2. School of Mathematics and Physics, Lanzhou Jiaotong University, Lanzhou, Gansu 730030, China;
3. School of Automation and Electrical Engineering, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China
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.