Tunable Diode Lasers (TDL) for Spectroscopy and Environmental Monitoring

The current status of III-V semiconductor diode lasers emitting between 1 -5 μm wavelengths to be used as light sources for absorption spectroscopy is reviewed. The emission wavelength of the laser is chosen to coincide with the primary absorption line of a molecule or one of its many overtones. The lasers, with a single longitudinal mode emission, are wavelength tuned over several angstroms by modulating the drive current of the device. This sweeping of the wavelength leads to the nomenclature tunable diode laser or TDL. Single mode distributed feedback (DFB) strained layer InGaAs(P) lasers grown on InP substrates with emission wavelengths from 1.2 to 2.06 μm have been developed at JPL, and several devices will be used for planetary atmospheric studies for the first time. TDLs at the wavelengths of 1.37 μm, 1.43 μm, and 2.04 μm for the detection of water, carbon dioxide and CO₂ isotopes respectively have been fabricated for implementation in the Mars Volatiles and Climate Surveyor (MVACS) instrument as part of Mars '98 flight. The performance of antimonide-based quantum well lasers emitting in the 2 to 5 μm range has improved significantly in recent years. Pulsed performance at temperatures over 200°K and CW performance over 150°K has been reported at wavelengths over 3 μm. Further reduction of the non-radiative Auger recombination to improve the CW operation of the antimonide-based lasers at higher temperatures is currently under investigation by several groups.