Let’s return now to the four basic functional requirements of any fiber distribution management system: terminations, splicing, slack storage, and housing of passive optical components.
In order for the signal to get from one fiber to another, the cores of the two fibers need to be joined, and brought into near-perfect alignment.
The measurements that determine the quality of the junction are insertion loss and return loss. Insertion loss (IL) is a measure of the power that is lost through the junction (IL=-10log(Pout/Pin)), where P is power.
An insertion loss value of 0,3dB is equivalent to about 7-percent of the power being lost.
Return loss (RL) is a measure of how much power is reflected back to the source from the junction (RL = 10log (Pin/Pback)).
A return loss value of 57dB is equivalent to 0,0002-percent of the light being reflected back. There are two means of joining fibers in the industry today: connector terminations and splices.
Connector termination in fiber optics refers to the physical joining, using a mechanical connector, of two separate fibers, with the goal of having 100-percent signal transfer. Connector terminations used for junctions are meant to be easily reconfigurable, to allow easy connection and reconnection of fibers.
There are several fiber connectors available in the industry today; the most commonly used singlemode types are SC, FC and LC. Typical singlemode ultra polish connectors (UPC) will provide insertion loss values of <0,3dB and return loss values of >52dB, while singlemode angled polish connectors (APC) have insertion loss values of <0,2dB and return loss values of >55dB.
Reliable operation of connectors depends on the proper geometry of the convex polished ferrule endface. The following parameters are routinely checked by interferometric inspection: radius of curvature, apex offset, fiber projection/undercut, polishing angle.A connector is installed onto the end of each of the two fibers to be joined.
Singlemode connectors are generally factory-installed, to meet requirements for optical performance and long-term reliability. The junction is then made by mating the connectors to each side of an adapter. The adapter holds the connectors in place and brings the fibers into alignment (see Figure 1).
The adapters are housed within a termination panel, which provides a location to safely house the adapter/connector terminations and allows easy access to installed connectors. Fiber termination panels typically house from twelve to 144 terminations.
Termination panels should adapt easily to any standard style of connector/adapter. This allows easy future growth and also provides more flexibility in evolving network design. Fiber cable management within the termination panel is critical.
Cable management within a termination panel must include proper bend radius protection and physical routing paths. The fibers should have bend radius protection along the route from the adapter port to the panel exit location.
The path the fiber follows in getting to the panel exit should also be very clear and well defined. Most cable management problems in termination panels arise from improper routing of patch cords. Improper fiber routing within the panels can make access to installed connectors very difficult, and can cause service-affecting macro-bends on adjacent fibers. Connectors should also be removable without the use of special tools, which can be costly and easily lost or left behind.
Proper fiber cable management in the termination panel improves network flexibility, performance and reliability while reducing operations costs and system reconfiguration time.When fiber is used in the local serving loop, such as in PON networks or fiber-fed digital loop converters (DLCs), backup fibers run to the optical network unit (ONUs) or to the DLCs.
These fibers are provided in case a technician breaks the active fiber or damages the connector during installation and maintenance. In the event of such an occurrence, the signal has to be rerouted from the original active fiber to the backup fiber.
This rerouting is done at the OSP termination panel within the ODF. While the fiber appearances on the termination panel are generally located either adjacent to each other or within a few terminations of each other, this reconfiguration should not jeopardize the integrity of the other installed circuits.
If installed fibers must be moved in order to access the target connector, then the probability of inducing a bending loss in those adjacent fibers is increased. And that loss could be enough to cause a temporary service outage.
These effects are especially pronounced in CATV systems, in which the system attenuation is adjusted to an optimal power level at the receiver to provide the best picture quality. Enabling easy access to individual terminations without disturbing other fibers is an important feature of a termination panel.
Reliable optical networks require clean connectors. Any time one connector is mated to another, both connectors should be properly cleaned and inspected. Dirty connectors are the biggest cause of increased back-reflection and insertion loss in connectors, including angled polish connectors.
A dirty ultra polish connector with a normal return loss of >55dB can easily have >45dB reflectance if it is not cleaned properly. Similar comparisons can be made with angled polish connectors. This can greatly affect system performance, especially in CATV applications where carrier-to-noise ratios (CNR) are directly related to signal quality. In order to ensure that both connectors are properly cleaned, the termination panel must allow them both to be easily accessed.
This easy access has to be for both the patch cord connector and the equipment or OSP connector on the back side of the termination panel. Accessing these connectors should not cause any significant loss in adjacent fibers. A system that allows uncomplicated access to these connectors has much lower operating costs and improved reliability. Without easy access to connectors, technicians will take more time to perform their work, delaying implementation of new services or redeployment of existing services.
Dirty connectors can also jeopardize the long-term reliability of the network, because dirt and debris can be embedded into the endface of the connector, causing permanent, performance-affecting damage.
For further information on fiber distribution and management methods, consider the SP4420 Fiber Optic Infrastructure Specialist course.