FR2943433A1 - Acousto-optics filtering method for e.g. acousto-optic tunable filter, involves forming reflexions on one another of reflective faces of acousto-optic perpendicular to axes common to all curves - Google Patents

Abstract

The method involves utilizing a piezo-electric tranducer (T1) for generating a transverse acoustic wave whose energy is propagated in a colinear manner to energy of incidental optical wave (Oi1). The waves have multiple reflexions on one another of reflective faces (AB, BC, CD, OF, EA) of a bi-refringent crystal acousto-optic perpendicular to symmetry axes common to a curve of acoustic slownesses and curves of optical indices ordinary and extraordinary of the crystal. The acousto-optics is made of tellurium dioxide, calomel and potassium dihydrogen phosphate. An independent claim is also included for a device for acousto-optics filtering, comprising reflective faces.

Description

The present invention relates to a method and a device for acousto-optical filtering of great length of optical and acoustic interaction.

This method applies in particular to devices whose configuration is such that the direction of propagation of the energy of the optical beam (referred to as the direction of the optical beam and characterized by the Poynting vector) coincides with the direction of propagation of the optical beam. energy of the acoustic beam (called direction of the acoustic beam). This condition ensures the greatest possible interaction length between the optical and acoustic waves, which is favorable for interaction efficiency. Moreover, when these devices 20 are used for a spectral filtering function of the optical wave, this condition promotes obtaining a high spectral resolution.

In general, it is known that the conditions leading to colinear interaction in a birefringent material have been discussed by V.B. Voloshinov 25 in: Close to collinear acousto-optics interaction in Paratellurite, Optical Engineering, 31 (1992), p. 2089. It is well known that when the crystal used is highly anisotropic and birefringent, the collinearity of the acoustic and optical beams does not imply the collinearity of the acoustic and optical wave vectors. French patent FR 9610717 of P. 30 Tournois Device for controlling light pulses by an acousto-optical programmable device and the publication of D. Kaplan and P.

Theory and performance of the acousto-optic programmable dispersive filter used for femtosecond laser pulse shaping, J. Phys. IV, 12 (2002), Pr5-69 / 75, describe the use of a collinear configuration for performing programmable spectral filtering in amplitude and phase of laser pulses. Tellurium dioxide or Paratellurite, a highly anisotropic material, is the most used for this application.

The lengthening of the length of the device to increase the interaction length is technologically limited by the existing capacities to date of crystal growth. With regard to tellurium dioxide, for example, the available devices are limited to a few centimeters in length. One solution is to fold the beams by optical and acoustic reflections on the crystalline faces that maintain the optical and acoustic collinearity. Nevertheless, the anisotropic nature of the crystal and the very large difference in wave and beam vector directions will, in general, have the effect that collinear beams before reflection will no longer collinear after reflection.

The only crystalline classes that satisfy these requirements are those for which the optical and acoustic axes of symmetry are combined, such as, for example, the tetragonal crystalline classes 422, 4 / mm and 4 / 2m.

The materials combining this condition and the adequate performance for such an application are: tellurium dioxide (TeO2), mercury halides (Hg2Cl2, Hg2Br2, Hg2I2), and KDP; among these materials, only tellurium dioxide (TeO2), Calomel (Hg2Cl2), and KDP are currently susceptible to industrial exploitation.

The subject of the invention is therefore a method and a device for acousto-optical filtering of great length of optical and acoustic interaction; it proposes, for this purpose, the use of a birefringent acousto-optical crystal whose propagation speed of the acoustic waves is the lowest possible, which acousto-optical crystal comprises on one of its faces, a piezoelectric transducer intended generating a transverse acoustic wave whose energy is propagated collinearly to the energy of an incident optical wave, all along the path of said incident optical wave, in the above-mentioned birefringent acousto-optical crystal, knowing that the transverse acoustic wave and the incident optical wave traverse a path comprising multiple reflections on one or the other of the reflective faces of the birefringent acousto-optical crystal perpendicular to the axes of symmetry common to the curve of the acoustic slowness and to the curves of the indices ordinary and extraordinary optics of said acousto-optical crystal.

One embodiment of the method according to the invention will be described below, by way of non-limiting example, with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation for an anisotropic crystal of the curve of acoustic delays and curves of the ordinary and extraordinary optical indices defining the composition of the acoustic and optical wave vectors, characteristic of the acousto-optical interaction; FIG. 2 is a schematic representation of a first example of reflection of the acoustic and optical beams on an oblique plane, parallel to the axis Oy of the birefringent crystal;

FIG. 3 is a schematic representation of a second example of acoustic and optical beam reflection on a plane parallel to the Oy and Ox axes of the birefringent crystal; FIG. 4 represents a first example of a schematic representation of an acousto-optical filter structure of great length of optical and acoustic interaction in the tellurium dioxide; FIG. 5 represents a second example of a schematic representation of an acousto-optical filter structure of great length of optical and acoustic interaction in the Calomel, and

FIG. 6 shows a third example of a schematic representation of an acousto-optic filter structure of great length of optical and acoustic interaction in the KDP.

In the example shown in FIG. 1, the schematic representation of the curves of ordinary and extraordinary optical indices (upper dials) and the curve of acoustic delays (lower dials) shows, in the orthonormal system defined by the Ox and Oz axes. birefringent crystal, the acoustic wave and incident optical wave vectors, respectively ka and ko; the acoustic wave vector ka makes an angle 0a with the axis Ox; the incident optical wave vector 20 is at an angle θ with the axis Ox. The optical Poynting vector Ko is collinear with the incident optical wave vector ko; the acoustic Poynting vector Ka is parallel to the optical vector Ko and therefore has an angle θ with the axis Ox.

In the example shown in FIG. 2, the schematic representation of a first example of reflection of the acoustic and optical beams on an oblique plane plane parallel to the Oy axis of a birefringent crystal of Tellurium dioxide (TeO2) shows, in the orthonormal system defined by the Ox and Oz axes of the birefringent crystal, the directions of the incident optical and acoustic energies 30 and the directions of the optical and acoustic energies after reflection on an oblique plane P. In the present case, the plane P is parallel to the axis Oy and is at an angle of 45 ° with respect to the axis Ox; the incident optical and acoustic energies Eoi, Eai, are at an angle of 60 ° with respect to the Ox axis; the reflection of the optical energies Eor and acoustic Ear is not done according to the same directions. In the example shown in FIG. 3, the schematic representation of a second example of reflection of the acoustic and optical beams on a plane parallel to the Oy and Ox axes of the birefringent telluric dioxide (TeO 2) crystal, shows, in FIG. orthonormal system defined by the Ox and Oz axes of the birefringent crystal, the directions of the incident optical and acoustic energies and the directions of the optical and acoustic energies after reflection on a plane P. In the present case, the plane P is parallel to the axis Oy and Ox axis; the incident optical and acoustic energies Eoi, Eai, are at an angle of 60 ° with respect to the Ox axis; the reflection of the optical and acoustic energies Eor, Ear, is in the same direction.

In the example shown in FIG. 4, an acoustic-optical filter structure of great length of optical and acoustic interaction involves an acousto-optical crystal of tellurium dioxide (TeO2), represented schematically by its polygonal section PO1 by a plane perpendicular to the axis Oy and called plane of propagation P.

The orientation of the acousto-optic crystal is defined by its two axes [110] and [001]. Since the propagation plane P is orthonormed respectively along Ox and Oz, the axis Ox is parallel to the axis [110], and the axis Oz is parallel to the axis [001].

The optical propagation angle θ with respect to the direction Ox is chosen according to a functional criterion. In the example of FIG. 4, it is chosen to maximize the merit factor of the diffraction efficiency M2 given approximately by the formula: M2 = no3 ne3 p2 / p V3, in which: no, ne p, p and V are respectively the ordinary index, the extraordinary index, the effective elasto-optical coefficient, the crystal density and the phase velocity of the transverse acoustic waves in the direction Oa which corresponds to the direction 9 of propagation of acoustic energy.

The phase velocity of the transverse acoustic waves V is given by: V = [Vx2 cos' 9Q + VZ2 sine 0.] I / 2, with: tape, = (VX / V) 2. tan9, which is the alignment condition of the optical and acoustic energies, VX being the acoustic velocity according to Ox, and VZ being the acoustic velocity according to Oz.

For the TeO 2, Hg 2 Cl 2 and KDP crystals, the angles 9 are respectively close to 60 °, 50 ° and 45 °.

The crystals considered have a prismatic shape, defined by their polygonal cross section by a plane parallel to the plane of propagation P and by the direction common to their edges perpendicular to the plane of propagation. The faces of interest of these crystals are those parallel to the edges and containing a given segment of the cross section. In the following, we designate the crystals considered by their cross section and the crystal faces by the corresponding segment of cross section.

In the first example of FIG. 4, the crystal PO1 of TeO2, defined by the polygon PO1 with vertices ABCDEF, comprises a first face AB containing the segment AB, A along the axis Oz close to the point O and B along the axis Ox close to the point 0, a second face BC along the axis Ox, then a third face CD perpendicular to the axis Ox, then a fourth face DE perpendicular to the axis Oz, then a fifth face EF making an angle 01 with the normal to the axis Oz, then a sixth face FA closing the polygonal section PO1.

The face AB of the crystal PO1 constitutes an input face Fei on which is applied at a point MO, perpendicular to said input face Fei, an incident optical beam O; i, polarized perpendicular to the propagation plane P containing said polygonal section PO1; the incident optical beam Or as well as the corresponding wave vector koi are collinear with the normal to the face AB.

A transducer Ti, located on the face FA, generates a transverse acoustic beam, whose vibrations are perpendicular to the plane of propagation P; this acoustic beam ends at the point MO of said input face Fei, then is reflected so that the corresponding acoustic Poynting vector is perpendicular to the aforesaid face AB.

Thus, the reflected acoustic beam and the aforesaid incident optical beam OH travel through a first collinear interaction zone Z1 between the point MO of the face AB and a reflection point M1 on the face DE, then traverse a second collinear interaction zone. Z2 between the point M1 on the face DE and a reflection point M2 on the face CD, then traverse a third colinear interaction zone Z3 between the point M2 on the face CD and a reflection point M3 on the face BC, then traverse a fourth collinear interaction zone Z4 between the point M3 on the face BC and a point M4 on the face EF, which constitutes the output face Fsi of the reflected optical beam OS1.

In each of the interaction zones, the colinear propagation direction is 0 or -6; given the elements defined above, the incident ordinary optical wave vector ko1 makes an angle θ close to 60 ° with the axis [110]. The interaction length of the optical and acoustic waves has been multiplied by a factor close to 3 with respect to the length of the crystal whose height is defined by the distance between the aforementioned faces BC and DE.

In the example shown in FIG. 5, an acoustooptic filter structure of great length of optical and acoustic interaction involves an acousto-optical Calomel crystal (Hg2C12) schematically represented by its polygonal section PO2, situated in the propagation plane P. The orientation of the acousto-optic crystal is defined by its two axes [110] and [001]. Since the propagation plane P is orthonormed respectively along Ox and Oz, the axis Ox is parallel to the axis [110], and the axis Oz is parallel to the axis [001]. The crystal PO2 comprises a first face AB, A along the axis Oz close to the point O and B along the axis Ox close to the point 0, a second face BC along the axis Ox, then a third face CD perpendicular to the Axis Ox, then a fourth face DE perpendicular to the axis Oz, then a fifth face EF 20 making an angle O2 with the normal to the axis Oz, then a sixth face FA closing the polygonal section PO2, of vertices ABCDEF.

The face AB of the crystal PO2 constitutes an input face Fe2 on which is applied at a point MO, perpendicular to said input face Fe2, an incident optical beam Oi2, polarized perpendicularly to the propagation plane P containing said polygonal section PO2 ; the incident optical beam Oe and the corresponding wave vector ko2 are collinear with the normal to the face AB.

A T2 transducer, located on the FA face, generates a transverse acoustic beam whose vibrations are perpendicular to the plane of propagation P; This acoustic beam ends at the point MO of said input face Fez, and is then reflected so that the corresponding acoustic Poynting vector is perpendicular to the aforesaid face AB.

Thus, the reflected acoustic beam and the aforesaid incident optical beam 012 travel through a first collinear interaction zone Z1 between the point MO of the face AB and a reflection point M1 on the face DE, then traverse a second interaction zone. colinear Z2 between the point M1 on the face DE and a reflection point M2 on the face CD, then traverse a third collinear interaction zone Z3 between the point M2 on the face CD and a reflection point M3 on the face BC and then traverse a fourth colinear interaction zone Z4 between the point M3 on the face BC and a point M4 on the face EF, which constitutes the exit face Fs2 of the reflected optical beam Os2.

In each of the interaction zones, the colinear propagation direction is 8 or -8; given the elements defined above, the incident ordinary optical wave vector ko1 makes an angle 8 close to 50 ° with the axis [110].

The interaction length of the optical and acoustic waves has been multiplied by a factor close to 3 with respect to the length of the crystal whose height is defined by the distance between the said faces BC and DE.

In the example shown in FIG. 6, an acousto-optical filter structure of great length of optical and acoustic interaction involves an acousto-optic crystal of KDP, represented schematically by its polygonal section PO3, situated in the propagation plan P; the proposed structure is different from those previously described, given the characteristics of anisotropy and birefringence of this material. The orientation of the acousto-optic crystal is defined by its two axes [100] and [001]. The propagation plane P being orthonormed respectively according to Ox and Oz, the axis Ox is parallel to the axis [100], and the axis Oz is parallel to the axis [001]. The crystal P03 comprises a first face AB, A along the axis Oz and B along the axis Ox, a second face BC perpendicular to the axis Ox, then a third oblique face CD at an angle θ with the normal to the axis Oz, then a fourth face DE perpendicular to the axis Oz, then a fifth face EA 10 closing the polygonal section P03, vertex ABCDE.

The face AB of the crystal P03 constitutes an input face Fe3 on which is applied at a point M0, perpendicular to said input face Fe3, an incident optical beam O13, polarized perpendicular to the propagation plane P containing said polygonal section P03. ; the incident optical beam Oi3 and the corresponding wave vector ko3 are collinear with the normal to the AB face.

A transducer T3, located on the CD face, generates a transverse acoustic beam whose vibrations are perpendicular to the plane of propagation P; this acoustic beam ends at the point MO of said input face Fe3, then is reflected so that the corresponding acoustic Poynting vector is perpendicular to the aforesaid face AB.

Thus, the reflected acoustic beam and the aforesaid incident optical beam Oi3 traverse a first collinear interaction zone Z1 between the point MO of the face AB and a reflection point M1 on the face BC, then traverse a second interaction zone. colinear Z2 between the point M1 on the BC face and a reflection point M2 on the DE face, then travel through a third collinear interaction zone Z3 between the point M2 on the DE face and a reflection point M3 on the EA face then traverse a fourth collinear interaction zone Z4 between the M3 point on the EA face and a M4 point on the BC face, and then traverse a fifth colinear interaction zone Z5 between the M4 point on the BC face. and a point M5 on the face AB, which constitutes the output face Fs3 of the reflected optical beam Os2, said output face Fs3 being merged with said input face Fei.

In each of the interaction zones, the collinear propagation direction is 8 or -0; in view of the elements defined above, the incident ordinary optical wave vector kol makes an angle θ close to 45 ° with the axis 10 [100].

The interaction length of the optical and acoustic waves has been multiplied by a factor close to 5 with respect to the length of the crystal whose height is defined by the distance between the aforementioned faces BC and EA. According to the three examples cited above, the solution consisting in folding the beams by optical and acoustic reflections on the crystalline faces of the birefringent acousto-optical crystal makes it possible to multiply by a factor close to or even greater than 3 with respect to the length of said crystal; this method 20 thus allows the significant increase in the length of optical and acoustic interaction while respecting the economic constraints related to the production of such crystals.

Advantageously, the aforesaid reflective faces (AB, BC, CD, DE, EA) may or may not comprise thin dielectric layers or thin metal films.

Advantageously, the aforesaid piezoelectric transducer (T1, T2, T3) intended to generate a transverse acoustic wave will be a transducer welded to one face (FA, CD) of the birefringent acousto-optical crystal (PO1, PO2, PO3). A first application of this acousto-optical acousto-optical long-distance acousto-optic filter, according to the invention, relates to the stents of frequency drift lasers such as those described in the article by D. Strickland and G. Mourou: Compression of amplified chirped optical 5 pulses, Optics Communications, 56 (1985), p.219, which make it possible to generate short pulses of very high power. In this type of laser, a programmable expander in amplitude and phase of long extension time is desirable to compensate for amplitude and phase defects of the compressors. A second application of this acousto-optical acousto-optical long-distance acousto-optic filter, according to the invention, relates to spectral analyzers which use fast and compact Acousto-Optic Tunable Filters (AOTF) acousto-optical filters. In this type of filter, a large length of acousto-optical interaction makes it possible to significantly increase the spectral resolution of these filters.

A third application of this acousto-optical acousto-optical long-distance acousto-optic filter, according to the invention, relates to the generation of multiple short light pulses, adjustable time spacings over a very long duration, obtained by programming. simultaneous multiple acoustic signals in the acousto-optical filter.

25 30

Claims (10)

REVENDICATIONS1. A method and a device for acousto-optical filtering of great length of optical and acoustic interaction comprising a birefringent acoustooptical crystal (PO1, P02, P03) whose acoustic wave propagation speed is as low as possible, which acousto-optical crystal (PO1, P02, P03) comprises on one of its faces (FA, CD), a piezoelectric transducer (T1, T2, T3) intended to generate a transverse acoustic wave whose energy is propagated collinearly to the energy of an incident optical wave (Oil, ° 12, O13), all along the path of said incident optical wave (Oil, Oie, O13), in the aforesaid birefringent acousto-optical crystal (PO1, P02, P03 ), characterized in that the transverse acoustic wave and the incident optical wave (On, Oie, Oi3) traverse a path comprising multiple reflections on one or the other of the reflecting faces (AB, BC, CD, DE , EA) of the birefringent acousto-optical crystal (PO1, P02, P03) perpen diculaires to axes of symmetry common to the curve of acoustic slowness and the curves of the ordinary and extraordinary optical indices of said acousto-optical crystal (PO1, P02, P03). 2. Method according to claim 1, characterized in that the aforesaid acousto-optic crystal (PO1, P02, P03) is part of the tetragonal crystalline classes 422, 4 / mmm, and 4 / 2m. 3. Method according to claims 1 and 2, characterized in that the aforesaid acousto-optic crystal (PO1) is tellurium dioxide (TeO2). 2943433 - 14 - 4. Method according to claims 1 and 2, characterized in that the aforesaid acousto-optic crystal (PO2) is Calomel (Hg2C12). 5 5. Method according to claims 1 and 2, characterized in that the aforesaid acousto-optical crystal (PO3) is KDP. 6. Application of the method according to any one of the preceding claims to the production of stents of frequency drift lasers. 7. Application of the method according to any one of the preceding claims to the production of AOTF acousto-optical filter spectrum analyzers. 15 8. Application of the method according to any one of the preceding claims to the production of multiple short light pulse generators of adjustable time spacings. 9. Device for implementing the method according to claim 1 for acousto-optic filtering of great length of optical and acoustic interaction, characterized in that the aforesaid reflective faces (AB, BC, CD, DE, EA). include thin dielectric layers or thin metal films. 10. Device according to claim 9, characterized in that the aforesaid piezoelectric transducer (T1, T2, T3) for generating a transverse acoustic wave is a transducer welded on one side (FA, CD) acousto-optical crystal birefringent (PO1, PO2, PO3). 10