Testing Effects of Micro- and Macrobending

The proper installation of cables and patch cords and the routing of optical fibers in splice trays is crucial to meet the performance expectations of today’s high-speed fiber optic systems. However, these proper practices can sometimes be forgotten with the emergence of bend insensitive or bend tolerant fibers (ITU-T G.657).  While legacy systems or systems containing non-bend insensitive (ITU-T G.652) fiber are known to be prone to the effects of micro- and macrobending (especially at longer wavelengths), bend insensitive fiber is supposed to be generally quite tolerant to these effects. However, the term “bend insensitive” fiber is used to describe many modern fiber products and is sometimes not completely accurate. Real world tests indicate that it is often quite susceptible to microbending.

When testing a fiber optic system, either with an optical loss test kit or an OTDR, it is imperative that the technician test at two or more wavelengths. The shorter wavelength sets a baseline and the longer wavelength detects the absence or presence of either a macro- or microbend.  The longer the waveform, the more susceptible that wavelength becomes to bending effects. As an analogy, a shorter wavelength (car) and a longer wavelength (semi-truck) will travel effectively down a straight road.  However, a wavelength travelling down a fiber that has been pinched due to improper installation or bent at too tight of a radius will have the same reaction as a car and semi-truck travelling on a windy and tight cornered road.  The car will be able to navigate those turns while the semi-truck may hit the guardrails… or worse, leave the road completely.

When testing optical fibers with an optical loss test kit, higher loss values at 1550 nm or 1625 nm than at 1310 nm most often indicates that micro/macro bends are present in the fiber optic link.  Testing a fiber optic link with an OTDR will not only detect a micro/macro bend but will also indicate where and how much loss is incurred at that event on the fiber optic link. 

Optical Loss Test Example of Micro and Macrobending

2016-05-May-IC.png

A reference is set at 1310 nm and 1550 nm using two patch cords, a bulkhead adapter, light source, and power meter.



2016-05-May-IC-2.png

The power meter is zeroed to reflect 0 dB of loss between the test cables at both 1310 nm and 1550 nm.



2016-05-May-IC-3.png

The system is tested using both 1310 nm and 1550 nm.  Notice the increased attenuation exhibited at 1550 nm.
This is indicative of a macro/micro bend in a fiber optic link.

OTDR Example of Micro and Macrobending

2016-05-May-IC-4.png

Fiber span measured at 1310 nm
Loss values for event #4 at 1310 nm = 0.079 dB


2016-05-May-IC-5.png

Fiber span measured at 1550 nm
Loss values for event #4 at 1550 nm = 1.677 dB


The span is tested at both 1310 nm and 1550. The increased attenuation caused by the micro/macro bending losses at 1550 nm is clearly visible on the traces.

In conclusion, testing a fiber optic link at multiple wavelengths with either an optical loss test set or and OTDR is vital to correctly determining the effects of microbending/macrobending on today’s fiber optic links. This gives the technician a clear understanding of where (OTDR) and how much loss (OLTS and OTDR) is generated by these events.