U.S. patent application number 10/381050 was filed with the patent office on 2004-02-12 for optical method for light diffraction, corresponding optical system and device.
Invention is credited to Jourdain, Erick, Lepere, Didier.
Application Number | 20040027647 10/381050 |
Document ID | / |
Family ID | 8854691 |
Filed Date | 2004-02-12 |
United States Patent
Application |
20040027647 |
Kind Code |
A1 |
Lepere, Didier ; et
al. |
February 12, 2004 |
Optical method for light diffraction, corresponding optical system
and device
Abstract
The invention concerns a light diffraction optical method
covering a wide wavelengths range, a optical system and a optical
measuring device corresponding. According to the optical method, at
least one incident light beam (21) is sent to a surface of an
optical system (1), according to a angle of incidence (.alpha.)
relative to the normal line (20) to the surface, the optical system
comprising a Bragg reflector (2) and a grating (3) engraved on the
Bragg reflector (2) on the surface. At least one returned beam (22)
is sent by the optical system according to an angle of diffraction
(.beta.) relative to the normal line after diffraction. The
wavelengths and the angle of incidence of at least one of the
incident beams are such that this incident beam is diffracted by
the Bragg reflector and/or by the grating.
Inventors: |
Lepere, Didier; (Sevres,
FR) ; Jourdain, Erick; (Bagnolet, FR) |
Correspondence
Address: |
ARENT FOX KINTNER PLOTKIN & KAHN
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Family ID: |
8854691 |
Appl. No.: |
10/381050 |
Filed: |
September 11, 2003 |
PCT Filed: |
September 26, 2001 |
PCT NO: |
PCT/FR01/02985 |
Current U.S.
Class: |
359/322 |
Current CPC
Class: |
G02B 5/1838
20130101 |
Class at
Publication: |
359/322 |
International
Class: |
G02F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 26, 2000 |
FR |
0012237 |
Claims
1. A light diffraction optical method wherein: at least one
incident light beam (21, 25) having at least a wavelength is sent
onto a surface (8) of an optical system (1) having a normal axis
(20), according to a direction of Incidence forming an angle of
incidence (.alpha.) relative to said normal axis (20), said optical
system (1) comprising a Bragg reflector (2) and a grating (3)
engraved on the Bragg reflector (2) at said surface (8), and at
least one returned beam (22, 26) is collected by the optical system
(1) according to at least one diffraction direction forming an
angle of diffraction (.beta.) with respect to the normal axis (20),
after diffraction of the incident beam (21, 25) by the optical
system (1), characterised in that said wavelengths and said angle
of incidence (.alpha.) of at least one of the incident beams (21)
are such that said incident beam (21) is diffracted by the Bragg
reflector (2) and/or by the grating.
2. An optical method according to claim 1, characterised in that
the Bragg reflector is a crystal.
3. An optical method according to one of the claims 1 or 2,
characterised in that said angle of incidence (.alpha.) is ranged
between 5.degree. and 80.degree..
4. An optical method according to any of the claims 1 to 3,
characterised in that said wavelengths are ranged between 0.1 nm
and 0.7 nm.
5. An optical method according to any of the claims 2 to 4,
characterised in that the Bragg reflector (2) is composed of
oriented silicon monocrystals (111).
6. An optical method according to any of the previous claims,
characterised in that the grating (3) is covered with a metallic
layer (4).
7. An optical method according to any of the previous claims,
characterised in that said wavelengths and said angle of incidence
(.alpha.) of at least another of the incident beams (25) are such
that said incident beam (25) is diffracted by the grating (3).
8. An optical method according to claim 7, characterised in that
said angle of incidence (.alpha.) of said other incident beam (25)
is at least equal to 70.degree..
9. An optical method according to one of the claims 7 or 8,
characterised in that said wavelengths of said other incident beam
(25) are comprised between 0.6 nm and 150 nm.
10. An optical method according to any of the previous claims,
characterised in that said incident beams are polychromatic
luminous beams.
11. An optical method according to any of the claims 1 to 9,
charact rised in that said incident beams are monochromatic
luminous beams.
12. An optical system (1) comprising a Bragg reflector (2),
characterised in that it comprises a diffraction grating (3)
engraved on the Bragg reflector.
13. An optical system according to claim 12, characterised in that
the Bragg reflector is a crystal.
14. An optical system (1) according to claim 13, characterised in
that the Bragg reflector (2) is composed of a silicon
monocrystal.
15. An optical measuring device comprising: an optical system (1)
according to one of the claims 12 to 14, means for lighting (51)
said surface (8) of the optical system (1) by means of at least one
incident light beam (21, 25), means for collecting (52) at least
one returned beam (22, 26) by the optical system (1) after
diffraction of said incident beams (21, 25) by the optical system
(1), and rotary means (53) relative of the optical system (1) with
respect to said incident beams (21, 25).
16. A use of the optical measuring method according to any of the
claims 1 to 11 or of the device according to one of the claims 12
to 15, for a primary calibration for measuring wavelengths or for
dispersion in a monochromator.
Description
[0001] This invention concerns a light diffraction optical method,
as well as corresponding optical system and device.
[0002] The range of application of the invention also relates to
wavelengths measurements taken on monochromatic luminous beams as
well as the dispersion of polychromatic beams, for example in a
monochromator.
[0003] The use of diffraction grating for the diffraction of light
is well known. Thus, in the vacuum ultraviolet range (wavelengths
greater than 0.6 nm) or VUV, an Incident beam is sent
conventionally under a grazing incidence to a diffraction grating
and a returned beam is collected for a given order of diffraction,
according to an angle of diffraction complying with the laws of the
gratings (diffraction of Fresnel). The gratings used, for example
for a synchrotron radiation VUV, are typically engraved in
materials such as silicon or SiC CVD, by ionic engraving or
rolling.
[0004] The wavelength range that may be covered by such gratings is
somehow limited Internally. Indeed, at constant angle of incidence,
the reflectivity decreases as a function of the wavelength.
Satisfactory efficiency of diffraction can not thus be easily
obtained below 0.6 nm.
[0005] For diffractions at lower wavelengths, other systems, such
as diffracting crystals, are implemented. In such a crystal, having
a given distance between reticular planes, incident beams are
diffracted according to the Bragg law. The useful wavelength range
is delineated above by the distance between the reticular planes,
and it is moreover limited in that the angular range is generally
comprised between 5.degree. and 85.degree. for practical reasons.
For instance, for an oriented silicon monocrystal (111), for which
the double distance between reticular planes is equal to 0.627 nm,
the wavelength range varies between 0.055 nm and 0.625 nm.
[0006] One may also perform Bragg reflectors by stacking of thin
layers on a substrate. They also diffract the light according to
the Bragg law. Below, by Bragg reflector is meant a crystal or a
stack of layers on a substrate.
[0007] Thus, to cover a wavelength range comprised for example
between 0.1 nm and 2 nm, it is necessary to use two devices and two
implementations completely distinct, one of them enabling to work
in the VUV range (diffraction grating) and the other in the X-ray
range (diffraction crystal). Still, it appears useful in some cases
to work in a range covering both these ranges, for example for a
monochromator collecting a synchrotron radiation beam.
[0008] The invention concerns a light diffraction optical method
implementing a diffraction grating, enabling to cover an extended
spectral range, extending for example from the vacuum ultraviolet
to hard X-rays. More accurately, the optical measuring method of
the invention enables to take into account wavelengths ranging from
0.1 nm to 20 nm or more, by means of a single device, easily and
economically.
[0009] The invention also concerns an optical system and an optical
measuring device having the advantages aforementioned.
[0010] To this end, the invention relates to an optical measuring
method wherein:
[0011] at least one incident light beam having at least one
wavelength is sent onto a surface of an optical system having a
normal axis, according to a is direction forming an angle of
incidence relative to said normal axis, such optical system
comprising a Bragg reflector and a grating engraved at the surface
of the Bragg reflector, and
[0012] at least one returned beam is collected by the optical
system according to a diffraction direction forming an angle of
diffraction with respect to the normal axis, after diffraction of
the incident beams by the optical system.
[0013] According to the invention, the wavelengths and the angle of
incidence of at least one of the incident beams are such that this
incident beam is diffracted by the Bragg reflector and/or by the
grating.
[0014] Thus, the Bragg reflector whereon is engraved the grating,
is used directly for the diffraction at low wavelengths.
[0015] The method of the invention thus enables to provide a
dissociation of the Fresnel diffraction (on the grating) and of the
Bragg diffraction (in the Bragg reflector), thereby providing
double operation in the VUV range (grating) and in the range of the
X-rays (Bragg reflector).
[0016] The method of the invention thus enables to simplify
considerably the measuring protocol and to provide smaller
equipment.
[0017] Advantageously, for this incident beam diffracted by the
Bragg reflector:
[0018] the Bragg reflector is a crystal,
[0019] the angle of incidence is comprised between 5.degree. and
80.degree. and/or
[0020] the wavelengths are ranged between 0.1 nm and 0.7 nm.
[0021] In a preferred embodiment, the crystal is formed of an
silicon monocrystal (111) and the grating is directly engraved on
this crystal. Such a substrate proves particularly suitable to
fulfil the double function of crystal diffraction and of grating
diffraction.
[0022] The grating is advantageously covered with a metallic layer.
The efficiency of the grating is thereby increased.
[0023] Preferably, the wavelengths and the angle of incidence of at
least another of the incident beams are such that this incident
beam is diffracted by the grating.
[0024] The double function of the optical system is thereby
provided: diffraction by the crystal and diffraction by the grating
Advantageously, for this other incident beam diffracted by the
grating:
[0025] the angle of incidence is at least equal to 70.degree.
and/or
[0026] the wavelengths of the other incident beam are comprised
between 0.6 nm and 150 nm.
[0027] In a first implementation of the method, the incident beams
are polychromatic luminous beams. Then, advantageously, the method
is implemented in a monochromator, the optical system serving as a
dispersive element and followed by a selection slit.
[0028] In a second implementation of the method, the incident beams
are monochromatic luminous beams. The optical system may thus be
used as primary calibration means for measuring wavelengths.
[0029] The invention also concerns an optical system comprising a
Bragg reflector.
[0030] According to the invention, it comprises a diffraction
grating engraved on the Bragg reflector.
[0031] Preferably, the Bragg reflector is a crystal and the crystal
is composed of a silicon monocrystal.
[0032] This optical system enables to implement the method of the
invention.
[0033] The invention also relates to an optical measuring device
comprising:
[0034] an optical system according to the invention,
[0035] means for irradiating the surface of the optical system by
means of at least one incident beam of light,
[0036] means for collecting at least one returned beam by the
optical system after diffraction of the incident beams by the
optical system, and
[0037] rotary means relative of the optical system with respect to
the incident beams.
[0038] The invention also applies to the use of the method or of
the device according to the invention for primary calibration for
measuring wavelengths (monochromatic luminous beam) or for
dispersion in a monochromator or spectrograph (polychromatic
luminous beam).
[0039] This invention will be understood better and illustrated by
means of the following embodiments, without limitation thereto,
with reference to the appended drawings whereon:
[0040] FIG. 1 is a schematic diagram illustrating an optical system
used in the method according to the invention (the scales are not
respected for better visibility);
[0041] FIG. 2 shows a profile, measured with a scanning tunneling
microscope (STM) of a modulation recorded on an optical system used
to implement an optical measuring method according to the
invention;
[0042] FIG. 3 is a principle diagram of embodiment of the optical
measuring method according to the invention in diffraction crystal
mode;
[0043] FIG. 4 is a principle diagram of the embodiment of the
optical measuring method according to the invention in a mode
diffraction grating;
[0044] FIG. 5 represents for the optical system of FIG. 2 and in
diffraction crystal mode, the reflectivity as a function of the
difference of the angle of incidence to the angle of Bragg for a
wavelength equal to 0.154 nm;
[0045] FIG. 6 represents for the optical system of FIG. 2 and in
diffraction grating mode, the efficiency as a function of the
incidence angle for the orders 1 and -1, for a wavelength equal to
1.33 nm;
[0046] FIG. 7 represents for the optical system of FIG. 2 and in
diffraction grating mode, the efficiency as a function of the angle
of incidence for the orders 1 and -1, for a wavelength equal to
1.55 nm; and
[0047] FIG. 8 shows an optical measuring device used to implement
an optical measuring method according to the invention.
[0048] An optical system 1 (FIG. 1) comprises a Bragg reflector 2
and a grating 3 engraved on the substrate of the Bragg reflector 2
at a surface 8 of the optical system 1. The grating 3 is covered
with a metallic layer 4, for example composed of a layer of 10 nm
of gold.
[0049] The Bragg reflector 2 is advantageously composed of a
silicon monocrystal (111). It is super-polished, with a slope error
of a few tens of arc s conds and a roughness of a few .ANG.. This
polishing enables the operation of the optical system 1 in grazing
reflection, for use in vacuum ultraviolet implementing a
diffraction by the grating 3.
[0050] In an alternative embodiment, the Bragg reflector 2 is a
stack of layers. It may be itself placed on a substrate.
[0051] The diffraction grating 3 is for example inscribed by
holographic recording and ionic machining. It comprises lines 5
(FIG. 2) whereof the depth is for example smaller than 10 nm, which
makes it a very little modulated grating. The profile of the
grating 3 can be obtained by scanning tunneling microscopy, in
height (depth of engraving), width and length (respectively axes
11, 12 and 13, in nm). The density N of lines of the grating 3 per
mm is for example equal to 1200.
[0052] For the lines 5, different shapes (sinusoidal, triangular,
and square) and different density laws (constant or variable) may
be used.
[0053] The optical system 1 is used to cover a spectral range from
the vacuum ultraviolet to hard X-rays. Thus, according to a first
embodiment (FIG. 3), an incident beam 21 is sent, having a
wavelength smaller than 0.6 nm on the surface 8. The optical system
1 having a normal axis 20 to the surface 8, the incident beam 21
forms relative to this normal axis, an angle .alpha. and with
respect to the reticular planes 6 of the Bragg reflector 2 (i.e.,
in such case, relative to the surface 8), an angle .theta.. The
angle .alpha. ranges preferably between 5.degree. and
80.degree..
[0054] The optical system 1 then behaves like a conventional
diffraction crystal, the beams diffracted 22 by the Bragg reflector
2 forming with the normal axis 20, an angle .beta. equal to the
angle .alpha. (FIG. 3). This system 1 may also be used as a
wavelength calibration or as a diffracting element of an X-ray
monochromator.
[0055] The absence of perturbations of the Bragg diffraction in the
Bragg reflector 2 by the grating 3 may be explained in that the
depth of the lines 5 of the grating is sufficiently small relative
to the depth of penetration of the incident beam in the Bragg
reflector 2, when such incident beam has wavelengths which are
sufficiently small (in particular X-rays).
[0056] The spectral range covered is given by the Bragg law:
.lambda.=2d sin .theta.
[0057] d designating the distance between the reticular planes.
Consequently, for the Bragg reflector 2 of the example (silicon
monocrystal (111), the double 2d of the distance is equal to 0.627
nm.
[0058] Thus, the angle .alpha. being comprised between 50 and
80.degree., the useful wavelengths range extends approximately from
0.1 nm to 0.625 nm.
[0059] Good results can also be obtained while using an oriented
silicon monocrystal (311).
[0060] According to a second embodiment, the optical system 1 is
caused to operate as a diffraction grating in the vacuum
ultraviolet range. Thus, (FIG. 4) an incident beam 25 is sent at a
wavelength greater than 0.6 nm. This incident beam 25 forms with
the normal axis 20, an angle .alpha. enabling to provide high
efficiency in the order of diffraction used, advantageously greater
than or equal to 70.degree., so that the incident radiation is
quasi a grazing one. High efficiency is thereby maintained. The
incident beam 25 then interacts with the diffraction grating 3 and
generates diffracted beams 26 forming angles .theta. with the
normal axis 20, such angles of diffraction .beta. depending on the
order of diffraction considered (the diffracted beam 26 represented
on FIG. 4 corresponds for example to the order -1).
[0061] The correct behaviour of the optical system 1 has been
checked for both operating modes, respectively in Bragg diffraction
and in Fresnel diffraction. For the tests performed, the density of
lines 5 by mm is equal to 1200 and the depth of the lines 5 is
equal to 7.2 nm.
[0062] Thus, the response of the optical system 1 has been tested
for a fixed wavelength (0.154 nm) as a function of the incidence
angle on an X-ray tube with a goniometer .theta.-2.theta.. In such
an arrangement, when the angle of incidence varies by
.DELTA..theta., the detector is rotated by 2.DELTA..theta., in
order to comply with the law of Bragg. On FIG. 5, as a function of
the difference of the incidence angle to the Bragg angle (given by
the law of Bragg, axis 14, in arc-seconds), the reflectivity is
carried forward for the wavelength of 0.154 nm. It can be noted
that the curve 31 obtained has a width at half the maximum (FWHM)
smaller than 20 arc-seconds, whereas the result is equivalent to
that obtained with conventional silicon crystals. This validates
the use of the optical system 1 in the X-ray range.
[0063] In order to test the operating mode in diffraction grating,
two monochromatic beams 25 have been sent in succession at two
distinct wavelengths. For each of them, the efficiency of the
grating 3 was measured in the order -1 and +1 as a function of the
angle of incidence .alpha.. Moreover, k designating the order of
diffraction, .lambda. representing the wavelength and N being the
number of lines 5 per mm of the grating 3, the detector was placed
at such at an angle that the law of the gratings is complied
with:
sin .alpha.+sin .beta.=kN.lambda..
[0064] For the wavelength of 1.33 nm (FIG. 6), the efficiency is
represented (i.e. the ratio of the intensity of the flux of the
diffracted beam 26 to the intensity of the flux of the incident
beam 25, axis 17) as a function of the angle of incidence (axis 16,
in degrees). For the orders -1 and 1, respectively sets of points
41 and 43 are obtained. They are compared respectively to
theoretical curves 42 and 44 calculated on the basis of the
parameters of the grating 3, derived from the measurements realised
with the scanning tunneling microscope. Similarly, for a wavelength
of 1.55 nm (FIG. 7), respectively sets of points 45 and 47 are
plotted as well as the corresponding theoretical curves 46 and 48
for the diffraction orders -1 and +1.
[0065] It can be observed that the measurements obtained are very
close to the theoretical curves, which validates the operation of
the optical system 1 in diffraction grating mode, the latter
diffracting the radiation with notable efficiency.
[0066] One may also use the optical system 1 in an optical
measuring device (FIG. 8) comprising means for irradiating 51 the
surface 8 of the optical system 1 and means for collecting 52 beams
returned by the optical system 1 after diffraction of the incident
beams. Such device also comprises rotary means 53 relative to the
optical system with respect to the incident beams, acting on the
optical system 1 and/or on the orientation of the incident beams 21
or 25, in order to produce relative rotation 54.
[0067] For instance, with the irradiating means 51, a monochromatic
luminous beam is emitted and thanks to the rotary means 53, the
optical system 1 is oriented with respect to the incident beam in
crystal diffraction mode (angle of incidence a comprised between
5.degree. and 80.degree.) or in grating diffraction mode (angle of
incidence .alpha. advantageously greater than or equal to
70.degree.), according to the range to which belongs the wavelength
of the beam processed. One then proceeds conventionally in crystal
operation or in grating operation with the collection means 52.
[0068] In another example, the irradiating means 51 are the output
of a synchrotron producing a polychromatic energy beam, and the
optical system 1 as well as the collection means 52 are used as a
monochromator. The collection means 52 comprise notably a silt for
selecting wavelengths. The device is th n implem nted in grating
diffraction mode or in crystal diffraction mode, according to the
wavelength(s) studied.
* * * * *