GB2262360A - Optical fibre termination and laser doppler velocimeter incorporating same - Google Patents

Abstract

An optical fibre termination device in which a fibre end face 7 is mounted on the axis of a lens unit 9 so that if laser light were emitted by the fibre it would be focused to a remote point on the axis but for inherent divergence of the beam. This divergence is gradual and causes a pseudo focus 15, a 'waist' so-called. The fibre and lens unit are rigidly fixed together so that the waist moves as one with the fibre. A pinhole plate 17 is positioned in the plane of the waist, the pinhole just embracing the waist. Coupling to the fibre can then be achieved by focusing an external beam on to the pinhole. By arranging the fibre end face close to the lens unit, the waist is made considerably larger than the fibre core diameter and focusing of the external beam is simplified. A viewing eye piece (19, Fig 2) can be temporarily positioned to give a magnified view of the pinhole and the accuracy of focus of a laser beam on the pin hole. A further aperture tailored to the outgoing beam diameter can be mounted beyond the pinhole to exclude all but axial incoming beams. In a laser doppler velocimeter incorporating such a termination, signal processing may be suppressed in the absence of scattering particles (Fig 6) and further, the probe volume, at the foci of projection and reception assemblies may be back projected on to a screen to check their coincidence (Fig 7). <IMAGE>

Description

Optical Fibre Terminations and Laser Doppler Velocimeters Employing such Terminations This invention relates to optical fibre terminations and to Laser Doppler Velocimeters employing such terminations, the terminations being particularly designed to facilitate the efficient coupling of light into the fibre either from a laser or from a remote focus. In principle it could also be adapted for coupling light from one fibre into another.

The coupling of light to and from fibres presents considerable problems in alignment and stability. Their small core diameters are difficult to see and are demanding of very precise focusing equipment.

An object of the present invention is to provide an optical fibre termination which is easy to use while giving good coupling efficiency with inherent optomechanical stability.

In a particular Laser Doppler Velocimeters embodiment of the invention a further object is to enable such an optical fibre termination, having an external focus, to have the location of this focus at least monitored and preferably controlled accurately.

According to the present invention a fibre-optic termination comprises, in sequence on an optical axis, an optical fibre having an end face, a lens unit, and a pinhole plate, the fibre end face and the pinhole being in a permanently conjugate arrangement with respect to the lens unit, the pinhole having a diameter greater than that of the fibre core and such as just to embrace the waist of a laser beam projected by the optical fibre and lens unit, and the arrangement being such that light can be coupled into the fibre end face by focusing the light on to the pinhole.

The fibre end face, lens unit and pinhole are preferably fixedly mounted in an integrated assembly.

There may be included optical magnifying means arranged to provide a magnified image of the pinhole on a viewing axis transverse to the optical axis. The magnifying means is preferably adapted to be detachably mounted with respect to the pinhole for use in alignment of an external beam to be focused on to the pinhole.

The fibre-optic termination may include a baffle plate having an aperture on the optical axis, the baffle plate being positioned on the side of the pinhole remote from the lens unit and the baffle plate aperture having a diameter greater than that of the pinhole and such as to accommodate the local diameter of a laser beam emanating from the optical fibre and lens unit.

According to another aspect of the invention, a reference beam laser doppler velocimeter in which a laser beam source is directed to focus a laser beam at a probe volume and reception means is disposed to collect light scattered from the probe volume and transmit it to photo-detector means by way of a fibre-optic termination as aforesaid, the axis of the laser beam source and the reception means being arranged at an acute angle, is provided for setting up purposes with a projection lens system at the rear of the probe volume relative to the laser beam source and reception means, to project an image of the probe volume on to a screen, the reception means being adapted to transmit a laser beam to the probe volume illuminated by the laser beam source and the test laser beam providing an indication of the degree of coincidence of the focus of the laser beam source and the focus of the reception means at the probe volume.

According to a further aspect of the invention, a reference beam laser doppler velocimeter includes a fibre-optic termination as aforesaid, light collecting means for collecting light scattered from any particle at a probe volume of the velocimeter and focusing it on to the pinhole of the termination, the light collecting means having a greater numerical aperture than that of the fibre-optic termination to produce an annuls of light which is not acceptable to the termination, means for collecting at least part of the annulus of light to indicate the presence of a particle at the probe volume, and means for generating a laser reference beam for mixing with doppler -shifted light scattered from said particle, and means responsive to the presence of the annul us of light to permit processing of particle velocity information.

The reference beam laser doppler velocimeter optionally includes optical amplifying means coupled into the path of the doppler-sifted signal from the reception means prior to mixing with the reference beam.

An optical fibre termination in accordance with the invention, and relevant parts of a laser doppler velocimeter, will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 is a diagrammatic sectional view of the basic termination; Figure 2 shows a modification including an auxiliary viewing module; Figure 3 shows a further modification of the module of Figure 2 for non mechanical coupling of the termination to an external source; Figure 4 is a schematic layout of an arrangement for launching a laser beam into an optical fibre termination; Figure 5 is a physical embodiment of the arrangement of Figure 4.

Figure 6 shows an application of the arrangement to a reference beam laser doppler velocimeter showing means for triggering the reference beam only in the presence of a light scattering particle; and Figure 7 shows diagrammatically the arrangement of a laser doppler velocimeter employing features useful in setting up the equipment.

Referring to Figure 1, an optical fibre cable 1 comprises a single fibre encased in a protective sheath. The end portion 3 of the cable is stripped of its sheath and encased in a moulded cylindrical block 5 which engages the sheath at one end to support the fibre and is ground perpendicular to the fibre axis at the other end.

The block 5 with its exposed fibre end-face 7 is fixed permanently to the housing 11 of a lens unit 9 which may comprise two or more lenses but constitutes a positive lens. The fibre end face 7 is accurately located on the optical axis of the lens system at a position 13 outside the lens unit left-hand focus so that the conjugate focal position 15 is at a finite but greater distance outside the right-hand focus. The positions 13 and 15 may suitably be approximately 3 and 30 millimetres from the principal planes of the lens system.

If a laser beam were emitted by the fibre it would therefore produce a geometric beam waist at the position 15.

It is essential to the correct operation of the arrangement that the fibre end face at 13 is fixedly positioned with respect to the lens unit 9, so that the position of the supposed beam waist at 15 is also permanently fixed in relation to the fibre end face. The greater spacing of the beam waist 15, than the fibre end face 7, from the lens unit ensures that the beam waist diameter at 15 is greater than that of the fibre end face. A suitable diameter ratio is 10 :1.

At the beam waist position 15 is a pinhole plate 17, the pinhole 18 being centred on the axis and of such diameter as just to embrace the beam waist of the supposed beam emerging from the fibre at the frequency in question. The pinhole may just intercept the beam waist but only to the extent of a predetermined maximum beam attenuation.

If the fibre termination shown in Figure 1 is to be used to couple to an identical termination the beam waist is preferably common to the two devices. In this case the pinhole plate 17 must be able to abut the opposing pinhole plate and the two plates be sufficiently thin that together they do not obstruct the beam. The two terminations may be completely identical and have a shared device for locking them together, or they may have male and female thread formations (eg a standard union coupling) together with dowels or key formations to locate the two pinholes in register.

In other applications, eg in coupling a fibre to a laser source, as illustrated in Figures 5 and 6, other measures have to be taken to focus a beam on to the pinhole (where the fibre termination is receiving a beam).

Figure 2 shows a modification of the termination of Figure 1 in that it includes optical magnifying means to provide a magnified image of the pinhole on a transverse axis. This magnifying means consists of an optic module 19 which is coupled to the housing 11, eg by a union screw coupling. A magnifying lens or lens combination 21 is mounted in the wall 23 of the module and focuses on the pinhole 18 by way of a mirror 25.

The housing 23 has a substantial front aperture 27 permitting access for a focused beam from an external source (not shown). The external source is adjusted (or the fibre termination is adjusted) to direct the beam on to the pinhole 18. The module 19 enables the operator to make this adjustment easily. The fibre termination and external source are then locked in their relative positions and the optic module 19 may be removed.

Figure 3 shows a further modification of the optic module 19. In order to ensure that the beam from the external source (eg in Figure 2) is not merely focused on the pinhole 18 but is also aligned with the optic axis of the fibre termination, a baffle 29 is interposed 'upstream' of the pinhole 18 (with respect to an incoming beam), the baffle 29 having an aperture 31 which just embraces the above hypothetical outgoing beam from the lens unit 9 and pinhole 18 at the particular position of the baffle. An incoming beam which is not axially aligned will therefore not intercept the pinhole 18 centrally and this will be detected by observation through the viewing module 21.

The chief advantage of the device lies in the relative ease with which light can be aligned with and focused on the pinhole rather than on the fibre core itself. It is easier for two reasons: (1) The pinhole is larger than the fibre core. It is therefore easier to see and furthermore, adjustment mechanisms required for alignment can be relatively coarse.

(ii) The material around the pinhole is opaque and scatters light, as opposed to the glass cladding around optical fibres. Errors in alignment and focusing can thereby be viewed readily and then corrected.

As shown in Figures 2 and 3, an optical magnifier can be used as an aid to alignment.

A second advantage concerns the coupling of intense light from powerful sources, particularly some lasers, into a fibre. In the absence of the pinhole, damage to the fibre termination can occur if the light is misaligned with the core. When the pinhole is in place such misdirected light is always prevented from reaching the fibre.

The addition of a baffle as shown in Figure 3 also prevents light which is misaligned in angle from the cone of acceptance of the fibre from reaching the fibre.

The assembly may be used in any system which requires light to be coupled into or out of an optical fibre. The following are some examples: Multimode Fibres Some types of extrinsic fibre optic sensors require an optical reception system to collect light from a well defined region near a point whilst rejecting all other light. This can be accomplished conveniently, using an optical fibre as shown in Figure 4, by matching the etendue of the fibre to the diameter of the focal region and the aperture of the collection lens.

The properties of the assembly described above can be used to advantage in constructing a ruggedised system for launching light from a laser into a multi-mode fibre, for example in transporting the very high power from lasers used in medical or industrial applications.

Single Mode Fibres As in the case of the multimode fibre, the assembly can also be used to collect light efficiently from an external focal region whilst rejecting unwanted background. In this case however the coherence of the collected light is maintained. It can be optically amplified and/or used directly in optical signal processing.

The assembly is likely to find application in a wide range of extrinsic fibre optic systems for example.

Laser Doppler Velocimetry LIDAR (Radar at optical frequencies) Optical signal processing Scanning optical microscopes Optical interferometry Holography Light scattering studies Medical laser systems Industrial laser systems Laser Light Launching The assembly can also be used to advantage in constructing a rugged system for launching light from a laser into a single mode fibre. Figures 5 and 6 show one such arrangement. Figure 5 shows the various components for focusing and aligning a laser beam on to the core of the fibre in a fibre optic termination as described above. A focusing lens 43 provides the main optical power external to the termination 20. It is placed so that the laser beam is focused at the pinhole plate 17.

A low power beam steering lens 45 has its axis fixed substantially parallel to the main optical axis but is adjustable in two (X and Y) directions in its own plane. Such adjustment has the effect of changing the lateral position of beam focus at the pinhole plate 17. By separating lenses 43 and 45 by the focal length of lens 43 this focal position adjustment can be made without changing appreciably the angle at which the laser light is incident at the fibre termination. A parallel-sided glass plate 47 is adjustable in tilt about orthogonal axes both orthogonal to the main optical axis.

The input and output beams at this plate 47 are always parallel but the output beam can be displaced laterally by tilting the plate. This has the effect of changing the angle of incidence of the laser light at the pinhole plate without affecting the position of the focus.

Finally, beam matching lenses 49 and 51 provide fine adjustment to the beam waist at the pinhole 18. Adjustment of these lenses 49 and 51 displaces the beam focus from the pinhole 18 which displacement can be corrected by movement of the whole termination 20. Alternatively, but more expensively, the beam matching combination might include a zoom lens to permit axial movement of the beam matching assembly while leaving the beam focus at the pinhole 18.

The method used to mount these elements is important for stable operation. Elements of high optical power are positioned with coarse adjustments which are then securely bolted in place. Fine adjustments are carried out with elements of low optical power.

When polarized beams are used and the orientation of the polarization plane is important the termination 20 includes a keyed locking ring to determine the angular orientation of the termination relative to the mounting base.

Figure 6 shows the fibre-optic termination 20 in a reference beam type lase doppler velocimeter application. In such an application the external beam focused on the fibre end is in fact derived from light scattered from a particle passing through a focal region corn only referred to as the 'probe volume', located at 33.

A light collecting lens system 35, possibly with a zoom facility to achieve optimum magnification and focusing of the scattered light on the fibre, is positioned in relation to the fibre optic termination 20 to collect the scattered light (indicated at 38) and focus it at the pinhole 18. This may be effected with the assistance of the optical module 19 in one or other of its forms as shown in Figures 2 and 3. The cone of light (38) collected by the light collecting system 35 produces a cone of light 39 which is slightly wider than that, 41, which can be accepted by the fibre 3 to ensure that the full capacity of the fibre termination is employed.

This cone 41 corresponds to an incident cone 40. Thus, the light collecting system 35 has a greater numerical aperture than that of the ternination 20. This feature is taken advantage of, as will be explained.

The probe volume 33 is illuminated by a focused beam (not shown) from a laser 22. The axis of the laser beam and the axis of the reception head shown in Figuure 6 are required to intersect at the probe volume and are arranged to do so at an acute anle, example, 20 -302. As is well known, a particle moving through the probe volume illuminated by a laser beam will scatter the incident light, the frequency of the scattered light being doppler shifted in dependence upon the particle velocity relative to the incident and reflection (scatter) directions. The light received by the fibre optic termination 20 thus has a doppler shifted frequency. In the reference beam laser doppler velocimeters the doppler shift is determined by mixing the doppler shifted signal with a sample of the original laser signal.If the two signals are simply added together and detected by a photo-detector the beat frequency is determined and the particle velocity component detected.

The reference beam 24 is added to the signal beam 26 in a signal detection and processing unit 28. This enables the doppler shifted signal to be amplified in an optical amplifier 30. Such optical amplification improves the signal-to-noise ratio and consequently the efficiency of detection. The amplifier 30 may, in a modification, comprise frequency selective filters or absorption filters, which may facilitate the use of, for example, a Fabry-Perot etalon to measure the doppler shift directly.

Reverting now to the use of the surplus annular beam width between the light collector optics 35 and the pinhole 18, an annular mirror 32 is arranged, inclined to the axes, to divert any scattered light that is not accepted by the fibre-optic termination 20.

In many applications of a reference beam laser doppler velocimeters the scattering particles occur both randomly and discretely. However, the reference beam is present continuously, giving rise to background noise and making detection of the particles difficult. It is a feature of the present invention that the annular beam diverted by the mirror 32 is used to suppress the generation of such noise in the absence of a scattering particle. This is achieved by disabling the signal acquisition system in the absence of the annul us of light from a particle currently passing through the probe volume. Since no signal processing takes place in these periods, noise that would otherwise arise from the reference beam is avoided.

The annular beam is applied to a photo-detector 34 the output from which (not shown) is employed to switch the signal acquisition on and off. In a possible alternative scheme the reference beam itself is suppressed except in the presence of a scattering particle and resulting annular beam.

An optical system 36 may be provided in the path of the annular beam. This system may comprise beam steering mirrors and a lens focusing system with stops and baffles for stray light rejection.

Alternatively it may take the form of a short multimode fibre optic link.

Referring now to Figure 7, this shows a reference beam laser doppler velocimeters including a projection assembly 57 and a reception assembly 59. These are arranged with their axes at an acute angle, intersecting at a probe volume 33. The reception assembly 59 comprises a collection lens 61, interference filter 63 and focusing lens 65, followed by a complete unit as shown in Figure 1, this unit being demountable and accurately and repeatably fixed to the rest of the assembly.

In the path of the doppler shifted signal, a beam combiner 71 is placed obliquely to introduce a reference beam 24 into the signal beam path. The reference beam is derived from the laser 22 by way of a single-mode fibre, collected by a lens 67 and re-directed by a mirror 69. This reference beam may be controlled as in Figure 6 to arise only in the presence of a scattering particle. The signal path, comprising doppler-shifted signal and reference beam combined, is aplied to signal detection unit 28.

The projection assembly 57 is coupled to the laser output as before.

It is clearly important that the focus of the incident beam from source 57 and the focus of the reception unit 59 should coincide to define the probe volume as accurately as possible. This is facilitated according to a feature of the invention as follows. A projection lens (or lens assembly) 73 is mounted with the probe volume just outside one of its foci, the lens being positioned to the rear of the probe volume relative to the general direction of the projection and reception assemblies. For the purpose of aligning the projection and reception foci the reception assembly is made to transmit a test laser signal, in the opposite direction to the normal received signal, and thus produce a focused test signal at the probe volume. During this test procedure a composite, magnified image 75 is produced on a screen 77. A lack of coincidence between the foci of the projection and reception assemblies will then be apparent. Either one of the two assemblies can then be adjusted to produce coincidence or, more conveniently, a beam steering lens may be incorporated in the signal path of the reception assembly or in the projection assembly, similar to lens 45 in figure 4. Lateral movement of the lens 45, in e.g. x and y co-ordinates, will cause corresponding movement of the reception focus.

Claims (11)

1. A fibre-optic termination comprising, in sequence on an optical axis, an optical fibre having an end face, a lens unit, and a pinhole plate, the fibre end face and the pinhole being in a permanently conjugate arrangement with respect to the lens unit, the pinhole having a diameter greater than that of the fibre core and such as just to embrace the waist of a laser beam projected by the optical fibre and lens unit, and the arrangement being such that light can be coupled into the fibre end face by focusing the light on to the pinhole.

2. A fibre-optic termination according to Claim 1, wherein the fibre end face, lens unit and pinhole are fixedly mounted in an integrated assembly.

3. A fibre-optic termination according to Claim 1 or Claim 2, including optical magnifying means arranged to provide a magnified image of the pinhole on a viewing axis transverse to said optical axis.

4. A fibre-optic termination according to Claim 3, wherein said magnifying means is adapted to be detachably mounted with respect to said pinhole for use in alignment of an external beam to be focused on to the pinhole.

5. A fibre-optic termination according to any preceding claim, including a baffle plate having an aperture on said optical axis, the baffle plate being positioned on the side of said pinhole remote from said lens unit and the baffle plate aperture having a diameter greater than that of said pinhole and such as to accommodate the local diameter of a said laser beam emanating from said optical fibre and lens unit.

6. A fibre-optic termination substantially as hereinbefore described with reference to any of Figures 1 2 and 3 of the accompanying drawings, or an arrangement for launching a laser beam into an optical fibre as described with reference to Figure 4 and Figure 5

7. A reference beam laser doppler velocimeter in which a laser beam source is directed to focus a laser beam at a probe volume and reception means is disposed to collect light scattered from the probe volume and transmit it to photo detector means by way of a fibre optic termination according to any of Claims 1-5, wherein the axis of the laser beam source and the reception means being arranged at an acute angle, the velocimeter being provided for setting up purposes with a projection lens system at the rear of the probe volume relative to the laser beam source and reception means to project an image of the probe volume on to a screen, the reception means being adapted to transmit a laser beam to the probe volume to define the reception means focus, the projected image of the probe volume illuminated by said laser beam source and said test laser beam providing an indication of the degree of coincidence of the focus of the laser beam source and the focus of the reception means at the probe volume.

8. A velocimeter according to Claim 7 including in said reception means beam steering lens means movable in a plane transverse to the axis of the reception means to enable the reception means focus to be steered in a parallel plane to tend to bring it into coincidence with the laser beam source focus.

9. A reference beam laser doppler velocimeter including a fibre -optic termination according to any of Claims 1-5. 7 and 8 light collecting means for collecting light scattered from any particle at a probe volume of the velocimeter and focusing it on to the pinhole of the termination, said light collecting means having a greater numerical aperture than that of said fibre-optic termination to produce an annul us of light which is not acceptable to said termination, means for collecting at least part of said annul us of light to indicate the presence of a particle at said probe volume and means for generating a laser reference beam for mixing with doppler-shifted light scattered from a said particle, and means responsive to the presence of the annul us of light to permit processing of particle velocity information.

10. A reference beam laser doppler velocimeter according to Claim 7 or Claim 8, including optical amplifying means coupled into the path of the doppler-shifted signal from said reception means prior to mixing with the reference beam.

11. A reference beam laser doppler velocimeter incorporating a reference beam triggering arrangement substantially as hereinbefore described with reference to Figure 6 of the accompanying drawings.

120 A reference beam laser doppler velocimeter incorporating back projection monitoring of the probe volume substantially as hereinbefore described with reference to Figure 7 of the accompanying drawings.