NZ602603B2 - Spectrometric instrument - Google Patents
Spectrometric instrument Download PDFInfo
- Publication number
- NZ602603B2 NZ602603B2 NZ602603A NZ60260312A NZ602603B2 NZ 602603 B2 NZ602603 B2 NZ 602603B2 NZ 602603 A NZ602603 A NZ 602603A NZ 60260312 A NZ60260312 A NZ 60260312A NZ 602603 B2 NZ602603 B2 NZ 602603B2
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- New Zealand
- Prior art keywords
- observation
- beamsplitter
- interferometer
- face
- angle
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- 230000005855 radiation Effects 0.000 claims abstract description 42
- 230000003287 optical effect Effects 0.000 claims abstract description 37
- 230000003595 spectral effect Effects 0.000 claims description 12
- 238000000034 method Methods 0.000 claims description 8
- 239000000523 sample Substances 0.000 claims description 8
- 238000012937 correction Methods 0.000 claims description 4
- 230000002596 correlated effect Effects 0.000 claims 1
- 230000000875 corresponding effect Effects 0.000 claims 1
- 238000013461 design Methods 0.000 description 10
- 238000006073 displacement reaction Methods 0.000 description 4
- 230000000694 effects Effects 0.000 description 4
- 238000001914 filtration Methods 0.000 description 3
- 238000005070 sampling Methods 0.000 description 3
- 238000004611 spectroscopical analysis Methods 0.000 description 3
- 125000004122 cyclic group Chemical group 0.000 description 2
- 230000001419 dependent effect Effects 0.000 description 2
- 230000005684 electric field Effects 0.000 description 2
- 239000000835 fiber Substances 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 1
- 238000004891 communication Methods 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- 230000002452 interceptive effect Effects 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000013307 optical fiber Substances 0.000 description 1
- 230000010355 oscillation Effects 0.000 description 1
- 239000013074 reference sample Substances 0.000 description 1
- 238000001228 spectrum Methods 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B9/00—Measuring instruments characterised by the use of optical techniques
- G01B9/02—Interferometers
- G01B9/02015—Interferometers characterised by the beam path configuration
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
- G01J3/453—Interferometric spectrometry by correlation of the amplitudes
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
- G01J3/28—Investigating the spectrum
- G01J3/45—Interferometric spectrometry
- G01J3/453—Interferometric spectrometry by correlation of the amplitudes
- G01J3/4535—Devices with moving mirror
Abstract
Disclosed is a spectrometric instrument (2) comprising a scanning interferometer (4,6,8), a monochromatic optical radiation source (12) and an observation optical radiation source (16). The interferomenter has a beamsplitter (4) for dividing incident optical radiation into a reflected beam and a transmitted beam. The monochromatic optical radiation source (12) is arranged to launch a reference beam into the interferometer (4,6,8) along a first propagation path (14) to be initially incident on a first face (4') of the beamsplitter (4). The observation optical radiation source (16) is arranged to launch an observation beam (18) into the interferometer (4,6,8) along a second propagation path (20) to be initially incident on the first face (4') of beamsplitter (4) and overlap the reference beam at the first face (4'). The radiation sources (12,16) cooperate to generate a first angle (?) between respective first (14) and second (20) propagation paths at the first face (4') which is larger than a divergence half-angle (?) of the observation beam (18). ansmitted beam. The monochromatic optical radiation source (12) is arranged to launch a reference beam into the interferometer (4,6,8) along a first propagation path (14) to be initially incident on a first face (4') of the beamsplitter (4). The observation optical radiation source (16) is arranged to launch an observation beam (18) into the interferometer (4,6,8) along a second propagation path (20) to be initially incident on the first face (4') of beamsplitter (4) and overlap the reference beam at the first face (4'). The radiation sources (12,16) cooperate to generate a first angle (?) between respective first (14) and second (20) propagation paths at the first face (4') which is larger than a divergence half-angle (?) of the observation beam (18).
Description
Description
Spectrometric Instrument
The present invention relates to a spectrometric instrument comprising a
scanning interferometer and more particularly comprising a ng
interferometer operating according to the Michelson principle or a principle
derived there from (generally referred to in this specification as a
“Michelson type” interferometer).
Known scanning interferometers, such as those ofthe son type,
generally comprise a beamsplitter (typically also including a compensator)
and two or more reflectors, such as mirrors or reflectors, with at least
one ofthe reflectors being arranged to be reciprocally translatable.
Collimating lenses or other optics may also be associated with the
interferometer but are not fundamental to its operating principle which
relies ially on the presence of a beamsplitter and relatively movable
reflectors.
It is tood that a scanning interferometer refers to an optical
arrangement in which a beam is first split by a beamsplitter into two
components which are subsequently recombined to interfere with one
another after each having traversed a different path that is delimited by a
respective one of a pair of relatively moveable reflectors. Information may
then be derived from the spectral ts of the interference which
relates to a property of a sample with which the beam has interacted.
When such an interferometer is, for example, employed in a spectrometric
ment for optical spectroscopy, an observation beam consisting of
relatively broad band radiation in a wavelength region of interest is
launched into the interferometer to strike the litter. In this context
the term “launched” refers to a beam being itted from a last optical
element, such as a light source, a fiber optic end, a lens or other optical
t which may affect the beam path or shape. This observation beam
is split into ially two parts of equal intensity at the beamsplitter. A
first beam is reflected by the beamsplitter and travels along a first ‘arm’ of
the interferometer to the first reflector from where it is reflected back to the
beamsplitter. A second beam is transmitted through the beamsplitter and
travels along a second ’arm’ to the second reflector from where it is also
reflected back to the beamsplitter to p the reflected first beam. The
retardation, 6, is the difference between the optical path lengths of the two
arms and depending on the retardation each wavelength of the spectral
source may interfere destructively or constructively when the
back-reflected light in the two arms overlap on the beamsplitter. The
intensity pattern of the overlapping, interfering light as a function of
retardation is known as an interferogram. The erogram is recorded by
a detector as the one or more tors are moved to create cyclic
excursions of the related optical path and hence a cyclic l path
length difference between the first and the second beams. As a result of
this each ngth in the observation beam is modulated at a different
frequency. Spectral ation may then be extracted from this
observation interferogram by numerically performing a Fourier transform
(FT).
When recording an observation interferogram, particularly when using the
led Fast FT technique, the sampling ofthe output of the associated
detector at exact equidistant positions of the translatable reflector is critical
for avoiding error.
It has become a well established practice in FT spectroscopy to use a
monochromatic source of radiation of known ngth, A, such as a
laser, to generate a reference beam. This reference beam is employed in
the scanning interferometer to determine the required exact equidistant
positions and one such FT interferometer is sed in US 6,654,125.
Here, as is common, the reference beam is launched into the scanning
interferometer simultaneously with the ation beam and is made to
follow a light path through the optical components of the interferometer
that is substantially parallel to that followed by the observation beam. As
with the observation beam the reference beam is split into two beams of
substantially equal intensity by the litter. A reference erogram
is generated by the two back-reflected portions of reference beam upon
their overlap at the beamsplitter to be detected by an associated detector.
This reference interferogram is sinusoidal having a period of oscillation on
the retardation axis aper, that is directly related to the wavelength as: ape.-
=N2 (1)
Since the ngth of the nce beam is accurately known then
periodically occurring features, such as zero crossing positions, of the
nce interferogram can be employed to accurately determine the
incremental displacement and/or velocity of the translatable tor in the
interferometer. Thus the sampling time for the observation interferogram
may be accurately determined.
A problem associated with the known scanning erometer design is
that the launch of the reference beam into the interferometer either
requires additional optical components or obstructs the observation beam
path. The reference beam may, for example, be launched by using
periscope mirrors or through a hole in any collimating optics for the
observation beam. In both cases however, a part of the observation beam
is blocked. Alternatively, the nce beam may be launched into the
interferometer using a dichroic mirror but this also gives rise to a reduction
in the total power ofthe observation beam through the interferometer and
also requires space in the observation beam path.
According to a first aspect of the present invention there is provided a
spectrometric instrument comprising: a scanning interferometer having a
beamsplitter for dividing nt l radiation into a reflected beam
and a transmitted beam; a romatic optical radiation source for
launching a reference beam into the interferometer to be initially incident
on a first face of the beamsplitter; an observation optical radiation source
for launching an observation beam into the interferometer to be initially
incident on the first face of beamsplitter and overlap the reference beam at
the first face; wherein the radiation sources cooperate to generate a first
angle n propagation paths of the two beams at the first face which
is larger than a co-planar divergence half-angle ofthe observation beam.
It is well known that all radiation beams have a divergence angle which
describes the extent of a widening ofthe beam with distance. It may be
considered, for example, as the angle n two directions on te
sides of an axis ofa light beam parallel to the beam path and in the same
plane as the axis at which the light intensity typically equals a stated
percentage of a reference intensity. If the beam has been collimated using
a lens or other focusing element, the divergence expected can be
calculated in a known manner from two ters: the diameter, D, of
the est point on the beam before the lens, and the focal length of
the lens, If The divergence half-angle is, as its name s, an angle
whose magnitude is halfthat of the divergence angle.
Thus by introducing the nce and ation beams into the
interferometer such that the angle between their directions of propagation
at the first face ofthe beamsplitter on which they are both initially incident
is larger than the co-planar divergence half angle of the observation beam,
it is possible to launch the reference beam from outside of the observation
beam to overlap with the observation beam at the first face of beamsplitter
without the need of any additional optical components; without obstructing
the ation beam and without the need for increasing size of
beamsplitter and the other l components.
Moreover, the angling of the beam paths ing to the present
invention provides a spatial filtering ofthe reference beam and the
observation beam so that an instrument may be designed in which
background radiation at an associated detector due to the other beam is
significantly reduced or even eliminated.
Usefully, a er is employed to extract spectral information from the
observation interferogram recorded by an associated detector and is
specifically adapted to compensate mathematically for wavelength errors
introduced in the spectral information due to the relative angling of the
reference and observation beams according to the present ion. This
correction ofthe ngth scale which is applied in the computer
provides an increased accuracy of the measurements made using the
interferometer.
According to a second aspect ofthe present invention there is provided a
method of operating a spectrometric ment having a scanning
interferometer according to the first aspect of the present invention
comprising the step of simultaneously launching a reference beam and a
divergent observation beam towards the beamsplitter to be initially incident
at a first face thereof, the beams being launched to provide at the
beamsplitter at a first angle between their optical paths greater than a
divergence half-angle of the observation beam.
An embodiment of the invention will now be described by way of example
only and with reference to the drawings of the anying figures of
which: Fig. 1 illustrates a sectional view in the X/Y plane of son type
interferometer according to the present invention; Fig. 2 illustrates a
sectional view in the Y/Z plane of the son type interferometer of Fig.
1; Fig. 3 illustrates graphically design criteria constraints on the
interferometer illustrated in Figs. 1 and 2; and Fig. 4 illustrates a sectional
view of a further embodiment of a Michelson type interferometer according
to the present invention.
Consider now an exemplary embodiment of a spectrometric instrument 2
according to the present invention which, as is illustrated in Figs. 1 and 2,
is presently configured to comprise a Michelson type scanning
interferometer. As the l principle of operation of such a scanning
interferometer is well known it will be described here only in such detail as
is necessary for an understanding of the present invention. The
exemplified scanning interferometer comprises a beamsplitter, here a
circular beamsplitter 4, and two reflectors which are here in the form of
circular plane-mirrors 6,8. One of the mirrors 6 is mounted for reciprocal
ation (illustrated by the double headed arrow) over a distance shown
as 2L and the other mirror 8 is fixed. The beamsplitter 4 is, in the present
embodiment, enclosed in an interferometer housing 10 together with the
two tors 6,8. Also comprising the exemplified instrument 2 are a
monochromatic optical radiation source 12 for generating a nce
beam and launching it generally along a propagation path 14,
uninterrupted by additional optical ts, towards a first face 4’ ofthe
beamsplitter 4 of the interferometer ) and an observation optical
radiation source 16 for generating a divergent observation beam 18 and
ing it towards the first face 4’ of the beamsplitter 4 of the
interferometer (4,6,8) generally along a ation path 20 between the
WO 50172
source 12 and the beamsplitter 4 without passing through additional
optical elements which would affect the direction of propagation (i.e.
propagation path) of this beam 18. It will be iated that should other
embodiments of an instrument according to the present invention comprise
optical elements or other components interposed between the s
12,16 and the beamsplitter 4 which may alter either of the propagation
paths 20,14 then the propagation paths according to the present invention
will be the direction of propagation of the appropriate beam between the
last of such an optical element and the litter 4. The term ‘launch’
will be interpreted accordingly.
As is known, the beamsplitter 4 is considered the first element of the
scanning interferometer (4,6,8) and is constructed so that an incident
beam will be divided into beams of substantially equal intensity to traverse
a transmitted beam path 22 and a reflected beam path 24. The moveable
mirror 6 is disposed relative to the litter 4 to return the beam
traversing the transmitted beam 22 path back to the beamsplitter 4 as it is
reciprocally translated. The other, fixed, mirror 8 is disposed relative to the
beamsplitter 4 to return the beam traversing the reflected beam path 24
back to the litter 4 to overlap with the ed beam following the
transmitted beam path 22 and thereby an interferogram is generated for
each ofthe reference beam from the reference beam source 12 and the
observation beam 18 from the observation source 16.
Corresponding nce beam and ation beam radiation detectors
26 ,28 respectively are also provided as a part of the spectrometric
instrument 2. The reference beam radiation detector 26 is disposed in the
interferometer housing 10 to detect a nce interferogram generated
from the reflected components of launched reference beam which
traverses a reference beam path 36. The ation beam radiation
detector 28 is likewise disposed in the interferometer housing 10 to detect
an ation interferogram generated from the reflected components of
the ed observation beam which traverses an observation beam path
34. Usefully and according to an embodiment ofthe present invention the
reference beam radiation detector 26 may be located outside of the
observation beam which traverses the beam path 34 from the beamsplitter
4 towards the observation beam detector 28. This allows the available
radiation which is incident on the observation beam detector 28 to be
maximised and provides for a spatial filtering of the observation beam path
34 and the reference beam path 36 at the respective detectors 28,26. This
spatial filtering effect is advantageous in that background noise in the
respective detectors 28,26 caused by light from the other beam (i.e. light
from the observation beam incident on the reference detector 26 and Vice
versa) is substantially reduced and may even be eliminated.
These reference beam and observation beam ors 26,28 are, in the
present ment, all d within the interferometer housing 10 but it
will be appreciated that one or more of these may be located outside the
g 10 and optically coupled, for example by means of suitable optical
fibers, into the housing 10. Similarly one or both the monochromatic optical
radiation source 12 and the observation optical radiation source 16 may be
located outside of the housing 10 and optically coupled into it so as to
follow the beam paths as illustrated in Figs. 1 and 2 and as described
herein.
A data processor, such as a suitably programmed computer 30, may be
operably connected to each of the reference beam and observation beam
radiation detectors 26, 28 to receive signals representative of the
tive detected reference interferogram and observation erogram
and to process these signals in order to obtain spectral information from
the ation erogram, typically by subjecting the observation
interferogram to a Fourier analysis. In the present embodiment the
computer 30 is illustrated as being a single device but it will be appreciated
that in the present context computer is to be taken to mean one or more
s configured using conventional mming and electronic
ering techniques to tically perform the desired calculations.
Any one or more of such one or more devices which constitute the data
processor 30 may be al with the housing 10 or may be provided
external the housing 10 in local (as illustrated via fixed connection) or
remote communication (such as via a telecommunications link, intranet or
internet connections).
When the spectrometric instrument 2 is used in optical spectroscopy a
transparent or translucent cuvette or other sample holder 32 may be
located in the observation beam path 20 and here is configured so as not
to alter the general direction of the beam path 20 between the source 16
and the first face 4’ of the beamsplitter 4. In the present embodiment, and
as an example only, the sample holder 32 is located before the
beamsplitter 4 (in the direction of propagation ofthe observation beam 18
along the path 20) but it may be d after the beamsplitter 4 or even
located before the litter 4 outside ofthe housing 10 if the
observation optical radiation source 16 is also located outside of the
housing 10. n wavelengths of the observation beam 18 will interact
with sample material in the holder 32 more than others. This produces a
ngth dependent variation in intensity ofthe observation beam 18
which is characteristic of the material in the sample holder 32. This
al information may be extracted from a deconvolution of the
observation interferogram, such as by means of a Fourier transformation,
in the computer 30.
The present uration has an advantage that the displacement ofthe
transmitted n 22 of the nce beam across the beamsplitter 4
(the walk-off) as the moveable mirror 6 is reciprocally translated is
minimised as compared with other relative ations of the
monochromatic optical radiation source 12 and the observation ion
source 16. It will however be appreciated that other relative orientations of
the sources 12, 16, about the Y axis (here equivalent to the propagation
path 20) may be employed without departing from the invention as
claimed.
Not all ofthe design variables ofthe interferometer (4,6,8) are
independently selectable and the interferometer 2 of Figs. 1 and 2 may be
designed having regard to the design criteria discussed in the following:
Consider the observation beam 18 that is being launched into the
interferometer (4,6,8) to be initially incident at the first face 4’ of the
beamsplitter 4 from the source 16 which, in the present embodiment, is
configured and orientated such that the beam divergence is rical
about a general direction of the beam propagation 20 (such as defined by
the direction of propagation ofthe beam centre or of the maximum ofthe
beam power distribution). This observation beam 18 has a divergence
half-angle, d, with respect to this general direction of beam propagation
. Simultaneously with this the reference beam is being launched into the
interferometer (4,6,8) along the reference beam path 14 to be initially
incident at the first face 4’ of the beamsplitter at an angle, 6, to the
propagation path 20 ofthe observation beam 18 in the plane (here, as
illustrated the Z-X plane) containing the divergence ngle or where,
according to the present invention, 6>d. Displacement of the moving mirror
6 varies between —L and +L. Thus, the total displacement of the mirror 6 is
Ltot=2L and the retardation varies between -2L and 2L. The maximum
retardation is omax=2L
When the retardation ofthe interferometer (4,6,8) is zero, the returned
components of the reference beam will have a maximum overlap on the
litter 4. However, since 9 is non-zero the returned components of
the reference beam will move away from one another on the beamsplitter
4 when the absolute value ofthe retardation increases above zero. This is
the so-called walk-off effect. At the largest absolute retardation, amax the
distance between the centres of the returned reference beam components
is: 2L sin(9) =6max sin(9) (2)
The amplitude ofthe nce interferogram is given by the p
integral of the electric field strength distribution of the two components of
the ed reference beam, which means that the amplitude is constant
only if dref>> omaxsin(e), where dref is the full width at half maximum
(FWHM) of returned reference beam (ie that traversing the n of
ation path 22 between mirror 6 and beamsplitter 4) on the
beamsplitter 4. The p ofthe magnitudes ofthe electric field strengths
will be reduced due to the walk-off effect, as the two returned beam
components move apart on the beamsplitter 4. Preferably, the
monochromatic ion source 12 is a laser source generating a
reference beam having a single spatial mode and a beam waist which is
located on the first face 4’ of the beamsplitter 4. In this manner the phase
front of the reference beam is made substantially parallel which maximises
the spatial nce and hence ses the tolerable walk-off.
If the reference beam is generated having a high spatial coherence, for
example a single mode or a diffraction limited beam, then beam walk-off
will mainly effect the ude ofthe reference interferogram. In practice,
a certain amplitude envelope on the nce interferogram is acceptable,
and the requirement on the returned reference beam size, dref, may be
d to: dref>86maxsin(9) (3), where a is an empirically determined
constant, selected such that the signal to noise ratio at the or 26 is
sufficient to permit determinations based on periodically repeating
features, typically zero-crossing determinations, to be made from the
reference interferogram.
From experiments on a ular configuration of the invention illustrated
in Figs. 1 and 2 and by way of example only, it was found that 8:50 was a
reasonable value, taking into account typical tolerances in optics and
construction. For example if the returned reference beam size is dref=2
mm and 6:10 degrees, the maximum retardation, amax, should be less
than 0.23 mm, to maintain a sufficient amplitude envelope ofthe reference
interferogram.
Another important design constraint exists between the ence
ngle, or, of the observation beam 18, the required spectral resolution
of the spectrometric instrument 2, AV, and the maximum wavenumber, V
max, at which this resolution AV is to be ed. The resolution is
inversely proportional to the maximum retardation such. This may be
defined as: 6max=1/(AV) (4), the upper limit of the ation beam
divergence may be expressed as: amax=(6mameax)-1/z (5)
Thus, if, for example, smax=o23 mm (as above) and typically the
maximum wavenumber Vmax=3000 cm'1 the maximum acceptable beam
divergence is omax=0.085 rad (or 4.9 degrees). The obtained resolution in
this case is 22cm'1 — limited by the mirror movement.
The example above illustrates the possibility of configuring a scanning
interferometer ) with a reference beam having an incidence angle 6
2012/057631
at the first face 4’ of the beamsplitter 4 which is larger than the observation
beam ence half-angle, d, and still obtaining the resolution limited by
the mirror movement (retardation). However, it may also be seen that this
kind of design is urable for achieving a high resolution as may be
iated from a consideration of the following: Following the example
above, the incidence angle ofthe reference beam may be reduced to 6:1
degree, to allow for a maximum retardation of 2.3 mm which corresponds
to an improved resolution of 2.2 cm'1. However, the requirement on the
upper limit of observation beam divergence is now Gmax=0.027 rad (or 1.5
degrees), such that . This means that the design of Figs. 1 and 2
cannot be realized, or that the maximum solid angle of the observation
beam cannot be utilized. In the latter case, the light energy hput is
reduced which reduces the signal-to-noise ratio on the detector.
The two examples described above are illustrated in the general plot in
Figure 3. The x-axis shows the incidence angle ofthe laser reference
beam and divergence angle of the observation beam respectively, and the
y-axis shows the corresponding maximum retardation, assuming a laser
spot size of d=2 mm and a maximum wavenumber of Vmax=3000 cm'1.
The low and high resolution designs described above are shown with
dashed lines. The plot shows that for the ters used here, it is only
possible to have a laser (reference source) incidence angle larger than the
divergence angle, if the m ation is smaller than
approximately 1 mm. For a larger retardation, Le. a higher resolution, it is
not possible to take advantage of the full solid angle of the observation
beam 18.
Another potential limitation in the accuracy ofthe interferometer 2
according to the present invention, is the apparent shift ofthe wavelength
as given by the period of the nce interferogram compared to the
physical wavelength, A, of the monochromatic reference beam. With an
incidence angle of 6 the retardation ofthe reference beam is a factor of
'1 longer than the movement of mirror 6. Thus the reference
interferogram will contain a factor of cos(6)'1 more zero-crossings (or other
periodically occurring features) than for a zero degree angle of incidence
reference beam and will look like a source with a wavelength of (cos(6) 0
Since, from the design of the interferometer, 6 is known with a high
accuracy such that a tion factor may be readily calculated in order to
compensate for this apparent wavelength shift.
In an embodiment of the present ion this correction factor is
employed in the computer 30 when determining the sampling time for the
observation interferogram.
It is known from, for example US2008/0290279, to correct the wavelength
scale of al information extracted from the interferogram based on
measurements of a reference sample having a spectra pattern comprising
features with known characteristic wavelength(s). In that publication the
spectral pattern associated with 002 in air within the interferometer is
employed for this purpose and is recorded as a component of the
observation interferogram. Thus according to the present invention
correction ofthe wavelength scale within the computer 30 may be done
using one or both spectral patterns from nce samples and a factor
dependent on the nce angle, 6, ofthe reference beam at the
beamsplitter 4.
A r exemplary embodiment of a spectrometric instrument 38
according to the present invention is illustrated in Fig. 4. The spectrometric
instrument 38 is lly similar in uction to that instrument 2
rated in Fig. 1 and comprises a beamsplitter 40, a fixed mirror 42 and
a moveable mirror 44 which are configured in a Michelson type
interferometer geometry as described above in respect of the instrument 2
of Fig. 1. In the present embodiment the beamsplitter 40, and mirrors 42,
44 are co-planar with an observation optical radiation source 46 (here
comprising an emission source 48 and a co-operable concave focussing
element 50) and a nce radiation source 52 (such as a
monochromatic laser radiation source). Here the radiation sources 46, 52
are, together with associated observation beam detector 54 and reference
beam radiation detector 56 (and, as illustrated in the present ment
a sample cuvette 58 and suitably programmed computer 30) are located
external of an interferometer housing 60 in which the beamsplitter 40 and
mirrors 42, 44 are located. In one realization ofthe present embodiment
according to Fig. 2 one or more of the sources 46, 48 and detectors 54, 56
will be optically coupled to the interferometer housing via fiber optic cables
or other suitable waveguides (not shown) to allow for a more flexible
spectrometric instrument 38 configuration.
As also described in relation to the instrument 2 of Fig. 1, here the
monochromatic reference radiation source 52 generates a reference beam
and launches it along a ation path 62 within the interferometer
housing 60 which is uninterrupted by additional optical elements that
would cause a deviation in the propagation path 62 to initially strike a first
face 40’ of the litter 40. The observation optical radiation source 46
generates a divergent observation beam 64 to traverse a propagation path
66 and initially strike the first face 40’ of the beamsplitter 40 in the
presence ofthe reference beam. The observation beam 64 which is
launched into the interferometer (40,42,44) has a ence half-angle or
with respect to its propagation path 66 and the propagation path 62 of the
reference beam is provided at an angle 6 to the propagation path 66 ofthe
observation beam 64, where according to the present invention e>o.
The ometric instrument 38 according to the second embodiment of
the present invention has been realized with the following design
parameters:
ation Source 46: Mirror 50 focal , f, = 14mm Emission Source
48 er, d = 2mm Divergence angle, 2o, = d/f = 8.20 Divergence
half-angle, or, = 4.10
Laser, Monochromatic Reference Source 52: Incidence angle, 6, = 180
Interferometer 40, 42,44: Max optimal retardation, amax = .24mm =
0.048mm Max(observation) wavenumber, Vmax= '1 Resolution
limited divergence, omax= (2*O.O24*3300)'1/2=4.6O Assuming 8:10, then
from on (3), dref= 1.5mm
Thus omax>o and the laser spot size is larger than 1.5mm as is required
by the present invention.
WO 20121150172 peTIEP2012/057631
Claims (8)
1. A spectrometric instrument (2;38) comprising: a scanning interferometer (4,6,8; 40,42,44) having a beamsplitter (4;40) for dividing incident optical radiation into a reflected beam and a transmitted beam; a monochromatic optical radiation source (12;52) for ing a reference beam into the interferometer (4,6,8; 40,42,44) along a first propagation path (14;62) to be lly incident on a first face (4';40') of the beamsplitter (4;40); an observation optical radiation source (16;46) for launching an observation beam (18;64) into the interferometer (4,6,8;40,42,44) along a second propagation path (20;66) to be lly incident on the first face (4';40') of beamsplitter (4;40) and overlap the reference beam at the first face (4';40'); wherein the radiation sources (12;16;52;46) cooperate to generate a first angle (8) between respective first (14;62) and second ) propagation paths at the first face (4',40') which is larger than a ence half-angle (0) of the observation beam (18;64).
2. A spectrometric instrument (2;38) as claimed in Claim 1 further comprising a reference detector (26;56) for detecting a reference interferogram generated from the launched nce beam and observation detector (28;54) for detecting an observation interferogram from the launched observation beam (18;64) wherein each detector (26;28;56;54) is located outside the beam path (36;34;62;64) of the other beam.
3. A spectrometric instrument (2;38) as d in Claim 2 further comprising data processor (30) ly connected to receive an output from each of the detectors (26;28;56;54) corresponding to the detected interferograms n the data processor (30) is specifically adapted to s the received outputs to correct for errors in spectral information extracted from the observation interferogram detected by the observation detector (28;54) resulting from having launched the reference beam at the first angle (8).
4. A ometric instrument (2;38) as d in Claim 3 n the reference beam has a beam diameter and monochromatic radiation source (12;52) is configured to launch the reference beam at the first angle (8) correlated with the beam er to achieve a degree of overlap at the first face (4';40') of the beamsplitter (4;40) selected to provide a minimum signal to noise ratio of the output from the reference detector (26) as the mirror (6) is translated sufficient to enable determination within the data sor (30) of ically repeating features from the reference erogram.
A method of operating a spectrometric instrument (2;38) as claimed in Claim 1 comprising the step of: simultaneously launching a nce beam from the monochromatic radiation source (12;52) and a divergent observation beam (18;64) from the observation optical ion source (16;46) along respective propagation paths (14;20;62;66) towards the first face (4’;40’) of the beamsplitter (4;40) of the interferometer (4,6,8; 40,42,44), the reference beam being launched along its propagation path (14;62) to be incident at the first face (4’;40’) at a first angle (6) with respect to the propagation path (20;66) of the observation beam which is greater than a ence half-angle (or) of the observation beam (18;64).
A method as claimed in Claim 5 r comprising the step of processing in a data processor (30) an interferogram obtained from the observation beam (18;64) to correct spectral information derivable rom for errors resulting from having launched the reference beam at the first angle (6).
A method as claimed in claim 6 wherein the correction comprises compensating for the reference beam having an apparent wavelength which differs from an actual wavelength by a factor of cos(6).
8. A method as claimed in any of the claims 5 to 7 further comprising the steps of passing the observation beam ) through a sample material; and processing in the data processor (30) the interferogram obtained from the observation beam (18;64) to extract spectral information characteristic of the sample material. W0 50172 W0 50172
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
EP2011056934 | 2011-05-02 | ||
EPPCT/EP2011/056934 | 2011-05-02 | ||
PCT/EP2012/057631 WO2012150172A1 (en) | 2011-05-02 | 2012-04-26 | Spectrometric instrument |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ602603A NZ602603A (en) | 2014-06-27 |
NZ602603B2 true NZ602603B2 (en) | 2014-09-30 |
Family
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