Multi-mode and single-mode of optical modules

The single-mode and multi-mode in the optical module actually only refer to the type of optical fiber. LC/PC refers to the type of connector, single-mode, multi-mode will have a standard on the module, single-mode SM, single-mode is used for long-distance, and the fiber is yellow. Multimode MM, multimode is used for short distance, the fiber is orange.

Generally, the wavelength window used for multimode is 850nm, while the wavelength window for single mode is mainly around the 1310nm and 1550nm windows. Due to the serious dispersion between modes, multi-mode is only used for short-range, that is, SR. The single-mode tube modules are mostly used for long-distance transmission such as LR, ER/ZR, etc.

Multimode optical fibers are mostly used in networks with relatively low transmission rates and relatively short transmission distances, such as local area networks. Such networks usually have many nodes, many joints, many detours, and the amount of connectors and couplers used is large. The unit The fiber length uses a large number of light sources, and the use of multimode fiber can effectively reduce network costs. Single-mode optical fiber is mostly used in lines with long transmission distance and relatively high transmission rate, such as long-distance trunk line transmission, metropolitan area network construction, etc.

There are several ways to classify optical fibers, according to the transmission mode of light in the optical fiber: single-mode optical fiber and multi-mode optical fiber.

The core diameter of the multimode fiber is 50~62.5 μm, the outer diameter of the cladding is 125 μm, the core diameter of the single mode fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The working wavelength of the optical fiber is short wavelength 0.85μm, long wavelength 1.31μm and 1.55μm. Optical fiber loss generally decreases as the wavelength increases. The loss of 0.85μm is 2.5dB/km, the loss of 1.31μm is 0.35dB/km, and the loss of 1.55μm is 0.20dB/km. This is the lowest loss of optical fiber. The wavelength is 1.65 Losses above μm tend to increase. Due to the absorption of OHˉ, there are loss peaks in the ranges of 0.90~1.30μm and 1.34~1.52μm, and these two ranges have not been fully utilized. Since the 1980s, single-mode fiber tends to be used more, and the long wavelength 1.31 μm is used first.

multimode fiber

Multi Mode Fiber (Multi Mode Fiber): The central glass core is relatively thick (50 or 62.5 μm), which can transmit multiple modes of light. However, its intermodal dispersion is relatively large, which limits the frequency of digital signal transmission, and it will be more serious with the increase of distance. For example: 600MB/KM optical fiber has only 300MB bandwidth at 2KM. Therefore, the transmission distance of multimode fiber is relatively short, generally only a few kilometers.

single mode fiber

Single Mode Fiber (Single Mode Fiber): The central glass core is very thin (core diameter is generally 9 or 10 μm), and only one mode of light can be transmitted. Therefore, its intermodal dispersion is very small, suitable for long-distance communication, but there are still material dispersion and waveguide dispersion, so single-mode fiber has higher requirements on the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability better. Later, it was found that at the wavelength of 1.31 μm, the material dispersion and the waveguide dispersion of the single-mode fiber are positive and negative, and their sizes are exactly equal. This means that at a wavelength of 1.31 μm, the total dispersion of the single-mode fiber is zero. From the point of view of the loss characteristics of the optical fiber, 1.31 μm is just a low loss window of the optical fiber. In this way, the 1.31μm wavelength region has become an ideal working window for optical fiber communication, and it is also the main working band of practical optical fiber communication systems. The main parameters of 1.31μm conventional single-mode fiber are determined by the International Telecommunication Union ITU-T in the G652 recommendation, so this fiber is also called G652 fiber.

In single-mode fiber, intra-mode dispersion is the main limiting factor for bit rate. Because it is relatively stable, if necessary, the dispersion can be compensated by adding a certain length of "dispersion compensating single-mode fiber". Zero dispersion compensation fiber is to use a section of fiber with a large negative dispersion coefficient to compensate the fiber with higher dispersion at 1550nm. The dispersion of the optical fiber near 1550nm is small or zero, so that the optical fiber has a higher transmission rate at 1550nm.

In single-mode fibers, another dispersion phenomenon is polarization mode dispersion (PMD), which cannot be compensated for because it is unstable.

multimode fiber

In multimode fiber, modal dispersion and intramodal dispersion are the main factors affecting bandwidth. The PCVD process can well control the refractive index distribution curve and give an excellent refractive index distribution curve. For graded multimode fiber (GIMM), it can limit the mode dispersion and obtain high mode bandwidth.