Hollow fiber is a fiber that mainly guides waves in the hollow region, and only a small part of the light propagates in solid fiber material (usually glass). According to the basic physical mechanism of guided waves in optical fibers, this is impossible to achieve: because, under normal circumstances, the refractive index of the fiber core needs to be greater than the refractive index of the surrounding cladding medium, but it is impossible to obtain glass with a refractive index less than air or vacuum Materials, at least in the optical region, there is no such material. However, a different wave guide mechanism is used here. Based on the photonic band gap, a photonic crystal fiber with a certain structure can be used. This fiber is called a photonic bandgap fiber. The name of air-guided optical fiber is not very accurate, because in fact, air is not provided to guide light.
The advantage of hollow fiber is that light propagating mainly in the air will greatly reduce the nonlinear effect, so the damage threshold is higher. In fact, even if the transparency of the glass material to light of a certain wavelength is relatively poor, guided waves can be realized. For example, in Er:YAG lasers, high energy pulses at 2940 nm can be obtained . In addition, the high light intensity in air or other gases can also be used to achieve Raman laser or higher harmonic generation.
One problem with hollow fiber is that its propagation loss is much higher than solid core fiber, especially when single-mode guided waves are required. However, there are ways to suppress losses.
Another problem is that the photonic bandgap guided wave mechanism works in a limited wavelength range. If Kagomé lattice design hollow fiber is used, then the wavelength range can be greatly expanded. The working principle of this design is not completely clear. However, by changing the structure near the core, the propagation loss of the fiber can be greatly reduced.