Cascaded Random Fiber Laser Based on Hybrid ... - IEEE Xplore

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 13, JULY 1, 2014

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Cascaded Random Fiber Laser Based on Hybrid Brillouin-Erbium Fiber Gains Changqing Huang, Xinyong Dong, Member, IEEE, Shuyuan Zhang, Nan Zhang, Member, IEEE, and Perry Ping Shum, Member, IEEE Abstract— We demonstrate a stable, cascaded random fiber laser with a half-open cavity, which is based on hybrid Brillouinerbium fiber gains and uses Rayleigh scattering as random distributed feedback. Up to four cascaded stimulated Brillouin stokes lines with frequency spacing of about 11 GHz have been generated. The peak power difference between the first and fourth stokes lines is 2.808 dB. The number of output stokes lines can be controlled by the 980 nm pump power. Peak power discrepancy between the odd and even stokes lines is avoided using the scheme of half-open cavity. The optimized Brillouin pump power is reduced to 1–2 mW and the multiwavelength output is flattened due to the employment of erbium-doped fiber. Index Terms— Fiber lasers, stimulated Brillouin scattering, Rayleigh scattering, random distributed feedback.

I. I NTRODUCTION

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N RECENT decades, random lasers (RLs) have attracted a lot of research interest for their potential applications in optical sensing, laser imaging, spectroscopy and medical sciences [1]–[4]. RLs employ multiple scattering in gain media, rather than regular resonator cavity, to obtain lasing emission, that may result in angular dependence of emission spectra and high threshold power [5]. Optical fibers, as a kind of well-known waveguides with two dimensional confinements, were chosen to improve random lasing performances [6]–[8]. For instance, Turitsyn reported a random fiber laser (RFL) based on the random distributed feedback (RDFB) from Rayleigh scattering (RS), which was Manuscript received December 29, 2013; revised April 4, 2014; accepted April 23, 2014. Date of publication May 1, 2014; date of current version June 10, 2014. This work was supported in part by the National Basic Research Program of China (973 Program) under Grant 2010CB327804, in part by the Zhejiang Provincial Natural Science Foundation of China under Grant LY13F050004, and in part by the Zhejiang Province Public Technology Applied Research Project under Grant 2013C31061. C. Huang is with the Institute of Optoelectronic Technology, College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China, and also with the School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 (e-mail: [email protected]). X. Dong is with the Institute of Optoelectronic Technology, College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, China, the School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, and also with CINTRA, Nanyang Technological University, Singapore 639798 (e-mail: [email protected]). S. Zhang, N. Zhang, and P. P. Shum are with the School of Electrical and Electronics Engineering, Nanyang Technological University, Singapore 639798, and also with CINTRA, Nanyang Technological University, Singapore 639798 (e-mail: [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2014.2321386

amplified by distributed Raman gain in a conventional optical fiber of total length of 83 km [9]. Pang reported a coherent Brillouin random fiber laser by using RS as RDFB and stimulated Brillouin scattering (SBS) as gain [10]. In 2011, Vatnik firstly reported the cascaded generation of second Stokes wave in RFL operating at 1.2 μm, based on Raman gain and RS-RDFB [11]. The threshold powers of 1st Stokes and 2nd Stokes lines are about 1.6 W and 6.5 W, respectively. Based on similar principle, Zhang proposed RFLs of 1st order and 2nd order Raman-based random lasing with lower threshold powers by using a half-open fiber cavity or mixing dispersion compensated fiber (DCF) and single mode fiber (SMF) [12], [13]. Third order random lasing using Raman gain has been reported by Wang [14]. However, cascaded generation based on Raman gain have obvious disadvantages such as high threshold power (more than 1 W for 2nd Stokes line), limited Stokes lines (no more than 3 Stokes lines) and large wavelength spacing (about 100 nm). On the other hand, SBS was widely used to achieve cascaded multiwavelength fiber lasers with precise and stable wavelength spacing and plenty of Stokes lines [15], [16]. By combining RS-RDFB, Raman gain and SBS, multiwavelength Brillouin-Raman random fiber lasers have been reported [17]–[19]. For multiwavelength fiber lasers, output power flatness is necessary for the dense wavelength division multiplexing (DWDM) applications. Some efforts have been done to decrease the peak power discrepancies and improve power flatness [19]. The half-open cavity design in Ref. [19] was found to have nearly no peak power discrepancy between the odd and the even Stokes lines, while that happened in the open cavity designs [17], [18]. Cholan proposed a ring cavity at one end of DCF to form a half-open cavity and employed four wave mixing (FWM) in DCF to lower the peak power difference between the first and the fifth output channel to 4.59 dB [20], in which, the output power contains the transmitted BP power. In this Letter, we report a cascaded random fiber laser (CRFL) with a half-open cavity, which is based on the RS-RDFB interacting with both the nonlinear gain from SBS and the linear gain from erbium-doped fiber (EDF). Evolvement of the cascaded lasing output with both powers of Brillouin pump (BP) and 980 nm EDF pump is investigated. Up to 4 laser lines with frequency spacing of ∼11 GHz have been achieved. II. E XPERIMENTAL S ETUP The experimental setup for the proposed CRFL is schematically shown in Fig. 1. A 1550 nm distributed feedback

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Fig. 1. Schematic setup of the proposed CRFL. BP: Brillouin pump; CIR: circulator; WDM: wavelength division multiplexer; EDF: erbium-doped fiber; SMF: single mode fiber; RFL: random fiber laser.

laser diode (DFB-LD) with approximately 5 MHz linewidth provides BP through circulator 1. A 1 m long EDF with peak core absorption of 80 dB/m at 1530 nm acts as linear gain medium, which is pumped by 980 nm laser diode through a 1550 nm/ 980 nm wavelength division multiplexer (WDM). A 10 km SMF (G. 652, labeled as SMF 1) acts as nonlinear gain medium to initiate SBS. After being amplified by EDF, BP light can initiate the back-propagating 1st Brillouin Stokes wave in the SMF 1. The counterclock-wise Stokes wave, together with Rayleigh scattering of BP, is amplified by EDF and then enters into another 20 km SMF (G.652, labeled as SMF 2) through circulator 2, which works as the seed of the following generation of new Brillouin Stokes lines in SMF 2. RS of the 1st Stokes wave and newly generated high order Stokes wave are partly sent back to SMF 1 via circulator 2. The remaining forward light is detected by the optical spectrum analyzer (OSA) with a spectral resolution of 0.01 nm and power meter. The RFL output is performed with angle polished connector to avoid Fresnel reflection. Compared with the full-open cavity with RDFB on both sides, the half-open cavity employs RDFB only on one side. In the proposed setup, the components of CIR1, WDM, EDF, SMF 1 and CIR 2 form a ring structure. The half-open cavity consists of RDFB of SMF 2 and the feedback of the ring structure. III. R ESULTS AND D ISCUSSIONS The output spectra of the proposed CRFL for different 980 nm pump power and fixed BP of 2 mW were shown in Fig. 2(a). When the 980 nm pump power is 36 mW, the output spectrum consists of only one BP-induced Rayleigh backscattering. When the 980 nm pump power increases to 100 mW, there are three peaks being observed, refering to BP-induced Rayleigh backscattering light, amplified spontaneous 1st Brillouin Stokes line and amplified 1st Brillouin anti-Stokes line. The background noise is due to the amplified spontaneous emission (ASE) of EDF. One can see from Fig. 2(a) that random 1st Stokes lasing based on SBS is initiated with the 980 nm pump power of 128 mW. The spectrum near the 1st Stokes line contains several peaks as a result of the competitive interaction between different modes [21]. Note that there are no any random lasing being detected by the OSA when the pump power is lower than 121 mW. The pump power of 128 mW is slightly higher than the threshold power measured by OSA. Fig. 2(b) shows the output spectra of the RFL over time for 980 nm pump power of 128 mW and BP of 2 mW. The curves were measured without

Fig. 2. (a) Output spectra for different 980 nm pump power and (b) output spectra over time for 980 nm pump power of 128 mW with fixed BP of 2 mW. The curves in Fig. 2(b) were measured without intervals and shifted 15 dB per division vertically.

intervals. For clarity, the plots in Fig. 2(b) were shifted 15 dB per division vertically. The results show that output wavelength and relative intensities of the peaks vary over time, which indicates that the generated RFL is unstable when the 980 nm pump power is slightly above the threshold power. With further increase of the 980 nm pump power to 150 mW, well above the threshold power, output spectrum of 1st Stokes line becomes smooth and only one peak with a linewidth of 0.024 nm and Brillouin Stokes frequency downshift of about 11 GHz can be seen. The linewidth measurement is really limited by the 0.01 nm resolution of the used OSA, and the real linewidth should be narrower. Stability measurement of the 1st Stokes output spectra being taken over more than 20 minutes reveals that there are almost no observable fluctuations in both the peak power and central wavelength over the measured period, which indicates that the 1st Stokes lasing is stable. When the 980 nm pump power is equal to 178 mW, the 2nd stimulated Brillouin Stokes lasing is newly generated, which has several peaks with output wavelength and relative intensities changing stochastically, whereas the 1st Stokes output spectra keep stable, with a linewidth of 0.024 nm. The 2nd Stokes lasing with frequency downshifted 11 GHz relative to the 1st Stokes line becomes stable with the 980 nm pump power of 227 mW. When the 980 nm pump power is 250 mW, in addition to stable 1st and 2nd Stokes lasing output, the 3rd stimulated Brillouin Stokes line is newly initiated and unstable, which becomes stable when the 980 nm pump power reaches 285 mW. The 3rd Stokes line has a linewidth of 0.022 nm and frequency downshifted 11 GHz relative to the 2nd Stokes line. It is noted that there is only 1st order stimulated Brillouin Stokes being detected if SMF 2 is taken away from the schematic setup in Fig. 1. Higher order Stokes lines are initiated through SMF 2. The proposed CRFL is based on RDFB via Rayleigh scattering and such laser is considered to be random fiber lasers [9], [10].

HUANG et al.: CASCADED RANDOM FIBER LASER BASED ON HYBRID BRILLOUIN-ERBIUM FIBER GAINS

Fig. 3. (a) Output spectra of the CRFL for high 980 nm pump power and (b) output power of the CRFL versus 980 nm pump power with fixed BP of 2 mW.

However, evaluation of the randomness of random fiber lasers needs further research. One can see from Fig. 2(a) that intensity of 1st Stokes line doesn’t change obviously after 2nd Stokes line is initiated. Similarly, intensity of 2nd Stokes line doesn’t vary obviously after 3rd Stokes line occurs. The results indicate the power saturation of the lower order Stokes line when the higher order Stokes lines are initiated. Fig. 3(a) gives output spectra of the proposed CRFL for high 980 nm pump power. There are 4 stimulated Brillouin Stokes lines being obtained when the 980 nm pump power is higher than 385 mW. By using the SMF 1, we can effectively eliminate the transmitted BP power from the output power, while that is failed in Ref. [20]. Intensity of higher order Stokes waves and anti-Stokes waves increases with the 980 nm pump power, which is due to the process of four-wave mixing (FWM) [22] and thus prevents the presence of higher stimulated Brillouin Stokes lines. More results show that every newly initiated SBS line is unstable and their wavelength and intensity vary obviously over time, till the higher-order Stokes line is initiated. By using the half-open cavity, no obvious peak power discrepancy between the odd and even Stokes lines can be detected, which is different from that employing full-open cavity [17], [18]. The 1st Stokes line and the 4th Stokes line record peak power of −4.04 and −6.848 dBm, respectively, when the 980 nm pump power is 425 mW. The peak power difference between the 1st and 4th Stokes lines is only 2.808 dB, which is smaller than that of 3.15 dB between the 1st and 4th Stokes lines reported by Cholan [20]. The proposed CRFL with stable and flattened multiwavelength output may find potential applications in the DWDM system. For the purpose of comparison, we have built up a Brillouin-erbium fiber laser with a full open linear cavity but obtained only two stimulated Brillouin Stokes lines even if the 980 nm pump power is higher

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than 400 mW. That may relate to the weak feedback for the full-open cavity random fiber laser design. Fig. 3(b) shows the CRFL output power versus the 980 nm pump power. The 980 nm pump powers are specific to this particular erbium fiber and are not general to other similar laser configurations. The threshold pump power is 110 mW, which is a little different from the power value when the unstable 1st Stokes line is firstly detected by the OSA. The maximal output power of the proposed CRFL is about 6 mW. There exists an unconspicuous turning point of output power for the 980 nm pump power of 180 mW, due to the initiation of the 2nd SBS line. In order to better investigate the output properties of the CRFL, the dependence of the 1st, 2nd, 3rd and 4th Stokes lines on the 980 nm pump power was measured by the OSA and also displayed in Fig. 3(b). The results show different order unstable Stokes lines can exist within the 980 nm pump power of about 14 mW, while such Stokes lines can become stable output when the 980 nm pump power increases. The low order Stokes line keeps stable if only the new higher Stokes line is initiated. Therefore, one can control output wavelength number and stability of the proposed CRFL by controlling the 980 pump power. The low output power of about 6 mW can be explained by (1) conversion efficiency from the 980 nm pump power to Er3+ excited state; (2) conversion efficiency from Er3+ excited state to amplified BP; (3) conversion efficiency from amplified BP to the 1st Stokes line in the SMF 1; (4) weak RDFB via Rayleigh scattering in SMF 2; (5) insertion loss of optical components. To identify the role of EDF in the proposed setup, we removed the EDF, WDM and 980 nm pump and then inserted an erbium-doped fiber amplifier (EDFA) and an isolator between BP and circulator 1 to overcome low maximal output power of 20 mW for DFB-LD. The results show the threshold powers for the 1st, 2nd, 3rd and 4th stimulated Brillouin Stokes lines are 12 mW, 31.6 mW, 73 mW and 610 mW, respectively. Despite of low threshold power for the first 3 Stokes lines, the 4th Stokes line of 610 mW is very high due to power saturation of SBS. The 1st Stokes line and the 4th Stokes line record peak power of −5.444 and −9.704 dBm, respectively. The peak power difference between the 1st and 4th Stokes lines is 4.26 dB. Compared with the Brillouin random fiber laser with only one output wavelength reported in Ref. [10], the scheme of hybrid Brillouin-erbium fiber gains decreases the BP power of Stokes lines and achieved four output wavelength. What’s more, the peak power difference between the 1st and 4th Stokes lines decreases from 4.26 mW to 2.808 mW due to the employment of erbium-doped fiber. Fig. 4 gives output spectra of the CRFL for different BP power with fixed 980 nm pump power. For the 980 nm pump power of 150 mW, the 1st spontaneous Brillouin Stokes line is obtained when BP is 0.2 mW, whereas the 1st SBS line is initiated when BP is 1 mW. When BP is 20 mW, the 2nd stimulated Brillouin scattering is excited. For the 980 nm pump power of 400 mW, there are 4 stimulated Stokes lines when BP is 0.2 mW. Higher order Stokes lines and anti-Stokes lines have optical signal-to-noise ratio (OSNR) of less than 5 dB. When BP increases to 1 mW and 2 mW, there are up to 5 and 4 main stimulated Stokes lines, respectively. More than

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Fig. 4. Output spectra of the proposed CRFL for different BP power with 980 nm pump power of 150 mW (a) and 400 mW (b). ASE: amplified spontaneous emission.

10 higher order Stokes lines or anti-Stokes lines have ONSR of more than 10 dB. However, number and OSNR of higher order Stokes lines and anti-Stokes lines decrease with BP power when BP power is higher than 3 mW. Only 3 stimulated Stokes lines can be generated for BP of 20 mW. More results indicate that when BP power is between 0.2 to 20 mW, the increase of BP power is helpful for the formation of 1st, 2nd and 3rd Stokes lines, whereas is disadvantageous for the generation of higher order Stokes and anti-Stokes lines. The effect of BP power on the output spectra can be explained as follows: it is easier for high BP power to reach the threshold power of the1st Stokes line in SMF 1 after being amplified by EDF. The newly generated 1st Stokes line has higher power and thus can initiate 2nd and 3rd Stokes lines. However, high BP power and high 1st Stokes power can lead to serious depletion of the excited Er3+ ions in EDF, which is disadvantageous to initiate higher order Stokes lines and generate FWM process in SMF 2. For BP power of 0.2 mW, low OSNR of output spectra for the 980 nm pump power of 400 mW is due to high background noise based on ASE of EDF, which can be justified in Fig. 4. The optimized BP power in the CRFL is between 1 mW and 2 mW. IV. C ONCLUSIONS In conclusion, we demonstrate a simple and cascaded random fiber laser based on hybrid Brillouin-erbium fiber gains with a half-open cavity. The CRFL has up to 4 output wavelengths and maximal output power of 6 mW under 2 mW BP pump power. The peak power difference between the 1st and 4th Stokes lines is only 2.808 dB. There is no peak power discrepancy between the odd and the even Stokes lines due to the half-open cavity. The BP power is optimized between 1 mW and 2 mW to achieve more Stokes lines and higher OSNR. The presence of EDF decreases the BP power and flattens the multiwavelength output. R EFERENCES [1] B. Redding, M. A. Choma, and H. Cao, “Speckle-free laser imaging using random laser illumination,” Nature Photon., vol. 6, no. 6, pp. 355–359, Apr. 2012. [2] Z. N. Wang et al., “Long-distance fiber-optic point-sensing systems based on random fiber lasers,” Opt. Exp., vol. 20, no. 16, pp. 17695–17700, Jul. 2012.

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