All-fiber absolute temperature sensor using an unbalanced high-birefringence Sagnac loop

We demonstrate a highly sensitive temperature sensor based on a stress-induced high-birefringence-fiber Sagnac loop that uses a Nd-doped-fiber amplified spontaneous emission source. Relative temperature sensing is done in the spectral domain by shifts of a resonant wavelength lambda(r) and absolute temperature sensing by changes in separation between resonances Delta lambda. The measured relative change of these parameters with temperature in the range 15-110 degrees C, is (1/lambda(r))(delta lambda(r)/delta T) = -(1/Delta lambda)(delta Delta lambda/delta T) approximate to (1/Delta n)(delta Delta n/delta T)(-0.94 +/- 0.02) x 10(-3)/K, with measured fiber birefringence Delta n = 8 x 10(-4). This gives a wavelength-shift sensitivity of -1.00 nm/K at 1.065 mu m and a resonance separation sensitivity of 0.006 nm/K for Delta lambda = 6.8 nm. This telemetric point sensor has a loop length of 80 m, an operational bandwidth of more than 50 nm, and a temperature accuracy of better than 1 degrees C. (C) 1997 Optical Society of America.