Abstract

Delayed optical feedback can be easily generated in all class of coherent light emitters when part of the output radiation is re-coupled into the laser cavity, following interaction with any external objects (e.g., reflecting or diffusive targets). Semiconductor lasers have exhibited the largest sensitivity to optical injection and a number of applications based on self-mixing interferometry in diode lasers have been demonstrated [1]. However, the relatively large spectral width of diode lasers is a limiting factor in high resolution measurements. On the other hand, gas lasers having a narrower linewidth and a prospective higher signal-to-noise ratio are restricted to the very weak feedback regime because of the high reflectivity of their output coupler mirror and poor efficiency [2].

Distinctively, Quantum Cascade Lasers (QCLs) promise to merge the opportunity offered by the two types of laser in terms of spectral purity [3], narrow intrinsic linewidth [4], and high quantum efficiency [5]. Also, QCLs are the most reliable light sources for feedback interferometric sensing across the mid-infrared and Terahertz (THz) region of the electromagnetic spectrum, due to the high output power levels over a wide range (1–5 THz and 15–100 THz) of operating wavelengths. Moreover, QCLs show an intrinsic stability of the continuous-wave emission in the presence of optical re-injection, tolerating strong optical feedback levels without exhibiting dynamical instabilities such as mode-hopping, intensity pulsation, or coherence collapse [6]. This unique behaviour of QCLs can be ascribed to the high value of the photon to carrier lifetime ratio (i.e., to an ultrafast electron relaxation time in the excited subbands) and to the negligible linewidth enhancement factor. Innovative sensing applications attained by coherently interfering the QCL radiation into the optical cavity will be discussed, spanning from free-carrier density imaging in semiconductors, detecting a variety of kinetic entities, characterizing laser parameters and optically induced THz metamaterials.

[1] D. M. Kane and K. A. Shore, Unlocking Dynamical Diversity—Optical Feedback Effects on Semiconductor Diode Lasers (John Wiley and Sons, 2005). [2] S. Donati, J. Appl. Phys. 49(2), 495–497 (1978). [3] S. Bartalini, S. Borri, P. Cancio, A. Castrillo, I. Galli, G. Giusfredi, D. Mazzotti, L. Gianfrani, and P. De Natale, Phys. Rev. Lett. 104, 083904 (2010). S. Vitiello, L. Consolino, S. Bartalini, A. Taschin, A. Tredicucci, M. Inguscio, and P. De Natale, Nat. Photonics 6(8), 525–528 (2012). Razeghi, S. Slivken, Y. Bai, B. Gokden, and S. Ramezani Darvish, New J. Phys. 11, 125017 (2009). P. Mezzapesa, L. L. Columbo, M. Brambilla, M. Dabbicco, S. Borri, M.S. Vitiello, H. E. Beere, D. A. Ritchie, and G. Scamarcio, Opt. Express 21(11), 13748–13757 (2013).