What is Single Mode and Multimode Fiber
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This tutorial introduces step-index and graded-index glass fiber. The V parameter and how to determine multimode and single mode fiber. It also shows common graded-index multimode fibers on the market today.
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What is Single Mode and Multimode Fiber
- 1. What is a Multimode Fiber?
- 3. Light Guiding in Step-Index Fiber Total Internal Reflection Critical Angle
- 4. Modes in Step-Index Fiber V > 2.405 ---- Multimode Fiber V < 2.405 ---- Single Mode Fiber
- 8. More Technical Details on Fiber Optics For Sale web site http://www.fiberoptics4sale.com/wordpress/basic-optics-for-optical-fiber/
Editor's Notes
- Today we will explain how optical fibers guide light, and what is exactly a multimode fiber.
- First let’s look at a glass fiber’s structure. It is composed of three cylindrical parts. The center is called fiber core, the next layer is called cladding, and the outer most layer is called buffer coating. The buffer coating does not affect fiber’s light guiding property, it’s there for mechanical protection. So we will focus only on fiber core and cladding. Let’s look at the refractive index profile of the fiber. The x-axis shows fiber radius, the y-axis shows refractive index. The core usually has a refractive index of 1.48, the cladding has a refractive index of 1.46. So the core has a slightly higher refractive index and step down to lower refractive index at the cladding. This is why it is called step-index fiber.
- When a light ray passes from a higher refractive index material to a lower refractive index material, light refraction happens. The light was bent farther away from the normal line. Normal line is the vertical line perpendicular to the interface. When the light ray incidents at the interface at an angle larger than a specific angle, the light ray is totally reflected back into the higher refractive index material, no light can go into the lower refractive index material. This is called Total Internal Reflection. Step-index fiber uses Total Internal Reflection to confine the light within the core. So they will not escape from the core. The light ray experiences multiple total internal reflections and is guided along the fiber. As shown in the right side picture.
- We have just talked about the light guiding principle of a step-index glass fiber. Now let’s talk about the fiber modes in a step-index fiber. What is a mode? A mode in a fiber basically means a particular light ray travelling in the fiber at a particular angle. The light ray must meet some requirement in order to be able to be confined within the core and travel down the fiber. Here is the V-number graph for a step-index fiber. I am only showing the conclusion here without any mathematical steps. The lines in the graph are different modes. V number is determined by the construction of the fiber, such as its core radius, the light wavelength, the core refractive index, the cladding refractive index, and so on. From the graph, we can see, when V number is less than 2.405, only one fundamental mode can exist, this is called single mode fiber. When V number is larger than 2.405, multiple modes can exist, this is called multimode fiber. The larger the V number, the more modes can propagate within the fiber. V number can be changed by changing the fiber’s design, such as a larger core size, different core and cladding refractive index numbers, etc.
- Here let’s look at multimode and single mode fibers from the perspective of light rays travelling down the fiber. Multimode fibers typically have larger core size, such as 62.5 micron and 50 micron, so light rays can enter the fiber at difference angles, all larger than the critical angle to meet total internal reflection requirement. The top figure here shows different modes traveling at different angles. Single mode fiber has a core so tiny, typically 8um, that only a single light ray can travel at the center of the core. The right side figure shows different modes’ transverse power distribution pattern at the far end of the fiber. The fundamental mode is the only mode that can propagate within a single mode fiber. However, in a multimode fiber many modes can propagate and the end result is typically an overlap of all modes.
- However, step-index profile multimode fibers have their own problems. Typically, because of modal dispersion, they can not carry signals at higher data rates. So graded-index multimode fibers were invented. Instead of a sharp step down from the core refractive index to the cladding as in step-index fiber, in graded-index fiber, fiber core has the highest refractive index at the center, and then refractive index gradually decreases until it is the same as the cladding at the edge of the core. In graded-index multimode fibers, the light ray is not total internal reflected, but rather it is bent gradually back to the fiber core by refraction. This is caused by the gradual decreased refractive index profile of the fiber. Graded-index MM fibers can compensate for modal dispersion and so they have much higher bandwidth than step-index MM fibers. Nowadays most modern multimode fibers are made as graded-index fibers.
- There are typically three graded-index multimode fibers on the market now, the 62.5um core, the 50um core, and the 50um core laser optimized fiber. The 62.5um core fiber used to be the most common mm fiber. It is called OM1 fiber and has the lowest bandwidth among all three. It can carry Gigabit Ethernet up to 300 meters with 850nm VCSEL laser. The 50um core was developed to replace 62.5um and has higher bandwidth. It is called OM2 fiber. OM2 fiber can carry Gigabit Ethernet up to 600 meters with 850nm VCSEL laser. The newest graded-index mm fiber is the 50um Laser Optimized MM fiber. It is called OM3 fiber and has the highest bandwidth. This fiber is optimized to work with 10 Gigabit Ethernet and 850nm VCSEL laser. It can carry Gigabit Ethernet up to 1100 meters, and 10 Gigabit Ethernet up to 550 meters. For new deployments, you should choose either 50um OM2 fiber, or 50um Laser Optimized OM3 fiber. The cost difference is minimum, but OM3 fibers offer much higher bandwidth for future expansion.
- We have one much more detailed tutorial about single mode and multimode fibers online. That tutorial discusses all the in and out of step-index and graded-index fibers. Visit it and you will be glad that you did.