WO2018146456A1 - Interféromètre compact - Google Patents

Interféromètre compact Download PDF

Info

Publication number
WO2018146456A1
WO2018146456A1 PCT/GB2018/050275 GB2018050275W WO2018146456A1 WO 2018146456 A1 WO2018146456 A1 WO 2018146456A1 GB 2018050275 W GB2018050275 W GB 2018050275W WO 2018146456 A1 WO2018146456 A1 WO 2018146456A1
Authority
WO
WIPO (PCT)
Prior art keywords
detector
diffraction grating
interferometer
diffraction
grating
Prior art date
Application number
PCT/GB2018/050275
Other languages
English (en)
Inventor
Elin MCCORMACK
Hugh Mortimer
Kate RONAYNE
Original Assignee
United Kingdom Research And Innovation
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by United Kingdom Research And Innovation filed Critical United Kingdom Research And Innovation
Publication of WO2018146456A1 publication Critical patent/WO2018146456A1/fr

Links

Classifications

    • 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
    • G01J3/12—Generating the spectrum; Monochromators
    • G01J3/18—Generating the spectrum; Monochromators using diffraction elements, e.g. grating
    • 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
    • G01J3/44—Raman spectrometry; Scattering spectrometry ; Fluorescence 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
    • G01J3/4531—Devices without moving parts

Definitions

  • the present invention relates to a compact interferometer.
  • the interferometer is based on a static Fourier transform interferometer that does not require moving parts to produce an interferogram. As a result the interferometer can be rugged, compact and lightweight.
  • FT spectrometers are considered to have a higher throughput and resolving power compared to diffractive spectrometers.
  • One type of FT spectrometer is the static instrument, where the optical components are fixed and arranged so that an entire interferogram is recorded onto an array detector without any scanning of the mirrors. This gives the instrument two main advantages. The first is that it can be rugged, compact and lightweight, and the second is that it gives higher temporal resolution because the interferogram can be recorded rapidly before any experimental scene changes.
  • the optical path difference between the two beams is created by the movement of one mirror relative to the other.
  • the beams created by the interferometer are parallel to each other when they overlap at the detector, creating an interference pattern that is determined by the temporal position of the mirrors which creates the optical path difference between the two beams.
  • the beams overlap at an angle at the detector and the path difference is created spatially by the different paths taken by the beams within the interferometer. The result is that the path difference increases across an area where a detector can be placed, which is usually an array detector.
  • the spectral resolution in both cases is inversely proportional to the optical path difference. Therefore in the Michelson interferometer the spectral resolution limit is obtained by the maximum distance travelled by one mirror, whereas in a static
  • the limit is determined by the size (width and number of pixels) of the array detector. This is due to the Nyquist theorem that states that the highest spatial frequency that can be resolved on the detector requires a minimum of two pixels per interferometer fringe.
  • the Sagnac interferometer comprises a beamsplitter and a pair of mirrors.
  • the beamsplitter divides the light, as for the Michelson interferometer, but instead of the two beam portions travelling along linear legs and being reflected directly back to the beamsplitter, they are instead reflected around a loop.
  • the beam portions travel along similar cyclic paths but in opposite directions. For this reason the Sagnac interferometer is sometimes known as a common path interferometer.
  • the two beam portions exit via the beamsplitter and recombine at a detector to produce an interference pattern.
  • WO 2011/086357 describes a Sagnac type interferometer such as that shown in figure 1.
  • the interferometer comprises a beamsplitter 1 10 and a pair of mirrors 121 and 122, similar to the discussion above.
  • the beamsplitter divides the input beam 105 into first and second beam portions. This may, for example, be by part of the input beam being transmitted (t) 132 through the beamsplitter and part of the beam being reflected (r) 131 by the beamsplitter.
  • the reflected beam portion 131 travels towards first mirror (M1) 121 where it is reflected towards second mirror (M2) 122 and back towards the beamsplitter (BS) 1 10 where it is reflected towards the detector (D).
  • the transmitted beam portion 132 travels towards second mirror (M2) 122 where it is reflected towards first mirror (M1) 121 and back towards the beamsplitter (BS) 110 where it is transmitted towards the detector (D).
  • the first and second mirrors 121 and 122 are curved mirrors which provide focussing of the two beam portions towards the detector 150.
  • a spatial interferometric signal is formed at the detector.
  • WO 201 1/086357 is hereby incorporated by reference herein. As this is a static interferometer the resolution is, as discussed above, determined by the size (width and number of pixels) of the array detector.
  • the present invention aims to increase spatial resolution by using a spatial heterodyne technique on a static interferometer.
  • a static interferometer creates an optical path difference between two beam portions such that as the two beam portions approach each other at an angle, for example as they approach a detector, a variation in path difference results across the detector, thereby producing an interference pattern.
  • the technique overcomes the limit of the detector by heterodyning the
  • the two beam portions derived from a static interferometer are directed to the diffraction grating which is preferably located at an image plane of the two beam portions, such as where the detector was placed when no heterodyning is present.
  • the detector is located behind the diffraction grating such that the two beam portions pass through the diffraction grating before arriving at the detector.
  • the diffraction grating produces diffraction orders or diffraction spectral orders for each of the two beam portions.
  • the angle of incidence of the beam portions at the diffraction grating determines the output angle of the diffraction spectral orders.
  • a diffraction spectral order of the first beam portion can be made to overlap a diffraction spectral order of the second beam portion.
  • the overlapping diffraction spectral orders produce an interference pattern at the detector which has a lower spatial frequency. This means a given detector may achieve greater spectral resolution than without the heterodyning.
  • Heterodyning is generally considered to be the process of mixing a high frequency with a second high frequency to produce a lower frequency output.
  • the interference pattern (having a first spatial frequency) that would be produced at the image plane can be considered to be mixed with the diffraction grating (which has lines at a second spatial frequency).
  • the resulting heterodyned interference pattern has a lower spatial frequency.
  • the spatial resolution may be increased by increasing the angle of incidence of the beam portions at the diffraction grating.
  • the present invention is suited to static interferometers which do not require lenses to generate an image plane at the diffraction grating, or lenses to image the diffraction orders on to the detector. Lenses will likely introduce optical aberrations and distortions to the system reducing the resolution, and thereby counteracting at least some of the benefit gained by the heterodyning technique.
  • the interferometer is preferably a static Fourier transform interferometer.
  • the present invention provides a static interferometer comprising path differencing optics arranged to divide an input beam into first and second beam portions and respectively direct the first and second beam portions along first and second paths to a diffraction grating, the static interferometer arranged such that first and second beam portions are incident on the diffraction grating at a non-zero interaction angle relative to each other, and interference at the diffraction grating represents path difference variations between the first and second beam portions across the diffraction grating, the static interferometer further comprising a detector arranged to receive at least part of the first and second beam portions after passing through the diffraction grating.
  • the interaction angle of the first and second beam portions at the diffraction grating may be such that a diffraction spectral order of the first beam portion overlaps or interferes across the detector with a diffraction spectral order of the second beam portion.
  • the overlapped diffraction spectral orders provide a heterodyned interference pattern across the detector.
  • static interferometer is used to mean an interferometer in which an interferogram can be recorded without having to move one or more optical components relative to other components.
  • a Michelson interferometer is not a static interferometer.
  • static interferometers the pair of beam portions overlap at an angle at the detector and a path difference is created by the different paths taken by the beam portions in the interferometer. The result is that the path difference varies across the detector. For example, the path difference may vary monotonically across the detector.
  • the static interferometer may further comprise at least one focussing element arranged along the first and second paths, the focussing element providing convergence to the first and second beam portions.
  • the focussing element is preferably provided as part of the path differencing optics and may comprise one or more curved mirrors.
  • the static interferometer may be a common path interferometer in which the path differencing optics direct the first and second beam portions in opposite directions around a cyclic path.
  • the cyclic path may be defined by at least two mirror regions curved in the plane of the cyclic path, such that at the diffraction grating overlap of the first and second beam portions includes path difference variations between the beam portions across the diffraction grating.
  • the diffraction grating may be arranged between the cyclic path and a detector.
  • the first and second beam portions may be incident at the diffraction grating at a non-zero interaction angle relative to each other.
  • a diffraction spectral order of the first beam portion may overlap or interfere across the detector with a diffraction spectral order of the second beam portion.
  • the overlapped diffraction spectral orders may provide a heterodyned interference pattern across the detector.
  • the at least two mirror regions curved in the plane of the cyclic path may provide convergence to the first and second beam portions.
  • the diffraction grating is preferably located at an image plane of the two beam portions.
  • the static interferometer may comprise path differencing optics comprising: a first Wollaston prism arranged to perform the division of the input beam into the first and second beam portions, a second Wollaston prism arranged to direct the first and second beam portions to the diffraction grating, and a polarizer between the second Wollaston prism and the diffraction grating.
  • the Wollaston prisms and polarizer may be arranged such that the first and second beam portions overlap at the diffraction grating.
  • the diffraction grating may be arranged between the second Wollaston prism and a detector.
  • the first and second beam portions may be incident at the diffraction grating at a non-zero interaction angle relative to each other.
  • a diffraction spectral order of the first beam portion may overlap across the detector with a diffraction spectral order of the second beam portion.
  • the overlapped diffraction spectral orders may provide a
  • the detector may be located adjacent to, or proximate to, the diffraction grating such that multiple diffraction spectral orders produced from the first beam portion and multiple diffraction spectral orders produced from the second beam portion are incident across the detector.
  • the detector may be placed spaced less than 1- 2mm or less than 500 ⁇ from the diffraction grating, such as at 200 ⁇ from the diffraction grating.
  • the detector may be parallel to the diffraction grating.
  • the detector may be located spaced from, or distal to, the grating such that only a single diffraction spectral order produced from the first beam portion and only a single diffraction spectral order produced from the second beam portion are incident across the detector.
  • the detector may be placed spaced at least 2-3mm from the diffraction grating, such as at 3- 10mm from the diffraction grating.
  • the detector may be placed at up to 20mm from the diffraction grating.
  • the detector may be parallel to, or tilted with respect to, the diffraction grating.
  • the above arrangement of a detector spaced from, or distal to, the grating may be duplicated to use two detectors.
  • the detector mentioned above may be a first detector located spaced from the grating such that a single diffraction spectral order produced from the first beam portion and a single diffraction spectral order produced from the second beam portion are incident across the first detector
  • the common path interferometer may further comprise a second detector located spaced from the grating such that another single diffraction spectral order produced from the first beam portion and another single diffraction spectral order produced from the second beam portion are incident across the second detector.
  • the diffraction grating is preferably a transmission diffraction grating.
  • the static interferometer may be further arranged such that the first beam portion and the second beam portion are incident at the diffraction grating at equal and opposite angles. In any case, by “non-zero interaction angle” we mean that the first and second beam portions arrive at the diffraction grating at different angles to each other.
  • the static interferometer may be further arranged such that a zeroth or first diffraction spectral order of one of the first beam portion and the second beam portion exits the diffraction grating at substantially the same angle as the first diffraction spectral order of the other of the first beam portion and second beam portion.
  • the static interferometer may be arranged such that a first diffraction spectral order of the first beam portion and a first diffraction spectral order of the second beam portion exit the diffraction grating at substantially the same angle as each other.
  • the angle may be substantially normal to the diffraction grating.
  • the detector may be arranged substantially parallel to the diffraction grating.
  • the static interferometer may be arranged such that a zeroth diffraction spectral order of the first beam portion and a first diffraction order of the second beam portion exit the diffraction grating at substantially the same angle as each other.
  • the angle may be offset from normal to the diffraction grating.
  • the detector may be arranged tilted with respect to the diffraction grating.
  • the static interferometer may be arranged such that a first diffraction spectral order of the first beam portion and a first diffraction order of the second beam portion exit the diffraction grating at substantially the same angle as each other, said angle being substantially normal to the diffraction grating, and a first detector is arranged substantially parallel to the diffraction grating, and a zeroth diffraction spectral order of the first beam portion and a second diffraction order of the second beam portion exit the diffraction grating at substantially the same angle as each other, said angle being offset from normal to the diffraction grating, and a second detector being arranged tilted with respect to the diffraction grating.
  • the static interferometer further comprising one or more diffraction gratings comprising a set of two or more regions of different spacings of grating lines, wherein the regions of different spacings of grating lines are arranged to be swapped into the image plane of the two beam portions for analysis of two different input beam wavelengths
  • the detector may comprise a pixel array, such as a linear array or an array having pixels in two-dimensions.
  • the static interferometer may further comprise an analyser coupled to the detector or detectors and arranged to perform a Fourier transform on data received from the detector(s) to provide an indication of one or more wavelengths or frequencies present in the input beam.
  • the present invention also provides a Fourier transform Raman spectrometer, comprising: a source for producing a beam of radiation, such as a laser, the source arranged to direct the beam to a sample space for receiving a sample to be analysed.
  • the beam, or portions of the beam are scattered by the sample. Scattering may, for example, take the form of backscatter, transmitted scatter or off-axis scattering.
  • the resulting scattered light from the interaction of the beam with the sample is collected (or at least part of it is collected) and forms the input to an interferometer , wherein the interferometer is a static interferometer as set out above; and the spectrometer further comprises an analyser coupled to the detector and arranged to process data received from the detector as result of collecting the scattered light.
  • the detected light may be Raman scattered light, that is, radiation which is inelastically scattered or shifted to higher (Stokes) or lower (anti-Stokes) frequencies. These frequencies correspond to vibrational and/or rotational transitions in the molecules in the sample. The frequency shifts may correspond to the vibrational frequency of molecular bonds and can be used to uniquely identify the chemical composition of the sample.
  • the analyser may be configured to perform a Fourier transform on data received from the detector so to provide an indication of the wavelength or frequency of Raman transitions occurring in the sample as a result of the beam of radiation.
  • this setup there may be included a method of filtering to exclude the Rayleigh or elastically scattered light, typically using a filter, such as a notch filter or bandpass filter.
  • a filter such as a notch filter or bandpass filter.
  • this filtering can be done in the time or space domain.
  • the detected inelastically scattered light may be expressed in relative
  • wavenumbers i.e. the magnitude of shift in radiation from the incident beam.
  • Current FT Raman collects light over a wider spectral range and therefore introduces noise into the system. Inelastic scattering is a very weak effect and therefore reducing sources of noise is important.
  • the ability to heterodyne and collect data over a smaller wavelength range is important as provided by the design of Fourier Transform spectrometer in this application.
  • the present invention provides a common path interferometer arranged to divide an input beam into first and second beam portions directed in opposite directions around a cyclic path to form an interference pattern at a diffraction grating.
  • the cyclic path may be defined by at least two mirror regions curved in the plane of the cyclic path, such that the interference pattern at the diffraction grating represents path difference variations between the first and second beam portions across the diffraction grating in a direction in the plane of the cyclic path.
  • the diffraction grating may be arranged between the cyclic path and a detector.
  • the first and second beam portions may be incident at the diffraction grating at an interaction angle relative to each other such that a diffraction spectral order of the first beam portion overlaps across the detector with a diffraction spectral order of the second beam portion.
  • the interaction angle between the first and second beams is preferably nonzero.
  • the overlapped diffraction spectral orders provide a heterodyned interference pattern across the detector.
  • figure 1 is a schematic diagram of a common path static interferometer according to the prior art
  • figure 2a is a schematic diagram of a common path static interferometer of figure 1 adapted for spatial heterodyning;
  • figure 2b is a ray-tracing diagram of the arrangement of figure 2a;
  • FIGS. 3a-3d are schematic diagrams showing different diffraction order and detection schemes used for the interferometer of figures 2a and 2b;
  • FIGS. 4a and 4b are diagrams showing simulated interference patterns for the configurations shown in figures 3a-3d;
  • figure 5 shows simulated interferogram patterns across a detector placed at two positions shown in 4(a) for the first configuration of figures 3a-3d;
  • figure 6a is an interferogram derived from a tungsten halogen source and figure 6b is a background subtracted interferogram at the array detector;
  • figure 6c is an interferogram derived from a mercury argon source and figure 6d is a background subtracted interferogram at the array detector;
  • figure 8 is a graph showing experimental spectra for a range of mercury
  • figure 9 is a block diagram of a static interferometer arranged for spatial heterodyning
  • figure 10 is a Wollaston prism based interferometer arranged for spatial
  • figure 1 1 is a block diagram of a Raman spectrometer utilizing the spectrometer arrangement of figure 10. Detailed Description
  • Figure 2a shows a modified arrangement of the interferometer of figure 1.
  • an input bean from a light source is directed at a beam splitter.
  • the beam splitter generates a reflected beam, indicated by Beam A, which in the arrangement shown travels clockwise to mirror M 1 and then to mirror M2 before being reflected again by the beamsplitter towards a diffraction grating TG and detector D.
  • the beamsplitter also generates a second beam, indicated by Beam B in figure 2a, which travels anti-clockwise to mirror M2 and then to mirror M1 before being transmitted by the beamsplitter towards the diffraction grating TG and detector D.
  • the mirrors M1 and M2 are concave cylindrical mirrors.
  • a diffraction grating TG is placed at the image plane produced by the combined action of the two mirrors.
  • the diffraction grating here is a transmission diffracting grating having equally spaced diffraction lines. Other diffraction gratings such as reflective or blazed gratings may be used.
  • a detector is placed directly behind the diffraction grating, other arrangements are possible, as described herein.
  • the mirrors and beamsplitter are arranged, similarly to figure 1 , to overlap and interfere Beam A and Beam B at the image plane.
  • the diffraction grating is arranged at the image plane in place of the detector to spatial heterodyne the interference pattern.
  • the diffraction grating is arranged to receive Beam A and Beam B and produce diffraction orders of Beam A and Beam B.
  • spatial heterodyning of the interference pattern is achieved.
  • the spatial heterodyning may be achieved by the interference of the diffracted orders at the detector.
  • a beam incident at a transmission grating with angle, ⁇ , will create diffraction orders, m.
  • the specific orders created, and their relative intensities, will depend on the grating's groove or line density, shape, and depth.
  • the positive and negative diffraction orders have equal weight.
  • the groove shape affects the relative intensities of the orders.
  • the angle can be set so that certain diffracted orders produced by the diffraction grating are co-linear with each other.
  • Equation 2 Equation 2 where ⁇ 0 is the wavelength, and ⁇ 0 is the angle which is required in order to achieve co- linear beams.
  • Equation 3 Equation 3
  • the detector is placed at a distance of 200 ⁇ from the diffraction grating.
  • the figure shows that at smaller z (the upper trace in figure 5), both the lower (heterodyned) and higher (non-heterodyned) frequencies are present, whereas only the heterodyned frequency is present at larger z (the lower trace in figure 5).
  • the presence of both lower and higher frequencies is not a problem as the lower frequency can be isolated following the Fourier transform processing of the interferogram.
  • the detector can be placed parallel to the grating at larger z (see figure 4b).
  • the intensity of the signal will depend on the distribution of light in the diffracted orders.
  • the experimental setup can be used at a specific range of wavelengths for one grating. However, if a completely different wavelength region is to be studied, a diffraction grating with a different line spacing must be used, and, depending on the frequency required at the detector, the angle between the two beams may be changed accordingly.
  • FIG. 2a An incoming ray from a fibre optic (Thorlabs BF20HSMA01) is expanded (Thorlabs BE02-05-A) before being split by a beam splitter (Thorlabs BSW27).
  • One beam (Beam A) is reflected by the beam splitter and travels clockwise onto two concave cylindrical mirrors (Thorlabs CCM254-200-P01), before being reflected again by the beam splitter towards the detector.
  • the second beam (Beam B) travels anti-clockwise and is transmitted by the beam splitter towards the detector.
  • a symmetric transmission grating (Edmund Optics Transmission Grating Beamsplitter, 46-069, 80 grooves mm -1 ) is placed.
  • the detector (Mightex SME-B050-U) was placed directly behind the transmission grating, at a distance of ⁇ 1 mm.
  • the detector has an array of 2560 ⁇ 1920 pixels (horizontal ⁇ vertical).
  • the interaction angle between the two beams was changed by moving Mirror 1 (M 1 ) along the x direction, and rotating Mirror 2 (M2) in the xy plane to ensure that the beams overlap uniformly at the grating. Due do the paths taken by the beams, both beams A and B have their focal point close to the beam splitter.
  • Interferograms may be produced by the addition of the intensity in all vertical detector pixels (perpendicular to path and diffraction or interference direction) for both beams, l AB . An integration time of 4 s was used. A linear detector array comprising a single line of pixels may instead be used. The integration time will depend on the time needed to collect sufficient data.
  • Background interferograms may be recorded to create the background intensities l A and l B for both beams by the separate blocking of the beams.
  • the blocking may occur at the focal points near the beamsplitter.
  • FIG. 6a and 6c Examples of signals acquired on the array detector are shown in figure 6a and 6c.
  • a filter centred at 580 nm (Thorlabs FB580-10) was placed after both a tungsten halogen source (Ocean Optics LS-1 ) and a mercury argon source (Ocean Optics HG-1 ).
  • Figure 6a shows the signal from the tungsten halogen source, with the mercury argon source blocked.
  • Figure 6c shows the corresponding result for the mercury argon source with the tungsten halogen source blocked.
  • the corresponding background-subtracted interferograms are shown in figures 6b and 6d respectively.
  • the absorption of the tungsten halogen source is dominated by the filter which has a large full width at half maximum (FWHM) of 10 nm, resulting in the interferogram detected having a fixed width.
  • FWHM full width at half maximum
  • the image of the transmission grating can be seen in the detector images (the higher frequency lines), although this frequency is removed with the subtraction of the background in the tungsten halogen interferogram (figure 6b), but is clearly still present in the mercury argon interferogram (figure 6d). This may be due to the signal acquired being less intense, so that the background subtraction is not as efficient.
  • the top trace in figure 6d represents a non- heterodyned simulated interferogram.
  • the middle and bottom traces are respectively the heterodyned simulated interferogram and the experimental interferogram.
  • the heterodyned interferogram matches the experimental, clearly showing the lower spatial frequency (heterodyned) pattern obtained with the inclusion of the grating.
  • results shown in the figures include no apodization in order to show the true width of the corresponding peaks.
  • detector calibration has not been performed to take into account e.g., shot noise, thermal effects, pixel sensitivity variation, which would affect the relative intensities of the interferograms.
  • the emission from the yellow mercury doublet was investigated by using a mercury argon source and a 580 nm filter.
  • the simulated interferogram has been produced by taking into account only the frequencies from the doublet, and the overall shape and frequency of the interferogram matches the experimental interferograms well.
  • Figure 7c shows the corresponding experimental Fourier transformed spectra together with a Gaussian fit.
  • the expected result is that known from literature.
  • the other peaks are incorrect by ⁇ 0.05%, demonstrating that the frequency calibration is acceptable.
  • the curved mirrors will produce wavefronts that will be curved, not parallel, and therefore the overlap between the beams will occur at a range of angles at the transmission grating, and may affect the interference pattern to a small degree.
  • the setup may be modified depending on the requirements. For example, if two wavelengths that differ by 2k are to be explored simultaneously, this can be achieved at one 0, by the placement of a detector at an angle, such as at Detector position 2 of Configuration I and one detector parallel but spaced from the grating such as in
  • two custom gratings may be interchanged close to the detector at one 0, to focus on each one separately.
  • two or more gratings could be provided on a movable piece such that different gratings could be rotated in to, or slid into position as required.
  • the ability to interchange gratings provides an advantage over prior art devices in which gratings are integrated into the instrument, so that it is inherently hard to change them.
  • Different wavelength regions may be explored by changing the diffraction grating and the ⁇ ,.
  • An additional feature is the possibility to increase the bandwidth range (with the corresponding decreasing of the spectral resolution) by rotating the grating from having the rulings parallel to the interferometer fringes, to an angle between them. Rotation of the grating effectively changes the line spacing of the grating thereby changing the resolution and bandwidth, so using a grating which allows its angle to be changed allows the resolution and bandwidth to be varied.
  • the advantages of the arrangement set out in this disclosure are that it can be used with any type of static interferometer where two beams overlap at an angle onto a focal plane array e.g., Sagnac interferometers (discussed above), or Wollaston-based polarisation division interferometers (discussed below).
  • Sagnac interferometers discussed above
  • Wollaston-based polarisation division interferometers discussed below.
  • the addition of one grating limits costs and the weight of the instrument, and the grating can be small as it is placed at the image plane of the beams. In comparison to prior art methods the resolution doesn't depend on the number of grating lines, and the system is easier to align as only one grating needs to be tilted to adjust the heterodyning frequency. Transmission gratings are less sensitive to angle changes in comparison to reflective gratings, making the placement of the grating easier in the experimental setup.
  • the arrangement set out in this disclosure can be used with any type of static interferometer where two beams overlap at an angle onto a focal plane array detector.
  • An example schematic arrangement is shown in figure 9.
  • a beam is incident on optics 910.
  • the optics divide the beam into first and second beam portions.
  • the beam portions travel along different paths and are then output spaced apart from each other. As they are output there is an angle between the beams such that they are directed to overlap.
  • the paths lengths travelled by the beams will be different and have a path length variation across the plane.
  • Interference fringes may be formed at the plane of overlap.
  • Diffraction grating 920 is placed at the plane of overlap.
  • the diffraction grating will be at the image plane of the beam portions.
  • the beam portions passing through diffraction grating will be diffracted into diffraction spectral orders governed by the grating equation (Equation 1).
  • the optics 920 may be arranged such that a diffraction spectral order of the first beam portion overlaps, or follows a substantially similar trajectory, as a diffraction spectral order of the second beam portion.
  • a detector 930 such as an array detector is arranged behind the diffraction grating. The actual positioning of the detector 920 will depend on the diffraction spectral orders used, as set out above.
  • the detector may be placed parallel to and adjacent to the grating (Configuration I, Detector position 1) or may be placed spaced from the grating and tilted with respect to the grating (Configuration II, Detector position 2). If first orders are used from both beam portions (Configuration II), then the detector may be placed parallel to the grating and either adjacent to the grating or spaced apart from the grating.
  • An example of another type of interferometer that can be used with this technique is a Wollaston prism based interferometer, such as that shown in figure 10.
  • a Wollaston prism is a rectangular block made from a pair of wedge-shaped prisms joined along their hypotenuse.
  • the wedges are of birefringent material with optic axes parallel to external faces of the block, but the optic axes of the two wedges are arranged perpendicular to each other.
  • the interferometer is arranged with a polarizer 1010 at 45° to the optic axes of the first Wollaston prism 1020.
  • the input beam is incident on the polarizer 1010. Some of the intensity of the input beam may be lost by the use of the polarizer, depending on the polarisation orientation of the input beam. If the input beam polarisation is already at 45° then substantially all of the intensity will be transmitted to the first Wollaston prism 1020. As the beam passes through the Wollaston prism the portion of the input beam polarized parallel to the optic axis will travel a different optical path length to the portion of the input beam polarized perpendicular to the optic axis.
  • the differently polarized portions of the beam will leave the first Wollaston prism 1020 spaced apart and at different angles.
  • the beam portions will propagate towards and into the second Wollaston prism 1030.
  • the second Wollaston prism will be arranged in the reverse configuration to the first prism to at least partly bring the beam portions back together.
  • the beam portions departing the prism 1030 are on convergent paths.
  • the beam portions pass through a second polarizer 1040 arranged after the second prism 1030 and having its polarization direction oriented parallel to the first polarizer 1010. Behind the second polarizer the beam portions overlap and may form an interference pattern.
  • the arrangement may also include focussing elements, such as lenses, or more preferably curved mirrors.
  • a first focussing element may be arranged at the input to the first polarizer 1010. The first focussing element may be provided to collimate the beam through the prisms. If the input beam is already collimated the first focussing element may not be needed.
  • a second focussing element may be arranged between the second polarizer 1040 and diffraction grating 1050. The second focussing element is configured to produce an image plane at the diffraction grating such that fringes occur on the diffraction grating. Operation of the interferometer, use of diffraction orders and location of the detector array is as set out in the above embodiments.
  • the detector which may be a detector array or pixel array, generate signals representing the overlapped diffraction patterns from first and second beam portions.
  • the detector may comprise a linear array of pixels, that is a line of pixels, or may be a two-dimensional array of pixels such as being x pixels long and y pixels wide.
  • the linear pixel array is arranged such that the two diffraction patterns from the beam portions are spread across the detector. For the two-dimensional array the same is true, but the pixels in the transverse direction at a given length position may have the detected intensities summed to together.
  • the integration time should be set to provide a large enough signal above the noise floor, but not too large such that desired temporal changes are lost. An integration time of from a few hundred milliseconds to the order of seconds is usually appropriate.
  • analyser may include a computing device or processor.
  • the analyser may display the combined diffraction pattern based on the intensity received at each detector pixel.
  • the analyser may provide an output indicative of the wavelengths or frequencies included in the input beam by performing a Fourier transform of the signals received from the detector.
  • the signal may comprise a number of signals one for each of the pixels in the detector array.
  • the analyser may perform the Fourier transform using a fast Fourier transform (FFT) algorithm.
  • FFT fast Fourier transform
  • the analyser may also filter out frequencies that are not of interest and may perform Gaussian fits of data to provide spectral data.
  • the frequencies or wavelengths of the input beam that have been determined may be output on a display, stored or used for further processing.
  • Figure 1 1 shows schematically an embodiment of a Raman spectrometer which uses the interferometer of figure 9.
  • the cyclic interferometer of figure 2 or the Wollaston prism based interferometer of figure 10 may be used.
  • Figure 11 shows a light source 11 10 such as a laser arranged to form a beam of probe light or radiation.
  • Delivery optics may be arranged to direct the beam of probe light at a sample 1 120 in a sample space.
  • Collection optics (not shown) may be arranged to collect light scattered from the sample 1120. The scattered light may be backscattered light, transmitted light or off-axis scattered light.
  • Interferometer optics 910 such as that shown in figure 9 then receive the collected light as an input beam.
  • the input beam is divided into two beam portions, and directed along paths to impart a path difference between the two beam portions.
  • the beam portions are directed at a diffraction grating 920.
  • the beam portions overlap at the diffraction grating and may form an interference pattern.
  • the path difference of the two beam portions varies across the diffraction grating because of their different angles of incidence.
  • the techniques described for the above embodiments may be used to overlap diffraction spectral orders from the two beam portions at the detector 930.
  • An analyser 1 130 is configured to process signals received from the detector to perform spectral analysis on the collected light.
  • the light source 11 10 may typically be a near infrared laser (but may instead be in the visible), emitting a beam of probe light in the near infrared region of the electromagnetic spectrum, for example at a wavelength of around 800 nm.
  • the delivery optics may be provided by one or more suitable optical fibres and/or lenses arranged to form a beam of probe light at the sample.
  • the collection optics may also be provided by one or more suitable optical fibres and/or lenses to define the collection region at the sample and to collect probe light from this region and deliver it to the interferometer optics 910.
  • the delivery and/or collection optics may be omitted in compact embodiments, for example directing light from the source directly at the sample, and receiving light at the
  • interferometer optics directly after scattering from the sample in sample space.
  • the probe beam induces Raman scattering within the sample.
  • the detected spectral features include Raman spectral features resulting from Raman scattering within the sample 1120.
  • the Raman scattering spectral features may include Stokes features which result from loss of energy of a probe light photon during Raman scattering, and anti-Stokes features which result from gain of energy of a probe light photon during Raman scattering.
  • the wavelength shift (often discussed in Raman spectroscopy in terms of wavenumber shift for
  • static interferometers such as those described here, do not usually have the resolution for application to Raman spectroscopy.
  • the use of spatial heterodyning makes this possible.
  • the use of spatial heterodyning also limits the bandwidth which is also helpful for Raman spectroscopy.
  • Raman spectroscopy the frequency ranges or wavelength shift of interest is usually relatively small, whereas static interferometers have a wide range extending from zero up to their maximum resolvable frequency.
  • the use of spatial heterodyning overcomes this by limiting the bandwidth to limited range away from zero frequency.
  • the combination of the heterodyne technique with that of the spectrometer of figures 2a and 2b has particular advantages.
  • the spectrometer of figures 2a and 2b has a wide acceptance angle compared to that of conventional spectrometers used for Raman spectroscopy, thereby providing an increased etendue.
  • Conventional dispersive Raman spectrometers use a narrow slit to limit the optical extent of the source and so improve the spectral resolution of the spectrometer.
  • the use of the heterodyne technique in combination with that of the spectrometer of figures 2a and 2b provides an enhanced area over which light can be collected, that is enhanced etendue, and so may allow the slit to be removed.
  • a notch filter may be provided before the spectrometer to remove the source light, or the source light could be removed in the time or space domain.
  • the ability of the heterodyne technique in combination with that of the spectrometer of figures 2a and 2b allows the spectrometer to concentrate on a wavelength or frequency spread of interest for Raman sensing.
  • some conventional spectrometers detect frequencies from zero to a maximum frequency fmax, but because of this their resolution is low.
  • the arrangement may detect over a limited bandwidth such as 100 or 200 nm around a frequency of interest and with a higher spectral resolution.

Landscapes

  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Spectrometry And Color Measurement (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)

Abstract

La présente invention concerne un interféromètre compact. L'interféromètre est agencé pour diviser un faisceau d'entrée en première et deuxième parties de faisceau dirigées dans des directions opposées autour d'un trajet cyclique pour former un motif d'interférence au niveau d'un réseau de diffraction. Le motif d'interférence au niveau du réseau de diffraction représente des variations de différence de trajet entre les première et deuxième parties de faisceau. Le réseau de diffraction est agencé entre le trajet cyclique et un détecteur, les première et deuxième parties de faisceau étant incidentes au niveau du réseau de diffraction à un angle d'interaction non nul l'une par rapport à l'autre de sorte qu'au niveau du détecteur, un ordre spectral de diffraction de la première partie de faisceau chevauche un ordre spectral de diffraction de la deuxième partie de faisceau. L'interféromètre produit une résolution spatiale accrue en utilisant le réseau de diffraction pour effectuer un hétérodynage spatial. L'invention concerne en outre d'autres modes de réalisation d'interféromètre.
PCT/GB2018/050275 2017-02-07 2018-01-31 Interféromètre compact WO2018146456A1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB1701998.5A GB201701998D0 (en) 2017-02-07 2017-02-07 Compact interferometer
GB1701998.5 2017-02-07

Publications (1)

Publication Number Publication Date
WO2018146456A1 true WO2018146456A1 (fr) 2018-08-16

Family

ID=58462516

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/GB2018/050275 WO2018146456A1 (fr) 2017-02-07 2018-01-31 Interféromètre compact

Country Status (2)

Country Link
GB (1) GB201701998D0 (fr)
WO (1) WO2018146456A1 (fr)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2020075869A1 (fr) * 2018-10-12 2020-04-16 英弘精機株式会社 Lidar d'observation météorologique
US11531280B2 (en) 2018-08-29 2022-12-20 Asml Holding N.V. Compact alignment sensor arrangements
WO2023166569A1 (fr) * 2022-03-01 2023-09-07 国立大学法人東北大学 Dispositif spectroscopique, procédé spectroscopique, dispositif d'analyse de diffusion raman, dispositif d'analyse spectroscopique de luminescence et dispositif d'observation harmonique

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20120268745A1 (en) * 2011-04-20 2012-10-25 Arizona Board Of Regents On Behalf Of The University Of Arizona Ultra-compact snapshot imaging fourier transform spectrometer
US20130188181A1 (en) * 2011-10-18 2013-07-25 Stanley Michael Angel Systems and Methods for Spatial Heterodyne Raman Spectroscopy
US9046412B2 (en) * 2010-01-18 2015-06-02 The Science And Technology Facilities Council Compact interferometer spectrometer

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US9046412B2 (en) * 2010-01-18 2015-06-02 The Science And Technology Facilities Council Compact interferometer spectrometer
US20120268745A1 (en) * 2011-04-20 2012-10-25 Arizona Board Of Regents On Behalf Of The University Of Arizona Ultra-compact snapshot imaging fourier transform spectrometer
US20130188181A1 (en) * 2011-10-18 2013-07-25 Stanley Michael Angel Systems and Methods for Spatial Heterodyne Raman Spectroscopy

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
KNUTTEL A ET AL: "Stationary depth-profiling reflectometer based on low-coherence interferometry", OPTICS COMMUNICATIONS, ELSEVIER, AMSTERDAM, NL, vol. 102, no. 3-4, 1 October 1993 (1993-10-01), pages 193 - 198, XP024456335, ISSN: 0030-4018, [retrieved on 19931001], DOI: 10.1016/0030-4018(93)90380-N *
T. H. BARNES ET AL: "Heterodyned photodiode array Fourier transform spectrometer", APPLIED OPTICS, vol. 25, no. 12, 15 June 1986 (1986-06-15), WASHINGTON, DC; US, pages 1864, XP055469060, ISSN: 0003-6935, DOI: 10.1364/AO.25.001864 *

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US11531280B2 (en) 2018-08-29 2022-12-20 Asml Holding N.V. Compact alignment sensor arrangements
WO2020075869A1 (fr) * 2018-10-12 2020-04-16 英弘精機株式会社 Lidar d'observation météorologique
JPWO2020075869A1 (ja) * 2018-10-12 2021-09-02 英弘精機株式会社 気象観測用ライダー
US11650323B2 (en) 2018-10-12 2023-05-16 Eko Instruments Co., Ltd. Meteorological lidar
JP7477919B2 (ja) 2018-10-12 2024-05-02 英弘精機株式会社 気象観測用ライダー
WO2023166569A1 (fr) * 2022-03-01 2023-09-07 国立大学法人東北大学 Dispositif spectroscopique, procédé spectroscopique, dispositif d'analyse de diffusion raman, dispositif d'analyse spectroscopique de luminescence et dispositif d'observation harmonique

Also Published As

Publication number Publication date
GB201701998D0 (en) 2017-03-22

Similar Documents

Publication Publication Date Title
US10732044B2 (en) Apparatus, systems, and methods for talbot spectrometers
US11029258B2 (en) Optical phase measurement method and system
US7777895B2 (en) Linear-carrier phase-mask interferometer
US10386174B2 (en) Three-dimensional interferometer, method for calibrating such an interferometer and method for reconstructing an image
US9778105B2 (en) Static interferometer with step-style reflective element
KR102383467B1 (ko) 스냅샷 엘립소미터
JP6622733B2 (ja) 偏光解析装置および光スペクトラムアナライザ
US20230063843A1 (en) Method and apparatus for high performance wide field photothermal imaging and spectroscopy
WO2018146456A1 (fr) Interféromètre compact
US7466421B2 (en) Diffractive interferometric optical device for measuring spectral properties of light
US20050117169A1 (en) Interferometric optical system
US8279449B2 (en) All-reflective, radially shearing interferometer
CN108362381B (zh) 一种宽视场大孔径空间外差干涉成像光谱仪
Przygodda et al. Interferometric observation at mid-infrared wave-lengths with MIDI
JP2007298281A (ja) 被検体の面形状の測定方法及び測定装置
Cai et al. Spatial heterodyne spectrometer based on the Mach–Zehnder interferometer
US10175115B2 (en) Wavefront sensor and method for determining differences in piston and tilt existing between several light beams
JP2008514945A (ja) コヒーレンス分光分析装置
JP7283170B2 (ja) フランコン型サバール板を使用した分光測定装置における特有ノイズの防止方法、分光測定装置及び分光測定方法
US10571442B2 (en) Sagnac fourier spectrometer (SAFOS)
Li et al. Design of orthogonal tunable spatial heterodyne spectrometer based on prism dispersion
Palásti et al. Optical modeling of the characteristics of dual reflective grating spatial heterodyne spectrometers for use in laser-induced breakdown spectroscopy
Liu et al. A simple real-time method for checking parallelism between the two gratings in Talbot interferometry
Bai et al. Design and evaluation of spatial heterodyne Raman spectrometer at 785 nm excitation wavelength
Feng et al. Experimental research for broadband spatial heterodyne spectroscopy

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 18703830

Country of ref document: EP

Kind code of ref document: A1

NENP Non-entry into the national phase

Ref country code: DE

122 Ep: pct application non-entry in european phase

Ref document number: 18703830

Country of ref document: EP

Kind code of ref document: A1