The commonly employed technique for analysing the state of a fibre optic is to test it with an Optical Time Domain Reflectometer (OTDR). The OTDR uses backscattered light of the fibre to imply loss. The OTDR works is similar way to RADAR, sending a high power laser light pulse down the fibre and looking for the return signals from backscattered light in the fibre itself or reflected light from the connector or splice interfaces. By measuring the time it takes for the reflected light to return to the source and knowing the refractive index of the fibre, it is possible to calculate the distance to the reflection point.
When this instrument is connected to one end of any fibre optic system up to 250 km in length, within a few seconds, it is able to measure the overall loss, or the loss of any part of a system, the overall length of the fibre and the distance between any points of interest.
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Operating Principle of OTDR
As light travels along the fibre, a small proportion of it is lost by Rayleigh scattering. As the light is scattered in all directions, some of it just happens to return back along he fibre towards the light source. The returned light is called backscatter. The backscatter power is a fixed proportion of the incoming power and as the losses take their toll on the incoming power, the returned power also diminishes as illustrated in the figure below:
The OTDR can continuously measure the returned power level and hence deduce the losses encountered on the fibre. Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fibre and hence causing a corresponding change in backscatter power. The position and the degree of the losses can be determined.
OTDR Components
Generally, an OTDR system has the following components:
Timer
The timer produces a voltage pulse which is used to start the timing process in the display at the same moment as the laser is activated.
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Pulsed Laser
The laser is switched on for a brief moment, the ‘on’ time being between 1 ns and 10 ms. The wavelength of the laser can be switched to suit the system to be investigated.
Directive Coupler
The directive coupler allows the laser light to pass straight through into the fibre under test. The backscatter from the whole length of the fibre approaches the directive coupler from the opposite direction. In this case, the mirror surface reflects the light into the avalanche photodiode (APD). The light has now been converted into an electrical signal.
Amplifying and Averaging
The electrical signal from the APD is very weak and requires amplification before it can be displayed.
Display
The amplified signals are passed on to the display. The display is either Cathode Ray Tube (CRT) like an oscilloscope or a computer monitor, or a liquid crystal display. They display the returned signals on a simple XY plot with the range across the bottom and the power level in decibels up the side.
Data Handling
An internal memory or external storage device can be used to store the data for later analysis. The output is also available via an RS-232 link for downloading to a computer. Additionally, many OTDRs have an on board printer to provide hard copies of the information on the screen. This provides helpful ‘before’ and ‘after’ images for fault repair as well as a record of the initial installation.
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