WO2007077431A1

WO2007077431A1 - A wavefront variation device

Info

Publication number: WO2007077431A1
Application number: PCT/GB2006/004949
Authority: WO
Inventors: Carl Paterson; John Stephen Massa
Original assignee: Imperial Innovations Ltd.
Priority date: 2005-12-30
Filing date: 2006-12-29
Publication date: 2007-07-12
Also published as: GB0526609D0

Abstract

A wavefront variation device comprises a refractive index medium cell (310) having a deformable reflective variation surface (300). The cell is arranged to pass a wavefront to be varied through a refractive index medium to the variation surface.

Description

A WAVEFRONT VARIATION DEVICE

The invention relates to a wavefront variation device.

Dynamic wavefront correction is known using an adaptive optical system for example in the form of a deformable mirror of the type shown in Fig. 1. The mirror includes an optically flat substrate 100 having a reflective coating 102 and a plurality of actuators 104 comprising, for example piezoceramic discs. The substrate 100 comprises a continuous sheet of material manufactured to have a low residual surface deformation (for example of the order of the wavelength of the wavefront to be corrected). The reflective coating is a high reflectivity coating. The actuators 104 provide controlled deformation of the surface across part or all of the mirror.

The arrangement shown in Fig. 1 provides aberration correction for example when aberrations are introduced into a wavefront by other optics in the system. The aberration is detected in any appropriate manner and the deformable mirror is deformed using the actuators 104 to cancel out the effect of the aberration. One known deformable mirror is described in US 20050023573 in which the actuator comprises an integrated circuit piezoelectric actuator.

In the simple implementation shown in Fig. 2, a deformable mirror is deformed as shown. The maximum deformation stroke of the mirror in the fundamental mode is the maximum displacement of the centre of the mirror (along the optical axis) with respect to the edge of the mirror and is shown at 200. The stroke is related to the maximum optical path difference (OPD) and hence phase difference that can be introduced across a wavefront reflected from the mirror by:

OPD = 2 x (stroke) x n. (1)

where n is the refractive index of the incident medium.

However, a problem with known systems is that only a limited stroke is available and/or significant and potentially damaging deformation of the mirror is required to obtain large strokes.

The invention is set out in the claims. Because the wavefront correcting reflector is placed in a refractive index medium cell, a high refractive index medium can be introduced, lengthening the effective stroke without requiring increased physical deformation of the mirror. In addition coolant fluid can be passed through the fluid cell.

An embodiment of the invention will now be described by way of example, with reference to the drawings of which:

Fig. 1 is a schematic sectional side view showing a prior art deformable mirror;

Fig. 2 shows the mirror of Fig. 1 in a deformed configuration;

Fig. 3 shows a deformable mirror according to the present invention;

Fig. 4 shows the deformable mirror of the present invention in conjunction with a fluid circuit; and Fig. 5 shows a Fabry-Perot interferometer according to another aspect of the invention.

In overview a wavefront variation or correction device such as a deformable mirror comprises a refractive index medium cell such as a fluid cell or chamber which contains a high refractive index medium such as a fluid. A wavefront to be corrected is received in the fluid cell, passes through the fluid and is reflected by a deformable reflective correction surface comprising a deformable mirror. The deformable mirror can be deformed by appropriate actuators to correct an incoming wavefront, but using a reduced stroke as a result of the effect of the high refractive index fluid on the OPD, increasing the value of n in equation (1) above.

Referring to Fig. 3 a deformable mirror shown generally as 300 includes a flexible substrate 302, a reflective coating 304 and a plurality of actuators 306 arranged to provide physical deformation. The mirror 300 forms part of a fluid cell having a front window 308 which is transparent at least to the wavelength of the wavefront to be corrected. The fluid cell defines a chamber 310 in which a high refractive index (greater than 1) liquid is present. The cell is closed by a gasket 312 sealing the interface between the mirror 300 and the window 308. It will be noted that the mirror is shown in Fig. 3 in a deformed configuration from an undeformed state in which front and rear surfaces are generally flat and parallel to one another. It will be noted that the mirror can be deformed in any appropriate manner by the actuators 306 and that the mirror can take any appropriate shape in its undeformed configuration as well.

Referring to Fig. 4 the cell is designated generally 400 and forms part of a closed correction system designated generally 402. The correction system includes a beam splitter 404 such as a partially-silvered mirror, a wavefront measurement device such as a sensor 406 and a fluid recirculation system shown generally at 408. The beam splitter 404 is provided in the path of an outgoing wavefront 410b, to be corrected and transmits part of the wavefront 410c towards the wavefront sensor 406 and part to the system output. The wavefront sensor 406 passes a detection signal to a processor 412 programmed to act as a wavefront reconstructor and arranged to control the actuators 302 on the deformable mirror 300. The fluid recirculation circuit comprises a fluid reservoir 414 and inlet and outlet conduits to and from the fluid cell 400 respectively.

In operation, the correction system receives a wavefront 410a to be corrected. The incoming light can be from any system requiring correction such as a laser system. The uncorrected wavefront 410a is reflected by the cell 400 at 410b partially deflected tat 41Oe to wavefront sensor 406 which can be, for example, a phase detector capable of identifying phase errors across the wavefront. The sensor 406 sends a signal to the processor 412 which controls the actuator 302 (Fig. 3) to deform the mirror 300 in a complementary manner, in particular to correct the phase errors across the incoming wavefront in a known manner. Subsequently the incoming wavefront 410a is received in the fluid cell, reflected back in a corrected form and then transmitted by the beam splitter 404 to provide a corrected output beam.

As a result a closed loop system is provided whereby the wavefront sensor 406 compares a sensed wavefront with a desired wavefront and adjusts the actuators accordingly to provide the desired wavefront either according to a pre- determined value, or by incrementally changing actuator values until the desired wavefront is converged on as is well known in closed loop theory.

It will be noted that the beam splitter can be arranged to take off a small proportion of the light so as not to noticeably affect the output beam, or can have appropriate coatings to reflect a non-needed wavelength and transmit a required wavelength. For example in infrared astronomy the beam splitter can be configured to reflect visible light towards the wave front detector but transmit infrared light as the system output. In this manner it is ensured that intensity of the desired wavelength is not diminished in the detection process.

It will be appreciated that the fluid cell can be any appropriate shape in its undeformed position. For example the fluid cell may be generally cuboid having front entrance and rear reflective faces as described above.

Although the fluid cell 400 can be statically filled, in the embodiment shown in Fig. 4, fluid is constantly circulating through the cell using recirculation circuit 408. In particular an appropriate inlet and outlet can be provided in the gasket 312 which can provide cooling. This can be of particular significance in high power applications such as high power lasers.

It will be understood by the skilled reader that individual components such as the reconstructor 402, wavefront sensor 406, beam splitter 404 and fluid recirculation circuit 408 can be adopted using any conventional device. For example Shack-Hartmann HASO wave front sensors available from Imagine Optic, France, and components of the type set out in J. W. Hardy, "Adaptive Optics for Astronomical Telescopes", Oxford University Press 1998, ISBN 0- 19-509019-5 may be used.

Similarly the deformable mirror may be of any conventional type but packaged into a fluid cell as described above. The deformable mirror may be deformed using piezoelectric actuators, electrostatic techniques, voice coil actuators, magnetostrictive effects or any other appropriate technique.

It will be seen that when the mirror is deformed, the front plate or window 308 of the cell remains rigid and the gasket, in the preferred embodiment composed of compliant epoxy, will flex to allow the rear reflective face to bend unimpeded by connection at its periphery to the rigid front window. The OPD introduced across the wavefront is therefore increased by a factor of n compared to a conventional deformable mirror where n corresponds to the refractive index of the fluid in the fluid cell 400. For example water has a refractive index of approximately 1.35 giving an effective increase in stroke of 35% and other liquids are available having refractive indices as high or higher than as 2, giving a 100% increase in effective stroke.

The front window of the fluid cell can be made of any appropriate material for example optically flat, rigid glass. The glass may have anti-reflective optical coatings to reduce reflective losses. The cell is sealed by any appropriate gasket for example a compliant epoxy seal having appropriate valving for inlet and outlet to a fluid circuit.

Any appropriate high reflective index fluid can be used or indeed, for static applications, any appropriately flexible high refractive index medium. For example refractive index matching or immersion liquids of the type available from Cargille Laboratories NJ, USA or even a refractive index gel can be adopted.

The operation of components of the system, in particular wavefront correction can be according to any appropriate known approach.

As a result of the arrangement provided an increased optical path difference can be introduced by the deformable mirror when compared to other technologies providing improved aberration correction. In addition, cooling of the mirror is provided by the recirculation circuit when the correction device is used for high power application such as the inside of a cavity of a laser.

In accordance with a further embodiment a Fabry-Perot interferometer is shown in Fig. 5 and designated generally 500. The interferometer includes a deformable mirror 502 having a partially reflective coating 504 and deformable by actuators 506 in the manner described above. The interferometer 500 further includes a front plate 508 having a partially reflective inner face 510.

The operation of Fabry-Perot interferometers is well known and is described in summary only here. In particular, typically, Fabry-Perot interferometers include a pair of partially reflective surfaces whose separation determine the wavelengths at which interference fringes appear allowing various applications for example analytical and tuning applications. According to the embodiment shown in Fig. 5, the mirror 502 can be partially reflective for example by including an appropriate partly absorptive material for the reflective coating. By variably deforming the mirror across its face using the actuators in the manner generally described above, the path difference can be varied spatially across the face of the interferometer as a result of which the design can be tuned W

8

to different wavelengths at different points in space. As a result a spatially dynamically variable Fabry-Perot interferometer is provided. It would be noted that no refractive index medium is required in this embodiment although in some implementations there may be benefits in having a refractive index medium in addition.

It will be seen that the approach has applications in any area where optical wavefront correction is required such as astronomy microscopy, ophthalmology, free-space or laser propagation telecommunications and laser systems such as high power laser systems for industrial or scientific application. It will further be seen that the mirror can be of any appropriate type and deformed in any appropriate manner, and any appropriate refractive index and fluid cell geometry and fabrication can be adopted.

It will be noted that in addition to correction of wavefronts, other variation of wavefronts may be achieved for example where it is desired to apply a specific profile to a wavefront.

It will further be seen that the refractive index medium can have a wavelength dependent refractive index, for example in the form of a dispersive refractive index medium as a result of which the effective path length for a given stroke will vary between wavelengths providing improved phase variation control.

Claims

1. A wavefront variation device comprising a refractive index medium cell having a deformable reflective variation surface in which the cell is arranged to pass a wavefront to be varied through a refractive index medium to the variation surface.

2. A device as claimed in claim 1 in which the refractive index medium cell includes a wavefront receiving window as a front face and the deformable reflective variation surface as a rear face for reflecting a wavefront passed through the front face.

3. A device as claimed in claim 1 or claim 2 further comprising piezoelectric actuators associated with said variation surface for deformation thereof.

4. A device as claimed in any preceding claim further comprising a compliant seal sealing the variation surface in the refractive index medium cell.

5. A device as claimed in any preceding claim in which the refractive index medium is a fluid cell.

6. A device as claimed in claim 5 further comprising a fluid in the fluid cell.

7. A device as claimed in claim 6 in which the fluid has a refractive index greater than one.

8. A device as claimed in claim 6 or claim 7 in which the fluid is a dispersive fluid.

9. A device as claimed in any of claims 6 to 8 in which the fluid is a coolant fluid.

10. A device as claimed in any preceding claim comprising a wavefront correction device.

11. A device as claimed in any preceding claim further comprising a wavefront measurement device arranged to receive the wavefront to be varied and a controller arranged to receive a detection signal from the wavefront measurement device and to control deformation of the deformable reflective variation surface to vary the detected wavefront.

12. A device as claimed in claim 11 in which the measurement device and controller provide closed loop deformation of the variation surface.

13. A system as claimed in any preceding claim further comprising a fluid circulation circuit arranged to circulate fluid through the fluid cell.

14. A wavefront variation system including an optical apparatus producing an optical beam having a wavefront to be varied and a wavefront variation device as claimed in any preceding claim arranged to receive the wavefront.

15. A system as claimed in claim 14 in which the optical apparatus comprises at least one of an astronomy, microscopy ophthalmology, free-space or laser beam propagation telecommunications, or high power laser apparatus.

16. An interferometer comprising opposed first and second at least partially reflective surfaces in which at least one of the surfaces is variably deformable across its face.

17. A method of varying a wavefront comprising receiving a wavefront to be varied in a refractive index medium cell having a deformable reflective surface and deforming the reflective variation surface to vary the wavefront, wherein the surface is deformed to vary an optical path difference as a function of the refractive index of the medium in the cell.

18. A method as claimed in claim 17 further comprising circulating fluid as a refractive index medium through the refractive index medium cell.

19. A method of controlling an interferometry device having first and second at least partially reflective surfaces comprising the steps of variably deforming at least one surface across its face.

20. A device, system or method substantially herein described with reference to the drawings.