## 1 Introduction  
Recently, considerable work has been done employing Mach-Zehnder fiber interferometers (MZFIs) based on two concatenated optical fiber tapers, which have been developed to enhance the lasing performance and functionalities in fiber laser systems. The filter operates by interfering with core and cladding modes. In the first taper, the core mode couples to higher-order cladding modes, acquiring a phase delay due to refractive index differences. The second taper reintroduces the cladding mode, forming a comb-like spectral pattern [1, 2]. Typically, the MZFI is included as an intra-cavity filter in a fiber ring laser configuration. The tailorable filtering properties of MZFIs are explored in various forms. The first publication that employs an MZFI by concatenating two abrupt tapers separated by a section of SMF to demonstrate a tunable fiber laser was reported by [PERSON] et al. in 2010 [3]. The tapers were fixed on positioning stages in that experiment, and adjustable bending was applied to the first taper. The amplified spontaneous emission of an erbium-doped fiber amplifier (EDFA) was reshaped into a comb-like transmission spectrum, and only higher-intensity peaks were amplified. As the taper's bending angle was increased, the lasing wavelength decreased consistently, tuning within the \(\sim\)1550-1605 nm wavelength range. Pursuing this tendency, the same group reported the impact and optimization of the waist diameter of tapered fibers and filter loss in an erbium-doped fiber ring laser (EDFRL), achieving a step-tunable performance from 1547.3 nm to 1563.4 nm [4]. Spectral features depend on taper dimensions and separation length. In this way, fiber spectral manipulation and laser wavelength performance rely, among other factors, on the variable that tunes the fiber filter, i.e., bending, strain, twist, or change in the external refractive index. For example, bending induces stress, thereby altering the effective refractive index of the interacting core and cladding modes and the geometrical dimensions influencing spectral transmission. Changes in the surrounding media also impact the filter's effective refractive index of the modes supported by the fiber structure. These factors have been utilized to demonstrate improved fiber lasers. For example, an EDFRL with a modified surrounding refractive index of the MZFI was reported in [5]. This modification involved gradually heating a glycerol solution to change the refractive index, achieving a wavelength tuning of 12 nm. A study in [6] also explored the immersion of an MZFI in combination with a fiber Bragg grating for a liquid-level fiber laser sensor, resulting in dual-wavelength modulation. Conversely, switchable multi-wavelength lasers can be developed by leveraging the comb-like transmission spectrum. For instance, as described in Ref. [7], the realization of switchable multi-wavelength laser emission was successfully demonstrated by implementing a displacement S-bent MZFI configuration. This led to the simultaneous generation of four laser lines within an erbium ring cavity. Additionally, a tunable dual-laser emission spanning a range from 1520 nm to 1530 nm has been documented utilizing dual tapering of a 3 m section of Er-doped fiber [8] or a dual-wavelength single longitudinal mode from 1550 nm to 1554 nm EDFRL with Rayleigh backscattering assistance [9], and the use of mechanically bent in-line tapered MZFI to serve as a variable attenuator for dual wavelengths (1546.23 nm and 1554.76 nm) in an EDFRL [10]. Proposed experimental configurations also offer the capability for wavelength selectivity by tuning or generating multi-wavelength emissions ranging from \(\sim\)1528 nm to 1562 nm. The proceeding was realized by superimposing two discrete free spectral ranges (FSR) and modifying the curvature of two MZFIs with dissimilar lengths between tapers connected in series [11].