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摘要:
为了改善全光纤马赫-曾德尔干涉仪(MZI)型梳状滤波器(interleaver)的输出特性，提出了由一个2×2光纤耦合器和一个带自反馈光纤谐振腔的光纤耦合器级联组成的改善型的全光纤MZI型梳状滤波器。根据其结构，利用光纤传输理论和矩阵理论推导出了该器件的输出表达式，并进行了数值模拟分析。结果表明：该器件在合理参数的情况下，利用带自反馈光纤谐振腔引入的相位调节效应，使其25 dB截止带宽为46.7 GHz，约占50 GHz频率间隔的93.4%，输出光谱形态近似于方波。当存在传输损耗时，两束干涉光信号的幅度不存在明显的差异，降低了传输损耗对滤波器消光特性的影响，与传统的光纤谐振环辅助非平衡MZI型梳状滤波器相比，不仅降低了传输损耗对滤波器消光特性的影响，而且还将所需的光纤耦合器数目减少至2个，使其在未来的密集波分复用系统中能发挥重要作用。
Abstract:In order to improve the transmission performance of all-fiber Mach-Zehnder interferometer (MZI), a novel all-fiber MZI interleaver is proposed and discussed in this paper. All-fiber interleaver consists of one 2×2 fiber coupler and one coupler with self-feedback fiber ring resonator. According to its structure, the output expression of the device is deduced by using optical fiber transmission theory and matrix theory, and numerical simulation analysis is performed. The results show that the device adopts the phase adjustment effect introduced by the self-feedback optical fiber resonator with reasonable parameters, and its 25 dB cutoff bandwidth is 46.7 GHz which accounts for 93.4% of the 50 GHz frequency interval. The output spectrum is similar to the square wave output. This device requires 2 fiber couplers, which is less than the number of couplers needed for the conventional all-fiber MZI-interleaver. When considering the existence of transmission loss, there is no difference in the amplitude of the two interference optical signals, which reduces the influence of transmission loss on the extinction characteristics of the filter. Compared with the conventional unbalanced MZI type interleaver with an optical fiber resonant ring, the structure is simple and compact, and has a certain anti-deviation ability. It also reduces the difficulty of making the device, which makes it play an important role in the future of dense wavelength division multiplexing systems.
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Key words:
- fiber optical coupler /
- interleaver /
- Mach-Zehnder interferometer /
- fiber ring resonator
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Overview: Since the passband shape of the conventional Mach-Zehnder interferometer (MZI)-interleaver is a cosine type. Its peak characteristics and the passband width don't meet the actual needs. In practice, it is easy to generate large insertion loss and crosstalk. The researchers proposed multi-level MZI cascaded all-fiber interleaver and optical fiber resonator-assisted non-equilibrium MZI-interleaver. Both of these proposals can improve response passband flatness. However, the former improvement scheme caused an increase in the total interference arm length of the MZI-interleaver, which will seriously affect the simplicity and performance stability of the all-fiber MZI-interleaver structure, at the same time, increase the practical production difficulties. The latter improvement is to couple the fiber cavity into a certain interference arm of the MZI to form an asymmetric structure of the fiber resonator-assisted unbalanced MZI-interleaver. The fiber cavity is very beneficial for improving the performance of the all-fiber MZI-interleaver. However, this fiber resonator assisted unbalanced MZI-interleaver has a structural asymmetry. The actual optical path of the transmitted signal is much larger than the optical path in the other interfering arm without the optical fiber resonator due to the action of the optical fiber resonator. Obviously, the amplitude of the two optical signals in the two arms will be greatly different when they reach the output coupler due to existence of transmission loss. It can be known from the interference principle that when there are amplitude differences between the two optical signals that interfere with each other, the output spectral extinction characteristics are bound to deteriorate, resulting in a large increase in crosstalk interference between channels. In order to improve the transmission performance of all-fiber MZI, a novel all-fiber MZI-interleaver is proposed and discussed in this paper. An all-fiber MZI-interleaver consisting of a 2×2 fiber coupler and a fiber coupler cascade with a self-feedback fiber cavity is designed, by selecting reasonable coupler parameters and using the phase adjustment effect introduced by the self-feedback fiber resonator. According to its structure, the output expression of the device is deduced by using optical fiber transmission theory and matrix theory, and numerical simulation analysis is performed. The results show that its 25 dB cutoff bandwidth is 46.7 GHz which accounts for 93.4% of the 50 GHz frequency interval. The output spectrum is similar to the square wave output. Compared with the conventional unbalanced MZI type interleaver with an optical fiber resonant ring, the structure is simple and compact, had a certain anti-deviation ability and reduced the number of fiber couplers require to 2, which makes it play an important role in dense wavelength division multiplexing systems in the future.
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图 1 带自反馈腔的2×2光纤耦合器的结构
Figure 1. Structure of a 2×2 fiber coupler with self-feedback cavity
图 2 自带反馈光纤谐振腔的MZI型梳状滤波器结构图
Figure 2. MZI interleaver structure with feedback fiber resonator
图 3 k0 = π/4, k1 = π/0.72时，interleaver的输出光谱图
Figure 3. The calculated transmission spectra of the proposed interleaver using these parameter k0 = π/4 and k1 = π/0.72
图 5 当(a) k0 = π/4+Δk0, k1 = π/0.72+Δk1; (b) k0 = π/4-Δk0, k1 = π/0.72+Δk1; (c) k0 = π/4+Δk0, k1 = π/0.72-Δk1; (d) k0 = π/4-Δk0, k1 = π/0.72-Δk1时，本设计的interleaver(虚线)与文献[5](实线)输出光谱的比较
Figure 5. Comparision of responses of the proposed interleaver (dotted) with interleaver reported in ref.[5] (solid). (a) k0 = π/4+∆k0, k1 = π/0.72+∆k1; (b) k0 = π/4-∆k0, k1 = π/0.72+∆k1; (c) k0 = π/4+∆k0, k1 = π/0.72-∆k1; (d) k0 = π/4-∆k0, k1 = π/0.72-∆k1
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