• Read the Scientific American Amateur Scientist column on the HeNe laser (Sept. 1964), specifically on how to fill the tube with gases, optimal pressures, etc. As well, read the follow-up in Dec. 1965 which deals with gas and contamination issues.
• Read the outline on Sam's Laser FAQ on the HeNe laser. Specifically look at the "Gas Fill" section
• Review operating procedures of a turbomolecular-pump based vacuum system from your previous lab.
• Review operation of the various gauges and valves in the system, including the RGA
• See Vacuum Notes:Pressure Reduction on this page regarding maintaining gas ratios

The tube will have a local isolation valve (called the tube valve) in series with it as per the main diagram on the turbomolecular pumping system description (found on the main page for this course). This is to ensure the tube may be stored at high vacuum to keep it free from contaminants. Connect the tube valve to the isolation valve for the system if not already done. Open the isolation valve and evacuate the manifold and the connecting vacuum line between the laser tube and the system. Now open the laser tube valve to evacuate it as well.

Purity is critical to allowing the relatively low-gain HeNe to operate so a cycle of cleaning the tube is required before the final gas fill. Begin by powering the tube and observing the discharge (if any). If a glow discharge begins, continue pumping until it becomes a thin violet glow followed by a dim red glow and finally extinguishes (you can guess the pressure at this point knowing the minimum pressure for a discharge from Lab #1). Pump for a few more minutes then close the evacuate valve on the system to isolate the manifold and tube from the pump.

Fill the laser tube with research-grade helium to a pressure of approximately 10 torr. Start the power supply and observe the tube glowing - the colour may well change as it operates. Allow the tube to run for one minute. Now, start the RGA and take a sample of the gas in the manifold (keeping the RGA head pressure under 10^-5 torr) - call it "first flush". The gas mixture may well show a quantity of undesired impurities as well as the helium injected into the system - many impurities will have evolved from the laser tube itself as it heats. Stop the RGA and turn OFF the filament.

Evacuate the tube to ultimate vacuum again. The flushing procedure must be repeated a minimum of three times to reduce the quantity of impurities in the system. It is not necessary to take an RGA scan each time.

Now fill the laser tube with the required gas mix. Close the manifold valve and overfill with He and Ne gases in a 2:1 or other suitable proportion (see the Vacuum Notes on the course home page regarding this). The manifold & tube are usually filled with 20-30 torr of helium first then an additional 10-15 torr of neon. Start the power supply and reduce the pressure gradually until the tube begins to operate (around 4 torr). Once the tube operates, stop reducing pressure and note the power output (in mW). Reduce the pressure in 0.2 torr increments and note the power at each pressure until lasing stops - this allows you to determine the optimal gas pressure for this tube. If the laser fails to operate at all, the current gas mix can be flushed and another attempt made to fill the tube.

With the laser operating now, perform an RGA analysis on the (fresh and working) gas mix to verify it's contents and assess impurity levels - call this scan "working mix". Compare to the original "first flush" - You expect, of course, to find helium and neon gases in the mix but check specifically for O₂, O, H₂O (water vapour), and N₂. Verify the He:Ne ratio as well.

Repump and power the tube at optimal gas pressure and operate for 10 minutes. During this time, observe the optical power output on the meter at intervals of no less than one minute and graph it over time. Run a final RGA scan ("final") to see what types of gases evolved which caused the power output to drop.

• Compare the residual gases before (fresh) and after (working) flushing. Were there less impurities in the second flush? If more flushes were done would you expect even less?
• Compare the residual gases before long-term operation (working) and after (final) for the tube gas mix. What impurities developed and where did these impurities come from? Assuming they were liberated solely during tube operation discuss tube materials as a possible source for contamination.

For this experiment, an abbreviated lab report is required as follows:

• A one-page outline of the experiment itself (Purpose, How it was done)
• A one-page background outlining the issues of gas purity as it affects the HeNe laser as well as the expected gas mixture (there is a relationship between tube diameter, etc ... research this and outline it here!) Most of this information will come from the SciAm article and/or SAMs Laser FAQ.
• Observations of the gas mix before and after flushing and a paragraph explaining the differences between the two (i.e. explain how the flushing process affected the gases in the tube and which gases were important to get rid-of to ensure lasing action).
• Observations of the power output over time and a paragraph explaining the observations (i.e. how the laser power varied).
• Observations of the gas mix after the power has dropped and a paragraph explaining how impurities affect the power output. Explain WHERE these impurities likely evolved from (i.e. what components of the tube harbour impurities).

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