Transformation between directional couplers and multi-mode interferometers based on ridge waveguides.

Area of Science:

• Photonics and Optical Engineering
• Integrated Optics
• Waveguide Devices

Background:

• Directional couplers (DC) and multi-mode interferometers (MMI) are fundamental optical components.
• MMI devices are known for compactness and polarization-insensitivity, desirable traits in integrated photonics.
• The structural relationship between DCs and MMIs has been historically recognized but not fully elucidated.

Purpose of the Study:

• To present a unique comparison of ridge-type directional couplers (DC) and multi-mode interferometers (MMI).
• To elucidate the transformational relationship and continuous evolution between DCs and MMIs.
• To explore the design trade-offs and similarities between these two optical device types.

Main Methods:

• Structural transformation analysis of directional couplers.
• Theoretical modeling of light propagation and interference in transformed structures.
• Comparative analysis of design parameters and device characteristics.

Main Results:

• Demonstrated the continuous evolution from two-mode coupling (DC) to multimode interference (MMI).
• Showcased that DCs can be engineered to exhibit MMI features like compactness and polarization-insensitivity.
• Identified that DC design requires intricate control of numerous parameters, unlike the more robust MMI design.

Conclusions:

• The MMI is a structural evolution of the DC, sharing a common lineage.
• Directional couplers can be designed to incorporate the advantageous features of multi-mode interferometers.
• MMI design offers greater robustness and simplicity compared to the complex parameter control needed for advanced DC designs.