The wavelength range l =3-4 mm contains the strongest
absorption bands of methane (CH4), formaldehyde (H2CO), and other hydrocarbons,
whose intensities exceed by one or two orders of magnitude the intensities of overtones in
the near-IR region where it is common to detect gases. Because of this, the application of
tunable lasers in the range of l =3-4 mm considerably expands the potentialities of high resolution
diode-laser spectroscopy and precise atmospheric trace gas analysis.
Tunable diode lasers are realized on the base of InGaAsSb/InAsSbP DH with 10-20 mm wide
mesa
stripes. Threshold currents Ith=7-20 mA (77 K), single-mode power of
several milliwatts in the cw regime, the current tuning rate 30- 200 cm-1/A,
and linewidth less than 5 MHz are characteristic features of 3000-3300 nm lasers
Top fig.: Current dependencies of cw output power and spectral purity
for laser with stripe width w=20 mm and cavity length L=140 mm at T=77, 100 K (a) and temperature dependence of the threshold
current and differential quantum efficiency (b)
Bottom fig.: Mode chat of a single-mode laser (a); oscillograms of a
pump pulsed current (b, left scale), signals from a photodetector measuring the radiation
transmitted through a Fabry-Perot etalon and a gas cell filled with a 1% CH4-N2
mixture at atmospheric pressure (b, right scale); measured transmission spectra for l %
mixtures of CH4 with N2 at pressures of 10, 50, and 150 hPa and
HITRAN data for the mixture at 10 hPas (c).
