Optical device and method for near and / or remote imaging, computer program product, data processing system, and signal processing unit

Abstract

The invention relates to a method and to an optical device (100) for near and / or remote imaging, having a housing (40) with a central connection unit (38), comprising at least three interchangeable optical modules (10) of the same type, of which each module (10) comprises a laser source (18) for feeding a first optical signal (41, 42, 43, 44) into an optical fibre (34) and a photodetector (28) for receiving a second optical signal (51, 52, 53, 54) via the optical fibre (34), wherein the optical modules (10) are electrically coupled (30) to the central connection unit (38), and also relates to a signal processing unit (30) for processing signals from the photodetectors (28) of the optical modules (10). The signal processing unit (30) is electrically coupled to the central connection unit (38). The invention also relates to a computer program product, to a data processing system (200), and to a signal processing unit.

Description

[0001] Description

[0002] Optical device and method for near and / or far imaging, computer program product, data processing system and signal processing unit

[0003] State of the art

[0004] The invention relates to an optical device and a method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, as well as a computer program product for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air using such a method, a data processing system and a signal processing unit.

[0005] Laser Doppler anemometry (LDA) is an established measurement technique for determining flow velocities and has wide-ranging applications in various fields of science and technology. In the context of aviation, LDA is being tested as an alternative optical air data sensor. Here, the technology offers the possibility of directly acquiring atmospheric parameters such as wind speeds. Highly coherent lasers, for example, those based on laser crystals, fibers, or semiconductor materials, are used as light sources.

[0006] A key element for the successful application of LDA in aviation is considering the aerosol concentration. Aerosols act as scattering particles for the laser beam and are therefore crucial for the frequency and accuracy of measurements. To account for this, adaptive algorithms and filtering techniques are used to detect signals even at very low aerosol concentrations. In wind tunnel applications, a technique called particle seeding is typically employed.

[0007] For example, DE 10 2021 100 788 A1 describes an optical device for near and far imaging for laser Doppler anemometry, comprising an optical unit with at least one first optical lens and at least one second optical lens, wherein the at least one first optical lens and at least one second optical lens are arranged along an optical axis. The optical device further comprises a transmitting and receiving unit with at least one, in particular substantially point-shaped, transmitting and receiving element for transmitting and receiving light rays through the optical unit, wherein a light transmission surface of the at least one transmitting and receiving element is arranged in or intersects at least one flat or curved surface.

[0008] EP 2 659 276 B1 further describes an optical angle-of-attack detector based on light detection and distance measurement for controlling an aerodynamic surface of a wind turbine generator.

[0009] Disclosure of the invention

[0010] The object of the invention is to provide an improved optical device for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air. A further object is to provide an improved method for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air.

[0011] Another task is to create a computer program product for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, using such a method.

[0012] Another task is to create a data processing system for executing such a procedure.

[0013] Another task is to create a signal processing unit for executing such a procedure.

[0014] The problems are solved by the features of the independent claims. Favorable embodiments and advantages of the invention become apparent from the further claims, the description, and the drawings.

[0015] According to one aspect of the invention, an optical device for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air, is proposed, comprising a housing with a central connection unit, comprising at least three identical interchangeable optical modules, each module comprising at least one laser source for injecting a first optical signal into an optical fiber and at least one photodetector for receiving a second optical signal via the optical fiber, wherein the at least three optical modules are electrically coupled to the central connection unit. Furthermore, the device comprises a signal processing unit for processing, in particular electrical, signals from the photodetectors of the at least three optical modules, wherein the signal processing unit is electrically coupled to the central connection unit.

[0016] In particular, near and / or far imaging can include Doppler evaluation of optical signals.

[0017] The proposed optical device enables the implementation of a vector-based LDA with at least three, typically four, optical modules, each optical module being self-contained and modularly interchangeable, realizing a complete LDA measurement channel. A measurement channel comprises at least one laser source and one photodetector.

[0018] The optical modules and / or measurement channels can be advantageously grouped together.

[0019] Advantages lie in the compact size and redundant design, allowing a defective LDA channel to be easily replaced. This also provides a safety feature, as the signals from the other LDA channels can be used if, for example, a laser source fails.

[0020] Due to its low weight, size, and the low power consumption of the required hardware, the optical device is particularly suitable for use as an airborne LDA system in aviation. Its high measurement rates and accuracy make it advantageous for determining the velocity and direction of aerosol ensembles under varying aerosol concentrations.

[0021] Advantageously, it is possible to measure the sign of a Doppler frequency shift without, for example, using an acousto-optic frequency shifter.

[0022] Furthermore, this method allows for sensitive detection of particle scattering through optimally adapted real-time data processing to maximize the measurement rate.

[0023] The proposed optical device is suitable for applications of flow measurement technology, particularly vectorial flow measurement, where optical access is possible, for example, via a window. Its advantages are especially evident in aircraft-borne applications, such as fixed-wing aircraft, rotary-wing aircraft, or drones.

[0024] In a favorable embodiment of the optical device, the at least three optical modules can each be optically connected via optical fibers to a common optical measuring head located outside the housing. This optical connection to a separate optical measuring head, acting as an optical probe for simultaneous imaging through an external window from at least three, typically four, fiber ends into the measuring volume, can be achieved with a favorably chosen imaging ratio of, for example, 1:2 to 1:10. The measuring head can, for example, include imaging optics and / or a small telescope.

[0025] According to a favorable embodiment of the optical device, the respective optical module can further comprise: a control unit, a laser power supply for the electrical supply of the laser source, wherein the laser power supply is controlled by the control unit, a direction-selective element which forwards the first optical signal into the optical fiber and / or into the at least one photodetector and forwards the second optical signal into the at least one photodetector, an optical superposition unit for superimposing the first and second optical signals, an optical input and / or output.

[0026] The optical module can advantageously include a control unit, which may be a microcontroller or a single-board computer. Furthermore, a laser current driver can be implemented, which can be controlled by the control unit. The laser source can be a master oscillator laser. The direction-selective element can be, for example, a circulator or a polarization beam splitter. The optical superposition unit can be an interferometer. The photodetector can optionally be equipped with an optical quadrature demodulator. In a favorable embodiment of the optical device, the optical superposition unit can include a compensating fiber to compensate for the optical path length of the interferometer arms of the optical superposition unit.In this way, by choosing a suitable laser source and compensating fiber in the interferometer, coherence length optimization can be carried out to match the optical modules to each other.

[0027] With a favorable design of the optical device, the respective optical module can further comprise: an optical amplifier for amplifying the first optical signal, and a thermal control system for the laser source, the thermal control being controlled by the control unit. A compact design can advantageously be achieved.

[0028] With a favorable design of the optical device, the central connection unit can be a computer system. In particular, the central connection unit can be a backplane computer system. For example, such a computer system, based on the so-called VPX standard, can be used to connect electronic components and link the optical modules.

[0029] With a favorable design of the optical device, the signal processing unit can include an FPGA and / or an ASIC for real-time digital signal processing. Alternatively or additionally, the at least three optical modules can be electrically coupled to an analog-to-digital converter, which is itself electrically coupled to the signal processing unit. For data processing, a field-programmable gate array (FPGA)-based digital signal processing (DSP) can be advantageously employed. It enables real-time processing and analysis of the acquired data. This allows for the advantageous use of modern algorithms, as well as Fourier transforms and spectral analysis, for signal processing.

[0030] With a favorable design of the optical device, the optical fiber connections can be spliced. Spliced ​​fiber connections can thus be advantageously used within the optical modules and / or measurement channels for a reliable and low-loss connection of the optical fibers.

[0031] With a favorable design of the optical device, the photodetector can be a pin photodiode. Such pin photodiodes (pin = positive intrinsic negative) can be advantageously used for radiation measurement and as receivers in fiber optic transmission technology.

[0032] In a favorable embodiment, the optical device can comprise at least four identical optical modules. In this way, in a vectorial LDA application requiring three measurement channels, at least one optical module can be provided as redundancy in case of a module failure. In a favorable embodiment, the optical device can be configured for vectorial laser Doppler anemometry. In particular, the optical device can be configured to determine the true airspeed, angle of attack, and sideslip angle of an aircraft during airborne applications. Advantageously, laser Doppler anemometry can thus be tested as an alternative optical air data sensor. The technology offers the possibility of directly acquiring atmospheric parameters such as wind speeds.

[0033] In a favorable embodiment of the optical device, the optical measuring head can be configured to emit at least three, preferably four, first optical signals, wherein the three, preferably four, first optical signals are each emitted at a non-zero angle to one another. Advantageously, vectorial LDA measurements can thus be performed by determining Doppler shifts.

[0034] According to a further aspect of the invention, a method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, is proposed using an optical device, comprising at least: emitting at least three first optical signals from laser sources of at least three optical modules via optical fibers; receiving at least three second optical signals generated by scattering the first optical signals; superimposing the second optical signals with their respective first optical signals to form at least three superimposed signals; receiving the at least three superimposed signals by means of photodetectors of the at least three optical modules, each of the optical modules having at least one photodetector; and digitizing the at least three superimposed signals to form at least three digitized signals.Simultaneous processing of at least three digitized signals using a signal processing unit and forwarding of a processing result to a central communication unit; and determination of a scattering event from the result.

[0035] The proposed method enables vectorial LDA measurements with at least three, typically four, optical signals, where each signal represents a full LDA measurement channel.

[0036] The optical signals or measurement channels can be conveniently grouped together.

[0037] Advantages lie in the compact design and redundant configuration, allowing for easy replacement of a defective LDA channel. This also provides a safety benefit, as the signals from other LDA channels can be used if, for example, one signal fails. The method can be advantageously used in aviation, for instance, via an airborne LDA system.

[0038] Due to its high measurement rates and accuracy, this method can be advantageously used to determine the speed and direction of aerosol ensembles under varying aerosol concentrations.

[0039] Advantageously, it is possible to measure the sign of a Doppler frequency shift without an acousto-optic frequency shifter.

[0040] Furthermore, this method allows for sensitive detection of particle scattering through optimally adapted real-time data processing to maximize the measurement rate.

[0041] The proposed method is suitable for applications of flow measurement technology, particularly vectorial flow measurement, where optical access is possible, for example, via a window. Its advantages are especially evident in aircraft-borne applications, such as fixed-wing aircraft, rotary-wing aircraft, or drones.

[0042] In a favorable embodiment of the method, at least three first optical signals from laser sources of at least three optical modules can be emitted via optical fibers and a common measuring head, and at least three second optical signals, generated by scattering the first optical signals, can be received via a common measuring head. An optical measuring head separate from the LDA measuring system is used as an optical probe for simultaneous imaging through an external window. The measuring head can, for example, include imaging optics and / or a small telescope.

[0043] In a favorable embodiment of the method, a number M of Fourier transforms can be applied in parallel to the individual digitized signals to determine a spectral power density, whereby the length of each Fourier transform is increased by a factor of 2', where M is a natural number and i = 1, ..., M. Probability density distributions for efficient signal processing can be obtained using the Fourier transforms. Advantageously, M can be at least 2 or 3.

[0044] In a favorable embodiment of the method, the digitized signals can be multiplied by two time-domain window functions, offset by half a window width. From the digitized signals multiplied by the window functions, spectral power densities can then be determined using two Fourier transforms, each offset by half the length of the Fourier transform. The window functions advantageously reduce spectral leakage.

[0045] In a favorable embodiment of the method, the window functions can be selected such that, for an overlap of the window functions by half the window width, the temporal change in the amplitudes of the digitized signals multiplied by the window functions is minimized. This allows for the advantageous minimization of noise components in the signals. In a favorable embodiment of the method, the digitized signals can be multiplied sequentially by the two window functions. Subsequent averaging of the digitized signals then allows for the advantageous minimization of noise components in the signals.

[0046] With a favorable implementation of the method, the digitized signals can be multiplied over time by the two window functions. This allows for the advantageous minimization of noise components in the signals.

[0047] With a favorable implementation of the method, the two spectral power densities can be averaged to obtain a single averaged spectral power density. This leads to a significant modification of the noise statistics, which has a beneficial effect on the detectability of events.

[0048] With a favorable embodiment of the method, the scattering event in the two spectral power densities can be determined using a threshold value. In this way, it is advantageous to decide which signal segments contain a scattering event, so that only these signal segments are passed further through the processing chain.

[0049] With a favorable implementation of the method, the threshold can be chosen to remain constant over time. Alternatively or additionally, the threshold can be chosen to be frequency-dependent. This allows interfering frequencies in the probability density distribution to be advantageously suppressed. With a favorable implementation of the method, the threshold can be chosen adaptively as a function of frequency. Alternatively or additionally, the threshold can be dynamically adjusted. Such a threshold determination can, for example, be more resistant to so-called clutter or interference.

[0050] With a favorable embodiment of the method, a Doppler frequency of the scattering event can be determined from the averaged spectral power density, from which a line-of-sight velocity can be calculated. This makes the method advantageously suitable for the application of an air data sensor in aviation.

[0051] According to a further aspect of the invention, a computer program product for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, is proposed using a device described above. The computer program product comprises at least one computer-readable storage medium containing program instructions that are executable on a computer system and cause the computer system to execute such a method. Advantageously, the optical device can thus be used with the method for an air data sensor in aviation.

[0052] According to a further aspect of the invention, a data processing system for executing a data processing program is proposed, which includes computer-readable program instructions for carrying out a method described above for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air. Advantageously, the optical device with the method can thus be used for an air data sensor in aviation. According to a further aspect of the invention, a signal processing unit is proposed, which includes computer-readable program instructions for carrying out a method described above for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air.

[0053] Depending on the design, the signal processing unit can be implemented as an integrated microelectronic component, in particular as an FPGA and / or as an ASIC.

[0054] Advantageously, the evaluation of the optical signals acquired by the optical device can be performed in the signal processing unit. The corresponding signal processing algorithms, such as Fourier transforms, windowing, averaging, and threshold detection, can be implemented on an integrated microelectronic component, particularly an FPGA and / or an ASIC. This allows the optical device to be implemented in a very compact and cost-effective manner. Therefore, the optical device can be advantageously used in conjunction with this method for an air data sensor in aviation.

[0055] drawing

[0056] Further advantages will become apparent from the following description of the drawings. The figures illustrate exemplary embodiments of the invention. The figures, the description, and the claims contain numerous features in combination. It is advantageous for those skilled in the art to also consider the features individually and combine them into meaningful further combinations. The following are shown as examples:

[0057] Fig. 1 shows a system architecture of an optical device for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, according to an embodiment of the invention;

[0058] Fig. 2 shows a block diagram of an optical module of the optical device according to Fig. 1;

[0059] Fig. 3 shows a schematic representation of the installation situation of an optical device in an aircraft in a front view of the aircraft;

[0060] Fig. 4 shows a schematic representation of the installation situation of the optical device according to Fig. 3 in a side view of the aircraft;

[0061] Fig. 5 shows a flowchart for real-time data processing according to the method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, according to an embodiment of the invention;

[0062] Fig. 6 shows a digitized signal with noise and a processed signal without noise, together with a layer of two window functions;

[0063] Fig. 7 shows a processed signal without noise and two window functions; Fig. 8 shows a probability density distribution for an exponential distribution and for an averaged signal;

[0064] Fig. 9 shows a cumulative probability density distribution for an exponential distribution and for an averaged signal; and

[0065] Fig. 10 processed measurement data together with a time-constant threshold function and a dynamically adjusted threshold function.

[0066] Embodiments of the invention

[0067] In the figures, similar or equivalent components are numbered with the same reference symbols. The figures merely show examples and are not to be understood as limiting.

[0068] The directional terminology used below, including terms like "left," "right," "above," "below," "in front," "behind," "after," and the like, serves only to improve the understanding of the figures and is in no way intended to limit their generality. The components and elements depicted, their interpretation, and their use may vary according to the considerations of a person skilled in the art and be adapted to the specific applications.

[0069] Figure 1 shows a system architecture of an optical device 100 for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air, according to an embodiment of the invention. The optical device 100 can, for example, be configured for vectorial laser Doppler anemometry. The optical device 100 comprises a housing 40 with a central connection unit 38. The central connection unit 38 can be configured as a computer system 200, in particular as a backplane computer system 200, for example according to the VPX standard.

[0070] The housing 40 contains four identical optical modules 10, which are electrically coupled to the central connection unit 38. Each module 10 (see Figure 2) comprises at least one laser source 18 for injecting a first optical signal 41, 42, 43, 44 into an optical fiber 34 and at least one photodetector 28 for receiving a second optical signal 51, 52, 53, 54 via the optical fiber 34.

[0071] The photodetector 28 can, for example, have a pin photodiode and / or optionally be designed with an optical hybrid.

[0072] Furthermore, a signal processing unit 30 for processing, in particular electrical, signals from the photodetectors 28 of the individual optical modules 10 is arranged in the housing 40. The signal processing unit 30 is also electrically coupled to the central connection unit 38.

[0073] The signal processing unit 30 may expediently include an FPGA and / or an ASIC for digital real-time signal processing.

[0074] The optical modules 10 are each optically connected via optical fibers 34 to a common optical measuring head 50 located outside the housing 40. Optionally, in an embodiment not shown, the fibers 34 can also be distributed instead of being located in the optical measuring head 50, for example, on the surface of a missile on which the device 100 is mounted. Furthermore, an analog-to-digital converter 32 is arranged in the housing 40, to which the optical modules 10 are electrically coupled. The analog-to-digital converter 32, in turn, is electrically coupled to the signal processing unit 30.

[0075] Optical fiber connections of the optical fibers 34 can advantageously be spliced.

[0076] Figure 2 shows a block diagram of one of the four identical optical modules 10 of the optical device 100 according to Figure 1.

[0077] The optical module 10 comprises a control unit 12, a laser power supply 16 for the electrical supply of the laser source 18, and the laser source 18 itself. The laser power supply 16 is controlled by the control unit 12.

[0078] Optionally, the optical module 10 can have a thermal control 14 for the laser source 18, wherein the thermal control 14 is controlled by the control unit 12.

[0079] The laser source 18 emits a first optical signal 41 to the optical measuring head 50, which is located outside the optical module 10 and also outside the housing 40 of the optical device 100. The measuring head 50 can, for example, comprise an imaging optic and / or a small telescope.

[0080] Optionally, the first optical signal 41 can be amplified by means of an optical amplifier 22. The optical module 10 further comprises a direction-selective element 24, which directs the first optical signal 41 into the optical fiber 34. The second optical signal 51 is then transmitted via the direction-selective element 24 from the measuring head 50 through an optical superposition unit 26 to the at least one photodetector 28. The direction-selective element 24 can, for example, be configured as a circulator or a polarization beam splitter.

[0081] An optical superposition unit 26 is arranged as an interferometer in the optical module 10, which superimposes the first and second optical signals 41, 51. The superimposed signal 61 is then directed to the photodetector 28. The first optical signal 41 is fed to the optical superposition unit 26 via a compensating fiber 20 to compensate for the optical path length of the interferometer arms of the optical superposition unit 26. Advantageously, the length of the compensating fiber 20 can be selected such that both interferometer arms of the optical superposition unit 26 are of similar length with respect to the measurement volume, for example, so that the difference between the two interferometer arms is less than 1 / 10 of the coherence length. The compensating fiber 20 can expediently be arranged within the optical superposition unit 26.

[0082] Alternatively, the first optical signal 41 can also be guided into the optical superposition unit 26 via the direction-selective element 24. In a further embodiment, not shown, a back reflection prior to the exit of the first optical beam 41 from the end of the optical fiber 34 can be used as a reference signal for the interferometric measurement in the optical superposition unit 26. The back reflection can, for example, be used in a measuring head 50 as a reference signal for the interferometric measurement in the optical superposition unit 26.

[0083] The optical module 10 has an optical input and / or output 36 for routing the first optical signal 41 to the measuring head 50 and for routing the second optical signal from the measuring head 50.

[0084] According to the proposed method for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air with the optical device 100, four first optical signals 41, 42, 43, 44 are emitted from the laser sources 18 of the four optical modules 10 via the optical fibers 34. This can be done, for example, via the optical measuring head 50, if present.

[0085] Subsequently, four second optical signals 51, 52, 53, 54, generated by the scattering of the first optical signals 41, 42, 43, 44, are received and forwarded via the direction-selective element 24 to the optical superposition unit 26. This can, for example, be done via the optical measuring head 50, if present.

[0086] In the optical superposition unit 26, the second optical signals 51, 52, 53, 54 are superimposed with their respective first optical signals 41, 42, 43, 44, which can be supplied from the laser source 18 via compensating fibers 20, to form four superimposed signals 61, 62, 63, 64. The four superimposed signals, of which only 61 is indicated, are received by the respective photodetectors 28 of the four optical modules 10.

[0087] The four superimposed signals 61 are digitized into four digitized signals 71, 72, 73, 74.

[0088] The four digitized signals 71, 72, 73, 74 are then processed simultaneously by the signal processing unit 30, and the result of the processing is forwarded to the central connection unit 38. In the central connection unit 38, which can be configured as a computer system 200, a scattering event 80 (see Figure 10) is determined from the result.

[0089] Figure 3 shows a schematic representation of the installation situation of an optical device 100 in an aircraft 300 in a front view of the aircraft 300 and Figure 4 shows a side view of the aircraft 300.

[0090] The optical device 100 is located in the nose of the aircraft and is not visible from the outside. Figures 3 and 4 show only the first optical signals 41, 42, 43, 44, which are each emitted from the optical measuring head 50 at a non-zero angle to each other. A possible wind flow 46 is shown in Figure 4 at a non-zero angle to a longitudinal axis 310 of the aircraft 300.

[0091] The optical device 100 can thus be used in airborne applications, for example as an air data sensor in the aircraft 300, to determine true airspeed, angle of attack, and sideslip angle. Figure 5 shows a flowchart for real-time data processing according to the method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, according to an embodiment of the invention.

[0092] The flowchart describes the real-time signal processing chain within the FPGA / ASIC of the signal processing unit 38, which scans the signal from the photodetector 28 for light pulses from scattering events. This allows for a reduction in data bandwidth from greater than 1 Gbit / s to data rates below 10 Mbit / s for typical atmospheric conditions. This, in turn, reduces the demands on subsequent processing units. Furthermore, statistically optimized detection of the scattering events with an adjustable false alarm rate can be achieved.

[0093] The superimposed signal 61, 62, 63, 64 is, after digitization in the analog-to-digital converter 32, divided into the digitized signal 71, 72, 73, 74 and distributed among at least one, typically three, event detectors, and processed by them in parallel. Each event detector performs two Fourier transforms 65, 66 on the data stream, offset by exactly half the length 67 of the Fourier transform 65, 66, and the data stream is multiplied beforehand by a window function 75, 76. The window function 75, 76 can advantageously be chosen such that for an overlap of 50% it achieves an amplitude flatness of 100%, i.e., for an overlap of the window functions 75, 76 by half the window width 77, a temporal change of amplitudes of the digitized signals 71, 72, 73, 74 multiplied by the window functions 75, 76 is minimized.The digitized signals 71, 72, 73, 74 are each multiplied by two time window functions 75, 76, which are offset by half a window width 77. From the digitized signals 71, 72, 73, 74 multiplied by the window functions 75, 76, spectral power densities 82, 83 are determined using the two Fourier transforms 65, 66, where the Fourier transforms 65, 66 are each offset by half a length 67 of the Fourier transform 65, 66.

[0094] The digitized signals 71, 72, 73, 74 can advantageously be multiplied successively, for example in a time-shifting manner, by the two window functions 75, 76.

[0095] The results of the two Fourier transformations 65, 66 are subjected to an averaging process 94 and a threshold detection 95.

[0096] The two spectral power densities 83, 83 are averaged together to form an averaged spectral power density 84.

[0097] The scattering event 80 (Figure 10) can then be determined in the two spectral power densities 82, 83 by means of a threshold value 86, 87. The threshold value 86 can be chosen to be constant over time and / or frequency-dependent.

[0098] Alternatively, the threshold value 87 can be adaptively selected depending on the frequency and / or dynamically adjusted. The Fourier transforms 65, 66 are first applied to the individual digitized signals 71, 72, 73, 74 with a length 67 of the Fourier transforms 65, 66. This procedure is then performed in parallel with M further Fourier transforms 65, 66, each with a length 67 increased by a factor of 2', where M is a natural number and i = 1, ... M.

[0099] The results of the respective threshold detections 95 of the M signal processing processes are combined in the frequency estimation 96 for the determination of the Doppler frequency.

[0100] Figure 6 shows signal values ​​78 of a digitized signal 71 with noise (dashed thin line) and a processed signal 81 without noise (solid thick line) together with the position of two window functions 75, 76 as a function of time 70. The window width 77 is the same in each case.

[0101] It can be seen that the relative position of a scatter signal can be random relative to the overlapping windows 75, 76. This illustrates the extraction of the signal for processing. Immediately before each window 75, 76 lies the next window function 75, 76 without a data gap.

[0102] Figure 7 shows a processed signal 81 without noise (solid line) and two window functions 75, 76 (dotted line and dashed line, respectively). A value 79 of the window functions 75, 76 is plotted as a function of time 70. The figure illustrates, for a signal 81 without noise, what the window function 75, 76 might look like when multiplied by the signal 81. The window function 75, 76 serves to reduce spectral leakage.

[0103] The spectral power density is determined from the Fourier-transformed signals, and the two spectra are averaged (averaging process 94 in Figure 5). This leads to a significant modification of the noise statistics, which has a beneficial effect on the detectability of events. If the noise of the signal in the time domain is assumed to be normally distributed white noise (e.g., shot noise), the distribution of the noise in the spectral power density can be described by an exponential distribution. For threshold-based detection (threshold detection 95 in Figure 5), this distribution is disadvantageous because the probability of high noise values ​​asymptotically approaches zero exponentially, but false detections cannot be ruled out. Simply averaging two spectra modifies the probability distribution to a chi-squared distribution. 2 -Distribution with four degrees of freedom.

[0104] Figure 8 shows a probability density distribution 88 for an exponential distribution 97 and for an averaged signal 84. Figure 9 shows a cumulative probability density distribution 89 for the exponential distribution 97 and for the averaged signal 84. Figures 8 and 9 illustrate the probability density distribution 88 and the cumulative probability 89 as a function of the spectral noise power density 91 for the cases of the exponential distribution 97 (no averaging) and the averaging of two spectra 82, 83 to obtain an averaged

[0105] Probability density 84 (see Figure 5) according to a Chi 2 A distribution with 4 degrees of freedom. An expected value of 85 is shown as a dotted line. The shape of the probability density distribution 88 changes significantly and leads to a faster asymptotic decline.

[0106] This is more clearly expressed in the cumulative probability density 89 in Figure 9. This value can be interpreted as the proportion of values ​​that are statistically present up to this value. Here, too, it is evident that the cumulative probability density 89 approaches the value 1.0 more quickly when two spectra 82 and 83 are averaged to obtain an averaged probability density 84, which reduces the probability of false positives.

[0107] The data processed in this way can be further processed by a threshold detection 95 based on a so-called CFAR detector (constant false alarm rate) to decide which signal sections contain a scattering event and only send these through the processing chain.

[0108] Figure 10 shows processed measurement data 84 as a spectral power density 92 versus frequency 90, together with a time-constant threshold function 86 and a dynamically adapted threshold function 87. The figure illustrates two possible variants of threshold detection. In the simple case, a time-constant threshold 86 is used, which can be selected as a function of frequency to suppress interfering frequencies.

[0109] A second variant uses more complex CFAR algorithms that adapt to the specific spectral shape and are more resistant to certain forms of so-called clutter or interference, dynamically adjusting the threshold 87.

[0110] If a signal spectrum is identified as a scattering event 80 in this way, it is forwarded to frequency estimation 96 (Figure 5). Here, the Doppler frequency can be determined from the spectral signature 92, from which a line-of-sight velocity can be directly calculated. The Doppler frequency of the scattering event 80 can conveniently be determined from the averaged spectral power density 84.

[0111] The proposed method can advantageously be implemented in a computer program product comprising at least one computer-readable storage medium, which contains program instructions executable on a computer system 200 and which cause the computer system 200 to execute the method. The computer system 200 can, for example, comprise or be integrated into the central communication unit 38.

[0112] Advantageously, a data processing system 200 can be used to execute a data processing program that includes computer-readable program instructions for carrying out the procedure. The data processing system 200 can, for example, comprise or be integrated into the central connection unit 38 of the optical device 100 (see Figure 1).

[0113] Furthermore, the signal processing unit 30 of the optical device can include 100 computer-readable program commands to carry out the above-described method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air.

[0114] Advantageously, the evaluation of the optical signals 51, 52, 53, 54 recorded by the optical device 100 can be carried out in the signal processing unit 30. The corresponding signal processing algorithms, such as Fourier transforms, windowing, averaging, and threshold detection, can be implemented on an integrated microelectronic component, in particular an FPGA and / or an ASIC.

[0115] Reference sign

[0116] 10 optical module

[0117] 12 Control unit

[0118] 14 Thermal control

[0119] 16 Laser power supply

[0120] 18 Laser source

[0121] 20 compensating fiber

[0122] 22 optical amplifiers

[0123] 24 direction-selective element

[0124] 26 optical superposition unit

[0125] 28 Photodetector

[0126] 30 Signal processing unit

[0127] 32 Analog-to-Digital Converters

[0128] 34 optical fibers

[0129] 36 optical input / output

[0130] 38 central connection unit

[0131] 40 cases

[0132] 41 first optical signal

[0133] 42 first optical signal

[0134] 43 first optical signal

[0135] 44 first optical signal

[0136] 46 Wind flow

[0137] 50 optical measuring head

[0138] 51 second optical signal

[0139] 52 second optical signal

[0140] 53 second optical signal

[0141] 54 second optical signal

[0142] 61 superimposed signals

[0143] 65 Fourier transform

[0144] 66 Fourier transform

[0145] 67 Length Fourier transform Time Digitized signals Digitized signals Digitized signals Digitized signals Window function Window function Window width Signal value Value of the window function Scattering event Signal Spectral power density Spectral power density Averaged spectral power density Expected value Threshold Threshold Probability density Cumulative probability density Frequency Spectral noise power density Spectral power density Averaging Threshold detection Frequency estimation Exponential distribution Optical device Computer system Aircraft Longitudinal axis

Claims

Claims 1. Optical device (100) for near and / or far imaging, in particular for determining the velocity and direction of aerosol ensembles in air, comprising a housing (40) with a central connection unit (38), comprising at least three identical interchangeable optical modules (10), each module (10) comprising at least one laser source (18) for injecting a first optical signal (41, 42, 43, 44) into an optical fiber (34) and at least one photodetector (28) for receiving a second optical signal (51, 52, 53, 54) via the optical fiber (34), wherein the at least three optical modules (10) are electrically coupled to the central connection unit (38), and a signal processing unit (30) for processing, in particular electrical, signals from the photodetectors (28) of the at least three optical modules (10), wherein the signal processing unit (30) is electrically coupled to the central connection unit (38). is.

2. Optical device according to claim 1, comprising at least three optical modules (10) each being optically connected by means of the optical fibers (34) to a common optical measuring head (50) arranged outside the housing (40).

3. Optical device according to claim 1 or 2, wherein the respective optical module (10) further comprises: a control unit (12), a laser power supply (16) for supplying electrical power to the laser source (18), wherein the laser power supply (16) is controlled by the control unit (12), a direction-selective element (24) which transmits the first optical signal (41, 42, 43, 44) into the optical fiber (34) and / or into the at least one photodetector (28) and transmits the second optical signal (51, 52, 53, 54) into the at least one photodetector (28), an optical superposition unit (26) for superimposing the first and second optical signals (41, 42, 43, 44; 51, 52, 53, 54), an optical input and / or output (36).

4. Optical device according to claim 2, wherein the optical superposition unit (26) has a compensating fiber (20) for compensating an optical path length of interferometer arms of the optical superposition unit (26).

5. Optical device according to one of the preceding claims, wherein the respective optical module (10) further comprises: an optical amplifier (22) for amplifying the first optical signal (41 , 42, 43, 44), a thermal control (14) for the laser source (18), wherein the thermal control (14) is controlled by the control unit (12).

6. Optical device according to one of the preceding claims, wherein the central connecting unit (38) is a computer system (200), in particular wherein the central connecting unit (38) is a backplane computer system (200).

7. Optical device according to one of the preceding claims, wherein the signal processing unit (30) comprises an FPGA and / or an ASIC for digital real-time signal processing and / or wherein the at least three optical modules (10) are electrically coupled to an analog-to-digital converter (32) which is electrically coupled to the signal processing unit (30).

8. Optical device according to one of the preceding claims, wherein the optical fiber connections of the optical fibers (34) are spliced.

9. Optical device according to one of the preceding claims, wherein the photodetector (28) is a pin photodiode.

10. Optical device according to one of the preceding claims, comprising at least four identical optical modules (10).

11. Optical device according to one of the preceding claims, configured for vectorial laser Doppler anemometry, in particular configured to determine a true airspeed, angle of attack and sideslip angle of an aircraft (300) in airborne use.

12. Optical device according to claim 10 or 11, wherein the optical measuring head (50) is configured to emit at least three, preferably four, first optical signals (41, 42, 43, 44), wherein the three, preferably four, first optical signals (41, 42, 43, 44) are each emitted at a non-zero angle to each other.

13. Method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air with an optical device (100) according to one of the preceding claims, at least comprising Emitting at least three first optical signals (41 , 42, 43, 44) from laser sources (18) of at least three optical modules (10) via optical fibers (34); Receiving at least three second optical signals (51, 52, 53, 54) resulting from the scattering of the first optical signals (41, 42, 43, 44); Superimposing the second optical signals (51 , 52, 53, 54) with the respective first optical signals (41 , 42, 43, 44) to form at least three superimposed signals (61 , 62, 63, 64); Receiving the at least three superimposed signals (61 , 62, 63, 64) by means of photodetectors (28) of the at least three optical modules (10), wherein each of the optical modules (10) has at least its photodetector (28); Digitizing the at least three superimposed signals (61 , 62, 63, 64) to at least three digitized signals (71 , 72, 73, 74); Simultaneous processing of at least three digitized signals (71 , 72, 73, 74) by means of a signal processing unit (30) and forwarding a result of the processing to a central connection unit (38); Determining a scattering event (80) from the result.

14. Method according to claim 13, wherein the emission of at least three first optical signals (41, 42, 43, 44) from laser sources (18) of at least three optical modules (10) via optical fibers (34) and a common measuring head (50) and the reception of at least three, by scattering the first optical signals (41, 42, 43, 44) created, second optical signals (51 , 52, 53, 54) and a common measuring head (50).

15. Method according to claim 13 or 14, wherein to determine a spectral power density (82, 83) a number (M) of Fourier transforms (65, 66) is applied in parallel to the individual digitized signals (71 , 72, 73, 74), wherein a length (67) of the Fourier transforms (65, 66) is each extended by a factor of 2', wherein M is a natural number and i = 1 , ... , M.

16. Method according to any one of claims 13 to 15, wherein the digitized signals (71, 72, 73, 74) are multiplied by two temporal window functions (75, 76) which are offset by half a window width (77), wherein spectral power densities (82, 83) are determined from the digitized signals (71, 72, 73, 74) multiplied by the window functions (75, 76) by means of two Fourier transforms (65, 66), wherein the Fourier transforms (65, 66) are each offset by half the length (67) of the Fourier transform (65, 66).

17. Method according to claim 16, wherein the window functions (75, 76) are selected such that for an overlap of the window functions (75, 76) by half the window width (77) a temporal change of amplitudes of the digitized signals (71 , 72, 73, 74) multiplied by the window functions (75, 76) is minimized.

18. Method according to claim 16 or 17, wherein the digitized signals (71 , 72, 73, 74) are successively multiplied by the two window functions (75, 76).

19. Method according to one of claims 13 to 18, wherein the digitized signals (71 , 72, 73, 74) are multiplied over time by the two window functions (75, 76).

20. Method according to any one of claims 13 to 19, wherein the two spectral power densities (82, 83) are averaged together to form an averaged spectral power density (84).

21. Method according to one of claims 13 to 20, wherein the scattering event (80) in the two spectral power densities (82, 83) is determined by means of a threshold value (86, 87).

22. Method according to claim 21, wherein the threshold (86) is chosen to be constant over time and / or wherein the threshold (86) is chosen to be frequency-dependent.

23. Method according to claim 21, wherein the threshold (87) is selected adaptively depending on the frequency and / or wherein the threshold (87) is dynamically adjusted.

24. Method according to one of claims 13 to 23, wherein a Doppler frequency of the scattering event (80) is determined from the averaged spectral power density (84), from which a line-of-sight velocity is determined.

25. Computer program product for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, with a device according to one of claims 1 to 12, wherein the computer program product comprises at least one computer-readable storage medium which includes program instructions that are executable on a computer system (200) and cause the computer system (200) to execute a method according to at least one of claims 13 to 24.

26. Data processing system (200) for executing a data processing program comprising computer-readable program instructions for executing a method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, according to at least one of claims 13 to 24.

27. Signal processing unit (30) for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, comprising computer-readable program instructions for carrying out a method for near and / or far imaging, in particular for determining the speed and direction of aerosol ensembles in air, according to at least one of claims 13 to 24.

28. Signal processing unit (30) according to claim 27, configured as an integrated microelectronic component, in particular FPGA and / or ASIC.

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