Although optical networks have been in existence for many years now, optical communication technologies are continuously evolving. Keeping in sync, optical testing is looking for high-speed, high-performance, all-in-one type and accurate test and measurement solutions that are upgradeable to 40G or 100G in the future
SHWETA DHADIWAL BAID
APRIL 2011: The need for high-speed connectivity and higher capacity will never cease. The launch of 3G and LTE has fuelled this need for high-speed and high-bandwidth backhaul. To meet this demand, optical networks based on fibre-optics technology provide a stable and upgradable system.
Optical systems have already been dominating the core networks. “Due to their reliability and bandwidth offering, optical networks are making inroads to support the passive optical network technologies in the access networks. Many enterprise networks and residential customers are now connected to core networks via optical networks,” shares Mohmedsaeed Mombasawala, general manager-applications, Agilent Technologies.
The increased service needs based on Triple Play in the user segment are fuelling the usage of optical-fibre systems in access networks as well.
There are two kinds of test and measurement requirements for optical communications: Optical network testing during installation and commission of networks, and testing for R&D and manufacturing. From development to deployment, the testing needs are very intensive and knowledge-oriented. Optical testing is looking for high-speed, high-performance, all-in-one type and accurate test and measurement solutions that are upgradeable to 40G or 100G in the future.
Optical communication technologies
An optical communications system is based on the priciple of transmission of a signal through an optical fibre to a distant receiver. The electrical signal is converted into optical at the transmitter and back into the original electrical signal at the receiver.
Russel Taws, market manager, JDSU, explains, “The fibre is classified into multi-mode and single-mode based on the way in which the light travels through it. Multi-mode fibres are used to transmit data over relatively short distances (200 km) and are used in all telecommunication systems for voice, data, video and mobile.”
2. End-to-end optical link loss
3. Rate of attenuation per unit length
4. Attenuation contribution to splices, connectors and couplers (events)
5. Length of the fibre or distance to an event
6. Linearity of fibre loss per unit length (attenuation discontinuities)
7. Reflectance or optical return loss
8. Chromatic dispersion
9. Polarisation-mode dispersion
10. Attenuation profile
“The capacity of a single-mode fibre communication system may be further increased by injecting multiple signals of slightly differing wavelengths. This is called wavelength-division multiplexing (WDM) and is an important technique to increase the system bandwidth,” explains Taws.
Fibre-optic networking is more than a decade old and existed alongwith synchronous optical networking (SONET) and synchronous digital hierarchy (SDH), which today goes up to over 10 GB.
In one of the interviews, Kumar N. Sivarajan, co-founder and chief technology officer, Tejas Networks, states that the recent development is dense wavelength-division multiplexing (DWDM), which includes the use of multiple frequencies or wavelengths for different signals from the infrared light portion of the optical spectrum. Today, optical networking has become synonymous with DWDM.
Optical testing challenges
Optical communications is a fairly stable technology with established processes and best practices. Therefore the initial ramp-up needed for manpower to operate this technology is also well-defined and easily available.
However, optical communication faces other types of issues. Madhukar Tripathi, manager-telecom segment, Anritsu, shares, “The major disadvantage is that it is very expensive to construct the fibre-optics for optical networks and to join the fibre-optical cables as compared to the copper cables.”
Mombasawala says, “The laying of optical cables, securing rights-of-way, frequent maintenance due to dig-outs and the necessity of the backup routes for network resiliency are some of the challenges faced in optical networks.”
“The testing of optical networks during installation and commissioning includes link verification, link characterisation, system tests like bit error rate (BER) measurements, alarms and transmission power measurements. This type of testing is very repetitive and needs standard tools like optical-time domain reflectometer (OTDR), BER meter and power meter, while R&D tests demand sound technical knowledge and skills to test the systems,” he adds.
Expressing his views on the challenges faced in India, Taws says, “One of the main challenges we have in India is ensuring that the optical connections are free of dust and dirt during installation and maintenance. Contamination is the main reason for trouble in optical networks. A single particle mated into the core of a fibre can cause significant back reflection, insertion loss and equipment damage. Visual inspection is the only way to determine whether fibre connectors are truly clean before mating them.”
Key measurements in optical communication
In order to qualify an optical fibre or an optical fibre system for proper transmission, several key measurements are performed. “Critical measurements such as OTDR, chromatic dispersion (CD), polarisation-mode dispersion (PMD), loss test set and optical return loss can be made quickly and accurately to maximise the network performance and accelerate deployment of new services,” shares Tripathi. Many basic and advanced systems are available to perform these tests.
In newer systems designed for high-speed communications, the common test areas include coherent detection, which involves measurement of various link impairments such as CD, PMD loss and polarisation-dependent loss (PDL). Even the effect of non-linear link impairments can be qualified by using a technique based on error vector magnitude.
Mombasawala explains, “Optical modulation analysis includes tests like optical I-Q diagram, I-Q eye diagram, constellation diagram, EVM, phase-error analysis, and bit- or symbol-error physical-layer analysis.” In waveform measurements, various tests like accurate analysis of optical waveforms, basic eye-diagram and pulse waveform characterisation, jitter analysis and channel impedance characterisation are performed.
Optical testing equipment
Fibre-optical network testing involves a variety of equipment and applies to multiple functionalities and applications. Testing solution providers offer a range of equipment for the optical communications industry, for R&D, production, installation, monitoring and manufacturing.
Some of the common tools include OTDRs, BER sets, optical spectrum analysers (OSAs), power meters and light sources. Besides these, there are advanced-level tests, particularly in the areas of R&D and verification where complex and accurate test instruments are required.
Optical time-domain reflectometer. It is an optoelectronic instrument used to characterise an optical fibre. An OTDR injects a series of optical pulses into the fibre under test. It may be used for estimating the fibre’s length and overall attenuation, including splice and mated-connector losses. It may also be used to locate faults, such as breaks, and measure optical return loss.
Test and measurement (T&M) vendors provide a range of OTDRs in hand-held, bench-top and OEM module configurations. Some companies provide specialised OTDRs known as ‘coherent OTDR.’ These are used for locating faults in under-sea optical cables of up to 10,000 km between two countries or continents.
Optical spectrum analyser. Also known as spectrometer, it is used to measure the properties of light over a specific portion of the electromagnetic spectrum. It also allows characterisation of sources and DWDM systems for their power, bandwidth and performance.
Optical power meter. T&M vendors provide a range of optical power meters including single-channel and multi-channel solutions for both hand-held and benchtop applications.
Optical component analyser. It is used to measure CD, PMD and PDL of the optical fibre and components. The lightwave component analyser is the instrument used to test all the relevant opto-electronic S-parameters and electro-optical components in 10 Gbps and 40 Gbps, fibre channel and CATV transmission systems as well as radio-over-fibre.
Bit-error rate test (BERT) sets. While dealing with any digital communications system, measurement of BER is very critical. Highly accurate and reliable BERT sets are used in high-speed, high-bandwidth optical communications.
Optical loss test sets. Optical test sets are multi-purpose optical measuring instruments that provide superior accuracy and reliability supporting reference light sources.
Optical signal sources/laser sources. Optical light sources are used to evaluate a wide range of optical devices and systems including WDM.
Multi-layer network test platform. It is designed to maximise the network performance and accelerate deployment of new services. Critical network parameters and performance can be monitored and network impairments quickly identified to accelerate deployment of new services. Results can then be easily documented by the powerful Windows-based platform simplifying and reducing the non-revenue-generating activities associated with operating an optical network. A modular design with different-size bay adaptors allows you to customise your test set as needed, adding further value.
Optical test peripherals. There are many application-specific peripherals for optical network testing, including video inspection probes, optical fibre identifiers and visual fault locators.
Optical sampling modules in oscilloscopes. For transmitter and receiver testing of optical network element designs up to 100 Gbps, companies provide optical sampling modules that support up to 80GHz bandwidth.
Fibre inspection and test tools. Optical fibre inspection is critical for successful delivery of optical communication. Some specialised microscopes provide direct view of end-face cleanliness and core condition when inspecting fibre in the field. Vendors provide complete solutions for inspecting, cleaning and testing fibre connectors with a combination of all the necessary equipment that technicians need to ensure the integrity of optical connections.
Visual fault locator. It is an essential tool that quickly and easily locates problem areas in fibre cables. By pinpointing the exact location of fibre damage, technicians can diagnose, troubleshoot and fix the problem efficiently. These rugged units are a safe, economical and non-destructive way to identify live optical fibre. They use local detection technology, which employs a macro-blend method, eliminating the need to open the fibre at the splice point for identification.
Focus of T&M vendors
There have been several technological developments in optical communication systems. T&M technologies are being designed accordingly to address their needs.
“First, you must certify the physical media (for attenuation, PMD, CD, etc) to know whether the link would work with the desired technology (SDH, Ethernet). The second step is to test the specific technology and rate that you deployed. For example, in the case of SDH/SONET you can perform BER test, while for Ethernet/IP/FC you can run RFC2544 tests (up to layer 3) or some tests including higher layers,” explains Damian Brazionis, product manager, Tecnous S.A.
5. General Photonics
7. Rohde & Schwarz
According to Mombasawala, “Another important development is in the area of enhancing the optical communication capacity. Due to the bandwidth limitation of the fibre at 10 Gbps, transmission at 40G and 100G is not easy. There are non-linear effects in the fibre and components that start influencing the system. To counter this, a new transmission method called ‘optical modulation’ and similar to the RF modulation scheme is designed.” Systems based on optical modulation require special test equipment like modulation analyser that allows various tests based on I-Q modulation for 40G and 100G systems.
To provide uninterrupted telecommunication link, the fibre-optic system requires continuous monitoring. Taws shares, “Test equipment can be integrated into an automated monitoring system and connected to a network operation centre. The system monitors the network continuously, alerting technicians and managers of faults as they occur.”
“New remote fibre test systems are composed from a series of remote test units, a central server and multiple client stations, and machines using associated Web-based applications. Fibres are monitored in real time, 24×7 according to user-programmable schedules. Data polled from RTUs in the field is mapped to the central database and combined with routing records and geographical information, enabling maintenance teams to access precise fault-location details,” adds Taws.
As the technologies continuously evolve, there is a need for sustainable test systems. Tripathi shares, “Older equipment are upgraded to meet the new generation’s network requirements. At some point, however, really old equipment are beyond upgradablility for today’s applications. This is when the telecom companies have to opt for newer equipment.”
Optical network in India
India is already making significant use of fibre-optics, with a high growth expected in the next few years. There are a number of telecom service providers in India with optical networks, several manufacturers, system integrators and service companies.
“India has approximately 1,000,000 km of optical fibre network operational using GPON, SDH and DWDM as a choice of technologies for transmission of information,” informs Sandeep Gupta, vice president, Ericsson India.
“Many mobile operators own India-wide optical network. Optical fibre has reached the villages in India. This will help to meet the broadband demand of the market,” shares Tripathi.
Mombasawala adds, “The digital microwave radio links used earlier by the operators are transitioning towards the optical-fibre links now. This way, there is deep penetration of optical networks within the country.” All the overseas communication links too are now based on fibres.
As the demand for high-speed data services increases, there lies a huge requirement of reliable optical networks and reliable optical testing solutions.
The author is a senior technology journalist at EFY