Hollow Fiber Optical Transmission

While hollow photonic crystal fibers are not yet able to challenge conventional optical fibers in the field of long-distance communications, they are superior to conventional optical fibers in several other important applications, perhaps the most noteworthy of which is laser beam transmission. An important advantage of hollow photonic crystal fibers over conventional fibers is their high damage threshold. Because only a very small amount of light actually travels through the glass, the energy-transfer capability of hollow photonic crystal fibers is far superior to that of conventional optical fibers.

Fig. 2 Electron micrograph of a cross-section of a low-loss hollow-core photonic crystal fiber applied to the communications band. This fiber has a minimum loss of 1.7 dB/km at 1550 nm.

Another difference between them is that hollow-core photonic crystal fibers have lower optical nonlinearities, which is a result of the fact that there is little cross-talk between the light and the glass. Crucially, the nonlinear refractive index of the gas in the core is about 1,000 times smaller than that of solid silicon, and that gas makes the nonlinear properties of hollow photonic crystal fibers three orders of magnitude smaller than those of conventional fibers. As a result, both continuous waves and short pulse sequences can be transmitted at very high power in hollow photonic crystal fibers without spectral distortion. In fact, hollow photonic crystal fibers can be designed in such a way that the nonlinear properties of the entire fiber are determined by the nonlinearities of the gas or glass inside the core. Alternatively, it can be filled with other gases in addition to air, allowing complete control of the nonlinear characteristics of the fiber as a whole.

Fig. 3 Increasing the core of a hollow photonic crystal fiber, (shown in Fig. 2) reduces loss, but also introduces more surface film crossings, which cause many spikes in the loss spectrum. A smaller core has a wider bandwidth and smoother spectrum, but the loss increases.

It's worth noting that when the pulse width is less than 1 ps, new constraints start to become important. The intrinsic bandwidth of the pulse begins to match the width of the low-loss window of a hollow photonic crystal fiber. In addition, group velocity dispersion in hollow photonic crystal fibers means that pulses less than 1ps propagate only a few meters in the fiber before significant dispersion occurs. Importantly, however, the low nonlinearity of hollow photonic crystal fibers is such that such dispersion is not accompanied by significant spectral distortion, even for pulses with pulse widths of 100 fs and peak powers at the level of typical mode-locked laser oscillators.

With conventional optical fibers, a pulse that short would travel only a few millimeters before it would be quickly split by the nonlinear effects and dispersion*** together. The low nonlinearity of hollow photonic crystal fibers means that as long as the linear dispersion in the fiber can be properly compensated for, for example by pre-chirping the pulse with a piece of glass before coupling into the fiber, then the pulse is perfectly capable of propagating several meters in a hollow photonic crystal fiber. Another possibility is to balance the linear dispersion by using the low nonlinear properties of the hollow photonic crystal fiber, so that the pulse will be able to propagate as a soliton in the hollow photonic crystal fiber. Previously, fiber solitons have been observed at relatively low power levels in the 1500 nm band using conventional fibers. However, hollow photonic crystal fibers can propagate high-intensity pulses with peak powers of up to several megawatts over a wide range of wavelengths.