Patent Description:
In microscopic and spectroscopic applications, acousto-optic tunable filters are conventionally used for filtering selected wavelengths from light which is generated by broad-band laser sources such as supercontinuum lasers. The optical filter function of such devices can be easily changed by applying a specific radio frequency signal to a piezoelectric transducer which generates an acoustic wave in the filter crystal. The acoustic wave causes refractive index fluctuations in the filter material, these fluctuations acting as a diffraction grating. Thus, the light propagating through the filter material is diffracted in case that the light wavelength matches the diffraction grating. The bandwidth of the output light transmitted through the filter is determined by the specific filter design which may take into account parameters of such as the orientation of the filter crystal with respect to the incident light direction and the length of the transducer.

Existing filter designs are excellent in terms of diffraction efficiency, but somewhat at the expense of a significant wavelength-dependent angular spread of the output light emitted by the acousto-optic tunable filter. A wavelength-dependent angular spread may be tolerated to some extent in a number of applications. However, specifically in high resolution imaging applications such as confocal microscopy it is desirable to have almost perfectly collinear light output from the acousto-optic tunable filter over the entire spectral bandwidth.

Furthermore, in some applications as for instance in Raman spectroscopy it is essential to illuminate a specific target region of the sample with two or three different wavelengths at the same time. However, since existing filter designs tolerate a significant wavelength-dependent angular spread of the output light, it is still difficult to implement an efficient sample illumination using a single acousto-optic tunable filter in those applications.

Document <CIT> discloses an optical filter device according to the preamble of claim <NUM>. The device includes an acousto-optical element <NUM> which is arranged in a beam path of an incident light beam. The device is configured to reduce the chromatic spread angle of the light diffracted by the acousto-optical element. For this, the device includes two focusing optical systems which are arranged upstream and downstream of the acousto-optical element, respectively. The upstream focusing optical system is arranged such that the light rays of the incident light beam are bundled into a focus within the acousto-optical element.

Document <CIT> discloses an arrangement for multi-color illumination comprising an acousto-optical tunable filter that is configured to diffract two light components of incident illumination light into different diffraction order directions.

It is an object to provide an optical filter device and a method enabling an acousto-optic tunable filter to simultaneously illuminate a specific target region of a sample with multiple wavelengths.

An optical filter device for illuminating a sample comprises a light supply unit configured to generate input light of a predetermined spectral bandwidth in a collinear manner. The optical filter device further comprises an acousto-optic tunable filter configured to receive the input light and to emit diffracted output light including multiple wavelengths from the spectral bandwidth at exit angles towards the sample, said exit angles defining an angular spread among the multiple wavelengths of the output light. The angular spread among the multiple wavelengths of the output light is determined by an angular characteristic varying from a minimum wavelength to a maximum wavelength. The angular characteristic has an extremum at an intermediate wavelength between the minimum wavelength and the maximum wavelength to provide at least one group of different wavelengths having the same exit angles.

Conventional filter designs usually have angular characteristics which vary monotonically from a minimum wavelength to a maximum wavelength. In contrast, the new filter design proposes an angular characteristic having an extremum between the minimum wavelength and the maximum wavelength which enables different wavelengths to have the same exit angle when being emitted from the acousto-optic tunable filter. Thus, the acousto-optic tunable filter is enabled to simultaneously illuminate a specific target region of a sample with multiple wavelengths.

Accordingly, the optical filter device can be used advantageously in microscopic applications where it is important to illuminate a specific point in the sample with different wavelengths at the same time. Taking into account the specific conditions in various microscopic applications, the inventor realized that an acousto-optic tunable filter can be designed such that, on the one hand, the exit angle is the same with respect to at least two wavelength and, on the other hand, any limitations that may result from the new filter design compared to conventional designs are not detrimental in the specific applications. In particular, the inventor recognized that it is possible to achieve the intended angular filter characteristic while limiting the light entry into the acousto-optic tunable filter to a collinear entry, and a collinear light entry over a certain spectral bandwidth is provided anyway in the applications under consideration. As result, an improved optical filter device is provided for applications with collinear light entry without having to accept any limitations which would be relevant for these applications.

Furthermore, providing the angular characteristic with an extremum allows to reduce the wavelength-dependent angular spread among the multiple wavelengths emitted from the acousto-optic tunable filter. Preferably, the acousto-optic tunable filter is formed in accordance with a filter design determining the angular acceptance range with respect to the input light and the angular spread among the multiple wavelengths of the output light. The filter design may deviate from a parallel-tangent condition to reduce the angular spread among the multiple wavelengths of the output light while limiting the angular acceptance range with respect to the input light over the spectral bandwidth thereof to collinear light incidence.

Conventionally, an acousto-optic tunable filter is designed to operate in the so-called parallel-tangent condition. A filter design which is based on the parallel-tangent condition yields an acousto-optic tunable filter with a wide angular acceptance range which corresponds to the field of view (FOV) of the filter. As known in the art, the parallel-tangent condition means that the tangents to the index surfaces on the wave-vector surfaces of the incident and the diffracted polarizations at the points of intersection of the corresponding waves are parallel to each other. In this respect, reference is made e.g. to [<NUM>], <NPL>; [<NUM>], <NPL>); and [<NUM>], <NPL>, <FIG> (https://www. com/App-Manual_pdf//AOTF-Notes. Such a conventional filter design satisfying the parallel-tangent condition has the advantage of a wide angular acceptance range. However, the inventor realized that this advantage comes at the expense of a relatively large angular spread of the output light over the spectral bandwidth given by the input light, and the inventor further realized that the angular spread may be detrimental in specific applications as for example in confocal microscopy. Surprisingly, the inventor found out that a new filter design deviating from the ubiquitous parallel-tangent condition enables the wavelength-dependent angular spread of the output light to be significantly reduced compared to conventional filter designs that satisfy the parallel-tangent condition. When deviating from the parallel-tangent condition, it must be accepted that the angular acceptance range of the acousto-optic tunable filter with respect to the input light is reduced compared to the conventional filter design. Therefore, the solution presented herein proposes that the angular acceptance range of the acousto-optic tunable filter is essentially limited to a collinear light incidence. Thus, a limitation to collinear light incidence can be easily implemented in a number of possible applications, in particular in confocal microscopy.

It is to be noted that the technical teaching proposed herein is not limited to a filter design which deviates from the parallel-tangent condition as described above. A deviation from the parallel-tangent condition is to be considered merely as an example for a suitable design. Any other filter design can be applied as long as the acousto-optic tunable filter provides the desired angular characteristic by limiting the light entry over the spectral bandwidth to a collinear input.

Preferably, the filter design is determined in relation to a reference filter design satisfying the parallel-tangent condition to reduce both the angular acceptance range with respect to the input light and the angular spread among the multiple wavelengths in comparison with the reference filter design. Thus, it is been found that even a small deviation from the parallel-tangent condition reduces the wavelength-dependent angular spread of the output light significantly.

Preferably, the filter design comprises at least one filter parameter selected from a group including an orientation of the light entrance surface, an orientation of the light emission surface, an orientation of the transducer surface, and a dimension of the transducer. By modifying one or more of the above filter parameters as compared to the conventional filter design a significant reduction of the angular spread among the multiple wavelengths filtered out from the spectral bandwidth of the input light can be achieved.

According to a preferred embodiment, the at least one group is a pair of different wavelengths or a triple of different wavelengths. When using two different wavelengths per group at the same exit angle, the acousto-optic tunable filter may be regarded as an achromatic configuration. Further, when using three different wavelengths per group at the same exit angle, the acousto-optic tunable filter may be regarded as an apochromatic configuration.

In this respect, it is to be noted that achromatic and apochromatic designs are known in the field of optical lenses. In optical lenses, due to the dependence of the refractive index of the glass medium on the incident wavelength, the effective focal length is not the same for all wavelengths. In other words, chromatic aberration occurs. To correct for this chromatic aberration, achromatic and apochromatic lens configurations use two and three types of glass materials with different refractive indices, respectively. While the new filter design also achieves such an achromatic or apochromatic effect, it is not necessary to combine multiple filter materials to achieve this effect. Rather, the effect is achieved in the new filter design due to the wavelength-dependent refractive index and the wavelength-dependent birefringence of the filter crystal material which may be tellurium dioxide.

Preferably, the minimum wavelength and the maximum wavelength define an operating range in which the acousto-optic tunable filter is operated for tuning the wavelengths of the output light. The operating range may be determined by the control unit.

Preferably, the angular acceptance range with respect to the input light is limited to to ±<NUM> milliradians, preferably +/- <NUM> milliradian relative to a nominal direction of light incidence.

According to a preferred embodiment, the optical filter device comprises a control unit which is configured to tune the acousto-optic tunable filter for illuminating the sample with at least one group of different wavelengths at the same exit angle.

Preferably, the control unit is configured to select the at least one group of different wavelengths based on the angular characteristic in response to a user input. Thus, a user is enabled to freely select suitable wavelengths which are collinearly emitted from the acousto-optic tunable filter.

The control unit may further be configured to tune the acousto-optic tunable filter simultaneously to the different wavelengths of the at least one group. Thus, it is ensured that the different wavelengths of the output light are irradiated at the same time onto the sample.

In a preferred embodiment, the optical filter device comprises a first light reflecting element which is configured to deflect the at least one group of different wavelengths output by the acousto-optic tunable filter into a predetermined target direction. Thus, all wavelength groups are enabled to propagate along the same direction towards the sample despite the fact that these groups are emitted from the acousto-optic tunable filter at the different exit angles.

Preferably, a memory may be provided to store calibration data which represent an amount for actuating the first light reflecting elements depending on the exit angle assigned to the at least one group of different wavelengths for deflecting said group into the predetermined target direction, wherein the control unit is configured to control the first light deflecting element based on the calibration data. The first light deflecting element may be formed from a galvanometer mirror.

According to a further embodiment, a second light deflecting element may be provided which deflects the at least one group of different wavelengths output by the acousto-optic tunable filter onto a predetermined target position on the sample. The second light deflecting element may be used to scan the sample with the multiple wavelengths of the output light which are emitted from the acousto-optic tunable filter at the same exit angle.

Preferably, the optical device comprises an illumination objective configured to illuminate the sample with the output light emitted by the acousto-optic tunable filter. As the new filter design ensures that multiple wavelengths of the output light are irradiated in a collinear manner onto the sample, a lateral shift of the illumination focus can be prevented from occurring even if the illumination objective has a high magnification.

The light supply unit may comprise one or more supercontinuum laser sources, for example white light lasers, without being limited thereto.

According to another aspect, a microscope is provided which comprises the optical filter device described above.

Further, a method for illuminating a sample is provided, comprising the steps of generating input light of a predetermined spectral bandwidth in a collinear manner, and using an acousto-optic tunable filter for receiving the input light and emitting diffracted output light including multiple wavelengths from the spectral bandwidth at exit angles towards the sample, said exit angles defining an angular spread among the multiple wavelengths of the output light. The angular spread among the multiple wavelengths of the output light is determined by an angular characteristic varying from a minimum wavelength to a maximum wavelength. The angular characteristic has an extremum at an intermediate wavelength between the minimum wavelength and the maximum wavelength to provide at least one group of different wavelengths having the same exit angle.

Preferably, the sample may be illuminated with the output light emitted from the acousto-optic tunable filter in order to perform confocal microscopy, multiphoton microscopy, Coherent Anti-Stokes Raman (CARS) microscopy, Stimulated Raman Scattering (SRS) microscopy, Sum-Frequency Generation (SFG) microscopy, Difference-Frequency Generation (DFG) microscopy, and/or Frequency-Modulated Stimulated Raman Scattering (FM-SRS) microscopy without being limited thereto.

The afore-mentioned microscopic applications have in common that a specific target region of the sample must be illuminated simultaneously with different wavelengths. Therefore, the optical filter device can be used advantageously in these applications to provide the required sample illumination.

<FIG> shows a microscope <NUM> which includes an optical filter device <NUM> according to an embodiment. The microscope <NUM> may be configured as a confocal microscope which is used to acquire an image of a sample <NUM> which is arranged on a microscope stage <NUM>. It is to be noted that <FIG> shows only those components which may help to understand the technical teaching disclosed herein. In particular, <FIG> focuses on those components which are used to illuminate the sample <NUM>. Needless to say that the microscope <NUM> may comprise additional components, in particular those components which are required for image acquisition as for example one or more image sensors / light sensors which are not shown in <FIG>.

The optical filter device <NUM> of the microscope <NUM> comprises a light supply unit <NUM> which is configured to generate input light <NUM> which is directed to an acousto-optic tunable filter (AOTF) <NUM> which includes a tunable piezoelectric transducer <NUM>. According to the embodiment shown in <FIG>, the light supply unit <NUM> may comprise one or more broadband laser sources, for example a supercontinuum laser source <NUM> which is configured to emit the input light <NUM> with a broad spectral bandwidth. Just as an example, it may be assumed that the light supply unit <NUM> is capable of providing a spectral bandwidth which ranges from <NUM> to <NUM>.

The optical filter device <NUM> further comprises a control unit <NUM> including a memory <NUM> in which various control data may be stored. The control unit <NUM> can be used to control an overall operation of the optical filter device <NUM>. Specifically, the controller <NUM> may control the light supply unit <NUM> and the piezoelectric transducer <NUM> of the AOTF <NUM>. In addition, the control unit <NUM> may control first and second light deflection elements <NUM>, <NUM> to deflect output light <NUM> emitted from the AOTF <NUM> as described in more detail below. For actuating the light deflecting elements <NUM>, <NUM> suitable drive means such as motors may be provided.

According to the embodiment shown in <FIG>, the optical filter device <NUM> serves to spectrally filter the illumination light which is irradiated onto the sample <NUM> through an illumination objective <NUM>. The illumination light is formed from the output light <NUM> which is emitted from the AOTF <NUM> and directed by means of the light deflecting elements <NUM>, <NUM> towards the sample <NUM>. In the present embodiment, the AOTF <NUM> enables filtering of desired spectral components from the broad spectral bandwidth of the input light <NUM> provided by the light supply unit <NUM>. The operating principle for filtering the input light <NUM> based on wavelength-dependent light diffraction is illustrated in <FIG> showing the AOTF <NUM> alone.

The AOTF <NUM> comprises a filter body <NUM> formed from a crystal such as tellurium dioxide (TeO<NUM>) without being limited thereto. The filter body <NUM> has a light entrance surface <NUM> which is configured to receive the input light <NUM> within an angular acceptance range A. In <FIG>, an arrow designated with reference sign <NUM> indicates a nominal direction of light incidence. The angular acceptance range A is defined by an angular tolerance ± α by which the light incidence is allowed to deviate from the aforementioned nominal direction. The specific value of the angular tolerance α is predetermined in accordance with the specific filter design based on which the AOTF <NUM> is manufactured. As explained below in more detail, according to the embodiment, the angular acceptance range A is limited to collinear light incidence over the entire spectral bandwidth of the input light <NUM>. Accordingly, it is to be noted that the width of the angular acceptance range A (and correspondingly the angle tolerance α) is shown exaggeratedly large in <FIG>. More specifically, the angular acceptance range A with respect to the input light <NUM> may be limited over the entire spectral bandwidth to ± α = +/-<NUM> milliradians, preferably +/-<NUM> milliradian relative to the nominal direction of light incidence.

The filter body <NUM> of the AOTF <NUM> comprises a transducer surface <NUM> on which the piezoelectric transducer <NUM> is attached. Further, the filter body <NUM> has a light emission surface <NUM> through which the spectrally filtered output light <NUM> is emitted from the AOTF <NUM>.

The optical filter function of the AOTF <NUM> can be easily changed by applying a suitable radio frequency (RF) signal to the transducer <NUM>. For this purpose, the control unit <NUM> shown in <FIG> may determine the RF signal e.g. in response to a user input and apply the RF signal to the transducer <NUM>. It is to be noted that the control unit <NUM> may simultaneously apply a plurality of RF signals to the transducer <NUM> so that the output light <NUM> comprises multiple wavelengths from the spectral bandwidth of the input light <NUM>, each wavelength being assigned to one of the RF frequency signals applied to the transducer <NUM>. For the sake of simplicity, it is assumed that <FIG> illustrates a case where only one wavelength is selected from the broad spectral bandwidth provided by the input light <NUM>.

In principle, the AOTF <NUM> operates as known in the art for spectrally filtering the input light <NUM>. Thus, upon receiving the RF signal from the control unit <NUM>, the transducer <NUM> of the AOTF <NUM> generates an acoustic wave which propagates through the filter body <NUM>. When propagating through the filter body <NUM>, the acoustic wave causes fluctuations of the refractive index in the crystal material of the filter body <NUM>, these fluctuations acting as a diffraction grating with respect to the light transmitting the filter body <NUM>. By tuning the RF signal and thus also the frequency of the acoustic wave, a desired wavelength can be filtered out from the spectral bandwidth of the input light <NUM> through acousto-optical diffraction.

In the example of <FIG> it is assumed that the input light <NUM> entering the filter body <NUM> through the light entrance surface <NUM> is unpolarized light. It is further assumed that the output light <NUM> which is used for illuminating the sample <NUM> is given by a diffracted beam of first order having a specific polarization, e.g. horizontal (or vertical) polarization. In this case, a light beam <NUM> shown in <FIG> represents a diffracted beam of first order having the other polarization, i.e. vertical (or horizontal) polarization. Further, light beams to <NUM>, <NUM> represent light beams which are not diffracted, i.e. beams of zero order having vertical and horizontal polarizations, respectively. As can be seen in <FIG>, the light beams <NUM>, <NUM>, <NUM>, <NUM> are emitted from the AOTF <NUM> at different exit angles so that they are spatially separated from each other. Since only the output light <NUM> is used for illumination, the light beams to <NUM>, <NUM>, <NUM> may be absorbed by suitable means such as a light trap (not shown).

Before continuing the description of the embodiments disclosed herein, a comparative example is explained in the following with reference to <FIG>.

When designing AOTF crystals, conventionally the parallel-tangent condition is applied. In AOTF crystals that have been designed to satisfy the parallel-tangent condition, the tangents on the wave-vector surfaces of the incident and the diffracted polarizations at the points of intersection of the corresponding waves are parallel to each other. AOTF crystals satisfying the parallel-tangent condition are very tolerant to variations in the angle of light incidence. This ensures that the maximum diffraction efficiency does not change significantly even if the angle of the incident light beam with respect to the light entrance surface is different from a nominal or design value. In other words, if the parallel-tangent condition is satisfied, the angular acceptance range is relatively large.

<FIG> shows a comparative example with an AOTF <NUM>. As in the embodiment, the AOTF <NUM> according to the comparative example comprises a filter body <NUM> which may be formed from a TeO<NUM> crystal or any other suitable material. Furthermore, as in the embodiment, the AOTF <NUM> comprises a light entrance surface <NUM>, a transducer surface <NUM> on which a piezoelectric transducer <NUM> is attached, and a light emitting surface <NUM>. However, in contrast to the embodiment, the AOTF <NUM> according to the comparative example is manufactured based on a conventional filter design which satisfies the parallel-tangent condition. Consequently, an angular acceptance range A' (corresponding to an angular tolerance α') is relatively large.

However, <FIG> also shows, as realized by the inventor, that satisfying the parallel-tangent condition in order to achieve the relatively large angular acceptance range A' may come at the expense of a relatively large angular spread C' among the multiple wavelengths of the output light <NUM> over the entire spectral bandwidth which is provided by the input light <NUM>. The aforementioned angular spread C' is determined by the different exit angles at which the multiple wavelengths of the output light <NUM> are emitted from the light emission surface <NUM> of the AOTF <NUM>. The greater the spectral bandwidth of the input light <NUM>, the greater the angular spread C' of the output light <NUM>, provided that the entire spectral bandwidth shall be usable. As explained below in more detail, deviating from the conventional filter design satisfying the parallel-tangent condition may be one option among several that allow to reduce the angular spread of the output light <NUM> (indicated as C in <FIG>) at the expense of limiting the angular acceptance range with respect to the input light <NUM> (indicated as A and α in <FIG>). Again, the chromatic spreads C, C' and the widths of the angular acceptance ranges A, A' are shown exaggeratedly large in <FIG> to qualitatively illustrate the proportions between the comparative example and the embodiment. In particular, <FIG> serves to illustrate that the chromatic spread C is significantly reduced in relation the comparative example which satisfies the parallel-tangent condition.

For a commercially available AOTF operating e.g. in a range of <NUM> to <NUM>, the difference in the exit angles between <NUM> and <NUM> is about <NUM> microradians as shown in <FIG> is a graph illustrating an angular characteristic W1 of the AOTF <NUM> that determines the angular spread C' among the multiple wavelengths of the output light <NUM>. The angular characteristic W1 in <FIG> indicates how the exit angles of the output light <NUM> vary depending on the wavelengths. In <FIG>, the abscissa of the graph designates the wavelengths of the output light <NUM> while the ordinate of the graph designates the exit angles of the output light <NUM> relative to the light emission surface <NUM>. (The absolute values of the exit angles are not essential in this context. It is rather the relative values that matter. ) The graph of <FIG> shows that the exit angles of the output light <NUM> monotonically vary by a relatively large amount between a minimum wavelength and a maximum wavelength.

A large angular spread C' may cause significant problems in applications where the light emission from the AOTF is desired to be as collinear as possible over the entire spectral bandwidth, such as in confocal microscopy. <FIG> illustrates an example in which the output light <NUM> emitted from the AOTF <NUM> includes a red spectral component 126r, a green spectral component <NUM>, and a blue spectral component 126b. It shall be assumed that the different spectral components126r, <NUM>, 126b are directed into the illumination objective <NUM> having an exemplary magnification of <NUM>. Considering an angular spread of <NUM> microradians in this example, a lateral focal shift of <NUM> is caused in a focal plane <NUM> of the illumination objective <NUM>. Such a lateral focus shift is close to the size of a typical point-spread function (PSF). As a result, image quality may deteriorate in case of a large angular spread.

In order to solve the aforementioned problem, the embodiments disclosed herein may be based on a filter design which deviates from the parallel-tangent condition to some extent. For this purpose, the filter design may be determined in relation to a reference design satisfying the parallel-tangent condition to reduce both the angular acceptance range A with respect to the input light <NUM> and the angular spread C among the multiple wavelengths of the output light <NUM> when compared with the reference filter design. Generally, the filter design based on which the AOTFs according to the present embodiments are manufactured may comprise at least one filter parameter selected from a group including an orientation of the light entrance surface <NUM>, an orientation of the light emission surface <NUM>, an orientation of the transducer surface <NUM>, and a dimension of the transducer <NUM>.

More specifically, the filter design applied to the embodiments may provide for a reorientation of internal crystal angles slightly away from the parallel-tangent condition. For instance, by varying the angle of light incidence by <NUM>°, the angle of the light entrance surface <NUM> by <NUM>° and the angle of the transducer surface <NUM> by less than <NUM>° from the respective ideal values of the reference filter design satisfying the parallel-tangent condition, an AOTF design can be achieved which has a significantly smaller angular spread C among the multiple wavelengths of the output light <NUM> as compared to the reference filter design. Surprisingly, small variations in the above filter parameters have been found to produce significant reductions of the angular spread C. As already mentioned above, reducing the angular spread C while violating the parallel-tangent condition comes at the expense of limiting the angular acceptance range A with respect to the input light <NUM>. However, in applications such as confocal microscopy it is possible to generate the input light <NUM> over the entire spectral bandwidth in a collinear manner so that there is no disadvantage when limiting the angular acceptance range A of the AOTF.

It should be noted again that the above filter parameters may also be selected such that they do not necessarily deviate the from parallel-tangent condition. Thus, strictly limiting the light entry to a collinear entry throughout the entire spectral bandwidth may give sufficient design freedom even within the parallel-tangent condition framework to achieve the desired angular characteristic with two or more wavelengths at the same exit angle.

Hereinafter, with reference to <FIG>, embodiments are disclosed having angular characteristics which enable the angular spread C of the output light <NUM> to be significantly reduced compared to a conventional filter design.

<FIG> is a graph showing an angular characteristic W2 according to an embodiment which is to be compared with the angular characteristic W1 of the conventional filter design shown in <FIG>. As in <FIG>, the angular characteristic W2 according to the embodiment of <FIG> varies from a minimum wavelength of <NUM> to a maximum wavelength of <NUM>. The entire variation of the angular characteristic W2 from <NUM> to <NUM> is less than <NUM> microradians which is a reduction in the angular spread by almost a factor of <NUM> compared to the conventional filter design of <FIG>. The angular characteristic W2 has an extremum (minimum) at an intermediate wavelength which is in a range between <NUM> to <NUM>. In other words, while the angular characteristic W1 of the conventional filter design shown in <FIG> monotonically varies from the minimum wavelength to the maximum wavelength, the angular characteristic W2 according to the embodiment shown in <FIG> at first decreases from the minimum wavelength to the intermediate wavelength where the extremum of the characteristic W2 is located and then increases from the intermediate wavelength to the maximum wavelength. Accordingly, the new filter design deviates from a monotonically varying angular characteristic enabling the angular spread C to be reduced.

<FIG> is a graph showing an angular characteristic W3 according to another embodiment. The angular characteristic W3 varies from a minimum wavelength of <NUM> to a maximum wavelength of <NUM>. The entire variation of the angular characteristic W3 is less than <NUM> microradians. The angular characteristic W3 has an extremum (maximum) at an intermediate wavelength which is in a range between <NUM> to <NUM>. Accordingly, while the angular characteristic W1 of the conventional filter design shown in <FIG> monotonically varies from the minimum wavelength to the maximum wavelength, the angular characteristic W3 at first increases from the minimum wavelength to the intermediate wavelength where the extremum of the characteristic W3 is located and then decreases from the intermediate wavelength to the maximum wavelength. Accordingly, the new filter design deviates from a monotonically varying angular characteristic which enables the angular spread C to be reduced.

<FIG> is a graph showing an angular characteristic W4 according to another embodiment. The angular characteristic W4 varies from a minimum wavelength of <NUM> to a maximum wavelength of <NUM>. The entire variation of the angular characteristic W3 is less than <NUM> microradians. The angular characteristic W4 has an extremum (maximum) at an intermediate wavelength which is in a range between <NUM> to <NUM>. Accordingly, while the angular characteristic W1 of the conventional filter design shown in <FIG> monotonically varies from the minimum wavelength to the maximum wavelength, the angular characteristic W3 at first increases from the minimum wavelength to the intermediate wavelength where the extremum of the characteristic W3 is located and then decreases from the intermediate wavelength to the maximum wavelength. Accordingly, the new filter design deviates from a monotonically varying angular characteristic which enables the angular spread C to be reduced.

As explained above, each of the angular characteristics W2 to W4 shown in <FIG> have an extremum at an intermediate wavelength within a wavelength range where the AOTF <NUM> is operated. Therefore, each angular characteristic W2 to W4 provides a plurality of pairs P1, P2, P3, each pair including two wavelengths having the same exit angle when being emitted from the AOTF <NUM> as indicated in <FIG> by dashed lines, respectively.

<FIG> shows the AOTF <NUM> when designed to implement one of the angular characteristics illustrated in <FIG>. As can be seen in <FIG>, each group P1, P2 and P3 includes two wavelengths illustrated as solid and dashed lines, respectively, which are emitted from the AOTF <NUM> at the same exit angle. In other words, while there is still some angular spread among the multiple wavelengths of the output light <NUM> (albeit significantly reduced compared to the conventional filter design), the angular characteristic enables the wavelengths of each group P1, P2, P3 to exit from the AOTF <NUM> in a perfectly collinear manner.

<FIG> is a graph showing an angular characteristic W5 according to another embodiment. The angular characteristic W5 of <FIG> is an example where not only one extremum but several extrema are present.

The angular characteristic W5 according to the embodiment of <FIG> varies from a minimum wavelength of <NUM> to a maximum wavelength of <NUM>. The angular characteristic W5 has a first extremum (minimum) at a first intermediate wavelength which is between <NUM> and <NUM>. Further, the angular characteristic W5 as a second extremum (maximum) at a second intermediate wavelength which is between <NUM> and <NUM>. Accordingly, the characteristic W5 decreases from the minimum wavelength to the first intermediate wavelength and then increases from the first intermediate wavelength to the second intermediate wavelength. Finally, the angular characteristic W5 decreases from the second intermediate wavelength to the maximum wavelength.

As the angular characteristic W5 of <FIG> has both a minimum and a maximum at the first and second intermediate wavelengths, respectively, the angular characteristic W5 provides a plurality of triples T1, T2, T3, each triple including three wavelengths having the same exit angle when being emitted from the AOTF <NUM> as indicated in <FIG> by dashed lines.

<FIG> shows the AOTF <NUM> when designed to implement the angular characteristic illustrated in <FIG>. As can be seen in <FIG>, each triple T1, T2 and T3 includes three wavelengths illustrated as solid, dashed and dash-dotted lines, respectively, which are emitted from the AOTF <NUM> at the same exit angle. Accordingly, while there is still some angular spread among the multiple wavelengths of the output light <NUM> (albeit significantly reduced compared to the conventional filter design), the angular characteristic W5 ensures that the wavelengths of each group T1, T2, T3 exit from the AOTF <NUM> in a perfectly collinear manner.

<FIG> is a graph showing the angular characteristics W1 to W5 described above in combination. As can be seen in <FIG>, the angular characteristics W2 to W5 which are based on the new filter design deviating from the parallel-tangent condition exhibit a significantly smaller angular spread compared to the conventional filter design which satisfies the parallel-tangent condition. Further, it is evident from <FIG> that the new filter design is suitable to cover a broad spectral bandwidth from <NUM> to <NUM> including visible and near infrared light.

By providing pairs or triplets of wavelengths having the same exit angle it is possible to illuminate the sample <NUM> with two and three wavelengths, respectively, which are impinging a target region of the sample <NUM> perfectly collinear to each other. For this purpose, the control unit <NUM> may be configured to tune the AOTF <NUM> simultaneously to the wavelengths of one pair P1, P2, P3 or one triple T1, T2, T3 so that these wavelengths irradiate the target region of the sample <NUM> simultaneously. The control unit <NUM> may be further adapted to select one pair P1, P2, P3 or one triple T1, T2, T3 in response to a user input.

As already mentioned above with reference to <FIG>, the first and second light reflecting elements <NUM>, <NUM> can be used to direct the output light <NUM> onto the sample <NUM>. Thus, the first light reflecting element <NUM> may be actuated via the control unit <NUM> to deflect the respective group P1-P3, T1-T3 of different wavelengths into a predetermined target direction. This ensures that all groups P1-P3, T1-T3 propagate along the same direction towards the sample <NUM> despite the different exit angles at which they are emitted from the AOTF <NUM>. When propagating along the predetermined target direction, the output light <NUM> may fall onto the second deflecting element <NUM> which is activated via the control unit <NUM> to deflect the output light <NUM> onto a predetermined target position on the sample <NUM>. For instance, the second light reflecting element <NUM> may be formed from a scanning device which is configured to scan the sample <NUM> with the output light <NUM>.

The memory <NUM> of the control unit <NUM> may store calibration data such as a calibration table. Based on the calibration data, the control unit <NUM> actuates the first light deflecting element <NUM> to deflect the respective group P1-P3, T1-T3 of different wavelengths into the predetermined target direction. The calibration data may represent an amount for actuating the first light deflecting elements <NUM> depending on the exit angle at which the respective group exits the AOTF <NUM>.

The new filter design which enables two or three different wavelengths to be emitted from the AOTF <NUM> at the same exit angle can be used in applications which require multiple wavelengths be incident on the sample <NUM> at the same time and at the same location. One example is Coherent Anti-Stokes Raman (CARS) microscopy where two different wavelengths λ1 and λ2 are directed simultaneously towards the sample. The difference between inverse of the wavelengths λ1 and λ2 is proportional to the vibrational frequency difference: <MAT>.

In the above relationship, Δv designates the (vibrational) frequency difference, and θ designates the unique exit angle at which the wavelengths λ1 and λ2 are emitted from the AOTF <NUM>.

The frequency difference corresponds to the energy of a vibrational mode in the sample under investigation. When the two wavelengths λ1 and λ2 impinge the sample at the same time and at the same location, a strong non-linear optical effect occurs in the sample. As a result, a new wavelength is generated at <NUM>/(<NUM>/λ1- <NUM>/λ2). For efficient generation of this new wavelength, it is imperative that the two incident wavelengths strike at the same target point and along the same direction in the sample.

<FIG> and <FIG> illustrate how a suitable difference of wavelengths λ1 and λ2 can be selected to match a vibrational mode to be excited in a CARS experiment. <FIG> shows wavelengths λ1 and λ2 which depend on a unique exit angle at which they are emitted from the AOTF <NUM>. The difference of the inverse wavelengths <NUM>/λ1 - <NUM>/λ2 should match the vibrational mode to be excited which is represented in <FIG> by a corresponding wavenumber. As can be seen in <FIG>, the vibrational mode depends on the exit angle at which the pair of wavelengths λ1, λ2 exit the AOTF <NUM>. Accordingly, by selecting a suitable pair of wavelengths λ1, λ2 at a unique exit angle, a specific vibrational mode can be excited. The first light deflecting element <NUM> can then be used to redirect the pair of wavelengths at an appropriate angle towards the sample.

In order to perform an experiment as explained above with reference to <FIG> and <FIG>, in principle any of the angular characteristics W2 to W4 as shown in <FIG> can be applied as these angular characteristics allow to selectively create pairs of wavelengths which are emitted from the AOTF <NUM> at unique exit angles. In contrast, the angular characteristic W5 of <FIG> may be applied to perform an experiment in which it is required to illuminate the sample simultaneously with three different wavelengths λ1, λ2 and λ3 at the same exit angle. An example for such an experiment can be found in the field of Stimulated Raman Scattering (SRS).

When detecting SRS signals, one often comes across a methodology which is called Frequency-Modulated Stimulated Raman Scattering (FM-SRS). For SRS signal detection, one of the two laser fields is amplitude modulated at a certain frequency. Due to the interaction with the sample, this amplitude modulation gets transferred to the laser field. The signal detection involves extracting the amplitude of this modulation transfer. In FM-SRS, instead of amplitude modulating the first laser field, a modulation of the frequency of the laser field is performed. This involves interleaving optical pulses of two different wavelengths at a certain frequency. The two wavelengths in the first laser field are chosen such that the second laser field interacts only with one of the two wavelengths in the first laser field to generate the SRS signal. The advantage of FM-SRS over the amplitude-modulated SRS version is that the intensity of the laser fields at the sample remains constant. This helps to avoid other undesired non-linear effects such as cross-phase modulation from being detected along with SRS. To perform FM-SRS, it is imperative that three different wavelengths with a predefined energy difference exit the AOTF and impinge the microscopic sample collinearly.

As shown in <FIG>, three wavelengths λ1, λ2 and λ3 can be selected at the same exit angle θ such that the wavelengths λ1, λ2 and λ3 are spaced apart by predetermined frequency differences Δv<NUM> and Δv<NUM> according to following relationships: <MAT> <MAT>.

It is to be noted that the new filter design is not limited to the above applications. For instance, generating two or three output wavelengths which are emitted from the AOTF in a perfectly collinear manner may also be useful for instance in Sum-Frequency Generation (SFG) microscopy and Difference-Frequency Generation (DFG) microscopy.

Claim 1:
An optical filter device (<NUM>) for illuminating a sample (<NUM>), comprising
a light supply unit (<NUM>) configured to generate input light (<NUM>) of a predetermined spectral bandwidth,
an acousto-optic tunable filter (<NUM>) configured to receive the input light (<NUM>) and to emit diffracted output light (<NUM>) including multiple wavelengths from the spectral bandwidth at exit angles towards the sample (<NUM>), said exit angles defining an angular spread (C) among the multiple wavelengths of the output light (<NUM>),
wherein the angular spread (C) among the multiple wavelengths of the output light (<NUM>) is determined by an angular characteristic (W2-W5) which varies from a minimum wavelength to a maximum wavelength and indicates how the exit angles vary depending on the wavelengths,
characterized in that the light supply unit (<NUM>) is configured to generate the input light (<NUM>) of the predetermined spectral bandwidth in a collinear manner, and
wherein the angular characteristic (W2-W5) has an extremum at an intermediate wavelength between the minimum wavelength and the maximum wavelength to provide at least one group (P1-P3, T1-T3) of different wavelengths having the same exit angle.