Definition.
There are fibers with more than one guided wave pattern in each polarization direction.
Multimode fiber refers to an optical fiber that can support multiple transverse guided wave modes for a given optical frequency and polarization. The number of guide modes depends on the wavelength and refractive index distribution. For stepped index fibers, the relevant quantities are the core radius and the numerical aperture, which together determine the v-value. If the v-value is large, the number of patterns is proportional to v2. In particular, when the fiber has a relatively large core (right side of Figure 1), the number of supported modes is very large. Such fibers can propagate light with very poor beam quality (e.g., from high-power diode arrays), but in order to maintain the beam quality of a higher brightness light source, it is preferable to use fibers with a small core and medium-sized numerical aperture, although it is difficult to achieve effective coupling into the fiber.
Figure 1 Singlemode fiber (left) has a much smaller core than cladding, while multimode fiber (right) has a large core.
Compared to standard single-mode fibers, multimode fibers have a large core area and a larger numerical aperture, such as 02-0.3。The latter enables a stable guided wave, even in the presence of large bending, but at the same time with greater propagation losses even in the absence of bending, since irregularities in the core-cladding cross-section cause scattering of light. The refractive index distribution is usually rectangular and sometimes parabolic (shown below).
The basic performance indicators of multimode fibers include the core diameter and the outer layer diameter of the multimode fiber. The common types in fiber optic communication are 50 125 m and 625 125 m fibers, i.e. core diameters of 50 m and 62 m, respectively5 m with a cladding diameter of 125 m. This fiber can support hundreds of guide modes. There are also large-core fibers with a core diameter of several hundred microns.
It is relatively easy to inject light into a multimode fiber because there is a greater tolerance for the position and angle of incidence of the incident light when it enters than on a singlemode fiber. However, the spatial coherence of the optical fiber output light is reduced, and it is difficult to control the outgoing field distribution.
Figure 2 is a plot of the electric field distribution of the step index fiber guide mode, and the field distribution at a certain wavelength is only calculated. The fundamental mode (LP01) is close to the Gaussian intensity distribution, and other higher-order modes have more complex spatial distributions. Each mode has a different value. Any mode field distribution can be seen as the superposition of some guide modes.
The total electric field distribution at any one location of the multimode fiber comes 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, so that destructive or constructive interference of different modes occurs at specific points on the fiber. Both power and phase are determined by the condition of the light as it enters the fiber, and the relative phase (and therefore the interference condition) is constantly changing due to the propagation constant as it is mode-dependent. As a result, the intensity distribution changes over time and changes significantly over propagation lengths of less than 1 mm. In addition, the relative phase changes when the incident conditions, the degree of bending, the fiber is stretched, the wavelength or temperature is changed, and so on.
It is important to note that for a light with a wide bandwidth (e.g., for a white light source), such a complex distribution will not be produced when its intensity is detected without interfering with the individual components of the spectral spectrum. Because the intensity distribution of each wavelength component is different, the contributions of the different wavelengths cancel each other out. The longer the fiber, the smaller the average spectral bandwidth required.
Figure 2 Electric field amplitude distribution for all guided wave modes in an optical fiber. Typically, the light entering the multimode fiber excites the superposition of different modes.
Beam quality requirements for coupling opticals into multimode fibers.
It is easier to incorporate light into a multimode fiber than on a singlemode fiber, especially if the multimode fiber supports many guide modes. In order to achieve effective coupling, the following two conditions need to be met:
1. The incident light can only be incident on the core, not the cladding.
2. The angle between the direction of incident light propagation and the axis of the optical fiber 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 at the same time. For ultra-Gaussian beams, the maximum m2 factor that effectively delivers the beam into the multimode fiber can be estimated according to the following formula:
If both the beam shape and angular distribution (i.e., the distribution in Fourier space) satisfy the effective incidence conditions, the above equation holds. For Gaussian beams, the m2 factor is smaller. For more details, please refer to the literature [3].
Multimode fibers are used to transmit lasers.
Multimode optical fibers are used to transmit the light in the laser light source to the place where it is needed, especially when the beam quality of the light source is poor or the fiber core area is relatively large in the case of high power. For example, light is transmitted from various steady-state high-power lasers to a material processing table for cutting or welding, where a robot can move the laser head at the fiber optic cable port. Fiber-coupled high-power diode arrays and diode stacks are also multimode fibers because the beam quality is far from the diffraction limit. Fiber coupling is very useful because it separates the pump diode from the cooling unit at the laser head of the diode-pumped solid-state laser. However, fiber-coupled laser diodes are expensive and can cause significant loss of brightness depending on the beam shaper.
In these applications, the number of guide modes is usually not greater than the number required to effectively enter the fiber, otherwise the power of the laser radiation will be distributed over the unwanted mode, and therefore the beam quality and brightness will be reduced.
In practice, simple step-refractive index fibers are often used. Its numerical aperture is fixed at some standard value, such as 022, and the core diameter is usually selected based on the beam quality of the light source. Common values for core diameters are 50, 100, 200, 400, 600 and 800 m.
Multimode fiber optics are used for optical communications.
Figure 3 Graded refractive index fiber with a parabolic refractive index distribution. The most common defect is the presence of a refractive index depression in the middle, which is often encountered in general manufacturing methods.
For short-distance fiber optic communications, multimode fibers are often chosen over single-mode fibers because a simple light source can be effectively coupled into the multimode fiber (e.g., light-emitting diodes, LEDs) and the alignment requirements are not very high (e.g., in fiber connectors). However, the data transmission rate and transmission distance of this fiber are limited, mainly because of the phenomenon of inter-mode dispersion: the group speed is related to the propagation mode, so the propagation of ultrashort pulses in multimode fibers is divided into several pulses with different propagation speeds, which may eliminate the transmitted signal. This effect can be greatly reduced by using a parabolic refractive index multimode fiber (graded refractive index fiber, Figure 3), which results in a larger bandwidth distance product. However, there are still some inevitable flaws. Some ISO standards (e.g., OM1, OM2, and OM3) quantify the level of inter-mode dispersion, as well as the limitation on the transmission bandwidth (or bandwidth distance product). The best performance can be obtained with OM350 125-M optimized fibers, which have a very precisely controlled refractive index distribution. The data emitter typically contains an 850 nm VCSEL.
For long-distance data transmission, single-mode fiber is more suitable because there is no inter-mode dispersion, but the system cost is much higher.
The International Telecommunication Union (ITU) has developed various standards for different kinds of fiber optics for fiber optic communications. The standard for multimode fiber is:
Active multimode fibers.
Some high-power fiber amplifiers use multimode fiber because of their larger mode area. By entering the elementary mode with the incident signal light and minimizing the inter-mode mixing, an output close to the diffraction limit can be obtained.
Materials and manufacturing methods.
Many materials can be used to make multimode fibers. The most common multimode glass fiber is a quartz fiber, in which a pure quartz core is surrounded by a doped substance that reduces the refractive index (e.g., fluorine). Alternatively, the core can be doped, such as germanium, to increase its refractive index. Especially for large core diameter fibers, the plasma external deposition (POD) method can effectively fabricate fluorine-doped reduced refractive index cladding fibers around a pure quartz core.
There are other glass materials, such as fluoride and sulfide glasses that can conduct longer wavelengths of light, as well as polymers (polymer fibers, POFs). These materials require corresponding manufacturing techniques.
Photonic crystal fibers (PCFs) can also be employed, which can be made of different glasses and can contain air cladding to achieve very high numerical apertures.