Abstract
We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.
Highlights
Laser-based spectroscopy for the detection and monitoring of trace gases is established for many regions of the electromagnetic spectrum with applications in biochemical, environmental, astronomical, industrial, and security sensing [1,2,3,4,5]
We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser
The absolute frequency calibration was done by a comparison with a CH3OH spectrum, which allows for the determination of the frequency tuning of the quantum-cascade laser (QCL) which is approximately 8 MHz/mA
Summary
Laser-based spectroscopy for the detection and monitoring of trace gases is established for many regions of the electromagnetic spectrum with applications in biochemical, environmental, astronomical, industrial, and security sensing [1,2,3,4,5]. Wavelength and frequency modulation techniques are frequently used for improving the detection sensitivity in laser absorption spectrometers [6,7,8]. In both cases, the frequency of the laser is modulated by some means, for example by using an external acousto-optical modulator or by modulating the driving current of the laser. The term wavelength modulation is used when the modulation frequency is smaller than the width of the investigated molecular absorption line. At infrared and visible wavelengths, detector-limited sensitivities as low as 10−7−10−10 have been demonstrated with FM spectroscopy and lead-salt or GaAlAs diode lasers [3, 9]. The technique was extended to an infrared quantum-cascade laser (QCL) [10]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
