System and a method for an optical speckle noise reduction

Abstract

The invention relates to a system for an optical speckle noise reduction. The system comprises an at least one section of an optic fiber of the first type having the first type of light propagation parameters set and a coherent light source optically coupled with the input of the at least one section of the optic fiber of the first type. The system further comprises an at least one section of an optic fiber of the second type having the second type of light propagation parameters set optically coupled in series with the optic fiber of the first type wherein the optic fiber of the first type differs from the optic fiber of the second type in at least one of the light propagation parameters. The invention also relates to a method for an optical speckle noise reduction of a light beam using the disclosure system.

Description

[0001] System and a method for an optical speckle noise reduction

[0002] Technical Field

[0003] The present invention relates to an optic speckle noise reduction system, in particular to a passive optic fiber system for an optic speckle noise reduction and to a method of optical speckle noise reduction.

[0004] Background

[0005] A known state of the art way of removing this unwanted phenomenon of optic speckle noise is introducing a time-varying illumination pattern to force the speckle pattern to vary from picture to picture and thus be easily differentiated from a static real image after comparing many image realizations. One of embodiments of such a system has been described in a patent EP 3 627 093 Bl. However, this method requires active electro-optic devices which are complicated, costly, and difficult to engineer into a medical-grade imaging unit.

[0006] A method of passive decreasing the speckle noise presence in an image was published in an article "Multimode fiber as a tool to reduce cross talk in Fourier-domain full-field optical coherence tomography" (Optics Letters, Vol. 47, Issue 4, pp. 838-841, 2022). The idea of using a long multimode fiber is based on the fact that various transverse modes in a multimode fiber experience different travel velocity, so after propagation in a sufficiently long fiber the modes should become separated in time and their interference would not take place. However, this method is only a proof-of-principle and does not cause an efficient speckle noise removal. Some modes are degenerated in the sense of their velocity so do not get separated easily with this method and there is no good control over the dispersion of velocities. Increasing the fiber length would help separate the modes further but, on the other hand, would introduce too much separation for the others. Typical fiber lengths used in this reported experiment are of the order of 100-300 meters.

[0007] None of the documents in the prior art discloses a solution in which the optical speckle noise reduction is removed using a passive solution not requiring additional power source or active optical signal filtering or processing. What is more, thanks to the invention, there is a possibility to obtain controlled modes separation of an optic signal using simplified system and method.

[0008] Brief description of the invention

[0009] The essence of the invention is a system for an optical speckle noise reduction. The system comprises an at least one section of an optic fiber of the first type having the first type of light propagation parameters set and a coherent light source optically coupled with the input of the at least one section of the optic fiber of the first type. The system further comprises an at least one section of an optic fiber of the second type having the second type of light propagation parameters set optically coupled in series with the optic fiber of the first type wherein the optic fiber of the first type differs from the optic fiber of the second type in at least one of the light propagation parameters.

[0010] Preferably, the system further comprises plurality of sections of the optic fiber of the first type and plurality of sections of the optic fiber of the second type coupled in series alternately so that adjected sections differ from each other in at least one of the light propagation parameters.

[0011] Preferably, the system further comprising an at least one section of an optic fiber of the third type having the third type of light propagation parameters set coupled in series with the section of the fiber optic of the first type and / or with the section of the fiber optic of the second type.

[0012] Preferably, the system further comprising a plurality of sections of an optic fiber of different types each having their light propagation parameters set coupled in series with each other wherein adjacent sections have different light propagation parameters.

[0013] Preferably, the optic fiber light propagation parameters set comprise fiber type, transverse mode, light propagation velocity, core type, core geometry, index type, fiber material.

[0014] Preferably, the sections are coupled by fusion-splicing.

[0015] Preferably, sections are coupled with industrial fiber connectors retaining the possibility to disconnect the fibers when needed. Preferably, sections are coupled using free-space fiber coupling, allowing the light to leave one section and use a focusing optical element to couple the beam into another section.

[0016] The invention relates also to the method for an optical speckle noise reduction of a light beam using the disclosed system comprising the following steps: the light beam from the coherent light source enters the section of the optic fiber of the first type; the light beam is transmitted through the section of the optic fiber of the first type and leaves the section of the optic fiber of the first type; the light beam enters the section of the optic fiber of the second type via the coupling; the light beam is transmitted through the section of the optic fiber of the second type leaves the section of the optic fiber of the second type.

[0017] Brief Description of the Drawings

[0018] Preferred embodiment of the present invention is presented in a more detailed way with reference to the attached drawing, in which:

[0019] Figure 1 is a scheme of a system for an optical speckle noise reduction comprising two sections of the optic fiber of the first type, one section of the optic fiber of the second type coupled using fusion-splicing.

[0020] Figure 2 is a scheme of a system for an optical speckle noise reduction comprising two sections of the optic fiber of the first type, one section of the optic fiber of the second type coupled using industrial fiber connectors.

[0021] Figure 3 is a scheme of a system for an optical speckle noise reduction comprising two sections of the optic fiber of the first type, one section of the optic fiber of the second type coupled using free-space fiber coupling.

[0022] Detailed Description

[0023] In the full-field OCT (optical coherence tomography) regime which is an OCT method using a 2D detector (typically a camera) and a simultaneous illumination of the entire imaged area, as opposed to the scanning OCT regime which is using a focused beam illuminating a single pixel of the image at a time, it is often the case that an excessive speckle pattern is created in an image. This is a result of the light source coherency - well-defined and constant phase relationships give rise to an interference pattern (speckle) which is an unwanted phenomenon, being a source of artificial features in the image.

[0024] The technical reason of the speckle pattern creation is a phenomenon of an optical crosstalk, i.e. fact of simultaneous presence of backscattered signal from entire illuminated area (e.g. human eye) on the camera and, as a result of scattering, some photons hiting the camera pixels not corresponding to the points from which the light has been originally scatered. The effect is an artificial patern of points with increased or decreased light intensity compared to the ideal imaging conditions.

[0025] According to the preferred embodiment of the invention, a system for an optical speckle noise reduction comprises an at least one section of an optic fiber of the first type having the first type of light propagation parameters set. Light propagation parameters set comprises fiber class type (e.g. optical, photonic crystal, polarization-maintaining), supported transverse mode type (single mode, double mode, multimode), light propagation velocity, core type (e.g. hollow or solid), index type e.g. gradient-index (an optical fiber which core has a refractive index that decreases continuously with increasing radial distance from the optical axis of the fiber) or a stepindex fiber (a uniform index of refraction in the core, and a lower index in the surrounding cladding), material (type of glass or plastic, potentially doped) and effective core diameter (or size in general if not circular, e.g. polygonal or elliptic) . System also comprises a coherent light source optically coupled with the input of the at least one section of the optic fiber of the first type. Optical coupling enables light beam to enter the optic fiber. In this example any form known couplings 3 can be used in the preferred example of the disclosure a fusion-splicing coupling is used (Fig. 1). In another example of the system industrial fiber connectors are used. This kind of coupling retains the possibility to disconnect the fibers when needed (Fig. 2). Alternatively, a free- space fiber coupling is used (Fig. 3). This coupling allows the light to leave one section or light source and with use of a focusing optical element (mirror, lens) to couple the beam into the optic fiber via its end (input). Different type of coupling can be used in one system. In the example the system further comprises second section of an optic fiber of the second type 2 having the second type of light propagation parameters set. The second section is optically coupled 3 in series with the first section of the optic fiber of the first type 1. Optical speckle noise reduction system has the optic fiber of the first type 1 different from the optic fiber of the second type 2 in at least one of the light propagation parameters. In this example of the disclosure the first section of the optic fiber of the first type 1 has different transverse mode velocity characteristics than the second section of the optic fiber of the second type 2. In this example two different optic fibers sections with different characteristics are connected in series, so the illuminating light (light beam) travels through these fibers one after another. In the first section some light transverse modes are separated, and others are still travelling together. Such beam is then transferred to the second section using coupling. In the second section light beam propagates in different way further separating the modes which were not separated in the previous fiber sections. Using a number of different fiber sections eventually makes it possible to separate all the modes in space. The light beam travels through the optic fiber of the first type where some different transverse modes separate form others (spatial separation occurs due to light propagations parameters of each transverse mode in the optic fiber differences e.g. different transverse mode propagation velocity). Some modes have very similar propagation parameters in the optic fiber in such a case mode separation is impossible or not efficient enough (e.g. requires very long optic fiber section). In preferable example of the disclosure light beam travels through more than one section of an optic fiber of certain type. In one section some modes can be easily separated and those which cannot be easily separated travel further in unseparated manner to another section (via coupling 3) of an optic fiber of other type. In the second section light propagations parameters of others transverse mode differs and some other transverse modes become easy to separate from the beam. If necessary, the system comprises more sections having different light propagations parameters from the rest (each section filters or separates certain transverse modes). In another example of the disclosure the system comprises more than one section of an optic fiber having the same light propagations parameters or even being the same optic fiber which are arranged alternately with an optic fiber section (or sections) having different light propagations parameters than neighbouring optic fiber section. The level of transverse modes separation (optical speckle noise reduction) increases with the number of optic fiber sections having different (or alternated) light propagations parameters. In another example of the disclosure the system comprises at least two different types of optic fibers with matched light propagation conditions, i.e. supporting a low-loss transmission of the illuminating light. There are three main groups of distinct fiber families (but the example is not limited to these): a multimode step-index fiber (traditional), a Gradient-index fiber (GRIN) and Photonic crystal fiber (PCF), either with a solid core or a hollow core. The fibers used do not need to be limited to circular geometry, other shapes are also possible, e.g. rectangular.

[0026] In this example of the system an efficient speckle noise removing set of fibers comprises 5 sections of optic fibers, three sections are multimode fiber type, and two sections are GRIN fiber type. Sections ale coupled alternately so that adjected sections differ from each other. In this example following combination of sections is disclosed:

[0027] 1. 100 m of a multimode fiber

[0028] 2. 10 m of GRIN fiber

[0029] 3. 100 m of a multimode fiber

[0030] 4. 10 m of GRIN fiber

[0031] 5. 100 m of a multimode fiber

[0032] This is to illustrate the example only and other sets of fiber types and their lengths are possible.

[0033] This passive multiple-fiber system makes it practical to include the system as a compact spool inside a market-grade full-field OCT device to efficiently remove the speckle noise from the images.

[0034] According to yet another example of the method of an optical speckle noise reduction is disclosed. The method comprises following steps: the light beam from the coherent light source enters the section of the optic fiber of the first type 1; the light beam is transmitted through the section of the optic fiber of the first type 1 and leaves the section of the optic fiber of the first type 1; the light beam enters the section of the optic fiber of the second type 2 via the coupling; the light beam is transmitted through the section of the optic fiber of the second type 2 leaves the section of the optic fiber of the second type 2.

Claims

Claims1. A system for an optical speckle noise reduction, comprising an at least one section of an optic fiber of the first type having the first type of light propagation parameters set and a coherent light source optically coupled with the input of the at least one section of the optic fiber of the first type characterized in that the system further comprises an at least one section of an optic fiber of the second type (2) having the second type of light propagation parameters set optically coupled (3) in series with the optic fiber of the first type (1) wherein the optic fiber of the first type (1) differs from the optic fiber of the second type (2) in at least one of the light propagation parameter.

2. The system according to claim 1 comprising plurality of sections of the optic fiber of the first type (1) and plurality of sections of the optic fiber of the second type (2) coupled in series alternately so that adjected sections differ from each other in at least one of the light propagation parameters.

3. The system according to claim 1 or 2 further comprising an at least one section of an optic fiber of the third type having the third type of light propagation parameters set coupled in series with the section of the fiber optic of the first type (1) and / or with the section of the fiber optic of the second type (2).

4. The system according to claim 1, 2 or 3 further comprising a plurality of sections of an optic fiber of different types, each having different light propagation parameters set, coupled in series with each other wherein adjacent sections have different light propagation parameters.

5. The system according to claim 1,2,3 or 4 wherein optic fiber light propagation parameters set comprise fiber type, transverse mode, light propagation velocity, core type, core geometry, index type, fiber material.

6. The system according to any of claim from 1 to 5 wherein optic fiber sections are coupled (3) by fusion-splicing.

7. The system according to any of claim from 1 to 6 wherein optic fiber sections are coupled (3) with industrial fiber connectors retaining the possibility to disconnect the fibers when needed.

8. The system according to any of claim from 1 to 7 wherein optic fiber sections are coupled(3) using free-space fiber coupling, allowing the light to leave one section and use a focusing optical element to couple the beam into another section.

9. A method for an optic speckle noise reduction of a light beam using the system according to claims 1-8, characterized in that it comprises following steps: the light beam from the coherent light source enters the section of the optic fiber of the first type (1); the light beam is transmitted through the section of the optic fiber of the first type (1) and leaves the section of the optic fiber of the first type (1); the light beam enters the section of the optic fiber of the second type (2) via the coupling; the light beam is transmitted through the section of the optic fiber of the second type(2) leaves the section of the optic fiber of the second type (2).

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