Asymmetric Chirped Fiber Bragg Grating for Stable Pump Laser Locking
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Solution Overview
Problem
Existing fiber Bragg gratings used with laser diodes in optical fiber amplifiers do not effectively manage coherence and power stability, leading to interference and reduced output power due to dominant laser modes and spectral hole burning.
Innovation Solution
Implementing an asymmetric chirped fiber Bragg grating with a reflectivity profile skewed to the short wavelength side, which broadens the bandwidth and shifts the central wavelength away from dominant modes, reducing coherence and enhancing power stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional fiber Bragg grating is used to lock the laser diode to a pump wavelength, then the laser diode can be stabilized to a specific wavelength, but dominant laser modes and spectral hole burning occur leading to coherence issues and power instability
Solution Approach 1:
The patent applies asymmetry by designing a fiber Bragg grating with an asymmetric reflectivity profile where the reflectivity is intentionally skewed toward the short wavelength side rather than being symmetric around the central wavelength. This asymmetric configuration suppresses dominant laser modes and reduces spectral hole burning, thereby improving power stability while maintaining wavelength locking capability
Solution Approach 2:
The patent implements local quality by creating different reflectivity characteristics at different wavelength regions within the grating. The short wavelength region has higher reflectivity compared to the long wavelength region, allowing different parts of the spectral envelope to be treated differently to suppress harmful modes while maintaining overall wavelength stabilization
2Reliability
If the fiber Bragg grating bandwidth is increased to capture more laser modes, then power sharing across modes improves and coherence is reduced, but the wavelength locking precision may be compromised
Solution Approach 1:
The asymmetric reflectivity profile concentrated on the short wavelength side enables the grating to provide both wide bandwidth for multiple mode capture and precise wavelength locking. The asymmetric shape creates a broader effective bandwidth while the skewed profile maintains a well-defined locking point, resolving the trade-off between bandwidth and precision
3Ease of manufacture
If a symmetric reflectivity profile is used in the fiber Bragg grating, then the manufacturing and design are simpler, but spectral hole burning and dominant modes cause coherence and power stability issues
Solution Approach 1:
The patent deliberately introduces asymmetry in the reflectivity profile to solve the power stability problem. While asymmetric gratings are more complex to manufacture than symmetric ones, the patent provides specific fabrication approaches using standard UV inscription techniques with appropriate phase mask designs, making the asymmetric configuration achievable with conventional manufacturing equipment
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 asymmetric chirped fiber Bragg grating reduces coherence and interference, stabilizes output power, and increases spectral width, thereby improving the performance of optical fiber amplifiers by suppressing spectral hole burning and enhancing power sharing across multiple modes.
Implementation Method 1
the refractive index variations have a first reflectivity for a short wavelength region of the fiber Bragg grating
Implementation Method 2
fiber Bragg grating configured to lock the pump light to the pump wavelength
Implementation Method 3
The refractive index variations have a chirped period changing spatially along a length of the fiber Bragg grating
Data Source
AI summary
A fiber amplifier to amplify seed light has a laser diode, an optical fiber segment, and a doped fiber. The laser diode generates pump light at a pump wavelength from an end facet, and optical fiber segment in optical communication with the pump light has a fiber Bragg grating (FBG) configured to lock the pump light from the end facets to the pump wavelength. The pump light from the laser diode interact with an active dopant of the doped fiber and can thereby amplifies the seed light. To provide less coherent light and improve stability of the laser diode over operation conditions, variations in refractive index in the FBG have a chirped period changing linearly along a length of the FBG. The chirped period shifts the reflectivity asymmetrically from a central wavelength region of the FBG, such as blue-shifting the reflectivity for a short wavelength.