Operating Characteristics of Helium-Neon Lasers

Mallory Fischer, Eastern Michigan University;
John Noe, Laser Teaching Center, Stony Brook University

Helium neon (HeNe) lasers can be found in labs and classrooms everywhere. They provide highly collimated beams with excellent quality at modest power levels up to tens of milliWatts. The most common wavelength is 632.8 nm (red), but several other visible wavelengths are also available. In the laser, an electrical discharge excites helium atoms to long-lived states which then transfer their energy to the neon atoms through collisions, thus maintaining the population inversion needed for stimulated emission. Each laser requires a power supply that is able to provide both the proper operating current and a starting voltage, which can be up to five times the operating voltage. Operating voltage scales roughly with tube length, while operating current depends mostly on the bore diameter of the capillary tube that contains the excited plasma.

The LTC currently has a family of thirteen HeNe laser heads and seven power supplies, most of which were obtained as surplus at very low cost. The power supplies each contain a sealed "brick" which provides a regulated current over a range of voltages. In some of the units the current is fixed and in others it can be adjusted with a screwdriver trimpot. While all lasers are physically compatible with all power supplies (all have the same Alden-type high-voltage connectors) only some combinations are electrically compatible.

In the first part of this project all available information on each laser and power supply was collected and compiled in a spreadsheet. Lasers and supplies were assigned reference numbers and letters respectively, and a compatibility matrix was created to provide a quick reference for recommended combinations.

More recent work has concentrated on measuring the V-I characteristic of each laser, and the dependence of laser output power P on current I. Digital meters can be damaged by high voltage transients, so two analog meters obtained from surplus equipment were configured to read 0-10 kV and 0-10 mA, respectively. These meters can easily be inserted between the laser and power supply using Alden patch cables. The output power of each laser peaks at an optimum current that can be found from the P-I curve; operating at a higher current just shortens tube life. V-I curves are expected to display the negative resistance phenomenon characteristic of a gas discharge.

This work was supported by a grant from the National Science Foundation (PHY-0552521)

Mallory Fischer
June 2007
Laser Teaching Center