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

2. Development of a Q-switched pulse laser

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 cm
Round Trip Loss= 5%
What is the optimum Reflection for rear mirror?

5. Design parameters for Q-sw

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

7. Slow Q-Switching

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

12. Mechanical Q-Sw sample:

13. 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

14. Electro-Optical Q-Switching

15. 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.

16. 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]

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

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-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]

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

21. 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.

22. 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.

23. PASSIVE Q-SWITCHING