Competitive market pressures demand that networks are continuously upgraded and maintained to ensure the delivery of higher-speed, higher-quality applications and services to customers.
The constant evolution of high-speed optical fiber systems raises the level of testing, accuracy, and precision required on the fiber infrastructure to support these applications and services. By offering the industry’s most complete range of fiber test solutions, VIAVI Solutions reduces the complexity around these new network architectures and makes them easier to test.
Fiber testing is critical today for nearly every type of network. Proficiency is required by fiber optic installers, contractors, project managers, technicians and engineers that need to understand, apply, and correctly measure and record the performance of fiber infrastructures.
Fiber optics support much of the world’s Internet, phone and television data transmission today. As these networks continue to expand and user load continues to increase, the development of standardized fiber testing practices becomes increasingly important.
The Origins of Fiber Testing
The transmission of an optical signal through a thin glass “fiber” is not a new concept. Over 100 years ago, experiments demonstrated the ability of light to travel through a curved glass substrate and retain most of its original intensity. By the late 1960’s, laser optics, ultra-transparent glass fibers, and digital signaling combined to form the foundation of the fiber optic communication networks that we know today. By the 1990’s, fiber optic networks could already carry up to 100 times more information than traditional cable with electronic amplifiers.
Fiber optics work by converting electronic/binary information into optical signals in the form of digital light pulses. These signals can be transmitted through long fiber optic runs to a receiver at the far end of the line, where the signal is converted back into its original binary form. This is the readable format for computer systems and devices. To verify and maintain the integrity of these optical signals throughout long distance runs and complex networks, and keep pace with the increases in bandwidth, fiber testing processes must continually evolve.
Fiber Testing Essential Concepts
The application of fiber optic communication may seem elegant in its simplicity, but fiber optic cable testing requires an understanding of some basic principles that differentiate fiber testing from its analog wire testing predecessor. These important concepts provide insight into why and how fiber testing is performed.
The Three C’s of an Optical Fiber
The construction of fiber optic cabling provides the foundation for application of innovative test practices to fiber testing. The basic elements of an optical fiber are sometimes referred to as the “three C’s”:
- Core: The center of the fiber cable, made of specially treated glass or plastic. This is the medium for light transmission throughout the span of the cable, so it must be as pure and clean as possible.
- Cladding: An additional layer made of material similar to the core, but with a lower refractive index to facilitate reflection of the light source back into the core continually.
- Coating: The outer layer of the cable which wraps, protects, and insulates the core and cladding.
Problems in a fiber optic transmission could be related to any combination of these three fiber components performing at less than an optimal level. Effectively testing a fiber optic network requires ensuring that these elements, along with all connections, splices and terminations, are free from defects or contamination and are properly installed.
Fiber Continuity Test
Fiber continuity testing answers a simple yes or no question regarding the connection between one end of a fiber and the other. This definition applies to optical fiber as well as wire cabling or any other medium.
When testing fiber networks, a simple light source connected to one end of the cable can be used verify transmission to the opposite end. This type of fiber test is only intended to detect gross fiber defects, since no relative measurements or diagnostics are included. Fiber continuity testing can also be used to determine whether the right fiber optic cable is connected to a patch panel location.
A Visual fault locator (VFL) uses laser light to test fiber continuity as well as detecting fault conditions. The intense light source will be visible through the coating at the location of any fiber breaks or defective splices.
A power measurement is a test of the signal strength from the transmitter. The power itself is unrelated to the fiber integrity, so a reference cable can be used to measure the power directly. The output can be quantified in terms of the heating power of the light. Since most applications utilize very low power levels, semiconductor detectors are often used to measure power levels. Optical power can be measured in “dBm” units, where the “m” represents 1 milliwatt and the “dB” refers to decibels.
Optical Loss in Fiber
If the actual power from the source can be verified, the fiber cable can be tested for optical loss or insertion loss. As the name implies, this is a measure of how much optical power is being lost over the length of a cable. For example, if three gallons of water were poured into a drain pipe, but only one gallon came out the opposite end, you would know you lost two gallons.
Optical loss is measured in decibels (dB). This unit of measure is most often associated with sound measurement, but the interpretation for optical applications is identical, since dB is only used to measure ratios. The measured optical power is compared to the actual (reference) optical power. Since the dB is on a logarithmic scale, larger differences in power measurements can be represented on a smaller relative scale, similar to the Richter scale used for earthquakes. The more negative the dB level, the higher the loss.
Fiber Optical Speed
Using light as a data transportation medium, one might infer that all data travels at the absolute speed of light as well. Although the speed of optical data transport is extremely fast, not all “speeds of light” are created equally.
Many factors contribute to optical speed, including transmission wavelength, internal reflection and refraction. Since multimode fiber is much thicker in diameter than single mode, the higher internal reflection will reduce the speed despite the overall increase in bandwidth.
Testing Fiber for Optical Loss
To test a fiber for optical loss, you will need to connect to a test source to provide an optical light standard, as well as a launch cable to provide a calibrated “0 dB loss” reference. A power meter at the opposite end of the circuit will measure the light source with and without the fiber under test to quantify the loss in dB of the fiber itself.
Other methods of optical loss testing include both launch cables and “receive” cables connected to the power meter. This method is the standard test for loss in an installed cable plant, and includes the loss measurements at both test cable connection ends. For this reason, ensuring all connections are extremely clean is an important aspect of any fiber test.
An Optical Time Domain Reflectometer (OTDR) can also be used to test fiber optical loss in a different way. Utilizing high intensity laser light emitted at a pre-defined pulse interval through a connecting cable at one end of the fiber optic cable run, the OTDR instrument analyzes the backscatter of light returning to the source location. This one-ended fiber test method can be used to quantitatively analyze the loss, as well as pinpointing the loss locations.
Testing Fiber for Optical Speed
As demands on the fiber optic infrastructure continues to increase, even incremental improvements in optical speed and bandwidth become essential. Accurately measuring the speed of a fiber optic network is therefore another important aspect of fiber testing. Since an OTDR can be used to test both the intensity and time stamp of light reflected back to the source, this tool can also be utilized to test optical speed. If the expected time sequence for a given frequency differs from what was expected, the transmission speed may be compromised by defects in the core, cladding or connections.
Conversely, the index of refraction for a given fiber type can be used to predict the speed of light through the fiber, using the OTDR. This value can then be used to determine the length of the fiber, based on the time sequence between transmission of the pulse and reflection back to the source.
Fiber Optic Cable Testing Best Practices
Testing fiber optic networks is an essential part of fiber optic installation, as well as ongoing maintenance. Following some fundamental fiber testing best practices will lead to safer, more efficient, and more accurate test results.
The importance of cleanliness in fiber installation and testing cannot be overstated. A fiber optic microscope can be used verify the cleanliness of the core and connecting ferrules. Specialized cleaning materials are recommended for proper cleaning of fiber optic connections. This same attention to cleanliness should be applied to reference cables and test equipment connections.
The use of integrated optical loss test sets (OLTS) is considered a good practice for ensuring compatibility between the light source and the optical power meter. A common misconception is that an OTDR is capable of replacing the OLTS for fiber optical loss testing. Although the OTDR is an invaluable tool, the OLTS method is still considered to be the most accurate.
One benefit of fiber optics over conventional cabling is the lack of electrical conductivity, and therefore improved technician safety. When testing fiber with a VFL, however, eye safety is extremely important. Since a VFL utilizes a high intensity laser light source, neither the source nor the fiber core illuminated by the VFL should be viewed directly with the naked eye.
Proper planning and preparation are basic best practices applicable to any organized endeavor, including fiber testing. Cleaning and pre-testing equipment, ensuring equipment is calibrated, studying the proposed layout, and assembling a complete and comprehensive test tool kit are additional ways to carry out the most effective and accurate fiber test.
Fiber Test Awards
Contact a Fiber Test Product Expert
Related Product Categories
Related Product Families
Fiber Testing Blogs