Lasers
HeNe | CO₂ | Argon Ion | Excimer

Of all the lasers I've worked with, I've probably used the HeNe for the majority of the time. In our lab kits, we had standard HeNe lasers (632.8nm JDS Uniphase) to do most of our experiments. The labs that were done in first and second year with the general kits built a base of knowledge on optics and the nature of light. In later courses, we used HeNe lasers to calculate the gain per meter of our laser, and built a complete cavity by manually aligning mirrors. It was this basis that helped me excel in projects working at Niagara College in the summer of 2003. Below are some examples of research I have done thus far.

Direct Gain Measurement Lab
My professor came to me with an idea for a research project. Our program purchased about 10 bare Melles-Griot HeNe tubes, which were mainly used for a HeNe alignment lab. As shown in the picture below a bare tube was mounted and used as the 'gain' medium. A 'source' laser was taken and aligned such that the beam went directly through the gain medium.

Copyright (c) 2003 Mark Csele
To be published in Fundamentals of Light and Lasers, Wiley, 2004
Image with Gain Medium Off

The gain of the laser is now easy to calculate, as it's a ratio of the power out with the gain medium switched on, and the gain medium switched on.
In this picture a green HeNe is shown as the source laser (for asthetic purposes), however, in a class lab setup a regular red HeNe would be used.

Cycling Characteristics of a HeNe laser
An experiment was done to show how a unstabalized HeNe laser 'cycles' its output power levels as it runs and heats up. With a HeNe laser, the more it heats up the more stable it becomes (much like a lot of equipment). Results were taken with an OPHIR power meter hooked up to a computer that logged the power readings over set periods of time. The laser that was used was a JDS Uniphase HeNe (632.8nm) rated at 4 mW, it was manufactured in 2001. Before each of the graphs shown here, the laser was at room temperature (not warmed up from previous use). The results are below as follows (click the image for a hi-res version):

After 20 minutes of operation, click this image to see a zoomed in view of the power output

Notice how there is a ripple in the graph, this becomes much more evident around the 6 minute mark. You can see that the power 'cycles', in the next picture the laser has been running for a longer period of time.

After 2 hours of operation, click this image to see a zoomed in view of the power output

After two hours it takes much longer for the tube to cycle its power. After a couple of hours the tube is much more warm and is producing a much more stable output.

This is the HeNe for 5 minutes after running for 16 hours

Obviousally the laser has become much more stable over 16 hours, this is why people usually leave their HeNe on overnight if their going to be doing an experiment with it the following day, and proof why HeNe tubes should be warmed up to give a much more consistent power output.

A way around this 'cycling' of power is to use a 'stabalized' laser, this type of laser basically has a heater around the tube that warms everything up. The heater on this laser would normally be left on constantly, and the power output of the laser would be close to the consistency (if not better) to the diagram above.

Lining up a HeNe Laser Cavity
This is something we did as a lab in second year, and was done again in the summer time, when we took some explanatory pictures of how to line up the optics. Below are a couple of pictures we took of the setup, you can click on the picture for a hi-res version.

Copyright (c) 2003 Mark Csele
To be published in Fundamentals of Light and Lasers, Wiley, 2004

Copyright (c) 2003 Mark Csele
To be published in Fundamentals of Light and Lasers, Wiley, 2004

With the experiment shown above, it is also possible to force different transverse modes; this is done by altering the cavity alignment to produce different modes. The most widely used (and recognized) mode is TEM₀₀ mode, this is a gaussian beam profile which produces a single 'dot', obviously other modes are possible, they are shown below.

TEM₀₀ mode

TEM₀₁ mode

TEM₀₂ mode

TEM₁₁ mode

All pictures of TEM modes:
Copyright (c) 2003 Mark Csele
To be published in Fundamentals of Light and Lasers, Wiley, 2004