SPECTROMETER

Abstract

A spectrometer for temporally separating electromagnetic radiation (10) includes a cavity (105) having first and second reflecting mirrors (1, 2, 4, 5). The first mirror (1, 2) has an aperture (8) arranged to allow electromagnetic radiation (10) to be input into the cavity (105). The spectrometer also includes an imaging device (3) between the first and second mirrors (1, 2, 4, 5) that defines an optical axis of the cavity (105) and performs spatial Fourier transforms of the electromagnetic radiation (10). The first and/or second mirrors (1, 2, 4, 5) has a normal that is arranged at a non-parallel angle to the optical axis, such that the position and/or angle of incidence of electromagnetic radiation (10) on the second mirror is shifted after each round trip. The second mirror (4, 5) allows a wavelength component (14) of the electromagnetic radiation to be output from the cavity (105) when the position and/or angle of incidence of the electromagnetic radiation on the second mirror (4) after one or more round trips of the cavity (105) exceeds a threshold.

Claims

1. A spectrometer for temporally separating input electromagnetic radiation, the spectrometer comprising: a cavity comprising a first mirror and a second mirror arranged to reflect input electromagnetic radiation therebetween, wherein the first mirror comprises an aperture arranged to allow electromagnetic radiation to be input into the cavity through the aperture; and an imaging device arranged in the path taken by the electromagnetic radiation between the first and second mirrors, wherein the imaging device defines an optical axis of the cavity and the imaging device is arranged to perform a spatial Fourier transform of the input electromagnetic radiation from the aperture in the first mirror onto the second mirror and to perform a spatial Fourier transform of the reflection of the electromagnetic radiation from the second mirror back onto the first mirror; wherein at least a portion of one or both of the first and second mirrors has a normal that is arranged at a non-parallel angle to the optical axis of the cavity, such that the position and/or angle of incidence on the second mirror of electromagnetic radiation input into the cavity through the aperture in the first mirror is shifted after each round trip of the electromagnetic radiation through the cavity; and wherein the second mirror is arranged to allow a wavelength component of the electromagnetic radiation to be output from the cavity when the position and/or angle of incidence of the electromagnetic radiation on the second mirror after one or more round trips of the cavity exceeds a particular threshold such that different wavelength components of the input electromagnetic radiation are output from the cavity after completing a different number of round trips of the cavity.

2. A spectrometer as claimed in claim 1, wherein the spectrometer is configured to measure the wavelength components of electromagnetic radiation having wavelengths between 200 nm and 2 m.

3. (canceled)

4. A spectrometer as claimed in claim 1, 2 or 3, wherein the spectrometer comprises an angular separation device arranged to angularly separate electromagnetic radiation that is incident upon it and to direct the angularly separated electromagnetic radiation through the aperture and into the cavity, and wherein the angular separation device is arranged to angularly separate the incident electromagnetic radiation dependent upon the wavelength components of the electromagnetic radiation.

5. (canceled)

6. A spectrometer as claimed in claim 4, wherein the spectrometer comprises an input imaging device arranged to focus the angularly separated electromagnetic radiation from the angular separation device onto the aperture of the cavity such that the electromagnetic radiation angularly diverges in the cavity.

7. A spectrometer as claimed in claim 1, wherein the electromagnetic radiation is arranged to be directed into the cavity at a non-parallel angle to the optical axis of the cavity.

8. A spectrometer as claimed in claim 1, wherein the first mirror comprises two separate portions defining the aperture therebetween, and wherein one of the portions of the first mirror has a normal that is arranged at a non-parallel angle to the optical axis of the cavity.

9. A spectrometer as claimed in claim 1, wherein the aperture is longitudinally extended in a direction perpendicular to the optical axis of the cavity and/or the aperture is offset by a distance from the optical axis of the cavity in a direction perpendicular to the optical axis.

10. (canceled)

11. (canceled)

12. A spectrometer as claimed in claim 8, wherein the aperture is longitudinally extended in a direction perpendicular to the optical axis of the cavity, and wherein the two portions of the first mirror are rotated with respect to each other about an axis that extends in a direction perpendicularly to the aperture in the first mirror and perpendicularly to the optical axis of the cavity.

13. (canceled)

14. A spectrometer as claimed in claim 1, wherein the aperture is longitudinally extended in a direction perpendicular to the optical axis of the cavity, wherein the second mirror comprises two portions, wherein one of the portions of the second mirror has a normal that is arranged at a non-parallel angle to the optical axis of the cavity, and wherein the two portions of the second mirror are rotated with respect to each other about an axis that extends in a direction perpendicularly to the optical axis of the cavity and parallel to the direction in which the aperture in the first mirror is longitudinally extended.

15. A spectrometer as claimed claim 1, wherein the cavity is arranged such that the round trip of the electromagnetic radiation through the cavity involves the electromagnetic radiation passing via the imaging device four times to return to substantially the same position on the second mirror.

16. A spectrometer as claimed in claim 1, wherein the second mirror is arranged to allow a wavelength component of the electromagnetic radiation to be output from the cavity when the position of incidence of the electromagnetic radiation on the second mirror after one or more round trips of the cavity exceeds a particular position threshold.

17. A spectrometer as claimed in claim 16, wherein the second mirror comprises a razor edge that defines the position threshold or a gradient spectral filter comprising a plurality of spectral edges that is arranged to transmit a plurality of wavelength components of the electromagnetic radiation when the wavelength components exceed respective position thresholds on the second mirror.

18. A spectrometer as claimed in claim 17, wherein the razor or spectral edge forms a straight line that extends in a direction that is perpendicular to the direction in which the electromagnetic radiation is shifted after each round trip through the cavity.

19. A spectrometer as claimed in claim 1, wherein the second mirror is arranged to allow a wavelength component of the electromagnetic radiation to be output from the cavity when the angle of incidence of the electromagnetic radiation on the second mirror after one or more round trips exceeds a particular angular threshold.

20. A spectrometer as claimed in claim 19, wherein the second mirror comprises an angle-dependent spectral filter arranged to transmit a plurality of wavelength components of the electromagnetic radiation when the wavelength components exceed respective angular thresholds on the second mirror.

21. A spectrometer as claimed in claim 1, wherein the second mirror is arranged to allow a wavelength component of the electromagnetic radiation to be output from the cavity when the position and angle of incidence of the electromagnetic radiation on the second mirror after one or more round trips of the cavity exceed particular position and angular thresholds.

22. A spectrometer as claimed in claim 21, wherein the second mirror comprises a gradient dielectric filter arranged to transmit a plurality of wavelength components of the electromagnetic radiation when the wavelength components exceed respective angular and position thresholds on the second mirror.

23. A spectrometer as claimed in claim 1, wherein the spectrometer comprises a detector arranged relative to the cavity such that the wavelength components output from the cavity are incident upon the detector.

24. (canceled)

25. A spectrometer as claimed in claim 23, wherein the detector is connected to a processing system arranged to receive signals from the detector, and to determine from the signals the time delays between the arrival of the wavelength components at the detector.

26. A spectrometer as claimed in claim 25, wherein the processing system is arranged to use the determined time delays to determine the wavelengths of the wavelength components output from the cavity.

Description

[0101] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0102] FIG. 1 shows a schematic overview of a spectrometer according to an embodiment of the present invention;

[0103] FIGS. 2a, 2b and 2c show the details of a cavity to be used in the spectrometer shown in FIG. 1;

[0104] FIG. 3 shows a schematic overview of a spectrometer according to another embodiment of the present invention;

[0105] FIGS. 4a, 4b and 4c show the details of a cavity to be used in the spectrometer shown in FIG. 3;

[0106] FIG. 5a shows an exemplary test spectrum for measurement by a spectrometer according to an embodiment of the present invention;

[0107] FIG. 5b shows the output of a detector of the spectrometer as captured by a data processor of the spectrometer;

[0108] FIG. 5c shows the calibration curve for the detector;

[0109] FIG. 5d shows the relative efficiency of the detector; and

[0110] FIG. 5e shows the determined spectrum compared to the test spectrum.

[0111] Preferred embodiments of a spectrometer in accordance with the present invention will now be described, which measures the spectrum of input electromagnetic radiation by converting the different wavelength components of the input electromagnetic radiation into different temporal components. Such a spectrometer has many different applications that each require the spectrum of, e.g. a pulse of, electromagnetic radiation to be measured. For example, lidar systems, remote detection of chemical traces and the characterisation of quantum light sources, may involve multiple, faint and/or pulsed light sources that are desired to be characterised.

[0112] FIG. 1 shows a schematic overview of a spectrometer 101 according to an embodiment of the present invention. In this embodiment the spectrometer 101 comprises a diffraction grating or prism 103 that is arranged to receive input electromagnetic radiation from a source of electromagnetic radiation 102 to be measured by the spectrometer 101. The spectrometer 101 also comprises an imaging system 104 that is arranged between the diffraction grating or prism 103 and the main cavity 105 of the spectrometer 101 that performs the temporal separation of the electromagnetic radiation input into the spectrometer 101, as will be described below.

[0113] The output of the cavity 105 is coupled to a detector 107, e.g. an avalanche photo diode, via an imaging system 106, with the detector 107 providing measurement data for analysis to a data processor 108.

[0114] A cavity 105 suitable for use with the spectrometer 101 shown in FIG. 1 will now be described with reference to FIGS. 2a, 2b and 2c which show the details of a cavity 105 to be used in the spectrometer 101 shown in FIG. 1. FIG. 2a shows a perspective view of the cavity 105; FIG. 2b shows a side view of the cavity 105;

[0115] and FIG. 2c shows a plan view of the cavity 105.

[0116] The cavity 105 comprises two sets of mirrors 1, 2, 4, 5 arranged at and defining each end of the cavity 105. Each set of mirrors 1, 2, 4, 5 have their reflective faces generally facing those of the other set, i.e. the two sets of mirrors 1, 2, 4, 5 are approximately parallel. A lens 3 is arranged in the optical path between the two sets of mirrors 1, 2, 4, 5, with the lens 3 defining an optical axis 6 of the cavity 105. The two sets of mirrors 1, 2, 4, 5 are arranged at a focal length f either side of the lens 3, with the two sets of mirrors 1, 2, 4, 5 oriented substantially in a plane perpendicular to the optical axis 6 of the cavity 105.

[0117] The first set of mirrors 1, 2 includes an upper portion 1 and a lower portion 2 that are separated by a distance that defines an aperture 8 for the input of electromagnetic radiation 10 (from the source of electromagnetic radiation 102) into the cavity. The aperture 8 forms a slit that is longitudinally extended in a direction perpendicular to the optical axis 6 of the cavity 105. The upper edge of the aperture 8, formed by a razor edge 10 at the lower edge of the upper portion 1 of the first set of mirrors 1, 2, is offset from the optical axis 6 by a distance in a direction perpendicular to the direction in which the aperture 8 is longitudinally extended (and thus the lower edge of the aperture 8 is defined by the upper edge of the lower portion 2 of the first set of mirrors 1, 2).

[0118] The upper portion 1 of the first set of mirrors 1, 2 lies in a plane perpendicular to the optical axis 6 of the cavity 105. The lower portion 2 of the first set of mirrors 1, 2 is rotated by an angle to the upper portion 1 about an axis perpendicular to the optical axis 6 and to the direction in which the aperture 8 is longitudinally extended, such that the normal to the lower portion 2 is at an angle to the optical axis 6, as can be seen from the plan view in FIG. 2c.

[0119] The second set of mirrors 4, 5 includes an upper portion 5 and a lower portion 4. The lower portion 4 of the second set of mirrors 4, 5 lies in a plane perpendicular to the optical axis 6 of the cavity 105. The upper portion 5 of the second set of mirrors 4, 5 is rotated by an angle to the lower portion 4 about an axis that defines the boundary between the upper and lower portions 5, 4 of the second set of mirrors, such that the normal to the upper portion 5 is at an angle to the optical axis 6, as can be seen from the side view in FIG. 2b. The optical axis 6 of the cavity 105 is perpendicular to and passes through the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors. The axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors is also parallel to the direction in which the aperture 8 is longitudinally extended.

[0120] The upper portion 5 of the second set of mirrors 4, 5 has a razor edge 12 extending along one side of the upper portion 5 in a direction perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors.

[0121] Operation of this embodiment of the spectrometer will now be described with reference to FIGS. 1, 2a, 2b and 2c.

[0122] The source of electromagnetic radiation 102 to be measured is arranged to be incident upon the diffraction grating or prism 103 such that the electromagnetic radiation 102 is angularly separated (within a plane) into a plurality of different wavelength components 14.

[0123] The diffraction grating or prism 103 is arranged relative to the source of electromagnetic radiation 102 such that the resultant angularly separated wavelength components 14 of the electromagnetic radiation are incident upon an imaging system 104 which images (focusses) the electromagnetic radiation to be incident through the aperture 8 in the first set of mirrors 1, 2 and into the cavity 105 of the spectrometer 101, at a position that is offset by a distance d from the optical axis 6 of the cavity 105 in a direction perpendicular to the optical axis 6 and the direction in which the aperture 8 is longitudinally extended. The cavity 105 (and the aperture 8 in particular) is arranged such that the plane in which the angularly separated wavelength components 14 of the electromagnetic radiation that are imaged onto the aperture 8 of the cavity 105 is oriented at an angle to the optical axis 6 of the cavity 105. This plane containing the angularly separated wavelength components 14 passes (parallel) through the aperture 8 of the cavity with the wavelength components 14 passing close to the razor edge 10 of the upper portion 1 of the first set of mirrors 1, 2, offset by a distance d from the optical axis 6 of the cavity 105.

[0124] Owing to the wavelength components 14 of the input electromagnetic radiation being angularly separated and then imaged onto the aperture 8 of the cavity 105, it will be appreciated that the wavelength components 14 diverge as they enter the cavity 105 and travel towards the lens 3. When the different wavelength components 14 pass through the lens 3, owing to their angular separation and the first and second sets of mirrors 1, 2, 4, 5 each being arranged at a distance of a focal length f of the lens either side of the lens 3, the different wavelength components 14 are spatially Fourier transformed to different positions on the lower portion 4 of the second set of mirrors 4, 5. The different positions on the lower portion 4 onto which the different wavelength components 14 are spatially Fourier transformed are spaced from each other in a direction parallel to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors but at the same distance from the optical axis 6 in a direction perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors.

[0125] As shown in FIG. 2b, the wavelength components 14 are incident upon the lower portion 4 of the second set of mirrors 4, 5. The position at which the wavelength components 14 are spatially Fourier transformed on the lower portion 4 of the second set of mirrors 4, 5 is offset by a distance f from the optical axis 6 and the wavelength components 14 are incident upon the lower portion 4 of the second set of mirrors 4, 5 at an angle d/f to the normal of the lower portion 4 (in planes perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors).

[0126] Following reflection from the lower portion 4 of the second set of mirrors 4, 5, the different wavelength components 14 pass through the lens 3 and are spatially Fourier transformed to a position on the lower portion 2 of the first set of mirrors 1, 2. The position at which the wavelength components 14 are spatially Fourier transformed on the lower portion 2 of the first set of mirrors 1, 2 is offset by a distance d from the optical axis 6 and the wavelength components 14 are incident upon the lower portion 2 of the first set of mirrors 1, 2 at an angle to the normal of the lower portion 2 (in a plane perpendicular to the direction in which the aperture 8 is longitudinally extended).

[0127] Owing to the lower portion 2 of the first set of mirrors 1, 2 being at an angle to the upper portion 1, the angle of each of the wavelength components 14, relative to the direction of the optical axis 6 and in a plane perpendicular to the planes of the upper and lower portions 1, 2 of the first set of mirrors, when reflected from the lower portion 2 of the first set of mirrors 1, 2, is rotated by 2 compared to the corresponding angle when the wavelength components 14 entered the cavity 105, as is shown in FIG. 2c.

[0128] Following reflection from the lower portion 2 of the first set of mirrors 1, 2, the different wavelength components 14 pass through the lens 3 and are spatially Fourier transformed to different positions on the upper portion 5 of the second set of mirrors 1, 2 (again, owing to the wavelength components 14 of the input electromagnetic radiation being angularly separated). The different positions on the upper portion 5 onto which the different wavelength components 14 are spatially Fourier transformed are spaced from each other in a direction parallel to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors but at the same distance from the optical axis 6 in a direction perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors.

[0129] As shown in FIG. 2b, the wavelength components 14 are incident upon the upper portion 5 of the second set of mirrors 4, 5. The position at which the wavelength components 14 are spatially Fourier transformed on the upper portion 5 of the second set of mirrors 4, 5 is offset by a distance f from the optical axis 6 and the wavelength components 14 are incident upon the upper portion 5 of the second set of mirrors 4, 5 at an angle d/f to the direction of the optical axis 6 (in planes perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors).

[0130] Owing to the upper portion 5 of the second set of mirrors 1, 2 being at an angle to the lower portion 4, the angle of each of the wavelength components 14, relative to the direction of the optical axis 6 and in a plane perpendicular to the planes of the upper and lower portions 5, 4 of the second set of mirrors, when reflected from the upper portion 5 of the second set of mirrors 4, 5, is rotated by 2, as is shown in FIG. 2c, compared to the angle that the wavelength components 14 would have been reflected from the upper portion 5 had it been coplanar with the lower portion 4.

[0131] Following reflection from the upper portion 5 of the second set of mirrors 4, 5, the different wavelength components 14 pass through the lens 3 and are spatially Fourier transformed to a position on the upper portion 2 of the first set of mirrors 1, 2. The position at which the wavelength components 14 are spatially Fourier transformed on the upper portion 1 of the first set of mirrors 1, 2 is offset by a distance d+2f from the optical axis 6 and the wavelength components 14 are incident upon the upper portion 1 of the first set of mirrors 1, 2 at an angle to the normal of the upper portion 1 (in a plane perpendicular to the upper portion 1), as shown in FIG. 2b.

[0132] It will be appreciated that the wavelength components 14 now take a very similar path through the cavity 105, being reflected off the mirror portions 1, 2, 4, 5, compared to when the wavelength components 14 first entered the cavity 105 through the aperture 8, except that they start from a position further offset from the optical axis 6 (by a distance d+2f compared to d when they entered the cavity 105, owing to the reflection from the angled upper portion 5 of the second set of mirrors 4, 5) and the angle of each of the wavelength components 14, relative to the direction of the optical axis 6 and in a plane perpendicular to the planes of the upper and lower portions 1, 2 of the first set of mirrors is rotated by 2 compared to the corresponding angle when the wavelength components 14 entered the cavity 105, owing to the angled lower portion 2 of the first set of mirrors 1, 2.

[0133] As the wavelength components 14 are directed at the same angle to the optical axis 6 as they were when they first entered the cavity 105, they will be incident upon the lower and upper portions 5, 4 of the second set of mirrors in the same positions in the plane perpendicular to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors, e.g. as viewed from the side in FIG. 2b. However, owing to the rotation of the wavelength components 14 through the angle 2 in the plane perpendicular to the planes of the upper and lower portions 1, 2 of the first set of mirrors, the positions at which the wavelength components 14 are incident upon the upper and lower portions 5, 4 of the second set of mirrors are each shifted by a distance 2f (in a direction parallel to the to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors) compared to the position when the respective components 14 were previously incident on these upper and lower portions 5, 4 of the second set of mirrors, as shown in FIG. 2c.

[0134] As can be seen, for each round trip of the wavelength components 14 of the input electromagnetic radiation through the cavity 105 (i.e. involving a reflection from each of the mirror portions 1, 2, 4, 5), the position of incidence of each wavelength component 14 is shifted by a distance 2f in a direction parallel to the to the axis forming the boundary between the upper and lower portions 5, 4 of the second set of mirrors and perpendicular to the razor edge 12 of the upper portion 5 of the second set of mirrors 4, 5. Thus each round trip moves the wavelength components 14 closer to the razor edge 12 until they exceed the position of the razor edge 12 and are thus output from the cavity 105.

[0135] The output wavelength components 14 are imaged by an imaging system 106, arranged beyond the razor edge 12 of the upper portion 5 of the second set of mirrors 4, 5, onto a detector 107 (e.g. an avalanche photo diode), which detects the arrival of each wavelength component 14.

[0136] Owing to the angular separation of the wavelength components 14 when they are input into the cavity 105 and thus the different wavelength components 14 being spatially Fourier transformed to different positions on the portions of the second set of mirrors 4, 5 (in a direction perpendicular to the razor edge 12 of the upper portion 5 of the second set of mirrors 4, 5) the different wavelength components 14 will exceed the position of the razor edge 12 after a different number of round trips through the cavity 105. This will therefore introduce a time delay between the different wavelength components 14. (The time delay is slightly over 8f/c, where f is the focal length of the lens and c is the speed of light.)

[0137] A signal, corresponding to the detection of arrival of each wavelength component 14 by the detector 107, is sent from the detector 107 to the data processor 108 for each detected wavelength component, with the data processor 108 being arranged to produce a time stamp for each arrival time of the wavelength components 14. These arrival times are then converted into wavelengths using a calibration of the spectrometer and their relative intensities determined using the relative efficiency of the spectrometer (as will be described later with reference to FIG. 5).

[0138] The Applicant also envisages a number of variants to the embodiment of the cavity shown in FIGS. 2a, 2b and 2c, which will now be described.

[0139] In one variant to the cavity shown in FIGS. 2a, 2b and 2c, the second set of mirrors 4, 5 is replaced with a single planar mirror that is arranged perpendicular to the optical axis, thus eliminating the angle of the upper portion of the second set of mirrors in the previous embodiment. As the angle was previously used to offset the reflected electromagnetic radiation the input aperture of the cavity, the aperture in this variant of the spectrometer is replaced with a reflective shutter, to prevent the reflected electromagnetic radiation from being incident upon the aperture and therefore escaping from the cavity. The shutter is configured to open to allow a pulse from the source of electromagnetic radiation into the cavity and to close such that the reflected electromagnetic radiation is reflected from the shutter upon incidence.

[0140] Operation of this variant of the cavity is the same as described above for the embodiment shown in FIGS. 2a, 2b and 2c, except that when the electromagnetic radiation is incident upon the (upper portion of the) second mirror, no angular shift of 2 is introduced, but instead the electromagnetic radiation is reflected back towards the shutter, where it is incident at the same position for each round trip.

[0141] In another variant of the cavity shown in FIGS. 2a, 2b and 2c, or to the variant described above, the razor edge 12 of the upper portion 5 of the second set of mirrors 4, 5 is replaced with an (upper) mirror portion that is arranged as a gradient spectral filter. This gradient spectral filter is arranged to transmit different wavelength components 14 of the electromagnetic radiation incident upon the (upper) mirror portion when the position of incidence of the different wavelength components 14 exceed different respective position thresholds on the mirror (in a direction parallel to the direction in which the input aperture is longitudinally extended) and otherwise reflect the wavelength components 14 of the electromagnetic radiation. Thus the (upper) mirror portion is arranged to transmit different wavelength components 14 at different positions, which introduces a time delay between the different wavelength components 14 of the input electromagnetic radiation.

[0142] Operation of this variant of the cavity is the same as described above for the embodiment shown in FIGS. 2a, 2b and 2c (and the variant above), except that the different wavelength components of the input electromagnetic radiation are output from the cavity when they are shifted to pass different particular positions on the (upper portion of the) second mirror such that the different wavelength components are transmitted through the mirror at different positions and thus at different times. Thus it will be appreciated that in this variant, like the embodiments described below, it is not necessary to angularly separate the electromagnetic radiation input into the cavity; it is possible to temporally separate the wavelength components by inputting the electromagnetic radiation in a, e.g. collimated, beam. Furthermore, the imaging device positioned after the exit from the cavity should be arranged to collect light from different positions, e.g. to focus all the wavelength components to a single point. A lens positioned at a distance one focal length from both the exit of the cavity and the detector would be suitable for this.

[0143] A further embodiment of the spectrometer according to the present invention will now be described, in which the input electromagnetic radiation is not angularly separated before being input into the cavity.

[0144] FIG. 3 shows a schematic overview of a spectrometer 201 according to this further embodiment of the present invention. The spectrometer of this embodiment is very similar to the embodiment shown in FIG. 1, but instead of a diffraction grating or prism, the spectrometer 201 comprises a collimator 203 that is arranged to receive input electromagnetic radiation from a source of electromagnetic radiation 202 to be measured by the spectrometer 201. The collimator is positioned between the source of electromagnetic radiation 202 and the main cavity 205 of the spectrometer 201 that performs the temporal separation of the electromagnetic radiation input into the spectrometer 201, as will be described below.

[0145] The same as for the embodiment shown in FIG. 1, the output of the cavity 205 is coupled to a detector 207, e.g. an avalanche photo diode, via an imaging system 206, with the detector 207 providing measurement data for analysis to a data processor 208.

[0146] A cavity 205 suitable for use with the spectrometer 201 shown in FIG. 3 will now be described with reference to FIGS. 4a, 4b and 4c which show the details of a cavity 405 to be used in the spectrometer 401 shown in FIG. 3. FIG. 4a shows a perspective view of the cavity 205; FIG. 4b shows a side view of the cavity 205; and FIG. 4c shows a plan view of the cavity 205.

[0147] Similar to the cavity shown in FIGS. 2a, 2b and 2c, the cavity 205 in this embodiment comprises two sets of mirrors 51, 52, 54, 55 arranged at and defining each end of the cavity 205. The arrangement of these mirrors is the same as in the previous embodiment, with a lens 53 positioned in the optical path between them and defining an optical axis 56 of the cavity, except that the lower portion 52 of the first set of mirrors 51, 52 is coplanar with the upper portion 51 of the first set of mirrors, i.e. not at an angle as for the cavity of the previous embodiment.

[0148] A further difference to the cavity shown in FIGS. 2a, 2b and 2c is that, for the cavity 205 shown in FIGS. 4a, 4b and 4c, the upper portion 55 of the second set of mirrors 54, 55 does not comprise a razor edge but instead comprises an angle dependent spectral filter. This spectral filter is arranged to transmit different wavelength components of the electromagnetic radiation incident upon the upper portion 55 of the second set of mirrors 54, 55 when the angle of incidence of the different wavelength components exceed different respective angular thresholds on the upper portion 55 of the second set of mirrors 54, 55. Thus the upper portion 55 of the second set of mirrors 54, 55 is arranged to transmit different wavelength components at different positions, which introduces a time delay between the different wavelength components of the input electromagnetic radiation.

[0149] Operation of this embodiment of the spectrometer 205, which is similar to the previous embodiment, will now be described with reference to FIGS. 3, 4a, 4b and 4c.

[0150] The source of electromagnetic radiation 202 to be measured is arranged to be incident upon the collimator 203 such that the electromagnetic radiation 202 is collimated into a beam of electromagnetic radiation 64 (i.e. with all of the different wavelength components of the electromagnetic radiation 64 collinear in this beam).

[0151] The collimator 203 is arranged relative to the source of electromagnetic radiation 202 such that the resultant collimated beam of electromagnetic radiation 64 is incident through the aperture 58 in the first set of mirrors 51, 52 and into the cavity 205 of the spectrometer 201. The cavity 205 (and the aperture 58 in particular) is arranged such that the beam of electromagnetic radiation 64 is oriented at an angle to the optical axis 56 of the cavity 205 (in a plane perpendicular to the plane of the first set of mirrors 51, 52). The beam of electromagnetic radiation 64 is also arranged to pass through the aperture 58 of the cavity close to the razor edge 60 of the upper portion 51 of the first set of mirrors 51, 52, i.e. offset by a distance d from the optical axis 56 of the cavity 205.

[0152] Similar to the previous embodiment, the beam of input electromagnetic radiation 64 then passes through the lens 53 and is spatially Fourier transformed onto the lower portion 54 of the second set of mirrors 54, 55 at a position that is offset by a distance f from the optical axis 56 of the cavity 205 and (as shown in FIG. 4b) at an angle d/f to the normal of the lower portion 54 (in a plane perpendicular to the axis forming the boundary between the upper and lower portions 55, 54 of the second set of mirrors).

[0153] Following reflection from the lower portion 54 of the second set of mirrors 54, 55, the beam of electromagnetic radiation 64 passes through the lens 53 and is spatially Fourier transformed to a position on the lower portion 52 of the first set of mirrors 51, 52. The position at which the beam of electromagnetic radiation 64 is spatially Fourier transformed on the lower portion 52 of the first set of mirrors 51, 52 is offset by a distance d from the optical axis 56 and the beam of electromagnetic radiation 64 is incident upon the lower portion 52 of the first set of mirrors 51, 52 at an angle to the normal of the lower portion 52 (in a plane perpendicular to the direction in which the aperture 58 is longitudinally extended).

[0154] In this embodiment the upper and lower portions 51, 52 of the first set of mirrors are coplanar and perpendicular to the optical axis 56, so no angular shift is introduced upon this reflection.

[0155] Following reflection from the lower portion 52 of the first set of mirrors 51, 52, the beam of electromagnetic radiation 64 passes through the lens 53 and is spatially Fourier transformed to a position on the upper portion 55 of the second set of mirrors 51, 52. The position on the upper portion 55 onto which the beam of electromagnetic radiation 64 is spatially Fourier transformed is offset by a distance f from the optical axis 56 and the beam of electromagnetic radiation 64 is incident upon the upper portion 55 of the second set of mirrors 54, 55 at an angle d/f to the direction of the optical axis 56 (in a plane perpendicular to the axis forming the boundary between the upper and lower portions 55, 54 of the second set of mirrors), as shown in FIG. 4b.

[0156] Owing to the upper portion 55 of the second set of mirrors 51, 52 being at an angle to the lower portion 54, the angle the beam of electromagnetic radiation 64, relative to the direction of the optical axis 56 and in a plane perpendicular to the planes of the upper and lower portions 55, 54 of the second set of mirrors, when reflected from the upper portion 55 of the second set of mirrors 54, 55, is rotated by 2, as is shown in FIG. 4c, compared to the angle that the beam of electromagnetic radiation 64 would have been reflected from the upper portion 55 had it been coplanar with the lower portion 54.

[0157] Following reflection from the upper portion 55 of the second set of mirrors 54, 55, the beam of electromagnetic radiation 64 passes through the lens 53 and is spatially Fourier transformed to a position on the upper portion 52 of the first set of mirrors 51, 52. The position at which the wavelength components 14 is spatially Fourier transformed on the upper portion 51 of the first set of mirrors 51, 52 is offset by a distance d+2f from the optical axis 56 and the beam of electromagnetic radiation 64 is incident upon the upper portion 61 of the first set of mirrors 61, 62 at an angle to the normal of the upper portion 61 (in a plane perpendicular to the upper portion 61), as shown in FIG. 4b.

[0158] It will be appreciated that the beam of electromagnetic radiation 64 now takes a very similar path through the cavity 205, being reflected off the mirror portions 51, 52, 54, 55, compared to when the beam of electromagnetic radiation 64 first entered the cavity 205 through the aperture 58, except that it starts from a position further offset from the optical axis 56 (by a distance d+2f compared to d when it entered the cavity 205, owing to the reflection from the angled upper portion 55 of the second set of mirrors 54, 55). As can be seen from FIG. 4c, in the plane parallel to the optical axis 56 and the aperture 58 the beam of electromagnetic radiation 64 is not shifted but simply is reflected back and forth along the optical axis 56, owing to the first set of mirrors 51, 52 being coplanar and perpendicular to the optical axis 56.

[0159] Thus, owing to the rotation of the beam of electromagnetic radiation 64 through the angle 2 by the angled upper portion 55 of the second set of mirrors 54, 55, the angle at which the beam of electromagnetic radiation 64 is incident upon the upper portion 55 of the second set of mirrors 54, 55 is shifted by an angle 2 for each round trip of the electromagnetic radiation 64 through the cavity 205, as shown in FIG. 2b.

[0160] The angle dependent spectral filter on the upper portion 55 of the second set of mirrors 54, 55 transmits electromagnetic wavelength when, for a particular wavelength component, the angle of incidence on the upper portion 55 of the second set of mirrors 54, 55 exceeds a threshold angle. Thus each round trip moves the angle of incidence of the beam of electromagnetic radiation 64 closer to the threshold angle for transmission for each wavelength component in the beam of electromagnetic radiation 64 (these threshold angles being different for each different wavelength component) until they exceed their angle of incident and are thus transmitted through the spectral filter of the upper portion 55 and output from the cavity 205.

[0161] The output wavelength components are imaged by a lens 206, arranged beyond the upper portion 55 of the second set of mirrors 54, 55, onto a detector 207 (e.g. an avalanche photo diode), which detects the arrival of each wavelength component 64.

[0162] Owing to the different threshold angles required for transmission through the spectral filter on the upper portion 55 of the second set of mirrors 54, 55 for the different wavelength components of the beam of electromagnetic radiation 64, the different wavelength components will exceed these respective threshold angles after a different number of round trips through the cavity 205. This will therefore introduce a time delay between the different wavelength components.

[0163] The signals from the detector 207, corresponding to the detection of each of the wavelength components in the beam of electromagnetic radiation 64 that are output from the cavity 205, are sent from the detector 207 to the data processor 208 to be processed in the same manner as the first embodiment described above, i.e. to be converted into wavelengths and relative efficiencies.

[0164] As with the first embodiment (shown in FIGS. 1, 2a, 2b and 2c), there are a number of variants to the embodiment of the cavity shown in FIGS. 4a, 4b and 4c that will now be described.

[0165] In one variant to the cavity shown in FIGS. 4a, 4b and 4c, the lower portion 52 of the first set of mirrors 51, 52 is oriented at an angle to the upper portion 51 about an axis perpendicular to the optical axis 56 and the aperture 58, i.e. the first set of mirrors 51, 52 is arranged as shown in FIGS. 2a, 2b and 2c. In addition, the angle dependent spectral filter on the upper portion 55 of the second set of mirrors 54, 55 is replaced with a position dependent (i.e. gradient) spectral filter. This spectral filter is arranged to transmit different wavelength components of the beam of electromagnetic radiation 64 incident upon the upper portion 55 of the second set of mirrors 54, 55 when the position of incidence of the different wavelength components exceed different respective position thresholds on the upper mirror portion 55 (in a direction parallel to the direction in which the input aperture 58 is longitudinally extended) and otherwise reflect the wavelength portions of the beam of electromagnetic radiation 64. Thus the upper portion 55 of the second set of mirrors 54, 55 is arranged to transmit different wavelength components at different positions, which introduces a time delay between the different wavelength components of the input beam of electromagnetic radiation 64.

[0166] Operation of this variant of the cavity is similar to that described above for the embodiment shown in FIGS. 4a, 4b and 4c, except that the different wavelength components of the input electromagnetic radiation are output from the cavity when they are shifted to exceed different particular positions on the upper portion 55 of the second set of mirrors 54, 55 such that the different wavelength components are transmitted through the mirror at different positions and thus at different times.

[0167] The operation of the data processor 108, 208 (as shown in FIGS. 1 and 3) for both embodiments and their variants described above will now be described with reference to FIG. 5, along with the calibration of the detector 107, 207 (as shown in FIGS. 1 and 3). FIG. 5a shows an exemplary test spectrum for measurement by the spectrometer, FIG. 5b shows the output of the detector as captured by the data processor, FIG. 5c shows the calibration curve for the detector, FIG. 5d shows the relative efficiency of the detector and FIG. 5e shows the determined spectrum compared to the test spectrum.

[0168] FIG. 5a shows an exemplary test spectrum 301, as measured by an optical spectrum analyser, for use in calibrating the spectrometer according to embodiments of the present invention. To calibrate the spectrometer the test spectrum 301 was input into the spectrometer, with the time delay outputs 302 of the different wavelength components, as captured by the detector and determined by the data processor, shown in FIG. 5b.

[0169] FIG. 5c shows the same test, as performed as described above in relation to FIGS. 5a and 5b, repeated for a number of test spectra of different wavelengths, in order to calibrate the time delays of the different wavelength components output from the cavity and measured by the detector to the actual wavelengths of these components (by determining a calibration curve 303) through the different points representative of the test spectra.

[0170] Using the same test spectra, the relative efficiency 304 of the spectrometer was determined, as is shown in FIG. 5d, for each of the wavelength components. This was done by comparing the intensity of the different wavelength components as measured by the detector (i.e. as shown in FIG. 5b) to the intensity of the corresponding wavelength components in the input electromagnetic radiation (i.e. as shown in FIG. 5a).

[0171] The result, using the calibration between the time delay and the wavelength, and the relative efficiency of the spectrometer, is shown in FIG. 5e, which shows the comparison between the determined wavelength of the wavelength components 305 (binned data) and the input test spectrum 301 (line data).

[0172] It can be seen from the above that in at least preferred embodiments of the invention, a spectrometer is provided that performs temporal separation of electromagnetic radiation input into the cavity using a free space spectrometer. This helps to allow a greater bandwidth spectrum of electromagnetic radiation to be analysed. The spectrometer enables spectra of electromagnetic radiation to be measured efficiently owing to the temporal separation of the different wavelength components of the spectra into multiple different time bins. Thus it may not be necessary to use a detector that is able to measure a range of frequencies at different spatial positions, but instead, a single channel detector may be able to be used that has a fast repetition rate and high sensitivity. This then enables electromagnetic radiation from correlated or dynamic systems to be measured more efficiently by the spectrometer according to embodiments of the present invention. Furthermore, the spectrometer of the present invention may be tuned easily, e.g. by the angle(s) of the mirror(s) that are chosen.

[0173] The work leading to this invention has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme (FP7/2007-2013) under REA grant agreement n627372 and n300820. The work leading to this invention has received funding from the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement n600645.