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Frequency-Noise Dynamics of Mid-Infrared Quantum Cascade Lasers

By measuring the frequency-noise power spectral density of a cryogenically-cooled mid-infrared quantum cascade laser, we investigate the different contributions to the noise spectrum and identify the main differences with respect to standard bipolar semiconductor devices. In particular, the existence of a thermal cut-off on the 1/f noise allows to identify the current fluctuations through the heterostructure as the physical mechanism, intrinsic to the device, at the basis of the measured flicker noise. This result, marking the difference with bipolar semiconductor devices, is confirmed analyzing the laser frequency response to a modulation of the driving current.

The v3 band of 14C16O2 molecule measured by optical-frequency-comb-assisted cavity ring-down spectroscopy

The infrared spectrum of the rare isotopologue has been investigated in the range 2190–2250 cm−1 with a frequency-comb-referenced cavity ring-down spectrometer. Thirty-three ro-vibrational transitions of the ν3 fundamental band have been detected. Their absolute frequency was measured with a relative uncertainty ranging from to . The experimental frequencies were fitted to the conventional Hamiltonian of a linear molecule and a new set of spectroscopic parameters for the fundamental vibrational state has been improved for this species.

Absolute frequency measurements of CO2 transitions at 4.3 μm with a comb-referenced quantum cascade laser

The infrared spectrum of the (0111–0110) ro-vibrational band of 12C16O2 in the range 2306–2312 cm−1 is investigated with saturated-absorption sub-Doppler spectroscopy. The absolute frequencies of six transitions belonging to the P-branch of this band are measured by recording their Lamb-dip features in a pump-probe detection scheme employing a room-temperature quantum cascade laser. The laser is phase-locked to a subkilohertz-linewidth difference-frequency-generated radiation source, which is referenced to an optical frequency comb synthesiser. The achieved relative uncertainties range from 1×10−11 to 5×10−11, improving by three to four orders of magnitude the previous tabulated values for such frequencies. Moreover, thanks to this precision level, self-pressure-shift coefficients due to collisional processes of CO2 molecules are reported for the first time.

Tunable Microcavity-Stabilized Quantum Cascade Laser for Mid-IR High-Resolution Spectroscopy and Sensing

The need for highly performing and stable methods for mid-IR molecular sensing and metrology pushes towards the development of more and more compact and robust systems. Among the innovative solutions aimed at answering the need for stable mid-IR references are crystalline microresonators, which have recently shown excellent capabilities for frequency stabilization and linewidth narrowing of quantum cascade lasers with compact setups. In this work, we report on the first system for mid-IR high-resolution spectroscopy based on a quantum cascade laser locked to a CaF2 microresonator. Electronic locking narrows the laser linewidth by one order of magnitude and guarantees good stability over long timescales, allowing, at the same time, an easy way for finely tuning the laser frequency over the molecular absorption line. Improvements in terms of resolution and frequency stability of the source are demonstrated by direct sub-Doppler recording of a molecular line.

High Precision Measurements on Helium at 1083 nm

We present a review of the helium spectroscopy, related to transitions between 23 S and 23 P states around 1083 nm. A detailed description of our measurements, that have produced the most accurate value of the 23 P 0–23 P 1 fine structure interval, is given. It could produce an accurate determination (34 ppb) of the fine structure constant α. Improvements in the experimental set up are presented. In particular, a new frequency reference of the laser system has been developed by frequency lock of a 1083 nm diode laser to iodine hyperfine transitions around its double of frequency. The laser frequency stability, at 1 s timescale, has been improved of, at least, two orders of magnitude, and even better for longer time scales. Simultaneous 3 He—4 He spectroscopy, as well as absolute frequency measurements of 1083 nm helium transitions can be allowed by using the I2-locked laser as frequency standard. We discuss the implication of these measurements for a new determination of the isotope and 23S Lamb shifts.

A New Precision Spectroscopy Based Method for Boltzmann Constant Determination and Primary Thermometry

We propose a new independent thermometry method, line strength ratio thermometry (LRT), based on optical spectroscopy measurement of the line strength intensity ratio R between pairs of molecular transitions. Due to strong dependence of R on kT, a given measurement uncertainty δR for R reflects in a small uncertainty of kT determination. By assuming experimental uncertainties of R and T to be those reported in literature, we foresee a k determination at the 5 ppm level, which is better than the most precise k determination by using Doppler broadening thermometry (DBT). In the frame of a new definition of the SI Kelvin unit, based on k as fixed constant, once the k constant is exactly established, LRT is proposed as a high resolution noncontact thermometry technique for absolute temperature measurements of gas samples at the ppm level.

Probing and controlling the comb features of a THz QCL

Single mode THz QCLs have already shown their huge potential for metrological grade THz spectroscopy [1, 2] while, on the contrary, continuous-wave multimode broadband operation has only recently been demonstrated [3]. These sources show a comb emission regime with up to 600 GHz spectral coverage. This kind of sources are very promising for metrological-grade applications, such as high precision spectroscopy, but their comb-like features and controllability have not been yet characterized.

Microcavity-stabilized quantum cascade laser

A CaF2 whispering gallery mode microresonator is used for frequency stabilization and linewidth narrowing of a mid-IR quantum cascade laser. Sub-Doppler recording of a CO2 transition demonstrates the suitability of the system for high-resolution spectroscopy.

THz technologies for sensing and non-destructive testing

Progress in the far infrared part of the spectrum now allows to make metrological measurements of molecular transition frequencies and may help to develop novel coherent set-ups for imaging.

Precise Measurements of Molecular Lineshapes with Direct Comb Spectroscopy

Environmental related applications, require a precise measurement of the molecular lineshapes. We demonstrate this can be done by using a direct comb spectroscopy that we applied to CO