GB2260052A

GB2260052A - Particle anemometry

Info

Publication number: GB2260052A
Application number: GB9219403A
Authority: GB
Inventors: David Macalpine Livesley; Clive Rowley Negus
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1991-09-28
Filing date: 1992-09-14
Publication date: 1993-03-31
Anticipated expiration: 2012-09-14
Also published as: GB9120687D0; GB2260052B; GB9219403D0

Abstract

Anemometry apparatus 10 comprises a laser 12, rotatable radial diffraction grating 22, lenses 27 and 32, and two glass blocks 30, each with a coating creating a dichroic mirror on one surface (35, fig 2; 55 fig 3). The blocks 30 ensure that the emergent beams of respective colours 23, 24 are spatially separated but accurately parallel. All four beams 23, 24 intersect at a common probe volume 14 and produce vertical fringes of one colour and horizontal fringes of the other colour. The blocks 30 enable a robust, compact, and simple apparatus 10 to be provided which can measure velocity components in two directions simultaneously. <IMAGE>

Description

Particle Anemolnetry This invention relates to an apparatus or monitoring particles using scattered laser light, so as to be able to determine their sizes and/or their velocities.

A variety of techniques are known in which two coherent laser beams are caused to intersect so as to define a probe volume (in which there may be a fringe system), and light scattered by particles such as aerosol droplets passing through the probe volume is detected.

Analysis of the detected light enables the particle's size and/or velocity to be determined, this general technique being referred to as laser Doppler anemometry. Such techniques are disclosed for example in US 4 540 283 (Bachalo), US 4 854 705 (Bachalo), and GB 2 237 950 A (UKAEA; Gillespie et al). In principle, the frequency of the scattered light signal (the Doppler frequency) enables the component of a particle's velocity perpendicular to the fringes to be determined, though not its direction. One way in which the direction can be found is to cause the fringes within the probe volume to move, and this can be achieved by use of a rotating annular diffraction grating whose lines are radial.However if it is desired to determine the magnitude and direction of two orthogonal components of a particle's velocity an approach which avoided the need to use two rotating diffraction gratings would be cheaper and simpler.

According to the present invention there is provided an apparatus for monitoring discrete particles, the apparatus comprising means for generating two parallel light beams of different wavelengths, means for splitting each beam into two spatially separate coherent light beams, and means for causing both pairs of coherent light beams to intersect and so to define a common probe volume, also comprising two transparent blocks through which the liglt beans pass between the splitting neans and the probe volume, each block having a coating on at least one surface forming a dichroic mirror end being such that when light beams of different wavelengths are incident on the block the beams emerging therefrom are spatially separated but with each beam parallel to its incident direction.

The use of transparent blocks with a dichroic mirror coating to bring about spatial separation of beams of different wavelengths provides a simple and robust means for separating the beams, and ensures the incident and emerging beams remain parallel even if the blocks become slightly displaced in use.

Preferably the blocks are arranged such that the planes defined by the pairs of intersecting coherent light beams are different. In the preferred embodiment the beam splitting means comprises a diffraction grating of annular form with radial lines, and the apparatus includes means to rotate the grating about its centre. This will enable the directions of two components of a particle's velocity to be determined, these two components corresponding to the two planes defined by the pairs of intersecting coherent light beams. Preferably the intersecting beams are in orthogonal planes, so that the components of velocity are at right angles.

Alternatively the blocks may be arranged to change only the separation of the pairs of coherent light beams, so the intersecting beams remain in a common plane, the light beams of different wavelengths intersecting at sufficiently different angles that the corresponding fringe separations would be different. This would enable a single component of velocity to be measured, but over a wider velocity range.

It should be appreciated that, in cach pair 0 coherent intersecting light beams, the beams may be oF the same polarization so that a fringe pattern is formed in the probe volume, or may be polarised in orthogonal planes. In the latter case the beams cannot interfere in the probe volume; nevertheless, as described in Gn 2 237 950 A, Doppler signals can be obtained from light scattered out of the probe volume by a particle.

The invention will now be further described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a perspective diagrammatic view of an apparatus for monitoring particles; Figure 2 shows a side view of a glass block of the apparatus of Figure 1; Figure 2a shows a modification to the glass block of Figure 2; and Figure 3 shows an alternative to the glass block of Figure 2.

Referring to Figure 1, an apparatus 10 for monitoring particles is shown diagrammatically. Light beams from a laser 12 are caused to intersect at a probe volume 14 by an optical system 13 whose optical axis 15 is shown by a chain-dotted line; any particles moving in the probe volume 14 scatter the light to be detected by sensors 16 (only two are shown). The detector system and signal analysis system associated with the sensors 16 are not shown.

The laser 12 emits both blue and green light, of wavelengths 488 and 514 nm respectively, as a beam. 20. This is incident on an annular diffraction grating 22 xzith racial lines, such that the beam. 90 is split up into blue beans 23 and green beams 24 on each side of the optical axis 15, all lying in a common, horizontal, plane. Only the first order beans are shown; other orders are cut out by a stop 26. The grating 22 is connected to an electric motor 25 so it can be rotated, the axis of rotation being parallel to the optical axis 15. The deviated and dispersed beams 23 and 24 are all collimated by a collimator lens 27, so as to be incident on apertures (not shown) in a plate 28 carrying two glass blocks 30.The blocks 30, described in greater detail below, allow the green light beams 24 to pass substantially straight through, whereas the blue light beams 23 undergo reflections so as to emerge parallel to their incident paths, but above and below the optical axis 15 respectively (i.e. in a vertical plane), the separation between the blue beams 23 being the same before and after passing through the blocks 30. A focussing lens 32 then causes the blue beams 23 and the green beams 24 to intersect at the probe volume 14, so that within the probe volume 14 are horizontal blue-light fringes, and vertical green-light fringes. Hence both horizontal and vertical components of a particle's velocity through the probe volume 14 can be determined.

It should be understood that a practical monitoring apparatus may incorporate many features in addition to those shown in Figure 1, which is intended only to illustrate the principle of operation. For example the laser 12 might be coupled to the optical system 13 by a fibre-optic cable; additional lenses may be provided; and means, such as a half-wave plate, may be provided to ensure the light beams have a desired plane of polarization. The optical system 13 causes the beams 23 and the beams 24 to intersect at their narrowest points. All four beams 23 and- 24 must therefore be accurately parallel when incident on the focussing lens 32, or they may not intersect, or at any rate not at their narrowest points.

emerging no to figure 2 there is shown of the glass blocks 30, in a side > ew. The block 30 consists of a parallelogran-shaped block 3t whose end faces 35, 36 are inclined at 45" to the plane of the block 34. One end face 35 is provided with a coating so it reflects blue light and transmits green light, so acting as a dichroic mirror. A 45" triangular glass prism 38 is glued onto the coated end face 35.An incident beam 40 of blue and green light is hence separated into two single-colour emerging beams: a green beam 41 which passes through the coating on the face 35 and through the prism 38, and a blue beam 42 which is reflected off the coated face 35, and undergoes total internal reflection off the other inclined face 36 before emerging from the block 34. It is important that the inclined end faces 35, 36 of the block 34 are accurately parallel, as this ensures the outgoing blue beam 42 is accurately parallel to the incident blue light (in beam 40); this is achieved even if the beam 40 is not incident exactly normal to the surface of the block 30.

In practice it may be found that not all the green light is transmitted through the coating, and that some is reflected along with the blue beam 42. This may be eliminated by passing the blue beam 42 through a blue filter (not shown) after it has emerged from the block 30.

Alternatively a coating may be provided on the part of the surface of the block 34 through which the beam 42 emerges, this coating allowing blue light to be transmitted and reflecting any green light. A further alternative is as shown in Figure 2a, wherein both the inclined end faces 35, 36 are provided with the coating which reflects blue light and transmits green light, and a second glass prism 38 is glued onto the other end face 36.

An alternative glass block 50 is shown in Figure 3.

This consists of a single rectangular glass block 52 whose top and bottom surfaces are accurately parallel, the incident blue and green light 40 being incident at an angle to the normal. The bottom surface (as shown) is provided with a coating 54 which transmits green light and reflects blue light, apart from the region onto which the light 40 is to be incident. The top surface (as shown) is provided with a coating 55 which transmits green light and reflects blue light (so as to act as a dichroic mirror) in the region where the blue and green light 40 is to be incident, and is provided with a totally reflecting coating 56 on much of the rest of its area. (The coatings 54, 55 and 56 are exaggerated in thickness for clarity).

Hence, just as with the blocks 30 described above, the incident blue and green light 40 is separated into two single-colour emerging beams: a green beam 41 which passes through the coating 55, and a blue beam 42 which is reflected off the coating 55, and reflects several times between the coatings 54 and 56, before emerging through the uncoated part of the top surface. The multiple reflecticns off the coating 54 ensure the purity of the emerging blue beam 42.

Furthermore the emerging beams 41 and 42 are parallel to the corresponding colours in the incident light beam 40; this is so even if the orientation of the block 50 is slightly changed.

The apparatus 10 of Figure 1 may also incorporate means (not shown) to change the separation of the pairs of light beams 23 and 24, for example from 20 mm to 40 mm, so changing their angles of intersection at the probe volume 14, the separation of the fringes, and hence the size range of the apparatus 10. This separation-changing means may consist of a support plate (not shown) between the plate 28 and the focussing lens 32, carrying four glass blocks 34 with end faces 35, 3S inclined at 45" (as described in relation to Figure 2, but without any dichroic mirror costings and without any triangular prism 39), the support plate being rotatable between a first position in which the light beams 23 and 24 pass straight through it, and a second position in which each light beam 23 and 24 passes through a glass block 34 to emerge further from the optical axis 15.

Claims

Clai.IS

1. A apparatus for monitoring discrete particles, the apparatus comprising means for generating two parallel light beams of different wavelengths, means for splitting each beam into two spatially separate coherent light beams, and means for causing both pairs of coherent light beams to intersect and so to define a common probe volume, also comprising two transparent blocks through which the light beams pass between the splitting means and the probe volume, each block having a coating on at least one surface forming a dichroic mirror and being such that when light beams of different wavelengths are incident on the block the beams emerging therefrom are spatially separated but with each beam parallel to its incident direction.

2. An apparatus as claimed in Claim 1 wherein the planes defined by the pairs of intersecting coherent light beams are different.

3. An apparatus as claimed in Claim 2 wherein the said planes aLe orthogonal.

4. An apparatus as claimed in Claim 1 wherein the pairs of intersecting coherent light beams lie in a common plane.

5. An apparatus as claimed in any one of the preceding Claims wherein the beam splitting means comprises a diffraction grating of annular form with radial lines, and the apparatus includes means to rotate the grating.

6. An apparatus as claimed in any one of the preceding Claims wherein at least one of the blocks is such that one of the light beams propagates once along the length of the block before emerging therefrom.

7. An apparatus as claimed in Claim 6 wherein the said one of the blocks comprises transparent means defining parallel faces adjacent to opposite ends of the block and inclined at substantially 45" to the direction of the light beams, the surface of the inclined face on which the light beams are first incident being provided with a coating so as to form the dichroic mirror, the transparent means also defining a face adjacent to one end of the block orthogonal to the direction of the incident light beams and onto which both the light beams are incident, and a face adjacent to the opposite end of the block orthogonal to the direction of one emerging light beam, and a transparent triangular prism with one face fixed to the inclined face on which is the dichroic mirror, and defining another face orthogonal to the direction of the incident light beam, through which the other light beam emerges.

8. An apparatus as claimed in Claim 7 wherein the surface of the other inclined face is provided with a coating so as to form another dichroic mirror.

9. An apparatus as claimed in any one of Claims 1 to 5 wherein at least one of the blocks defines parallel opposed front and rear surfaces inclined to the incident light beams, the front and rear surfaces being coated such that one of the light beams is repeatedly reflected by the front and rear surfaces before it emerges from the block.

10. An apparatus as claimed in Claim 9 wherein the coating on the rear surface of the block forms another dichroic mirror.

11. An apparatus as claimed in any one of the preceding Claims also including means for displacing a light beam so it emerges parallel to but laterally displaced from its initial direction, the displacing means comprising a transparent ember defining opposed parallel faces from which the light beam is reflected on passage through the transparent ember.

12. An apparatus as claimed in Claim 11 wherein the transparent member defines parallel end faces inclined at .45 to the incident direction of the light beam, and faces adjacent to each end face but orthogonal to the incident direction through which the light is incident, and from which the light emerges, respectively.

13. An apparatus as claimed in Claim 11 or Claim 12 wherein the displacing means is movable between an operative position, and an inoperative position.

14. An apparatus for monitoring discrete particles substantialy as hereinbefore described with reference to, and as shown in, Figure 1, and Figure 2 or Figure 2a or Figure 3 of the accompanying drawings.