here’s more explanation on this at your request: You have asked how to generate a passband signal using laser diode, which includes a spectrum of frequencies (or wavelengths) and not just one frequency (which is the laser’s frequency). Here’s my answer: What laser actually does is upconverting the baseband signal that you have generated before. Now if that baseband signal has a bandwidth of 20nm, after it is upconverted by laser it still retains its bandwidth. I hope I have been more clear this time.