Hi Gaganpreet,
You can design a Single mode fiber with zero attenuation. In Optical Fiber Properties, Main tab – select attenuation data type – constant, attenuation – 0 db/km, other factors like dispersion, non linearity doesn’t affect the attenuation. But in input source, even though if you are giving power = 0dbm (i.e 1 milli watts), when modulating the input signal you will get some power value at the output. When the signal travels in the fiber, it won’t get attenuated.
I have attached a picture. You can take a look at it.
The 1st optical power meter = output from source
The 2nd optical power meter = modulated output of the input signal
The 3rd optical power meter = output from Single mode fiber.
Note that modulated output and Optical fiber output are same (attenuation = 0 db/km in optical fiber).