Coherent phase lock of a 9 μm quantum cascade laser to a 2 μm thulium optical frequency comb

Abstract

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1 μm to a Tm-fiber laser frequency comb centered at 2 μm, with an integrated residual phase error of 0.9 rad (30 mHz to 1.5 MHz). This resulted in a QCL linewidth reduction from 525 to 25 kHz at 1 ms observation time, limited by the linewidth of the free-running frequency comb.

An experimental study of noise in mid-infrared quantum cascade lasers of different designs

“…Since their first demonstration in 1994 [1], they have been widely used in numerous applications, primar-ily in the fields of molecular spectroscopy and trace gas sensing, but also in free-space mid-infrared optical com-munications and in infrared countermeasure systems for civil and military aircrafts. Novel applications of QCLs in very high-resolution spectroscopy and optical metrology have emerged in the last years, in particular in combination with optical frequency combs [2][3][4][5], which are generally more demanding in terms of low frequency-noise and nar-row spectral linewidth. QCLs have the potential to achieve very narrow linewidths with an intrinsic value of a few hundreds hertz only [6,7] resulting from their close-to-zero Henry's linewidth enhancement factor [8].…”

Frequency-comb-calibrated Doppler broadening thermometry

“…It is phase locked to the Tm:fiber frequency comb via a sum-frequency generation (SFG) scheme (see Ref. [8] for details). To this purpose, a 5-mm-long AgGaSe 2 crystal has been selected for its high nonlinearity, low spatial walk-off, and broad transparency range.…”

“…In such tight locking conditions, the QCL frequency could be precisely and repeatedly tuned over several GHz by varying the comb repetition rate. Moreover, the QCL emission linewidth could be reduced down to 25 kHz at 1-ms observation time [8]. The NH 3 absorption was probed by sending a small fraction of the QCL beam through a 11-cm-long cell, whose transmittance was synchronously monitored by a nitrogen-cooled MCT detector followed by a lock-in amplifier.…”

“…In their apparatus, an ECDL at 1.5 μm was phase locked to the comb, and continuous frequency scans around a given 13 C 2 H 2 line were performed by tuning the repetition rate, thus demonstrating a precision in Doppler-width retrieval of 1200 ppm. The present paper deals with the extension of this approach to the mid-infrared, taking the advantage of a new referencing scheme allowing a quantum cascade laser to be phase locked to a near-infrared frequency comb [8].…”

Frequency Noise and Linewidth of Mid-infrared Continuous- Wave Quantum Cascade Lasers: An Overview

“…However, there has been a growing interest during the last couple of years for this topic, ranging from basic studies of the frequency noise in QCLs at either cryogenic 19 or room temperature, 20,21,24 to studies of its dependence as a function of the laser temperature, 22 and investigations of its possible origin, 22,23 as well as studies in relation to the frequency stabilization of mid-infrared QCLs. [30][31][32][33] In this chapter, we present an overview of experimental results obtained in recent years on the frequency noise of QCLs. The overview is based on a compilation of both our own work and studies from other laboratories.…”