Fiber-Optic Performance Factors

During the course of a normal fiber installation, you must be aware of a few factors that can negatively affect performance. They are as follows:

• Attenuation
• Acceptance angle
• Numerical aperture (NA)
• Modal dispersion
• Chromatic dispersion

Attenuation

The biggest negative factor in any fiber-optic cabling installation is attenuation, or the loss or decrease in power of a data-carrying signal (in the case of fiber, the light signal). It is measured in decibels (dB or dB/km). In real-world terms, a 3dB attenuation loss in a fiber connection is equal to about a 50 percent loss of signal. Figure 1 graphs attenuation in decibels versus percent signal loss. Notice that the relationship is exponential.

 
Figure 1: Relationship of attenuation to percent signal loss of a fiber-optic transmission

Cables that have higher attenuation typically have lower maximum supportable distances between transmitter and receiver. Attenuation negatively affects transmission speeds and distances of all cabling systems, but fiber-optic transmissions are particularly sensitive to attenuation.

Many different problems can cause attenuation of a light signal in an optical fiber, including the following:

• Dirty fiber end-faces (accounts for 85 percent of attenuation loss issues)
• Excessive gap between fibers in a connection
• Improperly installed connectors leading to offsets of the fiber cores when mated
• Impurities in the fiber
• Excessive bending of the cable
• Excessive stretching of the cable (note, however, that only bending causes light loss, not stretching; stretching the cable can cause the fiber to bend, causing light loss or attenuation)

Acceptance Angle

Another factor that affects the performance of a fiber-optic cabling system is the acceptance angle of the optical-fiber core. The acceptance angle (as shown in Figure 2) is the angle through which a particular (multimode) fiber can accept light as input.

 
Figure 2: An illustration of multifiber acceptance angles

The greater the acceptance angle difference between two or more signals in a multimode fiber, the greater the effect of modal dispersion. The modal-dispersion effect also has a negative effect on the total performance of a particular cable segment.

Numerical Aperture (NA)

A characteristic of fiber-optic cable that is related to the acceptance angle is the numerical aperture (NA). The NA is calculated from the refractive indexes of the core and cladding. The result of the calculation is a decimal number between 0 and 1 that reflects the ability of a particular optical fiber to accept light.

A value for NA of 0 indicates that the fiber accepts, or gathers, no light. A value of 1 for NA indicates that the fiber will accept all light it's exposed to. A higher NA value means that light can enter and exit the fiber from a wide range of angles, including severe angles that will not reflect inside the core, but be lost to refraction. A lower NA value means that light can enter and exit the fiber only at shallow angles, which helps assure the light will be properly reflected within the core. Multimode fibers typically have higher NA values than single-mode fibers. This is a reason why the less focused light from LEDs can be used to transmit over multimode fibers as opposed to the focused light of a laser that is required for single-mode fibers.

Modal Dispersion and Bandwidth

Multimode cables suffer from a unique problem known as modal dispersion, which is similar in effect to delay skew, relative to twisted-pair cabling. Modal dispersion, also known as differential mode delay (DMD), causes transmission delays in multimode fibers. Modal dispersion is the main property affecting the multimode fiber's bandwidth.

Here's how it occurs. The modes (signals) enter the multimode fiber at varying angles, so the signals will enter different portions of the optical core. In simple terms, the light travels over different paths inside the fiber and can arrive at different times as a result of different refractive indexes over the cross-sectional area of the core (as shown in Figure 3). The more severe the difference between arrival times of the modes, the more delay between the modes, and the lower the bandwidth of the fiber system.

 
Figure 3: Illustration of modal dispersion

In Figure 3, mode A will exit the fiber first because it has a shorter distance to travel inside the core than mode B. Mode A has a shorter distance to travel because its entrance angle is less severe (i.e., it's of a lower order) than that of mode B. The difference between the time that mode A exits and the time that mode B exits is the modal dispersion or DMD. Modal dispersion gets larger, or worse, as the fiber transports more modes or if there is an imperfection in the graded index refractive index profile. Multimode fiber with a 62.5 micron core carriers more modes than 50 micron fiber. As a result, 62.5 micron fiber has higher modal dispersion and lower bandwidth than 50 micron fiber. Fiber manufacturers pay careful attention to manufacturing the refractive index profiles of graded-index multimode fibers in order to reduce modal dispersion and thereby increase bandwidth. Differential mode delay is measured by fiber manufacturers and is directly related to bandwidth.

Chromatic Dispersion

The last fiber-optic performance factor is chromatic dispersion, which limits the bandwidth of certain single-mode optical fibers. Contrary to popular belief, lasers cannot emit one pure wavelength of light. Lasers emit a distribution of wavelengths with the characteristic laser wavelength in the center. (Granted, the distribution is very tight and narrow; it is a distribution, nonetheless.) These various wavelengths of light transmitted by the laser spread out in time as they travel through an optical fiber. This happens because different wavelengths of light travel at different speeds through the same media. As they travel through the fiber, the various wavelengths will spread apart (as shown in Figure 4). The wavelengths will spread farther and farther apart until they arrive at the destination at completely different times. This will cause the resulting pulse of light to be wider, less sharp, and lower in overall power. As consecutive pulses begin to overlap, the signal detection will be compromised. Multimode fiber can also have this problem, and this combined with modal dispersion lowers the bandwidth of multimode fibers compared to single-mode, which is mainly impaired only by chromatic dispersion.

 
Figure 4: Single-mode optical fiber chromatic dispersion