VCSEL Cavity Photon Lifetime via Refractive Index Layer
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Solution Overview
Problem
Existing methods for adjusting the cavity photon lifetime in vertical cavity surface emitting lasers (VCSELs) are complex, uncontrollable, and often result in unintended destruction of the VCSEL surface, making them unsuitable for high-volume manufacturing due to nonuniform etching and surface damage.
Innovation Solution
A method involving the deposition of an additional layer with a refractive index smaller than the layer stack, controlled by measuring and adjusting the design parameter H(fR) through small-signal response analysis, allowing for precise optimization of the cavity photon lifetime without destructive etching processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If wet chemical etching is used to adjust cavity photon lifetime, then etching rate can be controlled by solution concentration, but the etching rate becomes nonuniform and time-dependent due to sensitivity to concentration, temperature, pH, and solution movement
Solution Approach 1:
The patent replaces the chemical etching process with a physical deposition process. Instead of using wet chemical etching solutions that are sensitive to concentration, temperature, and movement, the invention deposits a hard mask layer (such as silicon nitride or silicon oxide) through physical vapor deposition or chemical vapor deposition. This physical process provides uniform, controllable, and repeatable results without the instability inherent in wet chemical etching.
Solution Approach 2:
The patent changes the approach from modifying the cavity by removing material (etching) to modifying the optical properties by adding a layer with different refractive index. By depositing a hard mask layer with a different refractive index than the semiconductor layers, the cavity photon lifetime is adjusted through the added optical path difference, avoiding all the problems associated with chemical etching parameter control.
2Manufacturing precision
If digital etching with alternating oxidation and etching steps is used, then precise oxide thickness control is achieved, but surface roughness, surface defects, and geometric proximity effects render the technique unreliable
Solution Approach 1:
Instead of removing material through digital etching (oxidation followed by acid etching), the patent inverts the approach by adding a protective hard mask layer through deposition. This deposition process is not affected by surface roughness or defects in the same way etching is, and it avoids the geometric proximity effects that plague digital etching near metal contacts and aperture edges.
Solution Approach 2:
The hard mask layer acts as an intermediary that modifies the optical properties of the cavity without directly attacking or modifying the semiconductor surface. This intermediary layer approach avoids the direct interaction between etchants and the semiconductor surface that causes surface damage, while still achieving the desired adjustment of cavity photon lifetime.
3Manufacturing precision
If dry etching is used to adjust mirror surface, then cavity photon lifetime can be controlled, but very low etching rates and extremely precise control of etching progress are necessary
Solution Approach 1:
The patent replaces the mechanical/chemical removal process of dry etching with a physical deposition process. By depositing a hard mask layer, the cavity photon lifetime is adjusted through the added optical path, eliminating the need for extremely slow and precisely controlled material removal. The deposition process is inherently more productive and easier to control with high precision.
4Manufacturing precision
If additional layers are deposited on top of the reflector, then cavity photon lifetime can be optimized, but the refractive index must be smaller than the layer stack which limits material choices
Solution Approach 1:
The patent changes the approach from directly etching the semiconductor mirror to depositing a hard mask layer with different optical properties. By selecting materials like silicon nitride or silicon oxide that have lower refractive indices than the semiconductor layers, the cavity photon lifetime is optimized through the refractive index contrast and added optical path, providing both precision and material versatility.
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 improved laser performance by increasing small-signal modulation bandwidth, reducing energy consumption, and enhancing output power, while being nondestructive and suitable for high-volume production.
Implementation Method 1
an additional layer on top of the layer stack at the light output side, said additional layer forming an output interface of the laser, wherein the refractive index of the additional layer is smaller than the smallest refractive index of the refractive indices of said layer stack
Data Source
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Figure 2a~2c
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AI summary
An embodiment of the invention relates to a vertical cavity surface emitting laser comprising a first reflector, a second reflector comprising a layer stack of semiconductor or isolating layers, an active region arranged between the first and second reflectors, and an additional layer on top of the layer stack at the light output side, said additional layer forming an output interface of the laser, wherein the refractive index of the additional layer is smaller, equal to or larger than the smallest refractive index of the refractive indices of said layer stack.