EP2201420A2 - System for structured illumination of a sample - Google Patents

Abstract

The invention relates to a structured illumination system (8) for the three-dimensional microscopy of a sample (14), that comprises : beam generation means (9, 10) adapted for generating coherent light beams ( I N, I-N, I O); a lens (12) having a rear focal plane (PFA), and arranged so that the sample (14) can be placed in the focalisation plane (PMP) of the lens; focalisation means (19) arranged for focusing the light beams in the rear focal plane (PFA) so that the beams interfere in a collimated manner in the focalisation plane (PMP) of the lens (12); characterised in that the beam generation means (9, 10) includes a light space modulator (10) programmed for diffracting a light signal (22) in order to generate at least two different diffracted beams (( I N, I O), ( I-N, IO)) that are not symmetrical relative to the lens optical axis, wherein the light space modulator includes a calculator adapted for applying a constant phase term to each of said at least two coherent beams.

Description

 STRUCTURED ILLUMINATION SYSTEM OF A SAMPLE

The invention relates to a structured illumination system for three-dimensional microscopy of a sample.

The publication of Wilson et al. "Confocal Microscopy by Aperture Correlation", 1996, describes such a structured illumination system for three-dimensional microscopy of a sample.

In this publication, the illumination of the sample is structured by a movable grid. Such structuring allows an improvement in the axial resolution of the system compared to a so-called conventional microscope but it does not improve the lateral resolution.

The invention more particularly relates to a structured illumination system for three-dimensional microscopy of a sample comprising:

beam generation means arranged to generate coherent light beams;

an objective having a rear focal plane, the objective being arranged so that the sample is likely to be positioned in the objective focusing plane;

focusing means arranged to focus the light beams in the rear focal plane so that the beams interfere in a collimated manner in the focus plane of the objective.

Such a system of structured illumination of a sample is notably known from the publication of Gustaffson et al. "Extended resolution fluorescence microscopy", 1999.

In this document, as illustrated in FIG. 1, the illumination system 1 comprises a light source 2, a diffraction grating 3 forming the beam generation means, the diffraction grating 3 being coupled to a filter 7 so as to generate only two coherent diffracted light beams 4a and 4b from the light source 2, a lens 5 arranged to focus the coherent beams 4a and 4b in the rear focal plane PFA of a microscope objective 6, a sample 18 being positioned in the focus plane PMP of the objective 6. The coherent beams 4a and 4b are diffracted beams having symmetrical orders p and

-P-

Such a structured illumination system for the three-dimensional microscopy of a sample has the particular advantage of having an improved lateral resolution and allowing measurements in the field.

However, it has the disadvantage of having an axial resolution identical to that of a so-called conventional microscope, that is to say of the order of 0.4 micrometer.

An object of the invention is to improve the axial resolution in such a structured illumination system for three-dimensional microscopy of a sample.

The application US 2007/0171519 aims to achieve this goal and describes a structured illumination system for three-dimensional microscopy of a sample as described above in which the coherent beams are such that two of the coherent beams are unsymmetrical relative to each other. to the optical axis of the lens. In particular, the application 2007/0171519 uses beams of diffraction orders 1, 0 and -1 to improve the axial resolution of the illumination system.

However, this document uses a simple grid to generate the diffracted beams, and it does not describe how axial displacement of the sample is performed for axial measurements.

However, conventionally, the movements are made by mechanical systems, for example piezo-electric, which cause significant loss of precision that reduce the resolution of the system.

An object of the invention is to provide a structured illumination system for three-dimensional microscopy of a sample to improve the axial resolution compared to a conventional illumination system.

The original approach underlying the invention is to simulate the axial displacement of the sample without mechanical displacement.

Another object of the invention is to provide a structured illumination system for the three-dimensional microscopy of a sample for improving both the axial resolution and the lateral resolution with respect to a conventional illumination system. that is to say to achieve axial resolutions of the order of 0.3 micrometers, and lateral resolutions of the order of 0.1 microns.

Another object of the invention is to achieve such performance while maintaining an acquisition rate compatible with the study of dynamic phenomena, that is to say greater than or equal to 1 Hertz.

At least one of these aims is achieved through a structured illumination system for three-dimensional microscopy as described above, wherein the beam generating means comprises a spatial light modulator programmed to diffracting a light signal so as to generate at least two coherent diffracted beams not symmetrical with respect to the optical axis of the objective, the spatial light modulator comprising a calculator arranged to apply a constant phase term to each of said at least two coherent beams not symmetrical with respect to the optical axis.

Thanks to the invention, and in particular to the fact that the coherent beams generated by the generating means comprise at least two beams that are not symmetrical with respect to the optical axis of the objective, the sample is illuminated by coherent but non-coherent beams. symmetrical in the axial direction. Therefore, it is possible, by a modification of the distance between the objective and the sample or an axial displacement of the illumination in the sample, to use this non-symmetry, in particular from the difference in illumination between two axial positions of the sample. This thus makes it possible to improve the axial resolution with respect to the Gustaffson et al. mentioned above, and in particular to obtain an axial resolution of less than 0.3 micrometers. According to the invention, this axial displacement is simulated by means of the spatial light modulator by applying a phase term to each of the coherent beams. Thus, the measurement does not require any mechanical displacement and the mechanical losses are therefore reduced.

The two beams that are not symmetrical with respect to the optical axis of the objective are in particular diffracted diffraction beams of n and p diffraction order with n + p different from 0.

In addition, thanks to the invention, the lateral resolution maintained around 0.1 micrometers, as in the Gustaffson et al. above.

Finally, since the system according to the invention is an imaging system full field, a rate greater than 1 Hz can be obtained.

According to the invention, a three-dimensional structuring of the illumination has thus been realized, which makes it possible to carry out an axial coding of the illumination in addition to the lateral coding known per se.

Advantageous embodiments of the invention will now be described.

According to one embodiment of the invention, the beam generating means may comprise a light source capable of generating a light signal.

Preferably, the beam generating means can be arranged to generate three coherent beams, at least two coherent beams among the three coherent beams being unsymmetrical with respect to the optical axis of the objective. In the context of the invention, the generation of these three coherent beams makes it possible to obtain a structuring of the field symmetrical with respect to the optical axis of the objective and a gain of a factor of two in axial resolution with respect to a lighting with only two coherent beams not symmetrical.

Preferably, the beam generation means are arranged so that the three coherent beams generated by diffraction correspond to two diffracted beams of symmetrical diffraction orders and to a zero diffraction order beam. In this case, the symmetric order beams N and N form, with the zero diffraction order beam, two pairs of beams that are not symmetrical with respect to the optical axis that can be used in the context of the invention.

In particular, the two diffracted beams of diffraction order symmetric diffraction orders can be chosen such that the intensity of the zero diffraction order beam is equal to four times the intensity of one of the order diffracted beams. symmetrical diffraction. Indeed, it is demonstrated that the improvement of the axial resolution is the best in this case.

In order to allow lateral measurements in a plane xOy perpendicular to the optical axis z of the objective, the computer can be programmed so as to phase out one of said at least two coherent beams unsymmetrical with respect to the other. This embodiment has the advantage of allowing a lateral displacement of the interference pattern in the sample and thus simulates the lateral displacement of a diffraction grating by programming the spatial light modulator. It thus makes it possible to carry out lateral measurements without physical displacement of the components of the system.

An embodiment of the invention will now be described with reference to the appended figures in which:

FIG. 2 represents a structured illumination system for three-dimensional microscopy of a sample according to one embodiment of the invention;

FIG. 3 shows the light beam paths in a system as illustrated with reference to FIG. 2.

In the figures, identical references, unless otherwise indicated, relate to similar technical characteristics.

As illustrated in FIG. 2, according to a first embodiment of the invention, a structured illumination system 8 for the three-dimensional microscopy of a sample comprises a light source 9 able to generate a coherent and polarized light signal. linearly. The light source 9 is for example a laser. It also comprises a spatial light modulator 10 in transmission or reflection programmed to generate, from the light signal received from the source 9, three diffracted beams of respective intensity IN, IN and LO. The diffracted beams IN and LN are N symmetric order beams, and the beam Io corresponds to a zero order beam.

The beams Io and IN are non-symmetrical with respect to the optical axis of the lens. Similarly, the beams I ₀ and L _N are unsymmetrical with respect to the optical axis of the objective.

The order N of the diffracted beams IN and LN is chosen so that the intensity of the zero-order beam is equal to four times the intensity of these beams, ie I ₀ = 4IN = 4LN. Indeed, it is demonstrated that the improvement of the axial resolution is the best in this case.

In order for the light signal received from the source 9 to adapt to the size of the spatial light modulator 10, it is possible to position a not shown afocal device between the source 9 and the spatial light modulator 10 so as to widen the signal bright, for example five to ten times.

At the output of the spatial light modulator 10, the diffracted beams pass through a new afocal device 11 and are focused in the rear focal plane PFA of a microscope objective 12 by an achromatic doublet 19. The magnification factor of the afocal device 1 1 is chosen so that the diffraction angles are adapted to the diameter of the objective 12 of the microscope 1 3.

A sample 14, for example in the form of a fluorescent point object, is positioned in the PMP focusing plane of the objective 12 of the microscope 13 at which the three diffracted beams IN, IN and Io interfere.

The system 8 described above allows a three-dimensional structuring of the illumination to the sample 14. In fact, the use of three diffraction orders reveals a Talbot effect in the field transmitted by the objective.

In operation, to perform measurements in the lateral plane xOy, the spatial light modulator 10 is programmed so as to laterally shift the interference pattern generated by the three diffracted beams in the objective focusing plane. This programming makes it possible to simulate a lateral displacement of a diffraction grating generating the three diffracted beams without the modulator physically moving. Following this simulated lateral displacement, the position of the fluorescent point object is measured in a manner known per se by combining several images or their Fourier transform for different real or simulated positions of the diffraction grating, for example by difference between the intensity the fluorescence received for two real or simulated positions of the diffraction grating.

In addition, in operation, in order to carry out measurements in the axial direction z, the optical distance between the sample 14 and the objective 12 is modified. To this end, it is possible to use a piezoelectric plate 15 positioned at the level of the sample. 14 or axially moving the objective 12. According to the invention, it is also possible to simulate this displacement by programming the spatial light modulator 10 to apply a constant phase term to all the pixels of the modulator 10, which consequently applies a constant phase term to the diffracted beams.

As before, and thanks to the well-known Talbot effect, one measures the axial position of the fluorescent point object, for example by difference between the intensity of the fluorescence received for two real or simulated distances between the objective 12 and the sample 14.

In FIG. 3, the path of the light beams is illustrated by the illumination system according to the invention. According to the invention, the light source 9 generates a light signal in the form of a plane wave 22. This plane wave is diffracted by means of beam generation 21 for generating only three diffraction orders lo, IN and IN. These beam generation means correspond to the spatial light modulator 10 described with reference to FIG. 2. The diffracted beams I ₀ , IN and LN are still plane waves. The focusing means 19, for example corresponding to a lens or achromatic doublet 19 described above converge these beams towards the back focal plane PFA of the objective 12 of microscope 13. In this way, the beams lo, IN and _N L interfere in the PMP focusing plane in the form of plane waves and measurements relating to a sample 14 positioned in the PMP focusing plane can be realized.

Variants of the invention are now described.

Beam generating means for generating three coherent beams that interfere at the objective focus plane have been described above. The effect of improving the axial resolution is then optimal for beams IN, IN and lo, the orders N and -N being chosen as before so that

However, it is understood that the effect of improving the axial resolution is obtained when the diffracted beams are such that at least two coherent beams that are not symmetrical with respect to the axis lens optics interfere at the focus plane PMP of the objective 12. Indeed, in this case, the axial symmetry of the beams is broken and it is possible, by moving the sample 14 with respect to the 12 or by simulating this displacement as mentioned above, to obtain information on the axial position of the sample 14.

On the contrary, it can be noted, with reference to FIG. 1, that a displacement of the sample 18 with respect to the objective 6 would have no effect within the framework of the system 1 described in the publication of Gustafsson et al. since the two beams 4a and 4b are symmetrical with respect to the optical axis of the lens. Such axial displacement therefore makes it impossible to perform precise axial measurements.

It has been demonstrated that the invention described above makes it possible to improve the axial resolution by a factor of the order of 10 relative to the so-called conventional microscopy.

In addition, the use of a spatial light modulator as described with reference to FIG. 2 has the advantage of avoiding the mechanical displacements of the system and thus of improving the accuracy of the measurements.

The system 8 described above can also easily fit on an existing microscope 13, in parallel with the existing illumination system.

In this case, in the embodiments described above, the microscope 13 may be an existing microscope with an existing objective 12, and the illumination system is adapted so that the beams IN, IN and Io are focused in the PFA rear focal plane of the objective 12. A new microscope having a improved resolution allowing full field observation.

Claims

1. Structured illumination system (8) for three-dimensional microscopy of a sample (14) comprising: - beam generation means (9, 10) arranged to generate coherent light beams (IN, I-N, IO); - a lens (12) having a rear focal plane (PFA), the lens being arranged so that the sample (14) is capable of being positioned in the focus plane (PMP) of the lens; focusing means (19) arranged to focus the light beams in the rear focal plane (PFA) so that the beams interfere in a collimated manner in the focusing plane (PMP) of the objective (12), characterized in that the beam generation means (9, 10) comprises a spatial light modulator (10) programmed to diffract a light signal (22) so as to generate at least two coherent diffracted beams ((IN, IO), ( IN, IO)) unsymmetrical with respect to the optical axis of the objective, the spatial light modulator comprising a computer arranged to apply a constant phase term to each of said at least two coherent beams. 2. System according to claim 1, wherein the beam generating means comprise a light source (9) capable of generating the light signal (22). The system of claim 1 or 2 wherein said at least two non-symmetric coherent beams are diffracted beams of respective diffraction order n and p with n + p different from zero. 4. System according to one of the preceding claims, wherein the beam generating means (10, 9) are arranged to generating three coherent beams (IN, IN _> IO), at least two coherent beams (IN, IN, IO) among the three coherent beams being unsymmetrical with respect to the optical axis of the objective. 5. System according to the preceding claim, wherein the beam generating means (10, 9) are arranged so that the three coherent beams generated by diffraction correspond to two diffracted beams of symmetric diffraction orders and a command beam. zero diffraction. 6. System according to the preceding claim, wherein the two diffracted beams of symmetrical diffraction order are of equal intensity, and the two symmetrical diffraction orders are chosen so that the intensity of the zero diffraction order beam is equal to four times the intensity of one of the diffracted beams of symmetrical diffraction order. 7. System according to one of the preceding claims, wherein the computer is further arranged to phase out one of the two coherent beams unsymmetrical with respect to the other. 8. Microscope comprising a structured illumination system (8) according to one of the preceding claims.