Tunable fiber lasers have the advantages of good beam quality, high integration, and adjustable output wavelength, and they are widely used in fields such as optical fiber communication and optical fiber sensing. The fiber filter is one of the key components of tunable fiber lasers. Among the various filters currently used, multimode interference filters have the advantages of simple structure, convenient implementation, flexible tuning methods, and convenient spectral range design. The structures of multimode interference filters based on multimode fibers, no-core fibers, multi-core fibers, tapered fibers, and other special fibers are introduced in this paper. The working principles and tuning methods are analyzed and the research progress of tunable fiber lasers based on these filters is summarized. Finally, the development trend of tunable fiber lasers based on multimode interference filters is discussed. The rapid development and applications of multimode interference filters can help improve the performance of continuous and pulse lasers as well as promote the practicality of tunable fiber lasers.

A novel vernier effect filter is designed utilizing two cascaded Mach–Zehnder interferometers (MZIs). Integrating the filter into an erbium-doped fiber laser (EDFL), the tunability of laser wavelength is achieved. Each MZI comprises two sequentially interconnected 3 dB optical couplers (OCs), where the incoming light is initially split into two arms at the first OC and subsequently recombined at the second OC. Interference occurs due to the optical path difference between these two beams. Notably, the two MZIs exhibit closely matched free spectral ranges (FSRs), leading to the formation of a broadened envelope in the superimposed spectrum. By delicately adjusting the optical path difference between the two arms of one MZI, a little drift of the interference spectrum is induced. This small amount of drift, in turn, triggers a significant movement of the envelope, giving rise to the so-called vernier effect. Integrating the vernier effect filter into an EDFL, the wavelength of the fiber laser can be tuned from 1542.56 nm to 1556.62 nm, with a tuning range of 14.06 nm. Furthermore, by employing a high-precision stepper motor, a remarkable tuning accuracy of 0.01 nm is attainable. The side mode suppression ratio of all wavelengths is above 55 dB. In comparison to reported tunable fiber lasers utilizing MZI filters, the proposed fiber laser in this study offers enhanced precision and a more user-friendly tuning process.

An interference filter is designed by cascading two Mach-Zehnder interferometers (MZIs), which is then utilized in the construction of a wavelength interval adjustable dual-wavelength Erbium-doped fiber laser (EDFL). Each MZI consists of two 3 dB optical couplers (OCs) connected in series. The input light is split into two arms at the first OC and recombined at the second OC. The first MZI (MZI 1) has a smaller free spectral range (FSR) and is used for selecting the output wavelength of the laser. The second MZI (MZI 2) has a larger FSR and forms an envelope structure on the comb-like spectrum of MZI 1. By adjusting the optical path difference between the two arms of MZI 2, the FSR of the envelope can be changed, thereby altering the interval of the output wavelengths. The implemented filter is inserted into the EDFL to achieve stable dual-wavelength output. By stretching one arm of MZI 2, the interval of the dual-wavelength can be adjusted from 4.64 to 13.94 nm, with the side-mode suppression ratio of over 44 dB for all wavelengths.

A wavelength-switchable ytterbium-doped mode-locked fiber laser is reported in this article. Two Mach–Zehnder interferometers (MZIs, denoted as MZI1, MZI2) with close free spectral ranges (FSRs) are connected in series to form a Vernier effect sensor. By utilizing the filtering effect of the Vernier effect sensor, the wavelength-switchable output of an ytterbium-doped mode-locked fiber laser is realized. When the 3 dB bandwidth of the Vernier effect filter is set to be 5.31 nm around 1073.42 nm, stable dissipative solitons are obtained. Stretching MZI1 horizontally, the central wavelengths of the pulses can be switched among 1073.42 nm, 1055.38 nm, and 1036.22 nm, with a total tunable central wavelength range of 37.2 nm. When the 3 dB bandwidth of the Vernier effect filter is set to be 4.07 nm, stable amplifier similaritons are obtained. Stretching MZI1 horizontally, the central wavelengths of the pulses are switchable among 1072.71 nm, 1060.15 nm, 1048.92 nm, and 1037.26 nm, with a total tunable central wavelength range of 35.15 nm. Compared with traditional fiber interference filters, the Vernier effect filter has a higher sensitivity, making wavelength switching more convenient and providing a wider tuning range for the ytterbium-doped mode-locked fiber laser.