OPTICAL SPECTROMETER AND METHOD FOR SPECTRALLY RESOLVED TWO-DIMENSIONAL IMAGING OF AN OBJECT

Abstract

The disclosure relates to an optical spectrometer (1) for spectrally resolved two-dimensional imaging of an object (0), comprising a dispersing device (2) arranged to disperse radiation from object (0), a multi-lens array arrangement (3) arranged to receive the dispersed radiation from the dispersing device (2), a two-dimensional detector (4) arranged to receive the dispersed radiation as directed by the multi-lens array arrangement (3), wherein the optical spectrometer (1) further comprises a collimating arrangement (5) for collimating the radiation from object (0) before the radiation reaches the dispersing device (2), the collimating arrangement (5) comprising a diffusing plate (6) for diffusing the radiation and an optical micro-channel component (7) arranged to receive the diffused radiation comprising a plurality of parallel and linear optical micro-channels directed towards the dispersing device (2). The disclosure further relates to a method for spectrally resolved two-dimensional imaging of an object (0).

Claims

1. An optical spectrometer for spectrally resolved two-dimensional imaging of an object, comprising a dispersing device arranged to disperse radiation from the object, a multi-lens array arrangement arranged to receive the dispersed radiation from the dispersing device, a two-dimensional detector arranged to receive the dispersed radiation as directed by the multi-lens array arrangement, wherein the optical spectrometer further comprises a collimating arrangement for collimating the radiation from the object before the radiation reaches the dispersing device, the collimating arrangement comprising a diffusing plate for diffusing the radiation and an optical micro-channel component arranged to receive the diffused radiation comprising a plurality of parallel and linear optical micro-channels directed towards the dispersing device.

2. The optical spectrometer according to claim 1, wherein the micro-channel component comprises capillary arrays.

3. The optical spectrometer according to claim 1, wherein the micro-channel component is a fiber-optical plate, with a numerical aperture significantly smaller than the numerical aperture of the lenses of the multi-lens array arrangement.

4. The optical spectrometer according to claim 1, wherein the multi-lens array arrangement is oriented at an angle with respect to the dispersing device.

5. The optical spectrometer according to claim 4, wherein the angle is 17-27 degrees.

6. The optical spectrometer according to claim 1, wherein the micro-channel component is arranged with its micro-channels parallel to a propagation path of the radiation and/or with its micro-channels arranged perpendicular to the diffusing plate and the dispersing device.

7. The optical spectrometer according to claim 1 wherein the dispersing device is a component comprised in the group consisting of a transmission grating, a holographic grating only transmitting the first diffraction order, and a grism.

8. The optical spectrometer according to claim 1, wherein the collimating arrangement further comprises a collimating lens between the object and the diffusing plate with a narrower distribution for increasing the transmission through the collimating arrangement.

9. The optical spectrometer according to claim 1, further comprising a lens or a lens package for directing radiation from the object onto the collimating arrangement.

10. A method for spectrally resolved two-dimensional imaging of an object, comprising the step of, dispersing radiation from the object by a dispersing device, receiving the dispersed radiation from the dispersing device at a multi-lens array arrangement arranged to, receiving the dispersed radiation as directed by the multi-lens array arrangement at a two-dimensional detector, wherein method further comprises the steps of collimating the radiation from the object before the radiation reaches the dispersing device by a collimating arrangement, the collimating arrangement diffusing the radiation by a diffusing plate, and collimating the diffused radiation by an optical micro-channel component comprising a plurality of parallel and linear optical micro-channels directed towards the dispersing device.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0025] The above objects, as well as additional objects, features and advantages of the present invention will be more fully appreciated by reference to the following illustrative and non-limiting detailed description of example embodiments of the present invention, when taken in conjunction with the accompanying drawings.

[0026] FIG. 1 shows a schematic view of an optical spectrometer for spectrally resolved two-dimensional imaging of an object.

[0027] FIG. 2 shows the schematic view of an optical spectrometer with some added components to increase signal strength and spectral resolution; and

[0028] FIG. 3 is a schematic view to illustrate orientation of the multi-lens array arrangement with respect to the dispersing device.

[0029] FIG. 4 shows block flow chart representing the method according to the present invention.

DETAILED DESCRIPTION

[0030] The present invention will now be described with reference to the accompanying drawings, in which currently preferred example embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the invention to the skilled person.

[0031] FIG. 1 shows an optical spectrometer 1 for spectrally resolved two-dimensional imaging of an object 0. A lens 8 images the radiation from the object 0 onto a collimating arrangement 5 for collimating the radiation from object 0 before the radiation continues to a dispersing device 2. The collimating arrangement 5 comprises a diffusing plate 6 for diffusing the radiation and an optical micro-channel component 7 arranged to receive the diffused radiation. The optical micro-channel component 7 comprises a plurality of parallel and linear optical micro-channels directed towards the dispersing device 2. Only radiation traveling in the direction of the parallel linear optical micro-channels will pass the optical micro-channel component 7. The radiation will thus be parallel and collimated after the optical micro-channel component 7.

[0032] The collimated radiation will thereafter arrive at the dispersing device 2 arranged to disperse the radiation. The dispersed radiation continues to a multi-lens array arrangement 3 arranged to receive the dispersed radiation from the dispersing device 2. As seen in FIG. 1 the multi-lens array arrangement 3 is placed very close to the dispersing device 2 to make the design of the optical spectrometer compact and avoid blurring of the spectrally dispersed image. A two-dimensional detector 4 is arranged to receive the dispersed radiation as directed by the multi-lens array arrangement 3. The two-dimensional detector 4 is arranged at a predetermined distance from the multi-lens array arrangement 3, wherein the predetermined distance is related to the focal length of the lenses in the multi-lens array arrangement 3 so that the dispersed radiation will be in focus on the two-dimensional detector 4.

[0033] The micro-channel component 7 may comprise capillary arrays as e.g. in so called black glass. The micro-channel component 7 may, however, also be a fiber-optical plate, with a numerical aperture of significantly smaller than the numerical aperture of the lenses of the multi-lens array arrangement 3 so as to not affect the resolution of the two-dimensional detector 4 and the spectrometer. According to some embodiments, the optical micro-channel component 7 is further arranged to spectrally filter the radiation. The micro-channel component 7 is further arranged with its micro-channels parallel to a propagation path of the radiation and/or with its micro-channels arranged perpendicular to the diffusing plate 6 and the dispersing device 2.

[0034] The multi-lens array arrangement 3 is preferably oriented at an angle with respect to the dispersing device so as to not have the dispersed radiation spectrum from one lens of the multi-lens array arrangement 3 to overlap the one from a neighboring lens. This is illustrated in FIG. 3. The angle may be 17-27 degrees, preferably 22.5 degrees. The diffusing plate 6 is arranged to distribute radiation in a Gaussian or square distribution, preferably a wider than the numerical aperture of the imaging system 8.

[0035] As can be seen in FIG. 3, the orientation of the multi-lens array arrangement 3 at an angle with respect to the dispersing device 2 makes the individual spectrum from each lens in the micro-lens arrangement 3 to not overlap as they are projected to different levels of the two-dimensional detector 4 as a result of the angled orientation. FIG. 3 also illustrates how the object is imaged, with a two-dimensional map where each “pixel” is a spectrum. The imaged object can thereby be spectrally analyzed for each spatial location. The resolution of the spectral two-dimensional image is limited by the density of lenses on the micro-lens arrangement 3.

[0036] The spectral resolution will now be discussed with some examples. According to the Rayleigh criterion, two wavelengths can be distinguished if their separation on the sensor is more than the point spread function of the micro-lens. The grating law for first order gives that,

sin(α)=λ/α, where α is the grating period.

[0037] The focal spot size is: w.sub.0=1.22*λf/D, where D is the micro lens diameter.

[0038] Given that two wavelengths can be distinguished if Δx, the separation between these wavelengths at the sensor is larger than the focal spot size, i.e. Δx>w.sub.0j. Therefore, two wavelengths can be distinguished if they are diffracted by the grating at angle of a and a′ separated by,

α−α′=w.sub.0/f=1.22*λ/D and, α=λ/a.

[0039] For example, setting α=3.333 μm, λ=0.8 μm, f=3.7 mm, and D=300 μm we get a resolution of Δλ=1.22*λα/D=11 nm.

[0040] By the formula for the spectral resolution Δλ=1.22*λα/D two options are available for increasing the resolution. Either the micro-lens diameter can be increased at the cost of a reduction of the spatial resolution, or the grating period a can be decreased, however optical aberrations in the lens quickly emerge and limits the possibilities. A further option is to use a grism 10, i.e. the combination of a prism 11 and a grating 2.

[0041] The dispersing device 2 may as mentioned above be a transmission grating 2, a holographic grating only transmitting the first diffraction order or a grism 10 for increased resolution. An embodiment using a grism 10 is disclosed in FIG. 2. The grism 10 comprises a prism 11 and a grating 2 and will increase the spectral resolution of the optical spectrometer by a factor 3. FIG. 2 further discloses a collimating arrangement 5 comprising a collimating lens 9 between the object 0 and the diffusing plate 6 with a narrower distribution for increasing the transmission through the collimating arrangement and thereby increase the signal strength. This embodiment will thus increase both the resolution and the signal strength compared to the embodiment shown in FIG. 1.

[0042] With reference to FIGS. 1-3, the multi-lens array arrangement 3 is a Shack-Hartmann micro-lens array, an achromatic multi-wave lateral shearing interferometer grating or any wavefront sensitive component. The two-dimensional detector 4 is typically a Charge Coupled Device (CCD) array, a Complementary Metal-Oxide-Semiconductor (CMOS) detector, an Indium Gallium Arsenide (InGaAs) detector or any other type of light sensitive detector.

[0043] With reference to FIG. 4, the second aspect of this disclosure shows a method for spectrally resolved two-dimensional imaging of an object 0, comprising the step of, 51 dispersing radiation from object 0 by a dispersing device 2, S2 receiving the dispersed radiation from the dispersing device 2 at a multi-lens array arrangement 3 arranged to, S3 receiving the dispersed radiation as directed by the multi-lens array arrangement 3 at a two-dimensional detector 4, wherein method further comprises the steps of a S4 collimating the radiation from object 0 before the radiation reaches the dispersing device 2 by a collimating arrangement 5, the collimating arrangement 5 diffusing S4-1 the radiation by a diffusing plate 6 and collimating S4-2 the diffused radiation by an optical micro-channel component 7 comprising a plurality of parallel and linear optical micro-channels directed towards the dispersing device 2.

[0044] The person skilled in the art realizes that the present invention is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. For example, further additional, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.