Introduction
One of the most important tools in a fibre engineers toolkit is an Optical Time Domain Reflectometer (OTDR). OTDR measurements are used to diagnose faults, such as breaks and bends, in fibre optic cables.
But what can affect the accuracy of your OTDR's readings? In this blog we will look at some of the key areas that can impact your results.
The Type of Fibre
Different types of optical fibres have different attenuation, dispersion, and reflectivity characteristics, which can impact the accuracy of OTDR measurements.
Attenuation
Attenuation refers to the loss of signal strength as light travels through a fibre. Different types of fibre have different attenuation characteristics, which can impact OTDR readings. For example, single-mode fibres have lower attenuation than multimode fibres, so they can be measured over longer distances without suffering significant signal loss.
Dispersion
Dispersion refers to the way that different wavelengths of light travel at different speeds through a fibre. Different types of fibre have different dispersion characteristics.
Reflectivity
Reflectivity refers to the ability of a fibre to reflect light back towards the source. Different types of fibre have different reflectivity characteristics.
Fibre length
The length of a fibre can affect OTDR readings in a number of ways.
Attenuation
As the length of the fibre increases, the signal power transmitted through the fibre can decrease due to attenuation. This can result in lower OTDR readings at longer distances.
Dispersion
Dispersion is the spreading out of a signal as it travels down a fibre. As the length of the fibre increases, dispersion can become more significant, resulting in distortion of the signal, and potentially affect OTDR readings.
Reflectance
The reflectance of a fibre varies depending on its length. As the length of the fibre increases, the number of splices, connectors, and other components in the fibre can increase, which can affect the amount of light that is reflected back to the OTDR.
Noise
Longer fibres can be more susceptible to noise, which can affect the accuracy of OTDR readings.
Connector Quality
Connector quality can have a significant impact on OTDR readings. Any imperfections in the connectors can cause signal loss, reflections, and other issues that can affect the accuracy of OTDR measurements.
Insertion Loss
When a signal is transmitted through a fibre, some of the signal power is lost at each connector or splice. This loss is called insertion loss, and it can vary depending on the quality of the connectors being used. High-quality connectors generally have lower insertion loss.
Reflections
When a signal encounters a connector or splice, some of the light can be reflected back towards the OTDR. This reflection can interfere with the original signal and cause errors in OTDR measurements. High-quality connectors are designed to minimise reflections.
End-face contamination
Any contamination on the end-face of a connector can cause signal loss, reflections, or other issues that can affect OTDR readings. High-quality connectors are designed to minimise end-face contamination.
To ensure that connector quality does not affect OTDR readings, it is important to use high-quality connectors and to inspect and clean connector end-faces regularly. It is also recommended to use connector adapters or reference cables to properly align the connectors and ensure accurate measurements.
Bend Radius
The bend radius is the minimum radius that a fibre can be bent without damaging the fibre.
Attenuation
When a fibre is bent beyond its minimum bend radius, the signal can experience higher levels of attenuation. This can result in lower signal power and lower OTDR readings.
Dispersion
When a fibre is bent, the signal can also experience higher levels of dispersion, which can cause distortion of the signal.
Reflectance
When a fibre is bent, it can cause reflections at the bend point. These reflections can interfere with the original signal and affect OTDR readings.
To minimise the effects of bend radius, it is important to adhere to the recommended minimum bend radius for the fibre being used. This may involve using specialised bend-insensitive fibres or careful handling techniques to avoid excessive bending.
Signal-to-noise ratio (SNR)
The signal-to-noise ratio (SNR) can have a significant impact on OTDR readings. The SNR is a measure of the signal strength relative to the background noise in the fibre. A higher SNR indicates a stronger signal and less background noise, while a lower SNR indicates a weaker signal and more background noise.
Accuracy
A higher SNR generally results in more accurate OTDR measurements. This is because a stronger signal can overcome background noise and provide a more reliable measurement of the fibre characteristics.
Resolution
A higher SNR can also improve the resolution of OTDR measurements. This is because a stronger signal can provide more detail about the fibre characteristics and help to distinguish small changes in the fibre.
Sensitivity
The sensitivity of an OTDR can be affected by the SNR. A lower SNR may require a higher sensitivity setting, which can increase the likelihood of errors in the measurement.
To optimise SNR and ensure accurate OTDR readings, it is important to use appropriate launch and receive cables, as well as appropriate measurement settings on the OTDR. It may also be necessary to take steps to reduce background noise, such as minimising ambient light or electromagnetic interference in the fibre.
Dead Zones
Dead zones can have a significant impact on OTDR readings. A dead zone is an area in the fibre where the OTDR cannot accurately measure the characteristics of the fibre. This can be caused by several factors, such as fibre bends, splices, connectors, or other obstructions in the fibre.
Inaccuracy
Dead zones can result in inaccurate OTDR measurements, as the OTDR cannot accurately measure the characteristics of the fibre in these areas. This can lead to errors in the measurement and potentially affect the reliability of the results.
Resolution
Dead zones can also affect the resolution of OTDR measurements, as the OTDR cannot provide detailed measurements of the fibre characteristics in these areas. This can result in a loss of detail and potentially affect the accuracy of the measurements.
Sensitivity
The sensitivity of an OTDR can be affected by dead zones. A larger dead zone may require a higher sensitivity setting, which can increase the likelihood of errors in the measurement.
To minimise the effects of dead zones on OTDR readings, it is important to use appropriate launch and receive cables and to optimise the measurement settings on the OTDR. It may also be necessary to use specialised equipment, such as an event dead zone eliminator or a launch box, to reduce the dead zones and improve the accuracy of the measurements.
Temperature
Temperature can affect OTDR readings in several ways.
Refractive Index
The refractive index of a fibre is temperature dependent, which can affect the speed of light in the fibre. This can cause a shift in the position of OTDR reflections and affect the accuracy of the measurements.
Attenuation
Temperature changes can also cause changes in fibre attenuation. As the temperature increases, the fibre attenuation typically increases, which can result in lower signal power and lower OTDR readings.
Sensitivity
The sensitivity of an OTDR can also be affected by temperature. A change in temperature can cause changes in the noise floor, which can affect the sensitivity of the OTDR and potentially affect the accuracy of the measurements.
To minimise the effects of temperature on OTDR readings, it is important to ensure that the fibre is within the recommended temperature range for the type of fibre being used. It may also be necessary to use specialised equipment, such as temperature-controlled launch boxes or receive cables, to help stabilise the temperature of the fibre during measurements.
Wavelength
The wavelength of light used in an OTDR measurement can have a significant impact on the OTDR reading. The wavelength determines how the light interacts with the fibre and how the reflections are detected.
Attenuation
The attenuation of light in a fibre is wavelength dependent. The attenuation is typically lower at longer wavelengths, so OTDR measurements made at longer wavelengths may have a longer range and better sensitivity.
Resolution
The resolution of OTDR measurements is inversely proportional to the wavelength of light used. This means that shorter wavelengths provide better resolution than longer wavelengths. However, shorter wavelengths also have higher attenuation, which can reduce the measurement range.
Reflectivity
The reflectivity of a fibre’s connectors and splices is also wavelength dependent. This means that different wavelengths of light will produce different levels of reflection at the same connector or splice.
To optimise OTDR readings, it is important to use the appropriate wavelength for the specific measurement and fibre type. For example, shorter wavelengths may be preferred for testing shorter lengths of fibre, while longer wavelengths may be preferred for longer fibre lengths.
Conclusion
To ensure you are getting accurate OTDR readings, it is important to consider the factors that can affect them and take appropriate measures, such as using high-quality connectors, avoiding excessive bending of fibres, and using appropriate pulse widths and wavelengths for the specific fibre type being measured.
Calibration and regular maintenance of equipment also plays an important part for continually obtaining reliable OTDR readings.
If you want to take a closer look at our range of OTDRs click here.
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