Cladding-Pumped Optical Waveguide Reducing Photo Darkening
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Solution Overview
Problem
High power optical fibre lasers experience rapid photo darkening due to high pump and signal power intensities, leading to increased absorption and degradation, making it difficult to maintain consistent output over extended periods.
Innovation Solution
A cladding-pumped optical fibre waveguide design with reduced guided signal overlap, where the rare earth doped material is distributed in inner and outer cladding regions, minimizing the overlap between the signal core and the active material to reduce photo darkening effects.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If the rare earth doped material is concentrated in the signal core to maximize amplification, then the gain is improved, but the photo darkening increases due to high optical flux
Solution Approach 1:
The waveguide structure is segmented into distinct functional regions: the signal core for light guidance, the inner cladding for pump light absorption, and the outer cladding for mechanical protection. The rare earth doped material is specifically located in the inner cladding region rather than the signal core, separating the amplification function from the signal transmission function. This segmentation reduces the overlap between the signal optical flux and the rare earth ions, thereby reducing photo darkening while maintaining amplification capability.
Solution Approach 2:
The patent transitions from a traditional single-clad fiber structure to a double-clad waveguide structure, adding an additional spatial dimension for light management. The inner cladding is designed with a larger mode field area than the signal core, creating a spatial separation in the radial dimension. This dimensional change allows the pump light to propagate in the inner cladding while the signal light propagates in the smaller signal core, reducing their interaction and minimizing photo darkening effects.
2Reliability
If the signal core area is reduced to improve beam quality, then the single mode operation is improved, but the optical flux increases leading to faster degradation
Solution Approach 1:
The waveguide is segmented into a small signal core for high quality beam output and a larger inner cladding for pump light absorption. The rare earth doped material is placed in the inner cladding, creating a spatial separation that reduces the optical flux density on the doped material. This segmentation allows the signal core to maintain a small area for good beam quality while the doped inner cladding handles the high power pump light, reducing photo darkening and extending device lifetime.
Solution Approach 2:
The inner cladding acts as an intermediary structure between the pump light source and the signal core. It absorbs the pump light through the rare earth ions and provides gain to the signal light in the core, while its larger area distributes the optical flux over a broader region. This intermediary role protects the signal core from direct exposure to high intensity pump light, reducing photo darkening and extending the operational lifetime of the device.
3Productivity
If the pump power is increased to maintain output power, then the productivity is improved, but the photo darkening accelerates due to higher intensities
Solution Approach 1:
The patent uses the double-clad structure to add a spatial dimension for pump light propagation. The inner cladding has a larger mode field area than the signal core, allowing pump light to propagate in this extended radial dimension. This dimensional expansion distributes the pump power over a larger area, reducing the optical flux density on the rare earth ions and slowing down photo darkening, even at high productivity levels.
Solution Approach 2:
The patent changes the spatial distribution parameters of the optical fields by designing the inner cladding with a larger mode field area than the signal core. This parameter change in the waveguide geometry allows the pump light to be distributed over a larger volume, reducing the intensity parameter (power per unit area) that drives photo darkening, while still maintaining high overall productivity through sufficient pump power coupling.
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 design maintains the lasing or amplifier output power within 15% of the initial level for 10,000 hours or more, significantly reducing photodarkening-induced degradation and ensuring stable performance in applications like materials processing and surgery.
Implementation Method 1
the wavelength λpump is resonant with an absorption band of said rare earth doping
Implementation Method 2
create an inversion in a rare earth doped optical fibre laser
Implementation Method 3
absorption effects at gratings or inside the core can result in unwanted degradation of the device within a time frame which is substantially shorter than the lifetime required
Data Source
AI summary
The invention relates to a high power amplifier waveguide for amplifying an optical signal wherein photo darkening due to high optical flux is reduced considerably. This is achieved by providing a cladding pumped amplifier waveguide wherein the optical mode overlap to the active material of the waveguide is low and/or wherein the active material is distributed over a large cross sectional region of the waveguide.