Fiber optic temperature sensor using a grating on an angled fiber tip

Abstract

A fiber optic temperature sensor intended to sense temperatures up to 1400°C was investigated experimentally. A key element of the sensor is a grating on the 45°-angled tip of a single mode fiber. When light propagating in the fiber reaches the tip, it is split into two parts, which traverse different paths in the fiber. Some of the light is diffracted back down the fiber by the grating, while another portion of the light is reflected by the 45°- angled fiber surface, then by the fiber-air interface, and finally is directed down the fiber by a second reflection at the 45°- degree surface. The two waves interfere and temperature is determined by measuring the reflected optical power. Details of the grating fabrication process have been explained. A test set up to observe the response of the sensor to changes in temperature in a furnace was built and the test results are in general agreement with prediction. The reflected power is observed to vary approximately as a sinusoidal function as expected for an interferometric sensor. The observed temperature change for a p radian phase shift is 531°C versus a predicted value of 575°C.