Large mode field fiber is defined as a fiber whose core diameter is larger than that of pure single-mode fiber and only one or several spatial propagation modes are allowed.

Large mode field optical fiber technology approach

Taking a comprehensive view of the research situation of large mode field fiber laser, it can be concluded that there are three technical approaches to achieve high beam quality laser output of large mode field fiber: fiber structure design, mode selection control and mode conversion method.

Single mode output fiber based on fiber structure design

This kind of large mode field fiber realizes direct single-mode output mainly by changing the refractive index distribution of the core or cladding. The usual method is to increase the fiber core diameter and reduce the fiber core numerical aperture to meet the normalized frequency requirements of single-mode transmission. However, due to the limitation of the optical fiber's vapor deposition processing technology, the numerical aperture of ordinary step optical fiber is difficult to be less than 0.05, and the corresponding single-mode optical fiber core diameter is 17 μ M (corresponding to wavelength 1.1 μ m)。 In order to obtain single-mode fiber with larger core diameter, some people propose to change the refractive index distribution of the fiber core to flat mode, gradual change, composite and other distributions, or set periodic structure, leakage structure and other refractive index distribution in the cladding, so that the fiber can achieve the basic mode output equivalent through leakage or coupling; It has also been proposed to use the elastic optical effect of residual thermal stress to form an extremely low refractive index difference to achieve the fundamental mode output of large diameter during the fiber processing.

Multimode fiber based on mode control and selection

Although direct single-mode output can be achieved through the structural design of the fiber, the numerical aperture of large mode field fiber in this case is usually low and the bending resistance is poor. For example, the previous 100 μ The m-rod photonic crystal fiber, with numerical aperture NA=0.03, can only be used when it is kept straight. A slight bending will bring fatal damage to it, which is very unfavorable to the application of actual fiber lasers. Moreover, due to the limitation of materials and current fiber doping process, the minimum numerical aperture of ordinary doped silica fiber core is about 0.05. If a larger core diameter is required, multimode output is necessary, which is not easy to meet in many applications. In order to achieve the output of fundamental mode, mode control and selection methods are usually used to suppress the output of higher-order modes as much as possible. The commonly used control methods are: bending mode selection, light cone mode selection, gain control mode selection, cavity mirror mode selection, etc.

Mode conversion method

S. The mode conversion mechanism proposed by Ramachandran et al. uses long-period fiber gratings and high-order mode fibers (HOF) to realize the conversion between high-order modes and low order modes, so that the light field mainly exists in the form of high-order modes with large mode field area, and large mode field area (up to 2100 μ M ^ 2 or even 3200 μ M ^ 2) to avoid the influence of nonlinear effects.