WO2000009978A1

WO2000009978A1 - Apparatus for measuring a light beam wavelength

Info

Publication number: WO2000009978A1
Application number: PCT/FR1999/001967
Authority: WO
Inventors: Christian Chappuis; Michel Chevalier; Philippe Cormont; François VIALA
Original assignee: Commissariat A L'energie Atomique; Compagnie Generale Des Matieres Nucleaires
Priority date: 1998-08-12
Filing date: 1999-08-11
Publication date: 2000-02-24
Also published as: JP2002522782A; FR2782383A1; EP1019688A1; FR2782383B1

Abstract

The invention concerns an apparatus for measuring a light beam wavelength comprising at least a Fizeau wedge (18, 20) delimited by first and second plates, means (30, 32) for high resolution detection of interference fringes resulting from the light beam reflections on the internal surfaces of the plates, means (72, 74, 76) for processing electric signals, generated by the detecting means, for computing the beam wavelength, and means for transforming the beam into a slightly divergent spherical light wave and for illuminating the Fizeau wedge with said wave.

Description

APPARATUS FOR MEASURING THE WAVELENGTH OF A BEAM

LUMINOUS

DESCRIPTION

TECHNICAL AREA

The present invention relates to an apparatus for measuring the wavelength of a light beam.

It applies in particular to laser beams, for example to those used in spectroscopy or for isotopic separation by laser.

STATE OF THE PRIOR ART

Apparatus for measuring the wavelength of a light beam is already known from document (1) which, like the other documents cited below, is mentioned at the end of this description.

We also know from documents (2),

(3) and (4), such an apparatus which comprises a Fizeau wedge (“Fizeau wedge”) having an input blade with parallel faces, this Fizeau wedge being used in reflection. This device makes it possible to measure the wavelength of a light beam absolutely and precisely, but poses a problem of reliability of the measurements. Document (5) proposes to improve the accuracy of the device known from documents (2) to

(4) by modifying the shape of the Fizeau wedge. Document (6) discloses the existence of problems of parasitic reflections on the Fizeau corner of the apparatus known from documents (2) to (4) and, to solve these problems, proposes to use a Fizeau corner having an entry blade with non-parallel faces.

STATEMENT OF THE INVENTION

The present invention relates to a device for measuring the wavelength of a light beam, which is reliable and simpler to manufacture than the devices known from documents (5) and (6), the problem of stray reflections. not being moreover posed in document (5).

The authors of the present invention have discovered that the lack of reliability of the device known from documents (2) to (4) was linked to parasitic reflections on the entrance face of the corner of Fizeau. To solve this reliability problem, the invention proposes an easy solution to implement from the technological point of view from the configuration described in documents (2) to (4), namely to illuminate the corner of Fizeau d 'a device of the kind described in these documents by a spherical and slightly divergent light wave. This is by no means obvious in view of the state of the art because, according to the principle of Fizeau interferometers, it is necessary to illuminate the corner of Fizeau with a plane wave, any deviation from such illumination resulting in the creation of aberrations. Specifically, the present invention relates to an apparatus for measuring the wavelength of a light beam, this device comprising an interferometric measurement assembly comprising:

- At least one corner of Fizeau delimited by first and second fixed, transparent blades and slightly inclined with respect to each other, the light beam crossing the corner of Fizeau according to an optical path of determined length and penetrating this corner of Fizeau by the first plate, the external face of this first plate being treated with anti-reflection, and - means for detecting, at high resolution, interference fringes resulting from reflections of the light beam on the respective internal faces of the first and second plate, these detection means generating electrical signals corresponding to these interference fringes, the apparatus also comprising means for processing these electrical signals, these processing means being able to calculate the wavelength of the light beam at from the positions of the axima of the electrical signals, this apparatus being characterized in that the interferometric measuring assembly further comprises optical transformation means provided for transforming the light beam into a slightly divergent spherical light wave and for illuminating the corner of Fizeau with this wave so as to eliminate the measurement uncertainties resulting from parasitic reflections on the first plate.

Anti-reflection treatment of the external face of the first plate does not completely eliminate stray reflections for all wavelengths. The measurement uncertainties due to these parasitic reflections greatly decrease when lighting the corner of Fizeau with the slightly divergent spherical wave. Admittedly, such an illumination causes a modulation of the amplitude of the interference fringes at a spatial frequency much higher than that generated by the illumination by a plane wave. This frequency is sufficient so that the average value of the pitch of the fringes is no longer affected by this modulation.

According to a preferred embodiment of the apparatus which is the subject of the invention, the optical transformation means comprise:

- means for forming, from the light beam, a beam diverging from a point, and - collimation means provided for receiving this diverging beam from a point and transforming it into the slightly diverging spherical light wave, these collimating means having a focal point slightly spaced, by a determined distance, from said point.

The collimating means can comprise a collimating lens but they preferably comprise a spherical or parabolic mirror. The means for forming the beam diverging from said point may comprise a mask pierced with a hole, this hole being placed at the focal point of an objective such as a microscope objective.

(receiving the beam whose wavelength is to be measured), but they preferably comprise an optical fiber intended to transport the light beam, said point being located at an end face of this optical fiber. In order to overcome the problem of the variation of the pitch of the interference fringes with the aberrations of the wavefront incident on the corner of Fizeau, the detection means are preferably placed in a particular plane, called "plane of section "or" shearing plane ", corresponding to the corner of Fizeau. Regarding this plan, consult documents (1), (3) and (4).

Preferably, in order to measure the wavelength of the light beam in an absolute manner with a very high precision, capable of being equal to 6 × ¹⁰ -8, the device object of the invention comprises two corners of Fizeau intended to be illuminated by ^• the spherical wave light diverging slightly, the detection means being provided for detecting the interference fringes corresponding to the two corners of Fizeau. Such precision of 6xl0 ^{~ 8} is required to measure the wavelength of a laser source in pulsed regime, in the case of isotopic separation by laser or spectroscopy.

Also preferably, said interferometric measurement assembly further comprises means for compensating for the chromaticism capable of being introduced by the first plate corresponding to each corner of Fizeau.

The detection means can advantageously comprise a strip of photodiodes for each corner of Fizeau.

To overcome variations in the refractive index of air, each corner of Fizeau is preferably under vacuum ("vacuum"). To do this, it is possible to create a vacuum in the space between the two blades corresponding to each corner of Fizeau. However, according to a preferred embodiment, which is particularly advantageous when the interferometric measurement assembly is compact, the device which is the subject of the invention furthermore comprises a sealed enclosure provided with means for evacuating it and enclosing the measurement assembly. interferometric.

Preferably, with a view to long-term stability of the device which is the subject of the invention, it further comprises a base, which is provided to support the interferometric measurement assembly, and means for mechanical stabilization of this base.

In view of this long-term stability, it is also preferable for the apparatus which is the subject of the invention to further comprise means for maintaining the interferometric measuring assembly at a constant temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which:

FIG. 1 is a schematic perspective view of a particular embodiment of the wavelength measuring device object of the invention and

• Figure 2 is a schematic and partial top view of this device. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The device according to the invention shown in these Figures 1 and 2 is intended to measure the wavelength of a light beam 2 emitted by a laser 4. A part 6 of this beam 2 is taken for example by means of a semi-reflecting mirror 8 placed at 45 ° to the beam 2. The light beam formed by this part 6 is ^' used to measure the wavelength' of the beam 2 with the apparatus. This beam 6 is injected via a lens 10 into one end of an optical fiber 12, preferably single-mode. The other end of the optical fiber 12 is mounted in a tip 14 ("ferrule"). The apparatus of FIGS. 1 and 2 comprises a spherical or parabolic mirror 16, two superimposed corners of Fizeau 18 and 20, two mirrors 22 and 24, two superimposed compensation blades 26 and 28 and two arrays of photodiodes 30 and 32. The mirror 16, the corners of Fizeau 18 and 20, the mirrors 22 and 24, the compensation blades 26 and 28 and the photodiode arrays 30 and 32 are placed on a base 34.

The end piece 14 of the optical fiber 12 is fixed in a support 36 comprising lower 38 and upper 40 blades provided with V-shaped grooves facing each other. The end piece is immobilized between these two grooves when the upper blade 40 is fixed, for example by screws 41, to the lower blade 38. The support 36 also includes a base 42 on which the two blades 38 and 40 are fixed. base 42 is mobile according to three degrees of freedom on the base 34 facing the mirror 16: in the example shown the base 34 comprises a groove 44 perpendicular to the axis X of the mirror 16 and the base 42 is capable of sliding in this groove 44 (along the arrow fl); the base 42 is also adjustable in height (along the arrow f2) relative to the base 34 for example by removable shims 46 of small thickness, interposed between this base and the bottom of the groove; in addition, this groove has a sufficient width to be able to vary the translational position of this base 42 in the groove 44 (which is symbolized by the arrow f3).

The support 36 is adjusted so that the axis Y of the end of the fiber placed in the end-piece 14 coincides with the axis X of the mirror 16 and that the point of intersection of the axis Y with the face of this end is offset longitudinally by a few hundred micrometers, typically 600 μ, from the focal point F of this mirror 16. The divergent light beam coming from this end is then reflected by the mirror 16 in the form of a spherical light wave forming a slightly divergent beam 48 (instead of being precisely collimated if this point of intersection was exactly at focus F). The two blades 38 and 40 are thin enough (each of them for example has a thickness of two millimeters) to intercept only a very small part of the slightly divergent beam 48 reflected by the mirror 16. The end of the fiber optic placed in the tip 14 is between the mirror 16 and the two superimposed corners of Fizeau 18 and 20 as seen in FIG. 1. The slightly divergent beam 48 reflected by the mirror 16 therefore reaches these two corners of Fizeau. More precisely, the upper part 48s (respectively lower 48i) of this beam 48 is reflected by the upper corner of Fizeau (respectively lower 18) towards the plane mirror 24 (respectively 22).

FIG. 2 schematically shows the lower Fizeau corner 18 but this diagram is valid for the upper Fizeau corner 20. It can be seen that each Fizeau corner is delimited by two blades 49 and 50 of silica. The blade 49 constitutes the input blade of the corner of Fizeau and has parallel faces and a thickness for example equal to 10 mm. The external face or entry face 52 of this strip 49, the face which receives the beam reflected by the mirror 16, is covered with an anti-reflection layer, not shown. The respective internal faces 54 and 56 of the two plates 49 and 50 have a flatness of λ / 100 peak to peak, λ being the wavelength of a test laser (633 nm). In addition, these internal faces 54 and 56 form an acute angle α of small value between them, typically 2/1000 radians. Wedges of thickness 58 and 60 in zerodur type glass are interposed between the two blades 49 and 50 and have a thickness of the order of 1 mm for the upper Fizeau corner 20 and 20 mm for the lower Fizeau corner 18. These shims of thickness 58 and 60 are fixed to the blades 49 and 50 by molecular adhesion.

The shims 58 and 60 of the same corner have of course not exactly the same thickness, because the angle α is not zero.

The upper Fizeau corner 20 is fixed to the lower Fizeau corner 18 by molecular adhesion of their respective blades 50. In addition, the corner of Lower fizeau 18 is fixed to the base 34 by molecular adhesion of the corresponding blade 50. The two blades 50 can have a great thickness equal for example to 25 mm. Light from corner 18

(respectively 20) and reflected by the mirror 22

(respectively 24) passes through the compensating blade 26

(respectively 28). These compensating plates correct the chromatism introduced by the plates 49 from the two corners of Fizeau. This chromaticism correction can be improved by introducing chromaticity coefficients for the whole apparatus, coefficients which can be calculated during the calibration phase of this apparatus. The light waves reflected by the internal faces 54 and 56 of each corner of Fizeau 18 or 20 interfere with each other to form, after passing through the corresponding compensating plate 26 or 28, interference fringes on the strip of corresponding photodiodes 30 or 32. Each photodiode strip is placed on the shearing plane of the corner of

Corresponding section (see the documents on this subject

(1), (3) and (4): remember that for a corner of

Given this, the shearing plane is the plane in which the images of the points of the internal faces 54 and 56 are merged where the beam 48 is reflected, such as for example the points A and B in FIG. 2). The orientations and relative positions of the corners of Fizeau, of the plane mirrors, of the compensating plates and of the arrays of photodiodes (intended to capture respectively the interference fringes which correspond to them) are provided for this purpose. The heights and relative arrangements of the mirrors 22 and 24 are provided so that the lower beam parts 48i and upper 48s, which are reflected by the two corners of Fizeau, reconstitute the beam 48 beyond the mirror 22. Next, each of the parts lower and upper of the beam 48 crosses the corresponding compensating plate and reaches the corresponding array of photodiodes.

In the example shown in FIG. 1, the bars 30 and 32 are mounted on a block 62 crossed by a lower slot 64, one end of which receives the lower part 48i of the bundle 48 and the other end of which faces the bar of corresponding photodiodes 30 for transmitting thereto the part 48i via the slot 64. This block 62 is provided with another slot 66 situated above the slot 64 and designed to receive the upper part 48s of the beam 48. This slot 66 opens, in block 60, onto a plane mirror 68 at 45 ° which also receives this upper part and reflects it towards the corresponding photodiodes array 32 through another slot 70 of block 62.

The mirror 16, the mirror 22, the mirror 24 and the compensation blade 26 are made of silica and fixed to the base 34 of silica by molecular adhesion. The blade 28 is made of silica and fixed by molecular adhesion to the blade 26. The base 34 is placed on three shock absorbers of which only two are shown in FIG. 1 and symbolized by arrows M. These shock absorbers form a stable support "line, point , plan ". All of these three shock absorbers absorb external vibrations and eliminate mechanical stresses. This ensures very good mechanical stability for the base 34 and the components thereon. Other devices are of course possible to ensure the mechanical stability of the base 34. As a purely indicative and in no way limiting, the arrays of photodiodes are arrays of 1024 pixels, of the kind which are marketed by the company Reticon. The pixels of these bars have a height of 2.5 mm for a width of 25 μm, which provides sufficient sensitivity so as not to have to use focusing lenses.

The electrical signals respectively supplied by the photodiodes arrays are characteristic of the corresponding interference fringes. The electrical signal supplied by the bar 30 (respectively 32) is sent to an electronic card 72 (respectively 74) for amplifying this signal. The signals thus amplified are sent to a computer 76 which calculates the wavelength of the light beam 2 emitted by the laser 4 using the algorithm relating to the apparatus described in documents (2) to (4) to which we are report to. This computer is provided with means 78 for displaying the calculations.

It is specified that the use of the two corners of Fizeau allows a measurement of wavelength much more precise than if one used a single corner of Fizeau since the interval comprised between the two blades of the corner of upper Fizeau is much larger than the corresponding interval of the lower Fizeau corner, about 20 times larger in the example described. Lower Fizeau corner cooperates with the corresponding photodiodes strip to measure the pitch of the fringes, hence an approximate value of the wavelength of the beam 2 emitted by the laser 4. By means of this approximate value, the fringes are numbered. The numbers depend on the thicknesses crossed. This leads to a second estimation of the wavelength with improved precision. With this second estimate, we know how to number the fringes corresponding to the upper Fizeau corner whose thickness is greater.

The base 34 as well as all the elements which are on this base are placed in a sealed enclosure 80 provided with pumping means 82 to evacuate it. Means 84 for measuring the pressure in this enclosure are also provided. It is specified that this enclosure 80 (the use of which is facilitated by the compact arrangement of the device of FIG. 1) has sealed passages for pumping and for measuring the pressure, for the passage of the optical fiber 12 , for the connections of the photodiodes arrays to their electronic cards and for the connection of a temperature probe to regulation means which are discussed below.

The interior of the enclosure 80 is maintained at a stable temperature better than one tenth of a degree. This temperature is monitored by a temperature probe 86 placed on the base 34, in the enclosure 80. The temperature stability is obtained by placing the vacuum enclosure 80 in a second thermal regulation enclosure, not shown. This thermal regulation is carried out using heating resistors 88 which are between the enclosure 80 and the thermal regulation enclosure and the temperature 86. A set temperature is indicated to an electronic regulator 90 which starts the operation of the heating resistors as soon as the temperature measured by the probe 86 becomes lower than the set temperature. An electronic device 92 called a "dimmer" is connected between the regulator 90 and the resistors 88 and enables these resistors to be started gradually.

The long-term stability of the device of FIGS. 1 and 2 is ensured by the vacuuming of the components placed on the base 34. It is preferable that the electronic amplification cards 72 and 74 are also in the enclosure 80 under empty in view of this stability. Temperature regulation and vibration absorption also contribute to this long-term stability. This stability eliminates the need to regularly calibrate the device. It is specified that the latter is calibrated from interference fringes relating to the emission of laser sources whose spectral characteristics are known.

If one wishes an accuracy lower than 6xl0 ^-8 on the measurement of the wavelength, for example an accuracy of 3xl0 ^"6 , it is possible to simplify the apparatus by using only one corner of Fizeau as well as the mirror plan, the compensating plate and the array of photodiodes corresponding to this corner of Fizeau.

In addition, instead of the temperature stabilization means mentioned above, the device can be placed in an air-conditioned room, measure the temperature in the latter and correct wavelength measurement as a function of the temperature measured.

The invention is not limited to measuring the wavelength of a laser beam. It also applies to the measurement of the wavelength of a light beam which is not perfectly coherent. The invention is also not limited to measuring the length of a perfectly monochromatic light beam. It also applies to measurements of light sources with several lines by appropriately modifying the algorithm.

The beam whose wavelength is measured can be visible or invisible (infrared and ultraviolet). In addition, it can be continuous or impulse.

The documents cited in this description are as follows:

(1) J.J. Snyder, Laser wavelength meter, Laser Focus, vol.18, 1982, p.55

(2) J.J. Snyder, Apparatus and method for determination of wavelength: US patent 4,173,442

(3) J.J. Snyder, Algorithm for fast digital analysis of interference fringes, Applied Optics, vol.19, n ° 8, 1980, p.1223

(4) MB Morris, TJ Mellrath and JJ Snyder, Fizeau wavemeters for pulsed laser wavelength measurements, Applied Optics, vol.23, n ° 21, 1984, p.3862 (5) RP Hackel and M. Feldman, Wavelength meter having elliptical wedge: US patent 5,168,324

(6) R. Spolaczyk and K.E. Elssner: patent DD290051.

Claims

1. Apparatus for measuring the wavelength of a light beam (2), this apparatus comprising an interferometric measurement assembly comprising: - at least one Fizeau wedge (18, 20) delimited by first and second fixed blades , transparent and slightly inclined with respect to each other, the light beam passing through the corner of Fizeau along an optical path of determined length and penetrating into this corner of Fizeau by the first strip, the external face of this first strip being anti-reflective treatment, and - means (30, 32) for detecting, at high resolution, interference fringes resulting from reflections of the light beam on the respective internal faces of the first and second plates, these detection means generating corresponding electrical signals at these interference fringes, the apparatus also comprising means (72, 74, 76) for processing these electrical signals, these processing means being able to calculate the wavelength of the light beam from the positions of the maxima of the electrical signals, this apparatus being characterized in that the interferometric measurement assembly further comprises optical transformation means (12, 16) provided for transforming the light beam into a slightly diverging spherical light wave and to illuminate the corner of Fizeau with this wave so as to eliminate the measurement uncertainties resulting from parasitic reflections on the first plate.

2. Apparatus according to claim 1, in which the optical transformation means comprise:

- means (12) for forming, from the light beam, a beam diverging from a point, and

- collimation means (16) provided for receiving this diverging beam from a point and transforming it into the slightly diverging spherical light wave, these collimation means having a focal point (F) slightly spaced, of a determined distance from said point.

3. Apparatus according to claim 2, wherein the collimating means comprise a spherical or parabolic mirror (16).

4. Apparatus according to any one of claims 2 and 3, wherein the forming means comprise an optical fiber (12) provided for transporting the light beam, said point being located at an end face of this optical fiber.

5. Apparatus according to any one of claims 1 to 4, wherein the detection means (30, 32) are placed in the shearing plane corresponding to the corner of Fizeau (18, 20).

6. Apparatus according to any one of claims 1 to 5, comprising two corners of Fizeau (18, 20) intended to be illuminated by the slightly divergent spherical light wave, the detection means (30, 32) • being provided for detect the interference fringes corresponding to these two corners of Fizeau.

7. Apparatus according to any one of claims 1 to 6, in which the interferometric measuring assembly further comprises means (26, 28) for compensating for the chromaticism capable of being introduced by the first plate (49) corresponding to each corner of Fizeau (18, 20).

8. Apparatus according to any one of claims 1 to 7, wherein the detection means comprise a strip of photodiodes (30, 32) for each corner of Fizeau (18, 20).

9. Apparatus according to any one of claims 1 to 8, wherein each corner of Fizeau (18, 20) is vacuum.

10. Apparatus according to claim 9, further comprising a sealed enclosure (80) provided with means (82) for evacuating it and enclosing the interferometric measurement assembly.

11. Apparatus according to any one of claims 1 to 10, further comprising a base (34), which is provided to support the interferometric measurement assembly, and means (M) for mechanical stabilization of this base.

12. Apparatus according to any one of claims 1 to 11, further comprising means (86, 88, 90, 92) for maintaining the interferometric measurement assembly at a constant temperature.