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Ultralong bulk semiconductor optical amplifiers (bulk UL-SOAs) have tremendous four-wave mixing (FWM) efficiencies so that due to two continuous wave (CW) input signals, a broad mode comb at the output of the UL-SOA can be obtained. When using tunable single mode lasers as input signals, the generated FWM mode comb is tunable in the mode spacing and the wavelength. As a result in time domain, widely tunable pulses in the repetition rate and the carrier frequency are obtained. Since the phase signature of the FWM modes is not linear over the whole mode comb, different filtering techniques can be applied for filtering modes with linear phase in order to efficiently generate short pulses. With the optimal filtering technique, pulses with full width at half maximum of less than 2 ps and a pulse repetition rate of 20 GHz can be obtained.