The working thory of Optical Transmitter/ Receiver
The most important optical components in opticaltransmitter is semiconductor laser, in fact, it is a laser diode (Laser Diode,LD), of course, do not use the laser diode, but the use of semiconductor lightemitting diode (Light Emitting Diode, LED).
1310nm optical transmitters generally usedirect modulation (VSB - Amplitude modulation, VSB-AM), its function is toconvert electrical signals into optical signals, the injected laser power toachieve it through an external circuit. It is set to the bias circuit toprovide the best bias power supply for the laser bias current, laser poweroutput automatic control circuit, to ensure the stability of the output opticalpower, should be designed optical power and laser temperature, such asmicrocomputer automatic control of the optical transmitter working state.
Lasers are widely used as the opticaloscillator (mamely, light-emitting devices), it is to rely on the energy stateof the laser medium material itself into the interaction with light.
To make the laser work, there must be acertain size of the current, have a certain relationship between the currentsize and light intensity, light intensity of a sharp increase when increasingthe current, this already can make the laser work, If continue to increase thethreshold current, it will form the output of the saturated zone. Thesaturation region current reaches a certain value, it will make the decline inthe quality of the transmission signal or even damage the laser diode. n termsof the optical fiber transmission power required, in the linear region severalMW of output power to meet the requirements of long-distance transmission ofsignals and information. The quality of light transmission In addition to thelight intensity the amount of spectrum and noise. Multi-wavelength spectrum ofhigh-quality analog signal transmission is not suitable, even with the thesinglemode way to work, its luminous lines width, the narrower the width of thelight waves become more pure, the more temporal coherence,good coherence oflight waves. The good coherence of light waves without using lenses and otherdevices will converge to a small point of light, but also more suitable forfiber optic incident.
The optical receiver’s main component isoptical detector which is of high sensitivity photodiode (PIN), photodiodephotoelectric effect of semiconductors to complete the detection of the opticalsignal, the optical signal to restore the RF television signal, then the RFsignalqualified RF signal amplification, and the AGC level control output afterprocessing for network distribution.
The main technologies are C/N\ C/CTB\ C/CSO. Thesethree indicators are all determined by the performance of the photoelectricconversion module. In the case of the same optical power input, convert theoutput RF level there are various sizes, high conversion efficiency ofphotovoltaic modules, its output level is high, the indicators of the C / Nvalue, and vice versa.C / N value indicators deteriorated. While the C / CSO,C, / CTB linearity of the two technical indicators by the photovoltaic modules,high-quality photovoltaic modules in the same C / CSO, C / CTB indicators, thereceived power allows a wider range.