Tunable fiber lasers have wide applications in optical fiber sensing, optical communications, optical spectroscopy and microwave photonics systems.

Tunable filters are the key components including Mach–Zehnder interferometers, fiber Bragg gratings, Sagnac loop mirrors, Fabry–Perot interferometers, various compounded fiber filters, and so on. However, these tuning approaches by changing external perturbations are complicated, discrete tuning and easy to damage the device.
     
A research team led by Prof. Dr. DONG bo from Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences (CAS) proposes a feasible scheme for tunable erbium-doped fiber laser based on an Er/Yb codoped phosphosilicate fiber (EYDF) based modal interferometer (MI) filter. The MI has the single mode fiber (SMF)-EYDF-EYDF-SMF structure with taper splice joints. Employing its pump induced refractive index change characteristics, tunable fiber laser can be achieved. The result was published in JOURNAL OF LIGHTWAVE TECHNOLOGY.

Schematic of the tunable erbium-doped fiber ring laser (Image by XIOPM)

The tunable fiber laser has an additional pump signed as pump 2 before the MI. The MI filter has the pump induced refractive index change characteristics, and the wavelength tuning is achieved by changing the pump power injected into it.

Moreover, the wavelength changes linearly with the power of pump 2 and is continuous tuning, which effectively improves the compactness of the system.

The notable discovery offers a new avenue to achieve tunable fiber laser based on fiber MI. The laser is more simple, stable and repeatable.