Reference Guide To Fiber Optics
|Topic: Testing Connectors in Fiber Optics||Table of Contents: |
The FOA Reference Guide To Fiber Optics
Connector and Splice Loss Testing
In fiber optics, a single connector has no loss. The "loss of a connector" is defined as a "connection loss" caused by a mated pair of connectors. The lab method used to establish the average loss value of a connector design is shown below. The loss of connectors on a patchcord or short cable is given by FOTP-171 and the loss of an installed cable plant is measured by OFSTP-14 (MM) or OFSTP-7 (SM.)
In order to establish a typical loss for connectors, it is necessary to test all connectors in a standardized fashion. Measurements of connector or splice losses are performed by measuring the transmitted power of a short length of cable and then inserting a connector pair or splice into the fiber and measuring the change of loss as a result of adding a connection. This test ( designated FOTP-34 by the TIA) can be used for both multimode and singlemode fiber, but the results for multimode fiber are very dependent on mode power distribution.
FOTP-34 has three options in modal distribution: 1)EMD (equilibrium modal distribution or steady state) , 2)fully filled, and 3)any other conditions as long as they are specified. Besides mode power distribution factors, the uncertainty of the measured loss is a combination of , inherent fiber geometry variations, installed connector characteristics, and the effects of the splice bushing used to align the two connectors.
This test is repeated hundreds or thousands of times by each
connector manufacturer, to produce data that shows the repeatability
of their connector design, a critical factor in figuring margins
for installations using many connectors. Thus loss is not the
only criteria for a good connector, it must be repeatable, so
its average loss can be used for these margin calculations with
some degree of confidence.
Connector and Splice Durability
Another factor important to a connector is the durability of the design, shown by its ability to withstand many matings without degradation in loss. Testing connector durability is simply a matter of repeated mating and demating of a connector pair while measuring loss. Since the loss is a function of both connectors and alignment sleeve, it is helpful to determine which are the contributors to degradation. Plastic alignment sleeves, when used with ceramic connectors, for example,will usually wear out much faster, shaving plastic off onto the connector ferrules and causing increased loss and return loss. When testing durability, periodic inspection of the connector end faces and ferrules with a microscope to determine wear or contamination is very important.
Splice durability is one of withstanding many cycles of environmental
stress, since splices are often used in splice enclosures in pedestals
or mounted on poles where they are exposed to the extremes of
climatic changes. Manufacturers usually test a number of splices
through many environmental cycles and accelerated aging to determine
their durability. Such tests may take years.
If you have ever looked at a fiber optic connector on an OTDR, you are familiar with the characteristic spike that shows where the connector is. That spike is a measure of the reflectane or optical return loss of the connector, the names used for the amount of light that is reflected back up the fiber by light reflections off the interface of the polished end surface of the connector and air. It is called fresnel reflection and is caused by the light going through the change in index of refraction at the interface between the fiber (n=1.5) and air (n=1).
That return spike is one component of the connector's loss, representing about 0.3 dB loss for a non-contact or air-gap connector (two air/glass interfaces at 4% reflection each), the minimum loss for non-contacting connectors without an index-matching fluid. But in high-bit rate singlemode systems, that reflection can be a major source of bit-error rate problems. In some singlemode systems, the reflected light interferes with the laser diode chip, causes mode-hopping and can be a source of noise. Minimizing the light reflected back into the laser is necessary to get maximum performance out of high bit rate laser systems, especially the AM modulated CATV systems. In multimode systems, reflections can add to background noise in the fiber.
Since this is more a problem with singlemode systems, manufacturers have concentrated on solving the problem for their singlemode components but multimode connectors benefit also. Several schemes have been used to reduce back-reflections, mainly reducing the gap between connectors to a few wavelengths of light using a physical contact (PC) polish on the end of the connector ferrule, which reduces the fresnel reflection. The usual technique involves polishing the end surface of the fiber to a convex surface or at a slight angle to prevent direct back reflections. Another , even more effective solution, is to polish the end of the singlemode connector ferrule at a small angle (about 8 degrees) to cause any reflected light to be absorbed in the fiber cladding. These are called angle-polish connectors (APC) and are widely used for CATV and high big rate digital systems.
Measuring this back reflection per standard test procedure EIA FOTP-107 is straightforward. This test setup can be used with a bare fiber output into which a connector pair is installed (analogous to a FOTP-34 connector insertion loss test) or with a connectorized output for testing preconnectorized jumpers (like FOTP-171).
For the EIA FOTP-107 test procedure, one needs a calibrated coupler which can be used to inject a source into the test cable or pigtail and measure the light reflected back up the fiber, along with a standard power meter and laser source. The coupler split ratio must be calibrated to know how much of the return signal goes to the power meter and how much is diverted to the source side of the coupler to calculate the total amount of back reflection. Due to the dynamic range required to measure return losses in the range of -25 to -60 dB, a high power laser source is generally necessary, and the source must be stable enough to allow making accurate measurements over relatively long times required for the experiment. To measure the connector reflection, the far end of the cable must be terminated by surrounding the fiber in an index-matching gel or epoxy.
To measure return loss, measure the amount of power transmitted to the end of the cable (Pout) and the power reflected back up through the coupler test port (Pback) with a fiber optic power meter. To calibrate out any crosstalk in the coupler or the back reflections of any intermediate connectors or splices, dip the connector end being tested in an index matching fluid (alcohol works well and isn't messy to clean up) and record the power at the coupler test port (Pzero). If the coupler split ratio is Rsplit (the fraction of the light that goes to the measurement port when transmitting in the back direction), the return loss is :
State-of-the-art connectors will have a return loss of about 40-60 dB, or about one-ten thousandth to one millionth of the light is reflected back towards the source. Measurements setups need to be carefully controlled to get valid data. The test connector being used to test other connectors or jumper cables must be kept clean and periodically repolished to insure as perfect a surface finish as possible. Purists will note that measurements of Pout ignore the fresnel reflection from the end of the test cable fiber and perhaps even the window of the detector, which can add a few percent to the errors.
The measurement of optical return loss is not a precise measurement. The coupling ratios are hard to calculate, reflections in the coupler and connectors are hard to zero out, any dirt or wear on the test connector will affect measurements and the dynamic range of the measurements are so large that uncertainties of up to +/-1 dB are common.
Like all fiber optic power measurements, instrument makers have been guilty of providing too much instrument resolution than that warranted by the uncertainty of the measurement. Manufacturers also offer dedicated instruments to measure ORL, but the measurement can be made easily with a standard meter and laser source.
While the techniques mentioned above refer to testing connector or splice return loss using sources and power meters, the techniques also refer to testing connectors on jumper cables. However, they do not refer to tests on connectors or splices which are installed in a link. Once they are installed, testing should be done with an OTDR.The spike seen when viewing a connector on an OTDR can be measured with respect to the backscatter signal on the OTDR. Most OTDRs are calibrated to make this measurement directly. The coupler/laser source/meter technique (sometimes referred to as "optical continuous wave reflectometry" or OCWR) cannot be used once the components are part of a cable plant, since the continuous backscatter from the lengths of fiber masks the effects of any one component. Besides, if you have a problem, you need to know where it is, and only the OTDR can give that information.