Will the beam quality deteriorate after the laser passes through the optical fiber?

When the light beam passes through the optical fiber, the quality of the light beam may be better or worse. To understand this conclusion, some basic understanding of optical fiber theory is required. It is recommended to find a special book to read, here are some synopses. For optical fiber, it is usually described by the theory of "guided wave optics". In this case, guided wave equations and coupled mode equations are used to simulate; but in the case of large core diameter and high numerical aperture, the theory of "light optics" can also be used Description, at this time, you can use the "ray tracing" model to simulate (zemax, the basic function of tracepro software). An optical fiber is an optical waveguide that can accommodate a series of guided wave modes, and can only accommodate a limited number of modes. Here are some examples (3 guided modes)

Different guide molds have different spot sizes and divergence angles, so they have different beam qualities. Guided modes can be arranged in order according to their effective refractive index. Generally, the first guided mode is most like a Gaussian beam and therefore has the best beam quality. The beam quality of other higher-order guided modes is generally getting worse. Any light beam injected into the fiber is mathematically equivalent to decomposing into each guided mode and a series of radiation modes. In the end, the radiation mode disappears and only the guided mode remains.

Therefore, if the injected fiber is a single-mode fiber (only one guided mode can be accommodated), the output beam quality will be excellent, theoretically close to a Gaussian beam, but because of the disappearance of the radiation mode, power will be lost. If the injected fiber is a super-multimode fiber, even if it is a Gaussian beam injection, as long as it is not well matched, many guided modes will be excited, resulting in poor output beam quality. It is estimated that the simulated fiber is a super-multimode fiber (the core diameter is much larger than the wavelength, or the numerical aperture is large), and there should be no clear relationship between the input and output beam quality.

The possibility of causing poor beam quality is:

1. Fiber bending will produce dispersion. If the fiber is bent excessively, it will not only excite higher-order modes in the fiber, but also cause the failure of the total internal reflection condition and cause mode leakage.

Dispersion: This is caused by many other reasons such as uneven refractive index of the fiber. Laser is usually a band, although it is very narrow, it still scatters. .

Attenuation: All media will attenuate the power of light, which can be understood as the medium absorbs light.

2. The uneven refractive index distribution of different parts of the optical fiber caused by the manufacturing process of the optical fiber will cause Rayleigh scattering.

3. Too high input light energy will cause nonlinear effects.