Photonic crystal fiber (PCF)

A photonic crystal fiber, also called a microstructured or holey fiber, replaces the doped cladding of a conventional fiber with an array of air holes, usually hexagonal, running the whole length of the fiber. The holes are drawn from a preform of stacked capillaries and can be made a few hundred nanometers to several micrometers across. Because the hole size $d$ and spacing $\Lambda$ can be chosen freely, they control the effective cladding index, the mode size and the dispersion over a far wider range than doping allows. There are two guidance mechanisms: solid-core fibers guide by a form of total internal reflection, the holes lowering the average cladding index; hollow-core fibers guide light in air by a photonic bandgap or by antiresonant reflection from thin glass walls.

The solid-core design has a property no step-index fiber shares. In a conventional fiber the V-number grows as the wavelength shrinks, so every fiber becomes multimode at short enough wavelengths. In a photonic crystal fiber the light at short wavelengths is confined more tightly to the glass between the holes, raising the effective cladding index along with the core's, and for small holes, $d/\Lambda$ below about 0.4, the fiber stays single-mode at every wavelength: the endlessly single-mode fiber. The same scaling lets the core be made large, tens of micrometers, while still single-mode, which is used in large-mode-area fiber lasers to lower the intensity.

At the other extreme, a core of 1 to 2 µm surrounded by large holes gives strong confinement and a small effective area. The nonlinear coefficient $\gamma = 2\pi n_2/(\lambda A_\text{eff})$, with $n_2 \approx 2.6 \times 10^{-20}$ m²/W for silica, is then 0.077 W⁻¹m⁻¹ for an effective area of 2 µm² at 1060 nm, against 0.0013 W⁻¹m⁻¹ for standard single-mode fiber with 80 µm² at 1550 nm, a factor of 58. The small core also contributes strong waveguide dispersion, which moves the zero-dispersion wavelength down to 800 nm or 1 µm, where titanium-sapphire and ytterbium lasers operate. The combination made photonic crystal fibers the medium for supercontinuum generation and for the octave-spanning spectra used to stabilize frequency combs.

Photonic crystal fibers cost more than standard fiber, splice less easily because the holes collapse under heat, and have higher loss than conventional fiber at telecom wavelengths, so they are used where their geometry matters: nonlinear optics, high-power delivery and amplification, sensing with gases or liquids filled into the holes, and polarization-maintaining designs made birefringent by an asymmetric hole pattern.

References: J. C. Knight, T. A. Birks, P. St. J. Russell, D. M. Atkin, Opt. Lett. 21, 1547 (1996); T. A. Birks, J. C. Knight, P. St. J. Russell, Opt. Lett. 22, 961 (1997); P. Russell, Science 299, 358 (2003).