Fiber optical channel monitoring – Grass Valley Python 3G User Manual
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Python 3G Operation
Fiber Optical Channel Monitoring
Fiber Optical Channel Monitoring
The Python 3G provides direct digital readout of the Fiber Optic Link signal strength for
signals received at the unit. This readout is presented in units of dBm. It is useful to
understand both the dB or decibel and the dBm or decibel referenced to one milliwatt.
The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of a
physical quantity (usually power or intensity) relative to a specified or implied reference
level. Since it expresses a ratio of two quantities with the same unit, it is a dimensionless,
relative unit. A decibel is one tenth of a bel, a seldom-used unit. Typically dB has been
employed in Audio Measurement and Fiber Optics among many uses.
Proper measurement of signal strength requires an absolute measurement and the dBm
provides this measurement. Since it is referenced to the milliwatt, it is an absolute unit,
used when measuring absolute power. By comparison, the decibel (dB) is used for
quantifying the ratio between two values, such as signal-to-noise ratio
The Python 3G operates within a defined range of Fiber Optic Link signal strength. The
minimum recommended signal strength is -20 dBm or better. Typically the system should
operate at levels between -8 dBm and -20 dBm. The standard laser output strength is -6
dBm. Cable length affects signal strength as does the number of connections between the
two Python 3G units. Any use of repeaters of cable bulkhead connector will produce a
minimal signal loss through each connection.
The optical output from each transmitter is generated by an infrared laser diode that is
coupled to a CWDM and onto the fiber. User connections on the Python 3G are made at the
bulkhead ST type connectors on the front and rear panels. Operation is intended for use on
single mode fiber. Since the CWDM output of the Python is the aggregate output of all 8 or
16 optical transmitters inside the frame, the total optical power output on a single fiber
optic cable will be in the +4 to +8 dBm range. Standard practice of NEVER looking directly
into a fiber should be followed at all times.
The maximum fiber distance is defined by the optical loss margin. The RX signal must be -
20 dBm or better. Losses on single mode fiber are approximately 0.5 dB/km or less. CWDM’s
account for about 5dBm of loss per pair and must be considered when computing your link
loss budget.
The integrated optical power meter will show the received optical power for each receiver
but note that this figure is post CWDM.
You should read the Using Fiber Optics Guide for information on how to manage and
deploy your fiber optics cabling, safety precautions, tips & tricks, and recommendations for
creating complex fiber optic networks. You can find a copy of this document on the
Support portal (see