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Mid-infrared Quantum cascade lasers (QCLs) opens the way for many applications thanks to “fingerprint region of molecules”. In recent years, the emergence of external cavity broadband wavelength-swept QCLs have been providing new benefits not found in FTIR such as compactness and faster wavelength scanning due to MOEMS technology.

Broadband, high-resolution, heterodyne, mid-infrared absorption spectroscopy was performed with a high-speed quantum cascade (QC) detector. By strictly reducing the device capacitance and inductance via air-bridge wiring and a small mesa structure, a 3-dB frequency response over 20 GHz was obtained for the QC detector, which had a 4.6-μm peak wavelength response. In addition to the high-speed, it exhibited low noise characteristics limited only by Johnson–Nyquist noise, bias-free operation without cooling, and photoresponse linearity over a wide dynamic range. In the detector characterization, the noise-equivalent power was 7.7 × 10−11 W/Hz1/2 at 4.6 μm, and it had good photoresponse linearity up to 250 mW, with respect to the input light power. Broadband and high-accuracy molecular spectroscopy based on heterodyne detection was demonstrated by means of two distributed-feedback 4.5-μm QC lasers. Specifically, several nitrous oxide absorption lines were acquired over a wavelength range of 0.8 cm−1 with the wide-band QC detector.

Laser processing of polymeric materials by means of 100-mW class QCL lasers with emission wavelength of 7.728 and 4.329 μm were examined. Polymeric materials show absorption bands based on transitions between vibronic levels in mid-infrared (MIR) wavelength. Since such absorption bands are very sharp, resonant conditions with laser wavelength are critical. Quantum cascade laser (QCL) is a promising laser source for mid-infrared laser processing; emission wavelength can be customized by changing the heterostructure. In this work, we have employed 100-mW class QCLs and applied for focused irradiation at several polymeric materials, polypropylene (PP), polycarbonate (PC), polyacetal (POM).

Thanks to their compactness and low-power consumption, Interband Cascade Lasers (ICLs) are emerging sources for mid-infrared (MIR) molecular sensing below 6 mu m. 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.

The infrared diode laser spectrum of the ν3-ν4 difference band of 14NO3 has been observed in the 690 cm−1 region. By fixing the lower state (v4 = 1) constants to the previously determined values, molecular constants in v3 = 1 have been determined for the first time : the vibrational energy E3 = 1054.13679(69) cm−1, the rotational constants B3 = 0.4562291(41), C3 = 0.2274567(53) cm−1, the first-order Coriolis coupling constant Cζ3 = 0.0032073(99) cm−1, and the spin–orbit interaction constant aeff = –0.03979(70) cm−1, with one standard deviation in parentheses. The ν3 fundamental band has been detected with weaker intensity than the ν3 + ν4 band by a factor of ~ 2000.

A mid-infrared spectroscopic system using a high-speed wavelength-swept and pulsed quantum cascade laser (QCL) for healthcare applications such as blood glucose measurement is proposed. We developed an attenuated total reflection measurement system comprising the QCL with a micro-electromechanical system (MEMS)-scanning grating, hollow optical fibers, and InAsSb detector and tested its feasibility for healthcare applications. A continuous spectrum was obtained by integrating comb-shaped spectra, the timing of which was slightly shifted. As this method does not require complex calculations, absorption spectra are obtained in real-time. We found that the signal-to-noise ratio of the obtained spectrum had been improved by increasing the number of spectra that were integrated into the spectrum calculation. Accordingly, we succeeded in measuring the absorption spectrum of a 0.1% aqueous glucose solution. Furthermore, the absorption spectra of human lips were measured, and it was shown that estimation of blood glucose levels were possible using a model equation derived using a partial least squares regression analysis of the measured absorption spectra. The spectroscopic system based on the QCL with MEMS-scanning grating has the advantages of compactness and low cost over conventional Fourier transform infrared-based systems and common spectroscopic systems with a tunable QCL that has a relatively large, movable grating.

Narrow-linewidth lasers are key elements in optical metrology and spectroscopy. Spectral purity of these lasers determines accuracy of the measurements and quality of collected data. Solid state and fiber lasers are stabilized to relatively large and complex external optical cavities or narrow atomic and molecular transitions to improve their spectral purity. While this stabilization technique is rather generic, its complexity increases tremendously moving to longer wavelenghts, to the infrared (IR) range. Inherent increase of losses of optical materials at longer wavelengths hinders realization of compact, room temperature, high finesse IR cavities suitable for laser stabilization. In this paper, we report on demonstration of quantum cascade lasers stabilized to high-Q crystalline mid-IR microcavities. The lasers operating at room temperature in the 4.3-4.6 μm region have a linewidth approaching 10 kHz and are promising for on-chip mid-IR and IR spectrometers.

Since its first invention and demonstration [1], saturated-absorption cavity ring-down spectroscopy (SCAR) of CO2 gas samples at 4.5 μm wavelength has been approaching accelerator mass spectrometry (ams) precision in radiocarbon measurements. The first intercomparison between SCAR and ams measurements was performed with modern (fermentation of brown cane sugar) and fossil (high-purity industrial gas cylinder) carbon dioxide samples [2]. These successful results have triggered further optimization of the data fitting procedure [3] and significant upgrades of the SCAR experimental apparatus, leading to an improved performance of the spectrometer in terms of precision and repeatability [4]. Recently, an independent research group has reproduced SCAR spectroscopy, assessing the working limits of this techniques in terms of saturation parameter with measurements on a different molecular species [5].

A CaF 2 whispering gallery mode microresonator is used for frequency stabilization and linewidth narrowing of a mid-IR quantum cascade laser. Sub-Doppler recording of a CO 2 transition demonstrates the suitability of the system for high-resolution spectroscopy.

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.

We report high performance λ~8.6 μm m quantum cascade lasers based on 5 quantum well single phonon-continuum (SPC) depopulation scheme, grown by metal organic vapor-phase epitaxy. Devices with SPC depopulation structures exhibit high device performances in pulsed and continuous wave (CW) operation due to a large dipole matrix element (3.0 nm) and short depopulation time (0.22 ps). A 4 mm-long, 8 μm-wide, high reflective (HR)-coated buried-hetero-structure laser with 33 cascade stages, for active region, demonstrates a high output power of 313 mW, a threshold current density of 1.35 kA/cm2, and slope efficiency of 547 mW/A at 30°C in CW operation. The maximum CW lasing temperature of the device is observed to be 120°C. Furthermore, by using a higher number of cascade stages (N=70) in a 4 mm-long, 22 m-wide, HR-coated ridge laser, a high peak power of 6.3 W is obtained together with a high wall-plug efficiency (WPE) of 10.8%, a slope efficiency of 2.7 W/A, and a threshold current density of 0.76 kA/cm2 in pulsed operation at 300 K. The measured WPE of over 10% at room temperature is close to the theoretical values.

We report the experimental linewidth enhancement factor (α-factor) results on mid-infrared InGaAs/InAlAs quantum cascade lasers (QCLs) obtained using the self-mixing method. The waveforms appearing under self-mixing conditions were different from those originally predicted, which assumed that the gain was symmetric around the gain peak and that α=0. The signs of the QCL α-factors were identified to be negative and asymmetric to the gain peak by using the nonsymmetrical mirror drive, and by comparing their wave patterns to those of conventional 1.55-µm laser diodes. The nonparabolicity of the subbands was a possible cause of the α-factor's asymmetry. The subbands are no longer parallel if the lasing energy is restricted by the nonparabolicity, which produces inhomogeneous broadening and an asymmetric optical gain spectrum with a long wavelength tail. Taking the nonparabolicity into account, we determined the QCL gain spectrum and the dependence of the α-factor on the spectral position relative to the gain peak. The dependence of the α-factor on detuning was in reasonable agreement with the calculation that accounts for the asymmetry in gain.

We have developed a theory of the intrinsic linewidths of laser output of single-mode quantum-cascade (QC) lasers in mid-infrared and terahertz (THz) ranges. In the theoretical treatment, the concept of an effective coupling efficiency of spontaneous emission, given by a fractional rate of spontaneous emission coupled into a lasing mode to total nonlasing relaxation, is introduced to clarify a hidden reason for the narrowness of the linewidths. A narrow linewidth (12-kHz) reported with a frequency-stabilized 8.5- distributed-feedback QC laser is successfully interpreted in terms of an extremely small effective coupling efficiency of spontaneous emission, caused by ultrafast nonradiative scatterings. The present theory predicts the presence of a minimum ldquolinewidth floorrdquo in a high-injection-current region and the independence of linewidth on detuning between gain-peak and emission wavelengths. The theoretical treatment is expanded to derive the further modified Schawlow-Townes formula including the line-broadening by black body radiation in a THz QC laser. The linewidth of a THz QC laser is predicted to be considerably broadened by black body radiation.

An ultra-sensitive and selective quartz-enhanced photoacoustic spectroscopy (QEPAS) combined with a high-finesse cavity sensor platform is proposed as a novel method for trace gas sensing. We call this technique Intra-cavity QEPAS (I-QEPAS). In the proposed scheme, a single-mode continuous wave quantum cascade laser (QCL) is coupled into a bow-tie optical cavity. The cavity is locked to the QCL emission frequency by means of a feedback-locking loop that acts directly on a piezoelectric actuator mounted behind one of the cavity mirrors. A power enhancement factor of ∼240 was achieved, corresponding to an intracavity power of ∼0.72 W. CO2 was selected as the target gas to validate our sensor. For the P(42) CO2 absorption line, located at 2311.105 cm(-1), a minimum detection limit of 300 parts per trillion by volume at a total gas pressure of 50 mbar was achieved with a 20 s integration time. This corresponds to a normalized noise equivalent absorption of 3.2 × 10(-10) W cm(-1) Hz(-1/2), comparable with the best results reported for the QEPAS technique on much faster relaxing gases. A comparison with standard QEPAS performed under the same experimental conditions confirms that the I-QEPAS sensitivity scales with the intracavity laser power enhancement factor.

We investigated the linewidth enhancement factor α of a Fabry–Perot quantum cascade laser by self-mixing interferometry. For the first time, the theoretically predicted characteristic waveform resulting from a small α of about -0.3 could be obtained when the optical feedback parameter was greater than one. The shape of this waveform was totally different from that of a conventional distributed-feedback laser diode with a large α. We demonstrated that subband nonparabolicity is one of the reasons why quantum cascade lasers have a small, but non-zero, α value.

We investigated the potential of a compact and high-power quantum cascade laser (QCL) in the 5.7 μm wavelength range for less-invasive laser angioplasty. Atherosclerotic plaques consist mainly of cholesteryl esters. Radiation at a wavelength of 5.75 μm is strongly absorbed in C=O stretching vibration mode of cholesteryl esters. Our previous study achieved to make cutting differences between a normal artery and an atherosclerotic lesions using nanosecond pulsed laser by difference-frequency generation (DFG laser) at the wavelength of 5.75 μm. For applying this technique to clinical treatment, a compact laser device is required. In this study, QCL irradiation effects to a porcine normal aorta were compared with DFG laser. Subsequently, QCL irradiation effects on an atherosclerotic aorta of myocardial infarction-prone Watanabe heritable hyperlipidemic rabbit (WHHLMI rabbit) and a normal rabbit aorta were observed. As a result, the QCL could make cutting differences between the rabbit atherosclerotic and normal aortas. On the other hand, the QCL induced more thermal damage to porcine normal aorta than the DFG laser at the irradiation condition of comparable ablation depths. In conclusion, the possibility of less-invasive and selective treatment of atherosclerotic plaques using the QCL in the 5.7 μm wavelength range was revealed, although improvement of QCL was required to prevent the thermal damage of a normal artery.

Summary form only given. A homogeneous broad gain bandwidth quantum cascade laser for external cavity is demonstrated. Spectrally homogeneous gain of anticrossed dual-upper state (DAU) design provide wide and stable wavelength tunability with external cavity configuration.Quantum cascade (QC) lasers have become the standard laser light sources for sensing applications in midinfrared region. Distributed feedback (DFB) QC-lasers which allow spectral single-mode emission are the most suitable light sources for applications where high-accuracy is requested. However, narrow wavelength tuning range of the DFB-QC-lasers (-5 cm-1) is a limitation of widespread in sensing applications. External cavity configuration is an alternative approach which allows wide wavelength tuning and various types of QC-gain media for the external cavity are proposed and demonstrated. Most of these are heterogeneous structure (severalwavelength-stack gain media) to extend gain bandwidth of the QC-lasers. Spectrally homogeneous broadband quantum cascade laser was developed and broad tuning was realized with external cavity mode. A unique homogeneous broadband design of QC-laser; anticrossed dual-upper state (DAU) design was employed [1-4] and tuning range of 321 cm-1 (from 6.0 gm to 7.42 gm) in pulsed operation was achieved. Even though the DAU structure is designed to be identical wavelength, its gain bandwidth is compatible to several-wavelength stack gain media. The DAU design also has the unique feature of weak dependence on operating voltage and insensitivity to temperature change [5]. The DAU is promising design for gain media for broadly tunable QC-system.

We evaluated the utility of a compact and high-power quantum cascade laser (QCL) in the 5.7 μm wavelength range for less-invasive laser angioplasty. Atherosclerotic plaques mainly consist of cholesteryl esters. The wavelength of 5.75 μm is well absorbed in C=O stretching vibration mode of cholesteryl esters. Our previous study achieved to make cutting differences between a normal tunica intima of an artery and an atherosclerotic lesions using a nanosecond pulsed laser by difference-frequency generation (DFG laser) at the wavelength of 5.75 μm. For realizing a clinical application of this technique, a compact laser device is required. In this study, QCL irradiation effects to a porcine normal aorta were compared with DFG laser. In addition QCL irradiation effects to an atherosclerotic aorta of myocardial infarction-prone Watanabe heritable hyperlipidemic rabbit (WHHLMI rabbit) and a normal aorta were observed. As a result, the QCL could make cutting difference between the rabbit atherosclerotic aorta and the normal aorta. On the other hand, the QCL induced more thermal damage to porcine normal aorta than the DFG laser at the irradiation condition of comparable ablation depth. In conclusion, the possibility of less-invasive and selective treatment of atherosclerotic plaques using the QCL in the 5.7 μm wavelength range was revealed, although improvement of QCL was required to prevent the thermal damage of a normal artery.

Laser angioplasty is one of the catheter treatments of atherosclerosis that is suitable for difficult-to-treat lesions such as in-stent restenosis and total occlusions. In clinical practice, excimer laser coronary angioplasty has been applied. However, conventional XeCl excimer laser (wavelength: 308 nm) have the risk of injuring normal vessels [1]. Therefore, safer laser devices have been required.

The infrared spectrum of the (01(1)1-01(1)0) ro-vibrational band of (CO2)-C-12-O-16 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 1x10(-11) to 5x10(-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

The frequency-noise power spectral density of a roomtemperature distributed-feedback quantum cascade laser emitting at lambda = 4.36 mu m m 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. (C) 2011 Optical Society of America

Relative intensity noise of a quantum cascade laser under optical feedback was first evaluated. Noise smaller than that of a conventional laser diode was due to a small line enhancement factor and long cavity length.

We report a high performance operation of quantum cascade lasers based on Single Phonon-Continuum depopulation (SPC) structures. The lasers exhibit low CW threshold current densities and high characteristic temperatures in the wide wavelength range between 4.5 and 10.8 μm. An 8.2 μm laser, despite a bare ridge structure which is extremely simple configuration without any intentional thermal dissipation equipments and any HR coatings, demonstrates the high device performance: a low threshold current density of 1.66 kA/cm 2 and a high maximum output power of 76 mW (from one facet) at room temperature in CW operation. Our shortest wavelength 4.5 μm laser with HR coating reveals a low threshold current density of 1.7 kA/cm 2 and maximum output power of 161 mW at room temperature in CW operation. For long wavelength, we present the first room temperature, CW operation of DFB QCL with top grating. The DFB laser emits ~9.6 μm single mode spectra at temperatures between -5 °C and 50 °C. The wide tuning range is obtained to be from 1031 to 1039 cm -1 .

We demonstrate room temperature, CW operation of a 5.2 mum quantum-cascade laser with single phonon resonance-continuum depopulation structure, grown by MOVPE. The laser exhibits a simple ridge structure without high reflection coatings.

We propose a new quantum-cascade laser structure with single phonon resonance-continuum depopulation and demonstrate room temperature, CW operation of the proposed 7.9 mum quantum-cascade laser. The laser grown by MOVPE emits a CW output power of 36 mW at 30degC. The proposed structure may lead to successful commercial mass-production of the lasers.

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. (C) 2014 Optical Society of America

Broadband spectral tuning in the long wavelength range (greater than 10 μm) was demonstrated with an external-cavity quantum cascade laser. The tunable wavelength of the laser ranged from 9.5 to 11.4 μm (176 cm(-1); corresponding to 18% of the center wavelength) in continuous wave (cw) operation at room temperature, without any anti-reflection coating. The gain chip based on the anti-crossed dual-upper-state (DAU) design provided a cw lasing up to 300 K, with a low threshold current density of 2.1 kA/cm2. The highly stable broadband spectral tuning and high laser performance were enabled by the spectrally homogeneous gain profile of the anti-crossed DAU active region.

Molecules such as water, proteins and lipids that are contained in biological tissue absorb mid-infrared (MIR) light, which allows such light to be used in laser surgical treatment. Esters, amides and water exhibit strong absorption bands in the 5–7 μm wavelength range, but at present there are no lasers in clinical use that can emit in this range. Therefore, the present study focused on the quantum cascade laser (QCL), which is a new type of semiconductor laser that can emit at MIR wavelengths and has recently achieved high output power. A high-power QCL with a peak wavelength of 5.7 μm was evaluated for use as a laser scalpel for ablating biological soft tissue. The interaction of the laser beam with chicken breast tissue was compared to a conventional CO 2 laser, based on surface and cross-sectional images. The QCL was found to have sufficient power to ablate soft tissue, and its coagulation, carbonization and ablation effects were similar to those for the CO 2 laser. The QCL also induced comparable photothermal effects because it acted as a pseudo-continuous wave laser due to its low peak power. A QCL can therefore be used as an effective laser scalpel, and also offers the possibility of less invasive treatment by targeting specific absorption bands in the MIR region.

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33 mu m. We achieved a laser optical power buildup factor of similar to 500, which corresponds to an intracavity laser power of similar to 0.75 W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4 s integration time, corresponding to a noise equivalent absorption coefficient of 1.4 x 10(-8) cm(-1) and a normalized noise-equivalent absorption of 3.2 x 10(-10) W cm(-1) Hz(-1/2). (C) 2014 AIP Publishing LLC.

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250 kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1 kHz, and its frequency is stabilized with an absolute traceability of 2 x 10(-12). This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1 kHz. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4799284]

A wide wavelength tuning of an external cavity quantum-cascade laser (QCL) based on the anticrossed dual-upper-state to multiple-lower-state design is demonstrated in continuous wave (cw) operation at room temperature. The tuning ranges of 321 cm-1 (Δλ/λ~22%) in pulsed operation and 248 cm-1 (Δλ/λ~17%) in cw operation are achieved, despite employment of the active region with translational symmetry. The present tuning range in cw operation substantially exceeds the values obtained with the QCLs based on conventional broadband active region designs. In addition, the continuous, single mode tuning is realized with its widely homogeneous gain spectrum.

The linewidth enhancement factor (α-factor) of a commercially available 6.1 μm InGaAs/InAlAs distributed-feedback quantum cascade laser capable of continuous-wave operation with a Peltier cooler and its current dependence were investigated. α tended to increase slowly with an increase in current. α, probably affected by detuning, was still smaller than that of conventional 1.55 μm semiconductor laser diodes.

The magnitude of the relative intensity noise (RIN) of a 5 mum distributed-feedback quantum cascade laser (DFB-QCL) was compared with a conventional 1.55 mum DFB laser diode (LD). The RIN for the DFB-QCL at a frequency of 1 MHz was 157 dBm at 10 mW light output, about 5 dB higher than that of the DFB-LD, which could almost be fully explained by the semi-classical noise model. The resonant tunnelling induced noise, which might be a cause of the RIN degradation, was not observed.

We propose a new quantum-cascade laser structure with single phonon resonance-continuum depopulation and demonstrate room temperature, continuous-wave operation of the proposed 7.9μm quantum-cascade laser. The laser grown by metal organic vapor-phase epitaxy emits a cw output power of 36mW at 30°C, exhibiting a threshold current density of 2.23kA∕cm2. The cw operation is reported for higher temperatures up to slightly above 60°C. The proposed structure may lead to a successful commercial mass production of the lasers.

Quartz-enhanced photoacoustic spectroscopy (QEPAS) and cavity-ehnanced spectroscopy are merged in a novel gas sensor. Thanks to the intracavity power build up, sensitivity is increased by more than a factor 100 with respect to standard QEPAS. © 2014 OSA.

A compact and simple laser spectroscopy apparatus at 4.5 ?m, based on saturated-absorption cavity ring-down (SCAR), has approached the ultimate sensitivity of accelerator mass spectrometry, measuring radiocarbon dioxide concentration down to few parts per quadrillion.