All-Fiber Bandpass Spectral Filter Based on Negative Curvature Hollow-Core Optical Fiber (NC-HCF)

Nowadays high-power fiber lasers with ultrashort pulse duration are widely used in different fields of science and technology. Spectral filters are used for spectral filtration in master oscillators, after amplifiers and to remove Raman spectral part from the output signal in ultrashort pulse lasers. The process of modern fiber filter fabrication is time-consumable and involves quite expensive technologies. A new costless and easy method to process hollow-core anti-resonant fibers into fiber filters is presented in [1]–[2]. It shown that it is possible to control the position and width of transmission bands by heating and tapering of HCFs. Such a method allows to fabrication of a wide range of spectral filters using only one piece of HCF. However, the spectral filters presented in [1] could not be used in fiber lasers, because of the big core diameter of the used HCF.