What is the Diameter of Fiber Optic Cable?

fiber size diameter

What is the Diameter of Fiber Optic Cable?
Fiber optic is generally divided into three layers: core, cladding and coating. The standard cladding diameter for most optical fibers is 125um, and the standard outer protective layer diameter is 245um. The core diameter of multimode fiber is generally 50um or 62.5um, and the core diameter of standard single-mode fiber is 9um. The diameters of the core and cladding play an important role in the specifications of the fiber optic cable. For example, we usually call a multimode fiber with a core diameter of 50um and a cladding diameter of 125um a 50/125 fiber.

fiber diameter

What is the Principle behind Optical Fiber Communication?
The realization of optical fiber communication is based on the principle of Total Internal Reflection. When light propagates into the center of the fiber, the refractive index of the fiber core is higher than that of the cladding, and the loss of the core is lower than that of the cladding, so that the light will undergo total reflection, and its light energy is mainly transmitted in the core. Due to successive total reflections, light can be transmitted from one end to the other.

fiber total internal reflection

Does Diameter Affect Fiber Optics Cable?
The diameter of the fiber is vitally important because that determines the mode of the fiber and whether it can sustain Total Internal Reflection for its entire length.

Light is injected into the fiber at a specific incident angle, and full emission occurs between the fiber and the cladding. When the diameter is small, only one direction of light is allowed to pass through, which is a single-mode fiber. When the fiber diameter is large, it can allow light to enter and propagate at multiple incident angles, and it is called a multimode fiber.

Imagine you’re dropping a marble down a twisty pipe. If the pipe is very wide, then the marble might come to a twist where it hits the pipe pretty much head-on. If the pipe is narrow, you never get the head-on collision, you only get continued glancing blows (or rolling). That’s what we want. To a photon, those glancing blows are Total Internal Reflections. And a head-on crash is death.

Small Diameter VS Large Diameter: How to Choose?

  • Transmission Distance and Application
    Each fiber has its own specific application area. The smaller the outer diameter of the fiber, the more careful handling is required during assembly. Based on physical principles, fibers with smaller core diameters have higher data rates and longer transmission distances.
    The smaller diameter of the fiber makes the reflections tighter. The number of light reflections that occur as light travels through the core is reduced, reducing attenuation and allowing further propagation of the signal. Single-mode fiber allows only one mode of light to propagate, allowing the optical signal to travel farther. Because there is no inter-mode dispersion or the inter-mode dispersion is very small, single-mode fiber can transmit a distance of 40 kilometers or more without affecting the signal. Smaller-diameter fiber is generally used for long-distance data transmission, and is widely used in telecommunications companies, cable TV providers, etc.
    long distance communication
    Large core diameter allows different modes of light to be transmitted on one fiber. Under multimode transmission, due to the larger core size, more optical energy is coupled at the beginning of the fiber, but the attenuation along its length will be higher. Signal quality degrades over long distances, so large core diameter is often used for short distances, audio/video applications and local area networks (LANs).
    Data Center
  • Cost
    If small diameter fiber has higher bandwidth and longer distances, why large diameter fiber is needed? Cost may be the key to this issue. Since the core diameter is too small, it is difficult to control the beam transmission, so a laser is required as the light source. Because optical transceivers are very expensive, the cost of using small-diameter optical fibers will be higher than the cost of large-diameter optical fiber cables. Pulling hot glass into a strand makes optical fibers. The thinner the strand of glass, the more likely it is to break during manufacturing. Thus, thicker glass is cheaper to make. This thicker fiber is large diameter multi-mode fiber. This fact drives most data centers to use large-diameter fiber to save costs.

Which diameter fiber optic to choose, depends on the specific needs.

Echo Huang

Echo Huang is an expert wordsmith and marketing professional at Bonelinks with more than 8 years of experience in high technology businesses – fiber optics, IoT, and telecommunication. She is very glad to share industry knowledge and communicate with others.

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