System and method for determining a transmission characteristic of a multimode fiber

Abstract

System (100) for determining a transmission characteristic of at least one multimode fiber (50), the system (100) comprising: a light source (10) designed to generate a light signal (11); an illumination beam path (21) configured to couple the light signal (11) into a proximal end (51) of the at least one multimode fiber (50); a focusing means (30) configured to vary a focal plane of the light signal (11) around a distal end (52) of the multimode fiber (50); and a reference mask (53) arranged near the distal end (52) of the multimode fiber (50), wherein the focusing means (30) is further designed to adjust the focal plane of the light signal (11) selectively to the reference mask (53) or to an object to be imaged (60) arranged near the distal end (52), and wherein the reference mask (53) has a predetermined spatially resolved reflectivity, transparency, absorption and / or polarization and / or is designed as a predetermined interference grating, and wherein the reference mask (53) is printed, etched or engraved on or into a cross section of the multimode fiber (50) or is attached to the distal end (52) of the multimode fiber (50).

Description

Subject of the invention

[0001] The present invention relates to a system for determining a transmission characteristic of a multimode fiber, preferably for in-situ determination and, if necessary, compensation of the transmission characteristic of the multimode fiber in the context of a medical or medical-technical application. The present invention further relates to a method for determining a transmission characteristic of a multimode fiber, preferably for in-situ determination and, if necessary, compensation of the transmission characteristic. Technological background

[0002] The use of technological aids is an integral part of modern medicine. Both imaging techniques and robotic systems are now used as routinely in surgery as they are in diagnostics. The use of imaging techniques enables the visualization and discrimination of a wide variety of structures within the patient, and the image data obtained from the patient can be used advantageously in diagnostics, as well as in therapeutic and surgical procedures.

[0003] For imaging in medical applications, probes or endoscopes are often used, which can be minimally invasively inserted into a patient's structure to be examined. From a medical perspective, it is important to keep the size of the probes or endoscopes used as small as possible to avoid harming the patient. At the same time, the required optical properties of the optical instrument and its imaging behavior limit miniaturization.

[0004] For example, but not only in ophthalmological applications or procedures, endoscopes can be used that have at least one optical fiber, in particular at least one multimode fiber. The multimode fiber serves at least to deliver light to the structure under examination and is elastic or flexible within certain limits. The use of multimode fibers in medical probes or endoscopes advantageously enables their further miniaturization and facilitates the insertion of these devices.

[0005] Light propagation in multimode fibers (MMFs) has long been researched and can be theoretically described by the concept of the transmission matrix. The transmission matrix describes the relationship between the input signal and the output signal of the multimode fiber in a spatially resolved manner, for example, per fiber or according to a differently defined two-dimensional grid. In practice, however, the transmission properties of multimode fibers, i.e. their transmission matrix, fluctuate depending on environmental conditions. This complicates the use of multimode fibers in medical applications, where precise knowledge of the transmission properties of the components used can be critical to the success of the application, for example, diagnosis and / or surgical intervention.

[0006] It is known from the prior art that the transmission properties of a multimode fiber can be determined by measuring a known reference signal after it exits the fiber. However, this only allows calibration of the fiber prior to use, since one end of the fiber is usually inaccessible during use, for example, located inside a patient's body. Therefore, methods have been developed to theoretically model the transmission properties of multimode fibers under the influence of various environmental conditions. One such method is described, for example, in WO 2016 / 193718 A1, according to which the light propagation in curved fibers and the influence of fiber deformation (bending) on the transmission properties of the multimode fiber can be predicted.However, such modeling requires knowledge of fiber deformation, which can be problematic during use of the fiber, for example in a patient's body.

[0007] US 2011 / 0137126 A1 discloses an endoscope with a light source and a fiber bundle, wherein a light phase modulator is arranged between the light source and the fiber bundle and adjusts the relative phase of the light incident from the light source before it enters the fiber bundle. US 2015 / 0015879 A1 discloses a system with a multimode waveguide and means for wavefront shaping, wherein a specific pattern is projected onto the proximal end of the calibrated waveguide to generate a light pattern at the distal end, and wherein an image is generated from light backscattered from the sample and retransmitted via the waveguide. From GU, YG [et al.]: Design of flexible multi-mode fiber endoscope. In: Optics Express, Vol. 23, 2015, No. 21, pages 26905-26918, a design for an MMF endoscope with a partial reflector at the distal end of the multimode fiber is known, by means of which a calibration of the MMF endoscope should be possible.WO 2019 / 140 144 A1 discloses the use of optical fibers in medical devices, such as endoscopes, catheters or trocars, whereby a transfer function of the optical fiber is said to be determinable even without access to the distal fiber end solely by interaction with the proximal fiber end.

[0008] The object of the present invention is to overcome or at least reduce the disadvantages of the prior art and to provide an improved system and an improved method for determining a transmission characteristic of a multimode fiber. Description of the invention

[0009] The object of the invention is achieved by the subject matter of the independent patent claims. Preferred developments are the subject matter of the subclaims.

[0010] A first aspect of the present disclosure relates to a system for determining a transmission characteristic of a multimode fiber and for determining an image signal of an object under examination. The system comprises a light source configured to generate a light signal, preferably a laser source. The light source is particularly preferably an RGB laser. The light source is further preferably configured to emit a polarized light signal, for example, a linearly polarized light signal. It is also preferably a (diode-pumped) single-frequency laser source.

[0011] The system according to the present disclosure comprises at least one multimode fiber with a proximal end and a distal end. The multimode fiber is preferably a very thin multimode fiber, in particular with a diameter of 50 µm to 200 µm. The multimode fiber used for illumination (of the examination object or the reference mask) has, for example, a core made of pure silicon dioxide, which is surrounded, for example, by a cladding of fluorine-doped silicon dioxide. The length of the multimode fiber can be several centimeters, for example between 5 and 50 cm, preferably between 10 and 30 cm. In addition, the multimode fiber is preferably mechanically flexible, which enables diverse applications, for example in an endoscope. The system further comprises an illumination beam path designed to couple the light signal into a proximal end of the multimode fiber.In other words, the illumination beam path is optically arranged between the light source and the proximal end of the multimode fiber and can comprise one or more optical elements. The distal end of the multimode fiber can be aligned with an object under examination and is arranged, for example, in an endoscope.

[0012] The system further comprises a focusing means configured to vary a focal plane of the light signal around a distal end of the multimode fiber. In other words, the focal plane of the light signal can be varied by means of the focusing means within a region arranged at the distal end of the multimode fiber. In particular, the focal plane can be selectively positioned by means of the focusing means within a region inside the multimode fiber near the distal end of the multimode fiber, and the focal plane can be selectively positioned by means of the focusing means within a region outside the multimode fiber near the distal end of the multimode fiber.In this context, “close” describes a distance that is small compared to an extension of the multimode fiber along a longitudinal axis of the multimode fiber, preferably a distance of a few millimeters, preferably of less than 20 mm, particularly preferably of less than 10 mm and also preferably of less than 5 mm.

[0013] The system according to the present disclosure further comprises a reference mask arranged near the distal end of the multimode fiber. The reference mask is preferably a component of the multimode fiber and, for example, is formed within the multimode fiber or is monolithically connected to the multimode fiber. Likewise preferably, the reference mask is arranged outside the multimode fiber and, for example, is arranged together with the multimode fiber in an optical and / or medical instrument. The instrument is, for example, an endoscope, particularly preferably a holographic endoscope. The reference mask is preferably arranged within a tip of the endoscope directly adjacent to a distal end of the multimode fiber.

[0014] According to the invention, the focusing means is designed to adjust the focal plane of the light signal coupled into the multimode fiber selectively to the reference mask or to an examination object to be imaged, preferably arranged near the distal end of the multimode fiber. If the focal plane is adjusted to the reference mask, an image (signal) of the reference mask can advantageously be captured or determined using the system according to the present disclosure. If the focal plane is adjusted to the examination object, an image (signal) of the examination object can advantageously be captured or determined using the system according to the present disclosure.The system according to the present disclosure advantageously enables, by selectively capturing the reference image, a determination of a transmission property, for example represented by a transmission matrix, of the multimode fiber in situ, that is to say during use of the system according to the present disclosure, in particular during use of the system according to the present disclosure for image generation, wherein the distal end of the multimode fiber is not accessible and, in the case of an endoscope, is located, for example, in the body of a patient.

[0015] According to the invention, the reference mask has a predetermined spatially resolved reflectivity, transparency, absorption, and / or polarization. Alternatively or additionally, the reference mask is designed as a predetermined interference grating. According to the invention, the reference mask is printed, etched, engraved on or into a cross-section of the multimode fiber, or alternatively, attached to a distal end of the multimode fiber. Preferably, the reference mask is designed such that the reference mask located in the focal plane of the light signal has a predetermined influence on an image signal acquired with the system according to the present disclosure. Likewise preferably, the reference mask is designed such that the reference mask located outside the focal plane of the light signal has a negligible influence on an image signal acquired with the system according to the present disclosure.Thus, by varying the focal plane using the focusing means, preferably by pre-shaping the wavefront using a micromirror actuator (see below), either an image signal from the reference mask influenced by the transmission properties of the multimode fiber can be captured, or an image signal from the object under examination that is largely undisturbed by the reference mask (but also influenced by the transmission properties of the multimode fiber) can be captured. The image signal captured by the reference mask allows conclusions to be drawn about the transmission properties of the multimode fiber, which can be used to correct the image signal captured from the object under examination.

[0016] According to the state of the art, it was previously necessary to optically characterize the optical properties of a multimode fiber prior to its use, for example, in an endoscope, in a dedicated calibration setup. Due to the separate setup required for this, characterization of the multimode fiber could not be performed during use, for example, during a medical procedure. However, the optical properties of a multimode fiber vary from fiber to fiber and are also, to a certain extent, dependent on environmental parameters acting on the multimode fiber, such as temperature, pressure, fiber geometry in space (curvature), pressure on the fiber, etc.

[0017] During a medical application, these environmental parameters typically vary, for example due to body temperature, localized pressure on the fiber, bending of the fiber, etc. This accordingly affects the optical transmission properties of the multimode fiber, so that the original characterization is no longer accurate. This can adversely reduce the image quality, performance, and reliability of a system comprising the multimode fiber. The system according to the present disclosure advantageously enables characterization of the optical properties of the multimode fiber by continuously or semi-continuously detecting the previously known or predetermined reference mask using the multimode fiber during use of the multimode fiber.Since the reference mask is positioned outside the focal plane of the light signal for or during the acquisition of the image signal, the reference mask does not adversely affect the imaging of an object under examination. This advantageously enables, as described in more detail below, monitoring changes in the optical transmission properties of the multimode fiber and, preferably, detecting a changed environmental parameter and / or adapting (modulating) the light signal to compensate for the changed transmission properties.

[0018] In a preferred embodiment of the system according to the present disclosure, the focusing means is a micromirror actuator arranged in the illumination beam path between the light source and the proximal end of the multimode fiber. Such a micromirror actuator is often also referred to as a "digital micromirror device" (DMD) and preferably has a plurality of individual mirror elements that can be controlled individually. The micromirror actuator can preferably be controlled by a control unit. The micromirror actuator is, for example, a ViALUX V-7001 DMD chip or a comparable micromirror actuator known to those skilled in the art.

[0019] In a particularly preferred embodiment of the system according to the present disclosure, the micromirror actuator is designed to reflect and reshape a wavefront of the light emitted by the light source. In other words, the micromirror actuator is preferably designed to variably preshape the wavefront reflected by the micromirror actuator. In the system according to the present disclosure, the light signal with a preshaped wavefront (modulated light signal) is then coupled into the multimode fiber, in particular its proximal end, for illumination. The reshaping of the wavefront advantageously causes the variation of the focal plane, in particular its adjustment either to the reference mask or the object under examination. Thus, the properties of the light that is coupled out again from the distal end of the multimode fiber can ultimately be specifically adjusted.Particularly preferably, the micromirror actuator is configured to (adjustably) preshape the wavefront of the light from the light source reflected by the micromirror actuator in such a way that the number of light spots, the spot size, the focus, the light intensity, the aperture, and / or the position of the at least one light spot can be adjusted at the distal end of the multimode fiber. The micromirror actuator is preferably adjusted via a correspondingly configured control unit of the system according to the present disclosure.

[0020] Particularly preferably, the micromirror actuator is configured to (adjustably) preshape the wavefront of the light from the light source reflected by the micromirror actuator in such a way that the position of the light spot can be variably adjusted without having to move the multimode fiber. In particular, the position of the light spot (focal point of the output light signal) is variably adjustable in three spatial directions, wherein adjusting the position in an (xy) plane enables scanning of the light spot over an object under examination, and adjusting the position along an optical axis (z-axis) enables adjusting the focal plane of the light signal at the distal end of the multimode fiber.Likewise, the micromirror actuator is preferably configured to (adjustably) preshape the wavefront of the light from the light source reflected by the micromirror actuator in such a way that a planar illumination of the examination object arranged at the distal end of the multimode fiber is achieved. The system according to the present disclosure thus advantageously enables various medical applications, such as imaging applications and / or invasive treatments with laser light.

[0021] The system according to the present disclosure further comprises an imaging beam path configured to capture an image signal. The image signal is optionally the image signal of the reference mask or the object under examination. In a preferred embodiment, the imaging beam path is configured to capture an image signal coupled out of the proximal end of the multimode fiber. In other words, the light signal emitted by the light source and modulated by the focusing means is coupled into the proximal end of the multimode fiber, and the image signal is coupled out of the same proximal end of the multimode fiber. In a likewise preferred embodiment, the system according to the present disclosure comprises a first multimode fiber and a second light guide, preferably also a multimode fiber.The first multimode fiber is designed to transmit the illumination signal, and the light signal emitted by the light source and modulated by the focusing means is coupled into its proximal end, and the illumination signal is coupled out of its distal end selectively onto the object under examination or the reference mask. The second multimode fiber is preferably designed to transmit the image signal, and the light signal (image signal) scattered by the object under examination and / or the reference mask is coupled into its distal end, and the image signal is coupled out of its proximal end into the imaging beam path. In the embodiment with two multimode fibers, the multimode fibers are preferably identical, and / or the reference mask is preferably arranged outside the multimode fibers or arranged in one of the multimode fibers.In a system with two multimode fibers, the transmission characteristics of the system consisting of both multimode fibers can be determined by imaging the reference mask. The imaging beam path preferably has a detector for capturing the image signal. The detector is preferably designed as a (spectrally resolving) photodetector.

[0022] In a further preferred embodiment of the system according to the present disclosure, it further comprises a compensating optical system that can be introduced into the illumination beam path. The compensating optical system is preferably a spatially adjustable optical system that enables spatially resolved manipulation of the light signal. The transmission, absorption, reflection, polarization, scattering, and / or attenuation of the compensating optical system can preferably be adjusted in a spatially resolved manner. As described above, the light propagation through the multimode fiber can be described theoretically and / or empirically by a transmission matrix that describes the linear relationship between specific quantities (e.g., a 2D matrix) of coupled-in input light signals and coupled-out output light signals.The transmission characteristic that can be determined with the system according to the present disclosure is preferably the transmission matrix or at least a part of the transmission matrix. In other words, the system according to the present disclosure enables monitoring of the transmission characteristic of the multimode fiber and preferably the determination of its (time-dependent) transmission matrix. The compensating optical system is preferably designed to influence the transmission characteristics, preferably the transmission matrix, in a spatially resolved manner. Thus, the compensating optical system can advantageously be used to manipulate the transmission characteristics, preferably the transmission matrix, of the overall system and thus, for example, to compensate for determined changes in the transmission characteristics, preferably the transmission matrix, of the multimode fiber.The compensating optical system preferably comprises a spatially resolved adjustable filter, such as a matrix filter, a microfilter array and / or a microlens array or the like.

[0023] In a further preferred embodiment, the system according to the present disclosure further comprises at least one polarization filter that can be or has been introduced into the illumination beam path and / or the imaging beam path. As described above, determining the transmission properties of the multimode fiber based on the image signal of the reference mask (within certain limits and assuming boundary conditions) enables conclusions to be drawn about the transmission matrix of the multimode fiber, in particular about changes in the transmission matrix of the multimode fiber. Determining the transmission properties of the multimode fiber based on the image signal of the reference mask also enables (within certain limits and assuming boundary conditions) conclusions to be drawn about environmental parameters of the multimode fiber, in particular about the geometry of the multimode fiber.The introduction of one or more polarization filters into the illumination beam path and / or into the imaging beam path advantageously improves the possibility for such conclusions, for example because mechanical deformations of multimode fibers affect the polarization properties of multimode fibers.

[0024] In a further preferred embodiment, the system according to the present disclosure further comprises an interferometric reference arm that is coupled or can be coupled into the imaging beam path. The interferometric reference arm is preferably designed to interferometrically superimpose a non-modulated reference light signal generated by the light source with the image signal modulated by the multimode fiber (and optionally the micromirror actuator) in order to subsequently detect the interference signal. This advantageously enables the determination of (relative) phase information of the image signal, which can advantageously also enable or improve the above-mentioned conclusions about the transmission matrix of the multimode fiber, in particular about changes in the transmission matrix of the multimode fiber.

[0025] In a further preferred embodiment, the system according to the present disclosure comprises a control unit configured to control the focusing means. The control unit is configured, in particular, to control the focusing means to adjust the focal plane of the light signal to the reference mask or to the object under examination (which is arranged near the distal end of the multimode fiber and is to be imaged). Furthermore, the control unit is preferably configured to control the micromirror actuator, in particular to control the micromirror actuator to specifically deform the wavefront reflected by the micromirror actuator. As described above, the system according to the present disclosure enables monitoring of the transmission properties of the multimode fiber and preferably the determination of its (time-dependent) transmission matrix.Preferably, the control unit is further configured to control the micromirror actuator depending on the determined transmission properties of the multimode fiber, for example, to compensate for a determined change in the transmission properties of the multimode fiber. This advantageously enables control or improvement of the optical properties of the overall system.

[0026] In a likewise preferred embodiment, the system according to the present disclosure is, at least partially, integrated into a surgical and / or medical instrument, for example, an ophthalmic surgical instrument. In particular, at least the multimode fiber and the reference mask are integrated into the instrument, but the focusing means (the micromirror actuator) and / or its control unit are also preferably integrated into the instrument. Likewise preferably, the light source is also integrated into the instrument. The surgical and / or medical instrument, particularly preferably an ophthalmic instrument, is preferably an endoscope, a pointer, and / or a laser scalpel. Likewise preferably, the instrument has an operating element with which a user can make inputs for the control unit and thus the micromirror actuator, the light source, etc.

[0027] The functionalities of the control unit of the system according to the present disclosure can be implemented by electrical or electronic parts or components (hardware), by firmware (ASIC), and / or by executing a suitable program (software). The functionalities of the control unit are preferably realized or implemented by a combination of hardware, firmware, and / or software. For example, individual components of the control unit for executing individual functionalities are designed as a separate integrated circuit or arranged on a common integrated circuit. The individual functionalities of the control unit are further preferably designed as one or more processes that run on one or more processors in one or more electronic computing devices and are generated when one or more computer programs are executed.The control unit is designed to cooperate with the other components, in particular the light source, the focusing means, and the detector, to implement the functionalities of the system described herein according to the present disclosure. It will also be apparent to those skilled in the art that the functionalities of multiple computers (data processing devices, control units, control devices) can be combined or combined in a single device, or that the functionality of a specific data processing device can be distributed across a plurality of devices to implement the functionalities of the control unit according to the invention.

[0028] In a likewise preferred embodiment of the system according to the present disclosure, said system is partially integrated into a surgical microscope. The surgical microscope has an interface for user input and a display means, or is connected to these. The surgical microscope preferably further comprises a surgical microscopy system with a camera, in particular a main observer camera or an environment camera. The control unit of the surgical microscope is preferably designed as a control unit of the system according to the present disclosure and is in particular designed to carry out the method according to the invention, as described below, on the basis of commands stored on a memory unit of the surgical microscope. In the context of the present disclosure, a surgical microscope is understood in the broadest sense to be a microscope suitable for use during an operation.The surgical microscope is also called a medical (technical) microscope or medically approved microscope.

[0029] A further aspect of the present disclosure relates to a method for determining a transmission property of a multimode fiber, in particular for determining a transmission property of a multimode fiber in situ, such as a transmission property in the form of a transmission matrix or a part thereof. The method comprises a step of coupling a light signal into a proximal end of the multimode fiber. The light signal is preferably generated by means of a light source and coupled into the proximal end of the multimode fiber via an illumination beam path arranged between the light source and the proximal end of the multimode fiber. The light source is preferably a laser light source, for example an RGB laser. Furthermore, the illumination beam path preferably comprises a focusing means, particularly preferably a micromirror actuator.Also preferably, the illumination beam path comprises a compensating optical system, one or more polarization filters and optionally further optical elements, such as lenses and mirrors.

[0030] The method further comprises the step of adjusting a focal plane of the light signal to a reference mask arranged near the distal end of the multimode fiber. The reference mask has a predetermined spatially resolved reflectivity, transparency, absorption and / or polarization or is designed as a predetermined interference grating, wherein the reference mask is printed, etched or engraved on or into a cross-section of the multimode fiber (50) or is attached to the distal end (52) of the multimode fiber (50). The reference mask is preferably arranged within the multimode fiber or monolithically connected thereto and preferably has a predetermined spatially resolved reflectivity, transparency, absorption and / or polarization. Alternatively or additionally, the reference mask is designed as a predetermined interference grating.The reference mask is, for example, printed, etched, engraved, or the like on or into a cross-section of the multimode fiber. The reference mask is also preferably attached to a distal end of the multimode fiber and has the aforementioned properties. The focal plane of the light signal is preferably adjusted by means of a focusing means, in particular a micromirror actuator, arranged in the illumination beam path of the system. The focusing means is preferably controlled by a control unit.

[0031] In a further step of the method, an image signal from the reference mask is captured, preferably an image signal coupled out from the proximal end of the multimode fiber. The reference mask is preferably designed such that the reference mask located in the focal plane of the light signal has a predetermined influence on the image signal captured in the method according to the present disclosure. The captured image signal from the reference mask thus corresponds to a known reference and, in particular, has a previously known pattern, for example, an interference pattern, intensity pattern, polarization pattern and / or color pattern or the like. The image signal is simultaneously influenced by transmission properties of the multimode fiber.

[0032] In a further step of the method according to the present disclosure, the transmission property of the multimode fiber is determined based on the extracted image signal of the reference mask and based on information about a previously known property of the reference mask. Preferably, the transmission property of the multimode fiber is determined based on a comparison of the extracted image signal of the reference mask and the information about a previously known property of the reference mask. According to the invention, the previously known property of the reference mask is the predetermined spatially resolved reflectivity, transparency, absorption, polarization, and / or predetermined interference property of the reference mask.For example, the reference mask can have spatially resolved etchings, engravings, coatings, or the like, which influence the transmission of light in a spatially resolved manner, so that the reference mask produces a characteristic spatially resolved reference pattern in a light signal focused on the mask and transmitted through the mask, reflected by the mask, or scattered by the mask. Alternatively or additionally, the information on the previously known property of the reference mask is a reference image signal of the reference mask. The reference image signal is preferably acquired under defined calibration conditions, for example, using a well-characterized (non-bent) multimode fiber under well-defined environmental parameters.

[0033] In particular, the information relating to a previously known property of the reference mask relates to an image signal of the reference mask that is expected or can be expected under certain conditions, for example calibration conditions, i.e., for example, an image signal of the reference mask acquired using a specific and non-bent multimode fiber, at a specific temperature and / or at a specific pressure. For example, the reference image signal is preferably acquired under defined calibration conditions, for example, using a well-characterized (non-bent) multimode fiber under well-defined environmental parameters. In this embodiment, a comparison of the reference image signal and an image signal of the reference mask acquired in the method according to the present disclosure is also carried out, and the transmission properties of the multimode fiber are determined based on such an image comparison.Such a comparison allows for the determination of deviations between the captured image signal and the expected / expected image signal of the reference mask. These deviations are attributable to the current transmission properties of the multimode fiber. Based on the determined deviations, conclusions can be drawn about the current transmission properties (a current transmission matrix) of the multimode fiber. Based on the determined transmission properties of the multimode fiber, the image signal captured by the object under investigation can be advantageously corrected, the transmission properties of the overall system can be manipulated (e.g., to compensate for the determined deviation), and / or conclusions can be drawn about the current state of the multimode fiber.Preferably, a comparison is made between a plurality of reference image signals and a plurality of acquired image signals of the reference mask, wherein the plurality of images have been acquired under different conditions, for example at different frequencies, polarizations, phases or the like of the excitation light signal (of the light source).

[0034] In a preferred embodiment of the method according to the present disclosure, a focal plane of the light signal is further adjusted to an examination object to be imaged, which is arranged near the distal end of the multimode fiber. In other words, in this step, the examination object is exposed and imaged using the multimode fiber. In particular, in this embodiment, an image signal of the examination object is captured, in particular an image signal of the examination object coupled out from the proximal end of the multimode fiber. The image signal is preferably also captured by means of the multimode fiber and an imaging beam path arranged at the proximal end of the multimode fiber, in particular a detector arranged in the imaging beam path. The detector is preferably a photodetector.The reference mask is preferably designed (and arranged) such that the reference mask located outside the focal plane of the light signal has a negligible influence on an image signal acquired in the method according to the present disclosure. Thus, according to this method step, an image signal of the object under examination is acquired that is largely undisturbed by the reference mask. However, the image signal is influenced by the previously determined transmission properties of the multimode fiber; therefore, the method preferably further includes a (computational / mathematical) correction of the acquired image signal based on the determined transmission properties of the multimode fiber.

[0035] In a likewise preferred embodiment of the method according to the present disclosure, a variation in the transmission property of the multimode fiber is determined. In other words, a continuous or semi-continuous determination of the transmission properties (transmission matrix) of the multimode fiber is carried out, as described above, and, based thereon, a determination of a temporal profile of the transmission properties (transmission matrix), i.e., the variation thereof. In an application, for example an endoscopic medical application, a variation in the transmission properties occurs due to changes in the environmental parameters of the endoscope or the multimode fiber. These environmental parameters can relate, for example, to the ambient temperature, the ambient pressure, fiber aging, or even a curvature of the multimode fiber.According to this embodiment of the method according to the present disclosure, the determined variation is further compensated. Such compensation is achieved, for example, by controlling a micromirror actuator used to adjust the focal plane of the light signal, as described above. By controlling the micromirror actuator, the wavefront of the reflected light can be specifically pre-shaped, whereby a variation in the transmission properties (transmission matrix) of the multimode fiber can be at least partially compensated. Alternatively or additionally, the determined variation is compensated by introducing a compensating optical system, as described above, into an illumination beam path designed to couple the light signal into a proximal end of the multimode fiber.Also preferably, according to this embodiment, an environmental parameter of the multimode fiber is determined based on the determined variation in transmission properties. For example, deformations, such as bends, of the multimode fiber influence the transmission properties, and thus, conclusions about the deformation of the multimode fiber can be drawn based on the variation in transmission properties. Particularly in medical applications, the actual geometry of the fiber in the body is important information for the user and can therefore be determined.

[0036] Further preferred implementations of the method according to this disclosure correspond to further preferred embodiments of the system according to the present disclosure and realize the same advantages as the embodiments.

[0037] A further aspect of the present disclosure relates to a computer program comprising instructions which, when executed by a computer, in particular by a control unit as described above, cause the system as described above (in particular a system with a control unit) to carry out the inventive method as described above. The computer program preferably comprises instructions which, when executed by a computer, in particular a control unit as described above, cause the system as described above to carry out the inventive method according to one of the preferred embodiments as described above. The computer program according to the invention is preferably stored in a volatile memory, for example a RAM element, or in a non-volatile storage medium, such as a flash memory.

[0038] A further aspect of the present disclosure relates to a multimode fiber for use in a system as described above and / or in a method as described above. The multimode fiber has a proximal end designed to couple in a light signal and a distal end designed to couple in an image signal scattered by an object under examination. The distal end of the multimode fiber is formed from an optically inactive material. The optically inactive material is in particular an optically transparent material (at least in the relevant frequency ranges) and is preferably formed or connected to the multimode fiber monolithically. The distal end is further designed, in at least part of its cross-section, as a reference mask, as described above.In particular, the distal end has a predetermined, spatially variable reflectivity, transparency, absorption, and / or polarization in at least part of its cross section, and / or the distal end is configured as a predetermined interference grating in at least part of its cross section. The reference mask is printed, etched, or engraved on or into a cross section of the multimode fiber, or is attached to the distal end of the multimode fiber. Preferably, the reference mask is configured in or on the optically inactive material.

[0039] Further preferred embodiments of the invention emerge from the remaining features mentioned in the subclaims and the figures explained below. The various embodiments of the invention mentioned in this application can be advantageously combined with one another, unless otherwise stated in individual cases. Description of the characters

[0040] The invention is explained below in exemplary embodiments with reference to the accompanying drawings. They show: Fig. 1 is a schematic representation of a distal end of a multimode fiber according to an embodiment; Fig. 2 is a schematic diagram of a system according to an embodiment; Fig. 3 a schematic representation of a system according to another embodiment; and Fig. 4 a schematic flow diagram of a method according to an embodiment.

[0041] Fig. 1 shows a schematic representation of a distal end 52 of a multimode fiber 50 according to one embodiment. The multimode fiber 50 has a proximal end (not shown) and the distal end 52 shown. The multimode fiber 50 has, for example, a pure silicon dioxide core with a diameter of 200 µm surrounded by a fluorine-doped silicon dioxide cladding with an outer diameter of 220 µm in an optically active region 55. The refractive index contrast between the core and cladding materials results in a nominal numerical aperture, NA, of 0.22, so that the multimode fiber 50 supports approximately 17,000 guided modes per orthogonal polarization state at a wavelength of 532 nm.At the distal end of the multimode fiber 50, an optically inactive (transparent) material 54 is arranged, in particular a material that is transparent at a wavelength of 532 nm, i.e. with a transmission at this wavelength of at least 85%, preferably at least 90%, particularly preferably at least 95%, and further preferably at least 99%. The optically transparent material 54 is preferably formed monolithically with the remaining multimode fiber 50, in particular the optically active region 55, for example by varying the melt during drawing of the multimode fiber. Alternatively, the optically inactive material 54 can also be monolithically connected to the remaining section of the multimode fiber 50, thereby avoiding the formation of an interface and reflection of light signals at the interface.

[0042] A reference mask 53 is arranged within the optically inactive material 54. The reference mask is arranged at a distance of less than 100 µm, preferably at a distance of less than 50 µm, and particularly preferably at a distance of less than 20 µm, from the distal end 52 of the multimode fiber 50. The reference mask 53 is preferably introduced as a spatially resolved manipulation of the optically inactive material 54 into at least part of a cross-section of the multimode fiber 50 in the region of the optically inactive material 54. For example, the optical properties of the material 54 are manipulated in a spatially resolved manner by localized constructive superposition of two light signals, for example laser beams, so that the multimode fiber 50 has spatially resolved optical properties.For example, a transmittance or polarization of the multimode fiber 50 in the region of the material 54 depends on the specific position on a cross-section of the multimode fiber 50. If a light signal is focused onto this cross-section of the multimode fiber 50, a predetermined manipulation of the light occurs through the reference mask 53 of the multimode fiber 50.

[0043] Fig. 2 shows a schematic representation of a system 100 according to an embodiment. The system 100 has an illumination beam path 21 for coupling a light signal 11 into a proximal end 51 of a multimode fiber 50, in particular a multimode fiber as described with reference to Fig. 1 configured multimode fiber 50. The system 100 has a light source 10 for generating an initially unmodulated light signal 11.1, wherein the light source in Fig. 2 is shown as part of the illumination beam path 21, but can also be arranged outside the illumination beam path 21. The light source 10 is preferably a laser source, for example an RGB laser source.

[0044] The illumination beam path 21 in Fig. 2 further comprises a focusing means 30, which is irradiated by the light source 10 with the unmodulated light signal 11.1. The focusing means 30 is designed to generate the modulated light signal 11.2 from the light signal 11.1. The focusing means 30 is, for example, a micromirror actuator, a microlens array, and / or a filter matrix. The modulated light signal 11.2 passes through a beam splitter 44 and is coupled into the proximal end 51 of the multimode fiber 50. The focusing means 30 is designed to adjust a focal plane of the light signal 11.2, in particular in the region of a distal end 52 of the multimode fiber 50. An examination object 60 is arranged at a distance from the distal end 52 of the multimode fiber 50 and is irradiated with light coupled out of the distal end 52 of the multimode fiber 50. The working distance between the distal end 52 of the multimode fiber 50 and the examination object 60 is 15 mm.

[0045] The light scattered by the examination object 60 is coupled as an image signal 12 into the distal end 52 of the multimode fiber 50 and coupled out of the proximal end 51 of the multimode fiber 50. An imaging beam path 22 guides the image signal 12 coupled out of the multimode fiber 50 to a detector 40, where it is detected. The imaging beam path 22 has, for example, the aforementioned beam splitter 44. The system 100 further comprises a control unit 70, which is connected to the light source 10, the detector 40, and the focusing means 30 for data communication (represented by dashed lines). The control unit 70 is particularly designed to control the focusing means 30 to adjust the focal plane of the modulated light signal 11.2 optionally onto the examination object 60 or onto a reference mask 53 arranged near the distal end 52 of the multimode fiber 50, or to position it along the optical axis (z-axis) of the multimode fiber 50. The control unit 70 is configured, for example, to effect this optional positioning of the focal plane by modulating the light signal 11.2. The control unit 70 is configured to detect an image signal 12 of the reference mask 53 by means of the detector 40 when the focal plane of the light signal 11.2 is positioned on the reference mask 53, and is configured to detect an image signal 12 of the examination object 60 that is largely undisturbed by the reference mask 53 by means of the detector 40 when the focal plane of the light signal 11.2 is positioned on the examination object 60.

[0046] Fig. 3 shows a schematic representation of a system 100 according to another embodiment. The system 100 according to Fig. 3 agrees in many aspects with the Fig. 2 and a repeated description of identical components is omitted. In the system 100 of Fig. 3, the focusing means 30 is designed as a micromirror actuator 31. The micromirror actuator 31 is designed to reflect the light signal 11.1 coming from the light source 10 and thereby generate the modulated light signal 11.2. For this purpose, the individual mirror elements (indicated by the grating) of the micromirror actuator 31 are controlled by a control unit, for example the control unit 70 of Fig. 1, to preshape the wavefront of the light 11.2 reflected by the micromirror actuator 31. This makes it possible to adjust the focal plane of a light signal coupled out of the distal end 52 of the multimode fiber 50. In particular, it is possible to adjust the number, spot size, focus (z), light intensity, aperture, and / or position (x, y) of light spots coupled out of the distal end 52 of the multimode fiber 50. Thus, scanning the light spots over the examination object 60 is possible, as is selectively adjusting the focal plane to the examination object 60 or the reference mask 53.

[0047] By setting the focal plane to the reference mask 53, an image signal 12 of the reference mask 53 can be detected by means of the detector 40. By comparing this image signal 12 of the reference mask 53 with information about a previously known property of the reference mask 53, for example with a reference image signal of the reference mask 53, a transmission property of the multimode fiber 50 can be deduced, in particular a current transmission matrix of the multimode fiber 50. By means of a compensating optical system 32 arranged in the illumination beam path 21, for example a further micromirror actuator, a microlens array and / or a filter matrix, a control unit, for example the control unit 70, can be used to Fig. 1, the light signal 11.2 can be modulated depending on the determined transmission characteristic of the multimode fiber 50. The control unit 70 can also be used to modulate the light signal 11.1 by the micromirror actuator 31 depending on the determined transmission characteristic of the multimode fiber 50, for example, to compensate for a temporal variation in the determined transmission characteristic of the multimode fiber 50.

[0048] In order to determine relevant transmission properties of the multimode fiber 50 (isolated) based on the image signal 12 of the reference mask 53, polarization filters 33, 41 are arranged in the illumination beam path 21 and the imaging beam path 22. The spatial geometry, in particular a current curvature of the multimode fiber 50, has a strong influence on the polarization properties of the light signals guided through the multimode fiber 50. The polarization filters 33, 41 allow the isolation of this additional helpful information and thus support the drawing of conclusions about the current geometry of the multimode fiber 50 based on the image signal 12 of the reference mask 53. For a similar reason, an interferometric reference arm 43 can be coupled into the illumination beam path 21, through which the unmodulated light signal 11 coming from the light source 10 is transmitted.1 can be superimposed by means of a beam splitter 45 with the image signal 12 of the reference mask 53 coupled out of the multimode fiber 50. Thus, relative phase information between the light signal 11.1 and the image signal 12 of the reference mask 53 can be determined, which also allows for improved conclusions to be drawn about the transmission properties of the multimode fiber 50. Furthermore, a polarization filter 42 is also arranged in the interferometric reference arm 43 in order to combine the determination of phase information and polarization information.

[0049] Fig. 4 shows a schematic flow diagram of a method for determining a transmission characteristic of a multimode fiber 50 according to one embodiment. The method is described below with reference to the system 100 as shown in Fig. 3. The method comprises a step S100 of coupling a light signal 11, in particular the modulated light signal 11.2, into a proximal end 51 of the multimode fiber 50. In a further step S200 of the method, a focal plane of the light signal 11 is adjusted to a reference mask 53 arranged near the distal end 52 of the multimode fiber 50. This is done, for example, by modulating the light signal 11.2 using the micromirror actuator 31. In step S300 of the method, an image signal 12 of the reference mask 53, which is coupled out from the proximal end 51 of the multimode fiber 50, is detected, in particular with the detector 40. Finally, in step S400 of the method, the transmission property of the multimode fiber 50 is determined based on the coupled-out image signal 12 of the reference mask 53 and based on information about a previously known property of the reference mask 53.The determination in step S400 is carried out by a control unit, for example the control unit 70 in the . Fig. 2, on which, for example, a reference image signal of the reference mask 53 is stored as information on the previously known property of the reference mask 53. In a preferred embodiment of the method, in a step S500, the focal plane of the light signal 11, in particular of the modulated light signal 11.2, is adjusted to the examination object 60 to be imaged, which is arranged near the distal end 52. The focal plane is preferably adjusted by means of the modulation of the light signal 11.2 by the micromirror actuator 31. In a step S600, an image signal 12 of the examination object 60 is then detected by means of the detector 40, which image signal is only minimally influenced by the reference mask 53 arranged outside the focus. List of reference symbols 10 Light source 11.1 Light signal (unmodulated) 11.2 Light signal (modulated) 12 image signal 21 Illumination beam path 22 Imaging beam path 30 focusing agents 31 Micromirror actuator 32 compensating optical system 33 polarizing filters 40 detector 41 polarizing filters 42 polarizing filters 43 interferometric reference arm 44 beam splitters 45 beam splitters 50 multimode fibers 51 proximal end 52 distal end 53 Reference mask 54 optically inactive material 55 optically active material 60 Subject of investigation 70 Control unit 100 systems S100 Coupling of a light signal S200 Setting a focal plane of the light signal on the reference mask S300 Acquiring an image signal of the reference mask S400 Determining the transmission characteristics of the multimode fiber S500 Setting a focal plane of the light signal on the object under investigation S600 Acquiring an image signal of the object under investigation

Claims

[1] System (100) for determining a transmission characteristic of at least one multimode fiber (50), the system (100) comprising: a light source (10) designed to generate a light signal (11); an illumination beam path (21) configured to couple the light signal (11) into a proximal end (51) of the at least one multimode fiber (50); a focusing means (30) configured to vary a focal plane of the light signal (11) around a distal end (52) of the multimode fiber (50); and a reference mask (53) arranged near the distal end (52) of the multimode fiber (50), wherein the focusing means (30) is further designed to adjust the focal plane of the light signal (11) selectively to the reference mask (53) or to an object to be imaged (60) arranged near the distal end (52), and wherein the reference mask (53) has a predetermined spatially resolved reflectivity, transparency, absorption and / or polarization and / or is designed as a predetermined interference grating, and wherein the reference mask (53) is printed, etched or engraved on or into a cross section of the multimode fiber (50) or is attached to the distal end (52) of the multimode fiber (50). [2] System (100) according to claim 1, wherein the focusing means (30) comprises a micromirror actuator (31) arranged in the illumination beam path (21) between the light source (10) and the proximal end (51) of the multimode fiber (50). [3] System (100) according to claim 2, wherein the micromirror actuator (31) is designed to reflect and reshape a wavefront of the light signal (11.1) emitted by the light source (10), and the reshaping of the wavefront causes the variation of the focal plane. [4] System (100) according to one of the preceding claims, further comprising an imaging beam path (22) configured to detect an image signal (12), comprising a detector (40) for detecting the image signal (12). [5] System (100) according to one of the preceding claims, further comprising a compensating optical system (32) which can be introduced into the illumination beam path (21). [6] System (100) according to one of the preceding claims, further comprising at least one polarization filter (33, 41, 42) which can be introduced into the illumination beam path (21) and / or the imaging beam path (22) according to claim 4. [7] System (100) according to claim 4, further comprising an interferometric reference arm (43) which can be coupled into the imaging beam path (22). [8] System (100) according to one of the preceding claims, further comprising a control unit (70) designed to control the focusing means (30) for selectively adjusting the focal plane of the light signal (11) to the reference mask (53) or to the object to be imaged (60) arranged near the distal end (52). [9] Method for determining a transmission characteristic of a multimode fiber (50), the method comprising the method steps: Coupling (S100) a light signal (11) into a proximal end (51) of the multimode fiber (50); Adjusting (S200) a focal plane of the light signal (11) to a reference mask (53) arranged near the distal end (52) of the multimode fiber (50), which reference mask has a predetermined spatially resolved reflectivity, transparency, absorption and / or polarization or is designed as a predetermined interference grating and which is printed, etched or engraved on or in a cross section of the multimode fiber (50) or is attached to the distal end (52) of the multimode fiber (50); Detecting (S300) an image signal (12) of the reference mask (53) coupled out from the proximal end (51) of the multimode fiber (50); and Determining (S400) the transmission property of the multimode fiber (50) based on the coupled-out image signal (12) of the reference mask (53) and based on information about a previously known property of the reference mask (53), wherein the previously known property of the reference mask (53) is the predetermined spatially resolved reflectivity, transparency, absorption, polarization and / or predetermined interference property of the reference mask (53) and / or wherein the information about the previously known property of the reference mask (53) is a reference image signal of the reference mask (53). [10] The method of claim 9, further comprising the steps of: Adjusting (S500) a focal plane of the light signal (11) to an object to be imaged (60) arranged near the distal end (52); and Acquiring (S600) an image signal (12) of the object under examination (60). [11] Method according to one of claims 9 and 10, Determining a variation in the transmission characteristic of the multimode fiber (50); and Compensating the determined variation by controlling a micromirror actuator (31) used to adjust the focal plane of the light signal (11) and / or by introducing a compensating optical system (32) into an illumination beam path (21) designed to couple the light signal (11) into a proximal end (51) of the multimode fiber (50); and / or Determining an environmental parameter of the multimode fiber (50) based on the determined variation of the transmission property. [12] Computer program comprising instructions which, when executed by a computer, in particular a control unit (70) of a system (100) according to claim 8, cause the system (100) according to one of claims 1 to 8 to carry out a method according to one of claims 9 to 11. [13] Multimode fiber (50) for use in a system (100) according to one of claims 1 to 8 and / or in a method according to one of claims 9 to 11, comprising a proximal end (51) for coupling in a light signal (11) and a distal end (52) for coupling in an image signal (12) scattered by an object under examination (60), wherein the distal end (52) is formed from an optically inactive material (54) which, in at least part of its cross section, has a reference mask (53) with predetermined locally variable reflectivity, transparency, absorption and / or polarization and / or is designed as a predetermined interference grating, wherein the reference mask (53) is printed, etched or engraved on or in a cross section of the multimode fiber (50) or is attached to the distal end (52) of the multimode fiber (50).

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