A novel non-feedback precoder circuit for high-speed parallel optical differential quadrature phase shift keying (DQPSK) modulation is proposed. The alternative control signal is introduced in replacement of the conventional feedback one, which eliminates the speed limitation due to the electronic propagation delay. The proposed precoder consists of four differential encoders, an exclusive OR gate, a cross switch, and delay lines. It is demonstrated by a true pseudo random bit sequence (PRBS) transmission at 20 Gb/s.