Abstract
High-resolution spectroscopy not only can identify atoms and molecules but also can provide detailed information on their chemical and physical environment and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications, and its frequency is not directly referenced to primary frequency standards. In this work, we injection lock a QCL operating at 2 THz to a compact fiber-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, <100 Hz. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be −75 dBc/Hz at 10 kHz offset from the 2 THz carrier.
Highlights
The locking of one oscillator to another when there is a degree of coupling between them is a universal phenomenon that was first observed by Huygens in 1665 and later described by Adler [1]; it applies well to mechanical, electronic, and optical oscillators
A weaker “master” oscillator with desirable temporal and spectral characteristics is used to control the output of a more powerful “slave” oscillator by coupling, or “injecting,” the “master” oscillation into the “slave” oscillator. This technique has become ubiquitous in many areas of science and technology, and inhigh-resolution spectroscopy and for the distribution of frequency standards
We demonstrate injection locking of a quantum cascade laser (QCL) operating at a frequency around 2 THz to an all-fiber telecommunications-frequency comb, which has great potential for compact integration in photonic circuits
Summary
The locking of one oscillator to another when there is a degree of coupling between them is a universal phenomenon that was first observed by Huygens in 1665 and later described by Adler [1]; it applies well to mechanical, electronic, and optical oscillators. Our near-IR comb source uses a narrow linewidth C-band laser to seed an amplified fiber loop incorporating a high-speed phase modulator, driven by a microwave frequency synthesizer that determines the comb line spacing (see Supplement 1) [27].
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