Research On 266nm Ultraviolet Laser

266nm ultraviolet laser has become a new research direction because of its wide application in material processing,photolithograph,microlithograph,and medical and scientific research field.In this paper,266nm quadrupled Nd:YAG ultraviolet laser used in fingerprint representing has been deeply studied from theory and experiment..The main contents are:Firstly,the development of ultraviolet laser at home and abroad during the last ten years is reviewed and various schemes to generate ultraviolet laser and the corresponding achieved performance index are analyzed in detail.On the basis of the literature,the scheme of Xe-lamp pumped and Cr⁴⁺:YAG passively Q-switched Nd:YAG laser is adopted,its repetition rate is adjustable from 1,3,5,10 to 20Hz.Then the laser is extra-cavity frequency doubled with KTP crystal and quadrupled with BBO crystal to obtain 266nm ultraviolet laser.Secondly,the principle of Cr⁴⁺:YAG passive Q-switching is discussed.The operating and optimal model for passively Q-switched laser is established by using slow-recovery saturable absorber with excited-state absorption.The operating rate equation of passively Q-switched laser is also established.Then Q-switching criterion which must be satisfied to obtain good giant-pulse output and analytical expression of output characteristic parameter for passively Q-switched laser are prescnted.The optimal operating conditions for passively Q-switched laser by selecting suitable absorber parameter and output mirror transmission under certain pumping conditions are also solved.Then the phase-matching mode and angle of uniaxial and biaxial crystals are discussed. On the basis of frequency-doubling efficiency formula,the factors to influence the efficiency and the methods to improve the efficiency are analyzed.Calculation theory of allowed parameter during the three-wave-interaction in nonlinear crystal is discussed as well.Then the phase-matching mode is presented and angle for the frequency-doubling crystal is calculated based on the formulas and curves of phase-matching angle and effective nonlinear coefficient. The walk-off angle during frequency doubling is also analyzed. Basing on passive Q-switching and frequency-doubling theory,we have designed and developed an ultraviolet laser generating 266nm laser.The laser is Cr⁴⁺:YAG passively Q-switched and Xe-lamp pumped Nd:YAG laser,whose repetition rate is adjustable from 1,3, 5,10 to 20Hz;the initial transmission ratio of 3mm Cr⁴⁺:YAG crystal is 30%.Then the laser is extra-cavity frequency doubled with KTP crystal and quadrupled with BBO crystal to obtain the mixture of three wavelengths.Beam splitting prism has been designed by using software to draw figure to obtain 266nm ultraviolet laser.The optical conversion efficiency from 1064nm to 532nm is 44%,and maximum output energy per pulse is 54.2mJ;the optical conversion efficiency from 532nm to 266nm is 23%,and maximum output energy per pulse at 266nm is 12.5mJ,pulse width of 266nm laser is 12ns.Combined with ultraviolet integral receiver and fingerprint detector,the designed 266nm ultraviolet laer has been applied as light source to ultraviolet laser fingerprint detecting system.The fingerprint extraction rate reaches 65%,and fingerprint 30 days before still can be detected.