Abstract
The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 microm has been long-term stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal. Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz.
Highlights
The most recent trace-gas sensing and frequency metrology applications are increasingly looking toward the mid-IR spectral region, known as the molecular “fingerprint” region, pushed by the presence of the fundamental ro-vibrational transitions of many simple molecules like CO2 and CH4 that are relevant for atmospheric implications.Difference-frequency generation (DFG) of tunable light has proven to be a suitable techniqueReceived 21 Apr 2008; revised 12 Jun 2008; accepted 20 Jun 2008; published 18 Jul 2008 21 July 2008 / Vol 16, No 15 / OPTICS EXPRESS 11638 for mid-IR trace-gas detection [1], high-resolution spectroscopy [2] and, after the introduction of optical frequency-comb synthesizer (OFCS) [3, 4], precise absolute frequency measurements [5, 6]
The frequency of a DFB quantum cascade laser (QCL) emitting at 4.3 μm has been long-term stabilized to the Lamb-dip center of a CO 2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal
Thanks to the non-linear sum-frequency generation (SFG) process with a fiber-amplified Nd:YAG laser, the QCL mid-infrared (IR) radiation has been linked to an optical frequency-comb synthesizer (OFCS) and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz
Summary
The most recent trace-gas sensing and frequency metrology applications are increasingly looking toward the mid-IR spectral region, known as the molecular “fingerprint” region, pushed by the presence of the fundamental ro-vibrational transitions of many simple molecules like CO2 and CH4 that are relevant for atmospheric implications (greenhouse effect, pollution monitoring). The high versatility of these sources has allowed us to perform both sensitive detection on N2O and CH4 by frequency-modulation spectroscopy [23] and Doppler-limited comb-assisted absolute frequency measurements of CO 2 transitions [24]. These experiments pointed out the benefits coming from a frequency stabilization of the QCL: in particular, a higher precision in frequency measurements can be achieved by locking the laser to a narrow reference and averaging the counts over long timescales.
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