Comcast LT1550-E, Instruction Manual

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LT1550-E
www.comcast-sa.com
Stimulated Brillouin Scattering (SBS) occurs, when the optical input power is
greater than a certain threshold value. SBS generate the lower frequency
backscattered light which will attenuate the transmission light and return to
the laser while destroying its performance. Causing optical power fluctuation,
generates large noise, and seriously deteriorates the system carrier to noise
ratio (CNR). To improve the SBS threshold, LT1550-E series optical transmitter
adopts SBS control technology which is independent researched and
developed by ourselves. The threshold value can be set up to 19dBm.
The optical modulator has a two-way optical signal output. Parts of that signal
are routed to an InGaAs photodiode. This detection of the optical signal has
two functions:
1) Detect whether the laser is normal working. Once the output optical power
is 2dB lower than standard power, alarm will be set off.
2) Detect CSO distortion to optimize the bias point of the optical modulator.
For working normal the detector circuit needs at least two carrier signal inputs
with an interval of 24MHz. There is a CSO initialization program in the boot
process. If the CSO install failed, the RF indicator will flash red, see details in
6.2 Troubleshooting.
Block Diagram