What are Multi-Mode (mm) Fibers?
Multi-mode (mm) fibers have large optical cores that can carry multiple modes, or paths, of light. Their main applications include telecom and audio/video links. Some specialty optical fibers are also available as mm, e.g., for medical and laser beam delivery tasks.
Large Core and High Numerical Aperture (NA)
Multi-mode fibers have a larger core diameter than single-mode fibers, which allows for multiple modes of light to propagate through the fiber at the same time. This larger core diameter also means that multi-mode fibers are capable of transmitting light over longer distances than single-mode fibers, making them ideal for use in local area networks (LANs) and other data/telecom applications where longer transmission distances are required.
The most common type of multi-mode fiber is 62.5/125 micron fiber, which has a core diameter of 62.5 microns and a cladding diameter of 125 microns. This fiber is used in a variety of applications, including Ethernet networks, fiber-to-the-home (FTTH) systems, and other short-haul communication networks. In recent years, advances in multi-mode fiber technology have enabled the development of higher-capacity multi-mode fibers that can support greater bandwidths and hence higher data transfer speeds. For example, the development of 50/125 micron fiber, which has a smaller core diameter and higher bandwidth than traditional 62.5/125 micron fiber, has made it possible to transmit data over longer distances and support higher data transfer rates.
In addition to its large core diameter, multi-mode fiber is also characterized by its high numerical aperture (NA), which is a measure of the ability of the fiber to capture and guide light. A high NA means that multi-mode fiber is able to accept light from a wider range of angles, making it more flexible and versatile than single-mode fiber.
In short-haul communications applications, multi-mode fibers can deliver larger bandwidths and hence higher data transfer rates.
Some Different Applications
Multi-mode fibers are also commonly used in industrial and medical communications applications due to their high tolerance for harsh environments and their ability to transmit large amounts of data over short distances. For example, multi-mode fibers can be used in high-temperature and high-radiation environments, such as in nuclear power plants or in high-energy particle accelerator systems, where traditional communication systems may not be able to function effectively.
Furthermore, multi-mode fibers are also used in the aerospace and defense industries, where they provide a secure and reliable means of transmitting sensitive information and data. For example, multi-mode fibers can be used in military communication networks, where they are used to transmit secure voice and data communications.
In the medical field, multi-mode medical fibers are found in a variety of applications, including endoscopy and laparoscopy procedures, where they are used to transmit light and images from the inside of the body to the outside. They are also employed in medical imaging devices, such as computed tomography (CT) scanners, where they are used to transmit data from the imaging sensors to the processing units.
Larger Cores Handle Higher Power
Power-handling is an important characteristic of multi-mode fibers, as it determines the amount of optical power that can be transmitted through the fiber without causing damage to the fiber or degrading the signal quality. The power-handling capability of a multi-mode fiber is determined by several factors, including the core diameter, numerical aperture (NA), and the properties of the material used to make the fiber.
In general, multi-mode fibers have higher power-handling capabilities than single-mode fibers due to their larger core diameter and higher NA. The larger core diameter of multi-mode fibers allows for the distribution of the optical power over a larger area, reducing the amount of power that is concentrated in any one area and reducing the risk of damage to the fiber. The higher NA also means that multi-mode fibers are able to capture and guide more light, allowing them to transmit higher levels of optical power without damaging the fiber.
Multi-Mode Specialty Optical Fibers
In addition to passive fibers used to transmit information, there are many other types of optical fibers, collectively referred to as Specialty Optical Fibers. These fibers support a wide variety of applications, including laser system components, laser beam delivery, material processing, surgery, spectroscopy, LiDAR, metrology, and more. Many of these are produced in both multi-mode and single-mode formats. They include both passive and active fibers, where active fibers are fibers doped with rare earth metal ions which generate and/or amplify light.
For high power applications such as cutting and welding metal for example, specialty optical fibers are sometimes used to deliver the power from a fiber laser to the cutting/welding head. These laser beam delivery fibers are designed to handle up to several kilowatts of power without premature darkening or other power-related degradation. Coherent NuMKW fibers are standout examples of this class of fibers.
Some Practical Caveats
Despite its many advantages, multi-mode fiber also has some limitations that must be considered when selecting it for a particular communications application. For example, multi-mode fiber is subject to modal dispersion, which occurs when different modes of light travel at different speeds through the fiber and cause the signal to spread out in time and become distorted. This can result in significant signal loss and degradation, particularly over longer distances.
It is also important to note that the power-handling capability of a multi-mode fiber can be affected by the presence of bends or other forms of mechanical stress in the fiber. Bends and other forms of mechanical stress can cause microbend loss, which can result in significant signal loss and degradation. To minimize the effects of microbend loss, multi-mode fibers are typically installed in protective tubes or other forms of protective covering to prevent damage to the fiber and maintain their power-handling capabilities.
Summary
Multi-mode fiber is a popular choice for a variety of applications due to its cost-effectiveness, versatility, ease of use, and ability to handle higher power. In many cases, the use of multi-mode fiber can provide an excellent balance of performance, cost, and reliability, making it an ideal choice for a wide range of applications.