RAYLEIGH BACKSCATTER EFFECTS ON 1550-NM CATV DISTRIBUTION-SYSTEMS EMPLOYING OPTICAL AMPLIFIERS

We analyzed the applicability of externally modulated 1550-nm laser tr ansmitters for trunking and distribution of AM CATV channels using pow er and in-line EDFA's. The distribution of multiple AM CATV channels o ver long fiber spans is degraded by the presence of Rayleigh backscatt er-induced low-frequency interferometric noise. When the laser source is modulated externally, the low-frequency interferometric noise is mi xed and translated around the AM carriers. Furthermore, when isolators are not used with the optical amplifiers, the low end of the broadcas t channels could be severely degraded due to doubly amplified Rayleigh backscatter. Employing narrow-linewidth semiconductor or Nd:YAG laser sources at the transmitter will lower the tail of the low-frequency i nterferometric noise level but wilt increase the translated noise peak level at each AM carrier. Therefore, the standard CNR measurement tec hniques, which assumes the noise spectrum is flat, may not reveal the correct video picture quality seen at the customer premises. In this a nalysis, we compared NCTA RF CNR and baseband video SNR results using CCIR recommended unified weighting filter. We determined that for lase r linewidth less than 1 MHz and with long fiber spans, baseband video SNR as opposed to RF CNR measurements should be used to characterize t he performance of AM-VSB CATV broadcast distribution systems. Finally, an experimental 78-channel AM-VSB CATV distribution system is constru cted employing two EDFA's simulating head-end and hub sites and we com pared RF CNR and baseband video SNR measurements using a 700-kHz linew idth externally modulated 1550-nm DFB transmitter.