Why Is SMF More Efficient Over Long Distances Than MMF?

Single-mode fiber is more efficient than multimode fiber over long distances because it significantly reduces modal dispersion and typically provides lower attenuation. Its small core allows light to travel primarily in a single propagation mode, keeping the optical signal more concentrated as it travels. This makes SMF suitable for high-speed, long-distance communication networks.

How Does Fiber Optic Transmission Work?

Information is transmitted in the form of light pulses through the glass or plastic core of the cable. The core is covered with cladding having lower refractive index, which enables the light to stay inside the fiber due to total internal reflection.

The main distinction between the single-mode fiber and multimode fiber is the number of propagation modes of light which can transmit in the core of the fiber.

Single-mode fiber usually has the core diameter of 8-10 micrometers. Due to the small core, it is possible to transmit light only in a single propagation mode.

The core of multimode fiber is usually 50 or 62.5 micrometers wide. Thus multiple light modes can propagate simultaneously in this kind of fiber.

These differences seem trivial, but they play the key role with increasing the transmission distance.

Why Is SMF Better for Long-Distance Transmission?

1. SMF Has Much Less Modal Dispersion

Modal dispersion is the main physical cause of the superiority of SMF on long distance.

In MMF different light modes can propagate in different ways. Some modes use rather direct path, while others travel longer way through the core of the fiber.

Despite that light pulses are generated simultaneously, their traveling via various routes results in different time of arrival.

With increasing of the distance, the difference in the arrival time becomes considerable. As a result, light pulse gets spread.

At last, it can result in overlapping of neighboring pulses making it harder for the receiver to distinguish them. It leads to reducing the bandwidth and the distance at which the MMF can transmit the signal.

SMF does not have this issue. Due to the small core size and thus presence of only the fundamental propagation mode during the ordinary operation, there is no multiple modal paths and corresponding modal delay.

This is the main reason for preferring SMF for long distance communication.

2. SMF Supports Higher Bandwidth on Distance

The absence of modal dispersion in the SMF makes it capable of keeping the signal on the distance significantly longer.

This feature becomes extremely important with the increasing of data rates of network. A fiber that is good for short-distance 10 Gbps connection can perform worse on long distance.

SMF is widely used in:

1. Telecommunications
2. Fiber-to-the-home networks
3. Metro area networks
4. Telecom backhaul
5. Data center interconnection
6. Backbone networks
7. Submarine communication systems

SMF also supports such technologies as wavelength division multiplexing, when several optical wavelengths transmit through the single fiber. In this way, it becomes possible to provide enormous capacity without using any new physical route.

3. SMF Can Have Lower Attenuation

Attenuation is the reduction of the optical power due to passing light through the fiber.

SMF usually operates at 1310 nm and 1550 nm wavelengths. The fiber, which operates on the wavelength of 1550 nm, can have particularly low attenuation that is crucial for long distance communication.

Low attenuation means that the optical signal can pass farther before being amplified or regenerated.

MMF usually operates at 850 nm wavelength in the short reach applications. The practical distance is limited not only by the attenuation, but also by the modal dispersion and bandwidth characteristics of the fiber and transceiver.

Thus, SMF has an important advantage, if the network should transfer lots of information on several kilometers distance or more.

SMF vs MMF

This comparison does not mean that SMF is better all the time. Both fibers are created for different needs of the networks.

Advantages and Disadvantages of SMF

The main advantages of single-mode fiber are:

1. Excellent performance on long distance
2. Very low modal dispersion
3. High potential of the bandwidth
4. Low attenuation on the common telecom wavelengths
5. Good support of the high speed optical communication
6. Compatibility with the WDM and other high capacity technologies

There are some disadvantages too.

The small core of the fiber requires more precise alignment of the fiber and optical components. The optical transceivers and other network equipment can become more expensive in some applications.

For the long distance telecom network, however, the higher performance can become the decisive factor.

Advantages and Disadvantages of MMF

MMF has its own strengths.

Its larger core makes coupling light into the fiber easier. This can simplify installation and provide greater tolerance for alignment errors. MMF systems can also be cost-effective for short-distance networks, particularly when relatively inexpensive short-reach optical transceivers are used.

The main disadvantage is modal dispersion. Several propagation modes transmit through the fiber at different speed and along the different paths. The increasing of the distance results in increasing of the pulse spreading.

Thus, MMF is suitable mainly for the short distance communication.

When Should You Choose SMF or MMF?

The transmission distance should become one of the first criteria for the selection.

The SMF is usually appropriate for:

• FTTH and FTTx networks
• Operator networks
• Connections between the buildings
• Metro networks
• Long distance backbone links
• High capacity data center interconnection
• 5G backhaul networks
• Industrial long distance communication

The MMF is used for:

• Enterprise LAN
• Connections inside the building
• Short distance data center links
• Connections from server to switch and elsewhere

In practice, I would not recommend choosing fiber only according to the cable price. The whole system should be considered, including transceivers, connectors, patch cords, installation requirements, transmission distance, data rate and future network expansion.

The Key Difference Is Modal Dispersion

The difference in the performance of the SMF on the long distance is not only in the small core of the fiber.

The small core affects the way of light propagation through the fiber.

The MMF lets multiple modes to propagate in the fiber. They can travel along the different paths and arrive with the different time, causing the modal dispersion and limiting the bandwidth with the increase of the distance.

The SMF restricts the propagation to the single primary mode. It greatly reduces the modal dispersion and makes it possible for optical signals to travel farther without losing the quality.

That is why the SMF has become the standard choice for long distance optical communication, while the MMF is still appropriate for many short distance networks.

For the network designer and installer, the correct choice of the fiber type from the very beginning can also help to avoid unnecessary equipment updates later. Huijue Network provides various FTTx products, including fiber optic cables, patch cords, pigtails and PLC splitters for different network architectures and requirements.