High-Resolution and Large-Dynamic-Range π-Phase-Shifted FBG Sensor by a Current Modulated Diode Laser

We present a high-resolution and large dynamic range
$\pi $
-phase-shifted fiber Bragg grating (
$\pi $
-PSFBG) temperature sensor. A current modulated laser diode with a linearized optical frequency sweep is used to interrogate the sensor. The sensor is comprised of a
$\pi $
-PSFBG sensing head and an HCN gas cell. The gas cell functions as an absolute frequency reference to compensate for the laser frequency drift. The reflection spectrum is sent to a lock-in amplifier to generate the error signal, proportional to the first order derivative of the spectrum. The center frequency difference that carries sensing information between the
$\pi $
-PSFBG and HCN gas cell is calculated by cross-correlation of their error signals. A sinusoidal modulation enables to move the data processing from low-frequency stage to high-frequency stage, by phase-sensitive detection and low-pass filtering, the interferences caused by laser intensity noises and electronic devices’ noises are significantly reduced. In the experiment, a frequency resolution of 3.696 MHz with a 55 GHz mode-hop-free tuning range, is achieved, corresponding to a temperature resolution of 2.6 mK with a dynamic range from 5 to 40 °C.