PHTN1300: Lasers and Light Sources
Lab #3B Helium-Neon Lasers (2017F)

Introduction

Students will study the quantum mechanics of the helium-neon gas laser with the application of such 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"!)

SAFETY WARNING

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!)

Prelab

The Experiment

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.

McPherson 2016 1m Monochromator

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.

Entrance Slit for the Monochromator 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.

Step Motor Driver 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).

Wavelength Dial 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.

Measuring HeNe tube spontaneous emission lines
Setup for measuring HeNe spontaneous emissions

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.

Assignment

The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, the student's name and ID number, and the names of your lab partner(s).

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 (with the title "Question 2") and question 3 starts at the top of a different page (with the title "Question 3"), etc. You'll have, therefore, at _least_ fourteen pages in this report.

The lab must be submitted in a report cover (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 outline, used for all condensed labs in this course, will result in deduction of marks

To be done individually ...

  1. 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.

    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 on this question where a numerical answer is given but no calculations are shown.

  2. Compare the intensity of the transitions of neon used in this experiment in a pure neon discharge, and the spontaneous emissions from a HeNe laser discharge (with the HeNe laser lasing - the Siemen's tube). To do this, for each of the three pairs of transitions (where the lower level of the 5s->3p transition serves as the upper level for the 3p->3s transition), calculate the ratio of emission intensities from the 5s->3p transition over the 3p->3s transition as a percentage.
    Now Organize data as a table with columns showing the pair of wavelengths involved, and the relative intensity of the 5s->3p transition (as a percentage ratio of the 3p->3s transition). There will be two tables, one for pure neon, one for neon with helium added as a pump gas.
  3. Describe why, in a pure neon discharge, one expects transitions in the 3p1 to 3s1 series to have higher intensities than those of the 5s1 to 3p1 series. To understand and explain this, consider the distribution of energy in such a discharge (covered in chapter 3 and 4 of Csele).
  4. Describe the role of helium in the helium-neon laser discharge and detail how the presence of helium in the system allows the excitation of energy levels which lead, for example, to higher relative intensities in the 5s1 to 3p1 set and ultimately to gain in the visible laser transitions like the 632.8nm transition (in short, why helium is required in the HeNe laser ... be sure to reference the exact energy levels and how energy levels in neon are affected).