FR3146358A1 - Mach-Zehnder interferometric device - Google Patents

Abstract

The invention relates to a Mach-Zehnder interferometric device (1), comprising: at least a first prism (2) and a second prism (3), the prisms (2, 3) being assembled together so as to form a monobloc, the prisms (2, 3) forming a beam-splitting interface (4) between them, the beam-splitting interface (4) being parallel to a first external face (5) of the first prism (2) and a second external face (6) of the second prism (3), at least one of the prisms (2, 3) comprising an input face (11) for receiving an incident light beam, the beam-splitting interface (4) being configured to split the incident beam into two partial beams, the first and second external faces (5, 6) each being configured to fully or partially reflect one of the partial beams, and the prisms (2, 3) each comprising an output face (15, 16) allowing the output of a first and second light beam. of a second outgoing light beam (20, 21), respectively; a spatial filter (9) configured to form a reference wave from one of the partial beams, the device (1) being configured to interfere the reference beam and one of the partial beams to form the outgoing beams (20, 21). Figure for abstract: [ Fig. 3 ]

Description

Mach-Zehnder interferometric device

The present invention relates to a Mach-Zehnder interferometric device.

The field of the invention is, in a non-limiting manner, that of adaptive optics and optical metrology components.

State of the art

Mach-Zehnder interferometers are used in many fields. An incident light beam is divided into two different light beams of the same intensity by a beam splitter. After their propagation on two different optical paths, the beams are recombined by another beam splitter. Interferences can then be observed at the output of the interferometer.

These Mach-Zehnder interferometers can be used to perform phase measurements of an electromagnetic wave. For example, surface measurements of a transparent object introduced into the optical path of one of the two beams make it possible to assess its flatness or sphericity. Mach-Zehnder interferometers are notably implemented in the context of wavefront analysis, by measuring the light source itself. This requires the creation of a phase reference obtained by spatial filtering. These devices are used, among other things, in the field of adaptive optics, to be able to eliminate the deformations of the wavefronts undergone following disturbances (for example, atmospheric or due to optical aberrations). This type of wavefront analyzers are then used, for example, in telescopes. Classic Mach-Zehnder type devices are difficult to implement and are very sensitive to any variation in the air index between the 2 arms.

It is an aim of the invention to propose a solid and compact Mach-Zehnder type interferometric device.

Another aim of the present invention is to provide a Mach-Zehnder type interferometric device which can be implemented in wavefront analysis systems for adaptive optics components.

It is also an aim of the present invention to propose an interferometric device whose implementation is easy, reliable and does not suffer from alignment problems.

• au moins un premier prisme et un deuxième prisme,
  • les prismes étant assemblés ensemble de façon à former un monobloc, les prismes formant une interface séparatrice de faisceaux entre eux, l’interface séparatrice étant parallèle à une première face externe du premier prisme et une deuxième face externe du deuxième prisme,
  • au moins l’un des prismes comprenant une face d’entrée pour recevoir un faisceaux lumineux incident, l’interface séparatrice étant configurée pour diviser le faisceaux incident en deux faisceaux partiels, les premier et deuxième faces externes étant chacune configurée pour réfléchir intégralement ou partiellement l’un des faisceaux partiels, et
  • les prismes comprenant chacun une face de sortie permettant la sortie d’un premier et d’un deuxième faisceaux lumineux sortant, respectivement ;
• un filtre spatial configuré pour former une onde de référence à partir d’un des faisceaux partiels,
  le dispositif étant configuré pour faire interférer le faisceaux de référence et l’un des faisceaux partiels pour former les faisceaux sortants.

At least one of these goals is achieved with a Mach-Zehnder interferometric device, comprising:

• at least a first prism and a second prism,
  • the prisms being assembled together so as to form a single block, the prisms forming a beam-splitting interface between them, the beam-splitting interface being parallel to a first external face of the first prism and a second external face of the second prism,
  • at least one of the prisms comprising an input face for receiving an incident light beam, the splitter interface being configured to split the incident beam into two partial beams, the first and second external faces each being configured to fully or partially reflect one of the partial beams, and
  • the prisms each comprising an exit face allowing the exit of a first and a second outgoing light beam, respectively;
• a spatial filter configured to form a reference wave from one of the partial beams,
  the device being configured to interfere the reference beams and one of the partial beams to form the outgoing beams.

The interferometric device according to the invention makes it possible to measure with nanometric precision the wave front reflected or transmitted by an optical or non-optical surface, which makes it possible to measure its shape and/or the refractive index of the material which constitutes it.

The solid-state Mach-Zehnder interferometric device can be used as a wavefront analyzer, especially for adaptive optics systems, for example for Very Large Telescopes .

Advantageously, the first external face can be provided with a phase shift layer of thickness λ/8.

In this case, the outgoing beams are 180° out of phase with respect to each other, allowing a linear simplification of the phase measurement at small phase shifts.

According to other examples, the thickness of the phase shift layer deposited on the first external face can vary between 0 and λ/2.

According to one embodiment, the beam-splitting interface between the first and second prisms may comprise a monolayer of thickness λ/4.

A monolayer is particularly suitable when a low coherence light source is used.

Alternatively, the beam splitter interface between the first and second prism may comprise a multilayer of thickness λ/4.

A multilayer is particularly suitable when a monochrome light source is used.

Advantageously, the first and second external faces parallel to the beam splitter interface are provided with reflective or semi-reflective layers.

This treatment prevents losses related to the transmission of light beams to the outside of the device. On one of the external faces, the reflective or semi-reflective layer allows spatial filtering to be carried out.

Advantageously, the output faces can each be provided with an anti-reflective layer.

The light power transmitted outside the device is thus optimized.

According to an advantageous embodiment, the at least two prisms can be assembled to each other by molecular adhesion.

No adhesive or other means of fixation is then necessary to assemble the prisms. Also, this adhesion technique does not modify the difference in optical paths traveled by the reference beam and one of the partial beams. It is therefore particularly suitable when the interferometric device is implemented with a low coherence light source, such as a white source.

Alternatively, the two prisms can be joined together using an index-matching glue.

Since the adhesive layer has a certain optical thickness, this adhesion alternative is only suitable when the device is implemented with a monochrome light source having a long coherence length.

According to an advantageous implementation mode, the entry and/or exit faces of the prisms can be inclined relative to the separating interface by an angle of 45°.

In this way, the incident and outgoing beams have an incidence and exit angle close to 0.

According to one embodiment of the device, the spatial filter may comprise at least one reflective or semi-reflective pellet deposited on the external face of the second prism.

According to examples, the at least one reflective patch may have an elliptical, square, circular, or other two-dimensional shape.

The reflective patch can be of the same nature (material, thickness) as the reflective or semi-reflective layer deposited on the external face of the first prism.

Advantageously, the prisms of the device according to the invention can be manufactured in BK7 or silicon, or any other low-dispersion optical material.

Description of figures and embodiments

• la est une représentation schématique d’un dispositif interférométrique selon un mode de réalisation ;
• la montre schématiquement un filtre spatial du dispositif selon un mode de réalisation; et
• la est une photographie d’un dispositif interférométrique selon un mode de réalisation de l’invention.

Other advantages and characteristics will appear on examining the detailed description of non-limiting examples, and the attached drawings in which:

• there is a schematic representation of an interferometric device according to one embodiment;
• there schematically shows a spatial filter of the device according to one embodiment; and
• there is a photograph of an interferometric device according to one embodiment of the invention.

It is understood that the embodiments which will be described below are in no way limiting. In particular, it will be possible to imagine variants of the invention comprising only a selection of features described below isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the state of the prior art.

In the figures, elements common to several figures may keep the same reference.

There is a schematic representation of a Mach-Zehnder type interferometric device according to an embodiment of the present invention.

The device 1 comprises two prisms 2, 3, called prisms. In the embodiment shown in the , prisms 2, 3 are identical. They each have the shape of a plate with two parallel ribs. The first prism 2 and the second prism 3 form the basis of the Mach-Zehnder type interferometer. The two prisms 2, 3 are assembled so as to form an interface 4 between them.

Preferably, the prisms 2, 3 are assembled to each other by molecular adhesion.

Alternatively, the 2,3 prisms can also be assembled using an index-matching adhesive. Since the adhesive adds an additional thickness to the thickness of the prisms, the optical path is extended for the beams passing through the adhesive layer. This adhesion alternative is therefore particularly suitable when the device is implemented with a monochrome light source having a long coherence length.

The material of the prisms 2,3 is a weakly dispersive material at the implemented wavelength of the device 1. For example, for wavelengths between approximately 600 and 950 nm, the material can be silicon or BK7, or any other suitable weakly dispersive material.

The interface 4 between the two prisms 2, 3 is a splitter interface constituting a light beam splitter. As shown in the , the separating interface comprises a layer 17 of thickness λ/4. According to one example, for a wavelength of 850 nm, this layer can be a monolayer of Ti ₃ O ₅ .

Still referring to the , the external face 5 of the first prism 2, parallel to the separating interface 4, is coated with a reflective layer 7. This first external face 5, called longitudinal, acts as a mirror thanks to the reflective layer 7.

The reflective layer can be, for example, an Ag layer with a reflectivity R > 95% at a wavelength of 850 nm.

The first external face 5 is also provided with a phase shift layer 18 of thickness λ/8. This may be, for example, a layer of SiO ₂ for an operating wavelength of 850 nm.

The longitudinal external face 6 of the second prism 3, also parallel to the separating interface 4, comprises a spatial filter 9. In the example shown in the , the spatial filter 9 consists of a reflective patch. The reflective patch 9 may be of the same nature as the reflective layer 7 of the first external face 5, that is to say, of the same material and/or of the same thickness.

An example of a spatial filter 9 is illustrated in the , showing the external face 6 of the second prism from below. Here, the spatial filter 9 consists of an elliptical reflective pellet.

According to other examples, the pellet can have a circular, square, oval, oblong apodized shape, etc.

Of course, the spatial filter can take other forms. In particular, it can be combined with a phase shift.

When implemented, a light beam 10 enters the device through one of the side faces 11 of the first prism 2 (the one that does not include the spatial filter 9). According to one example, the light beam has an aperture number 60 and is focused on the spatial filter 9. The beam 10 is divided into two partial beams 13, 14 by the beam splitter 4. One of the partial beams is reflected by the splitter 4 and then reflected by the external face 2 acting as a mirror; it constitutes the object beam. The other partial beam is transmitted by the splitter 4 and then reflected at the spatial filter 9. The beam reflected at the spatial filter 9 constitutes the reference beam. Interferences between the object beam and the reference beam can be detected at each of the two output faces 15, 16 (beams 20, 21). With the devices according to the embodiment as shown in the , signals 20, 21 of the two outputs are 180° degrees out of phase with each other.

The longitudinal external faces 5, 6 as well as the separating interface 4 between the prisms 2, 3 must be perfectly parallel to each other. Preferably, the parallelism defect between these optical surfaces must be less than 0.02 mrad. Also, the thickness of the prisms 2, 3 between the separating interface 4 and their external face 5, 6, respectively, must be the same for both prisms, preferably with a difference between the two thicknesses of less than 0.001 mm. The thickness may be, for example, approximately 20 mm.

To meet these two constraints, the two prisms 2, 3 can, for example, be obtained from a single plate of material with parallel faces.

There shows a photograph of a Mach-Zehnder type interferometric device according to an embodiment of the present invention. A ruler placed next to the device allows the dimensions of the device to be appreciated.

For example, the separating interface 4 of the device 1 may have a length (in the plane of the sheet) of approximately 80 mm, the external faces 5, 6 parallel to the separating interface 4 may each have a length of approximately 40 mm, with a thickness and depth of each prism 2, 3 of approximately 20 mm. These values may of course be adapted according to the desired application of the device.

In the embodiments shown in Figures 1 and 3, the lateral faces 11, 12, 15, 16 of the two prisms 2, 3, including the entry face and the two exit faces described above, are each oriented at an angle of approximately 45° relative to the separating interface 4. The entry face 11 and the exit faces 15, 16 comprise an anti-reflective treatment, preferably broadband.

At least all optical surfaces, i.e. reflecting or transmitting light beams during the implementation of the device, must be polished so as to have a roughness lower than λ/10, in order to guarantee good measurement quality.

• la caractérisation de surfaces optiques de très haute précision, telles que les miroirs de télescopes et les segments de miroirs de grands télescopes,
• le contrôle des processus de polissage mécanique ou chimique de surfaces optiques de précision ou encore la surface de substrats (wafer) dans le secteur de la microélectronique et des semi-conducteurs,
• les systèmes d'imagerie longue portée, satellites d'observation et systèmes de communications optiques par satellite, nécessitant des corrections de faisceaux optiques pour les liaisons entre satellites ou avec la surface terrestre,
• les systèmes dans le domaine de la métrologie XUV et des lignes de lumière avec les extrêmes ultraviolets et les miroirs à rayons X ou dans le secteur de la microphotonique et de l'optique intégrée.

The invention can be used in many interferometric systems for various applications, including:

• the characterization of very high precision optical surfaces, such as telescope mirrors and mirror segments of large telescopes,
• the control of mechanical or chemical polishing processes of precision optical surfaces or the surface of substrates (wafer) in the microelectronics and semiconductor sector,
• long-range imaging systems, observation satellites and optical satellite communications systems, requiring optical beam corrections for links between satellites or with the Earth's surface,
• systems in the field of XUV metrology and beamlines with extreme ultraviolet and X-ray mirrors or in the sector of microphotonics and integrated optics.

Finally, the invention can be advantageously used in the field of cellular biology, in particular for phase imaging of cells or 2-photon light sheet excitation microscopy (SPIM, Selective Plane Illumination Microscop y ).

Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention.

Claims (8)

Mach-Zehnder interferometric device (1), comprising:

• at least a first prism (2) and a second prism (3),
  • the prisms (2, 3) being assembled together so as to form a single block, the prisms (2, 3) forming a beam-splitting interface (4) between them, the beam-splitting interface (4) being parallel to a first external face (5) of the first prism (2) and a second external face (6) of the second prism (3),
  • at least one of the prisms (2, 3) comprising an input face (11) for receiving an incident light beam, the splitter interface (4) being configured to split the incident beam into two partial beams, the first and second external faces (5, 6) each being configured to fully or partially reflect one of the partial beams, and
  • the prisms (2, 3) each comprising an exit face (15, 16) allowing the exit of a first and a second outgoing light beams (20, 21), respectively;
• a spatial filter (9) configured to form a reference wave from one of the partial beams,

the device (1) being configured to interfere the reference beams and one of the partial beams to form the outgoing beams (20, 21). Device (1) according to the preceding claim, characterized in that the first external face (5) is provided with a phase shift layer of thickness λ/8. Device (1) according to claim 1 or 2, characterized in that the beam-separating interface (4) comprises a monolayer of thickness λ/4. Device (1) according to any one of the preceding claims, characterized in that the first and second external faces (5, 6) parallel to the separating interface (4) are provided with reflective or semi-reflective layers. Device (1) according to any one of the preceding claims, characterized in that the output faces (15, 16) are each provided with an anti-reflective layer. Device (1) according to any one of the preceding claims, characterized in that the at least two prisms (2, 3) are assembled to each other by molecular adhesion or by means of an index-adapting glue. Device (1) according to any one of the preceding claims, characterized in that the inlet and/or outlet faces (11, 15, 16) are inclined relative to the separating interface (4) at an angle of 45°. Device (1) according to any one of the preceding claims, characterized in that the spatial filter (9) comprises at least one reflective or semi-reflective pellet deposited on the external face of the second prism (3).