Conceptual Design of Multimode Interference-Based Photonic Crystal Mach-Zehnder Interferometer (De)Interleavers
Research Background and Problem Statement
With the rapid development of modern optical communication technology, wavelength division multiplexing (WDM) systems play a core role in achieving high-capacity, multifunctional optical networks. Among these, (de)interleavers, as key components of wavelength demultiplexing structures, can efficiently separate multiple wavelength signals, thereby providing greater flexibility in network design and supporting higher channel counts. However, traditional Mach-Zehnder Interferometer (MZI) designs have significant drawbacks in input and output couplers, particularly due to the strong wavelength dependence of the coupler structure, which limits their performance. Additionally, achieving a flat transmission spectrum and low crosstalk remains an important challenge in current research.
To address these issues, a research team from Shahid Beheshti University in Iran proposed a novel Photonic Crystal Mach-Zehnder Interferometer (PC-MZI) design based on Multimode Interference (MMI). This study aims to overcome the limitations of traditional MZIs in input/output couplers by optimizing the structural design of PC-MZIs and developing high-performance (de)interleavers suitable for Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) networks.
Source of the Paper
This paper was co-authored by Masoud Kamran and Kambiz Abedi, both from the Faculty of Electrical Engineering at Shahid Beheshti University. The paper was submitted on June 10, 2024, accepted on December 29, 2024, and will be published in the journal Optical and Quantum Electronics, article number 57:162, DOI: 10.1007/s11082-024-08021-y.
Research Details and Workflow
a) Research Process and Methods
1. Design Goals and Basic Principles
The main objective of this study is to design a PC-MZI (de)interleaver based on multimode interference to achieve a flat passband response, steep spectral edges, low power loss, and low crosstalk. The research is divided into two main parts: the first part designs a PC-MZI (de)interleaver with Y-splitter input and MMI output; the second part further extends this design, proposing a PC-MZI (de)interleaver based on input/output MMI.
2. Structural Design and Parameter Optimization
The researchers used a two-dimensional photonic crystal (PC) structure, with a lattice composed of silicon cylindrical rods in air. The dielectric constant of the rods is εrods = 11.85, and the radius is rd = 0.16a (where a is the lattice constant). To operate within a specific wavelength range, the study selected a working point of a/λ0 = 0.42, corresponding to a lattice constant of a = 0.65 µm and a rod radius of 108 nm.
The study employed the Finite-Difference Time-Domain (FDTD) method for simulation analysis, focusing on the behavior of TE-polarized light. The simulation domain was excited by a continuous wave source, simulated using a Gaussian beam profile. The time step was δt = 0.04 fs, and the simulation duration was 3 ps (C-band) or 5 ps (L and E bands), covering approximately 301 periods of the light source.
3. Experimental Design and Key Steps
The research included the following main steps: 1. Input/Output Coupler Design: The study compared three primary input/output coupler structures: Y-splitter, waveguide-type coupler, and MMI-type coupler. Ultimately, the MMI-type coupler was chosen due to its wider range of interference management and higher control over transmission characteristics. 2. Delay Line and Phase Modulator Design: By introducing delay lines of different lengths and a phase shifter (PS), the phase difference between the two optical signals was controlled. 3. Multimode Interference Region Design: The MMI region was formed by removing five consecutive rows of rods, with a length determined by the working point set at lmmi = 26a. Output single-mode waveguides were used to extract the desired output signals. 4. Performance Evaluation and Optimization: The device’s performance was optimized by adjusting various structural parameters, such as delay line length, MMI length, and effective refractive index.
4. Innovative Experimental Methods and Algorithms
The research team developed a method based on the Particle Swarm Optimization (PSO) algorithm to optimize the design parameters of photonic crystal waveguides. Additionally, the study utilized the self-imaging principle and multimode interference effects to achieve a flat passband response.
b) Main Results and Data Analysis
1. Performance of 4-, 6-, 8-, and 16-Channel (De)Interleavers
The study successfully designed and simulated 4-, 6-, 8-, and 16-channel PC-MZI (de)interleavers. The specific results are as follows: - 4-Channel Device: Central wavelengths were 1.521 µm, 1.541 µm, 1.554 µm, and 1.569 µm, with adjacent channel spacing of 10 nm and non-adjacent channel spacing of 20 nm. Power loss near 1.55 µm was 0.05 dB, and channel isolation was -24 dB. - 6-Channel Device: Central wavelengths ranged from 1.538 µm to 1.578 µm, with 1 dB bandwidths of 4.2 nm to 5 nm and 3 dB bandwidths of 7.6 nm to 8.5 nm. Channel isolation ranged from -12 dB to -26 dB. - 16-Channel Device: Central wavelengths ranged from 1.526 µm to 1.604 µm, with 1 dB bandwidths of 2 nm to 4.5 nm and 3 dB bandwidths of 3.9 nm to 6 nm. Adjacent channel spacing was 11 nm, and non-adjacent channel spacing was 22 nm.
2. Flat Passband and Steep Spectral Edges
All designs exhibited flat passband responses (shape factor > 0.5) and steep spectral edges (roll-off rate of 12–30 dB/nm). Additionally, the overall power loss of the devices was less than 3 dB, and channel isolation was below -14 dB.
3. Significance of Results and Logical Relationships
These results demonstrate that the PC-MZI design based on MMI effectively addresses the limitations of traditional MZIs in input/output couplers while meeting the demands of high-performance (de)interleavers for DWDM and CWDM networks.
c) Research Conclusions and Value
This study successfully developed a PC-MZI (de)interleaver based on multimode interference, achieving a flat passband response, low power loss, and low crosstalk. The design has significant application value in DWDM and CWDM networks, especially in high-density wavelength division multiplexing systems. Moreover, the optimization methods and design ideas proposed in this study provide new directions for the future development of photonic crystal devices.
d) Research Highlights
- Innovative Design: For the first time, MMI was applied to the input/output coupler design of PC-MZIs, solving the wavelength dependence issue of traditional MZIs.
- High-Performance Metrics: Achieved flat passband response, steep spectral edges, and low power loss.
- Wide Application Potential: The design is suitable for DWDM and CWDM networks, providing technical support for future high-capacity optical communication systems.
e) Other Valuable Information
The research team emphasized that the layout area of all designed demultiplexers is less than 1.4 × 10⁻³ mm², demonstrating extremely high integration and compactness. Additionally, the FDTD simulation method and PSO optimization algorithm used in the study provide important references for future similar research.
Summary and Significance
This paper not only showcases the design and optimization process of a PC-MZI (de)interleaver based on multimode interference but also provides new solutions for high-performance wavelength demultiplexing technology in the field of optical communications. The scientific value of the research lies in addressing key issues in traditional MZI designs, while its application value is reflected in providing reliable technical support for next-generation high-capacity optical networks.