The OTDR takes advantage of the scattering of light in optic fiber to make measurements. This includes a constant low level of backscatter created by the fiber called Rayleigh backscattering and a high-reflection peak at the connection point called Fresnel reflection.
Using these two methods, the OTDR can detect a range of physical events along the length of a link. These include connectors, splices and any other discontinuity that doesn’t totally break the fiber.
The Light Pulse
An otdr measures the light pulse that travels down an optical fiber. It does this by sending out a pulse, measuring the time it takes to go out, and then analyzing the light that comes back.
The light pulse is then processed by an otdr's algorithm to identify intrusion events. The algorithm is based on Dempster-Shafer evidential theory, which is derived from physics and applied experimentally.
Its output is then filtered and stored in a memory called a flash. This is then compared with the flashes in a flash file. If the two flashes are different, a false alarm will be created.
OTDRs use a pulse width multiplexing method that helps to reduce nuisance alarms. This multiplexing is done by setting a different width on each test pulse and then combining the data from all of the tests. This allows the otdr to see more events and reduce false and missing alarms.
In addition, it also enables the otdr to measure chromatic dispersion (CD), which is a type of scattering in which light waves change their wavelength depending on where they are reflected from. This information can help to determine the quality of an optical fiber.
A key factor that affects the accuracy of the otdr's measurement is its pixel resolution. This resolution is estimated based on laboratory-measured pixel angular resolutions ameas, which are high-biased due to the effects of laboratory atmosphere; however, asymmetries, which reveal actual defects in CCD/lens construction or mounting, can also affect the otdr's measurements.
We estimate that the pixel side-to-side ground range resolution of the OTD sensor, based on uniform estimates of ai,j from laboratory-measured pixel angular values, is typically 20 km to 40 km. These are estimates assuming that the pixel field of view is nonoverlapping and that the CCD design is perfect.
The diurnal lightning cycle over land is very pronounced, and sampling bias is likely to be present in the OTD's data due to aliasing from this cycle. Microlab-1 revisits a given equatorial earth coordinate and local hour every 55 days, so if the OTD's measurements are not smoothed over sufficiently long time scales, aliasing from this cycle can be quite severe.
Backscatter
Backscatter is an important part of how an OTDR works. It is the amount of light that is reflected or scattered back from the outgoing test pulse and then measured by the sensor on the OTDR. The otdr uses this information to determine loss.
One of the most important sources of error that can occur when testing is a difference in the backscatter coefficient from each fiber. This can be due to mismatches in the two fibers that are mated together. This will cause a gain in the power at the splice point, which looks like a "gain" on the trace and can be interpreted by the otdr as a splice loss.
In addition to this, a mismatch between the splice and the connector or splitter can also lead to loss measurement errors. This can be particularly common in long-fiber systems.
The dynamic range of the Palm OTDR, which is determined by the difference in the backscatter signal at the front end of the fiber and the noise floor at the far end, also plays an important role in determining loss accuracy and visibility. This is mainly due to the fact that backscatter levels can vary significantly over time. This is why it is essential that the OTDR be calibrated to a set level.
A simulated optical fiber backscatter waveform generator (OFBSG) can be used to generate these simulated backscatter signals that can then be coupled into the OTDR so that length accuracy, loss accuracy and dynamic range can be assessed. This will also help to ensure that the OTDR is operating within its intended scope.
Another way that a simulated backscatter can be produced is by using an otdr to send a series of different pulses into a fiber. This can be done for testing purposes or to show more details in a longer fiber.
The simulated backscatter can be generated by the OTDR itself or can be derived from a software application. This can be useful for testing in areas that are not able to be accessed by a real backscatter device.
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Reflection
Reflection is a key part of learning, and it can be used to help people improve their overall understanding of a certain topic. However, it's important to keep in mind that different types of reflection work well for different people. Consequently, you should make sure to choose the reflective practice that's right for your students and incorporate it into your teaching methods accordingly.
For example, some people might benefit from superficial reflection, where they simply identify the fact that they've done something wrong, while others might need to engage in deep reflection, where they can learn more about what went wrong and why it happened. In addition, you should avoid forcing reflection on people, as this can lead to ineffective reflection and a number of other issues.
One common issue with OTDRs is ghosting, which occurs when light pulses are sent down fiber that get reflected back up to the OTDR detector and then down the same fiber a second time, creating a false trace. This is a problem because it can hide real events that could be causing trouble.
Another common issue with OTDRs is loss errors, which occur when the light pulses that are sent down the fiber are influenced by different backscatter coefficients. This causes the percentage of light that is sent back to the OTDR to change. If two fibers are connected together with a fusion splice, for example, the percentage of light that is sent back will be lower than it would be if they were separated by a longer distance.
These differences in backscatter cause the loss of the OTDR to display a gain instead of a loss, resulting in confusion for OTDR users. This is a big source of error for the field, and it can be especially hard to resolve when two splices or connectors are placed in close proximity.
Luckily, most Mini OTDRs offer a comparison feature where you can copy one trace and paste it on another to compare them. This is especially useful if you're trying to confirm that your data collection method is accurate, as it allows you to check for a difference between two traces.
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Attenuation
The Attenuation is the amount of energy absorbed and deflected as it travels through a material. OTDRs measure attenuation and event loss by injecting an optical pulse into the fiber, then analysing the backscattered and reflected signal it produces. This allows the technician to fully characterise a fibre link and determine its performance.
An OTDR is a powerful little (or sometimes large) piece of kit that can give an in-depth view of a fibre link. It measures attenuation, event loss, reflectance and ORL allowing the technician to gain an overall view of the fibre link.
OTDRs have been used in fibre testing for over 20 years and have become commonplace on fibre networks around the world. They are easy to use and are much more reliable than traditional insertion loss tests.
To measure attenuation, an OTDR uses a time domain reflectometer that generates a short light pulse into the end of a fibre and then analyse the signal it returns. It then calculates the attenuation and event loss between the two markers, enabling technicians to identify any physical events that may be present along a fibre link.
The backscatter signals that return to the OTDR are a combination of Rayleigh scattering and Fresnel reflection. These small-amplitude reflections are triggered by the light leaving the glass core of the fibre and passing between air gaps within connectors or index matching gels in mechanical splices, for example.
When these events occur, they can be detected by the 7 inch multifunction OTDR and appear as spikes on its trace. This can be a very useful feature in detecting the loss and physical events such as splices and connectors that have been added to a fibre.
One important thing to remember is that OTDRs have a limited dynamic range, this means that they can only see a certain number of event points before they are unable to distinguish them from the background. This can be influenced by the pulse width, the refractive index of the fibre and the distance between the markers.
The shortest dead zone (EDZ) is usually 1 meter for single-mode OTDRs, and for multimode instruments it can be as low as 20 cm. This dead zone is the minimum distance for an OTDR to detect a consecutive non-reflective event following a reflective event.