THE IMPORTANCE OF THE MODE FIELD DIAMETER OF OPTICAL FIBERS

As we explored in previous articles about the basics of optical fibers and their specifications, optical fibers are defined by many features related to both their physical composition and the optical parameters of the light guide. At the end of the day, their main application is simple: to transport light through their core.

A particularly important factor to consider when optical fibers are used in a system is the dimension of the distinct intensity pattern of the transported light beam. This can have a significant impact on power limits, losses between components and even long-term reliability. In single-mode or few-mode fibers, the Mode Field Diameter (MFD) is a parameter often used to describe this intensity profile. Let us look at a simplified overview of what the Mode Field Diameter is.

What is the Mode Field Diameter?

When light propagates in a single-mode fiber, its intensity profile is like a Gaussian curve, with a bell shape.

A single-mode fiber can only guide in fundamental mode, provided the light’s wavelength is higher than what is known as the cut-off wavelength. Using a simple physical uniform core profile model (step-index), the intensity pattern of the light will be defined by the core size, the numerical aperture, and the light propagation wavelength. The Mode Field Diameter (MFD) is used to characterize the size of this intensity profile and can be represented by the diameter of the intensity at an e^-2 level (around 13.5%) for fundamental mode.

From the image above, we notice that the tails of the intensity pattern are “guided” outside the core area, meaning the power is not strictly contained within the core. We will see that depending on the numerical aperture and operating wavelength, the difference between the MFD and the core size can go from negligible to very significant for a fiber of a given core size.

A quick word on multimode fibers, since what we described was for a single-mode fiber: When the operating wavelength is below the cut-off wavelength of the fiber design, the guiding properties enable more modes (2, 4, 5 and higher) to be guided by the core; these are called Higher Order Modes or HOMs. The MFD definition here is only valid for fundamental mode. However, when controlled carefully in the system design, the light will be guided mostly in that fundamental mode. In that case the MFD will still be a representative parameter.

Going further, in highly multimode fibers there are so many possible propagation modes that no significant amount of power is contained within the fundamental mode. In that case, MFD is generally not a useful parameter, and the numerical aperture and other optical parameters of beam quality are used in addition to the core size to understand the light intensity profile.

The impact of the MFD on splice loss

The consideration of the MFD, and not only the physical dimensions of their cores, is particularly important to minimize fiber splice loss between two different fibers. A fiber splice is the connection of two fibers, achieved by quickly fusing them together using heat. The goal is to have their glass structure fused together solidly by minimizing the impact on the core and fiber shape, and by maximizing the energy transmission from one fiber to the other. For example, standard automated machines use an electrode arc to achieve this process.

Our intuition when creating a splice between two different fibers could be to consider only the core physical diameters, thinking that the splice will only be correct if they are the same. As we explained earlier, the MFD, representing the actual size of the transported light in fundamental mode, can be different to the core size due to the NA (for the same core, a lower NA means a higher MFD). To optimize splice loss, it is the MFD that should be as close as possible between the two fibers.

That can lead to situations where the splice is not optimal, even if the two fibers look like they should match together and have the exact same core size. Therefore, to confirm if two different fibers have a good MFD match, we need to consider the proper combination of core size and numerical aperture. The following figure shows a situation in which two fibers with the same core size will lead to splice loss because of the NA difference.

As we can see, for fundamental mode propagation, it is extremely important to consider the MFD when connecting two different fibers in an optical fiber device or system. While this model is best applied in single-mode fibers, where the MFD mismatch will correctly estimate the expected splice loss, it is also particularly useful in few-mode fibers where most of the power is in fundamental mode.

Understanding all parameters when buying an optical fiber

The impact of the MFD when estimating splice loss is a very direct example of when it is important to consider the right specification to compare two fibers, as other fiber parameters can be misleading.

While we will not go into details here and may explore those subjects in future articles, there are other situations in which the MFD specification is important to consider. For example, the combination of the MFD, numerical aperture (NA) and the operating wavelength will give insight into the bend loss of a fiber, and on the power limitation of the fiber.

When evaluating splice loss between two different Coractive fibers or other aspects related to the MFD, remember that our experts are happy to help!

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