Fiber Optic LC Connector and Propagation Modes

A fiber optic connector is a mechanical device attached to the end of a fiber optic cable to allow easier splicing and connection or disconnection to a similar device. The connectors mechanically couple and align fiber paths to allow light to pass through. A connector marks the specific location in the fiber optic data link where signal power can be lost and the Ber may be affected by a mechanical connection.

What is an LC connector?

An LC connector is a fiber optic connector that looks like a small SC connector. It is often used with single-mode and multimode fiber optic cables.

LC connectors are built with a plastic housing. Their ceramic ferrules facilitate accurate alignments. This type of connector is fitted with a locking tab and is rated for 500 mating cycles.

Matching LC connectors have a typical insertion loss of 0.25 dB. However, from a design’s point of view, a loss margin of 0.5 dB (the vendor recommendation for LC connectors) is advised.

Propagation Modes

Fiber optic cables generally have two propagation modes, namely single mode and multiple mode. Each mode performs differently in terms of attenuation and time dispersion.

To have a better understanding of the difference between the two, the term “mode of propagation” should be defined first.

Light can be viewed either as a wave phenomenon or a particle phenomenon. Particles include photons and solitons. Solitons are localized waves that demonstrate particle-like behavior. As a light wave passes down a fiber optic cable, it exhibits certain modes, which are simply variations of the light’s intensity. In any given fiber optic cable, the number of existing modes varies with the cable dimension and the variation of the indices of refraction.

The various propagation modes include multimode step index, single-mode step index, single-mode dual-step index, and multimode graded index.

  • Multimode Step Index
  • A fiber optic cable exhibiting multimode propagation with a step index profile is distinguished by higher attenuation and greater time dispersion relative to other propagation modes. It is also the least expensive and the most widely used in premises environment. Cables exhibiting this propagation mode are most ideal for linked lengths of over five kilometers, and may be fabricated from glass, plastic, and PCS.

    Their MMF core diameter is usually 50 or 62.5m.

  • Single-Mode Step Index
  • A fiber optic cable exhibiting single-mode propagation with a step index profile is characterized by minimum attenuation and time dispersion. However, this silica-made fiber optic cable is the most expensive when used in the premises environment; thus, it is mostly used with metropolitan and wide-area networks. It is also used by local area networks as they continue to extend over greater distances.

    Its core has an extremely small diameter, which typically ranges from 8 microns to 10 microns. Its standard cladding diameter is 125 microns.

  • Single-Mode Dual-Step Index
  • A fiber optic cable exhibiting single-mode propagation with a dual-step index profile is set with dual clad fiber, also known as depressed-clad fiber. It has the advantage of extremely low macrobending losses, two zero-dispersion points, and a lower dispersion over a wider wavelength range.

  • Multimode Graded Index
  • A fiber optic cable exhibiting multimode propagation with a graded index profile is characterized with levels of attenuation and limited time dispersion. Its core fibers have a higher refractive index, which steadily diminishes as they extend outward from the cylindrical axis. The core and the cladding are treated as a single graded unit.

The price of fiber optic cables exhibiting this propagation mode is midway between the costs of the most expensive and the least costly.

Graded index fiber optic cables often have core diameters of 50, 62.5, and 85 microns; whereas, the cladding diameter is usually 125 microns. This type of cable is often used in premise data communications applications.

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