On-Chip Polarizer Using Mode-Selective Expander Geometry
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Solution Overview
Problem
Current photonic integrated circuits face challenges in suppressing higher-order waveguide modes and discriminating between TE and TM polarization modes due to limitations in directional couplers and MMI couplers, which result in high insertion loss and low tolerance to fabrication inaccuracies and wavelength variations.
Innovation Solution
A low-loss, high-extinction-ratio optical waveguide polarizer is integrated into photonic integrated circuits using a mode-selective expander (MSE) with a core and outer waveguiding region, where the MSE preferentially expands light of the second mode into the outer region while propagating the first mode, minimizing crosstalk and noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If directional couplers or MMI couplers are used as polarization splitters, then TE and TM modes can be split in space, but the device suffers from high sensitivity to wavelength variations and fabrication inaccuracies, resulting in low TM/TE extinction ratio across wide wavelength ranges
Solution Approach 1:
The patent changes the geometric parameters of the waveguide, specifically introducing an asymmetric cross-section with different widths in the propagation direction versus the coupling direction. This parameter change creates a mode-selective expander that differently expands TE and TM modes, achieving high polarization extinction ratio while maintaining tolerance to wavelength variations and fabrication inaccuracies
Solution Approach 2:
The patent employs asymmetric waveguide geometry where the waveguide width in the propagation direction differs from the width in the coupling direction. This asymmetry causes differential expansion of TE and TM modes into the cladding region, enabling effective polarization filtering while being less sensitive to manufacturing variations and wavelength changes
2Manufacturing precision
If sufficiently narrow waveguides are used to suppress higher-order modes, then single-mode operation is achieved, but both TE and TM fundamental modes are supported, requiring additional discrimination mechanisms
Solution Approach 1:
The patent merges mode suppression and polarization discrimination into a single integrated structure. The asymmetric waveguide geometry simultaneously suppresses higher-order modes and discriminates between TE and TM fundamental modes, eliminating the need for separate discrimination mechanisms while maintaining single-mode operation
3Measurement precision
If asymmetrical Mach-Zehnder Interferometer or adiabatic couplers are used as waveguide polarizers, then polarization discrimination is achieved, but the device length becomes large and insertion loss for TE mode increases
Solution Approach 1:
The patent extracts the polarization discrimination function from long, complex interferometric structures and implements it in a compact waveguide section. By taking out the essential function of differential mode expansion and implementing it directly in the waveguide geometry, the device achieves polarization discrimination with significantly reduced length and lower TE mode insertion loss
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces mode and polarization crosstalk, achieving ultra-low loss for desired modes and high polarization extinction ratios, enhancing signal quality and noise reduction in photonic integrated circuits.
Implementation Method 1
the core and/or outer waveguiding regions are configured to preferentially expand the light of the second mode from the core waveguiding region into the outer waveguiding region
Data Source
AI summary
A low loss high extinction ratio on-chip polarizer is disclosed. The polarizer includes an input waveguide taper having an outer waveguiding region that widens in the direction of light propagation along at least a portion of the taper length, and a core waveguiding region that narrows in the direction of light propagation along at least a portion of the taper length, so as to selectively squeeze out light of undesired modes into the outer regions while preserving light of a desired mode in the waveguide core. An integrated light absorber/deflector may be coupled to the outer waveguiding regions.