Cascaded Fiber Gain Stages With Low-Loss Multimode Splicing

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Solution Overview

Problem

High power continuous-wave fiber sources face challenges in maintaining single-mode performance due to increased fiber core size, which leads to excess splice loss and multimode propagation, necessitating innovative solutions to reduce optical loss and maintain beam quality.

Innovation Solution

The use of an optical gain fiber with a specific core and cladding numerical aperture, optically coupled to a multimode fiber with a larger core diameter and numerical aperture, stabilizes the beam parameter product and reduces optical loss by selecting core diameters and numerical apertures to exceed the unstable threshold, thereby achieving low-loss coupling.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If fiber core diameter is increased to prevent non-linear effects and increase pump absorption, then power handling capability is improved, but splice loss increases due to modal mismatch

Engineering Contradiction:

Improvepower handling capabilityVSAvoidsplice loss

Core Design Contradiction:

PowerVSLoss of energy

Solution Approach 1:

The patent changes the numerical aperture parameter of the receiving fiber to be larger than that of the transmitting fiber. This parameter change allows the receiving fiber to better accommodate the modal distribution from the larger core transmitting fiber, reducing splice loss while maintaining the power handling benefits of large core diameter

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent applies different numerical aperture characteristics to different parts of the fiber system - the transmitting fiber has one numerical aperture optimized for power handling, while the receiving fiber has a larger numerical aperture optimized for low-loss coupling. This local differentiation resolves the contradiction between power capability and coupling efficiency

Inventive Principle:

Principle #3Local quality

2Productivity

If fiber core diameter is increased to increase pump absorption, then amplification efficiency is improved, but beam quality deteriorates due to multimode propagation

Engineering Contradiction:

Improveamplification efficiencyVSAvoidbeam quality

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The patent segments the fiber system into two distinct functional sections: a large core transmitting fiber for efficient pump absorption and amplification, and a receiving fiber with larger numerical aperture that accommodates the multimodal beam while maintaining stable propagation. This segmentation allows each section to be optimized for its specific function without compromising the other

Inventive Principle:

Principle #1Segmentation

3Manufacturing precision

If numerical aperture is decreased to maintain single-mode propagation, then beam quality is improved, but coupling efficiency deteriorates

Engineering Contradiction:

Improvebeam qualityVSAvoidcoupling efficiency

Core Design Contradiction:

Manufacturing precisionVSLoss of energy

Solution Approach 1:

Instead of decreasing the numerical aperture to maintain beam quality, the patent inverts the approach by increasing the numerical aperture of the receiving fiber. This inversion allows the receiving fiber to better match the modal distribution from the transmitting fiber, improving coupling efficiency while the beam quality is maintained through stable multimode propagation

Inventive Principle:

Principle #13The other way round (Inversion)

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 results in substantially reduced optical loss, typically below 0.5% of total optical power, while maintaining stable beam parameter products and preserving fundamental mode propagation, even at high power levels, thus enhancing the reliability and performance of cascaded fiber laser systems.

Implementation Method 1

the multimode fiber being optically coupled to the optical gain fiber so as to receive an optical beam propagating in the optical gain fiber

Methodology Applied

Scientific EffectOptical coupling: Refraction

Implementation Method 2

the core and cladding defining an optical gain fiber numerical aperture... the core and cladding of the multimode fiber defining a multimode fiber stable numerical aperture

Methodology Applied

Scientific EffectTotal internal reflection: Total Internal Reflection

Data Source

PatentUS11811186B2Fiber source with cascaded gain stages and/or multimode delivery fiber with low splice loss

Publication Date: 2023.11.07 NLIGHT INC

AI summary

An apparatus includes an optical gain fiber having a core, a cladding surrounding the core, the core and cladding defining an optical gain fiber numerical aperture, and a multimode fiber having a core with a larger radius than a radius of the optical gain fiber core, a cladding surrounding the core, the core and cladding of the multimode fiber defining a multimode fiber stable numerical aperture that is larger than the optical gain fiber numerical aperture, the multimode fiber being optically coupled to the optical gain fiber so as to receive an optical beam propagating in the optical gain fiber and to stably propagate the received optical beam in the multimode fiber core with low optical loss associated with the optical coupling.