Free­-running and Q­-switched Nd:YAG laser, laser amplifier and second harmonic generation

Goals

• Set-up and run Nd:YAG laser in free-running mode. Measure energetic, temporal, and spatial laser beam chracteristics.
• Set-up and run Nd:YAG laser in Q­-switch mode. Compare single- and multi-mode regimes. Measure and compare energetic and temporal laser beam chracteristics.
• Measure amplification of optical pulses generated in Q­-switched mode.
• KDP crystal synchronism setup and measure of second harmonic generation efficiency in Q­-switch mode.

Instructions

A. Laser characteristics in multi­-mode free­-running mode

1. Fig. 1.1: The laser kit configuration block scheme for experiments
   Components on the scheme on fig. 1.1 are marked: 1 – He­-Ne laser, 2 – aperture, 3 – prism, 4 – resonator back mirror (100% reflectivity), 5 – aperture, 6 – laser crystal, 7 – resonator output mirror, 8, 9, 10 – beam splitter, energy meter, photodiode (these accessories are not on the draw), 11 – LIF crystal for passive Q­-switching, 12 – amplifier laser crystal, 13 – KDP crystal, 14 – filter absorbing 1.06 μm. (experiments start without crystal 11)
   Set up the center of laser crystal into the He­-Ne laser beam.
2. Set up the laser resonator by beam coupling.
3. Run the laser (according to additional oral tutor instructions) and tune it on maximum output energy and as circular profile as possible by finely tilting resonator mirrors.
4. Record the output beam profile on the photo paper (use black paper for free­-running mode and gray paper for Q­-switched mode) and measure its area S.
5. Read the oscillator source voltage (the conversion table is on the source side) and compute the energy stored in the capacitors (C = 100 μF) representing pumping energy E_b.
6. Find the threshold pumping energy E_th for free­-running mode.
7. Measure the output energy E_out as a function of pumping energy.
8. Observe the generated radiation temporal profile on oscilloscope and estimate generation time τ_FR for various pumping energies:
   1. just above E_th
   2. for middle of interval used E_b
   3. for maximal E_b
   Rem.: for free­-running mode the τ_FR is the complet pulse envelope time

B. Laser characteristics in Q­-switched mode

9. Insert the LIF crystal inside the resonator (using altitudinal feed) to have the He­-Ne laser beam in the center and along the crystal axis.
10. Setup laser amplifier active element to optical axis of justified oscillator. Setup laser generation in transversally multimode regime (without tranversal limitation of resonator)
11. Measure input and output amplifier energy for variable oscillator pumping and compute amplification coefficient G.
12. Repeat instructions 4-9 for Q­-switched mode.
13. Switch to Q­-switched mode.
14. Adjust synchronism angle to optimum by maximum SHG energy in KDP crystal.
15. Adjust angle back and measure conversion efficiency as a function of input energy.

Requested results

1. Two tables for both modes, independent variable E_b, dependent output energy E, efficiency η=E/E_b, surface density of energy W=E/S, power P_avg=E/τ.
2. Graphs E(E_b); W(E_b); τ(E_b); P_str(E_b), in each graph two curves (free­-running and Q­-switched).
3. Two G(E_in) curves in one graph for both laser operation modes.
4. The graph of second harmonic output energy and conversion efficiency as a function of input energy.

References

Yariv, A., Quantum Electronics, chapters 7.3 – 7.4

rare­-earth lasers

MEOS - Q­-switch theory (pages 9,10)

https://www.tau.ac.il/~lab3/72_Laser_res/yag_manual.pdf

Annex

Move cursor over graph to read data. Data by Petr Navrátil, 2014.