Testing and troubleshooting fiber-optic connectors

April 25, 2018
There are four different methods to facilitate network connections, each with their own advantages and disadvantages: Pre-terminated cables, mechanical connectors, cutting a patch cord in half and splicing each half jumper, and splice-on connectors. But the key to success with any connection method is properly cleaning and inspecting the ferrule end face to minimize loss and reflectivity.

As fiber reaches deeper into telecommunication networks, the need for maintaining the integrity of these networks becomes increasingly important. It is imperative that best practices are employed and that high-quality components are used throughout the installation and verification process of fiber networks. No matter how technologically sophisticated the network, basic components of the system can severely impair connectivity or create a system failure. One of the most basic components of fiber-optic networks is the lowly connector. The connector is typically given the least amount of thought, except to seek out cost "savings"; however, this humble component can have a large impact on the reliability of the network.

There are four different methods to facilitate network connections, each with their own advantages and disadvantages: Pre-terminated cables, mechanical connectors, cutting a patch cord in half and splicing each half jumper, and splice-on connectors. But the key to success with any connection method is properly cleaning and inspecting the ferrule end face to minimize loss and reflectivity.

Know Your Options

Splice-on connectors provide a low insertion loss, low return loss approach and do not require a splice tray, as the splice protector is housed in the strain relief of the connector. Since the connection point in a splice-on connector is a fusion splice there is no mechanical integrity issue and the long-term performance is not a factor. The insertion loss of a fusion splice is typically <0.02 dB, which is much less than the specification for a typical connector at 0.3 dB. If a mated connector is clean and in good condition the insertion loss should be <0.1 dB.

Mechanical connectors can have a maximum insertion loss of 0.75 dB (including the loss from the ferrule connection) but the main concern with mechanical connectors remains the potential for a high return loss. Mechanical connectors have a possible failure point where the field fiber is connected to the ferrule fiber within the connector, which can also cause high insertion and return losses. Mechanical connectors should only be used in emergency restoration situations and be subsequently replaced at a later date.

Figure 1. Image of a typical LC splice-on connector.

Mated Angle Polished Connectors (APCs) will have a typical return loss (reflectivity) of approximately -70 dB. If the field fiber separates from the fiber stub inside of a mechanical connector, the return loss could increase significantly. An open glass to air interface is commonly referred to as a 4% reflection and is quantified as -14.4 dB. This would cause a very high reflective event and a high insertion loss. If a large reflection such as this is present in the network, the ability of the fiber link to reliably transport data can be severely impaired.

Trouble tips

Over 80% of network failures are attributed to improper cleaning of connector end faces (or to not cleaning them at all). A fiber link that is not clean will not transmit data efficiently or will have a total failure due to high insertion loss and/or a high return loss. Return loss is the measure of the reflectivity of the connector. Reflections are the enemy of fiber-optic networks and need to be minimized at any and every opportunity. Today's high-bandwidth applications such as 4K TV require fiber links that have low reflectivity to reliably transport the content to the consumer. To insure contaminated or damaged connectors are not installed in the network, the technician needs to inspect before every connection is made, clean if necessary, inspect again and then if the connector is deemed to be clean and in good condition, make the connection.

If a connector is not cleanable or is damaged it should be replaced to avoid compromising data transference. This must also be repeated for the bulkhead connector. Failing to perform the cleaning and inspecting process with both the connector ferrule and the bulkhead ferrule is only doing half of the job and most certainly will cause a failure.

Figure 2. Example test result of connector ferrule that passes IEC specifications.

If the above preventative guidelines have been followed, the connections should be successful. But, "things happen," at which time the technician needs to be able to diagnose the failure that is caused by a connector. One troubleshooting method is to use a visual fault locator (VFL). When a ferrule of a connector is inserted into the VFL, the red laser light emitted should be efficiently coupled into the fiber. If the light "explodes" at the connector, this indicates that the connector is contaminated or damaged. This is a highly subjective analysis on the part of the technician and is not the recommended method to troubleshoot. If connection to the VFL is good, the red laser light will be able to propagate down the fiber and the technician will be able to also identify kinks in the fiber and or discontinuities.

The most powerful tool in the technician's toolbox is the optical time-domain reflectometer (OTDR). Today's OTDRs can pinpoint the location, loss and reflectivity of each event and the optical return loss (ORL) of the entire fiber link.

When testing, the first step is to make sure that the bulkhead and launch cable connections are clean and not damaged. This insures that the probe pulse emitted from the OTDR is effectively coupled to the fiber link under test. Once the field fiber is connected, the technician must set the range of the OTDR to capture the entire length of the fiber. The optimum setting would be 75% fiber link and 25% noise after the end of the fiber. The pulse width is usually automatically set to accommodate the range selected, so the technician can now initiate a measurement.

Once the measurement is complete, the various events are annotated. Sometimes the technician will need to adjust the pulse width and averaging times to obtain a better idea of what each fault is. Increasing the averaging time will smooth the OTDR trace and allow for a clearer representation of very small events. Increasing the pulse width will couple more energy into the fiber under test but will result in poorer resolution. Decreasing the pulse width will improve the resolution but also reduce the maximum distance that the OTDR can probe. A wider pulse width will also increase the dead zone of the OTDR. The event dead zone of the OTDR is the ability of the OTDR to resolve between two reflective (Fresnel) events. The attenuation dead zone is the ability of the OTDR to measure a non-reflective event (Raleigh) after a reflective (Fresnel) event.

Figure 3. An OTDR measures the length to a fault
or the length of singlemode and multimode fiber-optic cables

In the case of a faulty connector, the event will likely show a reflective component (Fresnel) and loss component (Raleigh).

Figure 4. Example OTDR image of a lossy connector
(IL = 1.09 dB; RL = -35.7 dB).

This requires the technician to be able to carefully setup the OTDR to minimize the dead zone to enable effective measurement and diagnosis of the performance of a faulty connector. If the pulse width is set too wide, the faulty connector may never be "seen" because that connector was too close to another event and will be hidden by the dead zone of the OTDR.

As fiber networks are tasked with transmitting more data, it is imperative that the technician cleans and inspects every connector before they are connected to the network. This is necessary to ensure that all future high-bandwidth applications operate reliably. The ability of the technician to effectively measure the performance of an optical connector using an OTDR is necessary to enable quick identification and repair of the fiber link back to operational status.

Keith Foord is product manager, fiber optics at Greenlee Communications.