OTDR is the usual method to measure optical fiber cables respectively complete optical fiber path ways.
A light impulse is sent at the measurement position into the measured optical glass-fiber cable. Because of the back scattering of the sent light impulse light is reflected back into the measurement device. This back scattered light is analysed over amount (level) and time. Because light is constantly travelling with the speed of light, the arriving time of any event is a direct measure of the travel length of light and therefore a direct measure of the distance within the cable or light path way. The amount of light scattered back to the measurement device is relatively small, about one-millionth of the sent light impulse. To get reliable measurement results the quality and condition of the measurement devices are very critical.
|Optical Time Domain Reflectometry - OTDR|
The sent light impulse has a certain amount of energy and a certain length (pulse width). The wider the sent pulse is the higher is the sent energy. But higher emitted energy by a wider pulse results in a lower time scale measurement resolution, a shorter puls results in a higher time scale (and therefore cable length) resolution. A pulse width of one nanosecond provides a resolution on 10 centimeter, a pulse width of 100 nanosecond provides a resolution of 10 meter. The wider the pulse is the more energy can be transmitted. Every 10 times the pulse width results in 5 dB higher level resolution: a pulse with 100 nanosecond has a 5 dB higher level resolution than a 10 nanosecond pulse.
The back scattered light, displayed as a slope, is a direct measurement for the damping of the glass-fiber. It is calculated by the ratio of 'delta dB' to 'delta m' (see below). Different cable types behind each other will be shown as different slopes. But if the first fiber has a higher loss than the one afterwards, the percentage of light from the OTDR test pulse will diminished already and the measured loss will include the actual loss plus a loss error caused by the lower back scatter level. This results in a larger loss displayed than actually happen. On the other hand, going from a lower-loss glassfiber to a higher-loss glassfiber, the back scattering goes up and the result shows the measured loss less than it actually is.
The measured distance is calculated by the following formula:
|typical measurement with one connector and the polished open end of the fiber|
|the back scattering is a direct measurement for the damping of the fiber. It is calculated by the ratio of 'delta dB' to 'delta m'|
|mechanical splice or connector||termal splice||termal splice and two different cable types|
|open cable end polished open cable end broken|
Because the reflected, back scattered, signal is very small, the OTDR receiver circuit is very sensitive. In case of a large temporary reflection the receiver will be saturated or overloaded. In this case the receiver requires a certain time to recover and is unable to measure until the overload is diminished. To overcome strong reflections from the very first connection to the measurement device, often a so-called "launch cable" is used. This cable and the connectors must be of excellent quality and has a length of 500 or 1000 meters. With such a 'launch cable' the actual measurement starts at 500 respectively 1000 meters!