Detectors

The detector serves the opposite function from the source: It converts optical energy to electrical energy. The output circuitry of the receiver amplifies the signal and accurately reproduces the original digital signal. A variety of detector types are available. The most common is the photodiode, which produces current in response to incident light. Two types of photodiodes used extensively in fiber optics are the PIN photodiode and the Avalanche (APD) photodiode. They usually involve the following considerations:

• SENSITIVITY: How well does it receive incoming light signal, especially weak ones?
• SPEED: How fast does it respond to light pulses? How fast does it turn off and on?
• COMPLEXITY: Does it require a complex electronic bias circuit?
• COMPATIBILITY: Does it respond well to the wavelengths received?
• COST: Do the increased benefits justify the cost?

When light falls on the diode it creates current in the external circuit. Absorbed photons excite electrons from the valence band to the conduction band, a process known as intrinsic absorption. The result is the creation of an electron hole pair. These carriers, under the influence of the bias voltage applied to the diode, drift through the material and induce a current in the external circuit. For each electron hole pair thus created, an electron is set flowing as current in the external circuit. As a result, the output current of the detector is proportional to the input light intensity.

PIN PHOTODIODE

The PIN photodiode has a lightly doped intrinsic layer which separates the more heavily doped p-material with free electrons or p-material with holes. Although the intrinsic layer is actually lightly doped positive, the doping is light enough to allow the layer to be considered intrinsic (neither strongly n or p-type). The name of the diode comes from this layering of materials: Positive, Intrinsic, Negative (PIN).

Since the intrinsic layer has no free carriers, its resistance is high, and electrical forces are strong within it. The resulting depletion region is very large in comparison to the size of the diode. The PIN diode works like the pn diode. The large intrinsic layer, however, means that most of the photons are absorbed within the depletion region. The result is improved efficiency in incident photons, creating external current and faster speed. Carriers created within the depletion region are immediately swept by the electric field toward their p or n terminal. The PIN photodiode provides no gain. Also, it must receive a fairly strong signal, due to its characteristics of not being very sensitive. However, the PIN photodiode has several advantages. It is easy to use, has a fast response time, and is fairly inexpensive. All detectors require bias voltage, and the PIN photodiode only requires biasing of 5 volts.

AVALANCHE PHOTODIODE

For a PIN photodiode, each absorbed photon ideally creates one electron hole pair, which sets one electron flowing in the external circuit. In this sense we can loosely compare it to a LED. There is basically a one-to-one relationship between photons and carriers and current. In a Laser, a few primary carriers result in many emitted photons. In an Avalanche Photodiode (APD), a few incident photons will set a number of carrier electrons in motion, a phenomenon known as the avalanche effect, and produce an appreciable external current (or current gain). The structure of the APD creates a very strong electrical field in a portion of the depletion region. Primary carriers, the free electrons and holes created by absorbed photon, within this field are accelerated by the field, thereby gaining several electron volts of Kinetic energy. A collision of these fast carriers with neutral atoms causes the carrier to use some of its energy to raise a bound electron from the valence band to the conduction band. A free electron and hole appear. Carriers created in this way, through collision with a primary carrier, are called secondary carriers.

This process of creating secondary carriers is known as collision ionization. A primary carrier can create several new secondary carriers, and secondary carriers themselves can accelerate and create new carriers. The whole process is called photo multiplication, which is a form of gain. The multiplication or avalanche factor varies with the bias voltage. Because the accelerating forces must be strong enough to impart energies to the carriers, high bias voltages (several hundred volts) are required to create the high field region. The APD is about 10 times more sensitive and can respond better to faster incoming light signals than the PIN photodiode. The APD’s increased sensitivity makes it more expensive than the PIN. In addition the APD is very sensitive to variations in temperature and requires cooling devices and compensating circuitry.

A fiber optic data link also includes passive components other than an optical fiber. Figure 1-1 does not show the optical connections used to complete the construction of the fiber optic data link. Passive components used to make fiber connections affect the performance of the data link. These components can also prevent the link from operating. Fiber optic components used to make the optical connections include optical splices, connectors, and couplers. Chapter 4 outlines the types of optical splices, connectors, and couplers and their connection properties that affect system performance.

Proof of link performance is an integral part of the design, fabrication, and installation of any fiber optic system. Various measurement techniques are used to test individual parts of a data link. Each data link part is tested to be sure the link is operating properly. Chapter 5 discusses testing methods and measurements used to qualify a fiber optic link and measure performance.