Interband Cascade Lasers are becoming more and more attractive sources for sensing applications due to their direct mid-IR emission and low power consumption. In many demanding applications of precision spectroscopy and imaging, the laser frequency and intensity noise represent the ultimate limiting factor for the final sensitivity. Here, we first characterize the response of a DFB ICL to a frequency modulation and measure its tuning coefficients. A frequency noise analysis of the ICL is then provided, through experimental investigation of the frequency noise power spectral density, with a particular attention to the technical noise contribution. A possible scheme for frequency stabilization and linewidth narrowing is presented through frequency locking to a high-Q factor microresonator.
We demonstrate the full frequency stabilization of a Quantum Cascade Laser Frequency Comb by using radio-frequency injection for locking the mode spacing and frequency locking to a crystalline microresonator for stabilizing the offset frequency. A final mode linewidth of 30 kHz over 2 ms is achieved.
In many precision sensing applications, the final detection sensitivity is tightly related to the intensity noise of the laser source, which might represent the ultimate limit to the sensor performance. In this framework, we present here the intensity noise characterization of three different mid-infrared semiconductor devices (two quantum cascade lasers and one interband cascade laser). A fast homemade balanced detection system is used to measure the intensity noise of the emitted radiation over a broad Fourier-frequency range, facilitating the observation of shot-noise-limited radiation under specific measurement conditions and detection efficiency. This study allows for a direct performance comparison of the most widespread laser sources in mid-infrared sensing systems.
Highly sensitive trace-gas sensors are required in a large range of applications, such as biological, environmental, industrial, and fundamental physics. Photoacoustic spectroscopy has the advantages of compactness and robustness and is characterized by a high degree of flexibility in its configuration, in particular in the selection of the laser source and the transducer. Here we report the experimental characterization of new silicon-based Micro electro-mechanical systems (MEMS) structures to be applied as acoustic-to-voltage transducers in a photo-acoustic-based sensor. In our setup, a 4.5 μm continuous wave quantum cascade laser is used to address strong N2O roto-vibrational transition, and the detection of MEMS oscillations is performed via a balanced interferometric readout.
In the race toward increasingly high-performance trace-molecule sensors, one of the most significant steps forward in the last decade for photoacoustic sensors was their combination with high-finesse optical cavities. Validated with different configurations, this technique demonstrated enhanced sensitivities below the part-per-trillion level (ppt) and record dynamic ranges. Here we present our advanced cantilever-based photoacoustic setup, based on a custom-made silicon cantilever embedded in a doubly-resonant configuration. The combination of a high-quality-factor acoustic resonator and a high-finesse optical cavity allows a final sensitivity enhancement by several orders of magnitude. The sensor was tested on strong N2O transitions around 4.5 μm wavelength with a continuous-wave quantum cascade laser.
Laser sources, since their invention, have proved to be the right solution in practically all conceived applications. Recently, the so-called second quantum revolution and quantum technologies like sensing, computing, simulation or communication are triggering a new generation of sub-classical sources to tackle such novel and challenging applications. First concepts and experimental results aimed to endow quantum cascade lasers and other infrared sources with truly quantum properties will be shown.
Interband Cascade Lasers (ICLs) are gaining field in molecular sensing thanks to their compact sizes and low-power consumption. They emit in the 3-6 μm range, and are valid alternative to QCLs especially for on-field measurement. Their suitability for high-resolution spectroscopy and metrology critically depends on their frequency stability and linewidth.
Here we investigate this issues, reporting experimental studies of the frequency noise and intrinsic linewidth of an ICL at 4.6 μm. The main differences with respect to other semiconductor lasers are discussed, as well as quantum-limited operation achievable using low-noise current drivers and frequency locking to high-performance optical resonators.
We describe the development and the first characterization of a compact trace-gas sensor based on cantilever photoacoustic spectroscopy (CEPAS). The sensor was characterized in order to find the optimal operating parameters (pressure, molecule absorption line and laser modulated frequency). N2O was selected as test molecule. A quality factor of 200 at 10 mbar of cell pressure were determined. Furthermore, the first test measurements showed a minimum detection level of hundreds of ppb with integration time of 30 ms.
Thanks to their compactness and low-power consumption, Interband Cascade Lasers (ICLs) are emerging sources for mid-infrared (MIR) molecular sensing below 6 µm. Understanding their noise features is of fundamental importance for applications like high-sensitivity and high-resolution spectroscopy. It could unveil details of their intrinsic physical behavior and, similarly to what happened for Quantum Cascade Lasers (QCLs), lead to the development of frequency and phase stabilization techniques for linewidth reduction. In this manuscript, we discuss the importance of full frequency noise characterization for ICLs, pointing out the main similarities and differences with respect to QCLs, and we show preliminary noise measurements. The frequency noise spectrum is analyzed and discussed, and the laser linewidth over different timescales calculated.
Interband and Quantum Cascade Lasers are key sources for MIR molecular sensing. Understanding their noise features and stabilizing their emission is of fundamental importance for applications like precision spectroscopy and metrology. High-Q crystalline Whispering Gallery Mode Resonators have proven to be powerful tools for characterization and stabilization of lasers from the UV to the MIR. Here, we report our recent results on Whispering Gallery Mode Resonators used for frequency characterization, stabilization and linewidth narrowing of Interband and Quantum Cascade Lasers. These results pave the way to new classes of compact MIR sources usable in Space missions, Metrology and Fundamental Physics.
We report on a metrological-grade mid-IR source with a 10–14 short-term instability for high-precision spectroscopy. Our source is based on the combination of a quantum cascade laser and a coherent radiation obtained by difference-frequency generation in an orientation-patterned gallium phosphide (OP-GaP) crystal. The pump and signal lasers are locked to an optical frequency comb referenced to the primary frequency standard via an optical fiber link. We demonstrate the robustness of the apparatus by measuring a vibrational transition around 6 μm on a metastable state of CO molecuels with 11 digits of precision.
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.
Orientation-patterned gallium phosphide (OP-GaP) crystals are used here for the first time for the generation of continuous-wave coherent mid-infrared radiation around 5.85 μm by difference frequency generation (DFG) of a Nd:YAG laser at 1064 nm and a diode-laser at 1301 nm. We provide the first characterization of the linear, thermo-optic and nonlinear properties of OP-GaP in a DFG configuration, and we derive an effective nonlinear coefficient deff = 17 pm/V (3) for first-order quasi-phase-matched OP-GaP. This novel nonlinear material can be used to referenced the mid-IR light to a frequency standard by locking the pump and signal laser to a near-IR optical frequency comb.
Narrow-linewidth lasers are key elements in optical metrology and spectroscopy. From their spectral purity, the measurements accuracy and the overall quality of collected data critically depend. Crystalline micro-resonators have undergone an impressive development in the last decade, opening new ways to photonics from the mm to the μm scale. Their wide transparency range and high Q-factor make them suitable for integration in compact apparatuses for precision spectroscopy from the visible to the mid-IR. Here, we present our recent results on frequency stabilization and linewidth narrowing of quantum cascade lasers using crystalline Whispering Gallery Mode Resonators for mid-IR precision spectroscopy.
We report on three different quartz enhanced photoacoustic (QEPAS) sensors operating in the near-IR, mid-IR and THz
spectral ranges, employing quartz tuning forks of different sizes and shapes. To test our sensors in the near-IR we used a
diode laser working at 2.7 μm, while in the mid-IR we employed a quantum cascade laser (QCL) operating at 7.9 μm,
fiber-coupled to the QEPAS cell. In the THz range we employed a QCL emitting at 2.95 THz. H2S absorption features
with line-strength up to 10-20 cm/mol were selected and QEPAS normalized noise-equivalent absorption in the 10-10
W•cm-1•Hz-1/2 range was achieved..
We report on an innovative quartz enhanced photoacoustic (QEPAS) gas sensor operating in the THz spectral range,
employing a custom quartz tuning fork (QTF) with the two prongs spaced by ~800 μm. To test our sensor we employed
a quantum cascade laser light source and selected a methanol rotational absorption line falling at 131.054 cm-1 (~3.93 THz), with line-strength S = 4.28•10-21 cm. The sensor operated at 10 Torr pressure on the QTF first flexion resonance
frequency at 4245 Hz. We achieved a QEPAS normalized noise-equivalent absorption of 2•10-10 W·cm-1•Hz-1/2 comparable with the best result of mid-IR QEPAS systems.
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.
We will report here on the design and realization of optoacoustic sensors based on an external cavity QCL laser source
emitting at 10,54 μm, fiber-coupled with a QEPAS spectrophone module. SF6 has been selected as the target gas. Single
mode laser delivery through the prongs of the quartz tuning fork has been realized using a hollow waveguide fiber with
internal core size of 300 μm. The achieved sensitivity of the system was 50 part per trillion in 1 s corresponding to a
record for QEPAS normalized noise-equivalent absorption of 2,7•10-10 W•cm-1•Hz-1/2.
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 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.
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