Fibre optic long period grating-based humidity sensor probe using a Michelson interferometric arrangement.

Long period gratings (LPG)-based sensors are widely used due to their high sensitivity to a range of suitable measurands. However, a typical configuration involving a single grating-based LPG sensor system frequently has the disadvantage of the probe being used in transmission mode. Further, the broad bandwidth of the attenuation bands formed by the propagation mode coupling between the core and the cladding modes constitutes a difficulty when the device is used as a conventional sensor probe. To overcome these limitations, a Michelson interferometer-type sensor configuration has been developed, using a LPG grating pair formed by coating a mirror at the distal end of the LPG. This sensor configuration is more convenient to use and is able to overcome the limitations of the single LPG sensor due to the shifts in the attenuation bands being more easily detectable. As part of work to develop an optimum configuration for the probe, a series of tests has been carried out to determine the most suitable distance between the LPG and the mirror coating for a typical grating (LPG) of period 250 μm. The sensor thus developed was then calibrated for use for temperature and refractive index measurement. In this paper, the design of a LPG coated with polyvinyl alcohol (PVA) as a relative humidity (RH) sensor and used in the Michelson interferometer configuration is reported. PVA, which swells with the increase of RH in its surroundings, causes a change in the refractive index of the coated polymer layer which was observed by tracking the response of the resonance loss band of the LPG to varying RH levels. The distal end of the fibre containing the LPG has been coated with a silver mirror in order to track the shift of the resonance loss band in reflection and to achieve a high resolution. The sensor was evaluated by being subjected to humidity changes over a RH range from 20% to 85% and an excellent detection performance was obtained while retaining the sensitivity of the PVA coated sensor