Hi Gurleen,
I don’t know if I have actually got your question well, but 0 dBm power cannot be mentioned as zero power because it is not. If you mean to say that for 0 dBm the system in the paper has fwm products and your implementation of the same system does not, then that is the issue. I see that 0 dBm has been mentioned for the INPUT power in the paper, so you have applied a power but due to the small amount of it (or any other reason) it has not produced the result.
Regards