Testing evaluates the performance of fibre-optic components, cable plants and systems. For every fibre-optic cable plant, testing for continuity and polarity, end-to-end insertion loss and then troubleshooting any problems is necessary.
An optical-fibre cable, also known as fibre-optic cable, is like an electrical cable but contains one or more optical fibres that are used to carry light (signals). An optical-fibre cable consists of a core and a cladding layer. Signal transmission takes place in this type of cable due to total internal reflection, which happens due to the difference in the refractive index between the fibre material and air or any other surrounding material.
In September 2012, NTT Japan demonstrated a single fibre cable that was able to transfer one petabit per second over 50 kilometres. A single, modern fibre cable contains up to 1000 fibres, with tetrabytes per second of potential bandwidth.
Today, optical communication is preferred over wireless transmission because of its large bandwidth and low propagation attenuation. Also, it costs less.
Advantages of an optical-fibre cable are:
- Speed. A fibre-optic cable network operates at a high speed, in gigabytes.
- Bandwidth. It provides large computing capacity.
- Distance. Signals can be transmitted without needing to be refreshed or strengthened.
- Resistance. It offers greater resistance to electromagnetic noise from radios, motors or other nearby cables.
- Maintenance. It requires extremely low maintenance.
- It is less bulky, lighter and more flexible than conventional cables.
- It can carry large amounts of data.
Its applications include:
- Cable TV
- Computer network
- Surgery and dentistry
- Lighting and decorations
- Mechanical inspection
- Military and space applications
- Automobile industry
Accurate testing of cables ensures that no good cables are rejected, or bad ones are accepted. Industries spend a lot of time and effort to develop better standards that ensure accurate testing. But these standards are generally written for manufacturers and not customers.
One of the measurements required to confirm the quality of installation is optical loss or insertion loss of each fibre in the cable. Loss of measurement is made on end-to-end basis in a permanently-installed cable plant. For this, a test source and optical power meter—sometimes called optical loss test set—and reference set cables are used. Sometimes, an optical time-domain reflectometer is also used.
The right tools for accurate optical-fibre cable testing
A lot of time and cost can be saved if the contractor and installers know the proper measurements that need to be done, how to make these measurements correctly, which tools to use and keep these tools in good condition—tools must be calibrated from time to time.
All errors that occur in an optical-fibre cable can be broadly classified into random errors and systematic errors.
Fibre attenuation measurement is essential. Attenuation is the loss of optical power as a result of absorption, scattering, bending and other loss mechanisms as light travels through fibre. Cut-back method is often used for measuring total attenuation of the optical-fibre cable. Modal bandwidth testing in a multi-mode optical-fibre cable is also essential.
Sometimes, chromatic dispersion occurs because index of refraction is a function of wavelength and different wavelengths of light travel through fibres at different speeds. Chromatic dispersion measurement can be made by wavelength selectable losers.
Optical-fibre cable geometry measurements are also important. For this, input end of the fibre is overfilled and mode-filtered. Output end of the fibre is viewed with a video camera. Image from the camera is sent to a computer, which analyses the same to identify the edges of core and cladding.
The following optical-fibre cable geometries are measured in the optical-fibre cable industry:
- Cladding diameter
- Core diameter
- Cladding non-circularity
- Core non-circularity
- Core-cladding non-circularity
Instruments used in the optical-fibre cable industry are summerised in the table above.
A submarine communication cable is a cable laid on the seabed between land-based stations to carry telecommunication signals across stretches of oceans and seas. Modern under-sea optical-fibre cables carry digital data, which include telephone, Internet and private data traffic.
AFL is a leading optical-fibre cable manufacturing company. Its cables were recently deployed through McMurdo ice shelf and into Ross Sea in Antarctica as a part of the study to measure the effects of warming of oceans on glacial ice. In such projects, frequent testing of optical-fibre cables is essential.
Majority of the Internet flows through under-sea cables and very little passes through satellite. India receives its data connections from three major landing ports, namely, Mumbai, Tamil Nadu and Kerala. Internet data reaches India through a network of under-sea submarine optical-fibre cables owned by telecom giants such as Singtel, Etisalat, France Telecom and China Telecom. These cables are sometimes damaged by earthquakes, ship anchors and fishing trawls, and often need repair and maintenance. Repairing under-sea optical-fibre cables is like repairing punctured tyres.
Specifications of optical-fibre cables
Some factors that must be considered while selecting an optical-fibre cable are mechanical, environmental, dimensional characteristics with building codes and transmission equipment.
Normally, optical-fibre cables are classifieds into single-mode and multi-mode. However, these can also be categorised as outdoor, indoor and outdoor/indoor.
Optical-fibre cable specifications include attenuation and bandwidth, which are currently specified at two operating wavelengths for each fibre type. Performance is specified at 850nm, 1310nm and 1550nm for single-mode. Mechanical specifications include characteristics such as tensile strength, resistance to crush, impact and twist. Environmental considerations include temperature ranges for operation and storage. These also include sensitivity to moisture and sunlight, protection from lightening and rodent attack.
Commercially, two types of multi-mode optical-fibre cables are popular, each having a cladding diameter of 125 micron-metres but differing in core diameter. One has a core diameter of 50 micron-metres and the other of 62.5 micron-metres. These cables are called 50/125 and 62.5/125 micron-metre optical-fibre cables.
Loose tubes are individually coloured for ease of identification, and individual fibres are colour-coded.
Creating test environments for optical-fibre cables
According to a market research report, the optical-fibre cable test equipment market can be classified based on the following:
- Type. Optical time-domain reflectometer, optical power meter, optical loss test set, remote fibre test system
- Form factor. Handheld, bench type
- Application. Installation, manufacturing, research
- End user. Telecom, cable TV, data centres
The total market is expected to reach US$ 902.2 million by 2020 at an estimated CAGR of 4.86 per cent, from 2015 to 2020.
Optical-fibre cables are becoming an integral part of telecommunications. Need for maintenance and integrity of these networks are becoming increasingly important. Over 80 per cent of network failures happen due to improper cleaning of connectors at end-faces. An optical time-domain reflectometer can be used to measure the length of fault in single-mode or multi-mode optical-fibre cables.
Today, alarming changes are occurring in the design, installation and maintenance of optical-fibre cable networks, just as in the test and measurement (T&M) of optical-fibre cables.
An optical time-domain reflectometer cannot be used for short cables, as in LAN or building environment, as it is likely to show ghosts due to reflection at connectors. A light test set is easy to use and economical. An optical power meter is used to accurately measure the power of optical-fibre cable equipment or the power of signals passed through optical-fibre cables.
An optical-fibre cable network consists of such elements as fibres, connectors, splices, LEDs, laser sources, detectors and receivers. All of these need to be tested accurately for the efficient working of optical-fibre cable networks. The most important test is the insertion loss test of an installed optical-fibre cable network using a light source performance meter or an optical loss test set. A visual fault locator uses a visible laser to detect faults. A red laser light is powerful enough for continuity checking, to trace it for several kilometres.
A remote fibre test system automates the process of detecting, locating and repairing faults on the outside of the fibre-optic plant. A remote fibre test system can monitor the fibre network 24 hours a day. The system automatically provides fast and precise notifications about any network problems. It detects and locates faults along the cable and sends clear alarm reports to the right personnel, at the right time.
Measuring losses in optical-fibre cables
There are two methods for measuring losses in optical-fibre cables, as follows:
- Single-ended loss method, which makes use of only the launch cable
- Double-ended loss method, where the receiver cable is also attached to the test meter
Various losses that occur in optical-fibre cables include:
- Attenuation loss, which is also known as signal loss or fibre loss; it is the loss of light energy as light pulses pass from one end to the other end of the cable
- Light absorption loss, which is also known as material absorption loss
- Scattering loss
- Bend loss, which includes micro-bending and macro-bending losses
- Optical-fibre cable joining loss
Devices in the market
“Key growth drivers of the T&M industry are introduction of new telecom technologies, explosion of data/video services, roll-out of 4G services, new quality of services (QoS), conformity to radiation specifications, spectrum management, etc,” opines Vishnu Goel, an optical-fibre cable industry expert.
Recently, Anritsu introduced access master MT 9085 series of handheld testers that provide accuracy and broad measurement capability during installation and maintenance of optical-fibre cables in LTE and 5G backhaul, as well as metro and core networks. It incorporates a 20.3cm (8-inch) coloured touchscreen for enhanced operation, as well as integrates. Anristu’s fibre visualiser as a standard feature.
ED-4100 model by Euro Digital, available for ₹ 65,000, supports touch-sensitive LCD screen and multi-mode measurements.
Smart glass fibre OLTS-85/85 P by Viavi Solutions allows smart testing, and promotes fast and easy Tier 1 fibre testing based on TIA/ISO/IEC standards. It comes with dedicated multi-mode, single-mode and quad versions.
Challenges and improvement areas
An optical-fibre cable system is like a commodity. Challenges in T&M in the optical-fibre cable industry exist for both in-process and final quality testing, especially in standard telecom and datacom fibres. These challenges are mostly related to productivity, and are not technical in nature. This is due to availability of more-efficient and less-costly T&M equipment. Further, computer-based and Internet of Things (IoT) technologies give extremely precise and fast measurement possibilities.
It is true that test equipment purchased almost a decade ago are now obsolete. Many optical-fibre cable industries are now losing experienced test personnel due to newer trends. These days, it is easy to connect optical-fibre cables temporarily to test equipment. Automated test software and management information systems have further eased the T&M of optical-fibre cables.
“The IoT offers new possibilities for optical-fibre cable T&M industries,” says Henry Yaffe, founder and president of New Ridge Technologies. Through the IoT, it is possible to replace bulky T&M instruments with low-cost, ubiquitous, portable and smart devices. IoT-enabled optical spectrum analysers (such as NRT-8000 by New Ridge Technology Co.) are being greatly used by optical-fibre cable companies.
Due to the emergence of non-conventional, speciality optical-fibre cable manufacturers, high-quality unique measurement requirements exist. Optical-fibre cable manufacturers need more automated test equipment.
Vinayak Ramachandra Adkoli is BE in industrial production. He was lecturer in mechanical department for ten years in three different polytechnics. Now, he is a freelance writer and cartoonist.