Abstract
We present pure amplitude modulation (AM) and frequency modulation (FM) achieved electrically in a quantum cascade laser (QCL) equipped with an integrated resistive heater (IH). The QCL output power scales linearly with the current applied to the active region (AR), but decreases with the IH current, while the emission frequency decreases with both currents. Hence, a simultaneous modulation applied to the current of the AR and IH sections with a proper relative amplitude and phase can suppress the AM, resulting in a pure FM, or vice-versa. The adequate modulation parameters depend on the applied modulation frequency. Therefore, they were first determined from the individual measurements of the AM and FM transfer functions obtained for a modulation applied to the current of the AR or IH section, respectively. By optimizing the parameters of the two modulations, we demonstrate a reduction of the spurious AM or FM by almost two orders of magnitude at characteristic frequencies of 1 and 10 kHz compared to the use of the AR current only.
Highlights
A wide range of applications make use of modulated laser sources, notably in optical communications and sensing
We present pure amplitude modulation (AM) and frequency modulation (FM) achieved electrically in a quantum cascade laser (QCL) equipped with an integrated resistive heater (IH)
The FM transfer functions are very comparable to those previously reported for a similar IH-QCL [28]
Summary
A wide range of applications make use of modulated laser sources, notably in optical communications and sensing. Wavelength and frequency modulation spectroscopy (WMS/FMS) methods are widely used to detect small absorption features in trace gas laser spectroscopy sensing They consist in modulating the wavelength/frequency of a laser and detecting the harmonic signals of an atomic or molecular transition obtained when the laser frequency is scanned through the transition. It leads to the generation of derivative-like signals of the probed absorption profile in WMS [1,2] or to the possibility to access both the absorption and dispersion of a gaseous sample in FMS [3]. In a different area of gas sensing, light modulated at audio frequencies (in amplitude or wavelength) and absorbed in a gaseous sample can excite acoustic waves, which are exploited for sensitive trace gas monitoring in various photoacoustic spectroscopy (PAS) methods [8,9]
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