Fiber-to-Rod Multimode Amplifier for High-Power Single-Mode Lasers
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Solution Overview
Problem
High power fiber laser systems face challenges such as nonlinear effects, loss of fundamental mode power to high order modes, pump brightness limitations, and excessive heat generation, which restrict power scaling and require bulky, cumbersome designs that are not suitable for field use.
Innovation Solution
A compact, monolithic fiber-to-rod fiber booster stage with a continuous multimode core and cladding, where the core expands to a bottleneck shape, allowing counter-propagating pump light coupling and minimizing nonlinear effects, and a coreless termination block to manage high power density, enabling efficient absorption of pump light and emission of high power in a fundamental mode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the core diameter of SM/LM active fibers is increased to achieve high power levels, then power output is improved, but beam quality degrades due to increasing high order modes
Solution Approach 1:
The amplifier is divided into two distinct sections: a single-mode section for signal input and a multimode section for power amplification. This segmentation allows each section to operate in its optimal mode regime, with the single-mode section preserving beam quality and the multimode section enabling high power output through larger core diameter.
Solution Approach 2:
A mode-matching element (mode converter) is introduced as an intermediary component between the single-mode and multimode fiber sections. This converter transforms the single-mode field distribution into a multimode field distribution that optimally excites the fundamental mode of the large-core fiber, enabling efficient power transfer while maintaining beam quality.
2Power
If clad pumping is used to achieve high powers, then power output is improved, but the length of active fibers must be increased which lowers the threshold for nonlinear effects
Solution Approach 1:
The amplifier is divided into two distinct sections: a single-mode section for signal input and a multimode section for power amplification. This segmentation allows each section to operate in its optimal mode regime, with the single-mode section preserving beam quality and the multimode section enabling high power output through larger core diameter.
Solution Approach 2:
The numerical aperture of the multimode fiber is greatly reduced compared to conventional designs. This parameter change increases the mode field diameter, which improves pump light coupling efficiency and reduces the required amplifier length, thereby raising the threshold for nonlinear effects while maintaining high power output capability.
3Manufacturing precision
If a double clad fiber with mode converter is used, then beam quality is improved, but the system becomes bulky and not rugged for field use
Solution Approach 1:
The single-mode and multimode fiber sections are permanently joined through fusion splicing to form a single integrated amplifier assembly. The mode-matching element is also permanently coupled to the fiber ends. This merging eliminates the need for separate, adjustable mode converters and creates a compact, ruggedized device suitable for field deployment.
Solution Approach 2:
The mechanically adjustable mode converter is replaced with a permanently spliced fiber assembly that has built-in mode-matching characteristics. This substitution eliminates mechanical adjustment mechanisms, reducing device complexity and improving ruggedness while maintaining beam quality through optimized fiber geometry and splicing.
4Manufacturing precision
If tapered fiber is used as mode matching element, then mode conversion is achieved, but splice losses and beam distortion are sharply increased
Solution Approach 1:
Instead of using expensive, complex tapered fiber assemblies that are difficult to splice, the invention uses standard single-mode and multimode fibers with optimized parameters that can be directly fusion spliced. The mode matching is achieved through the inherent field distribution properties of the fibers and the splicing process itself, eliminating the need for specialized tapered components.
5Manufacturing precision
If fiber rod with small NA core is used, then fundamental mode output is maintained, but pump light coupling efficiency is limited
Solution Approach 1:
The amplifier is divided into two distinct sections: a single-mode section for signal input and a multimode section for power amplification. This segmentation allows each section to operate in its optimal mode regime, with the single-mode section preserving beam quality and the multimode section enabling high power output through larger core diameter.
Solution Approach 2:
The numerical aperture of the multimode fiber is greatly reduced compared to conventional designs. This parameter change increases the mode field diameter, which improves pump light coupling efficiency and reduces the required amplifier length, thereby raising the threshold for nonlinear effects while maintaining high power output capability.
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 system achieves kW average and MW peak powers with improved beam quality and reduced nonlinear effects, maintaining a compact and portable design suitable for field use without splice losses or heat-related hazards.
Implementation Method 1
The continuous core is doped with one or more light absorbers
Implementation Method 2
at least one reflective element spaced from output ends of respective amplifier and pump fiber and configured to couple pump light into core of amplifier in direction counter to signal propagation direction
Implementation Method 3
The output diffraction limited signal beam is further guided through a central opening of mirror 22 in the signal propagating direction, as will be disclosed in detail herein below.
Implementation Method 4
a monolithic fiber-to-rod fiber amplifier directly delivering signal light in substantially a fundamental mode to a laser head over free space
Data Source
Figure 1~4
AI summary
A high power single mode ("SM") laser system includes an amplifier configured with a monolithic fiber to rod fiber waveguide which is structured with a multimode ("MM") core and at least one cladding surrounding the core. The MM core is configured with a small diameter uniform input region receiving and guiding a SM signal light, a mode-transforming frustoconica! core region expanding outwards from the input region and a relatively large diameter uniform output position. The high power laser system is further structured with a MM pump light delivery fiber having a numerical aperture NA2, which is at most equal to that one of the output core portion. The amplifier and pump light output fiber traverse an unconfined deli very cable and terminate upstream from a mirror which is configured to focus the incident pump light into the core of the amplifier in a counter-propagating direction. The mirror is further structured with an opening aligned with the optical axis of the amplifier and configured to provide a lossless passage of amplified signal light in a propagating direction.