Femtosecond laser inscribed small-period long-period fiber grating coated with a nano-assembled polyelectrolyte film for independent sensing of ammonia gas and temperature

We propose and demonstrate a novel ammonia gas and temperature sensor based on a femtosecond laser inscribed small-period long-period fiber grating (SP-LPG) coated with a nano-assembled polyelectrolyte thin film composed of poly(diallyldimethylammonium chloride) (PDDA)/poly(acrylic acid) (PAA). The PDDA/PAA film is deposited onto the SP-LPG via layer-by-layer electrostatic self-assembly. The SP-LPG is inscribed in a single mode fiber with a period of 30 mu m and the total length is only 3 mm. The designed period is much smaller than that of the conventional long period fiber grating (LPFG) of several hundred microns, so we can not only see high-order cladding modes in the transmission spectrum, but also a series of Bragg resonance peaks in the reflection spectrum. The concentration of ammonia gas is detected by the wavelength shift of the transmission resonance dip. The ambient temperature is monitored by the wavelength shift of the Bragg resonance peak. Because the Bragg resonance peak is insensitive to refractive index, we can measure ammonia gas and temperature simultaneously. The experimental results show that the detection limit of the sensor is less than 0.1 ppm. The sensor is sensitive only to ammonia when exposed to different gases. The temperature sensitivity of the sensor is about 8.7 pm/degrees C from 30 degrees C to 70 degrees C. Compared with traditional LPFG, SP-LPG has a more compact structure and the special ability to monitor temperature in real time, which makes the device more advantageous in practical applications.