DFB Laser Waveguide Structure for Fiber Coupling
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Solution Overview
Problem
Conventional high power DFB laser chips face limitations in output power, slope efficiency, far field aspect ratio, relative intensity noise (RIN), and linewidth, which affect their performance in optical communication systems, particularly in coupling efficiency into circular waveguides like optical fibers.
Innovation Solution
The DFB laser design incorporates a waveguide structure with a compositionally different waveguide layer in the n-doped cladding layer and a diffraction grating in either the p-doped or n-doped cladding layer, along with a hetero-waveguide stack and partially corrugated grating, optimizing the refractive index and grating placement to reduce mode intensity and loss, and using anti-reflective and highly reflective mirrors with controlled reflectivity to enhance performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If conventional high power DFB laser chip design is used, then output power can reach ~200 mW, but slope efficiency is limited to ~0.2 W/A and far field aspect ratio is 2:1 which is suboptimal for fiber coupling
Solution Approach 1:
The patent changes the physical parameters of the waveguide structure by introducing a compositionally different waveguide layer with optimized thickness (0.7 to 1.5 of the guided wavelength) and refractive index. This parameter optimization enables the mode intensity to be significantly reduced in the lossy p-doped cladding layer, achieving slope efficiency exceeding 0.3 W/A while maintaining high output power
Solution Approach 2:
The patent employs composite material structure by combining the compound semiconductor substrate with a waveguide layer of different composition (e.g., InGaAsP on InP). This composite structure creates the desired refractive index profile that confines the optical mode effectively, reducing loss in the p-doped cladding layer and improving slope efficiency
2Power
If conventional DFB laser design is used, then output power can be increased, but relative intensity noise (RIN) and linewidth increase resulting in unacceptably high amplitude and phase noise
Solution Approach 1:
The patent optimizes the waveguide layer thickness to 0.7 to 1.5 of the guided wavelength, which changes the modal distribution parameters. This parameter optimization reduces the mode intensity in the p-doped cladding layer where loss and noise are generated, enabling high output power with acceptable RIN and linewidth performance
3Power
If conventional DFB laser design is used, then high power operation is achieved, but far field aspect ratio remains 2:1 which is not optimal for coupling into circular waveguides
Solution Approach 1:
The patent changes the vertical dimension parameter by introducing a waveguide layer with optimized thickness (0.7 to 1.5 of the guided wavelength). This parameter change modifies the far field radiation pattern to achieve an aspect ratio closer to 1:1, which is optimal for coupling into circular waveguides like optical fibers
4Measurement precision
If grating is placed in the p-doped cladding layer, then wavelength selection is achieved, but mode intensity and loss in the active layer increase
Solution Approach 1:
The patent introduces the waveguide layer as an intermediary structure between the substrate and the p-doped cladding layer. This intermediary waveguide layer with compositionally different material and optimized thickness acts as a barrier that reduces the mode intensity penetrating into the lossy p-doped cladding layer, thereby reducing overall loss while maintaining the wavelength selection function of the grating
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 design achieves higher output power, improved slope efficiency, reduced RIN, and lower linewidth, enabling better coupling efficiency into optical fibers while maintaining low noise performance and extended laser length without significant degradation in slope efficiency.
Implementation Method 1
a waveguide structure including a waveguide layer in the n-doped cladding layer that significantly reduces the mode intensity and thus the loss in the p-doped cladding layer and the active layer
Implementation Method 2
The diffraction grating acts as the wavelength selective element for at least one of the minors and provides the feedback, reflecting light back into the cavity to form the resonator
Implementation Method 3
the diffraction grating includes a grating layer having a periodic refractive index which is different from the refractive index of the adjacent layers
Implementation Method 4
The DFB laser has mirrors that can include an anti-reflective (AR) minor on a first end of a length of the DFB laser and a highly reflective (HR) minor opposite the AR minor on a second side of the length
Data Source
AI summary
A distributed feedback (DFB) laser includes a substrate of a compound semiconductor material, and quantum-well (QW) active layer(s) overlying the substrate. A p-doped cladding layer including the compound semiconductor material is on one side of the active layer and an n-doped cladding layer is on the other side. A grating is in one of the cladding layers configured to select an operating wavelength for the DFB laser. A waveguide structure in the n-doped cladding layer includes a waveguide layer of a first composition compositionally different from the compound semiconductor material having an optical thickness of 0.7 to 1.5 of the guided wavelength. The waveguide structure can further include a hetero-waveguide stack including a plurality of alternating compositional layers beyond the waveguide layer each having a thickness between one quarter and one half the guided wavelength alternating the compound semiconductor material with a second composition defining a composition wavelength.