Single-Frequency Fiber Amplifier SBS Suppression
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Solution Overview
Problem
High-power fiber lasers with narrow linewidths are limited by stimulated Brillouin scattering (SBS), which reduces output power and beam quality, particularly in applications requiring kilowatt power levels and narrow linewidths like LIDAR and gravitational-wave detection.
Innovation Solution
The use of gain fibers with different absorptions and core sizes, along with varying glass chemistry, to suppress SBS by reducing the effective fiber length and avoiding Stokes peak overlap, combined with a cladding light stripper to remove pump and high-angle signal light, allows for high power and narrow linewidth operation without degrading beam quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the fiber length is increased to absorb more pump light and achieve higher gain, then the output power increases, but the stimulated Brillouin scattering (SBS) threshold decreases due to longer effective fiber length
Solution Approach 1:
The gain fiber is divided into multiple segments with different core diameters and glass chemistries. The first segment has larger core diameter and higher pump absorption, while subsequent segments have smaller core diameters. This segmentation allows each segment to contribute differently to gain and SBS suppression, achieving both high output power and high SBS threshold.
Solution Approach 2:
Different segments of the gain fiber are assigned different local properties: the first segment has larger core diameter and higher absorption for efficient pump capture, while subsequent segments have smaller core diameters for better beam quality and SBS suppression. The glass chemistry is also varied locally to shift Stokes peaks and prevent overlap, creating optimal conditions for high-power narrow-linewidth operation at each position along the fiber.
2Use of energy by moving object
If the core diameter is increased to improve pump absorption efficiency, then the gain per unit length increases, but the beam quality deteriorates
Solution Approach 1:
The gain fiber is segmented into multiple sections with progressively smaller core diameters. The first segment uses larger core diameter to maximize pump absorption efficiency, while subsequent segments use smaller core diameters to maintain good beam quality. This segmentation allows the system to capture pump energy efficiently in early segments while preserving beam quality in later segments.
Solution Approach 2:
Each segment of the gain fiber is optimized with specific local properties: larger core diameter in the first segment for high pump absorption, and progressively smaller core diameters in subsequent segments for improved beam quality. This local optimization of structural parameters resolves the contradiction between absorption efficiency and beam quality.
3Measurement precision
If the linewidth is narrowed to achieve single-frequency operation, then the spectral purity improves, but the SBS threshold decreases
Solution Approach 1:
The glass chemistry is varied locally in different segments of the gain fiber to create different acoustic velocities and shift Stokes peaks to non-overlapping frequencies. This allows the system to operate with narrow linewidth for spectral purity while the Stokes peaks from different segments do not overlap, maintaining high SBS threshold even with narrow linewidth operation.
Solution Approach 2:
The system changes the physical-chemical parameters of the gain fiber, specifically the glass composition, to modify the acoustic velocity and shift the Stokes peak frequencies. By adjusting the glass chemistry in different segments, the Stokes peaks are separated in frequency, allowing narrow linewidth operation without SBS feedback, thus maintaining both spectral purity and high SBS threshold.
4Use of energy by moving object
If the ratio of core to cladding diameter is increased to achieve higher absorption, then the pump absorption efficiency improves, but the mode area increases leading to worse beam quality
Solution Approach 1:
The gain fiber structure is segmented into multiple sections, each with optimized core-to-cladding diameter ratios. The first segment has a larger ratio for high pump absorption efficiency, while subsequent segments have smaller ratios to maintain smaller mode areas and better beam quality. This segmentation allows the system to achieve high overall absorption while preserving beam quality through progressive core diameter reduction.
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 increases the power threshold for SBS onset, enabling high-power output while maintaining good beam quality, suitable for applications requiring kilowatt power and narrow linewidths.
Implementation Method 1
The first gain fiber has a first core diameter and a first absorption at a pump beam wavelength and guides and amplifies a signal beam. The second gain fiber is coupled to the first gain fiber and has a second core diameter smaller than the first core diameter and a second absorption at the pump beam wavelength lower than the first absorption. The second gain fiber further guides and amplifies the signal beam.
Implementation Method 2
The cladding light stripper can include a bare fiber waveguide with an etched outer surface coupled to an etched coreless endcap. The base fiber waveguide and/or the etched coreless endcap has a first refractive index surrounded circumferentially by a cladding with a second refractive index higher than the first refractive index.
Implementation Method 3
Stimulated Brillouin scattering (SBS) is a major obstacle to attaining high-power fiber lasers with narrow linewidths. SBS occurs when a signal-beam photon is converted into a phonon and a lower-frequency scattered photon via the optical fiber's χ(3) nonlinearity. The power threshold for SBS onset, also called the SBS critical power, depends on the mode area of the signal beam, the effective fiber length, and the SBS gain coefficient.
Data Source
AI summary
Stimulated Brillouin scattering (SBS) limits the maximum power in fiber lasers with narrow linewidths. SBS occurs when the power exceeds a threshold proportional to the beam area divided by the effective fiber length. The fiber lasers disclosed here operate with higher SBS power thresholds (and hence higher maximum powers at kilohertz-class linewidths) than other fiber lasers thanks to several techniques. These techniques include using high-absorption gain fibers, operating the laser with low pump absorption (e.g., ≤80%), reducing the length of un-pumped gain fiber at the fiber output, foregoing a delivery fiber at the output, foregoing a cladding light stripper at the output, using free-space dichroic mirrors to separate signal light from unabsorbed pump light, and using cascaded gain fibers with non-overlapping Stokes shifts. The upstream gain fiber has high absorption and a larger diameter for high gain, and subsequent gain fiber has a smaller diameter to improve beam quality.