PHTN1400: Principles Of Laser Systems
Pulsed Gas Lasers

Introduction

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 | PART B: TEA Carbon-Dioxide Lasers

PART A: Excimer and N2 Lasers

Excimer Laser

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

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

PreLab for Part A Only

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) - you will need to read the SOP to determine how this 'boost bottle' arrangement works. Evacuate the laser head using the attached vacuum pump, isolate it (i.e. with the tube evacuated and no vacuum pump connected), fill the boost bottle with nitrogen from the supply tank, and release into the laser head. Measure the resulting pressure increase at the laser tube in torr on the attached gauge. 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).

In summary ... to determine the amount of nitrogen added by each boost bottle

Close the READ VACUUM valve to isolate the vacuum gauge on the left side (which will be damaged by high pressures). Fill the evacuated tube with pure helium to a pressure of 15 psi as read on the right-side pressure gauge. Turn the coolant water ON and then the laser ON (and 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 Internal, and set the Internal Rate to 40Hz (2.0 times 20).

Never turn the high voltage ON unless the tube is pressurized to at least 15psi. Always turn the high voltage OFF when the tube is evacuated.

Although zero output is expected with 0% nitrogen in the mixture (the tube is currently filled with "pure" helium), there may be some residual nitrogen in the system and so lasing may or may not occur at this point. Run the laser with a charge of 15psi of helium for three minutes then TURN OFF the high voltage, evacuate the laser head again, and refill to a pressure of 30psi of pure helium. The previous (15psi) helium gas fill then becomes a flush which will purge residual nitrogen from the tube.

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.

With 30psi of pure helium in the tube (which was just flushed and purged), record the power with no nitrogen in the system (it should be less than 2mW, and be sure to zero the meter first). Now add one 'boost' bottle of nitrogen to the pure helium (at 30psi) in the laser tube 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).

Ultimately, you will have a chart of '# boosts' vs. 'Output Power'.

Part II: Gas Lifetimes

Pump-out the gas mixture by evacuating the tube (according to the SOP) and pump all residual gas from the tube. 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, un 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.

The effect you see is analogous to passivation which is discussed in the text (Csele, section 10.12).

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 a few minutes after which time the UV radiation will have bleached dye molecules in the paper leaving a distinct mark which may be measured with a ruler. Place paper in the path of the output beam immediately in front of the laser and close the shutter to hold it in place, then operate the laser for a few minutes.

When done, leave the laser pressurized and shutdown the laser according to the SOP (including the cooling water).

PART B: TEA Carbon-Dioxide Lasers

Lumonics TEA-203 CO2 Laser

PreLab for Part B Only

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 absolutely 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 ....). Peak voltage can be measured by the scope directly using the MEASURE feature.

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).

Lumonics TEA-203 CO2 LaserEvacuate the laser tube (see the complete procedure in the SOP) via the orange valve on the left. 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 on the controller. Now, pressurize the laser to an absolute pressure of 1" Hg or 25 torr (CAREFUL when using this gauge as it reads "backwards" ... 30" is really 0 torr so 29" on the gauge is 25 torr, our starting pressure). Fire the laser and record the pulse energy at this pressure - fire the laser at least four times at each unique pressure and take an average energy (be aware that at extreme pressure limits the laser can 'misfire' and these readings can be discounted).

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 (using the evacuate valve) 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 (i.e. before the lens) at a piece of thermal paper to see the effect. Now, insert a ZnSe 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.

Lumonics TEA-203 CO2 Laser - Air Breakdown

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 with the gas mix 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:

Hand In a WORD PROCESSED (not handwritten) lab report with contents as outlined below.

Lab Report The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, and the student's name and ID number

Answer each question as "1", "2", etc with each new question starting on a NEW PAGE so that question 2 starts on the top of a new page and question 3 starts at the top of a different page, etc. Where a question has multiple parts (e.g. 3a, 3b, 3c ...) answer each in a separate paragraph with a title identifying the question in the form "3a., 3b., 3c. ...". Do NOT answer an entire question (e.g. question 3) as a single paragraph.

This format will assist you in ensuring EACH and EVERY question is answered since marks cannot be given for work not completed, nor would it be expected that you could complete the TEST QUESTIONS which will most certainly be similar to those you see here! (Hint !)

The lab must be submitted in a report cover (preferably either a three-hole punched cover or one with a clamp on the left side, not a binder), and NEVER as a stapled mass of loose papers!

Failure to follow this simple format, used for all condensed labs in this course, will result in deduction of marks

  1. A one-page explanation of the experiment (both parts) including an explanation of what an E/P ratio is. Be specific in explaining how E/P ratio is determined experimentally.

  2. Part A (using the Excimer-500 laser with a nitrogen gas mix operating at 337nm)

  3. 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)
  4. A graph showing output power (Y-axis) versus partial-pressure and concentration of nitrogen (show both on the X-axis). The x-axis should have two axes which co-incide (and only one trace on the graph). Alternately two graphs can be used however be sure each fills the entire x-axis and is not "crushed" to the left side making it inaccurate.
  5. At the maximum power found, compute the partial pressure (in torr) of the nitrogen component(show calculations) and answer the following (being sure to LABEL the answers as "4a", "4b", and "4c").
  6. 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.

  7. Part B (using the Lumonics TEA-203 carbon-dioxide laser)

  8. A table of data (with columns showing total laser Pressure in torr, detector reading in Volts, and pulse energy in Joules), PLUS a graph of Energy (Joules) versus Pressure (Torr) for the Lumonics TEA laser
  9. At the observed peak energy output, compute the E/P ratio for the Mark V gas mix employed in this laser (show calculations)
  10. 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

  11. Putting it all together

  12. The Lumonics Excimer-500 _is_ specified for use with a carbon-dioxide mixture (along with a simple change of optics). Calculate the expected operating pressure using the Mark V mixture in the Excimer-500 laser from part A (show all calculations and assumptions).