A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation in photonic crystal fiber

The rise in output power from rare-earth-doped fiber sources over the past decade, via the use of cladding-pumped fiber architectures, has been dramatic, leading to a range of fiber-based devices with outstanding performance in terms of output power, beam quality, overall efficiency, and flexibility with regard to operating wavelength and radiation format. This success in the high-power arena is largely due to the fiber’s geometry, which provides considerable resilience to the effects of heat generation in the core, and facilitates efficient conversion from relatively low-brightness diode pump radiation to high-brightness laser output. In this paper we review the current state of the art in terms of continuous-wave and pulsed performance of ytterbium-doped fiber lasers, the current fiber gain medium of choice, and by far the most developed in terms of high-power performance. We then review the current status and challenges of extending the technology to other rare-earth dopants and associated wavelengths of operation. Throughout we identify the key factors currently limiting fiber laser performance in different operating regimes—in particular thermal management, optical nonlinearity, and damage. Finally, we speculate as to the likely developments in pump laser technology, fiber design and fabrication, architectural approaches, and functionality that lie ahead in the coming decade and the implications they have on fiber laser performance and industrial/scientific adoption.

High power fiber lasers: current status and future perspectives [Invited]

The history, fabrication, theory, numerical modeling, optical properties, guidance mechanisms, and applications of photonic-crystal fibers are reviewed

Photonic-Crystal Fibers

The diffraction tomography theorem is adapted to one-dimensional length measurement. The resulting spectral interferometry technique is described and the first length measurements using this technique on a model eye and on a human eye in vivo are presented.

Measurement of intraocular distances by backscattering spectral interferometry

Photonic crystal fibers are a new class of optical fibers. Their artificial crystal-like microstructure results in a number of unusual properties. They can guide light not only through a well-known total internal reflection mechanism but using also photonic bandgap effect. In this paper different properties possible to obtain in photonic crystal fibers are reviewed. Fabrication and modeling methods are also discussed.

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Photonic Crystal Fibers

A pump combiner with single-mode PM signal feed-through designed for an air-clad photonic crystal fiber is demonstrated. Signal coupling is realized by a novel taper element allowing single-mode guidance at a taper ratio of 3.7.

Pump combiner for air-clad fiber with PM single-mode signal feed-through

Generation of wide super continua several hundred nanometers wide is possible by pumping in the normal dispersion regime as the nonlinear effects broaden the spectra beyond the zero-dispersion wavelength. When pumping far away from the zero-dispersion wavelength, only self phase modulation (SPM) and Raman scattering contributes to the broadening leading to smooth stable spectra of limited width. Pumping closer to the zero-dispersion wavelength forms very broad spectra, but they are far more sensitive to fluctuations in pumping power and coupling efficiency between fiber and laser. The flattest and broadest spectra are achieved by pumping just below zero-dispersion wavelength and increase in power leads to flatter and broader spectrum. Furthermore, pumping close to the zero-dispersion wavelength enables generation of spectral components at shorter wavelengths below 750 nm through a four-wave mixing process.

Pumping wavelength dependence of super continuum generation in photonic crystal fibers

The invention relates to a high power amplifier waveguide for amplifying an optical signal wherein photo darkening due to high optical flux is reduced considerably. This is achieved by providing a cladding pumped amplifier waveguide wherein the optical mode overlap to the active material of the waveguide is low and/or wherein the active material is distributed over a large cross sectional region of the waveguide.

Improved cladding-pumped optical waveguide

We demonstrate several miniature continuum laser sources by combining compact Q-switched lasers with specially designed photonic crystal fibers. A continuum spanning two octaves is obtained with a 1-Watt diode pump.

Miniature supercontinuum laser sources

Supercontinuum generation in photonics crystal fibers (PCFs) pumped by CW lasers yields high spectral power density and average power. However, such systems require very high pump power and long nonlinear fibers. By on/off modulating the pump diodes of the fiber laser, the relaxation oscillations of the laser can be exploited to enhance the broadening process. The physics behind the supercontinuum generation is investigated by sweeping the fiber length, the zero dispersion wavelength, and the fiber nonlinearity. We show that by applying gain-switching a high average output power of up to 30 W can be maintained and the spectral width can be improved by 90%. The zero dispersion wavelength should be close to but below the pump wavelength to achieve the most visible light. By increasing the nonlinearity the fiber length can be reduced from 100 m to 25 m and the efficiency of visible light generation is improved by more than 200%. Keywords: Supercontinuum generation, photonic crystal fiber, fiber laser, gain-switching *crla@fotonik.dtu.dk; phone +45 45256352; www.fotonik.dtu.dk and nktphotonics.com

Kim P. Hansen

Papers

Pump combiner for air-clad fiber with PM single-mode signal feed-through

Pumping wavelength dependence of super continuum generation in photonic crystal fibers

Improved cladding-pumped optical waveguide

Miniature supercontinuum laser sources

Photonic crystal fibers for supercontinuum generation pumped by a gain-switched CW fiber laser