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Photonic crystal fiber (PCF) is an optical fiber based on the concept of photonic crystal proposed in 1987 and put into practice in 1995. Photonic crystal fiber is a new kind of optical fiber. Its structure and light guiding mechanism are different from those of ordinary optical fiber, presenting many characteristics that are difficult to be realized in traditional optical fiber, and therefore have received extensive attention. The principle of optical waveguide transmission in the optical fiber core diameter in the photonic crystal fiber is: total reflection and photonic band gap (PBG).

In the case where the core diameter of the optical fiber is hollow, a PBG optical fiber similar to the semiconductor energy gap is generated according to the arrangement of the periodic structure of the cladding. In the case of a solid core diameter, insertion of a high refractive index core rod in the cladding generates anti-resonance reflections. This fiber is also known as a porous, microstructured fiber.

As an example of the application of total reflection, holes are left in the cladding of the conventional step-type silica optical fiber to become a holey fiber. The voids provided on the cladding will change the refractive index of the cladding. Light waves will be totally reflected on the core diameter of the optical fiber and on the boundary of the cladding. Light waves can only be transmitted in the core diameter of the optical fiber. This is also the principle of the traditional single-mode fiber, which has the good advantage of reducing bending loss and controllable zero-dispersion wavelength. At present, the holey fiber has achieved a level of loss close to that of a conventional fiber and has begun to provide commercial use.

Based on the above principle, it can be said that this is a photonic band gap fiber (PBF). Having a hollow core diameter and cladding refractive index periodic structure can be distinguished by providing an air-hole optical fiber PBF on the cladding, and an optical fiber in which a high- and low-refractive-index layer symmetric to the axis is periodically arranged on the cladding ( Columnar fiber). If a high-refractive-index solid core rod is inserted in the cladding, light waves can propagate in the core diameter of the optical fiber, which is called all-solid-state optical fiber or optical polarization mode fiber.

In the case where the core diameter of the optical fiber is hollow, if light can be effectively confined to the core diameter, low loss and low dispersion can be realized, and the influence of nonlinearity can be reduced, so that high-power optical communication can be transmitted. Increasing the transmission optical power is crucial for improving the transmission distance.

A photonic crystal fiber has a structure in which pores are present in quartz glass (cladding). With the change of the pore array structure and the size of the pores of the PCF, the characteristics different from those of the ordinary optical fiber will be exhibited, that is, manifested in various aspects such as high nonlinearity, dispersion controllability, polarization wave holding, and single mode work. . If the PCF optical fiber light transmission principle is classified, it can be divided into refractive index waveguide type PCF fiber and photonic bandgap fiber.

The refractive index waveguide type PCF fiber usually has a fiber core diameter of glass and voids in the cladding. In the conventional fiber, the refractive index of the fiber is lower than that of the core of the fiber by adding a compound in the cladding portion, and the light wave is limited to propagate in a core diameter with a higher light refractive index. The actual refractive index of the PCF fiber due to the void in the cladding portion is caused. The refractive index is lower than the core diameter of the fiber, so that total reflection of light is achieved.

A PBF fiber is a closed optical fiber formed by a photonic bandgap structure. The core diameter is hollow, two-dimensional holes are arranged in the cladding, and the reflected light is confined within the core diameter of the fiber to propagate. The fiber structure requires strict periodicity, and the size of the hole must be uniform. A photonic crystal fiber like this, by making the actual refractive index difference between the cladding and the core diameter different, it is more advantageous than the ordinary optical fiber, and can be set freely, even in the short wavelength range, can also constitute a single mode fiber, at the same time Large core single mode fiber can also be realized. Further, the waveguides in the optical fiber are dispersed over a wide range to realize various kinds of optical fibers such as zero-dispersion in a short wavelength band and polarization-mode-maintaining optical fibers having a non-circular core diameter. This will greatly increase the fiber transmission path.

At present, countries such as the United States and Japan have photonic crystal fiber optic cables, which will allow fiber optic cable communication to enter a new phase.