Patent Description:
To solve these problems, a technology called adaptive optics is used which, by means of deformable optics, can compensate for these distortions.

The elements of an adaptive optics system are therefore a deformable device, an aberration measurement sensor and a control system.

This technology is used successfully in telescopes, greatly improving the resolution thereof.

In case of other optical instruments, such as, for example, microscopy, it is not possible, or very inconvenient, to use a wavefront sensor.

Therefore, wavefront correction strategies based on optimization algorithms have been used.

This type of algorithms aims at improving the quality of an image by searching for the shape of the deformable element which compensates for the contribution given by the present aberrations. The advantage of this technique is that it does not require a wavefront sensor. The disadvantage is that the optimization operation is very slow, and being based on algorithms of a stochastic nature, the result is not always repeatable. It is also strongly dependent on the type of image which is being captured.

To solve this limitation, a type of correction based on wavefront measurement has recently been implemented. This technique is based on the measurement thereof, by means of the acquisition of images taken by partially limiting the aperture of the pupil, i.e., by creating sub-pupils in different positions thereof. Thereby, the image on the system sensor will translate by an amount equal to the average gradient of the wavefront inside the sub-pupil. Therefore, with a series of acquisitions synchronized with the displacement of the sub-pupil, it is possible to sample the gradient of the wavefront and then measure the distortion thereof. Therefore, having a wavefront modulator available, it is possible to compensate for such distortion.

<CIT> presents a wavefront measurement system which comprises an aperture placed in the optical beam.

In the article <NPL>, the authors add to the standard architecture of a Shack Hartmann (SH) sensor a mask which can cover a lenslet every two.

A first implementation of this method is described in ("<NPL>) and <CIT>). In this implementation, a liquid crystal modulator is used both to segment the pupil of a microscope into sub-pupils and to independently control the wavefront within the sub-pupil itself. By virtue of the very high spatial resolution of the wavefront modulation of the liquid crystal panels, this technique has the advantage of being capable of dividing the light beam of the microscope into sub-beams whose phase can be controlled independently.

The main disadvantage is that the liquid crystal modulator, being divided into cells of micrometric size (in general, approximately from <NUM> to <NUM> microns), generates a series of diffracted beams which propagate at different angles at the exit from the modulator. In fact, in this type of systems, it is essential to use an occlusion, an optical stop, to block such beams (called "field stop"), as described in [<CIT>] and in <NPL>), which would otherwise overlap the image on the detector, ruining the contrast thereof. The elimination of the diffracted orders occurs by positioning the field stop in an image plane. This is achieved by positioning, after the modulator, a pair of lenses in the focal plane of which the field stop is positioned. Therefore, although this solution is very interesting, it always involves a considerable amount of space.

For example, in case of microscopy, it is not possible to position the entire correction system inside the microscope lens system. Therefore, this invention has only been implemented on microscopes designed and realized to incorporate this technique. Furthermore, it can not be conceived as a device to be added to existing microscopes. In fact, it is not possible to implement it on microscopes of the commercial type, in which the internal parts of the microscope are not accessible and/or modifiable.

A further limitation of this embodiment is that the liquid crystal modulators with the highest optical efficiency (<NUM>-<NUM>%) are of the reflective type (LCoS, Liquid Crystal on Silicon) while transmissive liquid crystal modulators (Spatial Light Modulators) have a much lower optical efficiency and are therefore not used in microscopy.

Furthermore, this invention is limited to the use with microscopy systems employing laser sources. In fact, liquid crystal modulators work well with monochromatic sources and with a well-defined light polarization. However, they do not work with wide light spectra, such as, for example, conventional bright field microscopes or optical coherence tomography.

It is the object of this invention to integrate the wavefront measurement and correction element in a single device.

Such object is achieved by the optical system according to claim <NUM>.

The structure comprises measuring means configured to measure the displacement of the image on the detector following a change in the position of the light beam occlusion. Furthermore, the structure comprises controllable actuator means, to actuate the wavefront modulator depending on the displacement of the image detected by the measuring means.

Such device will not require external elements, such as, for example, a diffracted order removal system, and can be inserted in an optical system in series with other elements of the system without altering the properties and functioning thereof, while adding the aberration correction function. For example, it can be inserted inside the lens system of a microscope. Therefore, it can be used as instrumentation in any type of microscope which is already assembled.

Hereinafter, reference will be made to the Figures shown in the drawings to describe the present invention in detail, in accordance with several preferred embodiments.

With the aid of the accompanying drawings, the technical contents and detailed description of the present invention are reported below on the basis of preferred embodiments, not intended to limit the application purpose thereof.

In this invention, a deformable lens and a variable iris are coupled. "Variable iris" means an occlusion of an optical beam which leaves a portion thereof free; such occlusion can change position and shape within the beam. The variable iris is suitable to create sub-pupils. "Sub-pupils" mean a portion of the optical beam of any shape, size and position; such portion of the beam can be selected on the optical pupil plane but also on any other plane lying within the light beam of the system. The wavefront modulation can occur by changing the shape of one or both surfaces of the lens. For example, see: <NPL>). Other deformable lenses can exploit other principles, such as electrowetting (<CIT>) or the use of transparent elastic membranes (<CIT>).

<FIG> is a simplified diagram of a generic image system. By way of non-limiting example, in case of a microscope, element <NUM> is the lens system, <NUM> is the "tube lens" and <NUM> a detector. Element <NUM> is the device for measuring and correcting aberrations. Device <NUM> can be conveniently positioned in the pupil of the system but, in some cases, also in other positions within the optical path. The diagram is shown by way of example and is representative of, but not limited to, white light microscopy techniques such as "bright field", fluorescence, light sheet, single molecule, structured light, etc. It can also be applied to other image acquisition techniques of the scanning type, such as, by way of non-limiting example, to microscopy of the confocal, two-photon, multi-photon, STED, optical coherence tomography types. <FIG> shows how, in a very convenient manner, the correction system <NUM> can be integrated within the lens system <NUM> of the optical system.

<FIG> show sections of a possible implementation. Inside the device <NUM>, a deformable lens <NUM> is placed, positioned along the optical axis <NUM> of the system. Such lens can change the shape thereof, so as to compensate for the distortions present in the optical path, whether generated in the sample or from imperfections or misalignments of the optical components. Facing the lens, there is a sheet of opaque material with one or more holes (variable iris or sub-pupils) <NUM> with a diameter smaller than the diameter of the pupil of the system <NUM>. A handling system can change the position of the variable iris, therefore illuminating a different sub-pupil <NUM> so as to scan all or part of the pupil of the system. During the scanning operation, the detector <NUM> captures the images of the sample and determines the displacement thereof in relation to a reference image. This image is the one relating to a sub-pupil placed on the optical axis.

<FIG> shows this principle. For a sample in which there are no aberrations, the wavefront from any single point in the image is spherical <NUM>. Therefore, by passing through the lens system, it produces a beam parallel to the optical axis <NUM>. The light is then filtered by the variable iris <NUM>, exclusively passing through the sub-pupil <NUM>. With the deformable lens in a flat position, the beam <NUM> propagates parallel to the axis without undergoing deviations.

<FIG> shows the case in which there are distortions of the spherical wavefront (solid line <NUM>) which deviate from the ideal one (dashed line <NUM>). Therefore, the light passing through the sub-pupil <NUM> will no longer propagate parallel to the optical axis but at a certain angle. The greater the distortion of the wavefront, the greater this angle will be. The slope of the wavefront in the position of the sub-pupil <NUM> will be measurable by the translation of the image on the detector. The measurement of these displacements is directly proportional to the gradient of the wavefront. It is therefore possible to deform the lens so that it compensates for this distortion and improve image quality. <FIG> shows this situation in which the deformable lens <NUM> assumes a shape such as to compensate for the deformation of the wavefront and to ensure that the beams emitted from a point propagate parallel to the optical axis. In this case, the variable iris is configured so as to leave the whole optical aperture open (therefore, it has not been shown in the Figures) so as to capture the image with the correct wavefront.

This operation can be repeated several times to improve the quality of the correction. In general, <NUM> to <NUM> iterations are required to achieve a good level of correction.

The variable iris can be formed by a sheet of opaque material, with suitable holes, preferably round, but possibly of any shape, which select a smaller part of the pupil. <FIG>, <NUM> shows the device which integrates both the variable iris <NUM> and the deformable lens <NUM>. By way of explanation, the aperture <NUM> shows a variable iris position. <FIG> shows an exemplary diagram showing the variable iris <NUM> and the optical pupil of the system. The variable iris has the task of moving the sub-pupil so as to entirely or partially scan the pupil.

In a preferred embodiment (<FIG>), the variable iris consists of a disc <NUM> made of opaque material, and a deformable lens positioned adjacent thereto as shown in <FIG>. The disc can, for example, be made of metal or plastic material, metal deposition on glass, etc. The disc is designed so that, with a simple rotation about the pin <NUM>, it is possible to fully scan the pupil.

In order to make a very compact perforated disc and, at the same time, use only one rotation operation, <FIG> shows that with <NUM> holes on the disc <NUM> placed in radial coordinates different with respect to the rotation axis and suitably spaced, it is possible to scan the entire pupil in <NUM> different positions as shown in <FIG>. The dashed lines in <FIG> show the trajectory of each hole during the rotation of the disc <NUM>. In fact, the sub-pupils obtained with the holes <NUM> of the disc are arranged so as to completely scan the aperture of the pupil by simply rotating the disc <NUM>. Scanning can occur with partial overlapping of the sub-pupils without overlapping. Furthermore, the aperture <NUM> allows to capture the correct image at the end of the correction procedure. <FIG> shows a three-dimensional representation of the device. The adaptive lens <NUM>, positioned centered with respect to the optical axis <NUM> and concentric with respect to the aperture <NUM> of the variable iris. In this position of the variable iris it is possible to capture images following the correction with adaptive optics. The variable iris can rotate about the rotation axis <NUM> thereof so that the pupil can be scanned by the sub-pupils <NUM>. <FIG> shows the pupil of the system <NUM> and how the irises <NUM> are capable of fully scanning the entire surface by rotating the disc <NUM>.

The correction procedure, which is not covered by the subject-matter of the claims, is shown in <FIG>. Initially, the procedure involves the characterization of the wavefront deformation functions given by each individual actuator of the deformable lens. This is achieved by activating each actuator independently and by measuring the generated wavefront. The deformation functions generated, also called influence functions in adaptive optics, are stored in a matrix called the matrix of influence functions "A". Within this matrix, each column contains the deformation of a single actuator. This information can also be obtained by activating multiple actuators simultaneously, by means of the technique called Hadamard matrix characterization.

The characterization can be carried out on the microscope by means of the technique described above, i.e., by moving the variable iris or on a separate optical system equipped with a wavefront sensor (for example with a "Shack-Hartmann" sensor).

The matrix of influence is used to create a model of the deformable lens in which the shape of the lens, or the wavefront generated therefrom, can be calculated as: F(x,y)=A*c, where "F" is the shape of the lens, "A" is the matrix of influence functions and "c" is the vector containing the control value given to each lens actuator.

The second operation is that of measuring the aberration of the system, which is carried out with the lens in a flat condition (all actuators off) or actuated on a previous position.

From the "Aber(x,y)" measurement of the wavefront, it is possible to calculate the control vector c to be given to the actuators of the deformable lens by means of the following calculation: c = - A-<NUM> * Aber(x,y). Thereby, the lens deforms in the opposite manner with respect to the aberration, thus compensating for the effects thereof. This measurement and correction operation can be performed several times to improve the degree of correction.

This type of aberration correction can be used for the correction of any optical system. For example, in microscopes, as explained above, of the image detection type with camera, and of the scanning type, such as, by way of non-limiting example, microscopes of the confocal, two-photon, optical coherence tomography, STED types and others. It can also be used for correcting aberrations in laser systems, without the use of wavefront sensors and by measuring aberrations in the experimental chamber.

In other embodiments, the variable iris and the deformable lens can be conveniently placed outside the pupil of the optical system. For example, they can be placed in an image plane of a plane on which system aberrations arise.

In another embodiment, which is not covered by the subject-matter of the claims, the deformable lens and the variable iris can not be adjacent but placed on different positions in the optical system. By way of non-limiting example, they can be placed in mutual image planes.

In another embodiment, the variable iris can be placed together with more than one deformable lens to increase the degree of correction.

Claim 1:
Optical system comprising:
- a lens system (<NUM>),
- a detector (<NUM>) suitable to capture an image of a sample, the image being formed on the detector by a light beam coming from the sample along an optical path, and
- a structure (<NUM>) for measuring and correcting aberrations of the optical system, wherein said structure comprises, within the optical path, a variable iris (<NUM>), the variable iris (<NUM>) being suitable to create sub-pupils (<NUM>, <NUM>), each sub-pupil (<NUM>, <NUM>) being a portion of the light beam, and the variable iris (<NUM>) being suitable to change the position of a light beam occlusion which allows the portion of the light beam to pass towards the detector (<NUM>),
wherein the structure (<NUM>) is configured to measure a wavefront aberration by measuring a displacement of the image on the detector following a change in the position of the light beam occlusion,
wherein the structure (<NUM>) comprises, within the optical path, a wavefront modulator; characterised in that the wavefront modulator is a deformable lens (<NUM>, <NUM>, <NUM>);
the structure being configured to correct the wavefront aberration by operating the wavefront modulator as a function of the displacement of the image on the detector (<NUM>); said image being related to a said sub-pupil (<NUM>, <NUM>) ;
wherein the wavefront modulator and the variable iris (<NUM>) are adjacent.