What is the difference between singlemode and multimode of optical fiber?

There are many classifications of optical fibers; The commonly said single-mode optical fiber and multi-mode optical fiber are classified according to the transmission mode of the optical fiber;

From a physical point of view, the comparison has the following differences:

The core diameter of multimode fiber is 50 m and the outer diameter of the cladding is 125 m, and the core diameter of single-mode fiber is 125 m. The working wavelengths of optical fibers are short wavelengths, long wavelengths, and more. The loss of optical fiber generally decreases with the lengthening of the wavelength, and the loss is , the loss is , and the loss is , which is the lowest loss of the optical fiber, and the loss above the wavelength tends to increase.

Due to the absorption of OH and the fact that there are loss peaks in the range, both ranges are underutilized. Since the 80s, there has been a tendency to use single-mode fibers, and long wavelengths have been used first.

Multi-mode fiber Multi-mode fiber: The center glass core is thicker (50 or more, which can transmit multiple modes of light. However, the intermodal dispersion is large, which limits the frequency at which the digital signal can be transmitted, and it becomes more severe with distance.

For example, a 600 km fiber will only have 300 MB of bandwidth at 2 km. Therefore, the distance of multimode fiber transmission is relatively close, generally only a few kilometers.

Single-mode fiber Single mode fiber: The center glass core is very thin (generally 9 or 10 m in diameter) and can only transmit one mode of light. Therefore, its inter-mode dispersion is very small, which is suitable for long-distance communication, but there are also material dispersion and waveguide dispersion, so that the single-mode fiber has higher requirements for the spectral width and stability of the light source, that is, the spectral width should be narrow and the stability should be good.

Later, it was found that at the wavelength, the material dispersion and waveguide dispersion of the single-mode fiber were positive and negative, and the magnitude was exactly the same. This means that at wavelengths, the total dispersion of a single-mode fiber is zero. From the point of view of the loss characteristics of the optical fiber, it happens to be a low-loss window of the optical fiber.

In this way, the wavelength region has become a very ideal working window for optical fiber communication, and it is also the main working band of the practical optical fiber communication system. The main parameters of 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.

Differences in practical applications:

Most of the multi-mode fibers are in the 850nm range, and the transmission distance is relatively short, because the attenuation is very strong; Single-mode fiber, on the other hand, can travel long distances.

Only the base mode is transmitting in the single-mode fiber, so roughly speaking, the mode field diameter is the diameter of the fundamental mode spot on the receiving end face of the single-mode fiber, and it can also be roughly considered that the mode field diameter d is similar to the core diameter of the single-mode fiber.

When the normalized frequency v of the optical fiber is less than its normalized cut-off frequency vc, single-mode transmission can be realized, that is, only the fundamental mode is transmitted in the optical fiber, and the rest of the higher modes are all cut off. That is to say, in addition to the parameters of the fiber, such as the core radius, the numerical aperture must meet certain conditions, and in order to achieve single-mode transmission, the wavelength of the light wave must be greater than a certain value, that is, c, which is called the cut-off wavelength of the single-mode fiber.

Single-mode optical fiber has low loss and long transmission distance, up to 120 kilometers;

Multi-mode optical fiber has large loss and short transmission distance, up to 2 kilometers.

The main difference is the core diameter.

The core diameter of a single mold is generally 7 10um

Multimode is 50 or.

1. Definition of single film and multiple film of light:

<> 2. The difference between the two:

1. The fiber core of single-mode optical fiber is correspondingly thinner, the transmission frequency bandwidth, the capacity is large, and the transmission distance is long, but the laser source is required, and the cost is high, and it is usually used in the environment between buildings or in a geographically dispersed environment;

2. The core of multi-mode optical fiber is thick, the transmission rate is low, the distance is short, and the overall transmission performance is poor, but the cost is low, and it is generally used in the environment of buildings or geographical proximity.