In the field of fiber-optic communication, further optical transmission and better communication quality presuppose the modulation of the optical signal, that is, to load the information to be transmitted into the optical signal by modulation for long-distance transmission. Therefore, a variety of modulation methods have been developed, such as differential phase-shift keying (DPSK) and on-off keying (OOK). With lower requirements on the optical signal-to-noise ratio, DPSK modulation shows superb performance in fiber-optic communication systems and has also become a research hotspot for fiber-optic communication technologies in recent years. DPSK signals are generally demodulated by a Mach-Zehnder interferometer. However, demodulation systems based on the traditional Mach-Zehnder interferometer are strict with the light source, modulation code rate, and arm length difference. Such a system requires a narrow-linewidth light source and is easily affected by temperature. To solve the above problems, this paper proposes a DPSK demodulation system based on a novel Mach-Zehnder interference structure. Compared with the traditional demodulation system, the novel demodulation system has higher robustness to arm length difference mismatch, better temperature stability, and lower requirements on the coherence properties of the light source, enabling wide-linewidth light sources to be applicable to DPSK demodulation systems as well.
This paper adopts the research method of combining simulation with experiment. Specifically, the transmission process of an optical signal in the system is derived mathematically, and the demodulation principle of the novel Mach-Zehnder interference structure is thereby described in detail. Then, a demodulation system based on the novel Mach-Zehnder interference structure is built in simulation software to verify whether the simulation results are consistent with the theoretical derivation results and prove the correctness of the system principle. Furthermore, the Q value of the system is calculated, and the novel demodulation system is compared with the demodulation system based on the traditional interferometer in terms of the light source, arm length difference mismatch, and temperature. The simulation results show that the novel demodulation system provides better performance. Finally, the feasibility of the proposed system is verified by building an experimental system to determine whether the system can achieve DPSK modulation and demodulation, and the temperature stability of the system is tested.
The novel demodulation system shows excellent performance in temperature, and its temperature stability is better than that of the traditional demodulation system (Fig. 5). In terms of arm length difference mismatch and the code rate, the mismatch ratio the novel demodulation system allows is higher than 20%, while that of the traditional demodulation system is required to be within 10%. This result proves that the novel demodulation system is more robust (Fig. 4). The novel demodulation system can use a wide-linewidth light source (Fig. 8), while the traditional counterpart can only use a narrow-linewidth light source. This contrast represents a significant advantage of the novel demodulation system. It is also the main purpose of the novel demodulation system proposed in this paper and is used to reduce the system's requirements on the coherence properties of the light source. The experimental results show that the system can achieve DPSK demodulation of optical signals with a wide-linewidth light source (Fig. 9). Moreover, the Q value of the proposed system is above 10 (Table 1) when the ambient temperature is within 0-70 ℃, indicating that the system is feasible.
This paper proposes a DPSK demodulation system based on a novel Mach-Zehnder interference structure and presents a mathematical model of optical signal transmission in the system. To prove the correctness of the system principle and the mathematical model, the paper combines simulation with experiments. The simulation results show that the bit error rate of the system is smaller than 2.38×10-154 and thus meets the requirement that the bit error rate of a communication system should be smaller than 10-12. In addition, the DPSK modulation and demodulation experiment proves that the proposed system is feasible, and the experimental results show that the system successfully modulates and demodulates the m sequence. The temperature experiment proves that the Q value of the system is above 10 when the ambient temperature is within 0-70 ℃, indicating reliable temperature stability of the system. Finally, the light source used in the experiment is a wide-linewidth light source, and its coherence length is smaller than the arm length difference, which proves that the novel demodulation system can reduce the system's requirement on light source linewidth. In summary, the novel demodulation system has higher mismatch robustness, better temperature stability, and lower requirements on the coherence properties of the light source.
