Why does a channel need to have bandwidth requirement for digital data transmission?
so how many bits we can transmit is really determined by the torch, not the bandwidth of the channel(fiber optics)


The "channel" includes everything: sender, receiver, and medium. Realistically light communications are limited by how fast you can turn the light on and off (known as "rise time" and "fall time"), and how fast the sensor can respond to that.

Diffraction may become important as well; the front of the wavefront can interfere with itself, so what starts out as a clean sharp rise at the transmitter may smear out along the way.