Best Practices for Field Testing Fiber Optic Cables
Fiber optic cable provides a low loss medium for high-speed communications. While the continuous fiber cable itself is low loss, terminations at each access point provide a potential Achilles heel. The biggest cause of signal loss across fiber optic connectors is contamination. Poor installation practices in pathways and enclosures can also affect the signal loss of the fiber.
The growing prevalence of fiber requires network technicians have a general understanding of fiber optic cable testing to enable them to troubleshoot or qualify cables. A majority of issues can be identified with two steps – cleaning/inspection of
connector end-faces and loss testing.
The TIA/TSB 140 standard requires testing each fiber link with an Optical Loss Test Set (OLTS) kit for Attenuation. This standard considers an Optical Time Domain Reflectometer (OTDR) optional. An OLTS tester, using the appropriate reference method, will yield the most accurate loss reading. However, the OLTS tester can not characterize the fiber to show the good or bad quality of the fiber spans, connections, and splices.
Clean & Inspect all Connectors
Fiber Size: A quick note regarding size helps to visualize the impact of dust and dirt. An average human hair is around 85 microns in diameter. The core, or signal carrying portion, of the most widely used optical fibers is either 62.5 microns or 9 microns. It does not take many dust particles to block a 9 micron “window”. A blocked window means light is blocked and the network experiences signal loss.
Inspection: A fiber optic microscope with proper magnification is necessary to view fiber connector ends for cleanliness. Typical magnifications used are 200x and 400x. When selecting a fiber optic microscope, keep safety in mind. Some microscopes are all optical and can direct laser light into a user’s eye. Others are available with no optical path to the eye. While all manufacturers caution users not to view live fibers, it is easy for a technician to inadvertently view a live fiber because laser emissions are invisible to the human eye.
Cleaning: Properly armed with a fiber optic microscope, one can determine if connectors are dirty, cracked, or pitted. If the fiber end is cracked or pitted, the connector will need to be re-polished or replaced. More likely, the connector end will be dirty. One’s first inclination may be to wipe the fiber end on a shirtsleeve and clean it like a set of glasses. This is not helpful. Lint, dust, and other particles big enough to block a 9 micron light path are on clothes regardless of how clean they may look.
Proper cleaning of fiber optic connectors requires fiber optic grade cleaning materials. Materials vary from optical grade wipes to cassette cleaners such as the widely used Cletop brand cleaners. Technicians often use these materials alone to “dry clean” connectors. While this is often effective, wiping a connector end-face across a dry wipe can add a static
charge to the end-face. During the summer months when the relative humidity is high, static charge may not present much issue. However, dry winter months – especially in northern climates – and air-conditioned areas allow static
charge to be a significant hindrance to the connector cleaning process.
Static charge build up on connectors attracts charged particles to the connector endface. This can effectively negate the dry cleaning process. A recent study by iNEMI, documented the tendency of particles to be drawn to the foremost part of connector end-faces during mating and unmating. Since fiber optic connectors are polished to have a radius end-face with the fiber core at the center, static charge causes particles to migrate to the optimal signal blocking position – not good.
Static charge is not ESD. Optical fiber is glass. Single fiber connector ferrules are ceramic. There is no conductive path from the connector end-face to the fiber jacket. Thus, ESD protection such as wrist or ankle straps will not eliminate static charge from connector end-faces. How does one minimize static charge then?
Fiber optic cleaning fluids are static dissipative. With an optical-grade cleaning wipe, a technician can wet a corner of the wipe with an appropriate cleaning fluid and drag the connector end-face from the wet area into the dry area. The fluid will neutralize static charge and help to release particles from the end-face.
IPA Alcohol has long been the fiber optic cleaning standard. Technicians regularly have 99% IPA in a pump dispenser bottle. IPA is static dissuasive. However, it is far from an ideal optical cleaning fluid. To begin with, it is hydrophilic – meaning it absorbs water. IPA in a dispenser bottle will absorb atmospheric moisture over time. This is bad for the cleaning process for two reasons. First, water slows down the drying process and requires more air to evaporate off all the fluid. Since most fiber optic technicians are not working in a clean room environment, more air brings more dust particles. Water absorption into IPA is also bad as water can leave streaks on the connector end-face.
There are safer cleaning solutions alternatives available today which dry faster than alcohol, do not absorb water, are non-hazardous, and are not regulated. As always, it pays to do your homework and not just grab the “ole standby”.
When cleaning connectors it is important to clean both connectors on jumpers and the connector behind the panel. The connector on the backside of a patch panel may not be accessible, or it could be risky to try to get to a back connector when there is live traffic on adjacent fibers. In this situation, a common option is to use a fiber optic cleaning stick to clean the end-face inside the adaptor. One inserts the stick until it makes contact with the end-face. Then the stick is twisted approximately ten times and discarded. The twisting motion can generate a static charge on the end-face. Therefore, a quick drying static dissipative cleaning fluid is a plus.
Testing Fiber Optic Cables:
Measuring Loss Using an OLTS – Always Required
Read TIA-568B or TIA/TSB 140 from cover to cover and you will find it requires you to make only one type of measurement, insertion loss, to certify a fiber optic cable. Also known as “dB loss”, “attenuation”, or simply “loss”, insertion loss is singled out by the TIA because it can be impacted by poor installation practices. For example, a poorly
polished or dirty connector can cause high loss, or a cable pulled around a corner that does not meet minimum bend radius specifications may exhibit high loss, especially on single-mode fibers at 1550 nm.
TIA-standards specify that you must measure loss using an optical power meter and the proper light source to certify an optical fiber cable. Multimode fiber loss measurements must be made using an LED source. Single-mode fiber loss measurements require a laser source.
Light sources and optical power meters are available as low-cost, stand-alone units (Figure 1), or they may be integrated into “smart” optical loss test sets (Figure 2), which offer additional features such as dual-fiber testing, length measurement, and pass/fail analysis. Either type of tester will provide accurate standards-compatible results, providing you use proper reference setting and connector cleaning procedures.
TIA-standards also specify maximum lengths for horizontal and backbone optical fiber cables, which you must verify as part of the certification process. However, the TIA-568B standard does not require that you optically measure cable length. You may use a tape measure or simply refer to length markings on the cable itself.
Bottom line: to certify an optical fiber cable you will need either an optical power meter and compatible light source(s), or an OLTS main-remote pair. Other test equipment may be very useful as discussed next, but nothing else is required.
OTDR Testing– Optional, but a good practice
TIA-standards also contain component specifications including maximum loss values for connections, splices, and optical fiber segments. Many installers find it a good practice to measure the loss of each connection and splice, and check cables for “macro-bends” and other defects for quality assurance purposes. The type of fiber tester normally used for these functions is an Optical Time Domain Reflectometer or OTDR.
OTDRs operate like radar. They generate short pulses of light and then sample the light backscattered by fiber segments and reflected by connections and other events. This allows the user (or OTDR event table software) to estimate the loss slope or “attenuation” of fiber segments and the insertion loss of individual connections and splices.
The setup used to measure the loss of an optical fiber cable with an OTDR is shown in
Figure 3. To measure the loss of the first and last connection in each fiber link or link
segment under test you must use a launch and receive cable respectively. Launch and
receive cables may also be called “launch reels”, “pulse suppressors”, or “test cables”.
Typically, these test cables include 100 or more meters of fiber in a ring-shaped or
rectangular case, terminated by jumpers that will mate with the fiber under test.
Using an OTDR to Measure End-to-End Loss
To illustrate the advantages of an OTDR, consider a 100 m (328 ft) backbone cable with
the following component loss values:
- Equipment room connection (1.2 dB)
- Splice (0.1 dB)
- Fiber (0.1 dB)
- Telecom closet connection (0.3 dB)
OTDR Trace Showing a Bad Connection
According to the TIA-standards, the maximum acceptable loss for this cable is 1.5 dB of connection loss (0.75 dB for each connection) plus 0.3 dB of splice loss and 0.1 dB of fiber loss (1 dB / km) for a total of 1.9 dB. Since this cable has an overall (end-to-end) insertion loss of 1.7 dB, it probably would be certified by an OLTS measurement. But as shown in Figure 4, an OTDR trace of this cable would reveal that the telecom closet connection has a loss of 1.2 dB, which exceeds the TIA-standard specified maximum of 0.75 dB. An OTDR can indicate and localize problems that would often be missed by an OLTS or optical power meter / light source kit.
In cases where an OLTS can detect a fault, for example the “infinite” loss caused by an open connection or fiber break, it cannot tell you where the fault is located. An OTDR trace, on the other hand, will locate such events, as illustrated in Figure 5.
OTDR Trace Showing a Break in the Fiber
An OTDR test can not replace an OTLS measurement. The OLTS test is required by the TIA standards, and OTDR measurements can slightly under-estimate loss, especially on multimode fibers. An OLTS test would indicate fiber mis-matches of 50/125 and 62.5/125um fibers with a high loss reading. An OTDR which uses a laser and does not fill the outer modes may not pick up the core mismatch.
OTDR tests in many cases offer additional information that can help you detect and proactively fix problems often missed by OLTS tests. And on some jobs you may be required to test all optical fiber cables using an OTDR by a quality-conscious customer who has received advice from a detail-oriented consultant. On other jobs, having an OTDR characterization of the fiber(s) at the time you signed-off on the project can protect the installer from damages caused by installers that come in later and pull other cable(s) into the pathways that may damage the fiber, causing macro- or micro-bends or breaks.
Dust, dirt, oils, and other common contaminates, along with poor installation practices can cause hours of grief for network service technicians. The good news is proper cleaning tools and good test techniques allow trained technicians to effectively remove contaminates, find and correct fiber problems, and get networks back in service. Whether or not you do OTDR testing is up to your schedule, budget, and often your customer.
Recommended Approach to Cleaning, Inspecting and Testing:
- Always presume connectors are dirty prior to mating.
- A fiber optic termination has two connectors – always clean both! Cleaning one and remating it to a dirty connector is not productive.
- Use optical quality cleaning materials to clean fiber end-faces.
- Use an optical cleaning fluid to minimize the static build up.
- Use a fiber optic microscope with built in eye-protection to inspect connectors prior to mating.
- Light Source & Power Meter or OLTS kits test for Attenuation.
- OTDR test to characterize the installation and get sign-off.