Plasma Deposition for Optical Preforms with Thermal Gradient Control
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Solution Overview
Problem
The existing plasma outside deposition (POD) methods for producing optical preforms with fluorine-doped quartz glass cladding layers face challenges in achieving axially homogeneous fluorine doping due to temperature gradients, which can lead to inadequate vitrification and mechanical stress, resulting in preform cracking.
Innovation Solution
The method involves using thermal elements to counteract excessive cooling at one turning point by applying direct or indirect heating at the other turning point, maintaining a higher mean temperature and reducing the axial temperature gradient, allowing for higher fluorine incorporation and reduced stress risks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the plasma torch is reversingly moved along the core rod to deposit fluorine-doped SiO2 layer by layer, then the cladding glass layer is formed with fluorine doping, but axial temperature gradients cause non-uniform fluorine incorporation and preform cracking
Solution Approach 1:
The patent applies local quality by introducing a thermal element that selectively heats the end portions of the core rod where excessive cooling occurs during plasma torch reciprocation. This creates a localized temperature adjustment at specific axial positions (the turning points) to compensate for the non-uniform cooling effects, thereby achieving more uniform fluorine incorporation along the entire axial length of the preform.
Solution Approach 2:
The thermal element provides preliminary anti-action by pre-heating the end portions of the core rod before the plasma flame reaches them during the reciprocating motion. This anticipatory heating counteracts the excessive cooling that would otherwise occur at the turning points, preventing temperature gradients that lead to non-uniform doping and potential cracking.
2Quantity of substance
If the core rod temperature is reduced to increase fluorine doping, then fluorine incorporation improves, but vitrification of the deposited layer becomes inadequate
Solution Approach 1:
The thermal element creates local quality by maintaining higher temperatures specifically at the end portions of the core rod where the plasma flame temporarily resides during reciprocation. This localized heating ensures adequate vitrification at these critical positions while allowing the overall process temperature to be optimized for fluorine incorporation, resolving the contradiction between fluorine doping efficiency and vitrification quality.
Solution Approach 2:
The thermal element performs preliminary action by pre-heating the end portions of the core rod before plasma deposition occurs at these locations. This ensures that when the plasma flame reaches these areas, the substrate is already at an appropriate temperature for both fluorine incorporation and proper vitrification of the deposited cladding layer.
3Productivity
If the plasma torch moves quickly to increase productivity, then deposition speed improves, but temperature distribution becomes more non-uniform
Solution Approach 1:
The thermal element acts as an intermediary by introducing an additional heat source that mediates the temperature distribution along the core rod. This intermediary heating mechanism compensates for the non-uniform temperature effects caused by faster plasma torch movement, allowing high deposition speeds to be maintained while achieving more uniform axial temperature distribution and fluorine doping.
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 achieves a high fluorine concentration in the cladding glass layer with a refractive index reduction of over 27×10−3, while minimizing the risk of preform cracking by flattening temperature gradients and maintaining a higher mean temperature, enabling efficient vitrification with a lower plasma flame temperature.
Implementation Method 1
a plasma torch is used that is fed with a silicon compound, oxygen and a fluorine compound and is reversingly moved along the core rod rotating about its longitudinal axis
Implementation Method 2
SiO2 particles are formed by means of a plasma torch in the presence of fluorine and are deposited layer by layer on the cylinder outer surface
Implementation Method 3
are vitrified to form the cladding glass layer
Implementation Method 4
The deposition process is carried out within a vertically oriented chamber with movable core rod which is moved upwards and downwards along its entire length to be coated along the stationary plasma torch
Implementation Method 5
thermal elements to counteract excessive cooling at one turning point by applying direct or indirect heating at the other turning point
Implementation Method 6
maintaining a higher mean temperature and reducing the axial temperature gradient
Data Source
AI summary
In plasma deposition processes for producing an optical preform according to the POD method, a cladding glass layer composed of fluorine-doped quartz glass is produced by means of a plasma torch on a cylindrical substrate body composed of quartz glass, said substrate body rotating about the longitudinal axis thereof. In this case, the plasma torch performs a reversing relative movement between two turning points (A; B) along the substrate body. In order, proceeding therefrom, to achieve a high fluorine doping in conjunction with a dopant distribution that is as uniform as possible axially, the invention proposes that a heat element has a heating effect on the region of one turning point (A; B) when the plasma torch is situated in the region of the other turning point (B; A).