DE102010021476B4 - Method and device for absolute length and distance measurement with continuous, tunable THz radiation - Google Patents

Abstract

Method for measuring lengths and distances using continuous, coherent electromagnetic waves in the frequency range 50 GHz to 10 THz, characterized in that the distance to be measured is traversed by such a wave that the frequency of the wave is varied, and that the desired Size is determined from the proportionality constant that exists between the frequency change and the phase position of the wave after it leaves the path.

Description

classification

The task arises mainly from the field of THz technology and optical technologies as well as absolute interferometry.

description

The object of the invention is a method and a device for measuring lengths, distances or positions (possibly also material thicknesses or refractive indices of media) using harmonic electromagnetic waves in the frequency range between a few hundred and a few thousand gigahertz (“terahertz radiation”).

(Darin bedeutet c₀ die Lichtgeschwindigkeit).The method is based on the relationship between the frequency v of the electromagnetic wave and its phase position φ after passing through the distance to be measured or the object or medium to be examined. For this purpose, the frequency is changed by a certain amount Δν, called the frequency deviation, and the resulting change in the phase position Δφ is registered. If D denotes the geometric length of the path traveled and n the refractive index of the medium penetrated, there is the following linear relationship between frequency and phase change: $$\mathrm{\Delta \phi \; =}2\mathrm{\pi }\mathrm{\Delta }\nu n\mathrm{D.}/{\text{c}}_0$$

(Here, c _{0 means} the speed of light).

In the method to be protected, the coefficient 2πnD / c _{0 is} determined, from which, if n is known, the variable D can be calculated (possibly also vice versa).

Und daraus $$nD=n^{\text{V}}{D}^{\text{V}}\cdot \mathrm{\Delta \phi /\Delta }{\mathrm{\phi }}^{\text{v}}.$$

With this method, the frequency deviation Δν has the character of a scale embodiment. If a calibration is difficult to perform with the required long-term stability, it is advantageous to determine Δν with every measurement. In particular, the device can consist of two similar arrangements, the quantity nD being known in one of the two (possibly after a calibration). If the two similar arrangements are referred to as measuring and comparison arrangements, and if the variables belonging to the comparison arrangement are designated with superscript V, then a phase change results in the comparison arrangement analogous to the above equation $$\mathrm{\Delta }{\mathrm{\phi }}^{\text{V}}=2\mathrm{\pi }\cdot \mathrm{\Delta \nu }\cdot n^{\text{V}}{\mathrm{D.}}^{\text{V}}/c_0$$

And it $$n\mathrm{D.}=n^{\text{V}}{\mathrm{D.}}^{\text{V}}\cdot \mathrm{\Delta \phi \; /\; \Delta }{\mathrm{\phi }}^{\text{v}}.$$

The determination of nD is thus attributed to a ratio formation, the optical length of the comparison distance n ^V D ^V now serving as a scale.

The proportionality of the phase change to the coefficient 2πnD / c ₀ leads to a periodic signal form whose period is given by P = c ₀ / (nD). If the frequency is detuned by one or more period lengths P, the optical path length D can also be determined from the determined signal period: nD = n ^V d ^V P ^V / P.

The method based on ratio formation has the advantageous property that if the two arrangements are exactly identical, all systematic measurement errors must disappear for reasons of symmetry. In addition, it is not necessary to determine the refractive indices n and n ^V for the measurement of D if the measurement and comparison sections run in the same medium. The ratio n / n ^V is then close to 1 and is particularly insensitive to environmental influences.

One possibility for generating variable-frequency terahertz radiation is to superimpose the fields of two continuously emitting, frequency-adjustable lasers and to convert the resulting beat into an electromagnetic wave by means of a non-linear optical or electrical effect (see e.g. publication 8). Their frequency is then equal to the difference between the optical frequencies. The same is changed by detuning at least one of the two lasers.

In this case, the frequency deviation Δν of the individual lasers can be determined by optical interferometers.

The phase position of the terahertz wave can be determined, for example, by what is known as coherent detection in a photoconductive antenna. This generates an electric current as a signal, the magnitude of which is proportional to the product of the terahertz field strength and the (time-dependent) optical power of the beat field. In this case the current intensity is a periodic function of the terahertz frequency v. A change in the phase position of the terahertz wave by Δφ results in an equal change in the phase in the current signal, so that Δφ can be determined by a continuous current measurement.

Periodic modulation of the terahertz frequency by a frequency deviation Δν is also particularly suitable. In this case, a lock-in Amplifier circuit can be used to evaluate the amplitude and phase position of the THz wave. The modulation frequency serves as a reference signal.

Contemplated Publications

1. 1) Continuous wave terahertz spectrometer as a noncontact thickness measuring device, Wilk et al, Applied Optics, 2008
2. 2) Continuous-wave all-optoelectronic terahertz imaging, Siebert, et al., APPLIED PHYSICS LETTERS, 2002
3. 3) patents U.S. 6,178,002 and DE 197 43 493 A1
4. 4) Free-space electro-optic detection of continuous-wave terahertz radiation, Nahata, et al., Appl. Phys. Lett. 75.2524 (1999)
5. 5) patent US 2008/0 137 068 A1
6. 6) patent US 2006/0 214 107 A1
7. 7) patent US 7 214 940 B2
8. 8) patent US 2009/0 180 122 A1

State of the art

1. 1. Dauerstrich-THz-Systeme, bei denen über eine optische Schwebung zweier Lasermoden eine monofrequente THz-Welle konstanter Frequenz erzeugt wird und bei denen eine Phasenänderung des Signals durch eine Variation der optischen Weglänge (Druckschrift 1) oder durch Modulation der Phase einer der beiden Lichtwellen (Druckschrift 8) bewirkt wird
2. 2. Bildgebende Dauerstrich-THz-Systeme mit konstanter Frequenz, bei denen eine Probe mechanisch durch einen THz-Fokus gefahren wird um diese räumlich zu charakterisieren. (Druckschrift 2)
3. 3. Verfahren zur Messung von Eigenschaften einer Substanz unter Verwendung von gepulster THz-Strahlung (Druckschriften 5 und 7; die Möglichkeit zur Verwendung kontinuierlicher Strahlung zu diesem Zweck wird in den Druckschriften vermutet.)
4. 4. Verfahren zur Detektion von Substanzen oder Gegenständen, wobei die Variation bestimmter Frequenzen die Unterscheidung von Objekten aufgrund ihres unterschiedlichen Abstandes von der Vorrichtung erlaubt (Druckschrift 6)

The following arrangements are known:

1. 1. Continuous wave THz systems in which a mono-frequency THz wave of constant frequency is generated via an optical beat of two laser modes and in which a phase change of the signal is caused by varying the optical path length (document 1) or by modulating the phase of one of the two Light waves (document 8) is effected
2. 2. Imaging continuous wave THz systems with constant frequency, in which a sample is moved mechanically through a THz focus in order to characterize it spatially. (Document 2)
3. 3. Method for measuring properties of a substance using pulsed THz radiation (publications 5 and 7; the possibility of using continuous radiation for this purpose is assumed in the publications.)
4. 4. Method for the detection of substances or objects, whereby the variation of certain frequencies allows the differentiation of objects due to their different distance from the device (document 6)

Disadvantages of the methods described

All known length measurement methods that use continuous THz waves are based on the fact that the signals are a periodic function of the variable to be determined (length or distance; possibly thickness, refractive index), with the period being half or a whole wavelength of THz radiation is. The association between the signal value obtained and the variable to be determined is therefore ambiguous. The previous methods therefore only allow changes to be made in the lengths sought. For practical use, this means that a reflector has to be moved along it in order to measure a route, the phase being determined continuously and full 2π cycles ("increments") counting and adding. The absolute value can only be determined in the event that the length is known in advance down to one wavelength.

Another disadvantage is that the determination of the signal phase requires a change in the same by mechanical movement of parts of the arrangement (e.g. a reflector). The arrangements are therefore mechanically complex, sensitive and the processes are comparatively slow overall.

In the known arrangements according to document 1, the ability to determine the specimen thickness or to determine the refractive index thus proves to be inadequate. The absolute determination of a length or a distance is also insufficient.

With the known arrangements according to document 2, the possibility of spatial sample characterization proves to be insufficient. The cause is the large amount of time required for mechanical sample movement and the large number of measuring points.

The arrangement according to document 6 allows objects to be differentiated or sorted according to their distance, but has the disadvantage that it is not possible to measure the distance itself because the necessary scale representation is missing.

Removal of the disadvantages according to the invention

The object of the invention is to avoid the disadvantages of the known solutions and to combine the advantages of various known arrangements. Technical embodiments relating to the invention are described in the following examples. They are characterized by the fact that no mechanical movement of components is required and that the quantities sought can be determined absolutely (i.e. not ambiguously) without prior knowledge of them.

The example arrangements have in common that they have at least two interferometers, at least one of which is implemented in the form of a THz interferometer. Furthermore, at least one of the interferometers is used as a reference interferometer.

By tuning the difference frequency between two laser beams, which is in the THz frequency range (50 GHz to 10 THz), the exact change in frequency of the difference frequency can be measured using the reference interferometer. With this knowledge, with simultaneous evaluation of the measurement of further interferometer arms, which are implemented in the form of THz interferometers driven by means of the laser radiation, the respective absolute optical length of the respective arm can be determined.

First embodiment

The object of the first exemplary embodiment is to determine the absolute sample thickness or the refractive index in the THz frequency range or the composition by using a THz absolute distance interferometry. For this purpose, the sample is introduced into at least one THz interferometer in a transmission or reflection geometry. The sample is characterized by tuning the THz frequency and simultaneously recording the interference signal.

Figures 1 to 3 describe the arrangement according to the first embodiment.

A reference interferometer can be implemented in a particularly suitable embodiment in the form of an optical interferometer. In order to precisely stabilize the frequency of the lasers (L1 and L2), both beams are passed through the beam splitter BS1 and BS2 (Beam Sampler 1, 2) one (preferably small) part is decoupled and one interferometric control unit each IC1 and IC2 (Interferometric Control 1, 2) supplied. These units measure the current laser frequency with high accuracy and speed, so that a frequency detuning with interferometric accuracy can take place via feedback to the control unit of the laser. (see also publication 3).

After the superimposition in a beam splitter (XI), at least one, better from both, partial beams can be made via further beam splitters BS3 and BS4 (Beam Sampler) further partial beams (each with the beat wave length) decoupled from which the heterodyne frequency in the detection units ( HCl and possibly HC2 ) can be measured. In particular, if the two lasers have the same or almost the same frequency, this beat frequency is low and can be determined with very high accuracy. This serves as an absolute reference for the difference frequency, while that in the control units IC1 and IC2 contained interferometer serve as a relative reference. A beat frequency that is both absolutely and relatively very precisely defined can thus be achieved. Both lasers can now be tuned away from each other by several terahertz. A direct measurement of the beat frequency then eludes the electronic possibilities. But about the interferometer units IC1 and IC2 In principle, however, an arbitrarily wide detuning can be precisely measured. The absolute heterodyne frequency can be determined instead of with the detection units ( HCl and possibly HC2 ) can also be determined from the THz measured variable by, with simultaneous detection of the THz signal, a defined optical path change with a delay unit ( VZ ) is caused. It is possible to use fiber couplers ( FC1 and FC2 ) to couple in fibers.

A THz interferometer is connected to a THz emitter ( RA ), preferably a photoconductive THz antenna or a nonlinear crystal. A tunable, continuous wave laser radiation, consisting of the superposition of two laser modes with a difference frequency in the THz frequency range, is directed onto the THz emitter. This beat frequency radiation is converted into a continuous and continuously tunable coherent THz wave (THzR) with the help of a non-linear electro-optical process (e.g. with a photoconductive semiconductor antenna structure or in a non-linear crystal).

The THz wave is generated using a THz lens ( Tbsp ) or reflective optics collimated or focused and directed onto the sample. Possibly. this can be passed through further THz lenses and / or reflective optics.

Will a sample ( T ) introduced into the path of the THz wave, the THz wave interacts with it.

With further THz lenses ( DL ) and / or reflective optics, the THz wave is directed to a THz detector ( RA ) directed.

The detection of the THz radiation transmitted through the medium to be measured and / or reflected by it is converted into a photocurrent by a further photoconductive antenna structure, together with a partial beam of the excitation light in an electro-optical mixing process, which is generated by a current-voltage converter TIA (Transimpedance Amplifier) amplified and the signal processing unit SP (Signal Processing) is supplied. Alternatively, the electro-optical effect in a non-linear crystal, for example zinc telluride, can be used to detect the measurement signal by what is known as electro-optical scanning (document 4).

Since the relative phase of the THz beat frequency of the excitation light and the THz radiation hitting the receiver are included directly in the generated measurement signal, it is a coherent detection, corresponding to the detection in an optical Mach-Zehnder interferometer ( MZI ).

The (also non-integer) number of measured interference structure phases Δφ during the tuning Δν corresponds exactly to the (effective, optical) difference in length of the interferometer arms. If the actual interferometer _arm lengths L are known, the “optical” length L _OPT = L _P * n _P (λ) of an introduced test body with the geometric length L _P and the refractive index n _P (λ) can be determined . The resulting optical interferometer length is L '= L + (n _P (λ) -1) L _P.

Since the wavelengths λ ₁ and λ _{2 of} the tunable cw lasers with the laser frequencies ν ₁ = c / λ ₁ and ν ₂ = c / λ ₂ (and the resulting THz frequency ν _T = (ν ₂ -ν ₁ ) / 2) is normally determined and stabilized with reference to an optical interferometer, the ADI measurement thus represents a return of the measured path length in the THz interferometer to the interferometer used for laser stabilization in the optical range.

If a sample is introduced into the corresponding THz interferometer arms, the absolute optical thickness of the sample can be determined with this method. Thus, if the material thickness is known, the refractive index of the sample, if the refractive index is known, the material thickness and if a mixture of several substances is measured, the material composition can be determined if the material thickness and the respective refractive indices are known. When the measured signal amplitudes are analyzed at the same time, the absorption coefficient of the sample can also be determined.

Second embodiment

The second exemplary embodiment is based on the object of determining the distance or the position or the surface properties of a sample in reflection geometry. The THz interferometer is formed by the path: THz emitter, sample, THz detector. The determined interferometer length is therefore identical to twice the sample distance to the THz transmitter and receiver unit.

Figures 4 to 5 describe the arrangement according to the second embodiment.

Third embodiment

The third exemplary embodiment is based on the task of using several THz interferometers at the same time to generate a multi-channel THz system which manages with a single detector. Setting different absolute path lengths in the interferometer arms results in measurement signals that can be separated by means of oscillation analysis, which make it possible to characterize several interferometer arms at the same time. Either an antenna array, a plurality of separate THz antennas or a single THz antenna whose radiation is split by THz beam splitters (THz-ST) can be used as the emitter. If the THz interferometers are arranged next to and / or on top of one another, it is possible to carry out a spatial characterization of one and / or more samples with a single THz detector and a single measurement.

Figure 6 describes the arrangement according to the third embodiment.

Figure list

• Fig. 1: An arrangement for absolute distance interferometry with THz radiation, fiber-based
• Image. 2 An arrangement for absolute distance interferometry with THz radiation, free-beam design
• Image. 3 An arrangement for absolute distance interferometry with THz radiation, combination of free-beam design and fiber outputs
• Image. 4 Measurement of a target in reflection
• Image. 5 Measurement of a target in reflection with a combined transmitting and receiving antenna
• Image. 6 multi-channel THz absolute interferometer

Claims (10)

Method for measuring lengths and distances using continuous, coherent electromagnetic waves in the frequency range 50 GHz to 10 THz, characterized in that the distance to be measured is traversed by such a wave that the frequency of the wave is varied, and that the desired Size is determined from the proportionality constant that exists between the change in frequency and the phase position of the wave after it leaves the path. Device consisting of a continuous frequency-tunable THz source (THz transmitting unit), a phase-sensitive THz receiving unit, which transforms the received radiation into an electrical signal with respect to the emission phase, and an evaluation unit, which the method according to Claim 1 realized. Device according to Claim 2 , in which the light waves emitted by two continuous wave lasers are superimposed and the resulting beat is converted into the wave used for measurement by a photoconductive antenna or a non-linear optical element. Device according to Claim 3 characterized in that the wavelengths of one or both lasers or their wavelength difference is measured by an optical interferometric arrangement. Device according to Claim 4 characterized in that the wavelengths of one or both lasers or their difference is stabilized by means of a feedback to the laser control with respect to the optical interferometric arrangement. Device according to Claim 3 , 4th or 5 characterized in that the lasers are distributed feedback (DFB) laser diodes. Device, which two or more arrangements according to one of the Claims 2 to 6th contains, characterized in that the length or distance measurement according to the separation of the signals of the individual arrangements Claim 1 can take place at the same time in several places. Method in which by measuring several distances according to Claim 1 at different points of an object whose shape is determined. Device according to one of the Claims 2 to 7th , which is characterized by a reflection geometry. Device according to one of the Claims 2 to 7th characterized in that the phase position is determined by means of a phase-sensitive rectification (lock-in amplifier circuit).

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