Fiber Optic Wiring Troubleshooting Facts Flashcards
Connectors
For light to pass through a fiber optic connector, the fiber within the jack must line up perfectly with the fiber in the connector. Several issues can occur when you are working with fiber optic connectors.
Using the wrong connector will result in misaligned fibers, disrupting the light signal even if the connector is successfully locked into the jack.
Dirty cables and connectors can also impede or disrupt the light signal. It’s important that the cables and connectors are kept clean. Several cleaning methods can be used with fiber optic connectors:
For connectors where the ferrule protrudes out of the connector, such as the FC connector, you can wipe the end of the ferrule with a lint-free cloth that has a small amount of denatured alcohol applied. Immediately wipe the ferrule dry with a dry lint-free cloth.
For fiber optic connectors where the end of the ferrule is less accessible, you must use a specialized cleaning tool. Some cleaning tools allow you to plug the fiber optic cable into the tool and clean it by pumping the tool’s handle.
To clean the jacks on fiber optic network interfaces, you can
Polishing
Whenever a connector is installed on the end of fiber optic cable, a degree of signal loss occurs. This is called insertion loss.
Additionally, some of the light that is lost is reflected directly back into the cable, toward the source. This is called back-reflection, or optical return loss (ORL). It can corrupt the data being transmitted and even damage the transmitter.
For a connector to work properly, there must be as little insertion loss and ORL as possible. The better the polish on the connector, the better the light will pass through without reflection.
Fiber optic equipment manufacturers rate connectors using the following polish grading designations.
Physical contact (PC) polishing is usually used with single-mode fiber. The ends of the fiber are polished with a slight curvature so that when the cable end is inserted into the connector, only the cores of the fiber touch each other.
Super physical contact (SPC) and ultra physical contact (UPC) polishing use a higher grade of polish and have more of a curvature than PC polishing. This further reduces ORL reflections.
Angled physical contact (APC) polishing is used to reduce back reflection as much as possible.
An APC connector has an eight-degree angle cut into the ferrule.
The angle cut prevents reflected light from traveling back into the fiber.
Any reflected light is bounced into the cable cladding instead.
You can use angle-polished connectors only with other angle-polished connectors.
Using an angle-polished connector with a non-angle-polished connector causes excessive insertion loss.
APC connectors are colored green to prevent them from being mixed with non-APC connectors.
Damaged and mismatched cables
Several issues can occur when you are working with fiber optic cabling.
Fiber optic cabling is much less forgiving of physical abuse than copper wiring. The fiber core is fragile and can be easily damaged by rough handling. For example, bending a fiber cable at too tight of a radius will break the core.
Wavelength mismatch causes serious issues with fiber optic cables. You cannot mix and match types of cable.
For example, if you connect single-mode fiber to multi-mode fiber, you will introduce a catastrophic signal loss of up to 99%.
Even connecting cables of the same type that have different core diameters can cause a loss of up to 50% of the signal strength.
Media adapters and transceivers
Many network switches and routers allow you to insert a transceiver such as a gigabit interface converter (GBIC) in an empty slot to convert the interface from copper wiring to fiber optic. Other devices use a small form-factor pluggable (SFP) transceiver to accomplish the same goal.
Several issues can occur when using these and other fiber optic media adapters:
Some GBIC/SFP modules use multimode fiber; others use single-mode. Make sure that you use the correct type of fiber optic cable and connector required by the specific adapter.
Media adapter modules malfunction on occasion. If you have lost connectivity on one of these links, ensure that the adapter module is working correctly.
Signal loss
Light signals being transmitted through a fiber optic cable experience attenuation, or signal loss, as they pass through the cable due to:
Reflection - A measurable amount of light is reflected when it hits the ends of the cable.
Much of a cable’s reflection loss occurs at each cable connection.
When the light hits the boundary between the core and the cladding, it is reflected back into the core.
There is minor loss to the signal when this occurs, but it contributes to overall signal loss.
Refraction - If the light hits the boundary between the core and the cladding at too steep of an angle, the light is refracted into the cladding instead of reflected back into the core, causing signal loss.
Some fiber optic cables are doped with impurities near the edge of the fiber so that the signals are bent instead of reflected back to the center of the core.
The loss due to this refraction is minor when compared with the benefits of confining the light to the center of the core.
Scattering - Impurities in the fiber core can cause light to scatter.
Some of the light continues down the fiber.
The light that is scattered backwards contributes to the signal loss.
Absorption - Impurities in the fiber can also absorb the light, converting it to another form of energy, such as heat. This is a major cause of signal loss.
Several physical cable attributes can contribute to signal loss:
Cable length - While higher quality cables carry light signals further, the longer the cable, the more signal absorption and the greater the signal loss.
Connectors - Every connector causes some level of signal loss, mostly due to reflection. While patch cables at each end of a run are to be expected, minimize any other connections.
Splices - There are tools that you can use to splice a cut fiber optic cable. However, the signal loss from a splice is comparable to the signal loss from a connector.
Bends - Micro bends in the cable due to things such as temperature change or manufacturing anomalies can cause signal loss.
While you have little control over micro bends, even macro bends that can’t be detected by the human eye can contribute to signal loss.
The straighter the fiber optic cable, the less the signal loss.
You can estimate how much signal loss (measured in dB) you should reasonably expect in a given run of fiber optic cabling.
Signal loss is calculated by summing the average loss of all the components used in the cable run to generate an estimate of the total attenuation that will be experienced end-to-end.
This estimate is called a loss budget .
When calculating a loss budget for a segment of fiber optic cable, use the following guidelines.
Connectors: 0.3 dB loss each.
Splices: 0.3 dB loss each.
Multi-mode cabling:
3 dB loss per 1000 meters when using an 850 nm light source.
1 dB loss per 1000 meters when using a 1300 nm light source.
Single-mode cabling:
0.5 dB loss per 1000 meters when using a 1310 nm light source.
0.4 dB loss per 1000 meters when using a 1550 nm light source.
The total attenuation should be no more than 3 dB less than the total power at the transmission source. This is called the link-loss margin.
For example, if the total power output at the transmission source of a cable run is 15 dB, then the total attenuation over the cable run should not exceed 12 dB.
This ensures that the cable will continue to function as its components (such as the LED light transmitters and connectors) degrade with age and use.
