CO2 Laser
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All pages in this lab. Note To print Full Lab Write-up click on each link below and print separately
'Warning From The Staff of the 111-Lab DO NOT BURN any burnable material like PAPER with the CO-2 Laser.
We know you may want to this but it is a safety hazard. You will receive an Grade of 'F' on the experiment if you do so.'
I. CO2 Laser
II. Pre-Labs must be printed separately CO2 Pre-Lab .PRINT, FILL THIS OUT, Get it Signed by 111-Staff, turn in your Signed Pre-Lab Sheet with your report, to the 111-Lab Staff.
III. Optics Tutorial
Reprints and other information can be found on the Physics 111 Library Site
Contents |
Before The Lab
Before using the apparatus in this experiment, you must complete training in the safe use of lasers detailed on the Laser Safety Training page. This includes readings, watching a video, taking a quiz, and filling out a form.
Discuss the Physics about this experiment with the faculty or the GSI's in the 111-Lab before starting.
You should keep a laboratory notebook. The notebook should contain a detailed record of everything that was done and how/why it was done, as well as all of the data and analysis, also with plenty of how/why entries. This will aid you when you write your report.
View the CO2 Lab video, discuss the pre-lab questions with an instructor, and have the CO2 Pre Lab Questions and Staff Sign Off Sheets.....PRINT, FILL THIS OUT, Get Signed and Turn it in.
. You may find it useful to review principles of optics in the Optics Tutorial.
Prerequisite Reading Materials
There are many books on lasers, and a number of modern optics books have brief chapters on the subject. The following is a selection of some useful references. A few are in the Engineering Library.
1. A. Bloom, Gas Lasers, 1968; a good, readable book, especially on resonators. \#TK7871.3.B55
2. C. Garrett, Gas Lasers, 1967: a basic treatment of cavities, excitation mechanisms, and other topics. \#TK7871.3.G3
3. B. Lengyel, Lasers, 2nd ed., 1971: a very good introduction to lasers in general, with a good section on CO2 lasers. \#TK7871.3.L4 †
4. D. Sinclair, Gas Laser Technology, 1969: gives a readable theoretical treatment of laser physics. \#TK7871.35.S54
5. C. Patel, "High Power Carbon Dioxide Lasers", Scientific American, August. 1968: an excellent introduction to the CO2 laser. \#T1.S5 †
6. A. Levine and A. DeMaria, Lasers, vol. 3, 1971: contains a comprehensive and detailed review article and lists many references. \#TK7872.L3A1.L2†
7. A. Levine, Lasers, vol. 2: contains a review article by Patel. \#TK7872.L3A1.L2
8. A. Yariv, Quantum Electronics, 2nd ed., 1975: practical laser physics in more mathematical detail; the place to look when you need a formula, for example, to describe resonator properties. \#TK7870.Y3
9. C. Willett, Introduction to Gas Lasers: Population Inversion Mechanisms, 1974. Gives detailed theoretical and experimental results, with a section devoted to CO2 lasers. \#QC3.I625
10. R.J. Pressley, CRC Handbook of Lasers, 1971. Has a very brief description of CO2 laser spectroscopy and lists wavelength data and references. \#TK7871.3.C17
11. G. Herzberg, Molecular Spectra and Molecular Structure, vol. 2, 1945. The classic work on spectroscopy. Lists wavelength data and references. \#QC451.H455
12. F. Llewellyn, The Glow Discharge, 1966. \#QC711.L56
13. A. Howatson, An Introduction to Gas Discharges, 2nd ed., 1976. \#QC711.H78
14. CO2 Beam Finder Specifications and Instructions
† at least part of the reference is included in the collection of reprints.
15. CO2 Laser Reprints and Equipment Specs in the Physics Library and online on 111-Lab share.
Introduction
The central piece of equipment and object of study in this experiment is a continuous-wave (CW) carbon dioxide laser. It is of relatively low power as CO2 lasers go, but is still capable of 10 watts CW output, or roughly 1000 times the power of a small He-Ne laser. It operates in the infrared at a wavelength near 10 ?m, so the laser radiation is invisible. These two facts, plus the presence of high voltage, make it potentially very hazardous. SAFETY MUST BE YOUR MAJOR CONCERN AT ALL TIMES. Safety measures are detailed in the next section. If you ever have any doubt about some procedure, consult a professor, GSI, or Don Orlando.
Safety Measures
- Never operate anything, especially alone, unless you know exactly what you are doing. Anytime you don't understand something, ask for assistance. Protective eye goggles are available for UV and reflected IR.
- The high voltage should never be turned on unless
- all connections are secure;
- the water is flowing.
- The gas is flowing through the laser!
- The laser beam must never be uncontrolled. This means:
- When powering up the laser, the output should be blocked with the power detector and a firebrick; always include the firebrick.
- Beam finders, optical pieces, and anything else must be inserted in the beam with great care, in such a way that there can be no damage to the object or the person, and that the beam is not reflected or scattered in an unintended direction;
- Place firebricks behind mirrors and detectors as backup protection.
- Never let the beam probes set in the laser beam for more than 5 seconds, keep them moving in and out of the beam.
- NO UNAUTHORIZED PERSONS ARE ALLOWED AROUND THE APPARATUS, no casual visitors nor anyone not enrolled in the 111 Lab. Step outside when you talk to them.
- In the event of an accident, the first thing to do is shut off the high voltage at the current regulator and the high voltage power supply. Then shut off BOTH WATER VALVES. (BLUE AND RED handles on wall by door. BLUE first; RED second.) Then call for help from 111-lab personnel.
Equipment
The table on which the CO2 laser sits is an optical table resting on four air filled legs to isolate the laser from room and building vibrations. The table is always filled with 50 pounds of regulated air pressure. If an air line breaks please quickly call for help the main shut off vavle is in room 286 behind the Muon experiment, Labled CO2 Air
1. Output-Mirror assembly: Mirror is ZnSe, 91% reflectivity, 10 m radius of curvature.
2. Horizontal and Vertical output mirror adjustments
3. NaCl Brewster-angle windows
4. 75 grooves/mm grating in a swivel mount.
5. Grating adjustment
6. Totally reflecting back mirror with Hor. and Vert. adjustments
Operation of the Laser
Before you start, familiarize yourself with the various pieces of apparatus. Look at the power meter to see how it operates and turn it off when not in use. (It is battery operated) Identify the HeNe or Diode laser used for alignment, firebrick, spectrum analyzer for the CO2 spectrum, gas filling system with tank, regulators, valves, pressure gauges, high voltage power supply and current regulator. When you work, keep the working surface neat and free of unused apparatus. This is good laboratory practice and an important safety measure.
The procedure is to align the laser mirrors, pump out and flow with gas at about 10 mm Hg to start, turn on the power, adjust the mirrors for maximum power, take data, and shut off the power. Aligning and adjusting will take much more time than you expect, and may stretch out into several days. Don't get discouraged, but do ask for help after you have exhausted your skills. The laser produces about 10 Watts of power, be careful.
Alignment Procedure
You should never change the height or position of the glass CO2 laser tube. Moving it significantly throws it out of alignment, and this could take days to realign. Instead, change the laser system by varying the mirror orientations, grating orientation, and He-Ne laser orientation.
The idea is to make the two mirrors at the ends of the laser cavity reflect a beam back-and-forth many times without striking the walls of the tube. There are a few tricks in aligning this particular laser. Step by step, they are:
- Make sure that there is no high voltage at the electrodes of the laser tube by checking that the power supply is turned off.
- The Red Helium-Neon (HeNe) or Red Diode laser is setup alongside the cavity, and using the two mirrors shown, direct the beam down the center bore, at the same height, of the laser tube. There is a mirror on a rotation point to move in and out for the alignment of the CO_2 Laser using the Red aglignment laser. The Red laser beam should be positioned on the central portions of the mirrors, for adjustment purposes. At the start, block off the back mirror with a piece of paper so that reflections don't confuse matters. See figure 2 for a suggestion on the set-up. Also remove very, very, very, carefully the output coupler mirror and set it on the optical table in a safe place. Note: Do not ever attempt to clean this optic mirror.
Figure 2: Alignment Configuration
- Now direct the Red laser beam through the middle of the output mirror (the first mirror it passes through). You will see more than one dot reflecting back WHY ?
- Adjust the mirrors until the Red laser beam goes through the middle of the bore without reflecting off the walls of the tube. It may not look as if it goes through the middle of the Brewster windows, and it may not go exactly through the middle of the output mirror. Going down the center of the bore is the most important.
- Remove the paper blocking the back mirror and adjust the mirror so that the reflection is centered on the output port of the Red laser (it is easier to align if you place an iris with a small opening set in it at the output port of the Red alignment laser).
- Now Re-install the output coupler mirror and now adjust the output coupler mirror so that the inner surface reflection of that mirror (the bigger, dimmer one of the two) is centered on the back mirror reflection spot at the Red alignment laser. Fringes can usually be seen on the reflections when the two are aligned (this is a Fabry-Perot interferometer). Alignment is pretty much complete. It may take you a day or two to get to this point.
- Block off the output port of the Red laser with a firebrick to protect it from the CO2 beam. Place the power detector in front of the CO2 output port and place a firebrick behind the detector. Place the power meter in Auotmatic mode and it will change scales automatically on the power meter scale.
Pumping-Out and Filling the Laser Tube
1. Before you begin, ask a GSI or staff for help, the pumping system is a molecular sieve No cold trap of liquid Nitrogen (LN2)is used anymore. The molecular sieve keeps oil from migrating up into the laser system as you pump. Always keep a vacuum on the laser tube when not in use. There may be moisture in the laser tube that would cause the presure to be affected making it difficult to pump down properly. Note we now have a gas flowing system. Gas flows into valve #3 and metering valve #1 to the laser and then through #6 to the vacuum system. The vacuum goes to the #6 valve (see Fig 3), vary the metering valve #1 and Vacuum valve #6 opening to regulate the pressure flow. This metering valve does not close and is used to regulate the pressure gas flow to the laser. When the valves are all the way in (turned clockwise) they are closed. NOT The metering valve it is always open a little. As you back out the screws (by turning them counter-clockwise), gas flows. Valve #6 is a stop-cock, which is closed when the handle is perpendicular to the gas line.
2. Note you should look at the hook up for the vacuum system for the CO-2 laser. There has been changes and we use a flowing gas system. We first want to flush the system. Make sure that all valves are closed including the GAS TANK VALVE. Turn on the pump. Then open valve #3 and then the course valve #6. (This cleans anything out of the gas tank regulator.) Then open valve #1 to clean out the laser tube.
After the system is pumped clean, close both valves #6 and #3. The laser tube is now pumped out. As the system is not perfectly vacuum tight and the pump will not go to zero pressure, it is best to let the gas flow for a few minutes so that residual undesirable impurities are flushed.
3. Now just adjust the pressure flow to about 10 mm to start. While watching the pressure gauge, slowly adjust the metering valve \#1 and adjust it until the pressure settles between 10 and 15 torr (mm Hg). If you go back to step 2 to clean out the tube some more, make sure that the gas tank valve #3 is closed. Otherwise you'll evacuate the gas tank. Now you are ready to go.
Power-On and -Off, and Maximizing Laser Power
1. Check that all electrical connections are secure to the equipment rack and to the laser. (See appendix at back of this write-up).
2. Turn on the water as follows: There are two water valves by the door: the blue valve is in-flow, the red valve is out-flow. The idea is to protect the laser tube from the pressure of the water line. So when you turn the water off and on, make sure the out-flow valve is open whenever the in-flow valve is open. To turn the water on, start with both the red and blue valves closed.
Procedure for Water Turn: Turn on the water-supply faucet in 286 LeConte (across the hall, by the sink to your right as you walk in the door)-set the 'O' on the top of the faucet handle to around 4 o'clock (where 12 o'clock points towards the wall); Then, go back in the room with the laser, The water interlock box is on the wall over the safety glasses. Push and hold the button then open the Red and then open the BLUE valve, now the Red light on the box will come on, indicating water is flowing. This completes the water turn on procedure.
Put on your safety glasses located next to the water interlock Box. If the water stops flowing then this interlock will stop the water flow for safety. Alway remember to turn off the BLUE 1st and then the RED when leaving the Laser for the day as stated above.
3. Check that the gas valve between the laser tube/thermocouple and the rest of the pumping system (valve \#1) is closed. The high voltage will arc across the gas tubing if this valve is open, a potentially lethal hazard (at around 15,000 volts!). Turn regulator switch off then on quickly to disengage the interlock, i.e. the red light goes out. Always turn off the high voltage before you change the pressure.
4. Now for the high voltage (HV). Make sure that all of the plastic panels are securely fastened on the laser housing. The electrical connections are already made; High Voltage is connected to \#2 terminal at the center of the laser tube. The voltage is regulated by a two Current Regulators. How does the laser work then?? The current regulators maintains a constant current through the CO2 laser tube. The regulator controls the current by adjusting a current through a tetrode vacuum tube device and transistors inside the regulator. The current regulator is in series with the high voltage supply and the CO2 laser tube. Changing the high voltage gets you nowhere, since it is the current that is regulated. Also, the high voltage power supply meter reads the total supply voltage only, not the voltage across the tube. The voltage across the laser tube equals the high voltage meter reading minus the voltage across the current regulator. (The voltage drop across the regulator is between 2KV and 6KV, see the meter scale switch on the current regulator).
To start, turn on the two Regulators. Then make sure that the powerstat (variable transformer) knob on the High Voltage Power supply is all the way down to zero (CCW), then turn on the HV Power supply.
After approximately 30 seconds, press the HV interlock button on the Regulators to allow current to flow. If you have satisfied all of the interlocks, the red light will come on. Set the current so that it reads around 10 mA on each regulator. This current is the internal regulator idle current until the laser tube breaks down (passes current).
Slowly turn up the powerstat knob increasing the high voltage. You will see the voltage increase on the high voltage panel but not on the regulator panel. This is because the regulator does not allow current to flow until the voltage passes a threshold value. Continue to increase the voltage until you see the voltage reading on the regulator panel voltmeter jump up. Now the regulators allows current to flow through the laser tube (tube breaks down). You should at the same time see a pink-purple glow discharge in the laser tube itself. (This should happen at almost the maximum output voltage )
To turn off high voltage
a. Flip the both regulator power switchs off and then on. This cuts off HV power to the two halves of the laser tube.
b. Turn HV down to zero. DO NOT TURN OFF REGULATORS YET!
c. Wait until HV meter on High voltage power supply reads zero. Then shut off the HV power supply and regulators.
CAUTION: The high voltage bleeds off slowly, so all points are still electrically hot for several minutes. Don't touch them without supervision or prior instruction.
d. When you are done for the day, Shut off water, first the BLUE handle, then the RED handle, one right after the other, and then the supply faucet across the hall in room 286. Also turn off all power meters and detector and all other electrical devices except the vacuum pump, which you should leave running with all valves closed in the vacuum lines, etc.
5. If you have properly aligned the laser it should be now lasing, and you should see power on the power meter. Much more likely, though, is that you are not lasing yet, but are very close and need to make some fine adjustments. To start, record the micrometer settings on both the back mirror and on the output mirror so you can return to them if necessary. (Check equipment specs on how to read the micrometers)
With the back mirror fixed, slowly tweak (ADJUST) the output mirror and watch the power meter for an increase in power. Use some scientific method here-one approach is to leave the vertical micrometer fixed and vary the horizontal position, then change the vertical slightly and again vary the horizontal, "walking" through the various orientations. After each mirror movement, it takes several seconds before the discharge and lasing power settle down. A continuous movement of the mirror, no matter how slowly you turn the knob, will completely mask the lasing action. Turn the knob a tad, wait a couple of seconds, turn it again, etc. This is true for adjusting both mirrors - be patient and wait for the discharge to settle down.
If this doesn't work, return the micrometers to their original settings and then change the back mirror slightly. Then again walk through the output mirror orientations. You will need to repeat this process until you see a power reading on the power meter.
If this doesn't work, carefully shut off the high voltage SEE ABOVE "To turn off high voltage " and again turn on the 632nm at 6 mW laser and make sure that the beam travels down the center of the tube, and that the return beam enters the He-Ne laser at the same point that the exiting beam does. It really is crucial here that the beam travels down the center of the laser tube.
6. Assuming some output is obtained, maximize the power by adjusting the micrometers. Move them slowly, in small steps - step and wait, step and wait. It takes time for the power and power meter to adjust to changes. There is always some backlash. Try "walking" the beam across the bore by detuning the back mirror, and compensating for it in the front. Do this until a maximum is reached. You should be able to get at least 700 mW, and the more power you have the better-off you will be, especially when you try to use the grating. The laser is capable of putting out 10 watts. It may be desirable to write down these final micrometer settings for further use (like if the laser gets detuned somehow). If the laser doesn't lase the next time it is turned on, and everything else says it should, try adjusting the back mirror \[or grating\] first. Sometimes you will leave at 5 PM with the laser in operating condition, and will return the next day and find it not operating. Remember that temperatures of the laser parts have changed, and the adjustments may have changed, but these changes are slight, so have patience and make small adjustments while attempting to regain the proper operating conditions. You shouldn't have to start over. (Any problems, call the staff for sympathy and perhaps advice).
Experiments
Your report should have discussions and explanations of the data obtained in the following experiments.
Current-voltage Curve and Gas Pressure.
Measure the current-voltage characteristic curve of the glow-discharge of the laser tube as a function of gas pressure, for three different pressures. Limit the maximum pressure to 20 torr. Remember that when changing pressure you should have the high voltage off.
Power Threshold
Vary the input power to the discharge and observe when the laser starts lasing. How does the threshold depend on the gas pressure? How sensitive is it to the optical alignment? (Qualitative measurements in both cases).
Output Power Stability
How stable is the output power? Near the lasing threshold are the fluctuations larger or smaller ? (Qualitative measurements).
Laser Spectrum
Use the spectrum analyzer to observe which wavelengths are present. This is the most important part of the lab. You will need the following information.
Beam-Finding
- The beam-finders used in this experiment come in a variety of shapes and sensitivities. But they all work the same way: They are coated with a material which fluoresces when exposed to ultra-violet light. Then as the laser-light hits the fluorescing coating, wherever the beam touches it, it ceases to glow. It absorbs heat and can burn up easily. Do Not Hold it there too long.
- To use a beam finder, wear your safety glasses, turn on the UV lamp and point it at the beam finder. The beam finder will glow brightly. Slowly insert the beam finder into the beam region until a darkening of the surface is observed. That's it, you've found the beam.
- The higher the number, the more sensitive the beam finder. Beam finders \#7, and \#8 can easily be damaged by an incident laser beam of too great an intensity. If the beam spot turns completely dark remove the finder quickly. It should never be set up so you cannot remove it safely in less than 1 second.
- Toward that end, always use a firebrick as a backup surface.
Use caution when inserting anything in the beam, especially any reflective or flammable object.
The Spectrum Analyzer (Spectrometer)
- Before you remove the power detector from the output port place a firebrick behind the detector to stop it from leaving the table area.. Using the mirrors, direct the beam to near the spectrum analyzer input port. CAREFUL! Try not to direct the beam in unintended directions! Wearing the UV safety glasses, use the UV lamp and a beam finder to locate the position of the beam. Then adjust the mirror and spectrum analyzer positions such that the beam enters the input slit.
- Make sure that the spectrum analyzer is level and in the same plane as the laser beam, then turn it on. Note that this turns on a UV lamp inside the unit, so you must continue to wear your safety glasses. Also make sure that the entrance slit is all the way open (adjusted by a knob on the back of the device). Then either open the viewing-lid on the top of the unit just enough to see inside (keeping as much room light out as possible), or turn out the room lights and open it all the way.
- There is a beam finder inside the unit. Using the knob on the back, bring this beam finder into view and hold it perpendicularly to the horizontal. You should see a dark spot on the beam finder. Orient the beam or spectrometer so that this spot lies in the upper left quadrant of the beam finder brackets (see figure 4). Then lower the beam finder. Depending on which frequencies are lasing, you should see at least one spot or line under the appropriate wavelength indicator.
How many lines are lasing simultaneously? Does the lasing spectrum change when the input power is increased from the threshold value, or the gas pressure is changed, or the table is pressed (press down on it with your thumb)?
5. Replace the end mirror with the diffraction grating and measure the wavelengths and intensities of all possible lasing lines, as described below. First, shut off the laser power: for convenience we remind you of the procedure:
To turn off high voltage
a. Flip the regulator power switch off and then on. This cuts off power to the laser.
b. Turn HV down to zero. (CCW).
c. Wait until HV meter reads nearly zero and thenshut off the power supply and regulator.
CAUTION: The high voltage bleeds off slowly, so all points are still electrically hot for several minutes. Don't touch them without supervision or prior instruction.
Swing the grating into place taking care not to touch its expensive optical surface. The grating functions in place of the back-mirror and will allow you to align the laser for one lasing wavelength at a time. Once again you will use the Red alignment laser as an alignment tool. Rotate the mirror as before and set the laser up as before and turn it on. Place a card with a small hole in it between the back Brewster window and the grating inside the large plastic safety box. When the Red beam passes through the hole and hits the grating, you can see the reflected diffraction pattern on the card (see figure 5). The row of dots should be horizontal, and as you move the grating, different dots should fall on top of the hole. Note that the dots are alittle higher then the original laser beam. This is because the Grating is angled up a very small amount. You will need to adjust the output coupler mirror to compensate for this alignment. You should only have to adjust the grating by rotating its mount around the vertical axis (see figure 1, \#5); the other orientations are fixed by what you do with the HeNe. If you cannot make the HeNe interference pattern horizontal, it is possible that the grating is not aligned along its other axes and you should see the staff for assistance. It does not help to turn the small allen-head screw that rotates the grating within its mount, don't do it. (THIS WILL CAUSE A LOSS OF TIME FOR YOU)
Align the grating so that the reflected reference-dot (see figure 5) returns to the output port of the HeNe laser. This grating angle is very close to the proper alignment for the P(20) line. You will have to adjust the grating slightly later since the diffraction pattern of the HeNe is only a measuring tool and this reference dot does not coincide exactly with the CO2 P(20) line. Use a rubber band to hold the grating in position against the micrometer.
Again, protect the HeNe output port by blocking it with a fire brick. Turn on the CO2 laser. Use the power meter and spectrometer to locate each line of the lasing spectrum as the grating angle is changed, and measure its intensity. Keep the input-power constant during this process or you will be unable to compare intensities. You will have to develop a sensitive touch as you move the grating from one line to the next, tweaking it to maximize the output power before each measurement. Also, you will have to discern second order maxima from primary ones. This is by far the most time-consuming part of the lab AND THE MOST IMPORTANT PART. Have patience. There are two sets of lines, centered about 9.6 and 10.6 microns, perhaps 40 lines altogether.
- How is the threshold changed when the back mirror is replaced by the grating?
- Plot the maximum output power versus lasing wavelength.
6. Lasing efficiency. Measure the output power for the P(18) and P(20) lines (10.57 and 10.59 ?m) as a function of the input power. Plot the efficiency curves.
7. Be sure to incorporate the answers to the following questions into the appropriate places of your report:
- What is a laser? What are the characteristics of its radiation?
- What is the principle of the CO2 laser? Which are the relevant energy levels of CO2 molecules? How are they excited to the upper lasing levels? Which transitions are the lasing transitions?
- Energy storage and energy transfer. How do nitrogen and helium in the gas mixture help increase the laser output?
Apparatus Layout
The above picture is the room layout and air table for the Laser system.
This diagram or the one below; check them out which is in effect.
