Multimode Fiber

What is multimode fiber?

English name of multimode fiber: multimode fibers

Definition: An optical fiber with more than one guided wave mode in each polarization direction.

Multimode fiber refers to the fiber that can support multiple transverse guided wave modes under a given optical frequency and polarization. The number of guided modes depends on the wavelength and refractive index distribution. For step index fibers, the relevant quantities are core radius and numerical aperture, which together determine the value of V. If the V value is relatively large, the number of modes is proportional to V2. Especially when the optical fiber has a relatively large core (on the right side of Figure 1), the number of supported modes will be very large. This kind of optical fiber can propagate light with poor beam quality (for example, light from high-power diode array), but in order to maintain the beam quality of light source with high brightness, it is better to use optical fiber with small core and medium numerical aperture, although it is difficult to achieve effective coupling into optical fiber in this case.

Figure 1: Single mode fiber (left) has a much smaller core than the cladding, while multimode fiber (right) has a large core

Compared with standard single-mode fiber, multimode fiber has a large core area and a larger numerical aperture, such as 0.2-0.3. The latter can achieve stable guided waves. Even in the case of large bending, it also has greater propagation loss even in the case of no bending, because the irregularity at the core cladding section will cause light scattering. The refractive index profile is usually rectangular (see step index fiber), sometimes parabolic.

The basic performance indexes of multimode fiber include the core diameter and the outer sheath diameter of multimode fiber. Common types in optical fiber communication are 50/125 μ M and 62.5/125 μ M optical fiber, i.e. the core diameter is 50 μ M and 62.5 μ m. Cladding diameter is 125 μ m。 The fiber can support hundreds of guided modes. There are also large core diameter optical fibers, whose core diameter is several hundred microns.

It is relatively easy to inject light into a multimode fiber, because compared with a single-mode fiber, there is a greater tolerance for the position and incidence angle of the incoming light. However, the spatial coherence of optical fiber output light is reduced, and it is difficult to control the output field distribution.

The total electric field distribution at any position of multimode fiber is from the superposition of different modes. The intensity distribution is not only related to the power of all modes, but also to their relative phases. Therefore, different modes of cancellation or interference will occur at a specific position of the fiber. Both power and phase are determined by the situation when the light enters the fiber, and the relative phase (and therefore the interference condition) is constantly changing due to the mode dependence of the propagation constant. As a result, the strength distribution varies over time and varies significantly over a propagation length of less than 1 mm. In addition, as long as the incident conditions, bending degree or stretching of the fiber, wavelength or temperature are changed, the relative phase will change.

It should be noted that for a light with a very wide bandwidth (for example, for a white light source), if its intensity is detected without interfering with each component of the detector spectrum, such a complex distribution will not occur. Because the intensity distribution corresponding to each wavelength component is different, the contributions of different wavelengths will offset each other. The longer the fiber, the smaller the average spectral bandwidth required.

Compared with single-mode fiber, it is easier to incident light into multimode fiber, especially when multimode fiber supports many guided modes. In order to obtain effective coupling, the following two conditions need to be met:

The incident light can only enter the core, not the cladding.

The angle between the incident light propagation direction and the fiber axis cannot be greater than the arcsine value of NA.

If the M2 factor of the incident light is small enough, the above two conditions can be met simultaneously. For super Gaussian beams, the maximum M2 factor for effectively projecting the beam into the multimode fiber can be estimated according to the following formula:

If the beam shape and angular distribution (that is, the distribution in Fourier space) meet the effective incident conditions, the above formula can be established. For Gaussian beams, the M2 factor is smaller.

Multimode fiber for laser transmission

The multimode fiber is used to transmit the light in the laser light source to the place where it needs to be used, especially when the beam quality of the light source is poor or the area of the fiber core of the fiber is relatively large in the case of high power. For example, the light from various stable high-power lasers is transmitted to the material processing workbench for cutting or welding, where the robot can move the laser head of the fiber optic cable port. And the high-power diode array and diode stack coupled with optical fiber also use multimode fiber, because its beam quality is far from reaching the diffraction limit. Fiber coupling is very useful because it separates the pump diode from the cooling device at the laser head of the diode pumped solid-state laser. However, fiber coupled laser diodes are more expensive, and may cause serious brightness loss depending on the use of different beam shapers.

In these applications, the number of guided modes is usually not greater than the number required for effective injection into the fiber, otherwise the power radiated by the laser will be distributed to the unnecessary modes, so the beam quality and brightness will be reduced.

In fact, a simple step index fiber is usually used. Its numerical aperture is fixed to some standard values, such as 0.22, and the core diameter is usually selected according to the beam quality of the light source. Common values of core diameter are 50100200400600 and 800 μ m。

Materials and manufacturing methods

Many materials can be used to make multimode optical fibers. The most common multimode glass fiber is the quartz fiber, in which the pure quartz fiber core is surrounded by substances (such as fluorine) doped to reduce the refractive index. Alternatively, the fiber core can be doped, such as germanium, to improve its refractive index. Especially for large core diameter fibers, the plasma external deposition (POD) method can effectively produce fluorine doped fibers with low refractive index cladding around the pure quartz core.

There are other glass materials, such as fluoride and sulfide glass, which can conduct longer wavelengths of light, and polymers (polymer optical fiber, POF). These materials need corresponding manufacturing technology.

Photonic crystal fiber (PCF) can also be used, which can be made of different glasses, and can contain air cladding to achieve very high numerical aperture.

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