Optical component for an observation or measurement instrument for homogenizing non-uniformities of a spatial sample for multi-spectral observation

Abstract

Optical component for an observation or measurement instrument for homogenizing non-uniformities of a spatial sample for multi-spectral observation Optical component for an observation or measurement instrument in a spectral range, said optical component comprising a plurality of optical waveguides (8) of all constant sections between each an input and an output of the waveguide, with multimode behavior in said spectral range, and whose input ends are aligned along an axis (5) which is a median longitudinal axis of an optical slit (1) of said optical component, said sections each having two bases (B) opposite one another, substantially rectilinear and parallel to the axis (5) of the slit. Each of the sections has, to connect said two bases, at least one inclined face (6) or one curved face (7), distinguishing said section from a rectangle. Abstract figure: 7

Description

Title of the invention: Optical component for an observation or measurement instrument for homogenizing non-uniformities of a spatial sample for multi-spectral observation Technical background

[0001] The invention falls within the field of measuring instruments adapted to be directed towards a scene to be observed and exploiting electromagnetic waves in different ranges - we speak of multispectral or hyperspectral measurement.

[0002] Such instruments can be used to form images - the instrument is therefore a spectral imager - and / or to carry out measurements, which make it possible, for example, to recognize a chemical composition in the observed scene.

[0003] The measuring device can be carried on board a satellite and in particular observe the Earth and its atmosphere. The satellite can revolve around the Earth or be geostationary.

[0004] The acquisition mode of the measuring instrument concerned by the invention is often the scrolling linear field mode or, according to English terminology, the push-broom acquisition mode. A line of pixels is used to successively examine several positions along a trajectory axis called the along track axis (or ALT). But the instrument can also operate, in particular, according to a shooting mode called "step and stare". In this case, during the integration time of a line of samples on the ground, the line of sight of the satellite is fixed. Nevertheless, in this document, the devices and principles will be described with reference to a scroll or at least to a potential scroll, and it will be understood that such a scroll is not essential to the broadest definition of the invention.

[0005] The principle of the contemplated observation of a distant moving or fixed scene is in this context the classic use of a telescope which captures the light from the distant scene and focuses it, forming an image of the scene. But the focused light is processed by an optical slit defined by a longitudinal axis and constituting the input of a spectrometer.

[0006] This slit isolates a small slice of the scene and therefore acts as a spatial filter, defining the elementary unit of the image or measurement sample in the direction of the trajectory (along track) which is transverse to its longitudinal axis (axis called across track or ACT). The two axes are often perpendicular even if this is not essential and moreover the exact inclination can change over time.

[0007] The scrolling of the scene, when there is such scrolling, allows to observe it pro- progressively in its transverse dimension to the slot, along the trajectory.

[0008] At its exit from the slit, the light, thus spatially filtered, is brought to a disperser, which can use for example a prism or a diffraction grating, carrying out a dispersion perpendicular to the longitudinal direction of the slit. The light is then analyzed on a matrix detector, which creates electrical signals.

[0009] For a given position of the scene with respect to the slit, the light is, depending on the wavelength, projected at different positions in a direction transverse to the direction of the slit, and onto the detector. The latter converts the light intensity into a digital value.

[0010] The observed wavelengths can in particular range from near UV to near infrared (0.15 pm to 3 pm). It is desired to observe atmospheric gases, which have a particular spectral signature in the aforementioned wavelength range, thanks to which it is possible to evaluate their concentration.

[0011] In more detail, when the scene is thus scrolled and observed through a slit, it can be considered as an incoherent or weakly coherent sum of point sources distributed in x and y, the x dimension being parallel to the slit, and the y dimension, very limited but nevertheless existing - the slit having a non-zero width - being the dimension perpendicular to the slit.

[0012] However, it is desired that the response of the assembly, also called the response or transfer function of the instrument, does not depend on the scene and in particular on the geometry of the scene, or on the contrasts in it, linked to significant differences in light intensity or luminance. However, although the slit is narrow, it is not narrow enough for the entrance to the slit to be illuminated in a strictly homogeneous manner along the y direction.

[0013] For spectrally hyper-resolved spectro-imagers (typically less than 1 nm per spectral sample), the control of the instrument's spectral response must be excellent. An error of 1 to 2% in the knowledge of the instrument's response can in fact be sufficient to induce an error in determining the gas concentration equal to the desired precision of the instrument.

[0014] The spectral response of the RSI instrument or spectrometer can be written approximately:

[0015] RSI(x,y)= s (y) ® PSF (x,y) ® PRF (x,y) (the symbol ® represents the convolution) where

[0016] s(y) represents the slit function. This corresponds to the image of the illumination function of the slit on the detector. In this equation, it is assumed that it does not depend on x. On a uniform scene along the y axis, it is expressed with a gate function (or rectangle function) whose dimensions simply correspond to the size of the slit along the trace, multiplied by the transverse magnification of the spectrometer.

[0017] PSF, which stands for "point spread function", is the impulse response of the optical system downstream of the slit, which projects an image of it onto the detector or photosensor. It is a function which depends on the value of the field (x) but also on the value of the field (y) also called the spectral field.

[0018] PRF stands for "photo response function" and is the spatial radiometric response of the detection area used as a spectral sample. It can be associated as a first approximation with a gate function.

[0019] Through the RSI function above, we see that s(y) and therefore RSI varies depending on whether the scene is uniform along y or whether the scene only illuminates a fraction of the slit along y. The RSI function therefore depends on the scene, which is not desired.

[0020] In order to limit the impact of the spatial non-uniformity (for a given wavelength in the observation band) of the scene in terms of intensity on the RSI function, we want the slit to distribute its output illumination, whatever the distribution of its input illumination: we then speak of a homogenized scene. A homogenization function should preferably be carried out by the slit itself.

[0021] From the document Sentinel-5 / UVNS ICSO 2018- Proc. Of SPIE Vol. 11180 1118004-1, we know a principle of homogenization by mirror slit. This homogenization can be described as 1D homogenization, since there is homogenization according to one dimension, in this case the ALT along-track dimension or along the trace (along y), thanks to the propagation according to the propagation dimension of the light.

[0022] 1D homogenization based on mirrors, however, presents two problems: Grazing incidence reflections on the mirrors generate transmission losses and polarize the light, which is a problem for a spectro-imager, and homogenization is done along the y (spectral) axis but not along x. In fact, at the output of the homogenizer - that is to say at the output of the slit, we recover a source focused along y but defocused in x. This defocusing being unacceptable, it must be compensated by the optical solution. This is on the one hand a significant constraint on the global solution, and on the other hand an intrinsic limitation: ACT (across track, along x) variabilities of the scene impact the RSI via the PRF(x,y) parameter.

[0023] Another method of homogenization in a single direction, in this case along ALT (along y), using a restrictive astigmatic optic, is also known from WO2020027657A1.

[0024] In these two solutions, it is necessary to treat the optical conjugations ALT and ACT (along x) separately. There is no homogenization along the across-axis. track. This homogenization technique is therefore not very efficient. ACT variabilities (according to x) of the scene always impact the RSI via the PRF(x,y) parameter.

[0025] Also known, for example, from EP3936841_A1 is an assembly based on multimode optical fibers. It is known that optical fibers are commonly used optical waveguides. This document mentions rectangular fiber sections. The multimode character, at the wavelengths of interest for the spectro-imager, is also mentioned. It is linked to the transverse dimensions of the fiber.

[0026] In EP3936841_A1, homogenization according to ACT and ALT is sought by the length of the fiber and an assembly involving curves in the fiber, between its input and its output.

[0027] But long fibers are bulky and therefore impractical, particularly for use in an observation satellite.

[0028] On the imaging spectrometer of the C02M (Copemicus Anthropogenic Carbon Dioxide Monitoring) satellite, it is planned to integrate a two-dimensional uniformity homogenizing input slit consisting of an alignment of short multimode optical fibers with a cross-section, again rectangular. The rectangular shape maximizes the energy flux transported between two planes. This slit is called a two-dimensional slit homogenizer (2DSH). The fibers perform a piecewise homogenization of the input scene in the spectral and spatial directions. The slit is composed of an array of 120 rectangular fibers stacked along the field of view in the ACT direction. The light collected on this sample undergoes multiple reflections as it propagates along the fiber, which averages the scene contrast to produce a spatially homogeneous near-field image.Homogenization is performed in the spectral (ALT) and also spatial (ACT direction) direction piecewise on each ACT spatial sample. However, the fibers are relatively short, which does not facilitate the coupling between modes and therefore limits the homogenization performance in comparison with the principle used in EP3936841_A1. Definition of invention - associated advantages

[0029] To optimize these systems, it appeared desirable to optimize the homogenizer itself, by seeking to improve the homogeneity of the transfer function over the entire section of the optical waveguide.

[0030] The proposed solution consists in using optical waveguides, as in the prior art, similar to each other, with typical lengths of 1 to 15 cm between their input and their output, and whose section, invariant from the input to the output, derives in one way or another from a rectangle, the sides of the same dimension of the rectangles being placed parallel (i.e. the neighboring rectangles are juxtaposed by the long sides, or, as proposed in the prior art and as is advantageous, they are by the short sides).

[0031] This solution maximizes the power collected in the slit, which is treated in the form of a succession of slit sections, which are naturally successive parallelograms, and more simply successive rectangles.

[0032] Thus, an optical component is proposed for an observation or measurement instrument in a spectral range, the component comprising a plurality of optical waveguides of sections all constant between each an input and an output of the waveguide (geometrically, the waveguides are delimited by a closed generator forming a perimeter of the section and the section is invariant between the input and the output), with multimode behavior in said spectral range, and the input ends of which are aligned along an axis which is a median longitudinal axis of an optical slot of said optical component.

[0033] Said waveguide sections each have two bases opposite each other, substantially rectilinear and parallel to the axis of the slot - these two bases typically being the long sides of a rectangle, on the two sides of the slot. This is dictated by the very natural desire to occupy the space of the slot as best as possible and this has therefore materialized in the prior art by the strictly rectangular shape of the optical fibers used: their long sides constitute opposite bases parallel to the axis of the optical slot.

[0034] But in an original manner here, the optical component is such that each of the waveguide sections has, to connect said two bases and form the perimeter of the waveguide, at least one inclined face (if a polygonal shape is retained) or a curved face (if it is decided to introduce convexities or concavities), so that said section is distinct from a rectangle.

[0035] This allows, according to the inventors' work, to greatly improve the homogenization function, even with short optical waveguides kept rectilinear between their input and their output. Without wishing to be bound by a theory, it is stated that the proposed geometry allows the formation of ergodic modes of propagation of waves of said spectral range in fibers or other guides having this section geometry, these propagation modes favoring homogenization. More precisely, the proposed geometry (internal angles between successive plane facets and / or a curved face so that said section is distinct from a rectangle) fights against the segregation of propagation modes into two families, one along the large side of the rectangle and the other along the small side of the rectangle.

[0036] This is particularly original and changes the perspectives in the field of homogenizing slots, since we obtain a slot that is always just as effective for capture light, but which also homogenizes without using long optical fibers.

[0037] Indeed, while the geometry of the slot and the usual manufacturing techniques encourage the use of rectangular section waveguides to easily observe the entire scene and maximize the energy collected with an elementary waveguide geometry, the proposed solution deviates from this rectangular geometry, and makes it possible to obtain, in a remarkable and very original manner, a very good quality homogenization while maintaining very good energy collection and complete observation of the scene.

[0038] It is possible to use only a discrete deviation from the shape of a reference rectangle in which the waveguide is inscribed, to maintain good energy collection and retain common manufacturing processes. But as soon as a deviation is created, for example by a flat cutting a vertex of the rectangle, even if it is discrete, homogenization is observed. It is estimated, as mentioned above, that this is linked to the fact that the purely rectangular geometry has the defect of confining the modes into two families of modes, and that the deviation from this geometry releases the modes into a greater diversity, not limited to two distinct families.

[0039] Thus, the optical component comprising the asymmetry mentioned above is a 2D homogenizer which is based on multimode optical guides arranged along a line, so as to constitute a pixelated slit, each pixel of which homogenizes the input scene in an original manner in ALT and ACT.

[0040] The waveguides - optical fibers or integrated optical elements - can be guides with a globally rectangular functional section with a truncated corner (by a rectilinear boundary) and three right-angled corners, which forms a convex pentagon with three consecutive right angles.

[0041] But other solutions are used in variants, including trapezoids, hexagons, octagons, or parallelograms. Curvilinear and not necessarily rectilinear boundaries are also used in some variants for the functional section. Thus, instead of a truncation, a soft, rounded shape is used in some embodiments.

[0042] In addition, each of the sections is preferably convex.

[0043] Optionally and advantageously,

[0044] Optionally, the optical waveguides may have an elongated functional section along the axis of symmetry.

[0045] The length of the waveguides between their input and their output can, optionally, be between 1 cm and 15 cm.

[0046] The waveguides may optionally be made of posi- optical fibers data side by side between two planes of a chassis. Thus, as in the prior art, the waveguides can be optical fibers, notably silica-based, but other materials can be used, such as plastics.

[0047] The waveguides can alternatively be integrated optical elements.

[0048] The waveguides may optionally be kept straight over a major portion of their extent between their input end and their output end.

[0049] The ratio between the maximum dimensions of the functional section of the optical guides along the axis of the slot and along the direction perpendicular to said axis of the slot may optionally be of the order of 3, 2 or 1, in particular.

[0050] - the length of the optical guides between their input and their output can optionally be between 1 cm and 15 cm,

[0051] - the slot can optionally be 30 to 100 mm high,

[0052] - its construction can optionally result from the alignment of for example between 20 and 300 optical waveguides

[0053] - the dimension in the alignment direction of each of the waveguides can op tionally be 30 pm to 1 mm.

[0054] The invention also relates to a use of the optical component as presented above.

[0055] Thus, the component can be placed to prepare a spatial sample for an observation or measurement instrument by spectral dispersion.

[0056] The optical component can be used as a spatial filter in a dispersive, airborne or space-borne spectro-imager, constituting said observation or measurement instrument, and resolved for example to less than 10 nm per spectral sample.

[0057] The invention therefore relates to a dispersive, airborne or spatial spectro-imager, comprising an optical chain upstream of a dispersing element, said optical chain comprising an optical component as mentioned above.

[0058] The exact profile of the section (shape, dimension and therefore, if the shape is rectangular, length, width and dimension of the cutaway or truncation) as well as the length of the optical guides are adapted according to the instrumental parameters and the targeted performances.

[0059] The homogenization performance is significantly improved compared to the prior art, while the other performances are preserved or only marginally impacted compared to the solution of fibers with a strictly rectangular section.

[0060] The invention is used for all spectro-imaging type instruments requiring stability of the spectral response independent of the measured scene, whether or not they are dispersion systems.

[0061] The invention benefits from the advantages offered by short waveguides (compactness, low impact on the rest of the instrument design, no particular constraints regarding the layout) and has no particular weaknesses in the face of vibrations or other aggressive environments.

[0062] The invention allows for a homogenization performance superior to previous solutions.

[0063] It is preferable to avoid reintroducing a particular regularity by transforming the rectangle into a regular octagon or a circle. List of figures

[0064] [Fig.l] shows the useful section of an optical fiber such as optical fibers used in a spectro-imager according to the prior art.

[0065] [Fig.2] shows the light present at the output of the optical fiber of [Fig.l], on the right of the figure, for an illumination at the input shown on the left of the figure.

[0066] [Fig.3] shows the same as [Fig.2], but for a different input illumination.

[0067] [Fig.4] shows the distribution of light at the output of an optical fiber according to one embodiment of the invention.

[0068] Figures 5 and 6 show the same thing as [Fig.4] but for other embodiments of the invention.

[0069] Figures 7 and 8 show an assembly according to the invention, from two different views.

[0070] Figures 9 to 14 show variants falling within the scope of the invention. Detailed description

[0071] The invention relates to a spectro-imaging instrument, as shown schematically below, and whose conventional “slit (spatial filter)” component is replaced by a high-performance 2D homogenizing slit, thus making the spectral response of the instrument almost insensitive to the non-uniformities of the observed scene.

[0072] The instrument is therefore made up of the following elements:

[0073] - a telescope which forms the image of the observation scene at the entrance to the slit ho 2D mogenizer, in an intermediate focal plane,

[0074] - a 2D homogenizing slit on which the image of the scene is applied by the telescope and which transports and homogenizes the scene from the image focal plane of the telescope to the object focal plane of the spectrometer. Apart from beam homogenization in both directions (across track and along track), the optical characteristics of the prior art are retained, in particular the f-number - the geometric aperture - and the light spectrum. The slit is based on optical fibers or on an integrated optics component. It will be described below with silica optical fibers, but more generally, each fiber can be replaced by an optical waveguide which would be made other than from silica optical fiber, and in particular using integrated optical component technology. Optical fibers made from a material other than silica - typically plastic polymers - may also be used.

[0075] - a spectrometer comprising a dispersive element (a grating or a prism, for example) example) and which directs the waves leaving the slit (and which may have passed through free space) towards the detector (typically, upstream of the dispersive element, the light is collimated, then downstream of the dispersive element, it is focused),

[0076] - a detector receiving the scattered waves from the spectrometer and acquiring of the signal in two dimensions: spatial and spectral.

[0077] [Fig-1] The inventors were interested in an optical fiber with a rectangular section, whose section length to section width ratio is fixed (it is chosen for a given equipment, depending on the scene scrolling speed). This fiber section is shown in [Fig.l]. It has a 3 ratio between length and width: 300 pm versus 100 pm. The fiber has a length of 5 cm for the discussion. Nevertheless, the invention has a wider application than the geometry of the section of [Fig.l],

[0078] [Fig.2] The left and right parts of Figures 2 and 3 represent respectively the fiber input and the fiber output, in a situation of illumination by a given light, of wavelength, for the discussion of 1 pm, even if another wavelength value between ultraviolet and mid-infrared could have been chosen.

[0079] If the input scene is a 2-dimensional Gaussian, essentially a point, visible in [Fig.2] on the left side, placed on the longitudinal axis of the slit, and therefore of the fiber, then the energy at the fiber output is dispersed during the path from the input to the output throughout the section, in the form of small spots, as can be seen on the right side of [Fig.2].

[0080] [Fig.3] And if the Gaussian is shifted relative to the longitudinal axis of the fiber, as shown in [Fig.3], on the left side which visualizes the fiber input, then the output energy, shown on the right side, is also dispersed, but with a different distribution from that obtained on the light is centered at the input. In particular, more intense lines are present, which is not desirable for good homogenization.

[0081] Ultimately, as can be seen by comparing Figures 2 and 3, the geometry of the output is dependent on the geometry of the input, which is not desired, since we are seeking to obtain a transfer function independent of the scene.

[0082] [Fig.4] As seen in [Fig.4], the truncation of an angle of the rectangle, re presented schematically in the left part of the figure, with an angle of 30° on the long side of the rectangle, and 60° on the short side of the rectangle at mid-length of the rectangle. This results in a distribution of the flux at the fiber output - shown in the right part of the figure for a Gaussian excitation centered at the fiber input - which is more random and ultimately distributed uniformly, due to homogeneous dynamics.

[0083] The values ​​of 30° and 60° can be replaced by other values, for example 40° and 50°.

[0084] [Fig.5] As seen in [Fig.5], even in the case of slight truncation, re presented in the left part of the figure, with always an angle of 30° on the large side of the rectangle, and 60° on the small side of the rectangle but this time at approximately 1 / 7th of the length of the latter, a homogenization effect appears at the fiber output.

[0085] [Fig.6] Furthermore, as seen in [Fig.6], even with a length of pro pagation - the length of the fiber - reduced (compared to the results presented in the previous figures, for which the length of the fiber was 5 cm) of the order of 1 cm - in the left part of the figure - or 2 cm - in the right part of the figure - homogenization is observed.

[0086] [Fig.7] In [Fig.7], slot 1 consists of 100 to 120 fibers placed along a axis 5 which constitutes the longitudinal axis of slot 1.

[0087] Each individual fiber core constitutes a sub-slit, the short side of which defines the slit width and the long side the instantaneous field of view of a spatial sample in the ACT direction.

[0088] For each of the fibers, the wave transmission section, called functional section 10, is a pentagon derived from a rectangle with a large side of 300 pm and a small side of 100 pm. Axis 5 is the large median of each of the rectangles from which the pentagons derive.

[0089] Axis 5 constitutes the median axis parallel to the long sides of the rectangle from which the pentagon derives. These long sides are the bases B. The side of the pentagon which is inclined with respect to the bases B is an inclined face 6.

[0090] The corners of pentagons can be rounded. The truncation can typically be done at a little less than halfway down the short side of the rectangle (e.g. at 4 10ths), with an angle of 35° on the long side of the rectangle.

[0091] The fibers are each placed in an oblong sheath 20 with large planar lateral faces running along the long sides of the functional section 10 and small planar lateral faces running along the long sides of the functional section 10.

[0092] The large lateral faces and the small lateral faces of the oblong sheathing 20 are joined by tangent rounded surfaces surrounding, for three of them, the angles of the pentagon.

[0093] Inside the volume defined by the cladding 20, the circumferential intermediate volume 30 between the inner surface of the cladding and the outside of the rectangular trapezoidal section is made of silica with a dopant different from the silica of the functional section. tional 10, such that it transmits the waves while the circumferential volume does not transmit them.

[0094] The fibers are joined by the small lateral faces. The truncations of the functional section of the fiber are all placed at the same orientation - in [Fig.7] at the top right of the rectangles, so that each fiber is deduced from the neighbor by a translation along the longitudinal axis. However, the pentagons could for example be arranged head to tail.

[0095] A solidified fluid glue 40 surrounds the sheaths of the fibers thus arranged which are held between two planes of a frame.

[0096] [Fig.8] In [Fig.8], we see that the alignment of the optical fibers forming the slot 1 is maintained thanks to a mechanical chassis 100 taking into account the constraints of use of the equipment in its environment. The waveguides have a length which can be approximately 7 cm, and which can, beyond this value, be between 1 cm and 15 cm, for example.

[0097] The production of non-symmetrical optical fiber is done by the same process as for the symmetrical case: a preform of the desired geometry is produced and then a fiberizing tower is used which reproduces this shape on a much smaller scale.

[0098] [Fig.9] In [Fig.9], an embodiment of the optical waveguides with integrated optical component technology is presented. It is based on a substrate 50, a confinement medium 51 and a superstrate 52 (air or vacuum, for example), the confinement medium 51 being of higher optical index than the substrate and the superstrate. The manufacturing may include thin film depositions, masking, etchings (chemical, ionic, electronic, or by laser...), and / or the diffusion of ions locally in a glass. The geometries may include, among others, edges, ribs, strips, slots.

[0099] Without wishing to be bound by a theory, the following comments are made.

[0100] In multimode optical waveguides, the modes exhibit spatial distributions of field intensity that follow the symmetries of the guide geometry. When the geometry of the section is modified or when one moves away from these simple geometries, the distribution of the intensity of the modes may appear more homogeneous. Some theories speak of ergodic modes or speckle modes by analogy with the pattern of random speckles in size and intensity observed during the diffusion of light on a rough surface.

[0101] This behavior has been discussed theoretically, in the context of the study of waves of any kind which propagate in media exhibiting chaotic ray dynamics. In media whose transverse dimensions are large compared to the wavelength, the properties of the waves which propagate there can be described by a semi-classical approach or geometric approach to rays.

[0102] In the case of an optical fiber section having a simple geometry such as a circle, a square, etc., the dynamics of the rays can be perceived as regular. But if the geometry of the medium induces a chaotic dynamics of the rays, that is to say if the rays explore the medium randomly and if the evolution of the trajectory followed is sensitive to the initial conditions, it is possible that the field of modes of the standing waves which are established in the medium can be seen as the result of a random superposition of plane waves giving rise to ergodic behavior.

[0103] From a theoretical point of view, however, it is not possible to analytically solve the Helmholtz equation in the case of systems whose geometry induces dynamics that are not regular. The individual modes can only be calculated numerically using numerical tools such as mode solvers, and / or with numerical methods such as Beam Propagation Method to simulate the propagation of the field along the fiber.

[0104] When the longitudinal and / or transverse symmetry of the fiber is broken, the regular transverse modes present in a rectangular fiber likely become, with all due reservations, ergodic modes giving the light a statistically homogeneous spatial distribution of energy very early in its progression along the fiber, which makes it possible to avoid using a significant length of optical fiber.

[0105] [Fig. 10] [Fig. 10] shows in the upper part (line a) an implementation variant of the invention. The optical fibers are the same as those used in the embodiment of [Fig.7], but every other fiber has been rotated 180° around the center of the rectangle circumscribing the pentagon that it forms. They are joined by the short sides, and due to the alternation, the short sides of the fiber core without truncation are joined together and the short sides of the fiber core with truncation are joined together, but the truncations are positioned alternately on one face of the slot, then on the other.

[0106] [Fig. 10] shows in the lower part (line b) another alternative embodiment of the invention. The optical fibers are the same as those used in the embodiment of [Fig.7], but one fiber out of two has had its output swapped with its input. They are joined by the short sides, and because of the alternation, the short sides of the fiber core without truncation are joined together and the short sides of the fiber core with truncation are joined together, and the truncations are all positioned on the same face of the slot.

[0107] [Fig. 11] [Fig. 11] shows four other variants of implementation of the invention. The optical fibers are this time designed to have a fiber core which is a rectangle with several vertices truncated.

[0108] In the first line shown (line c), two consecutive vertices on a long side are truncated, the other two vertices being untruncated. Thus, there is one face inclined relative to the bases to connect the bases to each other on one side and another on the other.

[0109] In the second line shown (line d), two consecutive vertices on a short side are truncated, the other two vertices being untruncated. Thus, there are two faces inclined relative to the bases to connect the bases to each other on one side and none on the other.

[0110] In an embodiment not shown, two vertices opposite by a diagonal are truncated and the other two are not.

[0111] In the third example of the figure (line e), three vertices are truncated. In the fourth example (line f), all four vertices are truncated.

[0112] [Fig. 12] [Fig. 12] shows another alternative embodiment of the invention (line g). The optical fibers are this time designed to have a fiber core that is a rectangle with one vertex gently rounded. Thus, instead of a straight line defining angles at its ends as in the other embodiments presented above, a convex circular arc constituting a curved face 7 connects one of the sides to an adjacent side.

[0113] Instead of this convex curved shape, other convex curved shapes can be used.

[0114] [Fig. 13] [Fig. 13] shows another variant (line h). The waveguides are of parallelogram-shaped sections. The successive parallelograms are positioned with the corresponding sides parallel, here the short sides. Each parallelogram derives from a rectangle to which two right truncations have been applied to two opposite vertices, the truncations each extending to the vertex next to and closest to the truncated vertex. In line h the left end, respectively right, of a waveguide does not start under the right end, respectively left, of the following waveguide, as in lines a to g.

[0115] Thus, there is, as in line c, a face inclined relative to the bases to connect the bases to each other on one side and another on the other.

[0116] [Fig. 13] also shows another variant (line i) where the waveguides are similar to those of line h, but where to increase the collected energy, the left, respectively right, end of a waveguide starts under the right, respectively left, end of the following waveguide, which reduces the surface area of ​​the non-functional spaces of the slot.

[0117] Again, there is a face inclined relative to the bases to connect the bases to each other on one side and another on the other.

[0118] [Fig. 14] [Fig. 14] (line j) shows that the large dimension of the guide section of waves can be transverse to the longitudinal axis of the slit. As a result, in certain cases, the optical instrument (telescope and spectro-imager) presents an anamorphosis linked to the optical slit, but this anamorphosis can be taken advantage of, or conversely corrected.

[0119] In the preceding description, mention has been made of bases of the waveguides parallel to the longitudinal axis, and which in the prior art are the large sides of the rectangles. In the embodiment of [Fig.7], these are the bases of the pentagons (a large base and a small base).

[0120] It is hereby stated that the invention only requires that these bases exist in one way or another, that they are opposite each other and that they are only substantially parallel to each other and to the longitudinal axis of the slot. It is not necessary that they be strictly parallel to each other and to the longitudinal axis of the slot.

[0121] Thus, if the two bases exist but are not strictly rectilinear (due to a notch or a protrusion or even a slight incurvation) or strictly parallel to each other and therefore to the axis of the slit (due to a slight inclination), the invention can nevertheless be implemented, with possibly a small loss of captured light intensity.

Claims

Claims

1. Optical component for an observation or measurement instrument in a spectral range, said optical component comprising a plurality of optical waveguides (8) with multimode behavior in said spectral range, and whose input ends are aligned along an axis (5) which is a median longitudinal axis of an optical slit (1) of said optical component, the sections of said waveguides each having two bases (B) opposite each other, substantially rectilinear and parallel to the median longitudinal axis (5) of the slit, the optical component being characterized in that each of the sections has, to connect said two bases (B) and form the perimeter of the waveguide, at least one inclined face (6) relative to the bases (B) or a curved face (7), distinguishing said section from a rectangle.

2. Optical component for an observation or measuring instrument according to claim 1, characterized in that the optical waveguides (8) have a functional section (10) elongated along said median longitudinal axis (5).

3. Optical component for an observation or measuring instrument according to claim 1 or claim 2, characterized in that the length of the waveguides (8) between their input and their output is between 1 cm and 15 cm

4. Optical component for an observation or measuring instrument according to one of claims 1 to 3, characterized in that the waveguides consist of optical fibers (8) positioned side by side between two planes of a chassis (100).

5. Optical component for an observation or measuring instrument according to one of claims 1 to 4, characterized in that the waveguides are integrated optical elements.

6. Optical component for an observation or measuring instrument according to one of claims 1 to 5, characterized in that the waveguides (8) are kept rectilinear over a major part of their extent between their input end and their output end.

7. Optical component for an observation or measuring instrument according to one of claims 1 to 6, characterized in that each of the sections is a convex pentagon with three consecutive right angles.

8. Optical component for an observation or measuring instrument according to one of claims 1 to 7, characterized in that each of the

9. sections is convex. Dispersive, airborne or spatial spectro-imager, comprising an optical chain upstream of a dispersing element, characterized in that said optical chain comprises an optical component according to one of claims 1 to 7.

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