There are there factors when we calculate the optical transmission distance.
They are the optical power of the transmitter; the receive sensitivity of receiver; and the light attenuation coefficient of wavelength.
The formula is: Transmission distance=| Receiving sensitivity- TX power | / Light attenuation coefficient.
For example, if the TX power of optical module = -5dB, Receiving sensitivity = -30dB, the light wavelength is 1310nm and the related light attenuation coefficient is from 0.4db~0.45db. Generally, according to the maximum attenuation value, it is 0.45dB /km.
Because the attenuation coefficient is generally larger, so the final result can be rounded. It is 56km.