TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method of and an apparatus for forming and shifting a light intensity distribution in a focal area of an objective lens. Further, the present invention relates to a microscope comprising such an apparatus and an objective lens.

PRIOR ART

In STED fluorescence microscopy, in addition to fluorescence excitation light focused into an intensity maximum, fluorescence inhibition light is directed into a sample to be examined. An intensity distribution of the fluorescence inhibition light comprises an intensity minimum or zero point coinciding with the intensity maximum of the fluorescence excitation light and surrounded by intensity maxima. In the areas of these intensity maxima, the emission of fluorescence light by fluorescence markers included in the sample is inhibited by the fluorescence inhibition light. Thus, fluorescence light detected may only origin out of the minimum of the intensity distribution of the fluorescence inhibition light. This corresponds to a strong increase in spatial resolution of measuring the sample as compared to common laser-scanning-microscopy. A known method of forming the intensity distribution of the fluorescence inhibition light comprising the intensity minimum enclosed by intensity maxima is to deform or modulate plane wavefronts of incoming coherent fluorescence inhibition light such that, when the fluorescence inhibition light is focused into a same focal area as the fluorescence excitation light, the desired intensity distribution is formed as an interference pattern. One known suitable modulation of the plane wavefronts is a phase difference between a center part of a pupil of an objective focusing the fluorescence inhibition light and an outer ring part of this pupil of p qG l/2, i.e. of half the wavelength of the fluorescence inhibition light. This phase modulation results in two strong maxima of the fluorescence inhibition light on both sides of the central intensity minimum along the optical axis of the objective and a weaker ring-shaped maximum enclosing the minimum within the focal plane of the objective. Another known phase modulation is a so-called phase clock according to which a phase difference is introduced which, over a circle around the center of the pupil of the objective focusing the fluorescence inhibition light, increases from zero to 2p or l, i.e. the wavelength of the fluorescence inhibition light. The intensity distribution of the fluorescence inhibition light resulting from the phase clock is a donut extending along the focal plane and enclosing a zero point of the intensity of the fluorescence inhibition light within the focal plane of the objective.

It is also known to discretize the phase clock in that the phase of the fluorescence inhibition light is constantly delayed over pie segments of the pupil of the objective. Already with three pie segments of equal size the phase clock can be approximated such that a central intensity minimum is enclosed by intensity maxima in all directions within the focal plane of the objective. The approximation of the phase clock gets better with more pie segments, and with six pie segments of equal size the phase clock is already approximated quite well.

In order to implement a discretized phase clock, the wavefronts of the fluorescence inhibition light may be modulated using a segmented phase plate. If the wavefronts of the fluorescence inhibition light are modulated using a spatial light modulator, the modulation pattern may also be varied to, for example, compensate for aberrations of an optical system, adapt the pattern to the pupil of the objective and tune the pattern to reduce both the intensity of the fluorescence inhibition light in the minimum of its intensity distribution and the spatial dimensions of this minimum.

Similar light intensity distributions as they are suitable for the fluorescence inhibition light in STED fluorescence microscopy are, for example, used in MINFLUX microscopy, see WO 2018/069283 A1 , and other microscopic methods, see, for example, WO 2015/097000 A1 , WO 2015/052186

A1 and WO 2013/072273 A1. In these methods, the intensity distribution comprising the central intensity minimum or zero point enclosed by intensity maxima is an intensity distribution of fluorescence excitation light used without fluorescence inhibition light.

At least some of the methods referenced above would benefit from or even require that the central intensity minimum of the fluorescence inhibition light can be shifted with regard to the sample at both a high velocity and a high precision. With regard to STED fluorescence microscopy, this particularly applies to a method called Minfield STED, see WO 2017/153430 A1.

The standard means for shifting a light intensity distribution with regard to a sample in scanning fluorescence microscopy is a scanner consisting of rotating or tilting mirrors which are rotated or tilted by electric drives like piezo-electric drives or galvanometric drives, i.e. so-called piezos or galvos. The velocities achieved in shifting a light intensity distribution using these drives is low when compared to the accuracy achieved. This is due to the fact that the mass of the respective mirror has to be moved in changing directions.

WO 2017/153430 A1 not only discloses the use of galvo mirrors but also of electro-optical scanners and acousto-optical deflectors for scanning smaller partial areas of a sample with a light intensity distribution. Further, this document states that a device for scanning partial areas of the sample may be combined with an additional electro-optical or acousto-optical modulator as a phase shifter for shifting the zero point of the light intensity distribution of luminescence inhibition light. No details, however, are given with regard to how one electro-optical may be used as a phase shifter.

It is known that a spatial light modulator may be used to shift a position of a zero point of a light intensity distribution of fluorescence inhibition light within a partial area of a sample. The modulation pattern of a spatial light modulator, however, cannot be changed at a high frequency.

In US 2015/01 16807 A1 , temporal focal modulation technique (FFM) is described which is a method used in fluorescence microscopy in order to be able to switch rapidly between different focusing fields. FFM is based on the rapid switching of the optical phase in the pupil of an objective lens. Particularly, the phase is shifted in two half pupils. The switching between two focusing states, one having a zero point on the optical axis, the other having no zero point, is made by operating an electro-optical modulator (EOM) arranged in front of a birefringent phase plate. The birefringent phase plate either splits the pupil of the objective lens into two half pupils with a phase offset of l/2 or is not active at another polarization of the input light transmitted through the EOM and thus not splitting the pupil into two different halves. US 2015/01 16807 A1 further discloses a microscope in which at least one illuminating beam, in a partial area along the cross-section thereof, is phase-modulated with a modulation frequency. A microscope objective is provided for focusing the illumination beam into a sample. In the illumination beam path upstream of the microscope objective, a first polarization beam splitter is provided which generates at least first and second partial beam paths. A second polarization beam splitter is provided for rejoining the partial beams. In one of the two partial beam paths, a phase element is provided which has at least two areas causing different phase shifts. The phase element may be a spatial light modulator (SLM). Acousto-optical modulators (AOMs) may be arranged in both partial beam paths to quickly switch them on and off.

US 9,285,593 B1 discloses a beam shaping method and apparatus in which a phase shift function is introduced in a beam of input light. The phase shift function is introduced by a phase transforming optical system implemented in form of a plate or a telescope or at a collimator or integrated into the focusing optical system. The phase transforming optical system includes an aspheric optical surface providing the phase shift function with smooth phase transition. Any diffraction-limited optics with positive dioptric power may be applied as the focusing optical system.

OBJECT OF THE INVENTION It is the object of the present invention to provide a method of and an apparatus for forming and shifting a light intensity distribution in a focal area of an objective lens which allow both for a very high velocity and a very high precision in shifting the formed light intensity distribution. Further, the method and the apparatus shall be quickly adaptable to multiple wavelengths.

SOLUTION The object of the invention is solved by a method comprising the features of claim 1 and by an apparatus comprising the features of claim 20.

Preferred embodiments of the method and the apparatus according to the invention are defined in the dependent claims.

DESCRIPTION OF THE INVENTION The method of forming and shifting a light intensity distribution in a focal area of an objective lens according to the invention comprises directing a plurality of portions of coherent input light into a plurality of non-identical pupil areas of a pupil of the objective lens, and modulating at least one portion of the plurality of portions of coherent input light with regard to at least one of its phase and its amplitude. According to the present invention, the plurality of portions of coherent input light consists of discrete portions of input light, and at least one discrete portion of the plurality of portions of coherent input light is separately modulated by means of separate electro optical modulator which is by-passed by other portions of the plurality of portions of coherent input light.

In the method according to the present invention, the at least one portion of the plurality of portions of coherent input light is not modulated with regard to its phase or its amplitude by means of any modulator through which all other portions of the plurality of portions of coherent input light pass as well. Particularly, the at least one discrete portion of the plurality of portions of coherent input light is not modulated by an area of a spatial light modulator comprising other areas reflecting or transmitting the other portions of the plurality of portions of coherent input light.

Instead, a separate electro optical modulator is provided for the at least one discrete portion of the plurality of portions of coherent input light. Such an electro optical modulator may be controlled at a very high frequency as it has no moving parts of a relevant physical mass. Further, the electro optical modulator may be controlled at a high precision with regard to the at least one of the phase and the amplitude of the at least one discrete portion of the plurality of portions of coherent input light. Both a modulation of the phase and the amplitude of the at least one discrete portion of the plurality of portions of coherent input light may be used to shift the light intensity distribution formed as an interference pattern of the focused portions of the plurality of portions of coherent input light in the focal area of the objective lens. The phase of the at least one portion will determine where in the focal area a positive or negative interference with the other portions of the plurality of portions of coherent input light will occur. The amplitude of the at least one portion will determine to which extent the respective positive or negative interference will occur at the respective location in the focal area.

That the pupil areas of the pupils of the objective lens into which the discrete portions of coherent input light are directed are non-identical at least means that they are not all extending over a same area of the pupil. Preferably, the non-identical pupil areas are essentially non-overlapping or even not overlapping at all. This means that each discrete portion of the plurality of portions of coherent input light is directed into a pupil area not overlapping with any pupil area of another one of the discrete portions of coherent input light. In the following discussion of the preferred embodiment of the method according to the present invention, the term "portion of the plurality of portions of coherent input light" will sometimes be abbreviated by "portion". Similarly, "pupil areas of the plurality of non-identical pupil areas of the pupil of the objective lens" will sometimes be abbreviated by "pupil areas". Other similar abbreviations will be made as well.

In some embodiments of the method according to the present invention, the separate electro optical modulator used for modulating the at least one portion of coherent input light is by-passed by all other portions of the plurality of portions of coherent input light. Thus the separate electro optical modulator only modulates the at least one portion of coherent input light. Further, at least two, preferably at least three and sometimes at least four discrete portions of the plurality of portions of coherent input light may be differently modulated by means of at least two or at least three or at least four separate electro optical modulators. Each further electro optical modulator allows for differently modulating a further one of the discrete portion of coherent input light. Thus, each further electro optical modulator increases the variability in forming and shifting the light intensity distribution in the focal area of the objective.

If the at least two or at least three or at least four discrete portions of the plurality of portions of coherent input light are separately modulated by means of the at least two or at least three or at least four separate electro optical modulators, wherein each of the at least two or at least three or at least four separate electro optical modulators is by-passed by all other discrete portions of the plurality of portions of coherent input light, the respective electro-optical modulator only modulates the respective one of the discrete portions. This makes controlling the modulation much easier than with electro-optical modulators through which more than one of the discrete portions of coherent input light is passed and, thus, modulated as well.

Preferably, at least a half of all discrete portions of coherent input light are modulated by means of separate, i.e. individual electro-optical modulators. More preferably, all of the discrete portions of the plurality of portions of coherent input light are separately modulated by means of the separate electro-optical modulators. This means that there are as many separate electro-optical modulators as discrete portions of coherent input light. Particularly, the separate electro optical modulators may be integrated optical light modulators integrated into optical fibers each guiding one discrete portion of the plurality of portions of coherent input light. Such integrated optical light modulators are commercially available and may be used for implementing the present invention. When each discrete portion of the plurality of portions of coherent input light is guided by one fiber of a plurality of optical fibers, the coherent input light getting out of the ends of these fibers may be collimated and projected into the plurality of non-identical pupil areas. The plurality of portions of coherent input light are then guided by a bundle of optical fibers, and each fiber directly corresponds to one of the non-identical pupil areas. In the method according to the present invention, a beam of coherent input light may be split up to provide the discrete portions of the plurality of portions of coherent input light. This splitting up may be implemented by any suitable beam splitter.

In the method according to the invention, a fraction of each discrete portion of the plurality portions of coherent input light may be coupled out and projected onto a monitoring camera for monitoring the positions of the individual discrete portions of the plurality of portions of coherent input light in the pupil of the objective lens. In addition to their phases and amplitudes, the positions of the discrete portions of coherent input light in the pupil of the objective lens will determine the shape and the position of the light intensity distribution in the focal area of the objective lens. Thus, for forming and shifting the light intensity distribution in a controlled way by modulating at least one of the phases and amplitudes it is mandatory to keep the positions of the individual discrete portions in the pupil of the objective lens constant. To achieve this purpose, the positions have to be monitored.

Preferably, the pupil areas of the plurality of non-identical pupil areas are uniformly distributed around an optical axis of the objective lens. Then, the modulation of the at least one of the phases and amplitudes of the discrete portions will form or alter and shift the light intensity distribution in the focal area of the objective lens in a most predictive way.

With pupil areas uniformly distributed around the optical axis of the objective lens, the discrete portions of coherent input light may be provided with basic phase offsets. These basic phase offsets may be implemented by a control offset in controlling the respective electro optical modulators or by different optical path lengths, wave plates or the like.

With n pupil areas of the plurality of pupil areas, the basic offset between two discrete portions of coherent input light directed into two neighboring pupil areas of the plurality of non-identical pupil areas may, for example, be 360 n. In this case, the basic phase offsets provide for a phase clock and thus for a central intensity minimum surrounded by a donut-shaped intensity maximum of the light intensity distribution in the focal area of the objective lens. By modulating the at least one of the phases and amplitudes of the discrete portions, the position of the central intensity minimum may be laterally shifted with regard to the optical axis. If the phase offset between the two discrete portions of coherent input light directed into the two neighboring pupil areas of the plurality of pupil areas is m x 360°, m being an integer, the intensity distribution will have a central intensity maximum which may be shifted by modulating the at least one of the phases or the amplitudes of the discrete portions of coherent input light.

Particularly, the plurality of pupil areas may include pairs of pupil areas which are axially symmetrically arranged on opposite sides of the optical axis of the objective lens, wherein at least one of the two discrete portions of coherent input light directed into the two pupil areas of each of the pairs of pupil areas is separately modulated by means of one of the separate electro optical modulators. By means of this modulation, the intensity pattern formed of the two discrete portions of coherent input light of the respective pair is shifted in the direction of their distance across the optical axis. A phase offset between the two discrete portions of coherent input light directed into the two pupil areas of each of the pairs of pupils areas may be (2m+1 ) x 180° or m x 360° m x 360°, m being an integer.

In the method according to the invention, the modulation of the at least one portion of the plurality of portions of coherent input light with regard to at least one of its phase and its amplitude by means of the separate electro optical modulator may either be static during a certain process, like a measurement process, or it may be varied or changed during such a process.

For example, the discrete portions of coherent input light may be alternately separately modulated by means of the separate electro optical modulators such as to alternately provide for first basic phase offsets and second basic phase offsets between the discrete portions of coherent input light, the first basic offsets differing, from the second basic offsets. Thus, the method according to the invention may alternately provide two different light intensity distribution, one having a central intensity minimum and one having a central intensity maximum, for example. As the modulation of the discrete portions of coherent input light by means of the separate electro optical modulators may be altered very fast, it is possible to change between the different intensity distributions very fast.

Particularly, the plurality of the discrete portions of coherent input light may alternately be provided with a first wavelength and with a second wavelength of the coherent input light, the second wavelength differing from the first wavelength, and the discrete portions of coherent input light may be separately modulated by means of the separate electro optical modulators differently with the first wavelength and with the second wavelength of the coherent input light. Thus, for example, directly successive pulses of fluorescence excitation light and of fluorescence inhibition light may be formed into different intensity distributions in the focal area of the objective lens.

Further, the discrete portions of coherent input light may separately be modulated by means of the separate electro optical modulators to compensate for aberrations of the plurality of portions of coherent input light caused by the objective lens or any other optic through which the discrete portions of coherent input light pass on their way to the focal area. Thus, any astigmatism caused by these optics may be compensated for by simply slightly altering the modulation of the discrete portions of coherent input light by means of the separate electro optical modulators.

In one particular embodiment of the method according to the present invention, the light intensity distribution in the focal area of the objective is formed such as to display a local intensity minimum enclosed by intensity maxima, wherein at least two discrete portions of the plurality of portions of coherent input light are modulated such as to move the intensity minimum along a circle around the optical axis of the objective lens. This embodiment of the method according to the invention may particularly be used to implement a variant of MINFLUX microscopy in that photons emitted by a single fluorophore molecule located in the focal area are detected, wherein for each photon detected an associated position of the local intensity minimum in the focal area is registered, and wherein an average position of the registered position is calculated and taken as the position of the single fluorophore molecule in the focal area. This variant of MINFLUX microscopy in which the local intensity minimum of the light intensity distribution is moved along a circle and wherein photons emitted by a single fluorophore molecule located in the focal area are detected, wherein for each photon detected an associated position of the local intensity minimum in the focal area is registered and wherein an average position of the registered positions, is calculated and taken as the position of the single fluorophore molecule in the focal area is also to be regarded as an invention by its own, independently on how the light intensity distribution comprising the local intensity minimum is formed and shifted. Further, in this variant of MINFLUX microscopy the light intensity distribution in the focal area of the objective displaying the local intensity minimum enclosed by intensity maxima may either be an intensity distribution of fluorescence or luminescence excitation light as in usual MINFLUX microscopy, or an intensity distribution of fluorescence or luminescence inhibition light, optionally combined with a central maximum of fluorescence or luminescence excitation light. I n the method according to the present invention, the light intensity distribution may be superimposed with at least one further light intensity distribution of light incoherent with regard to the coherent input light of the light intensity distribution. This further light intensity distribution may have the same effect or function as the light intensity distribution. For example, both the light intensity distribution and the further light intensity distribution may either consist of fluorescence excitation or fluorescence inhibition light. Then, typically both the light intensity distribution and the further light intensity distribution will have a central local intensity minimum and these two local intensity minima will coincide. The light intensity distribution may enclose its local intensity minimum by a donut in a focal plane of the objective, whereas the light intensity distribution may enclose its light intensity minimum by two strong light intensity maxima spaced apart along the optical axis and a weaker ring around the optical axis.

With same functions of the light intensity distribution and the further light intensity distribution, the non-coherency serves for avoiding an unwanted interference between the two light intensity distributions.

As an alternative, the further light intensity distribution may have another effect or function than the light intensity distribution. For example, the further light intensity distribution may consist of fluorescence excitation light, whereas the light intensity distribution consists of fluorescence inhibition light. Then, the further light intensity distribution may have a central intensity maximum coinciding with a central intensity minimum of the light intensity distribution.

In any case, the further light intensity distribution may be a static light intensity distribution which is not shifted when shifting the light intensity distribution, because the further light intensity distribution has no structure over that distance over which the light intensity distribution is shifted at maximum in the focal area of the objective lens.

An apparatus for forming and shifting a light intensity distribution in a focal area of an objective lens according to the present invention comprises optics for directing a plurality of portions of coherent input light into a plurality of non-identical pupil areas of a pupil of the objective lens, and a modulator equipment for modulating at least one portion of the plurality of portions of coherent input light with regard to at least one of its phase and its amplitude. The modulator equipment includes at least one separate electro optical modulator for separately modulating one discrete portion of the plurality of portions of coherent input light, and is bypassed by others of the portions of the plurality of portions of coherent input light. Here, the fact that the pupil areas into which the optics direct the portions of coherent input light are "non-identical" has the same meaning as in the context of the method according to the present invention.

The separate electro optical modulator may particularly be arranged for being bypassed by all other portions of the plurality of portions of coherent input light. Further, the modulator equipment may include at least two or at least three or at least four separate electro optical modulators which are configured and arranged for differently modulating at least two or at least three or at least four discrete portions of the plurality of portions of coherent input light.

These at least two or at least three or at least four separate electro optical modulators may be configured and arranged for separately modulating the at least two or at least three or at least four discrete portions of the plurality of portions of coherent input light in that each of the at least two or at least three or at least four separate electro optical modulators is bypassed by all other portions of the plurality of portions of coherent input light.

Most preferably, the modulator equipment includes one separate electro optical modulator for each discrete portion of the plurality of portions of coherent input light, although the apparatus according to the present invention will already achieve a good performance with the modulator equipment including one separate electro optical modulator per pair of portions of the plurality of portions of coherent input light as the relative phase and amplitude of the two discrete portions of coherent input light of each pair of portions will be decisive. In a particular embodiment of the apparatus according to the present invention, the separate electro optical modulators are integrated optical light modulators integrated into optical fibers each configured for guiding one portion of the plurality of portions of coherent input light. As already mentioned above, such integrated optical light modulators are commercially available. These commercially available integrated optical light modulators can be used in the method according to the present invention without modification.

In the apparatus according to the present invention, each fiber of a plurality of optical fibers may be used to guide one discrete portion of the plurality of portions of the coherent input light. Then, a projection optic can be used to collimate the coherent input light getting out of the ends of the fibers of the plurality of optical fibers and to project its discrete portions into the pupil of the objective lens.

The separate electro optical modulators employed in the apparatus according to the present invention may particularly be selected from Pockels-cells and Kerr-cells. Generally, any other type of known electro optical modulator may also be used, particularly if it may be operated both quickly and precisely with regard to the modulation of the at least one of the phase and amplitude of the light modulated.

The apparatus according to the present invention may comprise a beam splitting device configured and arranged for splitting a beam of coherent light to provide the discrete portions of coherent input light. The beam splitting device may include at least one polarizing or non- polarizing beam splitter made as a cube or plate. A polarizing beam splitter may be combined with wave plates for adjusting the relative intensities and the polarizations of the discrete portions of coherent input light. The beam splitting device may also include at least one birefringent beam splitter other than a polarizing beam splitter or a birefringent beam displacer. This at least one birefringent beam splitter may, for example, be a Wollaston prism. Further, the beam splitting device may include at least one fiber optical beam splitter. The beam splitting device and also the optics directing the plurality of portions of coherent input light into the pupil of the objective lens may additionally include at least one birefringent device like, for example, a beam displacer for adjusting lateral distances between the discrete portions of coherent input light to lateral distances between the electro optical modulators and/or lateral distances between the non-identical pupil areas of the pupil of the objective lens. In one particular embodiment of the apparatus according to the present invention, a monolithic optical unit includes the beam splitting device, the modulator equipment and at least one part of the optics. In this monolithic optical unit, all relevant parts of the beam splitting device and the modulator equipment and the at least one part of the optics are fixed with regard to each other. With such a monolithic unit, adjusting the apparatus and keeping a proper adjustment of the apparatus of the present invention becomes much easier than with individual optical elements which are all moveable with regard to each other.

The apparatus according to the present invention may comprise a monitoring equipment including a monitoring camera and configured and arranged for coupling out and projecting a fraction of each discrete portion of the plurality of portions of coherent input light onto the monitoring camera for monitoring the positions of the individual discrete portions in the pupil of the objective lens. The pictures of the monitoring camera may be evaluated automatically and used for readjusting the positions of the individual discrete portions in the pupil of the objective lens, if necessary.

Particularly, the optics of the apparatus according to the present invention may be configured for directing the portions of the plurality of portions of coherent input light into the pupil areas such that the portions are uniformly distributed around the optical axis of the objective lens. With n non- identical pupil areas, a basic phase offset between two discrete portions of coherent input light directed into two neighboring pupils areas may then be 360 n or 7207n or 0°.

The apparatus according to the present invention may comprise a controller controlling the separate electro optical modulators. Particularly the controller may control the separate electro optical modulators such as to alternately separately modulate the discrete portions of coherent input light to alternately provide for first basic phase offsets and for second basic phase offsets between the discrete portions of coherent input light, the first basic offsets differing from the second basic offsets. In one embodiment, the apparatus according to the present invention comprises a light source alternately providing the plurality of the discrete portions of coherent input light with a first wavelength and with a second wavelength of the coherent input light, the second wavelength differing from the first wavelength. Then the controller may control the separate electro optical modulators to separately modulate the discrete portions of coherent input light differently with the first wavelength and with the second wavelength of the coherent input light. Further, the apparatus according to the present invention may comprise a controller controlling the separate electro optical modulators to further separately modulate the discrete portions of coherent input light to compensate for aberrations of the plurality of portions of coherent input light caused by the objective lens or any other optic. The respective controller of the apparatus according to the present invention may particularly comprise a data storage and control the separate electro optical modulators to separately modulate the discrete portions of coherent input light based on predetermined control data stored in the data storage. The predetermined control data allow for very quickly change the modulation of the discrete portions of coherent input light by means of altering the control of the separate electro optical modulators by means of the controller.

In one embodiment of the apparatus according to the present invention, the optics are configured for directing the plurality of portions of coherent input light into the plurality of non-identical pupil areas, the plurality of non-identical pupil areas including pairs of pupil areas which are axially symmetrically arranged on opposite sides of the optical axis of the objective lens. Then, the modulator equipment will include separate electro optical modulators arranged for separately modulating at least one of two discrete portions of the plurality of portions of coherent input light which are directed into the two pupil areas of each of the pairs of pupil areas.

A microscope according to the present invention comprises an apparatus according to the present invention, a detector configured and arranged for detecting photons emitted by a fluorophore molecule located in the focal area of the objective lens of the apparatus, and a register configured for registering an associated position of the local intensity minimum in the focal area for each photon detected. This microscope may be used to implement the method which has been described above and which is to be regarded as an invention by its own.

All the afore mentioned phase and amplitude settings of the portions of the coherent input light may be set and changed very fast to accommodate multiple wavelengths. For this purpose calibrated control values may be stored in a controller of the apparatus or microscope according to the present invention for very fast switching between different wavelengths of the coherent input light. Advantageous developments of the invention result from the claims, the description and the drawings. The advantages of features and of combinations of a plurality of features mentioned at the beginning of the description only serve as examples and may be used alternatively or cumulatively without the necessity of embodiments according to the invention having to obtain these advantages. Without changing the scope of protection as defined by the enclosed claims, the following applies with respect to the disclosure of the original application and the patent: further features may be taken from the drawings, in particular from the illustrated designs and the dimensions of a plurality of components with respect to one another as well as from their relative arrangement and their operative connection. The combination of features of different embodiments of the invention or of features of different claims independent of the chosen references of the claims is also possible, and it is motivated herewith. This also relates to features which are illustrated in separate drawings, or which are mentioned when describing them. These features may also be combined with features of different claims. Furthermore, it is possible that further embodiments of the invention do not have the features mentioned in the claims. The number of the features mentioned in the claims and in the description is to be understood to cover this exact number and a greater number than the mentioned number without having to explicitly use the adverb "at least". For example, if an element is mentioned, this is to be understood such that there is exactly one element or there are two elements or more elements. Additional features may be added to the features listed in the claims, or these features may be the only features of the respective method or product.

The reference signs contained in the claims are not limiting the extent of the matter protected by the claims. Their sole function is to make the claims easier to understand.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention is further explained and described with respect to preferred exemplary embodiments illustrated in the drawings.

Fig. 1 schematically depicts a first embodiment of an apparatus according to the present invention. Fig. 2 schematically depicts a variant with regard to parts of the embodiment of the apparatus according to Fig. 1.

Fig. 3 schematically depicts a second embodiment of the method according to the present invention. Fig. 4 schematically depicts an electro optical modulator used in the apparatus according to the present invention.

Fig. 5 schematically depicts a further embodiment of the apparatus according to the present invention.

Fig. 6 depicts a further embodiment of the apparatus according to the present invention additionally comprising monitoring equipment.

Fig. 7 schematically illustrates a variant of the embodiment of the apparatus according to the present invention of Fig. 6; and

Fig. 8 depicts an even further embodiment of the apparatus according to the present invention, which may be made as a monolithic unit.

DESCRIPTION OF THE DRAWINGS

The apparatus 1 depicted in Fig. 1 is provided for forming and shifting a light intensity distribution 2, 3 in a focal area 4 of an objective lens 5. Fig. 1 shows the two light intensity distributions 2 and 3 which may alternatively be formed and shifted by the apparatus 1 in a front view at an enlarged scale. The apparatus 1 comprises optics 6 for directing discrete portions 7 and 8 of coherent input light into non-identical pupil areas 1 1 and 12 of a pupil 15 of the objective lens 5 which is additionally depicted in Fig. 1 in a front view at an enlarged scale. The optics include a 50/50 beam splitter 16 and three full mirrors 17 to 19 here. The 50/50 beam splitter 16 splits a beam 20 of coherent light 20 emitted by a laser 21 into the two discrete portions 7 and 8 of coherent input light, and it separates the beam paths of the discrete portions 7 and 8. Separate electro optical modulators (EOMs) 22 and 23 are arranged in the beam paths of the discrete portions 7 and 8 for separately modulating the discrete portions 7 and 8 with regard to at least one of their respective phase and their respective amplitude. By means of this modulation, the light intensity distributions 2 and 3 can both be formed and shifted in the focal area 4. Particularly, the light intensity distribution 2 is formed, if the discrete portions 7 and 8 display a basic phase offset of 180° so that the light intensity distribution 2 displays a local intensity minimum 26 in its center which is delimited by two intensity maxima 27 in one lateral direction. The light intensity distribution 3 displaying a central intensity maximum 28 is formed with a basic phase offset between the two discrete portions 7 and 8 of 0° or 360°. By varying the phase offset and the relative amplitudes of the discrete portions 7 and 8, the light intensity distributions 2 and 3 may be shifted laterally within the focal area 4. Fig. 2 shows another embodiment of the apparatus 1 according to the present invention but only depicts those parts of the apparatus 1 from the laser 21 to the pupil 15. Here, the coherent beam 20 is coupled into an optical fiber 29 and split up into the discrete portions 7 and 8 by means of a fiber optical beam splitter 41. The electro optical modulators 22 and 23 are integrated electro optical modulators 30 integrated in optical fibers 31 and 32 guiding the discrete portions 7 and 8. The discrete portions 7 and 8 emerging out of the optical fibers 31 and 32 are collimated and projected into the pupil 15 by means of microlenses 37 and 38.

Fig. 3 shows an embodiment of the apparatus 1 in which the optics 6, similar to Fig. 2, are fiber optics. The fiber optics of Fig. 3, besides the fiber optical beam splitter 41 , include two further fiber optical beams splitters 42 and 43 so that four discrete portions 7 to 10 of coherent input light are guided by four optical fibers 33 to 36. One electro optical modulator 22 to 25 is integrated in each of the four optical fibers 33 to 36. The four discrete portions 7 to 10 of the coherent input light emerging out of the optical fibers 33 to 36 are collimated by four microlenses 37 to 40, and they are then directed into four non-identical pupil areas 1 1 to 14 of the pupil 15, which are uniformly distributed around the optical axis 44 of the objective lens 5. With a phase offset between the discrete portions 7 to 10 directed into directly neighboring pupil areas 11 to 14 of 90° and a phase offset between the discrete portions 7 to 10 directed into pupil areas 1 1 to 14 on opposite sides of the optical axis 44 of 180°, the light intensity distribution 2 is formed that displays a central intensity minimum 26 enclosed by a donut-shaped intensity minimum 27 in the focal plane of the objective lens 5. With a zero phase offset of the discrete portions 7 to 10 of 0°, the light intensity distribution 3 with the central intensity maximum 28 is formed. Both light intensity distributions 2 and 3 may be shifted in both lateral directions by means of modulating the phases and/or amplitudes of the discrete portions 7 to 10 with the electro optical modulators 22 to 25. Fig. 4 schematically shows one electro optical modulator 22 as including an amplitude modulator 45 and a phase modulator 46. In some embodiments of the apparatus 1 according to the present invention, the electro optical modulators 22 to 25 will only include the phase modulator 46, in some other embodiments they will only include the amplitude modulator 45. Fig. 5 shows a further embodiment of the apparatus 1 according to the present invention. Similar to that one of Fig. 1 , this embodiment is based on 50/50 beam splitters 16, 48 and 48 and full mirrors 17 to 19 and 49 to 53. The embodiment of the apparatus 1 of Fig. 5, however, provides four discrete portions 7 to 10 of coherent input light in four non-overlapping pupil areas 11 to 14 of the pupil 15 of the objective lens 5. Thus, the embodiment of the apparatus 1 according to Fig. 5 may provide the same light intensity distributions as that one according to Fig. 3.

This also applies to the embodiment of the apparatus 1 depicted in Fig. 6. Here, the 50/50 beam splitters 16, 47 and 48 according to Fig. 5 are replaced by polarization beam splitters 54 to 56. These beam splitters are combined with l/2 waveplates 57 to 61 which may be rotated about their optical axes for defining the relative amplitudes of the discrete portions 7 to 10 of coherent input light modulated by the electro optical modulators 22 to 25. By means of the mirrors 17 to 19 and 50 to 53 plus additional 50/50 beam splitters 62 to 64, the discrete portions 7 and 8 are recombined to be directed into the pupil 15 of the objective lens 5 and to also couple out and project fractions of each discrete portion 7 to 10 onto three monitoring cameras 65 to 67 for monitoring the positions of the individual discrete portions 7 to 10 in the pupil 15 of the objective lens 5. For this purpose, the cameras 65 to 67 are arranged in planes conjugated to the pupil 15. Further, the embodiment of the apparatus 1 according to Fig. 6 includes an adjustable polarizer 68 and a l/4 waveplate 69 at the entrance of the objective lens 5 to provide for optimum polarizations of the discrete portions 7 to 10 which are needed for forming the desired light intensity distributions 2 and 3 depending on the relative phase offsets of the discrete portions 7 to 10.

Fig. 7 shows an embodiment of the apparatus 1 combining features of the embodiments of Figs. 5 and 6. This embodiment comprises a minimum number of six 50/50 beam splitters 16, 47, 48, 62 to 64 and two full mirrors 17 and 50 to provide the same functions as the embodiment of Fig. 6 including the monitoring cameras 65 to 67. Fig. 8 shows an embodiment of the apparatus 1 in which the beam of coherent light 20 coming from the laser 21 is at first split-up by a birefringent beam displacer 71 in a first lateral direction and then by a second birefringent beam displacer 72 in a second lateral direction orthogonal to the first lateral direction. Beam cross sections 70 depicted in Fig. 8 are no parts of the apparatus but added to illustrate the effect of the beam displacers 71 to 74. The beam displacers 71 and 72 are combined with l/2 waveplates 57 and 58 for defining the relative amplitudes of the four discrete portions of coherent input light 7 to 10 emerging out of the second birefringent beam displacer 72. The discrete portions 7 to 10 of the coherent input light are then modulated by the separate electro optical modulators 22 to 25 which are depicted in a perspective view here. Afterwards, the lateral distances of the discrete portions 7 to 10 required by the lateral distances of the electro optical modulators 22 to 25 are reduced by two further birefringent beam displacers 73 and 74 in the first and second orthogonal lateral directions. With their reduced lateral distances the discrete portions 7 to 10 of the coherent input light may be directly projected into the pupil 15 of the objective lens 5 which is not depicted here. Beam cross sections 70 depicted in Fig. 8 are no parts of the apparatus but added to illustrate the effects of the beam displacers 71 to 74. The optical elements arranged downstream of the laser 21 from the l/2 waveplate 57 up to the birefringent beam displacer 74 may be combined into one monolithic unit 75 which needs no readjustment in the use of the apparatus 1. The beam displacers 71 and 72 used as beam splitters will differ from the beam displacers 73 and 74 used for reducing the lateral distances between the discrete portions 7 to 10 but not for completely merging these portions 7 to 10 of the coherent input light. All the beam displacers 71 to 74 may be made of a suitable optical material like calcite, quartz or the like. Instead of the beam displacers 71 to 74, pairs of Wollaston prisms may be used to achieve larger lateral distances or to reduce larger lateral distances between the discrete portions 7 to10 of the coherent input light. Note that Fig. 8 shall be understood as a simplified schematic showing the principles and that further birefringent elements for polarization control may be included. These elements may be common for some beams or act on single beams. Further the lateral beam distances may also be reduced by other, non-birefringent optical elements such as tilted optical flats. LIST OF REFERENCE NUMERALS apparatus

light intensity distribution

light intensity distribution

focal area

objective lens

optics

discrete portion of coherent input light

discrete portion of coherent input light

discrete portion of coherent input light

discrete portion of coherent input light

pupil area

pupil area

pupil area

pupil area

pupil

50/50 beam splitter

full mirror

full mirror

full mirror

beam of coherent light

laser

electro optical modulator

electro optical modulator

electro optical modulator

electro optical modulator

intensity minimum

intensity maximum

intensity maximum

optical fiber

integrated electro optical modulator

optical fiber

optical fiber optical fiber

optical fiber

optical fiber

optical fiber

microlens

microlens

microlens

microlens

fiber optical beam splitter fiber optical beam splitter fiber optical beam splitter optical axis

amplitude modulator phase modulator 50/50 beam splitter 50/50 beam splitter full mirror

full mirror

full mirror

full mirror

full mirror

polarization beam splitter polarization beam splitter polarization beam splitter l/2 waveplate l/2 waveplate l/2 waveplate l/2 waveplate l/2 waveplate

50/50 beam splitter 50/50 beam splitter 50/50 beam splitter monitoring camera monitoring camera monitoring camera polarizer l/4 waveplate beam cross section beam displacer beam displacer beam displacer beam displacer monolithic unit