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EP-4737870-A1 - A MULTI-CHANNEL OPTICAL SYSTEM FOR SPECTROMETRY, POLARIMETRY AND SPECTRO-POLARIMETRY AND AN OPTICAL INSTRUMENT

EP4737870A1EP 4737870 A1EP4737870 A1EP 4737870A1EP-4737870-A1

Abstract

The invention relates to a multi-channel optical system based on pupil division for spectrometry, polarimetry and spectro-polarimetry. In particular, the system comprises an entrance pupil for receiving an optical beam, the entrance pupil aperture allowing a multiple number of beam channels to pass. The optical system also comprises first optics for imaging the corresponding number of beam channels to an intermediate image, and a field stop spatially limiting the corresponding number of beam channels in the intermediate image. Further, the optical system includes second optics for imaging the spatially limited intermediate image to a pupil image, and a pupil separation module angularly separating the corresponding number of beam channels propagating from the pupil image. In addition, a multiple number of filters for filtering the corresponding number of beam channels is included.

Inventors

  • CARON, Jerôme Fabien

Assignees

  • Nederlandse Organisatie voor toegepast- natuurwetenschappelijk onderzoek TNO

Dates

Publication Date
20260506
Application Date
20241029

Claims (14)

  1. A multi-channel optical system for spectrometry, polarimetry and spectro-polarimetry, comprising: - an entrance pupil for receiving an optical beam, the entrance pupil aperture allowing a multiple number of beam channels to pass; - first optics for imaging the corresponding number of beam channels to an intermediate image; - a field stop spatially limiting the corresponding number of beam channels in the intermediate image; - second optics for imaging the spatially limited intermediate image to a pupil image; - a pupil separation module angularly separating the corresponding number of beam channels propagating from the pupil image, and - a multiple number of filters for filtering the corresponding number of beam channels.
  2. An optical system according to claim 1, wherein the pupil separation system includes a multiple number of faceted mirror elements, a multiple number of prisms optically interacting with the corresponding number of beam channels in the pupil image, or a phase plate with metamaterials.
  3. An optical system according to claim 1 or 2, wherein the multiple number of filters are located in the corresponding number of sub apertures of the entrance pupil aperture.
  4. An optical system according to claim 1 or 2, wherein the multiple number of filters are located in the pupil separation module.
  5. An optical system according to any of the preceding claims, wherein the field stop includes a field mask provided with a mask aperture.
  6. An optical system according to any of the preceding claims, wherein the field stop is arranged for preventing spatial overlap between respective beam channels in final images received by a detector.
  7. An optical system according to any of the preceding claims, wherein the number of the sub apertures in the entrance pupil aperture is at least two.
  8. An optical system according to any of the preceding claims, wherein the multiple number of filters are spectral filters, implementing the multi-channel optical system as a spectrometer.
  9. An optical system according to any of the preceding claims, wherein the multiple number of filters are polarimetric filters, implementing the multi-channel optical system as a polarimeter.
  10. An optical system according to any of the preceding claims 1-8, wherein the multiple number of filters include spectral filters and polarimetric filters, for spectro-polarimetry applications.
  11. An optical system according to any of the preceding claims, wherein the entrance pupil aperture is divided into a multiple number of virtual or materialized sub apertures for generating the corresponding number of beam channels.
  12. An optical instrument, comprising - a multi-channel optical system according to any of the preceding claims, and - a detector to receive the angularly separated final images.
  13. An optical instrument according to claim 12, further comprising focus optics to focus the separated beams from the multi-channel optical system in each of the channels onto the final images received by the detector.
  14. An optical instrument according to claim 12 or 13, adapted for remote sensing, microscopy or applications requiring measurement of samples at finite distances, such as in scientific research, industrial inspection, and other technical fields.

Description

The present invention relates to a multi-channel optical system for use in an optical instrument. Optical remote sensing is usually done by collecting some spectral, polarimetric or spectro-polarimetric information from light received from a target. In some measurement concepts the detection is done with filters, such as spectral or polarimetric filters. It is an object of the invention to provide an improved multi-channel optical system, in particular for use in an optical instrument for spectrometry, polarimetry or spectro-polarimetry. It is a particular object of the invention to provide a multi-channel optical system performing measurements with an improved accuracy. Thereto, according to an aspect of the invention, a multi-channel optical system is provided, comprising an entrance pupil for receiving an optical beam, the entrance pupil aperture allowing a multiple number of beam channels to pass, first optics for imaging the corresponding number of beam channels to an intermediate image, a field stop spatially limiting the corresponding number of beam channels in the intermediate image, second optics for imaging the spatially limited intermediate image to a pupil image, a pupil separation module angularly separating the corresponding number of beam channels propagating from the pupil image, and a multiple number of filters for filtering the corresponding number of beam channels. By allowing a multiple number of beam channels to pass, corresponding images of the same target can be projected next to each other on an instrument detector, each beam channel having its own filter. The multiple beam channels can be angularly separated using a pupil separation module. Further, by using a field stop at an intermediate image, spatial overlap between respective separated beams in the final images at the detector is prevented, though all separated beams still correspond to exactly the same part of an observed scene. The entrance pupil aperture may include a multiple number of sub apertures for generating the corresponding number of beam channels. The multiple number of sub apertures may be virtual. Then, just a single physical entrance pupil aperture is present forming a full, undivided pupil, allowing to pass the multiple number of beam channels that are separated by the pupil separation module, at the pupil image. Alternatively, the multiple number of sub apertures may be materialized, e.g. by the presence of filters. As a relatively high amount of beam light can be collected, in the absence of a split aperture, a relatively high signal-to-noise-ratio and a high accuracy can be achieved, while on the other hand, multiple images of the same object can be analyzed, side-by-side, with mutually different filters. Advantageously, each channel can have its own filter function that does not depend on the other channel filters. The filters may be completely different or may have a spectral overlap, sharing a wavelength range. In principle, the multi-channel optical system is a true imager as, in principle, a two-dimensional image of an observed scene is formed for each channel, without spectral dispersion. For spectral measurements, the multi-channel optical system is very promising because the filters can be placed in a pupil image. It means that in order to increase the quantity of light that is measured, thereby reducing detection noise and increasing the measurement precision, it is only necessary to increase the dimensions of the instrument pupil. The filters placed in sub apertures of the entrance pupil aperture or in the pupil separation module are usually very sensitive to the angle of incidence. When the pupil is enlarged the angles are unmodified. By comparison, classical instrument concepts, where the filters are placed directly on the detector, suffer from the fact that to increase the amount of detected light it is required to use a converging beam with larger angles at the detector. The larger angles are degrading significantly the filter spectral response, which cannot remain sharp. This is a very strong limiting factor for some types of measurements such as optical gas sensing where the filters try to capture very narrow spectral signatures. By using the multi-channel optical system according to the invention, the amount of collected light can be increased as much as the optics permits, reaching a very low noise level without degrading the measurement accuracy. Also for polarimetric measurements, the multi-channel optical system according to the invention is very promising, at least for the reason that the polarimetric filters can be placed in sub apertures of the entrance pupil aperture. They are then the first optical components in the system that interact with an incoming beam, which means the polarization created by the instrument optics does not perturb the polarimetric measurement. This allows to reach a very high accuracy. This is a significant advantage over most existing polarimeters where the f