Mode-Field Diameter

Mode-field diameter (MFD) describes the size of the light-carrying portion of the fiber. This region includes the fiber core as well as a portion of the surrounding cladding glass. MFD is an important parameter for determining a fiber's resistance to bend-induced loss and can affect splice loss as well. MFD, rather than core diameter, is the functional parameter that determines optical performance when a fiber is coupled to a light source, connectorized, spliced, or bent. It is a function of wavelength, core diameter, and the refractive-index difference between the core and the cladding. These last two are fiber design and manufacturing parameters.

Cutoff Wavelength

Cutoff wavelength is the wavelength above which a single-mode fiber supports only one mode or ray of light. An optical fiber that is single-moded at a particular wavelength has two or more modes at wavelengths shorter than the cutoff wavelength.

The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment. The longer the fiber, the shorter the effective cutoff wavelength. Or, the smaller the bend radius of a loop of the fiber is, the shorter the effective cutoff wavelength will be.

 

Environmental Performance

While cable design and construction play a key role in environmental performance, optimum system performance requires the user to specify fiber that will operate without undue loss from microbending.

Microbends are small-scale perturbations along the fiber axis, the amplitude of which are on the order of microns. These distortions can cause light to leak out of a fiber. Microbending may be induced at very cold temperatures because the glass has a different coefficient of thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable become more rigid and contract more than the glass. Consequently, enough load may be exerted on the glass to cause microbends. Coating, fiber ribbon, and cabling materials are selected by manufacturers to minimize loss due to microbending.

Specification Examples of Uncabled Fiber

To ensure that a cabled fiber provides the best performance for a specific application, it is important to work with an optical fiber–cable supplier to specify the fiber parameters just reviewed as well as the geometric characteristics that provide the consistency necessary for acceptable splicing and connectorizing.

Splicers and Connectors

As optical fiber moves closer to the customer, where cable lengths are shorter and cables have higher fiber counts, the need for joining fibers becomes greater. Splicing and connectorizing play a critical role both in the cost of installation and in system performance.

The object of splicing and connectorizing is to match, precisely, the core of one optical fiber with that of another in order to produce a smooth channel through which light signals can continue without alteration or interruption.

There are two ways that fibers are joined:

• splices, which form permanent connections between fibers in the system
• connectors, which provide remateable connections, typically at termination points

    Fusion Splicing

Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by creating a homogenous joint between the two fiber ends. The fibers are melted or fused together by heating the fiber ends, typically using an electric arc. Fusion splices provide the highest-quality joint with the lowest loss (in the range of 0.04 dB to 0.10 dB) and are practically nonreflective.

Mechanical Splicing

Mechanical splicing is an alternative method of making a permanent connection between fibers. In the past, the disadvantages of mechanical splicing have been slightly higher losses, less-reliable performance, and a cost associated with each splice. However, advances in the technology have significantly improved its performance. System operators typically use mechanical splicing for emergency restoration because it is fast, inexpensive, and easy.

Connectors

Connectors are used in applications where flexibility is required in routing an optical signal from lasers to receivers, wherever reconfiguration is necessary, and in terminating cables. These remateable connections simplify system reconfigurations to meet changing customer requirements.