NETWORK DESIGN. Generally, 1550nm external modulation optical transmitter output port will be contact to EDFA.

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1 NETWORK DESIGN 1550nm system can use to extend transmission distance, when signal is operated in 1550nm, the fiber should be considered of some factors, such as Chromatic Dispersion, SBS, SPM ( Self Phase Modulation ) and other non-linearity effect of optic fiber. The network we designed for you has supplied some simple rules of using the transmitter. SBS: It s a physical phenomena occurred after incident optical power over some threshold. That s namely when the incident optical turned to backscatter Stokes optical in optical fiber that made forward transmission signal optical attenuated by non-linear and the backscatter will be return to optical transmitter and cause input optical power wave also appear noise. That s result in optic waveguide and acoustic waveguide operation process and the SBS is get up in the 1550nm fiber optic long distance transmission. Generally, 1550nm external modulation optical transmitter output port will be contact to. The input optical power of optical amplifier should be 3 dbm. When the value of input optical power is 3dBm then its CNR will be deteriorate about 2dBm; When the value of input optical power is 6~7 dbm then its CNR will be deteriorate about 1dBm. If there are many city network branch headend it should be high signal strength and it may caused CNR, CSO and CTB index deteriorated. Usually, the distance of city network community antenna transmission center and branch headend should be within 20km. If the SBS isn t get up, it won t be deterioration. The practice shows that 20dBm incident optical power can transmit to 15km distance and 25dBm incident light power can transmit to 5km distance then distributing. You d better adopt low power as incident optical fiber. So, please, note the relation of saturation output power, SBS index and transmission distance when select or use optical amplifier.

2 The choice of SBS critical value * The longest optic fiber distance in system transmission If you want to transmit more than 100km, you need to select SBS restrain status. Why you need to do this is because of SPM & optic fiber Chromatic Dispersion. If bandwidth-extending status is not changed, when fiber length is longer, CSO performance will be worse. If you want to choose longer distance transmission, you need to select smaller line bandwidth. is much smaller, is much bigger, and 18dBm is the biggest. The following table shows the recommended fiber length in which SBS restraint status. Optic Fiber Length <60Km <70Km <120Km >120Km SBS Restrain Condition,, 18dBm, (CSO performance will be some different from the specification possibly) ** The biggest optic fiber Input power is restricted by SBS restraint status. If your power is bigger than the stipulated limit, it will occur some bad result. The low frequency band will have 1/f noise, low channel s CNR will not be good, HUM will also be worse than in normal status. The attached table shows the limit value of SBS restraint power. SBS restraint power 18dBm Limit of optic fiber input power Max.18.4dBm Max.16.4dBm Max.13.4dBm The general CATV operator will use high power (such as 22dBm and higher one), and use optic splitter to arrange the power transmitting in the optic fiber & cable. It is no problem to do this, because the optic fiber patch cord is short. But it is necessary to notice that SBS restraint setup rest with the worse situation, for example, the longest distance & the biggest fiber input power in the optic cable.

3 The correct link for transmitter & SBS restraint Receive SBS restraint 19dBm 40Km 60Km SBS restraint Incorrect link for transmitter & LAUNCH POWER ABOVE SBS LIMIT Transmitter rated at 100 Km, SBS restraint LAUNCH POWER ABOVE SBS LIMIT SBS restraint 22dBm 19dBm 19dBm FIBER DISTANCE ABOVE MAXIMUM TRANSMITTER RATING 40Km 60Km SBS restraint

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