An excellent resource on lasers: http://www.explainthatstuff.com/lasers.html
HeNe laser 632.8 nm, GaAs laser 904 nm
Excerpts from this presentation https://goo.gl/Fp5B60
Laser equipment is grouped into four FDA classes with simplified and well-differentiated safety procedures for each. Low power lasers used in treating sports injuries are categorized as Class I and II laser devices.
Class I or "exempt" lasers, are considered non-hazardous to the body.
All invisible lasers with average power outputs of 1 mW or less are class I devices.
Include the GaAs lasers with wavelengths from 820 to 910 nm.
Class II, or "low-power" lasers are hazardous only if a viewer stares continuously into the source.
Includes visible lasers that emit up to 1 mW average power, such as the HeNe laser.
HeNe (gas) lasers deliver a characteristic red beam with a wavelength of 632.8 nm.
Laser delivered in a continuous wave and has a direct penetration of 2 to 5 mm and indirect penetration of 10 to 15 mm.
GaAs (semiconductor) lasers are invisible and have a wavelength of 904 nm
Average power output of 0.4 milliwatts.
Direct penetration of 1 to 2 cm and an indirect penetration to 5 cm.
PhysioTechnology the only manufacturer in the United States that currently produces low power HeNe and GaAs lasers.
Absorption of HeNe occurs within first 2-5 mm of soft tissue with an indirect effect of up to 8-10 mm.
GaAs which has a longer wavelength directly absorbed at depths of 1-2 cm and has indirect effect up to 5 cm.
Better for treating deeper tissues.