Interferometric Photonic Sensor Bulk Surface Index Separation
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Solution Overview
Problem
Existing photonic sensors struggle to differentiate between changes in bulk and surface refractive indices, leading to ambiguous sensor signals when both effects occur simultaneously, especially under conditions of varying concentrations or refractive index changes in the ambient medium.
Innovation Solution
A hybrid interferometer design is employed, where one arm includes a waveguide and the other arm comprises a free-space segment through the ambient medium, using optical reflectors and coupling devices to separate the light path through the ambient medium, allowing for decorrelation of bulk and surface refractive index effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional Mach-Zehnder interferometer with both arms as waveguides is used, then the device structure is simple and compact, but it cannot differentiate between bulk and surface refractive index changes
Solution Approach 1:
The measurement arm is segmented into two functional sections: a waveguide section for surface interaction and a free-space section for bulk medium interaction. This segmentation allows independent detection of surface and bulk refractive index changes, resolving the ambiguity present in conventional unified waveguide structures.
Solution Approach 2:
Free space acts as an intermediary medium between the waveguide and the bulk ambient medium. The light propagates through this intermediary in the free-space section, enabling direct interaction with the bulk medium while being optically coupled to the waveguide section through mode conversion at the interfaces.
2Measurement precision
If the measurement arm is kept separate from the ambient medium by a thick surface coating, then surface adsorption can be detected, but bulk refractive index changes cannot be detected
Solution Approach 1:
The measurement arm is divided into distinct functional zones: the waveguide section with surface coating for adsorption detection, and the free-space section without coating for bulk medium detection. This spatial segmentation enables simultaneous detection of both surface and bulk phenomena.
Solution Approach 2:
Different sections of the measurement arm have different local properties: the waveguide section has a surface coating for selective molecular attachment, while the free-space section has no coating to allow direct interaction with the bulk medium. Each section is optimized for its specific detection function.
3Volume of moving object
If both arms are wound into a spiral to increase length, then the device remains compact, but the light path through the ambient medium is limited
Solution Approach 1:
The light path in the free-space section propagates in a different spatial dimension (through air above the substrate) rather than being confined to the planar waveguide layer. This dimensional change allows extended interaction length with the bulk medium without increasing the device footprint on the substrate.
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
This approach effectively separates bulk and surface refractive index changes, enhancing sensor accuracy and reducing ambiguity, even under conditions of high concentration variations or environmental changes.
Implementation Method 1
at least a first coupling device for coupling a guided propagation mode of a waveguide and a free propagation mode of an ambient medium
Implementation Method 2
an optical reflector configured to reflect the light wave toward a second coupling device
Implementation Method 3
an interferometer having a first arm and a second arm comprising respective optical waveguides
Implementation Method 4
an electromagnetic wave propagating through a dielectric waveguide generates an evanescent field that extends a distance called the penetration length Ld (or d1/e) around the waveguide
Data Source
AI summary
A photonic sensor including an interferometer having a first arm and a second arm including respective optical waveguides, wherein the first arm includes: at least a first coupling device for coupling a guided propagation mode of the waveguide and a free propagation mode of an ambient medium; and an optical system configured to direct the free propagation mode toward the or a second coupling device for coupling the free propagation mode of the ambient medium and a guided propagation mode of the waveguide; whereby a light wave traversing the first arm travels one portion of its path through the ambient medium.