Why Do We Receive Dead Zone in OTDR?

Dead Zone in OTDR can significantly impact the measurement of optical fibres. The Optical Time Domain Reflectometer (OTDR) is a popular tool used in the telecommunications industry to measure the quality and integrity of these fibres.

They works by sending a light pulse down a fibre and measuring the reflection that comes back. However, in some cases, the OTDR may fail to detect a portion of the fibre, creating a dead zone. These zones can occur due to several reasons, including fibre quality, equipment limitations, and measurement errors.

In this article, we will discuss the various reasons behind dead zones and ways to minimize them.

What is a Dead Zone in OTDR?

Before we dive into the causes of dead zones, let us first understand what it means. A dead zone in OTDR is an area of a fibre optic cable where the OTDR is unable to detect any reflections or signal. This is usually caused by a high attenuation or loss of signal strength in the fibre.

The dead zone typically occurs at the beginning of the fibre, where the OTDR is unable to distinguish between the reflection from the connector and the reflection from the fibre. Dead zones can also occur at the end of the fibre due to the reflection from the splice or termination point.

Length of the dead zone depends on several factors, such as fibre quality, wavelength, and pulse width.

Causes of Dead Zones in OTDR

Fibre Quality

Fibre quality is one of the significant factors that can cause dead zones in OTDR. A dead zone can occur due to a high attenuation coefficient in the fibre. Attenuation is the reduction in signal strength as it travels through the fibre.

High attenuation can be caused by various factors, including fibre bending, micro-bending, and macro-bending. In general, single-mode fibres have lower attenuation than multimode fibres, and therefore, dead zones are less likely to occur in single-mode fibres.

Equipment Limitations

Another factor that can cause dead zones is equipment limitations. The performance of an OTDR depends on the quality and specifications of the instrument. The length of the dead zone is directly proportional to the pulse width of the laser. A shorter pulse width results in a shorter dead zone, while a longer pulse width results in a longer dead zone.

Therefore, it is crucial to choose an OTDR with the appropriate pulse width for the fibre being tested. The quality of the OTDR’s launch cable and the connector can also affect the dead zone. Poor quality launch cables and connectors can cause high reflectance, which can lead to a longer dead zone.

Measurement Errors

Measurement errors can also cause dead zones in OTDR. It is essential to follow the proper testing procedures and techniques to ensure accurate measurements. The OTDR should be properly calibrated before testing, and the launch cable should be properly installed.

Inadequate calibration or installation of the launch cable can cause signal reflections, leading to an increased dead zone.

Minimizing Dead Zones in OTDR

Use High-Quality Fibre

Using high-quality fibre can help minimize dead zones in OTDR. Single-mode fibres typically have lower attenuation than multimode fibres and are less prone to dead zones.

It is also essential to ensure that the fibre is not damaged or contaminated, as this can affect the signal strength.

Choose the Right Equipment

Choosing the right equipment can also help minimize dead zones in OTDR. The OTDR should have the appropriate specifications for the fibre being tested, such as the appropriate pulse width and dynamic range. The quality of the launch cable and connector should also be taken into account.

Proper Calibration and Installation

Proper calibration and installation of the OTDR and launch cable are crucial in minimizing dead zones. The OTDR should be calibrated before testing, and the launch cable should be installed correctly.

The launch cable should also be of high quality and properly cleaned to prevent any signal reflections that can cause dead zones.

Use a Fibre Breakout Box

A fibre breakout box can help in minimizing dead zones in OTDR. A fibre breakout box is a device that allows a single fibre to be divided into multiple fibres, allowing for more accurate measurements.

By using a fibre breakout box, the reflection at the connector can be eliminated, resulting in a shorter dead zone.

Use Averaging and Smoothing Techniques

Averaging and smoothing techniques can also help in minimizing dead zones in OTDR. These techniques involve taking multiple measurements and averaging them to reduce the impact of any measurement errors or fluctuations in the signal. Smoothing techniques involve filtering out any noise in the signal to obtain a more accurate reading.

Conclusion

Dead zones in OTDR can be caused by various factors, including fibre quality, equipment limitations, and measurement errors.

Dead zones can be minimized by using high-quality fibre, choosing the appropriate equipment, proper calibration and installation, using a fibre breakout box, and using averaging and smoothing techniques.

It is crucial to follow the proper testing procedures and techniques to ensure accurate measurements and reduce the impact of dead zones.

By minimizing dead zones, the reliability and quality of the fibre optic network can be improved, leading to better communication and connectivity.