Hollow core photonic crystal fibers (HC-PCFs) find applications which include quantum and non-linear optics, gas detection and short high-intensity laser pulse delivery. Central to most applications is an understanding of the linear and nonlinear optical properties. These require careful modeling due to the multitude of lengthscales involved and non-standard variations in properties such as the mode-field distribution. Linear mode-solvers require many 100,000's of basis functions to resolve the field variations, and extra terms are often required in descriptions of nonlinear propagation. The intricacies of modeling various forms of HC-PCF are reviewed. An example of linear dispersion engineering, aimed at reducing and flattening the group velocity dispersion, is then presented. Finally, a study of short high intensity pulse delivery using HC-PCF in both dispersive and nonlinear (solitonic) regimes is given.
Hollow core photonic crystal fiber (HC-PCF), fabricated according to a nominally non-birefringent design, shows a
degree of un-controlled birefringence or polarization mode dispersion far in excess of conventional non polarization
maintaining fibers. This can degrade the output pulse in many applications, and places emphasis on the development of
polarization maintaining (PM) HC-PCF. The polarization
cross-coupling characteristics of PM HC-PCF are very
different from those of conventional PM fibers. The former fibers have the advantage of suffering far less from stressfield
fluctuations, but the disadvantage of a higher loss figure and the presence of interface roughness induced modecoupling
which increases in strength as birefringence reduces. Close to mode anti-crossing events of one polarization
mode, the PM HC-PCF is characterized by high birefringence, a high polarization dependent loss and an increased
overlap between the polarization modes at the glass interfaces. The interplay between these effects leads to a wavelength
for optimum polarization maintenance, λPM, which is detuned from the wavelength of highest birefringence. By a
suitable fiber design involving antiresonance of the core-surround geometry, λPM may coincide with a low-loss
wavelength for the signal carrying polarization mode.
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.