Liquid detection by InGaAsP semiconductor diode lasers with multiple short external cavities (MSXC)

The paper reports a scheme of multiple short external cavities (MSXC) to force lasers to operate in modes spaced by up to 72 nm and the application of the MSXC laser to the detection of liquids. The mechanism of mode selection is to use two cavities, one to choose the mode enhancement region in the gain profile and one to confine the enhancement to a single mode. A glass plate of thickness 150 μm is attached on the front facet of the laser by an index-matching gel to select a mode, and the lasing-mode region is chosen by a mirror positioned approx.10 μm behind the laser and moved by a piezo-electric translator (PZT). The spectral output of the laser as the mirror is moved through λ/2(approx.0.7 μm) is shown. Eighteen distinct modes with each mode spaced about four free-running laser modes apart cover a spectral range of 72 nm, which provides enough contrast for the detection of liquids.