Quantum cascade laser-pumped terahertz molecular lasers: frequency noise and phase-locking using a 1560 nm frequency comb

Abstract

The recent demonstration of a terahertz (THz) molecular gas laser pumped by a mid-infrared quantum cascade laser (QCL) has opened up new perspectives for this family of sources, traditionally relying on CO2-laser pumping. A so far open question concerning QCL-pumped THz molecular lasers (MLs) is related to their spectral purity. Indeed, assessing their frequency/phase noise is crucial for a number of applications potentially exploiting these sources as local oscillators. Here this question is addressed by reporting the measurement of the frequency noise power spectral density (PSD) of a THz ML pumped by a 10.3µm-wavelength QCL, and emitting 1mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080 th harmonic of the repetition rate of a 1560nm frequency comb. We find a frequency noise PSD < 10Hz 2 /Hz (-95dBc/Hz) at 100kHz from the carrier. To demonstrate the effect of the stability of the pump laser on the spectral purity of the THz emission we also measure the frequency noise PSD of a CO2-laser-pumped 2.5THz ML, reaching 0.1Hz 2 /Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the frequency comb harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the frequency comb repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.

Widely tunable compact terahertz gas lasers

“…In that limit, the only variation in PTHz from line to line and molecule to molecule comes from the terms nTHz, nIR, and Pth. Even if such an approximation is acceptable for operating a QPML over a narrow range of frequencies and pressures, as was done in previous work [2,5], it is not valid for estimating how the widely tunable QPML performance varies from molecule-to molecule and from line-to-line. Moreover, the approximation proposed in [4] requires that the pressure and/or cavity length be varied to achieve the 𝛼 => 𝐿 > 1 condition.…”

Maximizing Performance of Quantum Cascade Laser-Pumped Molecular Lasers

“…Unlike electronic sources, the Manley-Rowe effect in these QCLpumped molecular lasers (QPMLs) causes the power to increase with increasing frequency. Consequently, reports of power exceeding 1 mW [13] and tunability approaching 1 THz [12] from QPMLs foreshadow the potential of these sources to span the terahertz gap.…”

Noise Measurement and Reduction in Mode-Locked Lasers: Fundamentals for Low-Noise Optical Frequency Combs

“…A significant number of ongoing research topics, e.g., high-speed photonic analogto-digital conversion [140], dual-comb spectroscopic measurements [141][142][143][144][145][146][147], X-band ultra-low-noise microwave generation [148][149][150][151], and astro-combs [152][153][154][155], require frequency combs with large mode spacing, therefore novel optical frequency combs aim to boost the comb spacing over the GHz level. Representative techniques generating high-repetition-rate laser sources are QCL-based combs [156,157], microcombs [158][159][160], and EO-modulated combs [161,162]. The detailed study of noise behavior in these novel combs could be found in this literature [150,[162][163][164][165].…”