The road towards the realization of quantum cascade laser (QCL) frequency combs (QCL-combs) has undoubtedly attracted ubiquitous attention from the scientific community, as these devices promise to deliver all-in-one (i.e. a single, miniature, active devices) frequency comb (FC) synthesizers in a range as wide as QCL spectral coverage itself (from about 4 microns to the THz range), with the unique possibility to tailor their spectral emission by band structure engineering. For these reasons, vigorous efforts have been spent to characterize the emission of four-wave-mixing multifrequency devices, aiming to seize their functioning mechanisms. However, up to now, all the reported studies focused on free-running QCL-combs, eluding the fundamental ingredient that turns a FC into a useful metrological tool. For the first time we have combined mode-locked multi-frequency QCL emitters with full phase stabilization and independent control of the two FC degrees of freedom. At the same time, we have introduced the Fourier transform analysis of comb emission (FACE) technique, used for measuring and simultaneously monitoring the Fourier phases of the QCL-comb modes. The demonstration of tailored-emission, miniaturized, electrically-driven, mid-infrared/THz coverage, fully stabilized and fully-controlled QCL-combs finally enables this technology for metrological-grade applications triggering a new scientific leap affecting several fields ranging from everyday life to frontier-research.
The Quantum Cascade Laser is becoming a key tool for plenty of applications, from the IR to the THz range. Progress in nearby areas, such as the development of ultra-low loss crystalline microresonators, optical frequency standards and optical fiber networks for time&frequency dissemination, are paving the way to unprecedented applications in many fields. For the most demanding applications, a thorough control of quantum cascade lasers (QCLs) emission must be achieved. In the last few years, QCLs unique spectral features have been unveiled, while multifrequency, comb-like QCLs have been demonstrated. Ultra-narrow frequency linewidths are necessary for metrological applications, ranging from cold molecules interaction and ultra-high sensitivity spectroscopy to infrared/THz metrology. In our group, we are combining crystalline microresonators, with a combined high quality factor in the infrared and ultra-broadband spectral coverage, with QCLs and other nonlinear highly coherent and frequency referenced sources. Frequency referencing to optical fiber-distributed optical primary standards offers astonishing stability values of 10-16 @1-sec timescales in laboratory environments but several hundred kilometres far away from the primary clocks. A review will be given of the present status of research in this field, with a view to perspectives and future applications.
A distributed-feedback quantum-cascade laser working in the 4.3÷4.4 mm range has been frequency stabilized to the Lamb-dip center of a CO2 ro-vibrational transition by means of first-derivative locking to the saturated absorption signal, and its absolute frequency counted with a kHz-level precision and an overall uncertainty of 75 kHz. This has been made possible by an optical link between the QCL and a near-IR Optical Frequency Comb Synthesizer, thanks to a non-linear sum-frequency generation process with a fiber-amplified Nd:YAG laser. The implementation of a new spectroscopic technique, known as polarization spectroscopy, provides an improved signal for the locking loop, and will lead to a narrower laser emission and a drastic improvement in the frequency stability, that in principle is limited only by the stability of the optical frequency comb synthesizer (few parts in 1013). These results confirm quantum cascade lasers as reliable sources not only for high-sensitivity, but also for highprecision measurements, ranking them as optimal laser sources for space applications.
We present a new generation of compact and rugged mid-infrared (MIR) difference-frequency coherent radiation sources referenced to fiber-based optical frequency comb synthesizers (OFCSs). By coupling the MIR radiation to high-finesse optical cavities, high-resolution and high-sensitivity spectroscopy is demonstrated for CH4 and CO2 around 3.3 and 4.5 μm respectively. Finally, the most effective detection schemes for space-craft trace-gas monitoring applications are singled out.
The realization and control of radiation sources is the key for proper development of THz-based metrology. Quantum
Cascade Lasers (QCLs) are crucial, towards this purpose, due to their compactness and flexibility and, even more
important, to their narrow quantum-limited linewidth. We recently generated an air-propagating THz comb, referenced to
an optical frequency comb by nonlinear optical rectification of a mode-locked femtosecond Ti:Sa laser and used it for
phase-locking a 2.5 THz QCL. We have now demonstrated that this source can achieve a record low 10 parts per trillion
absolute frequency stability (in tens of seconds), enabling high precision molecular spectroscopy. As a proof-ofprinciple,
we measured the frequency of a rotational transition in a gas molecule (methanol) with an unprecedented
precision (4 parts in one billion). A simple, though sensitive, direct absorption spectroscopy set-up could be used thanks
to the mW-level power available from the QCL. The 10 kHz uncertainty level ranks this technique among the most
precise ever developed in the THz range, challenging present theoretical molecular models. Hence, we expect that this
new class of THz spectrometers opens new scenarios for metrological-grade molecular physics, including novel THzbased
astronomy, high-precision trace-gas sensing, cold molecules physics, also helping to improve present theoretical
models.
We report on a novel intracavity quartz enhanced photoacoustic (I-QEPAS) gas sensing technique taking advantage from
both the high Q-factor of standard tuning forks and the power build-up of a high-finesse optical resonator. This first
prototype employs a distributed feedback quantum cascade laser operating at 4.3 μm. CO2 has been selected as gas target. Preliminary results demonstrate an improved sensitivity, close to the cavity enhancement factor (500) times the optical coupling efficiency (about 0.5), with respect to standard QEPAS technique. The detection limit was pulled from 7
ppm (obtained with standard QEPAS) down to 32 ppb, corresponding to normalized noise-equivalent absorption in the
10-9 W•cm-1•Hz-1/2 range.
Recently, we have demonstrated that the "intrinsic" linewidth of Quantum Cascade Lasers (QCLs) can go beyond
the radiative lifetime of the upper level. This represents the first demonstration of a sub-radiative linewidth for
any laser. The intrinsic linewidth of a QCL can be as narrow as hundreds Hz, paving new ways for ultra-sensitive
and precise harnessing and detection of molecules. We are working towards full exploitation of such
intrinsic properties by designing appropriate phase-lock loops and enhancement-cavities for interaction with
molecules. Combination with optical-frequency-comb-synthesizers and appropriate spectroscopic techniques,
like saturated-cavity-ring-down-SCAR or polarization spectroscopy can provide unprecedented sensitivity and
frequency accuracy for molecular detection.
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 experimental 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.
We recently reported the first Doppler-limited absolute frequency measurement of CO2 transitions around 4.4 μm
wavelength, by linking a DFB Quantum Cascade Laser (QCL) to an Optical Frequency Comb Synthesizer
(OFCS). We further achieved sub-Doppler recording of these transitions, improving of about three orders of
magnitude the measurement precision. We are exploring techniques able to significantly reduce the QCL jitter,
in order to get metrological-grade QCLs for very demanding experiments in the frequency-domain. The latest
experimental results in our group will be reported.
We present a novel tunable highly-coherent tunable source of IR radiation, based on difference-frequency generation
inside a miniature Ti:sapphire ring laser cavity by means of periodically-poled LiNbO3. Single mode
operation of the Ti:sapphire laser is provided by injection locking from an external-cavity diode laser and a
tunable-laser-injected fiber amplifier provides the signal radiation for the mixing. Such a source can be used in
spectroscopic set-ups for high sensitivity molecular sensing. Additional features are provided by properly altering
key properties of the 1-D photonic crystal.
We illustrate some of the applications of coherent infrared spectrometers based on frequency conversion by difference frequency generation. We show that very high sensitivity molecular detection can be performed as well as sub-Doppler saturated spectroscopy. Moreover, we describe a setup that allows absolute frequency measurements in the infrared wtih metrological grade uncertainties.
We report a beam deflection technique that exploits electric-field controlled deflection and total internal reflection at the interface between two anti-parallel domains realized in a single crystal lithium niobate wafer. The LiNbO3 z-cut sample was 500-μm-thick and was photolithographically patterned and poled by means of an applied electric field, in order to realize two adjacent regions of opposite domain orientation. The boundary between these domains should be very regular and free from residual stress, but in practice, a small residual index difference exists at the interface. An electric filed Ez applied across the interface region, produces equal in magnitude, but opposite in sign, refractive index variations between the adjacent anti-parallel domains. For sufficiently large index variation, and for grazing incidence geometry, that is when the incidence angle is between 87° and 89°, we obtain a high efficient beam deflection. Furthermore, if the incidence angle approaches the limit angle, which is about 89°, the Total Internal Reflection (TIR) occurs, producing an abrupt beam switch from transmission to reflection, characterized with a theoretical 100% switching contrast. However, the residual interface stress generates significant Fresnel reflection from this interface at high grazing angles, limiting the switching contrast ratio achievable at 20 dB. We present data obtained for wavelengths of 632.8 nm and 4.5 μm; at the latter wavelength we demonstrated the possibility to perform amplitude modulation faster than mechanical chopping, in a spectral region where no Pockels cells are available.
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