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

Abstract

A novel Q-switch device enable 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 and 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 laster. This novel combination offers larger modulation loss than otherwise possible with the active modulator alone, and it allows for higher gain build-up 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 id 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.

What is claimed is:  
     
         1 . 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.    
     
     
         2 . The Q-switch device according to  claim 1 , wherein said active modulator is an electro-optic modulator.  
     
     
         3 . The Q-switch device according to  claim 1 , wherein said active modulator is an acousto-optic modulator.  
     
     
         4 . The Q-switch device of any of  claims 1  to  3  wherein said saturable absorber is a. Cr4+: YAG.  
     
     
         5 . The Q-switch device of any of  claims 1  to  4  wherein said active modulator is spaced from said saturable absorber.  
     
     
         6 . A Q-switched solid-state laser comprising, 
 a laser material in a resonant optical cavity;    a Q-switch 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 so as to, in use, reduce mode-beating noise.    
     
     
         7 . The laser of  claim 6  wherein 
 said laser material is side-pumped on a polished flat-sided face by a focused optical pump source having a beam height at said polished face approximately equal to the designed laser mode diameter of an intracavity laser beam;  
 said laser material has an absorption length for the focused optical pump source approximately equal to the radius of the designed laser mode;  
 said focused optical source provides a convergent pump beam upon entering said laser material;  
 a resonant cavity axis is folded by internal reflection within the said laser material, with a fold angle having an apex at a center of said pumped face and an angle of between 20 and 160 degrees; and  
 said side-pumping geometry is such that transverse gain distribution, integrated along said resonant optical cavity axis, has a nearly circular profile symmetry.  
 
     
     
         8 . The laser of  claim 6  or  claim 7  further comprising a pumping source, said pumping source being a semiconductor laser array or laser bar.  
     
     
         9 . The laser of  claim 7  wherein said active modulator is an electro-optic modulator.  
     
     
         10 . The laser of any of  claim 6  to  claim 9  wherein said active modulator is an acousto-optic modulator.  
     
     
         11 . The laser of any of  claim 6  to  claim 10  wherein said saturable absorber is a Cr 4+ : YAG.  
     
     
         12 . The laser of any of  claim 6  to  claim 11  wherein said active modulator is spaced from said saturable absorber.  
     
     
         13 . The laser of any of  claims 1  to  5  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.  
     
     
         14 . The laser of any of  claims 1  to  5  wherein said 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.  
     
     
         15 . 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.    
     
     
         16 . 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 multiplied by the length of the saturable absorber has a value at least that of one-quarter of the wavelength of the fundamental mode.