Assembly for performing optical coherence tomography

Abstract

The present invention relates to an assembly with an interferometer for performing optical coherence tomography, the interferometer being divided into two interferometer parts (1, 2) separated by a spatial distance, the two interferometer parts (1, 2) being movable relative to each other and optically connected to each other by a flexible light guide (3, 4, 5) bridging the spatial distance. According to the present invention, an assembly with an interferometer is provided, which is as insensitive as possible to the effects caused by bending of the tubular cable packet and allows an optimal signal-to-noise ratio or optimal image quality of the OCT images, characterized in that at least one first flexible light guide (3) is designed as a polarization-maintaining light guide consisting of two connected polarization-maintaining optical fibers (3 a, 3 b).

Description

[Technical Field]

[0001] The invention relates to an assembly according to the preamble of claim 1 . [Background technology]

[0002] The term optical coherence tomography (commonly abbreviated as OCT) is used to describe an imaging method that can obtain two- and three-dimensional images from organic tissues that scatter light.

[0003] In this method, typically broadband, short-coherence-length light is split into two partial beams by a beam splitter. The first partial beam is directed at the organic tissue under test, while the second partial beam passes through a reference section. The third partial beam, i.e., the light reflected from the organic tissue, interferes with the second partial beam in an interferometer.

[0004] The signal from the interferometer allows depth-resolved examination of the tissue, i.e., at the depth of the optical axis of the first partial beam. Furthermore, by scanning the organic tissue with the first partial beam in the planar or transverse direction, a three-dimensional image of the organic tissue, a so-called OCT image, can be obtained.

[0005] Against this background, an assembly for performing optical coherence tomography is known from US Publication No. 2014 / 0176960, in which a polarization-maintaining optical fiber without a polarization control unit is used. In this respect, interferometers with polarization-stabilized fibers are already known.

[0006] More generally, an interferometer assembly for performing optical coherence tomography can be divided into two different system components: the first system component comprises a camera head, which is typically located on a table and pointed towards the eye being examined; and the second system component comprises at least one power supply unit located below the table and away from the eye. This division is chosen to keep the size and weight of the camera head small, which is particularly preferred in hospitals.

[0007] A flexible hose cable package connects the power supply unit to the camera head, and when the camera head is manipulated to align it with the eye being examined, the hose cable package, and therefore the optical fiber guiding the two system components together in an optically guided manner, is bent in an uncontrolled manner.

[0008] Conventional light-guiding single-mode fibers (abbreviated as SMF) guide the components of a light beam only transversely to the direction of propagation. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Publication No. 2014 / 0176960 Summary of the Invention [Problem to be solved by the invention]

[0010] When such fibers are used to connect system components, bending of the hose cable package can cause unpredictable polarization changes in the interferometer, which can be accompanied by a significant loss of signal-to-noise ratio and therefore degradation of the OCT image quality. [Means for solving the problem]

[0011] The present invention is therefore based on the problem of defining an assembly with an interferometer that is as insensitive as possible to the effects associated with bending of the hose cable package and that achieves the best possible signal-to-noise ratio or the best possible image quality of the OCT image.

[0012] The present invention solves the above-mentioned problems by the features of claim 1.

[0013] First, it has been recognized that the above-mentioned problems can be addressed, among other things, if the entire interferometer, including the light source, detector, and electronics, is concentrated in a single housing and integrated into the camera head.

[0014] This concept does not provide a flexible hose cable package with light guides or optical fibers between the two system components. However, this leads to a bulky and heavy camera head. Furthermore, miniaturizing and integrating all the hardware components into a small camera is a major technical challenge.

[0015] Furthermore, it has been recognized that the interferometer can be split into two system components, providing a means to make the camera head very compact. Thus, the entire interferometer, including the light source, detector, and electronic components, can be integrated outside the camera head.

[0016] This solution requires a flexible optical fiber cable connection between the two system components, which can be subject to changes in polarization state and an associated loss of signal-to-noise ratio in the OCT image.

[0017] It is also recognized that the described splitting approach will require active adjustment of the polarization state in the system components by mechanical or electro-optical manipulation of the polarization to reduce loss in signal-to-noise ratio.

[0018] However, active polarization adjustment is typically a very slow process and must be performed relatively frequently.

[0019] The solution proposed by the present invention does not require frequent polarization adjustments and is therefore not impaired by the time-consuming procedure for performing them.

[0020] It has further been recognized that the entire interferometer, including the light source, detector(s), electronic components, etc., can be integrated into a package outside of the camera head. This approach requires a flexible fiber optic cable connection between the two system parts, which, as discussed, can be subject to changes in polarization state and associated loss of signal-to-noise ratio in the OCT image.

[0021] In this approach, the OCT signal is split into two separate components with different polarizations using a polarization-sensitive optical splitter, which can then be measured by two separate detectors. Any change in the polarization state in the interferometer results in a redistribution of the OCT signal across the two detectors without signal loss. However, this approach requires two photodetectors and associated electronics.

[0022] The measurement results will take up twice the amount of data generated by the solution according to the invention, however the invention results in lower hardware costs and smaller data volumes.

[0023] It has also been recognized that the entire interferometer can be integrated into a fiber optic (coupler) path, a so-called common-path interferometer.

[0024] In this type of interferometer, the arms of the interferometer, ie the sample arm and the reference arm, are combined into one optical path and are therefore less susceptible to loss of signal-to-noise ratio in the OCT image due to polarization changes.

[0025] However, it has been recognized that the design of a common-path interferometer is complex for optimal OCT detection at the interface of a bulk optics-based microscope or ophthalmoscope, due to the lack of a clear physical separation between the two arms of the interferometer.

[0026] However, OCT microscopes and ophthalmoscopes require the ability to change the length of the reference arm of the interferometer to accommodate the depth localization of the imaged object or to accommodate the object in the sample arm of the interferometer. This is not possible with common-path interferometers. Additionally, the split optical path in a common-path interferometer makes it more difficult to simultaneously minimize light loss in both arms, which always results in some loss in the signal-to-noise ratio of the OCT image.

[0027] The solution proposed by the present invention can preferably use a Mach-Zehnder or Michelson interferometer type configuration, which is clearly not related to the common-path interferometer problems mentioned above.

[0028] It has also been recognized that, according to one approach, an alternative fiber-based interferometer can be used that uses a polarization-maintaining optical fiber (PMF) instead of a single-mode fiber (SMF), where light passing through the PMF does not experience a change in polarization state when the PMF is bent or moved.

[0029] However, the physical properties of PMF, specifically the difference in refractive index between the two crystal axes, make the interferometer susceptible to ghosting artifacts resulting from interference between the optical signals guided through the two crystal axes.

[0030] To eliminate or mitigate these ghosting artifacts requires complex solutions that are impractical.

[0031] Nevertheless, the solution proposed by the present invention uses a fiber-based interferometer, which is primarily based on the use of numerous SMFs and specially designed light guides, to avoid the ghost artifacts mentioned above. Surprisingly, the ghost artifacts can be avoided by connecting two polarization-maintaining optical fibers to form a single light guide.

[0032] In this manner, the present invention provides a method for using flexible optical fiber connections to split an OCT interferometer into two interferometer components that are insensitive to manipulation of the fiber connection.

[0033] In accordance with the present invention, it is now recognized that it is feasible to split the interferometer into two interferometer components and to connect these components while maintaining polarization stability.

[0034] The present invention provides a polarization-stabilized interferometer configuration for light guide or fiber-based optical coherence tomography that can have a compact, freely movable sample arm.

[0035] According to the present invention, therefore, it is possible to create an assembly in which one interferometer component is designed as a manually portable, camera-based, or otherwise movable interferometer component, which has light weight, small, and compact dimensions since most of the heavy hardware components can be located in a separate, sealed housing away from the other interferometer component, preferably the test object.

[0036] Specifically, the assembly according to the invention comprises an interferometer for performing optical coherence tomography, the interferometer being divided into two interferometer parts spaced apart from one another, movable relative to one another and optically connected to one another by a flexible light guide bridging the spatial distance, at least one of the flexible light guides being designed as a polarization-maintaining light guide consisting of two polarization-maintaining optical fibers connected to one another.

[0037] This first light guide can consist of two polarization-maintaining optical fibers, so-called PMFs, connected together, with their respective crystal axes tilted at 90° to each other. Because the PMFs are connected with a 90° offset, there is cross-coupling between the crystal axes of the fibers. In this way, a first light guide, so-called xPMF or cross PMF, is created.

[0038] Surprisingly, this first light guide compensates for the birefringence behavior of its first component with the birefringence behavior of its second component. Surprisingly, therefore, the ghost pattern and fixed interference pattern artifacts that conventional PMFs cause in optical coherence tomography images are reduced. The first light guide maintains a stable polarization state like conventional PMFs, but can be freely handled or bent without causing the optical coherence tomography artifact problems that conventional PMFs cause.

[0039] A conventional single polarization-maintaining fiber (PMF or PM fiber for short) is designed as a single-mode fiber that guides linearly polarized light. As light passes through this fiber, its polarization is maintained. Such a fiber maintains a large birefringence, allowing light to travel along the fiber in two distinct, well-defined polarization modes with distinctly different phase velocities. Therefore, polarization-maintaining fiber typically has two principal or crystal axes—the slow axis and the fast axis—that can transmit polarized light while maintaining its polarization state. By tilting these two axes 90° relative to the two axes of another fiber, ghosting artifacts can be prevented.

[0040] Two polarization-maintaining optical fibers can be connected to each other by splicing. When two fibers are spliced, they are fused or welded to each other. Preferably, no additional objects are needed to create a connection between the fibers. A principle method for splicing two fibers is known from EP 0 427 705 A1. Preferably, the polarization-maintaining optical fibers are of equal length. The birefringences of two fibers of the same length cancel each other or compensate each other. The two polarization-maintaining optical fibers are arranged back to back and connected to each other at the splice point.

[0041] At least the second and / or third light guides are designed as one or more single-mode fibers. The second and / or third light guides can guide light from the imaging interferometer component to the detector of the other interferometer component. The light guided through these single-mode fibers does not need to be polarized, and is preferably not polarized. These single-mode fibers (SMF), unlike PMF, do not maintain the polarization state of the guided light.

[0042] Against this background, the light guides can be housed in a flexible hose cable that runs between the two interferometer components. The flexible hose cable surrounds all light guides, and possibly additional electrical cables, in a tubular manner and protects them from being torn off or damaged in any other way. The flexible hose cable consists of an elastomer or a very flexible plastic, so that the camera head can be easily moved by hand by bending and / or displacing the hose cable.

[0043] The first interferometer component can be assigned to the power supply unit, so that the relatively heavy and bulky power supply unit can be arranged as a fixed element on a stable support, preferably on the floor, while the other optical elements can be moved manually relative to the first interferometer component.

[0044] Against this background, the second interferometer part can be assigned to a camera head that is movable relative to the first interferometer part, so that the camera head can be easily aimed at the eye to be examined.

[0045] The first interferometer part may be housed in a first housing containing electronic components and a power supply unit, thereby allowing heavy loads to be housed in the first stationary housing.

[0046] The second interferometer component can be accommodated in a second housing that forms the camera head, which allows the camera head to be easily aimed at the eye to be examined. It is conceivable that the second housing forms the camera head and the second interferometer component is integrated in this housing and thus in the camera head.

[0047] The sample and reference arms, along which light is directed, can be located entirely within the second housing or within the camera head, so imaging is not hindered by movement of the camera head.

[0048] Preferably, assemblies of the type described herein are used in ophthalmology or ophthalmic examinations. [Brief explanation of the drawings]

[0049] [Figure 1] FIG. 1 is a schematic diagram of an assembly having two interferometer components spatially separated from one another. [Figure 2] 2 shows the assembly according to FIG. 1, with the interferometer components and the hose cable package shown separately. [Figure 3] 1 shows a schematic representation of the connection of two polarization-maintaining optical fibers of equal length, with their crystal axes tilted 90° relative to each other at the junction. DETAILED DESCRIPTION OF THE INVENTION

[0050] FIG. 1 shows an assembly comprising an interferometer for performing optical coherence tomography, the interferometer being divided into two interferometer parts 1, 2 at a spatial distance from each other, the two interferometer parts 1, 2 being movable relative to each other and optically connected to each other by flexible light guides 3, 4, 5 that bridge the spatial distance between the interferometer parts 1, 2.

[0051] At least the first flexible light guide 3 is designed as a polarization-maintaining light guide consisting of two mutually connected polarization-maintaining optical fibers 3a, 3b of equal length, so-called PMFs.

[0052] The first light guide 3 consists of two polarization-maintaining optical fibers 3a, 3b of equal length connected to each other, with their respective crystal axes 21, 22 tilted at 90° to each other at a junction 20. This is shown diagrammatically in Figure 3.

[0053] The two polarization-maintaining optical fibers 3a, 3b of equal length that form the first light guide 3 are connected to each other by joining them at a junction point 20. The junction point 20 is located in the center of the light guide 3, which is formed by the two polarization-maintaining optical fibers 3a, 3b of equal length that are arranged back to back.

[0054] Furthermore, the second light guide 4 and the third light guide 5 are designed as single-mode fibers. All light guides 3, 4, 5 are housed in a flexible hose cable 6 which runs between the two interferometer parts 1, 2 and together with the hose cable 6 form a flexible and deformable hose cable package. A junction point 20 is located in the middle or approximately in the middle of the hose cable 6.

[0055] The first interferometer part 1 is assigned to a power supply unit 7. The second interferometer part 2 is assigned to a movable camera head 8.

[0056] The first interferometer part 1 is housed in a first housing 9 which comprises electronic components and a power supply unit 7. The second interferometer part 2 is housed in a second housing 10 which constitutes or forms a camera head 8. The sample arm 11 and the reference arm 12 are located entirely within the second housing 10 or only within the camera head 8.

[0057] 1 specifically shows that the assembly is divided into two interferometer parts 1, 2, which are essentially assigned to a power supply unit 7 and a camera head 8, respectively. The first interferometer part 1 contains all the necessary electronic components, including a light source 13 or OCT light source, trigger circuitry 14 and clock circuitry 15, DAQ electronics and detectors 16, 17. Thus, the first interferometer part 1 contains all or nearly all the electronic components that can reasonably be assigned to the first interferometer part 1.

[0058] In particular, the first interferometer part 1 comprises an OCT light source, i.e. a laser light source emitting light with a wavelength of 1050 nm, known to those skilled in the art as a VCSEL swept source ("Vertical Cavity Surface Emitting Laser Swept Source"), a trigger circuit 14 designed as a scan trigger circuit, a clock circuit 15, i.e. a so-called K-clock circuit, DAQ electronics and a balanced detector 16 of the interferometer.

[0059] The second interferometer assembly 2 is integrated into the camera head 8 and essentially contains only the passive fiber components of the entire interferometer.

[0060] Three light guides 3, 4, 5 pass through a flexible hose cable 6 and together form an elastically deformable hose cable package for optically connecting the two interferometer components 1, 2. The first light guide 3 guides light from the light source 13, i.e., the OCT light source, to the camera head 8 and is made of two equal-length polarization-maintaining optical fibers 3a, 3b, i.e., PMFs.

[0061] PMF stands for "polarization-maintaining fiber" and is abbreviated as PMF. The first light guide 3 is made by joining two PMFs 3a and 3b of the same length together.

[0062] The second light guide 4 and the third light guide 5 guide light from the imaging interferometer assembly 2 in the camera head 8 to the first interferometer assembly 1 in which the power supply unit 7 is located or to the balanced detector 16 in the housing 1. The balanced detector 16 is insensitive to the polarization state of the received light, and the PIN diode detects only its intensity.

[0063] Thus, conventional single mode fibers, abbreviated as SMF, are used to optically connect the balanced detector 16 of the first interferometer component 1 to the two outputs of the 50 / 50 coupler 18 of the second interferometer component of the camera head 8, but SMFs introduce unknown changes in the polarization state when they are handled or bent.

[0064] Therefore, the second and third light guides 4, 5 can be freely manipulated without affecting the quality of the optical coherence tomography images. It is particularly advantageous to use these two light guides 4, 5 in combination with the first light guide 3, which maintains polarization stability.

[0065] Because the three light guides 3, 4, 5 are insensitive to changes in polarization state, or because the detection of their signals is insensitive to changes in polarization state, a hose cable package can contain all three light guides 3, 4, 5 and can be moved and handled freely without concern for affecting the optical coherence tomography signal quality.

[0066] The interferometer configuration described herein requires a one-time adjustment and is stable in operation thereafter. No periodic or real-time polarization optimization is required. Additionally, the space used by the camera head 8 is minimized. This optimizes the configuration of both the power supply unit 7 and the camera head 8.

[0067] FIG. 2 shows diagrammatically the two interferometer parts 1, 2 and the hose cable package 6, as well as the usual electronic, optical or optoelectronic components of an interferometer well known to those skilled in the art.

[0068] 3 shows, using a splice point 20, a schematic illustration of the need to align the crystal axes 21, 22 of two equal-length polarization-maintaining optical fibers 3a, 3b when they are spliced together. To achieve the birefringence compensation effect described herein, an intentional 90° offset is introduced at the splice point 20, as shown in FIG.

[0069] A PM fiber fusion splicer, not shown, typically includes a device for suitably rotating and aligning the polarization-maintaining optical fibers to be spliced relative to one another. [Explanation of symbols]

[0070] 1 First interferometer component 2 Second interferometer part 3 First Light Guide 3a, 3b Polarization-maintaining optical fiber 4 Second Light Guide 5. Third Light Guide 6 Flexible hose cable 7 Power Supply Unit 8 Camera Head 9 First Housing 10 Second Housing 11 Sample arm 12 reference arm 13 Light source 14 Trigger Circuit 15 Clock Circuit 16 Balanced Detector 17 Other detectors 18 50 / 50 coupler 19 Scanning Unit 20 junction 21 First Crystallographic Axis 22 Second Crystal Axis

Claims

1. 1. An assembly comprising an interferometer for performing optical coherence tomography, the assembly comprising: the interferometer is divided into a first interferometer part (1) and a second interferometer part (2) at a spatial distance from each other; the first interferometer component (1) and the second interferometer component (2) are movable relative to each other and are optically connected to each other by a first flexible light guide (3) that bridges at least the spatial distance; The first interferometer part (1) is housed in a first housing (9) containing electronic components and a power supply unit (7), The second interferometer component (2) is housed in a second housing (10) that includes a camera head (8); the second interferometer component (2) comprises a sample arm (11) and a reference arm (12) along which light is directed, respectively; the sample arm (11) and the reference arm (12) are disposed in the second housing (10) or in the camera head (8); the first flexible light guide (3) is designed as a polarization-maintaining light guide consisting of two polarization-maintaining optical fibers (3a, 3b) connected to each other, The crystal axes of the two polarization-maintaining optical fibers (3a, 3b) are arranged at an angle of 90° to each other. assembly.

2. Assembly according to claim 1, characterized in that the two polarization-maintaining optical fibers (3a, 3b) are connected to each other by a splice and / or are of equal length.

3. An assembly as described in claim 1 or 2, further comprising a second flexible light guide (4) and / or a third flexible light guide (5) bridging the spatial distance and optically connecting the first interferometer component (1) and the second interferometer component (2) to each other, characterized in that the second flexible light guide (4) and / or the third flexible light guide (5) are designed as one or more single-mode fibers.

4. 4. An assembly according to any one of claims 1 to 3, characterized in that the first flexible light guide (3) is housed in a flexible hose cable (6) extending between the first interferometer part (1) and the second interferometer part (2).

5. 5. An assembly according to any one of claims 1 to 4, characterized in that the first interferometer part (1) is assigned to a power supply unit (7).

6. 6. An assembly according to any one of claims 1 to 5, characterized in that the second interferometer part (2) is assigned to a camera head (8) which is movable relative to the first interferometer part (1).

Images