WO2012104515A1

WO2012104515A1 - Aotf device for the programmable acousto‑optical filtering of high spectral resolution and of large angular aperture in the infrared region

Info

Publication number: WO2012104515A1
Application number: PCT/FR2012/050100
Authority: WO
Inventors: Pierre Tournois; Daniel Kaplan
Original assignee: Fastlite
Priority date: 2011-01-31
Filing date: 2012-01-17
Publication date: 2012-08-09
Also published as: FR2971063A1; FR2971063B1

Abstract

The subject of the invention is an AOTF device for the programmable acousto‑optical filtering of high spectral resolution and of large angular aperture in the infrared region, comprising a birefringent acousto‑optical crystal TR, a transducer T ensuring the generation of a transverse acoustic wave, allowing a diffraction of an ordinary incident optical wave 0_o, of incidence θ _o, into an extraordinary optical wave O_d, of angle θ _d, knowing that the abovementioned diffraction is equivalent to a reflection close to the normal to the wave plane of the abovementioned incident optical wave O_o, the abovementioned angle θ _d of the diffracted optical wave O_d is equivalent to the abovementioned angle of incidence θ _o of the incident optical wave 0_o, plus a value lying between 160 and 180 degrees.

Description

AOTF DEVICE FOR PROGRAMMABLE ACOUSTOOPTICULAR FILTERING WITH HIGH SPECTRAL RESOLUTION AND LARGE ANGULAR OPENING IN THE FIELD

INFRARED.

The present invention relates to an Acousto Optic Tunable Filter (AOTF) device for programmable acousto-optic filtering of high spectral resolution and large angular aperture in the infrared range.

The device presented applies to acousto-optical filtering programmable for the medium-infrared range from a wavelength λ in the vacuum of 5 to 30 μπι.

An AOTF device is defined as a device that achieves by acoustooptic diffraction the selection of a single wavelength in the radiation of an incident optical beam. The choice of the wavelength is programmable by varying the frequency of the acoustic signal. To be used with a wide variety of non-collimated sources, the wavelength selection must be independent of the angle of incidence within a given angular aperture. Thanks to their angular aperture, AOTF filters can work with extended sources of white light and provide multispectral images of these sources.

These devices are distinguished from the AOPDF filters described by D.KAPLAN and P. TOURNOIS in: "Theory and Performance of the Acoustic Optically Programmable Dispersive Filter Used for Femtosecond Laser Puise Shaping" J. Phys. IV France 12, Pr5-69 / 75 (2002), which are designed to optimize the spectral resolution for a collimated source, such as a laser and have a low angular acceptance. The main use of these AOPDF filters is the phase and amplitude programming of ultrashort laser pulses.

An AOTF function is theoretically possible to implement simply by using the contra-directive diffraction pattern by a grating in an isotropic material. For such a diffraction, consider the wave vector k ₀ of an incident optical wave, the wave vector k _d of the diffracted wave and the wave vector K of the acoustic wave. All the vectors are parallel to the same one direction, the acoustic wave planes are normal to this direction, the wave vectors k ₀ and k _d being of opposite directions. The diffraction corresponds to a reflection of the incident optical wave by the acoustic network. The conservation of the wave vectors, the optical frequency and the isotropy of the material lead to the relation:

k _d -k ₀ = K

For a small epsilon deviation of the angle of the incident optical wave vector, the length of the vector K has a quadratic dependence on epsilon, which introduces an acoustic frequency dependence sufficiently low to allow the nonzero angular aperture necessary for the function of an AOTF. The length of the acoustic wave vector is twice the length of the optical wave vectors and therefore the acoustic wavelength is two times smaller than the optical wavelength. For optical wavelengths of the order of one micrometer in vacuum and therefore half a micrometer in a material with a typical optical index 2, the acoustic wavelength is a quarter of a micrometer. For standard acousto-optic materials, whose propagation velocities are greater than 1000 m / s, the corresponding acoustic frequencies are higher than 4 GHz. The acoustic attenuation lengths at these frequencies are of the order or less than millimeter, which prevents the practical use of the simple principle defined above; this principle could never be used to realize an AOTF.

In the state of the art, it is the use of anisotropic materials that has allowed the realization of AOTF filters. The optically anisotropic materials have distinct propagation modes having optical indices and different wave vectors for the same wavelength. The acousto-optical interaction thus makes it possible to couple two modes of close but distinct wave vectors. It is then a co-directive diffraction. In this case, the acoustic wave vector has a length that is much shorter than the length of the optical wave vectors. The acoustic frequency is lower and is not a limit to this type of application.

Two categories of AOTF filters in anisotropic materials have been proposed:

1) The so-called collinear filters presented by Harris et al. in US Patent 3,679,288 hereinafter referred to as Harris. In this type of configuration, the three directions of wave vectors, incident optical, diffracted optical and acoustic are parallel or quasi-parallel. For materials such as Lithium Niobate, it is shown that it is possible to satisfy these conditions while maintaining a weak dependence of the filtered wavelength as a function of the orientation of the incident beam.

2) The non-collinear filters have been proposed by [IC Chang "Noncollinear Acousto-Optic Filter with Large Angular Aperture" Applied Physics Letters 25, 370-372 (1974)] and US Patent 5,329,397 hereinafter referred to as Chang. These filters overcome the collinearity condition to allow the use of common performance materials such as Tellurium dioxide whose symmetry does not allow the configuration of collinear filters. Another advantage of non-parallelism is that the incident beam and the diffracted beam can be separated geometrically without the use of polarizers. The above publications and patents and similar literature publications relating to AOTFs in anisotropic materials all relate to co-directive diffraction patterns as shown for example in Fig. 3 of Harris and Figures 3 and 4 of Chang.

It should be noted that the spectral resolution of these co-directive configuration filters decreases when increasing the acoustic wavelength, the relative resolution being given in order of magnitude by the number of periods of the acoustic wave contained in the length interaction. If, for example, the acoustic wave vector is 10 times shorter than the optical wave vector, for an optical wavelength of 10 microns the acoustic wavelength is 100 microns and the number of periods in a wavelength is one centimeter is of the order of 100, corresponding to a resolution of 10 cm -1.

The present invention relates to optical devices in the infrared field (5 to 30 microns). Its principle is to use the contra-directive configuration, with an acoustic wave vector of the order of twice the optical wave vector, to obtain a high resolution. This configuration has never been used in the state of the art to realize AOTFs, because of the higher acoustic frequencies to which it leads. The acoustic frequency is reduced in the present invention by two factors:

the application at high optical wavelengths, and

the use of materials having low acoustic propagation velocities for the chosen orientation.

The mercury halides (Hg ₂ Cl ₂ , Hg ₂ Br ₂ , LfeLJ are tetragonal materials having acoustic propagation velocities in the [110] direction respectively of 347, 282, 253 m / s.

The order of magnitude of the frequencies at 10 microns is then 200 MHz, allowing acoustic attenuation lengths exceeding one centimeter. It should be noted that non-collinear AOTF type filters with co-directive configuration have been proposed Knuteson et al. in "Crystal growth, manufacture, and design of mercurous bromide acousto-optic tunable filters" Opt. Eng. 46, 064001 (2007) and of AOPDF type for sources collimated by P. Tournois: "Design of Acousto-Optic Programmable Filters in Mercury Halides for Mid-Infrared Laser Puise Shaping" Optics Comm. 281, 4054-4056 (2008).

For the contra-directive configuration, the order of magnitude of the acoustic wavelengths for an incident optical wave at 10 microns is then 2.5 microns and the number of period 4000 for one cm, ie a resolution of 0.25 cm -1, improved. by a factor of 40 relative to a non-collinear filter of the same length. Moreover, the use of tetragonal birefringent anisotropic materials will make it possible to incorporate the advantage of the selection of the diffracted beam by polarization and the geometrical direction.

As will be apparent in the analysis below, the detailed design of the device in a material requires a slight deviation from the perfect contra-directive configuration. The orders of magnitude discussed above, however, remain valid.

The halides of Mercury such as Calomel (Hg ₂ Cl ₂ ), Mercury Bromide (Hg ₂ Br ₂ ) and Mercury lodide (Hg ₂ I ₂ ) belong to the tetragonal crystalline class. Ordinary optical indices n ₀ and extraordinary n _e are respectively in the middle infrared: n = 1.898, 2.05 and 2.26 and n _e =

2,445, 2.75 and 3.21. The current extraordinary index n _d> along an axis which makes an angle 9 _d with the axis [110], is written: 2 cos 2 sin ² 6> (1)

For crystals of the tetragonal class, a pure transverse acoustic wave propagates in the plane containing the axes [110] and [001] with a polarization perpendicular to this plane, called the diffraction plane. Its speed is given by:

¾) = ^ ₀ cos ² ¾ + F ₀ ² ₀₁ sin ² ¾ (2)

V _m = 347, 282, 253 m / s and V _m = 1084, 1008, 871 m / s being the acoustic velocities respectively of Calomel, Mercury Bromide and Mercury Plodide along axes [110] and [001] and 9 _At the angle that the acoustic wave vector makes with the axis [110]. The propagation of acoustic energy is done at an angle β _Α such that

tan ^ = (F ₀₀₁ / K _No.) .Tan ^ ² (3) In the case of a counter-directive diffraction in such a material, an ordinary wave characterized by a wavevector k ₀ 9 ₀ direction with axis [110], and an extraordinary wave vector wave / ¾ direction ¾ with the axis [110], where / ¾ - θ ₀ is close to 180 degrees, are coupled by an acoustic wave whose length K wave vector is close to 2 times the length of k ₀ and whose direction is close to a parallel to k ₀ and / ¾.

Thus the optical wave resulting from the acousto-optical interaction between an incident optical wave and a transverse acoustic wave, is of polarization perpendicular to that of said incident optical wave and propagates in a contra-directive direction to the incident wave. The optical direction θ _α and the acoustic propagation direction Θ _Α set the optical direction 9 _d by the conservation condition of the wave vectors and the frequency. The detailed design of the device, according to the invention, therefore requires searching for the combinations θ ₀ and Θ _Α minimizing the dependence of the filtered optical frequency as a function of the optical orientations. The subject of the invention is an AOTF device for spectral high-resolution, high-angle programmable infrared optical acousto-optic filtering, comprising:

a birefringent acousto-optical crystal TR consisting of a tetragonal-type birefringent mercury halide having a minimum propagation velocity of less than 400 m / s in the crystalline direction

[110] perpendicular to the birefringence axis [001],

a transducer T ensuring the generation of a transverse acoustic wave of a wave vector K, propagating in a plane P containing the aforesaid crystalline direction [110] and the aforementioned birefringence axis [001], allowing a diffraction of a ordinary incident optical wave O ₀ , of incidence θ ₀ with respect to said crystalline direction [110], propagating in said plane P with a polarization perpendicular to said plane P, in an extraordinary optical wave O _d , of angle δ par by relative to said crystal direction [110], propagating in said plane P with a polarization in said plane P, or to produce an inverse diffraction of an extraordinary incident optical wave propagating in said plane P with a polarization in said plane P in an ordinary optical wave propagating in the plane P with a polarization perpendicular to said plane P,

knowing that :

the aforesaid diffraction is equivalent to a reflection close to the normal to the wave plane of the said incident optical wave O ₀ , the aforesaid angle θ _d of the diffracted optical wave O _d is equivalent to the aforesaid incidence θ ₀ of the an incident optical wave O ₀ , increased by a value between 160 and 180 degrees, and, the direction of propagation of the said transverse wave vector waveform K is chosen to be equal to the aforesaid angle of incidence 0 _O of propagation of the ordinary incident optical wave 0 ₀ with respect to the direction [110] multiplied by a factor η whose value is close to 0.8.

One embodiment of the device 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 of the wave vector diagram of the acousto-optical interaction in a contra-directive configuration; Figure 2 shows the equi- (/ X) curves in a diagram

(Θ ₀ , Θ _Α ) for the calomel;

FIG. 3 represents the variation of the quality factor M ₂ as a function of θ _α for the three halides;

4 shows the change in the filtered wavelength as a function of θ ₀ for three halides around the value of 9 ₀ optimizing M2; and FIG. 5 is a schematic representation of a filter structure

Programmable Acousto-Optics AOTF for the infrared range.

The example shown in Figure 1, relates to the diagram of the wave vectors of the interaction. From an orthonormal coordinate system Ox, Oy, the axes [1 10] and [001] of the birefringent acousto-optical crystal are represented, respectively along the axes Ox, Oy; the plane defined by the two axes [110] and [001] will be called diffraction plane P.

Let C _{0 be} the ordinary index indicator curve and C _e the extraordinary index indicator curve; these two curves C ₀ , C _e , respectively cut the axis [110] into n ₀ : ordinary index and n _e : extraordinary index.

Let k _{0 be} an optical wave vector characterizing an ordinary wave polarized perpendicularly to the diffraction plane, of origin O, ending at point A on the curve C ₀ , and making an angle θ ₀ with the axis [110] and K a counter-directive acoustic wave vector starting from A, making an angle Θ _Α with the axis [110] and of length AB, where B is the point of intersection with the curve Ce. The vector OB defines a vector k _d , which satisfies the conservation of the wave vectors. The vector acoustic wave K diffracts the ordinary vector wave k ₀ into an extraordinary vector wave / ¾ and vice versa. This configuration can be used to produce a filter having as input the ordinary wave and filtered output the extraordinary wave or vice versa. To fix the design of the AOTF, it is necessary to study the dependence of the filtering as a function of the direction of optical propagation. As such, the following formula given by P.Tournois in: "Design of Acousto-Optic Programmable Filters in Mercury Halides for Mid-Infrared Laser Puise Shaping" Optics Comm. 281, 4054 ^ 056 (2008), relates to the length of the acoustic wave vector K as a function of θ ₀ and Θ _Α .

where: S = {n - n) l2n (5) The filtered wavelength is given by the relation:

\ l λ characterizes the filtered optical frequency expressed in cm-1. Figure 2 shows the equi- (/ X) curves in a diagram {Θ ₀ , Θ _Α ) for the material Hg ₂ Cl ₂ .

In this diagram, the horizontal tangent points correspond to configurations where the filtered optical frequency is independent of the first optical order P (angle θ _α ).

In dashed lines in the figure, a line Δ, defined by Θ _Α = η.θ ₀ , is represented with η = 0.79. This straight line Δ joins the horizontal tangent points with a good approximation and can therefore be used as a design criterion for a high angular aperture AOTF.

An examination of similar diagrams for the Bromide and Mercury lodide materials leads to a similar conclusion, subject to η values of 0.76 and 0.72 respectively for the Mercury Bromide and Plodide. The merit factor M ₂ which characterizes the intensity of the acousto-optic interaction and which determines the diffraction efficiency is given by:

where: p = 7.19, 7.307, 7.70 g / cm are the densities of Calomel, Bromide and Mercury Plodide and p is the effective elasto-optical coefficient given by:

p = - [(p _n -p _n ) / 2] sm0 _o cos0 _A + p ₄₄ cos9 _o sm0 _Â (8) with: _Al = 0.551, p _n = 0M, p ₄₄ = 0 (9) In the example represented in FIG. 3, the variation of M ₂ as a function of θ ₀ , for the condition Θ _Α = η.θ ₀ , is indicated for the three halides using the values of η mentioned above. M ₂ passes, for θ ₀ = 25 ° approximately, by a maximum of: 53, 147 and 485 mm / GW respectively for the Calomel (curve C1), the Mercury Bromide (curve C2) and the Iodide of Mercury (curve C3). This 25 degree angle is a preferable choice for the invention.

For this particular angle θ ₀ of 25 °:

θ _ά = 195.70 °, 194.53 °, 193.01 °,

Θ _Α = 19.81 °, 19.06 °, 18.03 °,

Δ = 74.12 °, 77.24 °, 75.47 °,

V = 491.5, 423.5, 361.4 m / s and

(/ Λ) = 2100, 1997, 1938 m / s,

respectively for the three halides or the curves Ci, C ₂ , C ₃ respectively concerning Calomel (Hg ₂ Cl ₂ ), Mercury Bromide (Hg ₂ Br ₂ ) and Mercury Iodide (Hg ₂ I ₂ ).

In the example represented in FIG. 4, (f ₂ A) is represented as a function of <9 ₀ , around θ ₀ = 25 °, for the three halides, namely the curves Ci, C ₂ , C ₃ respectively concerning the Calomel (Hg ₂ Cl ₂ ), Mercury Bromide (Hg ₂ Br ₂ ) and Mercury Iodide (Hg ₂ I ₂ ).

A very wide independence of the filtered wavelength λ as a function of the angle of incidence exists around θ ₀ = 25 °.

In the example shown in FIG. 5, an AOTF device for spectrophotometer and infrared high-aperture programmable acousto-optical filtering is produced in a Calomel crystal. This device involves a birefringent acousto-optical crystal represented schematically by a rectangle trapezium TR, located in an acousto-optical interaction plane P, which is defined by the two axes [110] and [001] of said acoustic crystal. optical.

The AB side is parallel to Tax [001]; the sides BC and FA are parallel to the axis [110]; CF side 9 forms an angle _A of 19.81 ° with the line segment AB. The CF side comprises a piezoelectric transducer T of length CD. Said piezoelectric transducer T generates a transverse acoustic beam whose shear wave is polarized perpendicular to said interaction plane P; this beam is represented schematically by two straight line segments CC and DD '; point C is located on line segment AB, and point D 'is on line segment FA; the two line segments CC and DD 'make an angle β _Α of 74.12 ° with the line segment BC.

An incident ordinary optical beam O ' ₀ , parallel to the plane of interaction P, of polarization perpendicular to said plane P, is applied to an input face S of the acousto-optical crystal TR, which input face S is represented by the aforesaid line segment AB; the incidence θ ' _{0 of} said incident ordinary optical beam 0' ₀ is 53.33 ° with respect to the axis [110].

This incident beam 0 ' ₀ enters the acousto-optical crystal TR; a beam 0 ₀ inside the crystal is represented diagrammatically by two parallel line segments, respectively AE and A'D; the angle θ _{0 of} said optical beam 0 ₀ is 25 ° relative to the axis [110].

An extraordinary optical beam Ο ^ is generated by interaction with the above-mentioned acoustic beam in the acousto-optical crystal; the angle / ¾, represented by an angle (9 _d - π), of said optical beam O _d is 195.70 ° with respect to the axis [110]. The aforesaid extraordinary optical beam O _d , after interaction in the acousto-optical crystal is then extracted from the acousto-optical crystal, represented by an extraordinary optical beam 0 '_d; the angle θ ' _ά represented by an angle (θ' _ά - π), of said optical beam 0 ' _d is 225.25 ° relative to the axis [110].

Advantageously, this device, according to the invention, makes it possible to produce either a diffraction of an ordinary incident optical wave 0 ₀ , of incidence θ ₀ with respect to said crystalline direction [110], propagating in said plane P with a polarization perpendicular to said plane P, in an extraordinary optical wave O _d , of angle θ with respect to said crystalline direction [110], propagating in said plane P with a polarization in said plane P, namely a reverse diffraction of a wave extraordinary incident optical propagating in said plane.P with a polarization in said plane P in an ordinary optical wave propagating in the plane P with a polarization perpendicular to said plane P.

By way of exemplary embodiment, from a Calomel crystal of dimension allowing an acousto-optical interaction length of 1 cm, the acoustic frequency being 210 MHz for a wavelength of 10 microns, the density the power to be applied to the acoustic transducer for total diffraction of each of the wavelengths is about 2.5 W / mm ² at 5 μηι and 10 W / mm at 10 μηι.

The spectral resolution is: 0.5 nm to 5 μηι and 2 nm to 10 μηι is: 0.2 cm ^"1. Without changing this resolution, the angular acceptance angle is: 1.5 ° to 5 μιη and 3 ° to 10 μηι .

Claims

An AOTF spectrophotometer-programmable high-resolution acousto-optic filtering device with large angular aperture in the infrared range, comprising:

a birefringent acousto-optical crystal (TR) consisting of a tetragonal-type birefringent mercury halide having a minimum propagation speed of less than 400 m / s in the crystalline direction [110] perpendicular to the birefringence axis [001 ]

a transducer (T) ensuring the generation of a wave vector transverse acoustic wave (K), propagating in a plane (P) containing the aforesaid crystalline direction [110] and the aforementioned birefringence axis [001], allowing a diffraction of an ordinary incident optical wave (0 ₀ ), incidence (θ ₀ ) with respect to said crystalline direction [110], propagating in said plane (P) with a polarization perpendicular to said plane (P), in an extraordinary optical wave (O _d ), of angle (<¾) with respect to said crystalline direction [110], propagating in said plane (P) with a polarization in said plane (P), or to produce a diffraction inverse of an extraordinary incident optical wave propagating in said plane (P) with a polarization in said plane (P) into an ordinary optical wave propagating in the plane (P) with a polarization perpendicular to said plane (P),

characterized in that the aforesaid diffraction is equivalent to a reflection close to the normal of the wave plane of said incident optical wave (0 ₀ ), the aforesaid angle (<¾) of the diffracted optical wave (Oa) is equivalent at the aforesaid incidence (θ ₀ ) of the incident optical wave (0 ₀ ), increased by a value between 160 and 180 degrees, and in that the direction of propagation of the aforesaid transverse wave vector acoustic wave (K) is chosen equal to the aforementioned angle of incidence (θ ₀ ) of propagation of the ordinary incident optical wave (0 ₀ ) relative to the direction [110] multiplied by a factor η whose value is close to 0.8.

2. Device according to claim 1,

characterized in that the aforesaid transducer (T) is a piezoelectric transducer disposed on a surface of the birefringent acousto-optical crystal (TR), perpendicular to the aforesaid plane (P).

3. Device according to claim 1,

characterized in that the value of the factor η is 0.79 for a birefringent mercury halide of the tetragonal class such as Calomel (Hg ₂ Cl ₂ ).

4. Device according to claim 1,

characterized in that the value of the factor η is 0.76 for a birefringent mercury halide of the tetragonal class such as Mercury Bromide (Hg ₂ Br ₂ ).

5. Device according to claim 1,

characterized in that the value of the factor η is 0.72 for a birefringent mercury halide of the tetragonal class such as Mercury Iodide (Hg ₂ I ₂ ).

6. Device according to claim 1,

characterized in that the value of (6> ₀ ) is approximately equal to 25 degrees.