: Lasers and Light Sources
Helium-Neon Lasers (2016F)

Introduction

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"!)

HeNe laser tube
Here, a HeNe tube mounted in two three-point mounts is seen lasing. The power supply is the small block behind the unit on the breadboard - the thick red wire is the high-voltage output and connects to the anode of the tube via a series ballast resistance. It runs from a 12 volt lab-bench power supply. Note that there is output from both ends of the laser - this will be used to determine the reflectivity of each optic.

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)

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

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.

The Experiment, Part A (Electrical, Optical)

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:

HeNe laser tube with power supply
HeNe laser tube with power supply

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.

HeNe laser optic Now, measure the reflectivity of the OC optic provided (extracted from a Melles-Griot 05-LHR-097 tube above) as follows:


HeNe laser optic - measuring transmission

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)

Analysis (for Part A - MG Tube Only)

Now, for the 05-LHR-097 tube only, for which you were provided with separate optics, calculate the following parameters of the tube (in order):

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 Experiment, Part B (Quantum)

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 ...

    Electrical Characteristics (primarily from the prelab reading)
  1. What is the purpose of the ballast resistance in a HeNe supply? Make specific reference to the electrical characteristics of a gas discharge as compared to an "ohmic" resistance. What might happen to a power supply if the ballast resistor was omitted?
  2. Why is a high voltage (> 5kV) start pulse required (i.e. describe what happens in the plasma tube)? Describe another CW gas laser which has "start pulse" generator (an actual physical circuit designed exclusively to produce a start pulse) and a 'run' voltage much lower voltage than the start pulse (cite the source where you found this).
  3. Describe all optical elements in a typical HeNe laser tube using a diagram. Give specific information on typical parameters for the optical elements (such as %T, %R, Radius of curvature).
  4. Describe the electrical elements of the tube (the anode and cathode) and describe why they are constructed as they are (i.e. the size, material, and structure of these elements). Specifically, answer why one element is much larger than the other and why one is made of a specific type of metal.
  5. Why does the output power of a HeNe laser DECREASE, eventually reaching zero, as current through the plasma tube is increased? Be very specific and make reference to the quantum excitation and de-excitation mechanisms (and specific atomic neon energy levels affected) in the laser to explain this. You will need to explain populations of excited atoms at both the LLL and ULL and how these change as current is increased.

  6. Optical Characteristics (primarily from the lab)
    For all numerical solutions, use the accepted parameters (e.g. attenuation) for the HeNe laser from chapter 8 of Laser Modeling which lists parameters for all common lasers
  7. Show observations (with an explanation of how you accomplished this) of the forward vs. reverse output beam power for all three types of tubes examined. Include measurements of attenuation and gain lengths as observed in the lab.
  8. For the one tube with separate optics provided (for which you measured the reflectivity of the OC), calculate the intra-cavity power and then the reflectivity of the HR. Be sure to show calculations. Using these values, compute the threshold gain of the tube (this is the minimum gain the medium must supply to allow the tube to oscillate). Use xa and xg parameters as done in the lectures.
  9. How do mirrors for most lasers achieve high (>95%) reflectivity not possible with simple metal coatings alone? Describe the structure of these mirrors (include a diagram showing the "stack" of multiple layers used) and describe, in a paragraph, how dielectric mirrors (also called "dichroic" mirrors) work. Be sure to detail the principles of operation, the thickness of the layers, and why that thickness is a critical parameter.
  10. Using an estimate for the small-signal gain (g0) of the HeNe laser of 0.15m-1 (and attenuation as per Laser Modeling), knowing the length of the gain medium of each tube (as measured in the lab), and assuming an HR of 100%R, calculate the minimum required reflectivity of the OC for each tube to allow it to oscillate. To do this, set gth equal to 0.15m-1 and solve for the reflectivity of the OC (usually R1 in the gain equation). Report all three figures and show all work (again, use xa and xg parameters in the same method as outlined in lectures).
  11. Assuming the green (543.5nm) HeNe laser transition has a small-signal gain (g0) of only 0.06 times that of a red HeNe laser (see chapter 9 of Fundamentals by Csele, which was a required prelab reading), calculate the minimum reflectivity of the OC for a "GreeNe" assuming the same parameters (including HR reflectivity) as question 7 which pertains to the stronger red transition. (Expect a reflectivity much higher than required for the red transition but it must obviously be less than 100% - if it is not, you have a calculation error). Do this calculation only for the one tube for which separate optics were provided (M-G 097 tube).
  12. Regardless of the output line selected (632.8nm or 543.5nm), the infrared line at 3.39μm has a much higher gain (see Laser Modeling of Csele section 2.6). Since this infrared line shares an upper lasing level with all visible transitions, this infrared line must be INHIBITED from lasing by selecting optics with a reflectivity below that required for lasing threshold of that line. Calculate the maximum reflectivity of the OC for the tube above (assuming the HR is broadband and has almost perfect reflectivity at 3.39μm) in order to inhibit this line and hence allow visible transitions to oscillate.

  13. Quantum (primarily from the lab)
  14. 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.
  15. 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).
  16. 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).