WO1999004301A1

WO1999004301A1 - Scanning microscope with miniature head

Info

Publication number: WO1999004301A1
Application number: PCT/AU1998/000561
Authority: WO
Inventors: Martin Russell Harris
Original assignee: Optiscan Pty. Ltd.
Priority date: 1997-07-16
Filing date: 1998-07-16
Publication date: 1999-01-28
Also published as: GB2340332B; GB2340332A; AUPO790197A0; GB9924347D0; DE19882512T1

Abstract

The present invention provides a scanning microscope including: (1) a light source for supply of a light beam; (2) optical transmission means for transmitting the light beam from an entry end thereof proximate the light source to an exit end thereof proximate light focusing means, the light focusing means focusing the light emerging from the exit end to illuminate a point observation field on or within an object to be examined; (3) a detector to detect object emanated light emerging from the point observation field; (4) a scanner mounted in an optical head casing with the light focusing means, to cause the illuminated point observation field to scan over a two-dimensional cross section of the object such that an image of the object emanated light received by the detector over the cross section may be constructed; wherein the scanner includes counterbalancing elements to balance reactive forces caused by operation of the scanner, such that vibration of the light focusing means caused by coupling of the reactive forces to the light focusing means is reduced.

Description

SCANNING MICROSCOPE WITH MINIATURE HEAD

This invention relates to scanning microscope requiring miniature optical heads, in particular but not limited to laser scanning microscopes with miniature heads which can utilised as an endoscope.

In order to construct a scanning microscope which has an optical head small enough to be inserted in the body to act as an endoscope, whereby internal organs of the body may be inspected at a microscopic level, the optical head of the scanning microscope must be able to be located physically independently of the bulky light source and photo-multiplier tube, and also must be miniaturised.

U.S. patents 5,120,953 and 5,161,053 ("Harris" and "Dabbs" respectively) describe how a confocal microscope may be constructed using optical fibres to make the optical head geometrically independent from the light source and/or detector. Harris and Dabbs further disclose how a single fibre may be used to construct a microscope where the exit end of the fibre acts as both the source pinhole and return pinhole in a confocal microscope.

Harris also describes embodiments where the fibre tip is vibrated to produce a miniature optical head suitable for an endoscope application. One practical problem with such a design is that the scanning accuracy of the system is limited by kinetic energy transfer to (and losses from) the vibrating components of the scanning mechanism. An alternative endoscope design remotely locates the scanning apparatus, and transmits the light down a standard endoscopic optical fibre bundle. This can be implemented by positioning the imaging end of the fibre bundle at the microscope stage of a laser scanning confocal microscope, or by using special optical arrangements as described in US Patent 5,323,009. This results at the specimen end of the fibre bundle in a corresponding scanning of the illuminating spot. A disadvantage with such systems is that the optical resolution is limited by the discrete nature of the fibre cores in the endoscopic bundle.

It is an object of the current invention to provide a miniaturised optical head for a scanning microscope having improved theoretical maximum resolution and reduced vibrational effects .

Therefore in accordance with a broad aspect of the invention there is provided a scanning microscope comprising

(1) a light source for supply of a light beam;

(2) optical transmission means for transmitting the light beam from an entry end thereof proximate the light source to an exit end thereof proximate light focusing means, the light focusing means focusing the light emerging from the exit end to illuminate a point observational field on or within an object to be examined; (3) a detector to detect object emanated light emerging from the point observational field;

(4) a scanner mounted in an optical head casing with the light focusing means, to cause the illuminated point observational field to scan over a two- dimensional cross-section of the object such that an image of the object emanated light received by the detector over the cross section may be constructed; wherein the scanner comprises counterbalancing elements to balance reactive forces caused by operation of the scanner, such that vibration of the light focusing means caused by coupling of the reactive forces to the light focusing means is reduced.

Preferably, the scanner scans the point observational field in a raster fashion, the scanner having a fast scanner to scan over rows and a slow scanner operating in a transverse direction to the fast scanner to displace consecutive rows . The counterbalancing elements may balance the reactive forces associated with the fast scan.

Preferably, the scanner and the counterbalancing element form a resonant system having a Q value sufficiently high to enable low energy input requirements. Preferably too, the Q value is sufficiently low so that perturbations introduced by physical shock do not persist.

Preferably, the microscope comprises a fork with first and second tines , the tines being caused to vibrate by driving means in mutually opposite phases, the vibration of the first and second tines providing the fast scanner and the counterbalancing element. The exit end of the optical transmission means may be fixed to the first tine so as to follow the vibration of the first tine to provide the fast scanner. Alternatively, a mirror or mirrors may be fixed to the tines or one of the tines in the optical path of the light beam to provide the fast scanner. The tuning fork may be mounted to the optical head casing with a flexible compliant material to further reduce transfer of vibration to the optical head case.

The optical transmission means may comprise an optical fibre, and the exit end of the optical transmission means may be the exit end of the core of the optical fibre.

The fibre may have a core/cladding composition chosen to have an effective numerical aperture as high as possible and preferably greater than the nominal numerical aperture of 0.12 of current standard fibres. The slow scanner may be provided by movement of the tuning fork in a direction perpendicular to the fast scan vibration, such as by rotation of the tuning fork about an axis.

The microscope may be a confocal microscope. In this case, the optical transmission means may include an optical fibre (which may be single mode) and the object emanated light returning from the illuminated point observational field may return through the focusing means and enter the exit end of the optical fibre, being extracted from the optical fibre by confocal return light separator means . The confocal return light separator means may be an optical fibre coupler or a beam splitter. Additionally, as described in International Application PCT/AU96/00159 , the optical transmission means may include near confocal transmission means having a light collection end adjacent the exit end of the confocal transmission means to selectably collect light emanating from regions close to the point observational field. The near confocal transmission means may be provided by the cladding of the single mode optical fibre.

Alternatively, the microscope may be non-confocal . In this case, at least a portion of the object emanated light emanating from the point observational field may be collected and returned via means other than the exit end of the optical transmission means . Such a non-con ocal microscope may be adapted to two-photon microscopy.

Preferably, the movement of the slow scanner is damped to reduce coupling of vibration from the fast scanner or from mechanical perturbations to the head.

Preferably, the slow scanner may function by contraction and elongation of a wire whose dimensions are controlled by temperature variation caused by a varying electrical control current through the wire. Alternatively, the slow scanner may function by a hydraulic actuator mechanism connected by a fluid conducting tube to a hydraulic driver pump remotely located which pumps fluid into and out of the hydraulic actuator in the optical head case.

The driving means of the fast scanner may comprise a first electromagnet proximate the first tine and a second electromagnet proximate the second tine, the first and second electromagnets being driven by alternating currents of opposite phase. Energy to maintain the driving means may be delivered to the scanner head by means current-carrying wires or by pulses of electromagnetic radiation conveyed by an optic wave guide. The electromagnetic radiation may be laser pulses conveyed by an optical fibre impinging on a photocell in the head case which supplies current obtained by conversion of the light energy the electromagnets.

A region of the optical fibre proximate the exit end thereof may be manufactured with a reduced cladding diameter to minimise inertia of the first tine in embodiments where the fibre is mounted on the first tine.

Alternatively, the inertia may be reduced by hydrofluoric acid etching of the region of the fibre proximate the exit end.

Other preferred features of the invention will be apparent from the following description of preferred embodiments , where :

Figure 1 shows an optical head constructed in accordance with a preferred embodiment of the current invention.

Referring now to Figure 1 there is shown an optical head case 1 providing an endoscope head shown in juxtaposition to tissue 2 for ±n vivo observation. Optical transmission means 3 in the form of a single mode optical fibre passes down flexible endoscope tube 30 from a remotely located laser light source, detector and electronics . The remotely located component may be constructed in accordance with the single fibre embodiments described in US Patent 5,120,953 by Harris and need not be repeated here.

The single mode optical fibre passes through a rear wall 31 of the optical head case 1 and an exit end 33 of the optical transmission means 3 is attached to a first tine 5 of a tuning fork 4. The tuning fork 4 has slots 50 in each side to reduce transfer of higher mode vibrations from the tines to the mounting portion 9. The tuning fork 4 is driven by electromagnets 7 so that the first tine 5 and second tine 6 have mutually opposite vibration at a frequency of approximately 1000 Hz. The tines are machined to have as closely matching vibrational states as possible, preferably compensating for any extra inertia introduced by the optical transmission means 3.

The Q value of the vibration should be sufficiently high to allow low energy impulse to the electromagnet 7 but sufficiently low to damp transients caused by physical shocks to the case. The optimal Q value can be determined within these constraints by trial and error. The electromagnets 7 are supplied by sinusoidal or pulsed current through current of opposite phase carrying wires 8. The vibrations of the ends of the tines 5 and 6 have an amplitude less than 1mm. A portion of the laser light emerging from the fibre tip at the end of the vibrating tine may be used to provide positional feedback by impinging on an optic fibre 20, the tip of which contains a fluorescent material 21 and which partly occludes the laser beam at one side of the emergent cone of light as it scans the lens . The returning fluorescence light passes to a detector at the remote end (not shown) , which converts the magnitude of the light signal into an electrical signal. The level of the electrical signal is functionally related to the amount of the fluorescence- doped plastic lying within the cone of illumination, and therefore provides positional feedback to assist in control of the electromagnet 7 to standardise and control the fast scan vibration.

Second tine 6 provides the counterbalancing element to counterbalance the reactive forces of the vibration of the first tine 5. Since additional elements are attached to first tine 5, increasing the mass of that tine, similar compensating masses can be positioned appropriately on second tine 6 to ensure sufficient matching of the counter-balancing with the reactive forces of vibration of the first, tine 5. Alternatively, the first and second tines can be machined with the additional elements in place.

The exit end 33 of the optical transmission means projects a cone of illumination on to a light focusing means 34 which focuses the illumination into a diffraction-limited point observational field within the tissue 2. The vibration in the fast scan direction shown by arrow X of the first tine 5 is transformed into scanning along a single row across of a two dimensional area within tissue 2 along arrow Y.

The tuning fork 4 is mounted onto the optical head case by anti-vibration mounting means (not shown) . This may be interposed at the point of attachment 9 of the tuning fork to the optical head case 1 or it may be integral with the slots 50. The slow scan is provided by reciprocating rotation about an axis Z, preferably located so that the axis of slow scan rotation 2 and the axis of fast scan vibration intersect. The rotation is slow scanner 25, actuated by contraction and elongation of "Nitinol" wire, controlled by a second current carrying wire 25, through which a current is passed in a varying level to provide the slow scan reciprocation.

Further reduction in the inertia of the first tine may be achieved by hydrofluoric acid etching of a portion of the optical fibre 3 proximate the exit end, whereby a portion of the cladding has been reduced in diameter to reduce the inertia of the fibre.

Alternatively, the etching of the fibre may be in the form of a gradual taper, which has an additional benefit that the numerical aperture of the exit end of the fibre is thereby increased, allowing a reduction in the distance of the focusing optics 34 from the exit end 33.

It has been found that by providing a fast scan mechanism which has counterbalancing movements to balance the fast scan vibrations, transfer of vibrations to the optical head case 1 can be minimised, thereby improving the quality of the image produced.

Modifications may be made to the invention as would be apparent to a person skilled in the art of scanning microscope design. For example, the scope of the invention is not limited to confocal arrangements, and suitably miniaturised heads may be provided which make use of two photon fluorescence, which does not require a confocal return of illumination. Additionally, the cladding of the fibre 3 can be used to return near- confocal, as described in PCT/AU96/00159. However, in this case, etching of the exit end of the fibre to reduce inertia may need to be performed so as to produce a step rather than a taper. The use of a specially produced fibre with reduced diameter cladding, of total glass diameter about 25 microns is also contemplated. Further still , the provision of current-carrying wires 8 to drive electromagnets 7 may be avoided by the use of an optical wave guide in the place of the current-carrying wires , which projects light onto a photo detector within the head case 1 converting the light energy into electrical energy to energise the electromagnet 7. The second current-carrying wire 26 may similarly may be replaced by a hydraulic connection to operate an alternate hydraulic slow scan mechanism. In this manner, electrical connections passing down the flexible endoscope tube 30 may be avoided.

These and other modifications may be made without departing from the ambit of the current invention, the nature of which can be ascertained from the foregoing description and the drawing.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A scanning microscope including:

(1) a light source for supply of a light beam;

(2) optical transmission means for transmitting the light beam from an entry end thereof proximate the light source to an exit end thereof proximate light focusing means, the light focusing means focusing the light emerging from the exit end to illuminate a point observation field on or within an object to be examined;

(3) a detector to detect object emanated light emerging from the point observation field;

(4) a scanner mounted in an optical head casing with the light focusing means, to cause the illuminated point observation field to scan over a two-dimensional cross-section of the object such that an image of the object emanated light received by the detector over the cross section may be constructed; wherein the scanner includes counterbalancing elements to balance reactive forces caused by operation of the scanner, such that vibration of the light focusing means caused by coupling of the reactive forces to the light focusing means is reduced.

2. Λ scanning microscope as claimed in claim 1, wherein said scanner is operable to scan the point observational field in a raster fashion, the scanner having a fast scanner to scan over rows and a slow scanner operating in a transverse direction to the fast scanner to displace consecutive rows.

3. A scanning microscope as claimed in claim 2, wherein said counterbalancing elements balance the reactive forces associated with the fast scanner.

. A scanning microscope as claimed in any one of the preceding claims, wherein said scanner and said counterbalancing elements form a resonant system having a Q value sufficiently high to enable low energy input requirements .

5. A scanning microscope as claimed in claim 5, wherein said Q value is sufficiently low so that perturbations introduced by physical shock do not persist.

6. A scanning microscope as claimed in either claim 2 or 3, wherein said microscope includes a fork with first and second tines, the tines being operable to vibrate by driving means in mutually opposite phases, the vibration of the first and second tines providing the fast scanner and the counterbalancing element.

7. A scanning microscope as claimed in claim 6, wherein said exit end of the optical transmission means is fixed to said first tine so as to follow the vibration of said first tine to provide said fast scanner.

8. A scanning microscope as claimed in claim 6, wherein said microscope includes a mirror or mirrors fixed either to said tines or to one of said tines in the optical path of the light beam to provide said fast scanner.

9. A scanning microscope as claimed in any one of claims 6 to 8, wherein said fork is mounted to the optical head casing with a flexible compliant material to further reduce transfer of vibration to the optical head case.

10. A scanning microscope as claimed in any one of the preceding claims, wherein said optical transmission means comprises an optical fibre, and the exit end of the optical transmission means forms the exit end of the core of the optical fibre.

11. A scanning microscope as claimed in claim 10, wherein said fibre has a core/cladding composition chosen to have an effective numerical aperture as high as possible.

12. A scanning microscope as claimed in claim 11, wherein said effective numerical aperture is greater than the nominal numerical aperture of 0.12 of current standard fibres .

13. A scanning microscope as claimed in any one of claims 6 to 9, wherein said slow scanner is provided by movement of said fork in a direction perpendicular to the fast scan vibration.

14. A scanning microscope as claimed in claim 13, wherein said movement is rotation of said fork about an axis.

15. A scanning microscope as claimed in any one of the preceding claims, wherein said microscope is a confocal microscope .

16. A scanning microscope as claimed in claim 15, wherein said optical transmission means includes an optical fibre.

17. A scanning microscope as claimed in either claim 15 or 16, arranged so that said object emanated light returning from the illuminated point observational field returns through said focusing means and enters said exit end of said optical fibre, being extracted from the optical fibre by confocal return light separator means .

18. A scanning microscope as claimed in claim 17 , wherein said confocal return light separator means is an optical fibre coupler or a beam splitter.

19. A scanning microscope as claimed in any one of the preceding claims, wherein said optical transmission means includes near confocal transmission means having a light collection end adjacent to said exit end of said confocal transmission means to selectably collect light emanating from regions close to the point observational field.

20. A scanning microscope as claimed in claim 19, wherein said optical fibre is single moded and said near confocal transmission means is provided by the cladding of said single mode optical fibre.

21. A scanning microscope as claimed in any one of claims 1 to 14, wherein said microscope is non-confocal.

22. A scanning microscope as claimed in claim 21, wherein said microscope is arranged so that at least a portion of said object emanated light emanating from the point observational field is collected and returned via means other than the exit end of the optical transmission means.

23. A scanning microscope as claimed in claim 22, wherein said microscope is adapted to two-photon microscopy.

2 . A scanning microscope as claimed in either claim 2 or 3, wherein movement of said slow scanner is damped to reduce coupling of vibration from the fast scanner or from mechanical perturbations to the head.

25. A scanning microscope as claimed in either claim 2 or 3, wherein said slow scanner is operable to function by contraction and elongation of a wire whose dimensions are controlled by temperature variation caused by a varying electrical control current through the wire.

26. A scanning microscope as claimed in either claim 2 or 3, wherein said slow scanner is operable to function by a hydraulic actuator mechanism connected by a fluid conducting tube to a hydraulic driver pump remotely located which pumps fluid into and out of the hydraulic actuator in the optical head case.

27. A scanning microscope as claimed in claim 6, wherein said driving means of the fast scanner includes a first electromagnet proximate said first tine and a second electromagnet proximate said second tine, the first and second electromagnets being driven by alternating currents of opposite phase.

28. A scanning microscope as claimed in either claim 6 or 27, wherein energy to maintain the driving means is delivered to the scanner head by means of current-carrying wires or by pulses of electromagnetic radiation conveyed by an optic wave guide.

29. A scanning microscope as claimed in claim 28, wherein said electromagnetic radiation comprises laser pulses conveyed by an optical fibre impinging on a photocell in the head case which supplies current obtained by conversion of the light energy the electromagnets.

30. A scanning microscope as claimed in claim 10, wherein a region of said optical fibre proximate the exit end is manufactured with a reduced cladding diameter to minimise inertia of the first tine in embodiments where said fibre is mounted on or attached to said first tine.

31. A scanning microscope as claimed in claim 10, wherein inertia of said first tine is reduced by hydrofluoric acid etching of a region of the fibre proximate the exit end.