A direct-detected orthogonal frequency-division multiplexing (DD-OFDM) optical signal transmitter, comprising: • a first transmit chain operable for selectively performing dispersion pre-compensation digital signal processing; • a second transmit chain operable for selectively performing dispersion pre-compensation digital signal processing; • a first digital-to-analog converter,DAC,; • a second DAC; • a first electrical-to-optical (E/O) converter; and • a second E/O converter, wherein if the dispersion pre-compensation is not performed in the first transmit chain and in the second transmit chain then the first transmit chain outputs first signals to the first DAC and the second transmit chain outputs second signals to the second DAC, the first digital signals and the second digital signals comprising in-phase components but not quadrature phase components, the first DAC sending its outputs to the first E/O converter and the second DAC sending its outputs to the second E/O converter; and wherein if the dispersion pre-compensation is performed in the first transmit chain then the first transmit chain outputs third signals to the first DAC and fourth signals to the second DAC, the third signals comprising in-phase components and the fourth signals comprising quadrature components, the first and second DACs (116, 118) sending their outputs to the first E/O converter. • a first transmit chain operable for selectively performing dispersion pre-compensation digital signal processing; • a second transmit chain operable for selectively performing dispersion pre-compensation digital signal processing; • a first digital-to-analog converter,DAC,; • a second DAC; • a first electrical-to-optical (E/O) converter; and • a second E/O converter, wherein if the dispersion pre-compensation is not performed in the first transmit chain and in the second transmit chain then the first transmit chain outputs first signals to the first DAC and the second transmit chain outputs second signals to the second DAC, the first digital signals and the second digital signals comprising in-phase components but not quadrature phase components, the first DAC sending its outputs to the first E/O converter and the second DAC sending its outputs to the second E/O converter; and wherein if the dispersion pre-compensation is performed in the first transmit chain then the first transmit chain outputs third signals to the first DAC and fourth signals to the second DAC, the third signals comprising in-phase components and the fourth signals comprising quadrature components, the first and second DACs (116, 118) sending their outputs to the first E/O converter. wherein if the dispersion pre-compensation is not performed in the first transmit chain and in the second transmit chain then the first transmit chain outputs first signals to the first DAC and the second transmit chain outputs second signals to the second DAC, the first digital signals and the second digital signals comprising in-phase components but not quadrature phase components, the first DAC sending its outputs to the first E/O converter and the second DAC sending its outputs to the second E/O converter; and wherein if the dispersion pre-compensation is performed in the first transmit chain then the first transmit chain outputs third signals to the first DAC and fourth signals to the second DAC, the third signals comprising in-phase components and the fourth signals comprising quadrature components, the first and second DACs (116, 118) sending their outputs to the first E/O converter.