OSA/OFC/NFOEC 2011 OMO6.pdf OMO6.pdf Cooperative Light Scattering Effects in Optical Fibers, Lasers, and Amplifiers Andrei A. Fotiadi, Patrice Mégret Service d'Electromagnétisme et de Télécommunications, Faculté Polytechnique, Université de Mons, Boulevard Dolez 31,B-7000 Mons, Belgium. Also with Ioffe Physico-Technical Institute, Russian Academy of Sciences, Politekhnicheskaya 26, St.Petersburg, 194021 Russia, Andrei.Fotiadi@umons.ac.be Abstract: We overview Rayleigh, Brillouin and Raman scattering in optical fibers, fiber amplifiers and lasers focusing on their mutual cooperation dynamics and explaining associated effects such as multi-cascade Brillouin scattering, Brillouin self-pulsing and random Raman lasing. OCIS codes: (060.4370) Nonlinear optics, fibers; (060.3510) Lasers, fibers 1. Introduction Rayleigh, Brillouin and Raman light scatterings are widely used for many telecom applications. They have been intensively studied within single-mode optical fibers that are ideal (long and thin) media for effective light-matter interaction. Stimulated Raman scattering (SRS) originates from interaction of light with optical phonons and is commonly used for optical signal amplification. Similarly, Stimulated Brillouin scattering (SBS) results from interaction with acoustic phonons. For a monochromatic pump, the Brillouin gain factor is almost three orders of magnitude higher than the Raman gain, so SBS nonlinearity dominates in the fiber. Backward Rayleigh scattering (RS) is the part in the backward direction of the linear elastic light scattering of on “frozen” fiber index nonuniformities.. When external feedback is strongly prevented, Rayleigh backscattering can provide a feedback to the Stokes field generated through SBS or SRS leading to pronounced cooperative effects in optical fibers, fiber amplifiers and lasers. Some of these cooperative effects are described hereafter. 2. Cooperative Rayleigh - Brillouin process in optical fibers Cooperative action of the Brillouin and Rayleigh scatterings is illustrated in Fig.1 a. Pump power at ν L provides a strong gain for SBS Stokes wave at ν S = ν L − ∆ SBS where ∆ SBS is the SBS shift. This ν S wave propagates in backward direction in the presence of feedback at ν S caused by the double Rayleigh scattering. Single Rayleighbackscattered Stokes waves do not contribute to the SBS process, but double Rayleigh-backscattered Stokes radiation provides coherent feedback. In fact, an interferometer consisting of two distributed Rayleigh mirrors is formed into the fiber. Although the reflection coefficient of this distributed Rayleigh mirror in single-mode optical fiber is very small (~1/600 part of the total Rayleigh losses), the SBS amplification in optical fiber is great enough ~ exp {20} to compensate for these low reflections. The feedback caused by the double Rayleigh scattering demonstrates strong spectral selectivity and can provide random lasing of narrow spectral components inside the SBS line (Fig.1 b, c) [1] and modify the SBS statistics [2]. The physical mechanisms behind the RS-SBS lasing can be illustrated in terms of the cavity modes. In a first approximation, the double Rayleigh scattering can be considered as a linear coupling of many pumped Fabry-Perot cavities with varying length l (see Fig. 2). The cavity resonances can be thought of a result of superimposing a large number of resonance curves with spacing ∆ν ( l ) ≡ c nl for different modes that lie close to each other. In a linear system, the resonant properties never disappear and the resulting spectrum is left with residual resonances that have the greater magnitudes and narrower linewidths when more elementary cavities are superposed [3]. Therefore, when the Brillouin gain in the fiber starts to cancel out the fiber loss, the most resonant photons reach first the lasing conditions, leading to generation of narrow components in the emission spectrum. 3. Rayleigh scattering supports multi-cascade Brillouin scattering In spite of the fact that the threshold of first-order SBS in a single optical fiber is very low (under CW, singlefrequency pumping), generation of second-order SBS by first-order SBS (and hence the following SBS orders) is not usually achieved in the fiber through the classical SBS process. There are two reasons for that. Firstly, as the pump power grows above the first-order threshold, the SBS power distribution inside the fiber incredibly concentrates near the fiber input and so the effective length of the fiber available for SBS interaction decreases: the first Stokes power OSA/OFC/NFOEC 2011 OMO6.pdf OMO6.pdf accumulated in the fiber is not enough to generate next Brillouin order. Secondly, in contrast with the first-order SBS process that is initiated by monochromatic pump, the second order SBS is pumped by the first-order Stokes radiation that is essentially not monochromatic: its spectrum is only 4 times narrower than the SBS gain spectrum in the fiber [2]. The situation becomes very different in the presence Rayleigh backscattering (RS) that causes a drastic narrowing of the first-order Stokes spectrum through the cooperative Rayleigh-Brillouin process and a redistribution the Stokes power along the fiber making it less abrupt. The generation of low-threshold (~273 mW) second-order SBS in a standard 4.4-km-long telecom fiber (Corning SMF-28) reported with CW single-frequency pumping at 1.06 µ m [4] has been explained by cooperation of SBS and RS effects in the fiber [5]. 4. Brillouin instabilities in fiber lasers and passive Q-switching For active fiber configurations (optical amplifiers, lasers), generation of narrow-band spectral components due to Rayleigh feedback provokes avalanche-like Brillouin instabilities observed as multiple strong spikes in the spectral and temporal domains. The Brillouin instability occurs due to the cascaded Brillouin process (Fig.3 a). Occasional lasing of a narrow-band spectral component due to residual cavity resonance gives additional Brillouin gain for down shifted modes leading to their exponential growth. During this cascade process, power from high-frequency components is transmitted to lower frequency ones, providing a higher growing rate (and higher peak power) for the new frequency components, until generation of short pulses by the latest cascades exhausts the cavity power [2]. This described picture is typical for all fiber lasers employing distributed Rayleigh feedback. The use of the RS-SBS mirror based on a single-mode fiber is the simplest, completely passive and rather universal way to organize passive Q-switching in a fiber laser operating at any wavelength. In the all fiber spliced configuration shown in Fig.3, multicascade stimulated Brillouin scattering (SBS) generated through RS-SBS cooperative process results in generation of giant nanosecond pulse train with a record peak/average power contrast of ~500W/25mW [6, 7]. 5. Cooperative Rayleigh – Brillouin – Raman process in fibers. Amplification due to SRS provides much better conditions for spectrally uniform lasing in open laser cavity than traditional solid-state laser media as there is no gain heterogeneity and no spectral hole-burning. In the Raman pumped configuration shown in Fig.4, the Rayleigh scattering (RS) and stimulated Brillouin scattering (SBS) cascaded process both provide a dynamical feedback in the fiber, leading to generation of gigantic nanosecond pulses [8]. The spectrum generated in this configuration through cooperative RS-SBS-SRS process contains 200 – 250 Brillouin orders. Suppression of RS-SBS instabilities with generation of CW incoherent laser field has been demonstrated recently with ultra-long Raman fiber cavities [9, 10]. The reason of that is a smoothing of the resonances (Fig.2) due to the intense frequency perturbation by the Raman pump through the cross-phase modulation (XPM) [2]. 6. Conclusion Many specific features of the cooperative scattering processes in fibers have still to be explored. The reported results and the proposed concepts might have significant future impact in fiber optics (nonlinear fiber optics, Rayleigh and Brillouin fiber mirrors) and applications (fiber lasers, distributed fiber sensors, telecommunications). This work was supported by the European Regional Development Fund and the Walloon Region (Mediatic project), the Interuniversity Attraction Pole program VI/10 of the Belgian Science Policy and program “Scientific and ResearchEducational Cadres for Innovation Russia” of Russian Federal Agency on Science and Innovation. [1] A.A. Fotiadi, R.V. Kiyan, “Cooperative stimulated Brillouin and Rayleigh backscattering process in optical fiber.” Optics Lett. 23, 1805 1807 (1998). [2] A.A. Fotiadi, A.A. Fotiadi, “Random lasers: An incoherent fibre laser,” Nature Photonics 4, 204-205 (2010). [3] A.A. Fotiadi, R. Kiyan, O. Deparis, P. Mégret, M. Blondel, “Statistical properties of stimulated Brillouin scattering in singlemode optical fibers above threshold”, Optics Lett. 27, 83 – 85 (2002). [4] A.A. Fotiadi, G. Ravet, P.Mégret, M. Blondel, “Multi-cascaded SBS in an optical fiber supported by Rayleigh backscattering”, Proc. of SPIE Vol. 5480 Diode Lasers and Telecommunication Systems, ed. by N.N. Rosanov, S.A.Gurevich (SPIE, Bellingham, WA, 2004), pp.82-90. [5] T.H. Russell, W.H. Roh, “Threshold of second-order stimulated Brillouin scattering in optical fiber”, J.Opt.Soc.Am.B 19, 2341-2345 (2002). [6] A.A. Fotiadi, P. Mégret, M. Blondel, “Dynamics of self-Q-switched fiber laser with Rayleigh – stimulated Brillouin scattering ring mirror”, Optics Lett., 1078 – 1080 (2004). [7] A.A. Fotiadi, P. Mégret, “Self-Q-switched Er-Brillouin fiber source with extra-cavity generation of a Raman supercontinuum in a dispersion shifted fiber”, Opt.Lett. 31, 1621-1623(2006). [8] G. Ravet, A.A. Fotiadi, P.Mégret, M. Blondel, “Passive Q-switching in all-fibre Raman laser with distributed Rayleigh feedback”, Electronics Lett., 528 - 529 (2004). [9] S.K. Turitsyn t al. “Random distributed feedback fibre laser,” Nature Photonics 4, 231-235 (2010). [10] S.K. Turitsyn, “Random Distributed Feedback Fiber Laser,” OFC’2011, invited talk. OSA/OFC/NFOEC 2011 OMO6.pdf OMO6.pdf (a) Pump,ν L Stokes,ν S 1-x RS-Stokes,ν S Stokes,ν S + 2-x RS-Stokes,ν S l (c) Cavity Cavitypower power Optical power spectrum (b) -10 10 -6 -5 0 5 Frequency, MHz -4 -2 0 Frequency, MHz Fig.1. The scheme of cooperative Rayleigh-Brillouin process (a). Optical Brillouin power spectrum without (b) and with (c) Rayleigh feedback. Lasing spectrum, a.u. Lasing power, a.u (a) ν4 ∆ SBS ν3 ν2 ∆ SBS (b) ν1 ∆ SBS ∆ SBS ν0 Frequency, ∆SBS/div R0 = 1 WDM Output 1 c nL divper div. Fig.2. The resonance properties of the double Rayleigh scattering in the fiber with Brillouin gain, L is fiber length ν 1− ν 0− ν 1+ ν 0+ ν 2− R1 = 0.45 EDFA (c) ν Frequency, 40 ∆ pe40r 2 ν 2− SMF ν 0− ν 1+ − ν 1− ν 2 ν + 0 Coupler 10/90 ν 3+ ν 3+ ν 1+ Output 2 2 ms/div. Fig.3. Brillouin instability through generation of cascade SBS (a), the operation of Er/Brillouin laser [6] (b), a typical pulse train generated by the laser (c). (a) (b) Wavelength, nm Fig.4. Setup for observation of RS-SBS-SRS cooperative process in the fiber [8] (a) and a typical spectrum generated through the process (b)