Jes Broeng

Highly nonlinear photonic crystal fiber with zero-dispersion at 1.55 /spl mu/m

We demonstrate, for the first time, a highly nonlinear polarization maintaining photonic crystal fiber with zero dispersion at 1.55 /spl mu/m, nonlinear coefficient of 20 (Wkm)/sup -1/ and splice loss to standard technology fiber of 0.3 dB.

High-birefringent photonic crystal fiber

A highly birefringent photonic crystal fiber design is analysed. Birefringence up to 10/sup -3/ is found. Random fluctuations in the cladding design are analysed, and the fiber is found to be a feasible polarization maintaining fiber.

Amplification and ASE suppression in a polarization-maintaining ytterbium-doped solid-core photonic bandgap fibre

We demonstrate suppression of amplified spontaneous emission at the conventional ytterbium gain wavelengths around 1030 nm in a cladding-pumped polarization-maintaining ytterbium-doped solid core photonic crystal fibre. The fibre works through combined index and bandgap guiding. Furthermore, we show that the peak of the amplified spontaneous emission can be shifted towards longer wavelengths by rescaling the fibre dimensions. Thereby one can obtain lasing or amplification at longer wavelengths (1100 nm - 1200 nm) as the amount of amplification in the fibre is shown to scale with the power of the amplified spontaneous emission.

Applications and Future Perspectives

In the previous chapters of this book, we have introduced the fundamental properties of photonic crystal fibres ranging from fundamental definition of photonic bandgap structures over numerical modelling to fabrication of these new fibre types. We have also discussed the fundamental differences between index-guiding PCFs and bandgap-guiding PCFs, and some examples of fibre structures and properties have been presented. However, the research field is still very young, and numerous new results and applications appear as the fibres are tested and investigated by more research groups and companies. For this reason, it is a very significant challenge to try to give an up-to-date picture of the most relevant applications of photonic crystal fibres, and just between the time of writing this chapter and to the point, when the book is printed, novel findings will be added to the field. For this reason, our ambition with the present chapter is more modest, since we have chosen to present some of the applications and ideas for the PCF technology, which primarily provides a good impression of the wide range of possibilities provided by these waveguides rather than necessarily covering all of the latest research results.

Photonic crystal fiber modelling and applications

Photonic crystal fibers having a microstructured air-silica cross section offer new optical properties compared to conventional fibers for telecommunication, sensor, and other applications. Recent advances within research and development of these fibers are presented.

Fabrication of Photonic Crystal Fibres

The idea of producing optical fibres from a single low-loss material with microscopic air holes goes back to the early days of optical fibre technology, and already in 1974 Kaiser et al. [4.1] reported the first results on singlematerial silica optical fibres. In the early days — as well as today — the key issues have been to obtain a desired fibre structure for a given application, and maintain this structure for very long fibre lengths. It will, generally, be needed that the fibre attenuation is kept at a rather low level, and the acceptable attenuation level will be given by the specific application. In this chapter, we will address the fundamental issues of fabrication of photonic crystal fibres, by first discussing the most commonly used preform fabrication method. Secondly, we will report details about the fibre drawing and coating procedure. Furthermore, we will discuss how additional doping techniques are needed for providing hybrid fibre types (such as the holeassisted lightguide fibre (HALF) [4.6]) combining the approach of microstructuring with index-raised doped glass or active dopants such as rare-earth ions needed for new amplifiers and lasers. The chapter will also shortly address the issues of photonic crystal fibres in low-melting-point glasses and polymers.

Polarization properties of photonic crystal fibers

Photonic crystal fibers provide increased range of mode-field diameters for passive and active fibers. At present, single mode photonic crystal fibers with mode field diameters ranging from sub-micron to beyond 40 /spl mu/m have been demonstrated. For a number of applications, it is desirable to introduce polarization maintaining properties of such fibers. In this presentation, we report on the latest development within this area and explain the design and characteristics of different types of photonic crystal fibers with both polarization-maintaining and polarizing properties.

Macro-bending loss estimation for air-guiding photonic crystal fibres

Optical fibres operating by the photonic bandgap effect offers an alternative approach to evanescent field waveguides for opticai sensing applications. In addition to this, these photonic crystal fibres provide completely new waveguiding properties, which include different macro-bending loss performance compared to standard optical fibres. A first estimation of these new properties is demonstrated for air-guiding fibre designs by combining accurate vectorial mode analysis and an adaptation of more traditional macro-bending loss theory.

Photonic crystal fibers: a variety of applications

Photonic crystal fibres having a microstructured air-silica cross section offer new optical properties compared to conventional fibres. These include novel guiding mechanisms, new group velocity dispersion properties and new non-linear possibilities.

Amplification in hollow core photonic crystal fibers

Hollow core photonic crystal fiber (HCPCF) amplifiers, in which Er3+- or Yb3+- doped glass acts as the gain medium, are proposed as a means of achieving high power pulse amplification. Double-clad configurations are identified which capture multimode pump light up to an NA of around 0.33. The nonlinear and breakdown properties of a HC-PCF amplifier with a mode area of approximately 50μm2 are predicted to be comparable to those of a solid core fiber amplifier with a mode area of 1000μm2. Mode competition effects within the HC-PCF amplifier strongly degrade the output signal unless the net gains of the unwanted guided modes are below that of the signal mode. This can be achieved if the ratio of amplifier gain to scattering loss is larger for the signal mode than any of the undesired guided modes. Assuming loss is dominated by hole interface roughness scattering, and an even doping profile produces the gain, the ratios for the unwanted guided modes of a typical HCPCF geometry are calculated to be similar to that for the signal carrying mode. The mode competition also places a lower bound on the active fiber length, typically implying a longer length is required than in a solid core fiber amplifier. This adversely affects the device efficiency due to scattering loss of the pump field incurred at the air/glass interfaces. To achieve a clean mode output and acceptable efficiency, alternative designs for the HC-PCF will need to be developed.