Overview of cable TV Optical transceiver

- Jun 08, 2017 -

In the CATV system HFC (fiber coaxial hybrid structure), the optical transmitter of the direct-modulated DFB laser, because of the direct modulation of the laser, changes in the intensity of the light at the same time, which produces the additional frequency modulation, this phenomenon is also called "chirp" (chirp), which will further broaden the modulation edge band. When the spectrum signal produced by modulation is transmitted in the optical fiber, the phase is shifted by dispersion and the analysis shows that the receiving end is composed of 2 times distortion (CSO). So the direct modulated optical transmitter can only work at 1310nm wavelengths with very little fiber dispersion. The optical transmitter of direct-modulated DFB laser is simple in structure, low in cost, small in effect of SBS, and 3-order distortion is a widely used optical transmitter. The disadvantage is the phenomenon of chirp, 2-order distortion is larger. The 1550nm optical transmitter must be used in the way of outward. The output power of 1550nm optical transmitter is only about 4MW, although its power is small but because of the erbium-doped fiber amplifier (EDFA), the weak optical signal can be amplified to 200mW above, and the distortion effect is very small, so let a 1550nm transmitter with a EDFA work to get a long distance transmission potential of the light source, with EDFA can also be directly amplified in the line light signal without the light-electricity-light conversion, greatly lengthened the transmission distance. Wavelength accuracy of optical transmitters: the wavelength accuracy of optical transmitters provided by different manufacturers is usually from 15NMM. Due to the change of wavelength value, the output light power of EDFA and noise coefficient have certain influence, so when choosing optical transmitter, we should choose light transmitter with small wavelength tolerance.

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