Demonstration of an ultra-high frequency picosecond pulse generator ...

Demonstration of an ultra-high frequency picosecond pulse generator using an SBS frequency comb and self phase-locking Sébastien Loranger,1,* Victor Lambin Iezzi,1 and Raman Kashyap,1,2 1

Department of Physics Engineering, École Polytechnique de Montréal, 2900 Édouard-Montpetit, Qc, Montreal H3T 1J4, Canada 2 Department of Electrical Engineering, PolyGrames, École Polytechnique de Montréal, 2900 Édouard-Montpetit, Qc, Montreal H3T 1J4, Canada * [email protected]

Abstract: We propose a method to generate phase-locked pulses in the picosecond regime by using Stimulated Brillouin Scattering (SBS). The phase-locked comb is generated using only long length of fiber and a single frequency CW pump laser. We show that there is a phase relationship between multiple Stokes peaks in a cavity, which directly leads to pulsing without the need to add a mode-locking component. This generates highly coherent pulses in the order of ~10 ps. The repetition frequency, which is very stable is in the order of tens of GHz, is based on the SBS frequency shift and has a linear dependence with temperature (1 MHz/°C). Such a laser could therefore be used in high-speed optical clocks and optical communication system. This system allows the pulses to be generated at any wavelength by simply changing the pump wavelength. ©2012 Optical Society of America OCIS codes: (140.3538) Lasers, pulsed; (320.5390) Picosecond phenomena; (290.5900) Scattering, stimulated Brillouin; (060.5625) Radio frequency photonics.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

V. Lugovoi and A. Prokhorov, “Possibility of Generating Ultrashort Light Pulses in Lasers with Small Luminescence Line Width of the Medium,” JETP Lett. 15, 49–51 (1972). F. Korolev, O. Vokhnik, and V. Odintsov, “Mode locking and ultrashort light pulses in SMBS in an optical resonator,” JETP Lett. 18, 32–33 (1973). G. P. Agrawal, Nonlinear Fiber Optics 4th ed. (Elsevier, 2007), Chap. 9. V. Lugovoi, “Theory of mode locking at coherent brillouin interaction,” IEEE J. Quantum Electron. 19(4), 764– 769 (1983). E. M. Dianov, S. Isaev, L. Kornienko, V. Firsov, and Y. P. Yatsenko, “Locking of stimulated Brillouin scattering components in a laser with a waveguide resonator,” Sov. J. Quant. Electron. 19(1), 1–2 (1989). M. J. Damzen, R. A. Lamb, and G. K. N. Wong, “Ultrashort pulse generation by phase locking of multiple stimulated Brillouin scattering,” Opt. Commun. 82(3-4), 337–341 (1991). B. S. Kawasaki, D. C. Johnson, K. Hill, and Y. Fujii, “Bandwidth-limited operation of a mode-locked Brillouin parametric oscillator,” Appl. Phys. Lett. 32(7), 429–431 (1978). I. Bar-Joseph, A. Friesem, E. Lichtman, and R. Waarts, “Steady and relaxation oscillations of stimulated Brillouin scattering in single-mode optical fibers,” J. Opt. Soc. Am. B 2(10), 1606–1611 (1985). C. Montes, A. Mamhoud, and E. Picholle, “Bifurcation in a cw-pumped Brillouin fiber-ring laser: Coherent soliton morphogenesis,” Phys. Rev. A 49(2), 1344–1349 (1994). C. Montes, D. Bahloul, I. Bongrand, J. Botineau, G. Cheval, A. Mamhoud, E. Picholle, and A. Picozzi, “Selfpulsing and dynamic bistability in cw-pumped Brillouin fiber ring lasers,” J. Opt. Soc. Am. B 16(6), 932–951 (1999). H. Li and K. Ogusu, “Instability of Stimulated Brillouin Scattering in a Fiber Ring Resonator,” Opt. Rev. 7(4), 303–308 (2000). J. Botineau, G. Cheval, and C. Montes, “CW-pumped polarization-maintaining Brillouin fiber ring laser: I. Selfstructuration of Brillouin solitons,” Opt. Commun. 257(2), 319–333 (2006). J. Botineau, G. Cheval, and C. Montes, “CW-pumped polarization-maintaining Brillouin fiber ring laser: II. Active mode-locking by phase modulation,” Opt. Commun. 257(2), 311–318 (2006). K. O. Hill, D. C. Johnson, and B. S. Kawasaki, “CW generation of multiple Stokes and anti-Stokes Brillouinshifted frequencies,” Appl. Phys. Lett. 29(3), 185–187 (1976). S. Yamashita and G. J. Cowle, “Bidirectional 10-GHz optical comb generation with an intracavity fiber DFB pumped Brillouin/erbium fiber laser,” IEEE Photonic Tech. L. 10(6), 796–798 (1998).

#172295 - $15.00 USD

(C) 2012 OSA

Received 10 Jul 2012; revised 2 Aug 2012; accepted 3 Aug 2012; published 9 Aug 2012

13 August 2012 / Vol. 20, No. 17 / OPTICS EXPRESS 19455

16. Y. J. Song, L. Zhan, J. H. Ji, Y. Su, Q. H. Ye, and Y. X. Xia, “Self-seeded multiwavelength Brillouin-erbium fiber laser,” Opt. Lett. 30(5), 486–488 (2005). 17. Y. G. Liu, X. Dong, P. Shum, S. Yuan, G. Kai, and X. Dong, “Stable room-temperature multi-wavelength lasing realization in ordinary erbium-doped fiber loop lasers,” Opt. Express 14(20), 9293–9298 (2006). 18. M. N. Mohd Nasir, Z. Yusoff, M. H. Al-Mansoori, H. A. Abdul Rashid, and P. K. Choudhury, “Broadly tunable multi‐wavelength Brillouin‐erbium fiber laser in a Fabry‐Perot cavity,” Laser Phys. Lett. 5(11), 812–816 (2008). 19. S. Shahi, S. W. Harun, and H. Ahmad, “Multi-wavelength Brillouin fiber laser using Brillouin-Rayleigh scatterings in distributed Raman amplifier,” Laser Phys. Lett. 6(10), 737–739 (2009). 20. N. Ahmad Hambali, M. Al-Mansoori, M. Ajiya, A. Bakar, S. Hitam, and M. Mahdi, “Multi-wavelength Brillouin-Raman ring-cavity fiber laser with 22-GHz spacing,” Laser Phys. 21(9), 1656–1660 (2011). 21. N. A. M. Hambali, M. A. Mahdi, M. H. Al-Mansoori, A. F. Abas, and M. I. Saripan, “Investigation on the effect of EDFA location in ring cavity Brillouin-Erbium fiber laser,” Opt. Express 17(14), 11768–11775 (2009). 22. S. Randoux, V. Lecoeuche, B. Ségard, and J. Zemmouri, “Dynamical analysis of Brillouin fiber lasers: An experimental approach,” Phys. Rev. A 51(6), R4345–R4348 (1995). 23. V. Lambin Iezzi, S. Loranger, A. Harhira, R. Kashyap, M. Saad, A. S. L. Gomes, and S. Rehman, “Stimulated Brillouin scattering in multi-mode fiber for sensing applications,” in Workshop on Fibre and Optical Passive Components (WFOPC), (2011 7th Workshop on, 2011), 1–4.

1. Introduction The idea of using Stimulated Brillouin Scattering (SBS) to generate pulses was initially proposed by Lugovoi and Korolev [1, 2]. SBS is a 4-wave mixing process, which may be simply described by the interaction of three principle waves, conserving energy and momentum: an optical pump, an acoustic wave and a Stokes wave [3]. SBS has the property of generating a very narrow bandwidth counter-propagating wave at a constant frequency shift determined by material properties. In a favorable configuration, SBS can be cascaded to generate multiples Stokes with a certain phase relation [4] which can enable pulsing. Phase locking of different Brillouin components to generate pulses has been demonstrated by Dianov et al. [5] in a short multi-mode fiber (MMF) Fabry-Perot cavity using an Nd:YAG crystal as the gain medium as well as by Damzen et al. [6] in a hexane gas cell, also within a Fabry-Perot cavity. In the latter case, the cavity length was adjusted so that the modes matched the SBS elements. This was done without a saturable absorber or any other nonlinear component. In the last decades, there has been increasing interest in intra-Stokes cavity mode locking [7] and relaxation oscillations [8] to generate pulses. A three-waves model using a Hopf bifurcation was proposed [9] and demonstrated [10, 11]. Soliton pulses [12] of such modelocked lasers and phase modulation [13] were also explored recently. In this paper, we demonstrate for the first time to our knowledge, a self pulsing SBS laser in a long ring cavity of single mode fiber (SMF) using an erbium doped fiber amplifier (EDFA) as the gain medium, which has the advantage of a low threshold and tuneable wavelength. Therefore, contrary to the limitations predicted by Dianov et al. [5], we demonstrate that a minimum of coherence can be maintained between multiple Stokes orders to allow the generation of ps pulses. Our system gives rise to an SBS frequency comb, which has previously been widely explored for telecommunication application as multi-channel sources. The idea was first demonstrated by Hill [14] and since then many system have been proposed [15–20]. We will show here two equivalent systems to generate frequency combs of different frequency spacing and then show that each Brillouin laser does indeed generate a pulsed output of high stability. 2. Theory In order to generate a pulse using multiple Stokes emission, either a constant or linear phase relationship must be maintained between the multi-Stokes at the output of the system. We are therefore interested here in the phase relationship between the multiple Stokes emission and the pump wavelength. SBS is generated from the coupling of a narrow bandwidth pump of wavelength λp and an acoustic wave of frequency ΩB. From the conservation of energy and of momentum, we get the following relations respectively:

#172295 - $15.00 USD

(C) 2012 OSA

Received 10 Jul 2012; revised 2 Aug 2012; accepted 3 Aug 2012; published 9 Aug 2012

13 August 2012 / Vol. 20, No. 17 / OPTICS EXPRESS 19456

Ω B = ω p − ωs

ka = k p − ks

(1)

Where ka, kp and ks are respectively the acoustic, optical pump and optical Stokes wave vector. Considering the acoustic wave ka acting as a travelling Bragg grating, since the refractive index is modulated by the acoustic pressure waves, and using | k p |≈| ks | since the wavelengths are almost identical, we can link the pump wave vector kp to the Stokes wave vector ks by Bragg grating relations [3] with the following equation:

Ω B = v a k p = 2v a k p sin(θ / 2)

(2)

Where θ is the angle between scattered wave ks and the pump wave kp. Since in a singlemode fiber the only relevant directions are forward (θ = 0) and backward (θ = π), SBS will only occur in the backward direction owing to Eq. (2), with θ = π. Therefore, the wave vector of this Bragg grating, i.e. the acoustic wave ka, will be along the same axis. The Brillouin frequency shift corresponds to the acoustic frequency and can be explained by a Doppler shift of the pump frequency considering the propagating acoustic wave, as:

Ω B 2 n pv A (3) = 2π λp From the last relations of Eq. (1)-(3), we can see that the Stokes’ phase, i.e. its frequency ωs and wave vector ks, are linked to the pump’s phase by the acoustic frequency shift ΩB, which can be approximated to be constant for a small number of Stokes orders. Therefore, considering this approximation, we can safely assume a constant phase shift ∆ϕB from one Stokes wave (or seed) to another, ignoring dispersion for now. By considering now all the sources of phase shift in a long ring cavity of length L, the output phase for each Stokes j is: vB =

ϕ j = ϕ0 + ∆ϕBj + ∆ϕt + ∆ϕd = ϕ0 + j ∆ϕB + j Ω B

L ∆L ∂n j ΩB + ω0 c c ∂ω

(4)

Where ϕ0 is the output pump wavelength phase, ∆ϕt is the phase delay due to a different cavity length ∆L from even and odd Stokes, which travel in opposite directions and may not see the same length, and ∆ϕd is the first order dispersion delay for the length of cavity. Equation (4) is valid as long as the number of generated Stokes j orders is not too large, and the dispersion length is longer than the fiber used. The length of the fiber must be short enough so that the second order dispersion induced phase change between the Stokes waves remains