PHTN1432: Vacuum Systems and Thin Film Technology
Lab #3 - Reprocessing Laser Tubes (2017W)

Introduction

Reprocessing a HeNe Tube in the Specroscopy Lab
The laboratory setup for reprocessing a helium-neon gas laser tube. Here a Melles-Griot 05-LHR-099 tube is connected to our turbomolecular pumping system and filled with a precise mixture of helium and neon gases. Visible in this photo are the power meter and power supply for the tube. Behind this setup is the gas discharge for another lab in this course.

Purposes

This lab ties together the principles covered in the first two labs of this course, specifically vacuum purity and residual gas analysis, in a critical application - processing a helium-neon laser tube.

Pre-Lab (homework before the lab)

  • The following prelab assignment (worth 10% of the total lab mark), is due before the lab period. Late marks are not assigned if the prelab it is not received at the beginning of the lab: you lose 10% of the total lab marks immediately with no recourse if it is not received upon entering the lab period in which it is due (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).
    1. Using the empirical relationship in the "HeNe Notes" article (the same one from last term) determine the optimal helium and neon pressures for the laser in torr. Since we do not intend "long term" operation, use the optimal values for output power, not for longevity. You will need the bore diameter for the specific tube in this lab: for this, find the "e/2" beam diameter from this Listing of Melles-Griot Red HeNe Tubes at SAM's laser FAQ and multiply this figure by TWO to get approximate bore diameter (since the tube bore must be large enough to support a TEM00 mode). Show a complete set of calculations detailing how you arrived at these values since a numerical answer without calculations will be worth nothing.
    2. Using the "impurities" article, determine the top three impurity gases that affect the HeNe laser and outline these.
    3. What is the approximate level of the "worst" impurity that can be tolerated? Knowing the optimal tube pressure from the first question, express the "tolerable" pressure of this impurity in both partial pressure in the tube (in torr) and as a percent of total tube pressure (so that it can be observed on the RGA)
    4. .

    The Experiment

    The laser 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. Leave both valves open for the duration of this experiment.

    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 in air 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 (only the first flush).

    Now fill the laser tube with the required gas mix determined from your pre-lab research (an estimate of total pressure based on empirical data such as tube dimensions will suffice for this part of the experiment). Close the manifold valve and overfill with He and Ne gases in a 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 amount of neon (or you may fill the neon first as it is the smaller quantity, then add helium to meet the required ratio). Start the power supply and reduce the pressure gradually until the tube begins to operate (around 4 torr). If the laser fails to operate at all, the current gas mix can be flushed and another attempt made to fill the tube. Assuming the laser begins to operate, stop reducing pressure (the tube is presumably at the maximum operating pressure now).

    Now that the tube is operating, 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 as well as the pressure range over which the laser will operate. The laser has now ceased operating but there is still gas inside the manifold. Run a quick RGA scan to determine the ratio of helium:neon in the tube at this point (Your observations now include the maximum tube operating pressure, minimum pressure, and the He:Ne ratio for which these observations are valid). The RGA analysis on this (working) gas mix allows verification of the tube contents and assessment of impurity levels - call this scan "working mix". In the lab report you will compare it to the original "first flush" - You expect, of course, to find helium and neon gases in the mix but check specifically for the impurities identified in the prelab. Verify the He:Ne ratio as well.

    If necessary, refill the tube and repeat to obtain a table of pressure-vs-output power values.

    Repump and fill the tube with the same mixture at optimal gas pressure (determined above by observation) and operate for 10 minutes continuously. During this time, observe the optical power output on the meter at intervals of 30 seconds 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.

    Analysis and Discussion of Results (in the lab)

    Analysis (Homework)

    When evaluating a gas mixture, it is often required that a gas component be expressed as a ratio (e.g. He:Ne). In the case of a helium-neon gas mix, the procedure to produce a ratio is to divide the total helium pressure by the total neon pressure (addition of both stable isotopes) to obtain the ratio for helium. If, for example, the RGA showed 2E-5 torr of helium and 2E-6 torr of neon the ratio would be 10:1 for Helium:Neon (which is the most common way to express the gas mixture employed). Note that the total pressure at the RGA is NOT relevant!

    Now, if one wanted to know the actual partial pressure of neon in the tube, one can say that one part in ELEVEN is actually neon so if the tube pressure was 2.5 torr (as read from the manifold gauge), there is actually 2.5/11 or 0.227 torr of neon in the tube. This partial pressure is required for calculation of the E/P ratio as is the anode-to-cathode distance and the tube voltage.

    On E/P Ratios

    E/P ratios will be covered extensively in PHTN1400 (Laser Systems) under the topics of the CO2 laser as well as pulsed gas lasers. In a nutshell, the E/P ratio is a characteristic of each gas and relates the electric field (E, in Volts/cm) to pressure (P, in torr) and so has units of "Volts per cm-torr". For each gas, the E/P ratio can be researched or observed (as it is in this lab). If the E/P value for a gas such as neon is known, one can predict the optimal (partial) pressure for that gas in a discharge tube by knowing the voltage across the tube and the length of the discharge path (between anode and cathode).

    To determine the E/P ratio for neon, find the partial pressure of neon (by multiplying the total pressure by the ratio found on the RGA): it should at least be reasonably close to that determined in the prelab. Next divide the TUBE voltage (power supply voltage minus ballast resistor voltage drop) by the product of that neon pressure (in torr) times the distance between the anode and cathode (in cm). CHECK THE UNITS. Expect a value between 100 V/(cm-torr) and 300 V/(cm-torr).

    For the curious, the concept of the E/P ratio (and the actual number for neon) are covered in this article by Calvert on electrical discharges. It will be covered in the lasers course later in the term.

    Lab Report

    For this experiment, an abbreviated lab report is required (word processed, never hand-written) as follows. As usual, submit the lab report in a bound folder NOT simply a pile of loose, stapled papers! nor in a large binder. A title page is required and each major question must begin on a new page (so your lab report must be at least ten pages in length, but hopefully longer).

    1. A one-page outline of the experiment itself (Purpose, How it was done, the kinds of data collected)
    2. Observations (from the mass spectrometer - both graphical output showing pressure vs. atomic mass, and partial pressure data) of the impurities in the Helium / HeNe 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 eliminate to ensure lasing action - research this in the "impurity" article). To make an accurate comparison, normalize all pressure data against the pressure of helium in the tube (i.e. express the pressure of impurities as a percentage or parts-per-million of the helium pressure in the sample). In summary, you need:
      1. A graphical output (pressure on y vs. atomic mass on x) of the "first flush" gas mixture.
      2. A calculation of the amount of impurities (top three) as % of helium pressure based on pressure data collected from this "first flush". SHow one example calculation and ALL pressures involved (i.e. pressure of the impurity, pressure of the helium as observed using the RGA).
      3. A graphical output (pressure on y vs. atomic mass on x) of the "working mix" gas mixture.
      4. A calculation of the amount of impurities (top three) as % of helium pressure based on pressure data collected from this "working mix". SHow one example calculation and ALL pressures involved (i.e. pressure of the impurity, pressure of the helium as observed using the RGA).
      5. A paragraph explaining what happened between the two samples, and a comparison of which impurities decreased and by how much.
    3. Observations of the power output as it varies over total tube pressure (done in 0.2 torr increments according to the lab outline - present this in the form of a table of values as well as a graph with total tube pressure as the x-axis). Report the gas mixture (He:Ne ratio) for which these observations are valid, using the RGA data to determine the He:Ne ratio (and be sure to show pressures observed on the RGA then the calculated ratio).
    4. Observations of the power output over time (a graph, with time on the x-axis) and a paragraph explaining the observations (i.e. how the laser power varied).
    5. Observations of impurities in the gas mix (from RGA data) after the power has dropped (a mass spectrometer output, and a calculation of impurities in the identical manner as Q2 for comparison) and a paragraph explaining how impurities affect the power output. Explain (in a paragraph) WHERE these impurities likely evolved from (i.e. what components of the tube harbour these impurities).
    6. Knowing the observed tube pressures over which the laser operates - maximum, optimal, and minimum - calculate the partial pressure of neon in all three cases (since you know the ratio) and compute the E/P ratio of neon in the tube for all three cases. Present these observations in the form of a table and explain HOW you calculated the E/P ratio (including assumptions and how you arrived at these assumptions). One example calculation showing how the E/P ratio was calculated is required. Use RGA data from the mixture to determine the actual ratio of helium-to-neon and use this to compute the partial pressure of neon. Be sure to show all required values such as tube voltage calculations and measured distances.

    For ALL information provide a footnote as to WHERE you obtained it (e.g. the page on SAM's site, page in Csele, from Calvert's article, etc). No "Magic Numbers" - all values used for comparison must be referenced as to their source!