P. De Natale

Quantum cascade lasers: a versatile source for precise measurements in the mid/far-infrared range

We provide an overview of recent developments of quantum cascade lasers (QCLs), from the mid-infrared (mid-IR) to the far-IR (THz) range, with a special focus on their metrological-grade applications in a number of fields. A special emphasis on the physics of the QCLs allows underlining peculiar effects and device features recently unveiled that pave the way to novel demanding photonics applications.

Probing the ultimate limit of fiber-optic strain sensing.

Enhanced Strain Sensitivity The ability to measure tiny deformations in length is useful for many disciplines, from largescale structural engineering to DNA analysis with optical tweezers. The most sensitive strain sensors are those using optical interferometers, which can detect small changes at the scale of visible wavelengths. Using an optical frequency comb to stabilize the output of a diode laser, and as a highly accurate ruler to determine small changes in length of an optic fiber sensor, Gagliardi et al. (p. 1081, published online 28 October) showed that sensitivity can be enhanced by several orders of magnitude. Such combined technology should provide for a new generation of high-performance sensors. The precisely spaced teeth of an optical frequency comb can be used as a highly accurate strain gauge. The measurement of relative displacements and deformations is important in many fields such as structural engineering, aerospace, geophysics, and nanotechnology. Optical-fiber sensors have become key tools for strain measurements, with sensitivity limits ranging between 10−9 and 10−6ε hertz (Hz)–1/2 (where ε is the fractional length change). We report on strain measurements at the 10−13ε-Hz–1/2 level using a fiber Bragg-grating resonator with a diode-laser source that is stabilized against a quartz-disciplined optical frequency comb, thus approaching detection limits set by thermodynamic phase fluctuations in the fiber. This scheme may provide a route to a new generation of strain sensors that is entirely based on fiber-optic systems, which are aimed at measuring fundamental physical quantities; for example, in gyroscopes, accelerometers, and gravity experiments.

Observing the intrinsic linewidth of a quantum-cascade laser: beyond the Schawlow-Townes limit.

A comprehensive investigation of the frequency-noise spectral density of a free-running midinfrared quantum-cascade laser is presented for the first time. It provides direct evidence of the leveling of this noise down to a white-noise plateau, corresponding to an intrinsic linewidth of a few hundred hertz. The experiment is in agreement with the most recent theory on the fundamental mechanism of line broadening in quantum-cascade lasers, which provides a new insight into the Schawlow-Townes formula and predicts a narrowing beyond the limit set by the radiative lifetime of the upper level.

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.

Quantum Physics Exploring Gravity in the Outer Solar System: The SAGAS Project

We summarise the scientific and technological aspects of the Search for Anomalous Gravitation using Atomic Sensors (SAGAS) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015–2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements and technologies.

Phase-locking to a free-space terahertz comb for metrological-grade terahertz lasers

Optical frequency comb synthesizers have represented a revolutionary approach to frequency metrology, providing a grid of frequency references for any laser emitting within their spectral coverage. Extending the metrological features of optical frequency comb synthesizers to the terahertz domain would be a major breakthrough, due to the widespread range of accessible strategic applications and the availability of stable, high-power and widely tunable sources such as quantum cascade lasers. Here we demonstrate phase-locking of a 2.5 THz quantum cascade laser to a free-space comb, generated in a LiNbO(3) waveguide and covering the 0.1-6 THz frequency range. We show that even a small fraction (<100 nW) of the radiation emitted from the quantum cascade laser is sufficient to generate a beat note suitable for phase-locking to the comb, paving the way to novel metrological-grade terahertz applications, including high-resolution spectroscopy, manipulation of cold molecules, astronomy and telecommunications.

Frequency-comb-referenced quantum-cascade laser at 4.4 μm

We report what we believe to be the first absolute frequency measurement performed using a quantum-cascade laser (QCL) referenced to an optical frequency comb synthesizer (OFCS). A QCL at 4.43 microm has been used for producing near-infrared radiation at 858 nm by means of sum-frequency generation with a Nd:YAG source in a periodically poled lithium niobate nonlinear crystal. The absolute frequency of the QCL source has been measured by detecting the beat note between the sum frequency and a diode laser at the same wavelength, while both the Nd:YAG and the diode laser were referenced to the OFCS. Doppler-broadened line profiles of (13)CO(2) molecular transitions have been recorded with such an absolute frequency reference.

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.

Optical comb generators for laser frequency measurement

Thanks to the advent of optical frequency comb synthesizers, based on femto-second mode-locked lasers, the field of optical frequency metrology has experienced an extraordinary growth, producing outstanding results in precision spectroscopy and opening new perspectives in different metrological branches. After introducing the basic concepts, systems and experiments, we overview the most significant achievements in this fast expanding field.

Mid-infrared fibre-based optical comb

An optical frequency synthesizer (OFS), based on difference- frequency generation (DFG), is demonstrated at 3 µm. By mixing a near-infrared (IR) OFS and a tunable continuous-wave (CW) laser in a periodically poled lithium-niobate (PPLN) crystal, a new comb is created from 2.9 to 3.5 µm (in 180 nm wide spans) with a 100 MHz mode spacing. By phase-locking, the pump laser to the near-IR OFS, the mid-IR comb is directly linked to the Cs primary standard via a global positioning system (GPS) time receiver. Then, the compactness of our fibre-based set-up opens real perspectives for the realization of transportable mid-IR frequency standards.