LE-0800 Generation of Q-Switch Laser Pulses


  • Nd:Nd:YAG Laser

  • Rate Equation Model

  • Steady State Solutions

  • Time Dependent Solutions

  • Spiking

  • Q - Switch

  • Saturable Absorber

  • Pockels Cell

  • Laser Pulse Width

  • Peak Power

  • Repetition Rate

LE-0800 Generation of Q-Switch Laser Pulses with saturable absorberA. LE-0800 Generation of Q-Switch Laser Pulses with saturable absorber

LE-0800 Generation of Q-Switch Laser Pulses with saturable Absorber and SHGB. Combination of passive q-switch and intra-cavity frequency doubling crystal

LE-0800 Generation of Q-Switch Laser Pulses with Pockels's cellC. LE-0800 Generation of Q-Switch Laser Pulses with Pockels's cell

The use of short-pulse lasers enables the generation of high peak power pulses in a short time, which are useful for the investigation of non-linear effects and for the investigation of time dependent effects e.g. time resolved spectroscopy. In order to achieve extremely high peak power up to the Gigawatt range, laser systems are applied, which possess long lived excited states able to store energy and to emit it in an extremely short time. One of such lasers is e.g. the Nd:YAG laser. With q-switching in so called active or passive mode, it is possible to generate such short pulses. Here, in a first step the theory of laser operation with Nd:YAG is discussed and the steady state as well as time dependent solution of the four level rate equation is analyzed. A two level rate equation model is introduced to explain the saturation behavior of an optical absorber. A saturable absorber (QS) for passive q-switching is introduced. The dynamics of the pulse generation, like repetition rate, pulse width and peak power are determined. The experiment consists of the laser diode pumped Nd:YAG - laser as basic version with an additional passive q-switch (Cr:YAG) crystal. The time dependent signals are displayed and evaluated using an optional oscilloscope. Beside the generation of short pulses, the behavior of the Nd:YAG laser can also be the subject of additional investigations, like measuring the threshold, slope efficiency and so on. By using the optional Pockels cell (AQ) including the high voltage driver (QD), active q-switch can be performed and explored.

Principle of passive q-switch with saturable absorber A. Principle of passive q-switch with saturable absorber

The q-switch crystal is a saturable absorber whose absorption depends on the intensity of the incident light, the higher it is, the less the absorption will be. Placing such a crystal into the Nd:YAG laser cavity will prevent the laser to oscillate. However, the stimulated and spontaneous emission increases and reduces the absorption of the crystal to such an extend, that the laser reaches the threshold and emits a giant pulse. Immediately after the pulse ends, the crystal’s absorption goes up again and prevents any laser action, until the crystal becomes transparent again under the influence of the strong fluorescence light. In this way a periodic pulse emission is created. Since the occurrence of the laser pulse depends on the systems parameter and its dynamics the pulse cannot predicted and thus this method is termed as passive q-switching opposed to the active q-switching where the operator controls the pulse release.


Principle of passive q-switch with an intra-cavity KTP frequency doupler crystal B. Principle of passive q-switch with an intra-cavity KTP frequency doubler crystal

The Figure B shows the combination of a passive q-switch and an intra-cavity KTP for frequency doubling. Due to the very high peak intensity of the fundamental laser at 1064 nm, the peak intensity of the "green" radiation is significantly higher.


Principle of passive q-switch with a Pockels cell C. Principle of passive q-switch with a Pockels cell

The active q-switch consists of a DKDP crystal (potassium di-deuterium phosphate (KD*P = DKDP)). Applying a high voltage to it, a phase retardation results which value depends on the applied voltage. The Brewster plate forces the Nd:YAG laser to oscillate in a fixed polarization. If the retardation of the Pockels cell causes circular polarization, the losses at the Brewster plate prevent the laser oscillation.
LE-0800 Generation of Q-Switch Laser Pulses, consisting of:
Item Code Qty. Description
1 CA-0060 1 Infrared display card 0.8 -1.4 µm
2 CA-0080 1 Optics cleaning set
3 CA-0450 3 BNC connection cable 1 m
4 DC-0040 1 Diode laser controller MK1
5 DC-0120 1 Si-PIN Photodetector, BPX61
6 MM-0020 1 Mounting plate C25 on carrier MG20
7 MM-0060 1 Filter plate holder on MG20
8 MM-0090 1 XY adjuster on MG20
9 MM-0100 1 Target Cross in C25 Mount
10 MM-0462 1 Kinematic mirror mount M16, right
11 MP-0150 1 Optical Bench MG-65, 500 mm
12 OC-0060 1 Biconvex lens f=60 mm in C25 mount
13 OC-0170 1 Collimator 808 nm in C25 mount
14 OC-0950 1 Filter RG1000 50x50x3 mm
15 OC-1070 1 Laser mirror M16, ROC 100 mm, HR @ 1064 nm
16 OM-0052 1 Cylindrical Beam Expander 3x
17 OM-0624 1 Nd:YAG rod in 2 axes kinematic mount
18 OM-0660 1 Cr:YAG passive q-switch, 5 axis mount on MG20
19 OM-L520 1 Diode laser module 808 nm 1W
20 UM-LE06 1 Manual for Nd:YAG Laser
Required Option (order separately)
21 CA-0200 1 Oscilloscope 100 MHz digital, two channel
Option (order separately)
22 LE-0820 1 Active Q-switch Extension
23 OC-0400 1 Adjustable iris mounted in C25
24 OC-1060 1 Laser mirror M16, ROC 100 mm, T 2% @ 1064 nm
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