In Optical OFDM system such as DD-OFDM, does the higher frequency (as we move farther from the carrier, ex. (N)) is delayed after the fiber by Td= DL(delta lambda)? Or it will move faster and have a delay difference Td with the other OFDM subcarriers?

You can see the figure below and tell me which case is right (a) or (b)?

Best regards