Photonic Crystal Fibres (PCFs) have appeared as a new class of optical waveguides, which have attracted large scientific and commercial interest during the last years. PCFs are typically microstructured silica waveguides with a large number of air holes located in the cladding region of the fibre. The size and location of these air holes allows for a large degree of design freedom within optical waveguide design, and PCFs with properties tailored for fibre lasers, airguiding fibres, nonlinear fibres, hybrid fibres etc. have been demonstrated. Further, the existence of air holes in the PCF gives the possibility of propagating light through air, or alternatively allows access close to the fibre core for interactions with new materials placed in the air holes. This makes a well controlled interaction between light and material possible.
We report on the latest development within active photonic crystal fibers for high power lasers and amplifiers with special focus on how the fibers can be improved with both polarization-maintaining and polarizing properties. We describe rod-type fibers for which a record-high power extraction of 250W/m is achieved. Moreover, we describe how active characterization is used to optimize fibers for laser and amplifier sub-assemblies with respect to beam quality, efficiency and robustness. Finally, we illustrate how the fibers can be integrated with high NA tapers and passive air-clad fibers containing Bragg grating to form an all-fiber, alignment-free, high-power fiber laser subassembly.
Laser damage thresholds of 8μm- and 22μm-core diameter solid-core photonic crystal fibres (PCF) and hollow-core photonic band gap (PBG) fibres have been measured. The studies were carried out using a 1.06μm Nd:Yag laser (30nsec pulses at 10Hz), which is optimally coupled into these fibres by careful mode matching, providing a coupling efficiency greater than 90%. It has been shown that the damage threshold of the 8µm core PBG fiber occurs at pulse energies close to 1 mJ, equivalent to a fluence well in excess of 1kJ/cm2 propagating down the fibre. This is a factor of 4 larger than the damage threshold of a solid-core PCF of similar core diameter. In comparison, the damage threshold of the large-core PBG is smaller than that of the equivalent PCF. Theoretical modelling based only on the optical modal properties of the single mode PBG fibre shows that an enhancement of a factor of 25 should be obtainable. Thus there are different damage mechanisms potentially responsible for the fragility of larger core PBG fibres. In an experimental study of bend losses it has been found that it is possible to bend the 8μm PBG fibre up to the breaking point bend radius (<1mm). The critical bend radius for the 22μm core PBG is close to 2 mm, which is 50 times smaller than the critical bend radius of a 20μm core PCF.
In order to realize an efficient absorption measurement based evanescent-wave sensor, a long interaction length and a strong penetration of the optical field into the sample space is required. For an optical fiber based device, with a solid silica core immersed into a liquid sample, the strength of the evanescent field increases with decreasing core radius. When the core diameter is comparable to the wavelength of the light, a large fraction of the light propagates in the evanescent field. We demonstrate evanescent-wave sensing on aqueous solutions of fluorophore labeled biomolecules positioned in the air holes of a hollow-core photonic crystal fiber (PCF). The aqueous solutions can be positioned in close proximity to light guided in small cores without removing the coating and cladding, thus ensuring a very robust device. In order to make selective DNA detection, we coated the inside of the hollow-core PCF with a sensing layer, which by hybridization selectively immobilize specific molecules. A fluorescence measurement method, where a line-shaped laser beam expose the fiber from the side and excites the fluorophore molecules, was realized. The emitted fluorescence tunnels via the evanescent field into the fiber core(s) and is analyzed by a spectrometer at the fiber end.