Free Space Fibre-to-Fibre Coupling Using Below Cut-Off Wavelengths
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Solution Overview
Problem
Fibre-to-fibre coupling in optical devices is challenging due to changes in mode field diameter and numerical aperture with wavelength, leading to inefficiencies in collimating profiles and coupling efficiency across a broad spectral range.
Innovation Solution
Operating optical fibres below their cut-off wavelength, maintaining fibre lengths short enough to ensure single mode propagation, and using achromatic collimating lenses to stabilize the mode field diameter and numerical aperture, thereby desensitizing the system to wavelength changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional single mode fibres are operated above cut-off wavelength, then single mode propagation is achieved, but mode field diameter and numerical aperture change significantly with wavelength causing poor collimating profile and low coupling efficiency
Solution Approach 1:
The patent operates optical fibres below their cut-off wavelength rather than above it, fundamentally changing the operating parameter regime. This inversion of the conventional approach (operating below rather than above cut-off) results in fibres that maintain essentially single mode propagation while exhibiting reduced spectral variation in mode field diameter and numerical aperture, thereby resolving the contradiction between maintaining single mode operation and achieving broad spectral coupling efficiency
2Quantity of substance
If fibre length is increased, then more fibre can be used in the system, but higher order modes are more easily excited and single mode operation is compromised
Solution Approach 1:
By changing the operating wavelength parameter to below cut-off, the patent enables single mode operation to be maintained over extended fibre lengths. The reduced spectral variation in mode field diameter below cut-off wavelength decreases the susceptibility to mode coupling from bends and perturbations, allowing longer fibre lengths without compromising single mode operation
3Stability of the object's composition
If fibre is operated below cut-off wavelength, then spectral variation of mode field diameter is reduced, but multimode propagation may occur if fibre is too long or perturbed
Solution Approach 1:
Operating below cut-off wavelength fundamentally changes the propagation characteristics, reducing the spectral variation of mode field diameter and numerical aperture. This parameter change stabilizes the optical characteristics across the spectrum while maintaining essentially single mode propagation, provided fibre length and perturbation conditions are controlled
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 maintains essentially single mode propagation and reduces spectral variation of the mode field diameter and numerical aperture, enhancing coupling efficiency across an extended wavelength range.
Implementation Method 1
using achromatic collimating lenses to stabilize the mode field diameter and numerical aperture, thereby desensitizing the system to wavelength changes
Implementation Method 2
Single mode optical fibres are designed to support only a single propagation mode per polarization direction for a given wavelength
Data Source
AI summary
A method of free space fibre-to-fibre coupling includes providing a first optical fibre and a second optical fibre spaced apart from one another by a free space distance having a pair of spaced-apart achromatic collimating lenses within the free space distance. Essentially single mode light is launched from the first fibre at a wavelength λ or a range of λs that provides at least a portion of its light at wavelength(s) below a cut-off wavelength for the first fibre. The first optical fibre and second optical fibre are both <50 m and sufficiently short to provide essentially single mode operation. The essentially single mode light after the launching propagates as free space light through said free space distance and is received by and propagates in the second optical fibre as essentially single mode light.