Optical fiber laser salinity sensor based on multimode interference effect

An optical fiber laser salinity sensor based on multimode interference effect is proposed and demonstrated. The sensor head manufactured by splicing a section of no-core fiber (NCF) between two single-mode fibers possesses a transmission peak at a certain wavelength due to multimode interference effect. Salinity of the liquid it immerges can be measured from wavelength shift the transmission peak. Enhanced measurement accuracy is achieved by inserting the sensor head into the cavity of an erbium-doped fiber ring laser, because the laser's linewidth is much narrower than that of the transmission peak. Linear response with salinity sensitivity of 19.4 pm/% has been achieved and the measurement range is from 3.86% to 21.62%.