Large-Mode-Area Fiber Pumping for High Power
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Solution Overview
Problem
High power optical amplifiers and lasers using Er-Yb co-doped fibers face limitations due to high phosphorus content, which raises the core index, limits mode-field-area, and results in heat generation, while alternative designs with single-mode fibers have limited energy storage capacity and require complex bulk optical components.
Innovation Solution
A multimode, gain-producing optical fiber with a high brightness pump source that is core-pumped, using a fiber-based mode expander to increase the core area and ensure perfect overlap of pump and signal light, reducing nonlinear effects and ASE, and employing multiple pump sources for enhanced gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If Er-Yb co-doped fibers are used to achieve high pump absorption and gain per unit length, then pump absorption efficiency is improved, but the core index is raised by phosphorus content which limits the maximum achievable mode-field-area and makes the fiber highly multimoded
Solution Approach 1:
The patent extracts the phosphorus doping from the core region and relocates it to the cladding region. This separation allows the core to maintain a lower refractive index suitable for large mode-field-area operation, while the cladding provides the necessary pump absorption through phosphorus doping. The pump absorption function is thus extracted from the core and placed in the cladding.
Solution Approach 2:
The patent applies different doping strategies to different regions of the fiber: the core is kept undoped or lightly doped with phosphorus to maintain low refractive index and support large mode-field-area, while the cladding is heavily doped with phosphorus to provide high pump absorption. This local differentiation of doping concentration resolves the contradiction between pump absorption efficiency and mode-field-area.
2Productivity
If high concentration of core dopant is used to achieve high pump absorption over short fiber length, then pump absorption per unit length is improved, but nonlinear effects, amplified spontaneous emission (ASE), and signal re-absorption increase
Solution Approach 1:
The patent extracts the high concentration phosphorus doping from the core and places it in the cladding. This allows the core to maintain low dopant concentration, reducing nonlinear effects and ASE, while the cladding provides high pump absorption capability. The harmful effects are thus separated from the pump absorption function.
Solution Approach 2:
The cladding acts as an intermediary structure that enables high pump absorption without requiring high dopant concentration in the core. The phosphorus-doped cladding serves as a mediator that absorbs pump energy and transfers it to the core, allowing the core to remain lightly doped and avoid nonlinear effects.
3Use of energy by moving object
If Er-Yb co-doped fibers are pumped at wavelengths between 900 nm and 1000 nm, then pump absorption is achieved, but quantum efficiency for gain at 1500 nm is low and significant heat is generated
Solution Approach 1:
The patent changes the pump wavelength parameter from the conventional 900-1000 nm range to 1480 nm, which is closer to the signal wavelength of 1500 nm. This parameter change improves quantum efficiency by reducing the energy gap between pump and signal photons, thereby reducing heat generation while maintaining effective pump absorption through the phosphorus-doped cladding.
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
Achieves high peak power pulses with increased energy storage capacity and reduced nonlinear impairments, maintaining a clean beam profile and efficient energy transfer, as demonstrated by amplification of nanosecond pulses with record peak power and low M2 values.
Implementation Method 1
the core is doped with a gain-producing species and then pumped by an optical pump at a wavelength that is absorbed by the species
Implementation Method 2
pump light that is coupled directly into the core
Data Source
AI summary
Optical apparatus includes a multimode, gain-producing fiber for providing gain to signal light propagating in the core of the fiber, and a pump source for providing pump light that is absorbed in the core, characterized in that (i) the pump source illustratively comprises a low brightness array of laser diodes and a converter for increasing the brightness of the pump light, (ii) the pump light is coupled directly into the core, and (iii) the area of the core exceeds approximately 350 μm2. In one embodiment, the signal light propagates in a single mode, and the pump light co-propagates in at least the same, single mode, both in a standard input fiber before entering the gain-producing fiber, and a mode expander is disposed between the input fiber and the gain-producing fiber. In another embodiment, multiple pumps are coupled into the core of the gain-producing fiber. The pumps may generate light of the same wavelength or of different wavelengths. In accordance with a particular embodiment of our invention, we have demonstrated amplification of nanosecond optical pulses at 1545 nm in a single clad Er-doped fiber having a core area of 875 μm2; the core was pumped by a high brightness Raman laser at 1480 nm; and the pulses had a record peak power of several hundred kW.