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Final Report Summary - 3SMP (Advanced Snapshot Swept-Source Mueller Polarimeter)

A lecturer at the University of Brest (UBO), France, the fellow has developed a project with the Applied Optics Group (UoK, University of Kent, UK) to develop polarimetric fibred devices whose features are compatible with medical exploration. This requires that: 1) The devices have to measure the polarisation of light to provide information on the investigated tissue. 2) The measurement of the polarisation and the data processing are fast so that the device is able to give an image in real time. 3) The devices are fibred to be implemented, in a near future, in a hand held probe.

The fellowship has led to 5 studies that have covered all the aspects of this project, from the speed of the data processing to the strategies to obtain the polarimetric information through optical fibres.

Polarimetric prototype insensitive to the handling of the fibre by the user.
Polarimetric techniques present tremendous potential. However they are still limited in their scope, at least regarding their in vivo investigations. A first limitation is that complete determination of polarisation properties (via Mueller matrices) is performed by time-consuming sequential measurements. Another identified problem is that the invivo investigation relies on optic fibres which randomly change light polarisation, thus altering the polarimetric measurement of the sample.
The fellow's polarimetric prototype is based on a spectral coding on polarisation that permits to measure at high speed the full polarisation properties of the sample, and an interferometer that compensates the polarimetric distortion coming from the fibre in real time. This experimental study, focused on the calibration strategies, has paved the way to full-Mueller endoscopic imaging.

Polarisation-sensitive optical coherence tomography system tolerant to fibre disturbances.
Optical Coherence Tomography (OCT) is used to provide 3D real-time image of biological samples in depth. Being non-invasive and non-destructive, a number of OCT devices are increasingly used in hospitals for diagnosis assistance. For convenience, OCT devices are fibered so that the analysis of polarisation properties of the sample in depth is altered by the handle of the fibre.
Another polarimetric prototype, designed to measure retardance in depth, was built to be tolerant to fibre handling (Fig. 1). The generation and the analysis of polarisation are carried out in free space, at the output of the fibre that lights the sample.

Optical Module to transform any OCT device into a Polarisation-sensitive OCT.
Along the same lines, a theoretical study of the design of an optical module has been carried out. The purpose was to find a way to transform any OCT device into a Polarisation-sensitive OCT insensitive to fibre disturbances. Two key points are present in the design of this optical module: the same set of optical elements is used for the generation of the incident polarisation state on the sample and for the analysis of the polarisation state of the returned signal. This analysis is achieved by alternating between a 90 degree rotation and no rotation of the polarisation state of the returned signal, whilst projecting the resulting polarisation state against a linear polariser. In this way, the polarisation state of the light re-injected into the collecting fibre is always the same, and, since the polarimetric measurement has already taken place, anything that happens to the polarisation state after this point does not affect the measurement itself. Thus the polarimetric measurement of the sample is insensitive to the fibre.

Fast data processing for Polarisation OCT: Complex Master Slave Interferometry (CMSI)
The fellow has revisited the concept of Master Slave Interferometry initially developed by the host group so that the data processing takes the phase of the signal into account and uses it to provide information on the sample polarisation. The image in depth obtained by OCT is the result of calculations based on data resampling and Fourier transformation, which are time-consuming and limit the real-time aspect. This new tool is called Complex Master Slave Interferometry (CMSI) and is based on a discrete integral transform that integrates the characteristics of the interferometer measured in a calibration step.

CMSI for long axial images of the anterior segment of human eyes.
CMSI, combined with the coherence revival regime of a swept-source laser, was used to obtain long axial images of the anterior segment of human eyes (Fig. 2). CMSI has brought an improvement in the speed of displaying cross-sectional images at high rates without the need of extra hardware such as graphics processing units or field programmable gate arrays.
Moreover CMSI has been applied to an original snapshot polarisation sensitive OCT built with optical passive elements. This study is to be submitted in the near future to a journal in optics.

Finally, the fellowship has been a major contribution to the fellow's career development as shown by the following achievements:
(i) training by research to enhance his skills in Biophotonics and imaging technique. The fellow has 1) developed original OCT devices to image for instance the anterior segment of human eyes, 2) attended several talks on PGA and GPU computing, central nervous system, elements of histopathology in the module Biomedical Optics (University of Kent), 3) met specialists in OCT and in eye surgery during seminars organized at the University of Kent and even during an observation session of eye surgery in an operating theatre.
(ii) training in teaching in English at a research University level. The fellow has 1) taught polarisation for PhD students at the University of Kent and 2) supervised 2 PhD students and 3) attended 40 hour English Classes at UoK’s Centre for English and World Languages.
(iii) training in the development and management of an international project. The fellow has found a one month-grant at his own University for a colleague of the host laboratory to visit the University of Brest and collaborate on the development of a polarimetric nonlinear microscope.
(iv) writing a patent about CMSI: PATENT APPLICATION NUMBER GBPO: 1519055.6
(v) participating in prestigious international symposia (two presentations in SPIE BiOS at San Fransisco and one in Hamlyn Symposium in London) and publishing in high-quality peer-reviewed international Journals (5 papers whose impact factor is superior to 2)

All these experiences have permitted the fellow to obtain the French highest degree (Habilitation à Diriger des Recherches), a qualification that allows an academic to supervise PhD theses in March 2016.

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