Device for fourier transform spectrometry

Abstract

The invention relates to a device for Fourier transform spectrometry using a broadband, spatially coherent radiation source, comprising: a beam splitter for splitting a source beam into two beam arms; a beam combiner for recombining the beam arms to form a detector beam directed onto a detector connected to a computing unit; and a sample holder arranged in the beam path. The aim of the invention is to enable the measurement of strongly absorbing samples in a large wavelength range with constant quality. This aim is achieved in that the beam splitter is a wavefront beam splitter and in that the beam combiner is a transmission-free wavefront combiner for recombining the beam arms entering from different directions.

Description

[0001] Device for Fourier transform spectrometry

[0002] Technical area

[0003] The invention relates to a device for Fourier transform spectrometry using a broadband, spatially coherent radiation source comprising a beam splitter for splitting a source beam into two beam arms and a beam combiner for recombining the beam arms into a detector beam which is directed onto a detector connected to a computing unit and a sample holder arranged in the beam path.

[0004] State of the art

[0005] Fourier transform infrared spectrometers are known from the prior art. They split a source beam emanating from a heated blackbody into two beam branches by a semi-transparent mirror acting as a beam splitter. These beam branches are deflected by mirrors and recombined by the same semi-transparent mirror acting as a beam combiner into a detector beam, which is then fed to a detector. A processing unit converts the interferogram recorded by the detector into a spectrum.

[0006] A disadvantage of the state of the art, however, is that the semi-transparent mirror reduces the radiation intensity usable for the measurement and the transmission behavior of the semi-transparent mirror is wavelength-dependent, so that measurement inaccuracies must be accepted when using broadband radiation sources and correspondingly large measuring ranges.

[0007] Description of the invention

[0008] The invention is therefore based on the object of providing a device for Fourier transform spectrometry which enables the measurement of strongly absorbing samples in a wide wavelength range with consistent quality.

[0009] The invention solves the stated problem in that the beam splitter is a wavefront beam splitter and the beam combiner is a transmission-free, preferably reflective and / or achromatic wavefront combiner for recombination of the beam arms entering from different directions.

[0010] In contrast to amplitude division or recombination known from the prior art via guide elements penetrated by a measuring beam comprising the beam arms and the detector beam, the features of the invention allow the measuring beam to be guided essentially transmission-free, in particular reflectively and / or achromatically. Wavefront recombination occurs at the beam combiner with a dissipation rate of less than 10%, so that the amplitude and thus the radiation intensity remain approximately constant. Furthermore, due to the transmission-free, preferably reflective and / or achromatic guidance, media transitions and thus wavelength-dependent refraction or other intramedia effects are avoided, so that the entire spectral bandwidth of spatially coherent radiation sources, typically ranging from ultraviolet to terahertz radiation, can be utilized.In a preferred embodiment, the entire measuring beam, in particular the entire beam path comprising the source beam, the beam arms, and the detector beam, is guided without transmission, in particular reflectively and / or achromatically, for example via mirror surfaces coated, for example, with gold or silver and having a reflectivity of at least 90%. This allows the examination of samples with greatly differing absorption profiles without sample-specific adjustments, but also with greater layer thicknesses, which results in greater measurement accuracy due to the improved radiation-matter interaction. To generate an interferogram for a specific wavelength to be measured, the path length of one beam arm relative to the other beam arm can be varied in a conventional manner.The sample holder can be positioned anywhere along the beam path, preferably along the measurement beam. Positioning it within the measurement beam provides better measurement quality due to its proximity to the detector, while positioning it in one of the beam arms also allows for the measurement of the sample's dispersive properties. Suitable radiation sources with high spatial coherence include supercontinuum sources, quantum cascade lasers, optical parametric oscillators, laser-based emitters, synchrotron emitters, or other sources based on nonlinear conversion techniques. Although position and / or wavelength conversion techniques are required for the measurement of the sample,While a monochromatic reference laser can be provided in the conventional manner for determining the length of the beam arms, which is guided on a plane parallel to the beam path comprising the source beam and the measuring beam, a simplified structure results if the radiation source has a short-wave emission, which is optionally guided via a narrow-band spectral filter.

[0011] In order to further increase the radiation power usable for examining the sample, the beam splitter in a preferred embodiment has two mirror surfaces and deflects the source beam in two different directions. In contrast to slotted diaphragms known from the prior art for splitting the source beam, this measure results in the advantage that a larger cross-sectional area of ​​the source beam can be deflected into the beam arms. In a preferred embodiment, the mirror surfaces abut one another directly, so that the entire cross-section of the source beam is split between the two beam arms. Design advantages arise if a guide body provided with a reflective layer on at least two adjacent lateral surfaces is provided as the beam splitter, such as a coated prism.In a preferred embodiment, the two beam arms exiting the beam splitter extend in two opposite directions and preferably along the same propagation axis. For this purpose, the mirror surfaces of the beam splitter can be arranged at a right angle to each other.

[0012] To increase the radiation power available at the detector while simultaneously maintaining a simple structural design of the device according to the invention, it is proposed that the beam combiner have two mirror surfaces that combine the beam arms arriving from different directions into a single detector beam. This allows the two beam arms to be recombined and parallelized simultaneously, so that the total number of guide elements required for beam guidance can be reduced. Design advantages arise if the beam combiner is a guide body provided with a reflective layer on at least two adjacent lateral surfaces, such as a coated prism. In a preferred embodiment, the two beam arms arriving at the beam combiner extend in two opposite directions and preferably along the same propagation axis.For this purpose, the mirror surfaces of the beam combiner can be arranged at a right angle to each other. In a preferred embodiment, the beam splitter and the beam combiner can be designed as a common guide element, for example, as a one-piece guide body whose base forms a square, from which the lateral surfaces provided with a reflective layer protrude. In a particularly simple embodiment, this guide element can be a cube.

[0013] To increase the measurement speed without requiring adjustment speeds and the associated vibrations, the beam combiner can be mounted on a movable support to change the path lengths of the beam arms relative to each other. Particularly in conjunction with two beam arms arriving at the beam combiner from opposite directions, this makes it possible to change the length of both beam arms simultaneously and in opposite directions, thus resulting in an increased measurement speed relative to the adjustment speed. Preferably, the detector beam emerging from the beam combiner is deflected parallel to the direction of movement of the support, for example, by a mirror, so that no additional design effort results from the displacement of the support.In this context, it is recommended that the guide element for deflecting the detector beam in a direction parallel to the direction of movement of the carrier is also arranged on the carrier. It will be understood by those skilled in the art that the carrier is connected to a motorized actuator for rapid and repeatably precise movement.

[0014] To further increase the measurement speed while maintaining a simple design, the carrier can be displaced along the sections of the beam arms entering the beam combiner. This not only doubles the measurement speed and spectral resolution relative to the carrier's positioning speed, but also eliminates the need for additional guide elements for deflecting the beam arms in the beam combiner area.

[0015] Relative to the carrier's positioning speed, the measurement speed can be further increased while maintaining the same simple design conditions if the beam splitter is mounted together with the beam combiner on the movable carrier, and the beam arms running between the beam splitter and beam combiner are deflected by fixed mirror surfaces. Due to the resulting beam guidance, the measurement speed and the spectral resolution can be quadrupled relative to the carrier's positioning speed, because a displacement of the carrier causes a change in the length of both the branch of the respective beam arm running toward the respective fixed mirror surfaces and the branch running back.In this context, it is recommended that the source beam be deflected toward the beam splitter via a guide element arranged on the carrier, preferably from a direction parallel to the direction of movement of the carrier. This further improves the stability and alignment of the source beam split at the beam splitter.

[0016] Although the two partial beams of the detector beam assigned to the respective beam arm can also be superimposed in other ways to form an interference pattern at the detector, particularly advantageous conditions with regard to the radiation intensity arriving at the detector arise when the detector beam is directed onto the detector via a transmission-free guide element such as a concave mirror, in particular a parabolic mirror. This leads to a superposition of the previously collinearly aligned partial beams, so that a large-area, high-quality interferogram can be generated. In a preferred embodiment, the concave mirror, in particular the parabolic mirror, is arranged off-axis with respect to the optical axis of the detector beam.

[0017] Brief description of the invention

[0018] The drawing shows an example of the subject matter of the invention.

[0019] Fig. 1 shows a first embodiment of a device according to the invention in a schematic plan view and

[0020] Fig. 2 is a plan view corresponding to Fig. 1 of a second embodiment of a device according to the invention.

[0021] Ways to implement the invention

[0022] A device according to the invention for Fourier transform spectrometry comprises a beam splitter 1 for splitting a source beam 3 emitted by a continuously emitting radiation source 2 into two beam arms 4a, 4b. These beam arms 4a, 4b are recombined into a detector beam 6 via a beam combiner 5. This detector beam 6 is deflected onto a detector 7, which is provided with a computing unit

[0023] 8 in order to generate a spectrum from one or more interferograms produced at the detector 7.

[0024] As can be seen schematically in the drawing based on the beam path, beam splitter 1 is designed as a transmission-free wavefront beam splitter, and beam combiner 5 is designed as a transmission-free wavefront beam combiner. This means that the splitting of source beam 3 into the two beam arms 4a, 4b, as well as the recombination of the two beam arms 4a, 4b to form detector beam 6, occurs without transmission, i.e., without transmission of the respective beams through a guide element.

[0025] In a preferred embodiment, the beam splitter 1 has two mirror surfaces 9a, 9b which deflect the source beam 3 in two different, preferably opposite directions and which can, for example, be lateral surfaces of a guide body 10 provided with a reflective layer.

[0026] In an analogous manner, the beam combiner 5 can have two mirror surfaces 11 a, 11 b that recombine the beam arms 4 a, 4 b from two different, preferably opposite, directions to the detector beam 6.

[0027] In the embodiment shown in Fig. 1, the guide bodies 10 have a triangular base from which the mirror surfaces 9a, 9b and 11a, 11b protrude and are designed, for example, as prisms.

[0028] In the embodiment shown in Fig. 2, a cuboid-shaped guide body 10, for example, has a square base from which the mirror surfaces 9a, 9b and 11a, 11b protrude.

[0029] A sample holder 12 is provided in the beam path, preferably in the measuring beam comprising the beam arms 4a, 4b and the detector beam 6. To measure a sample arranged in the sample holder 12, as shown in Fig. 1, the beam combiner 5 is mounted on a carrier 13 which can be displaced along an adjustment direction 14 with the aid of an adjustment drive (not shown in detail). This displacement along the adjustment direction 14 results in a relative change in the path length of the beam arms 4a, 4b to one another, whereby an interferogram can be generated at the detector 7 with respect to the wavelength to be measured. In a preferred embodiment, the adjustment direction 14 runs parallel to the sections of the beam arms 4a, 4b entering the beam combiner 5.

[0030] In the embodiment of the device according to the invention shown in Fig. 2, not only the beam combiner 5 but also the beam splitter 1 is arranged on the common carrier 13.

[0031] In both embodiments, the beam arms 4a, 4b are deflected via mirror surfaces 15 fixed relative to the support 13, with each mirror surface 15 being deflected by 90°.

[0032] For structural simplification, a further deflection mirror 16 can be provided for deflecting the source beam 3 onto the beam splitter 1 and the detector beam 6 emanating from the beam combiner 5, which deflects the source beam 3 from a direction parallel to the adjustment direction 14 or the detector beam 6 into a direction parallel to the adjustment direction 14 by 90° each.

[0033] For superimposing the two partial beams of the detector beam 6, a concave mirror 17 can be provided which deflects the detector beam 6 from a direction parallel to the adjustment direction 14 onto the detector 7.

[0034] In the embodiment shown in Fig. 1, in addition to the radiation source 2, a monochromatic reference laser 18 is provided, the beam 19 of which is deflected into or out of a path parallel to the beam arms 4a, 4b via deflection mirrors 20 arranged outside the source beam 3 or detector beam 6 and which is directed onto a reference detector 21 for an exact path length determination.

[0035] In the embodiment of Fig. 2, this reference laser 18 is replaced by a short-wave emission of the radiation source 2.

Claims

Patent claims 1. Device for Fourier transform spectrometry using a broadband, spatially coherent radiation source (2) comprising a beam splitter (1) for splitting a source beam (3) into two beam arms (4a, 4b), a beam combiner (5) for recombining the beam arms (4a, 4b) to form a detector beam (6) which is directed onto a detector (7) connected to a computing unit (8), and a sample holder (12) arranged in the beam path (3, 4a, 4b, 6), characterized in that the beam splitter (1) is a wavefront beam splitter and in that the beam combiner (5) is a transmission-free wavefront combiner for recombining the beam arms (4a, 4b) entering from different directions.

2. Device according to claim 1, characterized in that the beam splitter (1) has two mirror surfaces (9a, 9b) deflecting the source beam (3) in two different directions.

3. Device according to claim 1 or 2, characterized in that the beam combiner (5) has two mirror surfaces (11a, 11b) combining the beam arms (4a, 4b) arriving from different directions to form a detector beam (6).

4. Device according to one of claims 1 to 3, characterized in that the beam combiner (5) is arranged on a movable support (13) for changing the path length of the beam arms (4a, 4b) relative to one another.

5. Device according to claim 4, characterized in that the carrier (13) is displaceable along the sections of the beam arms (4a, 4b) entering the beam combiner (5).

6. Device according to one of claims 4 or 5, characterized in that the beam splitter (1) is arranged together with the beam combiner (5) on the movable support (13) and the beam arms 4a, 4b running between the beam splitter (1) and the beam combiner (5) are deflected via fixed mirror surfaces (15).

7. Device according to one of claims 1 to 6, characterized in that the detector beam (6) is directed onto the detector (7) via a concave mirror (17).

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