Abstract
We study the frequency noise and the referencing to a near-infrared frequency comb of a widely tunable external-cavity quantum-cascade-laser that shows a relatively narrow free-running emission linewidth of 1.7 MHz. The frequency locking of the laser to the comb further narrows its linewidth to 690 kHz and enables sub-Doppler spectroscopy on an N2O transition of the ν1 band near 7.7 μm with sub-MHz resolution and absolute frequency calibration. The combined uncertainty on the measured transition center is estimated to be less than 50 kHz.
Highlights
The invention of quantum-cascade-lasers (QCLs) has made many commercial laser solutions available over a large part of the so-called fingerprint region, from 4 to 13.6 μm, which is of major relevance for trace gas detection at high chemical selectivity
QCLs have been largely used in the research arena for breath analysis [2], study of gas kinetics in combustion processes [3], investigation of plasma [4] or supersonic expansion of gases [5] and precision spectroscopy [6]
Among these, the coherent phase lock to an optical frequency comb (OFC) turned out to be highly powerful [11,12,13,14], as it allows precise tuning, absolute frequency calibration and line narrowing, all at once
Summary
The invention of quantum-cascade-lasers (QCLs) has made many commercial laser solutions available over a large part of the so-called fingerprint region, from 4 to 13.6 μm, which is of major relevance for trace gas detection at high chemical selectivity. Besides improving the noise level of current drivers, a variety of locking techniques have emerged to shrink their emission linewidth Among these (see review given in [6] and references therein), the coherent phase lock to an optical frequency comb (OFC) turned out to be highly powerful [11,12,13,14], as it allows precise tuning, absolute frequency calibration and line narrowing, all at once. EC-QCLs stand out because of their typically higher output power in a single longitudinal mode (up to 200 mW and beyond) and of their much larger tuning range (up to 100 cm−1), which is highly beneficial for multi-species gas sensing and for quantitative detection of complex molecules with unresolved rotational structure at environmental pressure and temperature Their large free-running emission linewidth, often found at the 1020 MHz level, kept them away from precision spectroscopic studies.
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