How to choose single mode fiber and multimode fiber

1. Multimode fiber

1. What is multimode fiber?

When the geometric size of the fiber (mainly the core diameter d1) is much larger than the wavelength of the light wave (about 1 μm), there will be dozens or even hundreds of propagation modes in the fiber. Different propagation modes have different propagation velocities and phases, resulting in time delay and widening of optical pulses after long-distance transmission. This phenomenon is called the modal dispersion of the fiber (also called intermodal dispersion).

Modal dispersion will narrow the bandwidth of multimode fiber and reduce its transmission capacity, so multimode fiber is only suitable for small-capacity fiber communication. In short, the multimode fiber has a large core diameter (62.5mm or 50mm), allows hundreds of modes to transmit, has large dispersion, and operates at a wavelength of 850nm.

2. Transmission distance of multimode fiber

Compared with twisted pair, multimode fiber can support longer transmission distance;

In 10mbps and 100mbps Ethernet, multimode fiber can support up to 2000 meters of transmission distance;

In the 1Gbps Gigabit network, the multimode fiber can support up to 550 meters of transmission distance;

In the 10Gbps 10 Gigabit network, the multimode fiber OM3 can reach 300 meters, and OM4 can reach 500 meters;

Singlemode fiber and multimode fiber

2. Single-mode fiber

1. What is single-mode fiber?

When the geometric size of the optical fiber (mainly the core diameter) can be close to the wavelength of light, such as the core diameter d1 in the range of 5-10 μm, the optical fiber only allows one mode (fundamental mode HE11) to propagate in it, and the remaining high-order modes are all cut off , such a fiber is called a single-mode fiber.

Since it has only one mode of propagation, the problem of modal dispersion is avoided, so the single-mode fiber has a very wide bandwidth, which is especially suitable for large-capacity optical fiber communication. Therefore, in order to achieve single-mode transmission, the parameters of the fiber must meet certain conditions. It is calculated by the formula that for a fiber with NA=0.12 to achieve single-mode transmission above λ=1.3μm, the radius of the fiber core should be ≤4.2μm, that is, its core diameter d1≤8.4μm.

Since the core diameter of the single-mode fiber is very small, more stringent requirements are placed on its manufacturing process.

2. Single-mode fiber transmission distance

The core diameter of the single-mode fiber is 8.3 μm, and the outer diameter of the cladding is 125 μm. The operating wavelengths of the single-mode optical module are 1310 nm and 1550 nm. Compared with the multi-mode fiber, the single-mode fiber can support longer transmission distance. Even 1G Gigabit Ethernet, single-mode fiber can support transmission distances of more than 5000m.

The devices used in the single-mode optical module are twice that of the multi-mode optical module, so the overall cost of the single-mode optical module is higher than that of the multi-mode optical module; the transmission distance of the single-mode optical module can reach 150 to 200km; the multi-mode optical module The transmission distance is only up to 2km.

Here we summarize the commonly used wiring distances:

Multi-mode and single-mode transmission distances under different networks and different wavelengths.

Why is the transmission distance of multimode fiber not as long as that of single mode?

The working wavelengths of the optical fiber are short wavelength 850nm, long wavelength 1310nm and 1550nm. The fiber loss generally decreases with the wavelength, and the loss at 850nm is 2.5dB/km. The multimode fiber operates at this wavelength, and the loss is too large.

The loss of 1310nm is 0.35dB/km, and the loss of 1550nm is 0.20dB/km. The working wavelength of single-mode fiber is also the lowest loss of fiber.

3. The difference between multi-mode fiber optic transceivers and single-mode fiber optic transceivers

the difference

Distance: less than 2KM for multi-mode, more than 100KM for single-mode

Wavelength: Multimode 850/1310NM, Singlemode 1310/1550NM

Multi-mode transceivers correspond to multi-mode fibers, and single-mode and single-mode correspond to each other, and cannot be mixed.

Currently on the market, multi-mode transceivers are cheap, basically about 200 yuan is very good, more than 300 enterprise-level transceivers are enough, and the bandwidth is 100Mbps.

The multi-mode transceiver receives multiple transmission modes, and the transmission distance is relatively close.

A single-mode transceiver only accepts a single mode. The transmission distance is relatively long.

Fourth, the use of optical modules

1. Can single-mode/multi-mode optical fibers be mixed with single-mode/multi-mode optical modules?

Answer: Single-mode/multi-mode optical fiber can be mixed with single-mode/multi-mode optical module. The results are shown in the following table. We can see that they cannot be mixed. The optical fiber and the optical module must be well matched before they can be used normally.

When a single-mode optical module transmits on multi-mode, there will be a lot of packet loss.

2. Can multimode fiber be used with single mode optical module? If not, what is the reason?

Answer: No. Multi-mode fiber is best used with multi-mode optical modules, because multi-mode and single-mode converters must have corresponding wavelengths and optical transceiver functions to achieve photoelectric conversion, so multi-mode fibers can be used with single-mode optical modules. The use cannot be guaranteed. Effect.

3. All the optical modules in our computer room are single-mode optical modules, and the optical fibers are multi-mode. Do all the optical modules in the computer room need to be replaced with multi-mode?

Answer: It is best to replace all optical modules with multi-mode optical modules. Single-mode and multi-mode optical modules cannot be mixed, because the core diameters of single-mode optical fibers and multi-mode optical fibers are very different, which will cause the insertion loss to be too large when the two are matched.

Five, the use of fiber optic cables

How to choose fiber optic cable?

The selection of optical cables is not only based on the number of optical fibers and the type of optical fibers, but also the structure and outer sheath of the optical cable according to the use environment of the optical cable.

1. When the outdoor optical cable is directly buried, loose armored optical cable should be selected. For overhead use, a loose-sheathed fiber optic cable with black PE outer sheath with two or more reinforcing ribs can be used.

2. When selecting the optical cable used in the building, the tight sleeve optical cable should be selected and attention should be paid to its flame retardant, toxic and smoke characteristics. Generally, the flame retardant but smoke-free type (Plenum) or the flammable and non-toxic type (LSZH) can be used in the pipeline or forced ventilation, and the flame-retardant, non-toxic and smoke-free type (Riser) should be used in the exposed environment.

3. When cabling vertically or horizontally in the building, you can choose the tight-sleeve optical cable, distribution optical cable or branch optical cable that is common in the building.

4. Select single-mode and multi-mode optical cables according to network application and optical cable application parameters. Usually indoor and short-distance applications are dominated by multi-mode optical cables, and outdoor and long-distance applications are dominated by single-mode optical cables.

In the connection of optical fiber, how to choose different applications of fixed connection and active connection?

The active connection of optical fibers is realized through optical fiber connectors. An active connection point in an optical link is a clear dividing interface. In the choice of active and fixed connections, the advantages of fixed connections are lower cost, less optical loss, but less flexible, while active connections are the opposite.

During network design, it is necessary to flexibly select the use of active and fixed connections according to the conditions of the entire link to ensure both flexibility and stability, so as to give full play to their respective advantages. The active connection interface is an important interface for testing, maintenance, and change. Compared with the fixed connection, the active connection is relatively easier to find the fault point in the link, which increases the convenience for the replacement of faulty components, thereby improving system maintainability and reducing maintenance costs.

Optical fibers are getting closer and closer to user terminals. What are the meanings of "fiber to the desktop" and what factors need to be paid attention to in system design?

In the application of "fiber to the desktop" in the horizontal subsystem, the relationship with copper cables is complementary and indispensable. Optical fiber has its unique advantages, such as long transmission distance, stable transmission, not affected by electromagnetic interference, high support bandwidth, and no electromagnetic leakage. These characteristics make optical fiber play an irreplaceable role in copper cables in some specific environments:

1. When the information point transmission distance is greater than 100m, if you choose to use copper cable. It is necessary to add repeaters or add network equipment and weak current rooms, thereby increasing costs and hidden troubles, which can be easily solved by using optical fibers.

2. There are a large number of electromagnetic interference sources in specific working environments (such as factories, hospitals, air-conditioning computer rooms, power computer rooms, etc.), and optical fibers can be free from electromagnetic interference and operate stably in these environments.

3. There is no electromagnetic leakage in the optical fiber, and it is very difficult to detect the signal transmitted in the optical fiber. It is a good choice in places with high confidentiality requirements (such as military, R&D, auditing, government and other industries).

4. In environments with high demand for bandwidth, reaching more than 1G, optical fiber is a good choice.

The application of optical fiber is gradually extending from the backbone or computer room to desktop and residential users, which means that more and more users who do not understand the characteristics of optical fiber are beginning to contact the optical fiber system. Therefore, when designing an optical fiber link system and selecting products, the current and future application requirements of the system should be fully considered, and compatible systems and products should be used to facilitate maintenance and management to the greatest extent possible, and to adapt to the ever-changing on-site actual conditions and user installation needs.

Can the FC connector be directly connected with the SC connector?

Yes, it's just a different connection method for two different types of connectors.

If you need to connect them, you must choose a hybrid adapter, use the FC/SC adapter to connect the FC connector and the SC connector on both ends respectively. This method requires that the connectors should be flat ground, and if you absolutely need to connect angled (APC) connectors, the second method of preventing damage must be used.

The second method is to use a hybrid patch cord and two connection adapters. Hybrid patch cords are those with different fiber optic connector types on both ends that will go where you need to connect, allowing you to use a generic adapter in the patch panel to connect to the system, but at the cost of the system attenuation budget. The increment of one connector pair.

The fixed connection of optical fibers includes mechanical optical fiber splicing and thermal fusion splicing. What are the selection principles of mechanical optical fiber splicing and thermal fusion splicing?

Mechanical optical fiber splicing, commonly known as optical fiber cold splicing, refers to a fiber splicing method that does not require a thermal fusion splicer, and uses simple splice tools and mechanical connection technology to achieve permanent connection of single-core or multi-core fibers. In general, mechanical splicing should be used instead of thermal fusion splicing when splicing optical fibers with a small number of cores scattered in multiple locations.

Mechanical fiber splicing technology was often used in engineering practices such as line repairs and small-scale applications in special occasions. In recent years, with the large-scale deployment of fiber-to-the-desk and fiber-to-the-home (FTTH), people realize the significance of mechanical fiber splicing as an important fiber splicing method.

For fiber-to-desktop and fiber-to-the-home applications with a large number of users and scattered locations, when the number of users reaches a certain level, the complexity of construction and construction personnel and fusion splicers cannot meet the time requirements for users to open services. The mechanical optical fiber splicing method provides the most cost-effective optical fiber splicing solution for large-scale deployment of optical fibers due to its simple operation, short personnel training period, and small equipment investment. For example, in high corridors, small spaces, insufficient lighting, inconvenient on-site power access, etc., mechanical optical fiber splicing provides a convenient, practical, fast and high-performance optical fiber splicing method for designers, construction and maintenance personnel.