Depolariser With Power Splitter And Rotator For Low DOP
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Solution Overview
Problem
Existing depolarizers for broadband light sources, such as superluminescent diodes, face challenges in achieving low degree of polarization (DOP) performance across a wide spectral range due to spectral modulations, leading to issues like birefringence when used in medical applications like catheters for artery diagnostics.
Innovation Solution
A depolarizer system with a power splitter that splits optical signals into two arms with an optical path difference greater than the coherence length, a polarisation rotator to orthogonalize the signals, and a variable optical attenuator to equalize power, achieving low DOP performance with lower insertion losses compared to traditional designs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If all-fibre Lyot-type depolarisers are used for broadband light sources, then depolarisation is achieved, but spectral modulations occur causing non-uniform DOP performance across the spectral range
Solution Approach 1:
The broadband spectrum is divided into multiple wavelength bands using a dispersive element (prism or grating). Each wavelength band is directed through a separate optical path with tailored depolarisation elements, allowing independent optimization for each spectral region. This segmentation eliminates spectral modulations by ensuring uniform depolarisation across all bands rather than treating the broadband light as a single entity.
Solution Approach 2:
Different depolarisation mechanisms or optical path configurations are applied to different wavelength regions. For example, specific birefringent elements or fibre orientations are selected for particular spectral bands to achieve optimal depolarisation performance locally, rather than using a uniform approach across the entire spectrum. This local optimization ensures consistent DOP performance throughout the broadband range.
2Reliability
If traditional depolariser designs are used, then depolarisation is achieved, but insertion losses are high
Solution Approach 1:
The patent replaces traditional mechanical depolarisation methods (such as rotating wave plates or mechanically adjustable birefringent elements) with a static optical system using dispersive elements and fixed optical paths. This substitution eliminates mechanical losses and reduces insertion loss while maintaining effective depolarisation. The system uses the natural dispersion of light combined with stationary optical components rather than moving parts that introduce additional losses.
3Adaptability or versatility
If broadband SLEDs are used as light sources, then wide spectral range is achieved, but high DOP causes birefringence issues in optical fibres
Solution Approach 1:
The depolarisation process is performed preliminarily, before the broadband light enters the optical fibre system. By removing polarisation effects upstream using the dispersive depolariser, the light is pre-conditioned to prevent birefringence issues from arising in the fibre. This preliminary action eliminates the need for downstream polarisation management and prevents harmful birefringence effects before they can occur.
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 proposed depolarizer design achieves low DOP performance over a wide spectral range with minimal insertion losses, suitable for broadband light sources, enhancing the reliability of medical and optical applications by reducing birefringence effects.
Implementation Method 1
an optical delay line arranged in one of the first arm and the second arm to introduce an optical path difference between the first and second arms that is at least 50 times greater than the coherence length of the light source
Implementation Method 2
a polarisation rotator arranged in the first arm and operable to rotate the polarisation state of the first optical signal portion to be orthogonal to that of the second optical signal portion
Implementation Method 3
a beam combiner arranged to receive and recombine at least the first optical signal portion after rotation by the polarisation rotator and the second optical signal portion to form an output optical signal
Data Source
Figure 1~2A
Figure 2B~3
Figure 4~5
AI summary
A depolariser for a broadband optical source to split the source beam by power, not by polarisation state, and route the components into respective light paths. A polarisation rotator arranged in one of the light paths rotates the polarisation state of that beam component to make it orthogonal to that of the other. The components are then recombined by a combiner and output. A variable optical attenuator is arranged in one of the light paths, which during operation is adjusted by a controller to maintain power equalisation between the light paths and hence depolarisation performance. The controller receives power measurements from the light paths and from after the combiner via respective sensors. With this feedforward design reminiscent of a Mach-Zehnder interferometer the light from a light source which generates highly polarised light can be depolarised in theory with zero insertion loss and in practice with losses of about 1 dB.