3a 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]
4Optimized 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?
13Mechanical 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
18Prelasing & 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]
20Prelasing & Postlasing Postlasing Results from Piezo-Optic Effects.[KD*P & LiNbO3]
21ACOUSTO-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.
22PASSIVE 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.