What Is a Multi-Mode Optical Fiber?

Fibers that transmit multiple modes at a given operating wavelength. According to its refractive index distribution, it is divided into abrupt type and gradual type. Ordinary multimode fiber has a numerical aperture of 0.2 ± 0.02, a core diameter / outer diameter of 50 m / 125 nu, and its transmission parameters are bandwidth and loss. As there are hundreds of transmission modes in multimode fiber, the propagation constants and group rates of each mode are different, which makes the fiber's bandwidth narrow, dispersion is large, and loss is also large. It is only suitable for short and medium distance optical fiber communication systems. .

Multimode fiber

Fibers that transmit multiple modes at a given operating wavelength. According to its refractive index distribution, it is divided into abrupt type and gradual type. Ordinary multimode fiber has a numerical aperture of 0.2 ± 0.02, a core diameter / outer diameter of 50 m / 125 nu, and its transmission parameters are bandwidth and loss. As there are hundreds of transmission modes in multimode fiber, the propagation constants and group rates of each mode are different, which makes the fiber's bandwidth narrow, dispersion is large, and loss is also large. It is only suitable for short and medium distance optical fiber communication systems. .

Multimode

There are basically two types of multimode

Relative to

The core wire diameter of a multimode fiber is 62.5m / 125m. Or 50m / 125m. Specifications (number of cores) are 2, 4, 6, 8, 12, 16, 20, 24, 36, 48, 60, 72, 84, 96 cores and so on. The outer sheath material of the cable is common type; common flame retardant; low smoke and halogen-free type; low smoke and halogen-free flame retardant type.

Market share in the 1990s

In the 1990s, multimode optical fibers have always occupied a stable share in the world's optical fiber market.

The new generation of multimode fiber is a 50 / 125m multimode fiber with a graded refractive index profile. The 50 m core diameter is used because the number of transmission modes in this fiber is about 1 / 2.5 of the transmission mode in 62.5 m multimode fiber. This can effectively reduce the modal dispersion of multimode fibers and increase the bandwidth. For 850nm wavelength, 50 / 125m can triple the bandwidth of 62.5 / 125m multimode fiber. According to the IEEE802.3z standard, at a rate of 1 Gbit / s, a 62.5 m core diameter multimode fiber can only transmit 270 meters; and a 50 m core diameter multimode fiber can transmit 550 meters. In fact, recent experiments have confirmed that using a 850nm vertical cavity surface emitting laser (VCSEL) as the light source, at a rate of 1Gbit / s, a standard multimode fiber with a core diameter of 50 m can transmit 1750 meters without error codes (the line contains 5 pairs of connectors) The new generation multi-mode optical fiber with a core diameter of 50 m can transmit 2,000 meters without error codes (the line contains 2 pairs of connectors).

Another reason for using a 50 m core diameter is that the advantages of a 62.5 m core diameter multimode fiber were previously valued by the people, which has become irrelevant with the advancement of technology. In the early and mid-1980s, LED light sources had low output power, large divergence angles, and large connector losses. The use of fibers with large core diameters and numerical apertures to maximize optical power injection must be considered. At that time, no one seemed to think that the LAN speed might exceed 100Mbit / s, that is, the bandwidth performance of multimode fiber is not outstanding, especially with VCSEL, optical power injection is no longer a problem. Core diameter and numerical aperture are no longer as important as before, and the transmission rate of 10 Gbit / s has become a major contradiction. 50 m core diameter multimode fibers that can provide higher bandwidth are much favored.

Past

According to the comparison between the laser and the arc tube described above, the multi-mode fiber uses a laser as a light source, and its transmission bandwidth should be greatly improved. However, preliminary experimental results show that simply replacing the LED as a light source with a laser will not only reduce the bandwidth of the system but also decrease it. Research by the IEEE expert group found that the bandwidth of a multimode fiber is also related to the mode power distribution or injection state in the fiber. In the manufacturing process of the preform, the axis of the optical fiber is prone to produce a refractive index depression. In the past, LED was used as the light source. It was over-filled launch (OFL-OverFilled Launch). All modes (hundreds) of optical fibers were excited, and each mode carried its own power. The distortion of the refractive index of the optical fiber center only affects the delay characteristics of a few modes, and the influence on the optical fiber mode bandwidth is relatively limited. The measured multimode fiber bandwidth is correct for systems using LEDs as light sources. In other words, the transmission rate and distance of the system can be estimated by using the measured bandwidth data. However, when a laser is used as the light source, the laser spot is only a few microns, and the divergence angle is smaller than that of the LED. Therefore, it only excites a few modes transmitting in the center of the fiber, each mode carries a considerable part of the power, and the refractive index of the center of the fiber is distorted The impact on the delay characteristics of these only and few modes has significantly reduced the bandwidth of multimode fibers. Therefore, the traditional overfill injection (OFL) method cannot be used to measure the bandwidth of a multimode fiber using a laser as a light source.

The new standard will use the restricted mode injection method (RMLRestricted Mode Launch) to measure the bandwidth of a new generation of multimode fiber. The bandwidth measured by this method is called "laser bandwidth" or "mode-limited bandwidth". The bandwidth previously measured using LEDs as light sources is called "overfill injection bandwidth". The two indicate the multimode fiber bandwidth when the laser and LED are used as the light source injection. Standards for limited-mode injection and multimode fiber laser bandwidth were drafted by the TIA FO-2.2.1 task force. The content is as follows:

FOTP-203 specifies the power distribution of a light source used to measure the bandwidth of a multimode fiber laser. It is required that after the light source is coupled through a short multimode fiber, its near-field intensity distribution should meet a luminous flux greater than 75% in the center 30 m and a luminous flux greater than 25% in the center 9 m. The new standard does not recommend using a VCSEL as a light source to measure the bandwidth. This is because the optical power distribution of VCSELs from different manufacturers is very different.

FOTP-204 specifies the use of mode-limited fiber to couple the light source to multi-mode fiber for laser bandwidth measurement. The mode-limited fiber is used to filter the overfill state and limit the excitation of the high-order mode of the multimode fiber. The mode-limited fiber is a graded-index multimode fiber with a core diameter of 23.5 m and a numerical aperture of 0.208. The refractive index gradient index of this multimode fiber is close to two. It should have a bandwidth greater than 700MHz.km under the conditions of 850nm and 1300nm over-full injection. The length of the mode-limited fiber should be greater than 1.5 meters to eliminate leakage mode and less than 5 meters to avoid transient loss. The core diameter of 23.5 m was chosen because the injection state produced by it is closest to VCSEL.

1.Single-mode transmission distance

2.Multi-mode transmission bandwidth is large

3. No dispersion occurs in single mode, and the quality is reliable

4. Single mode usually uses laser as light source, which is expensive, while multimode usually uses cheap LED

5.The price of single mode is relatively high

6, multi-mode is cheap, close-range transmission can