GB2504188A - Grism optical system - Google Patents

Abstract

A dispersion device 10 for use with a polychromatic beam has first and second identical transmissive grisms 11, 12 (prismatic grating) aligned back to back along optical axis 15 so their gratings 14 are opposite to, and face each other. A distance D separates the gratings 14. The distance D may be varied. A polychromatic beam sent along axis 15 are dispersed by the first grism 11 and collimated by the second grism 12 to provide a dispersed collimated output beam. The grisms 11,12 may be volume phase hologram grisms (21, 22, fig 2).

Description

Optical System

Introduction

The present invention relates to a dispersion device for outputting a dispersed collimated beam. The device may be used in combination with a 3D multi-colour imaging system to correct chromatic aberrations of multidimensional images.

Background

Advanced optical techniques often require controlling the chromatic shear of an input beam. This is the case in 3D imaging using off-axis Fresnel zone plates in order to control lateral smearing or in multi-photon processes for controlling a 3D intensity profile. In both these examples, a pair of reflective gratings and a folded optical path is used to achieve the desired chromatic shear. In this case, the amount of lateral chromatic shear is controlled by the distance between the gratings. Varying the lateral shear, for example, to accommodate a slightly-different optical system requires further adjustments of the angle and/or position of various optical components. This complicates the control system, making it hard to integrate into user instrumentation such as microscopes.

3D imaging using an off axis Fresnel zone plate can be used to perform 3D imaging in biological systems, see for example Blanchard P.M. and Greenaway A.H. Broadband simultaneous multiplane imaging", Opt. Commun. 183, 29-36, (2000). This allows for simultaneous focus of multiple object planes on a single image plane using a simple, on axis optical set-up. However, the Fresnel zone plate introduces a natural chromatic smearing in the non-zero diffraction orders. Although this smearing can be reduced using a narrow band filter of --10 nm, this approach also reduces available photon flux.

This limits the use of the technique as the source of photons arising from the sample, such as fluorescently-tagged live cells, is often intrinsically faint and may be limited by photophysics effects including bleaching, blinking and cyto-toxicity.

Summary of the invention

According to a first aspect of the invention, there is provided a dispersion device for use with a polychromatic beam, the device comprising: a first transmissive grism; and a second transmissive grism, wherein the first and second grisms are identical, and arranged to disperse a polychromatic beam to provide a dispersed collimated output beam.

The dispersion device of the invention can be incorporated into complex optical systems in order to achieve a variable chromatic shear on an input beam without changing the convergence properties of the beam (maintaining a collimated beam).

The beam trajectory is maintained for different chromatic shear (distances). This makes the optical system easier to align and more practical to operate and maintain even by untrained personnel.

The dispersion device may comprise means for varying a distance D between the first grism and the second grism.

The first and second grisms may each have a prism and a grating. The first grating may face the second grating.

The first and second grisms may be Volume Phase Hologram grisms.

According to a second aspect of the invention there is provided an imaging system for use with polychromatic light, comprising the dispersion device of the first aspect of the invention.

The imaging system may be a spatio temporal imaging system. The imaging system may be a 3D imaging system.

The imaging system may comprise a diffractive optical element at an output of the dispersion device, for example a Fresnel lens.

The dispersion device may be located between a pair of lenses. The distance between the first and second grisms may be chosen to equalize the diffraction angle at all wavelength. This provides a simple chromatically-corrected imaging system that does not require limiting a bandwith of a light source used with the system, such as a fluorescently labelled sample. In turn this allows making effective use of the amount of photons available.

According to a third aspect of the invention there is provided a laser machining/inscription device comprising the dispersion device of the first aspect of the invention.

The laser machining/inscription device may comprise a pulsed laser and an objective lens, wherein the dispersion device is between the laser and the objective lens and is arranged to separate the spectral components of the pulses in space before the pulses enter the objective lens so as to achieve temporal focusing. This provides a simple control system by means of which a laser pulse spatio-temporal intensity profile can be dynamically tuned.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which: Figure 1 is a top view of a dispersion device comprising a pair of identical grisms; Figure 2 is a top view of a dispersion device comprising a pair of identical Volume Phase Holographic grisms; Figure 3 is a top view of an experimental setup to measure chromatic shear; Figure 4 (a) is a montage showing separation between spots at 543 nm and 633 nm as a function of camera position; Figure 4 (b) is a plot of the separation between spots at 543 nm and 633 nm measured as a function of camera position and for different grism separations; Figure 4 (c) is a plot of chromatic shear as a function of grism separation; Figure 5 is a top view of an optical setup for chromatically-corrected, multi-colour, multi-plane 3D imaging; Figure 6 (i) is a false colour image from a bandpass-filtered white light source measured without a dispersive device; Figure 6 (ii) is a false colour image from a bandpass-filtered white light source measured with a dispersive device; Figure 6 (iii) is a false colour image from a bandpass-filtered white light source measured at optimal focus for each colour without a dispersive device.

Detailed description of the invention

Figure 1 shows a schematic of a dispersion device 10 that has a pair of identical, back to back transmissive grisms for producing a collimated beam with chromatic shear from a collimated polychrome input. The lateral shear between the polychrome components in the output beam is controlled by varying the separation between the grisms. The transmissive grisms may be of any suitable type, for example as described by Traub W.A., "Constant-dispersion grism spectrometer for channelled spectra", J.Opt.Soc.Am.A, 7(9), 1779-1791, (1990).

The dispersion device 10 comprises a pair of identical first and second transmissive grisms 11 and 12 respectively. Each grism 11, 12 has a right-angle dispersive prism 13 and a blazed transmission grating 14 on the hypotenuse face of the dispersive prism.

Each grating has exterior grating facets that are parallel to the input face of the prism.

The prism is made of N-BK7 and has a wedge angle of 18°8'. The grating is made of B270 with 300 grooves per mm and a blaze angle of 17°30'. With this combination, the grisms have a "straight-through" wavelength of 538.5 nm.

The first and second grisms are aligned back to back along an optical axis 15, so the grating of the first grism is opposite and facing the grating of the second grism. A distance D separates the two gratings. The grims are positioned so that their gratings are substantially parallel. In this configuration, the grating facets of the first grism are parallel to the grating facets of the second grism. Prism mounts (not shown) allow the faces of the grisms to be rotated in the horizontal direction with respect to the optical axis with an accuracy of -2 degrees. The prism mounts also allow the grism separation to be varied with an accuracy of approximately ±1mm, over a range from 3 to 155 mm.

The lower figure is selected to prevent contact of the fragile grating surfaces. Automatic control of the grism separation could easily be implemented using a motor driven translation system.

In use the dispersive effects of the prism acting through refraction are superimposed on the dispersive effects of the grating acting through diffraction. When a collimated polychromatic beam is sent along the optical axis 15 its central wavelength 16 passes through the first and second grism undeviated. Other wavelengths, such as the short 17 and a long 18 wavelengths of the polychromatic beam, are dispersed by the first grism and collimated by the second grism, resulting in a collimated output beam with chromatic shear (dispersion). The level of dispersion of the output beam is controlled by varying the distance between the two grisms. The greater the distance, the wider the lateral shear between the polychrome components in the output beam.

Ideally a grism should be made of a prism and a blazed transmission grating fabricated from the same material (i.e. they should have the same refractive index). In addition the grating blaze angle should be identical to the wedge angle of the prism. The grating is attached to one side of the prism in such a way that the exterior faces of the grating facets are parallel to the input face of the prism as shown in Figure 1.

An optical ray-tracing ray tracing simulation (Zemax) was performed to select a suitable combination of components. The model showed that an N-3K7 prism with a wedge angle of 21°30' combined with a 300 grooves per mm transmission grating would provide appropriate dispersion. From the commercial components available an 18°8' wedge-angle prism and a B270 transmission grating of 300 grooves/mm with a blaze angle of 17°30' (from Edmund Optics) were selected. The prisms and gratings were cemented together to make the grisms.

Figure 2 shows a schematic of an alternative dispersion device 20, that has a pair of identical, Volume Phase Hologram (VPH) grisms 21 and 22 (Glazebrook K, LDSS+÷ commissioning report', AAO Newsletter, No87, 1998) for producing a dispersed collimated beam with an un-deviated central wavelength 23. VPH grisms comprise a holographic grating 24 sandwiched between a pair of symmetric prisms 25, 26, and forming an apex (Hill G.J, Wolf M.J., Tufts JR. and Smith E.C., Volume Phase Holographic (VPH) Grisms for Optical and Infrared Spectrographs", SPIE 4842, 2003).

This design makes VPH grisms extremely robust if expensive and, for which optical efficiency of 73% at wavelength of 1 3pm was calculated. Efficiencies of 91.5% have been claimed at 2.2pm wavelength. The first and second VPH grisms 21, 22 are aligned along an optical axis, so that the holographic grating of the first VPH grism is substantially parallel to the holographic grating of the second VPH grism and the apex of the first VPH grism points in a direction opposite to the apex of the second VPH grism. A distance D separates the two gratings.

Figure 3 shows a top view of an experimental setup 30 for testing the chromatic shear produced by the dispersive device 10 described above. The setup comprises a dispersive device 10 optically aligned between a pair of lenses 36, 38 separated by 300 mm, a polychromatic light source and a COD detector (6.45 urn COD pixels Qimaging Retiga). Lenses 36 and 38 have a focal length of 30 mm and 250 mm respectively, with a diameter of a 25 mm. The polychromatic light source is formed by two lasers 32 of wavelengths 543nm and 633nm coupled into an optical fibre 34 (Thorlabs SM600 fiber, single mode at 633nm) via a beamsplitter and neutral density filters (not shown).

In use the polychromatic light from the fibre is collimated by the first lens 36 to provide a 3 mm diameter beam and refocused by the second lens 38 onto the CCD detector.

The black broken lines illustrate ray paths and position of focus if no grisms are used, and the black solid lines for use with grisms. The ray paths converge at an image plane denoted by, Al. Arrows indicate how the chromatic shear is changed by varying the grism spacing d. The maximum separation d, between the inner faces of the grisms is 155mm. The angle at which the laser beams come to focus is assessed by scanning the camera axially about the best focus', as indicated by Al.

Figure 4 (a) shows a false-colour montage displaying the separation of the image of the fibre source in each laser line 543 nm and 633 nm, as function of the camera position away from best focus. Images with both lasers incident were recorded as a function of COD / camera position as the camera was translated ± 10 mm either side from the best focus' image plane for maximum grism separation (false color image). The ND filters were adjusted such that both laser lines were closely matched in intensity and, in combination, and did not saturate the COD detector. As the COD translates through the best focus' image plane, there is a clear shift of the 633 nm spot relative to the 543 nm spot. The 543 nm spot remains largely fixed as it is close to the design wavelength of the grism pair. Figure 4(a) demonstrates the potential for chromatic tuning using an on-axis grism pair.

Figure 4 (b) shows the separation between spots at 543 nm and 633 nm as function of the camera position obtained for different grism separation distances. Results were obtained for a grism separation of 3 mm, 29 mm, 72 mm, 113 mm and 154 mm.

Figure 4 (c) shows chromatic shear as a function of grism separation. By repeating the measurements as a function of grism separation distance, the angle of arrival of chromatic components can be estimated and, using the focal length of the imaging lens, the spatial chromatic shear produced by the grism separation can be deduced. A shear between the laser beams of 0.0294 mm per mm separation is equivalent to 327 pm shear per nm bandpass per mm of grism separation.

Figure 5 shows a schematic of the grism system used to correct chromatic blur in a off axis Fresnel zone plate based 3D imaging system. The imaging system 40 comprises a dispersive device 10 optically aligned between a pair of achromatic lenses 42, 44, each with a focal length of 400mm, a polychromatic light source, a diffractive optic element 46 and a COD detector (not shown). The polychromatic light source is selected from the dual laser system previously described or from a multimode white light source, a 50 pm core fibre-launched white light filtered by a range of 10 nm bandpass filters with central wavelengths from 470 nm to 650 nm in 20 nm steps. The lenses are separated by a distance of 200 mm and form a unit magnification relay system with an effective focal length of 266 mm.

The diffractive optic element is an off-axis Fresnel zone plate, i.e. a phase grating with quadratic distortion that works as a lens by imparting a different focusing power in each diffraction order. The zone plate has a focal length in the +1 diffraction order at 633nm and has a nominal axial period 50 pm. The three focal points shown on the right illustrate the position of focus of the single fibre source in each diffraction order and when the single source position is fixed. On a single, flat camera plane each diffraction order is focussed on a different specimen plane and the three focal points are recorded simultaneously. The diffractive optic element is placed 266 mm from the second principal plane (133mm from the second lens) of the compound imaging system. In this configuration equal magnification images are obtained in each diffraction order, providing simultaneous multi-plane imaging due to the order dependent focusing power introduced by the DOE. Only the spacing between the two grisms is relevant, not their absolute position between the achromatic pair.

In use, the polychromatic light from the fibre is collimated by the first lens 42 and refocused by the second lens 44 onto the COD detector. Because a single polychromatic source is used it is necessary to scan the source position to record the 3D foci in the different diffraction orders. These images were recorded with and without using the grism system. When the grisms were introduced, the grism spacing / was chosen as accurately as possible to equalize the diffraction angles at all wavelengths.

The off axis Fresnel zone plate period is linearly chirped along one axis. By pre-dispersing the polychrome input beam using the dispersive device 10, each incident wavelength is positioned within the chirped zone plate to see' the same DOE structure measured in wavelength units. The pre-dispersion thus equalises the angle of diffraction for each wavelength, giving non smeared white light' images in all diffraction orders for broadband sources.

Figure 6 shows various images demonstrating the effectiveness of the dispersion device of the invention. In particular, Figure 6 shows images (i) at a single focus without grism-based correction, (ii) at a single focus with grism correction, (iii) without grism correction and with the camera re-focussed for each chromatic component (image positions unadjusted) and (iv) with grism correction and with the camera refocussed for each component and those imaged summed directly to form a false-colour composite (image positions unadjusted).

Figure 60) demonstrates, for the +1 diffraction order, the principle of the chromatic correction using 50 pm core fibre-launched white light filtered by a range of 10 nm bandpass filters from 470 nm to 650 nm in 20 nm steps. Figure 60) additionally shows the falsely colored image from focusing the system for wavelength, 560 nm, but without grism correction and without adjusting for the linearly wavelength dependent focusing power of the zone plate.

Figure 6 (ii) shows the falsely coloured image obtained by focusing the white light source for 560 nm central wavelength with grism correction, in the +1 diffraction order.

Chromatic smearing is effectively removed by the dispersive device 10 leaving a single un-smeared spot. The image quality is dominated by a residual A dependence of the focal length of the DOE. Figure 60) and (ii) demonstrate the principle of the grism-based chromatic correction. However a second-order X dependent focusing now dominates. In principle a chromatically aberrated imaging lens could be designed to mitigate this effect.

Figure 6 (Ui) shows the falsely coloured image obtained by focusing the system for 560 nm central wavelength without grism correction. But in this case the camera was re-focussed for each chromatic component (image positions unadjusted). When using the dispersive device, the uncorrected rainbow in Figure 6 (Vi) is transformed to a single white' spot in the grism corrected system (not shown). To achieve this, grism correction is performed with the camera refocussed for each component. Imaged are then summed directly to form a false-colour composite (image positions unadjusted).

The invention provides a simple, effective and robust optical system that uses back to back identical grisms to achieve a chromatic shear in a collimated polychrome input beam without change in beam convergence. The chromatic shear can be controlled simply by varying the separation of the grisms. The grisms are typically prisms with blazed transmission gratings cemented to their surfaces. Alternatively, each grism may be two prisms with volume phase holograms cemented between them.

The optical system of the invention uses a combination of prisms and gratings arranged to provide a dispersion system in a diffraction order arranged such that a central wavelength is undeviated. The chromatic shear can be controlled by varying the separation of the optical components along an undeviated optical axis.

The optical system of the invention can be used to control dispersion in 3D imaging using off-axis Fresnel lenses. Equally, the optical system of the invention can be used to control dispersion in a spatio-temporal imaging system.

The 3D imaging system described above demonstrates the ease with which the dispersive device 10 can be incorporated into complex optical systems in order to achieve a variable chromatic shear on an input beam without changing the convergence properties of the beam.

The 3D imaging system described above can be used to perform multi-colour fluorescence microscopy. For application in 3D live-cell microscopy of fluorescently-tagged live cells where the sources are intrinsically faint and the photon flux may be limited by the inherent photophysics (bleaching, blinking and cyto-toxicity), achieving the highest total optical throughput is very important. The limiting bandwidth of most fluorophores is c 100 nm. The simple chromatic correction of the system allows making effective use of the full fluorophore bandwidth.

The dispersive device 10 could also be used in laser inscription applications.

Combining the chromatic angle of arrival control achieved using the dispersive device with accurate chromatically-dependent timing of the spectral components in a femtosecond laser pulse, would provide a simple control system by means of which the 3D profile in multi-photon laser inscription could be dynamically tuned. In this case, because control of the axial focus quality is desired, a high numerical aperture objective lens would be used.

Using a pair of grisms (a grating combined with a prism) allows accurate control of an essentially linear chromatic shear through the simple mechanism of changing the distance of separation between two grisms. The pair of grisms can be used for the correction of lateral chromatic smearing in multi-plane imaging using Fresnel zone plate using the inherent chirp in the zone-plate structure to achieve a near-complete correction the principal chromatic defects in the 3D imaging.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.

Claims (13)

1. CLAIMS1. A dispersion device for use with a polychromatic beam comprising: a first transmissive grism having a first prism and a first grating; and a second transmissive grism having a second prism and a second grating, wherein the first and second grisms are identical, and arranged to disperse a polychromatic beam to provide a dispersed collimated output beam.
2. 2. A device as claimed in claim 1 comprising means for varying a distance D between the first grism and the second grism.
3. 3. A device as claimed in claim 1 or 2, wherein the first grating faces the second grating.
4. 4. A device as claimed in any of the claims 1 to 3 wherein first and second transmissive grisms are Volume Phase Hologram grisms.
5. 5. An imaging system for use with polychromatic light comprising a device as claimed in any of claims ito 4.
6. 6. An imaging system as claimed in claim 5 wherein the imaging system is a spatio temporal imaging system.
7. 7. An imaging system as claimed in claim 5 or claim 6 comprising a diffractive optical element at an output of the dispersion device.
8. 8. An imaging system as claimed in claim 7 wherein the diffractive optical clement is a Fresnel lens.
9. 9. An imaging system as claimed in claim 7 or claim 8 wherein the dispersion device is between a pair of lenses.
10. 10. An imaging system as claimed in any of claims 5 to 9 wherein the imaging system is a 3D imaging system.
11. 11. An imaging system as claimed in any of claims 5 to 10 wherein a distance between the first and second grisms is chosen to equalize the diffraction angle at all wavelength.
12. 12. A laser machining/inscription device comprising a device as claimed in any of claims 1 to 4.
13. 13. A laser machining/inscription device as claimed in claim 12 comprising a pulsed laser and an objective lens, wherein the dispersion device is between the laser and the objective lens and is arranged to separate the spectral components of the pulses in space before the pulses enter the objective lens so as to achieve temporal focusing.