An aluminum coated all-fiber mirror and a Mach–Zehnder interferometer for developing a switchable and tunable multi-wavelength laser

Throughout the last decade, several different types of optical setups have been proposed to obtain multi-wavelength erbium-doped fiber lasers (MWEDFLs), since these have interesting characteristics, such as high output power, high stability and are relatively inexpensive [1]. Because of these properties, recently MWEDFLs have found applications in numerous areas, for instance, in sensing, as transmitters in communication systems with wavelength division multiplexing (WDM) and spectroscopy of methane [2], [3], [4], [5], [6]. There are two principal challenges to face for obtaining a switchable and tunable MWEDFLs in a linear cavity. The first one is to suppress the unstable mode competition induced by the homogeneous broadening characteristic of erbium-doped fiber (EDF), and the second one is to achieve a standing wave [7], [8]. Hence, the key component of the MWEDFLs linear cavity is the design of the reflective mirrors. The mirrors can be implemented by using two principal optical devices, such as commercial broadband mirrors [9] or all-fiber mirrors [10], [11], [12], [13], [14]. Between them, all-fiber mirrors are particularly attractive for MWEDFLs application since they avoid the requirement of precise alignment of the central wavelength. Moreover, inside the all-fiber mirrors, the following tunable filters can be placed: fiber Bragg gratings (FBG) [10], Chirped FBG [11], Fabry–Perot interferometer [12], Mach–Zehnder interferometer (MZI) [13] and polarization maintained FBG [14]. Recently, an MWEDFL based on a pair of FBG in a Sagnac loop and an interleaver was achieved in [10], where the authors reported a single-, dual- and triple-wavelength emission located at 1530.22, 1531.17 and 1532.66 nm which can be switched by simply adjusting the polarization controllers. Additionally, a switchable laser capable to emit three lines based on a micro cavity Fabry–Perot interferometer as a reflective mirror has been proposed [12]. A similar structure of a Fabry–Perot interferometer was combined with a fiber loop mirror and an MZI to achieve an MWEDFL in [13], where the emitted laser lines could be switched from 1524 to 1534 nm. Other example of MWEDFL can be found in [14], where authors reported a single to quadruple switchable laser wavelength emission. In that work, the all-fiber mirrors were implemented by using a double-pass MZI and a polarization maintained FBG as wavelength selector.

In this paper, a switchable and tunable MWEDFL containing an MZI and an aluminum coated fiber cleaved end mirror in a linear configuration is presented. A fiber loop mirror included an MZI acting as a wavelength selective filter, and an aluminum coated cleaved fiber end acting as a full reflective mirror (FRM). While diverse fiber coating substances have been explored for multiple purposes, especially for saturable absorbers on pulsed fiber lasers, only a few works have implemented a metal coated fiber such as gold [15] and aluminum as a FRM for a compact laser setup. Thus, the proposed laser configuration can emit up to four lines simultaneously by changing the polarization state of the cavity modes. In addition, the laser lines can be tuned and switched within the complete C-band of erbium gain spectrum. Finally, high stability at room temperature and single-wavelength tuning was achieved.

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