Terahertz imaging through self-mixing in a quantum cascade laser

Abstract

Article:Dean, P, Lim, Y, Valavanis, A et al. (10 more authors) (2011) Terahertz imaging through self-mixing in a quantum cascade laser. Optics Letters, 36 (13 We demonstrate terahertz (THz) frequency imaging using a single quantum cascade laser (QCL) device for both generation and sensing of THz radiation. Detection is achieved by utilizing the effect of self-mixing in the THz QCL, and, specifically, by monitoring perturbations to the voltage across the QCL, induced by light reflected from an external object back into the laser cavity. Self-mixing imaging offers high sensitivity, a potentially fast response, and a simple, compact optical design, and we show that it can be used to obtain high-resolution reflection images of exemplar structures.

Terahertz quantum cascade lasers — The past, present, and potential future

“…But, more recently, it has been shown that the emitting QCL cavity itself can be used as a radiation selfdetector, providing system miniaturization, and absorptioncoefficient-sensitive reflection imaging (for a review, see [13]). This Ôself-mixingÕ approach has been highly successful, and it has been possible to demonstrate: displacement sensing interferometry [14]; two-dimensional imaging [15], imaging over distances exceeding 10 m [16]; and, swept frequency coherent imaging, enabling the measurement of porcine tissue [17], inter alia.…”

Biomedical applications of terahertz self-mixing interferometry

“…Of these, monitoring the laser terminal voltage is preferable as it obviates the need for an external terahertz detector. 3 The small voltage variation (referred to as the SM signal) depends on both the amplitude and phase of the electric field of the reflected laser beam. This results in a highly sensitive and compact sensing technique that can probe information about the complex reflectivity or refractive index of the external target.…”

Demonstration of the self-mixing effect in interband cascade lasers

“…21 The total amplitude change of the laser terminal voltage (3 lV peak-to-peak) as the target is displaced is an order of magnitude smaller than the typical signal obtained for in-plane diode lasers or vertical-cavity surface-emmiting lasers (which are both in the tens of microvolts 22,23 ) and several orders smaller than the terminal voltage change observed in terahertz QCLs (millivolts). 19,24 The experimental self-mixing waveform obtained in this way can be modelled through the use of the excess phase equation (that is, the steady-state solution to the Lang-Kobayashi rate equation model for a laser under optical feedback), when the stimulus is simply displacement linear in time. 7,25 The broken line in Fig.…”

“…As we have demonstrated in earlier work, self-mixing sensors are most sensitive when operated close to the laser threshold. 24 With low threshold currents and low bias voltages (leading to input-power requirements almost two orders of magnitude lower than those for QCLs) ICLs operated in CW at room-temperature are perfectly positioned to become the lasers of choice for MIR SM sensors. Our demonstration paves the way for the application of ICL-based SM systems to the identification or analysis of gases and materials at MIR wavelengths.…”