Fiber Optics Data Links

A fiber optic data link sends input data through fiber optic components and provides this data as output information. It has the following three basic functions:

• To convert an electrical input signal to an optical signal
• To send the optical signal over an optical fiber
• To convert the optical signal back to an electrical signal

A fiber optic data link consists of four parts—transmitter, optical fiber, connectors/splices, and receiver. Figure 1-1 is an illustration of a fiber optic data-link connection. The transmitter, optical fiber, and receiver perform the basic functions of the fiber optic data link. Each part of the data link is responsible for the successful transfer of the data signal. A fiber optic data link needs a transmitter that can effectively convert an electrical input signal to an optical signal and launch the data-containing light down the optical fiber. A fiber optic data link also needs a receiver that can effectively transform this optical signal back into its original form. This means that the electrical signal provided as data output should exactly match the electrical signal provided as data input.

The basic functions of a fiber optic data link are to convert an electrical input signal to an optical signal, send the optical signal over an optical fiber, and convert the optical signal back to an electrical signal.

The purpose of the transmitter is to convert an electrical waveform or digital data stream to the best optical signal for transmission through an optical fiber. There are three different types of optical transmitters; (1) light-emitting diodes (LEDs), (2) Vertical Cavity Surface Emitting Lasers (VCSELS) and (3) laser diodes.

Light Emitting Diodes (LED)

LEDs are relatively restricted in their range of possible applications because of their relatively low data rate and power levels. LEDs are utilized in Local Area Networks (LANS) where transmissions of less than two kilometers are required with data rates usually no more than 680Mbps/km. They are also used for control signals such as opening and closing valves and vent dampers using programmable logic controllers. Their expected operating life usually exceeds 100,000 hours or about ten years. They are simple in design, require only a few components to power, drive and monitor the device and because of their low bias voltage no cooling circuits are needed. The output power of the typical LED ranges from -15dBm to -20dBm. They operate at wavelengths of 850nm and 1300nm.

Vertical Cavity Surface Emitting Laser (VCSEL)

The VCSEL is a short range high data rate transmitter for fiber optic data links. A VCSEL because of the increased bandwidth and mode field diameter requires a 50 micron multimode laser optimized fiber as its transmission medium. The most common emission wavelengths of VCSELs are in the range of 750–980nm (often around 850nm). Data rates with VCSELs of 10Gbps can be reached over a distance of a few hundred meters.

Light Amplification by Stimulated Emission of Radiation (LASER)

Laser diodes come in many shapes, sizes and operating characteristics. Lasers provide stimulated emission rather than the simpler spontaneous emission of LEDs. The main difference between an LED and a Laser is that the Laser has an optical cavity required for lasing (See figure 1-3 below). This cavity, called the Fabry-Perot cavity, is formed by cleaving the opposite end of the chip to form a highly parallel, reflective mirror like finish.

At low drive currents, the LASER acts like a LED and emits spontaneous light. As the current increases it reaches the threshold level above which lasing action begins. Some of the photons emitted by the spontaneous action are trapped in the optical cavity, reflecting back and forth from end mirror to end mirror. If one of these photons influences an excited electron, the electron immediately recombines and gives off a photon. Remember that the wavelength of a photon is a measure of its energy. The photon created is a duplicate of the first photon. It has the same wavelength, phase, and direction of travel. In other words, the incident photon has stimulated the emission of another photon and in effect, it cloned itself. Amplification has occurred, and emitted photons have stimulated further emissions. Although some of the photons remain trapped in the cavity, reflecting back and forth and stimulating further emissions, others escape through the two cleaved end faces in an intense beam of light. Thus, the LASER differs from a LED in that LASER light has the following attributes:

NEARLY MONOCHROMATIC: The light emitted has a narrow band of wavelengths. It is nearly monochromatic, which means a single wavelength. In contrast to the LED, LASER light is not continuous across the band of its spectral width. Several distinct wavelengths are emitted on either side of the central wavelength (refer to Figure 1-4). COHERENT: The light wavelengths are in phase, rising and falling through thesine cycle at the same time. HIGHLY DIRECTIONAL: The light is emitted in a highlydirectional pattern with little divergence. Divergence is the spreading of a light beam asit travels from its source.

The LASER output power can be as high as 20mW. Not only is the light more powerful than a LED’s, but the narrow beam allows the greater percentage to be coupled into the fiber. The Laser can be turned on and off faster than a LED, making the LASER usable at data rates of 300 MHz and higher. Nevertheless, the LASER suffers a few drawbacks: first, it is very expensive. Second, it is temperature sensitive and requires more complex electronic circuitry to operate. Last, it is less reliable and has a shorter expected life time than an LED.