Tapered-Grating DFB Lasers for Stable Single-Mode Operation
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Solution Overview
Problem
Existing single-mode semiconductor lasers with uniform gratings often operate in one of two distinct modes due to variability in end-facet positioning, leading to reduced fabrication yield and instability, especially at higher powers, as they tend to hop between modes and experience spectral broadening.
Innovation Solution
Incorporating a tapered grating structure into the DFB waveguide, which includes a tapered grating portion sandwiched between two uniform grating portions, allowing for a single low-loss longitudinal mode operation independent of end-facet positioning, thereby reducing index and gain variations and improving stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a uniform grating structure is used in the DFB waveguide, then the fabrication process is simple, but the laser operation becomes unstable with mode hopping and spectral broadening due to end-facet positioning variability
Solution Approach 1:
The patent introduces a tapered grating portion with asymmetric ridge heights (gradually increasing from one end to the other) between two uniform grating portions. This asymmetric structure creates a unique longitudinal mode with superior confinement that is insensitive to end-facet positioning variations, thereby eliminating mode hopping and spectral broadening while maintaining fabrication feasibility through standard lithography and etching processes.
2Power
If the waveguide is made longer to achieve significant optical gain, then the gain is sufficient, but the free-spectral range becomes much smaller than the laser wavelength leading to multi-longitudinal-mode operation
Solution Approach 1:
The patent applies local quality by creating a specific tapered grating region with varying ridge heights in the central portion of the waveguide, while maintaining uniform gratings at the ends. This localized tapered structure provides strong longitudinal mode confinement and creates a unique lowest-order mode with superior confinement, enabling single-longitudinal-mode operation even in longer waveguides with sufficient optical gain.
3Productivity
If end-facet positioning is varied during fabrication, then manufacturing flexibility increases, but laser wavelength predictability decreases due to operation in different modes
Solution Approach 1:
The patent implements preliminary action by designing the tapered grating structure in advance that pre-determines the longitudinal mode characteristics. The asymmetric tapered ridges create a unique mode profile with superior confinement that is established during fabrication, making the laser wavelength predictable and consistent across all devices regardless of subsequent end-facet positioning variations during cleaving or cutting.
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 predictable single-mode operation across all devices with identical specifications, increasing fabrication yield and maintaining narrow spectral linewidth even at higher operating powers, compared to lasers with uniform or shifted defect gratings.
Implementation Method 1
a grating structure distributed along a sidewall of the waveguide, where the grating structure protrudes horizontally out of a side of the waveguide
Implementation Method 2
the waveguide includes a grating structure distributed along a sidewall of the waveguide
Implementation Method 3
a semiconductor active region; and a semiconductor cap stacked on top of the semiconductor active region, where semiconductor active region and semiconductor cap form a waveguide
Data Source
AI summary
Single-mode distributed-feedback (DFB) lasers including single mode DFB waveguides with tapered grating structures are provided herein. Tapered grating structures provide for single mode DFB waveguides with predictable single mode operation. Uniform grating structures may provide for single mode operation, however DFB waveguides implementing uniform grating structures may operate at one of two single modes. Advantageously, DFB waveguides with tapered gratings operate with a spectrally narrow single mode at the same predictable single mode for all DFB waveguides with substantially identical specifications. Such predictability may lead to increased yield during manufacture of DFB waveguides with tapered gratings.