We demonstrate influences of the power splitting ratio of the quadrature-arm to the input signal on both receiver sensitivity and residual phase error for a 10-Gb/s binary phase-shift keying coherent receiver based on an optical homodyne Costas loop. Fine adjustment of the signal power splitting ratio (Ks) is realized by tuning the polarization state of the input signal light of a dual-polarization optical 90-deg hybrid, leading to the precise control of the power distribution on two orthogonal states of polarization that are used for in-phase and quadrature arm, respectively. When the phase error is negligible (<10 deg) under different Ks, the sensitivity is improved by 2.65 dB and the Ks is optimized around 0.05. Based on loop bandwidth maintaining, the requirement on laser linewidth is also relaxed, i.e., 5.26 times larger linewidth is permitted at Ks = 0 . 05 than that without loop bandwidth maintaining. To fully utilize the signal light power and to avoid excess losses of the dual-polarization hybrid, homodyne Costas coherent receiver with a free-space optics-based 90-deg hybrid is also proposed. All experimental and theoretical results demonstrate the potential of approaching shot noise limited sensitivity for a Costas coherent receiver with an optimum Ks. It is significant to increase power budget and transmission span for satellite optical communication and free-space optical communication.
We demonstrate the coherent detection of 10 Gb/s return-to-zero (RZ) binary phase-shift keying (BPSK) signal based on a homodyne Costas optical phase-locked loop (OPLL). It demonstrates time misalignment tolerance of +/- 10% of the transmitted RZ-BPSK signal, i.e. -20 to +20 ps between the pulse carver and the phase modulator for 5 Gb/s RZ-BPSK signal, -10 to +10 ps or 10 Gb/s RZ-BPSK signal. Besides, the Costas coherent receiver shows a 2.5 dB sensitivity improvement over conventional 5 Gb/s NRZ-BPSK and a 1.4 dB over 10 Gb/s NRZ-BPSK only at the cost of slightly higher residual phase error. Those merits of sufficient tolerance to misalignment, higher receiver sensitivity, and low residual phase error of RZ-BPSK modulation are beneficial to be applied in free space optical (FSO) communication to achieve higher link budget, longer transmission distance.