What can you see with High Spatial Resolution RL?

Aida Rahim, PhD
Field Applications Engineer
Lightwave Division

Our previous blog post on return loss (RL) gave an overview of some examples of events along the optical network that cause high RL, and their effects. To recap, in technical terms, RL is the ratio of the light reflected back from a device under test, P_out, to the light launched into that device, P_in, usually expressed as a negative number in dB.

                RL = 10 log₁₀(P_out/P_in)

Compared to other reflectometry techniques used for measuring RL, the advantages of our OBR product line are high spatial resolution, high sensitivity, and near-zero dead-zone. Since measurements can be obtained at spacings as small as 10 microns over 30 m or a few mm over 2km, users can identify faults in their optical network that were previously unseen, or that would merge into nearby higher-reflection events.

Identify Unseated Connector

It is important to measure the RL of optical connections to verify the quality of the connection.  High RL can indicate a poor quality polish, a dirty connection, or a poor quality mate. In some cases, while the insertion loss (IL) of a connection can still be good, the RL can indicate a failure waiting to happen. For example, we measured the RL of a few angle polished LC connections at the end of a fiber spool. The figure below shows the reflected amplitude, zoomed in at approximately 150 m along the fiber assembly. The black trace was taken with the LC connectors at about 150.6 m properly seated while the red trace was taken with the connectors in contact but not securely locked. An RL measurement comparison reveals more than 8 dB difference where corresponding IL measurements show no change (-0.18 dB). The unseated connection would easily become disconnected during normal use.  By monitoring connector RL, the user can therefore identify faulty connections that would otherwise pass IL inspections, predicting and potentially mitigating catastrophic failure [1].

Good vs Bad Coupler

The example scans of couplers below further emphasize the benefits of measuring for both RL as well as IL when screening components or parts. The IL of the two couplers is nearly identical. A look at the RL, however, identifies a small reflection peak in the bad coupler, which indicates a crack in the coupler. While this RL is very low (typically around -80 dB), our experience has shown that such a coupler will break in the field, leading to complete failure.  High RL sensitivity and high spatial resolution are required to detect this small crack in the coupler.  Screening for this failure mode has saved our customers from experiencing equipment failures due to broken couplers.  

Where is the Fault?

When an event happens very close to a connector, having a near-zero dead-zone and high spatial resolution comes in very handy. It allows the user to accurately figure out on which side of a connection an event is taking place. Let’s look at the example of an FC/APC-FC/APC connector with a bent boot very close to the connector. Two measurements were taken, with either the FC boot bent on the left or on the right of the connector. The location of this bend can be easily seen in the amplitude trace as the broader peak on the left in the black trace and the broader peak on the right in the red trace.  Not only can you tell the difference between a bend and a connection by the width of the reflection peak, you can identify the precise location of the fault.  If this were a fault in a network on a small aircraft, for example, this could help identify which panel to remove to get access to repair the fault, saving time (and therefore $) in the repair process.

In summary, high resolution, accurate and sensitive RL measurements can be critical in identifying and precisely locating faults in an optical network that might otherwise go undetected or be poorly identified or localized.  This can help save time in finding and fixing problems in a network and also help to avoid catastrophic failures in the field due to faulty connections or components. 

References:

[1] Bos, J.J. et al., “Mode Conditioner and Portable High-Resolution Reflectometer for Maintenance and Diagnostics of Single and Multi-mode Avionic Fiber Networks,” Avionics Fiber-Optics and Photonics, 2011 IEEE