[0001] The present invention relates to a wavelength selector and a controllable

wavelength apparatus comprising a wavelength selector and a broad spectrum source.

Background to the Invention

[0002] In many metrology and microscopy techniques, it is desirable to illuminate a sample with light in one or more specific narrow wavelength bands. Examples include fluorescence microscopy, where multiple wavelengths are used to cause different parts of a test sample to fluoresce. It is known to provide multiple wavelengths in a number of ways. Light from different laser sources having different wavelengths can be used, but this is constrained by the relatively limited ranges within which the sources can be tuned. It is also known to provide a broad-spectrum or super-continuum source and select a desired wavelength range, either using a multilayer dichroic filter or acousto-optical tunable filters. Dichroic filter systems can select multiple wavelength bands or a single wavelength band with variable width and centre wavelength. Acousto-optical tunable filters can select multiple wavelength bands but each band is very narrow, transmitted power is reduced and acousto- optical tunable filters introduce loss of signal and beam distortions.

Summary of the Invention

[0003] According to a first aspect of the invention we provide a wavelength selector to receive light from a broad spectrum laser source, the selector comprising a first dispersion element, to receive a beam of light from a broad spectrum source and generate a dispersed beam, and a mask to selectively pass a part of the dispersed beam.

[0004] The wavelength selector may comprise a first focussing element to focus the dispersed beam.

[0005] The mask may be located at the focal plane of the focussing element.

[0006] The wavelength selector may comprise output beam optics to receive the passed part of the dispersed beam and generate an output beam. [0007] The output beam optics may comprise a second focusing element to receive the passed part of the dispersed beam and a second dispersion element to receive the focussed beam from the second focussing element.

[0008] The second focussing element and second dispersion element may be the same as the first focussing element and first dispersion element.

[0009] A mirror may be located behind the mask to reflect the passed part of the dispersed beam back through the mask.

[0010] The mirror may be disposed to provide a spatial offset between the input beam and output beam.

[0011] The first dispersion element may comprise a prism. [0012] The first focusing element may comprise a concave mirror.

[0013] The paths followed by light passing through the wavelength selector may have the same length and are independent of wavelength.

[0014] The output beam may have the same profile as the input beam.

[0015] According to a second aspect of the invention there is provided a controllable wavelength apparatus, the apparatus comprising a broad spectrum source and a wavelength selector according to the first aspect of the invention, wherein the wavelength selector is configured to receive a beam of light from the broad spectrum source.

[0016] The broad spectrum source may be a supercontinuum source.

Brief Description of the Drawings

[0017] An embodiment of the invention is described by way of example only with reference to the accompanying drawings, wherein;

[0018] Figure 1 is a general schematic view of a wavelength selector according to the present invention, [0019] Figure 2a is a schematic view of a particular configuration of a wavelength selector embodying the present invention, and

[0020] Figure 2b is a side view of an alternative arrangement of the wavelength selector of figure 2a.

Detailed Description of the Preferred Embodiments

[0021] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice.

[0022] Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and

terminology employed herein is for the purpose of description and should not be regarded as limiting.

[0023] Referring now to figure 1, a wavelength selector is generally shown at 10. As illustrated at 11, the wavelength selector 10 receives an input beam from a broad spectrum or supercontinuum source, such as a white light laser. Preferably the beam 11 is collimated, although if not it will be straightforward to include additional collimation optical components or vary the spacing of the other components described below.

[0024] The input beam is then passed through a dispersion element 12. As shown in the embodiment of figure 2, the dispersion element is preferably a prism . A reflection or transmission grating may alternatively be used, although a grating may be disadvantageous as light is scattered into higher order modes. The effect of the dispersion element is generate a dispersed beam 13 in which the different wavelengths in the input beam 11 are spatially distributed, in known manner. The dispersed beam 13 passes through a focusing element 14 which then focuses the beam at a focal plane 14a. A mask 15 is placed in the focal plane 14. The mask 15 includes one or more apertures to selectively pass a part of the light from the dispersed beam 13, and it will be apparent that particular wavelength bands or ranges of arbitrary centre and width can be selected by providing a mask with

appropriate apertures and locating the mask appropriately within the dispersed beam 13. Where the dispersed beam 13 has the light wavelength along varying one axis, ideally the apertures will comprise vertical slits extending perpendicular to that axis to maximise the light transmitted in the selected wavelength bands, although other shapes of apertures could be used.

[0025] The passed part 16 of the dispersed beam 13 then passes through a second focussing element 17, located at its own focal length from the mask 15. Second focussing element 17 focuses the passed part 16 on a second dispersion element 18, which acts to reverse the effect of first dispersion element 12 and generates an output beam 19.

[0026] It will be apparent that by selecting the appropriate focussing and dispersion elements, the output beam 19 will have essentially the same beam parameters as the input beam 11 but with light in only a specific range or ranges, as defined by the mask 15.

[0027] To permit finer control over the selected wavelengths, it is desirable that the width of the dispersed beam 13 be as large as possible, so that an aperture of a given size in the mask will allow a relatively smaller wavelength range to pass through. This can be achieved by lengthening the distance between the various components, in particular between the first dispersion element 12 and the mask 15, but a linear configuration as shown in figure 1 is potentially unwieldy for the purposes of designing a self-contained apparatus. One solution is to use plane mirrors to 'fold' the optical configuration of figure 1 such that the arrangement will fit into a smaller volume. In an alternative embodiment as shown in figure 2a, a single dispersion element and a single focusing element can act as both first and second dispersion elements and first and second focusing elements respectively. [0028] Referring to figure 2a, a further wavelength selector is generally shown at 20. Input beam 21 passes over an output mirror 22 and passes through dispersion element 23, which in this embodiment is a prism. As illustrated at 25, the prism 23 generates a dispersed beam, with longer wavelength light being refracted through a smaller angle than the higher wavelength components. The dispersed beam 25 strikes a focusing element, in this example a concave mirror 26. The concave mirror 26 may be spherical or cylindrical, and in this example has a focal length of 200mm. The concave mirror 26 focuses the dispersed beam and reflects it towards a mask 27 in front of a plane mirror 28. As in figure 1, the mask 27 comprises at least one aperture and is positioned to allow light in one or more selected wavelength ranges to pass. Light that passes through the mask 27 is reflected by the plane mirror 28, back through the mask 27 and back towards the concave mirror 26. The returned light is refocused by the concave mirror 26 and combined into an output beam 29 by the prism 23. To enable the output beam 29 to be offset from the input beam 21, the plane mirror 28 is slightly angled relative to the dispersed beam 25 so that the output beam 29 is spatially offset relative to the input beam 21. In this example, the output mirror 22 is positioned so that the output beam 29 strikes the output mirror and is directed to an output port. The mirrors 26, 28 may be silvered mirrors (or similar) which have high reflectivity over the entire output range of the supercontinuum laser.

[0029] The plane mirror 28 and mask 27 should be at the focal plane of the concave mirror, so that each spectral component is focussed at that plane. This ensures maximum sharpness of the wavelength cutoff, assuming that the incoming white light is a diffraction limited beam. Furthermore, the prism 23 is also at the focal point of the concave mirror 26, ensuring that the different wavelength components are parallel to the mirror 28 and mask 27. The sharpness of the frequency cutoff is given by dL = L/(W.D), where L is the

wavelength, dL the width of the cutoff, W the beam width at the prism and D the dispersion. It will be apparent that the resolution can be improved, that is dL reduced, by increasing the beam width at the prism or the dispersion. The beam width can optionally be increased by use of beam expanding optics if required, and the dispersion increased by using additional dispersion elements, such as one or more additional prisms.

[0030] The wavelength selector 20 further includes a plurality of beam blocks 30a, 30b, to block unwanted beams, reflected from a surface of the prism 23 or from multiple internal reflections within the prism 23. Beam block 30c defines a maximum wavelength by limiting the long-wavelength part of the dispersed beam 25. A part 29a of the returned beam 29 is reflected from surface 23a of prism 23, and advantageously can be directed to an auxiliary output 31 to permit the spectrum of the output beam to be monitored. Other blocks or monitoring sensors may be used as desired.

[0031] As will be apparent, in the embodiment of figure 2a the length of the path followed by the dispersed and returned beams varies with wavelength, with higher wavelengths following a shorter path. This introduces small wavelength-dependent differences between the beam properties of the spectral components of the recombined beam 29 which may be undesirable as the output of the device 20. To provide balanced paths, the apparatus 20 can alternatively be configured so the plane mirror 28 and mask 27 are vertically positioned above or below the dispersion element 23. Figure 2b shows a side of view of such an embodiment, with corresponding components to figure 2a having corresponding primed reference numerals. As seen in figure 2b, plane mirror 28' and mask 27' are located directly above prism 23'. Input beam 21' passes over output mirror 22', passes through prism 23' and is reflected by concave mirror 26'. The input beam 21' is angled relative to the concave mirror 26' such that the dispersed beam 25' is directed to the mask 27' and plane mirror 28' above the prism 23'. The plane mirror 28' is angled about a horizontal axis as seen in figure 2b, so that the returned beam is directed below the input beam 21' and hits the output mirror 22'. This ensures that the path lengths through the apparatus are the same length and are independent of wavelength.

[0032] Although the selector discussed above is particularly envisioned for use with infrared and optical wavelengths, it will be apparent that the wavelength selector can be used with any wavelength range from any suitable broad spectrum source. The output beam may have the same profile as the input beam, but contain only the selected wavelength ranges. The optical components can be adapted as necessary depending on the wavelengths of interest. The selected wavelength ranges are dependent on the physical dimensions of the apertures in the mask, and it will be apparent that multiple wavelength ranges of any desired width can be selected using an appropriate mask. The mask may be replaceable, so that a particular mask can be selected for a desired application, or may be movable, automatically or otherwise, and may have variable size apertures. The output beam can be transmitted directly to a microscope or other instrument or coupled into an optical fibre. Undesirable wavelength ranges will be completely blocked by using a suitable mask material.

[0033] Advantageously, a controllable wavelength apparatus can be provided using a wavelength selector as described herein together with a broad spectrum source, for example held in a single housing with a single controller.

[0034] In the above description, an embodiment is an example or implementation of the invention. The various appearances of "one embodiment", "an embodiment" or "some embodiments" do not necessarily all refer to the same embodiments.

[0035] Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment.

[0036] Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above.

[0037] Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined.