Ulrike Willer

An ultra-short fiber Bragg grating with a grating length of 0.2 mm and constant grating period (uniform FBG) is proposed as an integrated dispersive element for spectral analysis in a single-mode glass fiber. This dispersive element is used to set up a fiber optical spectrometer that demonstrated an average spectral resolving power of 825, a pixel resolution of 0.02 nm/pixel, and a 40 nm bandwidth covering the 810-850 nm wavelength range. Furthermore, the dependence of this dispersive element on light polarization was examined, which induces a reduction in the wavelength accuracy up to ±0.05 nm. Finally, the investigated spectrometer to function well as a part of a readout device for fiber Bragg grating sensors is presented. Short uniform Bragg gratings as dispersive elements allow the manufacturing of miniaturized, compact spectrometers without additional focusing components between the FBG and detector.

In this paper, we report on the progress of the auto-triggered quartz-enhanced photoacoustic spectroscopy (QEPAS) technique which operates without external frequency generators and ensures permanent locking to the current resonance frequency of the tuning fork. This is obtained by incorporating the tuning fork in an oscillator circuit that autonomously oscillates at the present resonance frequency that shifts with changing environmental conditions, e.g., density and viscosity of the surrounding gas, temperature, and pressure. Both, the oscillation amplitude as well as the frequency can be read from the oscillator circuit. The photoacoustic signal appears as an offset of the electrically induced signal amplitude. Since the sum amplitude depends on the phase relation between the electrical and photoacoustic driving forces, the phase is permanently modulated, enabling the extraction of the photoacoustic component by use of a second lock-in amplifier stage which is being referenced with the phase modulation frequency. The functionality of this method is demonstrated for methane detection in a carbon dioxide atmosphere in a concentration range from 0 to 100% and ammonia in synthetic air employing a pulsed mid infrared QCL around 1280 cm⁻¹. The gas mixtures are motivated by the demands in biogas-analysis.

In this review, two different methods for gas sensing applications are highlighted: QEPAS and a fiberoptic sensor relying on a change of refractive index when the target species is present.

A new sensor for methane and carbon dioxide concentration measurements in biogas plants is presented. LEDs in the mid infrared spectral region are implemented as low cost light source. The combination of quartz-enhanced photoacoustic spectroscopy with an absorption path leads to a sensor setup suitable for the harsh application environment. The sensor system contains an electronics unit and the two gas sensors; it was designed to work as standalone device and was tested in a biogas plant for several weeks. Gas concentration dependent measurements show a precision better than 1% in a range between 40% and 60% target gas concentration for both sensors. Concentration dependent measurements with different background gases show a considerable decrease in cross sensitivity against the major components of biogas in direct comparison to common absorption based sensors.