Measurement system and method for measuring a light source

Abstract

The invention relates to a measuring system for measuring a light source independently of polarization, comprising a camera (5) with a plurality of image sensors (12) arranged in an array and microscope optics (M), and to a method for measuring a light source independently of polarization. The object of the invention is to measure the optical power of a light source more easily and more accurately with a spatial resolution in the microscope field of view and largely independently of polarization. To this end, the invention proposes that the image sensors (12) are each assigned a linear polarizer (13), wherein the linear polarizers (13) are arranged in an array in front of the image sensors (12) and two or more, preferably four, polarizers (13) form an array group (13a), wherein the passing directions of the linear polarizers (13) placed side by side within the array group (13a) are twisted relative to one another, preferably by 45° or 90°. In the method according to the invention, the measurement signals of the image sensors (12) assigned to the polarizers (13) of the same array group (13a) are converted into optical power measurement values in order to achieve the desired polarization independence.

Description

Technical Field

[0001] This invention relates to a measurement system for measuring a light source independently of polarization, comprising a camera and microscope optics having multiple image sensors arranged in an array. Furthermore, this invention relates to a method of using said measurement system. Background Technology

[0002] The measurement system is used to measure a light source using a microscope and a camera, and (after appropriate calibration) determine the distribution of the absolute power of the light source. The light source can be, in particular, a VCSEL (vertical-cavity surface-emitting laser) element assembly, for example, in the form of a VCSEL array on a wafer. Here, the light emitted from a single VCSEL element is polarized, where the polarization direction is indeterminate or changes over time. Image sensors in known measurement systems using CMOS cameras are polarization-dependent. The indeterminate polarization of the light to be measured can introduce a systematic error of up to 10% when measuring optical power.

[0003] To eliminate or reduce the polarization of light, different types of depolarizers exist, which convert polarized light into unpolarized light. However, a drawback of depolarizers in VCSEL elements is that they often do not function adequately because the light source has an excessively narrow spectrum, resulting in residual polarization and / or, due to birefringence, failing to achieve the spatial resolution required for measuring incident light.

[0004] All known measurement systems either fail to provide satisfactory compensation for polarization characteristics or are excessively costly. Therefore, absolute power measurements with acceptable error estimates are impossible. At most, known measurement systems can only measure the relative power of VCSEL elements. Summary of the Invention

[0005] Therefore, the object of the present invention is to further improve the type of measurement system described at the beginning, so as to enable, with improved ease and as polarization-independent measurement of absolute power or radiation parameters related to absolute power, such as, in particular, the radiation density of the light source, while obtaining spatial resolution within the microscope's field of view.

[0006] Therefore, the present invention proposes a measurement system of the type described at the beginning, wherein linear polarizers are configured for image sensors, wherein the linear polarizers are arranged in an array in front of the image sensors, and two or more, preferably four polarizers form an array group, wherein the passing directions of the linear polarizers placed side by side within the array group are preferably twisted relative to each other by 45° or 90°.

[0007] Furthermore, the present invention proposes a method for measuring a light source independently of polarization using the aforementioned measurement system, wherein,

[0008] - The light source emits light, which is focused by microscope optics onto the image sensor of the camera.

[0009] - The light passes through a polarizer configured for the corresponding image sensor.

[0010] - and the light is detected by image sensors, wherein each image sensor converts the light incident on it into a measurement signal.

[0011] The image sensor's measurement signal is then converted into an optical power measurement, the image sensor being configured with polarizers in the same array group to compensate for polarization-related deviations in the optical power measurement.

[0012] - and an image of the distribution of the light source's light power, generated from the light power measurements of all array groups.

[0013] The polarization of light incident on a given image sensor is definitively determined using an array of polarizers preceding each individual image sensor. The polarization sensitivity of the image sensor is thus compensated, and the estimated measurement error is minimized by averaging the measurement signal. A polarization-independent value is obtained by averaging the measurement signal, and this value is correlated with the absolute power by a value obtained through calibration. The spatial resolution when measuring the light power distribution can be predetermined by combining the parameters of the microscope optics with those of the array group.

[0014] A 2x2 array of four polarizers is particularly advantageous here, with the passing directions of the polarizers twisted relative to each other by 45°, i.e., having passing directions of, for example, 0°, 45°, 90°, or 135°. The passing direction gives the direction of the electric field of the electromagnetic light wave perpendicular to the direction of the light path, through which the light wave can pass.

[0015] Rotating the polarizers placed side-by-side by 90° in their direction of travel is sufficient to cancel out polarization effects. A 45° rotation enables the measurement of polarization.

[0016] An advantageous further embodiment of the invention involves arranging microlenses in an array before the polarization filter. The microlenses optimally distribute incident light onto the photosensitive surface of each image sensor, thereby improving the sensitivity of the image sensor and reducing noise.

[0017] To further improve measurements, a beam splitter is used, which allows light from the light source to be supplied to both the camera and a spectral measurement device. Using a spectral measurement device allows for more accurate measurements of light intensity / power and the spectrum. Furthermore, the spectral measurement device can be used to calibrate the camera.

[0018] A spectroradiometer can be used, for example, as a spectral measurement device. Spectroradiometers are proven reliable through accurate and reliable measurement methods. The spectroradiometer can be designed to perform so-called point measurements, that is, measurements that are non-positionally resolved, unlike those taken by a camera. Individual VCSEL elements of a light source can be addressed, for example, by means of an aperture that can move laterally along the optical path and can be accurately measured by means of a spectroradiometer.

[0019] Instead of a spectroradiometer, the spectral measurement device can have an optical edge filter that can be tilted or moved into the beam path between the light source and the camera. Image capture by the camera is performed here without further image capture via the tilted optical edge filter. The absorption edge of the edge filter is located within the range of the (pre-known) average emission wavelength of the light source, thus allowing individual absorption values ​​to be assigned to each wavelength. The wavelength of each individual image point can be easily determined by comparing the measurement signals from two measurements based on known filter characteristic parameters. This is preferably done via software. In the VCSEL array serving as the light source, each individual VCSEL element can be identified by its position in the image, allowing individual emission wavelengths to be assigned to each VCSEL element. The measurement principle also operates, in principle, independently of the polarizer configured for the image sensors, i.e., through each measurement system comprising a camera with multiple image sensors arranged in an array, and in which an optical edge filter is positioned or moved into the beam path between the light source and the camera.

[0020] In a preferred embodiment of the invention, the microscope optics are configured to have at least one optical filter, such as a medium-density filter, so that the light emission intensity is matched with the sensitivity of the camera.

[0021] Furthermore, to achieve this purpose, the microscope optics feature tubular lenses. This enables microscopes with so-called "infinite optics," thus providing the flexibility to add intermediate elements (filters, beam splitters, etc.) into the beam path.

[0022] One drawback of the present invention is the need for interpolation between image sensors to obtain the full resolution of the array of components. This is not a fundamental drawback; such interpolation is common in typical RGB camera sensors. In a preferred design, the magnification and numerical aperture of the microscope optics are chosen such that the optical resolution is less than the geometric "digital" resolution, which is derived from the components and parameters of the array. This ensures that the Nyquist criterion is met, thus preventing information loss. Attached Figure Description

[0023] The invention will now be described in detail with reference to the accompanying drawings. In the drawings:

[0024] Figure 1a and 1b : Schematic 3D views of the measurement system according to the invention, with housing (a) and without housing (b);

[0025] Figure 2 : schematically showing the origin Figure 1b Detail area A;

[0026] Figure 3a and 3b The structure of a polarizer for use in a measurement system according to the invention is shown schematically.

[0027] Figure 4a and 4b The total power measurements with different polarizations are shown in the case of no polarization correction (4a) and in the case of polarization correction (4b).

[0028] Figure 5 The diagram illustrates the total power measurement using a conventional camera and a camera with a polarizer including polarization correction according to the present invention. Detailed Implementation

[0029] In the accompanying drawings, the housing of the measuring system according to the invention is indicated by reference numeral 1. The microscope objective lens 2 is arranged on the front side. Figure 1b The internal structure of the measurement system is shown with the housing 1 removed. Further components of the microscope optics M are arranged after the microscope objective lens 2; the individual components of these optics are further discussed below (see [link to documentation]). Figure 2 In addition, a beam splitter 3 is provided, which guides a portion of the light through a coupling input optics 4 into an optical fiber F, showing only a short segment of the fiber. The optical fiber guides the light into a spectroradiometer (not shown) for spectral measurement. Furthermore, a camera 5 is provided, which detects another portion of the light and measures the optical power at a position resolution.

[0030] Figure 2 Specifically, it shows that it comes from Figure 1b The microscope optics M. An optical filter 6 and a tube lens 7 are arranged after the microscope objective lens 2.

[0031] exist Figure 3a Three array arrangements (8, 9, 10) are shown, which are components of the camera 5. The front array arrangement 8 consists of microlenses 11. The rear array arrangement 10 consists of a single image sensor 12. The image sensor 12 is implemented, for example, as a CMOS sensor or a CCD sensor. A further array arrangement 9 is located between these two array arrangements 8 and 10. Array arrangement 8 consists of polarizers 13. The polarizers 13, placed side-by-side, have different aperture angles. Each polarizer 13 is equipped with an image sensor 12 and a microlens 11.

[0032] The polarizer 13 is additionally divided into 2x2 array groups 13a. The array groups 13a are schematically arranged in... Figure 3b As shown in the diagram, the passing directions of individual polarizers 13 in array group 13a are rotated by 45° relative to adjacent polarizers 13, and in this embodiment, they are 0°, 45°, 90°, and 135°.

[0033] When measuring a light source using the measurement system according to the invention, light is emitted by the light source. The light enters the measurement system through a microscope objective 2 and is guided to a beam splitter 3 via an optical filter 6 and a tube lens 7. The beam splitter 3 guides the light to a camera and, in parallel, to a spectroradiometer. In the camera, the light is directed to an image sensor 12 via a microlens 11 and a polarizer 13. The light is detected by the image sensor 12 and converted into an electrical measurement signal. The measurement signal from the image sensor 12, configured in a 2x2 array group 13a, is then converted into a power measurement value that eliminates polarization effects. This minimizes the influence of polarization of the light emitted by the light source, and the measurement result is almost independent of polarization. The polarization sensitivity of the image sensor 13 is compensated by this conversion, and the error estimation caused by polarization is minimized. Thus, the optical power can be accurately determined with position resolution. To determine the absolute power or radiation parameters related to the absolute power, such as, in particular, radiation density, calibration is required, for example, by pre-measuring a reference light source. The angle of light emission can be determined by changing the distance between the light source and the measurement system and observing the change in image size on the sensor array 10. This is particularly important when measuring VCSEL arrays. The measurement system simultaneously enables rapid, easy, and accurate measurement of the absolute power of individual emitters in the VCSEL array.

[0034] In another embodiment according to the invention, the polarizer 12 may be divided into 2x1 array groups, and the passing direction of the polarizers 13 in the array group may vary by 90°. Further variations are possible.

[0035] To convert the measurement signal into a probability measurement, a polarization-related correction coefficient can be used according to the present invention. The 2D information of polarization contained in each array group is used to find the accurate correction coefficient for each pixel.

[0036] Typical camera calibrations include bad pixel correction, dark current correction ("img_dark(x,y)"), flat field calibration ("img_ffc(x,y)"), and sensitivity correction ("sensitivity(lambda)").

[0037] img_cal(x,y)=(img_raw(x,y)-img_dark(x,y))*img_ffc(x,y)*sensitivity(lambda)

[0038] - "img_raw" is an image with raw camera pixels, just as seen by the camera.

[0039] - "img_dark" is the noise level of the camera, typically measured by the camera in a dark environment without light.

[0040] - "img_ffc" is a position-dependent correction factor due to defects in the optical components and changes in the camera's sensitivity.

[0041] - "sensitivity (lambda)" is a wavelength-dependent correction factor due to camera technology, the quantum efficiency of which is related to the wavelength of the incident light.

[0042] According to the present invention, the conventional correction is extended with a correction factor for polarization:

[0043] img_cal(x, y)=(img_raw(x, y)-img_dark(x, y))*img_ffc(x, y)*sensitivity(lambda)*polcorrection(x, y)

[0044] Polarization correction ("polcorrection(x,y)") is related to the polarization angle at position (x,y) and the degree of polarization at that position (x,y):

[0045] polcorrection(x,y)=A0(x,y)*cos(2*alpha(x,y)-alpha0(x,y))+Aoff(x,y)*DoP(x,y)

[0046] - "alpha(x, y)" describes the polarization angle at position (x, y) measured by the camera and the camera's polarization-sensitive pixels (array group).

[0047] - "DoP(x,y)" describes the degree of polarization at position (x,y) measured by the camera and the camera's polarization-sensitive pixels.

[0048] - "A0(x, y)" describes a position-sensitive array with zero-phase polarization.

[0049] - "alpha0(x,y)" describes zero-phase polarization (related to the polarization filter of the corresponding array group at position x,y of the sensor array);

[0050] - "AOff(x, y)" describes the position-dependent compensation amount of the amplitude.

[0051] Calibration of a camera with four different polarization orientations is performed in a further embodiment of the method according to the invention in the following steps:

[0052] 1. Bad Pixel Correction: The so-called cold and hot pixels in the camera are determined in the same way as in conventional methods. At least two images are taken, one in dark mode and one in bright mode, and individual pixel discrepancies are found.

[0053] 2. Dark Current Correction: The image was taken in a dark environment (as in the traditional method). This provides the value for dark current correction "img_dark(x, y)".

[0054] 3. Flat Field Calibration: Different flat field calibration images of light polarized in at least four different polarizations (e.g., 0°, 45°, 90°, 135°) are captured. Flat field calibration is performed for each polarization in the same manner as in conventional methods. The four flat field calibration images are used to correct each polarization filter of the camera. The complete image is thus calculated. This image provides the polarization-independent flat field calibration value “img_ffc(x, y)”.

[0055] 4. Since the polarizer is not ideal (manufacturing errors, etc.), possible compensation amounts can be calculated from four different polarization images. This provides compensation amounts (Aoff(x,y)) for each polarization calculation (alpha0(x,y)) and amplitude changes, which are related to the position on the camera (A0(x,y)) and possibly to the polarization of the light, said compensation amounts are also related to the degree of polarization.

[0056] 5. Use monochromatic light to measure the camera's sensitivity. This must be done across the entire wavelength calibration range and provide scalar coefficients for each wavelength. These scalar coefficients are needed for the camera's absolute calibration. This provides the sensitivity correction value "sensivity(lambda)".

[0057] exist Figure 4a and 4b The total power measurements are shown side-by-side with and without polarization correction. Measurements were performed by rotating the light source in 45° increments. The measurements were conducted using a polarized light source and a typical CMOS camera with microscope optics. Figure 4a As can be seen, simply rotating the polarization of the light source by 90° results in a difference greater than 10% in the total pixel count. This clearly demonstrates that the polarization of the light source cannot be ignored when measuring absolute power through a camera. Polarization dependence also cannot be ignored in the relative measurements of individual transmitters in a VCSEL array, as the possibility of polarization change for each transmitter occurs independently.

[0058] Figure 4b The results are shown by measuring the same light source using a camera with a polarization filter and with polarization correction performed as described above. Polarization dependence is almost no longer visible.

[0059] Figure 5 The measurement of a polarized light source is shown. The total power measured using a standard CMOS camera (solid line) and the total power measured using a camera with a polarizer (dashed line) and correction according to the present invention are shown. The polarizer is rotated using a Lambda / 2 plate. Measurement errors due to polarization are greatly reduced.

[0060] List of reference numerals

[0061] 1. Shell

[0062] 2. Microscope Objectives

[0063] 3-way beam splitter

[0064] 4 Coupled Input Optics

[0065] 5 cameras

[0066] 6 Optical Filters

[0067] 7-tube lens

[0068] 8-10 array arrangement

[0069] 11 microlenses

[0070] 12 Image Sensors

[0071] 13 Polarizer

[0072] 13a An array group consisting of polarizers 13

[0073] M-microscope optics

[0074] F-fiber.

Claims

1. A measurement system for measuring a light source in a polarization-independent manner, comprising a camera (5) having a plurality of image sensors (12) arranged in an array and a microscope optics (M). Its features are, Linear polarizers (13) are configured on the image sensor (12) respectively, wherein the linear polarizers (13) are arranged in an array in front of the image sensor (12), and two or more polarizers (13) form an array group (13a), wherein the passing directions of the linear polarizers (13) placed side by side in the array group (13a) are twisted relative to each other. The measurement system is configured to convert measurement signals from image sensors (12) configured for polarizers (13) in the same array group (13a) into optical power measurements, and to compensate for polarization-related deviations by averaging the optical power measurements. The conversion is performed using polarization-related correction coefficients. These coefficients are determined from 2D polarization information contained in the corresponding array group, taking into account values ​​obtained through pre-implemented calibration. The absolute optical power measurement is then obtained based on this calibration. The determined optical power measurements of all array groups (13a) thus enable the generation of a position-resolved image of the optical power distribution of the light source.

2. The measurement system according to claim 1, characterized in that, Microlenses (11) are arranged in an array in front of the polarizer (13), and microlenses (11) are respectively provided for the polarizer (13).

3. The measurement system according to claim 1 or 2, characterized in that, The image sensor (12) is designed as a CMOS sensor.

4. The measurement system according to claim 1 or 2, characterized in that, A beam splitter (3) is provided, wherein the light from the light source can be supplied to the camera (5) and simultaneously to the spectral measurement device by means of the beam splitter (3).

5. The measurement system according to claim 4, characterized in that, The spectral measuring device is a spectroradiometer (4).

6. The measurement system according to claim 1 or 2, characterized in that, An optical edge filter is provided that can be inserted into or moved into the beam path between the light source and the camera (5).

7. The measurement system according to claim 1 or 2, characterized in that, The microscope optics (M) has at least one optical filter (6).

8. The measurement system according to claim 1 or 2, characterized in that, The microscope optics (M) have a tube lens (7).

9. The measurement system according to claim 1 or 2, characterized in that, The magnification factor and numerical aperture of the microscope optics (M) are selected such that the optical resolution of the microscope optics (M) is less than the geometric resolution of the components of the array group (13a).

10. The measurement system according to claim 1 or 2, characterized in that, The calibration includes bad pixel correction, dark current correction, flat field calibration, sensitivity correction, and polarization correction.

11. The measurement system according to claim 1, characterized in that, Four polarizers (13) form an array group (13a).

12. The measurement system according to claim 1, characterized in that, The directions of passage are twisted relative to each other by 45° or 90°.

13. A method for measuring a light source independently of polarization using a measurement system according to any one of claims 1 to 12, wherein, - The light source emits light, which is focused by the microscope optics (M) onto the image sensor (12) of the camera (5). - The light passes through a polarizer (13) configured for the corresponding image sensor (12), and - The light is detected by the image sensors (12), wherein each image sensor (12) converts the light incident on it into a measurement signal. - Wherein, the measurement signals of the image sensors (12) configured for polarizers (13) in the same array group (13a) are then converted into optical power measurements, in which polarization-related deviations are compensated by averaging the optical power measurements of the image sensors within the same array group (13a), and The conversion is performed using polarization-related correction coefficients, which are determined from 2D polarization information contained in the corresponding array group, taking into account values ​​obtained through pre-implemented calibration. The absolute optical power measurement is then obtained based on the calibration. - A position-resolved image of the optical power distribution of the light source is generated from the optical power measurements of all array groups (13a).

14. The method according to claim 13, characterized in that, The light source being measured is an array of VCSEL elements.

15. The method according to claim 13 or 14, characterized in that, The measurements were performed with a spatial resolution of less than 1 µm.

16. The method according to claim 13 or 14, characterized in that, The light source emits light with a wavelength greater than 800 nm.

17. The method according to claim 13 or 14, characterized in that, The measurement signal of the image sensor (12) of the polarizer (13) configured in the same array group (13a) is converted into an absolute optical power measurement value based on a pre-implemented calibration.

18. The method according to claim 13 or 14, wherein, The calibration includes bad pixel correction, dark current correction, flat field calibration, sensitivity correction, and polarization correction.

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