In the process of rapid development of the network, optical fiber is widely used in the field of fiber optic communication due to its advantages such as fast transmission speed and large transmission capacity. According to the different optical transmission modes, optical fibers are divided into single-mode fibers and multi-mode fibers. Due to the significant differences in geometric and transmission characteristics of these two types of fibers, they also have significant differences in practical applications. Next, this article will provide a detailed introduction to the differences and usage of single-mode fiber and multimode fiber, guiding you in the correct selection and use of single-mode fiber.

1、 Basic structural differences between single-mode fiber and multimode fiber

Fiber optic generally consists of an outer sheath, cladding, fiber core, and light source structure. As shown in the following text, single-mode fiber and multimode fiber have the following differences in terms of outer sheath, core diameter, and light source:

Color difference of outer sheath

The color of the outer sheath of the optical fiber can quickly distinguish between single-mode and multimode fibers. According to the TIA-598C standard definition, the outer sheath color of OS1/OS2 single-mode fiber is yellow, while the outer sheath color of multimode fiber is orange or water green. Among them, OM1/OM2 multimode fiber uses orange outer sheath, OM3/OM4 multimode fiber uses water green outer sheath, and OM5 multimode fiber uses green outer sheath.

Figure 1: Comparison of Outer Sheath Colors between Single Mode Fiber and Multimode Fiber

Difference in diameter of fiber cores

There is a significant difference in core diameter between single-mode fiber and multimode fiber.

The core diameter of single-mode fiber (SMF) is generally 9 μ m. Due to its narrow core, it can only transmit one mode of light at 1310nm, 1550nm, and WDM wavelengths. As a result, single-mode fiber has small dispersion and high bandwidth, while multi-mode fiber (MMF) has a core diameter of 50/62.5 μ m and a core width. It can transmit multiple modes of light at a given operating wavelength (850nm or 1310nm wavelength). However, due to the hundreds of modes transmitted in multi-mode fiber, the transmission constants and group rates of each mode are different, resulting in narrow bandwidth, high dispersion, and high loss in multi-mode fiber.

Figure 2: Comparison of core diameters between single-mode and multimode fibers

Note: The standard cladding diameter for most optical fibers is 125 μ m, and the standard outer protective layer diameter is 245 μ m, without distinguishing between single and multiple modes.

Light source differences

Single mode fibers and multimode fibers use different light sources. Single mode fibers generally use laser light sources, while multimode fibers generally use LED light sources.

Figure 3: Comparison of Light Sources Used in Single Mode Fiber and Multimode Fiber

2、 The difference in transmission distance between single-mode fiber and multimode fiber

As is well known, single-mode optical fibers are commonly used for long-distance transmission, while multi-mode optical fibers are generally used for short distance transmission. However, different types of single-mode and multi-mode fiber optic applications have different transmission distances in Ethernet at different speeds, as shown in the table below:

Table 1: Comparison of Transmission Distance between Single Mode Fiber and Multimode Fiber

From the table, it can be seen that at 1G and 10G speeds, single-mode fiber has a much longer transmission distance than multimode fiber. So why not all data centers use single-mode fiber? This is because the construction of data centers mainly focuses on short distance transmission. In short distance transmission applications, the performance of multimode fiber is the same as that of single-mode fiber, but the cost is lower. Moreover, OM3/OM4 multimode fiber can support higher speeds. In today's era of high-speed networks, the demand for multimode fiber is also not to be underestimated.

3、 The cost difference between single-mode fiber and multimode fiber

The cost of single-mode fiber is higher than that of multimode fiber, mainly due to differences in equipment hardware, light sources, and material costs.

Similarly, the construction cost of multimode fiber optic systems is lower than that of single-mode fiber optic systems. Taking the HX Fiber solution as an example, the cost of a multi-mode transmission system (with a combination of multi-mode optical modules and fiber jumpers) ranges from 3300 yuan to 5300 yuan, while the cost of a single-mode transmission system (with a combination of single-mode optical modules and jumpers) usually exceeds 6700 yuan, with a price difference of over 1000 yuan between the two.

4、 Common Q&A on single-mode fiber and multimode fiber

① Can single-mode fiber and multimode fiber be mixed for use?

Due to the different transmission modes of single-mode fiber and multi-mode fiber, the two fibers cannot be directly connected together, otherwise it is easy to cause link loss and line jitter. But we can use conversion devices such as single mode to multi mode conversion jumpers (i.e. mode adjustment jumpers) to connect single mode and multi-mode links.

② Can multi-mode optical modules be used on single-mode fibers?

Multi mode optical modules cannot be used on single-mode optical fibers, which not only results in significant losses, but may also prevent communication. Due to the small diameter of single-mode optical fibers and the large divergence angle of multi-mode optical modules, the further the transmission distance, the greater the attenuation of the signal. It is possible that the signal has already decayed before reaching the transmission distance.

③ Can single-mode optical modules be used on multimode fibers?

Single mode optical modules can be used on multimode fibers, but fiber transceivers need to be used to convert fiber types. For example, by using fiber transceivers, the 1000BASE-LX single-mode optical module can operate on multimode fibers, as shown in the following figure. Fiber optic transceivers can also be used to solve the connection problem between single-mode and multi-mode optical modules.

④ How to choose between single-mode fiber and multimode fiber?

When choosing single-mode fiber and multimode fiber, we should consider the actual transmission distance and cost of the network. If the transmission distance does not exceed 550 meters, multi-mode optical fiber is sufficient. If the transmission distance reaches several thousand meters or more, single-mode optical fiber is the ideal choice.

Epilogue

As can be seen from the comparison above, the application fields of single-mode fiber and multimode fiber are different. Among them, single-mode fiber optic is mainly used in metropolitan area networks and PONs, while multi-mode fiber optic is mainly used in enterprise networks and data centers. The selection should be based on actual wiring needs.