Method for determining the refractive index distribution of a cylindrical optical object

Abstract

The present invention relates to a method for determining the refractive index distribution of an optical object (22) having a cylindrical surface (26) and a cylindrical longitudinal axis (25), the method comprising the following steps: (a) scanning the cylindrical surface (26) of the object (22) at multiple scanning positions (23) by means of a light beam (21) incident perpendicular to the cylindrical longitudinal axis (25); (b) capturing the position-dependent intensity distribution of the light beam (21) deflected in the optical object (22) by means of optical detectors (7; 8); (c) Determine the deflection angle of the zero-order beam at each scan position (23) from the captured intensity distribution, including eliminating the beam intensity of higher-order beams from the intensity distribution to obtain the deflection angle distribution of the zero-order beam, and (d) Calculate the refractive index distribution of the object (22) based on the deflection angle distribution, wherein method steps (a) and (b) are performed with beams having at least two different wavelengths, and in order to eliminate the beam intensity of higher-order beams, the intensities at the same position of the intensity distributions of the different wavelengths are mathematically processed with each other, more specifically multiplied and / or added together.

Description

Background Technology

[0001] This invention relates to a method for determining the refractive index distribution of an optical object having a cylindrical surface and a longitudinal cylindrical axis, the method comprising the following steps:

[0002] (a) The cylindrical surface of the object is scanned at multiple scanning positions using a light beam incident perpendicular to the longitudinal axis of the cylinder.

[0003] (b) The position-dependent intensity distribution of a beam of light deflected in an optical object is captured by means of an optical detector.

[0004] (c) Determine the deflection angle of the zero-order beam (I0) at each scan position from the intensity distribution (40a, 40b, 40c), including eliminating the beam intensities of higher-order beams (I1, I2, I3) from the intensity distribution (40a, 40b, 40c) to obtain the deflection angle distribution of the zero-order beam (I0), and

[0005] (d) Calculate the refractive index distribution of object (22) based on the deflection angle distribution.

[0006] Such cylindrical optical objects include, for example, fiber preforms, optical fibers, optical waveguides, or cylindrical lenses. One of the important characteristics of such objects is their refractive index (refractive coefficient) and its spatial distribution, particularly the radial refractive index distribution, which is also referred to below as the "refractive index distribution." For example, the refractive index distribution of a fiber preform thus determines the waveguide characteristics of the optical fiber drawn from it.

[0007] Known preform analyzers are used for apparatus-aided analysis of refractive index distribution. A focused beam (hereinafter also referred to as the "beam") is guided in a grid pattern through a cross-section of the object transverse to the longitudinal cylindrical axis of the optical object being measured (such as a preform of optical fibers), and the deflection angle of the refracted beam exiting the object is measured at the point of incidence relative to the beam direction. The transmission of the grid pattern of the cross-section at multiple scanning positions is referred to herein as a "scan." A set of different deflection angles measured perpendicular to the longitudinal cylindrical axis while scanning the beam is called the "deflection angle distribution." Based on the transverse measurement data of the deflection angle distribution, the refractive index distribution can be reconstructed in the irradiated volume region.

[0008] Existing technology

[0009] A method for reconstructing the radial refractive index distribution of a cylindrical optical preform from the deflection angle distribution and the associated incident point is known from EP 3 315 948 A1. The measured deflection angle distribution is thus processed by analyzing and determining extreme values, such as those appearing at, for example, the outer edges of the core or shell of the preform.

[0010] The precise determination of the refractive index distribution of an optical object is hampered by refractive index fluctuations at the microscopic scale. These fluctuations appear in the form of stripes or layers, which are formed during the production of synthetic glass via layer-by-layer deposition of carbon soot particles from the vapor phase. Methods used for layer-by-layer deposition are known as OVD (External Vapor Deposition), MCVD (Modified Chemical Vapor Deposition), PECVD (Plasma Enhanced Chemical Vapor Deposition), POD (Plasma External Deposition), and VAD (Vapor Axis Deposition).

[0011] The layers act as a transparent diffraction grating, at which a beam of light passing through the object is additionally diffracted. If the interlayer spacing is within the order of the beam's wavelength, the beam can interact with the layers and split into additional diffracted beams of different orders, with more or fewer diffracted beams. Each of these additional beams can then undergo further diffraction in a separate beam path, resulting in diffraction blurring due to different deflection angles and exit positions from the optical object.

[0012] Due to beam diffraction, a complex deflection pattern is obtained for each output beam. This pattern includes the primary beam (zero-order beam) that is only refracted but not additionally diffracted, and also includes higher-order beams diffracted by the diffraction. Therefore, the deflection angle of the diffracted beam relative to the undiffracted beam can be small, causing the corresponding exit positions to be close to or overlap each other. If beam diffraction effects dominate the deflection angle distribution, the intensity of higher diffraction modes can even exceed that of the primary diffraction modes. If applicable, it is difficult, if not impossible, to reconstruct the refractive index distribution from the deflection angle distribution, where the extent of layer damage analysis depends on the layer spacing and amplitude.

[0013] Increasing the measurement wavelength reduces beam diffraction effects. For this reason, US 5,396,323A proposes a technique for analyzing refractive index distribution, in which a beam with a long measurement wavelength, such as 3395 nm (i.e., in the infrared spectral range), is used to measure the deflection angle distribution.

[0014] According to US2016 / 123873 A1, a method for measuring the refractive index distribution of a cylindrical glass body of the aforementioned type is known. The aim is to measure a glass body with large fringes. For this purpose, a collimated beam originating from an illumination gap is used to scan the cylindrical surface of the glass body at multiple scanning positions, with the illumination gap focused on an imaging plane behind the glass body. An image of the illumination gap is captured by means of at least one detector, and those exit positions where the zero-order beam has been incident after passing through the compacted glass body from the scanning position are identified.

[0015] In the imaging plane, all beams exiting the vitreous body are minimum size. By capturing an image of the illumination gap in the imaging plane, it is easier to distinguish the zero-order beam from the higher-order diffracted beams.

[0016] The deflection angle of the zero-order beam and the corrected deflection angle distribution of the zero-order beam are determined by data from the scanning and exit positions. The refractive index distribution of the glass is reconstructed from the corrected deflection angle distribution using the Abelian transform.

[0017] Technical goals

[0018] Considering known measurement methods, the intensity of higher-order beams captured by a camera is eliminated. This elimination involves predicting the beam path of the zero-order beam through the glass body to be measured for each scan position. This prediction is based on the scan position of the incident beam on the cylindrical surface and the expected position of the zero-order beam incident on the optical detector. Based on this, analytical data of diffracted higher-order beams already detected by the detector are rejected.

[0019] To identify the exit position of the zero-order beam, two cameras are used, which simultaneously capture images of the gap using a beam splitter. The object planes of the cameras are positioned individually such that one object plane is in front of the imaging plane illuminating the gap, and the other object plane is behind the imaging plane. However, since neither camera's object plane is precisely located in the imaging plane, the cameras capture at least slightly distorted images of the gap.

[0020] To pre-determine the approximate incident point of the zero-order beam, a pre-scan is performed. This involves measuring a reference preform of similar size and refractive index distribution. The position of the zero-order beam's central axis, found in the pre-scan, is used to adjust the center position of the camera recording window and ensure that the center of the zero-order beam is approximately centered within the recording window for analyzing data. An alternative to the pre-scan is to use existing knowledge about the vitreous body and the general shape of the deflection angle function to determine the expected position of the zero-order beam.

[0021] For example, a laser diode is used as the beam source. The measurement wavelength of the radiation is in the visible wavelength range or in the near-infrared (NIR) or mid-infrared (MIR) range. Compared to NIR radiation, when using, for example, 3.39 μm MIR radiation, the diffraction angle of the higher-order beams increases, resulting in a larger gap between the intensity signal of the zero-order beam and the intensity signal of the diffracted higher-order beams, and adjacent signals are therefore easier for the camera to distinguish. On the other hand, the signal-to-noise ratio of the thermal detector used to capture MIR radiation is, in principle, worse than that of the NIR detector and the detector for visible light. Finally, if the periodic layers in the glass to be measured have a large spacing, for example, 14.1 μm, then MIR radiation is recommended, and if the periodic layers in the glass to be measured have a small spacing, for example, 6.7 μm, then NIR radiation is recommended.

[0022] In order to predict the beam path and appropriately select the measurement wavelength, known measurement methods require background knowledge about the vitreous body to be measured or about its production.

[0023] Using two cameras to identify the exit position of a zero-order beam is structurally complex and requires significant expense to adjust and process the captured data.

[0024] Therefore, the present invention is based on the purpose of specifying a method for determining the refractive index distribution of a cylindrical transparent object affected by a periodic layer, the method being carried out with little or no prior information about the object and its production.

[0025] Furthermore, the purpose of this invention is to design the determination of the deflection angle distribution to be structurally the simplest, including identifying the zero-order beam and eliminating signals from higher-order beams. Summary of the Invention

[0026] According to the aforementioned type of method, this objective is achieved by method steps (a) and (b), each of which is performed with a beam of different wavelengths, wherein a first position-related intensity distribution of a first beam having a first wavelength and at least one additional second position-related intensity distribution of a second beam having a second wavelength are obtained, and wherein the elimination of the beam intensity of the higher-order beam includes a comparison of the intensity of the first intensity distribution and the intensity of the second intensity distribution at the same scanning position.

[0027] Eliminating the beam intensity of higher-order beams is particularly important in locations where the intensity distribution is indistinct. This is typically the case, for example, in the case of optical preforms in regions of refractive index transition or jump, and in the case of production-related layer formation. Here, the deflection angle distribution can exhibit a distinct pattern, and the associated light intensity can exhibit a fan-out distribution.

[0028] To reduce and ideally eliminate this fan-out, in the method according to the invention, the optical object to be measured is scanned simultaneously or preferably continuously with at least two beams of different wavelengths. Here, a first position-dependent intensity distribution of a first deflected beam having a first wavelength λ1 and at least one additional second intensity distribution of a second deflected beam having a second wavelength λ2 are obtained. The two intensity distributions differ from each other, particularly at the locations of the maximum intensity of the higher-order beams, while the maximum intensity of the zero-order beam is substantially the same in the intensity distributions. This is due to the wavelength dependence of the aforementioned beam diffraction effect. Diffraction at the microscopic refractive index fluctuations at the first measurement wavelength differs from diffraction at the second measurement wavelength; conversely, the refraction of the beam in the object is practically independent of wavelength. By comparing the beam intensities measured at the same radial position (relative to the preform) of the first and second beams, the intensity of the higher-order beam can be identified and eliminated based on their displacement relative to each other. Conversely, by comparing the beam intensities measured at the same radial position (relative to the preform) of the first and second beams, the intensity of the zero-order beam can be identified because they do not have displacement relative to each other.

[0029] To identify and eliminate the beam intensity of higher-order beams, prior information about the internal structure of the optical object being measured is not required, especially knowledge about the periodicity of layers or beam path prediction, as well as complex ray tracing methods.

[0030] In a preferred procedure, the intensity values ​​at the same location of the first intensity distribution and the second intensity distribution are mathematically processed to eliminate the bundle intensity of higher-order bundles.

[0031] For example, the mathematical processing of intensity values ​​at the same location (which are intensity values ​​measured at the same scan position) is part of optical image processing. It includes one or more mathematical operations aimed at masking the beam intensity of higher-order beams. The mathematical operations preferably include processing of the intersection of intensities at the same location, specifically at least one multiplication and / or at least one addition of the intensities at the same location from a first intensity distribution and a second (and possibly additional) intensity distribution. By multiplication, the product of intensity values ​​at the same location is essentially the product of intersections, which can be very small if at least one factor is very small. Adding them together produces a sum of intensity values ​​at the same location, which is essentially a union, and the union can also be relatively small if both addends are small or at least one of the addends is small.

[0032] The result of the mathematical processing is the prepared intensity distribution, which has relatively small intensity values ​​in regions with significant relative displacement of the beam intensity at the same location, and high intensity values ​​in regions with no or minimal displacement, i.e., regions with local stable deflection angles of the zero-order beam.

[0033] To further improve the detection accuracy of the zero-order beam, the elimination of the beam intensity of the higher-order beam may include the complete or partial elimination of the intensity measurement of a first prepared intensity distribution and / or at least one additional second prepared intensity distribution that falls below an intensity threshold.

[0034] For example, the prepared intensity distribution is obtained by performing the mathematical operations described above on the intensity at the same location of the first and second intensity distributions. Intensity signals with low levels are mathematically suppressed or removed via an intensity threshold filter. Preferably, all intensity values ​​below the threshold are discarded before determining the deflection angle of the zero-order beam.

[0035] Therefore, the intensity threshold is preferably set to be less than 20% of the maximum intensity value of the light intensity distribution, and preferably less than 15%.

[0036] Setting a high fixed threshold, such as more than 20% of the maximum value, may result in the loss of important information. Therefore, in principle, a constant threshold should only be chosen as high as needed.

[0037] The comparison of the intensities of the first intensity distribution and the second intensity distribution at the same scan position for eliminating the beam intensity of higher-order beams preferably includes computer-aided image processing.

[0038] In a particularly preferred variation of the method, a line scan camera with only one photosensitive line sensor is used as an optical detector for capturing the beam intensity distribution according to method step (b).

[0039] Compared to area sensors, the datasets generated by line sensors with the same resolution are significantly smaller and can be read out and processed much faster.

[0040] Wire sensors advantageously have a length sufficient to capture the entire angular distribution of an object during a single scan. Wire sensors with a length of at least 40 mm, preferably at least 60 mm, have proven successful for this purpose.

[0041] The longer the sensor line, the larger the deflection angle that can be imaged, meaning the larger the refractive index jumps that can be captured in the refractive index distribution. Line sensors longer than approximately 80 mm are generally unnecessary, as a reduced mapping to the line sensor can be achieved using a lens in front of the line scan camera, albeit at the cost of lower resolution.

[0042] If color resolution is omitted and a monochrome line sensor is used as preferred here, the dataset to be processed remains particularly small.

[0043] If the line sensor operates at a low color depth (such as 8-bit bit depth), the amount of data to be processed will be further reduced. With low data volumes, an 8-bit depth can achieve a resolution of 256 luminance values, which is sufficient for this application.

[0044] Typically, a focused beam is used to scan the surface of an optical object. Focusing is usually achieved using a convex lens. However, the focal position thus depends on the wavelength of the measured radiation. Since different measurement wavelengths are used in the method, compensation measures must be taken when focusing with the aid of a convex lens to achieve a constant focal position. Alternatively and preferably, in order to scan a cylindrical surface according to method step (a), the beam is focused using a parabolic mirror.

[0045] Parabolic mirrors, particularly preferably so-called off-axis parabolic mirrors, are capable of achieving independent focusing of dispersion of light beams of different wavelengths.

[0046] In a preferred procedure, method steps (a) and (b) are performed using radiation of a first wavelength and at least one second wavelength, wherein the first wavelength and the second wavelength differ from each other by at least 50 nm and at most 400 nm, preferably by at least 80 nm and at most 300 nm.

[0047] In another preferred variation of the method, steps (a) and (b) are performed using radiation of a first wavelength, a second wavelength, and a third wavelength, wherein the third wavelength is longer than the first wavelength and shorter than the second wavelength, and the third wavelength differs from the first wavelength and the second wavelength by at least 50 nm and at most 400 nm, and preferably by at least 80 nm and at most 300 nm.

[0048] When using three different wavelengths, a practical advantage is that color images are typically stored, and especially processed using three color channels. Therefore, conventional image processing methods can be applied to evaluate the intensity signal.

[0049] The greater the wavelength difference between adjacent wavelengths, the more pronounced the relative shift in deflection angle or beam intensity of the same higher-order beam. On the other hand, it is advantageous if the same detector can be used for two or all measurement wavelengths, which is technically most easily achieved with a smaller wavelength difference.

[0050] Method steps (a) and (b) are preferably performed serially with radiation of a first wavelength and subsequently with radiation of a second and additional wavelengths.

[0051] Therefore, the terms "first wavelength" and "second wavelength" do not indicate which wavelength is shorter or longer. Serial processing facilitates the evaluation of intensity data through serial mathematical calculations.

[0052] A procedure has been established in which different wavelengths are in the wavelength range of 400 nm to 1600 nm, and preferably below 1100 nm.

[0053] For measurement wavelengths in the visible and near-infrared ranges, up to 1600 nm, preferably up to 1100 nm, sufficiently good signal-to-noise ratios and suitable light sources and detectors can be obtained. By definition, the near-infrared wavelength range begins at approximately 780 nm.

[0054] On the one hand, a minimum distance between adjacent measurement wavelengths is required so that the relative displacement of the deflection angle or associated beam intensity of the same higher order (e.g., corresponding first order) and thus the corresponding beam intensity splitting becomes visible. However, if the deflection angle of a higher order of one measurement wavelength approximately coincides with another higher order deflection angle of another measurement wavelength, it can prevent the assignment of the deflection angle or associated beam intensity to a particular measurement wavelength. For example, such “approximate coincidence” can occur if the measurement wavelengths in question have approximately the same least common multiple. A wavelength difference of less than 40 nm is defined herein as “approximately equal.” The risk of such “approximate coincidence” of diffraction beams of different higher orders (up to the maximum value of the third order) is reduced if at least one of the measurement wavelengths is selected from a wavelength range close to the upper limit of the detector’s spectral sensitivity. For measurement wavelengths in the visible and near-infrared ranges, the maximum value is 1600 nm, preferably 1100 nm, and given the detector’s spectral sensitivity in that wavelength range, these conditions are generally satisfied. In a preferred exemplary embodiment, the different wavelengths are selected from the following wavelength ranges: 635±50nm, 840±50nm, 970±50nm, 1040±50nm, and 1550±50nm.

[0055] It has also proven advantageous to focus the beam on a single point within the optical object during scanning of its cylindrical surface. When focused on a point within the volume of the cylindrical object, such as along the longitudinal axis of the cylinder, sharp transitions in the form of refractive index jumps are better imaged and evaluated compared to focusing outside the volume.

[0056] definition

[0057] The various method steps and terms described above are further defined below. These definitions form part of this specification. In the event of any factual conflict between any of the following definitions and the remainder of the specification, the definitions expressed in the specification shall prevail.

[0058] beam

[0059] The light beam incident on the cylindrical surface of the object to be measured during the scanning process is generated, for example, by the displacement of the light beam (such as a laser beam).

[0060] Deflection angle distribution Ψ(y)

[0061] The deflection angle is defined as the angle between the outgoing beam leaving the object to be measured and the incoming beam entering the optical object. A set of deflection angles measured during the scanning of the object due to the offset of the beam perpendicular to the longitudinal axis of the cylinder (in the y direction) results in a "deflection angle distribution Ψ(y)".

[0062] Beam intensity distribution

[0063] The deflection angle distribution can be represented as the local distribution of beam intensity captured by the optical detector during the scanning of the optical object to be measured. In this respect, the beam intensity distribution measured during scanning represents the deflection angle distribution. Due to diffraction effects and the formation of higher-order beams diffracted in one or more regions, the beam intensity distribution can be fan-out.

[0064] This fan-out is assigned to a specific radial position of the preform, and occurs when a detector at the same measurement position simultaneously captures the beam intensity at multiple points on its optical sensor.

[0065] Exemplary Implementation

[0066] The invention will now be explained in more detail with reference to exemplary embodiments and the accompanying drawings. The drawings show detailed embodiments.

[0067] Figure 1 A diagram illustrating the implementation scheme of a measurement system used to measure the distribution of deflection angles.

[0068] Figure 2 : A schematic diagram used to explain the implementation method of the measurement.

[0069] Figure 3 The beam intensity distribution measured on a preform with a core and shell produced by the OVD process for three different measurement wavelengths.

[0070] Figure 4 : Figure 3 The beam intensity distribution is detailed in the magnified representation, and

[0071] Figure 5 : A schematic diagram showing the original light distribution, obtained by exciting pixels of a line-scan camera at the same measurement locations ((a) and (b)), and the calculation processing ((c) and (d)) used for the original light distribution to eliminate parts of the diffracted radiation.

[0072] Figure 6A schematic diagram showing a comparison of refractive index distributions determined by means of methods according to the prior art and methods according to the invention, wherein destructive higher-order diffraction in the original data is identified and eliminated.

[0073] Embodiment 1 of the present invention relates to a method for determining the refractive index distribution of an optical object having a cylindrical surface and a longitudinal cylindrical axis, the method comprising the following steps:

[0074] (a) The cylindrical surface of the object is scanned at multiple scanning positions using a light beam incident perpendicular to the longitudinal axis of the cylinder.

[0075] (b) The position-dependent intensity distribution of a beam of light deflected in an optical object is captured by means of an optical detector.

[0076] (c) Determine the deflection angle of the zero-order beam (I0) at each scan position from the intensity distribution (40a, 40b, 40c), including eliminating the beam intensities of higher-order beams (I1, I2, I3) from the intensity distribution (40a, 40b, 40c) to obtain the deflection angle distribution of the zero-order beam (I0), and

[0077] (d) Calculate the refractive index distribution of object (22) based on the deflection angle distribution;

[0078] The method is characterized in that steps (a) and (b) are each performed with beams of different wavelengths (λ1, λ2, λ3), wherein a first position-related intensity distribution (40a) of a first beam having a first wavelength (λ1) and at least one additional second position-related intensity distribution (40b) of a second beam having a second wavelength (λ2) are obtained, and wherein the elimination of the beam intensity of the higher-order beams (I1, I2, I3) comprises a comparison of the beam intensity of the first intensity distribution (40a) and the beam intensity of the second intensity distribution (40b) at the same scanning position (23).

[0079] This method is used to determine the refractive index distribution of a cylindrical optical object, in an exemplary embodiment of an optical preform manufactured by means of an OVD method for drawing optical fibers, the optical preform having a distinct layered structure over a portion of its volume.

[0080] The cross-section of the preform is transmitted (scanned) by a beam in a grid pattern, and the deflection angles can be calculated based on the corresponding incident points of the beam on the cylindrical shell surface of the preform and the incident points of the beam on the optical sensor. A set of deflection angles of the scanning beam forms a deflection angle distribution, from which the refractive index distribution of the preform is reconstructed.

[0081] The deflection angle distribution was measured using a structurally improved commercial preform analyzer P-106 from York Technology Ltd. Figure 1 The optical structure is schematically illustrated. The analyzer has a cylindrical measuring cell 1 for receiving the cross-section of the preform to be measured and an immersion liquid surrounding the preform. The factory-supplied light source is replaced by three laser diodes 2a, 2b, and 2c, with specific emission wavelengths of 842 nm (2a), 977 nm (2b), and 1080 nm (2c), respectively. These measurement wavelengths are selected such that, within the limits of the spectral sensitivity of the online scanning camera 7, “approximate overlap” of different higher-order diffraction beams is excluded.

[0082] Laser diodes 2a, 2b, and 2c with different emission wavelengths are connected to a beam input component 4 via two Y-fiber bundles 3. This beam input component forms a structural unit with a beam-adjusting optics 5. The beam-adjusting optics are essentially used for dispersive independent focusing of measurement beams of different wavelengths at the same focal point. It consists essentially of two off-axis parabolic mirrors, called off-axis parabolic mirrors 5, and is configured such that the beam focus lies in the yz plane and within the longitudinal cylindrical axis of the measurement cell 1. The beam exiting the preform strikes a line-scan camera 7 with a line sensor 8. The line sensor 8 extends in the y-direction, as indicated by a Cartesian coordinate cross. The center of the line-scan camera 7 is ideally located on the optical axis 13. This allows for complete resolution of the maximum possible deflection angle in the deflection angle distribution.

[0083] The line scan camera 7 is a CMOS line scan camera with a monochrome sensor, commercially available from Teledyne e2V under the name UNIIQA+16K CL MONOCHROME. It features an 82mm sensor length and a horizontal resolution of 16,384 pixels, with a pixel size of 5μm and a color depth (luminance resolution) of 12 bits, but only 8 bits are used. The line scan camera has sufficient spectral sensitivity in the wavelength range of 400nm to approximately 1100nm.

[0084] The line sensor 8 of the line scan camera 7 detects the beam deflected in the y-direction, where the necessary dataset for this purpose remains manageably small, despite its large range of 82 mm sensor length (more than 1000 times smaller than when using a region scan camera). Due to this sensor length, even with relatively large refractive index jumps in the preform to be resolved, potential optics behind the measurement cell 1 can be eliminated. The line scan camera 7 reduces the range of measurement data to be evaluated to the necessary extent, resulting in a significant improvement in performance. The following uses... Figures 3 to 6 Describe the assessment in more detail.

[0085] The position of the measuring cell 1 can be changed relative to the optical axis 13. For this purpose, the measuring cell 1 is mounted on the displacement stage 9, and by means of it, it can be displaced perpendicular to the optical axis 13 in the direction (y-direction) indicated by the pointing arrow 10. The displacement stage 9 and the line scan camera 7 are connected to the computer 11 via the data cable 12.

[0086] Figure 2 The beam paths of beam 21 at the upper scanning position (a) and the lower scanning position (b) are schematically shown, wherein the preform 22 is inserted into the measuring cell 1. Figure 1 The light beam 21 entering the beam input component 4 strikes the cylindrical surface 26 and is refracted into the preform 22 at the incident position 23 in the direction of the preform's central axis 25. Upon exiting at the exit position 24, the light beam 21 is refracted again and reaches the line sensor 8 of the line scan camera 7. In this case, the photosensitive pixels of the line sensor 8 are at the same scanning position 23 (such as the scanning position 23 at a distance s from the center line M). Figure 2 A single beam intensity is captured at point b), where one or several adjacent pixels are excited. Alternatively, multiple beam intensities are captured at different points on the line sensor 8, where multiple pixels spaced apart from each other are excited. The latter occurs, for example, in cases where one or more higher-order beams are transmitted in addition to the zero-order light mode. The excited pixels deflect the beam intensity of beam 14, which is distributed along the length of the line sensor 8 and is referred to hereinafter as the "luminous pixel distribution". The line sensor 8 and the luminous pixel distribution extend in the y-direction (in... Figure 2 In the coordinate system ( ). In the following explanation of the distribution of luminous pixels, the name y 像素 It is also used in its extended direction.

[0087] By means of the grid pattern shift of the preform 22 perpendicular to the optical axis 13, the incident point of the beam is shifted along the preform 22 until its cross-sectional area is fully transmitted. At each shifted position, the line sensor 8 of the line scan camera 7 captures a new distribution of luminescent pixels, which in each case is formed by a deflected, non-diffracted zero-order beam and any deflected and diffracted higher-order beams. The preform 22 is also shifted in the y-direction (in... Figure 2 In the coordinate system). In order to coordinate with the extension direction "y 像素 In contrast, the direction of displacement is also called "y". 移位 ".

[0088] Typically, the deflection angle distribution Ψ(y) is shown in a two-dimensional intensity distribution diagram, where in the direction y 像素 Draw the distribution of luminous pixels on one axis, and along the y 移位The displacement position is plotted on another axis. The two-dimensional beam intensity distribution (which represents the one-dimensional deflection angle distribution Ψ(y) of the preform 22 as a whole) is plotted along y. 移位 This representation is achieved by connecting the distributions of all captured luminous pixels.

[0089] For example, Figure 3 Three of these schematic diagrams are shown. Beam intensity distributions 40a, 40b, and 40c represent the deflection angle distributions of both the intensity signals of the zero-order beam and the intensity signals of the higher-order beam.

[0090] The intensity distributions of these beams are evaluated to identify the intensity distribution of the zero-order beam and to eliminate signals attributable to higher-order beams. For this purpose, the same preform cross-section is continuously scanned with the beams of all laser diodes 2a, 2b, and 2c and their specific different emission wavelengths. This results in three raw beam intensity distributions recorded by the line scan camera 8 and stored by the computer 11.

[0091] For one of the measurement wavelengths mentioned above (diodes 2a, 2b, 2c), Figure 3 The three deflection angle distributions are shown in each case as beam intensity distributions 40a, 40b, and 40c captured by the line scan camera 7. Beam intensity distributions 40a, 40b, and 40c are typically contained within a single common record; however, for illustrative purposes, separate records for each color channel are shown here. The two-dimensional beam intensity distributions 40a, 40b, and 40c each include a horizontal direction (y... 像素 8000 pixels on the top and vertical direction (y 移位 The 12,000 pixels on the preform 22 simultaneously form a deflection angle distribution Ψ(y); the latter is largely mirror-symmetric about the centerline M. Each deflection angle distribution Ψ(y) shows an edge region, which is assigned to either the measuring cell material 41 or the oil immersion 42. The central core region 43 of the preform 22 consists of undoped quartz glass and is surrounded by a shell 44 made of fluorine-doped quartz glass. The radius of the preform 22 is indicated by a blocky arrow “r”. Within the “r” segment, each deflection angle or each beam intensity value is assigned to a specific incident point of the beam on the surface of the preform and a specific radial position of the preform being measured.

[0092] In the region of shell 44, the beam intensity distributions 40a, 40b, and 40c show regions 44a, 44b, and 44c marked by the frame, which are in the direction y 像素The light intensity distribution in the shell region exhibits a highly structured and widely fanned-out pattern, and does not allow for the identification of a clear and distinctive deflection angle distribution within this region. The fan-out of the light intensity distribution in the shell region results in diffraction of the corresponding beam 21 at the layered structure of the preform 22. The “light-emitting pixel distribution” captured by the line sensor 8 in these regions shows not only the intensity of a single beam (e.g., in the case of the core region 43), but also multiple beam intensities spaced apart from each other. This will be referenced below. Figure 4 and Figure 5 To explain in more detail.

[0093] Figure 4 The magnification of regions 44a, 44b, and 44c shown includes approximately 2500 to 6000 y-squares that will be assigned to shell region 44. 像素 The range of numbers and y from 8000 to 8500 移位 The recordings in each case show multiple intensity lines L0, L1, L2, which are assigned to the zero-order beam and the higher-order diffracted beam at the corresponding measurement wavelengths. Upon closer inspection, it can be seen that the distance between intensity lines L0, L1, L2 increases from region 44a via 44b to region 44c. This demonstrates the wavelength dependence of such spacing, or, respectively, the wavelength dependence of the location of the deflection angle distribution of the higher-order beams. Conversely, the location of the zero-order beam will be found independently of the measurement wavelength at the same location. In an exemplary embodiment, this is intensity line L0, which is in the range of approximately 3600 vertical pixels across all recordings. It is evident here that intensity line L0 is not in the middle of the diffraction order in which the beam appears. This is because the grating (i.e., the stripe structure or layer structure of the preform) by which the beam is diffracted is not ideal, but rather curved and non-periodic. In region 44a, horizontal auxiliary line 45a is drawn at a radial position rs (8300 pixels; where r = preform radius and s = distance between scan position 23 and preform centerline M), and other horizontal auxiliary lines 45b and 45c are drawn through regions 44b and 44c respectively at the same radial position rs (8300 pixels). 像素 Extending upwards and each spanning multiple light intensity lines L0, L1, L2. The light intensity distribution that can be measured along auxiliary lines 45a, 45b, 45c is referred to here as the "luminous pixel distribution".

[0094] The positional independence of the deflection angle distribution Ψ(y) of the zero-order beam, or more precisely, the positional independence of the light intensity line L0, can identify, mask, and eliminate other light intensity lines L1 and L2 (and any other light intensity lines), as described below. Figure 5The schematic diagrams in (a) and 5(b) schematically illustrate the distribution of the emitting pixels (I(λ1); I(λ2)) in each case for a specific measurement wavelength (λ1; λ2). The beam intensity I (relative units) is plotted relative to the position coordinate P, which represents the line sensor in the y-direction. 像素 The pixel sequence on the surface. The distribution of luminescent pixels Ι(λ1) can be, for example, along the auxiliary line 45a ( Figure 4 The measurement is performed, and the luminous pixel distribution I(λ2) can be measured, for example, along the auxiliary line 45b. The two luminous pixel distributions (I(λ1); I(λ2)) form the same radial position s of the online sensor 8. Figure 2 At point b), and in this respect, they belong to the "same location" beam intensity distribution. They are substantially different from each other in the positions of the intensity signals I1, I2, I3 of the higher-order diffracted beams. The positions of the maximum intensity of the higher-order diffracted beams, P1 to P5, are shifted relative to each other, while the intensity signal I0 of the zeroth-order beam that is only refracted is substantially at the same position in the intensity distribution, which is position P3 in this example.

[0095] Figure 5 (c) A schematic diagram illustrates the prepared luminescent pixel distribution (I(λ1)×I(λ2)), which is obtained by mathematical operations on the intensity values ​​at the same location of the first luminescent pixel distribution I(λ1) and the second luminescent pixel distribution I(λ2). The latter involves multiplication of the intensity values ​​at the same location of the first and second luminescent pixel distributions (I(λ1); I(λ2)). The multiplication results in a product of the intersection of the intensity values ​​at the same location, which, in the exemplary embodiment, is particularly high for the two already fairly high intensity signals I0, and relatively small for the intensity signals I1, I2, I3. The prepared luminescent pixel distribution (I(λ1)×I(λ2)) obtained after the first mathematical processing step has relatively small intensity values ​​in regions with perceptible relative displacement in the deflection angle, and relatively high intensity values ​​in regions with no or at most very little displacement, i.e., in regions with spatially stable deflection angles of the zero-order beam.

[0096] To further improve the detection accuracy of the zero-order beam, in the second mathematical processing step, the prepared luminous pixel distribution (Ι(λ1)×Ι(λ2)) is subjected to an intensity threshold filter, wherein intensity signals below level L are removed by calculation, which is defined as 10% of the maximum value of the prepared luminous pixel distribution (λ1(Ρ),λ2(Ρ)).

[0097] Figure 5(d) Schematically illustrates the luminescent pixel distribution (Ι(λ1)×Ι(λ2)+L) after the mathematical operation has been performed. Only a single peak remains, based on which the position of the deflection angle of the zero-order beam is determined and defined. In the luminescent pixel distribution (Ι(λ1)×Ι(λ2)+L) obtained after this preparation, the initially measured luminescent pixel distribution, which was widely fanned out on the layered structure of the preform 22 due to the diffraction of the beam 21, is replaced by a clear and distinct signal that reflects only the deflection angle of the zero-order beam at the radial measurement position s.

[0098] After correspondingly processing the beam intensity distributions 40a, 40b, and 40c at all radial positions, or those requiring such preparation, a prepared beam intensity distribution or deflection angle distribution Ψ with a unique intensity distribution of the zero-order beam is obtained. The radial refractive index distribution of the preform is thus determined using the known inverse Abelian transform. Figure 6 An example of this situation is illustrated in the schematic diagram, where the refractive index n (in relative units compared to undoped quartz glass) is plotted relative to the radial position P (in mm). The measured preform includes a core region 50, an inner shell region 51, and an outer shell region 52, each with a different refractive index. The schematic diagram contains two curves. Curve A shows the refractive index distribution determined using prior art, and curve B shows the refractive index distribution determined using the present invention. Thus, the preform is scanned with measurement wavelengths of 842 nm, 977 nm, and 1080 nm, and the intensity distribution subsequently obtained is modified using the first mathematical processing step (multiplication of intensity values ​​at the same location) and the second mathematical processing step (intensity threshold filter at 10% of the maximum intensity) described above. The refractive index distribution of the preform (as reflected in curve B and obtained after step (d) of the method according to embodiment 1) provides a good basis for further preparing the refractive index distribution using conventional methods, such as those described in EP 3 315 948 A1. In this method, the refractive index distribution is used to define orientation values, such as the orientation value of the layer radius of the preform or the orientation value of the refractive index of the layer. In contrast, for the refractive index distribution of curve A, the refractive indices in the inner shell region 51 and the core region 50 are too low, and the step refractive index distribution is not obvious. The result is distorted, which may also be due to horizontal displacement and deformation of the core caused by the underlying mathematics.

[0099] In the case of a non-radially symmetric refractive index distribution, the transformation from the measured deflection angle distribution is advantageously performed not by means of the inverse Abelian transform, but by means of the so-called inverse Radon transform. Therefore, the processing of the deflection angle distribution is performed as described in the example above. However, multiple deflection angle distributions are determined because the preform rotates about its longitudinal axis. The corresponding deflection angle distributions are combined and converted into a phase difference diagram, i.e., a so-called sine diagram. Applying the inverse Radon transform to the latter yields a 2D refractive index distribution.

Claims

1. A method for determining the refractive index distribution of an optical object (22), the optical object having a cylindrical surface (26) and a cylindrical longitudinal axis (25), the method comprising the following steps: (a) The cylindrical surface (26) of the object (22) is scanned at multiple scanning positions (23) by means of a light beam (21) incident perpendicular to the longitudinal axis (25) of the cylinder. (b) A first intensity distribution (40a) and at least one additional second intensity distribution (40b) of the beam (21) deflected in the optical object (22) are captured by means of an optical detector, wherein the first intensity distribution (40a) and the second intensity distribution (40b) are position-dependent; (c) Determining the deflection angle of the zero-order beam (I0) at each scan position (23) based on the first intensity distribution (40a) and the second intensity distribution (40b), including eliminating the beam intensities of higher-order beams (I1, I2) from the first intensity distribution (40a) and the second intensity distribution (40b) to obtain the deflection angle distribution of the zero-order beam (I0), and (d) Calculate the refractive index distribution of the object (22) based on the deflection angle distribution. Its features are, Method steps (a) and (b) are each performed using beams of different wavelengths (λ1, λ2), wherein a first intensity distribution (40a) of a first beam having a first wavelength (λ1) and at least one additional second intensity distribution (40b) of a second beam having a second wavelength (λ2) are obtained, and wherein the elimination of the beam intensity of the higher-order beams (I1, I2) comprises a comparison of the beam intensity of the first intensity distribution (40a) and the beam intensity of the second intensity distribution (40b) at the same scanning position (23); and In order to scan the cylindrical surface (26) according to step (a), laser diodes (2a, 2b) with different emission wavelengths (λ1, λ2) are connected to the beam input component (4) via a Y-fiber bundle (3), and the beam (21) is focused by means of a parabolic mirror (5); In order to eliminate the beam intensity of higher-order beams (I1, I2), the intensity at the same position of the first intensity distribution (40a) and the second intensity distribution (40b) are mathematically processed, wherein the mathematical processing involves at least one multiplication and / or at least one addition of the intensity at the same position of the first intensity distribution (40a) and the second intensity distribution (40b).

2. The method according to claim 1, characterized in that, The elimination of the deflection angle of the higher-order beams (I1, I2) includes the complete or partial elimination of the measurement of the intensity of the first intensity distribution (40a) and the second intensity distribution (40b) falling below the intensity threshold (L).

3. The method according to claim 2, characterized in that, The intensity threshold (L) is set to be less than the intensity distribution (I). λ1 (y); I λ2 (y) is 20% of the maximum strength value.

4. The method according to claim 3, characterized in that, The intensity threshold (L) is set to be less than the intensity distribution (I). λ1 (y); I λ2 The value is 15% of the maximum strength value of (y)).

5. The method according to any one of claims 1 to 4, characterized in that, The elimination of the beam intensity of the higher-order beams (I1, I2) includes computer-aided image processing.

6. The method according to any one of claims 1 to 4, characterized in that, A line scan camera (7) having only one photosensitive line sensor (8) is used as the optical detector, which is used to capture the first intensity distribution (40a) and the at least one additional second intensity distribution (40b) according to method step (b).

7. The method according to claim 6, characterized in that, A monochrome line sensor is used, which operates at a bit depth of 8 bits and has a length of at least 40 mm.

8. The method according to claim 7, characterized in that The monochrome line sensor has a length of at least 60 mm.

9. The method according to any one of claims 1-4, characterized in that, Method steps (a) and (b) are performed using radiation of a first wavelength (λ1) and at least one second wavelength (λ2), wherein the first wavelength (λ1) and the second wavelength (λ2) differ from each other by at least 50 nm and at most 400 nm.

10. The method according to claim 9, characterized in that, The first wavelength (λ1) and the second wavelength (λ2) differ from each other by at least 80 nm and at most 300 nm.

11. The method according to claim 9, characterized in that, Method steps (a) and (b) are performed using radiation of the first wavelength (λ1) and subsequently radiation of the second wavelength (λ2).

12. The method according to claim 9, characterized in that, Method steps (a) and (b) are performed using radiation of the first wavelength (λ1), the second wavelength (λ2), and the third wavelength (λ3), wherein the third wavelength (λ3) is longer than the first wavelength and shorter than the second wavelength (λ2), and the third wavelength (λ3) differs from the first wavelength (λ1) and the second wavelength (λ2) by at least 50 nm and at most 400 nm.

13. The method according to claim 12, characterized in that, The third wavelength (λ3) differs from the first wavelength (λ1) and the second wavelength (λ2) by at least 80 nm and at most 300 nm.

14. The method according to any one of claims 1-4, characterized in that, The different wavelengths (λ1, λ2) are in the wavelength range of 400 nm to 1600 nm.

15. The method according to claim 14, characterized in that, The different wavelengths (λ1, λ2) are in the wavelength range of 400 nm to 1100 nm.

16. The method according to any one of claims 1-4, characterized in that, The different wavelengths (λ1, λ2) are selected from the following wavelength ranges: 635±50nm, 840±50nm, 970±50nm, and 1040±50nm.

17. The method according to any one of claims 1-4, characterized in that, When scanning the cylindrical surface (26) of the object (22), the beam (21) is focused on a point on the optical object.

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