Availability of reliable flight sensor data and knowledge of the structural behaviour are essential for safe operation of the Ariane launcher. The Ariane launcher is currently monitored by hundreds of electric sensors during test and qualification. Fibre optic sensors are regarded as a potential technique to overcome limitations of recent monitoring systems for the Ariane launcher [1]. These limitations include cumbersome application of sensors and harness as well as a very limited degree of distributed sensing capability. But, in order to exploit the various advantages of fibre optic sensors (high degree of multiplexing, distributed sensing capability, lower mass impact, etc.) dedicated measurement systems have to be developed and investigated. State-of-the-art fibre optic measurement systems often use free beam setups making them bulky and sensitive to vibration impact. Therefore a new measurement system is developed as part of the ESAstudy [2].

For the purpose of structure and temperature monitoring of Ariane launchers, a new fiber optic sensor interrogation system is developed by our institute. It is based on a monolithic, modulated-grating laser diode that is capable of switching between any two output wavelengths within less than ten nanoseconds. This makes the system very versatile since different sensors within the connected measurement channels can be read out in arbitrary sequence. Therefore sampling rates of single sensors can be adapted according to their application independently. The connection between spectral and time domain during read out of sensors is clarified. Performance optimization due to “Time of Flight” measurement and its consequences for different sample sequences are described. Issues that occur because of switching between different output wavelenghts during operation of the system are described in detail and solutions based on software algorithms are presented.

The resolution of many optical measurement systems employing 1D or 2D array sensors (e.g. CCD-cameras) is limited by the pixel-resolution of the detector. Subpixel algorithms allow to exceed this limit.This paper aims at fiber Bragg grating interrogators, which acquire the sensor signal spectrometrically. The accuracy of measurement of these systems is strongly depending on the used algorithms.Five different algorithms for peak detection are described and compared in theory and experiment. Most of these algorithms can be found both in literature and in different software-libraries (e.g. MATLAB, Lab VIEW...)To compare the different algorithms, a single fiber Bragg grating sensor was used to acquire data with a spectrometric measurement system consisting of a 1D-CCD-line-array, a superluminescent light source and a reflection grating. The same data was fed the five algorithms, such that the output signals are comparable. To characterize the peak-search algorithms the standard deviation of the output data has been computed for different frame rates and variable wide regions of interest around a FBG-peak.As a result we can provida a concise recommendation which of the analyzed algorithms is suitable for an application in spectrometric fiber Bragg grating interrogators.

Thin film reflectometry is a common way to monitor film thicknesses in numerous processes. Semiconductor and optics producing industries, e.g., require film thickness observation. Spectral reflectometry interrogation is one of several methods, which can be assessed by various algorithms depending on resolution and measuring range demands. Incident angle is a crucial parameter to these interrogation methods. Light, under different incident angles, produces different spectral reflectivity functions. These are integrated by a detector, in this case a spectrograph. Therefore, one needs to minimize the range of incident angles onto the measured object in order to increase the resolution of film thickness. Lateral resolution may also be a crucial parameter in some processes. Lateral resolution is a function of the range of incident angle, it increases with increasing angle range. These two crucial facts result in an estimate of measuring error introduced by the angle range under a given maximal lateral resolution. A measuring setup for a range of incident angle measurements is presented, as well as an example interrogation of a thin film under different incident angles.