Random-lasing-based distributed fiber-optic ... - OSA Publishing

Random-lasing-based distributed fiber-optic amplification Xin-Hong Jia,1,2 Yun-Jiang Rao,1,* Fei Peng,1 Zi-Nan Wang,1 Wei-Li Zhang,1 Hui-Juan Wu,1 and Yun Jiang1 1

Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, Sichuan 611731, China 2 School of Physics & Electronic Engineering, Sichuan Normal University, Chengdu, Sichuan 610068, China * [email protected]

Abstract: The gain and noise characteristics of distributed Raman amplification (DRA) based on random fiber laser (RFL) (including forward and backward random laser pumping) have been experimentally investigated through comparison with conventional bi-directional 1st-order and 2nd-order pumping. The results show that, the forward random laser pumping exhibits larger averaged gain and gain fluctuation while the backward random laser pumping has lower averaged gain and nonlinear impairment under the same signal input power and on-off gain. The effective noise figure (ENF) of the forward random laser pumping is lower than that of the bi-directional 1st-order pumping by ~2.3dB, and lower than that of bi-directional 2nd-order pumping by ~1.3dB at transparency transmission, respectively. The results also show that the spectra and power of RFL are uniquely insensitive to environmental temperature variation, unlike all the other lasers. Therefore, random-lasing-based distributed fiberoptic amplification could offer low-noise and stable DRA for long-distance transmission. ©2013 Optical Society of America OCIS codes: (060.4370) Nonlinear optics, fibers; (190.5650) Raman effect; (060.3510) Lasers, fiber.

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Received 12 Dec 2012; revised 31 Jan 2013; accepted 18 Feb 2013; published 8 Mar 2013

11 March 2013 / Vol. 21, No. 5 / OPTICS EXPRESS 6572

Rev. A 84(2), 021805 (2011). 14. A. E. El-Taher, P. Harper, S. A. Babin, D. V. Churkin, E. V. Podivilov, J. D. Ania-Castanon, and S. K. Turitsyn, “Effect of Rayleigh-scattering distributed feedback on multiwavelength Raman fiber laser generation,” Opt. Lett. 36(2), 130–132 (2011). 15. Z. N. Wang, Y. J. Rao, H. Wu, P. Y. Li, Y. Jiang, X. H. Jia, and W. L. Zhang, “Long-distance fiber-optic pointsensing systems based on random fiber lasers,” Opt. Express 20(16), 17695–17700 (2012). 16. W. L. Zhang, Y. J. Rao, J. M. Zhu, Z. X. Yang, Z. N. Wang, and X. H. Jia, “Low threshold 2nd-order random lasing of a fiber laser with a half-opened cavity,” Opt. Express 20(13), 14400–14405 (2012). 17. Y. J. Rao, W. L. Zhang, J. M. Zhu, Z. X. Yang, Z. N. Wang, and X. H. Jia, “Hybrid lasing in an ultra-long ring fiber laser,” Opt. Express 20(20), 22563–22568 (2012). 18. X. H. Jia, Y. J. Rao, Z. N. Wang, W. L. Zhang, Y. Jiang, J. M. Zhu, and Z. X. Yang, “Towards fully distributed amplification and high-performance long-range distributed sensing based on random fiber laser,” OFS 2012, Proc. SPIE 8421, 842127, 842127-4 (2012). 19. J. Nuño, M. Alcon-Camas, and J. D. Ania-Castañón, “RIN transfer in random distributed feedback fiber lasers,” Opt. Express 20(24), 27376–27381 (2012).

1. Introduction The Erbium-doped fiber amplifier (EDFA) and distributed Raman amplification (DRA) are two major signal amplification technologies in modern optical fiber communication and sensing systems [1–3]. For EDFA and DRA, one of the basic challenges for further performance improvement arises from the trade-off between the amplified spontaneous (ASE) noise accumulation and nonlinear impairment. It is well known that conventional EDFA and DRA indicates pronounced gain fluctuation along the transmission fiber, giving rise to the larger nonlinear impairment for the regime with higher gain, and the deteriorated optical signal-to-noise ratio (OSNR) for the region with lower gain. It has been rigorously proved that, in case of transparency propagation, the best balance is attained as the spatial distribution of the gain keeps constant [4]. For this purpose, higher-order DRA using multiple pumps has been proposed [5], and 2nd-order amplification scheme based on an ultra-long cavity laser has been proposed and demonstrated [6–8]. An experimental demonstration of 8 × 40Gb/s unrepeated transmission over 320km has been reported very recently [9]. The random fiber laser (RFL) [10–19], designated as a milestone in laser physics and nonlinear optics, has been suggested for use in optical fiber communication and sensing firstly by S. K. Turitsyn et al. [10]. The real application of the RFL to distributed fiber-optic sensing has been demonstrated recently by the authors [18]. The relative intensity noise (RIN) transfer of the RFL was also analyzed very recently [19]. In this paper, the gain and noise characteristics of fully DRA based on the RFL have been experimentally investigated and compared with conventional bi-directional Raman amplification, for the first time. The temperature response of the RFL was also measured to confirm its stability for use in longdistance fiber-optic transmission. 2. Experimental setup The arrangement of the experimental system is shown in Fig. 1(a). Four amplification schemes were studied over 93km standard single mode fiber (SMF). The 1550nm signal from the distributed feedback (DFB) laser was injected into the left side of the SMF via a wavelength-division-multiplexer (WDM). For conventional bi-directional 1st-order pumping, a 1480 nm pump was injected to the two sides of the SMF via a 50:50 coupler. For the bidirectional 2nd-order pumping, a pair of 1454nm fiber Bragg gratings (FBGs) with 95% reflectivity were added to the two sides of the SMF, and a 1366 nm fiber Raman laser was used as the 2nd-order primary pump [6–8]. This structure was similar to the conventional 2ndorder pumping where no FBGs were used [8]. For the explored forward (backward) random laser pumping, the scheme was similar to that of the bi-directional 2nd-order pumping, except that the right (left) FBG was removed to avoid the facet-end feedback. Note that only one FBG was reserved in order to decrease the lasing threshold [16]. The 1454nm ‘modeless’ random laser is generated through the purely random Rayleigh distributed feedback and Raman amplification by 1366nm primary pump along the transmission span (due to the lack of closed facet-end feedback, the random lasing

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cavity is ‘mirrorless’ [10–19]). The 1550nm signal is further amplified by the 1454nm-band fully-distributed random laser pumping. In the experimental setup shown in Fig. 1(a), a power meter (PM) and an optical timedomain reflectometry (OTDR) were used to measure the input-output gain and gain distribution, respectively. An optical spectrum analyzer (OSA) was used to measure the noise figure (NF) according to [3]: NF ≡

SNR in P  1 =  1 + ASE  SNR out G  hνΔν 

(1)

where SNRin and SNRout are the input and output signal-to-noise ratios, respectively, G is the net gain, hν is the photon energy, PASE is the power of ASE in the resolution bandwidth Δν. The effective noise figure (ENF) was used to compare the noise performance directly with EDFA, defined as [3]: ENF ≡ NFexp( −α L)

(2)

where α is the fiber loss coefficient for signal, L is the span length. For the transmission span with the same signal input power and identical dispersion map, the accumulated nonlinear effect induced by self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave-mixing (FWM) can be quantified through the ratio RNL of averaged power [3]: RNL ≡

L Pave = L−eff1  G ( z )dz 0 Pref

(3)

where Pave and Pref are the averaged signal powers when pump is on and off, respectively, G(z) is the gain distribution. Leff is the effective length [3].

Fig. 1. (a) Experimental arrangement to measure the gain and noise figure for various pumping configurations. (b) Experimental setup for testing the temperature response of RFL.

The experimental setup for testing the temperature response of RFL over 100km SMF is shown in Fig. 1(b). The fiber was placed in a temperature controlled chamber, and the injected power of 1366nm pump was ~1.5W. The output power and optical spectrum were recorded every 20 minutes from −40 to + 40°C, by a 10°C step. 3. Results and discussions 3.1 Gain characteristics Figure 2(a) shows the measured on-off gain as function of input power of the primary pump for various pumping configurations. In the measurement procedure, the 1550nm signal power coupled into the fiber was attenuated to ~-35dBm to avoid the gain saturation. The primary pump power was adjusted to obtain the given on-off gain. It is found that, for the same on-off gain (>2dB), the required primary pump power for random laser pumping is larger than that of the conventional bi-directional 2nd-order pumping by ~2-2.5dB. The conventional bidirectional 1st-order pumping shows the lowest pump power requirement.

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Fig. 2. (a) Measured on-off gain as function of input power of primary pump for various pumping configurations. (b)Measured gain distribution for various pumping configurations at transparency transmission point (18.6dB on-off gain). (c) Ratio of averaged power when pump is on and off as a function of on-off gain, where the signal input power is the same.

Figure 2(b) gives the measured gain distribution for various pumping configurations at transparency transmission point (18.6dB on-off gain). It is shown that, the forward random laser pumping exhibits larger averaged gain and gain variation (~5.5dB); the backward random laser pumping shows smaller averaged gain and gain variation (~3.8dB); the bidirectional 2nd-order exhibits the lowest gain variation (~2.5dB). The ratio of averaged signal power as function of on-off gain, when pump is on and off, is shown in Fig. 2(c). The ratio has no obvious difference for 10dB onoff gain, the forward random laser pumping exhibits larger ratio by ~0-3dB with increased onoff gain. At transparency transmission point, the backward random laser pumping shows the smallest ratio (~4.4dB) and thus the lowest nonlinear impairment under the conditions of the same signal input power and on-off gain. 3.2 Effective noise figure Figure 3(a) shows the measured ENF as function of the on-off gain. It is observed that, for