Tip of the Week:
If your fiber network uses optical amplifiers then you will likely need to measure optical signal to noise ratio (OSNR).
OSNR is a ratio of the signal optical power to noise power in a fiber and is typically measured in unit dB. It is specified for fiber optic transceivers as a minimum value for a given bit error rate (typically 10-12). Typical minimum transceiver OSNR is 24 dB for OC192/10GigE transceivers and 18 dB for OC48 transceiver (refer to individual transceiver specs for correct value).
Optical amplifiers introduce noise into the fiber link and if the link design is not proper, this noise can overwhelm the signal and cause bit errors even if the optical signal power is well within specification. If your network does not contain optical amplifiers, then the fiber signal noise level is generally very low and OSNR does not need to be considered (except for some cases where WDM, DWDM and CWDM units are deployed).
OSNR is commonly measured with an optical spectrum analyzer (OSA), which by the way is very expensive. The OSA is also used to accurately measure signal power when noise is present. Measuring signal power with a standard power meter will provide an inaccurate reading since the meter will show both the combined signal and noise power.
A simple test that can be performed to illustrate amplifier noise, is to connect an optical power meter to an optical amplifier (EDFA) output port when it’s input port is not connected (note some optical amplifiers will automatically shut down if there is no input power). The power meter will read a noise power level even with no input into the amplifier. This is the normal noise generated by the amplifier and does not mean that the amplifier is defective.
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- “Planning Fiber Optic Networks”, ISBN 0-07-1499199 available from Amazon.com . Table of contents can be seen here : http://www.TelecomEngineering.com/products/resources/planningfibernetworks.htm ,
- -“Fiber Optic Installer’s Field Manualâ€, ISBN 0-07-135604-5 available from Amazon.com. Table of contents can be seen here http://www.TelecomEngineering.com/products/resources/field-manual.htm .
