Abstract:
A method for generating a laser output signal includes the steps of: generating an optical pump signal that is a sequence optical pulses each having a duration of about nτ f , where τ f  represents a flourescence lifetime of a laser dye and 3≦n≦25; directing the optical pump signal into an optical resonant cavity having a laser dye gain element that contains the laser dye for transforming the optical pump signal into an excited optical signal; resonating the excited optical signal in the optical resonant cavity; and emitting a portion of the excited optical signal from the optical resonant cavity.

Description:
BACKGROUND OF THE INVENTION 
     Dye lasers, and particularly organic dye lasers, have certain, unique features. Depending on the specific dye in the laser, the output wavelength is tunable over a bandwidth of approximately 100 nanometers. Dye lasers can operate from the ultraviolet to the infrared, and a single laser resonator cavity can be used to cover this entire wavelength range simply by changing dyes and coatings on the intra-cavity optical components. 
     Threshold pump power for solid-state dye lasers vary depending on the gain material, and the laser cavity design, but are typically several kilowatts. As laser diodes tend to be multi-watt devices, the concept of direct diode pumping of solid-state dye lasers is remote, requiring literally thousands of laser diodes. It would therefore be desirable a system and/or method by which fewer laser diodes would be able to achieve dye laser threshold. 
     SUMMARY OF THE INVENTION 
     The inventive concept may be implemented as a laser that includes: a first optically reflective element; a second optically reflective element opposed to and aligned with the first optically reflective element to define a laser cavity having an optical axis; a laser dye gain element having a laser dye and which is interposed between the first and second optically reflective elements along the optical axis for transforming an optical pump signal into a resonant optical signal; a laser diode system for generating and injecting the optical pump signal into the laser cavity along the optical axis, where the optical pump signal is a sequence of optical pulses having a duration of about nτ f , where τ f  represents a flourescence lifetime of the laser dye, and 3≦n≦25. 
     The inventive concept may also be implemented as a method for generating a laser output signal and includes the steps of: generating an optical pump signal that is a sequence optical pulses each having a duration of about nτ f , where τ f  represents a flourescence lifetime of a laser dye and 3≦n≦25; directing the optical pump signal into an optical resonant cavity having a laser dye gain element that contains the laser dye for transforming the optical pump signal into an excited optical signal; resonating the excited optical signal in the optical resonant cavity; and emitting a portion of the excited optical signal from the optical resonant cavity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an embodiment of a solid-state laser that is pumped by a laser diode. 
         FIG. 2  shows the waveform of an optical pulse that may be used to pump the solid-state laser of  FIG. 1 . 
         FIG. 3  shows a plot of the ratio of the concentration of excited state dye molecules produced after time t after the onset of an optical pump pulse to the steady state concentration of excited state dye molecules 
         FIG. 4  shows an embodiment of a nearly hemispherical optical resonant cavity. 
     
    
    
     Throughout the several views, like elements are referenced using like references. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , there is shown a laser  10  that includes a laser diode  12 , focusing lens  14 , first optically reflective element  16 , second optically reflective element  18 , and dye gain element  20 . The first and second optically reflective elements  16  and  18  are opposed and aligned so as to define an optical resonant, or “laser” cavity  15  having an optical axis a-a. Laser diode  12  generates an optical pump signal  22 , characterized by a wavelength λ 1 . The optical pump signal  22  includes a sequence of optical pulses  23  ( FIG. 2 ), where the pulses may have a periodicity P, and a pulse width or duration as described below. Optical pump signal  22  is focused by lens  14  and directed through first optically reflective element  16  and dye gain element  20 . Lens  14  is selected so as to be made of a material that is highly transparent to optical energy having a center wavelength of λ 1 . For example, fused silica is highly transparent to optical energy having a wavelength 650 nanometers (nm). Dye gain element  20  absorbs pump signal  22  and produces an excited optical signal  24  that resonates along optical axis a-a between the reflective surfaces  17  and  19  of reflective elements  16  and  18 , respectively. In another embodiment, either or both of reflective surfaces  17  and  19  may be flat or curved. By way of example, dye gain element  20  may be implemented so as to include a solid-state host material in which a dye is dissolved. Such solid-state host materials may be selected from the group that includes plastic, porous glass and sol-gels. Although only one laser diode  12  is depicted in  FIG. 1 , it is to be understood that laser  10  may be implemented using any appropriate number of laser diodes  12  which may be configured into an array or otherwise as required to suit the needs of a particular application. 
     Excited optical signal  24  is characterized by a wavelength λ 2  that is highly reflected by reflective surface  17  of optically reflective element  16 , but only partially reflected by reflective surface  19  of optically reflective element  18 . Thus, excited optical signal  24  resonates between optically reflective elements  16  and  18 , and gains energy so as to be “amplified” each time excited optical signal  24  passes through dye gain element  20 . Excited optical signal  24  may, therefore, be referenced as a “resonant” optical signal. Because surface  19  of optically reflective element  18  is only partially reflective of optical energy having a center wavelength of about λ 2 , a laser output signal  26 , which is a fraction, or portion of excited optical signal  24 , is emitted out of the optical resonant cavity  15  through optically reflective element  18  along optical axis a-a. 
     Still referring to  FIG. 1 , in one embodiment, dye gain element  20  may be made of a solid-state plastic host material such as modified polymethyl methacrylate (MPMMA) in which a laser dye is dissolved. Examples of laser dyes suitable for use in conjunction with gain element  18  include rhodamine  700 , oxazine  750 , DOTCI, and oxazine  725 . In one embodiment, the laser dye concentration in gain element  20  may be established so that the gain element  20  absorbs about 85% of the energy of optical pump signal  22 . 
     Referring to  FIG. 2 , optical pump signal  22  may include a series or sequence of pump pulses  23  having a periodicity P. Each pulse  23  has a pulse duration or width of about nτ f , wherein τ f  represents a fluorescence lifetime, or fluorescent time constant, of the laser dye in dye gain element  20 , and 3≦n≦25. Typical fluorescence lifetimes for laser dyes are about 4 nanoseconds. By way of example, P may be in the range of about 1 KHz to 1 MHz. 
       FIG. 3  is a graph of the equation: 
                   n   *       n   ss   *       =     1   -     ⅇ       -   t     /     τ   f             ,         
where n* represents the concentration of excited state dye molecules of a laser dye, and n ss * represents the concentration of excited state dye molecules that would exist if the optical pump pulse were infinitely long, and t represents the time during the pump pulse. In the case where t=3τ f , then
 
                 n   *       n   ss   *       =       1   -     ⅇ     -   3         =     0.950   .             
In the case where
 
               t   =     4   ⁢     τ   f         ,           ⁢       then   ⁢           ⁢       n   *       n   ss   *         =       1   -     ⅇ     -   4         =   0.982       ,         
and in the case where
 
               t   =     5   ⁢     τ   f         ,           ⁢       then   ⁢           ⁢       n   *       n   ss   *         =       1   -     ⅇ     -   5         =     0.993   .               
Thus, it may be appreciated that in applications wherein optical pump signal  22  has a pulse duration in the range of 3τ f  to 25τ f , then the concentration of dye molecules in the excited state is close to that of the steady-state value. Hereinafter each pulse  23  of optical pump signal  22  having a duration in the range of 3τ f  to 25τ f  is referenced herein as a “short optical pulse.”
 
     Laser diodes operate in a “quasi-continuous” wave mode after the first 50-100 ns after being turned on. But in the non-steady-state mode, for the first few tens of nanoseconds, a laser diode emits approximately 50 to 100 times the “quasi-continuous” wave power. Therefore, a 1 watt diode can produce 50 to 100 watts of short pulse power. Since fluorescence lifetimes of a laser dye are typically a few nanoseconds, for example 3-6 ns, an efficient directly diode pumped pulsed solid-state dye laser can be produced by short pulse laser dye excitation wherein each optical pulse is a “short optical pulse,” as defined above. 
     When dye gain element  20  is pumped by a sequence of short optical or excitation pulses  23 , the laser  10  may exceed the threshold for lasing. In contrast, operating the laser diodes  12  in a “quasi-continuous” wave mode may not produce enough power to exceed the lasing threshold of optical resonant cavity  15 . Therefore, by operating laser diodes  12  with a short pulsed optical pump signal, only 10-20 laser diodes  12  may be needed to exceed the lasing threshold for optical resonant cavity  15 , rather than needing thousands of one watt laser diodes operating in a quasi-continuous mode. 
     In  FIG. 4 , there is shown another embodiment of optical resonant cavity  15  wherein second optically reflective element  18  has a partially reflective surface  19  as described above, where concave surface  19  has a radius of curvature, r. Dye gain element  20  is positioned within a distance d 1  of the reflecting surface  17  of first optically reflective element  16 , where d 1  is minimized as far as is practical, as for example, where d 1 ≦200μ. Moreover, the distance d 2  between the center of curvature of reflective surface  19  and the reflective surface  17  of first optically reflective element  16  is within a few millimeters less than the radius of curvature r so that optical resonant cavity  15  provides a nearly hemispherical resonator. 
     Obviously, many modifications and variations of the laser diode pumped solid-state laser described herein 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 specifically described.