Presentation on theme: "Solid-State Laser Engineering By W.Koechner, 6 th revised and updated ed.2006 بسم الله الرحمن الرحیم"— Presentation transcript:
Solid-State Laser Engineering By W.Koechner, 6 th revised and updated ed.2006 بسم الله الرحمن الرحیم
Development of a Q-switched pulse laser
a typical cw pumped Nd:YAG laser example : L= 50 cm T = 10 % δ = 5% SO: τ c = 22 ns & Q= 39 × 106 * فاکتور Q به طول عمر فوتون و در نتیجه به مشخصات رزوناتور بستگی دارد. 2gl T + δ [Threshold Condition] τ c = t r /Loss =2L/c(T + δ) [Life Time of Photon in Resonator] Q 2πν 0 τ c [Quality Factor]
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? Optimized Design:
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:
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
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.
Electro-Optical & Passive Q-Sw [Pulsed Lasers] Visible & Near IR Acousto-Optical Q-Sw [CW Pumped Lasers] Visible & Near IR Rotating Prisms still uses. [E.g: 3um Er;Lasers] Mid IR Lasers
Prelasing & Postlasing Prelasing: 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]
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: 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.