Self-Mixing Interference Device With Integrated Waveguide
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Solution Overview
Problem
Existing self-mixing interference devices based on semiconductor lasers are limited by their height and cost, particularly when used in compact applications like mobile devices and trackballs, due to the need for a significant distance between the laser source and the measured object, which also reduces signal strength and requires complex optical elements for focusing.
Innovation Solution
The integration of a semiconductor laser source and photodetector with an optical waveguide structure on a substrate allows for reduced device height and cost by guiding laser radiation and backscattered light within the substrate, eliminating the need for external focusing elements and enabling two-axis motion sensing with a single VCSEL die.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a significant distance between the laser source and the measured object is used, then the self-mixing interference effect can be achieved, but the device height increases and signal strength decreases
Solution Approach 1:
The patent transitions from a vertical light path (requiring large device height) to a horizontal light path within the substrate plane. The waveguide structure guides light laterally through the substrate, allowing the round-trip length to be achieved in-plane rather than vertically, thus reducing device height while maintaining the necessary interaction distance for self-mixing interference.
Solution Approach 2:
The waveguide structure acts as an intermediary medium between the laser source and the target object. It guides the laser radiation and backscattered light through the substrate, enabling the light to travel the necessary distance for interference while confining the path within the substrate plane, thereby reducing the vertical profile of the device.
2Reliability
If a significant distance between the laser source and the measured object is used, then the self-mixing interference effect can be achieved, but the signal strength is reduced
Solution Approach 1:
The waveguide structure serves as an optical intermediary that guides and confines the laser radiation and backscattered light through the substrate. This confinement reduces light dispersion and loss over the transmission path, thereby maintaining signal strength despite the extended horizontal path length required for the self-mixing interference effect.
3Illumination intensity
If external focusing elements are used to regain signal strength, then the device complexity increases and manufacturing cost increases
Solution Approach 1:
The waveguide structure itself provides the light guiding and confinement function that would otherwise require separate external focusing elements. By utilizing the substrate's own material properties and waveguide structure to maintain signal strength, the patent eliminates the need for additional optical components, thereby reducing device complexity and manufacturing cost.
4Adaptability or versatility
If two laser sources are used to measure movement in two directions, then the measurement capability is improved, but the device complexity and cost increase
Solution Approach 1:
The patent divides the single laser source into two spatially separated laser sources (e.g., two VCSELs) that are integrated on the same substrate. Each laser source is coupled to its own waveguide structure, allowing independent measurement in orthogonal directions. This segmentation enables two-axis motion sensing while keeping the overall device integrated and manageable.
Solution Approach 2:
The substrate and waveguide structure serve multiple functions: they guide light for both laser sources, provide mechanical support, and enable integrated packaging. By making the waveguide structure universal and reusable for both laser sources, the patent reduces the overall complexity compared to having completely separate dual-laser systems.
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 enables a more compact and cost-effective self-mixing interference device capable of accurate 2D motion sensing with improved signal strength and reduced manufacturing costs, suitable for use in smaller devices like mobile phones and trackballs.
Implementation Method 1
The wave guide structure is optically connected to the laser source and designed to guide the laser radiation emitted by the laser source to an out-coupling area at the surface of the substrate and to guide a portion of the laser radiation back scattered from a target object outside of the substrate to re-enter the laser source
Implementation Method 2
a semiconductor laser source, in particular a VCSEL, arranged on a surface of the substrate and emitting laser radiation towards said surface
Implementation Method 3
a photodetector arranged to detect intensity variations of the laser radiation
Implementation Method 4
laser light which is scattered back from a target object and re-enters the laser cavity, interferes with the resonating radiation and thus influences the output properties of the device
Implementation Method 5
the laser can be operated with a defined current shape, e.g. a periodic saw tooth or triangular current, causing the output frequency to almost instantaneously follow these current variations due to the simultaneously changed optical resonator length. This change in resonator length is temperature induced, resulting from the dissipative heating by the laser current
Data Source
AI summary
The proposed self-mixing interference device comprises a substrate (1) with an integrated optical wave guide structure (3), a semiconductor laser source (2) arranged on a surface of the substrate (1) and emitting laser radiation towards said surface, and a photodetector arranged to detect intensity variations of the laser radiation. The wave guide structure (3) is optically connected to the laser source (2) and designed to guide the laser radiation emitted by the laser source to an out-coupling area at the surface of the substrate (1) and to guide a portion of the laser radiation scattered back from a target object (4) outside of the substrate (1) to re-enter the laser source (2). This self-mixing interference device can be realized with a lower total height compared to the known self-mixing interference devices.