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Tunable Erbium-Doped Fiber Laser Based on Random Distributed Feedback Volume 6, Number 5, October 2014 Lulu Wang, Member, IEEE Xinyong Dong, Member, IEEE Perry Ping Shum, Member, IEEE Haibin Su

DOI: 10.1109/JPHOT.2014.2352623 1943-0655 Ó 2014 IEEE

IEEE Photonics Journal

Tunable EDFL Based on RDFB

Tunable Erbium-Doped Fiber Laser Based on Random Distributed Feedback Lulu Wang,1,2,3 Member, IEEE, Xinyong Dong,1,2,4 Member, IEEE, Perry Ping Shum,2,3 Member, IEEE, and Haibin Su2,4 1

Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China CNRS International NTU THALES Research Alliance (CINTRA), Nanyang Technological University, Singapore 637553 3 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 4 School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 2

DOI: 10.1109/JPHOT.2014.2352623 1943-0655 Ó 2014 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received July 2, 2014; revised August 19, 2014; accepted August 20, 2014. Date of publication August 27, 2014; date of current version September 9, 2014. This work was supported by the National Basic Research Program of China (973 Program) under Grant 2010CB327804; by the National Natural Science Foundation of Zhejiang Province, China, under Grant Z13F050003; and by the Singapore A STAR “Advanced Optics in Engineering” Program under Grant 1223600006. Corresponding author: X. Dong (e-mail: [email protected]).

Abstract: A tunable erbium-doped fiber (EDF) laser based on random distributed feedback through backward Rayleigh scattering in a 20-km-long single-mode fiber and a tunable fiber Fabry–Perot interferometer filter is demonstrated. It has a broad wavelength tuning range up to 40 nm (from 1525 to 1565 nm) with a narrow linewidth of 0.04 nm. Due to the half-opened laser cavity design and the efficient gain from the pumped EDF, the measured threshold power is only 13 mW, and the pump efficiency is up to 14%. Index Terms: Erbium-doped fiber, random fiber laser, Rayleigh scattering, tunable laser.

1. Introduction Differing from normal fiber lasers, random fiber lasers based on random distributed feedback can operate without a precise cavity, leading to a unique advantage of simple technology and low production cost [1]–[3]. The laser output is of modeless characteristics that can suppress multi-modes interactions [4]–[7] and lead to low spatial and/or temporal coherence. Low coherent laser sources may find applications in some special occasions such as parallel imaging, in which highly coherent illuminated light is not preferred because they may introduce cross-talk and speckles [8], [9]. So far, most of the reported random fiber lasers are based on backward Rayleigh scattering (RS) in a long-distance optical fiber amplified through stimulated Raman scattering (SRS) effect [10]–[14]. As the Raman gain can provide a relatively broad and flat profile, tunable random fiber lasers based on SRS effect were also reported by using different tunable filters such as optical fiber Fabry–Perot (FFP) filters, optical fiber gratings and long-period grating-based Mach–Zehnder interferometer [15]–[17]. However, the gain efficiency of SRS in an optical fiber is relatively low and the backward Rayleigh scattering due to inhomogeneities within the glass structure of the fiber is extremely weak, making the laser operations difficult (with typical pump threshold of 1.5 W [15], [16]). On the other hand, erbium-doped fiber (EDF)based tunable lasers have attracted great attention in wavelength-division multiplexed (WDM)

Vol. 6, No. 5, October 2014

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Fig. 1. The proposed tunable EDFL. WDM, wavelength-division multiplexer; EDF, erbium-doped fiber; SMF, single-mode fiber; OC, optical coupler; FFP, fiber Fabry-Perot interferometer filter; and RDFB, random distributed feedback.

systems, instrument testing, optical sensors, and spectroscopy [18]–[21] due to their many advantages including low threshold, high optical signal-to-noise ratio (OSNR), narrow bandwidth, and wide wavelength tuning range. Low pump threshold random fiber lasers by using EDF as gain medium, instead of Raman gain, to collaborate with unique fiber gratings [22], [23] or Brillouin/Rayleigh scattering effects [24]–[27] have also been demonstrated recently. In this paper, we propose a widely tunable random fiber laser by using EDF as the gain media (instead of SRS amplification), tunable FFP filter and random distributed feedback via backward RS in a 20-km-long single-mode-fiber (SMF). Thanks to the half-opened cavity design and high efficiency gain of the EDF, the pump threshold power is only 13 mW, which is about two orders lower in magnitude than that of the previously reported tunable random fiber lasers [15]–[17]. The generated laser has a wide tuning range up to 40 nm (1525–1565 nm) with a narrow linewidth of about 0.04 nm.

2. Laser Design The experimental setup for the proposed tunable erbium-doped fiber laser (EDFL) based on random distributed feedback is shown in Fig. 1. We used an optical circulator-based loop mirror to form a half-opened laser cavity, which can reduce the pump threshold power significantly [13], [14]. In the loop mirror, a wavelength tunable FFP filter driven by a voltage controller and a 90/10 optical coupler (OC) with output coupling ratio of 10% were included. The FFP filter has free spectrum range of 78 nm and 3-dB bandwidth of 0.17 nm. A 2-m-long EDF with mode field diameter of 6 m and high peak absorption coefficient of 11 dB/m at 1480 nm was used to provide gain. The EDF was pumped by a 1480 nm laser with maximum power of 400 mW through a 1480/1550 nm WDM. A 20-km-long SMF (type G. 652) was connected to the EDF to provide random distributed feedback via backward Rayleigh scattering. An angled-polished connector was used in the other end of the SMF to eliminate unwanted Fresnel reflection. Laser output was measured from the both output ports by using an optical spectrum analyzer (OSA) with resolution of 0.02 nm and an optical power meter.

3. Experimental Results and Discussion When the EDF was pumped, population inversion of erbium ions generated amplified spontaneous emission (ASE), which underwent random distributed feedback through backward Rayleigh scattering in the long SMF and wavelength-selectively reflected by the optical circulator-based loop mirror. With increasing power of the pump laser, resonance happened when gain from the EDF overcame total loss in the laser cavity. By changing driving voltage applied to the FFP filter, we can tune the lasing wavelength within a wide range. Fig. 2 shows the laser output spectra measured when the output wavelength is tuned every 10 nm at a fixed pump power of 400 mW. The tuning range is up to 40 nm (1525–1565 nm), in accordance with gain profile of the EDF, and the maximum power fluctuation is only 1.5 dB. As total insertion losses of the FFP filter, OC and circulator are high, 7 dB, and only 10%


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Fig. 2. Laser spectra at different wavelengths measured from output ports (a) 1 and (b) 2 when pump power is 400 mW.

Fig. 3. Output spectra measured under different pump powers at 1550 nm from output ports (a) 1 and (b) 2.

intra-cavity laser power is coupled out from output port 1, the laser intensity from output port 1 is about 20 dB lower than that from output port 2. However, laser output from port 1 has OSNR up to 57 dB, which is 5 dB higher than that from output port 2 due to the high isolation of the FFP filter to the ASE noise spectrum of the EDF. And linewidth of the laser from the two output ports are much different, 0.04 nm and 0.23 nm for output ports 1 and 2, respectively. The obvious difference should be related to the filtering effect of the FFP filter, as well as the nonlinear spectral broadening effect on the laser from output port 2 when it propagates though the 20-km-long SMF. We fixed the laser wavelength at 1550 nm and measured the laser spectra from the two output ports at different pump powers ranging from 10 to 400 mW. The results, as shown in Fig. 3, indicate that laser oscillation starts when the pump power reaches 13 mW and there exists a main side-peak in the longer wavelength side corresponding to the Brillouin scattering effect (the wavelength separation is 0.088 nm) in most of the cases. The side-peak is 5.5 dB lower than the main laser peak in Fig. 3(a) when the pump power is 400 mW, while it merges with the main peak in Fig. 3(b) when the pump power reaches 200 mW due to the nonlinear spectral broadening effect in the long SMF. By using a FFP filter with bandwidth (currently 0.17 nm) close to or narrower than Brillouin frequency shift, the side-peak can be suppressed significantly. Fig. 4 shows the measured laser output powers from the two output ports against pump power at the wavelengths of 1525 and 1550 nm. The threshold pump power for the laser at 1525 nm is 25 mW, relatively higher than that at 1550 nm due to the relatively poorer gain efficiency at this wavelength. The output powers increase nearly linearly with the pump powers. The maximum output powers from the output port 1/2 at 1550 nm and 1525 nm are 0.63/53 mW and 0.32/20.6 mW, respectively. So the pump efficiency at 1550 nm and 1525 nm are 13.9% and 5.6%, respectively, which are comparable with that of the normal (not random) fiber lasers.


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Fig. 4. Output power against pump power at fixed wavelengths of 1550 and 1525 nm measured from output ports (a) 1 and (b) 2.

And the threshold pump power is about two orders lower in magnitude than that of the previously reported tunable random fiber lasers amplified by distributed Raman effect [15]–[17].

4. Conclusion A low-threshold, high-efficiency, narrow-linewidth and wide-wavelength tunable erbium-doped fiber laser based on random distributed feedback through backward Rayleigh scattering in a 20-km-long SMF and a FFP filter has been demonstrated. Due to the half-opened cavity design of the random laser and the high-efficiency gain from the pumped EDF, the threshold power is only 13 mW, which is about two orders lower in magnitude than that of the previously reported tunable random fiber lasers. The pump efficiency at 1550 nm is 13.9% and the wavelength tuning range is up to 40 nm (from 1525 nm to 1565 nm).

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