This research introduces a new non-destructive technique for the characterization of fiber Bragg gratings (FBGs) based on the analysis of the FBG diffraction profile via measuring its asymmetry and intensity. This approach enables the determination of such FBG parameters as an off-axis displacement, aberrations of the focusing system, outcoupling efficiency of refractive index modulation, grating length, and grating order. This proposed technique can significantly improve quality control in FBG manufacturing. The applicability of this technique is demonstrated on different types of fiber Bragg gratings written by point-by-point femtosecond laser writing.
Different modifications of quartz enhanced photoacoustic spectroscopy (QEPAS) are discussed. An analysis of the signal-to-noise ratio for the generated photoacoustic signal gives hints for an optimized geometry of tuning fork and acoustic resonator. Furthermore, simultaneous or alternate photoacoustic and electrical driving is discussed, leading the way to new detection schemes that are capable of measuring changes in molecular relaxation dynamics.
Arrayed waveguide gratings (AWG) originally designed as demultiplexing device and manufactured with well established silicon wafer technology are already used successfully as compact spectrometers with high resolution1. In this paper, the concept of a new design for a wavelength demultiplexing device based on tailor-made polymers is presented. The motivation for a new design is a smaller footprint of the device and the avoidance of bended waveguides and the associated losses. Extensive simulations were performed to optimize the design. Using microscope projection lithography and hot embossing a first polymer based device was realized. Its characterization and the achieved performance in terms of resolution and covered wavelength range will be discussed.
In this paper, the concept of a micro ring resonator formed of waveguides in off-the-shelf polymers is presented. Extensive simulations were performed to determine appropriate dimensions for the waveguide and the design of ring and coupling zone as well as for the estimation of losses. Based on the calculated parameters, a first polymer ring resonator was realized using microscope projection lithography.
A microring resonator is used as a photonic sensor device for the detection of the explosive trinitrotoluene (TNT).
Selectivity is achieved by coating the sensor chip with specially designed receptor molecules. The measurand is the shift
in resonance frequency of the microring resonator induced by the change in effective index of refraction of the
waveguide materials due to adsorption/intercalation of the analyte. The response is linear with concentration and
reversible, i.e. the TNT molecules desorb from the sensor surface when it is flushed with carrier gas. This enables online
measurements since the sensor can be used again after flushing and no sampling is needed. Insensitivity to other
substances is demonstrated. Some chemically similar molecules induce a shift also, but the sensitivity is much lower.
The sensing limit for TNT is determined to be 0.5ppb. Simultaneous operation of two ring resonators is demonstrated,
proving the capability of a multi species monitoring when the rings are coated with different receptor molecules.
Detection of explosives is an emerging task for maintaining civil security. Optical methods and especially tunable diode laser spectroscopy are discussed as means for providing fast and reliable data. Selective and sensitive detection is possible in the midinfrared spectral region; however, until recently, small and easy to operate laser sources were not readily available for applications outside the laboratory. The situation changes with the maturation of quantum cascade lasers (QCLs). We present detection methods based on photofragmentation and subsequent midinfrared detection of the fragments for the detection of nitrogen-based explosives. For this type of explosive, the very low vapor pressure makes the use of direct spectroscopic techniques extremely difficult, since the equilibrium concentrations are in the ppb to ppt range. Peroxide-based explosives like triacetone triperoxide possess a much higher vapor pressure, making direct absorption spectroscopy and also a quartz-enhanced photoacoustic spectroscopy sensor possible. The progress and challenges of the application of QCLs, also with respect to interferences with other molecules present, are discussed.
A fiber optic sensor for monitoring CO2 during sequestration as well as for sensing the explosive TNT is described. The
sensor is based on evanescent-field spectroscopy, functionality and sensitivity are characterized.
A fiber optical laser sensor is applied for online and in-situ concentration monitoring of volcano gases. This can help to better understand the mechanisms underlying volcanic activity and enable the establishment of novel warning systems.
A compact, tunable and single-mode laser for the mid-infrared (MIW) spectral range is developed by difference frequency generation (DFG) in AgGaS2 and two "off-the-shelf' diode lasers. The MIR laser light is coupled into a silverhalide fiber and at the end of the fiber an infrared detector is used to record the transmitted MIR laser light. If the index of refraction of the fiber material is higher than the one of the surrounding medium the light is guided through the fiber due to total reflection. There are two loss mechanisms that will attenuate the laser intensity when passing through the fiber: (1) The frustrated total reflection (FTR) and (2) the attenuated total reflection (ATR). The FTR is related to a change of the index of refraction while the ATR is related to a change of the absorption coefficient. When tuning the MIR laser over an absorption line of a molecule that is outside of the fiber both, FTR and ATR, contribute to the measured spectral line profile that is recorded by the infrared detector. Similar to the direct laser absorption spectroscopy the recorded line profiles in the case of the evanescent~field spectroscopy can be used for estimating concentrations of molecules. A practical application of the evanescent field fiber sensor is shown as H2S is measured online and insitu at the volcano "Solfatara" in Italy.
Tunable mid-infrared radiation in the spectral range between 6.8 micrometer and 12.5 micrometer is generated via nonlinear optical difference-frequency-generation in AgGaS2. The input radiation that is needed for this process is provided by two high-power single mode diode-lasers. The combination of this MIR-laser source with a MIR-detector and lock-in detection technique has very interesting applications in spectroscopy. The on-line performance of this MIR-spectrometer is shown as single rotational lines of molecules are probed in the gas phase and in the vapor extracted from contaminated soil samples.
Two cw-single mode diode-lasers with powers of 30 and 50 mW at the center wavelengths 682 and 791 nm are applied as signal and pump sources for difference frequency generation (DFG) in an AgGaS2 crystal with a length of 30 mm. For 90 degree type I phase matching tunable mid-infrared laser radiation is obtained in the spectral range between 4.9 and 5.1 micrometers , while the DFG-output power is 0.2 (mu) W. The performance of this diode-laser MIR-DFG spectrometer is shown as the absorption of CO for the P(28) rotational line around 2023 cm-1 is probed in a cell and on-line in the exhaust of an engine.
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