A quasi-cyclic low-density parity check (QC-LDPC) code is constructed by an improved stability of the shortest cycle algorithm for 160-Gb/s non-return zero differential quadrature phase shift keying (NRZ-DQPSK) optical transmission system with the fiber-based optical parametric amplifier (FOPA). The QC-LDPC code with stability of the shortest cycle reduces the bit error ratio (BER) to 10-14 and restrains the error floor effectively.