Full Poincaré generator for polarisation sensing

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Polarisation sensing is vital in many areas of research, with applications spanning from microscopy to aerospace technology. These industries often struggle to perform effective visual inspection due to low contrast or highly reflective imaging conditions. Relying on polarisation properties of light can help uncover hidden material properties or provide significantly enhanced visual clarity.

While the potential benefits of polarisation sensing are evident, traditional approaches to polarimetry may be cumbersome. Additionally, the accuracy of current methods is lower than desirable, so there is a huge appetite in the industry for improvements to measurement precision and system sensitivity.

Traditional polarisation sensing methods can be divided into two categories: “time-resolved”, where measurements are taken using a sequence of analysers in a time multiplexed manner, or “snapshot”, where different analysers are spatially multiplexed. Time-resolved measurements can be easier to implement, but snapshot methods are crucial for applications with rapidly changing inputs.

For both methods, the polarisation state of an incoming signal is determined by a matrix multiplication with a vector containing the set of intensity measurements.

A new method has been developed by researchers at the University of Oxford with significant benefits over the known techniques. The new method takes a different approach of using a complex optical element to provide – in effect – a complete set of polarisation elements. This system is known as a Full Poincaré Generator. The complex optical element provides an intensity pattern that is characteristic of the polarisation state. This intensity pattern can be interpreted using an algorithm such as a neural network.

The complex optical element that has enabled this development is a graded refractive index (GRIN) lens. GRIN lenses have radially changing birefringence resulting from the fabrication process. This can be seen as an undesirable property, but in this implementation, it is harnessed to great effect.

The performance of this new set-up is far superior to known methods of polarisation sensing.

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