T & M

Optical Fiber is Future Technology for Today and Tomorrow

Madhukar Tripathi

Associate Director - Optical Business & Marketing, Anritsu India Pvt. Ltd.

5G known as Fifth Generation of Mobile Network has increased expectation of customers along with mobile service providers for higher bandwidth, low latency, spectrum efficient network. All communication technologies (wired and wireless) have own merit and demerit. In view of the requirements of 5G expected in the long run regarding data rate, latency, the optical fiber is basically the most future-proof and scalable medium under wired communication network and now supplement to wireless technology. In addition, it is operationally cost-effective in the long term because there are no longer active network elements between the control center and the mobile radio station. From a technical point of view, a pure fiber optic connection of the mobile radio stations is the most efficient choice in the long run.

According to the International Telecommunications Union's (ITU) latest “Trends in Telecommunication Reform” report, ongoing capital investments related to fiber infrastructure are expected to total a staggering $144.2B between 2014 and 2019. One of the primary drivers for this immense capital investment into fiber infrastructure deployments comes out of thin air, in the form of tomorrow’s 5G radios.

Even for the sake of network management, the optical fiber connection of 5G locations is optimal. Due to the cellular nature of mobile phone networks, there are always interferences with neighbouring cells at the cell edge, which significantly impairs performance. This problem can be reduced with the introduction of 5G. However, this requires the fast and low-latency coordination of all neighbouring transmitting and receiving stations. For this reason, the optical fiber is the first choice from the technical point of view, as alternative backhaul technologies may lead to necessary compromises in the achievable coordination performance. Mobile service providers have used fiber for years, using the Common Public Radio Interface (CPRI) and soon enhanced CPRI (eCPRI), as well as other optical schemes and microwave links for fronthaul, midhaul, and backhaul. But 5G presents a very different challenge than its 4G predecessor, as it will require more fiber in more places to serve 5G’s massive number of small cells, while accommodating data-intensive applications such as gaming, 4K video streaming, and virtual reality. All these applications will generate orders of magnitude more data than seen today. This data need a most cost effective mechanism to transfer from one site to other, with in network in more efficient and faster manner and Optical Fiber is answer to this.

Optical Fiber Cable (OFC) is being produced more than ever now to ensure connectivity across buildings, cities, nations, and globe. Optical Fiber cables are tested for various stages during production. As per AMR Optical Fiber and Accessories Market in India to Hit $1.66 Billion by 2026, at 17.2% CAGR. Government projects like Bharatnet, Broadband for all, surge in adoption of Fiber To The Home (FTTH), FTTx connectivity by major telecom service providers (Reliance Jio, Airtel), modernization of communication network by Govt PSU’s and Statewide optical fiber deployment projects are creating demand for optical fiber cable.

TESTING OPTICAL FIBER CABLES

Manufacturer testing on fiber-optic cable falls into two general categories: production testing and characterization, or type, testing. These two kinds of tests are quite different, but each is useful in its own way.

Production testing:

the manufacturing line. Some production tests, in fact, are conducted online as the cable is being fabricated.

Production testing is meant for internal quality assessment or requirements and not necessary part od final OFC delivery to buyer. Mechanical test, geometrical test is also conducted in various phases in production environment. These tests ensure final cable assembly is meeting buyer’s requirement and meet industry standards. ITU -T has notified SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Transmission media and optical systems characteristics – Optical fibre cables vide document no 11/2009.

Recommendation ITU-T G.652 describes the geometrical, mechanical and transmission attributes of a single-mode optical fibre and cable which has zero-dispersion wavelength around 1310 nm. The ITU-T G.652 fibre was originally optimized for use in the 1310 nm wavelength region but can also be used in the 1550 nm region. This is the latest revision of a Recommendation that was first created in 1984 and deals with some relatively minor modifications. This revision is intended to maintain the continuing commercial success of this fibre in the evolving world of high-performance optical transmission system.

Type testing

The purpose of type testing is to ensure that the cable will function properly under a range of adverse mechanical and environmental conditions, not just when the final system tests are performed. In many cases, type testing is destructive, so it is not conducted on production cables. It is called "type testing" because selected cables of a particular type may be tested as representative of a whole family of cables of that type.

ITU-T standards, also known as ITU-T Recommendations, describe the geometrical properties and transmissive properties of multimode and single-mode fiber optic cables. Now there are seven common ITU-T Recommendations currently in effect at the date of its publication: ITU-T G.651.1, ITU-T G.652, ITU-T G.653, ITU-T G.654, ITU-T G.655, ITU-T G.656, and ITU-T G.657.

Common mechanical tests performed on optical cable include tensile strength, compressive loading (crush), repeated impact loading, torsion loading, flexing, and bending.

Every fiber in every cable should be measured for both optical loss and point discontinuities. Most fiber cable manufacturer provides Optical Loss/ Attenuation data with final product dispatch. This works as judgment and acceptance document for buyer and seller.

Optical Time Domain Reflectometer, commonly known as OTDR is most popular instrument used for this measurement. It is not easy to measure attenuation per meter in Short fiber cables.

Common OTDR wavelengths

The wavelength selection for cable manufactures can differ from the telecom operator requirements. Key wavelengths for the telecom operator are normally 1310, 1490, 1550nm for testing the network at operational wavelengths and 1625nm for confirming good installation practices (macro bending for example).

For the cable manufactures often 1383nm is more important than 1625nm to gain insight into any possible issues with the water peak or confirm the water peak as been removed on relative fibers. ITU-T standard G.652 talks about the different attenuation of fiber G.652C and G.652D both have reduced water peak requirements. The attenuation of these fiber types at 1383nm is required to be less than or equal to the highest attenuation between 1310 to 1625nm. As 1310nm has the highest attenuation across this spectrum people often refer to the loss per km at 1383nm having to be less than that at 1310nm. Full specification details can be obtained from the ITU web site at http://www.itu.int which should always be referenced for the latest and most accurate information.

OTDR Test Set up in OFC Plant:

Figure- 1 : Anritsu OTDR used for Testing Optical Fiber in cable plant.

Figure-2 OTDR Trace Basics

Figure-3 Bidirectional Loss Measurement using OTDR.

Optical Power Meter and Optical Light Source is used for loss measurement for short cables likes patch cord while OTDR is used for longer cables lengths. Optical loss can be a sign of a poorly designed, poorly manufactured, or otherwise defective cable. If a fiber is subjected to any undue mechanical stress during manufacture, it will be manifested as an increase in optical loss. A twisted, crushed, or pinched fiber will show a point loss or "step" at the location of the defect. It is therefore important that factory measurements include both end-to-end loss measurement and inspection for point discontinuities on every fiber. These measurements should be made and recorded at the wavelength(s) at which the installed system will operate.

Insertion Loss (IL) and Return Loss (RL) are two other parameters tested very often. Fiber Patch Cord manufacturers specify this parameter in final products packing. The insertion loss technique is more practical for field work. However, measurement uncertainty is compromised by connector loss uncertainty. It is commonly used in field situations where acceptable measurement performance is obtained regardless of connector performance.

Conclusion:

Figure 4- Optical Loss Measurement Set Up.

Return loss for the entire fiber under test, including fiber backscatter and reflections and relative to the source pulse, is called Optical Return Loss (ORL). There are many advances taking place in almost every area of optical communications, from optical modulators to switching, add/drop multiplexers, reconfigurable optical add-drop multiplexers, signal processing, detection schemes, optical network system architectures, and many more. Many of these developments will make it possible to bring the benefits of fiber to wireless fronthaul and backhaul, increasing throughput while keeping cost controlled.

Suitable testing of all types of cables (including Optical Fiber) are very important and priority factor to consider when designing mobile network (indoor and outdoor). Network performance, user experience depends on these OF cables. These cable works as nerve in mobile network and carry all data from one customer to other, connect us and much more. Reliable, high quality test instrument, easy GUI, automation make these testing easy and fast. Network keeps running due to Cables!