What are the geometric dimensions of an optical fiber?

OFSCN Global Technical Center OFSCN Technical Knowledge Base
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125/250 What does this mean? Is this a universal size for all fibers?

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In optical engineering and fiber optics, “125/250” refers to the most basic geometrical dimension specification of an optical fiber, measured in micrometers (\mu\text{m}).

1. Physical Meaning of “125/250”

A standard optical fiber, from inside out, typically consists of three concentric cylindrical layers: the core, the cladding, and the coating.

  • “125”: Refers to the outer diameter of the cladding being 125\ \mu\text{m}. The cladding is the critical physical boundary for light transmission in a fiber (confining the light signal within the core via total internal reflection) and is usually made of high-purity silica glass.
  • “250”: Refers to the outer diameter of the outermost coating layer being 250\ \mu\text{m} (typically between 245\ \mu\text{m} and 255\ \mu\text{m} in actual factory specifications). The coating (usually a polyacrylate material) primarily provides mechanical protection, preventing the inner silica glass cladding from being damaged by micro-scratches, moisture ingress, or bending stress.

In a complete geometrical parameter description, the core diameter is usually included. For example:

  • Single-mode fiber (SM) is typically denoted as 9/125/250, where the core diameter is approximately 9\ \mu\text{m}.
  • Multi-mode fiber (MM) is typically denoted as 50/125/250 or 62.5/125/250, where the core diameters are 50\ \mu\text{m} or 62.5\ \mu\text{m}, respectively.

2. Are these Dimensions Universal for All Fibers?

No.

While 125/250\ \mu\text{m} is the most common and widely adopted industry standard dimension in telecommunications and network communication (e.g., standard G.652D, G.657 single-mode fibers), the geometrical dimensions of optical fibers can vary significantly in Specialty Optical Fibers and applications such as sensing, high-power laser transmission, and aerospace.

(1) Small Diameter Fiber

In scenarios where installation space is extremely limited or ultra-small bending radii are required, small-diameter fibers with reduced cladding and coating diameters are used. For example:

  • 80/100 Size: Cladding diameter of 80\ \mu\text{m}, with a coating diameter of only 100\ \mu\text{m}.

(2) Specialty Coated Fibers (e.g., High Temperature, Metal Coated)

For sensing fibers requiring high-temperature resistance or improved strain transfer efficiency, the coating material is not the conventional thick polyacrylate. Instead, a very thin layer of polyimide or metal (like gold) is used. This significantly reduces the outer diameter of the coating layer:

  • 125/155 Size: The cladding remains standard at 125\ \mu\text{m}, but the coating diameter is reduced to 155\ \mu\text{m}.

(3) Large Core Fiber

Used for laser energy transmission (e.g., laser surgery, industrial cutting) or high-power laser guiding. Its geometrical dimension series is more diverse, for example:

  • Core/Cladding/Coating diameters of: 105/125/155\ \mu\text{m}, 200/220/245\ \mu\text{m}, 400/440/470\ \mu\text{m}, etc.

3. Examples of Related Official OFSCN® (Beijing Dacheng Yongsheng Technology Co., Ltd.) Specialty Fiber Products

Beijing Dacheng Yongsheng Technology Co., Ltd. offers fiber products in the standard dimensions mentioned above as well as various specialty geometrical dimensions to cope with different harsh physical environments:

For more engineering details on different geometrical dimensions and specialty fiber classifications, please visit:
OFSCN® Specialty Optical Fiber Product Classification