Controlling and Phase‐Locking a THz Quantum Cascade Laser Frequency Comb by Small Optical Frequency Tuning

Abstract

Full phase control of terahertz (THz)‐emitting quantum cascade laser (QCL) combs has recently been demonstrated, opening new perspectives for even the most demanding applications. In this framework, simplifying the set‐ups for control of these devices will help to accelerate their spreading in many fields. This study reports a new way to control the emission frequencies of a THz QCL comb by small optical frequency tuning (SOFT), using a very simple experimental setup, exploiting the incoherent emission of an ordinary white light‐emitting diode. The slightly perturbative regime accessible in these conditions allows tweaking the complex refractive index of the semiconductor without destabilizing the broadband laser gain. The SOFT actuator is characterized and compared to another actuator, the QCL driving current. The suitability of this additional degree of freedom for frequency and phase stabilization of a THz QCL comb is shown and perspectives are discussed.

Terahertz Quantum Cascade Lasers as Enabling Quantum Technology

“…The possibility to employ visible light as further actuator on QCL-FCs degrees of freedom has been also recently investigated by the small optical frequency tuning (SOFT) technique. [91] As a final note, a novel mechanism of FC generation in QCLs, suggested by studies performed in the mid-IR, [92] has been recently proposed and relies of the emission of optical modes separated by higher harmonics of the cavity free spectral range (FSR) (Figure 8a), rather than adjacent cavity modes as typical of fundamental comb regime (Figure 8b). Such a state, known as harmonic comb regime, can be reached by varying the QCL driving current so that it first reaches a state of high single-mode intracavity intensity and then, when its intensity is large enough, it is driven at an instability threshold caused by the 𝜒 (3) population pulsation non-linearity, favoring the appearance of modes separated by tens of FSRs from the first lasing mode.…”

“…The possibility to employ visible light as further actuator on QCL‐FCs degrees of freedom has been also recently investigated by the small optical frequency tuning (SOFT) technique. [ 91 ]…”

Toward new frontiers for terahertz quantum cascade laser frequency combs

“…This technique allows full phase stabilization of a QCL-FC, the emission linewidth of each comb mode being narrowed down to ∼2 Hz in 1 s, and a metrologicalgrade tuning of their individual modes frequencies. Currently, the use of visible light as an additional actuator on QCL-FC degrees of freedom is under investigation and has already taken to demonstration of phase stabilization of the QCL comb mode spacing [50].…”

Quantum cascade laser frequency comb locked with 200 mrad residual phase noise

“…Drive current is a conventional handle to modulate the QCL with bandwidths reaching up to 400 kHz [3]. Alternatively, light at another wavelength could control the QCL comb [4] as already demonstrated for THz QCL combs [5]. In this work, we take advantage of the mature near-infrared (NIR) technology to tightly-lock a MIR QCL comb to a distributed feedback (DFB) QCL using a NIR laser with a record stabilization bandwidth of 2 MHz and reach an integrated residual phase noise as low as 200 mrad.…”