Cheng received his MSc degree in physics from Delft Univer¬sity, The Netherlands in 1989. His thesis is "Fabrication of and investigation on gratings in optical waveguides". After his study he joined the Optics Group of the TNO. He is involved in researches in interferometric systems, non-linear optics, optical sensors for process control and fiber optic sensor systems. From 1994 to 2000, Cheng was responsible for the Fiber Optic (FO) hydrophone project which was funded by the Royal Netherlands Navy (RNLN). He is involved in a number of European Space Agency (ESA) projects/proposals for infrared single mode fiber development and structural health monitoring using Fiber Bragg Grating sensor. He has developed various fiber optic pressure sensor systems for applications varying from GPa measurement in explosion/detonation investigations to sub-mPa acoustic detection for Oil&Gas applications. His current research interests are fiber laser sensors, fiber interferometers, picostrain FBG sensor and special fiber optic sensor systems for extreme environmental conditions as space, nuclear fusion, lithography and oil&gas. He is (co)author of more than 60 conference papers. His research resulted in >10 patents including a high-speed interrogation system for Fiber Bragg Grating sensor array (“DEMINSYS”) and a fiber optic Vortex flow meter (“SmartFlow”) which are commercialized under license.
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Test results of lateral load insensitive FBGs embedded in composites to suppress spectral distortion
Within the current project, a multi-parameter FBG sensor array demonstrator system for temperature and strain measurements is designed, fabricated and tested under ambient as well as Thermal Vacuum (TV) conditions in a TV chamber of the European Space Agency (ESA), ESTEC site. The aim is the development of a multi-parameters measuring system based on FBG technology for space applications. During the TV tests of a Space Craft (S/C) or its subsystems, thermal measurements, as well as strain measurements are needed by the engineers in order to verify their prediction and to validate their models. Because of the dimensions of the test specimen and the accuracy requested to the measurement, a large number of observation/measuring points are needed. Conventional sensor systems require a complex routing of the cables connecting the sensors to their acquisition unit. This will add extra weight to the construction under test. FBG sensors are potentially light-weight and can easily be multiplexed in an array configuration.
The different tasks comply of a demonstrator system design; its component selection, procurement, manufacturing and finally its assembly. The temperature FBG sensor is calibrated in a dedicated laboratory setup down to liquid nitrogen (LN2) temperature at TNO. A temperature-wavelength calibration curve is generated. After a test programme definition a setup in thermal vacuum is realised at ESA premises including a mechanical strain transducer to generate strain via a dedicated feed through in the chamber. Thermocouples are used to log the temperature for comparison to the temperature FBG sensor. Extreme temperature ranges from -150°C and +70°C at a pressure down to 10-4 Pa (10-6 mbar) are covered as well as testing under ambient conditions. In total five thermal cycles during a week test are performed. The FBG temperature sensor test results performed in the ESA/ESTEC TV chamber reveal high reproducibility (within 1 °C) within the test temperature range without any evidence of hysteresis. Differences are detected to the previous calibration curve. Investigation is performed to find the cause of the discrepancy. Differences between the test set-ups are identified. Equipment of the TNO test is checked and excluded to be the cause. Additional experiments are performed.
The discrepancy is most likely caused by a ’thermal shock’ due to rapid cooling down to LN2 temperature, which results in a wavelength shift. Test data of the FBG strain sensor is analysed. The read-out of the FBG strain sensor varies with the temperature during the test. This can be caused by temperature induced changes in the mechanical setup (fastening of the mechanical parts) or impact of temperature to the mechanical strain transfer to the FBG. Improvements are identified and recommendations given for future activities.
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