Views:0 Author:Site Editor Publish Time: 2021-09-11 Origin:Site
In the HFC network, the CATV optical transmitter is one of the core equipment of the system. It is used to carry out optical modulation of the radio frequency CATV signal, realize the opto-electric conversion (E/O), and send a continuous, stable and reliable optical signal to the optical cable system. The following is an introduction to the working principle of CATV optical transmitters.
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Directly modulated CATV optical transmitter
Externally modulated CATV optical transmitter
Direct modulated CATV optical transmitters output 1310nm laser wavelength, most of which use DFB laser assembly, with an average life of 120,000 h. Machine with bias circuit, power supply, laser slow start circuit, power control and cooling control circuit, the distortion compensation circuit, overload protection, and a driver protection circuit, light detection circuit, using optical power detection and automatic power control of optical detector (PIN) chips, used for bidirectional automatic temperature control (ATC) of semiconductor refrigerator and thermistors.
The input signal of the optical transmitter is TV radio frequency signal, and the signal level is 75~90dBμV (subject to the specification). TV from front-end multiplex video signal mixer after entering the preamplifier output into the light transmitter input, in order to make the drive laser level stable, automatic electric control attenuation, and predistortion compensation and automatic power level control, and then to drive laser components, electro optical modulation, the signal light signal is converted to light intensity changes. An optical isolator at the output can greatly reduce the influence of the optical reflection wave on the laser. Finally, the optical signal is fed into the cable through the optical movable connector. There is a single chip microprocessor in the CATV optical transmitter, and all kinds of switches on the front panel are controlled by the microprocessor, in which the best working state data of the laser is stored, the laser can be slowly started, and the RF drive current can be automatically disconnected to protect the laser.
Externally modulated 1550nm CATV optical transmitter is mainly composed of semiconductor laser diode pump source, neodymium-doped yttrium aluminum garnet solid-state laser, optical isolator, external modulator, photodetector, thermoelectric cooler, compensating nonlinear CSO, CTB circuit, power supply, etc. It has two optical output channels (two ports). Compared with directly modulated CATV optical transmitters, externally modulated CATV optical transmitters have higher output power, up to 2×20mW, smaller CSO and larger C/CSO, up to more than 65dB. Therefore, externally modulated CATV optical transmitters are often used for long-distance transmission or large wired systems. External Modulation CATV optical transmitter generally has two light outputs, and the RF modulation of the two light channels is the opposite of each other. The receiving end adopts the supporting optical receiver of CATV, this optical receiver has two receiving channels, and the output end uses the power mixer to mix the opposing RF signals into one way, so that the nonlinear distortion cancels each other, and the technical performance is greatly improved
All the working parameters and monitoring functions of the laser in the CATV optical transmitter are under the control of the microcomputer. An LCD display on the front panel displays information related to the working status of the laser. Once the working parameters of the laser deviate from the allowable range set by the software, the power supply of the laser will be automatically switched off. At the same time prompt alarm, LCD display shows the cause of the fault.