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• Abstract

SECTION 1

## INTRODUCTION

The growing need for very fast information dissemination around the globe has necessitated the growth and exploration of new concepts in fiber optical communications system that is generally accepted as a dominant medium for high speed data/information transmission. This is due to the awesome bandwidth offered by the optical fiber. Dense wavelength division multiplexing (DWDM) systems that are based on the ability of an optical fiber to carry many different wavelengths of light simultaneously without mutual interference evolved to utilize the massive bandwidth of the optical fiber [1]. Fiber laser have been reported to be an attractive candidate to support the DWDM systems because of its ability to generate multiple wavelengths from a coherent single wavelength light source [2]. Among the different technologies of fiber laser design, the Brillouin-erbium fiber laser (BEFL) has been receiving a lot of attention from researchers for its several advantages and potential applications that include low threshold, narrow linewidth, inherent maintained Stokes signal spacing of $\sim$10 GHz [3], a high-power single-frequency laser [4], and optical generation of microwave/millimeter-wave carriers [5].

Multiple-wavelength BEFL (MWBEFL) that covers the conventional band (C-band) and the long-wavelength band (L-band) of the optical communication windows, utilizing ring and linear cavity resonators, have been demonstrated [6], [7], [8]. In an effort to increase the capacity utilization in the DWDM communication systems, researchers considered the L-band MWBEFL as an extension of the C-band MWBEFL. One of the early works on L-band MWBEFL was reported by Harun and Ahmed [8]. In the demonstrated work, 10 channels were obtained using dual ring cavity that utilized two erbium-doped fiber (EDF) and two 980-nm pumps. In another work, Harun et al. reported an L-band MWBEFL that utilized two EDFs in another ring cavity resonator that produced 5 channels [9]. Further increase in the number of output channels produced by a linear cavity L-band MWBEFL to 24 was reported by Haddud et al. in 2005 [10]. In this paper, however, two 980-nm pump lasers were employed in pumping the EDF. In the work reported by Al-Mansoori et al. in 2007, a linear cavity L-band MWBEFL was demonstrated [11]. In this paper, the MWBEFL was pumped by a 1480-nm laser, and the resonator was formed by two fiber loop mirrors at both ends of the laser's cavity. This architecture utilizes the high power conversion efficiency of 1480 nm pump power. In this way, the laser has a reduced threshold value, and thus, 23 Brillouin Stokes lines are generated.

Another approach for enhancing channels generation of L-band MWBEFL had been demonstrated, in which 27 lasing lines were observed. The researchers employed nonlinear amplifying loop mirror in the resonator [12]. Additionally, by utilizing double-pass Brillouin pump (BP) preamplification technique within the laser cavity, 30 lasing lines were obtained by L-band MWBEFL [13]. Recently, we reported widely tunable L-band MBEFL that generates 24 channels by utilizing polarization maintaining fiber (PMF) and two polarization controllers (PCs) placed in a nonlinear amplified fiber loop mirror (AFLM) filter [14], However, the use of PCs in the nonlinear AFLM makes the proposed laser cumbersome for any practical application.

In this paper, we report an efficient L-band MWBEFL that utilizes AFLM in the fiber laser structure. The proposed laser utilizes low pumping powers to generate higher number of lasing lines. We experimentally prove that the laser structure generates 55 lasing lines at BP signal wavelength of 1601.7 nm with corresponding power of −8.5 dBm and 1480-nm laser pump power of 33 mW. This was achieved through the manipulation of the EDF pump power in the AFLM (where the transmission and reflection function of the AFLM is associate with the EDF gain factor) with the assistance of the choice of BP signal wavelength and power such that the effect of gains depletion and saturation of Erbium and Brillouin, respectively, are reduced. To the best of our knowledge, the manipulation of the laser pump powers on the gains depletion and saturation of L-band MBEFL utilizing AFLM has not been investigated. Also, to the best of our knowledge, this is the highest number of lasing lines produced by a linear cavity, L-band MWBEFL.

SECTION 2

## LASER STRUCTURE AND PRINCIPLE OPERATION

The experimental setup of a multiwavelength BEFL utilizing AFLM is shown in Fig. 1. The laser cavity comprises AFLM, single-mode fiber (SMF) and a highly reflective mirror (M) spliced to one end of the SMF and an optical circulator. The AFLM is formed by a bidirectional erbium-doped fiber amplifier (EDFA) connected between the output ports of a 50/50 (3-dB) coupler that forms a fiber loop. The EDFA consists of EDF and a wavelength selective coupler (WSC) that was used to multiplex the pump and signal lights. A 1480-nm pump laser with a maximum power of 150 mW was used for the excitation of the $\hbox{Er}^{3+}$-ions. The Brillouin gain is generated in the SMF that is placed between the highly reflective mirror and the 50/50 coupler. An external tunable laser with 1 MHz laser linewidth was used to provide the BP signal, which is injected into the laser's resonator through port 1 of the circulator as shown in Fig. 1. The output of the laser was monitored through an optical spectrum analyzer (OSA) spliced to port 3 of the circulator. The OSA has its spectral resolution set at 0.015 nm.

Fig. 1. Multiwavelength BEFL with AFLM.

In order to design an efficient MWBEFL, the balance between cavity gain and loss must be considered. In the proposed laser structure, the linear gain is provided by a fixed length of EDF. In our case, we chose 10 m long EDF with characteristics of 900-ppm erbium ion concentration, a cutoff wavelength around 1420 nm, and an absorption coefficient of 19 dB/m at 1530 nm. On the other hand, there is a tradeoff between Brillouin gain and propagation loss, and thus, the SMF length must be optimized. In our experiment, seven different spools of SMF with length of 0.1 km, 0.5 km, 1 km, 4.7 km, 6.7 km, 11.5 km, and 12.8 km were tested. We found that there is a significant effect on the characteristics of the laser contributed by the change of SMF length from 0.1 km to 12.8 km. Short length of SMF requires higher laser power for a sufficient acoustic wave to be setup in the fiber to induce scattering. On the other hand, longer SMF length increases the cavity loss. Finally, we found that 6.7 km is the optimum SMF length to achieve the desired results.

SECTION 4

## CONCLUSION

We have successfully demonstrated an efficient linear cavity L-band MWBEFL that produced 55 stable output channels at low Brillouin and laser pump powers of −8.54 dBm and 33 mW, respectively. The laser structure has the unique advantage of utilizing AFLM and permits preamplification of the BP signal in the laser cavity before being transmitted into the Brillouin gain medium, the SMF. This arrangement has the advantage of utilizing low amount of BP and 1480-nm pump powers that can sustain a large number of lasing signals. The impacts of BP power, BP wavelengths, and 1480-nm pump power on the number of output channels was thoroughly investigated. We discovered that the utilization of AFLM with the manipulation of EDF pump power, BP signal wavelength and power reduces the effect of gains depletion and saturation of Erbium and Brillouin, respectively. Thus, enhances the capacity of L-band MBEFL and leads to the generation of a high number of output channels.

## Footnotes

Corresponding author: M. H. Al-Mansoori (e-mail: mmansoori@soharuni.edu.om).

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