Q-Switching بسم الله الرحمن الرحیم Solid-State Laser Engineering By W.Koechner, 6th revised and updated ed.2006

Development of a Q-switched pulse laser

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]

Optimized Design: For : Nd:YAG Laser: 100 mj ROD Diameter= 5 mm Resonator L = 30 cm Round Trip Loss= 5% What is the optimum Reflection for rear mirror?

Design parameters for Q-sw

Optimize Design E.g: Nd:YAG Laser: 100 mj ROD Diameter= 5 mm Resonator L = 30 cm Round Trip Loss= 5% So: And Optimum Reflectivity:

Slow Q-Switching

Different Q-Switch Techniques Mechanical Electro-Optical Acousto-Optical Passive

Mechanical Q-Sw sample:

Mechanical Q-Sw Advantages: Drawbacks: They are insensitive to polarization and therefore birefringence effects. more energy from the laser can be extracted multiple pulses. they require frequent maintenance because of the relatively short lifetime of the bearings

Electro-Optical Q-Switching

An important Qustion: Why we place Q-Sw assembly in front of Rear mirror? For Achieving a better hold-off of laser For decreasing ASE The peak power in this section is lower.

Visible & Near IR Electro-Optical & Passive Q-Sw [Pulsed Lasers] Acousto-Optical Q-Sw [CW Pumped Lasers] Mid IR Lasers Rotating Prisms still uses. [E.g: 3um Er;Lasers]

Electro-Optical Q-Sw Electro-Optic Q-Sw Pockels cells [in crystals] KDP & KD*P [KH2PO4] LiNbO3 Kerr cells [in liquids]

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-Sw Prelasing 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]

Prelasing & Postlasing Postlasing Results from Piezo-Optic Effects.[KD*P & LiNbO3]

ACOUSTO-OPTICAL Q-SW In low Gain cw-pumped Q-Switched lasers: Do not need high extinction ratio But do demand low insertion loss So using a AR coated fused silica [T>95%] as an acousto optic Q-Sw is the best choice.

PASSIVE Q-SWITCHING ADVANTAGES: DRAWBACKS: no H.V no need for fast driver no need for RF generator simple design, small and low cost DRAWBACKS: lack of a precision external trigger lower output because of residual absorption and high insertion loss.

PASSIVE Q-SWITCHING