ELECTRO-OPTIC DISTANCE MEASURING SOUNDER AND DISTANCE MEASURING METHOD

Description

[0001] The invention relates to an electro-optical rangefinder according to claim 10 and a method for measuring distance according to claim 1.

[0002] Various principles and methods are known in the field of electronic and electro-optical distance measurement. One approach involves emitting pulsed electromagnetic radiation, such as laser light, at a target whose distance needs to be determined, and subsequently receiving an echo from this target as a backscattering object. Visible light can be used to make the point targeted for measurement visible on the target object. The reflected optical radiation is converted into an electrical signal by a photosensitive element within the device. Often, optical components for beam shaping, deflection, filtering, etc., such as lenses, wavelength filters, mirrors, etc., are located in the optical transmission and reception paths.

[0003] The distance to the target being measured can be determined, for example, based on the pulse's transit time or transmission pattern, and / or the pulse's phase within a modulation cycle. Such laser distance meters have become standard solutions in many fields, such as geodesy and industrial surveying, for example, in the form of total stations, laser scanners, EDMs, or laser trackers. To achieve a correspondingly high accuracy in distance measurement, the requirements for temporal resolution are quite high due to the high propagation speed of optical radiation in free space. For example, at typical measurement distances, a temporal resolution with an accuracy of at least approximately 6.6 picoseconds is required for a distance resolution of 1 mm or significantly less.

[0004] For the clearest possible and therefore most precisely analyzable received signal, the highest possible output power is desirable. However, the electro-optical devices discussed here are subject to limitations regarding the emittable signal power. For example, when emitting laser light, eye safety determines a maximum permissible average signal power that may be emitted. To still obtain sufficiently strong signal intensities for measurement that are detectable by the receiver, pulsed operation is preferred. Short pulses with high peak power are emitted, followed by pauses without signal transmission. Thus, the reflected pulse component has a sufficiently high intensity to be evaluated against background interference and noise, especially in the presence of background light (sunlight, artificial lighting, etc.), with a high signal-to-noise ratio.

[0005] Well-known light sources with system-synchronous pulse or modulation sequences of high peak power include, for example, electronically pulsed laser diodes or superluminescent LEDs combined with optical amplifiers. All these light sources have the disadvantage that the pulses are several hundred picoseconds long and, when measuring on rough natural surfaces, generate distance measurement errors of typically 0.1 mm to 2 mm due to spatial and chromatic irregularities.

[0006] Q-switched solid-state lasers are well-known, with relatively short pulses down to the range of 200 ps and a pulse rate of a few MHz. A disadvantage, however, is the temporal noise of the pulse rate, which typically amounts to 1% to 5% of the pulse interval. Handling this irregularity in precise distance measurement systems requires a complex device and a correspondingly sophisticated evaluation method.

[0007] In the field of pulsed interferometric distance measurement, distance meters with stabilized frequency comb lasers are also known, e.g., from DE 10 2009 012 646 A1. These feature tunable resonators for high-precision distance measurements (ppm accuracy) to stabilize the pulse rate or pulse frequency—typically at pulse lengths around 100 fs—so that it is regulated with ppm accuracy, as well as CEP (carrier envelope phase) stabilization. This stabilization ensures constancy of the optical wavelength and phase. However, a disadvantage of such devices and methods is their high complexity and the associated high costs associated with manufacturing, use, and maintenance.

[0008] To determine the propagation time of the signal, one known method is the so-called phase measurement principle, which determines the signal propagation time by comparing the phase position of the amplitude modulation of the transmitted and received signal.

[0009] On the other hand, the so-called Time-of-Flight (TOF) method is known, which determines the time between the transmission and reception of a light pulse. This time measurement is based on the edge, the peak value, or another characteristic of the pulse shape. Pulse shape refers to the temporal profile of the light intensity of the received signal, specifically the received light pulse, as detected by the photosensitive element. The transmission time can be determined either by an electrical trigger pulse, the signal applied to the transmitter, or the aforementioned reference signal.

[0010] In distance measurement, ambiguities can arise if the signal propagation time exceeds the inverse of the pulse transmission rate, resulting in multiple identical signals traveling simultaneously between the device and the object being measured. This makes it impossible to unambiguously assign a received pulse to its corresponding transmitted pulse. Without further measures, it is therefore unclear whether the distance or the partial distance (the remainder divided by the transmitted pulse period) was measured.

[0011] To detect the backscattered pulse, two different approaches or a combination thereof are usually used.

[0012] In the time-of-flight method, based on the so-called threshold principle, a light pulse is detected when the intensity of the radiation incident on a detector of the distance measuring device exceeds a certain threshold. This threshold prevents background noise and interference from being falsely detected as the useful signal, i.e., as backscattered light from the emitted pulse.

[0013] The other approach of the time-of-flight method is based on sampling the backscattered pulse. This approach is typically used for weak backscattered signals (e.g., pulse signals), such as those caused by larger measurement distances, or generally to increase measurement accuracy. An emitted signal is detected by sampling the radiation captured by a detector, identifying a signal within the sampled area, and finally determining the signal's position in time. By using a large number of samples and / or summing the received signal synchronously with the emission rate, a useful signal can be identified even under unfavorable conditions, thus enabling the detection of larger distances or noisy or interference-laden background scenarios. With this approach of very precise temporal sampling,Following the sampling of the backscattered signal, the electrical signal generated by the detector is converted into a digital signal sequence using an analog-to-digital converter (ADC).

[0014] In fast analog-to-digital converters (ADCs), a high sampling rate combined with a high signal amplitude resolution (e.g., 1 GS / s, 14 bits) is achieved, for example, by generating multiple ADC conversion stages, by interleaving several slower ADC conversion stages, by pipelined quantization of the sampled signal amplitudes, or by a combination of multi-stage quantization of the signal samples from several ADC conversion stages. A disadvantage of such ADCs and such methods is the high complexity, which is also reflected in high manufacturing costs.

[0015] The object of the invention is therefore to provide a simple or simplified distance meter and distance measurement method with which highly precise, in particular absolute, distance measurements are made possible.

[0016] This problem is solved by realizing the characterizing features of the independent claims. Features that further develop the invention in an alternative or advantageous way are to be found in the dependent claims.

[0017] The invention relates to a method for distance measurement. In this method, pulsed optical radiation is emitted at a pulse rate preferably at least 1 MHz. Preferably, the radiation is generated by a frequency comb laser. A portion of the optical radiation reflected from a target object is then received by a photosensitive electrical receiver and converted into an electrical signal. This received signal, optionally filtered, is then sampled at a sampling rate to generate a digitized signal from the resulting sampling points. Finally, the digitized signal is evaluated to determine the signal propagation time between transmission and reception, thus enabling the distance to be calculated based on this propagation time.

[0018] The sampling rate is set as a function of the pulse rate, preferably being at least ten times the pulse rate. Sampling is performed over a plurality of received radiation pulses. For this purpose, the pulse rate is preferably measured, particularly continuously, and the sampling rate is set, particularly continuously, as a function of the measured pulse rate. Optionally, the resulting sampling rate is also determined absolutely using an external or independent time base (e.g., with a counter). Alternatively, the pulse rate and, adapted to it, the sampling rate are set based on predetermined clock rates, particularly continuously, where the clock rate is generated, for example, by an independent clock generator.

[0019] Preferably, the sampling rate is set in such a ratio to the pulse rate that the sampling rate and pulse rate are phase-locked to each other. Preferably, the sampling rate is set to be a non-integer multiple of the pulse rate, where, for example, the ratio of sampling rate to pulse rate is either: Abtastrate = Pulsrate * N + 1 / x or: Abtastrate = Pulsrate * N − 1 / x , where N is a natural number and x is the plurality of received radiation pulses.

[0020] Optionally, the sampling rate is significantly smaller than the bandwidth of the receiving unit (subsampling) and / or the pulse rate-dependent adjustment of the sampling rate is algorithmic, i.e., a computational, evaluation-side adjustment of the sampling rate - e.g. as resampling - to the pulse rate.

[0021] In a training course, the digitized signal is generated by accumulating the sampling points of the majority of the received radiation pulses, preferably without assigning any specific values ​​to the individual radiation pulses. Alternatively or additionally, the sampling points are used to optimize parameter values ​​of polynomials that describe the signal or its waveform, particularly in real time.

[0022] The sampling process can optionally be performed in such a way that the sampling points repeat after a certain number of repetitions, and a multiple of these repeating sampling points is used to generate the digitized signal. In other words, a sampling pattern repeats after a certain number of sampling points, so that the pulses are sampled at recurring intervals, and the data is then accumulated over several such repetitions. Thus, after x pulses, the sampling grid on the pulse shape repeats itself.

[0023] In a further training step, a (slight) asynchronicity between the sampling rate and the pulse rate is set by an additional fraction (y, e.g., expressed as ppm) greater than a multiple of the transmitted pulse rate, in particular at most one millionth, so that a (controlled) phase shift or sampling without repeating sampling points occurs. This phase shift via frequency offset can optionally also be combined with the sampling rate to pulse rate ratio described above, expressed formulaically as follows: Abtastrate = Pulsrate * N ± 1 / x * 1 + y

[0024] Optionally, the sampling rate is shifted by this fraction from sample to sample, accumulating over a time period TACC. Thus, with accumulation over a time period TACC, the sampling grid shifts by a time ΔT relative to the laser pulse. ΔT = = y * T Acc

[0025] As a further option, any ambiguities in the distance determination are resolved by means of discrete and / or analog modulation of the radiation pulse sequence, optionally including the determination of the number of radiation pulses between transmission and reception.

[0026] The invention further relates to an electro-optical rangefinder, in particular a laser rangefinder. The rangefinder comprises a radiation source for generating pulsed radiation with a pulse rate, as well as an optical lens and a photosensitive component, in particular a photodiode, for receiving a portion of the radiation reflected by a target object and converting it into a received signal, and an analog-to-digital converter (ADC) for digitizing the received signal by sampling at a sampling rate. Furthermore, the rangefinder comprises an electronic evaluation unit or evaluation electronics which, based on a signal propagation time, determines a distance between the rangefinder and the target object using the digitized received signal, wherein the distance is preferably measurable absolutely.

[0027] The rangefinder further features a sampling function, in the execution of which the sampling rate is set depending on the pulse rate, the sampling rate being preferably at least ten times higher than the pulse rate. Furthermore, the sampling for digitizing the respective received signal used for distance determination is carried out over a plurality x of received radiation pulses.

[0028] In a preferred embodiment, the radiation source is a free-running, frequency-unstabilized comb laser. Optionally, the comb laser is configured to generate pulses with a pulse duration between 100 fs and 10 ps and / or is configured as a micro-comb laser with at least one monolithic microresonator (optical whispering gallery mode resonator).

[0029] Preferably, the rangefinder is configured to detect the pulse rate, preferably continuously, e.g., by means of an internal radiation detector, and to adjust the sampling rate as a function of the detected pulse rate, in particular by means of a phase-locked loop (PLL), preferably continuously and / or asynchronously. Optionally, for adjusting the sampling rate based on the measured pulse rate, the rangefinder includes a first clock generator, in particular a phase-locked loop (PLL), which generates the sampling rate using the detected pulse rate, so that the laser serves as a low-noise time base for the first clock generator. The first clock generator then passes the pulse rate to another unit to measure the exact pulse rate of the laser. This is done, for example, in an FPGA, which is clocked by a precise time base (TCXO, OCXO). Knowing the precise sampling rate is essential for accurate distance determination.

[0030] Optionally, the rangefinder includes at least one independent clock generator, in particular a temperature-compensated (crystal) oscillator (TCO, TCXO, or OCXO), for generating an independent clock rate. When the sampling functionality is implemented, the sampling rate and the pulse rate can then be set, preferably continuously, based on the clock rate. Additionally or alternatively, the absolute frequency of the radiation can be continuously determined based on the clock rate, with the rangefinder being configured to continuously calculate a sampling-to-distance factor, thus enabling distance determination with a scale error of less than 10 ppm, but also less than 1 ppm.

[0031] Optionally, the photosensitive electronic component has a bandwidth in the gigahertz range and can optionally be configured as an avalanche photodiode (APD) or single-photon avalanche photodiode (SPAD) array. As a further option, the rangefinder includes a fiber optic radiation amplifier and / or an internal reference target, whereby an optical reference path for absolute distance measurement can be provided using the reference target.

[0032] Preferably, the rangefinder, for resolving ambiguities in distance determination, includes a modulator for modulating the laser pulses, in particular a variable optical attenuator (VOA) outside the laser cavity to generate an amplitude-modulated signal with a constant frequency. Alternatively or additionally, modulation is achieved using a pulse picker, which, for example, at an initial pulse rate of 10 MHz, only allows every ninth or tenth pulse to pass through to an optical amplifier or be emitted onto the target. As a further option for ambiguity resolution, the device includes an electro-optical phase shifter inside a cavity of the laser to generate at least two different pulse repetition frequencies.

[0033] Optionally, the rangefinder is configured for spectral interferometry. This option includes frequency tuning of the pulse rate during distance determination, enabling pulse overlap interference between a reference pulse and a received pulse, and / or spectral splitting of the received pulses using a dispersive element, in particular a grating or Virtual Imaged Phased Array (VIPA), and intensity measurement using a camera of the rangefinder, and / or reduction of the pulse interval using a receiver-side ring cavity of the rangefinder, preferably in the reference light path, and / or pulse stretching using an output Fabry-Perot cavity and grating of the rangefinder.

[0034] The inventive method and device offer the advantage that precise distance measurements, down to resolutions in the micrometer or nanometer range, are possible even with cost-effective components, especially a simple ADC and a simple frequency comb laser, even at large distances to the target (500 m or more). A simple ADC is characterized, among other things, by a comparatively low sampling rate, low power consumption, and systematic errors such as inaccurate internal calibration.

[0035] By coupling or adjusting the sampling rate to the pulse rate, high, or rather increased, final sampling rates or a high density of sampling points can still be provided, even at pulse rates in the MHz range, whereby, to further reduce the complexity of multiple sampling, it is possible to forego assigning sampling points to the "original" received signals.

[0036] Frequency comb lasers offer the advantage that, due to their very short pulse duration, the disruptive effects of speckle are drastically reduced compared to other lasers. This is particularly beneficial for targets with uneven surfaces, improving measurement precision and signal propagation time. The frequency comb laser in this device lacks both pulse rate stabilization and CEP stabilization, significantly reducing complexity, size, and cost. Frequency comb lasers without pulse rate and optical carrier phase stabilization have extremely short pulses and therefore exhibit low spatial and chromatic irregularities. The presented method of adjusting the sampling rate solves the problem that the laser pulse rate of an unstabilized frequency comb laser is inherently asynchronous with the ppm- or sub-ppm-accurate electronic master oscillator of a distance measurement system.

[0037] Furthermore, the present invention largely avoids distance jitter. This jitter is so minimal that interferometric distance measurements are possible even without active pulse-to-pulse stabilization or optical phase stabilization. The short laser pulses, e.g., in the femtosecond range, enable accurate single-shot measurements, even at short distances (e.g., less than 20 m) and with a small number of received photons per pulse. The shot noise level of a distance measurement is well below one micrometer.

[0038] The inventive method and the inventive device are described in more detail below by way of example, using specific embodiments shown schematically in the drawings, and further advantages of the invention are also discussed.

[0039] Specifically, we show: Fig. 1a,b each shows an exemplary embodiment of a distance meter or distance measuring method according to the invention; Fig. 2 shows a preferred embodiment of a freely oscillating frequency comb laser; Fig. 3 shows an example of setting the sampling rate to the pulse rate with accumulation of sampling points; Fig. 4 shows a further example of signal accumulation; Fig. 5 shows an example of an additional shift of the sampling rate relative to the pulse rate; Fig. 6 shows a first embodiment for resolving measurement ambiguities; and Fig. 7a,b,c shows further embodiments for resolving measurement ambiguities.

[0040] Figure 1aFigure 1 shows a first exemplary embodiment of a rangefinder 1 or range-measuring method according to the invention. The rangefinder 1 has a free-running, preferably single-mode fiber-based, frequency comb laser 2, which generates radiation pulses 12. The radiation 12 accordingly has a certain pulse rate, e.g., between 5 and 25 MHz, which is not (permanently) predetermined or fixed due to the free-running nature or the lack of frequency stabilization of the laser 2.

[0041] Preferably, the respective pulse duration is between 100 fs and 10 ps, ​​whereby the pulse length can be determined, for example, by means of dispersing elements. Optionally, the laser is configured as a micro-frequency comb laser with a monolithic microresonator for generating a soliton pulse train. As a further option, the laser 2 has an optical amplifier unit, e.g., fiber-based, such as an erbium-doped fiber amplifier (EDFA) with single-mode fiber (see also Figure 1b For peak power levels above 5 kW, fibers with a large core diameter, for example 25 µm (not shown), can be used to avoid unwanted pulse broadening or saturation. Other examples of optical amplifiers used are YDFA (ytterbium-doped fiber amplifiers) or those based on bismuth-doped glasses.

[0042] The laser pulses 12 are emitted via an optical lens arrangement 14 onto a target 100, with some of the pulses 12a being coupled out by means of a partially transparent mirror 6s, so that the radiation 12 is partially directed onto a photodetector 3, e.g., a GHz photodiode. Pulses 13 reflected from the target 100 are directed by means of a receiver unit, in this example by means of the lens 14 (or an additional receiver-lens arrangement) and the mirror 6s onto another photosensitive electrical component 4. The detector 4 preferably has a bandwidth in the gigahertz range and is, for example, an avalanche photodiode (APD) or a single-photon avalanche photodiode array (SPAD). The detector 4 is energy-sensitive, so that the electrons released by the ultrashort laser pulses are not lost but contribute to the electrical detector signal.

[0043] As a further option, not shown, the rangefinder 1 has a fiber optic, e.g., doped, radiation amplifier on the receiver side. The electrical received signal from the photosensor 4 is transferred to an analog-to-digital converter (ADC) 5, where it is digitized by sampling at a specified sampling rate. Optionally, the rangefinder 1 includes, in a manner known per se, a signal filter upstream of the ADC 5 to improve the signal-to-noise ratio.

[0044] The sampling rate is set depending on the pulse rate, preferably being at least ten times the pulse rate. In this example, this pulse-rate-dependent setting is achieved by continuously measuring the pulse rate using detector 3 and a counter 11. In this example, counter 11 is integrated into an integrated circuit (preferably a field-programmable gate array, FPGA) 8, to which the signal from detector 3 is fed. A TCX oscillator 7 with sub-ppm accuracy serves, for example, as a precise reference time base for counter 11. Alternatively, the pulse rate is measured by the ADC 5 itself. Optionally, the sampling rate can be smaller than the receive bandwidth.

[0045] In this example, depending on the measured value of counter 11, i.e., depending on the pulse rate, the sampling rate of the ADC 5 is set either phase-locked or (controlled or controllable) phase-shifted relative to the measured pulse rate by means of a synthesizer 10 and a phase-locked loop (PLL) 6. The laser 2 thus serves as a low-noise time base for the clock-generating PLL. The sampling rate is continuously controlled based on the current laser pulse rate.

[0046] In alternative possible embodiments with a regulated, stable (but therefore more complex) radiation source, a (continuous) measurement of the pulse rate can be dispensed with, and this can be determined, for example, only at the (first) start and possibly at longer intervals.

[0047] As an alternative to setting the sampling rate based on the measured pulse rate as described, both the sampling rate and the pulse rate can be set "externally," e.g., using an independent clock generator 7, so that the sampling rate is adapted to the pulse rate and, for example, asynchrony between the sampling rate and the pulse rate is present, whereby asynchrony also includes pseudo-asynchrony. Alternatively, any targeted or active control of the sampling rate and / or the pulse rate can be omitted, so that the sampling rate and pulse rate are (at least with a very high probability, or except for negligible moments) "randomly" different.

[0048] A clock generator 7 is optionally used to continuously determine the absolute frequency of the radiation 12 or 13 based on the clock rate, with the rangefinder 1 being configured to continuously calculate a sampling-to-distance factor. This option makes it possible to determine the distance to the target 100 with a scale error of less than 10 ppm or even less than 1 ppm.

[0049] The ADC 5 samples the received signal at a sampling rate set according to the pulse rate. The sampling to create a digitized received signal, from which the distance value to the target 100 is ultimately determined based on the signal propagation time between transmission and reception by means of an electronic evaluation unit 15, is carried out using several received radiation pulses 13.

[0050] The sampled values ​​of the ADC 5 from a plurality of pulses 13 are combined, specifically added, using a signal accumulator 9, without assigning sample points to individual pulses 13. Thus, all sample points of the plurality of pulses 13 are combined without assignment using the signal accumulator 9. This creates a kind of artificial sampling rate, which is a factor of x higher than the actual sampling rate of the ADC 5, where x represents the minimum number of sampled received pulses 13. After x pulses, the sample points on the signal pulses repeat, or the sampling pattern repeats.

[0051] Through the accumulation of samples, the analog received signal(s) 13 are ultimately sampled so densely that the desired distance can be determined very precisely from the resulting digitized signal. The number x can also be considered the length of the interval of the beat frequency between the two frequencies fs and flaser. x * S = N + 1 * P with N being a natural number or the least common multiple.

[0052] This type of sampling with accumulation averages out systematic errors such as crosstalk or ADC nonlinearities of the optical and electronic signal channels. Distance measurement is therefore free of interference and thus much more accurate than conventional distance measuring units. Optionally, the measured laser pulse rate, referenced to a TCXO or VCXO, also serves as a picosecond-accurate time base for the distance measurement.

[0053] As an alternative to accumulating the sampling points of the plurality x of pulses 13, the parameter values ​​of polynomials describing the digitized signal are optimized based on the sampling points of x pulses 13, particularly in real time. In this variant, the sampling points obtained from many pulses 13 thus serve to generate the digital signal by concretizing a mathematical description of the signal based on the sampled values, or in other words, by determining support values ​​for a functional representation of the desired signal, where the laser pulse rate specifies the intervals between the support values. An advantage of this type of algorithmic, laser pulse rate-dependent adjustment of the sampling rate is that the signal is already available as a function and no longer needs to be converted, resulting in a comparatively small sampling memory and independence from the sampling frequency.

[0054] As a further alternative to accumulating the sampling points of the plurality x of pulses 13, parameter values ​​are optimized based on the sampling points of x pulses 13, also in real time. These parameters describe the propagation delay using signatures derived from the received signals. This signal representation is purely mathematical, can be recorded in tabular form, and saves storage space because the sampling data is only stored temporarily.

[0055] In the exemplary embodiment according to Figure 1a The rangefinder 1 also has an internal reference target 101. This can be swivelled into the beam path, thus providing a reference light path of known distance. This enables the determination of the zero point of the distance measurement (e.g., on-site or immediately before the distance measurement to target 100), so that distances between the instrument's position and targets 100 can be measured absolutely.

[0056] As a further option, not shown, the rangefinder 1 is configured for spectral interferometry for distance determination. During distance measurement, the pulse rate is frequency-tuned, for example, so that pulse overlap allows interference between an internal reference pulse and a received pulse 13. Alternatively or additionally, the received pulses 13 are spectrally split using a dispersive element, e.g., a grating or Virtual Imaged Phased Array (VIPA), and / or intensity is measured using a camera integrated into the rangefinder 1. The rangefinder 1 also optionally features a ring cavity for reducing the pulse interval on the receive side and / or a high-loss Fabry-Perot cavity and grating for pulse stretching.

[0057] Figure 1bFigure 1 shows a second exemplary embodiment of a rangefinder. For simplification, in contrast to the Figure 1aThe illustration of the target and optics is omitted. In this example, the radiation 12 generated by the comb laser 2 is amplified by an optical amplifier 25, e.g., an EDFA, so that amplified radiation pulses 12' are emitted. The comb laser 2 thus serves as a seed laser for the optical amplifier 25. Furthermore, the comb laser 2, or more precisely the pulse rate it generates, serves as input for a phase-locked loop (PLL) 6 (for this purpose, a photodiode for laser radiation detection is integrated into the comb laser, for example). The PLL 6 is used to control or adjust the sampling rate of the analog-to-digital converter 5 as a function of the pulse rate. The target radiation pulses 13, detected by the photosensitive detector 4, are then digitized using the ADC 5 set in this way. The PLL 6 also serves as input for an FPGA 8 or the counter 11 integrated on it.In this example, a TCX oscillator 7 is again used as the precise reference time base for counter 11. Figure 2 Figure 1 schematically shows a preferred embodiment of a freely oscillating frequency comb laser 2. In the example, the fiber-based laser 2 has a pump diode 13 which couples the power via fibers 14 by means of a coupler 15 into the cavity consisting of a reflector 16 combined with a bonded semiconductor saturable absorber (SESAM) and a grid 17 as a second reflector.

[0058] For example, a section of the single-mode fiber 14 is erbium-doped. This amplifier unit, together with the nonlinear transmission of the SESAM, causes locking between the many spectral laser modes and results in a stable pulse rate of laser emission in the lowest-order eigenstate. The laser pulses 12 have a duration of less than 10 ps, ​​typically < 1 ps. The pulse rate is determined by the travel time across the optical path between the two resonator mirrors. The laser pulse rate can optionally be adjusted by changing the optical path length between the two mirrors. A percentage of the pulse power is routed to output 12 via the coupler 15.

[0059] As shown, laser 2 features neither pulse rate stabilization nor CEP stabilization (CEP: carrier envelope phase), which advantageously keeps complexity, size, and cost low. Furthermore, a fiber-based laser 2 as shown has high mechanical stability compared to solid-state free-beam lasers, e.g., against vibrations or shocks, which is particularly advantageous for mobile rangefinders.

[0060] Figure 3 This illustrates an example of setting the sampling rate S to the pulse rate P with accumulation A of sampling points 18a,b within the framework of the distance measurement method. In the three parts of the figure, the time axis is represented as the horizontal axis and the signal intensity as the vertical axis.

[0061] In the upper part of the Figure 3Two of the emitted pulses 12 are shown, which are emitted with a pulse rate P symbolized as a time interval, wherein the pulse rate P is preferably not fixed / stabilized as described.

[0062] In the middle part of the Figure 3 Two received pulses, or rather their respective corresponding received signals 13a, 13b, are represented from a plurality x of received pulses or received signals. These are sampled at a sampling rate S, symbolized as a time interval, so that sampling points are generated which encompass the relevant sampling points 18a, 18b, 18c ... 18x of the received signals 13a, 13b,..., 13x. The sampling rate S is set as a function of the pulse rate P such that, as shown, there is a shift of 1 / x between two received signal pulses. In the example, P is given by: S = P * (N + 1 / x), where N is a natural number and preferably at least equal to 100, e.g., 500, 1000, or more.

[0063] In other words, the sampling rate is a non-integer multiple of the pulse rate (phase rigidity), where, in this example, the offset is adapted to the number of sampling points 18a-18x for an accumulation of pulses 13a-13x. After a certain number x, the sampling points 18a-18x repeat with respect to their temporal position on the received signals. The sampling pattern repeats after x emitted signals. A measurement typically comprises several such cycles; a shortest meaningful high-quality measurement comprises exactly one cycle of a certain number x.

[0064] In the lower part of the Figure 3Figure A shows the accumulation of sampling points 18a-18x. Sampling points 18a-18x are combined to form a digital received pulse 13D. As mentioned above, this combination is unassigned; that is, the source (received pulse 13a, 13b, etc., or 13x) of each sampling point is not recorded. The combination is a purely sequential arrangement of the received signal pulses modulo x.

[0065] Optionally, the previously mentioned repetition of sampling points 18a-18x after the pulse plurality x is used to accumulate not just one "pass" of pulses x, but all sampling points Mx of a number M of repetitions. For example, with M=4 repetitions, this results in 4*x sampling points.

[0066] Figure 4 This shows another example of signal accumulation during ADC conversion. In the upper part of the Figure 4The individual digitized sampling pulses or digital signal vectors (DSVs) 13Dx are shown in chronological order, where x is seven in this example. These digital signals 13Dx are generated from the analog received pulses by sampling at a sampling rate S, which in this example is 320 megasamples per second (MS / s).

[0067] From these x digital pulses 13Dx, a total digital pulse 13D is generated by preferably unassigned accumulation (over a period of 4µs) (lower part of the Figure 4By combining all the sample points, a sample is obtained that corresponds to a sampling rate S*x, which in this example—since seven pulses 13Dx are used—is seven times the original sampling rate, i.e., 2.24 gigasamples per second. This approach, which provides a significantly increased sampling rate, enables precise distance measurement even with simple, inexpensive rangefinder components, primarily an ADC converter. Furthermore, systematic errors of the electronic receiving channel, which exhibit synchronous behavior, are averaged out by the asynchronous acquisition of the received signals.

[0068] Figure 5This illustrates how a further method for shifting the sampling grid can be implemented as an advanced option. This second method can be combined with the procedure described above, as well as being implemented as an independent shift of the sampling rate relative to the pulse rate. The shift is typically a fraction of the transmit pulse frequency, less than 1 / x, e.g., 0.5 ppm relative to the pulse rate. A combination with the method mentioned above can therefore be described, for example, as follows: S = P * N − 1 / x * 1 + y where, for example, y = 0.5ppm

[0069] However, as a standalone procedure, this can be described as follows: S = P * N * 1 + y .

[0070] For example, with a signal measurement or accumulation time of 15 ms, the sampling grid shifts relative to the pulse pattern by a total of 0.5 ppm * 15 ms = 7.5 ns. When the sample points are randomly combined into a single digitized 13Dx pulse, a slightly blurred or smeared pulse shape results. This causes the subsamples or 13Dx pulses to blend together after just a few signal pulses, ensuring sufficient reduction or elimination of converter errors such as gain errors, timing errors, or offset errors, which are particularly problematic with inexpensive analog-to-digital converters, even after short measurement times.

[0071] Figure 6An example of a further development of the rangefinder according to the invention. Shown are a laser 2 and fibers 14 for guiding the emitted laser light. Especially at high pulse rates, several light pulses are located between the measuring device and the target during distance measurement, which leads to distance ambiguities; thus, it is no longer possible to unambiguously assign a pulse to the time-of-flight measurement. In order to resolve or prevent such ambiguities, the rangefinder in the example of the Figure 6A (fiber optic) amplitude modulator 19 is used as a variable optical attenuator, which in this example is controlled via a coaxial cable 20. The laser pulse amplitudes are modulated (analogously) by the amplitude modulator 19 in such a way that individual laser pulses become distinguishable. Thus, modulation—in this example of the amplitudes—occurs such that laser pulses, pulse groups, or envelopes of pulse sequences are uniquely identifiable. The distance can therefore be determined unambiguously, even with thousands of pulses between the measuring instrument and the target.

[0072] The Figures 7a and 7b This shows another form of ambiguity resolution. In this example, a targeted selection is made from the pulses 12 generated by the laser 2. For this purpose, the device has a pulse picker 21 ( Figure 7a), which selects pulses 12 from the sequence of laser pulses 22 generated with the pulse rate, or suppresses a portion of the pulses 12. This discrete modulation thus creates a modified pulse sequence 22m.

[0073] Figure 7b shows such a modulated pulse sequence 22m from radiation pulses 12. The original pulse sequence (22 in Figure 7a ) is selectively interrupted by means of pulse picking (arrows 23) in such a way that clearly identifiable "packets" 24a and 24b of pulses 12 are generated, on the basis of which any ambiguities can be resolved or avoided.

[0074] Amplitude modulation according to Figure 6 and pulse picking according to the Figures 7a and 7bThese methods can also be combined. As a further alternative or additional training option, the number of pulses 12 between transmission and reception is determined. In another form, not shown, the laser 2 has an electro-optical phase shifter located inside the cavity instead of outside, allowing the pulse rate to be varied and at least two different pulse repetition frequencies to be provided. By measuring the received pulses with at least two such frequencies, ambiguities can be resolved—where the difference frequency of the two pulse repetition frequencies determines the maximum possible ambiguity distance—and the distance can be determined absolutely.

[0075] Figure 7c indicates in reference to Figure 1bA rangefinder 1 with pulse picker 21. The pulse picker 21 modulates the radiation 12 or pulse train 22 generated by the laser 2 before it is amplified by the optical amplifier 25, so that a modulated and amplified radiation pulse train 22m1' is emitted. The pulse picking is performed based on a signal from the PLL 6. For example, with an "original" pulse rate of 100 MHz generated by the laser 2, only every 100th pulse is allowed through, so that radiation 22 is emitted with an effective pulse rate of 1 MHz. If, for example, the (average) power of the laser is 1 mW, it is 1 µW after the pulse picker and is then amplified to 10 mW by the amplifier.

[0076] In this example, PLL 6 also controls the modulation of the generated pulse sequence 22 such that, in addition to the first pulse sequence 22m1, a second modulated pulse sequence 22m2 is generated, for example, with every 100th pulse, by allowing every 110th pulse to pass through. Thus, distance measurement is performed using a first pulse frequency of, for example, 1 MHz and a second frequency of, for example, 909 kHz. Evaluating the resulting synthetic intermediate frequency of 90.9 kHz resolves ambiguities at distances of up to 1650 m.

[0077] It is understood that these figures only schematically represent possible embodiments. The various approaches can also be combined with each other as well as with devices or methods of the prior art.

Claims

1. Method for distance measurement comprising ▪ emitting pulsed optical radiation (12) with a pulse rate (P, 22), in particular wherein the pulse rate (P, 22) is at least 1 MHz and / or the radiation (12) is generated by a frequency comb laser (2), ▪ receiving a portion of the optical radiation (13, 13a, 13b) reflected from a target object (100) with a photosensitive electrical receiving element (4) and converting said portion into an electrical receive signal (13a, 13b, 13c ... 13x), ▪ sampling the receive signal (13a, 13b, 13c ... 13x) with a sampling rate (S) to generate sampling points (18a, 18b, 18c ... 18x), ▪ evaluating a digitized signal (13D) to determine the signal propagation time between emission and reception to determine the distance based on the signal propagation time, characterized in that the sampling rate (S) is set dependent on the pulse rate (P, 22) and the digitized signal (13D) is generated by sampling receive signals (13a, 13b, 13c ... 13x) of a plurality x of received radiation pulses (13), wherein the ratio of the sampling rate (S) to the pulse rate (P, 22) is set such that the sampling rate (S) is a non-integer multiple of the pulse rate (P, 22).

2. Method according to Claim 1, characterized in that the sampling rate (S) is at least ten times as large as the pulse rate (P, 22).

3. Method according to Claim 1 or 2, characterized in that the pulse rate (P, 22) is measured and the sampling rate (S) is set dependent on the measured pulse rate (P, 22).

4. Method according to any one of the preceding claims, characterized in that the pulse rate (P, 22) and, adapted thereto, the sampling rate (S) are set on the basis of predetermined clock rates, generated in particular by an independent clock generator (7).

5. Method according to any one of the preceding claims, characterized in that ▪ for the ratio of sampling rate (S) to pulse rate (P, 22) either of the following applies: sampling rate = pulse rate* (N+1 / x) or: sampling rate = pulse rate*(N-1 / x), with N as a natural number, and / or ▪ the sampling rate (S) is less than the bandwidth of the receiving unit.

6. Method according to any one of the preceding claims, characterized in that the digitized signal (13D) is generated by the sampling points (18a, 18b, 18c ... 18x) of the plurality x of the received radiation pulses (13) ▪ being accumulated, in particular unassigned to the individual receive signals (13a, 13b, 13c ... 13x), and / or ▪ being used to optimize parameter values of at least one polynomial describing the signal (13D).

7. Method according to Claim 6, characterized in that the sampling points (18a, 18b, 18c ... 18x) repeat after the plurality x and Mx sampling points (18a, 18b, 18c ... 18X) are used for generating the digitized signal (13D), with M as a natural number.

8. Method according to any one of the preceding claims, characterized in that the sampling rate (S) ▪ is shifted in relation to the pulse rate (P, 22) by a fraction less than 1 / x, in particular not greater than one millionth, and / or ▪ is set algorithmically.

9. Method according to any one of the preceding claims, characterized in that in order to avoid ambiguities in the distance determination, a discrete and / or analogue modulation (23) of a radiation pulse sequence (22) is performed, wherein in particular, the number of radiation pulses (12, 13) between emission and reception is determined.

10. Electro-optical distance meter (1), in particular laser distance meter, comprising ▪ a radiation source for generating pulsed radiation (12) with a pulse rate (P, 22), ▪ a lens (14) and a photosensitive component (4), in particular a photodiode, for receiving a portion of the radiation (13) reflected from a target object (100) and converting said portion into a receive signal (13a, 13b, 13c ... 13x), ▪ an analogue-to-digital converter (5) for generating sampling points (18a, 18b, 18c ... 18x) by sampling the receive signal (13a, 13b, 13c ... 13x) with a sampling rate (S) and ▪ an electronic analysis unit, which determines a distance, in particular an absolute distance, between the distance meter (1) and the target object (100) on the basis of a signal propagation time of the digitized signal (13D), characterized in that the distance meter (1) has a sampling functionality, which when executed sets the sampling rate (S) dependent on the pulse rate (P, 22) and the digitized signal (13D) is generated by sampling the receive signals (13a, 13b, 13c ... 13x) of a plurality x of received radiation pulses (13), wherein the ratio of the sampling rate (S) to the pulse rate (P, 22) is set such that the sampling rate (S) is a non-integer multiple of the pulse rate (P, 22), in particular wherein the sampling rate (S) is at least ten times as large as the pulse rate (P, 22).

11. Distance meter (1) according to Claim 10, characterized in that the radiation source is a freewheeling, non-frequency-stabilized frequency comb laser (2), in particular ▪ designed to generate radiation pulses (12) with a pulse duration between 100fs and 10ps and / or ▪ designed as a microfrequency comb laser with a monolithic microresonator.

12. Distance meter (1) according to Claim 10 or 11, characterized in that the distance meter (1) is designed to measure the pulse rate (P, 22) and to set the sampling rate (S) dependent on the measured pulse rate (P, 22), in particular by means of a phase-locked loop (6) or a synthesizer.

13. Distance meter (1) according any one of Claims 10 to 12, characterized in that the distance meter (1) has at least one independent clock generator (7) for generating an independent clock rate, ▪ wherein when the sampling functionality is executed, the sampling rate (S) and the pulse rate (P, 22) are set based on the clock rate, and / or ▪ so that the absolute frequency of the radiation (12) can be determined continuously on the basis of the clock rate, and the distance meter (1) is designed to continuously compute a sample-to-distance factor so that the distance determination is enabled with a scale error of less than 10ppm.

14. Distance meter (1) according to any one of Claims 10 to 13, characterized in that ▪ the photosensitive electrical component (4) has a bandwidth in the gigahertz range, in particular wherein it is formed as an avalanche photodiode or single-photon avalanche photodiode array, and / or ▪ the distance meter (1) has a fibre-optic radiation amplifier and / or ▪ the distance meter (1) has an internal reference target (101), on the basis of which an optical reference path can be provided so that an absolute referencing of the distance determination is enabled.

15. Distance meter (1) according to any one of Claims 10 to 14, characterized in that to resolve ambiguities in determining the distance, the distance meter (1) has ▪ a modulator (19, 21) for modulating the laser pulses (12), in particular a variable optical attenuator (VOA) and / or a pulse picker, and / or ▪ an electro-optical phase shifter inside a cavity of the laser (2) for generating at least two different pulse sequence frequencies.

16. Distance meter (1) according to any one of Claims 10 to 15, characterized in that the distance meter (1) is designed for spectral interferometry as part of the distance determination, in particular wherein as part of the distance determination ▪ a frequency tuning of the pulse rate (P, 22) takes place so that interference between a reference pulse and a receive pulse (13, 13a, 13b) can be used by means of pulse overlap, and / or ▪ a spectral decomposition of the received pulses (13, 13a, 13b) takes place by means of a dispersive element, in particular a lattice or virtual imaged phased array, and intensity measurement by means of a camera of the distance meter (1) and / or ▪ a reduction of the pulse interval by means of a receive-side ring cavity of the distance meter (1) and / or ▪ a broadening of the pulse shape by means of an output-side Fabry-Perot cavity and lattice of the distance meter (1) takes place.