Tunable full waveband- and adjustable spacing multi-wavelength erbium-doped fiber laser based on controlling cavity losses through bending sensitive interferometric filter

Highlights

• A balloon-like-Mach-Zehnder interferometric comb filter is proposed.

• A tunable full waveband erbium-doped fiber laser is presented.

• Performance of adjustable spacing multi-wavelength fiber laser is presented.

• The effect of enhancing the pump power on wavelength tunability is investigated.

Abstract

In this work, tunable full waveband and adjustable spacing multi-wavelength erbium-doped fiber laser (EDFL) based on the interferometric filtering performance of a bent single-mode fiber (SMF) is experimentally demonstrated. The tunable full waveband EDFL operation with a wide wavelength-tunable range of 11.9 nm was realized by gradually varying the bent diameter of the interferometric filter structure, formed by bending a standard SMF into a balloon shape. Also, when the launched diode pump power was enhanced from 110 to 195 mW, the central wavelength of the EDFL was shifted from 1562.9 nm to 1559.5 nm. Moreover, by adjusting the polarization state inside the cavity, the wavelength operation was switched from a single to multi-wavelength operation. Dual-, triple-, and quadruple- wavelength emissions were achieved. To the best of our knowledge, this is the first time that a bending SMF into a balloon-like MZI has been employed as an all-fiber filter to manipulate the spectral output characteristic of EDFL. The proposed EDFL output characteristics of tunability and switchability can provide great potential in diverse photonics applications.

Introduction

In recent years, spectral manipulation of erbium-doped fiber lasers (EDFL) is an important issue, and efforts have been devoted on achieving a versatile performance such as tunability, switching, or multi-wavelength emission due to their wide broad range of applications such as in fiber communication systems, fiber optic sensors, etc. [1]. Tunable wavelength fiber lasers have been under intensive study in recent years due to their growing optical spectroscopy applications, optical sensors, and wavelength division multiplexing (WDM) [2], [3]. In certain applications, such as fabrication of light source for the wavelength routers of WDM network and photonics systems' characterization, switching the fiber laser's operation from one single lasing line to another is preferable [4]. On the other hand, in some applications, such as dense wavelength-division-multiplexing (DWDM) optical networks, signal processing, generation of soliton pulses, and multi-parameter measurement, a multi-wavelength operation is ideal [5], [6]. Moreover, in a practical view, a multi-wavelength fiber laser has more potential than a fixed wavelength fiber laser since the variable wavelength output with adjustable spacing may satisfy various applications such as in high-resolution spectroscopy, optical frequency metrology, microwave/Terahertz generation, optical sensing, and in fiber communications where the current WDM systems require various channels spacing depend on their specific applications [7], [8]. In addition, their advantages, like low operating costs, compactness, long operational lifetimes, and a high signal-to-noise ratio, are all benefits from these multi-wavelength fiber lasers [6], [9], [10].

The basic principle behind a tunable and multi-line output characteristic of EDFL at room temperature requires an optical filter to reduce the gain medium's mode competition [11], [12], [13]. However, two main concerns need to be carefully handled to achieve a stable operation of tunable and multi-wavelength EDFL operation: The strong homogeneous line broadening and the cross-gain saturation induced in the erbium-doped fiber (EDF) gain medium [14]. These limitations are still challenging to obtain stably tunable-, and multi-wavelength fiber lasers outputs [15], [16]. Therefore, high-performance optical filters with low cost, ease of handling, and fabrication to implement these types of EDFL output characteristics are of great interest [14]. Optical fiber comb filters are one form of these optical filters [17]. Various techniques to realize all-fiber comb filters have been reported, including the usage of tapered fibers [18], [19], fiber Braggs (FBGs) in combination with tapers [20], [21], [22], Sagnac interferometers (SI) [[23], [24]], and Fabry–Perot interferometers (FPI) [25], [26] have been proposed to towards tunable and multi-wavelength fiber EDFLs generation. Mach–Zehnder interferometers (MZIs) have many advantages over other types of interferometers, such as high reliability and environmental change insensitivity [27]. In this regard, many structures have been investigated in the literature for the construction of all-fiber MZI-based filters, such as single-mode multimode single-mode (SMS) fiber structure [28], tapered fibers [29], microfibers [30], and photonic crystal fiber [31]. Temperature, electrical, and mechanical approaches are used to manipulate and tune the spectral response of these MZI comb filters [32]. However, the manufacturing of such MZI structures can be complicated and expensive [[24], [33], [34]]. Recently, MZI featuring ease of fabrication, simplicity, and cost-effectiveness based on bending a single-mode fiber (SMF) into a balloon-like structure has gained a lot of interest in sensing applications [34], [35], [36], [37]. On the other hand, very recently, He et al. reported the possibility to generate individually tunable adjacent single wavelength, dual-wavelength, triple-wavelength, quadruple-wavelength, and quintuple-wavelength operation by appropriate adjustment of the polarization controllers (PCs) in the laser cavity [38]. Some researchers have proposed and demonstrated optically tunable comb filters whose characteristics can be adjusted by pump light strength [39], [40].

In this paper, a tunable full waveband and spacing adjustable multi-wavelength EDFL based on balloon-like bent SMF structure act as MZI based filter is experimentally demonstrated. First, the proposed filter's attenuation bands are reshaped by changing a traditional SMF's bending diameter from 40 mm to 1.5 mm, achieving a tunable full waveband lasing operation in the range of 1571.9–1560 nm. Second, the effect of enhancing the pump power on the wavelength tunability was investigated. The laser emission was tuned from 1562.9 nm to 1559.5 nm as the pump power was increased from 110 to 195 mW. Finally, the multi-wavelength generation with spacing tunability was achieved by adjusting the state of polarization inside the ring cavity. To the best of our knowledge, this is the first time that a bending SMF into a balloon-like MZI has been employed as an all-fiber filter to manipulate the spectral output characteristic of EDFL.