Abstract:
A device ( 100 ) for variable deflection of light is described, encompassing a micromechanical mirror arrangement ( 14 ) having a plurality of light-reflecting mirror actuators ( 18, 20, 22, 24, 26 ), and a control unit ( 32 ) with which the mirror actuators ( 18, 20, 22, 24, 26 ) are controllable into different reflection positions in order to vary the light deflection. The device ( 100 ) has a back-reflection structure ( 60 ), systematically adapted to the mirror arrangement ( 14 ), for reflecting back onto another portion of the mirror actuators ( 18, 20, 22, 24, 26 ), in targeted fashion, the light reflected onto the back-reflection structure ( 60 ) from one portion of the mirror actuators ( 18, 20, 22, 24, 26 ).

Description:
RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. DE 10 2011 052 336.7, filed Aug. 1, 2011, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a device for variable deflection of light encompassing a micromechanical mirror arrangement having a plurality of light-reflecting mirror actuators, and a control unit with which the mirror actuators are controllable into different reflection positions in order to vary the light deflection. 
     BACKGROUND OF THE INVENTION 
     Devices that make possible, at the end of the imaging beam path, a selective detection of individual spectral components of the detected light are often provided in microscopy, in particular confocal microscopy. Beam splitters are often used for this; these transmit a specific spectral component of the light while reflecting the other spectral components. The result is that, for example, the fluorescent radiation emitted from a sample being imaged can be selectively detected. 
     An alternative possibility for selective light detection is to pass the detected light firstly through a dispersing optical element, e.g. a prism, which refracts the detected light in wavelength-dependent fashion and thereby generates a spectrally dispersed, diverging light bundle. This light bundle is then delivered to a mirror slider apparatus that is constituted from multiple cascades of mirrors. Each of these mirror cascades is made up of two mirror elements that are separated from one another by a gap. A portion of the spectrally split light bundle passes through this gap, while the remaining portion of the light bundle is reflected at the mirror elements onto a further mirror cascade that in turn directs a specific spectral component onto another detector. 
     A mirror slider apparatus of this kind has the advantage, as compared with the beam splitters predominantly used in the existing art, that the spectral components to be delivered to the detectors can be modified in simple fashion by shifting the mirror elements. Examples of a mirror slider apparatus of this kind are described in DE 43 30 347 A1 and in DE 100 38 049 A1. 
     The mirror slider apparatuses have the disadvantage, however, that they are of comparatively complex construction in order to enable the desired flexibility in selecting the spectral light components to be detected. Precisely operating motors are therefore required in order to move the mirror elements into the respectively desired position. Displacement of the mirror elements is moreover comparatively time-consuming. 
     U.S. Pat. No. 6,396,053 B1 discloses a scanning microscope that contains a light deflection device that delivers the spectral components of a light bundle, spectrally dispersed by a prism, selectably to different detectors. This light deflection device encompasses a micromechanical mirror arrangement having a plurality of mirror actuators that can move individually into different reflection positions. For example, five different reflection positions, and thus five different deflection angles, are provided for each of these mirror actuators, in order to direct the light selectably onto the different detectors. This comparatively large number of different reflection positions that each individual mirror actuator can assume requires precise application of control to the respective mirror actuator; technical implementation of this previously known light deflection device is thereby made difficult. 
     Regarding the existing art, reference is furthermore made to so-called digital light processing (DLP) systems, which are utilized in projection technology, for example, for video projectors. These DLP systems likewise encompass a micromechanical mirror arrangement having a plurality of mirror actuators arranged in the form of a matrix. For each of these mirror actuators, however, provision is made only for exactly two reflection positions, between which the mirror actuator can move back and forth up to several thousand times per second. Each mirror actuator can represent one image point or pixel, the brightness of which is adjusted over the entire time during which the respective mirror actuator is located, within a predetermined time interval, in the one or the other reflection position. 
     Because of the short switching times, it would be desirable also to use the DLP technology explained above, for example, in confocal microscopy in order to deflect detected light onto different detectors. The only way to implement these short switching times, however, is by providing only exactly two defined reflection positions, which ultimately also make possible only two different deflection directions. In typical confocal applications, however, the deflection of light in more than two directions is desirable. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to further develop a light deflection device of the kind recited initially so as to make possible, with simple technical means, rapid and precise switching of the deflection directions. 
     The invention achieves this object in accordance with Claim  1  by means of a back-reflection structure, systematically adapted to the mirror arrangement, for reflecting back onto another portion of the mirror actuators, in targeted fashion, the light reflected onto the back-reflection structure from one portion of the mirror actuators. 
     The invention thus provides, in addition to the micromechanical mirror arrangement, a back-reflection structure which is adapted to the mirror arrangement in such a way that different deflection pathways for light can be implemented within the deflection device as a function of the different reflection positions of the mirror actuators, with the result that the light deflection brought about by the device according to the present invention can be controlled as desired. For this purpose, the back-reflection structure is arranged with respect to the mirror arrangement in such a way that it makes available, in interaction with the mirror actuators constituting the mirror arrangement, the desired deflection paths, which can be selected freely by means of a corresponding application of control to the mirror actuators. A “systematic adaptation” of the back-reflection structure to the mirror arrangement thus means a suitable spatial arrangement of the back-reflection structure relative to the mirror arrangement, by means of which the different deflection paths can be established. 
     The back-reflection structure according to the present invention can thus be embodied so that as a function of the reflection positions of the mirror actuators, multiple reflections between the mirror actuators and the back-reflection structure are possible, in the manner of a mirror cascade. It is conceivable, for example, for a respective mirror actuator, in a first reflection position, to send the light reflected onto it by the back-reflection structure back onto the back-reflection structure, and thus to retain the light within the unit constituted by the mirror arrangement and the back-reflection structure, while in a second reflection position it reflects the light out of that unit. Application of this principle allows the light deflection to be varied within broad limits. 
     The micromechanical mirror arrangement used is preferably a DLP module, in which the mirror actuators can be individually controlled with very short switching times that are on the order of milliseconds. The individual mirror actuators usually have edge lengths that are in a range of a few micrometers or hundred micrometers. A micromechanical mirror arrangement of this kind thus makes possible variable light deflection at high resolution in terms of space and time. In addition, it can be produced economically in the form of a compact MOEM component that has a long service life and is largely insensitive to temperature fluctuations and atmospheric moisture. The invention thereby makes available a digital light selector that operates precisely and rapidly. 
     Because the back-reflection structure, in interaction with the mirror arrangement constituted from the mirror actuators, makes possible multiple reflections within the device according to the present invention, the mirror actuators can also have only a few, preferably only exactly two, defined reflection positions in order to implement the desired plurality of deflection paths in the device. This simplifies the application of control to the mirror actuators, and moreover increases the precision with which light can be deflected. 
     The principle of the present invention—utilizing multiple reflections with incorporation of the back-reflection structure—encompasses an arbitrary reversibility of the light path on which the light is deflected. The device according to the present invention can thus be used, for example, to switch light that is emitted from different spatially distributed light sources in a (single) predetermined direction. It is likewise conceivable to switch light deriving from a single light source in different directions. Mixed forms of these two deflection principles recited above are also conceivable. 
     The mirror actuators are preferably elements of identical design having identical dimensions. They can, however, also have different dimensions, for example if the light-deflecting system according to the present invention has arranged in front of it a prism that spectrally disperses the light and thus converts it into a light bundle that diverges in wavelength-dependent fashion. In this case the wavelength-dependent spreading of the light bundle can be taken into account by a corresponding adaptation of the size of the individual mirror actuators. 
     The interaction according to the present invention of the rapidly switchable mirror actuators and the back-reflection structure facing toward them makes possible a flexible adaptation of the deflection device to the deflection geometry present in each case. It is conceivable, for example, for light firstly to strike the mirror arrangement centrally, and then to be conveyed alternatively in two directions by multiple reflections before emerging from the device. Light can likewise firstly strike the mirror arrangement at the edge, and then be conveyed in a predetermined direction. Further, the light to be directed can also be constituted from multiple light beams that are incident onto the deflection device all at the same angle or at different angles. The light beams reflected away from the deflection device can likewise have the same exit angles or different exit angles. 
     A preferred embodiment, in which the mirror actuators are each controllable into exactly two reflection positions, provides two stops for each mirror actuator in order to define the two reflection positions. The result is that the mirror actuators can be respectively switched rapidly and precisely into the desired reflection position in order to achieve the desired light deflection. 
     The back-reflection structure is preferably embodied in stationary fashion. The combination of switchable mirror actuators and the stationary back-reflection structure is favorable to particularly simple control of the micromechanical mirror arrangement in order to achieve the desired light deflection. Alternatively, the back-reflection structure can also be made up of movable elements. It is possible in particular also to provide a micromechanical mirror arrangement for the back-reflection structure. 
     In a preferred embodiment, the mirror actuators form multiple mirror rows lying parallel to one another. Each of these mirror rows can be used, for example, to deflect a specific spectral component of the light, or light beams incident from different directions, in the desired fashion. 
     The mirror actuators arranged in the respective mirror row have in their reflection positions, for example, identical tilt angles relative to a common reference plane of the mirror arrangement. The provision of identical tilt angles simplifies the construction of the micromechanical mirror arrangement. 
     Alternatively, however, the mirror actuators arranged in the respective mirror row can also each have, in their reflection positions, different tilt angles relative to a common reference plane of the mirror arrangement. It is thereby possible, for example, when light enters in centered fashion, to implement the multiple reflections in the unit constituted from the mirror arrangement and the back-reflection structure in such a way that all possible exit regions have the same light exit angle. 
     In a preferred embodiment, the back-reflection structure comprises multiple reflection elements and multiple transparent transmission elements, at least one of the transmission elements constituting a light entry region for the light to be deflected, while the other transmission elements constitute light exit regions for the light reflected at the mirror actuators. The light entering the light entry region is, in this context, selectably deflectable into one of the light exit regions as a function of the reflection positions of the light-reflecting mirror actuators. In this embodiment, the reflection elements in interaction with the mirror actuators provide for the multiple reflections within the unit constituted from the mirror arrangement and the back-reflection structure, while the transparent transmission elements make possible light entry into, and light exit from, that unit. 
     The reflection elements and the transmission elements are preferably arranged parallel to one another and alternatingly. The reflection elements and the transmission elements here constitute a kind of strip-shaped back-reflection pattern that can be adapted as desired to the geometry of the mirror arrangement, in which the mirror actuators are preferably arranged in parallel mirror rows. 
     In a preferred embodiment, the light entry region is arranged centeredly between the light exit regions. This configuration is usable, for example, when a central light incidence and a subsequent deflection from the center toward the edges of the device is desired. It is likewise conceivable, however, for the light entry region to be provided at the edge of the device, so that light is then redirected away from that edge in a predetermined direction. 
     The reflection elements and the transmission elements are preferably arranged in a plane spaced away from the mirror arrangement and extend, as projected onto the mirror arrangement, orthogonally to the mirror rows. In this configuration the elements constituting the back-reflection structure and the mirror rows are therefore arranged, so to speak, crosswise. This makes possible particularly simple systematic adaptation of the back-reflection structure to the mirror arrangement, for example in order to bring about the desired multiple reflections upon deflection of a light bundle that diverges as a result of spectral spreading. 
     By preference, at least one of the mirror actuators in the respective mirror row is directed, as projected onto the back-reflection structure, onto one of the transmission elements. In this embodiment the mirror actuators are, for example each arranged beneath one of the transmission elements that can serve as light entry regions or light exit regions. This makes possible a perpendicular light incidence into, or perpendicular light exit from, the deflection device; this simplifies, for example, alignment of the deflection device onto a light source or a detector. 
     A housing in which the mirror arrangement is accommodated, and a transparent cover which is mounted on the housing and on which the back-reflection structure is embodied, are preferably provided. The transparent cover is, for example, a glass plate on which the back-reflection structure is embodied in the form of a reflective coating. The back-reflection structure is preferably located on the underside of the cover facing toward the mirror arrangement. This is favorable to loss-free multiple reflections between the mirror arrangement and the back-reflection structure. The back-reflection can also, however, be arranged on the upper side of the cover, or outside the cover. The transparent cover is preferably anti-reflection-coated on both sides, so that undesired multiple reflections between the mirror arrangement and the back-reflection structure are avoided. 
     In a particularly preferred embodiment, a dispersion element is provided which, by spectral dispersion of the light, generates a light entry bundle in such a way that the light entry bundle has a light bundle cross section, elongated by the spectral dispersion, with which the light entry bundle is incident through the light entry region orthogonally to the mirror rows onto the mirror arrangement, the mirror rows each having a predetermined spectral region of the light entry bundle assigned to them. This embodiment can be profitably used in particular in a confocal microscope, so that the mirror slider apparatuses hitherto used therein can be replaced with the deflection device according to the present invention which makes possible particularly rapid and precise deflection of various spectral components of the light, for example onto different detectors. In this context, each mirror row receives a predetermined spectral region of the light entry bundle, which can then be directed, independently of the other spectral regions, selectably in a desired deflection direction. 
     The mirror rows are preferably grouped together into multiple groups that each generate, by deflection of the light entry bundle, an exit light bundle of a predetermined spectral composition that exits from one of the light exit regions. The fact that the mirror rows are combined into multiple (including non-continuous) groups, each individual group of which generates a light exit bundle of predetermined spectral composition deflected in a specific direction, allows the deflected light to be distributed as desired onto different detectors as a function of its spectral composition. In particular, in contrast to the mirror slider apparatuses hitherto used, it is no longer necessary for a contiguous wavelength region to be assigned to the individual detectors. In the fluorescence microscopy application, for example, it is possible to direct the entire fluorescence band onto a single detector, and at the same time to “cut out” from it the excitation wavelengths located in it. The deflection device according to the present invention further has the advantage, as compared with known mirror slider apparatuses, that each mirror actuator has assigned to it a fixed wavelength region that does not change in the context of a change in the wavelength assignment to specific detectors. Conversely, with mirror slider apparatuses in which the mirror pairs are moved in motorized fashion, individual motor steps can “get lost,” resulting in a shift of the spectrum. 
     The device preferably encompasses multiple detectors each for sensing at least one of the exit light bundles. As already indicated above, it is not necessary, for utilization of the deflection device according to the present invention, for the wavelength regions assigned to the individual detectors to be continuous, i.e. monotonically rising or falling. If a specific detector is particularly well suited for a specific application, for example in fluorescence lifetime microscopy or fluorescence correlation spectroscopy and/or when a specific method is being used, for example a gating method, it can then have any desired wavelength region assigned to it, regardless of where the detector is located in the system in question. This means that the device according to the present invention is capable of assigning each wavelength to the detector best suited for it. 
     According to a further aspect of the invention, an optical device, in particular a microscope and particularly preferably a confocal microscope, is provided, which device is equipped with a device of the kind described above for variable deflection of light. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described below in terms of an exemplifying embodiment, with reference to the Figures, in which: 
         FIG. 1  is a sectioned side view of a deflection unit that is part of a deflection device according to the present invention; 
         FIG. 2  is a perspective plan view of the deflection unit; and 
         FIG. 3  is a schematic depiction to illustrate an example of how the deflection device according to the present invention is used. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1 and 2  show deflection unit  10  in a sectioned side view and a plan view, respectively. Deflection unit  10  is part of a device that is used, for example, in a confocal microscope in order to deliver spectrally separated light to different detectors. 
     Detection unit  10  has a housing  12  in which a micromechanical mirror arrangement  14  is accommodated. Mirror arrangement  14  is, for example, a DLP component, and is made up of a plurality of mirror actuators arranged in a matrix shape, of which, purely by way of example,  FIG. 1  depicts five actuators labeled  18   a  to  26   a , and  FIG. 2  depicts fifteen actuators labeled  18   a  to  26   a ,  18   b  to  26   b , and  18   c  to  26   c . Be it noted in this context that the depictions in  FIGS. 1 and 2  serve merely for illustration. Considerably more mirror actuators are provided in a practical implementation, for example (depending on the application) in a range from several hundred to several thousand actuators. As shown in the perspective plan view of  FIG. 2 , the mirror actuators form mirror rows arranged parallel to one another, of which once again only three rows, labeled  30   a ,  30   b , and  30   c , are depicted in  FIG. 2 . 
     The statements that follow relate to mirror row  30   a  formed from mirror actuators  18   a  to  26   a . These statements apply correspondingly to the other mirror rows. 
     Mirror actuators  18   a  to  26   a  are each independently tiltable relative to a common reference plane E of mirror arrangement  14 . Provided for each mirror actuator  18   a  to  26   a  are two stops (not shown in  FIG. 1 ) that define two different reflection positions of the respective mirror actuator  18   a  to  26   a . In  FIG. 1 , one of the two reflection positions is in each case illustrated with a solid line, and the other with a dashed line. As is further evident from  FIG. 1 , mirror actuators  18   a  to  26   a  have, in their respective reflection positions, different tilt angles relative to reference plane E. A control unit  32  is provided for applying control to mirror actuators  18   a  to  26   a , which unit can cause each individual mirror actuator  18   a  to  26   a  to be impinged upon by a, for example, electrostatic displacement force in order to switch over the respective mirror actuator  18   a  to  26   a  between its two defined reflection positions. 
     A cover glass  34  that covers housing  12  is mounted on the upper side of housing  12 . Embodied on the lower side, facing toward mirror arrangement  14 , of cover glass  34  are elongated reflection elements  40 ,  42 ,  44 , and  46  that are arranged parallel to one another and, as projected onto mirror arrangement  14 , extend orthogonally to mirror rows  30   a ,  30   b , and  30   c . In the present exemplifying embodiment, reflection elements  40 ,  42 ,  44 , and  46  are reflective coatings that are evaporatively deposited onto cover glass  34 . Between reflection elements  40 ,  42 ,  44 , and  46  and on the edges of cover glass  34  (see  FIG. 2 ) are regions that do not have a reflective coating. These regions, labeled  48 ,  50 ,  52 ,  54 , and  56  in  FIG. 1 , are consequently transparent, and are referred to hereinafter as transmission elements. As shown in  FIG. 2 , transmission elements  48  to  56  also extend, as projected onto mirror arrangement  14 , orthogonally to mirror rows  30   a ,  30   b , and  30   c.    
     Reflection elements  40  to  46  and transmission elements  48  to  56  constitute a back-reflection structure, labeled generally as  60  in  FIG. 1 , which serves to make possible within deflection unit  10 , by multiple reflections of the incident light, different deflection paths for deflecting the light in a desired fashion. The interaction of the displaceable mirror actuators  18   a  to  26   a  and the back-reflection structure  60  facing toward them is explained below with reference to  FIG. 1 . 
     In the example shown in  FIG. 1 , deflection unit  10  is intended to deflect perpendicularly incident light selectably to different exit regions. It is assumed for the present example that the light enters, in the form of an entry light bundle  62 , into an elongated central light entry region that is constituted by transmission element  52 . Entry light bundle  62  is incident, inter alia, onto mirror actuator  22   a  which, depending on its reflection position, directs the light either onto reflection element  42  (to the left in  FIG. 1 ) or onto reflection element  44  (to the right in  FIG. 1 ). The light incident onto reflection element  42  or reflection element  44  is then reflected back to mirror actuator  20   a  or mirror actuator  24   a , respectively. The respective mirror actuator  20   a  or  24   a  then, depending on its reflection position, directs the light either onto reflection element  40  or  46 , respectively, or onto the respective transparent region  50  or  54  through which the light exits from deflection unit  10 . The two exit possibilities recited above are illustrated in  FIG. 1  by exit light bundles  64  and  66 . 
     Further exit light bundles  68 ,  70 ,  72 , and  74  can be generated in corresponding fashion by the arrangement shown in  FIG. 1  as a function of the reflection positions of the further mirror actuators  18   a  and  26   a.    
     Mirror row  30   a , depicted in section in  FIG. 1  and constituted by mirror actuators  18   a  to  26   a , is thus, in interaction with back-reflection structure  60  facing toward it, capable of selectably generating one of the exit light bundles  64  to  70  from entry light bundle  62 . The same is correspondingly true of mirror rows  30   b  and  30   c  shown in  FIG. 2 . Each mirror row  30   a ,  30   b ,  30   c  can thus deflect the light incident onto it, depending on the reflection positions of its mirror actuators, selectably in one of several possible directions. 
     In the present exemplifying embodiment, each mirror row  30   a ,  30   b ,  30   c  has assigned to it a predetermined wavelength region of the incident light. Deflection unit  10  is thereby capable of deflecting the light in different directions as a function of wavelength. This application is illustrated again in  FIG. 3 . 
       FIG. 3  schematically shows a deflection device, labeled in general as  100 , in which a prism  80  is arranged in front of deflection unit  10 . Prism  80  serves to spectrally disperse an incident light bundle  82  and thus to generate a divergent light bundle that has a light bundle cross section which is elongated as a result of the spectral dispersion. In  FIG. 3 , this spectrally dispersed light bundle cross section is illustrated by three sub-bundles  84 ,  86 , and  88 , each of which is intended to represent a spectral component of light bundle  82 . 
     The three sub-bundles  84 ,  86 , and  88  are incident onto deflection unit  10 , which ensures that sub-bundles  84 ,  86 , and  88  are deflected in different directions. In the example shown in  FIG. 3 , sub-bundle  84  is intended to be converted into an exit light bundle  90 , sub-bundle  86  into an exit light bundle  92 , and sub-bundle  88  into an exit light bundle  94 . Exit light bundles  90 ,  92 , and  94  are each selectably delivered by deflection unit  10  to one of several detectors  96 ,  98 ,  99 . In the example depicted in  FIG. 3 , one of the exit light bundles  90 ,  92 , and  94  is respectively incident onto one of the detectors  96 ,  98 , and  99 . This is to be understood, however, merely as an example. It is also possible, for example, for two of the three exit light bundles  90 ,  92 , and  94  to be directed onto one and the same detector. 
     In order to generate exit light bundles  90 ,  92 , and  94  in the manner illustrated in  FIG. 3 , the mirror rows can be grouped together in any desired combinations into groups that respectively generate one of the exit light bundles  90 ,  92 , and  94 . Provision is made in the present example that the light entry bundle constituted by sub-bundles  84 ,  86 , and  88  is incident, with its longitudinal bundle cross section generated by the spectral dispersion, in an orthogonal orientation onto the mirror rows. 
     PARTS LIST 
       10  Deflection unit 
       12  Housing 
       14  Mirror arrangement 
       18   a  to  26   a  Mirror actuators 
       18   b  to  26   b  Mirror actuators 
       18   c  to  26   c  Mirror actuators 
       30   a  to  30   c  Mirror rows 
       32  Control unit 
       34  Cover glass 
       40 ,  42 ,  44 ,  46  Reflection elements 
       48 ,  50 ,  52 ,  54 ,  56  Transmission elements 
       60  Back-reflection structure 
       62  Entry light bundle 
       64 ,  66 ,  68 ,  70 ,  72 ,  74  Exit light bundles 
       80  Prism 
       82  Light bundle 
       84 ,  86 ,  88  Sub-bundles 
       90 ,  92 ,  94  Exit light bundles 
       96 ,  98 ,  99  Detectors 
       100  Deflection device