Single mode fiber and multimode fiber can be easily discriminated from the size of the core. The core of a single-mode fiber is small, about 4 to 10 um, and only transmits the main mode. This completely avoids modal dispersion, making the transmission band wide and the transmission capacity large. This fiber is suitable for high-capacity, long-distance fiber-optic communications. It is an inevitable trend in the future development of optical fiber communication and lightwave technology.

Multimode fibers are further classified into multimode abrupt fibers and multimode graded fibers. The former has a larger core diameter and more transmission modes, so the bandwidth is narrower and the transmission capacity is smaller. The refractive index of the latter core decreases with increasing radius, and a relatively small modal dispersion can be obtained, so that the frequency band is smaller. Wide, transmission capacity is large, the latter is generally applied.

Since the transmission speeds of different modes of light in a multimode fiber are different, the transmission distance of the multimode fiber is short. Single-mode fiber can be used in non-relay optical communication.

In the theory of optical fiber communication, the optical fiber has single mode and multimode, and the difference lies in:

1. Single-mode fiber has a small core diameter (about 10m), allowing only one mode of transmission, low dispersion, and working at long wavelengths (1310nm and 1550nm), which is relatively difficult to couple with optical devices.

2. The multimode fiber has a large core diameter (62.5m or 50m), allowing transmission in hundreds of modes, large dispersion, and operating at 850nm or 1310nm. Coupling with optical devices is relatively easy.

For the optical module, strictly speaking, there is no single mode or multiple mode. The so-called single-mode, multi-mode module refers to the optical device used by the optical module to match the fiber to obtain the best transmission characteristics.

Generally there are the following differences:

1. Single-mode modules generally use LD or narrow-spectrum LEDs as the light source. The size of the coupling components is good with single-mode fiber, and can be transmitted over long distances when transmitting with single-mode fiber.

2. Multimode modules generally use lower cost LEDs as the light source, and the coupling components are sized to fit the multimode fiber.