Tunable multi-wavelength mode-locked thulium-doped fiber laser with precise controlled wavelength spacing via Mach-Zehnder interferometer

• An all-fiber Mach-Zehnder interferometer in a TDFL cavity resolves mode competition and ensures precise wavelength spacing. • Simulated FSR and arm length tuning in the Mach-Zehnder interferometer filter enable accurate, flexible spectral control. • A hybrid setup with MZI filter, PC, and BP achieves tunable multi-wavelength pulses. • This compact, cost-effective laser design enhances scalability and versatility for spectroscopy and diagnostics. This study presents an experimental demonstration of a multi-wavelength mode-locked Thulium-doped fiber laser (TDFL) utilizing a cost-effective, fully fiber-integrated Mach-Zehnder interferometer (MZI) as the spectral filter. The working mechanism of the MZI filter is analyzed, demonstrating its role in shaping the laser spectrum and providing precise bandwidth control. The MZI operates by inducing interference between its two arms, where the arm length difference ( ΔL ) determines the free spectral range (FSR). By adjusting ΔL , the FSR and, consequently, the pulse wavelength spacing can be precisely controlled. Simulated transmission spectra confirm the filter’s tunability and precision. The laser cavity integrates the MZI filter with a polarization controller (PC) and black phosphorus in a hybrid mode-locking scheme. The MZI acts as a comb filter, while the PC and black phosphorus facilitate mode-locked operation. Adjusting the PC alters the refractive index of the fiber, dynamically tuning the filtering wavelength within the cavity. This configuration enables a tunable single-wavelength output spanning 1842.1 nm to 1897.9 nm, covering a 55.8 nm range. By varying the pump power, the laser transitions from single-wavelength to four-wavelength mode-locking. This innovative design highlights the potential of the MZI filter in improving the performance and versatility of Thulium-doped fiber lasers, offering promising applications in wavelength-selective optical sensing and diagnostics.