# Tunable and Switchable Erbium-Doped Fiber Laser Using a Multimode-Fiber Based Filter
## 1 Introduction  
Fiber lasers have attracted intensive research interests in the past few decades and have become the fastest growing field in the laser community due to their wide industrial applications in telecommunications, material processing, and sensors, etc. In particular, switchable and tunable erbium-doped fiber lasers (EDFLs) that are able to switch from single-wavelength to multi-wavelength output, as well as tunable capability, have been attractive to applications, such as optical instrument testing. Due to the nature of the homogeneous broadening effect of the erbium doped fiber, it is challenging to achieve a stable multi-wavelength operation. Multiwavelength EDFLs have been demonstrated by using different schemes, for example, cascaded fiber Bragg gratings (FBGs) [1], four-wave-mixing of dispersion shifted fiber [2], nonlinear polarization rotation [3], and various fiber-optic interferometers (FOIs) [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Among them, much of the research attention has been recently paid to employ FOIs to generate multiwavelengths. An FOI can be constructed with a pair of long period gratings (LPGs) [4], sandwich-structured single-mode fiber (SMF) driven by an acoustic transducer [5], a fiber loop mirror (FLM) incorporated with one or two sections of high birefringent fibers [6; 7; 8; 9] or a combination with hollow-core Bragg fiber [10], and modal interference by using fiber tapers [11; 12; 13] or multimode fiber (MMF) [14; 15]. Although the switchable function from single- to multi-wavelength is readily realized by adopting a polarization controller (PC) [6; 8; 9; 11; 14] or a variable optical attenuator (VOA) [13; 15] in the cavity, it is highly desirable that, in the meantime, the laser wavelength and wavelength spacing could betunable for practical applications. Many efforts have been devoted to the tunable and switchable multiwavelength fiber lasers. [PERSON] et al. [10] demonstrated a switchable EDFL with a tunable range of 3.4 nm for single wavelength operation by using two PCs. With three PCs, [PERSON] et al. [7] realized a switchable EDFL, with a tunable wavelength spacing. Another approach by using a combination of a PC and a rotary fiber squeezer inserted with multi-mode fiber (MMF) has been implemented to attain a tunable multiwavelength thulium-doped fiber laser (TDFL) [16] and a wavelength spacing switchable TDFL [9]; nevertheless, the lasers suffer from a high insertion loss of the multimode interference filter in the rotary squeezer [9] and a relatively high power fluctuation for dual-wavelength operation [16]. [PERSON] et al. [17] recently reported a tunable multiwavelength EDFL with a polarization maintaining FBG (PM-FBG) and a Mach-Zehnder interferometer (MZI); although wavelength spacing can be changed by a variable fiber delay line (VDDL), the tuning range is limited by the bandwidth of the PM-FBG. [PERSON] et al. [18] demonstrated a tunable single and dual-wavelength EDFL using an FLM with a high-birefringence photonic crystal fiber (PCF); while a tuning range of 12 nm for the single wavelength mode was obtained by adjusting two PCs, the tuning at the dual-wavelength mode was not continuous. A pair of mismatching LPGs and an FLM with polarization-maintaining fiber (PMF) along with three PCs were employed to achieve a wide tuning range of 24 nm for both single and multiwavelength modes [19]. [PERSON] et al. [20] demonstrated a tunable single and multiwavelength EDFL based on an MZI formed by a SMF-PCF-SMF structure with a tuning range of 24 nm for single and multiwavelength modes by changing the curvature radius of the PCF-based filter. On the other hand, active approaches by a temperature controller [13], an acoustic transducer [5], or a voltage controlled mechanically induced LPG [21] also enable tunable functions; however, the system is complex and costly. In this paper, we proposed and demonstrated a tunable and switchable erbium-doped fiber laser only using one PC to realize tuning and switching functions.