A device for measuring high-resolution spatio-spectral images using multiple detectors and an inclined slit.

Abstract

The invention relates to an imaging spectrometer for beam measurement using a reflective inclined slit. The imaging spectrometer is characterized in that it comprises a reflective inclined slit. The transmitted portion of the beam is analyzed by the imaging spectrometer. The reflected portion is imaged using similar optics, allowing the determination of the percussion response of the optical system and the transmitted beam portion.

Description

Title of the invention: Device for measuring a high-resolution spatio-spectral image using multiple detectors and an inclined slit

[0001] The invention relates to the field of optical metrology.

[0002] It relates more particularly to an imaging spectrometer adapted to beams such as ultrashort laser sources for applications in the field of characterization of these light sources or for hyperspectral imaging.

[0003] In this document, the term imaging spectrometer means a spectrometer capable of forming an image of a transverse dimension at the input and spectrally resolved for each line of that dimension.

[0004] More particularly, the invention relates to an improved imaging spectrometer to form an image exhibiting high image quality over the entire image field while having very low spatial and spectral distortions, in order to obtain a high spectral and spatial resolution image in the entire image field even if it is very wide (>400mm2).

[0005] There are many optical spectrometer setups. An imaging spectrometer usually includes an entrance slit, a dispersive element, which is generally a diffraction grating or a prism, and various optical elements (lenses or mirrors) to form the image of the entrance slit on a detection device (2D detector array).

[0006] Certain application areas of spectrometry require spectrometric instruments with specific imaging properties. An imaging spectrometer must exhibit very good imaging quality, not only in the spectral dimension (which is the classic spectral resolution quality of a spectrometer) but also in the spatial dimension, i.e., good spatial resolution along the direction of the entrance slit.

[0007] It is well known to those skilled in the art that a spectrometer generally comprises: a. an entrance slit adapted to allow the upstream light beam to pass through and whose width contributes to the spectral resolution, b. a light collimation part which projects the image of the slit to infinity, c. angular dispersion means adapted to angularly disperse an incident light beam as a function of a plurality of wavelengths, d. an imaging part which images the initial slit on an image surface, e. a detector placed on this surface.

[0008] The collimation, dispersion and imaging parts are sometimes merged into a single component, for example a concave grating.

[0009] In the vast majority of cases, the detector is a CCD (“Charged-Couple Device”) or CMOS (“Complementary Metal-Oxide Semiconductor”) detection matrix whose surface is flat.

[0010] An imaging spectrometer must also have the following properties: a. a wide field of view, meaning that image quality is maintained even at significant slit heights, b. low distortion; for an imaging spectrometer, distortion can be spectral and / or spatial. c. a large numerical aperture to optimize resolution.

[0011] A first area requiring such spectrometers is the characterization of ultrashort lasers. The distinctive feature of these lasers is that their beam spectrum is broad, and their accurate measurement requires simultaneously measuring the spectral and spatial properties of the entire beam. Various methods for measuring these lasers utilize imaging spectrometers. A characteristic of these sources is that the beam has a small extent or a low M2 factor. This small extent means that the beam can be collimated with low residual divergence. In other words, the beam can be focused with a focal spot close to the diffraction limit. Another property of these beams that is unusual in spectrometry is their high illumination. Light collection is not a problem here.The numerical aperture is therefore important for optimizing resolution, not for coupling the largest possible portion of the beam to be characterized. However, knowing which part of the beam is actually coupled in the spectrometer is important for this type of measurement.

[0012] Most current commercial imaging spectrometers are based on catadioptric systems to cover a very wide wavelength range. They often allow the dispersion system to be changed and the measurement range to be adjusted. This flexibility necessitates a calibration method using a reference source to calibrate the wavelength scale.

[0013] The use of mirrors guarantees the independence of the collimation and imaging optical systems with respect to wavelength. Typical architectures are of the Offner or Czerny-Tumer type. Among these, the ISOPLANE model (registered trademark) from ROPER Scientific Inc., now part of Teledyne Imaging, can be distinguished as described in the following document: has. J.MC CLURE: “ANASTIGMATIC IMAGING SPECTROGRAPH”, WO 2013 / 106307 Al Although optimized to maintain good resolution and low field distortion, this spectrometer remains limited in aperture (f / 4.6) and its spatial resolution, characterized by its percussion response function (PSF, or Point Spread Function) or its equivalent in the spatial frequency domain, the modulation transfer function (MTF, Modulation Transfer Function), does not exceed 201 lines / mm at half-height at the center of the field, corresponding to an equivalent spatial resolution of approximately 25 pm. This resolution is suitable for older types of scientific CCD detectors with pixels larger than 1 µm and dimensions of approximately 20 x 30 mm² at most. These performance characteristics are described, for example, in the specification sheet for the Teledyne Imaging ISOPLANE 320A model.

[0014] Many other types of optical architecture are possible and increasingly use complex freeform optics or aspherical optics to correct aberrations as described in the review document: has. P.Mouroulis, RO Green: “Review of high fidelity imaging spectrometer design for remore sensing”, Opt.Eng. 57(4), 040901 (2018). But most architectures are optimized to cover a very wide spectral band and often with detectors having pixels of more than 1 Opm.

[0015] To best adapt the resolution of the spectrometer with modern detectors, in particular CMOS detectors with pixel sizes of less than 4pm for a total surface area greater than 20 x 30 containing more than 10 million pixels, it is well known to those skilled in the art that the optical system must be used on the axis and therefore refractive, as described in the document: a. DEBattey, JBSlater, R. Wludyka, H.Owen, DM Pallister, MD Morris: "Axial Transmissive f / 1.8 Imaging Raman Spectrograph with Volume-Phase Holography Filter and Grating", Applied Spectroscopy, Vol. 47, 1993, describing a spectrograph based on lens objectives for collimation and imaging, publication describing the Holospec f / 1.8i model of KAISER OPTICAL SYSTEM Inc ("registered trademark"). The resolution can then exceed 100 lines / mm even at the edge of fields, including for large fields.

[0016] This very high resolution over the entire field is particularly important in interferometric measurements where fringes must be resolved and exploited, especially in spectral interferometry.

[0017] To maintain good measurement dynamics, the optical setup must also avoid multiple reflections. To reduce the influence of these phenomena, it is known to those skilled in the art to use a grating calculated such that the spectrum it produces is inclined. Thus, the beams reflected from the sensitive surface of the detector do not return to the optics.

[0018] To optimize resolution, the slit must be very narrow. The challenge with such a narrow slit is then to couple the beam within it and determine which part of the beam has been coupled. However, to fully exploit the narrowness of the slit and the resolution of the optical system, knowledge of the percussion response, which combines the two, is invaluable. Indeed, precise knowledge of the percussion response allows for measurement data processing that improves the instrument's response.

[0019] The objective of the present invention is therefore to propose a solution to guarantee both optimal coupling, to estimate the proportion of the beam which is analyzed by the spectrometer, ideally the entire beam, and to measure the percussion response of the device.

[0020] To this end, the invention relates to an inclined slit imaging spectrometer comprising the following characteristics considered individually or in all their technically possible combinations: a. An imaging spectrometer characterized in that it comprises at least: i. A reflective inclined slit, which separates the incident beam into a beam transmitted through the slit and a beam reflected by the complementary reflective surface of the slit, ii. A collimation optic that transforms the beam transmitted through the diverging slit into a collimated transmitted beam whose rays are parallel, iii. A dispersive part generating collimated transmitted beams dispersed angularly at different angles depending on the wavelength from the collimated transmitted beam, iv. A focusing optic imaging the entrance slit at different positions on a first detector for angularly dispersed collimated transmitted beams. v. An imaging optics for the beam reflected by this slit onto a second detector. The slit's inclination is used both to fine-tune the slit width if needed and to image the reflection off this reflective slit to determine the coupled and uncoupled portions of the beam in the imaging spectrometer. This reflected portion is also used to determine the percussion response of the imaging spectrometer using imaging optics similar to those used in the imaging spectrometer for the transmitted beam, which images the slit on a second detector for the reflected beam. a. An inclined-slit imaging spectrometer as described above, characterized in that the inclined slit is composed of several elements allowing its width to be varied, the optical part of which consists of two blades, each having a part of its surface reflective and the other transparent. These two blades are pressed against each other to form a transparent slit whose width is adjusted by the movement of one blade relative to the other. b. An inclined slit imaging spectrometer as described above, characterized in that it includes an included calibration source. c. An inclined slit imaging spectrometer as described above, characterized in that it comprises after the dispersive part and before the first detector one or more beam splitters separating the collimated transmitted beams dispersed angularly focused on several detectors whose images are used to reconstruct a spatio-spectral image with higher resolution and / or better signal-to-noise quality than that obtained with a single detector.

[0021] In various possible embodiments, the present invention also relates to the features which will become apparent during the following description and which should be considered individually or in all their technically possible combinations: a. The inclined slit spectrometer includes a slit imaging device on a detector or screen, b. The angle of inclination of the slot is used to determine the actual width of the slot. c. The slit is composed of two reflective optics, one of which is fixed and the other movable relative to the latter, the reflective surface of which covers half of the surface with an extremely precise straight edge, the two surfaces constituting the masking parts of the slit, d. The beam imaging system on the slit is centered on the reflected incident beam, e. The system may also include beam focusing optics in the slit, which can be moved along the axis to optimize coupling. f. The dispersion section is framed by two mirrors allowing for fine adjustment of the input and output angles. g. It is possible to insert additional optics into the dispersion section, such as filters for calibration or spatial modulators to modify the percussive response of the device. h. The spectrometer has several channels obtained by replication and several detectors whose data are processed to obtain the final signal.

[0022] The invention will be described in more detail with reference to the accompanying drawings in which: a. Figures 1 and 2 are representations of a measurement of a beam by an imaging spectrometer in a schematic three-dimensional representation and top and side views according to the prior art; b. The [Fig.3a] is a representation of the slit of an imaging spectrometer in a schematic three-dimensional representation and top and side views according to the prior art; c. Fig. 3b is a representation of the inclined slit and its apparent opening for the transmitted beam in a schematic three-dimensional representation and top and side views; d. The [Fig.3c] is a representation of the inclined slit made up of two reflective blades on one half of their surface which combined allow to obtain a slit of variable width in a schematic three-dimensional representation and top and side views; e. The [Fig.4] is a representation in top and side views of an imaging spectrometer according to a first embodiment of the invention; f. The [Fig.5] is a representation in top and side views of an imaging spectrometer according to a second embodiment of the invention; g. The [Fig.6] is a top and side view representation of an imaging spectrometer according to a third embodiment of the invention; h. The [Fig.7] is a representation in top and side views of an imaging spectrometer according to a fourth embodiment of the invention.

[0023] Figure 1 is a schematic three-dimensional representation of a measurement of a beam (1) propagating along the optical axis z, almost collimated. This beam has an energy distribution along the transverse dimensions (x and y) and a spectral distribution in wavelength X. This beam is measured by a device (2) which allows its profile to be obtained along a spatial dimension, here x for example, and the spectral dimension. The resulting image (3) corresponds to the measured signal, i.e., the spectrospatial energy density of the beam. Ideally, the entire beam is measured, i.e., the energy along y is summed.

[0024] Figure 2 is a schematic representation of the top and side views of a measuring device such as that of Figure 1 with a state-of-the-art device. The beam to be measured (1) is entered at the input. This beam is coupled to the device (2), which includes a shaping optic 4 that allows the beam to be coupled into the slit (5). The slit (5) of the imaging spectrometer is very narrow along the y-axis and wide along the x-axis. The coupling optic (4) must focus along the y-axis and leave the beam collimated along the x-axis. In the state-of-the-art imaging spectrometer (20), this optic is not included. Depending on the dimension used for spectral dispersion (y), the beam is diffracted by the very narrow slit (<20 pm). It is then collimated by a first optic (6), called the collimation optic, angularly dispersed along the wavelength by one or more dispersive elements (7), and refocused by the imaging optic (8) to create the image of the slit on the detector (9). In the other dimension along x, there is no dispersion, and therefore the image obtained is that of the beam at the slit. If all the beam energy passes through the entrance slit (5), the measurement corresponds to the measurement of the spectro-spatial energy density I(A, X, y) summed along y, or more precisely, along the spatial frequency associated with y, kv: T ti A r+fy T, -, , x . If the slit is wide, then fv J y A, x) = J f A, X, ky) dky & y is large and all the energy is integrated. But in this case, the image of the slit on the detector is also large, and the spectral resolution is low. We want the highest possible spectral resolution. Therefore, we must use a slit width determined not by the incoming beam and its spatial frequency characteristics along y, but by the desired spectral resolution. In addition to high spectral resolution, the measurement bandwidth must be sufficient to cover the entire beam spectrum. The detector must therefore have a very large number of pixels. State-of-the-art detectors have more than 50 million pixels with a size of 3.76 µm or 3.45 µm, for example, the Sony IMX455, IMX461, and IMX661 CMOS detectors (registered trademark). This pixel size is much smaller than the 16 µm or larger previously used in the state of the art.The optimal resolution of the imaging spectrometer fully exploiting this resolution is therefore much higher than the state of the art defined by the ISOPLANE 320A. However, the size of the image of the slit on the detector is the result of the image of the slit convolved by the percussion response of the imaging system or its optical transfer function H described directly according to X: . I(Atx) = |E(Atx)| 2 = tfE^xïHtX-lxïdl] 2 where Ein is the field at the entrance slit. Without considering optical aberrations, we find that the resolution is directly limited by diffraction and the aperture of the imaging on the detector. The limit is therefore on the order of j .22^ where f is the focal length and D the diameter of the aperture. For measurements around 800 nm, we find that the aperture f / D must be less than f / 3.8 and the slit less than 5 pm. The slit therefore has a width close to, or even less than, the diffraction limit of the entrance optics, which causes a coupling problem and consequently the need to determine the proportion of the analyzed beam. Magnifying the imaging of the slit on the detector can be used to reduce the constraints on the slit width, but in this case, it is the slit length that poses a problem and must exceed 25 mm. Beyond 25 mm, the size of the incoming beam also becomes a problem per 1 pm.According to current best practices, a good compromise is a length of . A 25mm slit with 1 Opm resolution. Depending on the detector used, the ideal magnification of the optics covering the entire height of the detector is either close to 1 (for the 36 x 24mm IMX455 detector) or 0.75 (for the IMX461 and IMX661). The equivalent pixel width of the slit is then 3.5 pm or 2.6 pm, and therefore less than the 5 or 1 Opm per 25mm slits that will be used.

[0025] The size of the slit (5) is shown in [Fig. 3a] in three dimensions and then in front and top views. Its width along the y-axis must be extremely narrow (the size of a pixel), while its length along the x-axis must be the size of the complete detector, i.e., 24 mm or 35 mm. Without resorting to more complex technology than that used for the 1 µm and 5 µm slits, the present invention proposes an inclined slit mounting to reduce its projected width along the y-axis, as illustrated in [Fig. 3b]. The corresponding slit width is reduced by a factor cos(a), which is greater the larger the angle. To further reduce the slit width, it is also possible to assemble two optics to form a slit 5b, as shown in [Fig. 3c]. The two optics comprise a reflective area and a transmission area, as shown in inset 5b 1 of [Fig. 3c].Their fabrication requires high precision to obtain a perfectly straight and smooth demarcation between the two zones. This type of deposition can be achieved by skilled professionals using vacuum deposition in electronics. The two optics are assembled perfectly parallel and in contact, pressed against each other to form assembly 5b. This allows for a further reduction in the width of the entrance slit while maintaining a very long slit length. Other, more complex assemblies are also possible. This type of slit (5a, 5b, or others) is used in the various embodiments of an imaging spectrometer 40, 50, 60, and 70 shown in Figures 4 to 7. The effective width of the inclined slit then varies with the angle. Its dimensions are so small that its alignment and the optimization of the portion of the incident beam within the slit become difficult to achieve and estimate.

[0026] Thus, [Fig. 4] represents a possible embodiment of the invention which combines the use of a tilted slit (5a, 5b, or others) with a system for visualizing the beam reflected (6b, 8b, 9b) by the slit in parallel with the spatio-spectral measurement. The portion of the reflected beam corresponds to the beam spot on the slit without the portion transmitted by the slit, as shown in image (3b). The point (A) at the center of the slit is imaged at the center by the point (A') on the detector (9b). The optics used (6b, 8b) are similar to those used (6, 8) by the spatio-spectral part, allowing the percussive response of the system to be characterized. The dispersion part 7 is not identical. In the slit visualization part, this part (7b) comprises either a fine spectral filter centered on the band or no optics. This involves either attenuation or a spatial filter. The resulting response can then be compared with that of the portion of the beam transmitted through the slit. Combining these two measurements allows us to determine the portion of the beam transmitted through the slit and the percussive response of the device.

[0027] Figure 5 represents a possible embodiment of the invention that combines the embodiment of Figure 4 with an internal calibration source (11) that uses the other face of the inclined slit (5) to inject a beam of known calibrated spectrum into the device. The reflected portion is imaged in the system and used on the detector to adjust the optics and calibrate the percussion response and dispersion. This implementation, however, limits the calibration because the calibration source does not pass through the slit.

[0028] Figure 6 represents an embodiment in which the calibration source portion (11 and 110) is modified to follow the same path as the beam to be measured through the slit. The calibration and measurement of the percussion response are then under conditions identical to those of the measurement.

[0029] In [Fig. 7], the signal is separated onto another detection channel by a splitter blade (12) to allow the use of two detectors (9 and 9c) at the output. The two detectors improve the signal quality and resolution by processing the information obtained.

[0030] The various embodiments of the invention allow for the improvement of the use of an imaging spectrometer through the combined use of a reflective inclined slit whose apparent aperture varies with the angle of inclination. This slit thus makes it possible to obtain very small apertures in the direction used for wavelength analysis, while simultaneously providing a large length for the other transverse dimension, which corresponds to the size of the detector. The significant difficulty in coupling light in the extremely narrow slit thus obtained is compensated for by visualizing the slit in reflection. The portion of the beam reflected by the slit is indeed used. It is imaged with an optical device equivalent to that used for spectral analysis. The image obtained on the detector is used to optimize the coupling in the imaging spectrometer by minimizing the reflected energy, for example.But also to determine, by studying the image of the slit, the percussion response of the spectrometer. Since the optics are identical, the image obtained is the complementary image of that seen through the imaging spectrometer.

[0031] The present invention not only provides an improvement in image quality by using thinner slits and measuring the percussion response but also a simplification of the use of the device by allowing simplified alignment and verification of the part actually coupled in the spectrometer.

[0032] The use of the inclined slit also makes it easier to include a calibration source in the device itself, as illustrated in Figures 5 and 6. Calibration then ensures the best performance in terms of absolute spectral resolution for the spectrometer.

[0033] The use of several detectors at the output of the device, separating the beam with a beam splitter as shown in [Fig. 7], also improves the performance of the spectrometer by limiting the sampling effects related to the detectors. The use of several splitters and more than two detectors can then be considered.

Claims

Demands

1. An imaging spectrometer characterized in that it comprises at least: a. A reflective inclined slit (5), which separates the incident beam into a beam transmitted through the slit and a beam reflected by the complementary reflective surface of the slit b. A collimating optic (6) which transforms the beam transmitted by the diverging slit (5) into a collimated transmitted beam whose rays are parallel, c. A dispersive part (7) generating collimated transmitted beams dispersed angularly at different angles depending on the wavelength from the collimated transmitted beam, d. A focusing optic (8) imaging the entrance slit at different positions on a first detector (9) for angularly dispersed collimated transmitted beams. e. An imaging optic (6b,7b,8b) of the beam reflected by this slit onto a second detector (9b). The slit's inclination is used both to fine-tune the slit width if needed and to image the reflection off this reflective slit to determine the coupled and uncoupled portions of the beam in the imaging spectrometer. This reflected portion is also used to determine the percussion response of the imaging spectrometer using imaging optics (6b, 7b; 8b) similar to those used (6, 7, 8) in the imaging spectrometer for the transmitted beam, which image the slit on a second detector (9b) for the reflected beam.

2. An inclined slit imaging spectrometer according to claim 1, characterized in that the inclined slit is composed of several elements allowing its width to be varied, the optical part of which consists of two blades, each having a reflective portion of its surface and the other a transparent portion. These two blades are pressed against each other to form a transparent slit, the width of which is adjusted by the movement of one blade relative to the other.

3. Inclined slit imaging spectrometer according to claim 1, characterized in that it includes an included calibration source.

4. An inclined slit imaging spectrometer according to claim 1, characterized in that it comprises after the dispersive part and before the first detector one or more beam splitters separating the collimated transmitted beams dispersed angularly focused on several detectors whose images are used to reconstruct a spatio-spectral image with higher resolution and / or better signal-to-noise quality than that obtained with a single detector.

Images