II-type long-period fiber grating for inhibiting high-order resonance and scattering loss

Abstract

The invention provides a II-type long-period fiber bragg grating capable of inhibiting high-order resonance and scattering loss, which comprises an optical fiber, and a long-period fiber bragg grating which is linearly distributed along the axial direction of the optical fiber is inscribed in an area between the dead center of a fiber core of the optical fiber and the junction of a fiber core cladding through a femtosecond laser direct writing technology so as to inhibit the scattering loss; the refractive index modulation of the long-period fiber grating is sine-like along with the change of the length of the grating, so that high-order resonance is avoided, and the refractive index modulation caused by femtosecond laser is negative. According to the invention, scattering loss of a refractive index change area to a fundamental mode caused by femtosecond laser can be effectively reduced, and high-order resonance is effectively avoided.

Description

technical field [0001] The invention relates to the technical field of optical fiber gratings, in particular to a type II long-period optical fiber grating capable of suppressing high-order resonance and scattering loss. Background technique [0002] The long-period fiber grating can realize the coupling of the transmission mode in the same direction. Under the condition of satisfying the phase matching, the core mode can be coupled to the cladding mode through the long-period fiber grating for transmission. It can be used as a filter device in the fiber laser, such as pulling The Mann filter improves the performance of the laser. In addition, the cladding mode is more sensitive to external factors, and the long-period fiber grating also plays an important role in optical fiber sensing. [0003] There are many methods for preparing LPFGs, but all of them have some defects, such as: (1) LPFGs can be realized through a periodic stress field, but LPFGs do not exist without a st...

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Problems solved by technology

[0003] There are many ways to prepare LPFGs, but there are some defects, such as: (1) LPFGs can be realized through a periodic stress field, but in the absence of a stress field, LPFGs do not exist

(2) by CO 2 The laser can engrave a periodic structure on the surface of the fiber cladding to realize a long-period fiber grating, but the long-period fiber grating is usually relatively fragile due to the damage of the cladding structure, and when this grating is used in a high-power laser, the The pump light transmitted by the cladding will thus leak a lot

(3) Ultraviolet exposure method can write high-quality long-period fiber gratings, but the optical fiber needs to be loaded with hydrogen in advance, and requires cumbersome annealing treatment. If the treatment is not done properly, it will also affect the performance of the grating, thereby affecting the performance of the laser.

According to the properties of gratings, gratings written by femtoseconds are mainly divided into two categories. The first category is I-type gratings whose refractive index is modulated to be positive. Grating also degrades

The second type is highly localized type II grating with negative refractive index modulation. This type of grating is very stable. Even at a high temperature of 1000 ° C, the grating is also very stable, but this type of grating has a large insertion loss. For Some special applications, such as high-power fiber laser systems, require the grating to withstand a certain high temperature and require low insertion loss. Therefore, it is necessary to develop a type II grating with low insertion loss

In addition, the femtosecond laser is usually used to prepare long-period fiber gratings by writing line segments in the fiber core. The function of the coupling coefficient changing with the length of the grating is a square wave, and the high-order resonance brought by the harmonic components will also exist. This is also a technical problem that those skilled in the art need to solve

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