An investigation of pulsed gas lasers focussing on the gas mixture and matching of the E/P ratio for optimal laser output. This is a two week lab in which students in small groups will complete both parts alternately. One lab report is required for both parts, which have separate questions to be answered.

PART A: Excimer and N₂ Lasers

Housed in the high power laser lab (V15), our Lumonics-500 excimer laser is currently operating a mixture of helium and nitrogen gases.

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Introduction

This lab introduces students to the operating procedures of an excimer laser including mixing laser gases.

PreLab for Part A Only

• Print the LAB WORKSHEET for Part A
• Read chapter 10 in Fundamentals of Light and Lasers by Csele
• Review class notes on E/P ratio for gas discharges
• Review the Excimer Mix recipes that came with the laser
• Read the Standard Operating Procedure (SOP) for the Lumonics Excimer-500 laser

This laser is a class-IV laser producing approximately 400mW of power in the UV. Safety glasses suitable for a nitrogen (i.e. with an OD > 3 at 337nm) are required.

Part I: Optimizing Gas Mixtures

The purpose of this experiment is to determine the relationship between power output and gas mixture. Begin (with the laser turned OFF) by determining the amount (in percent) of nitrogen gas held in the 'boost' bottle (normally used for the halogen gas in an excimer). Evacuate the laser head, seal it, fill the boost bottle with nitrogen, and release into the laser head. Measure the pressure increase in torr. Add ten such 'boosts' and average to determine the precise amount added each time. Now evacuate the laser head again to ultimate vacuum (at least 5 minutes of pumping).

Fill with pure helium to a pressure of 30 psi (be sure to close the low-pressure gauge valve when filling the laser). Turn the laser ON (wait for the thyratron to preheat), switch the high voltage ON, and observe the output power with 0% nitrogen in the mix. In order to observe the output power set the HV Control to 35kV, Trigger Mode to External, and set the Internal Rate to 40Hz (2.0 times 20). To run the laser for power measurements switch the Trigger Mode to Internal (left position), read the power, and set the Trigger Mode to External again when done to stop the laser from firing. Be sure the power meter is set to a wavelength of 337.1 nm (unless a broadband meter is used). Note that although zero output is expected with 0% nitrogen in the mixture there may be some residual nitrogen in the system (did the vacuum gauge hit zero when you evacuated the laser head??) and so lasing may or may not occur.

The output of this laser is measured with the Melles-Griot broadband power meter. The apeture of the meter is smaller than the beam width so the meter will only read a percentage of the actual total output power however since the beam width is fixed the power metered will always be linearly proportional to the actual output power.

Now add one 'boost' bottle of nitrogen and observe the output power again. Continue increasing the concentration of nitrogen in the gas mix, one 'boost' bottle at a time, and observe the power until either forty boosts were used or the output power decreases to a level of no more than one-quarter of the peak output observed (or even ceases lasing entirely).

Part II: Gas Lifetimes

Pump-out the gas mixture by first opening the VENT valve to release pressure in the laser head to atmospheric levels, then closing the VENT valve, and then pumping (according to the SOP). Refill with He:N₂ mix to just below the optimal concentration value found in the last experiment (e.g. if it was found that eighteen boosts provided optimal output power, use sixteen for this experiment). Now run the laser at a repetition rate of 40Hz and take power readings of the output at one minute intervals (2400 shots per minute). Do this for ten minutes or until the output power peaks then falls again to half of the peak value. Plot power output (y) versus number of shots (x) for the sealed gas mixture.

Part III: Laser Parameters

While the laser is still operating using the above gas mix, determine the electrode spacing by measuring the width of the output beam close to the output apeture - this is required for E/P calculations. This may be done easily by allowing the beam of the working laser to irradiate paper for one minute after which time the UV radiation will have bleached dye molecules in the paper leaving a distinct mark which may be measured.

When done, leave the laser pressurized and shutdown the laser according to the SOP.

PART B: TEA Carbon-Dioxide Lasers

PreLab for Part B Only

• Read chapter 10 in Fundamentals of Light and Lasers by Csele
• Review class notes on E/P ratio for gas discharges
• Read the Standard Operating Procedure (SOP) for the Lumonics TEA-203 laser

This laser is a class-IV laser producing up to 6J of energy per pulse. Safety glasses suitable for a carbon-dioxide laser (10600nm) are required.

The UNFOCUSSED output of the laser is monitored with a Gentec QE-50 pyroelectric Joulemeter (with no attenuator) attached to an oscilloscope. The scope should be set to trigger on a rising edge and is calibrated for 3.43 V/J (but this varies by wavelength .... keep reading ....).

Calibration information for the Gentec Joulemeter is provided HERE. This allows correlation between the peak voltage observed on the scope and the energy per pulse. The observed voltage must be compensated for the wavelength used (10600nm - the Joulemeter is calibrated at 1064nm). The calibration certificate provided outlines the calibration factor required in terms of Volts (observed) per Joule (incident pulse energy).

Evacuate the laser tube (see the complete procedure in the SOP). Ensure compressed air is flowing into the spark gap and the gap is pressurized properly for 20kV operation. Now, set the HV for 20kV - laser voltage is read via a meter attached to the controller as follows:

Now increase the pressure in 1" / 25 torr increments (increase the pressure to 28" on the gauge, then 27", etc) recording the pulse energy at each pressure until a maximum absolute pressure of 10" (500 torr) is reached. Note that the pressure values employed in this lab exceed the nominal values specified in the SOP: do not exceed 20kV in order to prevent damage caused by arcing in the tube.

Reset the tube pressure to the optimal value found in this experiment. Ensure the large beam dump is in place about 50cm in front of the laser and will intercept the beam. Remove the Gentec Joulemeter. Be careful in this part of the experiment to prevent any unwanted reflections and be sure no part of your body intercepts the beam. Fire the unfocussed laser beam at a piece of thermal paper to see the effect. Now, insert a lens into the beam path and fire the laser while moving a piece of thermal paper away from the lens and towards the focal point, observing the effect.

The breakdown of air at the focus of the beam. With an output of about 6J and a discharge of 300ns, the resulting power is 20MW.

When done, purge the tube to atmospheric pressure and shut down the power supply as per the SOP.

Graph Energy (in J) on the Y-axis versus Tube pressure (in torr) on the X-axis. At the peak power output, compute the E/P ratio of the gas mixture employed (Praxair Mark V mix = 8% CO2, 4% CO, 0.5% H2, 16% N2, Balance He) knowing the electrode spacing is 4cm.

Lab Submission:

For this experiment, an abbreviated lab report is required (word processed, never hand-written) as follows. Submit the lab report in a folder or binder NOT simply a pile of loose, stapled papers! (a penalty applies for reports submitted without a folder)

1. A one-page explanation of the experiment including an explanation of what an E/P ratio is
2. A completed lab worksheet for Part A plus a determination of the amount of 'boost' added each time (in terms of both concentration in percent and partial pressure in torr)
3. A graph showing output power (Y-axis) versus partial-pressure and concentration of nitrogen (show both on the X-axis)
4. At the maximum power found, compute the partial pressure (in torr) of the nitrogen component(show calculations).
• How does this compare to the known optimal pressure of a low-pressure nitrogen laser design (which employs pure nitrogen alone)? Cite references for the pressure of a low-pressure laser
• At the maximum power found, compute the E/P ratio of the nitrogen component (show calculations).
• Compare the E/P ratio found in this experiment to the known E/P ratio for nitrogen as a percentage See the class notes for the accepted E/P values - cite the 'known' value
5. Using the given recipes for the Excimer-500 laser, as well as measured parameters (e.g. electrode spacing), compute the E/P ratio for the N₂⁺ species which emits at 427nm.
6. A graph of Energy (Joules) versus Pressure (Torr) for the Lumonics TEA laser
7. At the peak power output, compute the E/P ratio for the Mark V gas mix employed in this laser (show calculations)
8. Compare the E/P ratio found in this experiment to the known E/P ratio for a similar CO₂ mix (with the ratio of component gases as close as possible to the Mark V mix). Report the percentages of the components of both the Mark V mix and the comparison mix. See the class notes for the accepted E/P values - cite the 'known' value
9. The Lumonics Excimer-500 _is_ specified for use with a carbon-dioxide mixture (along with a simple change of optics). Calculate the expected pressure using the Mark V mixture in the Excimer-500 laser (show all calculations and assumptions).

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