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Self-Mixing Interferometry With Terahertz Quantum Cascade Lasers

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12 Author(s)
Valavanis, A. ; Sch. of Electron. & Electr. Eng., Univ. of Leeds, Leeds, UK ; Dean, P. ; Yah Leng Lim ; Alhathlool, R.
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Terahertz frequency quantum cascade lasers (THz QCLs) are compact sources of coherent THz radiation with potential applications that include astronomy, trace-gas sensing, and security imaging. However, the reliance on slow and incoherent thermal detectors has limited their practical use in THz systems. We demonstrate THz sensing using self-mixing (SM) interferometry, in which radiation is reflected from an object back into the QCL cavity, causing changes in the laser properties; the THz QCL thus acts simultaneously as both a source and detector. Well-established SM theory predicts a much weaker coupling in THz QCLs than in diode lasers, yielding a near-linear relationship between the phase of SM signals and the external cavity length. We demonstrate velocimetry of an oscillating reflector by monitoring SM-induced changes in the QCL drive voltage. We show that this yields data equivalent to that obtained by sensing the emitted THz power, thus allowing phase-sensitive THz-SM sensing without any external detector. We also demonstrate high-resolution SM-imaging at a round-trip distance of 21 m in air-the longest-range interferometric sensing with a THz QCL to date.

Published in:

Sensors Journal, IEEE  (Volume:13 ,  Issue: 1 )