Del Mar Photonics

Characterization of Terahertz Quantum Cascade Lasers for laser
spectroscopy applications
Neil Macleod, Damien Weidmann
Space Science and Technology Department, STFC, Rutherford-Appleton
Laboratory, Oxford OX11 0QZ, United Kingdom.
Paul Dean, Edmund Linfield
Institute of Microwaves and Photonics, School of Electronic and Electrical
Engineering, University of Leeds, Leeds, LS2 9JT, United Kingdom.
The development of compact, high power, single mode laser sources
provides the primary tool for high sensitivity molecular sensing across a
variety of disciplines including astronomy, atmospheric monitoring and trace
detection of illicit materials. Quantum Cascade Lasers (QCLs) [1,2] have
proved to be efficient sources of radiation throughout the mid infra-red
region (5-15 microns); the combination of compactness, high power, room
temperature operation and broad wavelength tuning range provides a
formidable spectroscopic and analytical tool, which has enabled the
development of high sensitivity molecular sensors operating both in-situ and
remotely.
The current challenge is to extend the capabilities of QCLs towards longer
wavelengths to cover the far infrared and the terahertz region of the
spectrum. Availability of reliable, single mode QCLs in this spectral window
(which contains pure rotational transitions) would drive the development of
laser-based spectro-radiometers. Molecular trace sensors active in this region
would yield lower detection limits, a significant decrease in the noise
temperature associated with heterodyne detection systems and high
resolution spectroscopic measurements on unstable radicals and weakly
bound molecular clusters. [3,4].
In the present work, a number of QCLs were examined with a particular
focus on spectral tuning range; it is this characteristic which distinguishes
QCLs from other laser sources of terahertz radiation. An optical cavity
created between the laser chip and the detector allowed tuning rates of the
order of 50-100 MHz/K to be determined. Peak power output in the milliwatt
range was observed for operating temperatures up to 100 K giving tuning
ranges greater than 0.1 cm-1, which are sufficient to cover entire absorption
bands at low pressures where Doppler broadening is dominant. Further
characterization of different QCLs was performed, including power,
operational parameters, and laser beam spatial profile.




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