TABLE OF CONTENTS Waguih Ishak
Agilent Technologies
Palo Alto, California
The development of high-power, narrow-linewidth semiconductor lasers and the invention and development of erbium-doped fiber amplifiers have impacted fiber-optic communications links in two important areas. First, new network configurations have emerged such as wavelength division multiplexing schemes. Second, long fiber links between repeaters are becoming more and more common. For example, the systems installed under the Mediterranean will use a 150-km repeaterless link. As these systems have become available, the demand on testing and monitoring the network has increased and the need for higher sensitivity, resolution, and SNR test equipment has also increased.
The optical time domain reflectometer (OTDR) is the most commonly used instrument to test a fiber-optic link. An OTDR is an instrument that characterizes optical fiber by launching a probe signal into the fiber under test and detecting, averaging, and displaying the return signal. The distance to a given feature (such as a splice, a bent, or a kink) is determined by simply timing the arrival of that part of the return signal at the detector.
A block diagram of a generic OTDR is shown in Fig. 25.1. An electronic probe signal generator is used to modulate the intensity of a laser. In a conventional OTDR, the probe signal is a single square pulse. In field portable instruments, a semiconductor laser is used as the source. The output of the laser is coupled into the fiber under test using a beam splitter...
