3 a typical cw pumped Nd:YAG laser example : τc = tr/Loss =2L/c(T + δ)[Life Time of Photon in Resonator]2gl ≈ T + δ[Threshold Condition]a typical cw pumped Nd:YAG laser example :L= 50 cm T = 10 % δ = 5%SO:τc= 22 ns & Q= 39 × 106* فاکتور Q به طول عمر فوتون و در نتیجه به مشخصات رزوناتور بستگی دارد.Q ≈ 2πν0τc[Quality Factor]
4 Optimized Design: For : Nd:YAG Laser: 100 mj ROD Diameter= 5 mm Resonator L = 30 cmRound Trip Loss= 5%What is the optimum Reflection for rear mirror?
13 Mechanical Q-Sw Advantages: Drawbacks: They are insensitive to polarization and therefore birefringence effects.more energy from the laser can be extractedmultiple pulses.they require frequent maintenance because of the relatively short lifetime of the bearings
18 Prelasing & Postlasing Extinction ratio= max intensity transmitted by Q-Sw/min intensity.Misalignment & weak voltage can decrease the Extinction ratio.Low extinction ratio prelasing damaging the ROD.Prelasing occurs when Gain>Loss introduced by Q-SwPrelasing occure when Q-Sw & polarizor do not act as a perfect shutter.It results a ‘seed pulse’ within a small area of rod where the main pulse will develop rapidly.[HOT SPOT]
21 ACOUSTO-OPTICAL Q-SW In low Gain cw-pumped Q-Switched lasers: Do not need high extinction ratioBut do demand low insertion lossSo using a AR coated fused silica [T>95%] as an acousto optic Q-Sw is the best choice.
22 PASSIVE Q-SWITCHING ADVANTAGES: DRAWBACKS: no H.Vno need for fast driverno need for RF generatorsimple design, small and low costDRAWBACKS:lack of a precision external triggerlower output because of residual absorption and high insertion loss.