Students will study the quantum mechanics, as well as the electrical and optical characteristics of a commercial helium-neon gas laser tube. Specific topics that will be investigated in the course of the lab are the characteristics of gas discharges (e.g. negative resistance and the need for ballast resistance), determination of the optical elements of a typical gas laser (e.g. cavity mirror reflectivities), computation of threshold gain for a given optical configuration, and an application of quantum mechanics principles including use of pumping to populate upper energy levels.
This is one of the two most important labs in this course (They might be called "Über labs"!)
This lab consists of two parts which will be performed alternately by different groups of students (i.e. you may be completing part B before part A)
The purpose of this lab is to expose students to techniques involved in using gas laser tubes and their related power supplies. Be aware that these lasers operate at 1500 VOLTS and the starting pulse for such a tube can reach 10,000 VOLTS. With currents limited to under 5mA, death is unlikely (possible, but unlikely) but PAIN IS LIKELY, so be careful when performing this lab experiment and AVOID touching the metal ends of the tube. Use the high-voltage shields provided to cover live anode terminals to prevent accidental exposure. Also, short-out the high voltage terminal to ground before touching the wiring since capacitors in the supply can store energy even after it is turned off (do this by attaching a jumper to the grounded cathode first then touching the other end of the jumper to the anode terminal briefly). Failure to short-out the HV terminal will result in the student learning what CAPACITANCE is the hard way (i.e. through a nasty jolt!)
|WARNING: Arrival to the lab without your parts kit will result in being marked ABSENT with the accompanying penalty (including a deduction in marks and being placed on course condition) - you will NOT be permitted time to "run home" to obtain your kit.|
Using the basic gain threshold equation from the text (equation 4.9.2 in Fundamentals of Light Sources and Lasers or 2.5 in Laser Modeling in which the gain and attenuation lengths are considered to be the same - a misconception that will be corrected later in this lab), and knowing the small-signal gain (g0) of the HeNe laser medium (found in table 8.1 on page 222 of Laser Modeling), compute (in the same manner as the end-of-chapter question 4.9 in FLL) the minimum reflectivity of the OC for both the longest and the shortest HeNe tubes which Melles-Griot manufactures (which range from 0.4mW to 35mW in output power). Attenuation can be found on pg 222 of the Laser Modeling text.
You will need to look at this Listing of Melles-Griot Red HeNe Tubes at SAM's laser FAQ for physical tube parameters.
As usual, show all work for both sets of calculations for marks (answers without calculations shown will receive a mark of zero).
As seen in this reference, the white-light HeCd laser has a complex quantum system. Understanding it, including application of quantum rules will help with understanding the HeNe and other lasers. This prelab assignment also serves as a review of basic quantum concepts covered so far in the course.
Compute the energy levels (in eV) of the upper and lower energy levels of the 442nm, red, and green transitions (eight levels in all).
Use the NIST eBook (which you used last year) to determine the energy levels of the Cadmium-ion (Use the NIST Levels Database and select the "Cd II" spectrum - type in "C" "d" "_" "i" "i" - representing Singly Ionized Energy Levels for Cadmium since it is an ion laser). Note that the levels given are relative to the Cd ion ground state so that any answer in eV must be added to the Cd+ ionization energy of 72540.07 cm-1 (8.995eV). Since energies in the NIST tables are provided in units of inverse cm, use a converter on the web to change all answer into units of eV. By comparing the resulting determined levels with the diagram provided some (but not all) key levels may be determined. Knowing the wavelength of transitions (from the diagram), the energies of all other levels can be determined. For example, the lower level of the 325nm transition (not shown, but easily determined) is found in the NIST tables to be 44136cm-1 so the actual level is 116676cm-1 or, converted to eV, 14.48eV. The upper level can then be found by adding the energy of the 325nm photon (also converted to eV). In the case of the green and red transitions, only the energies of the lower levels are available from the NIST handbook: these are the 4d105d [2D5/2] and 4d105d [2D3/2] energy levels (which are listed). Knowing the wavelengths of the transitions, deduce the energy levels as required (since the 4f and 6g levels are not even listed in the NIST tables ... without the presence of helium as is the case in this laser, the population of these levels would be incredibly small).
Now, draw a complete transition diagram like that provided but more complete and with all energy levels (of each and every level for each transition) shown in eV directly on the diagram - you may start with the diagram provided and simply add the energy levels in eV beside each individual level shown for Cadmium. Be sure to calculate energies to at least FIVE decimal places and be sure they make sense (i.e. they should be close to those on the side of the diagram provided).
Submit both the diagram above and a series of calculations showing how the four energy levels for the 635.5nm and 636.0nm red transitions, as well as the green transitions, were calculated. A zero will be received where a numerical answer is given but no calculations are shown.
Start the small HeNe laser at the bench (the commercial, packaged one, not the bare tubes used for the rest of this procedure) and allow it to stabilize during the first part of the experiment (below). You may close the shutter on the front of the laser for safety but leave the laser energized the entire time (read chapter 9 of FLL by Csele to see why this happens). This laser will be needed LATER in the experiment to measure the reflectivity of laser optics.
Install an unmounted Siemens LGR7641 HeNe laser tube in two three-point mounts on a breadboard as shown in the photo at the top of this page. Face the output end (with the warning label "Laserstrahl") of the tube towards a Gentec UNO power meter also on the breadboard. Turn on the meter and set for a wavelength of 633nm.
Now wire the laser power supply as follows:
Turn off the tube and repeat power measurements for the other two tubes - a Uniphase 098-1 and a Melles-Griot 05-LHR-097. The power supply we use (a 'block' type supply) is current regulated and so the current through each of the three tubes will be the same allowing a reasonable comparison of output powers. When turning the tube off, briefly short the tube with a piece of wire to discharge any capacitance in the supply.
Now, measure the reflectivity of the OC optic provided (extracted from a Melles-Griot 05-LHR-097 tube above) as follows:
The setup for measuring OC transmission. Note that the power meter must be over 30cm from the front of the preheated bench laser to avoid measuring spontaneous emission from the front of the laser (the "blue glow").
You will measure the optical parameters for only the Melles-Griot 05-LHR-097 tube (and will base gain calculations on that specific tube only)
For that one tube, then, you should now have values for %ROC, %RHR, and length of the gain medium (OC = Approx 99%, HR > 99.9%, but you will compute both as part of this lab using the procedure outlined above).
For the 05-LHR-097 tube only, compute the threshold gain of the laser (gth) in the same manner as example 2.1 in Laser Modeling by Csele. Gamma (γ) is attenuation of the medium and can found on page 222, R1 and R2 are the reflectivities of the OC and HR which you have computed/measured as part of this lab (and are dimensionless so that "1.00" represents 100% reflection), and xg is the length of the actual gain medium in m - you must measure this in the lab - it is the length of the inner glass tube (approx 1mm diameter bore) through which the discharge is confined and actual laser gain occurs (where the discharge is not confined, such as the space between this inner glass tube and the rear optic, no laser gain occurs).
The purpose of this section of the lab is to demonstrate how the presence of helium allows population of the ULL of neon required for the HeNe laser, and how a pure neon discharge does not allow adequate population of these levels.
Since there is only one MacPherson spectrometer in the lab to use, larger groups will be necessary for this part of the experiment.
In order to resolve the closely-spaced emission lines involved in this experiment accurately, the McPherson 1m monochromator must be used. This monochromator has a resolution of 0.05nm allowing details to be seen that were not apparent using the spectroscopes from earlier labs. In this case, gas discharge tubes will be used as sources. The tube is aligned so that its emissions fall onto the entrance slit for the spectrograph. Start with the slits (both entrance and exit/PMT) open 0.006" (i.e. one quarter turn from fully closed), the photomultiplier power supply switched on allowing it to warm-up (Read the notes on PMTs from the course home page), the meter switched on, and the shutter in front of the PMT open. Using the manual wavelength control, the monochromator is now scanned across major emission lines.
For ultimate accuracy, as required in this lab, slits must be closed as narrow as possible while still allowing enough light for a reasonable signal intensity - in this experiment the strong line of neon at 650.65nm (a non-lasing transition) should yield a signal of between 1V and 2V as read on the DMM attached to the output of the PMT. In order to read the intensity of a line, adjust the wavelength control carefully for maximum output. When the experiment is complete, the shutter on the PMT must be closed to protect its sensitive cathode from excessive light exposure.
Wavelengths may now be selected, if desired, using the step motor driver on the unit or manually using the wavelength selector knob. To use the drive, simply switch it ON and use the pendant control to move the wavelength up or down as required. The pendant switch has several positions: pressing lightly on the switch causes the wavelength indicator to move slowly while pressing it harder causes the wavelength indicator to move quickly. When you are "close" to the desired target wavelength, use the slow position to optimize for highest output signal from the PMT.
The driver has an auto-off feature: when the pendant is released the motor is held ON for one minute after which the drive is released and current flow ceases to the motor to avoid overheating. Meter readings must hence be taken within one minute of selecting the target wavelength before the drive releases for accuracy.
Additional Images of the Monochromator:
Details Of The Grating and the drive mechanism allowing scanning of wavelength ranges
Dispersion - a photograph of the light inside the monochromator after it has been dispersed by the mirrors and grating. White light was used here showing the visible spectrum. A particular wavelength of this spectrum is then selected by the exit slit and passed to the PMT for detection
As of 2013/10/22 the offset error is between 0nm and -0.20 nm (i.e. add 0.20nm to the desired wavelength to get the target wavelength for the monochromator dial so the 626.65nm line will be found around "626.85nm" on the indicator).
Begin by installing a pure neon discharge tube into the power supply and align the tube in front of the entrance slit. As usual, ensure the fluorescent lights in the lab are shut off and the incandescent room lights are employed. Adjust the wavelength control for the 650.65nm line and position the tube (left/right) for maximum signal. Adjust the entrance slit approximately 1/4 turn open and the PMT slit approximately 1/16 turn open. The signal for the 650.65nm line should now read approximately 2 Volts.
After reading the signal at 650.65nm, and without adjusting the slits or the lamp position, read the intensity of the following lines: 632.82nm, 629.37nm, 626.65nm, 614.31nm, 611.97nm, 594.48nm. Be careful to adjust the wavelength control for maximum output at each wavelength. Some of the wavelengths correspond to 5s1 to 3p1 laser transitions and others to 3p1 to 3s1 spontaneous transitions as outlined on the diagram provided in the prelab.
Next, mount a HeNe laser tube vertically on a retort stand so that emissions from the side of the discharge enter the monochromator. Adjust the wavelength for the 650.65nm reference line. Again, align the tube for a signal of about 1 to 2 Volts. Obtain the intensities of the same lines as previously measured using the pure neon tube.
To be done individually ...