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The main idea of OFDM is to eliminate inter-symbol interference, when the delayed symbols come from non-line-of-sight directions and alias to the next channel symbol. The second aim is to made a communication more reliable in case of deep fading on some frequencies within channel.
If these two effect occur in fiber optic channel then the implementation of OFDM seems to be reasonable (but I dont think so).
QPSK is easier to implement in fiber optical communications, while OFDM increases the channel capacity(there are studies showing transmission of 100Gbps) and, as Mityan said, eliminates the inter-symbol interference => decreases BER, but is much more complex for realization. So if you are looking for better channel capacity definitely OFDM, the fundamental problems of OFDM in optical communications are large sensitivity to phase and frequency noise and large peak-to-average power ratio.
A detailed explanation of the use of OFDM for optical communications is given in "OFDM for optical communications" by William Sheih and Ivan Djordjevic.