Large area average temperature sensing by the use of optical coherence domain reflectometry

Average temperature measurements are important for the oil tanks, oil-filled power transformers and buildings. A lot of temperature sensors based on a number of different measurement principles have been developed. In this paper, we report a novel temperature sensing system by the use of optical coherence domain reflectometry (OCDR). OCDR is a technique based on the low coherence interferometry. A CW low coherence source with a coherence length on the order of 10 μm is coupled to a fiber directional coupler and divided into the test arm and the reference arm. In the test arm, the light is divided into the sensing loop and sub-reference loop by the couplet and reflected by the loop ends (R1 and R2) with 4% reflectivity. The optical length of the test arm can be modulated by the PZT. The optical delay in the reference arm can be varied by translating the movable mirror. When it matches the delay for light reflected from R1 and R2 respectively, optical interference appears. The length difference between the sensing loop and the sub-reference loop is set to be less than 2 mm, but it is much longer than the coherence length of the light source. The signal detected by the photodetector is passed through a signal processing unit and the result is demonstrated in Fig.2 where the signal is a function of the mirror position. The peak R1 and R2 illustrate that the light reflected from the mirror matches that reflected from the loop ends R1 and R2 respectively. The distance between the two peaks illustrates the optical path difference between the sensing loop and the sub-reference loop. When the sensing loop is heated and the sub-reference loop kept under the same condition with the reference arm, the peak distance between R1 and R2 will be varied as a function of the temperature of the sensing fiber loop