Source Emitters

Optical emitters couple light into a fiber according to NA and core size. Using a light source not matched to a particular fiber's NA and core size will cause less than optimum light coupling for the system.

LEDs are relatively inexpensive, reliable and easy-to-use because their electronic circuitry is less complex than that required for a laser.

Semiconductor lasers and LEDs are both direct transducers from electrical to optical radiation. LEDs couple less power into the fiber because they emit the optical radiation over a broader angle area. The laser is a much more complicated structure due to the requirement for a small dual-face cavity. Also its output is temperature dependent and the lifetime is less than the LED.

Several different LED packaging styles are commercially available:

The LED or laser diode can be packaged so that the fiber cable plugs directly into the device package. An alternative is fastening the fiber directly to the chip and leaving the opposite end available for a connector.

Matched transmitter and receiver units, plus a wide variety of other fiber optic components ranging from discrete elements like LEDs, laser diodes, and detectors to complete rack-mounted modules are all readily available.

Detectors

Lightwave receivers use photodetectors, where the photons of light generate photoelectrons. A minimum average number of photons in each pulse is necessary to achieve a given error probability (21 photons for 10-9 error probability). Considerable amplification is necessary. For an avalanche-photodiode (APD) initial amplification is internal. For positive-intrinsic-negative detectors (PIN) this amplification is by external electronic amplifiers.

Optical Fiber Loss

We've already considered core size and numerical aperture as measures of fibers ability to accept the optical power. Now let's consider what happens to the optical signal once it's launched.

In coaxial cable, the higher the frequency, the more signal strength decreases with distance and this is referred to as attenuation. Fiber frequency is constant until it reaches its bandwidth limit. Thus optical loss is proportional to distance.

This attenuation within the fiber is caused by absorption and scattering of lightwaves due to chemical impurities and molecular structure. These fiber properties absorb or scatter the optical radiation so that it escapes the core and is lost.

Attenuation within a fiber is specified by the manufacturer at certain wavelengths: For example 3 dB/km at 850 nanometers. This is done because fiber loss varies with wavelength.

These wavelengths are measured in nanometers (nm)--billionths of a meter-- which represent the distance between two cycles of the same wave. Wavelength is a descriptive property of electromagnetic radiation. Light and infrared radiation are a portion of the total electromagnetic spectrum.

Microwaves, radar, television and radio operate in the longest wavelength areas. In between the ultraviolet and the microwave spectrums, we have fiber optic wavelengths, which are in the infrared spectrum.

Fiber Selection

Fibers are therefore optimized for operation at certain wavelengths. For example, less than 1 dB/km loss is attainable in 50/125 µm multimode fiber operating at 1300 nm, and less than 3 dB/km (50% loss) is attainable for the same fiber operating at 850 nm. The 50/125 nomenclature indicates both the outside diameter of the core (50 microns) and the cladding (125 microns).

The favorable transmission regions within the optical spectrum for a fiber are referred to as "windows". The 800 to 900 nanometer region is the first window, 1100 to 1300 nanometers is the second window, and the third window occurs at about 1550 nanometers. In these spectral windows fibers have very low attenuation. The lowest losses occur in the infrared region around 1300 nm and again around 1550 nm.

Great improvements have been made in all fiber types so that premium fibers exhibit losses of less than 0.5 dB/km at wavelengths of 1300 and 1550 nm. However, source emitters and detectors for these regions are currently more expensive.

If the fiber is to perform well, the source chosen should provide optical radiation at the specified wavelength, and the detector should be sensitive to the same wavelength.

In coaxial and other metallic cables, very high frequency signals tend to be attenuated rapidly with distance. As a result, amplifiers and equalizers are required at periodic intervals to build up signals to usable levels.

However, each time an analog amplifier is added, noise is introduced to the metallic system and the overall system signal-to-noise ratio degrades.

With optical communications, all of the light energy is at approximately the same frequency or wavelength. As a result, the attenuation of a specific wavelength is dependent only on distance. The chart below shows a comparison of attenuation differences between coaxial and fiber optic cable. The requirement for repeaters is, therefore, minimized and the need for equalizers is eliminated in fiber systems.

Connector Loss

Connector loss is a function of the physical alignment of one fiber core to another fiber core.

Scratches and dirt can also contaminate connector surfaces and severely reduce system performance, but most often the connector loss is due to misalignment or end separation.