Transfer matrix analysis of the unidirectional grating-assisted codirectional coupler.

Area of Science:

• Photonics
• Optical Engineering
• Waveguide Devices

Background:

• Unidirectional grating-assisted codirectional couplers (U-GACC) enable irreversible coupling of orthogonal waveguide eigenmodes.
• Current analysis relies on coupled mode theory, limiting its design utility.

Purpose of the Study:

• To analyze the U-GACC using the transfer matrix method (TMM).
• To elucidate the operational principles of U-GACC unidirectionality.
• To identify previously unrecognized design limitations.

Main Methods:

• Analysis of U-GACC using the transfer matrix method.
• Simultaneous modulation of real and imaginary parts of the refractive index in the coupling region.

Main Results:

• The transfer matrix method provides a clear explanation for U-GACC unidirectional operation.
• A fundamental limit to minimum crosstalk between outputs is identified for practical designs.
• This crosstalk limit is a novel finding for U-GACC devices.

Conclusions:

• The transfer matrix method is a more accessible design tool for U-GACC compared to coupled mode theory.
• Understanding the minimum crosstalk limit is crucial for optimizing U-GACC performance.
• This research advances the design and application of unidirectional optical couplers.