Device for determining distance

Abstract

Device for determining distance, with at least one light source for generating bichromatic light waves (E1, E2) with a first and second frequency (ω1, ω2), with a device for generating bichromatic reference waves (E Lo1 , E Lo2 ), which is designed in such a way that, starting from the bichromatic light waves (E1, E2), bichromatic reference waves (E Lo1 , E Lo2 ) shifted by a difference frequency (A C0M2 ) are generated with a first and second reference frequency (ω'1, ω'2), where the bichromatic light waves illuminate a scene or an object and are referred to as object waves (E Obj,1,2 ) be reflected upon, with a demodulator that uses the difference frequency (Δω M2 ) is operated, wherein the distance determination device is designed such that the demodulator is completely free from the object waves (E) on its receiving range. Obj,1,2) is illuminated, while the bichromatic reference wave (E Lo1 , E Lo2 ) only illuminate a first reception area of ​​the demodulator, wherein a second reception area is based on a monochromatic reference wave (E Lo1 ) is illuminated with a first frequency (ω'1), with an evaluation device designed in such a way that a distance is determined based on signals demodulated in the first and second reception range of the demodulator.

Description

[0001] The invention relates to a device for determining distance according to the preamble of the independent claims.

[0002] Patent CH 695 516 A5 discloses a method for optoelectronic distance measurement using a main light emitter, a reference light emitter, a main receiver, and a reference receiver. The light intensities of the main and reference emitters are simultaneously intensity-modulated at different frequencies. As a result, both receivers receive a signal mixture containing signal components from both emitters. These signal mixtures are converted into an intermediate frequency range containing two frequency components. The separation of the phase information contained in the simultaneously generated intermediate frequency signals is performed based on the different frequencies for comparative signal evaluation and determination of the measurement distance, thereby compensating for errors.

[0003] German patent DE 10 2014 004 697 A1 discloses a distance measurement system that combines a multi-wavelength holography module with a phase-time measurement module. The holography module comprises at least two holographic light sources for generating light beams of different wavelengths, a beam splitter for splitting the beams into object and reference light beams, and a diffraction element for dispersively tilting the reference light beams to acquire a holographic measurement data set. Simultaneously, a phase-time measurement module acquires a phase-time measurement data set using a modulated phase-time light beam. An evaluation module determines the final distance based on the combined data sets, thus linking a coarse and a fine measurement.

[0004] US Patent 2019 / 0310372 A1 discloses a method for Doppler correction in chirped optical distance measurement. A first set of distances is determined based on frequency differences between an optical return signal and an upward-chirped transmit signal, while a second set of distances is obtained from a downward-chirped transmit signal. To map the distances from both sets, a matrix containing values ​​for a cost function is determined, calculating a value for each distance pair. Using this matrix, a matching pair of distances is identified, and the Doppler effect and the corrected distance are determined from their combination.

[0005] US Patent 2016 / 0377721 A1 discloses a method for bandwidth compression of beat signals in laser distance measurement. In this method, at least two frequency-modulated laser detection subsystems, differing in their modulation characteristics such as center frequency or chirp rate, simultaneously generate separate high-frequency, range-determining beat signals for an object. These two high-frequency beat signals are then electrically mixed to generate a low-frequency beat difference signal. The distance to the object is then determined using this difference signal, thus eliminating the need to process the original high-frequency signals.

[0006] The object of the invention is to design a distance determination system that is robust against background light.

[0007] This problem is advantageously solved by the devices according to the independent claims.

[0008] Advantageously, a device for distance determination is provided, with at least one light source for generating bichromatic light waves with a first and second frequency, with a device for generating bichromatic reference waves, which is designed such that, starting from the bichromatic light waves, bichromatic reference waves are generated shifted by a difference frequency with a first and second reference frequency, where the bichromatic light waves illuminate a scene or object and are reflected as object waves, with a demodulator operated at the difference frequency, wherein the device for distance determination is designed such that the demodulator is completely illuminated by the object waves on its receiving area, while the bichromatic reference waves only illuminate a first receiving area of ​​the demodulator, wherein a second reception area is illuminated by a monochromatic reference wave with a first frequency, with an evaluation device designed in such a way that a distance is determined based on signals demodulated in the first and second reception range of the demodulator.

[0009] This approach has the advantage that the distance information can be obtained from the demodulated and therefore background-light-free signal.

[0010] Alternatively, and with the same effect, distance information can be obtained by separating the different measurement situations not spatially, but temporally.

[0011] Advantageously, a device for determining distance is provided for this purpose. with at least one light source for generating bichromatic light waves with a first and second frequency (ω1, ω2), with a device for generating bichromatic reference waves, which is designed such that, starting from the bichromatic light waves, bichromatic reference waves are generated shifted by a difference frequency with a first and second reference frequency, where the bichromatic light waves illuminate a scene or object and are reflected as object waves, with a demodulator operated at the difference frequency, wherein the device for distance determination is designed such that the demodulator is illuminated on its receiving range by the object waves and the bichromatic reference waves in a first measurement interval, wherein in a second measurement interval the demodulator is illuminated on its receiving range by the object waves and a monochromatic reference wave with a first frequency, with an evaluation device designed in such a way that a distance is determined based on the signals demodulated in the first and second measurement intervals.

[0012] The invention is explained in more detail using the figures.

[0013] They show schematically: Fig. 1 a coherent superposition of a monochromatic object wave and a frequency-shifted monochromatic plane local oscillator wave running obliquely to the z-axis on a CMOS image sensor in the (x,y)-plane at z=0. Fig. 2 Schematic representation of different spatial frequency spectra, Fig. 3 a bichromatic distance measurement, Fig. 4 a monochromatic distance measurement, Fig. 5 an alternative monochromatic distance measurement, Fig. 6 an example of a bichromatic distance measurement according to Fig. 3.

[0014] The core idea of ​​the invention is a coherent heterodyne-holographic superposition of an optical wave field and an obliquely incident plane reference wave with detection and reconstruction of the wave field.

[0015] In Fig. Figure 1 shows the coherent superposition of a monochromatic optical wave field and a monochromatic plane wave frequency-shifted by Δω = 2πΔv, which strike an image sensor obliquely to the light waves emanating from the object. The spatial frequency spectrum (v x , v y The object wave is concentrated around the z-axis, allowing calculations to be performed using a paraxial approximation. For the two positively assumed spatial frequency components (v xLO , v yLO) the plane reference or local oscillator wave should apply vxLO≫|vx|and vyLO≫|vy| The object wave is assumed to be a paraxial wave field. The following representation describes the situation in a scalar approximation, thus assuming, in particular, linear polarization of the electromagnetic fields.

[0016] The electric field strength at the angles (α x , α y ) with sin(αxLO)=vxLO / λ and sin(αyLO)=vyLO / λ (2) monochromatic plane reference or local oscillator wave propagating obliquely to the z-direction is, up to a constant phase factor, EL(x,y,z,t)=ELO exp(i(ω+Δω)t−2πi(vxLOx+vyLOy)−ikzz) (3) given, where E LO the real amplitude of the wave and k z = 2π v z denotes the wave vector component in the z-direction. The electric field of the object or signal wave can be written more generally as E(x,y,z,t)=E0(x,y,z) exp(−iφ(x,y,z)) exp(iωt), where E0 (x, y, z) = IE (x, y, z, t)I is the magnitude and φ(x, y, z) is the phase of the signal wave.

[0017] The superposition field in the sensor plane z = 0 is given up to a phase factor by Etot(x,y,z=0,t)=EL(x,y,z=0,t)+E(x,y,z=0,t) =ELO exp(i(ω+Δω)t−2πi(vxLOx+vyLOy)) +E0(x,y,z=0) exp(−iφ(x,y,z=0)) exp(iωt).

[0018] The intensity I(x, y, t), as the time-averaged optical energy flux density through the area z = 0, is proportional to the square of the magnitude of the electric field strength. In non-magnetic materials, with refractive index n0, speed of light in a vacuum c0, and permittivity ε0, the following relationship generally applies: I(x, y, t) = n0c0ε0 |E| 2 and thus follows I(x,y,t) / (n0c0ε0)=|Etot(x,y,z=0,t)|2=ELO2+E02(x,y,z=0) =ELO2+E02(x,y,z=0) +2 ELO Re{E0(x,y,z=0)exp(−2πi(vxLOx+vyLOy)−iφ(x,y,z=0))exp(iΔωt) =ELO2+E02(x,y,z=0) +2 ELO E0(x,y,z=0)cos(Δωt−2π(vxLOx+vyLOy)−φ(x,y,z=0)).

[0019] The first two terms are constant over time. The interesting third term describes the coherent heterodyne superposition of the object light wave with the frequency-shifted plane local oscillator wave and yields a high-frequency signal whose phase is identical to the phase of the object light wave and whose amplitude is determined by the local oscillator amplitude E. LO can be adjusted.

[0020] With the real part or in-phase component of the total location-dependent complex electric signal field strength Ertot(x,y,z=0)=ELO E0(x,y,z=0)cos(2π(vxLOx+vyLOy)+φ(x,y,z=0)) as well as the imaginary part or quadrature component of the position-dependent complex electric field strength Eitot(x,y,z=0)=ELOE0(x,y,z=0)sin(2π(vxLOx+vyLOy)+φ(x,y,z=0)) This results in strong local oscillator field strength. ELO2≫E02(x,y,z=0) the relationship I(x,y,t) / (n0c0ε0)=ELO2+2 ELO E0(x,y,z=0)cos(2π(vxLOx+vyLOy)+φ(x,y,z=0))cos(Δωt)+2 ELO E0(x,y,z=0)sin(2π(vxLOx+vyLOy)+φ(x,y,z=0))sin(Δωt) or written differently I(x,y,t) / (n0c0ε0)=EL02+2(Ertot(x,y,z=0)cos(Δωt)+Eitot(x,y,z=0)sin(Δωt)).

[0021] The detection process "downmixes" the complex amplitude distribution of the light wave E0 (x,y,z = 0) exp(-iφ(x,y,z = 0)) into the complex spatial distribution of the high-frequency signal, allowing it to be completely determined, i.e., in terms of magnitude and phase, for every point in the sensor plane (x,y,z=0). For small difference frequencies Δv < 20 Hz, the alternating component can be directly evaluated with a video measurement camera. It is important to note that the interference pattern to be recorded must not change during the exposure time of at least one period T = 1 / Δv. Consequently, significantly higher difference frequencies, preferably around Δv ~ 100 MHz, are required for recording moving objects, as offered in particular by synchronously time-integrating active pixel pmd-CMOS sensors. These sensors utilize spatiotemporal modulation of the generated photoelectrons and provide a time-dependent photocurrent characteristic i for each pixel.ph (x, y, t) and luminous flux P(x, y, t) = AI(x, y, t) incident on a pixel of the area A of the form iph(x,y,t)=(qη0ℏω+qη1ℏωcos(2πΔvt))P(x,y,t), where q denotes the electron charge and ℏω the photon energy, η0 the static and η1 the dynamic quantum yield, and it was assumed that in the pmd detector each pixel is modulated with the frequency Δv.

[0022] Note: The same transfer function can be achieved with a classic Active Pixel CMOS image sensor with an attached electroabsorption modulator!

[0023] Time averaging over m periods of the high-frequency signal according to <iph(x,y,t)> =(1mT)∫0mTiph(x,y,t)dt According to (10), modulating the pmd detector with cos(Δωt) yields the in-phase component <iphi(x,y,t)> =(n0c0ε0) (qη0ℏωELO2+qη1ℏωELOE0(x,y,z=0)cos(2π(vxLOx+vyLOy)+φ(x,y,z=0))) and accordingly, modulating the detector with sin(Δωt) yields the quadrature component. <iphq(x,y,t)> =(n0c0ε0) (qη0ℏωELO2+qη1ℏωELOE0(x,y,z=0)sin(2π(vxLOx+vyLOy)+φ(x,y,z=0)))

[0024] Without object light, the signal is simple: <iph0(x,y,t)> =(n0c0ε0)(qη0ℏωEL02).

[0025] Interestingly, a low dynamic quantum efficiency η1 is apparently caused by a strong local oscillator field E. LO can be compensated for.

[0026] The differential measurement signal grows proportionally with the square root of the local oscillator intensity at a constant field strength of the object wave. <iphi,q(x,y,t)> −<iph0(x,y,t)> ∝ELO

[0027] Taking into account (7) and (8), it can be shown that the real and imaginary parts of the electric field of the object light wave and the superimposed local oscillator wave can be easily determined from the measured sensor signals, since it holds that Error(x,y,z=0)=(<iphi(x,y,t)> −<iph0(x,y,t)> )Mω / (n0c0ε0qη1ELO) and correspondingly Eitot(x,y,z=0)=(<iphq(x,y,t)> −<iph0(x,y,t)> )Mω / (n0c0ε0qη1ELO).

[0028] For stationary wave fields, the measurement of the real and imaginary parts of the complex electric field strength can be performed sequentially in two steps. If the conditions change dynamically, for example due to object movement, beam splitters can be used to generate replicas of the object and reference fields. However, beam splitting introduces angle-dependent distortions, which, although correctable, require a considerable increase in the material cost of the detection process and ultimately make the method less attractive for practical applications. Therefore, the following section demonstrates how the complex electric field strength of the object light in the sensor plane E0 (x, y, z = 0) exp(-iφ(x, y, z = 0)) can be determined solely by measuring the in-phase component according to equation (17) (or alternatively from the quadrature component (18)).The method utilizes properties of two-dimensional analytical signals. In general, this is (v. x , v y The spatial frequency spectrum of the object light at the sensor's location in the plane z=0 is band-limited according to equation (1), because all waves emanating from a coherently illuminated object reach the finite area of ​​the sensor at a finite angle of incidence significantly less than 90°. Mixing with a plane reference wave with an even larger angle of incidence, i.e., a higher spatial frequency (v), results in a larger spatial frequency (v). xLO , v yLO ), ensures that only frequency components from the first quadrant (v) are present in the spatial frequency spectrum of the superposition signal. xLO > 0, v yLO > 0) are non-zero. Thus, the complex electric superposition field ES(x,y,z=0)=E0(x,y,z=0)exp(−2πi(vxLOx+vyLOy)−iφ(x,y,z=0)) a two-dimensional analytical signal, analogous to a one-dimensional signal with a one-sided spectrum. The real part of the superposition field (19) determines the spatial dependence of the measured signal according to (13) and (16). It holds that 2Re{ES(x,y,z=0)}=2{E0(x,y,z=0)cos(2π(vxLOx+vyLOy)+φ(x,y,z=0))} =exp(2πi(vxLOx+vyLOy)) E0(x,y,z=0) exp(iφ(x,y,z=0)) +exp(−2πi(vxLOx+vyLOy)) E0(x,y,z=0) exp(−iφ(x,y,z=0)) and the two-dimensional Fourier transform is F{Re{ES(x,y,z=0)}}(vx,vy) =δ((vx−vxLO,vy−vyLO)∗F{E0(x,y,z=0) exp(iφ(x,y,z=0))} / 2 +δ((vx+vxLO,vy+vyLO)∗F{E0(x,y,z=0) exp(−iφ(x,y,z=0))} / 2, where * denote the two-dimensional convolution and δ the two-dimensional δ-function. Due to condition (1) and the translation property of the δ-function, the first term on the right-hand side of equation (21) is only in the first quadrant (v). x > 0, v y > 0) the (v x , v y)-plane is non-zero and the same applies in the fourth quadrant (v x < 0, v y < 0) for the second term of the right-hand side of (21) is different. Using the signum function sgn(vx)={+1 for vx>00 for vx=0−1 for vx<0 This results in the interesting relationship (1+sgn(vx))(1+sgn(vy))F{Re{ES(x,y,z=0)}}(vx,vy) =2δ((vx−vxLO,vy−vyLO)∗F{E0(x,y,z=0)exp(iφ(x,y,z=0))}.

[0029] Inverse Fourier transform yields (according to (20)) F−i{(1+sgn(vx))(1+sgn(vy))F{Re{ES(x,y,z=0)}}(vx,vy)}(x,y,z=0) =exp(2πi(vxLOx+vyLOy)) E0(x,y,z=0) exp(iφ(x,y,z=0)), which allows the complex electric field distribution in the sensor plane to be determined taking into account (20) from measured photocurrent signals and the spatial frequencies known from the recording geometry (v xLO , v yLO ) can be determined according to (Check factor 2!) E0(x,y,z=0) exp(iφ(x,y,z=0))=exp(−2πi(vxLOx+vyLOy)) F−i{(1+sgn(vx))(1+sgn(vy))F{Re{ES(x,y,z=0)}}(vx,vy)}(x,y,z=0).

[0030] In Fig. Figure 2 shows various spectra schematically to illustrate the considerations described in equations (19) to (24) for determining the complex spatially dependent electric field strength from the measured distribution of the real part. The method is based on the processing of one-dimensional analytical time signals. Determining the real part of the spatially dependent field strength is a special feature, not only because it allows measurements down to the quantum noise limit in practice, but also because it enables excellent short-time or high-frequency measurements down to the single-digit nanosecond or gigahertz range, and is therefore well suited for recording moving objects.In simpler versions of the method, spatially resolved depth determinations in the z-direction, i.e. in the direction of view, are possible, for example using holographic two-wavelength methods or velocity measurements in the z-direction by exploiting the Doppler effect.

[0031] The complex electric field strength distribution E(x,y,z=0) measured by the pmd sensor in the plane z = 0 is related to the field strength distribution E(x,y,z=z0) in the plane z = z0 > 0 via the diffraction integral. In a paraxial approximation, the following holds: E(x0,y0,z=z0)=(exp(ikz0)iz0λ)∫∫−∞∞E(x,y,z=0)exp(ik2z0[(x0−x)2+(y0−y)2])dx dy, where k = 2π / 2 = ω / c0 is the vacuum wavenumber. A converging lens with refractive index n0, thickness Δz, focal length f in the sensor plane z = 0 results in an additional quadratic phase rotation with the amplitude transmission factor. t(x,y)=exp[ik(n0−1)Δz]exp(−ik2f[x2+y2]), where the first factor accounts for an additional constant phase shift on the optical axis. The electric field strength distribution in the rear focal plane of the lens, i.e., for z0 = f, is thus E(x0,y0,z=f)=(exp(ikf)iλf)exp[ik(n0−1)Δz]exp(−ik2f[x02+y02]) ∫∫−∞∞E(x,y,z=0)exp(−i2πλf[xx0+yy0])dx dy.

[0032] This is, except for the constant factor 1 / (iλf) and a phase factor that depends only on the coordinates (x0,y0), the Fourier transform of the field distribution in the lens plane z = 0. The intensity distribution in the focal plane, i.e., in the far-field image plane, is correspondingly... I(x0,y0,z=f)=|E(x0,y0,z=f)|2=1 / (λf)2|∫∫−∞∞E(x,y,z=0)exp(−i2πλf[xx0+yy0])dx dy|2.

[0033] The intensity distribution in other planes can be calculated similarly. From the measured field distribution E(x, y, z = 0), one obtains, so to speak, "subsequently" in every other image plane, thus allowing the object to be reconstructed in its entire depth.

[0034] The coherent recording of optical interference fields generally requires that the object and recording system remain stationary. More precisely, the interference pattern to be recorded should not smear during exposure. Displacements in the direction of the optical axis, i.e., in the z-direction, are therefore problematic. Δz≪λ To be required. For difference frequencies of Δv ≈ 100 MHz, integration times of mT = m / Δv ≈ 100 ns can be achieved. Thus, the following applies to the maximum permissible speeds in the z-direction. vzmT≪λ or (with λ = 1µm, m=10) v Z < 10 m / s = 36 km / h

[0035] On the other hand, movements in parts of the object occur at velocities of v Z = 10 m / s to Doppler frequency shifts of δv = vv Z / c0 = 10 MHz. Such axial velocities can be detected with detector systems tuned to modified difference frequencies (Δv + δv).

[0036] One challenge is capturing fine structures in the interference pattern in the (x,y) sensor plane, which is largely determined by the lateral extent of the illuminated object. The smallest structures Δx are generated by object rays incident on the sensor at the largest angle Δθ, measured to the optical z-axis. The superposition of these waves with the strong axial local oscillator wave produces patterns with a period Δx = λ / sin Δθ.

[0037] The considerations presented here demonstrate that coherent image acquisition systems already offer an attractive alternative for one-dimensional applications with near-point or only weakly divergent object illumination. These systems are characterized by extremely high receiver sensitivity, approaching the quantum noise limit. Object illumination is achieved with minimal intensity, resulting in extremely low energy consumption. This also ensures maximum eye safety. In addition to acquiring three-dimensional spatial coordinates (acquisition of the z-coordinate is not discussed here!), axial velocities can be directly detected as a function of location.

[0038] The analyses performed for the near-infrared range are fully transferable to the visible spectral range. In all the aforementioned ranges, diode laser systems with high electro-optical conversion efficiencies of over 30% and high coherence lengths of well over 100 m are available. The frequency shift required for the investigated heterodyne technology can be very effectively achieved by using acousto-optical modulators, which are also advantageously suited for generating short optical pulses and pulse trains with minimum pulse durations of 5 ns.

[0039] The object is illuminated with two spatially coherent light waves with different frequencies ω1 and ω2 but the same amplitudes E. 10(x, y, z) = |E1 (x, y, z, t)| and E2 (x, y, z) = |E2 (x, y, z, t)|, where the difference frequency (ω1 - ω2) / 2π ≈ 100 MHz is typically in the high-frequency range. We choose a plane reference or local oscillator wave that travels obliquely to the z-axis. EL(x,y,z,t)=ELO exp(i(ω1+Δω)t−2πi(vxLO1x+vyLO1y)−ikzLO1 z)+ELO exp(i(ω2+Δω)t−2πi(vxLO2x+vyLO2y)−ikzLO2z). and add more v yLO1 = v yLO2 = 0 and v xLO1 = v xLO2 = v xLO For the sake of simplicity, we consider a plane directly around the z-axis in the far field at z≈−z0 / 2,|z0|≫λ concentrated object from which to Fig. a backscattered paraxial wave emanates, which appears as a plane wave near the sensor at z=0. E(x,y,z,t)=|E0|exp(−ikz1(z−z0)+iω1t)+|E0|exp(−ikz2(z−z0)+iω2t) This can be approximated, and z0 takes into account the additional path the light has to travel to illuminate the object. The sensor dimensions are typically small compared to |z0|, and the intensity in the sensor plane z = 0 is I(x,y,t)= {EL(x,y,z=0,t)+E(x,y,z=0,t)}{EL(x,y,z=0,t)∗+E(x,y,z=0,t)∗}, where the superscript asterisk * denotes the notation for forming a complex conjugate. Calculation using (21) and (22) yields I(x,y,t)=ELO2(2+2 cos(ω2−ω1)t)+2ELO|E0|(cos(kz1z0−Δωt+2πvxLOx)+cos(kz1z0+(ω1−ω2−Δω)t+2πvxLOx ))+2ELO|E0|(cos(kz2z0−Δωt+2πvxLOx)+cos(kz2z0(ω2−ω1−Δω)t+2πvxLOx))+|E0|2(2+2 cos[(kz2−kz1)z0+(ω2−ω1)t]).

[0040] At the angular frequency Δω, the signal EΔω(t)=2ELO|E0|[cos(kz1z0−Δωt+2πvxLOx)+cos(kz2z0−Δωt+2πxLOx)]=2ELO|E0|[cos(kz1z0+2πvxLOx)c osΔωt+sin(kz2z0+2πvxLOx)sinΔωt]+2ELO|E0|[cos(kz2z0+2πvxLOx)cosΔωt+sin(kz2z0+2πvxLOx)sinΔωt]

[0041] Cross-correlation, i.e., multiplication by cos Δωt and time integration over an integer m of periods T = 1 / Δω, yields the signal (1mT)∫0mTEΔω(t)cosΔωt dt =2ELO|E0| cos[(kz2−kz1)z0 / 2] cos[2πvxLOx+(kz2+kz1)z0 / 2] ≈2ELO|E0| cos[(ω2−ω1)z0 / 2c0] cos[2πvxLOx+ω1z0 / 2c0], where c0 denotes the speed of light in a vacuum. On the synchronous sensor, according to the characteristic curve (11), a cosinusoidally modulated fringe pattern results on the uniform background intensity, which is determined primarily by the strong reference wave; more precisely, a beat signal in the x-direction, whose period is 1 / v. xLO depends on the spatial frequency of the reference wave, and its amplitude changes periodically with the depth coordinate z0 of the object. Accordingly, according to (13), the following relationship is obtained for the in-phase component of the measured photocurrent: <iphi(x,y,t)> =<iphi(x)> =(n0c0ε0)(qη0ℏω2 ELO2) +(qη1ℏω2 ELO|E0| cos(kz2−kz1)z0 / 2) cos[2πvxLOx+(kz2+kz1)z0 / 2)]).

[0042] Without object light, the photocurrent signal is determined solely by the strong local oscillator light. <iph0(x,y,t)> =(n0c0ε0)(qη0ℏω2 ELO2), which is to be assumed to be constant in time and space across the sensor area. The contrast K of the spatial beat signal in the x-direction is given by K(z0)=<iphi,max(x)> −<iphi,min(x)><iphi,max(x)> +<iphi,min(x)> =(|E0| / ELO)|cos((kz2−kz1)z0 / 2)|. Fig. Figure 3 shows a method for determining the depth coordinate of a point-illuminated, diffusely scattering object using a pmd image sensor. A spatially coherent bichromatic laser source is used for illumination. The scattered light is recorded using a heterodyne holographic method.

[0043] The contrast can be used to precisely determine z0, if, as in Fig. The process is schematically represented. The bichromatic light backscattered by the object is superimposed on a partial surface of the sensor with the bichromatic reference wave (21) and simultaneously on another separate part of the sensor with the monochromatic partial wave of (21), which is caused by ELmon(x,y,z,t)=ELO exp(i(ω1+Δω)t−2πi(vxLO1x+vyLO1y)−ikzLO1 z) is given. Evaluation with the modified reference wave as in (27) to (29) yields the superimposed wave (21) on the plane reference wave and evaluated simultaneously with (26). The modified signal corresponding to equation (26) is (1mT)∫0mTEΔω,mon(t)cosΔωt dt =ELO|E0| cos[2πvxLOx+kz1z0]=ELO|E0|cos[2πvxLOx+ω1z0 / 2c0].

[0044] The same applies to the measured photocurrent signal. <iphi,mon(x,y,t)> / (n0c0ε0) =(qη0ℏω2 ELO2)+(qη1ℏω2 ELO|E0|cos[2πvxLOx+kz1z0]), and with the modified contrast K mon (z0) = (|E0| / E LO) z0 can be determined from the ratio K(z0) / Kmon(z0)=|cos((kz2−kz1)z0 / 2)|=|cos[(ω2−ω1)z0 / 2c0]| determine. The contrast changes periodically with z0. The condition for maximum contrast is z. 0,2m = 2mπc0 / (ω2 - ω1), where m is an integer. Minimal contrast is found for z 0,(2m+1) = (2m + 1)πc0 / (ω2 - ω1).

[0045] The distance measurement method described for point-like object illumination can be easily extended to one-dimensional illumination patterns (stripes, lines) and two-dimensional area illumination. Spatially resolved distance determination remains guaranteed for convex objects as long as the waves incident on the sensor from the object and the plane reference wave satisfy condition (1).

[0046] Fig. Figure 4 shows a setup in which an object wave E reflected from an object Obj and a frequency-shifted reference wave E LoThe signal is directed towards a sensor and made to cause interference. The sensor, which is designed as a demodulator, demodulates the interference signal applied to it.

[0047] As already shown in formula (5): Etot=EObj+ELo Etot=aeiφ(x,y,z)e−iωt+Aeikre−iωt−iΔωt and for the intensity: I≈|E2|=a2+A2+Re 2aAeiφ(x,y,z)e−iωteikze−iωt−iΔωt with A >> a I≈A2+2A(x,y)cos(Δωt−φ(x,y)−kr)

[0048] According to the invention, to determine object distances, it is provided that the real part is detected with a sensor that demodulates the signal at the beat frequency Δω.

[0049] In the example according to Fig. 4. A laser emits coherent light with a frequency ω1 towards an object. To form a reference wave or frequency ω1', a portion of the light emitted by the laser is directed onto an optical modulator, here an acousto-optic modulator AOM, which shifts the incoming laser frequency by a modulation frequency Δω applied to the optical modulator. ω1'=ω1'+Δω

[0050] The sensor is designed as a synchronous demodulator and preferably operates according to a photon mixing principle or PMD principle and has an array of light time-of-flight pixels that detect the interference of the reference and object wave at the sensor in phase synchrony with the modulation frequency Δω.

[0051] Object distances can be determined based on the phase-synchronously determined signals.

[0052] The modulation frequency Δω is generated using a modulator designed in such a way that the modulation frequency Δω is provided to the optical modulator and the demodulator in phase synchrony or possibly shifted by a predetermined phase position.

[0053] Fig. Figure 5 shows a variant in which the frequency-shifted light wave is directed towards the object and the unshifted light wave serves as a reference.

[0054] Fig. Figure 6 shows a setup for bichromatic distance measurement according to Fig. 3. In the example shown, a first coherent light wave ω1 is generated using a laser, and a second coherent light wave with a second frequency ω2 is additionally generated via a downstream acousto-optic modulator 1. The acousto-optic modulator AOM1 is set to a modulation frequency Δω by means of a first modulator. M1applied in order to modulate a portion of the incoming light wave by the modulation frequency Δω of the AOM M1 The signal is shifted. At the output of the acousto-optic modulator AOM1, two light waves with different frequencies ω1 and ω2 are present, which illuminate an object. The two frequencies ω1 and ω2 advantageously differ by 100 to 1000 MHz; preferably, the frequency difference lies in the range of 200 to 400 MHz.

[0055] As an alternative to this frequency processing, two lasers with two different frequencies can also be used.

[0056] A portion of the light waves exiting the first acousto-optic modulator is directed to a second acousto-optic modulator AOM 2, which operates at a second modulation frequency Δω. M2 The modulator 2 is operated. The frequencies of the two light waves entering the AOM2 are changed by the frequency of the applied modulation frequency Δω. M2shifted and form the reference waves ω1', ω2', which are directed to the demodulator. The second modulation frequency Δω M2 The operating frequency range is advantageous from 1 to 100 MHz and particularly advantageous from 40 to 80 MHz. Depending on the design of the demodulator, higher modulation frequencies are also possible.

[0057] The other part of the light waves ω1, ω2 emerging from the first AOM1 is used to illuminate an object, whereby the object waves reflected by the object are directed to the demodulator in such a way that they interfere with the reference waves there.

[0058] The superposition of the two interference patterns is demodulated using the demodulator, and location information of the object is determined.

Claims

[1] Device for determining distance, with at least one light source for generating bichromatic light waves (E1, E2) with a first and second frequency (ω1, ω2), with a device for generating bichromatic reference waves (E Lo1 , E Lo2 ), which is designed in such a way that, starting from the bichromatic light waves (E1, E2), bichromatic reference waves (E Lo1 , E Lo2 ) shifted by a difference frequency (A C0M2 ) are generated with a first and second reference frequency (ω'1, ω'2), where the bichromatic light waves illuminate a scene or an object and are referred to as object waves (E Obj,1,2 ) be reflected upon, with a demodulator that uses the difference frequency (Δω M2 ) is operated, wherein the distance determination device is designed such that the demodulator is completely free from the object waves (E) on its receiving range. Obj,1,2) is illuminated, while the bichromatic reference wave (E Lo1 , E Lo2 ) only illuminate a first reception area of ​​the demodulator, wherein a second reception area is based on a monochromatic reference wave (E Lo1 ) is illuminated with a first frequency (ω'1), with an evaluation device designed in such a way that a distance is determined based on signals demodulated in the first and second reception range of the demodulator. [2] Device according to claim 1, wherein the distance is determined from a contrast difference of the signals demodulated in the first and second reception areas. [3] Device for determining distance, with at least one light source for generating bichromatic light waves (E1, E2) with a first and second frequency (ω1, ω2), with a device for generating bichromatic reference waves (E Lo1 , E Lo2), which is designed in such a way that, starting from the bichromatic light waves (E1, E2), bichromatic reference waves (E Lo1 , E Lo2 ) shifted by a difference frequency (A C0M2 ) are generated with a first and second reference frequency (ω'1, ω'2), where the bichromatic light waves illuminate a scene or an object and are referred to as object waves (E Obj,1,2 ) be reflected upon, with a demodulator that uses the difference frequency (Δω M2 ) is operated, wherein the device for distance determination is designed such that the demodulator is protected from the object waves (E) on its receiving range Obj,1,2 ) and the bichromatic reference waves (E Lo1 , E Lo2 ) is illuminated in a first measurement interval, in a second measurement interval the demodulator is exposed to the object waves (E) on its receiving range Obj,1,2 ) and a monochromatic reference wave (E Lo1) is illuminated with a first frequency (ω'1), with an evaluation device designed in such a way that a distance is determined based on the signals demodulated in the first and second measurement intervals. [4] Device according to claim 3, wherein the distance is determined from a contrast difference of the signals demodulated in the first and second measurement intervals. [5] Device according to any one of the preceding claims, with a first laser to generate a first coherent light wave (E1) with a first frequency (ω1), and with a second laser or a first acousto-optic modulator (AOM1), operated with a first modulation frequency (Δω) M1 ), to generate a second coherent light wave (E2) with a second frequency (ω2). [6] Device according to claim 5, wherein The scene is illuminated on the one hand by the bichromatic light waves or with the first and second light waves (E1, E2), and on the other hand by a second acousto-optic modulator (AOM2). wherein the second acousto-optic modulator (AOM2) is equipped with a second modulation frequency or the difference frequency (Δω) M2 ) is operated and two reference waves (E) are generated at the output of the second optical modulator (AOM2) Lo1 , E Lo2 ) with a first and second reference frequency (ω'1, ω'2) are available. [7] Device according to one of the preceding claims, wherein the demodulator is designed as a PMD sensor and is equipped with the second modulation frequency or difference frequency (Δω M2 ) is operated. [8] Device according to one of the preceding claims, wherein the frequency difference between the first and second light wave (E1, E2) is in a range of 100-1000 MHz. [9] Device according to one of the preceding claims, wherein the difference frequency (Δω M2 ), with which the demodulator is operated, lies in a range of 1-100 MHz.

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