Etched-Facet DFB Laser Dual Cavity Phase Control
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Solution Overview
Problem
Cleaved facets in semiconductor lasers limit the precision of facet positioning and orientation, leading to unpredictable phase variations between facets and gratings in DFB lasers, resulting in low yield and performance issues such as random phase variation and limited side mode suppression ratio.
Innovation Solution
The development of an improved structure and method for etched-facet semiconductor DFB lasers, where at least two DFB laser cavities are formed in close proximity with one cavity being slightly longer than the other, allowing for precise control of the phase difference between facets and gratings, thereby enhancing alignment accuracy and yield.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If cleaved facets are used to form laser facets, then the fabrication process is simple, but the positioning precision and orientation control of facets are limited to a few microns causing random phase variation
Solution Approach 1:
The patent transitions from mechanical cleaving to photolithographically defined etched facets, changing the fabrication parameter from simple cleaving to controlled etching processes. This enables precise control of facet position and orientation while maintaining manufacturability through standard semiconductor processing techniques.
Solution Approach 2:
The patent replaces the mechanical cleaving process with a photolithographic and etching-based system. Instead of physically breaking the semiconductor crystal along cleavage planes, the invention uses photoresist patterning and chemical etching to define facet locations with sub-micron precision, eliminating random phase variations.
2Manufacturing precision
If photolithographically defined etched facets are used, then facet positioning precision is improved, but the alignment accuracy between grating and facet remains insufficient to deterministically specify phase
Solution Approach 1:
The patent applies preliminary action by pre-defining the grating structure with a specific phase relationship to the etched facets during the photolithography process. The grating is patterned before facet etching, and the photomask design ensures that the grating lines are aligned at the desired phase (e.g., 90 degrees) relative to the eventual facet positions, eliminating the need for post-fabrication phase adjustment.
Solution Approach 2:
The patent introduces a photomask as an intermediary element that mediates the alignment between the grating and facets. The photomask contains alignment marks and grating patterns that are precisely positioned relative to the facet definition features, serving as a reference framework that ensures deterministic phase specification throughout the fabrication process.
3Device complexity
If single cavity DFB lasers are manufactured, then the device structure is simple, but the yield is limited due to unpredictable phase variation
Solution Approach 1:
The patent applies segmentation by dividing a single laser cavity into multiple discrete cavity sections, each with its own etched facets. By creating arrays of closely spaced cavities (e.g., 2-10 cavities per array) with controlled phase relationships, the system ensures that at least one cavity per array will have the desired phase for high yield, while maintaining relatively simple individual cavity structures.
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 significantly increases chip yield by ensuring that at least one of the two closely positioned DFB lasers operates with a high side mode suppression ratio, improving the reliability and performance of DFB lasers by achieving a desired phase range between facets and gratings.
Implementation Method 1
Distributed feedback (DFB) lasers use a distributed diffraction grating to generate a single wavelength output
Implementation Method 2
The cavities are configured such that the first cavity forms a first phase with a diffraction grating and the second cavity forms a second different phase with the diffraction grating
Data Source
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AI summary
A semiconductor chip has at least two DFB etched facet laser cavities with one set of facets with AR coatings and a second set of etched facets with HR coatings that have a different relative position with respect to the gratings. This creates a difference in the phase between each of the etched facets and the gratings which changes the operational characteristics of the two laser cavities such that at least one of the lasers provides acceptable performance. As a result, the two cavity arrangement greatly improves the yield of the fabricated chips.