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

M.L.V.Tse, P.Horak, F.Poletti, and D.J.RichardsonOptoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, United Kingdom. Email: mlt@orc.soton.ac.uk

What is a Holey Fiber?

Soliton Compression Theory:

Cored

Cladding Air holes

Conventional Optical Fiber: Holey Fiber:

Holey fiber basic parameters: • Hole size (d)• Hole-to-hole spacing or pitch () • Air-filling fraction (d/).

Advantages: • Small cross section Large nonlinearity • Dispersion control

Long optical pulses

Nonlinear tapered holey fiber

Dispersion, Dispersion Slope and Effective Area Contour maps: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 m can be achieved in holey fibers. Recently, this has been successfully demonstrated with femtosecond pulses at 1.06 m [2].

Here, we investigate numerically the adiabatic compression of solitons at 1.55 m 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/, ) grid. Compression factors >10 are achieved for optimum fiber parameters.

Contour map for dispersion (ps/nm/km), dispersion slope (ps/nm2/km) and effective area (m2) versus pitch and d/ for holey fibers of hexagonal geometry at 1.55 m wavelength.

Contour map for adiabatic compression factors versus pitch and d/ for holey fibers of hexagonal geometry at 1.55 m 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)

D= 0

D= 25

D= 50

D= 75

Aeff= 70

Aeff= 30

Aeff= 15

Aeff= 7

Aeff= 3

Ds= 0.05

Ds= 0

Ds= -0.2

• For given fiber parameters and pulse energy, the width of a fundamental soliton is

sol

eff

Ecn

DA

22

3

02

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 and d/. 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 m in dispersion-decreasing holey fibers,” Opt. Lett. 31, 3504 (2006).

(1)

• Adiabatic compression, Esol= constant.

• 0 D *Aeff

• In tapered holey fibers, (, d/)(z) D *Aeff(z)

Path 1

• D: 25 5 ps/nm/km

• Aeff: 70 7 m2

• 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

Path 2

• D: 25 5 ps/nm/km

• Aeff: 75 30 m2

• Ds~ 0 at fiber end

• Long fiber, (50 m), no loss

• Compression factor: 12.5

• Numerical simulation agrees with theory

Length Considerations

• Fiber loss soliton broadening

• Require short fiber length

• Trade-off with adiabaticity

• Optimize length using constant effective gain method

constz

A

Az

D

DD

cg

opt

eff

effopteff

2

1

2

122

20

Example: Path 2

Not optimized Optimized

Input pulse: 400 fs

Simulated spectrum, no Raman effect. Simulated spectrum, Ds= 0.