What is standard channel spacing for DWDM systems?*120 channels, 25 GHz60 channels, 200 GHz40 channels, 100 GHz30 channels, 150 GHz

The standard channel spacing for Dense Wavelength Division Multiplexing (DWDM) systems varies depending upon the system design and application. However, generally speaking:

1. For 120 channels, a channel spacing of 25 GHz is typically used. This tight spacing allows for a dense packing of channels, maximizing the use of available optical bandwidth.

2. For 60 channels, the common channel spacing is 200 GHz. This wider spacing reduces the potential for channel interference, making it suitable for longer distances and higher power signals.

3. For 40 channels, a 100 GHz spacing is commonly employed. This strikes a balance between density and performance, providing adequate separation while allowing for a manageable number of channels.

4. For 30 channels, a spacing of 150 GHz can be used. Like the 100 GHz spacing, it facilitates a decent level of distinction between channels while allowing a fair number of them to operate simultaneously.

In conclusion, among the options you provided, channel spacing varies based on the number of channels: 25 GHz for 120 channels, 200 GHz for 60 channels, 100 GHz for 40 channels, and 150 GHz for 30 channels. Each choice reflects trade-offs between channel count and performance, with tighter spacing enabling more channels but potentially increasing crosstalk and non-linear effects in the fiber.