Active linewidth-narrowing of a mid-infrared quantum cascade laser without optical reference.

We report on a technique for frequency noise reduction and linewidth-narrowing of a distributed-feedback mid-IR quantum cascade laser (QCL) that does not involve any optical frequency reference. The voltage fluctuations across the QCL are sensed, amplified and fed back to the temperature of the QCL at a fast rate using a near-IR laser illuminating the top of the QCL chip. A locking bandwidth of 300 kHz and a reduction of the frequency noise power spectral density by a factor of 10 with respect to the free-running laser are achieved. From 2 MHz for the free-running QCL, the linewidth is narrowed below 700 kHz (10 ms observation time).

Frequency noise of free-running 4.6 µm distributed feedback quantum cascade lasers near room temperature.

The frequency noise properties of commercial distributed feedback quantum cascade lasers emitting in the 4.6 µm range and operated in cw mode near room temperature (277 K) are presented. The measured frequency noise power spectral density reveals a flicker noise dropping down to the very low level of <100 Hz(2)/Hz at 10 MHz Fourier frequency and is globally a factor of 100 lower than data recently reported for a similar laser operated at cryogenic temperature. This makes our laser a good candidate for the realization of a mid-IR ultranarrow linewidth reference.

Optical phonon sidebands of electronic intersubband absorption in strongly polar semiconductor heterostructures.

We present the first evidence for a distinct optical phonon progression in the linear and nonlinear intersubband absorption spectra of electrons in a GaN/Al(0.8)Ga(0.2)N heterostructure. Femtosecond two-color pump-probe experiments in the midinfrared reveal spectral holes on different vibronic transitions separated by the LO-phonon frequency. These features wash out with a decay time of 80 fs due to spectral diffusion. The remaining nonlinear transmission changes decay with a time constant of 380 fs. All results observed are described by the independent boson model.

Free-running 9.1-microm distributed-feedback quantum cascade laser linewidth measurement by heterodyning with a C18O2 laser.

We report spectral linewidth measurements of a 9.1-microm 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 C18O2 R10 line) and then at a high injection current (beating with the C18O2 R8 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.

Chemical sensing with pulsed QC-DFB lasers operating at 15.6 micrometers.

Pulsed thermoelectrically cooled QC-DFB lasers operating at 15.6 micrometers were characterized for spectroscopic gas sensing applications. A new method for wavelength scanning based on repetition rate modulation was developed. A non-wavelength-selective pyroelectric detector was incorporated in the sensor configuration giving the advantage of room-temperature operation and low cost. Absorption lines of CO2 and H2O were observed in ambient air, providing information about the concentration of these species.

Photoacoustic spectroscopy with quantum cascade distributed-feedback lasers.

We present photoacoustic (PA) spectroscopy measurements of carbon dioxide, methanol, and ammonia. The light source for the excitation was a single-mode quantum cascade distributed-feedback laser, which was operated in pulsed mode at moderate duty cycle and slightly below room temperature. Temperature tuning resulted in a typical wavelength range of 3cm(-1)at a linewidth of 0.2cm(-1). The setup was based on a Herriott multipass arrangement around the PA cell; the cell was equipped with a radial 16-microphone array to increase sensitivity. Despite the relatively small average laser power, the ammonia detection limit was 300 parts in 10(9)by volume.

Assessment of acute mountain sickness by different score protocols in the Swiss Alps.

INTRODUCTION: Purpose of the present study was to evaluate the Lake Louise acute mountain sickness (AMS) score questionnaire at different altitudes and to compare it with the currently used clinical score and the environmental symptoms questionnaire AMS-C score. METHODS: We investigated 490 climbers who stayed over night at 4 huts in the Swiss Alps, located at the altitudes of 2850 m, 3050 m, 3650 m, and 4559 m. AMS was assessed using our previously described clinical score, the Lake Louise consensus AMS score questionnaire and the environmental symptoms questionnaire III. RESULTS: Below 4000 m, the prevalence of AMS, defined by symptoms that force a reduction in activity, was 7%; when assessed with the clinical score (score > or = 3) it was 22%; with the AMS-C score (score > or = 0.7) 4% and with the Lake Louise score (score > 4) 8%. At the altitude of 4559 m, the prevalence of AMS was 30%, 38%, 40%, and 39%, respectively. The standardized regression coefficients from multiple regression analysis (adjusted R2 0.65, p < 0.001) were 0.45 (p < 0.001) for the self-reported Lake Louise score, 0.48 (p < 0.001) for the sum of the points assigned in the clinical section of the Lake Louise questionnaire, and 0.05 (p = 0.27) for the AMS-C score. The sensitivity and specificity of the Lake Louise score > 4 was 78% and 93%, respectively. CONCLUSIONS: The Lake Louise consensus score is adequate and, compared with the AMS-C score, more effective for the assessment of acute altitude illness at different altitudes.

Theory of loss measurements of Fabry Perot resonators by Fourier analysis of the transmission spectra.

We present a detailed theoretical analysis for the determination of the total internal loss in Fabry-Perot resonators based on Fourier analysis of the emission or transmission spectrum. The observation of higher-order harmonics and their relative height in the Fourier-transformed spectrum allow us to quantify the total resonator loss. Because this new method considers both contrast and shape of the Fabry-Perot fringes it is especially well suited for the evaluation of high-finesse laser resonators such as those of vertical cavity surface-emitting lasers in terms of propagation loss/gain.

Treatment of acute mountain sickness by simulated descent: a randomised controlled trial.

OBJECTIVE: To evaluate the therapeutic efficacy of a portable hyperbaric chamber for treatment of acute mountain sickness. DESIGN: Controlled randomised trial over two mountaineering seasons. SETTING: High altitude research laboratory at 4559 m above sea level. SUBJECTS: 64 climbers with acute mountain sickness randomly allocated to different treatments. INTERVENTIONS: One hour of treatment in the hyperbaric chamber at a pressure of 193 mbar or 20 mbar as control or bed rest. MAIN OUTCOME MEASURES: Symptoms of acute mountain sickness before, immediately after, and 12 hours after treatment. Permitted intake of analgesic and antiemetic drugs in the follow up period. RESULTS: Treatment with 193 mbar caused greater relief of symptoms than did control treatment or bed rest. During the 12 hour follow up period intake of analgesics was similar (58-80% of subjects in each group). Symptom scores had improved in all subjects after 12 hours with no significant differences between groups. CONCLUSIONS: One hour of treatment with 193 mbar in a portable hyperbaric chamber, corresponding to a descent of 2250 m, leads to a short term improvement in symptoms of acute mountain sickness but has no beneficial long term effects attributable to pressurisation.