The dielectric materials with different dielectric constants are arranged in a certain periodic order. Due to the cyclical nature, the dispersion curve of the light waves propagating in the medium will form a band structure, and there may be a possibility that a band similar to a semiconductor band gap "Photon bandgap", light frequency in the band will be strictly prohibited in the ban. The periodic dielectric structure with photonic bandgas is usually a "photonic crystal". This was the concept proposed by John and Yablonovitch in 1987 to discuss the effects of periodic dielectric structures on the propagation of light. According to this feature, photonic crystals can effectively suppress spontaneous emission. If a certain degree of defect is introduced in the photonic crystal, the photon that coincides with the defect frequency may be confined to the defective position, and once the deviation is significant, the light is rapidly attenuated. The use of defects can be tied to a specific location in the light, light can not spread from any direction. Photonic Crystal Fiber (PCF) is a new type of fiber with this characteristic, composed of two-dimensional photonic crystals with lattice constant as the order of wavelengths, also known as microstructured optical fiber.
Photonic crystal fiber lasers have many excellent performance compared to conventional fiber lasers, such as non-cut-off single-mode transmission characteristics, adjustable dispersion characteristics, high nonlinearity and high birefringence characteristics. Photonic crystal fiber has high non-linear characteristics, such as self-phase modulation, cross-phase modulation, four-wave mixing, stimulated raman scattering, soliton self-frequency shift and supercontinuum generation. The non-linearity factor can be increased by reducing the effective core diameter and using a material with a high non-linear coefficient. Photonic crystal fiber can easily realize the large-scale long-area, inner-clad large numerical aperture, high rare earth doping and even multi-core design, effectively reduce the fiber laser high-power operation of the nonlinear effects and thermal effects, and to maintain good beam quality , Is difficult to achieve conventional fiber.
Denmark PhotonicCrystal Fibre is committed to the development of high-power photonic crystal fiber lasers, 2007, prepared double-clad ytterbium doped photonic crystal fiber, single fiber continuous output power has reached 2.5kw; United States Nufern company, prepared MM-EYDF- 12/130 double-clad erbium-ytterbium co-doped photonic crystal fiber with a working wavelength of 1550nm and a large core design for the development of high power pulsed fiber lasers and pulse amplifiers such as laser radars. Domestic photonic crystal fiber lasers in the field of research started late, domestic photonic crystal fiber high power laser reported is relatively small.