How do we impose Hermitian Symmetry in optical OFDM?
I don't understand your question about the IFFT step: as in &quot;normal&quot; OFDM, you send a full vector to the IFFT, as that is a vector-valued operation of vectors. The output are (interpreted as) time-domain samples.
In fact, you really shouldn't have put Step 3 and 4 as separate operations, and certainly not in that order: the input to the IFFT has to be hermitian symmetric to yield a real-valued signal (which is what you need if you don't want to implement a quadrature upconverter to mix your signal into a carrier frequency, so in all honesty, fig. 3 is misleading in that the $-\sin(2\pi f_c t)$ branch doesn't have anything to do if you ensure your IFFT input is hermitian symmetric.)

My confusion here is if we have to &quot;impose&quot; that certain carriers must take a given value (e.g. carrier 0 and carrier N/2 must equal 0) then isn't that somehow &quot;overwriting&quot; the value for that carrier that we have applied at the QAM modulation stage?

No, you're just mixing up what you're doing: you're not free to assign an arbitrary value to each subcarrier at will, you need to make sure the vector is hermitian symmetric. You hence only have half the subcarriers to choose freely, the other one is inherently set to the complex conjugate.
So, you're not &quot;overwriting&quot; anything. You're just only assigning your QAM symbols to half the subcarriers.