EP2691760A1

EP2691760A1 - Imaging thz measurement method and apparatus

Info

Publication number: EP2691760A1
Application number: EP12711880.0A
Authority: EP
Inventors: Christian Jansen; Maik Scheller; Martin Koch; Steffen Schumann
Original assignee: Philipps Universitaet Marburg
Current assignee: Philipps Universitaet Marburg
Priority date: 2011-03-31
Filing date: 2012-03-30
Publication date: 2014-02-05
Also published as: WO2012131085A1; EP2505986A1

Abstract

The present invention relates to an imaging THz measurement method, which uses at least one spectral spreading and/or spectral convergence component or uses at least one spectral spreading and/or spectral convergence component in combination with at least one imaging system to convert the frequency information into location information such that the measurement time is decreased and/or fewer mechanical components are needed.

Description

Imaging THz measurement method and device

The present invention relates to an imaging THz measuring method which, by using at least one spectral flattening and / or spectral-collecting component or by using at least one spectral flattening and / or spectral-collecting component in combination with at least one imaging system, converts the frequency into a location information, so that the measuring time is shortened and / or mechanical components are saved.

Description of the prior art

The THz range lies in the electromagnetic spectrum between the microwaves and the infrared radiation. This exposed position between optics and electronics opens up a multitude of metrological applications, for example in safety technology, in industrial process monitoring and in medical technology.

Of particular importance are, above all, imaging techniques. In many areas, systems working with non-ionizing THz waves could replace the currently used, but often critically evaluated X-ray scanners for radiation protection reasons. Industries that show great interest in using THz scanners include the food industry, pharmacology companies, and plastics processors. Imaging THz measurements provide, among other things, images that represent structural information from within a sample. The widespread use of THz scanners is currently hampered by the low measuring speed of imaging THz systems. Often, it is desirable to integrate THz scanners directly on conveyor belts in production lines to ensure 100% control and real-time process control. To meet these demands, however, the duration of an imaging measurement may only be one hundredth of the measurement time which is usual in systems according to the prior art. Furthermore, the costs for an imaging THz system are still quite high, because in addition to complex optics often complex mechanisms are used. According to the prior art, THz waves are focused by a suitable quasi-optical components to a smallest possible point. In most cases, the sample is then rasterized in the focal point by stepwise processing to produce a pictorial representation.

The most common and simplest way is to use a mechanical tracker which shifts the sample through the locally fixed focus point of the THz radiation. Another approach shifts the focus point of the THz radiation on a line instead of the sample by a special lens assembly is combined with mechanically adjustable tilting mirrors. Again, a complex and expensive mechanics is needed. The advantage of this technique is that it can be easily integrated into an assembly line. In this case, the conveyor belt feeds the sample from one line to the next, and the tilting mirrors drive each line. In this way, a non-contact quality control is possible. However, this method is slow as well and offers no significant speed advantage over the raster scan method in which the sample is rasterized by a mechanical process of the sample or focus point. Each point of the recorded image corresponds to a measurement of the THz radiation, therefore, previous methods are too slow for a real-time operation. In addition, they are complex and expensive in their construction, since mechanical components such as moving units or motorized tilting mirrors are used. task

The object is therefore to provide a device for carrying out imaging THz measurements and to develop a method for carrying out that reduces the measuring time of an imaging THz measurement and / or avoids mechanical components in order thus to reduce system costs.

Solution of the task

For this purpose, according to the invention, a device according to claim 1, which has spectrally flaring 3a and / or spectral collecting 3b components, and a method according to claim 13 and / or claim 14 for the transfer of a Frequency is used in a location information to simultaneously detect at least one line of the image in the imaging THz method using a pulsed system. Using a cw system (continuous wave system), an image line is thus measured by tuning the emitter frequency. Here, a spatial fanning of the THz beam in a spatial direction, in particular orthogonal to the THz propagation direction.

Exemplary embodiments of the spectrally flaring 3a and / or spectral collecting 3b components which are suitable according to the invention are THz diffraction gratings 11, in particular high-efficiency THz gratings with blaze technology 13, THz prisms 15 and THz waveguides 17. As THz diffraction gratings 11 and THz Blaze technique 13 diffraction gratings are particularly suitable for structures made of metals (for example, aluminum, copper, brass, steel) that are operated in reflection or components made of plastics that work in transmission. THz prisms consist for example of plastics, polymers, glasses, silicates and / or ceramics. Suitable THz waveguides 17 are in particular parallel-plate waveguides.

The THz system can be operated in two alternative ways, as a pulsed system or as a cw system.

A pulsed THz system uses electromagnetic pulses which, due to their short duration in the picosecond range, include frequency components from below 100 GHz to well above 1 THz. When using the pulsed system, the measurement of a THz pulse instead of a single pixel, a whole image line so that the measurement time of a THz image is significantly shortened by a sample. The individual spectral components are focused by a suitable imaging system, which consists of lenses, mirrors and / or diffractive optics, each at a point along the focus line, which corresponds to an image line. Thus, it is possible with this method to assign a location information to the individual frequency components. With pulsed systems it is thus possible to record a complete image line of the sample by means of a single measurement. This results in an enormous speed advantage of two to three orders of magnitude compared to conventional systems for image acquisition. An alternative usefulness of the presented method results in combination with cw-THz systems, that is, systems that only ever have one at a time Send out frequency. In conventional cw-THz imaging systems, the focus point on the focus line can only be adjusted by means of mechanical components, such as tilt mirrors or moving units. According to the invention, however, this is done by changing the frequency of the tunable cw-THz system. It eliminates the mechanical displacement of the focal point. In cw-THz systems, a frequency change is achieved in particular electronically and / or thermally and / or by a mechanical change in one element of the cw-THz system. This simplifies the structure by means of fewer components and enables shorter adjustment times of the focal point than by a mechanical change in the THz beam guidance. In particular, due to the steady progress in purely electronic systems and the resulting cost savings, such use of the method is very advantageous.

The frequency of a cw system is tuned in various ways, for example electronically or thermally. The measurement time of a single frequency point in a cw system is inherently shorter than that of a pulsed THz system. The advantage of the invention lies in the saving of mechanical components. If the frequency of the cw system is determined, the focal point of the imaging system moves along a line, preferably orthogonal to the THz propagation direction. Mechanical beam steering units such as e.g. Swivel mirrors or Galvoscanner are therefore superfluous. This makes the system more robust and less expensive.

If one considers the innovation brought about by the method according to the invention for imaging cw systems, above all the method of the focal point without mechanical components such as tilting mirrors is a breakthrough. On the one hand, significantly higher speeds can be achieved by electronically tuning the emission frequency, and on the other hand, there is no need for expensive precision mechanics. Through this construction, one achieves a transformation from any frequency to a location of the focus line.

In both modes of operation, either the measuring system or the sample is moved to advance to the next image line. Alternatively, another deflection optics is available, which also replaces this mechanical feed. Another possibility is to rotate the at least one spectrally flaring and / or spectral-collecting component 3 or the entire measuring system. This also rotates the focus line on the sample and thus the recording of a full picture allows. A mechanical feed is made, for example, by conveyor belts.

According to the invention, the THz radiation (defined below between below 30 GHz and above 100 THz) is divided at least at one point on the beam path between the THz emitter 1 and the sample, the beam splitting occurring before or after the spectral fan-out. Such a multi-channel system allows the acquisition of several image lines in the same time in which an image line is otherwise measured, or several samples simultaneously and has the advantage that only a THz emitter is needed for this purpose. However, this structure can also be used several times in one device, so that several THz emitters are used in each case for measuring a plurality of image lines. On the detector side, the THz beam is measured with several detectors. The beam splitting is effected by a beam splitter, for example made of highly resistive silicon, quartz glass or a polymer.

The advantages of the present invention can be described by three core features:

- In pulsed THz systems, an image can be captured several orders of magnitude faster. The systems become real-time compatible.

- When using tunable cw systems eliminated mechanical components, since the focus point is adjustable by tuning the frequency, which also in electronically tunable systems also a significant increase in speed is possible.

- Both pulsed and cw-based systems become simpler, more robust and less expensive because mechanical components are eliminated.

A first possibility of the construction consists in a transmission structure in which the THz radiation penetrates the sample. The frequency fanning and / or the frequency collection takes place both by reflection on, as well as by transmission through a spectrally flaring and / or spectral collecting component.

A second possibility of realization consists in a reflection structure in which the imaging information is obtained from the radiation reflected at a sample. Again, the frequency fanning and / or frequency collection takes place both by reflection on, as well as by transmission through a spectral flattening and / or spectral collection component.

The task of the at least one imaging system 5 is to focus the fanned-out radiation along a focal line 25 perpendicular to the optical axis and / or to direct it from the focus line 25 to the spectral-collecting component 3b. The imaging system 5 consists of lenses for the THz frequency range, mirrors or other imaging optics and combinations of these components. An implementation according to the invention represents an optically adapted f-0 lens system. An f-0 lens system corrects the error of field curvature. The focal point is independent of the angle of incidence on an image plane.

The second imaging system 6 is inserted between detector and spectral collecting component 3b and / or THz emitter 1 and spectral fan-out component 3a. Also this imaging system consists of lenses for the THz frequency range, mirrors or other imaging optics and combinations of these components.

A core component of the structure according to the invention are spectral fan-out 3a and spectral-collecting 3b components. Examples of this core component are according to the prior art (Figure 1):

1 1 THz diffraction grating (operated in transmission or reflection)

13 THz diffraction gratings with blaze technology (operated in transmission or reflection)

15 THz prism

17 THz waveguides

In 1 1 and 13, the frequency dependence of the diffraction of an electromagnetic wave is used to achieve the spectral fanning. Diffraction gratings with Blaze technology 13 have a sawtooth profile, which causes a displacement of the specular reflection. THz prisms 15 use materials that have a frequency-dependent refractive index. Thus, the different frequency components in the prism are broken in different directions. A likewise usable possibility for achieving a spectral fanning or spectral collection is the use of a THz waveguide 17. The waveguide offers different propagation speeds for different frequencies, so that a fanning out of the frequency components can be achieved analogously to the prism. A combination of these Embodiments in a device is possible. Furthermore, spectral collection and spectrally diffusing components can be constructed the same or different in one device.

THz emitter 1 and THz detector 2 consist of two separate components, alternatively they consist of a single component, a so-called transceiver. There are various possibilities of image data evaluation. Each detected frequency point corresponds to a location point. In the case of the pulsed system, a spectral transformation is applied to the time domain data (preferably an FFT / Fast Fourier Transform) to represent the data in frequency space. Optionally, a frequency-dependent normalization of the data is performed to compensate for system or sample-related, frequency-dependent distortions. The nonlinear relationship between frequency and location information shown in FIG. 8B is used to convert the frequency space into an equivalent space. The image is then compounded line by line and the respective received signal strength or phase information displayed in a false color image.

Optical, or quasi-optical, diffractive elements produce interference effects by changing the phase front of an incident electromagnetic wave. The diffraction patterns occurring are frequency-dependent and can be used, for example, to reflect broadband radiation as a function of their frequency at different angles. Diffractive elements can be produced in both transmission and reflection. Due to the increased efficiency (power in the diffraction orders to the total power) reflection gratings are to be preferred for the given application. Reflection gratings are THz diffraction gratings 1 1, which are operated in reflection.

According to the invention, gratings with angled reflection surfaces, so-called blaze gratings (diffraction gratings with blaze technique), are preferred embodiments of the THz diffraction grating 11, which have a fundamentally different efficiency distribution compared to conventional diffraction gratings. In Fig. 1 1 a is a schematic sketch of a rectangular grid (embodiment of the THz diffraction grating 1 1 in a rectangular shape) shown in Fig. 1 1 b, the corresponding Blazegitter shown (embodiment of the THz diffraction grating 1 1 in Blaze technique 13). In one embodiment, a grid with a gap width of 1 mm is used, the gap depth is in both cases 414 μιη, both for the embodiment in a rectangular shape and for the embodiment in blaze technique. The Blazegitter is made in this embodiment with a blaze angle of 22.5 °.

FIGS. 11c and 11d show the associated simulations at a frequency of 400 GHz. The simulations indicate the field distribution of the reflected wave plotted against the angle of reflection. In the simulations, the grid is illuminated parallel to the surface normal (0 °). The circle marks the diffraction order used in the experiment, which is approximately 48 ° for a case of 400 GHz. It can be seen that for the blaze grating the power of the incident wave is almost completely transferred to this diffraction order. In contrast, the rectangular grid reflects only a fraction of the power in the relevant diffraction order. Since the broadband THz sources used in the industrial environment are generally very weak, an efficient blazed grating is preferable for the invention.

Preferred angles for a blazed grating (blazed gratings) are 5 ° to 30 °. The gap widths depend on the usable frequencies. Preferred gap widths are 0.1 mm to 3 mm, very particularly preferably 0.5 mm to 3 mm.

The invention includes a method for measuring images with THz radiation with the described device, characterized in that the frequency is converted into a location information. Using a pulsed THz system, an image line is thus measured simultaneously, and in a cw system, an image line is thus measured without mechanical displacement by tuning the emitter frequency.

The method has, for example, the following steps:

The THz radiation is generated by a THz emitter 1.

The THz radiation is fanned out by a spectrally flaring component 3a in accordance with the frequencies contained.

The fanned THz radiation is focused by an imaging system 5 on a focus line 25.

The fanned-out THz radiation is reflected and / or transmitted through a sample. The fanned-out THz radiation reflected and / or transmitted by a focus line 25 of the sample is guided by an imaging system 5 onto a spectral-collecting component 3b.

The fanned-out radiation is combined by a spectral-collecting component 3b.

The THz radiation is measured by at least one THz detector 2.

• The measurement result is converted to the corresponding image line.

The method is alternatively used only on the emitter side with the following steps:

i) The THz radiation is caused by a spectrally flaring component

3a fanned out according to the frequencies contained. ii) The fanned out THz radiation is focused by an imaging system 5 on a focus line 25.

Alternatively, the method is only used on the detector side with the following steps:

iii) The fanned out THz radiation reflected and / or transmitted by a focal line 25 of the sample is guided by an imaging system 5 onto a spectral collecting component 3b.

iv) The fanned radiation is brought together by a spectral collecting component 3b.

Furthermore, by splitting the radiation several lines, also in different planes, are measured simultaneously.

embodiments

2: In a first exemplary embodiment, the device consists of a transmission structure in which the incident THz radiation 19 is fanned out by the at least one spectrally flaring component 3 a and focused on a focus line 25 by the imaging system 5. Wherein this focus line 25 is in the sample area and the THz radiation penetrates the sample. There follows another imaging system 5 and a spectral collection component 3b. In this arrangement, spectrally flaring 3a and spectral-collecting 3b components operated in reflection are used.

Fig. 3: In a second embodiment, the device consists of a transmission structure in which the THz radiation also penetrates the sample and spectral flattening 3a and spectral collecting 3b components operated in transmission are used. Shown is the incident THz radiation 19, the arrangement of the at least one spectrally expanding component 3 a, two of the imaging systems 5, the at least one spectral collecting component 3 b, and the emergent THz radiation 21.

4: In a third exemplary embodiment, the device consists of a reflection structure with reflection-driven component 3, which has a spectral-fan-out 3a and spectral-collecting 3b function. Shown are also an imaging system 5 and the focus line 25.

5: In a fourth exemplary embodiment, the device consists of a reflection structure with a transmission-operated component 3, which has a spectral-fan-out 3a and spectral-collecting 3b function. Shown are also an imaging system 5 and the focus line 25.

FIG. 6: In a fifth exemplary embodiment, a device is considered in which diffraction gratings with blaze technology 13 operated as a spectrally flaring and spectral-collecting component in reflection and as an imaging system 5 an f-lens system consisting of high-density polyethylene THz lenses is used. The device is constructed in transmission according to the schematic diagram in FIG. The diffraction gratings 13 used here behave in a spectral-broadening or spectral-collecting manner. By means of the lens system 5 used, the THz frequencies are focused, for example, from 300 GHz to 500 GHz on a focus line 25 orthogonal to the propagation direction. According to the invention, the THz frequency is in a range of less than 30 GHz and more than 100 THz. The diffraction grating 13 used reflects the THz radiation in different directions depending on the frequency. The fanned-out THz radiation is focused on a focus line 25 with the aid of the imaging system 5. After the focus line 25, the radiation diverges again and is guided by the reciprocally constructed system by means of an imaging system 5 and a spectral-collecting component, here a diffraction grating in blaze technique 13, onto a THz detector 2. This construction achieves a transformation from an arbitrary frequency to a location of the focus line 25. Thus, a spatially resolved measurement of an entire line within the measurement duration of a single THz pulse is possible. Considering a measurement without a sample in the system of FIG. 6, one arrives at the measurement result shown in FIG. Two frequency bands higher Signal strength are recognizable, the 1. and 2. diffraction order of the THz diffraction grating used with blaze technique 13 correspond.

The present invention converts the frequency to a location information. This relationship is shown in FIG. As shown in FIG. 8A, the THz detector 2 is moved along a focus line 25. In the process, the measurement data shown in FIG. 8B are recorded. It can be clearly seen that different frequency components are contained on the focus line as a function of the position of the THz detector 2, so that a conversion from frequency to position information is possible. In this exemplary embodiment (FIG. 8A), the spectrally flaring component 3a, here a blazed diffraction grating 13, and the associated method are used only on the emitter side, the emitter side including the region of the beam path and all components between the sample and the Focus line 25 and up to and including the THz emitter 1 are arranged.

FIG. 9 shows a THz image which was measured in a device according to the invention, as shown in the exemplary embodiment according to FIG. These are the aluminum foil letters "THz" glued to a sheet of paper, which was moved linearly through the focus line and evaluated line by line.

FIG. 10B shows a THz image which was measured in a device according to the invention and shows the structure of a plastic sample. 10A, a photograph of the plastic sample is shown in which the holes 26 and metal wires 27 present in the sample are identified.

In a further exemplary embodiment (not shown), the spectral-collecting component and the associated method are used only on the detector side, wherein the detector side includes the region of the beam path and all components which are arranged between sample or focus line 25 and up to and including the THz detector 2 ,

In another embodiment (not shown), the spectral flaring component follows directly after the THz emitter 1, i. The beam path goes directly from the THz emitter 1 to the spectrally flaring component 3a.

In another embodiment (not shown), the THz detector 2 directly follows the spectrally diffusing component 3a. In another embodiment (not shown), the spectrally diffusing component 3a consists of a THz waveguide 17 and the radiation is focused directly on the focus line 25 in the sample.

In a further exemplary embodiment (not shown), the radiation reflected and / or transmitted by the sample is guided directly onto a THz detector 2 by a THz waveguide 17, which acts as a spectral collecting component 3b. In a further exemplary embodiment (not illustrated), the radiation reflected and / or transmitted by the sample is passed directly from a THz waveguide 17, which acts as a spectral collecting component 3b, and a subsequent imaging system 6 onto a THz detector 2.

drawings

LIST OF REFERENCE NUMBERS

1 at least one THz emitter

2 at least one THz detector

3 component with spectral fan 3a and / or

Spectral collecting 3b function

5 at least one imaging system

6 a second imaging system

1 1 diffraction grating

13 diffraction gratings with blaze technique

15 prism

17 waveguides

19 THz emitter 1 generated incident THz radiation

21 failed THz radiation to the THz detector 2

23 lens system

25 focus line

26 holes

27 metal wires Fig. 1: Embodiments of the spectrally expanding component

FIG. 2: Transmission setup with reflecting spectrally expanding components FIG. 3: Transmission setup with transmissive spectrally expanding components

Fig. 4: Reflection structure with transmitting spectrally expanding components Fig. 5: Reflection structure with reflecting spectrally expanding components

Fig. 6: embodiment: schematic diagram

Fig. 7: Measurement result when measuring without sample

Fig. 8: Measurement setup and measurement data: Recording along an image line

Fig. 9: According to the invention detected THz image

10 shows another THz image of a sample acquired according to the invention

FIG. 11: Sketches and simulations of a rectangular grid (a), (c) and blazed grid (b), FIG.

(d) at 400 GHz

Claims

claims

1 . A device suitable for carrying out THz imaging measurements comprising at least one THz emitter (1) and at least one THz detector (2) and characterized in that it additionally has at least one spectrally flaring and / or spectrally collecting component (3).

2. A device according to claim 1, characterized in that the at least one spectral flattening and / or spectral collecting component (3), or the at least one spectrally flaring and / or spectral collecting component (3) in conjunction with at least one imaging system (5), so that it converts a frequency information into a location information.

3. A device according to one of claims 1 to 2, characterized in that the at least one spectrally flaring and / or spectral collecting component (3) or the at least one spectrally flaring component (3) are arranged in conjunction with at least one imaging system (5) in that a spatial fanning out of the THz beam lies in a spatial direction outside the propagation direction.

4. A device according to one of claims 1 to 3, characterized in that at least one spectrally diffusing component (3a) is provided on the emitter side and / or at least one spectrally collecting component (3b) on the detector side.

5. A device according to one of claims 1 to 4, characterized in that by the at least one spectrally flaring component (3a) or the at least one spectrally flaring component (3a) in conjunction with the at least one imaging system (5), a spatial fanning of the THz Beam is generated orthogonal to the THz propagation direction.

6. A device according to one of claims 1 to 5, characterized in that at least one imaging system (5), is made of a combination of lenses and / or mirrors and / or diffractive optics, so that the fanned out THz radiation on a focus line (25) is focused is arranged between the sample and the spectrally fanning and / or spectral collecting component (3) and / or a second imaging system (6) between the emitter (1) and the spectrally diffusing component (3a) and / or between the detector (2) and the Spectral collecting component (3b) is arranged.

7. A device according to one of claims 1 to 6, characterized in that a spectrally flaring and / or spectral collecting component (3) as a THz prism (15), as a THz waveguide (17), as a THz diffraction grating (1 1) and / or as a THz diffraction grating with Blaze technique (13) or as a combination of said elements is executed.

8. A device according to one of claims 1 to 7, characterized in that the THz emitter (1) is a pulsed THz system or a cw system (continuous wave / continuous wave system) is.

9. A device according to one of claims 1 to 8, characterized in that the THz radiation penetrates a sample (transmission structure) and / or is reflected on the sample (reflection structure).

10. A device according to one of claims 1 to 9, characterized in that the spectrally flaring and / or the spectral collecting component (3) is operated in reflection and / or in transmission.

1 1. A device according to any one of claims 1 to 10, characterized in that a beam splitter is arranged so that it divides the THz radiation at least one point on the beam path between THz emitter (1) and sample for the simultaneous measurement of several image lines and / or several samples.

12. A device according to one of claims 1 to 1 1, characterized in that THz emitter (1) and THz detector (2) are designed as one or more components and / or an imaging system (5) or more imaging Systems (5) is / are designed as first and second imaging system (6) and / or one or more components are designed as a spectral flattening and spectral collection component (3).

13. A method for carrying out imaging THz measurements with a device according to one of claims 1 to 12, characterized in that the frequency is converted into a location information, having following steps:

i) The THz radiation is fanned out by a spectrally flaring component (3a) according to the frequencies contained.

ii) The fanned-out THz radiation is focused on a focus line 25 by an imaging system (5).

14. A method for performing THz imaging measurements with a device according to one of claims 1 to 12, characterized in that the frequency is converted into a location information, comprising the following steps:

iii) The fanned THz radiation reflected and / or transmitted from a line of the sample is guided by an imaging system (5) onto a spectral collection component (3b).

iv) The fanned radiation is brought together by a spectral collecting component (3b).

15. A method for performing THz imaging measurements according to one of claims 1 to 14, characterized in that

Before step i) step a) takes place:

a) The THz radiation is generated by a THz emitter (1).

After step ii) and before step iii) step b) takes place:

b) The fanned THz radiation is reflected on a sample and / or transmitted through it.

After step iv) steps c) and d) take place:

c) The THz radiation is measured by at least one THz detector (2).

d) The measurement result is converted into the corresponding image line.