase of the two lights by using the PZT portion of the high reflectance (HR) 1.0 um optic, PZT and mount assembly  270 . 
     A similar process, if desired, may be repeated for the 1.5 um pulsed laser light  120 . In such case, as shown, the 1.5 um CW seed laser light  220  is directed into a 1.5 um seed collimator  400 , which then directs the 1.5 CW seed laser light  220  into a combination 1.5 um isolator and ½ waveplate  410 , the latter providing polarization and isolation functions. The 1.5 um CW seed laser light  220  is then injected into the 1.5 um cavity  110  by way of the HR 1.0 um/3.4 um/99% reflectance 1.5 um optic and mount assembly  320 . Also present, as shown, is a 50% reflectance 1.5 um optic and mount assembly  420  which allows leakage levels of both the 1.5 um pulsed laser light  120  and the 1.5 um CW seed laser light  220  to be routed into a 1.5 um phase detector  430 . The phase of the these two lights may then be measured, so that the electronics portion of the 1.5 um seed source and electronics module  210  ( FIG. 1 ) may be used to lock the phase of the two lights by using the PZT portion of the HR 1.5 um/AR 3.4 um optic, PZT and mount assembly  330 . 
     In addition, a side benefit of the present invention results from the 1.0 um and 1.5 um cavities overlap region  140  producing diagnostic leakage light  230  for all of the lasers. This leakage light may be analyzed using test equipment, such as an optical spectrum analyzer, or other diagnostic equipment  240  ( FIG. 1 ). A diagnostic collimator  420  may be used, if desired, to collimate the diagnostic light beams prior to reaching the diagnostic equipment. 
     The following table provides an exemplary list of the parts that may be used in the system shown in  FIGS. 1 and 2 . Some of the parts are optional as described above. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                  10 
                 800 nm Pulsed Pump Laser Diode Source 
               
               
                   
                   
                 &amp; Electronics 
               
               
                   
                  20 
                 800 nm Pulsed Pump Laser Light 
               
               
                   
                  30 
                 1.0 um Cavity 
               
               
                   
                  40 
                 1.0 um Rod 
               
               
                   
                  50 
                 Q-Switch (Pockels Cell) 
               
               
                   
                  60 
                 Q-Switch Driver Electronics 
               
               
                   
                  70 
                 Q-Switch Driver Signal 
               
               
                   
                  80 
                 1.0 um Pulsed Laser Light 
               
               
                   
                  90 
                 Optical Parametric Oscillator (OPO) 
               
               
                   
                   
                 Nonlinear Crystal 
               
               
                   
                 100 
                 Optical Parametric Amplifier (OPA) 
               
               
                   
                   
                 Nonlinear Crystal 
               
               
                   
                 110 
                 1.5 um Cavity 
               
               
                   
                 120  
                 1.5 um Pulsed Laser Light 
               
               
                   
                 130  
                 3.4 um Pulsed Laser Light 
               
               
                   
                 140  
                 1.0 um and 1.5 um Cavities Overlap 
               
               
                   
                 150  
                 OPO-OPA Thermal Controller 
               
               
                   
                 160  
                 OPO Thermal Control Signals 
               
               
                   
                 170  
                 OPA Thermal Control Signals 
               
               
                   
                 180  
                 Base MWIR Laser System 
               
               
                   
                 190  
                 1.0 um Seed Laser Source &amp; Electronics 
               
               
                   
                 200  
                 1.0 um Seed Laser Light 
               
               
                   
                 210  
                 1.5 um Laser Seed Laser Source &amp; 
               
               
                   
                   
                 Electronics 
               
               
                   
                 220  
                 1.5 um Seed Laser Light 
               
               
                   
                 230  
                 Diagnostic Leakage Light 
               
               
                   
                 240  
                 Optical Spectrum Analyzer or other 
               
               
                   
                   
                 Diagnostic Equipment 
               
               
                   
                 250 
                 800 nm Pump Fiber Mount 
               
               
                   
                 260  
                 800 nm Lens 
               
               
                   
                 270  
                 High Reflectance (HR) 1.0 um Optic, 
               
               
                   
                   
                 Piezoelectric Actuator (PZT), &amp; Mount 
               
               
                   
                   
                 Assembly 
               
               
                   
                 280 
                 1.0 um ½ Waveplate 
               
               
                   
                 290  
                 1.0 um Thin Film Polarizer (TFP) 
               
               
                   
                 300  
                 1.0 um ¼ Waveplate 
               
               
                   
                 310  
                 HR 1.0 um, AR 1.5/3.4 um Optic &amp; Mount 
               
               
                   
                   
                 Assembly 
               
               
                   
                 320 
                 HR 1.0/3.4 um, 99% R 1.5 um Optic &amp; 
               
               
                   
                   
                 Mount Assembly 
               
               
                   
                 330 
                 HR 1.5 um, AR 3.4 um Optic, PZT &amp; 
               
               
                   
                   
                 Mount Assembly 
               
               
                   
                 340 
                 3.4 um Lens 
               
               
                   
                 350  
                 1.0 um Seed Collimator 
               
               
                   
                 360  
                 1.0 um Isolator &amp; ½ Waveplate 
               
               
                   
                 370  
                 50% R 1.0 um Optic and Mount Assembly 
               
               
                   
                 380  
                 1.0 um Lens 
               
               
                   
                 390  
                 1.0 um Phase Detector 
               
               
                   
                 400  
                 1.5 um Seed Collimator 
               
               
                   
                 410  
                 1.5 um Isolator &amp; ½ Waveplate 
               
               
                   
                 420  
                 50% R 1.5 um Optic and Mount Assembly 
               
               
                   
                 430  
                 1.5 um Phase Detector 
               
               
                   
                 440  
                 Diagnostic Collimator 
               
               
                   
                   
               
             
          
         
       
     
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.