Laser-based homogeneous illumination for fluorescence microscopy and other applications

Abstract

To homogenize coherent light (5) emitted by a light source (4) with a luminous flux, the luminous flux is modulated with a modulation frequency of at least 100 kHz. The emitted light is split into at least two light components. These at least two light components are coupled into a multimode optical fiber (11) at different coupling angles. The at least two light components are guided together through the multimode optical fiber (11), and the homogenized light is coupled out of the multimode optical fiber (11).

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the reduction of speckles or granulation that form during illumination with coherent light, in particular from laser-based illumination systems.

[0002] More specifically, the invention relates to a method for homogenizing coherent light and a device suitable for carrying out such a method. STATE OF THE ART

[0003] Compared to other light sources, laser-based lighting systems offer high energy efficiency, good focusability, and high luminance. However, due to the coherence of the light, interference occurs, which becomes visible in lighting applications in the form of light granulation (brighter and darker areas). This light granulation is also known as speckles.

[0004] Gaussian beam profiles can be generated by propagating the coherent laser beam through a single-mode fiber (SMF). Beam-shaping devices, such as beam-shaping lenses and beam-shaping masks, are used to create a homogeneous beam profile. These beam-shaping devices transform a Gaussian input beam profile into a homogeneously illuminated area with steeply sloping edges, a so-called flat-top hat profile.

[0005] Multimode fibers (MMFs) have a larger core diameter than singlemode fibers, allowing them to propagate other modes with high efficiency in addition to the transverse mode (TEM00). Mode interference during propagation becomes visible as speckles in the fiber output image. However, compared to SMFs, MMFs offer higher coupling efficiency and the ability to generate different beam shapes using different fiber core shapes. For example, a square core produces a square beam shape.

[0006] Due to these advantages of MMF, various methods have been developed to suppress granulation, particularly for applications in super-resolution wide-field microscopy, astronomy, and telecommunications. These methods are based on the principle of temporal averaging of different, successively generated granulation patterns.

[0007] Different granulation patterns are generated using fast piezo-driven mirrors, by using a rotating diffusion disk before coupling the light into the MMF, or by mechanically or acoustically moving the MMF. If different patterns are generated within the camera's recording time, homogeneous illumination can be achieved by temporally integrating the individual patterns.

[0008] The homogenization achieved through temporal averaging far exceeds that achieved with beam-shaping lenses or masks and is usually more cost-effective to implement. Combined with the described beam shaping through the shape of the MMF fiber core, this allows the creation of a flat-top hat beam shape characterized by steep flanks and a particularly evenly illuminated central region.

[0009] In addition to the aforementioned use of diffusion plates for generating different granulation patterns, speckle reducers from Optotune Switzerland AG are known. These are based on moving diffuser polymer membranes (see also US Pat. No. 8,902,520 B2). The diffuser plate is a polymer film with two surfaces. Both surfaces are connected to electrodes. By applying a voltage difference between the two electrodes, the position of the polymer film can be shifted. By changing the voltage difference, the polymer film can be moved back and forth. If the polymer film is used as a diffuser element, different granulation patterns can be generated by the movement of the polymer film.

[0010] DE 10 2020 122 227 B3 describes a method for homogenizing the output beam profile of MMFs, ideally implemented in the form of an integrated photonic circuit. For this purpose, an input beam is split into several beam paths. Integrated directional couplers and resistance heaters temporally modulate the beam intensity or phase of the various beam paths. Subsequent superposition of the beam paths by coupling them into an MMF allows different granulation patterns to be generated, from which homogeneous illumination is generated through temporal averaging. This solution works independently of the light sources used and does not require modulation of the light sources, since the temporal modulation is implemented within the photonic circuit.

[0011] A previous publication by the inventor on granulation reduction, Schröder D, Deschamps J, Dasgupta A, Matti U, Ries J. Cost-efficient open source laser engine for microscopy. Biomed Opt Express. 2020 Jan 3;11(2):609–623. doi: 10.1364 / BOE.380815. PMID: 32206389; PMCID: PMC7041445, discloses the use of a vibration motor within a specially designed mount. This method is not suitable for short integration times below 100 µs.

[0012] DE 10 2009 059 894 A1 describes a device for homogenizing optical radiation using two consecutive microlens arrays. The arrangement comprises an imaging optics unit with a microlens array located in its focal plane. The optical elements are arranged such that a beam waist forms behind the microlens array. Behind the beam waist is another imaging optics unit with another microlens array located in its focal plane.

[0013] EP 2 591 875 A1 describes a system for beam shaping laser radiation that uses a beam transformation lens. The beam transformation lens forms a rectangular beam profile from a round input beam and homogenizes the illuminated area.

[0014] US Pat. No. 5,224,200 A proposes homogenizing light using a delay beam path before coupling it into a light guide. The process consists of two mirrors, a collimating lens, and a light guide. The mirrors are arranged so that an input beam is reflected multiple times between the two mirrors. One of the two mirrors is semitransparent and arranged so that several transmission beams are focused onto the light guide by the collimating lens. The transmission beams then interfere incoherently with each other, achieving a homogenizing effect.

[0015] WO 2010 086 336 A1 deals with the reduction of granulation patterns. It proposes inserting a mirror with a vibrating membrane into the beam path before or after propagation through a light guide in order to superimpose multiple granulation patterns by temporal averaging.

[0016] JP 2008 224 889 A proposes splitting the input beam into multiple beam paths using light splitters to homogenize coherent light. The resulting beam paths are directed to a common collimating lens using totally reflecting mirrors. No superposition of the beams occurs prior to collimation.

[0017] DE 10 2013 016 413 A1 also describes a method for homogenizing light. Two light guides are used, each end of which has optical funnels flanged flush with the core. An optical system focuses the light cone from the first light guide output onto the light input of the second light guide. This device is intended to homogenize not only the emission plane but also the emission direction.

[0018] DE 10 2009 010 693 A1 describes a device for homogenizing spaced-apart partial beams, as is the case in laser diode bars. This involves an array of refractive surfaces, through which the partial beams converge. A subsequent lens then superimposes the partial beams into a homogeneous beam.

[0019] To date, no technical solutions based on temporal averaging are known that achieve high homogenization even for short integration times below 100 µs. OBJECT OF THE INVENTION

[0020] The invention is based on the task of finding a solution for light homogenization that is compatible with both short integration times well below 100 µs and very long integration times in the range of several minutes. Furthermore, the design should eliminate mechanically moving or vibrating parts, thus ensuring a low-maintenance setup. SOLUTION

[0021] The object of the invention is achieved according to the invention with the features of the independent patent claims. Preferred embodiments of the invention can be found in the dependent patent claims. DESCRIPTION OF THE INVENTION

[0022] In a method according to the invention for homogenizing coherent light emitted by a light source with a light output, the light output is modulated with a modulation frequency of at least 100 kHz, preferably of at least 900 kHz, the emitted coherent light is divided into at least two light components, the at least two light components are coupled into a multimode optical fiber at different coupling angles, the at least two light components are passed together through the multimode optical fiber, and the homogenized light is coupled out of the multimode optical fiber.

[0023] In a device according to the invention with a light source that emits coherent light with a light output, a light output controller that is designed to modulate the light output with a modulation frequency of at least 100 kHz, preferably of at least 900 kHz, a beam splitting device that is designed to split the emitted light into at least two light components, a multimode light guide extending between two light guide ends and a beam coupling device that is designed to couple the at least two light components into the multimode light guide at different coupling angles at one of the two light guide ends, the homogenized light emerges from the other of the two light guide ends of the multimode light guide.

[0024] By temporally modulating the light source's light output, temporal modulation within the beam arms is eliminated, enabling a more cost-efficient implementation. At the same time, a higher modulation frequency also enables applicability for acquisition times as fast as the microseconds. The method according to the invention thus achieves homogenization without the use of moving mechanical components and is also compatible with short integration times. Furthermore, maintenance requirements are reduced.

[0025] The inventive method combines three measures for granulation reduction. Different granulation patterns are generated by continuously varying the output power of the coherent light source. Since this occurs electronically using direct modulation, short switching times of the light source of 1-2 ns are achievable. Due to these short switching times, it is possible to generate many different granulation patterns within just a few µs and use them accordingly for averaging. Both multiplication or superposition of switch-on trigger signals with noise signals and the periodic switching off of the laser diodes can be used to modulate the light output of the light source; fast electronic circuits can accelerate the modulation to the physical limits of the laser diodes.Furthermore, the fast switching times enable pulsed interleaved excitation applications of the inventive method without the use of additional optical elements. To further improve homogenization, the coherent light is split into at least two light components using a beam splitter in a subsequent step. Using phase plates, the polarization axis of one of the light components can optionally be changed relative to another. The beam arms are then recombined, optionally using mirrors or beam splitters. This setup can be realized cost-effectively using standard components. Finally, homogenization is further increased by coupling the beam arms into the multimode optical fiber at different angles.

[0026] By appropriately shaping the core of the multimode light guide, the output beam profile can be varied, resulting in a homogeneous illumination field with sharp profile edges after propagation. For many applications, such as microscopy, additional slit apertures are thus no longer required.

[0027] In addition to the modulation signal for granulation reduction, the light sources can also be controlled by additional control signals, such as the turn-on trigger signals. This can be achieved with a logic unit in which various external control signals and the modulation signal are superimposed by convolution or multiplication, and the resulting signal is used to control the light source.

[0028] To further reduce granulation, the polarization of one or more of the light components can be statically modulated, for example, using waveplates. This includes rotation by a fixed angle, conversion to circularly or elliptically polarized light, and modification of the helicity.

[0029] For further granulation reduction, it may be useful to combine other homogenization techniques, such as mechanical agitation of the light guide.

[0030] It is advantageous to use not just one but several light sources, for example, implemented as laser diodes that emit coherent light at different wavelengths. An electronic circuit can generate a corresponding signal for each laser diode, using logic gates and / or programmable microchips, in a compact and cost-effective manner to modulate its light output. By using a beam combiner between the laser diodes and the beam splitter, the light from all light sources is divided into the light components.

[0031] Advantageous further developments of the invention emerge from the patent claims, the description and the drawings.

[0032] The advantages of features and combinations of several features mentioned in the description are merely exemplary and can be effective alternatively or cumulatively, without the advantages necessarily having to be achieved by embodiments according to the invention.

[0033] With regard to the disclosure content – ​​not the scope of protection – of the original application documents and the patent, the following applies: Further features can be found in the drawings – in particular the illustrated geometries and the relative dimensions of several components to one another, as well as their relative arrangement and operative connection. The combination of features of different embodiments of the invention or features of different patent claims is also possible, deviating from the chosen references of the patent claims, and is hereby encouraged. This also applies to features that are illustrated in separate drawings or mentioned in their description. These features can also be combined with features of different patent claims.Likewise, features listed in the patent claims may be omitted for further embodiments of the invention, but this does not apply to the independent patent claims of the granted patent.

[0034] The number of features mentioned in the patent claims and the description is to be understood as meaning that exactly this number or a greater number than the stated number is present, without the need for the explicit use of the adverb "at least." Thus, for example, if reference is made to one phase plate, this is to be understood as meaning that exactly one phase plate, two phase plates, or more phase plates are present. The features mentioned in the patent claims may be supplemented by further features or may be the only features present in the subject matter of the respective patent claim.

[0035] The reference signs contained in the patent claims do not represent a limitation of the scope of the subject-matter protected by the patent claims. They serve solely to make the patent claims easier to understand. SHORT DESCRIPTION OF THE FIGURE

[0036] In the following, the invention is further explained and described with reference to a preferred embodiment shown in the figure. Fig. 1 is a block diagram of an embodiment of the method and device according to the invention. FIGURE DESCRIPTION

[0037] Fig.1 shows an embodiment of the method and device according to the invention as a schematic block diagram. Switch-on trigger signals 1 for switching on light sources 4 are combined with a modulation signal 2 from a signal generator in a logic unit to form a signal 3 for controlling the light sources 4 designed as laser light sources. Coherent light 5 from the individual light sources 4 is superimposed in a common beam path by a beam combining device 6. A beam splitting device 7 generates several light components and deflects them by totally reflecting mirrors 8. The polarization of one of the light components is changed using a phase plate 9. A beam coupling device 10 couples the light components into a multimode optical fiber 11 at different coupling angles.

[0038] The homogenized light exiting at the end of the light guide is collimated with an outcoupling optic 12. LIST OF REFERENCE SYMBOLS 1 power-on trigger signal 2 Modulation signal 3 Signal 4 Light source 5 coherent light 6 Beam combining device 7 Beam splitting device 8 mirrors 9 Phase plate 10 Beam coupling device 11 multimode optical fiber 12 Output optics QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 8 902 520 B2

[0009] DE 10 2020 122 227 B3

[0010] DE 10 2009 059 894 A1

[0012] EP 2 591 875 A1

[0013] US 5 224 200 A

[0014] WO 2010 086 336 A1

[0015] JP 2008 224 889 A

[0016] DE 10 2013 016 413 A1

[0017] DE 10 2009 010 693 A1

[0018] Cited Non-Patent Literature

[0000] Schröder D, Deschamps J, Dasgupta A, Matti U, Ries J. Cost-efficient open source laser engine for microscopy. Biomed Opt Express. 2020 Jan 3;11(2):609-623. doi: 10.1364 / BOE.380815. PMID: 32206389

[0011]

Claims

[1] Method for homogenising coherent light (5) emitted by a light source (4) with a light output, - the light output is modulated with a modulation frequency of at least 100 kHz, - wherein the emitted coherent light (5) is divided into at least two light components, - wherein the at least two light components are coupled into a multimode optical fiber (11) at different coupling angles, - wherein the at least two light components are passed together through the multimode optical fiber (11) and - wherein the homogenized light is coupled out of the multimode optical fiber (11). [2] Method according to claim 1, characterized by , - that the light output is modulated with a modulation frequency of at least 900 kHz. [3] Method according to claim 1 or 2, characterized by , - that the light source (4) is controlled to modulate the light output with a signal (3) which is a product of a modulation signal (2) and a switch-on trigger signal (1). [4] Method according to one of the preceding claims, characterized by , - that the polarization of one of the at least two light components is rotated by a fixed angle relative to the polarization of the other of the at least two light components. [5] Method according to one of the preceding claims, characterized by , - that the emitted light is circularly or elliptically polarized, wherein optionally the helicity of one of the at least two light components is reversed relative to the helicity of the other of the at least two light components. [6] Method according to one of the preceding claims, characterized by , - that the light guide (11) is moved mechanically and preferably shaken. [7] Method according to one of the preceding claims, characterized by , - that the coherent light (5) emitted by the light source (4) is combined in a common beam path with further coherent light (5) emitted by a further light source (4) with a different wavelength before the light is split into the at least two light components. [8] Device with - a light source (4) which emits coherent light (5) with a light output, - a light output control designed to modulate the light output with a modulation frequency of at least 100 kHz, - a beam splitting device (7) designed to split the emitted light into at least two light components, - a multimode optical fiber (11) extending between two optical fiber ends and - a beam coupling device (10) which is designed to couple the at least two light components into the multimode optical fiber (11) at different coupling angles at one of the two optical fiber ends, - wherein the homogenized light emerges from the other of the two light guide ends of the multimode light guide (11). [9] Device according to claim 8, characterized by , - that the light output control is designed to modulate the light output with a modulation frequency of at least 900 kHz. [10] Device according to claim 8 or 9, characterized by , - that the light power control is designed to control the light source (4) with a signal (3) which is a product of a modulation signal (2) and a switch-on trigger signal (1). [11] Device according to one of claims 8 to 10, characterized by - an output coupling optic (12) which is designed to collimate the homogenized light emerging from the other of the two optical fiber ends of the multimode optical fiber (11). [12] Device according to one of claims 8 to 11, characterized by - another light source (4) which emits further coherent light (5) with a different wavelength, and - a beam combining device (6) which is designed to combine the coherent light (5) emitted by the light source (4) with the further coherent light (5) in a common beam path before splitting the light into the at least two light components. [13] Device according to one of claims 8 to 12, characterized by - a phase plate (9) in the beam path of one of the at least two light components. [14] Device according to one of claims 8 to 12, characterized by - a vibrator acting on the multimode optical fiber (11).

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