I'm currently trying to build a circuit to transmit audio via laser. It works great and the quality is decent. But I don't get the reason for the diode. If I remove the diode and replace it with a normal wire the music is no longer properly transmitted. I know what a diode does but not how it is used in this special use case. Could anyone try to explain? The "Cont Freq" block is meant to show the music output from a headphone jack of a mobile. The two power sources both supply 5 V each, R4 = 660 Ohm and R2 = 220 Ohm.
It helps if you redraw it, though you are missing some grounds.
Note that diode D2 and the diode that is the Base-Emitter junction if Q1 are in parallel. This basically forms a kind of current mirror. Presumably R4 is set to bias them both appropriately.
Any current inserted from the signal source will then be converted into current through the laser diode.
If you replaced the diode with a wire it would short out the base of Q1 and it won't work.
Still, I'm surprised it works as well as you say it does.