Molecular relaxation effects in hydrogen chloride photoacoustic detection

A photoacoustic (PA) sensor has been developed to monitor hydrogen chloride at sub-ppm level in the 1740-nm region. The system was designed to control the process in the novel low-water-peak optical fiber manufacturing process. Relaxation effects in hydrogen chloride PA detection in oxygen-helium and nitrogen-helium gas mixtures are presented, showing that the generation of the PA signal is strongly affected by the ratio of these substances. In addition, the role of water vapor in the PA signal is investigate

Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation

A photoacoustic sensor has been developed for trace-gas monitoring using a near-infrared semiconductor laser emitting in the 2ν3 band of methane at 1.65μm. The apparatus was designed for on-line process control in the manufacturing of the novel low-water-peak fibres developed for optical telecommunications. The importance of collisional relaxation processes in the generation of the photoacoustic signal is reported in the particular case of CH4 detection in dry O2 and O2-N2 mixtures. The negative influence of these effects results in a strongly reduced and phase-shifted photoacoustic signal, induced by a fast resonant coupling between the vibrational states of methane and oxygen, associated with the slow relaxation of the excited oxygen molecules. An unusual parabolic response of the sensor with respect to the methane concentration has been observed and is discussed. Finally, the beneficial effect of several species, including water vapour and helium, acting as a catalyst to hasten the relaxation of the CH4-O2 system, is demonstrate

Ammonia trace measurements at ppb level based on near-IR photoacoustic spectroscopy

A photoacoustic sensor using a laser diode emitting near 1532nm in combination with an erbium-doped fibre amplifier has been developed for ammonia trace gas analysis at atmospheric pressure. NH3 concentration measurements down to 6ppb and a noise-equivalent detection limit below 3ppb in dry air are demonstrated. Two wavelength-modulation schemes with 1f and 2f detection using a lock-in amplifier were investigated and compared to maximise the signal-to-noise ratio. A quantitative analysis of CO2 and H2O interference with NH3 is presented. Typical concentrations present in ambient air of 400ppm CO2 and 1.15% H2O (50% relative humidity at 20°C) result in a NH3 equivalent concentration of 36ppb and 100ppb, respectivel

Antibiotica in de bodem. Een pilotstudie.

In het kader van een overeenkomst tussen Stichting Kennisontwikkeling Kennisoverdracht Bodem (SKB) en een breed consortium en klankbordgroep is een pilotstudie uitgevoerd naar veterinaire antibiotica in de bodem en het grondwater

Frequency comb offset dynamics of SESAM modelocked thin disk lasers

We present a detailed study of the carrier-envelope offset (CEO) frequency dynamics of SESAM modelocked thin disk lasers (TDLs) pumped by kW-class highly transverse multimode pump diodes with a typical M2 value of 200-300, and give guidelines for future frequency stabilization of multi-100-W oscillators. We demonstrate CEO frequency detection with > 30 dB signal-to-noise ratio with a resolution bandwidth of 100 kHz from a SESAM modelocked Yb:YAG TDL delivering 140 W average output power with 748-fs pulses at 7-MHz pulse repetition rate. We compare with a low-power CEO frequency stabilized Yb:CALGO TDL delivering 2.1 W with 77-fs pulses at 65 MHz. For both lasers, we perform a complete noise characterization, measure the relevant transfer functions (TFs) and compare them to theoretical models. The measured TFs are used to determine the propagation of the pump noise step-by-step through the system components. From the noise propagation analysis, we identify the relative intensity noise (RIN) of the pump diode as the main contribution to the CEO frequency noise. The resulting noise levels are not excessive and do not prevent CEO frequency stabilization. More importantly, the laser cavity dynamics are shown to play an essential role in the CEO frequency dynamics. The cavity TFs of the two lasers are very different which explains why at this point a tight CEO frequency lock can be obtained with the Yb:CALGO TDL but not with the Yb:YAG TDL. For CEO stabilization laser cavities should exhibit high damping of the relaxation oscillations by nonlinear intra-cavity elements, for example by operating a SESAM in the roll-over regime. Therefore the optimum SESAM operation point is a tradeoff between enough damping and avoiding multiple pulsing instabilities. Additional cavity components could be considered for supplementary damping independent of the SESAM operation point