ARRANGEMENT FOR CARRYING OUT OPTICAL COHERENCE TOMOGRAPHY

Description

[0001] The invention relates to an arrangement according to the preamble of claim 1.

[0002] Such an arrangement is already known from US 2007 / 0081166 A1. US 2004 / 0126048 A1 shows an arrangement with a polarization-maintaining sample arm and a polarization-maintaining reference arm. US 2003 / 0174339 A1 shows an interferometer in which at least one arm is configured as a fiber optic.

[0003] Optical coherence tomography (OCT) is an imaging technique that can be used to obtain two- and three-dimensional images of light-scattering organic tissues.

[0004] In this method, light with a wide bandwidth and a short temporal coherence length is typically split into two partial beams in a beam splitter. The first partial beam falls on the organic tissue under investigation, while the second partial beam travels through a reference path. A third partial beam, namely light reflected by the organic tissue, interferes with the second partial beam in an interferometer.

[0005] Signals from the interference allow the tissue to be examined with depth resolution, i.e., at the depth of the optical axis of the first partial beam. If the organic tissue is also scanned flatly or laterally with the first partial beam, three-dimensional images of the organic tissue, so-called OCT images, are obtained.

[0006] Against this background, US 2014 / 0 176 960 A1 discloses an arrangement for performing optical coherence tomography using polarization-maintaining, light-conducting fibers without a polarization control unit. Interferometers with polarization-stable fibers are already known.

[0007] In general, interferometer arrangements for performing optical coherence tomography can be divided into two different system components. The first system component comprises a camera head, which is typically positioned on a table and directed toward the eye to be examined.

[0008] A second system component includes at least one power supply unit located under the table and away from the eye. This arrangement is chosen to keep the dimensions and weight of the camera head small, which is particularly preferred in hospitals.

[0009] A flexible hose cable assembly connects the power supply unit to the camera head. When the camera head is manipulated to align it with the eye being examined, the hose cable assembly and thus also the light-conducting fibers that optically connect the two system components are bent in an uncontrolled manner.

[0010] Conventional single-mode fibers (SMFs) only transmit components of light rays that are oriented transversely to the direction of propagation.

[0011] If such fibers were used to connect the system components, bending the tubular cable package would introduce unpredictable polarization changes into the interferometer. This could result in a significant loss of the signal-to-noise ratio and thus a degraded image quality of an OCT image.

[0012] The invention is therefore based on the object of specifying an arrangement with an interferometer which is as insensitive as possible to the effects associated with the bending of a hose cable package and which realizes the best possible signal-to-noise ratio or the best possible image quality of an OCT image.

[0013] The present invention solves the above-mentioned problem by the features of claim 1.

[0014] Firstly, it was recognised that one way to address this problem was to enclose the entire interferometer, light source, detectors and electronics, combine them in one housing and integrate them into the camera head.

[0015] This concept would not require a flexible cable assembly with light-conducting or optical fibers between two system components. However, it would result in a bulky and heavy camera head. Furthermore, miniaturization and integration of all hardware components into a small-sized camera pose a significant technical challenge.

[0016] It was further recognized that a means could be provided to separate the interferometer into two system components, while maintaining a very compact camera head. The entire interferometer, including the light source, detector(s), electronic components, etc., would then be combined in a single package outside the camera head.

[0017] This solution would require a flexible cable connection with optical fibers between the two system parts and could suffer from changes in polarization states and the associated loss of signal-to-noise ratio in an OCT image.

[0018] It was also recognized that the described separation approach would require active alignment of the polarization state in a system part by mechanical or electro-optical manipulation of the polarization to reduce losses in the signal-to-noise ratio.

[0019] However, active polarization alignment is usually a very slow process and must be performed relatively frequently.

[0020] The solution proposed by the invention does not require frequent polarization alignment and therefore does not suffer from time-consuming procedures to perform it.

[0021] It has been further recognized that the entire interferometer, light source, detector(s), electronic components, etc., could be integrated into a single package external to the camera head. This approach would require a flexible optical fiber cable connection between the two system components and, as described, may suffer from changes in polarization states and the associated loss of signal-to-noise ratio in an OCT image.

[0022] This approach would allow OCT signals to be split into two distinct components with different polarizations using polarization-sensitive optical splitters and measured by two separate detectors. Any changes in the polarization state in an interferometer would result in a redistribution of the OCT signals across the two detectors without signal loss. However, this approach would require two optical detectors and the associated electronics.

[0023] The measurement results would require twice the data volume generated by the inventive solution. However, the invention leads to reduced hardware costs and a lower data volume.

[0024] It was also recognized that a so-called common-path interferometer could be used, which integrates the entire interferometer in a fiber optic (coupler) path.

[0025] This special type of interferometer integrates the arms of the interferometer, namely the sample arm and the reference arm, into a single optical path and is therefore immune to losses in the signal-to-noise ratio of an OCT image due to changes in polarization.

[0026] However, it was recognized that the design of the common-path interferometer is complicated to construct for optimal OCT detection in a bulk-optics based microscope or ophthalmoscope interface because there is no clear physical separation between the two arms of the interferometer.

[0027] However, for OCT microscopes and ophthalmoscopes, the length of the interferometer's reference arm must be adjustable to accommodate the depth localization of the imaged object or the object in the interferometer's sample arm. This is not possible with the common-path interferometer. Furthermore, the split optical path in the common-path interferometer makes it difficult to simultaneously minimize optical losses in both arms, which will always result in some loss of signal-to-noise ratio in the OCT images.

[0028] In the solution proposed by the invention, a configuration in the manner of a Mach-Zehnder interferometer or Michaelson interferometer can preferably be used, which do not involve the problems of the common-path interferometer described above.

[0029] It was also recognized that an alternative fiber-based interferometer approach could be used that uses polarization-maintaining optical fibers (PMFs) instead of single-mode fibers (SMFs). Light passing through PMFs is not subject to changes in polarization state when the PMF is bent or moved.

[0030] However, the physical properties of a PMF, in particular the difference between the refractive indices of the two crystal axes of this type of optical fiber, make the interferometer susceptible to ghost artifacts originating from interference between the optical signals transmitted across the two crystal axes.

[0031] Removing or mitigating these ghost artifacts requires complex solutions that are not practical.

[0032] The solution proposed by the invention, however, uses a fiber-based interferometer, which is primarily based on the use of a large number of SMFs and a specially designed optical fiber that does not generate the aforementioned ghost artifacts. Surprisingly, ghost artifacts can be avoided by combining two polarization-maintaining optical fibers into a single optical fiber.

[0033] The invention thus realizes a way by which an OCT interferometer can be split into two interferometer parts by means of a flexible optical fiber connection, which are insensitive to manipulation of the fiber connection.

[0034] According to the invention, it has been recognized that the division of an interferometer into two interferometer parts and the connection of these interferometer parts can be realized while maintaining polarization stability.

[0035] According to the invention, a polarization-stable interferometer configuration for fiber-optic or fiber-based optical coherence tomography is realized, which can have a compact and freely movable sample arm.

[0036] According to the invention, it is thus possible to create an arrangement in which one interferometer part is designed as a manually portable, camera-based, or otherwise movable interferometer part. This interferometer part has small and compact dimensions and low weight, since most of the heavier hardware components can be placed in the other interferometer part, preferably in a separate, enclosed housing that is remote from the object to be examined.

[0037] An arrangement according to the invention specifically comprises an interferometer for performing optical coherence tomography. The interferometer is divided into two spatially spaced interferometer parts. The two interferometer parts are movable relative to each other and are optically connected to each other by flexible optical fibers that bridge the spatial distance. At least one of these flexible optical fibers is designed as a polarization-maintaining optical fiber, which consists of two interconnected polarization-maintaining optical fibers.

[0038] This first optical fiber consists of two interconnected polarization-maintaining optical fibers, so-called PMFs, whose respective crystal axes are tilted by 90° relative to each other. The PMFs are connected at a 90° offset, creating cross-coupling between the fiber crystal axes. This creates the first optical fiber, a so-called xPMF or cross-PMF.

[0039] This first light guide surprisingly compensates for the birefringence behavior of its first component with the birefringence behavior of its second component. Therefore, ghost patterns and fixed pattern artifacts that occur with conventional PMFs in optical coherence tomography imaging are surprisingly mitigated. The first light guide maintains a stable polarization state like a conventional PMF, but can be manipulated or bent arbitrarily, yet does not produce artifact problems in optical coherence tomography that conventional PMFs would produce.

[0040] A typical single polarization-maintaining optical fiber (PMF or PM fiber for short) is designed as a single-mode fiber, guiding linearly polarized light. As the light passes through this fiber, the polarization of the light is maintained. Increased birefringence is maintained in such optical fibers, allowing light to travel along the fiber in two well-defined polarization modes with distinct phase velocities. Therefore, a polarization-maintaining fiber typically has two principal axes, or crystal axes—a slow and a fast axis—along which polarized light can be transmitted while maintaining its polarization state. By tilting these two axes by 90° relative to two axes of another optical fiber, ghost artifacts are avoided.

[0041] The two polarization-maintaining optical fibers could be connected to each other by splicing. When splicing two fibers, the fibers are fused or welded together. Preferably, no additional materials are required to create the connection between the fibers. EP 0 427 705 A1 discloses how the splicing of two fibers can be carried out in principle. Preferably, the polarization-maintaining optical fibers are of equal length. The birefringence of the two equally long fibers cancels or compensates for each other. The two polarization-maintaining optical fibers are arranged one behind the other and connected to each other at a splice point.

[0042] At least one second and / or third optical fiber is / are configured as a single-mode fiber or fibers. The second optical fiber and / or the third optical fiber can guide light from the imaging interferometer section to a detector in the other interferometer section. The light guided through these single-mode fibers need not be polarized, or is preferably unpolarized. These single-mode fibers, or SMFs for short, do not maintain a polarization state of the guided light, unlike PMFs.

[0043] With this in mind, the optical fibers could be housed in a flexible tubular cable that extends between the two interferometer sections. A flexible tubular cable surrounds all optical fibers and any other electrical wires in a tube-like manner, protecting them from tearing or other damage. The flexible tubular cable is made of an elastomer or a highly flexible plastic, allowing a camera head to be easily moved manually by bending and / or sliding the tubular cable.

[0044] The first interferometer part is associated with a power supply unit. This allows a relatively heavy and bulky power supply unit to be arranged as a stationary element on a stable base, preferably on a floor, while other optical elements can be manually moved relative to the first interferometer part.

[0045] With this in mind, the second interferometer section is assigned to a camera head that can be moved relative to the first interferometer section. This allows the camera head to be easily aligned to the eye being examined.

[0046] The first interferometer part is housed in a first housing, which includes electronic components and a power supply unit. This allows heavy objects to be accommodated in the first, stationary housing.

[0047] The second interferometer part is housed in a second housing, which encloses the camera head. This allows the camera head to be easily aligned with the eye being examined. It is conceivable that the second housing forms the camera head, and the second interferometer part is integrated into this housing and thus into the camera head.

[0048] The sample arm and the reference arm, along which the light is guided, are located entirely within the second housing or the camera head. This means that imaging is not disturbed by camera head movement.

[0049] The arrangement of the type described here is preferably used in ophthalmology or eye examination.

[0050] In the drawing show Fig. 1 a schematic representation of an arrangement with two spatially separated interferometer parts, Fig. 2 the arrangement according to Fig. 1 , where the interferometer parts and the hose cable package are shown separately, and Fig. 3 is a schematic representation of the splicing of two polarization-maintaining optical fibers of equal length, the crystal axes of which are tilted by 90° relative to each other at the splice point.

[0051] Fig. 1 shows an arrangement comprising an interferometer for carrying out optical coherence tomography, wherein the interferometer is divided into two interferometer parts 1, 2 which are spatially spaced from one another, wherein the two interferometer parts 1, 2 are movable relative to one another and are optically connected to one another by flexible light guides 3, 4, 5 which bridge the spatial distance between the interferometer parts 1, 2.

[0052] At least one first flexible optical fiber 3 is designed as a polarization-maintaining optical fiber, which consists of two interconnected polarization-maintaining optical fibers 3a, 3b of equal length, so-called PMFs.

[0053] The first optical fiber 3 consists of two interconnected polarization-maintaining optical fibers 3a, 3b of equal length, whose respective crystal axes 21, 22 are arranged at a splice point 20 tilted by 90° relative to each other. This is shown in Fig. 3 shown schematically.

[0054] The two equally long polarization-maintaining optical fibers 3a, 3b, which form the first optical fiber 3, are connected to each other by splicing at the splice point 20. The splice point 20 is located in the center of the optical fiber 3, which is formed by the two equally long polarization-maintaining optical fibers 3a, 3b arranged one behind the other.

[0055] Furthermore, a second optical fiber 4 and a third optical fiber 5 are designed as single-mode fibers. All optical fibers 3, 4, and 5 are accommodated in a flexible tubular cable 6, which extends between the two interferometer parts 1, 2, and together with the tubular cable 6, form a flexibly deformable tubular cable package. The splice point 20 is positioned in the center or approximately in the center of the tubular cable 6.

[0056] 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.

[0057] The first interferometer part 1 is housed in a first housing 9, which includes electronic components and the power supply unit 7. The second interferometer part 2 is housed in a second housing 10, which includes or forms the camera head 8. The sample arm 11 and the reference arm 12 are arranged entirely and only in the second housing 10 or in the camera head 8.

[0058] Fig 1 Specifically, it shows that the arrangement is divided into two interferometer parts 1, 2, each of which is essentially assigned to the power supply unit 7 and the camera head 8. The first interferometer part 1 contains all essential electronic components, which include the light source 13 or OCT light source, trigger circuits 14 and clocking circuits 15, DAQ electronics, and detectors 16, 17. The first interferometer part 1 therefore contains all or almost all electronic components that can be meaningfully assigned to the first interferometer part 1.

[0059] Specifically, the first interferometer part 1 contains the OCT light source, namely a laser light source known to those skilled in the art as a VCSEL swept source ("Vertical Cavity Surface Emitting Laser Swept Source"), which emits light with a wavelength of 1050 nm, the trigger circuit 14, which is designed as a scan trigger circuit, the clocking circuit 15, namely a so-called K-clock circuit, DAQ electronics and a balanced detector 16 of the interferometer.

[0060] The second interferometer part 2 is integrated into the camera head 8 and essentially contains only passive fiber components of the entire interferometer.

[0061] Three optical fibers 3, 4, and 5 run through the flexible tubular cable 6 and, together with it, form the flexibly deformable tubular cable package to optically connect the two interferometer parts 1, 2. The first optical fiber 3 guides light from the light source 13, namely the OCT light source, to the camera head 8 and is made of two equally long polarization-maintaining, light-conducting fibers 3a, 3b, namely the PMFs.

[0062] PMF stands for "Polarization-Maintaining Fiber" and is abbreviated to PMF. The first optical fiber 3 is created by splicing two PMFs 3a and 3b of equal length.

[0063] The second light guide 4 and the third light guide 5 guide light from the imaging interferometer part 2 in the camera head 8 to the balanced detector 16 in the first interferometer part 1 or housing 1, in which the power supply unit 7 is arranged. The balanced detector 16 is insensitive to the polarization state of the received light, and PIN diodes only detect its intensity.

[0064] Therefore, common single-mode fibers, abbreviated to SMF, are used for the optical connection of the balanced detector 16 in the first interferometer part 1 with the two outputs of the 50 / 50 coupler 18 of the second interferometer part 2 in the camera head 8, although the SMFs undergo unknown changes in polarization state when they are handled or bent.

[0065] It is therefore possible to manipulate the second and third light guides 4, 5 arbitrarily without fear of affecting the quality of an optical coherence tomography image. The use of these two light guides 4, 5 in combination with the first light guide 3, which maintains polarization stability, is therefore particularly advantageous.

[0066] Since the three light guides 3, 4, 5 are either insensitive to changes in the polarization state or the detection of their signals is insensitive to polarization changes, the hose cable package can contain all three light guides 3, 4, 5 and can be moved and handled arbitrarily without having to fear any effect on the signal quality of the optical coherence tomography.

[0067] The interferometer configuration described here requires a one-time adjustment and is stable during subsequent operation. It does not require periodic or real-time polarization optimization. Additionally, the space used in the camera head 8 is minimized. This optimizes the configuration of both the power supply unit 7 and the camera head 8.

[0068] Fig. 2 shows schematically 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 that are familiar to the person skilled in the art.

[0069] Fig. 3 shows schematically, using the splice point 20, that when splicing two equally long polarization-maintaining optical fibers 3a, 3b, it is necessary to coordinate the orientations of their crystal axes 21, 22 at the splice point 20. A deliberate offset of 90°, as in Fig. 3shown, is generated at the splice point 20 in order to achieve the birefringence compensation effect described here.

[0070] A PM fiber fusion splicer (not shown) typically includes a device for appropriately rotating and aligning polarization-maintaining optical fibers to be connected relative to one another. List of reference symbols:

[0071] 1First interferometer part 2Second interferometer part 3First light guide 3a, 3bPolarization-maintaining optical fiber 4Second light guide 5Third light guide 6Flexible hose cable 7Power supply unit 8Camera head 9First housing 10Second housing 11Sample arm 12Reference arm 13Light source 14Trigger circuit 15Clocking circuit 16Balanced detector 16 17Further detector 1850 / 50 coupler 19Scan unit 20Splice point 21First crystal axis 22Second crystal axis

Claims

1. An assembly comprising an interferometer for carrying out an optical coherence tomography, wherein the interferometer is divided into two interferometer parts (1, 2) at a spatial distance from each other, wherein the two interferometer parts (1, 2) are movable relative to each other and are optically connected to each other by flexible light guides (3, 4, 5) which bridge the spatial distance, wherein the first interferometer part (1) is accommodated in a first housing (9)comprising electronic components and a power supply unit (7), wherein the second interferometer part (2) is accommodated in a second housing (10) comprising a camera head (8), and wherein the sample arm (11) and the reference arm (12) are arranged in the second housing (10) or in the camera head (8),characterized in that at least a first flexible light guide (3) is designed as a polarization-maintaining light guide which consists of two polarization-maintaining light-guiding fibers (3a, 3b) connected to each other, the respective crystal axes of which are arranged tilted by 90° relative to each other.

2. The assembly as claimed in claim 1, characterized in that the two polarization-maintaining light-guiding fibers (3a, 3b) are connected to each other by splicing and / or are of equal length.

3. The assembly as claimed in any one of the preceding claims, characterized in that at least a second light guide (4) and / or third light guide (5) is or are designed as a single-mode fiber or as single-mode fibers.

4. The assembly as claimed in any one of the preceding claims, characterized in that the light guides (3, 4, 5) are accommodated in a flexible hose cable (6) which extends between the two interferometer parts (1, 2).