Splices And Connections Of Regular to BI Singlemode FIbers

Background

A Quiet But Important Change In The Fiber You Buy

Singlemode Fiber - moves toward being mainly G.657 Bend Insensitive Fiber

Below, FOA technical advisor Joe Botha provides some interesting data on the splicing compatibility of conventional G.652 singlemode fiber and G.657 bend insensitive (BI) fiber that showed excellent compatibility. That got us thinking. With so many cable designs today, like microcables or high fiber count cables, requiring bend-insensitive fibers, would it make sense to make all or most singlemode fibers as bend insensitive fiber?

We reached out to some of our contacts at fiber manufacturers and asked them. What we got was a good tutorial on BI fibers and an answer to our question. First the technical tutorial.

Conventional G.652 singlemode fiber has been around about 40 years and the standard for it is almost as old. The specifications are straightforward.

G.652 Singlemode
Outside Diameter: 125 µm
Mode Field Diameter (MFD): 8.6-9.2 µm @ 1310 nm
And several new specifications added more recently:
Low water peak. With maximum attenuation of 0.4 dB/km across a band from 1310 nm to 1625 nm
Minimum bend radius: 30mm

With the introduction of BI singlemode fiber, new standards were written as G.657 fiber with several grades, each having a minimum bending diameter and loss specification.

G.652 fiber bend radius 30mm
(The G.657 standard notes "ITU-T G.652 fibres deployed at a radius of 15 mm generally can have macrobending losses of several dB per 10 turns at 1625 nm.")

G.657 fiber (bending loss specs at 1550nm)

G.657.A1  bend radius 10mm, loss 0.75dB/turn

G.657.A2  bend radius 7.5mm, loss 0.5dB/turn

G.657.B3  bend radius 5mm, loss 0.15dB/turn (for special applications)

Designing singlemode fibers requires tradeoffs. A smaller mode field diameter will have better bend performance but higher attenuation. Larger MFD provides lower attenuation, and the majority of G.652 fiber, which is much of the installed base, is a MFD of 9.2 µm. Simply reducing MFD for better bend performance leads to mismatch losses when splicing or connecting fibers and causes OTDR tests with gainers, requiring time consuming bidirectional testing.

Corning data and graphic

The right way to create a BI singlemode fiber is to redesign it to get BI performance while maintaining a larger MFD for compatibility and lower attenuation. And that's what has already happened at some fiber manufacturers with standard 250 micron and smaller buffer coating fibers.

Here is what two say:

Corning: The industry is moving towards a G.657.A  specification in fiber, because the industry is moving towards smaller denser cables in the network & the bend resilience is a requirement for the cable design. The industry will not move wholesale towards a G.657.A2  specification because this is not necessary in all cases. There is no need to compromise on the 9.2 um MFD to get a G.657.A  fiber because Corning innovation delivers this, alongside the bend resilience in; SMF-28 Ultra and SMF-28 Contour fibers.

Worth reading: Corning ap note AN2020 on splicing compatibility.

OFS: The simple answer is most SMF is moving to G.657.A1. OFS AllWave+ and Corning’s Ultra fiber which are among the most deployed fibers in America right now are both examples of this trend.  There have been some modifications to the G.657 specification that puts more stringent boundaries on MFD to assure compatibility of BI fiber with standard G.652 fiber. Further ITU has studied the full set of transmission parameters for G.657A1 and A2 fibers and has stated that the products are fully compatible. That said, smaller MFD’s have better macrobend performance and as a result many of the more bend insensitive G.657.A2 and G.657.B3 fibers on the market may show artifacts in one way OTDR traces due to the MFD change.
 
So singlemode fiber is moving to being BI fiber, exactly what happened with 50/125 laser optimized fibers a decade ago. With most new fiber, compatibility is not an issue. But it is recommended to check with the cable manufacturer if you are not sure what fiber is being used in the cable you are purchasing.

This was a followup to the article below - both are important technical news.

The Curious Compatibility Issues Of Bend Insensitive FIber

Bend insensitive (BI) fiber (also referred to as reduced bend sensitive - RBS - fiber) has been around for more than a decade now, but it is still not well understood and there are some technical controversies associated with it. When first introduced, it was mainly for multimode fiber which can get a lot of bending stress in premises applications, especially around patch panels. It took little time before practically all MM fiber was BI.

That created the first controversies. There was much concern at the time over the testing of MM fiber with its dependence on modal distribution when testing loss. For many years, standards for testing called for using reference test cables with non-BI fiber. The problem was finding non-BI MM fiber cables. Eventually most standards changed to say use either fiber for reference test cables.

Singlemode fiber only recently joined this controversy. BI fiber (ITU-T G.657) was an essential component in making cables more densely packed with fibers. Not only did cables start using BI fibers to make cables more fiber dense, but they began using coatings of less than 200 microns on the fiber instead of 250 micron coatings to increase density even more. Many of today's cables, especially the high fiber count cables and those with flexible ribbons rely on BI fibers in their designs.

Then this month, FOA gets this inquiry: "I've currently got quite a few G652 launch boxes of various lengths but we are testing more and more G657 A1 fibre. My question is, are my G652 launch boxes compatible for testing G657A1 cable."

An ITU online publication says "ITU-T G.657.A1 and ITU-T G.657.A2 fibres are fully compliant with ITU-T G.652.D fibres."

FOA asked our technical advisor Joe Botha of Triple Play Fiber Optics about this compatibility. Joe has done studies on this issue for clients in Africa.
Here is what Joe tells us.

"To start with, they are perfectly compatible, with a marginal difference in MFDs i.e. only a slight gainer and additional loss is visible. With latest Corning fibers, the two MFDs are identical.
 
Years back, I was asked to do research on exactly this for a few FNO’s . Based on findings, the following recommendations are made:
Not too many are aware that the ~27m long fiber in a OTDR, is in fact G.652D - which is a good reason to use a G.652D launch patch cord and pigtail in your patch panel.
Where a fiber terminates in a patch panel, whether it’s a G.655, G.656, G.657, or G.652, etc, you should splice-on G.652D pigtails. The reasons for this are:
Your patch cords will in all likelihood be G.652D. And remember, the fiber in the OTDR is G.652D.
With different fiber types, one cannot obtain a perfect enough core alignment, like you would with a fusion spice, through a connector/mating adapter type connection."

From the trace below taken at 1310 nm, the splice loss of the joint between two ~25km fibers, one G.652 and the other G.657, is low and has minor directional differences. At 1550, the loss of the splice was too low in both directions for the OTDR to detect it.


So the answer to the original question is "Yes, you can use your G.652 launch cables with G.657 fibers. The bigger question of compatibility of the two fibers is "Yes, they are compatible."