850nm is multimode. In fiber optic communications, there are single mode and multi-mode optical fibers. Multimode optical fibers have a larger core diameter, allowing multiple modes of light to be transmitted through them, so more light signals can be transmitted. However, due to the different transmission paths of the light rays, dispersion and attenuation occur, limiting the transmission distance. 

Single mode fiber has a very small core diameter, allowing only one mode of light to travel through it. Because of the single light transmission path, no dispersion and attenuation phenomenon will occur, so it can be transmitted over longer distances, but the manufacturing cost is higher.

The 850nm wavelength region is usually used for multimode fiber optic communications, 1550nm is single mode, and 1310nm is available in both single mode and multimode. Referring to ITU-T, the attenuation of 1310nm is recommended to be ≤0.4dB/km, and the attenuation of 1550nm is ≤0.3dB/km, while the loss of 850nm is 2.5dB/km. Fiber optic loss generally decreases as the wavelength lengthens around the C-band (1525-1565nm), the 1550 nm central wavelength is usually used for multimode communication, but is more expensive to manufacture. The central wavelength of 1550 nm is often referred to as the zero-loss window, which means that the quartz fiber has minimal attenuation at that wavelength.

Light is defined by its wavelength, and in fiber optic communications, the light used is light in the infrared region, where the wavelength of light is greater than that of visible light. Typical wavelengths in fiber optic communications are 800 to 1600 nm, with the most commonly used wavelengths being 850 nm, 1310 nm, and 1550 nm.

In selecting the transmission wavelength, the main combined consideration is fiber loss and scattering. The goal is to transmit the most data by transmitting to the farthest distance with the least fiber loss. The loss of signal strength in transmission is attenuation. The degree of attenuation is related to the length of the waveform; the longer the waveform, the less the attenuation. The light used in fiber optics has longer wavelengths at 850, 1310, and 1550 nm, so the attenuation of the fiber is less, which results in less fiber loss. These three wavelengths also have almost zero absorption, making them the most suitable for use as a usable light source for transmission in optical fibers.