Solid-State Laser EngineeringBy W.Koechner, 6th revised and updated ed.2006

سم الله الرحمن بالرحیم

Development of a Q-switched pulse laser

a typical cw pumped Nd:YAG laser example :L= 50 cm T = 10 % δ = 5%SO:τc= 22 ns & Q= 39 × 106

Q* فاکتور

نتیجه به مشخصات رزوناتور بستگی دارد. به طول عمر فوتون و در

2gl ≈ T + δ [Threshold Condition]

τc = tr/Loss =2L/c(T + δ) [Life Time of Photon in Resonator]

Q ≈ 2πν0τc [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:

Slow Q-Switching

Different Q-Switch Techniques

MechanicalElectro-OpticalAcousto-OpticalPassive

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 laserFor decreasing ASEThe peak power in this section is lower.

Electro-Optical Q-Sw

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-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]

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 ratioBut do demand low insertion lossSo 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.

PASSIVE Q-SWITCHING