How Does an OTDR Work?

Unlike sources and power meters which measure the loss of the fiber optic cable plant directly, the OTDR works indirectly. The source and meter duplicate the transmitter and receiver of the fiber optic transmission link, so the measurement correlates well with actual system loss. The OTDR, however, uses a unique optical phenomenon of fiber to indirectly measure loss.

The biggest factor in optical fiber loss is scattering. In fiber, light is scattered in all directions, including some scattered back toward the source as shown here. The OTDR uses this "backscattered light" to make measurements along with reflected light from connectors or cleaved fiber ends.

The OTDR consists of a high power laser transmitter that sends a pulse of light down the fiber. Back-scattered light and reflected light returns to the OTDR through the fiber and is directed to a sensitive receiver through a coupler in the OTDR front end. For each measurement, the OTDR sends out a very high power pulse and measures the light coming back over time. At any point in time, the light the OTDR sees is the light scattered from the pulse passing through a region of the fiber. Think of the OTDR pulse as being a "virtual source" created by the scattering that is testing all the fiber between itself and the OTDR as it moves down the fiber. Since it is possible to calibrate the speed of the pulse as it passes down the fiber from the index of refraction of the glass in the core of the fiber, the OTDR can correlate what it sees in backscattered light with an actual location in the fiber. Thus it can create a display of the amount of backscattered light at any point in the fiber along its length.

There are some calculations involved. Remember the light has to go out and come back, so you have to factor that into the time calculations, cutting the time in half. One must also cut the loss in half since the light sees loss both ways. The power loss is a logarithmic function, so the power is measured and displayed in dB.

The amount of light scattered back to the OTDR is proportional to the backscatter of the fiber, peak power of the OTDR test pulse and the length of the pulse sent out. If you need more backscattered light to get good measurements, you can increase the pulse peak power or pulse width or send out more pulses and average the returned signals. All three are used in many OTDRs, with user control of some of the selections.

OTDRs are always used with a launch cable and may use a receive cable. The launch cable, sometimes also called a "pulse suppressor," allows the OTDR to settle down after the test pulse is sent into the fiber and provides a reference connector for the first connector on the cable under test to determine its loss. A receive cable may be used on the far end to allow measurements of the connector on the end of the cable under test also.

Information in the OTDR Trace

They say a picture is worth a thousand words, and the OTDR picture (or "trace" as they are called) takes a lot of words to describe all the information in it! Consider the diagram of a trace at the right.

The slope of the fiber trace shows the attenuation coefficient of the fiber and is calibrated in dB/km by the OTDR. In order to measure fiber attenuation, you need a fairly long length of fiber with no distortions on either end from the OTDR resolution or overloading due to large reflections. If the fiber looks nonlinear at either end, especially near a reflective event like a connector, avoid that section when measuring loss.

Connectors and splices are called "events" in OTDR jargon. Both should show a loss, but connectors and mechanical splices will also show a reflective peak. The height of that peak will indicate the amount of reflection at the event unless it is so large that it saturates the OTDR receiver. Then peak will have a flat top and tail on the far end, indicating the receiver was overloaded.

Sometimes, the loss of a good fusion splice will be too small to be seen by the fiber testing OTDR. That's good for the system but can be confusing to the operator. It is very important to know the lengths of all fibers in the network, so you know where to look for events and won't get confused when unusual events show up (like ghosts, we'll describe below.)

Reflective pulses can show you the resolution of the OTDR. You cannot see two events closer than is allowed by the pulse width. Generally, longer pulse widths are used to be able to see farther along the cable plant and narrower pulses are used when high resolution is needed, although it limits the distance the OTDR can see.

Making Measurements With The OTDR

Fiber Attenuation by Two Point Method.

The OTDR measures distance and loss between the two markers. This can be used for measuring the loss of a length of the fiber, where the OTDR will calculate the attenuation coefficient of the fiber or the loss of a connector or splice.

To measure the length and attenuation of the fiber, we place the markers on either end of the section of fiber we wish to measure. The OTDR will calculate the distance difference between the two markers and give the distance. It will also read the difference between the power levels of the two points where the markers cross the trace and calculate the loss or difference in the two power levels in dB. Finally, it will calculate the attenuation coefficient of the fiber by dividing loss by distance and present the result in dB/km, the normal units for attenuation.

In order to get a good measurement, it is necessary to find a relatively long section of fiber to give a good baseline for the measurement. Short distances will mean small amounts of loss, and the uncertainty of the measurement will be higher than if the distance is longer. It is also advisable to stay away from events like splices or connectors, as the OTDR may have some settling time after these events, especially if they are reflective, causing the trace to have nonlinearities caused by the instrument itself.

Fiber Attenuation by Least Squares Method

The OTDR measures distance and loss between the two markers but calculates the best fit line between the two points mathematically using the "least squares" method to reduce noise. When the markers are selecting the noisy part of the fiber trace, the least squares attenuation (2-pt LSA) tool can be applied to calculate the dB loss between the cursors. Look closely and you will see a thick grey line between the markers, indicating the best fit to the trace, averaging all the noise.

Splice Loss by Two Point Method

The OTDR measures distance to the event and loss at an event - a connector or splice - between the two markers.

To measure splice loss, move the two markers close to the splice to be measured, having each about the same distance from the center of the splice. The splice won’t look as neat as this, with the instrument resolution and noise making the traceless sharp looking, as you will see later on. The OTDR will calculate the dB loss between the two markers, giving you a loss reading in dB.

Measurements of connector loss or splices with some reflectance will look very similar, except you will see a peek at the connector, caused by the back reflection of the connector.

Splice Loss by Least Squares (LSA)

The OTDR measures distance and loss at an event - a connector or splice - between the two markers but calculates the best fit line between the two points using the "least squares" method to reduce noise.

If you noticed, the markers are separated by some distance, which includes the loss of some fiber on either side of the actual connector or splice Most OTDRs will calculate the loss for you by extrapolating the fiber traces on both sides of the event and calculating the loss without any influence from the fiber length. The mathematical method uses is called "Least Squares Approximation", hence the term "LSA" used by many OTDRs in their display and setup menus.
Setting LSA requires setting several markers - one on the peak, the two regular markers near the event and the two end markers which define the segments used for least-squares analysis. These segments should be long enough to allow good measurement but not so long as to approach other events.

Reflectance

The OTDR measures the amount of light that's returned from both backscatter in the fiber and reflected from a connector or splice. The amount of light reflected is determined by the differences in the index of refraction of the two fibers joined a function of the composition of the glass in the fiber, or any air in the gap between the fibers, common with terminations and mechanical splices.

This is a complicated process involving the baseline of the OTDR, backscatter level and power in the reflected peak. Like all backscatter measurements, it has a fairly high measurement uncertainty but has the advantage of showing where reflective events are located so they can be corrected if necessary.

By choosing the reflectance measurement and putting the right (blue) cursor on the peak of the reflection and the left (red) cursor just to the left of the reflection, the OTDR will measure the reflectance.

Comparing Traces

Comparing two traces in the same window is useful for confirming data collection and contrasting different test methods on the same fiber. Comparisons are also used to compare fiber traces during troubleshooting with traces take just after installation to see what has changed. All OTDRs offer this feature, where you can copy one trace and paste it on another to compare them. Here is an example of how you can use this feature.

Note that the two traces are taken from the same multimode fiber cable plant at different test wavelengths. The major difference in the slope of the traces displays the different attenuation coefficient of the fiber. The blue line (top) represents the attenuation coefficient of the cable in at 1300 nm, the green line (bottom) represents the same cable measured at 850 nm. There is also a noticeable difference in the reflectance at the splice. Variations in reflectance due to the wavelength difference is not unusual.

Other reasons you might want to compare two traces includes:

• Compare several fibers in the same cable to see if they are different.
• Traces taken at different times to see if the cable has changed.
• At different wavelengths, since fiber is more sensitive to stress at longer wavelengths, this allows finding stress points caused by installation.
• At different pulse widths (below) to decide which setting gives the best compromise between noise and resolution or to find events lost with wide pulse widths.

Read the complete solution of OTDR and optical power meter/RF Meter. Get the brand best OTDR from Candid Optronix Pvt Ltd.