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 $\mathrm{\Delta }\lambda$. The measured relative change of these parameters with temperature in the range 15–110 °C, is $\left(1/{\lambda }_r\right)\left(\delta {\lambda }_r/\delta T\right)=-\left(1/\mathrm{\Delta }\lambda \right)\left(\delta \mathrm{\Delta }\lambda /\delta T\right)\approx \left(1/\mathrm{\Delta }n\right)\left(\delta \mathrm{\Delta }n/\delta T\right)\left(-0.94\pm 0.02\right)\times {10}^{-3}/\mathrm{K}$, with measured fiber birefringence $\mathrm{\Delta }n=8\times {10}^{-4}$. This gives a wavelength-shift sensitivity of $-1.00 \mathrm{nm}/\mathrm{K}$ at $1.065 \mathit{\mu }\mathrm{m}$ and a resonance separation sensitivity of $0.006 \mathrm{nm}/\mathrm{K}$ for $\mathrm{\Delta }\lambda =6.8 \mathrm{nm}$. 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 °C.