Alternate intensity modulation of a dual-wavelength He-Ne laser for differential absorption measurements

A simple method is demonstrated for internal intensity modulation of the 3.391 and 3.392-micron emissions of a He-Ne laser with equal amplitudes which are 180 degrees out of phase to each other. A modulation amplitude of 0.7 mW peak-to-peak at 1 kHz for the individual emissions has been obtained from a laser plasma tube 50 cm long while maintaining the total-intensity modulation as low as 0.25 microwatts for a signal averaging time of 1 s. This laser source can greatly simplify the setup and improve the sensitivity of differential absorption measurements for the methane remote sensing.