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Views: 399 Author: Anna Publish Time: 2025-10-15 Origin: Site
1.1 What is Multimode Fiber?
1.2 Transmission Distance of Multimode Fiber
2.1 What is Single-Mode Fiber?
2.2 Single-Mode Fiber Transmission Distance
3. The Difference Between Multimode and Single-Mode Fiber Transceivers
4.1 Can single-mode/multimode fiber be mixed with single-mode/multimode optical modules?
4.2 Can multimode fiber be used with single-mode optical modules? If not, what is the reason?
4.3 Our computer room is equipped with only single-mode optical modules, but the optical fiber is multimode.
When the geometric dimensions of an optical fiber (primarily the core diameter d1) are significantly larger than the wavelength of light (approximately 1 μm), dozens or even hundreds of propagation modes can exist within the fiber. Different propagation modes have different propagation velocities and phases, resulting in time delays and broadening of optical pulses over long transmission distances. This phenomenon is called modal dispersion (also known as intermodal dispersion).
Modal dispersion narrows the bandwidth of multimode fiber, reducing its transmission capacity. Therefore, multimode fiber is only suitable for smaller-capacity optical fiber communications.
In short, multimode fiber has a large core diameter (62.5 mm or 50 mm), allows hundreds of modes to propagate, has high dispersion, and operates at a wavelength of 850 nm.
Compared to twisted-pair cables, multimode fiber supports longer transmission distances.
In 10 Mbps and 100 Mbps Ethernet, multimode fiber can support transmission distances up to 2000 meters.
In 1 Gigabit Ethernet, multimode fiber can support transmission distances up to 550 meters.
In 10 Gigabit Ethernet, OM3 multimode fiber can reach distances up to 300 meters, and OM4 multimode fiber can reach distances up to 500 meters.
When the fiber's geometric dimensions (primarily the core diameter) are close to the wavelength of light, such as when the core diameter d1 is between 5 and 10 μm, the fiber only allows one mode (the fundamental mode HE11) to propagate, while all other higher-order modes are blocked. This type of fiber is called single-mode fiber.
Because it only propagates in one mode, it avoids the problem of modal dispersion. Therefore, single-mode fiber has an extremely wide bandwidth and is particularly suitable for high-capacity fiber-optic communications. Therefore, to achieve single-mode transmission, the various optical fiber parameters must meet certain conditions. According to a calculation formula, for an optical fiber with an NA of 0.12, to achieve single-mode transmission at λ=1.3μm or above, the fiber core radius should be ≤4.2μm, meaning its core diameter d1 should be ≤8.4μm.
Due to the extremely small core diameter of single-mode fiber, more stringent requirements are placed on its manufacturing process.
Single-mode fiber has a core diameter of 8.3μm and an outer cladding diameter of 125μm. Single-mode optical modules operate at wavelengths of 1310nm and 1550nm. Compared to multimode fiber, single-mode fiber supports longer transmission distances. For 100Mbps Ethernet and even 1G Gigabit Ethernet, single-mode fiber can support transmission distances exceeding 5000m.
Single-mode optical modules use twice as many components as multimode modules, so their overall cost is higher. The transmission distance of single-mode optical modules can reach 150 to 200 km, while the transmission distance of multimode optical modules is only 2 km.
Here we summarize commonly used cabling distances:
Multimode and single-mode transmission distances at different wavelengths in different networks.
Why is the transmission distance of multimode fiber not as long as that of single-mode?
Optical fiber operates at wavelengths of 850 nm (short wavelength), 1310 nm (long wavelength), and 1550 nm (long wavelength). Fiber loss generally decreases with increasing wavelength. The loss at 850 nm is 2.5 dB/km, which is excessive for multimode fiber operating at this wavelength.
The loss at 1310 nm is 0.35 dB/km, and at 1550 nm is 0.20 dB/km. This wavelength is the operating wavelength of single-mode fiber and also has the lowest loss.
Price: Multimode is cheaper, single-mode is more expensive
Distance: Multimode is less than 2 km, single-mode can transmit over 100 km
Wavelength: Multimode 850/1310 nm, single-mode 1310/1550 nm
Multimode transceivers correspond to multimode fiber, and single-mode to single-mode; they cannot be mixed.
Currently, multimode transceivers are inexpensive on the market. Those around 200 yuan are quite good, and enterprise-grade ones over 300 yuan are sufficient. Both offer 100 Mbps bandwidth.
Single-mode transceivers, on the other hand, are less readily available and more expensive, costing around 400 yuan each. They offer 1000 Mbps bandwidth, significantly higher than multimode.
Although multimode is being phased out and is less commonly used now, it is still widely used in surveillance applications due to its lower price, primarily within cabling ranges under 500m. For short-distance transmission, we still recommend single-mode, although its performance is inferior to single-mode.
Multimode transceivers receive multiple transmission modes and have shorter transmission distances.
Single-mode transceivers receive only a single mode and have longer transmission distances.
The results of mixing single-mode/multimode fiber and single-mode/multimode optical modules are shown in the table below. We can see that they cannot be mixed. The fiber and optical module must be properly matched for proper operation.
Single-mode optical modules will experience significant packet loss when transmitting over multimode fiber.
No. Multimode fiber is best used with multimode optical modules. This is because the converters between multimode and singlemode must have corresponding wavelengths and optical transceiver capabilities to achieve optical-to-electrical conversion. Therefore, using multimode fiber with singlemode optical modules cannot guarantee effective operation.
It is best to replace all modules with multimode optical modules. Single-mode and multimode fibers cannot be mixed. The core diameters of single-mode and multimode fibers differ significantly, resulting in high insertion loss when the two are matched.
In addition to considering the number of fibers and the type of fiber, the cable structure and outer sheath should also be selected based on the environment in which the cable is used.
For direct burial of outdoor optical cables, loose-tube armored cables are recommended. For overhead installation, loose-tube cables with a black PE outer sheath and two or more reinforcing ribs can be used.
When selecting optical cables for use within buildings, tight-buffered cables should be used, and their flame-retardant, toxic, and smoke-resistant properties should be considered. Generally, flame-retardant but smoke-producing types (Plenum) or flammable, non-toxic types (LSZH) should be used in ducts or forced ventilation areas. Flame-retardant, non-toxic, and smoke-free types (Riser) should be used in exposed environments.
When laying cables vertically or horizontally within a building, tight-buffered cables, distribution cables, or branch cables commonly used within buildings can be used.
Select single-mode or multimode optical cables based on network applications and cable application parameters. Multimode cables are generally used for indoor and short-distance applications, while single-mode cables are preferred for outdoor and long-distance applications.
Flexible fiber connections are achieved through fiber optic connectors. A flexible connection point in an optical link is a clear dividing interface. When choosing between active and fixed connections, fixed connections offer advantages such as lower cost and reduced optical loss, but they also offer lower flexibility, while active connections offer the opposite. Network design requires flexible selection of both active and fixed connections based on the overall link situation, ensuring both flexibility and stability, thereby fully leveraging the advantages of each. Active connection interfaces are crucial for testing, maintenance, and modification. Active connections are easier to locate link faults than fixed connections, making it easier to replace faulty components, thereby improving system maintainability and reducing maintenance costs.
As optical fiber is increasingly connected to user terminals, what are the implications of "fiber to the desktop" and what factors should be considered in system design?
In horizontal subsystem applications, "fiber to the desktop" and copper cables are complementary and indispensable. Optical fiber has unique advantages, such as long transmission distance, stable transmission, immunity to electromagnetic interference, high bandwidth support, and no electromagnetic leakage. These characteristics make optical fiber irreplaceable to copper cables in certain specific environments:
When the transmission distance between information points exceeds 100m, copper cables are used. Fiber optics can easily solve this problem by requiring the addition of repeaters or network equipment and weak-current rooms, increasing costs and potential for failure.
Specific working environments (such as factories, hospitals, air-conditioning rooms, and power equipment rooms) are subject to numerous sources of electromagnetic interference. Fiber optics are immune to electromagnetic interference and operate stably in these environments.
Fiber optics do not experience electromagnetic leakage, making it extremely difficult to detect signals transmitted through them. It is an excellent choice for environments with high confidentiality requirements (such as the military, R&D, auditing, and government).
Fiber optics are an excellent choice for environments with high bandwidth requirements, reaching over 1G.
Fiber optic applications are gradually expanding from backbone networks or computer rooms to desktops and residential users. This means that more and more users who are unfamiliar with fiber optic characteristics are beginning to access fiber optic systems. Therefore, when designing fiber optic link systems and selecting products, full consideration should be given to current and future system requirements. Compatible systems and products should be used to maximize maintenance and management, adapting to ever-changing site conditions and user installation requirements.
