Device for shaping a laser beam for an atomic sensor having retroreflector prism, linear polarizer, and form birefringence plate

Abstract

Device for shaping a laser beam for an atomic sensor, comprising a retroreflector prism with an entrance area, an exit area and two reflection surfaces forming a right angle therebetween. The device for shaping includes a linear polarizer disposed upstream from the entrance area of the retroreflector prism for linearly polarizing a laser beam entering into the retroreflector prism by the entrance area, and a form birefringence plate disposed downstream from the exit area of the retroreflector prism to circularly polarize a laser beam exiting from the retroreflector prism by the exit area.

Claims

1. A laser beam shaping device intended in particular to be included in an atomic sensor such as an atomic clock, atomic magnetometer or atomic gyrometer by being combined both with a laser beam emission laser and also an alkaline vapor cell comprising a cavity filled with a gas into which a shaped laser beam could enter and a photo detector for receiving a laser beam having entered the cell, the shaping device comprises a retroreflector prism comprising an entrance area for an entering laser beam, an exit area for an exiting laser beam and also two internal laser beam reflection surfaces, said reflection surfaces forming a right, angle therebetween, wherein the shaping device further comprises a linear polarizer disposed upstream from the entrance area of the retroreflector prism so as to be traversed by a laser beam entering the retroreflector prism by the entrance area and able to linearly polarize said entering laser beam, and a form birefringence plate disposed downstream from the exit area of the retroreflector prism so as to be traversed by a laser beam exiting the retroreflector prism by the exit area and able to circularly polarize said exiting laser beam in order to obtain a shaped laser beam.

2. The device according to claim 1, wherein the linear polarizer comprises a sub-wavelength metal network, and wherein the sub-wavelength metal network of the linear polarizer comprises a plurality of metal plates extending parallel to each other along a metal plate direction and being juxtaposed one relative to the other along a passing direction of the metal network perpendicular to the metal plate direction.

3. The device according to claim 2, wherein the passing direction of the metal network is parallel to a direction of intersection between the respective geometric planes of extension of the two reflection surfaces.

4. The device according to claim 1, wherein the form birefringence plate comprises a zero-order sub-wavelength dielectric network, and Wherein the zero-order sub-wavelength dielectric network of the form birefringence plate comprises a plurality of dielectric plates extending parallel to each other along a dielectric plate direction and being juxtaposed one relative to the other along a dielectric network direction perpendicular to the dielectric plate direction.

5. The device according to claim 4, wherein the linear polarizer comprises a sub-wavelength metal network, wherein the sub-wavelength metal network of the linear polarizer comprises a plurality of metal plates extending parallel to each other along a metal plate direction and being juxtaposed one relative to the other along a passing direction of the metal network perpendicular to the metal plate direction, and wherein, the dielectric plate direction forms an angle of 45° with the metal plate direction.

6. The device according to claim 4, wherein the passing direction of the metal network is parallel to a direction of intersection between the respective geometric planes of extension of the two reflection surfaces, and wherein the dielectric plate direction forms an angle of 45° with the metal plate direction.

7. The device according to claim 1, wherein the linear polarizer and the form birefringence plate are disposed side-by-side on a glass plate—or wafer.

8. The device according to claim 1, wherein the entrance area and the exit area of the retroreflector prism form two portions of the retroreflector prism transmission surface and in which the linear polarizer and the form birefringence plate are placed facing said transmission surface, which are either formed on said transmission surface or formed on a glass plate—or wafer—placed facing said transmission surface.

9. An atomic sensor comprising a device for shaping a laser beam according to claim 1, combined, compactly, both with a laser beam emission laser and also an alkaline vapor cell comprising a cavity filled with a gas into which a shaped laser beam could enter and a photo detector for receiving a laser beam having entered the cell.

10. The atomic sensor according to claim 9, wherein a preferred direction of polarization of a laser beam emitted by the emission laser is substantially aligned with the passing direction of the linear polarizer metal network.

11. A process for fabrication of a device for laser beam shaping intended in particular to be included in an atomic sensor such as an atomic clock, atomic magnetometer or atomic gyrometer by being combined both with a laser beam emission laser and also an alkaline vapor cell comprising a cavity filled with a gas into which a shaped laser beam could enter and a photo detector for receiving a laser beam having entered the cell, wherein: a retroreflector prism is disposed comprising an entrance area for an entering laser beam an exit area for an exiting laser beam and also two internal laser beam reflection surfaces, said reflection surfaces forming a right angle therebetween, a linear polarizer is formed and disposed able to linearly polarize an entering laser beam and disposed upstream from the entrance area of the retroreflector prism so as to be traversed by a laser beam entering the retroreflector prism by the entrance area; and a form birefringence plate is formed and disposed able to circularly polarize an exiting laser beam and disposed downstream from the exit area of the retroreflector prism so as to be traversed by laser beam exiting the retroreflector prism by the exit area.

12. Process according to claim 11, wherein the linear polarizer is formed by depositing a metal layer on a glass plate—or wafer, in particular an aluminum or gold layer; and the form birefringence plate is formed on said glass plate—or wafer—either by depositing a layer of dielectric material, in particular a silicon nitride layer, on said glass plate—or wafer—or by etching said glass plate—or wafer; the linear polarizer is disposed upstream from the entrance area of the retroreflector prism and the form birefringence plate downstream from the exit area of the retroreflector prism by placing the glass plate—or wafer—facing a transmission surface of the retroreflector prism, where the entrance area and the exit area of the retroreflector prism form two portions of said transmission surface.

13. Process according to claim 11, wherein the linear polarizer is formed and disposed upstream from the entrance area of the retroreflector prism by depositing a layer of metal material, in particular a layer of aluminum or gold, on a transmission surface of the retroreflector prism, the entrance area of the retroreflector prism forming a portion of said transmission surface; and the form birefringence plate is formed and deposited downstream from the exit area of the retroreflector prism either by depositing a layer of dielectric material, in particular a layer of silicon nitride, on the transmission surface of the retroreflector prism, the exit area of the retroreflector prism forming a portion of said transmission surface, or by etching of said transmission surface.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the drawings:

(2) FIG. 1 is a schematic transverse section of an atomic sensor according to an embodiment of the invention, comprising a laser beam shaping device according to an embodiment of the invention, with which are combined a laser beam emission laser and also a cavity filled with a gas into which a shaped laser beam can enter and a photodetector receiving a laser beam having gone into the cell;

(3) FIG. 2 is a detailed perspective view of a linear polarizer for the laser beam shaping device from FIG. 1, comprising a metal plate metal network;

(4) FIG. 3 is a detailed perspective view of a form birefringence plate for the laser beam shaping device from FIG. 1, comprising a zero-order subwavelength dielectric network of dielectric plates; and

(5) FIG. 4 is a detailed perspective drawing of a laser beam shaping device from FIG. 1, on which the path of a laser beam is shown comprising portions of initial laser beam, entering laser beam, internal laser beam, exiting laser beam and shaped laser beam.

(6) In the various figures, the same references designate identical or similar items.

DETAILED DESCRIPTION OF THE DISCLOSURE

(7) FIG. 1 shows an example of an atomic sensor 1 according to the invention incorporating an alkaline vapor cell 2, a laser beam emission laser 3, for example a vertical cavity laser (VCSEL for Vertical Cavity Surface Emitting Laser) and a receiving photodetector 4 for a laser beam having gone into the cell 2. The laser beam is designated by 5.

(8) The atomic sensor 1 is for example a chip-scale atomic clock (CSAC) based on the principle of atomic resonance by coherent population trapping (CPT).

(9) In variants of the invention, the atomic sensor 1 could be an atomic magnetometer or an atomic gyrometer for example.

(10) The cell 2 in particular includes a gas cavity 6 provided with at least one optical window 7 and filled with a gas. The gas is for example a gas comprising an alkaline element in vapor phase like cesium or rubidium and, as applicable, a buffer gas. The term “gas” can designate a gas, vapor, or an alkaline vapor or a mixture of a gas and a vapor.

(11) The atomic sensor 1 further comprises a laser beam shaping device 8.

(12) The device 8 is inserted, in the path of the laser beam 5, between the laser 3 and the cell 2 and is able to receive a laser beam 5 from the laser 3 and to transmit a laser beam 5 entering into the gas cavity 6 by the optical window 7 of the cell 2.

(13) The shaping device 8 comprises a retroreflector prism 9, a linear polarizer 10 and a form birefringence plate 11.

(14) In FIGS. 1 and 4, the retroreflector prism 9 comprises an entrance area 12 for an entering laser beam 5b, and exit area 13 for an exiting laser beam 5d and also two reflection surfaces 14, 15 for an internal laser beam 5c.

(15) The reflection surfaces 14, 15 together form a right angle.

(16) It is understood by “the surfaces together form a right angle”, that the reflection surfaces 14, 15 extend respectively along respective geometric planes of extension P1, P2, such that said geometric planes of extension P1, P2 form a geometric right angle between them.

(17) In the embodiment from FIG. 1, the entrance area 12 and the exit area 13 form two portions of a transmission surface 16 of the retroreflector prism 9.

(18) The transmission surface 16 of the retroreflector prism 9 can for example be oriented at 45° to each of the reflection surfaces 14, 15.

(19) Here again, “the transmission surface is oriented at 45° to each of the reflection surfaces 14, 15” means that the transmission surface extends along a geometric plane of extension P3 and the reflection surfaces 14, 15 extend respectively along the respective geometric planes of extension P1, P2 such that the geometric plane of extension P3 of the transmission surface forms a geometric angle of 45° with each of the geometric planes of extension P1, P2 of the reflection surfaces 14, 15.

(20) It will be noted that, as much as the angle between the reflection surfaces 14, 15 must be precisely controlled for guaranteeing a retroreflection of the laser beam, the orientation between the transmission surface 16 and the reflection surfaces 14, 15 is less critical because at least for small angles it involves principally a spatial offset of the beam and not an angular error.

(21) Thus, for example, the angle between the reflection surfaces 14, 15 is advantageously close to 90° within ±15 arcsec (±0.004°).

(22) The path of the laser beam 5 in the retroreflector prism is illustrated in FIG. 4 and is broken down in the following way.

(23) In entering laser beam 5b passes into the retroreflector prism 9 by the entrance area 12 and encounters a first reflection surface 14 generating an internal laser beam 5c. The internal laser beam 5c propagates in the retroreflector prism until encountering a second reflection surface 15 and generating an exiting laser beam 5d leaving the retroreflector prism 9 by the exit area 13.

(24) The entering laser beam 5b and the exiting laser beam 5d are thus parallel to each other in a plane of section H of the retroreflector prism 9.

(25) Such a plane of section H of the retroreflector prism 9 is in particular a plane perpendicular to the direction of intersection I between the respective geometric planes of extension P1, P2 of the two reflection surfaces 14, 15.

(26) The retroreflector prism can for example be made of a material such as N-BK7, Soda-lime, N-BaK1 or even N-SF11.

(27) Advantageously, the retroreflector prism can be made of a material having a sufficiently high index of refraction for guaranteeing a total reflection of the incident laser beam even when it has a relatively high divergence, as shown in FIG. 1.

(28) The retroreflector prism can for example be made of N-BaK1 characterized by an index of refraction n=1.564 at an 852.1 nm wavelength; such an index of refraction guarantees a total reflection for a laser beam having an angle of incidence on the transmission surface 16 included between −8° and +8°.

(29) The linear polarizer 10 is shown in more detail on FIG. 2 and comprises a subwavelength metal network 17.

(30) This subwavelength metal network 17 comprises a plurality of metal plates 18. The metal plates 18 extend parallel to each other along a metal plate direction DM.

(31) The metal plates 18 are additionally juxtaposed one relative to the other along a passing direction DP of the metal network 17 perpendicular to the metal plate direction DM.

(32) “Subwavelength network” is understood in particular to mean that the metal plates are juxtaposed one relative to the others with a spatial period along the passing direction DP less than the wavelength of the laser beam 5, at normal incidence, divided by the index of refraction of the substrate supporting the metal plates, for example less than about 900 nm in the case of a cesium atomic clock.

(33) The metal plates 18 are for example made of aluminum or gold.

(34) Thus, the linear polarizer 10 can for example be formed by depositing a metal layer on a glass plate—or wafer—19, in particular an aluminum or gold layer.

(35) In a sample embodiment provided purely for information, the linear polarizer 10 is formed by depositing a 300 nm thick aluminum layer shaped for having metal plates 18 spaced apart from each other with a period of 200 nm and each having a length of 90 nm along the passing direction of the network 17, meaning having a filling factor (width/period) of 0.45.

(36) In a sample embodiment also provided purely for information, the linear polarizer 10 is formed by depositing a 350 nm thick gold layer shaped for having metal plates 18 spaced apart from each other with a period of 200 nm and each having a length of 157 nm along the passing direction of the network 17, meaning having a filling factor (width/period) of 0.45.

(37) Other sizes of the linear polarizer 10, the metal network 17 and metal plates 18 are obviously conceivable.

(38) The linear polarizer 10 is disposed upstream from the entrance area 12 of the retroreflector prism 9 so as to be traversed by a laser beam entering 5b the retroreflector prism 9 by the entrance area 12. The linear polarizer 10 is able to linearly polarize said entering laser beam 5b.

(39) More precisely, the linear polarizer 10 receives an initial laser beam 5a emitted by the laser 3 and transforms it into an entering laser beam 5b having a defined linear polarization.

(40) In fact, the component of the incident initial laser beam 5a which is parallel to the direction of the metal plate DM is converted into a displacement of electrons in the metal plates 18 and therefore absorbed by the linear polarizer 10, whereas the component of the instant initial laser beam 5a which is parallel to the passing direction DP of the metal network is left intact in the entering laser beam 5b.

(41) In a specific embodiment of the invention, the emitting laser 3, for example a vertical cavity laser (VCSEL for Vertical Cavity Surface Emitting Laser), can comprise a network which guarantees oscillation in a single polarization mode, in particular a fixed and known linear polarization. In this embodiment, the initial laser beam 5a can also have a linear polarization along a polarization direction DL set by the laser 3.

(42) It is important to note that the assembly constraints for the laser 3 in the atomic sensor 1 make the precise alignment of this polarization direction DL with the other elements of the atomic sensor like the form birefringence plate 11, the reflection prism 9 or the optical cell 2 very difficult.

(43) Advantageously, the linear polarization of the initial laser beam 5a, emitted by the emission laser 3, is substantially aligned with the passing direction DP of the metal network 17 of the linear polarizer 10, meaning that the polarization direction DL and the passing direction DP of the metal network are substantially aligned.

(44) “Substantially aligned” is understood to mean that the two directions are parallel or else close to parallel, for example forming an angle between them of less than 10°.

(45) Additionally, the linear polarizer 10 is placed facing the transmission surface 16 of the retroreflector prism 9, by being either disposed in said transmission surface 16 or formed on a glass plate—or wafer—19 in contact with said transmission surface 16.

(46) As shown in FIG. 2, the metal network 17 of the linear polarizer 10 can in particular extend in a geometric plane of extension PP, formed by the passing direction DP and the metal plate direction DM, such that said geometric plane of extension PP is parallel to the geometric plane of extension P3 of the transmission surface 16.

(47) The passing direction DP of the metal network 17 can in particular be parallel to the direction of intersection I between the geometric planes of extension P1, P2 of the two reflection surfaces 14, 15.

(48) In this way, the linear polarization applied to the entering laser beam 5b is maintained during reflections in the prism 9 and therefore maintained in the exiting laser beam 5d.

(49) Additionally, the form birefringence plate 11 is shown in more detail in FIG. 3. The form birefringence plate 11 is for example a form birefringence or artificial birefringence plate.

(50) For this purpose, the form birefringence plate 11 comprises a zero-order subwavelength dielectric network 20.

(51) This dielectric network 20 comprises a plurality of dielectric plates 21. The dielectric plates 21 extend parallel to each other along a dielectric plate direction DD.

(52) The dielectric plates 21 are additionally juxtaposed one relative to the other along a dielectric network direction DR perpendicular to the dielectric plate direction DD.

(53) “Subwavelength network” is here again understood in particular to mean that the dielectric plates are juxtaposed one relative to the others with a spatial period along the dielectric network direction DR less than the wavelength of the laser beam 5, at normal incidence, divided by the index of refraction of the substrate supporting the dielectric plates, for example less than about 900 nm in the case of a cesium atomic clock.

(54) “Zero order dielectric network”, or “zero order grating”, is understood to mean a dielectric network for which the period separating the dielectric plates is sufficiently small that the dielectric network does not generate propagating diffraction orders except order 0.

(55) With such a zero order dielectric network, the phase shift between the transverse electric component and the transverse magnetic component of an incident laser beam can be controlled and therefore the polarization of the exiting light as a function of the polarization of the entering light.

(56) The dielectric plates 21 are for example implemented in glass or silicon nitride (Si.sub.3N.sub.4).

(57) Thus, the form birefringence plate 11 can for example be formed by depositing a silicon nitride layer (Si.sub.3N.sub.4) on a glass plate—or wafer—19, for example the glass plate—or wafer—19 on which the linear polarizer 10 is formed.

(58) Alternatively, the form birefringence plate 11 can be formed of glass (silica), for example by etching of a glass plate—or wafer—19, for example the glass plate—or wafer—19 on which the linear polarizer 10 is formed.

(59) In an embodiment provided purely for illustration, the form birefringence plate 11 is formed by etching of a glass plate—or wafer—19 formed for having dielectric plates 21 with a thickness of 2710 nm separated from each other with a period of 580 nm, where the dielectric network 20 has a filling factor (width/period) of 0.464. The aspect ratio of such a network (thickness/width) is therefore about 10.

(60) In another embodiment also provided purely for illustration, the form birefringence plate 11 is formed by depositing a silicon nitride layer with a thickness of 598 nm shaped for having dielectric plates 21 separated from each other with a period of 580 nm, where the dielectric network 20 has a filling factor (width/period) of 0.331. The aspect ratio of such a network (thickness/width) is therefore about 3.

(61) Other sizes of the form birefringence plate 11, dielectric network 20 and dielectric plates 21 are obviously conceivable.

(62) The form birefringence plate 11 is disposed downstream from the exit area 13 of the retroreflector prism 9 so as to be traversed by a laser beam exiting 5d the retroreflector prism 9 by the exit area 13. The form birefringence plate 11 is able to circularly polarized said exiting laser beam 5d for obtaining a shaped laser beam 5e.

(63) With such a zero order dielectric network, the phase shift between the transverse electric component and the transverse magnetic component of an incident laser beam can be controlled and therefore the polarization of the leaving light depending on the polarization of the entering light.

(64) In fact, as detailed earlier, with such a dielectric network 20 the phase shift between the transverse electric component and the transverse magnetic component of an incident laser beam can be controlled and therefore the polarization of the exiting light as function of the polarization of the entering light. The dielectric network 20 therefore behaves like an anisotropic material, where the anisotropy does not arise from intrinsic properties of the material but from the periodic structure implemented on the surface thereof, which is sufficiently small for artificially manipulating the index of refraction.

(65) The dimensions of the dielectric network 20 are determined such that, for an exiting beam 5d having linear polarization, the polarization of the shaped beam 5e is circular, and for example has an ellipticity of the circular polarization less than 1.1. Such a polarization can be obtained with the dielectric network 21 examples detailed above.

(66) To adapt the dimensions of the dielectric network 20, the equations of effective medium theory (EMT) can for example be used. On this subject, also refer to the information in the document “Submicrometer periodicity gratings as artificial anisotropic dielectrics,” by D. C. Flanders, that appeared in the journal Appl. Phys. Lett. 42, 492-494 (1983).

(67) Additionally, the form birefringence plate 11 is placed facing the transmission surface 16 of the retroreflector prism 9, by being either disposed in said transmission surface 16 or formed on a glass plate—or wafer—19 in contact with said transmission surface 16.

(68) Thus for example, the linear polarizer 10 and the form birefringence plate 11 can be disposed side-by-side on the glass plate—or wafer—19.

(69) As shown in FIG. 3, the dielectric network 20 of the form birefringence plate 11 can in particular extend in a geometric plane of extension PL, formed by the dielectric plate direction DD and the dielectric metal direction DR, such that said geometric plane of extension PL is parallel to the geometric plane of extension P3 of the transmission surface 16.

(70) Additionally, the geometric plane of extension PL of the dielectric network 20 can be parallel to the geometric plane of extension PP of the metallic network 17.

(71) More specifically, the dielectric plate direction DD of the dielectric network 21 can form an angle of 45° with the metal plate direction DM.

(72) Similarly, given that the dielectric network direction DR and the dielectric plate direction DD of the dielectric network 21 are perpendicular, the direction of the dielectric network DR can form an angle of 45° with the direction of the metal plate DM.

(73) Or again, given that the passing direction DP of the metal plate and the metal plate direction DM of the metal network 17 are perpendicular, the passing direction DP of the metal plate can form an angle of 45° with the direction of the dielectric network DR or the direction of the dielectric plate DD.

(74) In these ways, the shaped laser beam 5e can be circularly polarized.