Fiber Optic Tutorials
Lightwave System Architectures
The preceding tutorials focused on the three main components of a fiber-optic communication system - optical fibers, optical transmitters, and optical receivers. In the following tutorials we consider the issues related to system design and performance when the three components are put together to form a practical lightwave system.
From an architectural standpoint, fiber-optic communication systems can be classified into three broad categories - point-to-point links, distribution networks, and local-area networks. This tutorial focuses on the main characteristics of these three system architectures.
Optical Receiver Performance
The receiver performance is characterized by measuring the BER as a function of the average optical power received. The average optical power corresponding to a BER of 10-9 is a measure of receiver sensitivity. The figure below shows the receiver sensitivity measured in various transmission experiments by sending a long sequence of pseudorandom bits (typical sequence length 215 - 1) over a single-mode fiber and then detecting it by using either a p-i-n or an APD receiver. The experiments were performed at the 1.3- or 1.55-μm wavelength, and the bit rate varied from 100 MHz to 10 GHz. The theoretical quantum limit at these two wavelengths is also shown in the figure.
Optical Receiver Sensitivity Degradation
The sensitivity analysis in the previous tutorial is based on the consideration of receiver noise only. In particular, the analysis assumes that the optical signal incident on the receiver consists of an ideal bit stream such that 1 bits consist of an optical pulse of constant energy while no energy is contained in 0 bits. In practice, the optical signal emitted by a transmitter deviates from this ideal situation. Moreover, it can be degraded during its transmission through the fiber link. An example of such degradation is provided by the noise added at optical amplifiers. The minimum average optical power required by the receiver increases because of such nonideal conditions. This increase in the average received power is referred to as the power penalty. In this section we focus on the sources of power penalties that can lead to sensitivity degradation even without signal transmission through the fiber. Several transmission-related power-penalty mechanisms are discussed in another tutorial.
Optical Receiver Sensitivity
Among a group of optical receivers, a receiver is said to be more sensitive if it achieves the same performance with less optical power incident on it. The performance criterion for digital receivers is governed by the bit-error rate (BER), defined as the probability of incorrect identification of a bit by the decision circuit of the receiver. Hence, a BER of 2 x 10-6 corresponds to on average 2 errors per million bits. A commonly used criterion for digital optical receivers requires the BER to be below 1 x 10-9. The receiver sensitivity is then defined as the minimum average received power 
required by the receiver to operate at a BER of 10-9. Since depends on the BER, let us begin by calculating the BER.
required by the receiver to operate at a BER of 10-9. Since depends on the BER, let us begin by calculating the BER.
Coherent Detection
It is clear from the discussion about optical receiver noise that, even though shot noise sets the fundamental limit, it is the thermal noise that limits a photodetector in practice. The use of APDs helps to reduce the impact of thermal noise to some extent, but it also enhances the shot noise. One may ask if it is possible to design a detection scheme that is limited by shot-noise alone. The answer is provided by a technique known as coherent detection, so called because it combines the incoming optical signal coherently with a CW optical field before it falls on the detector. An added benefit is that such a technique can also be used for systems that encode information in the optical phase (such as FSK and PSK modulation formats) because it converts phase variations into amplitude variations.