Designing Dispersion- and Mode-Area-Decreasing Holey Fibers for Soliton Compression

1. Designing Dispersion- and Mode-Area-Decreasing Holey Fibers for Soliton Compression M.L.V.Tse, P.Horak, F.Poletti, and D.J.Richardson Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, United Kingdom. Email: mlt@orc.soton.ac.uk Dispersion, Dispersion Slope and Effective Area Contour maps: • Length Considerations • Fiber loss  soliton broadening • Require short fiber length • Trade-off with adiabaticity • Optimize length using constant effective gain method Abstract Compression of soliton pulses propagating in optical fibers with decreasing dispersion is a well-established technique [1]. Using holey fibers it is possible to decrease dispersion (D) and effective mode area (Aeff) simultaneously, which potentially offers a greater range of variation in soliton compression factors. Moreover, soliton compression in new wavelength ranges below 1.3 mm can be achieved in holey fibers. Recently, this has been successfully demonstrated with femtosecond pulses at 1.06 mm [2]. Here, we investigate numerically the adiabatic compression of solitons at 1.55 mm in holey fibers which exhibit simultaneously decreasing in D and Aeff. We identify some of the limitations and propose solutions by carefully selecting paths in contour maps of D and Aeff in the (d/L, L) grid. Compression factors >10 are achieved for optimum fiber parameters. Aeff= 30 D= 50 Aeff= 15 D= 75 D= 25 Ds= 0.05 Aeff= 7 Ds= 0 Example: Path 2 D= 0 Not optimized Optimized Aeff= 3 Ds= -0.2 Aeff= 70 What is a Holey Fiber? Contour map for adiabatic compression factors versus pitch L and d/L for holey fibers of hexagonal geometry at 1.55 mm wavelength. (Normalized to the top left corner of the map, which has the largest value of D*Aeff) (green dotted line represents the single mode ‘SM’ and multi-mode ‘MM’ boundary) Contour map for dispersion (ps/nm/km), dispersion slope (ps/nm2/km) and effective area (mm2) versus pitch L and d/L for holey fibers of hexagonal geometry at 1.55 mm wavelength. Holey Fiber: Conventional Optical Fiber: d Core L Path 1 Cladding Air holes • Holey fiber basic parameters: • Hole size (d) • Hole-to-hole spacing or pitch (L) • Air-filling fraction (d/L). Input pulse: 400 fs • Advantages: • Small cross section  Large nonlinearity • Dispersion control Simulated spectrum, no Raman effect. Simulated spectrum, Ds= 0. Soliton Compression Theory: • D: 25  5 ps/nm/km • Aeff: 70  7 mm2 • Expected compression factor: 50 • Limitations: • Dispersion slope  ZDW close to soliton • Raman SSFS effect • Therefore, require paths that have Ds~0 near the end and a smaller Aeff ratio Conclusions We have investigated adiabatic compression of femtosecond solitons in silica holey fibers of decreasing dispersion and effective mode area. These parameters are directly related to the structural design parameters L and d/L. A compression factor of 12 has been obtained for low-loss fibers in the adiabatic regime. A method for minimizing the fiber length required for adiabatic compression in the presence of propagation losses is suggested. References [1] S. V. Chernikov, E. M. Dianov, D. J. Richardson and D. N. Payne, “Soliton pulse compression in dispersion-decreasing fiber,” Opt. Lett. 18, 476 (1993). [2] M. L. V. Tse, P. Horak, J. H. V. Price, F. Poletti, F. He, and D. J. Richardson, “Pulse compression at 1.06 mm in dispersion-decreasing holey fibers,” Opt. Lett. 31, 3504 (2006). Long optical pulses Nonlinear tapered holey fiber Path 2 • D: 25  5 ps/nm/km • Aeff: 75  30 mm2 • Ds~ 0 at fiber end • Long fiber, (50 m), no loss • Compression factor: 12.5 • Numerical simulation agrees with theory • For given fiber parameters and pulse energy, the width of a fundamental soliton is (1) • Adiabatic compression, Esol= constant. • t0 aD *Aeff • In tapered holey fibers, (L, d/L)(z)  D *Aeff(z)