This is the first time I ever used three traces at once for anything useful.
The top trace is the squarewave output right at the function generator. The rounded edges are really there. The function generator doesn't consider it important to make them super-sharp. I use it here because it's an easy way to get an approximate squarewave to modulate the laser beam (the DC offset is handy in this case).
The middle trace is the signal right at the laser diode, which it powers. You can see slight degradation of the signal, but it is not at all bad. The impedance of the laser diode is low, so you get surprisingly little problem with the 50-ohm generator and 50-ohm coax.
The modulated laser beam shines onto a photodiode, which is biased and fed to the input of the scope, producing the bottom trace.
These signals are all about 3 MHz.