Abstract:
Dual-cavity resonators that may be optimized for multiple functions (operating modes). The dual-cavity resonators provide a first set of operating modes that exhibits low repetition rates (5-20 Hz), high energy per pulse, and long, pulse-width, and a second set of operating modes that exhibits high repetition rates (100-2000 Hz), low energy per pulse, and short pulse-width.

Description:
BACKGROUND 
     The present invention relates generally to laser resonators, and more particularly, to an improved dual cavity laser resonator having multiple operating modes. 
     Multi-functionality is typically a requirement for low-cost laser sensors. Multi-functionality many times involves physically opposing laser requirements that are typically solved by compromising performance in any one function. It would be desirable to have a single laser that may be optimized to provide multiple functions. 
     More particularly, new laser sensors, such as those used in military systems, for example, require multiple-functionality from the laser transmitters used therein to reduce the size, weight, and cost of the sensors. When the requirements have physically opposing characteristics, one may take a compromising approach and not meet all relevant requirements, or use separate lasers optimized for each mode of operation. The first approach typically does not meet customers&#39; desires and/or requirements, and the second approach involves a design of a laser that is double the cost and size. It would therefore be desirable to have a single laser that can meet multi-functional requirements using a single laser transmitter. 
     Particularly, it would be beneficial to have a single laser resonator that provides one set of operating modes that provides low repetition rates (5-20 Hz), high energy per pulse, and long, pulse-width, and a second set of operating modes that provides high repetition rates (100-2000 Hz), low energy per pulse, and short pulsewidth. Accordingly, it would be advantageous to have an improved dual cavity multi-functional laser resonator that meets these diverse requirements. 
     SUMMARY OF THE INVENTION 
     The present invention provides for an innovative approach that implements a dual-cavity resonator that allows for a single laser to be optimized for multiple functions (operating modes). The dual-cavity resonator provides a first set of operating modes that exhibits low repetition rates (5-20 Hz), high energy per pulse, and long, pulse-width, and a second set of operating modes that exhibits high repetition rates (100-2000 Hz), low energy per pulse, and short pulse-width. The multi-cavity resonator is an elegant compact and inexpensive solution to implement such multi-functionality. Using the present invention, multi-functional requirements may be met using a single laser transmitter. The present invention may be advantageously employed in laser systems that require multi-mode operation. 
     In one embodiment, the dual cavity laser resonator comprises a diode-pumped slab laser, and first and second cavities that are selectively made operational by a spoiler. An electro-optical Q-switch and an output coupler are common to both cavities. 
     In another embodiment, the dual cavity laser resonator comprises a diode-pumped slab laser, first and second cavities, and a spoiler that selectively inhibits lasing in the selected cavity. The first cavity has a relatively long cavity length and comprises an electro-optical Q-switch and an output coupler. The second cavity has a relatively short cavity length and comprises a passive Q-switch and a partial reflector. The spoiler is selectively disposed in one of the cavities to inhibit lasing in the selected cavity. This embodiment does not require the spoiler to be critically aligned. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which: 
     FIG. 1 illustrates a first exemplary embodiment of a dual cavity laser in accordance with the principles of the present invention; and 
     FIG. 2 illustrates a second exemplary embodiment of a dual cavity laser in accordance with the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, FIG. 1 illustrates a first exemplary embodiment of a dual cavity laser  10 , or laser resonator  10 , in accordance with the principles of the present invention. In particular, FIG. 1 shows a dual cavity Nd:YAG slab laser  20  employing a flip in/out rotatable mirror  23 . The dual cavity slab laser  20  comprises two resonators  10   a ,  10   b  that share a common gain medium  22 , but have different bounce patterns. The operational lasing resonator  10   a ,  10   b  (or cavity  10   a ,  10   b ) is selected by inhibiting the non-lasing resonator  10   b ,  10   a  using the rotatable mirror  23 . 
     Implementations of the dual cavity laser  10  are described in detail below using specific operating parameters, but it is to be understood that the present invention is not limited to these specific operating parameters. In order to produce a laser transmitter capable of generating up to 100 mJ pulses at 10 to 20 Hz with long pulse-widths (15-20 ns) and 40 mJ at 100 Hz with short pulsewidth (&lt;8 ns, for example), the present invention comprises a diode-pumped slab laser 20 along with an output coupler  13 . The approach of the present invention extends existing single cavity resonator technology to provide a dual cavity resonator  10 , which allows the generation of either short (5-8 ns) low energy or long (15-20 ns) high energy pulses, depending on the chosen mode of operation. 
     FIG. 1 illustrates a first exemplary embodiment of a dual cavity laser  10  in accordance with the principles of the present invention. The dual cavity laser  10  comprises a diode-pumped, slab laser  20 . The diode-pumped, slab laser  20  comprises a diode pumped gain medium  20  that is common to both resonators  10   a ,  10   b . The gain medium  22  may comprise a Nd:YAG gain medium  22 . A plurality of pump diodes  24  are provided to pump light into the gain medium  22 . 
     The first embodiment of the dual cavity laser  10  comprises a rear high reflectance reflector  11  (HR) disposed at a first end thereof. An electro-optical Q-switch  12  and an output coupler  13  are disposed between the rear high reflectance reflector  11  and the gain medium  22 . Pulse control electronics  30  are coupled to the diode stack  24  and the electro-optical Q-switch  12 . The pulse repetition rate is controlled by the diode-pumping rate and the timing of the electro-optical Q-switch  12 . 
     A long pulsewidth high energy cavity  10   a  comprises one or more high reflectance reflectors  11  disposed on the opposite side of the gain medium  22  from the electro-optical Q-switch  12  adjacent a second end of the high energy cavity  10   a . A short pulsewidth, low energy/pulse, high repetition rate cavity  10   b  comprises a single high reflectance reflector  11  disposed on the opposite side of the gain medium  22  from the electro-optical Q-switch  12  at the second end of the cavity  10   b.    
     The optical switch  23  or flip in/out rotatable mirror  23  is disposed in the optical paths of the cavities  10   a ,  10   b . If the optical switch  23  or flip in/out rotatable mirror  23  is rotated out of the optical path, the short pulsewidth, low energy/pulse, high repetition rate cavity  10   b  is energized. If the flip in/out rotatable mirror  23  is rotated into the optical path, the long pulsewidth high energy cavity  10   a  is energized. Rotation is illustrated by the dashed arrow. 
     As is shown in FIG. 1, both cavities  10   a ,  10   b  share the rear high reflectance reflector  11 , electro-optical Q-switch  12 , and Nd:YAG slab gain medium  22 . If short pulse operation is desired, the optical switch  23  or rotatable mirror  23  is rotated out of the optical path, which causes laser light to propagate between the rear high reflectance reflector  11  and the high reflectance reflector II at the opposite end of the short pulsewidth, low energy/pulse, high repetition rate cavity  10   b . The short pulsewidth, low energy/pulse, high repetition rate laser beam is reflected off the output coupler  13  as a short pulsewidth, low energy, high repetition rate output beam. The resulting, 5-8 ns, 1.06 μm pulse may be used to pump a nonlinear crystal, such as a KTA OPO or other crystal, for example, to provide 1.5 μm short pulse generation. 
     For long pulse generation (15-20 ns), the long pulsewidth high energy cavity  10   a  is used to provide a longer cavity length. If long pulse operation is desired, the optical switch  23  or rotatable mirror  23  is rotated into the optical path, which causes laser light to propagate between the rear high reflectance reflector  11  and the high reflectance reflector  11  at the opposite end of the long pulsewidth high energy cavity  10   a . The long pulsewidth high energy laser beam is reflected off the output coupler  13  as an long pulsewidth high energy output beam. 
     One drawback of the simple approach shown in FIG. 1 is that optical switch  23  or rotatable mirror  23  when in the “on” position (long resonator mode as implemented in FIG. 1) must be critically aligned to the optical axis of the laser  10 . In other words, the final resting angle of the optical switch  23  or rotatable mirror  23  determines the direction of the laser beam out of the laser  10 . 
     Another implementation is one that does not require the optical switch  23  or rotatable mirror  23  to be critically aligned. One approach that achieves this is shown in FIG. 2, and is shown using specific numbers of components, but is not limited to the specific configuration that is shown. 
     Referring now to FIG. 2 it illustrates a second exemplary embodiment of a dual cavity laser  10  in accordance with the principles of the present invention. In the dual cavity laser  10  shown in FIG. 2, each cavity  10   a ,  10   b  has its own Q-switch  12 ,  14 . In the exemplary dual cavity laser  10  of FIG. 2, an electro-optical Q-switch  12  is used for the long pulsewidth high energy cavity  10   a , and a passive Q-switch  14  is used for the short pulsewidth, low energy/pulse, high repetition rate cavity  10   b . In the second embodiment of the dual cavity slab laser  10 , except for the gain medium  22 , the two cavities  10   a ,  10   b  do not share optical components. 
     More particularly, the long pulsewidth high energy cavity  10   a  of the second embodiment of the dual cavity slab laser  10  comprises high reflectance reflectors  11  at each end of the high energy cavity  10   a . An electro-optical Q-switch  12  is disposed adjacent one of the rear high reflectance reflectors  11 . An output coupler  13  is disposed between the electro-optical Q-switch  12  and the gain medium  22 . The gain medium  22  has a plurality of pump diodes  24  (diode stack  24 ) that couple pump light into the gain medium  22 . Pulse control electronics  30  are coupled to the diode stack  24  and electro-optical Q-switch  12 . The pulse control electronics  30  functions described in the discussion of FIG.  1 . 
     One or more high reflectance reflectors  11  is used on the opposite side of the gain medium  22  adjacent a second end of the long pulsewidth high energy cavity  10   a  to create a relatively long resonator path for the laser light produced by the long pulsewidth high energy cavity  10   a . A flip in/out spoiler  25  is selectively disposed in the long pulsewidth high energy cavity  10   a  in order to selectively inhibit lasing in the long cavity  10   a.    
     The short pulsewidth, low energy/pulse, high repetition rate cavity  10   b  is comprised of a high reflectance reflector  11  at one end and a partial reflector (PR)  26  at the other end of the cavity  10   b . The output from the short pulsewidth, low energy/pulse, high repetition rate cavity  10   b  is provided by the partial reflector  26 . A passive Q-switch  14  is disposed between the high reflectance reflector  11  and the gain medium  22 . The flip in/out spoiler  25  is selectively disposed in the short pulsewidth, low energy/pulse, high repetition rate cavity  10   b  in order to selectively inhibit lasing in the short cavity  10   b.    
     The primary advantage of using two cavities  10   a ,  10   b  is that each cavity  10   a ,  10   b  can be optimized for its specific mission. In particular, the temporal and transverse spatial profile (diameter) of the beam derived from the short cavity  10   b  can be tailored (as flat-topped as possible) for the purpose of efficiently pumping the nonlinear crystal, such as a KTA OPO or other crystal, for example, without having the additional burden of serving as a high quality, long pulse used for designation. After conversion to 1.5 μm, the beam may be combined with the 1.06 m beam using a dichroic element for collinear output. 
     An electronically controlled shutter may be used as the flip in/out spoiler  25  to inhibit (spoil) either the short or long cavity  10   b ,  10   a , depending on the choice of operating mode. Pulse control electronics for the diode stack  24  and electro-optical Q-switch  12  will dictate the output pulse formats for each mode of operation while maintaining constant heat load to the slab gain medium  22  so that vertical lensing in the slab gain medium  22  is invariant in all operating modes, regardless of average power requirements. 
     Thus, improved dual cavity multi-functional laser resonators have been disclosed. It is to be understood that the described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.