Abstract
We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL). The method is solely based on the modulation of the external cavity length, exploiting the intermediate optical feedback regime. While the QCL is operated in continuous-wave mode, optical feedback results in a change of the QCL frequency as well as its terminal voltage. The first effect is exploited to tune the lasing frequency across a molecular absorption line. The second effect is used for the detection of the self-mixing signal. This allows for fast measurement times on the order of 10 ms per spectrum and for real-time measurements of gas concentrations with a rate of 100 Hz. This technique is demonstrated with a mixture of D2O and CH3OD in an absorption cell.
Highlights
Spectroscopy at terahertz (THz) frequencies has attracted increasing interest in recent years [1,2]
We report on real-time gas sensing with a terahertz quantum-cascade laser (QCL)
We present a real-time external optical feedback (EOF) sensor based on a QCL operating at 4.7 THz in an intermediate feedback regime
Summary
Spectroscopy at terahertz (THz) frequencies has attracted increasing interest in recent years [1,2]. Since scattering processes typically occur on a ps time scale, the response time of QCLs to EOF is intrinsically suitable for very fast, real-time spectroscopic applications [12,16] Such a realtime EOF spectrometer based on a QCL has been theoretically proposed for high-speed material detection and identification [17]. The study comprises an experimental as well as theoretical investigation and demonstrates that THz spectroscopy through EOF is not limited to weak feedback This allows for short measurement times on the order of 10 ms for a single spectrum with a 200 MHz coverage. The results are compared to conventional direct absorption measurements
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