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Fiber-Optic Microprobe For Interferometric Measurements In Generators

In the framework of the development of an extrinsic type fiber-optic interferometric sensor, the design and the qualification tests of the sensing microprobe are presented. The microprobe is the end-termination of a fiber-optic instrument devised for the monitoring of the end-turn vibrations of large generators. Since the monitoring must be made in real operating conditions, the microprobe must survive in real harsh environment: 100°C average temperature, very high electromagnetic field (30000 Amp. average current), pure hydrogen at 4 atmosphere pressure, high mechanical noise. The microprobes were developed following two approaches: conventional and stacked micro-optics concept. Several thermal tests showed a better behaviour by the stacked-optic microprobe. During the test, autointerference modulation phenomena appeared on the control beams. A hypothesis of a polyetalon existence inside the microprobe was formulated and its correctness was supported by a computer simulation. Preliminary qualification tests of the whole interferometric instrument, ended by the stacked-optic microprobe, showed an excellent response over the whole thermal operating range.