Fiber Optic Fundamentals - PAGE 2.

Now that the CATV RF signals have been converted from coaxial cable to infrared laser light for transport via single-mode fiber, we turn our attention to the fiber path.   If you have a fiber optic guy who has worked with digital fiber networks for years putting in your fiber, you will probably have trouble with your fiber network from DAY ONE!  His fiber world has flat-finished BLUE or BLACK fiber connectors in it that are a source of optical reflections and they will destroy the signal quality of analog RF signals, but are fairly transparent to digital signals.  These BLUE connectors are SC-UPC ultra-polished connectors.  The mating surfaces are polished at a 90-degree angle to the fiber path, causing reflections of the laser light.

Consider the digital laser transmitters for a minute:  Light on, Light off - represents Ones and Zeroes for binary data transmissions.  This is 100% modulation of the laser light beam.  These types of systems may have a "small form-factor pluggable" (SFP) digital fiber transceiver module or "gigabit interface converter" (GBIC) transceiver with optical output power from 0 dBm to +5 dBm feeding digital fiber transceivers at the remote end of the fiber cable with a sensitivity down to -23 to -25 dBm.  With that much dynamic range in the optical loss budget, the optical reflections don't amount to much when it comes to telling the difference between "LIGHT ON or LIGHT OFF" at the digital fiber receiver at the other end.

Now consider the analog RF fiber optic transmitter.  With so many analog RF carriers, the total RF input power of all those channels must be considered and controlled to minimize the unwanted mixing of the separate RF channels (CTB & CSO).  Instead of 100% modulation possible with digital signals, the optical modulation index for analog signals can be in the range of 10% optical modulation index (OMI) or less, depending on how many signals are to be transmitted.  These ANALOG television and radio signals ARE NOT DIGITAL SIGNALS!  There can be combinations of amplitude-modulated, phase-modulated, or frequency-modulated RF signals.  With a large number of channels, 3% OMI is common.  With this limitation on the modulation, any REFLECTIONS that occur in the fiber path can severely degrade the performance at the receiving end.  The reflections cause the light rays to arrive at different times at the optical detector inside the receiver.  Remember, the coherent optical signals are the ones that perform best.

Where do these reflections occur?  If you have any FLAT-FINISHED optical connectors in your system, you will find out.  GREEN angle-polished connectors are REQUIRED in analog RF over fiber networks.

Angle-polished connectors shown above have reflections that are absorbed into the cladding of the fiber, not bounced back and forth as in the FLAT finished connectors shown in the figure seen to the LEFT.

Now imagine what happens when you try to plug a BLUE flat-finished fiber connector up against a GREEN angle-polished connector.   You could damage the ceramic material that surrounds the fiber, but more important is the fact that the fibers at the center of the connector will not physically touch each other as seen in the images above.  The air gap that exists between the two fiber ends will have a high attenuation, possibly 5 to 10 dB of optical light loss.  A 10 dB optical signal POWER loss will cause a 20 dB RF signal VOLTAGE loss at the optical receiver.  Power is a 10 LOG function when comparing two different power levels.  RF signal level meters use a 20 LOG function for comparing two different RF signals.

Angle-polished and flat-finished fiber connectors do not mix!  Make sure you know what kind of connector is on the other side of bulkhead connectors when building the fiber network.

The two Optical Time Domain Reflectometer (OTDR) traces below show the results of changing SC-UPC or other types of FLAT-end fiber optic connectors to SC-APC green angle-polished connectors.  Those spikes in the trace on the LEFT BELOW are the reflections along the fiber path where fiber patch bays have fiber connectors with flat-ends.  The same fiber path is shown on the right with fusion-splices and angle-polished factory-made connectors fusion-spliced to the LONG fibers at all locations.  Please take notice of the lower losses overall, AND the absence of the spikes that were the reflected optical energy degrading analog RF signals in the fiber optic cable.  Replacing the connectors allowed for CATV fiber optic signals to be used on the fiber path, and reduced the optical attenuation on the digital signals on other wavelengths also transmitted on this fiber path.  Lowering the fiber connection losses allowed for the installation of the optical MUX and DEMUX that combined the CATV wavelength at 1310nm to the link with existing multiple CWDM wavelengths carrying digital signals (one's and zeroes).

BLUE SC-PC or any FLAT-FINISHED CONNECTORS	GREEN SC-APC ANGLE-POLISHED CONNECTORS

Field-installable angle-polished connectors are for digital data fiber links ONLY, not for RF fiber links.  An actual OTDR trace is seen below where those connectors were installed on 680 feet of single-mode fiber. The reflections indicated as spikes on the trace destroyed the quality of the CATV signals.  These connectors require a cleaved fiber to be inserted into the connector where it touches the end of short internal fiber. That short fiber is angle-polished at the front face of the connector but has a FLAT end where the installed fiber touches the fiber imbedded in the connector.  It is those blunt fiber faces meeting inside the connector that creates the reflections seen in the OTDR trace below.

Using factory-made GREEN angle-polished fiber pigtail connectors that are FUSION-SPLICED to the long fibers between buildings or IDF closets is the only way to avoid reflections in a fiber optic network transmitting RF signals over single-mode fiber.

  (Updated on October 4, 2011 with OTDR trace of field-installable connector and text above.)