Fiber Optic ST Connector and Propagation Modes

A fiber optic connector is a mechanical device mounted at the end of an optical fiber to facilitate quicker connection and disconnection with a similar device. It mechanically couples and aligns the fibers to allow light to effectively pass through. It also indicates the area in the premises fiber optic data where there might be losses of signal power and the Ber might be affected by a mechanical link.

What is an ST connector?

An ST connector is a plug and socket type of connector that uses a keyed bayonet to lock it in place. This feature facilitates easier insertion and removal from a fiber optic cable. It comes in two push-in and twist type versions, namely ST and ST-II, which are keyed and spring-loaded. These types of connectors are used for both multimode and single-mode fiber optic cables.

ST connectors are constructed with a metal housing and plated with nickel. They feature ceramic ferrules and are rated for 500 mating cycles.

Matched ST connectors have a typical insertion loss of 0.25 dB. It is advisable to use a loss margin of 0.5 dB, the vendor recommendation for ST connectors.

What is the mode of propagation?

The mode of propagation can be better understood with the wave mechanics of light. Light can either be a wave phenomenon or a particle phenomenon that includes photons and solitons. When light is guided down a fiber optic cable, it exhibits different modes. These modes are essentially variations in the intensity of light over the cable cross-section and down the entire cable length. The number of modes existing on a given fiber optic cable depends on the dimension of the cable and the indices of refraction of both the cladding and the core.

The different propagation modes are multimode step index, single-mode step index, single-mode dual-step index, and multimode graded index.

  • Multimode Step Index
  • Fiber optic cables exhibiting multimode propagation with a step index profile are characterized with higher attenuation and greater time dispersion. Cables of this type are the least inexpensive and the most widely used in premises environment. These are most ideal for linked lengths that spread over five kilometers. They may be fabricated from glass, plastic, and PCS.

    Their standard MMF core diameter is 50 or 62.5m.

  • Single-Mode Step Index
  • Fiber optic cables demonstrating single-mode propagation with a step index profile are characterized by minimum time dispersion and attenuation. As it is the most expensive propagation mode to use in premises environment, these cables are predominantly used with metropolitan- and wide-area networks. They have also been used by local area networks as they continue extending over long distances.

    Their core diameter is exceedingly small, ranging from 8 microns to 10 microns. Their standard cladding diameter is 125 microns. These types of fiber optic cables are made of silica glass.

  • Single-Mode Dual-Step Index
  • Fiber optic cables exhibiting single-mode propagation with a refractive index profile, also called dual step index, are distinguished by the advantage of having extreme low macrobending losses, two zero-dispersion points, and a lower dispersion over a wider wavelength range. These cables are set with dual clad fiber, also referred to as depressed-clad fiber.

  • Multimode Graded Index
  • Fiber optic cables exhibiting multimode propagation with a graded index profile are distinguished by levels of attenuation and limited time dispersion. The fibers in the core have a higher refractive index, which gradually decreases as they extend outward from the cylindrical axis. The core and the cladding are treated as a single graded unit.

Cables of this propagation mode cost between the prices of the cables of other propagation modes.

They have core diameters of 50, 62.5, and 85 microns, while their cladding diameter is usually 125 microns. They are often used in premise data applications.