Abstract
We report the measurement of the frequency noise power spectral density (PSD) of a Terahertz (THz) molecular laser (ML) pumped by a mid-infrared (MIR) quantum cascade laser (QCL), and emitting 1 mW at 1.1THz in continuous wave. This is achieved by beating the ML frequency with the 1080th harmonic of the repetition rate of a 1560 nm frequency comb (FC). We find a frequency noise PSD < 10Hz2/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 0.1Hz2/Hz (-105dBc/Hz) at 40kHz from the carrier, limited by the frequency noise of the FC harmonic. Finally, we show that it is possible to actively phase-lock the QCL-pumped molecular laser to the FC repetition rate harmonic by controlling the QCL current, demonstrating a sub-Hz linewidth.
Highlights
A novel approach for THz molecular lasers (MLs) was demonstrated, where conventional CO2laser pumping is replaced by a mid-infrared (MIR) quantum cascade laser (QCL) [1]
With the laser studied in this work, we reach a wall plug efficiency of 10-4 in CW at 1THz, which is comparable to what is achieved with state-of-the art frequency multiplication chains and photomixers, and approximately a factor of ten higher than what can be reached at room temperature with intra-cavity difference-frequency generation in MIR QCLs [38,39,40,41]
The fact that QCLs provide a continuous coverage of the entire MIR spectrum, partially mitigates the intrinsically poor tunability of THz MLs by facilitating the access to molecular transitions close to the targeted THz spectral region
Summary
A novel approach for THz MLs was demonstrated, where conventional CO2laser pumping is replaced by a mid-infrared (MIR) QCL [1] This results in a much more compact, frequency agile and deployable source that has revived the interest for THz MLs, in particular for their exploitation as low-phase noise sources operating at room temperature in applications such as radio-astronomy, atmospheric science, free-space communications, highresolution spectroscopy or radar imagery [2,3,4,5,6,7,8]. Typical CO2-laser linewidths are in the ~1-100kHz range, whereas MIR QCLs linewidths are approximately 100 times larger (~1-10MHz) [9,10,11] Determining how this affects the spectral purity of QCLpumped MLs is the main purpose of this work. These results pave the way to the development of QCL-pumped MLs as compact, low-phase noise THz sources operating a room temperature
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