OTDR is connected to one end of any fiber optic system up to 250km in length. Within a few seconds, we are able to measure the overall loss, or the loss of any part of a system, the overall length of the fiber and the distance between any points of interest. OTDR is a amazing test instrument for fiber optic systems.

As light travels along the fiber, 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 the fiber towards the light source. This returned light is calledbackscatter as shown below.

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 shown in the following figure.

The OTDR can continuously measure the returned power level and hence deduce the losses encountered on the fiber. Any additional losses such as connectors and fusion splices have the effect of suddenly reducing the transmitted power on the fiber and hence causing a corresponding change in backscatter power. The position and degree of the losses can be ascertained.

The OTDR uses a system rather like a radar set. It sends out a pulse of light and ‘listens’ for echoes from the fiber.

If it knows the speed of light and can measure the time taken for the light to travel along the fiber, it is an easy job to calculate the length of the fiber.

Assuming the refractive index of the core is 1.5, the infrared light travels at a speed of

This means that it will take

This is a useful figure to remember, 5 nanoseconds per meter (5 nsm^-1).

If the OTDR measures a time delay of 1.4us, then the distance travelled by the light is

The 280 meters is the total distance traveled by the light and is the ‘there and back’ distance. The length of the fiber is therefore only 140m. This adjustment is performed automatically by the OTDR – it just displays the final result of 140m.

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.

The laser is switched on for a brief moment. The ‘on’ time being between 1ns and 10us. We will look at the significance of the choice of ‘on’ time or pulsewidth a little bit later. The wavelength of the laser can be switched to suit the system to be investigated.

The directive coupler allows the laser light to pass straight through into the fiber under test. The backscatter from the whole length of the fiber 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.

The electrical signal from the APD is very weak and requires amplification before it can be displayed. The averaging feature is quite interesting and we will look at it separately towards the end of this tutorial.

The amplified signals are passed on to the display. The display is either a CRT like an oscilloscope, or a LCD as in laptop computers. They display the returned signals on a simple XY plot with the range across the bottom and the power level in dB up the side.

The following figure shows a typical display. The current parameter settings are shown over the grid. They can be changed to suit the measurements being undertaken. The range scale displayed shows a 50km length of fiber. In this case it is from 0 to 50km but it could be any other 50km slice, for example, from 20km to 70km. It can also be expanded to give a detailed view of a shorter length of fiber such as 0-5m, or 25-30m.

The range can be read from the horizontal scale but for more precision, a variable range marker is used. This is a movable line which can be switched on and positioned anywhere on the trace. Its range is shown on the screen together with the power level of the received signal at that point. To find the length of the fiber, the marker is simply positioned at the end of the fiber and the distance is read off the screen. It is usual to provide up to five markers so that several points can be measured simultaneously.

An internal memory or floppy disk can store the data for later analysis. The output is also available via RS232 link for downloading to a computer. In addition, many OTDRs have an onboard printer to provide hard copies of the information on the screen. This provides useful ‘before and after’ images for fault repair as well as a record of the initial installation.

If we were to connect a length of fiber, say 300m, to the OTDR the result would look as shown in the following figure.

Whenever the light passes through a cleaved end of a piece of fiber, a Fresnel reflection occurs. This is seen at the far end of the fiber and also at the launch connector. Indeed, it is quite usual to obtain a Fresnel reflection from the end of the fiber without actually cleaving it. Just breaking the fiber is usually enough.

The Fresnel at the launch connector occurs at the front panel of the OTDR and, since the laser power is high at this point, the reflection is also high. The result of this is a relatively high pulse of energy passing through the receiver amplifier. The amplifier output voltage swings above and below the real level, in an effect called ringing. This is a normal amplifier response to a sudden change of input level. The receiver takes a few nanoseconds to recover from this sudden change of signal level.

The Fresnel reflection and the subsequent amplifier recovery time results in a short period during which the amplifier cannot respond to any further input signals. This period of time is called a dead zone. It occurs to some extent whenever a sudden change of signal amplitude occurs. The one at the start of the fiber where the signal is being launched is called the launch dead zone and others are called event dead zones or just dead zones.

As the launch dead zone occupies a distance of up to 20 meters or so, this means that, given the job of checking a 300m fiber, we may only be able to check 280m of it. The customer would not be delighted.

To overcome this problem, we add our own patch cord at the beginning of the system. If we make this patch cord about 100m in length, we can guarantee that all launch dead zone problems have finished before the customers’ fiber is reached.