All About Bend-Insensitive Optical Fibre Cable

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By using the magic of light, fibre optic cables have laid the foundations of global connectivity. But as the hunger for higher data transmission, better performance and speed, and large-scale communications infrastructure grows, we will need better versions of the humble optical fibre cable. This is where the bend insensitive fibre has emerged as a key difference maker.

In the following blog, we will cover various topics such as:

  • The process behind manufacturing and deploying optical fibre cable
  • Challenges in deployment
  • Bend-insensitive fibre, its various types and advantages

How is an Optical Fibre Cable made & deployed?

If you’re reading this blog on your broadband-connected smart device, then that’s been made possible only due to optical fibre cable. Wonderful, right? Now, can you imagine the sheer volume of fibre optic cables that have been deployed across the world since their advent in 1970s? 4 billion km – which is approximately the distance between wherever you are to the planet Neptune!

Essentially, an optical fibre cable consists of minutely thin strands of glass that carry information in the form of light pulses. This beam of light bounces off the walls of the cable at such shallow angles that it is completely reflected back inside. This phenomenon is called Total Internal Reflection. Thus, by zigzagging across the tubular cable, it reaches its destination where it is converted into electrical information understandable by a computer.

An optical fibre cable, even the bend insensitive fibre, is made of an extremely pure optical material such as glass (silica) that contains no impurities. First, a preform glass cylinder is created using a chemical process called modified chemical vapour deposition. Next, thin fibres are drawn out of the preform at high temperatures, with each spool containing almost 1.4 km of fibre. Thereafter, the fibre is tested for various parameters like tensile strength, refractive index profile and attenuation. Finally, multiple individual hair-thin optical fibres are bound around high-strength plastic carrier or a central steel cable. This forms the core of the cable, which is then surrounded by protective coverings of Kevlar, aluminium and polyethylene (i.e., the cladding). Voila!

And what about deployment? Well, fibre optic cables can be deployed in three ways: aerially, underground, or submarine. While aerial deployment consists of mounting them on poles, it is not considered ideal because of extra tension and other detrimental environmental factors. In contrast, fibre optic can easily withstand up to 200 pounds of pressure and thus can be buried deep underground. Undersea deployment is also common and is carried out via vessels.

Challenges in cable deployment in optical networks

The rate of optical fibre cable deployment across the world is increasing at a breakneck speed to keep pace with rising data demands. To adequately reach more subscribers and meet 5G and FTTx network demands, optical fibre deployments will have to be way denser. To do that, cable deployment in modern optical communication networks will have to mitigate two major challenges: limited space in distribution networks and tighter bends in subscriber loop.

Remember, the higher the count of fibres in a cable, the more information it can carry. This is prerequisite to service a larger number of subscribers via optical distribution networks. While it is possible to have, say, 1000 fibres in one larger diameter cable, such a deployment is thwarted due to limited space in currently installed ducts at distribution networks. Moreover, if attempting to replace these ducts with larger ones requires a lot of time, expense, and civil approvals.

Then there is the problem of increased optical power loss due to tighter bends in subscriber loop. This refers to the increase in the instances and degree of bending in the optical fibre cable in the last mile phase as it loops around a building and connects to the subscriber. Such distortion of the cable leads to attenuation, bending loss or even leakage of the optical signal.

How do we solve these challenges, then? The answer is installing micro cables and bend-insensitive fibre patch cables.

What is bend-insensitive fibre & its types?

As we’ve read already, fibre optic cables are sensitive to stress such as bending which is a major challenge during deployment. When they are bent beyond the minimum bend radius, light in outer core is not guided back in. Instead, it gets coupled into the cladding leading to signal loss.

This is where bend-insensitive fibre cable was invented to provide resistance to any bending-related damage and minimise optical signal loss. It consists of an additional layer of glass surrounding the core. This glass layer has a lower index of refraction that reflects the weakly guided light modes at the bent portion of the cable back into the core. Therefore, a bend insensitive fibre cable experiences minimised loss of transmitting light even if the cable is bent beyond the minimum bend radius.

In terms of properties, a bend-insensitive fibre cable is much more flexible than a regular optical fibre cable. It is often designed to last as high as 7500+ bending cycles. Moreover, its bend radius is also much smaller, thus giving it the ability to be tightly wound or wrapped around corners or equipment.

Like regular fibre optic cables, bend-insensitive fibre also has two major types: Single Mode Fibre (SMF) and Multi-Mode Fibre (MMF).

Bend Insensitive Single Mode Fibre (BISMF)

A bend insensitive single mode fibre cable only allows one type of light mode to travel through it at any given time. There are two classes of single mode bend insensitive fibre cables specified by the International Telecommunication Union (ITU): G.657.A and G.657.B.

Let’s compare their minimum bend radius with that of standard single mode G652 fibres which stands at 30 mm. G.657.A1 BISMF has a minimum bend radius of 10 mm, while its A2 version and G.657.B1 BISMF have a minimum bend radius of 7.5 mm! The B2 version is even more flexible with a measure of 5 mm – a whopping 6X difference compared to a regular single mode optical fibre cable. Due to this very reason, single mode bend insensitive fibre has huge scope in long-distance, high-density and low installation cost applications. It also suffers from much lower attenuation compared to multi-mode fibre.

Bend Insensitive Multimode Fibre (BIMMF)

As the name suggests, a bend insensitive multimode fibre cable allows multiple modes of light to travel through it at any given time. OM2, OM3 and OM4 are the laser optimised grades found in the category. Their special property is that, in tight bend circumstances, they display 10X less signal loss.

The BIMMF is also very flexible, with its minimum bend radius going as low as 7.5 mm. Bend insensitive multimode fibre is utilised in short-distance applications and is prevalent in data centres as well as intra-building architecture.

Advantages of Bend Insensitive Fibre Optic Cable

Compared to a regular optical fibre cable, bend insensitive fibre optic cable offers numerous advantages, the most obvious one being minimised attenuation and bending loss.

  • Flexibility during deployment – Given their immaculate flexibility, bend insensitive fibre optic cables can be deployed in high density settings where navigating tights corners and lack of space can be a challenge for regular cables. Thus, they are finding application in indoor fibre cable deployment as they are easy to loop around walls, ceilings, ducts, pillars and buildings without causing excessive micro or macro-bending.
  • Prevention of installation errors – Unlike a regular optical fibre cable, a bend insensitive fibre cable does not need to worry about installation damage caused due to steep bends, usually inside racks or panels and at entrance or exit points.
  • Higher performance – Since bend insensitive fibre optic cables preserve the performance of optical signal despite bending, they are being used to carry higher bandwidth signals. 
  • Higher resilience – Bend insensitive fibre already has a significantly smaller bend radius, and is also highly tough and resistant when it is clamped, stapled or tie-wrapped – something that would damage a regular fibre optic cable with ease.
  • Comparable manufacturing cost – Bend insensitive fibre cables cost only marginally more to manufacture compared to normal fibre cables, thus making the process quite economical.
  • No difference in splicing – There are no special splicing processes when it comes to bend insensitive fibre. It uses the same methods as are prescribed for any regular optical fibre cable. 

STL’s Bend Insensitive Fibre Solutions

STL, being the world leader in fibre optic solutions, has devoted a lot of R&D to develop cutting-edge bend insensitive fibre cable solutions. These cables are in the range of 250um and 200um Bend Insensitive Single Mode Fibre and Non-Zero Dispersion Shifted Single Mode Fibre (NZDSF) solutions that cater to all the requirements of 5G and FTTx applications. STL has also created and installer optical fibre cable named StellarTM – the world’s first G.657.A2 macro-bend insensitive fibre which is compatible with legacy networks comprising of G.657.A1 and G.652D. 

FAQs

How to calculate bend radius of fibre optic cable?

Bend Radius can be calculated using the following formula:

Minimum Bend Radius = Cable Outer Diameter X Cable Multiplier

For every fibre optic cable, its bend radius depends upon a host of factors such as: type of cable, its outside diameter, cable condition under stress (tensile load), cable condition without stress (no load), and its cable multiplier. Usually, the multiplier is 6X for 5000 V or less cables, and 8X for cables above that voltage.  

What’s the difference between micro and macro-bending?

Micro-bending refers to microscopic deviations that occur along the optical fibre axis during bending. Macro-bending, on the other hand, refers to bending in the cable large enough to be visible to the human eye. Both lead to damage to the fibre cable, thus resulting in some form of attenuation or leakage of light.

Is Bend Insensitive fibre compatible with regular fibre?

Bend-insensitive fibre is compatible with Single Mode Fibre (SMF). This is because, in SM, only one mode is guided in the core. Thus, the trench causes minimal impact on the system performance or measurement. However, bend-insensitive fibre’s compatibility with Multi-Mode Fibre (MMF) is yet to be proven with certainty.

Does bend-insensitive fibre reduce Macro bending losses?

Yes, bend-insensitive fibre can reduce macro-bending losses. In fact, these optical fibre cables were introduced for this very purpose, thus allowing them to be run around corners better or be fitted into smaller spaces.

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