Experimental investigation of unidentified helium–neon lasers

The helium–neon (HeNe) laser used to be a staple in every industrial or university lab having anything to do with photonics, as well as in numerous commercial products such as barcode scanners. They are now less common but still essential for many areas of research and education, and may turn up without any documentation. This paper describes an interesting experience of attempting to operate a HeNe laser without any prior information about its input–output specifications. A HeNe tube was encapsulated in a plastic cover with a copper-foil-wrapped power supply jack attached to it. It was like an optoelectronic gadget without any mention of its operating instructions, wiring, or power requirements. This HeNe laser was investigated as part of a research methods course case study. One week’s testing and investigation led us to conclude that the plasma tube was either defective due to mirror misalignment or internal contamination. As a result of this troubleshooting experience, a well-engineered investigation procedure was developed for testing HeNe lasers. A systematic approach helped to prioritize investigation of mirror alignment, mirror coating damage, possible debris inside the cavity, and power supply problems. General guidelines as well as this specific diagnostic experience can be very useful for laser technicians and engineers.