Model 922-MES
– User Manual
Focal Technologies Corporation
Page 7-2
A Moog Inc. Company
Document No. 922-0662-00, Rev 1.0
7.3 Optical Budget, Range, and Bit Error Rate (BER)
A variety of approved and tested SFP transceivers are available from Focal Technologies for maximum
distances from 10 km to 200 km. The optical power budget is dependent upon the SFPs selected, but typically
varies from 16 dB to 45 dB with an optical bit error rate (BER) of 1 x 10
-12
. Contact the factory for the current list
of available, API qualified transceivers.
Optical receivers will experience errors if the received optical power is too low (i.e. below the receive sensitivity
of the photo detector). Ensure the total optical losses of the components in the external cable system (jumpers,
cables, connectors, couplers, etc.) are less than the specified optical power budget of the SFP combination
used. For detailed measurements or trouble-shooting, a calibrated optical power meter is recommended.
Optical receivers can also saturate and experience errors if the received optical power is too high. This is more
likely to occur (especially in a lab environment) when using high power transceivers. Use a 5 or 10 dB fixed
attenuator in line with each fiber during bench tests or with short, low loss links to ensure a minimum level of
attenuation is present. A variable optical attenuator (VOAT) can be used for simulating system losses or testing
the maximum insertion loss supported. In some high power systems, receivers can actually be damaged by
excessive optical power, so a fixed attenuator is recommended even when using a VOAT to ensure a minimum
attenuation at all times.
Adding attenuators for bench testing simulates only the insertion loss of a field system. Dispersion effects are
not simulated with attenuators and should be verified with spools of test fiber matching the length of the system
being designed. Dispersion effects con incur a power penalty of 1-2 dB. Please contact the factory for assistance
with optical system analysis and recommendations.
7.4 Return Loss Tolerance
The optical system external to the card should have a minimum of 20 dB return loss. Excessive optical reflections
may degrade optical budget and/or reduce transceiver lifetime.
For systems with bidirectional optical transmission on a single fiber, sufficient reflected optical power on one
wavelength can interfere with received power on another wavelength and cause data errors. Return loss
specifications for systems with WDM or CWDM couplers must be determined based on a more detailed analysis
of the maximum link attenuation, optical levels, and filter characteristics, but typically 25-30 dB return loss is
sufficient to ensure reflected optical power does not cause link errors.
7.5 Optical Power
All optical power levels are within Class I limits (eye-safe), however as general safe practice, fiber connectors
should not be held close to the eye for inspection. Note that all of the SFP optical wavelengths used for Model
922 links are infrared and not visible, even at high power levels.
Active optical outputs should NEVER be viewed with a magnifying
device of any kind (e.g. fiber microscope).
Always turn off all transmitters before inspecting fibers, optical
connectors, or optical bushings.
