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DOCSIS Return | Dirty Fiber Optic Connectors

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Dirty Fiber

An often overlooked, but easy to resolve RF impairment are loose and/or dirty fiber optic connectors.  This issue often manifests itself as an increased noise floor when viewed on a spectrum analyzer.  The key to determining the difference between regular ingress and fiber connector noise is that the noise created by a loose or dirty fiber connector does not stop at the diplex filter roll-off frequency (typically 42 MHz in North America and 65 MHz Europe).  The image below shows a perfect example of this type of impairment:

Best practices recommend that you should always look at the return path spectrum up from 500 kHz to 200 MHz.  The image above is proof again of why this is a best practice.  Analog return path transmitters can transmit signals up to 200 MHz even.  It is important to keep this in mind, especially since we often have the mindset that the return path is limited to 42 MHz (65 MHz Europe) due to the low pass filter in a diplex filter.  We can take advantage of extra bandwidth of the return path transmitter to learn more about return path impairments, such as the one being displayed.

Identifying the Connector

Now the challenge is identifying the loose or dirty connector.  The headend is a good place to start since you have many fiber connections, either between fiber racks or equipment-to-equipment patching such as between the return path receiver and the fiber racks.  If you don’t have good headend documentation, you will need to identify Fiber. Fibers can easily and safely be identified either by proper labeling of the cables and connections or by using a Visual Fault Locator that uses a safe visible red laser that can go as far as 3Km. The laser will make the connector end easily visible to the naked eye with out looking directly at the fiber with a scope. It will also allow you to view light escaping the fiber at any point where the fiber may be compromised.  Once you have located the fiber that is associated with the noisy upstream, apply pressure and/or gently wiggle each connector while watching the return spectrum.  If you see the noise floor move up and down you likely have the culprit.

Inspecting and Cleaning the Connector

If the fiber is not loose, meaning snapping it into the connector does fix the noise floor, then the connector likely needs to be cleaned or repaired.  It’s also good practice to clean connectors if there is any question that they may be dirty.  Dirty fiber connections are the most common cause of light loss and signal degradation in the optical portion of the HFC plant.  This can be done with the right combination of cleaning products and tools. Cleaning supplies range from swabs (for cleaning fibers loaded in bulkheads or equipment receptacles) to connector cleaning pads, chemicals and tools.  By far the best cleaning device is the little green box with the auto-advancing cleaning tape.  This makes for a quick and effective cleaning job which can be done in just a few seconds – fast enough to not lose any cable modems in the process.

To do the job right, a visual inspection of the connector should be performed.  This will ensure that the connector is dust free and also verify that it is free of scratches, which no amount of cleaning will fix.  For visual inspection, make sure to verify the fiber is dead (that is no optical light).  Failure to identify if a fiber is live or dead can result in significant eye damage! The presence of light or laser light can easily be detected using a visible/ invisible light sensor. This will quickly and easily remove the liability associated with laser light eye damage.  Next, clean the fiber to remove any dirt.  Finally, visually inspect the fiber using a Fiber Optic Scope.  Scopes come in a variety of configurations and amounts of magnification. Most common are 200x and 400x magnification direct viewing or video interface. If you are simply trying to verify if the fiber is dirty or has a contaminant obstruction, a 200x scope is sufficient. If the need is for inspection of a fiber after or during termination, a 400x scope would be recommended. If you need to visually inspect a fiber that cannot be verified as live or dead, then a scope with a video interface should be used so that there is no direct eye exposure to the light on the fiber.

One More Thing

So it is likely that you will find some dirty or loose fibers in your headend.  Then you’ll see some noise floor improvement by cleaning or snapping them in place.  Keep in mind that there is at least one more fiber connection out in the field in fiber node you may need to check.  Also, noise floors can be elevated due to too much optical power hitting the return path receiver.  The signature will be similar to the dirty fiber one in that the noise floor is elevated past the diplex and extends to 200 MHz.  However the big difference with too much optical power is that the noise floor usually has a hump or increases and then drops back down.  This depends upon the intensity of the optical power.  An easy test is to take the fiber going into the return path receiver and wrap a couple turns of it around a pencil as you watch the spectrum analyzer.  If the noise floor above the diplex filter drops, then you have too much optical power.  You will need to disconnect this fiber and insert a fiber optic attenuator.

Mr. Volpe has over 25 years of communications industry experience. He is focused on the cable and telecom industry with deep technical and business skills. Mr. Volpe is currently the president and chief technologist of the Volpe Firm and holds an MSEE with honors.

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