The Fiber Optic Association

  The Fiber Optic Association, Inc.
the non-profit professional society of fiber optics

Reference Guide To Fiber Optics



Topic: Fiber Optic Instrumentation  Table of Contents:
The FOA Reference Guide To Fiber Optics


Overview of Fiber Optic Instrumentation

Testing fiber optic components and cable plants requires making several  measurements with the most common measurement parameters listed in the Table below. Optical power, required for measuring source power, receiver power and, when used with a test source, loss or attenuation, is the most important parameter and is required for almost every fiber optic test. Backscatter and wavelength measurements are the next most important and bandwidth or dispersion are of lesser importance. Measurement or inspection of geometrical parameters of fiber are essential for fiber manufacturers. And troubleshooting installed cables and networks is required.

Fiber Optic Testing Requirements

Test Parameter Instrument

Optical Power

(Source Output, Receiver Signal Level)

Fiber Optic Power Meter

Attenuation or Loss of Fibers, Cables & Connectors
(Insertion Loss) 

FO Power Meter & Source or
OLTS (optical loss test set)

Source Wavelength, Spectral Width FO Spectrum Analyzer

Backscatter For  Loss,  Length and  Fault Location)

Optical Time Domain Reflectometer(OTDR)
Fault Location

OTDR,

Visual Cable Fault Locator

Bandwidth / Dispersion

(MM:Modal & Chromatic,
SM: Chromatic and Polarization Mode)

Dedicated Bandwidth Testers
Reflectance OTDR,  OCWR (Optical Continuous Wave Reflectometer)
Fiber Geometry
(Core and cladding diameter, concentricity, etc.)
Various mechanical and optical inspection tools


Standard Test Procedures

Most test procedures for fiber optic component specifications have been standardized by national and international standards bodies, including TIA in the US and ISO/IEC internationally. FOA has its own standards for basic tests. Procedures for measuring absolute optical power, cable and connector loss and the effects of many environmental factors (such as temperature, pressure, flexing, etc.) are covered in these procedures. 

In order to perform these tests, the basic fiber optic instruments are the FO power meter, test source, OTDR, optical spectrum analyzer and an inspection microscope. These and some other specialized instruments are described below.

Fiber Optic Power Meters

Fiber optic power meter

Fiber optic power meters measure the average optical power out of an optical fiber. Power meters typically consist of a solid state detector (silicon for short wavelength systems, germanium or InGaAs for long wavelength systems), signal conditioning circuitry and a digital display of power. To interface to the large variety of fiber optic connectors in use, some form of removable connector adapter is usually provided.

Power meters are calibrated to read in dB referenced to one milliwatt of optical power. Some meters offer a relative dB scale also, useful for loss measurements since the reference value may be set to "0 dB" on the output of the test source. Occasionally, lab meters may also measure in linear units (milliwatts, microwatts and nanowatts.) Since all semiconductor detectors have a sensitivity that varies with the wavelength of the light it is measuring, power meters are calibrated at the typical wavelengths used in fiber optics, 850, 1300 and 1550 nm. Meters for POF systems are usually calibrated at 650 and 850 nm, the wavelengths used in POF systems.

Power meters cover a very broad dynamic range, over 1 million to 1 or 60 dB. Although most fiber optic power and loss measurements are made in the range of 0 dBm to -50 dBm, some power meters offer much wider dynamic ranges. For testing analog CATV systems or fiber amplifiers, on needs special meters with extended high power ranges up to +20 dBm (100 mW). Although no fiber optic systems operate at very low power, below about -50 dBm, some lab meters offer ranges to -70 dBm or more, which can be useful in measuring optical return loss or spectral loss characteristics with a monochromator source.

Power meters measure the time average of the optical power, not the peak power, so the meters are sensitive to the duty cycle of an input digital pulse stream. One can calculate peak power if one knows the duty cycle of the input, by dividing the average power by the duty cycle. For most loss measurements, one uses a test source with CW (steady state) or 2 kHz pulsed output. As long as the source modulation doesn't change, no compensation needs to be made. When testing link transmitter power or receiver sensitivity, it is necessary to establish a standard test pattern, generally a 50% duty cycle, called a square wave, to allow accurate measurement of transmitter output or receiver sensitivity.

FO power meters have a typical measurement uncertainty of +/-5%, when calibrated to transfer standards provided by national standards laboratories like the US National Institute of Standards and Technology (NIST). Sources of errors are the variability of coupling efficiency of the detector and connector adapter, reflections off the shiny polished surfaces of connectors, unknown source wavelengths (since the detectors are wavelength sensitive), nonlinearities in the electronic signal conditioning circuitry of the FO power meter and detector noise at very low signal levels. Since most of these factors affect all power meters, regardless of their sophistication, expensive laboratory meters are hardly more accurate that the most inexpensive handheld portable units. Meters should be recalibrated frequently by labs with NIST traceable calibration systems.   (Photo courtesy Advanced Fiber Solutions)

Fiber Optic Test Sources

fiber optic test source

In order to make measurements of optical loss or attenuation in fibers, cables and connectors, one must have a test source as well as a FO power meter. The test source must be chosen for compatibility with the type of fiber in use (singlemode or multimode with the proper core diameter) and the wavelength desired for performing the test. Most sources are either LED's or lasers of the types commonly used as transmitters in actual fiber optic systems, making them representative of actual applications and enhancing the usefulness of the testing. Some tests, such as measuring spectral attenuation of fiber requires a variable wavelength source, which is usually a tungsten lamp with a monochromator to vary the output wavelength.

Typical wavelengths of sources are 650 or 665 nm (plastic fiber), 820, 850 and 870 nm (short wavelength glass fiber ) and 1300 or 1310 nm and 1550 nm (long wavelength ). LED's are typically used for testing multimode fiber and lasers are used for singlemode fiber, although there is some crossover, especially in high speed LANs which use multimode fiber with lasers and the testing of short singlemode jumper cables with LED's. The source wavelength can be a critical issue in making accurate loss measurements on long links, since attenuation of the fiber is wavelength sensitive especially at short wavelengths. Thus all test sources should be calibrated for wavelength.

Test sources almost always have fixed connectors. Hybrid test jumpers with connectors compatible with the source on one end and the connector being tested on the other must be used as reference cables. This may affect the type of reference setting mode used for loss testing.

Other source-related factors affecting measurement accuracy are the stability of the output power and the modal distribution launched into multimode fiber. For extremely accurate measurements, the source may need optical feedback stabilization to maintain output power at a precise level for long times required for some measurements. Industry standards have requirements or recommendations on the modal output of test sources for multimode fiber that are aimed at the manufacturers of the test sources. Mode scramblers, filters and strippers may be required to adjust the modal distribution in the fiber to approximate actual operating conditions.  (Photo courtesy Advanced Fiber Solutions)

Optical Loss Test Sets/Test Kits

OLTS

The optical loss test set is an instrument formed by the combination of a fiber optic power meter and source which is used to measure the loss of fiber, connectors and connectorized cables. Early versions of this instrument were called attenuation meters. A test kit has a similar purpose, but is usually comprised of separate instruments and includes accessories to customize it for a specific application, such as testing a FO LAN, telco or CATV.

The OLTS may have several optional features that affect its use. Some have individual source outputs and meter inputs like a separate power meter and test source, but may have two wavelengths from one source output (MM: 850/1300, SM:1310/1550.) Some offer bidirectional testing on a single fiber and some have two bidirectional ports. Some manufactures of premises copper cabling testers offer modules to convert these testers to an OLTS, allowing fiber and copper testing with one instrument.

The combination OLTS instrument which contains both a meter and source may be less convenient than an individual source and power meter, since the ends of the fiber and cable are usually separated by long distances, which would require two OLTSs instead of one source and one meter. An OLTS often has a single port for bidirectional measurements also. This port usually has a fixed connector which may cause problems when testing cable plants with connector styles different than those on the instrument, requiring a 2- or 3-cable reference for loss testing which may not meet industry standards. The bidirectional port may also have problems meeting standards for modal power distribution in multimode fibers. Ask the OLTS manufacturer about these issues  before purchasing an instrument. (Photo courtesy EXFO)

Optical Time Domain Reflectometer

OTDR

The optical time domain reflectometer (OTDR) uses the phenomena of fiber backscattering to characterize fibers and installed cables, find faults and optimize splices. Since scattering is one of the primary loss factors in fiber (the other being absorption), the OTDR can send out into the fiber a high powered pulse and measure the light scattered back toward the instrument. The pulse is attenuated on the outbound leg and the backscattered light is attenuated on the return leg, so the returned signal is a function of twice the fiber loss and the backscatter coefficient of the fiber.

If one assumes the backscatter coefficient is constant, the OTDR can be used to measure loss as well as locate fiber breaks, splices and connectors. In addition, the OTDR gives a graphic display of the status of the fiber being tested. And it offers another major advantage over the source/FO power meter or OLTS, in that it requires access to only one end of the fiber.

The uncertainty of the OTDR measurement is heavily dependent on the backscatter coefficient, which is a function of intrinsic fiber scattering characteristics, core diameter and numerical aperture. It is the variation in backscatter coefficient that causes many splices to show a "gain" instead of the actual loss. OTDRs must also be matched to the fibers being tested in both wavelength and fiber core diameter to provide accurate measurements. Thus many OTDRs have modular sources to allow substituting a proper source for the application.

While most OTDR applications involve finding faults in installed cables or verifying splices, they are very useful in inspecting fibers for manufacturing faults. Development work on improving the short range resolution of OTDRs for LAN applications and new applications such as evaluating connector return loss promise to enhance the usefulness of the instrument in the future.

OTDRs come in three basic versions. Full size OTDRs offer the highest performance and have a full complement of features like data storage, but are very big and high priced. MiniOTDRs provide the same type of measurements as a full OTDR, but with fewer features to trim the size and cost. Fault finders use the OTDR technique, but greatly simplified to just provide the distance to a fault, to make the instruments more affordable and easier to use.   (Photo courtesy Advanced Fiber Solutions)

Visual Cable Tracers and Fault Locators

VFL

Many of the problems in connection of fiber optic networks are related to making proper connections. Since the light used in systems is invisible, one cannot see the system transmitter light. By injecting the light from a visible source, such as a LED or incandescent bulb, one can visually trace the fiber from transmitter to receiver to insure correct orientation and check continuity besides. The simple instruments that inject visible light are called visual fault locators.

If a powerful enough visible light ,such as a HeNe or visible diode laser is injected into the fiber, high loss points can be made visible. Most applications center around short cables such as used in telco central offices to connect to the fiber optic trunk cables. However, since it covers the range where OTDRs are not useful, it is complementary to the OTDR in cable troubleshooting. This method will work on buffered fiber and even jacketed single fiber cable if the jacket is not opaque to the visible light. The yellow jacket of singlemode fiber and orange of multimode fiber will usually pass the visible light. Most other colors, especially black and gray, will not work with this technique, nor will most multifiber cables. However, many cable breaks, macrobending losses caused by kinks in the fiber , bad splices etc. can be detected visually. Since the loss in the fiber is quite high at visible wavelengths, on the order of 9-15 dB/km, this instrument has a short range, typically 3-5 km.

Fiber Identifiers

ID

Telco technicians often need to identify a fiber in a splice closure or at a patch panel. If one carefully bends a singlemode fiber enough to cause loss, the light that couples out can also be detected by a large area detector. A fiber identifier uses this technique to detect a signal in the fiber at normal transmission wavelengths. These instruments usually function as receivers, able to discriminate between no signal, a high speed signal and a 2 kHz tone. By specifically looking for a 2 kHz "tone" from a test source coupled into the fiber, the instrument can identify a specific fiber in a large multifiber cable, especially useful to speed up the splicing or restoration process.

Fiber identifiers can be used with both buffered fiber and jacketed single fiber cable. With buffered fiber, one must be very careful to not damage the fiber, as any excess stress here could result in stress cracks in the fiber which could cause a failure in the fiber anytime in the future.

Measuring Fiber Bandwidth

Although fiber has a very high bandwidth, some applications actually approach its limits, requiring performance evaluation. Two factors limit multimode fiber bandwidth: modal dispersion and chromatic dispersion. Long singlemode links require concern over chromatic dispersion or polarization-mode dispersion. Specialized instruments are available for testing each of these specifications but are expensive and rarely used outside the laboratory.

O/E and E/O Converters

Optical to electrical (O/E) and electrical to optical (E/O) converters have other uses besides testing fiber bandwidth. O/E converters can be used with high speed oscilloscopes to analyze pulses in fiber optic links to see if the waveforms are of the proper shape. This means measuring rise and fall times of the pulse and the depth of modulation (the difference between the peak power of the pulse and the lowest power reached between pulses. They can be used for testing lasers and LEDs used in transmitters and link dispersion in long links. E/O converters are used to test receivers for bandwidth and margin, usually in conjunction with a bit error rate tester and attenuator.

Optical Continuous Wave Reflectometers (OCWR)

The OCWRor reflectance tester  was originally proposed as a special purpose instrument to measure the reflectance or optical return loss of connectors installed on patchcords or jumpers. Unfortunately, its purpose became muddled between conception and inception. As actual instruments came on the market, they had much higher measurement resolution than appropriate for the measurement uncertainty (0.01 dB resolution vs. 1 dB uncertainty), leading to much confusion on the part of users as to why measurements were not reproducible. In addition, several instruments were touted as a way to measure the optical return loss of an installed cable plant, obviously in ignorance of the fact that they would also be integrating the backscatter of the fiber with any reflections from connectors or splices. Since the measurement of return loss from a connector can be made equally well with any power meter, laser source and calibrated coupler, and an OTDR is the only way to test installed cable plants for return loss, the OCWR has seen little use in fiber optic testing.

Test Equipment For Long Distance Fiber Links

Long distance fiber links may suffer from chromatic dispersion or polarization mode dispersion. Generally they also use DWDM so need testing for spectral attenuation. These tests use very specialized instruments. Read more about these tests here.

Optical Fiber Analyzers

There are many parameters of optical fiber that require testing by the manufacturer. These include attenuation (as a function of source wavelength), bandwidth/dispersion, numerical aperture and all the physical dimensions such as core and cladding diameter, ovality, and concentricity. Automated laboratory instruments are available to measure all these parameters automatically, but many fiber manufacturers prefer to build their own. The most difficult part of fiber measurements is the fact that subtle differences in test setup and instrumentation can cause differences in measured values.

Visual Inspection with Microscopes

fiber optic inspection microscope

Cleaved fiber ends prepared for splicing and polished connector ferrules require visual inspection to find possible defects. This is accomplished using a microscope which has a stage modified to hold the fiber or connector in the field of view. Fiber optic inspection microscopes vary in magnification from 30 to 800 power, with 30-100 power being the most widely used range. Cleaved fibers are usually viewed from the side, to see breakover and lip. Connectors are viewed end-on or at a small angle to find polishing defects such as scratches.

Fiber Optic Talksets

While technically not an measuring instrument, FO talksets are useful for FO installation and testing. They transmit voice over fiber optic cables already installed, allowing technicians splicing or testing the fiber to communicate effectively. Talksets are especially useful when walkie-talkies and telephones are not available, such as in remote locations where splicing is being done, or in buildings where radio waves will not penetrate.

The way to use talksets most effectively is to set up the talksets on one fiber (or pairs appropriate) and leave them there while all testing or splicing work is done. Thus, there will always be a communications link between the working crew, which facilitates deciding which fibers to work with next. The continuous communications capability will greatly speed the process.

Recent developments in talksets include talksets for networking multi-party communications, especially helpful in restoration, and system talksets for use as intercoms in installed systems. There are also combination testers and talksets.

There are no standards for the way talksets communicate. Some use simple AM transmission, some FM and some proprietary digital schemes. Thus no two manufacturers' talksets can communicate with each other. Bellcore has addressed this matter in a technical advisory that proposes a FM method at 80 and 120 kHz, but it will take years before a standard has been set and manufacturers offer compatible instruments.

Attenuators

Attenuators are used to simulate the loss of long fiber runs for testing link margin in network simulation in the laboratory or self-testing links in a loopback configuration. In margin testing, variable attenuators are used to increase loss until the system has a high bit error rate. For loopback testing, an attenuator is used between a single piece of equipment's transmitter and receiver to test for operation under maximum specified fiber loss. If systems work in loopback testing, they should work with a proper cable plant. Thus many manufacturers of network equipment specify a loopback test as a diagnostic/troubleshooting procedure.

Attenuators can be made by gap loss, or a physical separation of the ends of the fibers, inducing bending losses or inserting calibrated optical filters. Both variable and fixed attenuators are available, but variable attenuators are usually used for testing. Fixed attenuators may be inserted in the system cables where distances in the fiber optic link are too short and excess power at the receiver causes transmission problems.

Reference Test Jumper Cables and Mating Adapters

reference test cables

In order to test cables in an insertion loss test, one needs to establish test conditions. This requires reference launch jumper cables to connect the test source to the cable under test and receive cables to connect the fiber optic power meter. For accurate measurements, the launch and receive cables must be made with fiber and connectors matching the cables to be tested and terminated carefully to ensure low loss. To provide reliable measurements, launch and receive cables must be in good condition and kept very clean. They can easily be tested against each other to insure their performance. Connector mating adapters are used to connect the cables under test to the launch and receive cables. Only the highest performance bulkhead splices should be used, and their condition checked regularly, since they are vitally important in obtaining low loss connections.

Additional Reading
Testing Installed Cable Plants  
Accuracy of fiber optic measurements  




 


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