Unitrek - optic fibre mode condition

Coupled Power Ratio

Coupled power ratio is the difference in dB of the power coupled from a fibre under test to both a similar MM fibre and a SM fibre. The rationale is the measurement is the difference between the total power in the fibre and the power in the central modes, so a fully filled fibre will have a greater dB difference in CPR.

CPR was divided into classes. The rated category values in dB for both 850nm and 1300nm into a 62.5/125 multimode fibre, are as follows:

In use a overfilled (Category 1) source with a mandrel wrap was specified for testing. CPR was used for almost 20 years until it was realised that it was subject to large errors in fibres which had central dips in the index profile, a common fault in poorly made fibres. It was determined that a better metric would be a profile created by an integral of the light included inside a given radius of the fibre, leading to the defining of encircled flux.

850nm:

Category 1 (overfilled): 25 ~ 29 dB
Category 2: 21 ~ 24.9 dB
Category 3: 14 ~ 20.9 dB
Category 4 (similar to typical VCSELs): 7 ~ 13.9 dB
Category 5 (very under filled): 0 ~ 6.9 dB

1300nm:

Category 1 (overfilled): 21 ~ 25 dB
Category 2: 17 ~ 20.9 dB
Category 3: 12 ~ 16.9 dB
Category 4: 7 ~ 11.9 dB
Category 5 (very under filled): 0 ~ 6.9 dB

Encircled Flux

Encircled flux (EF), defines the integral of power output of the fiber over the radius of the fiber. The EF is a better way to define mode fill for loss testing.

The EF can be measured using imaging techniques. The vertical (Y) scale shows the total power from the core of the fiber up to a point on the radius (in microns), so when one gets to 25 microns, one measures all the power. The shape of the curve is chosen to emulate an idealized source that is between underfill and overfill conditions.

Cladding Mode Strippers

Cladding mode strippers are used to remove any light being propagated in the cladding to insure that measurements include only the effects of the core. If you are using at least 1 meter of fiber, cladding modes will probably not be a factor in measurements. One can easily tell if cladding modes are a factor. Start with 10 meters of fiber coupled to a source and measure the power transmitted through it. Cut back to 5 meters and then 4, 3, 2, and 1 meter, measuring the power at every cutback. The loss in the fiber core is very small in 10 meters, about 0.03 - 0.06 dB. But if the power measured increases rapidly, the additional light measured is cladding light, which has a very high attenuation, and a cladding mode stripper is recommended for accurate measurements if short lengths of fiber must be used.

Mode Scramblers

Mode scrambling is an attempt to equalize the power in all modes, simulating a fully filled launch. This should not be confused with a mode filter which simulates the modal distribution of a fiber in equilibrium modal distribution (EMD). Both may be used together sometimes however, to properly simulate test conditions. Mode scramblers are easily made by fusion (or mechanical) splicing a short piece of step index fiber in between two pieces of graded index fiber being tested. Simply attaching a step index fiber to a source as a launch cable before a reference launch cable will also work.