Dual-channel Mach-Zehnder interferometer based on femtosecond laser writing waveguide for seawater temperature and salinity sensing

Based on femtosecond laser microprocessing technology, a dual-channel Mach-Zehnder interferometer (MZI) sensor is demonstrated and aimed at achieving the contemporaneously monitor of seawater temperature and salinity. Two symmetrical trapezoidal waveguides and dual microcavities are written in the no-core fiber (NCF) which is spliced between the two segments of single-mode fibers of the proposed dual-channel MZI sensor using femtosecond laser microprocessing technology. One of the microcavities is filled with ultraviolet glue for seawater temperature measurement. The other microcavity is unfilled for direct contact with seawater to achieve salinity measurement. After the theoretical derivation and experimental verification, the performances of the dual-channel MZI sensor are further illustrated. As a result, the temperature and salinity sensitivities of the proposed dual-channel MZI sensor are -1.646 nm/℃ in the range of 5-40 ℃ and -1.608 nm/‰ (equivalent to -8320.957 nm/RIU) within a scope of 5-40 ‰, respectively. The proposed dual-channel MZI sensor shows good stability and repeatability with a compact structure. Such a highly sensitive sensor with a wide measuring range paves a new way for dual-parameter measurement of seawater.