First, what is a Helium Neon Laser a.k.a HeNe Laser? Unlike modern laser "pointers," which use semiconductor diodes as the gain mediums, HeNe lasers use noble gases instead. Other gases commonly used to generate coherent light include Argon and Krypton ("ArKr"). By exciting the helium atoms in the tube, which collide with the neon atoms, the particles emit photons of a specific wavelength.
I decided to take a chance when I bought this used Melles Griot 543.5nm Helium Neon Laser, purchased from a member of the Laser Pointer Formus. This wavelength was supposed to produce a bright green laser, however, the unit was not guaranteed to work. Nonetheless, the prospect of a functional HeNe laser in my room was enough for me to try my luck with it.
Fortunately, the seller had already extracted the HeNe tube from the original housing.
The tube is composed primarily of glass, with a smaller tube inside where the light is concentrated and amplified when power is applied. It really is a beautiful device.
The big trick was going to locate a proper power supply..
These kinds of lasers require an extremely high voltage (but low current) to operate. A startup pulse of multiple kilovolts is required to light the tube. I came up with the requirements for this model of:
> 10kV startup pulse ~ 3000V output voltage < 10mA current draw
It turns out the best place to find a good power supply is eBay. I found a seller that sourced Voltex Inc. power supplies in various configurations, so I picked one up that fit the specifications I needed.
To connect the PSU to a power source (120 - 240v AC), I ended up using an old computer power cable I had lying around, soldering the wires together according to the specs sheet.
Now I had a power supply ready to go!
Next, I had to extract the wiring harness from the original case, including the ballast resistor potted inside the endcap. I took this out and added some extra length to the wires. Each end of the connectors have special terminations that fit onto the HeNe tube.
Then I connected the harness to the power supply with the supplied two-prong connector and snapped the two connectors onto the laser.
Only thing left to do, plug it in!
The PSU comes with a built in 5 second delay for safety. After making sure that both ends of the tube were isolated, I plugged in the power supply and counted to five. The process doesn't work 100% of the time, but most of the time there is a small 'click' and:
The inner bore where the light is being bounced back and forth can clearly be seen. Awesome!
But.. let's remember that this is a laser, not a desk lamp. We need output! And this is where my luck seems to have run out. These lasers have surface mirrors at both ends to amplify the light inside, and both need to be perfectly aligned for the light to bounce back and forth and not be wasted. At some point, the mirrors had gotten knocked slightly off, so 543.5nm output is periodic and unreliable, and at a fraction of the rated power for sure.
Below, the wasted light can be seen reflected off of the end of the tube...
All was not lost, however! By putting light pressure on one side of the tube, I could push the mirrors closer to alignment to achieve a decent output of a brilliant green color!
I believe this laser was worth every penny. It is a beautiful work of engineering and it's great to see it in action.