Introduction to Single-mode Fiber and Multimode Fiber

Multimode fiber has a core diameter of 5062.5μm and a cladding outer diameter of 125μm. Single-mode fiber has a core diameter of 8.3μm and a cladding outer diameter of 125μm. The operating wavelengths of optical fiber include short wavelength 0.85μm, long wavelengths 1.31μm and 1.55μm. Fiber loss generally decreases with increasing wavelength, with a loss of 2.5dB/km at 0.85μm, 0.35dB/km at 1.31μm, and 0.20dB/km at 1.55μm, which is the lowest loss of the fiber. Loss tends to increase at wavelengths above 1.65μm. Due to the absorption effect of OHˉ, there are loss peaks in the ranges of 0.901.30μm and 1.34~1.52μm, and these two ranges cannot be fully utilized. Since the 1980s, there has been a tendency to favor single-mode fiber, initially using the long wavelength of 1.31μm.

Multimode Fiber

Multimode Fiber: The central glass core is relatively thick (50 or 62.5μm), capable of transmitting multiple modes of light. However, its intermodal dispersion is relatively large, which limits the frequency of transmitted digital signals, and this becomes more severe with increasing distance. For example, a fiber rated at 600MB/KM will only have a bandwidth of 300MB at 2KM. Therefore, the transmission distance of multimode fiber is relatively short, generally only a few kilometers.

Single-mode Fiber

Single-mode Fiber: The central glass core is very thin (core diameter generally 9 or 10μm), capable of transmitting only one mode of light. Consequently, its intermodal dispersion is very small, making it suitable for long-distance communication. However, material dispersion and waveguide dispersion still exist, which imposes higher requirements on the spectral width and stability of the light source for single-mode fiber, namely narrow spectral width and good stability. It was later discovered that at the 1.31μm wavelength, the material dispersion and waveguide dispersion of single-mode fiber are one positive and one negative, with magnitudes exactly equal. This means that at the 1.31μm wavelength, the total dispersion of single-mode fiber is zero. In terms of fiber loss characteristics, 1.31μm happens to be a low-loss window of the fiber. Thus, the 1.31μm wavelength region has become a very ideal operating window for optical fiber communication, and it is also the main operating band of current practical optical fiber communication systems. The main parameters of conventional 1.31μm single-mode fiber are defined by the International Telecommunication Union ITU-T in Recommendation G652, hence this fiber is also called G652 fiber.

Single-mode fiber has a very small core diameter (8-10μm). The optical signal is transmitted as a single light ray at a single resolvable angle to the fiber axis, propagating in only a single mode, which avoids modal dispersion, resulting in wide transmission bandwidth, large transmission capacity, low optical signal loss, and low dispersion, making it suitable for large-capacity, long-distance communication.
Multimode fiber transmits optical signals as multiple light rays at multiple resolvable angles to the fiber axis, propagating in multiple modes simultaneously. Its diameter ranges from 50-200μm, and its transmission performance is inferior to that of single-mode fiber. It can be divided into multimode step-index fiber and multimode graded-index fiber. The former has a larger core, more transmission modes, narrow bandwidth, and small transmission capacity.