Brief Introduction on Receiver

The Receiver component serves two functions as shown in Figure 1-1. First, it must sense or detect the light coupled out of the fiber optic cable then convert the light into an electrical signal. Secondly, it must demodulate this light to determine the identity of the binary data that it represents. In total, it must detect light and then measure the relevant Information bearing light wave parameters in the premises fiber optic data link context intensity in order to retrieve the Source's binary data.

              
                 Figure 1-1: Example of Receiver block diagram - first stage

Within the realm of interest in this book the fiber optic cable provides the data to the Receiver as an optical signal. The Receiver then translates it to its best estimates of the binary data. It then provides this data to the User in the form of an electrical signal. The Receiver can then be thought of as an Electro-Optical (EO) transducer.

A Receiver is generally designed with a Transmitter. Both are modules within the same package. The very heart of the Receiver is the means for sensing the light output of the fiber optic cable. Light is detected and then converted to an electrical signal. The demodulation decision process is carried out on the resulting electrical signal. The light detection is carried out by a photodiode. This senses light and converts it into an electrical current. However, the optical signal from the fiber optic cable and the resulting electrical current will have small amplitudes. Consequently, the photodiode circuitry must be followed by one or more amplification stages. There may even be filters and equalizers to shape and improve the Information bearing electrical signal.

All of this active circuitry in the Receiver presents a source of noise. This is a source of noise whose origin is not the clean fiber optic cable. Yet, this noise can affect the demodulation process.

The very heart of the Receiver is illustratedas in figure 1-2. This shows a photodiode, bias resistor and a low noise pre-amp. The output of the pre-amp is an electrical waveform version of the original Information out the source. To the right of this pre-amp would be additional amplification, filters and equalizers. All of these components may be on a single integrated circuit, hybrid or even a printed circuit board.

The complete Receiver may incorporate a number of other functions. If the data link is supporting synchronous communications, this will include clock recovery. Other functions may include decoding (e.g. 4B/5B encoded information), error detection and recovery.

The complete Receiver must have high detectability, high bandwidth and low noise. It must have high detectability so that it can detect low level optical signals coming out of the fiber optic cable. The higher the sensitivity, the more attenuated signals it can detect. It must have high bandwidth or fast rise time so that it can respond fast enough and demodulate, high speed, digital data. It must have low noise so that it does not significantly impact the BER of the link and counter the interference resistance of the fiber optic cable Transmission Medium.

There are two types of photodiode structures; Positive Intrinsic Negative (PIN) and the Avalanche Photo Diode (APD). In most premises applications the PIN is the preferred element in the Receiver. This is mainly due to fact that it can be operated from a standard power supply; typically between 5 and 15 V. APD devices have much better sensitivity. In fact it has 5 to 10 dB more sensitivity. They also have twice the bandwidth. However, they cannot be used on a 5V printed circuit board. They also require a stable power supply. This makes cost higher. APD devices are usually found in long haul communications links.

The demodulation performance of the Receiver is characterized by the BER that it delivers to the User. This is determined by the modulation scheme - in premise applications - Intensity modulation, the received optical signal power, the noise in the Receiver and the processing bandwidth.

Considering the Receiver performance is generally characterized by a parameter called the sensitivity, this is usually a curve indicating the minimum optical power that the Receiver can detect versus the data rate, in order to achieve a particular BER. The sensitivity curve varies from Receiver to Receiver. It subsumes within it the signal-to-noise ratio parameter that generally drives all communications link performance. The sensitivity depends upon the type of photodiode employed and the wavelength of operation. Typical examples of sensitivity curves are illustrated in Figure 1-2.

In examining the specification of any Receiver you need to look at the sensitivity parameter.In a sense it represents optimum performance on the part of the photodiode in the Receiver. That is, performance where there is 100% efficiency in converting light from the fiber optic cable into an electric current for demodulation.

 

Figure 1-2: Receiver sensitivities for BER = 10^-9, with different devices.