Extinction ratio testers for FTTH networks may experience measurement limitations, such as blind zones, when testing very short fiber spans like 5 meters, due to device response time and optical pulse width.

Understanding Extinction Ratio in FTTH

The extinction ratio (ER) is the ratio of optical power transmitted for a logic '1' to that for a logic '0' in a digital optical signal. It is a key parameter for evaluating the performance of optical transmitters and the quality of fiber links, particularly in FTTH networks . Accurate ER measurements ensure proper biasing of lasers and help maintain low bit-error rates (BER) in the system.

Attenuation Blind Zone

A blind zone refers to a short distance near the tester or source where the device cannot accurately measure attenuation or extinction ratio. This occurs because the optical pulse has a finite width, and reflections or backscatter from very short fiber segments may overlap with the initial pulse, preventing the tester from resolving the signal correctly . For many handheld FTTH testers, the minimum measurable fiber length can range from a few meters to tens of meters, depending on the device and wavelength. For a 5-meter fiber span, the following factors are relevant:

• Pulse width and resolution: Shorter fibers require testers with narrow pulse widths to avoid overlap and ensure accurate ER readings.
• Device response time: Handheld testers like the EXFO FiberBasix 50 series can measure attenuation and ER, but very short fibers may fall within the blind zone, leading to inaccurate readings .
• Connector reflections: In short fibers, reflections from connectors can dominate the signal, affecting the measured extinction ratio .

Practical Recommendations

1. Use high-resolution testers: Devices with fast response and narrow pulse widths reduce the blind zone effect.
2. Minimize connector reflections: Ensure connectors are clean and properly mated to reduce measurement errors.
3. Consider reference patchcords: Using a known reference fiber can help calibrate the tester and compensate for short-span limitations .
4. Check device specifications: Some polarization extinction ratio meters, such as the OZ Optics DTS0068, include features to handle short fibers and maintain accuracy .

Summary

While FTTH extinction ratio testers are effective for standard fiber lengths, very short spans like 5 meters may fall within the attenuation blind zone, causing potential inaccuracies. Selecting a tester with appropriate pulse width, using reference fibers, and minimizing reflections are key strategies to obtain reliable measurements in short fiber links .