Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers

Abstract

optical frequency comb synthesizers have represented a revolutionary approach to frequency metrology, providing a grid of frequency references for any laser emitting within their spectral coverage. Extending the metrological features of optical frequency comb synthesizers to the terahertz domain would be a major breakthrough, due to the widespread range of accessible strategic applications and the availability of stable, high-power and widely tunable sources such as quantum cascade lasers. Here we demonstrate phase-locking of a 2.5 THz quantum cascade laser to a free-space comb, generated in a Linbo 3 waveguide and covering the 0.1-6 THz frequency range. We show that even a small fraction ( < 100 nW) of the radiation emitted from the quantum cascade laser is sufficient to generate a beat note suitable for phase-locking to the comb, paving the way to novel metrological-grade terahertz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and telecommunications.

Phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser using a room-temperature superlattice harmonic mixer

“…Those demonstrations can be mainly divided into a few cases: (a) phase locking of Fabry-Perot (FP) based QCLs with the use of a cooled superconducting detector as the mixing element [3][4][5] or by the use of a frequency comb generated from a mode-lock femtosecond laser. 6,7 The latter is operated at room temperature but requires relatively bulky and high power consumption electronics; (b) frequency locking of an FP or 3rd order DFB laser using a gas absorption line as the reference; 8 (c) frequency locking of an FP laser using a Schottky-diode harmonic mixer, 9 which was operated at room temperature, but requires high THz input power from the QCL in the order of several mW and has so far been demonstrated only below 3 THz.…”

Frequency Locking and Monitoring Based on Bi-directional Terahertz Radiation of a 3rd-Order Distributed Feedback Quantum Cascade Laser

“…In this way, to achieve frequency locking, part of the beam power is unavailable for the mixer. There have been many experiments to demonstrate the phase or frequency locking of a THz QCL [6][7][8][9][10][11][12]. For local oscillators operated at the high end of terahertz frequencies, such as for the astronomically important [OI] line at 4.7 THz, only two techniques are practically usable for frequency/phase locking since it can only be observed by an instrument in space.…”

Terahertz Frequency Metrology for Spectroscopic Applications: a Review

“…In fact, the intrinsic comb nature of pulsed THz sources used in time-domain spectroscopy has been recently demonstrated [31,32], and direct use of such sources as frequency Bruler^for a THz QCL has ever been reported in 2012, by phaselocking a single-frequency CW QCL emitting at 2.5 THz onto a single tooth of a THz FCS [33]. This allowed for not only the direct knowledge of the QCL absolute frequency, but also a narrowing of its emission linewidth down to around 130 Hz could be observed.…”

“…In [33], the THz comb generation is realized by optical rectification, in Cherenkov configuration [34], of a femtosecond mode-locked Ti/Sa laser in a single-mode waveguide. The waveguide is fabricated on a MgO-doped LiNbO 3 crystal plate [35].…”