Six basic fiber-optic cable tests
A half-dozen simple but rigorous tests, performed with an optical time-domain reflectometer and an optical power meter, characterize the optical transmission performance of fiber-optic cables
DANIEL E. BEOUGHER
A definitive repertoire of tests, known as the "essential six," can aid the inexperienced system engineer. Using an optical time-domain reflectometer test instrument, these tests analyze the operation of fiber-optic cables and their conveyance of transmitted light signals. The light signals may be used to carry digital, analog or lightwave signals, but the tests of the conduit are essentially the same.
Whatever the end use of the optical fiber cable, its fundamental signal-carrying characteristics relate to the nature of light. When it is being transmitted over a fiber medium, light tends to travel in a straight line. In addition, it refracts (bends) because of speed variations as it passes from one transmission medium to another. The backscatter effect--the tendency of light to spread or "bloom" in all directions--also affects its ability to carry information. All these light characteristics can impact the transmission of data-bearing signals through fiber-optic cable.
Fiber-optic cables are generally installed in the air, underground and in buildings. The cables are coupled to optical or electronic equipment or to other cables by splices or connectors. The OTDR verifies that the fiber is undamaged before and after installation, splicing and connectorization. Upon fiber-optic cable activation, the OTDR is used to check, test and analyze the fiber`s signal-transmission parameters.
Six basic test procedures measure distance, fiber loss, event loss, link loss, event-return loss and link-return loss. These procedures are essential because they are implemented at all four levels of fiber operation, including pre-installation, installation and acceptance, maintenance and restoration.
An OTDR can be used to accomplish the six test procedures. In addition, clean connections on the fiber media under test are imperative. All six test procedures will prove inaccurate or impossible to accomplish if the fiber-optic connectors are dirty.
The optical distance between one point and another depends on definition. For example, the distance could be the fiber-cable length between a transmitter and a receiver, or it could be the fiber-cable length between two splices. An OTDR is the test instrument used worldwide to measure optical test events either automatically or manually. Events are detected as disturbances in the OTDR`s relatively linear trace display.
To measure optical distance between two points, the OTDR launches a laser-generated light pulse down the fiber at the transmission end of the cable. The instrument then detects the backscatter returned from the fiber and any reflections from shiny surfaces. It measures the time taken by the light pulse to make the round trip on the fiber and calculates that time into distance.
One minor deviation in this test is the difference between real and apparent distance. The optical, or apparent distance, is the distance reading registered on the OTDR, and is always longer than the real distance. One reason for the distance difference results from the undulation of fiber as it resides within loose-tube cable, which adds to its length. Another reason involves buried cable as it winds within a trench, thereby producing a longer optical length.
The backscatter trace is a representation of the fiber itself. The slope of the backscatter trace discloses that less and less light is being reflected back as the length of the fiber increases. This slope represents fiber loss, a manufacturer`s specification. Typical fiber-loss measurements are given as the amount of light (in decibels) lost per kilometer. For example, a long-haul telephone fiber might lose 0.15 dB/km, whereas a multimode local area network fiber could lose 3 dB/km. Fiber loss is always measured along a featureless section of backscatter with no events to skew the calculation.
A test event is a disturbance that occurs above or below the backscatter baseline. Splices, connectors, bends and cracks are typical events that produce trace disturbances on the OTDR display. Normally (but not always), an event results in a loss of light. There are two types of events--reflective and non-reflective. The spikes along the baseline indicate a reflection. Because more photons appear and thus exceed the normal backscatter level, a mechanical splice or the end of the fiber is revealed. Other causes of reflections are connectors and fiber cracks.
A drop in backscatter level with no reflection indicates a fusion splice. In this case, all the reflective surfaces appear to be melted together. This condition reduces backscatter but creates no reflection. Other non-reflective events include macrobends and microbends.
A rise in the backscatter across an event is known as a gainer and is caused by splicing two fibers of different specifications.
Events that occur along the fiber become important when a fiber-loss budget is calculated. Only a finite amount of light is launched by the transmitter. Consequently, if the receiver does not receive enough light, a major cable problem has occurred.
Link loss is the total amount of light lost between two points. A link can be the distance between events or between two end points. Total link loss is typically specified when it directly affects the loss budget. If the link loss is a high value, then specific events are consuming light.
Return loss is essentially the light lost because of reflections back toward the transmission or source end. The shiny surfaces of connectors and mechanical splices reflect light. Some of this reflected light returns to the source. Any transmitted light that does not reach the end of the fiber is lost. An OTDR trace displays return loss as the height of a reflection.
Return loss is defined as the ratio in dB of the incident power to the reflected power. Return loss is always expressed as a positive number:
In contrast, reflectance is defined as the ratio of reflected power to the incident power or the inverse of the return-loss formula. When expressed in decibels, reflectance is a negative number. In addition, reflectance can be expressed in terms of density or as a percentage.
In reality, these terms mean noise. The reflected light travels back to the source, reflects off the input and makes another round trip. To a digital system, the reflected light looks like a bit error. To an analog system, such as cable TV, reflected light creates sparkle. The higher the reflection value, the more dramatic the noise level becomes.
Link-return loss is similar to link loss. It is the total amount of reflected light in the link. Therefore, link-return loss is often used as an acceptance test. If the total amount of return loss is below a certain level, the link is assumed not to contain a single event reflecting above specification.
These six essential tests should be used to test fiber during pre-installation, installation and acceptance, and for maintenance and restoration.
A pre-installation test should be performed when fiber-optic cable arrives from the vendor. This receiving type of test is important because it quickly and easily determines product acceptance or rejection before system usage.
The first pre-installation test should check for correct cable distance, confirming fiber length and integrity. The next or fiber-loss test confirms the manufacturer`s specifications and checks for material anomalies. If a fiber defect or problem arises, the cable can be readily returned to the vendor at this early receiving stage. Defects found after the fiber cable is used, installed or buried are expensive, time-consuming and labor-intensive. Consequently, tests at this stage are generally done at the request of the end user. The system supplier must substantiate and document that before installation, the fiber is free from defects. In this manner, a product test trail can corroborate that the fiber was received in acceptable condition before possible installation damage.
Installation and acceptance testing should involve all six tests. Link loss and link-distance measurements should be made first. If overall link loss measures too high, then the abnormal event loss is tracked. If link-loss levels are acceptable, then link-return loss and event return-loss tests should be performed. Collectively, all these tests should disclose whether the fiber will pass enough light for the receiver to operate properly, have minimal reflections to prevent noise and document the distances needed to establish proper access to the system at a later date.
A capable OTDR should be able to detect events on the fiber that cannot be detected visually. These events should be marked by the OTDR, and the data for each event must be retrievable.
Thorough test documentation is imperative; a discernible data representation of each fiber within the system--not just an image of multiple linkage--should be indicated. Test documentation can be stored in a floppy or hard-disk file, or printed as a hard copy with all test results labeled. The data points acquired by the OTDR can be retrieved and viewed immediately or later on an OTDR or a personal computer.
Maintenance testing consists of comparing archived test results with current tests. It may also consist of checking for degradation of connectors and splices as a result of daily mobility or of looking for environmentally induced changes such as macrobends and microbends. Comparing the latest test acquisitions with archived results can also be made with OTDR differential software.
Restoration testing attempts to discover why light is not getting to the receiver, or perhaps why the light is corrupted or degraded. Common light-transmission problems involve a high loss event, such as a broken fiber, a pulled connector or a failed splice. In some cases, a high return loss on a splice can cause a high bit-error rate.
Logical, proactive test processes help considerably in restoration testing. For example, a restoration testing sequence might first check the light level emanating from the transmitter. If the light level is acceptable, a check of the integrity of the fiber is in order. If the fiber checks out, an electronic problem might exist in the receiver. For testing purposes, optical fiber can be thought of as a light conduit; therefore, keep the tests simple. u
Daniel E. Beougher is training manager at Tektronix Inc. in Redmond, OR.