1550nm external transmitter | Vcast

Description

LT1550-E series optical transmitter is a 1550nm DFB laser external modulated

transmitter. It is specially developed for the CATV signal that satisfies HFC network, and the long-distance transmission of cable phone and cable data. Working principle

LT1550-E series transmitter has 7 function modules: RF control, DFB laser, optical modulator, SBS control, CSO control, communication/display control and

power supply. Automatic gain control circuit (AGC) or manual gain control circuit (MGC)

amplifies the RF signal. AGC or MGC control makes the optical modulator maintain a

suitable input level. Use the detected RF root-meansquare(RMS)-total power to

calculate the optical modulation index(OMI). In general we recommend using the AGC function, and special users can use the

MGC function to adjust the CNR/CSO/CTB performance indexes. The core of transmitter is the optical modulator. The 1550nm signal input the

optical modulator, make the laser intensity changed follow the external RF signal

voltage, and then generate the AM optical signal.

1550nm external modulated optical transmitter operating manual

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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.