Insertion Loss Method

Step-1, Set reference

The purpose of reference measurement is to cancel out, as much as possible, the losses caused by the various patch cables.

Contamination of the optical connector end face can cause significant change in optical power. It is important to inspect the end face of both connectors to verify they are clean and undamaged before mating.


Step-2, Measure the attenuation along a fibre link

It is important to note that significant variations can occur in attenuation measurements if precautions are not taken with the injection conditions.

Insertion Loss Test from PCD to Multiport

Step-1, Set reference

using two SCA-SCA patchcords and one through connector


Step-2, Measure the attenuation along a fibre link





Coupled Power Ratio

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

What often gets ignored when measuring CPR is at 850 nm, the SM fiber must be a 850 SM fiber with a core diameter of ~5 ┬Ám which is not a common fiber - not a regular 1300 nm SM fiber.

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

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

In use a overfilled (Category 1) source with a mandrel wrap was specified for testing (see below). CPR was used for almost 20 years until it was realized that it was subject to large errors in fibers which had central dips in the index profile, a common fault in poorly made fibers. 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 fiber, leading to the defining of encircled flux.

Encircled Flux

Recently, a more precise method of defining mode fill has been adopted. Encircled flux (EF), defines the integral of power output of the fiber over the radius of the fiber. When you look look at the graph below, consider that the vertical axis is the

total amount of optical power from the source coupled into a fiber core inside the radius shown in the horizontal axis. EF was defined during the development of 10 GB Ethernet as a way to define the light output from an ideal VCSEL source which concentrates more of its power in the center of the fiber than a LED. The EF definition was used for bandwidth simulation only at that point. Of course a real VCSEL may be (is likely to be) different, but this model allowed calculating the bandwidth of this ideal VCSEL in various types of fibers of various lengths to determine their capability of supporting 10 GBE. It was later decided that EF would be a better way to define mode fill for loss testing.

This method of measuring mode fill should be more precise than other methods like CPR. EF should be easier to measure using imaging devices that can be calibrated.

EF is a more sensitive way of defining power and it 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.

EF has become part of several new MM testing standards. It is intended to create a more reproducible modal condition for testing that is similar to the CPR/mandrel wrap method described below. However, data shows a close correlation between EF and the results of a mandrel wrap conditioner.

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. Most American fibers are "self-stripping"; the buffer is chosen to have an index of refraction that will promote the leakage of light from the cladding to the buffer. 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.

To make a cladding mode stripper, strip off the fiber's buffer for 2 to 3 inches (50 to 75 mm) and immerse the fiber in a substance of equal or higher index of refraction than the cladding. This can be done by immersing the fiber in alcohol or mineral oil in a beaker, or by threading the fiber through a common soda straw and filling the straw with index matching epoxy or an optical gel (Note: stripping the buffer away from the end of a fiber is easily done, using a chemical stripper. If the fiber cannot be chemically stripped, like those with Teflon buffers, check with the fiber manufacturer for instructions.) A caution. Do not stress the fiber after the mode stripper, as this will reintroduce cladding modes, negating the effects of the mode stripper. Mode stripping should be done last if mode scrambling and filtering are also done on a fiber under test.

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.