The level of crosstalk depends on the power per optical carrier launched into the fiber, subcarrier modulation frequency, optical carrier spacing and the number of optical carriers. Thus, you may always control crosstalk interference by launching appropriate optical carrier powers. For this application, a fortuitous property of crosstalk among DWDM optical channels is that the interference decreases with increasing frequency. The forward path frequency range experiences less crosstalk interference than is experienced at 5-42 MHz. Thus, by shifting the return band higher in frequency as is done in a block-converted system, more optical power may be launched into fiber for the same crosstalk level.
Crosstalk degrades as the number of optical carriers increases.
The relationships among crosstalk, optical carrier spacing, number of optical carriers and modulating frequency are more complex. Another design tradeoff is the choice of 1,550 nm DWDM optical transmitter technology. Because of laser chirp combined with the high chromatic dispersion of standard single-mode fiber at 1,550 nm, the directly modulated optical transmitter s useful frequency range is limited to an octave because of the generation of second order distortion. Therefore, it is limited to carrying nine 5-42 MHz returns. Application examples Crosstalk and other system parameters, as discussed earlier, limit the power per optical carrier launched into the fiber.