MEMS VCSEL With Embedded Photodetector for Tunable Depth Sensing
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Solution Overview
Problem
Conventional VCSELs used in self-mixing interferometry for eye-tracking and depth sensing have limited wavelength tunability and resolution due to their small cavity length, leading to reduced precision and increased phase error, especially when exposed to ambient light, which affects the accuracy of distance and velocity measurements.
Innovation Solution
A MEMS-based VCSEL with an integrated photodetector is developed, featuring a tunable wavelength operating in a single mode, where the MEMS structure replaces the upper or lower DBR, increasing wavelength tunability to tens of nanometers and incorporating a photodetector within the DBR to enhance signal-to-noise ratio and reduce phase errors, allowing for sub-100 micrometer depth resolution.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the cavity length of VCSEL is increased to improve wavelength tunability, then wavelength tunability is improved, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The patent employs a MEMS-based tunable mirror that dynamically adjusts the optical cavity length by moving the mirror position. This dynamic adjustment mechanism enables wavelength tuning without permanently increasing cavity complexity, as the mirror can be positioned at different locations to create variable cavity lengths for different wavelength operations.
Solution Approach 2:
The patent introduces a distributed Bragg reflector (DBR) as an intermediary component between the active region and the MEMS mirror. The DBR serves as a wavelength-selective mirror that works in conjunction with the MEMS mirror to define the optical cavity, enabling precise wavelength control while maintaining manageable device complexity through the use of specialized optical layers.
2Measurement precision
If the cavity length of VCSEL is increased to improve depth resolution, then depth resolution is improved, but phase error increases
Solution Approach 1:
The MEMS-based tunable mirror enables dynamic adjustment of the optical cavity length to optimize the balance between depth resolution and phase error. By tuning the cavity length according to specific measurement requirements, the system can achieve high depth resolution while maintaining acceptable phase error levels through adaptive optimization.
Solution Approach 2:
The patent changes the optical cavity parameters (length, refractive index distribution) through the MEMS mirror positioning and DBR design. By adjusting these parameters, the system optimizes the trade-off between depth resolution (which benefits from longer cavity) and phase error (which increases with longer cavity), achieving the best overall measurement performance.
3Device complexity
If conventional VCSEL design is used to maintain simple structure, then device complexity is low, but signal-to-noise ratio is reduced due to ambient light sensitivity
Solution Approach 1:
The patent incorporates a photodetector as an intermediary component integrated within the VCSEL structure. This photodetector directly detects the optical signal generated by the VCSEL, enabling immediate signal processing and discrimination from ambient light noise. The integration of this detection function within the VCSEL structure maintains relative simplicity while significantly improving signal-to-noise ratio.
Solution Approach 2:
The patent merges the light emission function (VCSEL) with the light detection function (photodetector) into a single integrated device. This combination allows the system to use the VCSEL as both the light source and the reference signal generator, while the integrated photodetector detects the interference pattern between the emitted light and reflected light, effectively rejecting ambient light noise through coherent detection.
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 MEMS-based VCSEL design significantly enhances the wavelength tunability and signal-to-noise ratio, improving the depth resolution and accuracy of eye-tracking and depth sensing applications, enabling high-resolution measurements in compact sensing architectures.
Implementation Method 1
The upper reflector may include a MEMS grating... a voltage may be applied to the MEMS architecture to adjust dimensions of the resonator and tune an operational wavelength
Implementation Method 2
a photodetector to detect power undulations resulting from coherent interference of backscattered light into the laser cavity
Implementation Method 3
an active region overlying the distributed Bragg reflector... configured to generate coherent light
Data Source
AI summary
A MEMS-based vertical cavity surface emitting laser (VCSEL) includes an embedded photodiode. A representative VCSEL includes a distributed Bragg reflector (DBR), a photodiode structure located within the distributed Bragg reflector, an active region overlying the distributed Bragg reflector, and a MEMS upper reflector disposed over the active region. The VCSEL may be configured as an optical sensor. The laser cavity and hence a working wavelength of the sensor may be tuned by modulating the MEMS reflector such as through an applied voltage.