Wavelength-Tunable Mode-Locked All-Fiber Laser Based on Humidity-Controlled Mach-Zehnder Interference

A wavelength-tunable passive mode-locked fiber laser based on a Mach-Zehnder interferometer (MZI) filter tuned by humidity variation is proposed. The all-fiber filter is constructed from an MZI that relies on cascading single-mode fiber (SMF)-strongly coupled seven-core fiber (SC–SCF)-SMF and is covered with an agarose (AG) film. For SC–SCF, the core spacing is small (11[Formula: see text][Formula: see text]m) and the diameter of the central core is the same as that of SMF, which provides extremely low insertion loss. By using the ability of AG to stretch or contract in response to humidity variation, the bending strain of MZI filters can be changed, which results in central wavelength variation, thereby achieving wavelength tunability of the wavelength-tunable laser. The experimental results indicate that continuous wavelength tuning of 34.44[Formula: see text]nm can be realized in the range of 35%–65% RH and the tuning efficiency is 1.35[Formula: see text]nm/% RH. In addition, the dual-wavelength mode-locking can be achieved by increasing the pump power at a specific relative humidity. With the advantages of simple structure, low cost, large wavelength tuning range and high tuning efficiency, our laser has a broad application prospect in the fields of optical communication, spectral analysis and biomedicine.