DE19513823A1 - Range finding for industrial use and for ultra short time laser spectroscopy - Google Patents

Abstract

The method applies the principle of transit time measurement of an optical measurement pulse reflected from a target object and received by a detector. The detector determines a measurement time which is coupled in a defined manner with the time of reception of the optical measurement pulse. The interval between the measured time and that of the transmission time of the reference time point coupled in a defined manner with the optical measurement point is used to determine the transit time. For measuring the transit times ultra-short pulses with a high optical power level and pulse sequence frequency with a low mean power are used.

Description

The invention relates to a distance measuring method based on the principle of transit time measurement an optical measuring pulse according to the preamble of claim 1 and devices for performing this method according to the preamble of claim 25.

As known from the relevant literature (see, for example, S. Sizgoric et al. "Medium-range, high accuracy laser ranges for industrial applications ", SPIE Vol. 663 Laser Radar Technology and Applications (1986) 90-96; R. Ahola et al., "A fast time-of-flight distance sensor for industrial applications ", SPIE Vol. 654 Automatic Optical Inspection (1986), 12-18 (Ref. 1)) slow laser pulses become relatively high for distance measurement Power with a relatively low pulse repetition frequency and fast on the other Laser pulses of relatively low power with a relatively high pulse repetition frequency used.

Measurement surfaces can have very low reflectivities, which is why with high optical Transmission powers must be worked. Single heterostructure laser or SH laser for short ("single heterostructure") have a relatively large active volume with a laser emission high energy; they are therefore preferably used in optical pulse radar systems. If individual chips are used to emit high optical pulse powers, then point such laser diodes generally have relatively wide emission apertures of typically 254 μm. This leads to the fact that the measuring spot imaged via a focusing optic in the measuring plane Diameter of a few millimeters is what two-dimensional and three-dimensional precision Measurement tasks with a required lateral resolution of one millimeter are not is sufficient. They are single-chip pulse laser diodes with a sufficiently small one Emission aperture known. Because of their smaller volume, they radiate one correspondingly lower optical nominal power, which is essential for the reliable operation of the Distance measuring device is often no longer sufficient.

A laser diode stack provides a higher optical than a single chip Power. A major disadvantage for use in precision distance measuring devices However, is that the individual laser diode chips of a stack at a fast Current pulse drive does not start to emit at the same time, d. H. the delay times of the individual chips are different despite their serial connection. Leading then in the case of receiver circuits with a sufficiently large bandwidth for typical step functions the leading edge of the laser pulse. Since the use of laser emission from the plant depending on the individual laser diodes, a reliable precise time can be determined not be carried out. The determination of the time-significant point is in particular especially critical if this falls within the signal level range of a step transition. So a laser diode stack is not suitable, ultra-short optical pulse rise times and in addition to generate ultra-short optical pulses.

The measurement uncertainty of distance measuring devices of a known type is particularly affected by influences parasitic effects, which are essentially due to crosstalk transmission-side power components due to the sensitive receiver part are. Such influences that can hardly be controlled require higher demands on the Goodness of the distance measurement a steeply rising pulse edge for determining the time-significant point. However, high power laser pulses generally do not show any  linear course of the pulse leading edge over time. After an initial steep climb follows a creep effect with increasing laser noise. Make low-frequency interference signals This is particularly noticeable with distance measuring devices based on the sampling principle work. In this case, these overlap the sampled pulses, so that a very exact timing is no longer guaranteed on the disturbed pulse edge.

Typical pulse rise times of a known type are in the order of 1.5 to 3 ns. The pulse widths are 5 to 20 ns. The pulse repetition frequencies are a maximum of 10 kHz. The radial measurement uncertainty of distance measurement with laser pulse time functions known type is typically in the centimeter range. To increase the measuring accuracy the known method of pulse integration applied. For example, in one Integration time of one second a measurement uncertainty of one millimeter is reached.

Higher pulse repetition frequencies of a few hundred kilohertz are achieved with laser diodes achieved, which produce peak output powers of typically <1 watt. From Ref. 1 is a 3-D rangefinder with a DH laser diode with a pulse power of 500 mW and a pulse repetition frequency of 1.6 MHz is known. The laser pulse width is 6 ns. Of the Laser pulse rise time of 1.6 ns corresponds to a distance difference of 24 cm. A small fluctuation of the trigger level of only 1% means with such slow ones Impulses a measurement uncertainty of 2.4 mm. The given low pulse power limits the Use appropriate rangefinders on relatively well reflecting measuring surfaces. With a measuring beam diameter of 4.9 mm, the given lateral measuring resolution is for Quality control tasks, for example, are not sufficient.

From the patents DE 29 08 854 C2, US 4344705, EUR 15566 and JAP 1305263 (Ref. 2) and the publication by G. Kompa, "Accurate optical pulse reflection measurement", IEEE Trans. Instrument. Meas. IM-33, No. 2, June 1984, 97-100 is a range finder ren known that the distance information does not have a single transmission and Receive pulse (real-time signals) wins, but according to the principle of sampling Procedure the determination of the time-significant points on sampled time-stretched Signals. The sampled signal is made up of a large number of real-time pulses derived. For a short measuring time of the distance it is therefore as long as possible To aim for pulse repetition frequency. A measurement was made within a measuring range of 2 m safety of 7 mm with a measuring time of 25 ms. Via pulse integration of 512 ms, a system-specific digital measurement resolution of 1 mm was achieved. For A laser diode stack is used for laser emission. The pulse shape of the emitted pulses is determined by the spiking of the semiconductor laser, the several strongly attenuated Have relaxation peaks. A further reduction in laser emissions to the Initial spike would significantly reduce the average optical power. This would have been Advantage that in compliance with the international laser protection regulations for a laser class 1 system, the pulse repetition frequency could be increased considerably, causing the Shortened measuring time or increased measuring accuracy via pulse integration.

The invention has for its object a method and a device of the beginning mentioned type so that even weakly reflecting surface elements of Objects can be measured one-dimensionally very precisely. Another one The object of the invention is to use both surface elements of measurement objects high radial as well as high lateral resolution very precisely in two and three dimensions measured.  

To achieve this object, the invention provides that in claim 1 (method) or Claim 25 (devices) laid down features.

A particular object of the invention is to provide a distance measuring method that is suitable for the precise two- or three-dimensional measurement of objects. Solved this task is characterized in that a laser emission with a small aper is used, the ultra-short pulses of high optical pulse power and high Im pulse repetition frequency has low average power. The ultra-short laser pulses allow a more precise individual pulse measurement that goes beyond the state of the art larger distances and shorter measuring times due to the much higher pulse train frequency. At shorter measuring distances, they are due to the high optical impulse Compared to known measuring methods also for measuring weakly reflecting Suitable measuring surfaces. The ultra-short laser pulses according to the invention allow in particular compliance with international standards, especially with a high pulse repetition frequency Laser emission limit values for eye safety, which is particularly the case with distance measuring devices, which according to the phase comparison method with optical continuous wave powers of up to some 10 mW work, can not be easily achieved.

The standard deviation of the measured distance is proportional to the increase time of the optical pulse and inversely proportional to the signal-to-noise ratio (SNR). Since the rise time is itself proportional to the receiver bandwidth (B), it follows that that the standard deviation is inversely proportional to the product of B and SNR. Taking into account further that the signal-to-noise ratio of a photo receiver proportional to the received power or transmit power and inversely proportional to the Root from the receiver bandwidth behaves (see K. Määttä et al., "Profiling of hot surfaces by pulsed time-of-flight laser range finder techniques ", Appl. Optics, Vol. 32, No. 27.1993, 5334-5347), it ultimately follows for the standard deviation that this is below the A requirement of sufficiently low background radiation is inversely proportional to the root from reception power and the receiver bandwidth. Usually the SH- Laser diodes in the quasi-static operating state with relatively long and slow ones Current pulses directly modulated. The emits after a current threshold value is exceeded Laser, the time course of the radiation intensity largely the time course corresponds to the pump current. The following breaks after the threshold current is undershot Laser emission from. Usual pulse rise times are on the order of a few Nanoseconds. A reduction in the rise time through the invention Maximum injection, d. H. Increasing the signal or receiver bandwidth would be immediate increase the measuring accuracy. Shorter optical pulses still allow under the From the point of view of laser safety, higher optical powers lead to a further one Reduce the standard deviation of the measured distance. The well-known Pulse modulation of conventional laser diodes cannot do this.

The semiconductor lasers used in distance measuring devices work in quasi-static Operation, d. H. the pump current pulse leading edge and the pump current pulse width are relative long, so that the time course of the intensity of the laser emission of the semiconductor laser directly from the course of the pump current via the quasi-static power-current Characteristic results. Laser activity begins as soon as the carrier density threshold is reached becomes. In addition, injected charge carriers contribute to laser emission. For generation high power, pulse currents of high amplitude are required in quasi-static operation.  

The invention is based on the idea of using a suitable measure to prevent that practically without delay the laser emission after reaching the critical charge carriers density begins. In addition to the critical charge carrier density, the Time range as many charge carriers as possible in the active area of the laser diode be pumped to then short and powerful when the laser condition is met Lead laser spike. This can be done by controlling the laser diode using a Pump current pulse of extremely short rise time, i. H. steep pulse leading edge reached be so that during the delay between reaching the Threshold value carrier density and the use of laser emission a larger number of Charge carriers are pumped into the active area of the laser diode. Such Current edge modulation leads to a pronounced transient response or spiking of the Laser diode (relaxation oscillation). To reduce the emission to a single optical spike to reduce, a sufficiently short pump current pulse is required. It should be noted, that the optical power is proportional to the net carrier density injected during the current pulse duration behaves, which limits the amplitude of the optical pulses leads.

Laser diodes have an optical resonator, which is a multi-layer material system represents. Different material layers form heterojunctions with good optical Wave guide properties. However, the same material layers with different doping show only weak index guidance of an optical wave. To the weak index guided laser resonators count u. a. also the rib waveguides with quasi-index guidance (see P.G. Eliseev et al., "Study of strained-layer InGaAs / GaAs SQW RW lasers including analysis of internal coupling of modes and antiguiding effects ", SPIE Proceedings Volume 2146-23).

Are weakly index-guided laser diodes according to claims 2 and 3 with a sufficiently fast and high pump current is applied, so can be according to the invention Claim 4 achieve that the laser emission in the index-guided nominal operating state is suppressed since the ultra-fast injected charge carriers break down the weak index jump of the optical waveguide leads. This allows over the In addition to the nominal carrier density, further charge carriers can be injected into the active zone. this has as a result that the wavelength-dependent material gain increases, the Maximum shifts to smaller wavelengths with increasing carrier density. To smaller ones Wavelengths the increase in carrier density also causes a growing index jump. With a sufficiently large modal gain, that of the material gain and the transversal Field distribution of the optical wave in the laser resonator depends on the fulfillment of the Laser condition the laser emission in the form of a single powerful laser spike. There both the temporal change in the material gain and that of the index profile for the Generation of the laser pulse are essential, this mode should be short according to claim 5 index-led Q switching. The main beam direction of laser emission takes place according to claim 6 perpendicular to the emission aperture.

A modulation variant according to the invention for generating ultra-short laser pulses for precise distance measurement is based on the fact that the injection rate of the charge carriers It is extremely high that a primary laser spike is emitted in the material gain switching mode (see E.L. Portnoi et al., "Switching phenomena in SH GaAs-AlGaAs laser diodes with saturable absorber created by implantation of heavy ions ", submitted for publication).  

In SH lasers, this leads to optical leakage waves, the main beam direction of which 7 deviate from the optical axis of the laser resonator. For DH lasers with quasi Indexing is due to the symmetrical arrangement of leaky waves on both sides the axis of symmetry, which according to claim 8 each as a transmission and reference signal can serve.

Other embodiments of semiconductor lasers that have comparable optical superspikes generate, are conceivable according to claim 9. It is of fundamental importance that the Laser emission is suppressed as long as possible after the start of electricity pumping to generate the highest possible carrier densities during this time by maximum injection. For example, the middle p-doped GaAs layer of a conventional SH laser be replaced by an AlGaAs layer with a very low Al content, so that in the Hibernation no significant positive jump in the refractive index of the pn transition exists. This would have the advantage that at moderate pump current amplitudes no lasers at all emission could occur; strong laser activity would then only exceed a maximum injection of charge carriers may be possible with wavelengths at which the carrier densities make a positive contribution to the real part of the refractive index.

The described different operating modes of the index-guided gain and Q Switching and material gain switching occur when the maximum injection of a classic SH Lasers not in their purest form; so during the emission of an ultra-short Laser pulse an initial Q switching into an index-guided gain switching. It is also possible that a material gain switching into an index-guided gain switching transforms. Such a combined operating mode leads on the one hand to the fact that the optical Spectrum of the ultra-short impulse becomes wider, and on the other hand to the fact that the Pulse width increased. According to claim 62, one can via targeted modifications classic SH laser, the respective accompanying parasitic emissions suppress. With the measure that according to claim 63 the step profile of a classic SH laser is replaced by a stair profile, the emission takes place in the pure material Gain switching. For example, the Al content of the middle material layer is conceivable Sh laser configuration to choose so that a negative jump in the refractive index of the pn transition arises. As a result, the critical carrier density at which Laser emission, is extremely high. An index-guided emission state is then locked out. In an advantageous development according to the invention according to claim 10 an SH laser is additionally provided with a saturable absorber. This leads to the SH laser emits in a pure index-guided Q-swichting mode. According to experience this measure leads in particular to a faster decay of the laser emission and thus further shortening the pulse width of the ultra-short laser pulses.

The refractive index changes in the semiconductor laser with the charge carrier density. Since the Charge carrier density varies with the change in current or the laser emission changes the charge carriers reduced density, the change in refractive index affects the propagation properties of the op table fashions and thereby causes a shift in the mode spectrum. The waves length shift Δλ is proportional to the change in charge density ΔN. In the event of ultra-short laser pulses absorb the charge density during the optical pulse due to stimulated emission, which means a chirp with unidirectional redshift exercise is linked. As in P.P. Vasil'ev, "Ultrashort pulse generation in diode lasers", Opti cal and quantum electronics, vol. 24, pp. 801-824, 1992, the inventions  According to the invention with semiconductor lasers generated ultra-short optical pulses according to claim 11 further shorten using known methods of optical pulse compression technology. At For example, the groups are in a medium with normal dispersion (e.g. optical fiber) speed of a short-wave signal is lower than that of a long-wave. It follows, that the transit time in the medium for a pulse of longer wavelength is smaller than that of one Pulse with a shorter wavelength of the optical carrier. There is therefore an optimal one Propagation distance, after passing through the rear chirp pulse part the front Part has caught up, and all wavelengths compressed to a shorter optical pulse are. The pulse compression leads to a steepening of the pulse edges and thus to one more precise determination of the time-significant point.

The ultra-short laser on which the distance measuring method is based according to claim 1 High repetition frequency pulses can also be according to claim 12 via mode-locked lasers be generated. Their optical performance is generally lower, so that Corresponding distance measuring devices are limited to the close range.

The detection of the powerful, ultra-short laser pulses requires broadband Receiver circuits, for example, based on III-V compound semiconductors, which Amplify ultra-fast laser pulses according to claim 13 true to the waveform. A quick one Receiver circuit consists, for example, of an MSM photodiode, a transimp dance amplifier and a main amplifier. From V. Hurm et al., "8.2 GHz bandwidth monolithic integrated optoelectronic receiver using MSM photodiode and 0.5 µm recessed gate AlGaAs / GaAs HEMTs ", Electronics Letters, 25th April 1991, Vol. 27, No. 9, 734-735, it is known that with relatively large-area MSM photodiodes of 100 × 100 µm² Receiver bandwidths of 8.2 GHz can be achieved. The use according to the invention Such distance measurement circuits require sensitive due to the lower ultra-fast photodiodes a correspondingly higher optical reception power, their provision is the subject of the inventive method. Known characteristics laser emission are not suitable for this. A higher reception sensitivity can be with a slightly reduced bandwidth with avalanche photodiodes. There are Avalanche photodiodes are known to have optimal circuit design and maximum Bias have a rise time of about 100 ps. If not you Rise time of the ultra-fast laser pulses, they mean opposite Receiver circuits of a known type, after all, an improvement by a factor of 15 to 30.

A reference signal is usually taken from the transmission beam for distance measurement and given as a time reference to the receiving diode, so that with the time-delayed Echo signal at the output of the photodiode, an electrical double pulse for evaluation is present.

The ultra-short optical pulses on which the invention is based are according to claim 1 essentially symmetrical triangular pulses with an approximately linear rise the leading edge and approximately linear drop of the trailing edge. Typical increase times are less than 50 picoseconds. Since the rise time (20% -80%) of the ultra short impulses approximately equal to the fall time (20% -80%) can, according to claim 14 the determination of the time-significant point in principle without loss of accuracy both on the leading edge and on the trailing edge or by averaging the Timing on the leading edge and trailing edge take place according to claim 15.  

A special feature of the invention follows from claim 16, according to which the time-significant Point regardless of the respective signal amplitude precisely by averaging the an essentially arbitrarily selectable fixed threshold value obtained signal times the leading and trailing edge is determined. Regardless of the threshold, the time average always the time of maximum signal amplitude and is therefore one clear time reference. Due to the high edge steepness of the pulses according to the invention can often be based on a common dynamic ratio threshold detector that is time-sensitive edge point when the 50% signal level is reached, be dispensed with. The unique time reference can also according to claim 64 via a differentiation of the ultra rapid impulses are obtained via the signal zero crossing.

Sampling technology enables very precise runtime measurement with low switching and expense. Using the sampling principle, the difficult high-resolution direct Time measurement on the fast real-time pulses can be bypassed. A basic one Problem with the known distance measuring device in Ref. 2 results from the use of two separate sampling stages, one for the reference signals and one for the measurement signals. A precise distance measurement with measurement uncertainties in the lower millimeter range or Submillimeter range sets an ideal temperature synchronization over the entire operation temperature range ahead, which is practically impossible to achieve. Beyond that it is Pairs of sampling units with approximately the same temperature drift with a high tech niche effort. Temperature stabilization with the help of Peltier elements is complex; the dissipation of temperature radiation within a closed Device housing is fundamentally critical. Therefore, according to claim 17, the one common photoelectric converter preferably received periodic optical Measurement signals and the optical reference signals determining the reference time better sampled by means of a common sampling unit and the time-significant points in the performed according to the invention on the sampled signals.

A particular problem with distance measurement results from the non-linearity of the photoelectric converter. Due to nonlinear effects, the dynamics of the Receive signal for measurement errors. This experience applies regardless of the visual Waveform. In particular, this effect occurs increasingly with avalanche photodiodes those with larger signal amplitudes gain due to saturation effects decreases. Assuming that the product of the gain and bandwidth is constant remains, the bandwidth increases with decreasing gain, i. H. the leading edge of the Detected pulse increases, so that the time-significant point becomes smaller Fair values shifts. With a constant measuring distance, this becomes a test object Measuring distances displayed, which depend on the respective reflectance of the measuring surface. It is therefore u. a. in European Patent Application 57447 (Ref. 3) found that a optimal measurement accuracy can only be achieved if the amplitudes of the optical received reference and measurement signals are as similar as possible. As you know, serve adjustable mechanical attenuators in the form of filter wheels or gray wedges or Electromagnetically operated fiber optic switches in the optical lines of the Reference and / or measurement signals. Such additional devices cause them due to their longer response times extend the measuring time. In addition, they increase the technical effort.

Another way to address the nonlinearity problem is to use devices  to be used with the weakest possible non-linear transmission behavior. Such Hardware-based optimization is lengthy. With the required high measuring accuracy in the sub-millimeter range, for example for quality control tasks remaining non-linear component parameters are often no longer tolerable.

It is therefore more advantageous according to claim 18, the non-linear properties of the used Components exactly through a metrological calibration process measured. A correction reference (zero error correction) can preferably be by equality the signal amplitude of the reference and measurement signal. The calibration The procedure is carried out, for example, in the form of a fixed one Retroreflector received optical measurement signal via an optical attenuator in intensity is gradually dampened. The calibration characteristic, which is particularly important for avalanche Photodiodes is strongly non-linear, is then due to the negative deviations from the Distance meter displayed distance values depending on the intensity of the Received signal given. The correction data are then in the microprocessor unit in analytical form or in the form of a look-up table. A measurement error correction additionally requires a unit for determining the respective measurement technology Signal amplitude.

Another preferred method of measuring error correction on which the invention is based according to claim 19 is the following. There are now high-frequency technology Methods of very precise metrological characterization, modeling as well Simulation of nonlinear components is available (see e.g. G. Kompa, Modeling of dispersive microwave FET devices using a quasi-static approach, for Publication in MMWCAE, John Wiley & Sons, 1995). The application on Photodio leads to nonlinear network models with nonlinear model elements. Over a The transmission behavior can be simulated with different reception levels determine amplitude-dependent signal propagation times with computer assistance; these can then be in the Timing device can be used for measuring error correction. To carry out the Measurement error correction is again the current information about the amplitude of the Receive signals required. The main advantage of this approach is that the non-linear behavior of each component or one is no longer necessary hardware linearization of each component individually by means of complex circuits, or by looking for replacement components with lower non-linear properties optimize, but rather the exact obtained through a calibration process Knowledge of the impact of the respective non-linear properties on the transit time measurement used to create a corresponding computer-aided correction program Measurement error correction of the measured distance, for example in the microprocessor unit to make. In contrast to the purely metrological calibration method according to claim 18 is the computer model generally valid, d. H. regardless of the respective Operating conditions, the microprocessor unit can make an error correction.

A computer-aided error correction of the non-linear transmission properties the well-known insertion of optical dampers, in particular, of photodiodes In principle, signal adjustment is superfluous. This eliminates the control time of the optical Damper, which significantly reduces the measurement time. The received measurement signals can thus directly according to claim 20 supplied to the photoelectric converter be, which significantly reduces the technical effort.  

A distance measuring device according to the invention can be used by means of a scanner 21 can be used for two- and three-dimensional (2D, 3D) measurement tasks. Here the spot diameter of the optical beam determines the lateral range resolution. The problem with these measurement tasks is the use of powerful laser diodes with wide emission apertures, such as given in Ref. 2, because it is due to the Optical imaging laws do not allow radiation at a distance of some Focus on a beam diameter of about one millimeter, which in As part of quality control tasks, a measurement spot value is often required. A 2D or 3D measurement with a high lateral resolution therefore requires according to claim 22 sufficiently small laser apertures, such as those with low SH lasers optical power are given. According to the invention, however, it can be considerable Increased performance through modulation of the laser diodes by means of maximum injection to reach. The application of the current modulation on which the invention is based Maximum injection enables laser emission in a superspike, its optical power is a multiple of the laser diode nominal power with a sufficiently small laser aperture.

A measurement spot minimization of large-area laser apertures can be achieved in that according to claim 23, a laser chip with a larger aperture by tapering an optical Waveguide is reduced to a smaller emission aperture. In a preferred one Embodiment, the optical waveguide taper consists of a tapered optical fiber.

According to claim 24, the scanning of measurement objects can also be done by means of a fiber optic Scanners according to a possible embodiment in DE 39 04 634C2.

The invention will now be described with reference to the accompanying figures in the description which follows explained. From the figures show:

Fig. 1 is a block diagram of a laser range finder known type,

Fig. 2 is a block diagram of a laser range finder according to the invention,

Fig. 3 SH band structure of a laser (A), refractive index profile and intensity distribution of the optical waveguide quasi-static or dynamic in current modulation of a known type (B),

Fig. 4 shows the wavelength-dependent variation of the real part of the refractive index of the laser active material (A) and the wavelength-dependent gain material (B) as a function of the injected carrier density of a SH-laser,

Fig. 5 shows the time course of an optical Super spikes according to the invention,

Fig. 6 shows the inventive pump current generator (A) and a detailed section (B) with discharge circuit and the laser diode,

Fig. 7 is further developments of the invention preferred of the discharge circuit with the laser diode: heat sink with a positive polarity (A), a heat sink with a negative polarity (B), an example of a microsystem structure (C),

Fig. 8, exemplary embodiments of the direct reference signal extraction according to the invention: Joint for reference and measurement signal effective photosensitive area (A), lateral coupling of the reference signal in the photodiode (B),

Fig. 9 equivalent circuit of a broadband photoreceiver switching circuit according to the invention (A), typical pulse shape of a received invention ultrashort laser pulse (B),

Fig. 10 is an equivalent circuit of a photodetector of known type (A) and an extended spare circuit according to the invention (B) for compensating background radiation effects,

Figure 11 is a simplified representation of the time distance measurement according to the invention in ultra-short pulses. Simple technical embodiment with the aid of an integrated package TDC 10000 (A), increased time resolution by means of a microwave oscillator (B)
Fig. 12 shows the block diagram of a known laser range finder according to the sampling principle

Fig. 13 is a block diagram of a laser range finder according to the invention according to the sampling principle

Fig. 14 is a graph for explaining the erroneous determination of the time-significant point at different signal levels due to non-linearity effects of the photoelectric converter.

Fig. 1 shows the simplified block diagram of a range finder known type. A from an oscillator 37 controlled pump current generator 1 with a typical pulse repetition rate of several kilohertz controls a laser transmitter 2 of which has a semiconductor laser source for providing a larger optical power is usually from several electrically in Series connected and stacked semiconductor chips (laser diode stack) exists. The major part 4 of the emitted laser radiation 75 is emitted onto the target 5 via an optical transmitter 3 , reflected there and fed as an optical measurement signal 6 via an optical receiver 7 to a photodiode 9 , which converts the optical measurement signal 78 into an electrical measurement signal 10 .

A small part of the optical transmission power 77 reaches the photodetector 9 (time reference) as a reference signal 14 via an optical coupler 15 . The double pulse occurring at the output is amplified via an amplifier 11 , a gate pulse generator stage 72 is fed and evaluated in a time measuring device 22 . The optical coupler is usually a beam splitter or a small-area deflecting mirror. It is also common to route the reference signal to the detector via an optical fiber 16 .

Usually laser pulses are used which have a pulse length of 5 to 20 ns exhibit. This means that the reference and measuring pulses change during a short Overlap measuring distance; a perfect pulse distance measurement is then no longer  possible. Remedy can then be created in that the reference pulse over a optical fiber of greater length is delayed so much that it also in the case of maximum Measuring distance arrives at the detector after the measuring pulse. Under certain circumstances, this can Lead fiber lengths of a few hundred meters. It is particularly disadvantageous that the long Reference fiber due to the finite radius of curvature a larger volume of the transmit / Receiving unit results. Furthermore, temperature-related changes in length of the Fiber practically uncontrollable transit time changes of the optical pulses, so that for a precise distance or transit time measurement according to the invention a longer optical Reference fiber must be avoided.

The photo receivers of known type used in distance measuring devices have Bandwidths of typically 250 MHz. The pulse edges for determining the time-significant points typically at 1.5 ns, so that in nominal operation by an SH- Laser diode did not provide optical pulses with a typical rise time <1 ns fully used, which in principle deteriorates the potential measurement accuracy means that this is directly proportional to the rise time. The aim of a radar concept it should rather be for the accurate distance measurement, the relevant pulse edges make sure that their rise times are as short as possible than the required measurement resolution. This would in particular address the known problems of dynamic timing omitted. Compared to known methods, the proposed solution comes here general goal very close.

FIG. 2 shows the simplified block diagram of a distance measuring device according to the invention for carrying out the method according to claim 1. The basic mode of operation is comparable to the illustration in FIG. 1. In addition, there are significant advantageous further developments, which are explained in more detail below.

According to the invention, the distance measuring device works with ultra-short laser pulses Pulse repetition frequency with the advantages of a higher measuring accuracy and a higher Measuring speed. Furthermore, the ultra-short pulses also allow measurement in the Close range, since there is no pulse overlap from a distance of a few millimeters occurs. This has the advantage that the usual reference fiber with temperature-related Changes in term can be omitted. The steeper pulse forms help that the Determination of the time-significant points can be done much more accurately. Overall lead the claims according to the invention thus to considerably improved distance measuring devices, which can be used in particular for 1-D, 2-D and 3-D quality assurance tasks.

The ultra-short laser pulses are generated with very little technical Effort preferably with inexpensive SH lasers. With a preferred one Execution of a distance measuring device according to the invention with a common one Integrated sampling stage for the measurement and reference signals become typical radial Measurement uncertainties in the micrometer range reached. Lateral resolutions are in the Order of 1 mm.

An essential inventive task is ultra-short for distance measurement Use laser pulses. The SH- and DH- mostly used in distance measuring devices Laser diodes work in quasi-static operation, i. H. the time course of the pump current is so slow that the intensity of the laser emission of the semiconductor laser changes accordingly  the power-current characteristic curve. Like the corresponding data sheets of the manufacturers can be removed from laser diodes, the required reference lies with DH lasers Pump current value at about 1 ampere / watt; for SH lasers this increases to about 4 Amps / watt. Leave high pump current values on the order of a few tens of amperes pump current generators containing mercury switches or thyristors, produce. However, their pulse repetition frequency is in the Hertz or lower kilohertz range limited. Avalanche transistors as fast switches are particularly popular in Pumpstromge Generators with lower switching power are widely used. For switching higher pump currents an arrangement of several transistors connected in parallel is often used, whereby the synchronization of the switching time of the individual transistors is difficult. The Pulse repetition frequencies of known distance measuring devices with higher output power are therefore often below one kilohertz. There are also occasionally higher ones Pulse repetition frequencies up to about 10 kHz have become known.

In dynamic operation with a distributed rising edge of the pump current pulse, the laser emission is no longer synchronized with the pump current. A typical transient behavior of the laser occurs, which is known as relaxation oscillation or spiking. The distance measuring device of known type described in Ref. 2 works with powerful optical transmission pulses, which consist of a few oscillation maxima or laser spikes.

FIG. 3 is a simplified representation shows the band structure of a SH-laser known type. A detailed description of Ppn Al _0.3 Ga is _0.7 As GaAs material system, for example, in HC Casey and MB Panish, heterostructure laser, Academic Press 1978. Both with quasi-static and dynamic modulation with current amplitudes up to about 3 times the threshold current, the injected charge carriers only make the asymmetrical refractive index profile 28 of the optical waveguide of the laser resonator caused by the AlGaAs / GaAs heterojunction 30 and by the pn junction 31 changed insignificantly. In the given multilayer material system, consisting of a p-doped AlGaAs layer 25 , a p-doped 26 and n-doped 27 GaAs layer, the emission wavelength for doping of known type is approximately 904 nm. It does not change significantly during the quasi static or dynamic operation. The transverse distribution of the intensity 29 of the optical wave in the laser resonator decreases sharply despite one-sided weak index guidance due to the relatively small jump in the refractive index of the pn junction outside the laser-active region 26 .

According to the invention, the distance measuring device works with optical pulses of high power and high pulse repetition frequency, which consist of ultra-short laser spikes which largely have the shape of symmetrical triangular pulses. According to the invention, this is achieved by dynamic maximum injection of charge carriers into the active zone of an SH laser, which is a preferred embodiment of a semiconductor laser with weak index guidance. For example, if the maximum permissible pump currents for an SH laser are 3 to 4 times the threshold current in nominal operation of known type, the current amplitudes for the maximum injection are many times higher. In the distance measuring device according to the invention, the pump current amplitude for the SH laser used corresponds approximately to 2.5 to 3 times the permissible maximum current value in quasi-static nominal operation. Due to the associated higher power loss, a significant reduction in the pump current pulse width is required, which is achieved by a compact hybrid circuit design of the pump circuit. The current pulse width is typically in the order of 2 ns with current amplitudes of a few 10 A. Because of the very short pump current pulses compared to distance measuring devices of a known type, far higher pulse repetition rates are possible. The pulse repetition frequencies are usually on the order of a few kHz. Ref. 2 reports a maximum frequency of 10.48 kHz. The distance measuring device on which the invention is based preferably operates at a frequency of 40 kHz. The given design allows operation above 100 kHz without further technical effort. When using substrate materials with an increased thermal conductivity to stabilize the temperature of the transistor switch in the pump circuit, repetition frequencies of a few 100 kHz can be achieved. Compared to known rangefinders, this means a significant increase in performance.

The ultra-short laser pulses consist of a single initial spike of the relaxation oscillation of the laser, the optical peak power of which exceeds the nominal laser power in quasi-static and dynamic operation by a multiple, for example by a factor of 10 to 15. In this case, the laser works in the index-guided gain Switching mode. Without pump current, the multilayer material system of the laser resonator is an optical waveguide with weak index guidance on one side. In quasi-static and dynamic nominal operation, the transmission behavior of the waveguide is hardly changed by the injected charge carriers. In the pump current modulation of a maximum injection according to the invention, the usual values of the charge carrier densities are considerably exceeded, so that the jump in refractive index 31 caused by the different doping of the GaAs material is reduced more and more with increasing charge carrier density. The index guidance of the pn transition thus decreases, with the result that the intensity distribution 29 expands increasingly into the passive region 27 , which results in a decrease in the so-called confinement factor and thus an increase in the quasi-static threshold current. If the pump current pulse is shaped in such a way that its rising edge is extremely steep, even after reaching the so-called critical carrier density, in which laser emission can occur after a certain delay, charge carriers can still be pumped vigorously into the laser-active region. The confinement factor continues to decrease with the injected charge carriers, but the material gain increases. If the modal gain from material gain and confinement factor exceeds the resonator losses, then an ultra-short laser emission takes place. Due to the much higher charge carrier density compared to the nominal operation, the laser intensity exceeds the nominal power many times over. In this index-guided gain switching operation of the SH laser according to the invention, the maximum amplitude of the pump current is limited by the fact that, with the injected charge carriers, an index guidance of the optical wave in the laser resonator - even if it is very weak - is just maintained. An indication of this gain switching laser mode according to the invention is the wavelength of the laser emission, which approximately corresponds to that in nominal operation.

Another preferred type of modulation of SH lasers on which the invention is based is the index-guided Q-switching mode, which requires current pulses with extremely fast current pulse rising edges and extremely high pump current amplitudes. In this case, the carrier density increases so much within the normal delay time between the current pulse and the laser emission that the indexing of the optical waveguide given at lower pump current pulses is initially canceled (see refractive index curve 67 in FIG. 3B). Fig. 4A shows the principle of wavelength-dependent course of which depends on the carrier density change in refractive index .DELTA.n. The carrier density n ₁ 32 represents the nominal operating state common in SH lasers. The change in refractive index caused by the injected charge carriers is negligible. The carrier density n₂ 33 describes a maximum injection of charge carriers with a serious impact on the refractive index profile and thus on the propagation properties of the optical waveguide of the laser resonator. Laser emission with the usual wavelength of approximately 904 nm is no longer possible since the injected charge carrier density makes a negative contribution to the real part of the refractive index 34 and the jump in the refractive index of the pn junction is reduced. Since the optical waveguide no longer exists, additional charge carriers can be injected. The material gain increases with the load carriers. As FIG. 4B illustrates, the material gain is also dependent on the wavelength. With increasing carrier density, the maximum of the gain shifts to smaller wavelengths. The product of material gain and confinement factor, the so-called modal gain, is essential for the onset of laser activity. Laser emission is then given in the index-guided Q-switching mode as soon as the modal gain more than compensates for the laser resonator losses. With the continuous charge carrier injection with suppressed index guidance, the material gain to smaller wavelengths increases more and more, at the same time the refractive index in the laser-active range increases in this wavelength range. After an abnormal delay time of a few nanoseconds, the SH laser finally emits at a wavelength that is smaller than the nominal operation. Due to the anomalous delay time, the stored charge density is particularly high, which leads to a superspike of the laser emission with an extremely high amplitude.

The delay time after a laser is powered up to its emission passes, as is known for a given pump current amplitude of the value of the threshold current dependent. Since this depends on the temperature of the laser, the delay time can be over temperature stabilization can be set in a controlled manner. Laser diodes with low Threshold current densities at maximum injection in index-guided gain switching mode work, can be according to claim 34 by temperature-controlled lifting the Simply convert the threshold current values into the index-guided Q-switching mode.

The optical superspike according to the invention shown by way of example in FIG. 5 by means of dynamic maximum injection differs fundamentally from the signal forms of the known type which are used in optical distance measuring devices. In particular, they have a much steeper rising edge, which, when used in distance measuring devices with single pulse evaluation of a known type, leads directly to a considerable increase in performance in terms of measuring accuracy and measuring speed.

The novel modulation of the dynamic maximum injection of typical SH laser diodes on which the invention is based means that these emit a multiple of their optical nominal power. The nominal power of an LD-60 is 2.3 watts. As shown in FIG. 5, the laser diode emits about 90 watts in the new operating state, ie an optical pulse power that is 39 times higher. Two goals are thus achieved at the same time: the bandwidth of the ultra-short optical signals according to the invention is far larger than the known type. With a far steeper optical pulse edge compared to known waveforms by a factor of 100 to 200 and power increases by a factor of 30 to 40, it is therefore obvious that the use of the ultra-short pulses for transit time measurement means a significant improvement in the measurement accuracy of the distance measurement.

The ultra-short transmit pulses of high optical pulse power can with a high Pulse repetition frequency can be emitted without the internationally recognized laser limit values for exceeding eye safety. So the pulse repetition frequency can be related to Increase measuring devices of a known type in a 1D measurement by a factor of 10 to 20. In order to can significantly shorten the measurement time from the point of view of eye safety will.

Pump circuits for laser diodes are usually built with components in conventional assembly and connection technology of the so-called "printed" type Circuits ". Such a conventional technique limits the packing density of the Pump circuit and leads in relation to a planar hybrid structure with unhoused Chip components for an electrically longer cable routing. The accompanying Parasitic electrical effects slow down the in the pump circuit occurring electrical switching processes and thus to a delayed discharge of the Charge storage. This inevitably leads to the required nominal pump currents to longer current pulses and thus wider laser pulses. Usual laser pulse widths are in the order of 5 to 10 ns. The eye safety of a laser removal messers requires that the average emitted optical power has a defined one Limit does not exceed. This is the reason why known measuring systems are relatively have a low pulse repetition rate of a few kilohertz.

Fig. 6 shows a practical embodiment of the pumping current generator 1 of the distance measuring device according to the invention with discharge circuit according to claim 38. It is an avalanche pulse generator; exemplary versions can be found in the patent specifications specified under Ref. 2 . The charge lines used there as charge stores are advantageous if current pulses with a pulse roof of defined amplitude are to be generated. However, it is disadvantageous that the voltage swing provided by the avalanche transistor is then only half as large as in the case of a charging capacitor. The dynamic maximum injection of charge carriers into the semiconductor laser according to the invention requires a special construction and connection technology that avoids parasitic inductive influences of any kind in the discharge circuit as far as possible. The circuit is therefore implemented in a preferred embodiment using hybrid technology. For the index-guided Q-switching effect according to the invention, it is essential that, in contrast to Ref. 2 , the laser diode in chip form is also integrated into the pump circuit, since the laser diode housing slows down the dynamic current pulse leading edge due to the parasitic housing reactances and prevents the Q-switching mode can. Furthermore, the superspike emission requires a single diode; a laser diode stack would lead to a stepped optical front edge that would no longer make it possible to determine the precise precise time. The diode laser chip is bonded to a heat sink (open heatsink), which is preferably at a positive potential with respect to ground, so that it is electrically insulated from the metallic base body of the substrate carrier of the discharge circuit. Due to the temperature stabilization of the laser diode, the heat sink must also be thermally insulated in order to keep heat losses as low as possible. According to claim 37, semiconductor sources in the form of laser diode arrays are also suitable which, because of their strong optical coupling, generate uniform, ultra-short optical pulses. The chip capacitor 52 is charged via the supply voltage at the contact 57 and the collector resistor 56 . Compared to a charging line, a charging capacitor enables a considerable reduction in the area of the pump circuit. The realization of the collector resistor by means of several chip resistors connected in series has the advantage that the average power loss is distributed over several resistors. The individual chips can thus be designed for correspondingly lower power values, which in turn has a favorable effect on the design. The output signal of the 40 kHz oscillator (not described in more detail), which functions as a trigger generator for the pump circuit, reaches the base of the avalanche transistor 64 via an input network comprising the resistor 59 and the capacitor 60 . Resistor 61 serves as reflection-free termination of the trigger signal and is connected to ground in terms of radio frequency via a capacitor 62 . The bias on contact 63 is used to adjust the base bias. In a preferred embodiment, the resistor 61 , which due to its low-resistance value does not lead to self-triggering of the transistor, is connected directly to ground. When the transistor is triggered, the capacitor 52 is discharged in the shortest possible electrical manner via the collector-emitter path of the transistor 53 and the laser chip 49 to the ground 47 . It is of particular importance that the discharge circuit is arranged in such a way that its minimum electrical length is limited only by the design of the component chips and by the line lengths required for wire bonding of the transistor and laser diode.

According to claim 39, in a further embodiment of the invention, laser diode 49 , transistor 53 and charge store 52 are arranged concentrated on a common heat sink 48 . On the one hand, this has the advantage of reducing the electrical connections of the discharge circuit, on the other hand, a simplified joint temperature control of the laser diode and avalanche transistor is possible.

Another embodiment of the discharge circuit with a laser diode according to the invention is shown in FIG. 7A according to claim 40. With such an optimal packing density, laser diode 49 , charging capacitor 52 and avalanche transistor 53 are preferably arranged in a row at a short distance from one another, so that the capacitor can be discharged via the transistor and the laser diode in the shortest possible way. The electrically conductive bonding surface 65 is electrically insulated from the heat sink 48 via a thin substrate, which is not further identified. The insulating substrate and conductive bonding surface can also be easily produced using sputtering techniques.

FIG. 7B shows a corresponding embodiment according to the invention of claim 41, if the cathode of the diode laser chip is located on the heat sink potential. In this case, the components are arranged in the order of the laser chip transistor capacitor. Of particular rem advantage shows that the heat sink is now at ground potential, so that a ground contact to other circuit parts of the distance measuring device can be produced in a simple manner. The complete integration of the laser diode and the discharge circuit is of great benefit for the dynamic maximum injection due to the further reduction of the parasitic circuit effects, on the other hand it leads to a miniaturization of the laser transmitter.

Fig. 7C shows another exemplary microsystem technical realization of the invention the transmitting unit in quasi-monolithic structure and Verbinungstechnologie (AVT) as, for example, in A. Fathy et al., "Miniature Gain block Satellite Communication Transceivers", MTT-S '87 565-567. According to the invention, the base body 99 consists here of silicon material, since this can be processed well micromechanically by means of dry etching techniques. For example, different depressions can be generated in a Si substrate. Good electrical insulation is also ensured by lightly applying an approximately 2 μm thick SiO₂ layer (quartz) 101 . Electrically conductive contact surfaces 48 and 65 are produced using sputtering techniques. After inserting the appropriate component chips such as laser diode 66 and transistor 53 , these can be glued with polyamide. The photosensitive polyamide can be structured easily, and thus represents a suitable mask material for the electrical contact 98 to be subsequently carried out.

According to claim 43, the optical reference signal according to the invention is obtained via the rear laser emission of a semiconductor laser chip. The optical reference signal can be supplied according to claim 44 via an optical fiber. In a particularly advantageous embodiment variant, the optical reference signal is fed directly to the photoelectric converter. Fig. 8A illustrates such an embodiment of an integrated transmitting and receiving unit of the distance measuring the invention. A special feature of the given arrangement is given in that the diode laser chip 66 and the chip 102 of the photodiode are arranged in the immediate vicinity in such a way that the parasitic radiation emerging at the rear aperture of the laser serves directly as a reference for the pulse transit time of the transmission pulse 77 . The optical coupler 15 shown in FIG. 1 and the reference fiber for the optical connection of the separate structural units of the laser transmitter and photodetector are thus omitted. In the arrangement according to the invention, these are integrated with each other in terms of microsystem technology in a structural unit.

While in Fig., The reference radiation through a diffuser 103 according to claim 47 or 48 is spread on the common radiation-sensitive area of the diode 102 8A, the coupling is carried out in Fig. 8B according to claim 49 in the lateral direction. The power of the rear laser radiation and the adjacent position of the photodiode lead to an electrical reference signal of sufficient amplitude despite the lower lateral photosensitivity.

The reception of ultra-short laser pulses requires corresponding broadband receiver circuits. FIG. 9A shows a possible embodiment according to claim 26 and FIG. 9B shows a preliminary output signal of an ultra-short optical pulse received at the input. The broadband amplifier preferably consists of a Si-PIN photodiode 79 , a trance impedance amplifier stage 80 with a feedback resistor R _F 81, a further amplifier stage 82 and an output driver 83 . The field effect transistor module used is, for example, the microwave HEMT of type JS8902-AS from Toshiba. Due to the circuit frequency bandwidth of a few gigahertz, the circuit is constructed in planar technology. The FET components are integrated into the planar circuit using "chip and wire" technology. An embodiment of the circuit in monolithic technology as MMIC is also conceivable.

In a further embodiment, due to the higher sensitivity in the 50 according to the invention a fast An avalanche photodiode according to claim 53 with rise times of the order of magnitude 100 ps used. Although they do not match the given range of ultra-short Optical pulses are adapted, the embodiment according to claim 17 enables the solution the measurement task of a very precise three-dimensional measurement of Area elements with a radial resolution in the submillimeter range.

Measurement on glowing surfaces represents a particular measurement problem. Because of the predominant background radiation, this results in an additional DC current component in the photodiode, which, based on the equivalent circuit diagram of an avalanche photodetector of the known type in FIG. 10A, on the series resistor R1 73 causes an additional voltage drop, as a result of which the set operating point of the diode 72 shifts and the multiplication factor M changes. This changes the signal amplitude and, due to the non-linear character of the photodiode, also the signal shape. A reduction in the resistance R1 can reduce the influence of the background radiation, but cannot completely eliminate it. In addition, R1 serves as a series resistor for current limitation and can therefore not be chosen to be as small as desired. FIG. 10B shows in accordance with claim 54 an extended photodetector circuit of the invention, the _v by appropriate correction of the detector bias U 71 compensates for the background radiation effect on the photodiode voltage U _APD. This is done via a controller unit 70 , which registers the instantaneous value of the working current I _{DC of} the photodiode via a measuring resistor R2 74 . Deviations from the target value are corrected by corresponding changes in the detector bias voltage U _v . For the characteristic of the controller, there is obviously the functional relationship between the control voltage U _R2 and the detector bias voltage U _v : U _v = U _APD + ((R1 + R2) / R1) U _R2 with R1 «R2. If the internal series resistance Rs of the photodiode is also to be taken into account, R2 must be increased by this resistance value. The series resistance can generally be neglected with Rs «R2. In a preferred embodiment of the distance measuring device according to the invention, the time constant is significantly faster than the repetition frequency of 40 kHz and much slower than the time duration of an optical pulse.

A serious advantage of this detector circuit according to the invention is that Interference filter to block the background radiation can be dispensed with. This in turn is a prerequisite for optical pulses whose optical bandwidth is not very is small, as is the case with chirped pulses, for example.

Due to the ultra-short pulse edges, the pulse distance measurement of reference and received signal (double pulse) can no longer be carried out with the known integrated circuits. ECL gates have typical rise times of 200 ps and are therefore not suitable for evaluating the ultra-short pulses. An embodiment of the distance measurement according to the invention, shown in simplified form, is shown in FIG. 11A. It is characterized by a very low technical effort with a relatively high measuring accuracy. The double pulse received by the photodetector 9 and amplified via the amplifier chain ( 80 , 82 , 83 ), consisting of the reference and measurement signal, is applied via a matched signal divider 84 to the positive input of an ultra-fast comparator, preferably using GaAs technology 85 given, on the other hand, a part is delayed on the negative input, which is biased via a reference voltage so that noise effects at the input of the comparator are suppressed. The comparator essentially contains an ultra-fast differential amplifier as described in principle in S. Feng et al., "A 4 Gs / s and 10 mV latched comparator in 0.5 µm GaAs HEMT technology", ESSDRC (1991) (Ref 3) is known. The circuit design in 0.5 µm FET technology allows the processing of signal frequencies up to about 4 GHz. Since the upper frequency limit is essentially dependent on the input capacitance of the FETs used, the embodiment of the comparator according to the invention provides microwave FETs with gate lengths of typically 0.15 to 0.25 µm, so that they are 2 times larger than Ref. 3 until 3 are faster. This makes it possible to trigger the ultra-short impulses properly. The time delay t _d1 determines the width of the rectangular pulses occurring at the output of the comparator.

According to claim 29, the time interval measurement of the pulses occurring at the output of the comparator 85 takes place in such a way that the shaped double pulse via a signal divider 86 is delayed on the one hand to the START input and, on the other hand, without delay to the STOP input of a highly accurate time difference meter 87 of the TDC type 10000 from the company MSC. The time interval measurement starts as soon as a start signal is present. Since the time delay t _{d2 is of} the order of magnitude of the pulse duration of an ultra-short pulse, a reference pulse first arrives at the STOP input and is ignored. The reference pulse also arrives at the START input with a time delay and starts the time measurement process. The time evaluation ends when the measurement signal arrives at the STOP input. The single-shot range measurement resolution of the integrated module in CMOS technology is 50 ps, which corresponds to a range resolution of 7.5 mm.

Fig. 11B shows a circuit variant according to the invention with high time resolution according to claims 30 and 31. The output signal of a microwave oscillator 88 with the frequency f₁ passes through a divider 89 to the mixer 90 , which has an intermediate frequency | f₁ with the frequency f₂ of a local oscillator 91 at the output -f₂ | provides. After passing a filter 92 , the signal is then amplified by means of an amplifier 93 . The output signal then corresponds to the signal of the oscillator 37 in FIG. 1. The signal of the microwave oscillator continues to reach a counter unit 94, which is not described in detail. Since the transmission signal frequency is synchronized with the oscillator signal, the maximum error in the time evaluation of the gate pulse corresponds to the time interval between two counting pulses.

FIG. 12 shows the block diagram of a known optical distance measuring device according to Ref. 2 , which works according to the sampling principle. A pump current generator 1 controlled by an oscillator 37 with a pulse repetition frequency (PRF) of 10.48 kHz controls a laser transmitter 2 , which consists of a plurality of electrically connected and stacked semiconductor chips (laser diode stack). Because of the greater power losses due to reflection at measurement planes with a low degree of reflection, single hetero structure lasers are preferred. The major part 4 of the emitted laser radiation 75 is emitted onto the target 5 via an optical transmission system 3 , reflected there and fed to the measuring channel 8 as an optical measurement signal 6 via an optical reception system 7 . A photodiode 9 converts the optical measurement signal 75 into an electrical measurement signal 10 . This is amplified via an amplifier 11 and fed to a measuring channel sampling stage 12 . In the case of an avalanche photodiode with an internal amplification, the photoelectric converter 9 and the amplifier 11 form a structural unit 23 . An optical interference filter 13 ensures that the background radiation is blocked. The distance of the measuring device to the measurement object, which obviously corresponds to twice the transit time of the transmission pulse from the measurement device to the measurement object, requires a time reference which defines the time of the transmission pulse transmission. Such an optical reference or start pulse 14 is obtained in that, as can be seen in FIG. 1, a small part of the optical transmission power 77 is passed via an optical coupler 15 via an optical reference fiber 16 to a further photo receiver 17 , which serves as a reference receiver becomes. Furthermore, the cascaded function blocks of the reference channel 45 correspond to those of the measuring channel. The unit 23 , which has an avalanche photodiode with internal amplification, can usually be replaced by a photodiode without internal amplification in the case of the reference receiver due to the sufficiently available optical reference power, so that the amplifier block 18 in the unit 24 is omitted. The time interval measurement is carried out on the sampled reference 20 and measuring pulses 21 in a time measuring device 22 . A particular problem arises with the known system concept that the sampling of the reference pulses and measuring pulses takes place by means of separate sampling stages 12 and 19 . Experience shows that temperature synchronization of the sampling stages is hardly possible over a wide temperature range. A temperature stabilization of the complete stages is quite conceivable, but hardly justifiable due to the considerable technical effort. It is therefore advantageous according to claim 50 to couple the reference signal obtained in FIG. 12 via an optical coupler into the measuring channel, preferably between interference filter 13 and photodiode 9 . In further embodiments, this can also be in front of the receiving optics 7 or in front of the interference filter.

As can also be seen from ref. 2 , the pulse width of the optical transmission signals is above one nanosecond. Given the given optical peak power of a few watts and the pulse repetition frequency of approximately 10 kHz, the laser radiation from the measuring device can be classified as eye-safe (class 1 laser system). Given the given optical pulse widths and the high repetition frequency required due to the sampling principle, the laser diode could only be driven with a current amplitude at which it emitted only 20 to 25% of its nominal optical power in order to comply with the safety regulations. A further shortening of the optical pulse is desirable, since this leads to a higher optical performance and a higher pulse repetition rate while maintaining eye safety and the measuring process is accelerated.

Fig. 13 shows an embodiment of the distance measuring according to the invention according to the preamble of claims 1 and 25 and 50. It corresponds to the system concept according to FIG. 12, but the reception of the optical reference and measurement signals is carried out according to the invention by means of a single photodiode and the sampling of the reference and measurement signals according to the invention via a common sampling stage, so that a temperature drift affects the electrical reference and measurement pulses in the same way and thus has no influence on the accuracy of the distance measurement. A suitable embodiment of a sufficiently broadband sampling stage is described, for example, in W. Thomann et al., "A 40 GHz / 9 ps sampling head for wideband applications", Proc. EuMC '91, 830-835. A transmission optics 3 , which preferably consists of two achromats, focuses the optical transmission beam 4 via a deflection mirror 35 and mirror scanner 36 in the measurement plane 5 . There the impulse is reflected diffusely. Part of the reflected radiation is preferably directed via an XY mirror scanner 36 onto a further two-lens arrangement 7, preferably made of achromatic lenses, and focused onto a photoelectric converter 23 , which preferably contains an avalanche photodiode. To generate the reference pulse, a small part of the optical transmission pulse is passed directly to the photodiode via an optical fiber 16 as a reference path. At the output of the photodetector 23 there are two electrical pulses in the form of a double pulse, the time interval between which is a measure of the distance to the object to be measured. The fast electrical pulses at the output of the photodiode are fed to the input of the sampling stage 12 via a bias network 43 , which provides the bias voltage for the photodiode. By means of an A / D converter 38 according to claim 51, the sampled analog double signal 44 is converted into a corresponding digitized signal and then transmitted according to claim 52 to a microprocessor unit 39 , which takes over the data evaluation, ie determines the time interval of the double pulses and their amplitudes. According to claim 14, the time is determined preferably by means of the pulse leading edges of the sampled reference and measurement signals. Another task of the microprocessor unit is to control the mirror scanner. From the determined radial distance information and the position of the scanner mirror, the microprocessor unit can calculate a three-dimensional surface contour of the measurement object, which is displayed graphically on the display unit 40 .

A particular problem of the transit time measurement arises from the signal dynamics of the measurement signals. Experience shows that the distance value displayed by the measuring device generally depends on the intensity or the amplitude of the received signal. This applies to any signal forms, in particular also to harmonic signals, which are used in the phase comparison process. This becomes clear, for example, by means of standardized, sampled received signals from an avalanche photodiode in FIG. 14. With the increase in the reception amplitude, the point in time defined by a 40% signal level shifts to shorter time values (t₃ <t₂ <t₁). In order to avoid such signal level-dependent transit time errors due to the non-linear properties of the photoelectric converter with large signal modulation as far as possible, the intensities or the amplitudes of the reference and measurement signals are adjusted to one another in a known manner with the aid of adjustable attenuators such as gray wedges and filter wheels Photoelectric converter for both signals is driven in the same way. Signal tracking always takes time and delays the dynamic measurement process. The technical implementation of electrical or optical control elements also means additional effort.

In a preferred embodiment, the signal level-dependent time shift which can be seen in FIG. 14 is error-corrected in the microprocessor unit due to the non-linear transmission properties of the photodiode according to claim 19 via a suitable non-linear network model which is obtained via the known process of experimental modeling.

The stored data at the output of the A / D converter correspond to the Equidistant times sampled signal values of the double pulse. A direct one Distance measurement on the time-discretized signals of the double pulse caused by the If a defined signal threshold value is exceeded in each case, the result is maximum evaluation error that is twice the time interval in succession Corresponds to sampling points. With a measuring range of, for example, 2 m and a required  A resolution of 1 mm should be at least 2000 sampling points. According to claim 55 the time-significant point is not determined directly via the time-discrete Signal data, but according to the invention indirectly via a linear regression. To do this preferably all sampling points on the respective pulse leading edge in the range of 10 and 90% of the signal amplitude lie over a best-fit line in the sense of the smallest Squares of errors adjusted. The intersection with the selected threshold then results an improved point compared to the direct time determination z 03498 00070 552 001000280000000200012000285910338700040 0002019513823 00004 03379. A subsequent integration via further measurement events enables an improved Distance measurement. For reasons of time, it is cheaper according to claim 56, the time Integration of the time-discretized signals before determining the time-significant points to make. In the exemplary embodiment of the distance according to the invention with a measuring range of 2 m, 1024 time-discretized sampling points and integration over 16 measurement events a measurement uncertainty of 200 to 300 micrometers reached.

The distance measuring device according to the invention works with a mirror scanner 36 for 3- D measurement. According to claim 24, a fiber optic scanner can be used, which is known in principle from PS 3904634C2. In an embodiment according to the invention, the transmission beam focusing and the reception of the reflected optical pulses are not carried out via an oscillating transmission and reception optics, but via a common static transmission / reception optics. Such an arrangement leads to larger reception apertures. An optical transmission / reception unit is also conceivable, which consists of a combination of an oscillating transmission fiber according to OS 3825081 A1 and a static reception optics. The vibrating fiber can be arranged coaxially or parallel to the optical axis of the receiver. In the parallel arrangement, the plane of vibration is tangent to the contour of the receiving lens. The beam focusing is preferably carried out with a resonating Selfoc lens. In the case of a coaxial arrangement, the receiving lens preferably has a gap for receiving the oscillating transmission fiber. Because of the shadowing effect, the fiber can also be arranged with a small offset from the optical axis of the receiver.

The change in position of the measuring spot caused by the oscillating transmission fiber leads to displacements of the imaging point on the photosensitive detector surface. Large diodes have long response times. According to the invention A photodetector according to claim 60 comprising a photodiode array consisting of several closely adjacent and linearly arranged and electrically isolated diode chips.

In a further embodiment variant according to claim 67, the direction of the Selfoc lens focused transmission beam via an oscillating small deflection mirror, which is attached to the end of the mechanical vibrator, for example, rotated by 90 ° so that in the resting phase of the oscillating transmission fiber the optical axes of the transmission beam and the receiving optics match. The beam deflection can also have a The fiber end is chamfered.

The determination of the transmitted beam direction is carried out by determining the position of the respective deflection of the transmission fiber, which is preferably deflected via a spring tongue becomes. Preferably, according to claim 61, the metal tongue via a light emitting diode illuminated and the reflected light from a position detector, for example of the type  S3274 received. The transmitter and receiver are preferably offset by 90 °. With a such a simple and inexpensive arrangement, the fiber position is high Accuracy can be determined.

Claims (67)

1. Distance measuring method according to the principle of the transit time measurement of an optical measuring pulse, which is emitted by a measuring device, reflected on a target object and received as an optical echo signal by the measuring device by means of a detector device which determines a measuring time linked to the receiving time of the optical measuring pulse in a defined manner , whose time interval to a reference time linked to the transmission time of the optical measuring pulse in a defined manner is measured and evaluated to determine the transit time, characterized in that ultra-short pulses of high optical pulse power and high pulse repetition frequency with a low average power are used for the transit time measurement . 2. Distance measuring method according to claim 1, characterized in that the ultra-short optical pulses are preferably generated with the aid of semiconductor lasers, the Laser resonator has an optical waveguide in the vertical or lateral direction has weak index management on one or both sides. 3. Distance measuring method according to claim 2, characterized in that the semiconductor preferably a single heterostructure or double heterostructure type laser turlasers with quasi-index management. 4. Distance measuring method according to claims 2 and 3, characterized in that by means of a dynamic maximum injection of charge carriers into the active zone of the Semiconductor laser the rate of change of the pump current is so large that within the Delay between the start of pumping and the potential onset of Laser emission in the normal index-guided operating state the charge carrier density in Laser resonator has grown so much that due to the change in the index profile a index-guided optical wave propagation in the laser resonator is suppressed. 5. Distance measuring method according to claim 4, characterized in that by means of dynamic maximum injection of charge carriers, the laser emission preferably in one index-guided Q-switching mode. 6. Distance measuring method according to claim 3, characterized in that the main beam Direction of the laser emission is preferably perpendicular to the emission aperture. 7. Distance measuring method according to claim 3, characterized in that the main beam direction of the laser emission is inclined to the normal of the emission aperture. 8. Distance measuring method according to claim 3, characterized in that the double heterostructure lasers with symmetrical quasi-index guidance laterally generated leaky waves as optical transmit and be used as optical reference signals. 9. Distance measuring method according to claims 1 to 4, characterized in that the Single heterostructure laser made of at least three material layers with different doping consists of a first heterojunction with a large index jump and a second  with a very small positive or negative or missing index jump. 10. Distance measuring method according to claims 62 and 1 to 4, characterized in that the single heterostructure laser additionally has a saturable absorber and in a pure index-guided Q-switching mode. 11. Distance measuring method according to claims 1 to 4 and 62, characterized in that the ultra-fast laser pulses have a chirp with red shift and by means of Pulse compression converted into a shortened optical pulse with increased amplitude will. 12. Distance measuring method according to claim 1, characterized in that for generation the ultra-short optical pulses serve mode-locked laser diodes. 13. Distance measuring method according to claims 1 to 12, characterized in that the ultra-fast laser pulses are true to the shape of the signal using broadband optoelectronic Converters converted into electrical signals and true to form over subsequent broadband be amplified. 14. Distance measuring method according to claim 1, characterized in that in the over a photoelectric converter obtained ultra-short electrical pulses Determination of the time-significant point on the pulse leading edge or on the pulse trailing edge or using both the leading and trailing edge he follows. 15. Distance measuring method according to claims 1 and 14, characterized in that in the case of the ultra-short electrical pulses, a time determination at least over one Averaging the time-significant times on the pulse leading and trailing edge he follows. 16. Distance measuring method according to claims 1, 14 and 15, characterized in that over a fixed threshold on both the front and back pulse edge of a pulse a time measurement point is obtained and the determination of the time signifi edge point by averaging the time measuring points on the pulse front and Pulse trailing edge occurs. 17. Distance measuring method according to claims 1 and 13, characterized in that those received from a common photoelectric converter are preferably periodic rule optical measuring signals and the optical determining the reference time Reference signals are sampled using a common sampling unit and the Determination of the time-significant points on the sampled signals in a corresponding manner Way as is done with the non-sampled signals. 18. Distance measuring method according to claims 1 and 17, characterized in that intensity-dependent internal transit times in the measuring channel, preferably the non-linear opto electrical transmission behavior of the photodiode, error correction via a calibration procedure be greeded.   19. Distance measuring method according to claim 18, characterized in that a Measurement error correction preferably via a non-linear computer model of the photodiode he follows. 20. Distance measuring method according to claims 1 and 17, characterized in that the measurement signals received via a focusing device preferably directly Photoelectric converters are supplied. 21. Distance measuring method according to claim 1, characterized in that in the beam lengang is also a 1-D or 2-D scanner. 22. Distance measuring method according to claim 21, characterized in that laser sources can be used with small laser apertures. 23. Distance measuring method according to claim 22, characterized in that the large flat emission aperture of a laser diode using a tapered optical waveguide, which preferably consists of a tapered optical fiber, is reduced in size. 24. Distance measuring method according to claim 21, characterized in that for topographical scanning a fiber optic scanner is used. 25. Distance measuring device according to the principle of the transit time measurement of an optical measuring pulse with a transmitter for emitting optical transmission pulses, a receiver for receiving the echo pulses generated by reflection of the transmission pulses on a target object and for receiving the reference pulses in a predetermined temporal relationship to the transmission pulses and with one Time measuring device for measuring the time interval between each reference pulse and an associated measuring pulse, for carrying out the method according to claim 1, characterized in that the ultra-short optical reference signal ( 14 ) and the ultra-short echo signal ( 78 ) obtained via an optical receiving system ( 7 ) placed on a common photodiode ( 9 ) and the signals are converted into electrical reference and measurement signals true to the shape. 26. Distance measuring device according to claim 25, characterized in that the ultra-short electrical reference and measurement signals occurring at the output of the photodiode ( 9 ) by means of a high-frequency broadband amplifier unit, consisting of a transimpedance amplifier ( 80 ), preamplifier ( 82 ) and main amplifier ( 83 ) , are amplified according to the shape of the waveform ( FIG. 9A). 27. Distance measuring device according to claims 25 and 26, characterized in that the ultra-short reference and measurement signals are converted into square-wave pulses by means of an ultra-fast comparator circuit ( 84 , 85 , 95 , 96 , 97 ) ( Fig. 11A). 28. Distance measuring device according to claim 27, characterized in that the comparator ( 85 ) has a differential amplifier with ultra-fast microwave FETs. 29. Distance measuring device according to claim 28, characterized in that the time interval measurement is carried out by means of a time measuring unit ( 86 , 87 , 98 ) ( Fig. 11A). 30. Distance measuring device according to claim 25, characterized in that the Pulse repetition frequency of the ultra-short pulses via a mixing process from one Microwave oscillator is obtained, which also serves as a counter for time evaluation of the The time interval between the reference and measurement signal is used. 31. Distance measuring device according to claim 30, characterized in that the counter is preferably realized in submicron FET technology. 32. Distance measuring device according to claim 25, characterized in that the generation of ultra-short optical impulses through a dynamic maximum injection of charge carriers preferably in the active zone of a single heterostructure laser diode. 33. Distance measuring device according to claim 32, characterized in that the laser emission preferably in index-guided Q-switching mode. 34. Distance measuring device according to claim 33, characterized in that by means of a Temperature control unit of the Q-switching laser mode is ensured. 35. Distance measuring device according to one of claims 32 to 34, characterized in that the semiconductor diode is integrated into the pump circuit in the form of a chip. 36. Distance measuring device according to one of claims 32 to 35, characterized in that the semiconductor laser source preferably consists of a single laser chip. 37. Distance measuring device according to claim 25, characterized in that the semiconductor laser source consists of several laser diodes, the laser emission of which in a coherent interaction stand. 38. Distance measuring device according to claim 25, characterized in that the discharge circuit of the avalanche pulse generator in planar technology from the elements charging capacitor ( 52 ), transistor ( 53 ), lead bridge ( 55 ), laser diode ( 49 ), metal contact ( 50 ) and lead bridge ( 51 ) exists, which are preferably arranged in the shortest possible electrical way in a rectangular manner ( FIG. 6). 39. Distance measuring device according to one of claims 25 and 38, characterized in that the planar discharge circuit of laser diode ( 49 , 66 ), transistor ( 53 ) and charge store ( 52 ) are constructed on a common heat sink ( 48 ). 40. Distance measuring device according to one of claims 25 and 38, characterized in that the elements of the discharge circuit in a heat sink with positive polarity, laser diode ( 49 ), capacitor ( 52 ) and transistor ( 53 ), arranged in series and electrically by the shortest route are connected ( Fig. 7A). 41. Distance measuring device according to one of claims 25 and 38, characterized in that the elements of the discharge circuit in a heat sink with negative polarity, laser diode ( 66 ), transistor ( 53 ) and capacitor ( 52 ), arranged in series and electrically by the shortest route are connected ( Fig. 7B). 42. Distance measuring device according to one of claims 25 and 38, characterized in that the laser transmitter ( 66 , 98 , 53 , 52 , 65 , 64 , 48 ) is constructed quasi-monolithically, preferably on a base body 99 made of silicon, which is used for planar recording by means of dry etching techniques the component chip ( 66 , 53 , 52 ) is structured in a suitable manner ( 100 ) and has an electrical insulation layer 101 ( FIG. 7C). 43. Distance measuring device according to one of claims 1 and 25, characterized in that the optical reference signal via the rear laser emission of a semiconductor laser chip is won. 44. Distance measuring device according to one of claims 1, 25 and 43, characterized in that that the optical reference signal via an optical fiber that is connected to the rear laser aperture of a semiconductor laser chip is coupled, is obtained. 45. Distance measuring device according to one of claims 1, 25 and 43, characterized in that that the rear laser emission as the reference signal directly the photoelectric Transducer is fed. 46. Distance measuring apparatus according to any one of claims 1, 25, 43, 45 and 66, characterized in that the laser transmitter with the discharge circuit (53, 52, 65) and laser diode (66) and the photodetector with photodiode chip (102) has a microsystem unit form Figure . 8A). 47. Distance measuring device according to one of claims 43 and 45, characterized in that the optical reference and measurement signals over a common photosensitive surface in electrical signals are converted. 48. Distance measuring device according to claim 47, characterized in that the rear Laser radiation from the laser diode chip via a scattering body onto the photosensitive Area is strewn. 49. Distance measuring device according to one of claims 43 and 45, characterized in that the reference signal laterally and the measurement signal normally in the photodiode chip is irradiated. 50. Distance measuring device according to claims 25, characterized in that the electrical reference and measurement signals sampled by means of a common sampling unit will. 51. Distance measuring device according to claim 50, characterized in that the scanned analog reference and measurement signals can be digitized using an A / D converter. 52. Distance measuring device according to claims 50 and 51, characterized in that the Time interval between a sampled digitized reference signal and the associated measurement signal A microprocessor unit is determined. 53. Distance measuring device according to claim 50, characterized in that the photoelectric converter is an ultra-fast avalanche photodiode chip.   54. Distance measuring device according to claim 53, characterized in that the detector bias voltage ( 72 ) of the photodiode, which results from the bias voltage U _v ( 71 ) and the resistors R₁ ( 73 ) and R₂ ( 74 ), tracked via a control device ( 70 ) ( Fig. 10B). 55. Distance measuring device according to claim 51, characterized in that the smoothing of the Pulse leading edges are preferably carried out via a linear regression. 56. Distance measuring device according to claim 55, characterized in that before Pulse edge smoothing is carried out by linear regression. 57. Distance measuring method according to claim 24, characterized in that the fiber-optic scanner has a common static transmission / reception optics. 58. Distance measuring method according to claim 24, characterized in that the fiber optic scanner is a vibrating transmission fiber that is coaxial or parallel to the optical Axis of the receiver is arranged, and has a static receiving optics. 59. Distance measuring method according to claim 58, characterized in that the Receiving lens a gap for receiving the vibrating fiber and for free radiation which has transmission pulses. 60. Distance measuring method according to claim 59, characterized in that the Detector area is an ultra-fast photodiode array that consists of several closely neighboring and linearly arranged and electrically isolated from each other diode chips. 61. Distance measuring method according to claim 25, characterized in that the Position determination of the optical fiber preferably based on the triangulation principle by means of a light emitting diode and a position detector. 62. Distance measuring method according to one of claims 1 to 5 and 9, characterized characterized that the combined occurring at maximum injection Operating modes can be converted into pure operating modes via targeted measures. 63. Distance measuring method according to claim 62, characterized in that a pure Material gain switching operating status via a stair profile of the refractive index curve is realized. 64. Distance measuring method according to one of claims 1 to 13, characterized in that the amplitude of an ultra-short pulse is defined as a time-significant point and this preferably on the differentiated pulse shape via a zero crossing detection is determined. 65. Distance measuring device according to one of claims 25 and 39 to 42, characterized characterized in that the laser diode chip and the discharge circuit on a common Heat sink are located, which is temperature controlled. 66. Distance measuring device according to one of claims 25, 43 and 45, characterized in that  that laser diode chip, discharge circuit and photodiode chip are integrated with one another Represent microsystem technical unit. 67. Distance measuring device according to one of claims 24, 58, 60 and 61, characterized characterized in that preferably by means of an oscillating deflecting mirror or Chamfer the fiber end by the direction of the transmitted beam emerging from the fiber is preferably rotated 90 °.