Résultat de la recherche
CO2 isotope sensor using a broadband infrared source, a spectrally narrow 4.4 μm quantum cascade detector, and a Fourier spectrometer
We report a prototype CO2 gas sensor based on a simple blackbody infrared source and a spectrally narrow quantum cascade detector (QCD). The detector absorption spectrum is centered at 2260 cm−1 (4.4 μm) and has a full width at half maximum of 200 cm−1 (25 meV). It covers strong absorption bands of two spectrally overlapping CO2 isotopomers, namely the P-branch of 12CO2 and the R-branch of 13CO2. Acquisition of the spectral information and data treatment were performed in a Fourier transform infrared (FTIR) spectrometer. By flushing its sample compartment either with nitrogen, dry fresh air, ambient air, or human breath, we were able to determine CO2 concentrations corresponding to the different gas mixtures. A detection limit of 500 ppb was obtained in these experiments.
Ozone detection by differential absorption spectroscopy at ambient pressure with a 9.6 µm pulsed quantum-cascade laser
We report direct absorption spectroscopic detection of ozone at ambient pressure with a pulsed, DFB quantum-cascade laser (QCL) tuned within 1044–1050 cm-1 by temperature scanning. Wavelength calibration curves were derived from FTIR and CO2 spectra and interpreted with respect to the heat transfer from the heterostructure to the sink. The laser linewidth (~0.13 cm-1 FWHM) was found to decrease with temperature, probably as a result of operation at constant current. Spurious spectral features due to baseline inaccuracies were successfully filtered out from the QCL O3 spectra using differential absorption. Reference O3 concentrations were obtained by applying the same method to UV spectra, simultaneously measured with a differential optical absorption spectrometer (DOAS). Column densities retrieved from QCL spectra are in fairly good agreement (±20%) with the DOAS values above 28 ppmm. The estimated QCL lowest detectable, absolute and differential absorptions, (7×10-3 and 2×10-3, respectively), entail effective detection limits of 14 and 25 ppmm, respectively. Ongoing improvements in the acquisition system should allow the achievement of detection limits at the level of commercial open-path DOAS systems (~2 ppm m) in the near future. Our results demonstrate the applicability of the differential absorption method to QCL spectroscopy at ambient pressure, and encourage its use for open path detection.
Continuous-wave operation of quantum cascade laser emitting near 5.6 µm
Buried heterostructure quantum cascade lasers emitting at 5.64 µm are presented. Continuous-wave (CW) operation has been achieved at –30°C for junction down mounted devices with both facets coated. A 750 µm-long laser exhibited 3 mW of CW power with a threshold current density of 5.4 kA/cm2.
High-frequency modulation of a quantum-cascade laser using a monolithically integrated intracavity modulator
We report a quantum-cascade laser monolithically integrated with an intracavity modulator which could be operated up to 1 GHz. In contrast to earlier approaches, where the radio frequency (RF) modulation signal was supplied to the entire cavity length of the laser structure, we drive only a relatively small 375-/spl mu/m-long section of the cavity. At the same time, a quasi-continuous-wave signal was supplied to the remaining 1125-/spl mu/m-long section. This modulation scheme resulted in smaller parasitic capacitance effects than what we reported previously, and enabled us to work with lower RF voltages and currents.
Microfluidic tuning of distributed feedback quantum cascade lasers
In this Letter, we report the tuning of the emission wavelength of a single mode distributed feedback quantum cascade laser by modifying the mode effective refractive index using fluids. A fabrication procedure to encapsulate the devices in polymers for microfluidic delivery is also presented. The integration of microfluidics with semiconductor laser (optofluidics) is promising for new compact and portable lab-on-a-chip applications.
Mid-infrared trace-gas sensing with a quasi- continuous-wave Peltier-cooled distributed feedback quantum cascade laser
A recently developed distributed feedback quantum cascade laser (QCL) capable of thermoelectric-cooled (TEC) continuous-wave (cw) operation and emitting at ~9 μm is used to perform laser chemical sensing by tunable infrared spectroscopy. A quasi-continuous-wave mode of operation relying on long current pulses (~5 Hz, ~50% duty cycle) is utilized rather than pure cw operation in order to extend the continuous frequency tuning range of the quantum cascade laser. Sulfur dioxide and ammonia were selected as convenient target molecules to evaluate the performance of the cw TEC QCL based sensor. Direct absorption spectroscopy and wavelength-modulation spectroscopy were performed to demonstrate chemical sensing applications with this novel type of quantum cascade laser. For ammonia detection, a 18-ppm noise-equivalent sensitivity (1 σ) was achieved for a 1-m absorption path length and a 25-ms data-acquisition time using direct absorption spectroscopy. The use of second-harmonic-detection wavelength-modulation spectroscopy instead of direct absorption increased the sensitivity by a factor of three, achieving a normalized noise-equivalent sensitivity of 82 ppbHz-1/2 for a 1-m absorption path length, which corresponds to 2×10-7 cm-1Hz-1/2.
Continuous wave operation of quantum cascade lasers
Continuous wave (CW) operation of quantum cascade lasers is reported up to temperatures of 312 K. The devices were fabricated as buried heterostructure lasers and episide-down mounted for improved heat dissipation. Fabry–Perot lasers emitted 17 mW of continuous optical power at 292 K and still 3 mW at 311 K at λ=9.12 μm. Distributed feedback quantum cascade lasers showed CW single-mode operation up to 260 K at λ=9.0 μm with a side mode suppression rate better than 30 dB.
Spectroscopic study of the ν1 band of SO2 using a continuous-wave DFB QCL at 9.1 µm
We report results of spectroscopic measurements with a continuous-wave distributed-feedback quantum-cascade laser (DFB QCL). Line intensities and self-broadening coefficients were measured in the ν1 band of SO2 between 1088 and 1090 cm-1. The self-broadening coefficients in this paper confirm the typical decrease of νself with increasing rotational quantum number K’’a. The line intensities determined here are smaller than those in the HITRAN 2000 database. Several lines found in this study were not present in the database.
Continuous-wave operation of quantum cascade laser emitting near 5.6
Buried heterostructure quantum cascade lasers emitting at 5.64 mum are presented. Continuous-wave (CW) operation has been achieved at -30degreesC for junction down mounted devices with both facets coated. A 750 mum-long laser exhibited 3 mW of CW power with a threshold current density of 5.4 kA/cm(2).
Free-running 9.1-μm distributed-feedback quantum cascade laser linewidth measurement by heterodyning with a C18O2 laser
We report spectral linewidth measurements of a 9.1-μm distributed-feedback quantum cascade laser (QCL). The free-running QCL beam was mixed with a waveguide isotopic C18O2 laser onto a high-speed HgCdTe photomixer, and beat notes were recorded from a radio-frequency spectral analyzer. Beating was performed at two operating conditions, first near the QCL laser threshold (beating with the C18O2R10 line) and then at a high injection current (beating with the C18O2R8 line). Overall, beat note widths of 1.3–6.5 MHz were observed, which proves that a free-running QCL can have a short-term spectral width near 1 MHz.