Iacopo Galli

Saturated-absorption cavity ring-down spectroscopy.

A novel approach to cavity ring-down spectroscopy with the sample gas in saturated- absorption regime allows to decouple and simultaneously retrieve empty-cavity background and absorption signal, improving both measurement sensitivity and resolution. OCIS codes: 300.6340 Spectroscopy, infrared; 300.6390 Spectroscopy, molecular; 300.6460 Spectroscopy, saturation The availability of a molecular-spectroscopy technique, able to combine the ultimate performance in terms of sensitivity, resolution and frequency accuracy, can be crucial in many fundamental physical measurements. Indeed, profiting from the strength and ease of saturation of many mid-IR ro-vibrational transitions, this technique could provide new insights in elusive quantum-mechanical effects encoded in molecules. Such a technique could also represent a major step forward in trace-gas sensing. Cavity ring-down (CRD) spectroscopy has already proven to be a good technique to directly provide a sensitive and quantitative measurement of gas absorption coefficient with a simple experimental set-up. In principle, it is not limited by amplitude noise of the laser source, but only by detection shot noise. However, variations of the empty-cavity decay rate always prevent to achieve this ultimate limit and to average measurements over long times. Other techniques (e.g. CRD heterodyne spectroscopy and NICE-OHMS) are even more sensitive than standard CRD, but they are more complex (frequency modulations and/or lockings are needed), less quantitative (calibration procedures are needed) and require fast and sensitive detectors, generally unavailable in the mid IR. We present a new spectroscopic technique, namely saturated-absorption cavity ring-down (SCaR), that improves the CRD sensitivity (1). We show that the progressive decrease of the saturation level during each SCaR event makes our technique very effective in identifying and decoupling any variation of the empty-cavity decay rate. Saturated absorption induces a deviation of the SCaR signal from the perfectly exponential behavior, making a detailed treatment of non-linear effects needed to fit experimental data to the underlying physics of matter-radiation interaction. We developed and tested a new model which is very effective in exploiting the SCaR spectroscopic technique. The experimental set-up (2) is based on a difference-frequency-generated CW coherent source widely tunable in the mid IR, with the near-IR pump/signal lasers phase-locked one another through a fs Ti:sapphire optical frequency comb (OFC). The 1-m-long cavity is formed by 2 high-reflectivity mirrors with 6-m radius of curvature and optical losses of 440 ppm around 2340 cm -1 . With this set-up we performed several spectroscopic measurements to test both sensitivity and resolution using the newly developed model.

Measuring frequency noise and intrinsic linewidth of a room-temperature DFB quantum cascade laser

The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 μm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

Lamb-dip-locked quantum cascade laser for comb-referenced IR absolute frequency measurements

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.

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 4.5 μm

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.

High-coherence mid-infrared frequency comb

We report on the generation of a frequency comb around 4330 nm with an unprecedented coherence of the single teeth. Generating the comb within a Ti:sapphire laser cavity by a difference-frequency process and using a phase-lock scheme based on direct digital synthesis, we achieve a tooth linewidth of 2.0 kHz in a 1-s timescale (750 Hz in 20 ms). The generated per-tooth power of 1 μW ranks this comb among the best ever realized in the mid-infrared in terms of power spectral density.

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.

Mid-infrared frequency comb for broadband high precision and sensitivity molecular spectroscopy

We report on the experimental demonstration of the metrological and spectroscopic performances of a mid-infrared comb generated by a nonlinear downconversion process from a Ti:sapphire-based near-infrared comb. A quantum cascade laser at 4330 nm was phase-locked to a single tooth of this mid-infrared comb and its frequency-noise power spectral density was measured. The mid-infrared comb itself was also used as a multifrequency highly coherent source to perform ambient air direct comb spectroscopy with the Vernier technique, by demultiplexing it with a high-finesse Fabry-Perot cavity.

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 2190–2250u2009cm−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.

Quantum cascade lasers for high-resolution spectroscopy

Despite the growing interest that quantum cascade lasers (QCLs) are gaining, they still present a few unclear aspects of their fundamental properties, such as spectral purity, that need to be deeply investigated when aiming to make these innovative laser sources suitable for high-resolution spectroscopy and metrology. This paper is a review of our efforts towards QCL-based high-resolution spectroscopy and of our experimental investigation of QCLs frequency noise, aimed to discover the ultimate performances attainable by QCLs and to develop the exper- imental techniques required to achieve them. Our results, confirmed by several independent measurements, show that QCLs have a very small intrinsic linewidth buried under a large frequency-noise background. The development of appropriate frequency stabilization techniques will make QCLs well suited for high-resolution spectroscopy and metrology in the mid and far IR. C 2010 Society of Photo-Optical Instrumentation Engineers.

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.