Usage in Computers

2. How It Works ...


2.1 Preview

2.4 Light Detector

2.2 Electrical Signal Transmitter(Driver)

2.5 Receiver

2.3 Light Source



2.1 Preview

A simple fiber optic system is called a transmission link. It consists of a transmitter with a light source, a length of fiber and a receiver with a light detector. The operation of a system is as follows. A digital or analogue signal is applied to a transmitter. Within the transmitter, the input signals are converted from electrical to optical energy by modulating an optical light source, which is normally achieved by varying current. The modulated light is launched into a length of fiber, where it reflects from wall to wall through the fiber core. At the opposite end of the fiber, a detector accepts the light signal and converts it back to an electrical signal. The electrical signal may now be conditioned to perform its original work. However, there are several components that make up a fiber optic system.

2.2 Electrical Signal Transmitter(Driver)

The purpose of the transmitter (driver) is to change the electrical signal into the required current to drive a low impedance light source. The electrical inputs are either digital or analogue. The choice of the converter should depend on the current requirement of the light source. If the signal is digital, the transmitter should consists of a high-speed pulser to turn the light source on and off. If the signal is analogue, the transmitter should be able to supply current to the light source to transmit the positive and negative alternations of the signal.


Analogue Transmitter

An ordinary analogue transmitter is illustrated in the figure on the right. R2 and R3 provide a voltage divider for the input signal. The potentiometer P1 and resistor R1 serve to set the operating point so that the positive and negative swings of the analogue input signal produce only a positive output. The output current never changes polarity, only its amplitude. RFB is the feedback resistance. The light emitting doide(LED) transmits light as current varies. Resistor RL provides current limiting and load.


LED Transmitter

A very basic LED transmitter consists of an inverter with an LED tied to its output is shown here in the figure on the left. The resistor RL serves as a current limiter. Signal pulses at the input direct pulse current to flow through the LED causing it to radiate.






Pulse Modulated Transmitter

The figure on the right shows a variable analogue signal input being directed through a pulse modulator. The modulator is usually one of these three types; PRM-pulse width modulation, PPM-pulse position modulation or PWM-pulse width modulation. Modulated output directs pulse current to flow through the LED, causing it to radiate. (These examples are greatly simplified for easier understanding.)

2.3 Light Source

The purpose of the light source is to launch a light signal into the optical fiber at an angle that provides maximum signal transfer. There are two basic light sources used in fiber optic electronics. There are the light emitting diode (LED) and the injection laser diode (ILD). Both of these units provide small size, brightness, low drive voltage and are able to emit signals at desired wavelengths. Each has characteristics that make them desirable or undesirable for different applications. The LED has longer life span, greater stability, wider temperature range and much lower cost. The ILD is capable of producing as much as 10 dB more power output than the LED. It can launch the light signal at a much narrower numerical aperture (NA), and therefore can couple more power through the optical fiber than the LED. The problem of using ILD is that its current range is extremely limited. Since some system operations vary greatly, the ILD must have compensation devices added to the electronics. This may make the cost prohibitive.

2.4 Light Detector

The light detector accepts a light signal from the optical fiber and converts it into an electrical current. There are three types of detectors in use: the phototransistor, the PIN diode and the avalanche diode (APD). The phototransistor is relatively inexpensive but has slow rise times and a limited bandwidth capability. Therefore, it's seldom used. The PIN diode, which contains positive, intrinsic and negative solid state layers in its construction, exhibits fast rise times and acceptable bandwidth parameters. It is reasonably priced. The APD is more expensive than the PIN diode since it provides greater receiver sensitivity. The APD also requires an auxiliary power supply.

2.5 Receiver

Voltage Drop Across Resistor

The function of the receiver is to accept low level power from the detector and convert it into a high voltage output. There are many ways to accomplish this. The figure on the left shows a detector current producing a voltage drop across a load resistance. The voltage drop is directed into amplifier. An output voltage representative of the transmitted signal is the result. Other electronics maybe added to the circuitry to maintain adequate response. A gain control may be used in the front end to vary the impedance of the receiver. The operational amplifier is used as a current to voltage converter.
Since the signal inputs are generally weak, shielding and power supply decoupling are a requirement to achieve sensitivity. Sensitivity is set by input noise. Noise, of course, leads to many errors in most digital systems and restricts signal to noise ratio (SNR) in analogue systems.