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WO1999012009A1 - Interferometre a laser - Google Patents

Interferometre a laser Download PDF

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Publication number
WO1999012009A1
WO1999012009A1 PCT/GB1998/002601 GB9802601W WO9912009A1 WO 1999012009 A1 WO1999012009 A1 WO 1999012009A1 GB 9802601 W GB9802601 W GB 9802601W WO 9912009 A1 WO9912009 A1 WO 9912009A1
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WO
WIPO (PCT)
Prior art keywords
pulses
sub
pulse
laser interferometer
light sensitive
Prior art date
Application number
PCT/GB1998/002601
Other languages
English (en)
Inventor
Boris Yu Shekunov
Original Assignee
Bradford Particle Design Plc.
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 Bradford Particle Design Plc. filed Critical Bradford Particle Design Plc.
Priority to AU88754/98A priority Critical patent/AU8875498A/en
Publication of WO1999012009A1 publication Critical patent/WO1999012009A1/fr

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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
    • G01J9/00—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength
    • G01J9/02—Measuring optical phase difference; Determining degree of coherence; Measuring optical wavelength by interferometric methods

Definitions

  • the present invention relates to an optical laser interferometer and to a method of interferometry .
  • the present invention is well suited for dynamic measurements of high pressure solutions and high velocity flows.
  • Optical interferometry is a well established technique widely used in metrology. Sensitivities of digital interferometers for precision measurements of surface profiles (at angstrom resolution) and displacements (at picometer resolution) can be considered to be at the frontier of present experimental possibilities.
  • interferometry in physical chemistry are based on a simple principle whereby the object of investigation (for example, a growing crystal surface, solution or gas flow, or particle assembly) is placed in an optical cell and included in one arm of a Michelson, Twyman-Green or Mach-Zender interferometer.
  • the distribution of the interferometric fringes corresponds to the surface profile and/or concentration or temperature gradients. Movement of the interferometric fringes gives information about the process dynamics.
  • Interferometry can be combined with the other optical methods such as laser scattering, particle imaging velocimetry and fluo.rescence spectroscopy, complementing all these techniques.
  • the major problem is, however, the relatively long acquisition time (typically, about a second) for a typical PSI algorithm to be performed. This does not allow for fast dynamic measurements.
  • crystallization kinetics in highly supersaturated solutions require at least millisecond resolution in order to distinguish small morphological details
  • the principal problem is that it is not possible to combine the high vertical resolution of current PSI ( ⁇ /1000- ⁇ /2000) with high temporal resolution (nanoseconds) .
  • in situ investigation into the dynamics of physio-chemical processes requires, firstly, special optical cells to study phenomena under high pressure and in high velocity flows, and secondly, new approaches in signal processing in order to provide real-time observation of surface morphology or flow gradients.
  • the horizontal resolution of the interferometry is limited by the wavelength of light and is, typically, in the order of microns. This resolution is inferior to that of atomic force microscopy, the technique recently applied for in situ surface measurements. In many cases, however, the fast and non-invasive nature, and flexibility of interferometry are more important.
  • K. Onuma, K. Tsukamoto and S. Nakadate 'Application of real time phase shift interferometer to the measurement of concentration field
  • J. Crystal Growth 129 (1993) 706 J. Crystal Growth 129 (1993) 706
  • K. Onuma, T. Kameyama, K. Tsukamoto 'In situ study of surface phenomena by real time phase shift interferometry'
  • J. Crystal Growth 137 (1994) 610 is a system which used a real-time processing and, most importantly, applied the PSI system to in situ measurements of solution concentration and surface crystallization kinetics.
  • the disadvantages of this system lie in its optical design, which is based on three-camera acquisition.
  • the acquisition (integration) time is defined by the camera frame rate (about 20 ms for the interlaced CCD) . All of those methods were based on a continuous laser source and therefore unable to perform fast successive measurements.
  • Interferometry has a significant potential as a versatile, non-invasive, in-line analytical tool which allows different physico-chemical phenomena to be investigated and technological processes to be optimised for the needs of the industry. Any system in which phase borders and certain inhomogeneity are present can be investigated.
  • the list includes surface crystallization kinetics in solution, gas, melt or gel; dielectro- and electrophoresis; transitions in colloid and liquid-crystal systems; flow dynamics and ballistics, combustion and plasma behaviour; mass- and heat-transfer.
  • the present applicant's interest in the development of the novel PSI system lies in the area of crystallization technology, particularly that related to the production of solid organic pharmaceuticals from supercritical fluids and high-pressure solutions. These pharmaceuticals are required for new, more efficient drugs which are designed for specific targeting options and controlled delivery.
  • New crystallization methods have to be developed to manufacture the drug particles. These are, for example, rapid expansion of supercritical solution (RESS) and solution enhanced dispersion by supercritical fluids (SEDS) , in which crystallization is produced by a rapid, uniform increase of supersaturation in SCF solution flow at pressures of 100-400 bars and very high, often supersonic, flow velocities.
  • the crystallization can also be initiated rapidly by changing pressure (a few kbar) in a hydrostatic high pressure cell. The high pressure and high velocity conditions introduce very fast changes in the solution bulk and on the growing crystal interface.
  • the main aim of this invention is to create an interferometric system for in situ applications in physical organic chemistry with emphasis on the investigation of crystallization phenomena under high-pressure and in supercritical fluids (SCF) .
  • This system will preferably be based on instantaneous phase-shift interferometry (PSI) and double-pulsed laser illumination.
  • PSI phase-shift interferometry
  • the interferometer will be able to obtain two successive high-resolution interferograms ( ⁇ /1000- ⁇ /2000) within 5 nanoseconds and a digital processor will enable online real-time mode of operation.
  • the present invention provides a laser interferometer including: laser means for provision of two light pulses separated by a time interval; phase modulator means including means for: splitting each pulse into a pair of sub- pulses, each sub-pulse of each pair being directed along a respective separate path, directing the sub-pulses in one of the paths through or onto a sample to be investigated, and subsequently recombining the respective pairs of sub-pulses; and signal decoder means for directing the recombined pulses via respectively separate paths onto respective light sensitive devices or respective locations of a light sensitive device.
  • phase modulator means including means for: splitting each pulse into a pair of sub- pulses, each sub-pulse of each pair being directed along a respective separate path, directing the sub-pulses in one of the paths through or onto a sample to be investigated, and subsequently recombining the respective pairs of sub-pulses; and signal decoder means for directing the recombined pulses via respectively separate paths onto respective light sensitive devices or respective locations of a light sensitive device.
  • phase modulator means includes a combination of two interferometers, geometrically similar to Mach-Zender interferometers although other devices may be used, for example, a Michelson or Twyman-Green interferometer.
  • the pulse modulator means includes means for shifting the polarisation of one sub-pulse of each pair of sub-pulses. This has the advantage that the subpulses can be recombined and directed together along a light path without interfering. In interferometers including such means, means are also provided for aligning the planes of polarisation of the recombined pairs of sub-pulses.
  • the means for shifting the polarisation includes a half-wave plate .
  • the light sensitive device is a charge coupled device and a plurality of such devices may be provided.
  • both of the pairs of sub-pulses are included in the same exposure of .the light sensitive device, i.e. there is no need to recharge the light sensitive device.
  • the sample to be investigated is contained within an optical crystallisation cell.
  • the present invention provides a method of performing interferometry including the steps of:
  • FIG. 1 is a schematic diagram of an interferometer according to the present invention
  • Figure 2 is a schematic diagram of a second embodiment of a phase modulator unit for use in the interferometer of Figure 1;
  • Figure 3 is a schematic diagram of the image processor of Figure 1;
  • Figure 4 shows a second embodiment of the laser system
  • Figures 5 and 6 show an example of results obtained using an embodiment of the present invention.
  • the interferometer consists of the following modules:
  • a dual-head Nd: YAG laser system (1) provides two laser pulses with the duration of e.g. about 5 ns and a variable delay time (e.g. as small as 1 ns) between the pulses. Such system is commonly used in particle imaging velocimetry (PIV) technique.
  • the laser wavelength is preferably the second harmonic (green, 532 nm) of the residual infra-red beam.
  • the polarisation of the two pulses may be orthogonal (achieved eg by rotating one of the laser heads or inserting a half-wave plate on one of the beams) ; this property being further used for discrimination of the two successive pulses (in the phase modulator) .
  • the double-pulsed laser beam is collimated and improved using a spatial filter (not shown in the figure) .
  • the delay/pulse generator (11) is connected to both the laser (1) and computer (12), providing timing/synchronisation of the whole optical system.
  • a single continuous laser can be used together with a beam chopper, as shown in Figure 4.
  • Fig. 4 illustrates use of an eg He-Ne (633 nm) continuos laser beam 40 modulated by a beam-chopper 42 which can be, for example, an electro-optical or mechanical switch.
  • a polarisation device 44 can be an electro-optical unit such as Pockels cell or mechanical unit such as rotating half-wave plate. Both devices are synchronised with each other to provide generation of the two perpendicularly polarised laser pulses.
  • phase modulator The optical layout of the phase modulator (2) is a combination of two interferometers, similar to Mach-Zender interferometers, defined by beamsplitters NPBl-PBl and PB1-NPB2 correspondingly (the term "interferometer” is not entirely correct here because the orthogonally polarised beams do not interfere before they enter the signal decoder
  • the phase modulator works as follows: the laser pulses (of both polarisations) are equally divided by the non-polarising beamsplitter NPB1 into the reference beam (NPB1-M1) and the informative beam (NPBl-crystallization cell-M2) .
  • NPB1-M1 the reference beam
  • NPB2 the informative beam
  • Ml Mirror 1
  • M2 Mirror 2.
  • the half-wave plate (21) in the reference beam may rotates the polarisation by 90°.
  • the first half wave plate 20 adjusts the polarisation angle o.f both pulses to be 45° in respect of the optical axis of the device. This matches the intensity of the information and reference beams when recombined.
  • the informative and reference waves of each pulse acquire a phase difference (defined by the chemical process in the optical cell) and arrive orthogonally polarised at the point of polarising beam splitter PB1.
  • Their further optical path depends on the initial polarisation of the laser pulse as illustrated in Fig. 1.
  • objective lenses 22,23 help produce a sharp image. If the initial pulse has its polarisation parallel to plane Ml- NPB1-M2 (dotted arrows) , both reference and informative waves are recombined in PB1-M3-NPB2 direction due to polarisation properties of the beamsplitter PB1.
  • the reference and j informative waves are recombined in PB1-M4-NPB2 direction.
  • the mirrors M3 and M4 are shifted to produce a lateral displacement o.f the two successive laser pulses of different polarisation (see Fig. 1) .
  • the first part of the system (NPBl-PBl) can be substituted by another phase-shifting device, for example, a Michelson or Twyman-Green interferometer as required.
  • another phase-shifting device for example, a Michelson or Twyman-Green interferometer as required.
  • Signal decoder (3) The function of this module is, firstly, to introduce a fixed phase shift between the informative and reference waves and, secondly, to align the polarisation vectors of these waves (s- and p-components) in the same direction of interference. The s-components and p-components interfere respectively.
  • a laser beam, exiting from the phase modulator (non- polarising beamsplitter NPB2) passes through the half-wave plates (31,32) which rotate the beam polarisations by 45°. This rotation orients the beam so that the polarisations are at 45° to the optical axes of the polarising beamsplitters PB2 and PB3.
  • PB2 analyses the p-components of the beam in one arm and s-components in the other arm.
  • PB3 analyses these polarizations.
  • Two signals produced by PB3 are sinusoidal and 180° out-of-phase to each other. These signals are also 90° out-of-phase with those signals from the first beam half (PB2) , therefore four interferograms, with 90° phase shift, are produced for each of the two initial laser pulses.
  • the two displaced interferograms on each CCD sensor correspond to the different laser pulses. These four CCD sensors could be replaced by a single CCD sensor.
  • This decoder may be a known optical unit.
  • Image processor (4) The interferometric pictures obtained are analysed using a 3+3 phase- shift algorithm, is explained mo-re clearly in Fig. 3, which shows that the algorithm is able to correct for linear phase shift error. Simple 3- and 4-step algorithms can also be performed using the same processor.
  • the image processor (4) consists of digital interface and look-up tables (memory devices) to perform all mathematical operations in tandem. High-resolution, 12-bit format pictures from Images 1-4 ( I 2 to I 4 )
  • phase-shift map ⁇ is transferred into the PCI frame grabber and computer (12), and further subjected to mathematical treatment such as phase map unwrapping (production of a smooth picture of surface or refractive index profiles) , then a graphical presentation (2-D profiles, 3-D profiles, colour picture of the gradients, topographic map) , and determining the kinetic data (rate, slope and concentration gradients versus time) for a double laser pulse.
  • phase map unwrapping production of a smooth picture of surface or refractive index profiles
  • a graphical presentation (2-D profiles, 3-D profiles, colour picture of the gradients, topographic map
  • determining the kinetic data rate, slope and concentration gradients versus time for a double laser pulse.
  • the double-pulse acquisition procedure will be repeated (the repetition rate of the laser is about 20 Hz) and new statistics accumulated concerning the dynamics of the process over a longer time-scale (several seconds) .
  • a supercritical fluid (SCF) crystallization unit consists of a steel cylindrical tube supplied with two co-axial glass/silica high-quality optical windows, designed for pressures up to 400 bars.
  • the entire tube is constructed on a modular principle in which the tube section with windows can be mounted in any position along the SCF flow (about 600 mm long) . As a result, the flow dynamics can be monitored along the whole flow length.
  • the tube is connected, through a nozzle, to a SCF extraction system. When the crystal surface is to be observed, the surface under investigation will be mounted parallel to the cell window and the whole tube filled with a supersaturated SCF solution.
  • FIG. 2 shows a second embodiment of a phase modulator (6) for use in place of the phase modulator 2 of Figure 1.
  • This phase modulator is for use where the sample to be investigated is not transparent and therefore the optical crystallisation cell 5 of Figure 1 is not appropriate.
  • a reflective sample cell 61 is used instead.
  • the difference of the optical layout in Fig. 2 from that of the transparent mode of operation (Fig. 1) is that the informative laser beam, after being produced by beamsplitter NPB10, is reflected back (from the surface under investigation) and then recombined with the reference beam (mirrors M10-M20) at the polarising beamsplitter PB10.
  • the half-wave plate in the reference beam rotates the polarisation by 90°.
  • the informative and reference waves of each laser pulse acquire a phase difference defined by the surface morphology of the sample.
  • the further optical path (PB10-NPB20 phase decoder) is the same as for the transparent mode of the device (Fig. 1) .
  • FIGs 5 and 6 show an example of results obtained using the present invention.
  • Figure 5 shows interferometric pictures of carbon dioxide flow obtained by depressurising liquid C0 2 (at pressure about 55 bar) into the air.
  • the polarising interferometer was arranged in the Mach-Zender configuration.
  • the four pictures differ by the phase shift which is 0, ⁇ /2, ⁇ and 3 ⁇ /2.
  • the left and right part of each picture are time-resolved (time interval 5 ms) and show a fluctuation of the jet caused by precipitation of dry carbon dioxide in the nozzle.
  • the right part of the figures also shows a development of density -.fluctuations within the jet (seen as dark and light fringes along the axis) .
  • I i r I 2 , I 3 , I 4 are the intensities of interferograms with the shift 0, ⁇ /2, ⁇ and 3 ⁇ /2 correspondingly (see Figure 5) and ⁇ (x, y) is the phase difference between the informative and reference beams.
  • Figure 6 shows a phase-wrapped picture obtained using interferograms in Figure a PPl i ⁇ g a four-step phase shifting algorithm.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

La présente invention concerne un interféromètre à laser comprenant: - un appareil laser (1) destiné à émettre deux impulsions lumineuses séparées par un intervalle; - un appareil de modulation de phase (2) comprenant un dispositif destiné: à diviser chaque impulsion en une paire de sous-impulsions, chaque sous-impulsion de chaque paire étant orientée le long d'une trajectoire séparée respective, à orienter les sous-impulsions dans l'une des trajectoires ou sur un échantillon à examiner, et à recombiner par la suite les paires respectives de sous-impulsions; et - un appareil de décodage de signaux (3) destiné à orienter les impulsions recombinées via les trajectoires respectivement séparées sur des dispositifs photosensibles respectifs ou sur des emplacements respectifs d'un dispositif photosensible.
PCT/GB1998/002601 1997-08-29 1998-08-28 Interferometre a laser WO1999012009A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU88754/98A AU8875498A (en) 1997-08-29 1998-08-28 Laser interferometer

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GBGB9718405.5A GB9718405D0 (en) 1997-08-29 1997-08-29 Laser Interferometer
GB9718405.5 1997-08-29

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WO1999012009A1 true WO1999012009A1 (fr) 1999-03-11

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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1639397A4 (fr) * 2003-02-27 2006-10-04 Bioscan Ltd Systeme permettant de recuperer un etat de polarisation d'entree de faisceau optique a une intensite proportionnelle
DE102005050151B3 (de) * 2005-10-19 2006-11-02 MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V. Verfahren und Vorrichtung zur Abtastung von sich periodisch wiederholenden Ereignissen
WO2006127199A1 (fr) * 2005-05-20 2006-11-30 The Boeing Company Émetteur-récepteur de diagnostic de front d’onde de mems optique et émetteur

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP0091826A2 (fr) * 1982-04-14 1983-10-19 The Board Of Trustees Of The Leland Stanford Junior University Capteur à fibre optique amélioré pour détecter de très petits déplacements d'une surface
GB2125957A (en) * 1982-03-18 1984-03-14 Plessey Co Plc Interferometers
US5585913A (en) * 1994-04-01 1996-12-17 Imra America Inc. Ultrashort pulsewidth laser ranging system employing a time gate producing an autocorrelation and method therefore

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2125957A (en) * 1982-03-18 1984-03-14 Plessey Co Plc Interferometers
EP0091826A2 (fr) * 1982-04-14 1983-10-19 The Board Of Trustees Of The Leland Stanford Junior University Capteur à fibre optique amélioré pour détecter de très petits déplacements d'une surface
US5585913A (en) * 1994-04-01 1996-12-17 Imra America Inc. Ultrashort pulsewidth laser ranging system employing a time gate producing an autocorrelation and method therefore

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
ONUMA K ET AL: "Application of real time phase shift interferometer to the measurement of concentration field", JOURNAL OF CRYSTAL GROWTH, APRIL 1993, NETHERLANDS, vol. 129, no. 3-4, ISSN 0022-0248, pages 706 - 718, XP002081463 *
ONUMA K ET AL: "In situ study of surface phenomena by real time phase shift interferometry", JOURNAL OF CRYSTAL GROWTH, APRIL 1994, NETHERLANDS, vol. 137, no. 3-4, ISSN 0022-0248, pages 610 - 622, XP002081462 *

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1639397A4 (fr) * 2003-02-27 2006-10-04 Bioscan Ltd Systeme permettant de recuperer un etat de polarisation d'entree de faisceau optique a une intensite proportionnelle
US7532390B2 (en) 2003-02-27 2009-05-12 Bioscan Ltd. System for recovering input polarization state of optical beam at proportional intensity
WO2006127199A1 (fr) * 2005-05-20 2006-11-30 The Boeing Company Émetteur-récepteur de diagnostic de front d’onde de mems optique et émetteur
US7379191B2 (en) 2005-05-20 2008-05-27 The Boeing Company Optical MEMS wavefront diagnostic transceivers and receiver
DE102005050151B3 (de) * 2005-10-19 2006-11-02 MAX-PLANCK-Gesellschaft zur Förderung der Wissenschaften e.V. Verfahren und Vorrichtung zur Abtastung von sich periodisch wiederholenden Ereignissen

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GB9718405D0 (en) 1997-11-05

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