What I've learnt so far is that, if we change a time-domain signal into frequency domain, its bandwidth shows the range of frequency of radio waves that required to generate such signal. However, for example in fiber optic,a 1550nm light is used to be the source and BPSK is used to modulate it, but if there is only a single frequency of light (1550nm) forming a signal, then what is the point of finding the bandwidth?
A single frequency is a pure sinusoid, and thus unmodulated (no BPSK). Once you add any modulation, you no longer have a pure sinusoid. The FT of this distorted sinusoid will have a non-zero bandwidth from the sidebands required to change the sinusoid from just pure to somewhat distorted (same as modulated).
It's not a single frequency. You also have sidebands.