PHTN1500 Advanced Laser Theory

PHTN1500 Advanced Laser Theory

Lab #1 - Lasing Threshold and Mathematical Model of a Solid-State Laser
(2011F)

Introduction

The threshold of pump power for a long-pulse YAG laser is determined and mathematical models developed to determine the required power under various optical configurations.

SAFETY WARNING
This lab uses a YAG laser with high pulse energies - enough to cause retinal damage with a single pulse. Safety glasses suitable for 1064nm YAG radiation must be worn at all times during the experiment. The laser must be considered in a "ready to fire" state at all times when either of the high voltage indicators is lit.

Prelab

Read chapter 12 from Csele on Solid State Lasers (esp. flashlamp pumping) and chapter 5 (5.10). In class, we will discuss the parameters of this specific laser and the mathematical model involved.
Download the Wheel Transmission data for the inserted-loss neutral-density filter.

Experiment

The laser system will be configured as seen here in this photo of the prototype setup. Turn the HeNe (alignment) power supply ON and ensure the output beam will strike the center of the detector.

This modified Control Laser CL-5 laser utilizes two Sorenson DCR300-1.5B power supplies, one for the trigger supply and the second for the main capacitor. The trigger supply is set for a constant 250 volts - this ensures a healthy trigger potential which will fire the lamp even when voltage at the main terminals is lower than the minimum specified for the lamp (250 VDC). Voltage across the main capacitor (1200 microFarads) may be varied which will, in turn, vary discharge energy according to the relation E=0.5*C*V².

The energy meter, pictured here, will capture the energy of a single pulse.

To measure the threshold of lasing:

Select the minimum inserted loss
Select ENERGY mode on the meter (the JOULES indicator will illuminate)
Adjust the main capacitor voltage for 200 volts
Press ENERGY RESET on the meter to zero it
Adjust the OFFSET to read zero (to compensate for the HeNe targeting beam)
Fire the laser. Pulse energy will be displayed

Now, Increase the voltage across the capacitor in 10V increments, RESET the meter, and fire the laser again until laser output is observed. Continue until maximum power supply voltage is reached (> 300V). Graph output pulse energy versus capacitor energy (in Joules) to determine the pumping threshold for this particular laser.

Now, set the filter wheel for a higher density (the wheel is inside the optical cavity of the laser between the rod and the HR). Repeat the experiment with the new inserted loss - obviously the threshold energy will be larger now. Repeat again with a higher loss until the laser finally ceases to oscillate even at maximum voltage.

Assignment

Hand In a WORD PROCESSED (not handwritten) lab assignment as follows:

To be done individually ...

Hand-in graphs of E_optical (Joules) vs. flashlamp energy (Joules) for each case (with various inserted losses). A single graph with multiple curves shown is also acceptable.
1. Identify the threshold pump energy on each graph.
2. Calculate g_th for each case of a different inserted loss and plot g_th versus pump threshold energy (Joules).
3. Research "slope efficiency" and compute it for the basic laser with minimum inserted loss.
Follow-through example 5.10.1 in Csele (Page 150). Compute the expected pump threshold energy as follows:
1. Develop an expression for ΔN of the laser with minimum inserted loss (use equation 5.8.1) - to generate this inversion the input energy is that which was determined experimentally in this experiment.
2. Modify the expression for ΔN which includes the inserted losses (by modifying the gain equation and computing ΔN). Compare this inversion to that determined in (a) above and multiply by the energy in (a) above by the same ratio of ΔN (with inserted loss)-to-ΔN to determine the predicted pump energy. This assumes that the inversion generated (ΔN) is proportional to pump energy.
3. Summarize the results in a chart: three columns with various losses, each column including predicted and actual threshold pump energy
1. As per the methodology of problem 4.3, compute the number of Nd³⁺ ions which must be excited to the upper level to obtain an inversion, factoring-in thermalization of the LLL as well
2. Compute the maximum theoretical available energy per pulse assuming 100% inversion (but taking thermalization of the LLL into account)
3. Compare the experimental value of ΔN_th as computed from the experiment data to the theoretical data. Ratioing the two figures, what actual percentage of available ions are used in the inversion in this operating four-level laser?
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