Suppression of mode-beating noise in a q-switched pulsed laser using novel q-switch device and applications

Abstract

A novel Q-switch device enables significant quality and value improvement for a Q-switched laser system by achieving a significant reduction of mode-beating noise during the pulsed output. The origin of mode-beating noise in a Q-switched laser is a result of high gain availability and amplification of competing standing-waves in formation, whose optical frequency is a product of natural selection via spatial hole burning in the gain medium. The novel Q-switch device employs an active, electro-optics or acousto-optics, Q-switch in combination with a saturable absorber device, to provide an optimized soft opening of the optical path and a controlled timing of a Q-switched laser. This novel combination offers larger modulation loss than otherwise possible with the active modulator alone, and it allows for higher gain build-up and energy extraction efficiency. Specifically, it will enable a low-voltage modulator (<100 V) for high gain (small-signal gain>10) and Q-switched operation at high repetition rate (>10 kHz). The combination is devised to slow down the signal build-up and to sweep the fundamental longitudinal mode frequency at least within the free spectral range of the resonator, such that it varies adiabatically during the Q-switched pulse formation. A laser geometry amenable to high gain and high power is proposed for use in conjunction with the proposed novel Q-switch device. The invention will enable the deployment of cost-effective Q-switched lasers operating in both single-longitudinal and single-transverse (TEM 00 ) mode.

1 . A Q-switched solid state laser comprising: 
 a laser material in a resonant optical cavity; and    a Q-switched device comprising an active modulator capable of producing optical loss at a controlled rate and a saturable absorber within said resonant optical cavity selected and arranged such that a change in refractive index of said saturable absorber during pulse rise time multiplied by the length of the saturable absorber has a value at least that of one-quarter of the wavelength of the fundamental mode.    
   
   
       2 - 4 . (canceled)  
   
   
       5 . The Q-switched device according to  claim 1 , wherein said active modulator is an electro-optic modulator.  
   
   
       6 . The Q-switched device according to  claim 1 , wherein said active modulator is an acousto-optic modulator.  
   
   
       7 . The Q-switched device of  claim 1  wherein said saturable absorber is a Cr4+:YAG.  
   
   
       8 . The Q-switched device of  claim 1  wherein said active modulator is spaced from said saturable absorber.  
   
   
       9 . The Q-switched device of  claim 1  wherein said saturable absorber has a thickness of between one to ten millimeters.  
   
   
       10 . A Q-switched solid state laser comprising: 
 a laser material in a resonant optical cavity;    a Q-switched device comprising an active modulator capable of producing optical loss at a controlled rate and a saturable absorber within said resonant optical cavity selected and arranged such that a change in refractive index of said saturable absorber during pulse rise time results in a frequency chirp of the fundamental mode sufficient to reduce mode-beating noise.    
   
   
       11 . The Q-switched device according to  claim 10 , wherein said active modulator is an electro-optic modulator.  
   
   
       12 . The Q-switched device according to  claim 10 , wherein said active modulator is an acousto-optic modulator.  
   
   
       13 . The Q-switched device of  claim 10  wherein said saturable absorber is a Cr4+:YAG.  
   
   
       14 . The Q-switched device of  claim 10  wherein said active modulator is spaced from said saturable absorber.  
   
   
       15 . The laser of  claim 10  wherein said frequency chirp of the fundamental mode is of the order of at least a few times the free-spectral-range of said resonant optical cavity.  
   
   
       16 . The laser of  claim 10  wherein said saturable absorber has a thickness of between one to ten millimeters.  
   
   
       17 . A Q-switched solid state laser comprising: 
 a laser material in a resonant optical cavity;    a Q-switched device comprising an active modulator capable of producing optical loss at a controlled rate and a saturable absorber within said resonant optical cavity selected and arranged such that a change in refractive index of said saturable absorber during pulse rise time results in a frequency chirp of the fundamental mode of the order of at least a few times the free-spectral-range of said resonant optical cavity.    
   
   
       18 . The Q-switched laser of  claim 17  wherein said saturable absorber has a thickness of between one to ten millimeters.