WO2024046676A1

WO2024046676A1 - Direct time-of-flight sensor and corresponding measurement method

Info

Publication number: WO2024046676A1
Application number: PCT/EP2023/071155
Authority: WO
Inventors: Alexandre Pollini; Javier MIGUEL SÁNCHEZ
Original assignee: Ams-Osram Ag
Priority date: 2022-08-31
Filing date: 2023-07-31
Publication date: 2024-03-07

Abstract

Direct time-of-flight sensor and corresponding measurement method The invention relates to a low cost and easy to manufacture direct time-of-flight sensor (2) with improved measurement precision, the sensor (2) comprising a light source unit (4) with a light source (10) capable of emitting a light pulse and a detector unit (6) with a light detector (12) capable of detecting the light pulse after reflection at a distant object, the sensor (2) further comprising a main clock (8) defining a measurement cycle within a main time reference (MTR), such that a start event (SE) specified in the main time reference (MTR) corresponds to triggering light pulse emission, wherein the light source unit (4) comprises a local oscillator (34) with a frequency higher than the frequency of the main clock (8), defining a light source time reference (LSTR), wherein the light source unit (4) further comprises circuitry for measuring a delay between light pulse triggering and actual light pulse emission and for generating according light source timestamps (TS-LS-START, TS-LS-STOP) relative to the light source time reference (LSTR), and wherein an evaluation unit (38) is configured to receive the light source timestamps (TS- LS-START, TS-LS-STOP) and to use them for determining a time-of- flight (ToF1) of the light pulse, taking into account said delay and/or local phase shifts. To improve the measurement precision, a local light source time clock architecture similar to the one in the detector is used.

Description

Direct time-of- f light sensor and corresponding measurement method

DESCRIPTION

TECHNICAL FIELD

The invention relates to direct time-of- f light sensor . It also relates to a corresponding method of measuring a time- of- flight of a light pulse .

BACKGROUND

A direct time-of- f light ( dToF) sensor includes a light source and a detector . Its operation principle is to emit a light pulse and detect the returning pulse , detected a while after it has been back reflected by an obj ect . The distance between the sensor and the obj ect is derived from the measurement of the pulse ' s time-of- f light .

The time-of- f light is determined from the subtraction of the arrival time of the received pulse determined by the detector and the pulse emission time .

The precision of the time-of- f light measurement depends on the precision and relative phase of the time references used respectively to determine the emission and the arrival times . To maximi ze the precision on the time-of- f light measure , these time references have to be controlled in frequency and in phase . The respective frequency and phase of the time reference used on both sides have to be synchroni zed or their respective di f ference in frequency and phase needs to be determined . Any synchroni zation deviations must be taken into account to determine the time-of- f light . To guarantee both frequency and phase precision, the same time reference of a few or few tens of MHz is used in practice by the light source and by the detector of dToF sensors . The short-term time frequency stability of time references used in dToF sensors over the integration times used in practice is precise enough for the targeted time or distance measurement precision (hundreds of picoseconds or millimeter ) . Hence , the frequency synchroni zation is straightforward .

The situation is di f ferent for the phase synchroni zation . It is challenging to achieve a proper phase synchroni zation of time reference signals that must be distributed to the light source and the detector . Reasons are multiple , they include :

• Routing constraints of the signals from the time reference to the latest stage of the light source emitting the pulse and the time measuring stage of the detector .

• Environmental influences : temperature variation ( e . g . , VCSEL emitter turn-on-delay) , power supplies variations and perturbations , interferences , etc .

• Variation over time as the time reference is ageing .

• IC manufacturing processes change from one lot to another .

All these possible causes af fect the synchroni zation of the phase of the time references of the light source and the detector .

The detector of a large maj ority of cost and performance competitive sensors uses single-photon avalanche diodes ( SPAD) associated with a time-to-digital converter ( TDC ) . Each TDC require its own high frequency local oscillator ( typically with a frequency of a few GHz ) . The functionality of a TDC is to timestamp the occurrence of an event or a photon detection . The timestamps are streamed toward a memory that accumulates them to build a histogram . Then, a calculator does peak detection on the histogram to determine the mean or median of the timestamps . With such a detector architecture , a few tens of picoseconds time precision should be achieved theoretically . The resulting time precision should lead to distance measurement precision in the millimeter domain or even below . However, this is hardly achieved in practice . PRIOR ART

EP 2 189 80 5 Al, titled "Optoelectronic sensor and method for measuring distance according to time-of-f light " , makes use of an artificial time shift addition between each of the light pulses. The time-of-f light precision achieved depends, among other factors, on the time shift defined. The present invention does not use an artificial time shift.

ON 113075679 A, titled "TOE ranging system", considers many of the technical problem aspects related to dToF sensors. To overcome the technical problem, this prior art makes use of a reference surface. The reflected light by the reference surface hits some pixels of the detector. The present invention does not use a surface reference.

US 7 636 150 Bl, titled "Method and system to enhance timing accuracy for time-of-f light systems", describes a feedback mechanism to adjust dynamically the phase of the emitted signal using delay and a phase locked loop. The present invention does not adjust dynamically the phase of the emitted signal.

SUMMARY

An objective underlying the present invention is to provide a low cost and easy to manufacture direct time-of-f light sensor with improved measurement precision. In particular, complicated methods of phase synchronization shall be avoided. Furthermore, a corresponding method of measuring a time-of-f light of a light pulse shall be given.

According to claim 1, the device-related objective is met by a direct time-of-f light sensor comprising a light source unit with a light source capable of emitting a light pulse and a detector unit with a light detector capable of detecting the light pulse after reflection at a distant obj ect , the sensor further comprising a main clock defining a measurement cycle within a main time reference , such that a start event speci fied in the main time reference corresponds to triggering light pulse emission, wherein the light source unit comprises a local oscillator with a frequency higher, preferably considerably higher, than the frequency of the main clock, defining a light source time reference , wherein the light source unit further comprises circuitry for measuring a delay between light pulse triggering and actual light pulse emission and for generating corresponding light source timestamps relative to the light source time reference , and wherein an evaluation unit is configured to receive the light source timestamps and to use them for determining a time-of- f light of the light pulse , taking into account said delay and/or local phase shi fts .

The according method is defined in claim 14 .

Herein and further below the expression ' considerably higher' is preferably to be understood as at least one power of ten higher, more preferably at least two powers of ten higher ( given same measurement units ) .

The invention is based on the consideration that until now the theoretical measurement precision is not achieved in practice , partly, because on the light source side , the real or ef fective emission time ( or phase ) of the light pulse is not timestamped with the same precision or same time granularity as the arrival time in the detector . The ef fective time of the pulse emission in the optical domain is therefore loosely measured in comparison with the arrival time .

To improve the measurement precision, one may take into consideration as much as possible the deviation of the relation between the phase of the time reference and the real-ef fective time of the pulse emission . Time measurements are then compensated according to calibrations based on factory pre-loaded parameters taking into account as many as possible of the phase de-synchroni zation causes quoted above .

Despite the calibration, the timestamp precision of the pulse emission time never achieves tens of picoseconds for all operating conditions over the li fetime of a product .

Therefore , the invention aims at actually measuring or estimating more precisely than before and taking into account the switch-on delay of the light pulse ( and thus the real or ef fective pulse emission time ) for each measurement cycle .

The proposed solution improves time-of- f light precision and additionally relaxes the hurdle of product calibration and layout design constraints by :

1 . Using a local light source time clock architecture , for example similar to the one used in detector .

2 . Using a timestamps exchange mechanism, wherein timestamps generated locally either in the light source and/or in the detector are trans ferred to an evaluation unit of the sensor, the evaluation unit collecting timestamps , preferably from both sides ( two-way exchange ) , and applying an algorithm generating phase or time correction parameters based on said timestamps exchange principle .

3 . Using correction parameters generated by said algorithm processing the timestamps from both sides to calculate more precise time-of- f lights . Summarizing, the main advantage of the proposed solution is the increased-accuracy determination of time-of-f light , and hence distances.

The proposed solution uses, on the light source side, a local time reference having a time granularity comparable to the one of the detector. This way the timestamps related to light pulses emission have the same or at least comparable time resolution as the timestamps related to time of arrival of the photons on the detector.

All the relevant timestamps are transferred to a central evaluation unit that is collecting them. On the basis of this information, this unit has the possibility to take into consideration the phase differences or time jitters appearing in the light source and the detector, and hence to calculate more precise time-of-f lights .

Implementing a time-of-f light sensor in such a way leads to additional advantages:

One advantage is the reduction of the design constraints of routing high-frequency (clock) signals from one point of the sensor (e.g., the detector) to another point (e.g., the light source driver) that need precise time synchronization between them.

Another advantage is the relaxation of the hurdle of the product calibration. As the light source and the detector benefit from a synchronization that is by default precise and permanent, all calibrations aiming at compensating desynchronization sources in standard products can be suppressed or simplified.

Further preferred embodiments, objectives and advantages may be concluded from the dependant claims.

Preferably, the detector unit has as similar architecture as the light source unit, wherein the detector unit comprises a local oscillator with a frequency higher, preferably considerably higher, than the frequency of the main clock, defining a detector time reference ( DTR) , wherein the detector unit further comprises circuitry for measuring the arrival of a reflected light pulse and for providing according detector timestamps relative to the detector time reference , and wherein an evaluation unit is configured to receive the detector timestamps and to use them for determining the time-of flight .

Preferably, the local oscillator of the detector unit has a frequency which has the same order of magnitude as the frequency of the local oscillator of the light source unit . The expression ' same order of magnitude ' is preferably to be understood such that both frequencies di f fer by a factor less than on power of ten, given same measurement units . That means that the local time granularity or resolution on both the detector side and the light source side are at least roughly the same and therefore comparable .

In a preferred embodiment the detector unit comprises a time- to-digital converter clocked by the local oscillator of the detector unit and configured to generate the detector timestamps . Correspondingly, the light source unit preferably comprises a time-to-digital converter clocked by the local oscillator of the light source unit and configured to generate the light source timestamps .

The time-to-digital converter of the light source unit preferably comprises a start signal input which is triggered by the start event in the main time reference . Furthermore , it comprises a stop signal input which in a first preferred variant is triggered by an electrical signal in an output stage of a light source driver for the light source . In a second preferred variant the stop signal input is triggered by an electrical signal generated by a photodetector facing the light source .

Preferably, the local oscillator of the light source unit has a frequency in the regime of GHz , in particular from 1 to 10 GHz . The same is true for the local oscillator of the detector unit , whereas the main clock has a preferred frequency in the regime of MHz , in particular from 1 to 20 MHz .

In a preferred embodiment the light source is a laser diode , in particular a vertical-cavity surface-emitting laser, whereas the light detector preferably is a single-photonavalanche diode .

What has been said with respect to the device may analogously be applied to the method, and therefore need not be further repeated here . Device embodiments and details have a counterpart in the method . Again, no explication is required here in view of the above description .

In summary, key elements of the invention are the use of a light source local time reference , preferably having a time resolution similar to the one of the detector, the trans fer of the timestamps measured in the light source and/or in the detector to an evaluation unit that can calculate more precise time-of-lights by taking into account the local phase shi ft or j itter of the light source and the detector .

BRIEF DESCRIPTION OF DRAWINGS

Further below, exemplary embodiments of the invention are discussed with reference to the accompanying drawings .

Fig . 1 shows a schematic diagram of a direct time-of- f light (dToF) sensor according to the invention . Fig . 2 shows a time diagram illustrating various time reference frames and events related to the operation of the sensor according to Fig . 1 .

Fig . 3 shows a time diagram illustrating a mapping between various time reference frames with corresponding timestamps exchanges .

DETAILED DESCRIPTION

Fig . 1 shows a schematic diagram of a direct time-of- f light (dToF) sensor 2 . The dToF sensor 2 comprises a light source unit 4 and a detector unit 6 . A common main clock 8 or main oscillator provides a main time reference MTR with a periodic clock signal to both the light source unit 4 and the detector unit 6 . The main clock 8 may be a separate component or unit , as indicated in the drawing . Alternatively, it may be integrated into the light source unit 4 or the detector unit 6. In Fig . 2 the periodic clock signal is exemplarily shown as a square or rectangular wave signal in the uppermost row of a corresponding time diagram . The frequency of the main time reference MTR is typically in the range from one to several MHz , in particular 1 to 20 MHz , with 10 MHz as a preferred value chosen in the example . The corresponding period time PT is therefore typically in the range of nanoseconds . One period of the main time reference MTR corresponds to one measurement cycle of the dToF sensor 2 . During one measurement cycle a light source 10 within the light source unit 4 is triggered to emit a short light pulse , and after reflection at a distant obj ect the reflected part of the light pulse is detected by a detector 12 of the detector unit 6 . The measured time-of- f light ToF, i . e . the time di f ference between pulse emission and reflected pulse detection, allows for determining said distance on the basis of the known light velocity .

In the example according to Fig . 1 the respective measurement cycle is initiated by the rising signal edge or flank of the square wave signal shown in Fig . 2 . Hence , said signal edge defines a start event SE at which the light source unit 4 is triggered to emit a light pulse , and at which the detector unit 6 is armed to detect the reflected light pulse somewhat later within the measurement cycle ( after the time-of- flight ) . After one period of the main time reference MTR, the next measurement cycle is initiated .

The light source unit 4 comprises a light source driver 14 and a light source 10 which, for example , may be or comprise a laser diode or any other suitable light source which is capable of emitting ultra-short light pulses with a pulse duration in the range of , for example , picoseconds . In particular, the light source 10 may be a vertical-cavity surface-emitting laser (VCSEL ) . In general , the pulse duration should be considerably shorter than the expected time-of- f light ToF to measure the latter accurately . The light source driver 14 is triggered by the start event SE speci fied above and is therefore wired or connected signalwise to the common main clock 8 .

The detector unit 6 comprises the actual light detector 12 for the reflected light pulse which may be or comprise a single photon avalanche diode ( SPAD) and corresponding analysis electronics or circuitry . Alternatively, any light detector or photodetector with suitable sensitivity with respect to the ultra-short light pulses may be used .

To facilitate an accurate measurement of the arrival time of the reflected light pulse , the output of the light detector 12 is coupled to a time-to-digital converter ( TDC ) 16 . Time- to-digital converters or time digiti zers are devices commonly used to measure a time interval and convert it into digital (binary) output . TDCs are therefore used to determine the time interval between two signal pulses ( known as start and stop pulse ) . In this case , the start event SE speci fied above , based on the main time reference MTR, acts as a start pulse . Hence , the start signal input 18 of the TDC 16 is wired or connected signal-wise to the main clock 8 . The corresponding stop pulse is provided by the signal output of light detector 12 when it is hit by the reflected pulse . Thus , the signal output of the light detector is wired or connected signal-wise to the stop signal input 20 of the TDC 16 . This way, the TDC' s 16 timestamp output 22 provides a digital start timestamp TS-D-START which corresponds to the start event SE , i . e . the initiation of the measurement cycle based on the main time reference MRT , and a digital stop timestamp TS-D— STOP which corresponds to the arrival of the reflected light pulse at the light detector 12 . The corresponding time di f ference calculated by subtractor 24 equals the raw (uncorrected) time-of- f light ToF .

To allow for a high time resolution of the arrival of the reflected light pulse at the light detector 12 , the TDC 16 is coupled to or comprises a local clock or local oscillator 26 which provides a periodic detector clock signal . The local oscillator 26 is preferably arranged in the immediate neighborhood of the TDC 16 , facilitating short signal paths . The frequency of this local clock signal is preferably at least one order of magnitude higher than the frequency of the main time reference MTR . Preferably, the frequency is in the range or regime of gigahertz , for example 4 GHz . This corresponds to a period time or time increment in the range of several tens or hundreds of picoseconds . The local oscillator 26 acts as an internal clock or clock generator of the TDC 16 , allowing for a time-resolved readout and analysis of the signal output from the light detector 12 . Owing to the statistical properties of the photons which constitute the light pulse , a time-resolved histogram of the photon countrate can be acquired for said time increments in the picoseconds regime . Typically, the arrival time of the received pulse is found or speci fied to coincide with the mean or the median of the photon count distribution . Therefore , the TDC 16 is able to provide the arrival time - and hence the (uncorrected) time-of- f light - with picoseconds accuracy, preferably as multiples of the period time of the local oscillator 26 . However, such accuracy is only apparent or meaningless , unless the emission time of the light pulse is known with a similar level of accuracy . The present invention is based on the reali zation that in practice the real or ef fective time of emission is delayed with respect to the triggering start event SE , in particular due to unavoidable signal (phase ) delays in the light source driver 14 , but possibly also due to the physical processes underlying light pulse creation in the light source 10 .

One way of dealing with this situation is to assess an average delay by way of calibration measurements and to account for the average delay by a generic correction function or parameters to be applied to the raw time-of- f light output of the detector unit 6 . Alternatively or additionally, correction functions or parameters may be derived by theoretical considerations and/or numerical simulations within a modeling framework .

Nevertheless , the achievable accuracy of such methods generally is still not comparable to the accuracy of the above-described measurement of the arrival time of the reflected light pulse at the light detector 12 . Furthermore , if parameters of operation or related boundary conditions change in an unforeseen manner, the applied generic correction might be wrong or misleading . Therefore , the present invention aims at actually measuring the precise emission time of the light pulse ( or the delay with respect to the triggering start event SE ) for each measurement cycle . The thus-derived information is then taken into account for a case-by-case correction of the measured raw time-of- f light value .

Speci fically, the light source unit 4 comprises a time-to- digital converter ( TDC ) 28 similar to the TDC 16 of the detector unit 6 . The TDC 28 comprises a start signal input 30 which is wired or connected signal-wise to the main clock 8 . Therefore , the start event SE speci fied in the main time reference MTR which triggers the light source driver 14 also triggers a time count in the TDC 28 . The corresponding stop signal which corresponds to the real or ef fective emission time of the light pulse leaving the light source 10 may be derived, for example , from the electrical domain of the last stage or output stage of the light source driver 14 (under the assumption that a further delay in the light source 10 is negligible , or manageable by generic corrections as described above ) , as indicated by the dashed connection line in Fig . 1 . In this case the connection line is preferably as short as possible . In an alternative embodiment , there may be a light or photodetector 40 such as a photodiode arranged in the immediate neighborhood of the light source 10 which with negligible or at least known or predictable delay captures the light pulse emerging from the light source 10 and thus provides a corresponding trigger signal emerging from the optical domain to the stop signal input 32 of the TDC 28 .

Similar to the configuration of the detector unit 6 , the TDC 28 of the light source unit 4 is preferably coupled to a local clock or a local oscillator 34 which provides a periodic detector clock signal . The local oscillator 34 is preferably arranged in the immediate neighborhood of the TDC 28 , facilitating short signal paths . The frequency of this local clock signal is preferably at least one order of magnitude higher than the frequency of the main time reference MTR . Preferably, the frequency of the local oscillator 34 in the light source unit 4 has the same order of magnitude as the frequency of the local oscillator in the detector unit 6 . It may be preferable but is by no means necessary that the frequencies of both local oscillators 26 and 34 are exactly the same . Thus , the frequency of the local oscillator 34 is preferably in the range or regime of gigahertz , for example 4 GHz . This corresponds to a period time or time increment in the range of several tens or hundreds of picoseconds . The local oscillator 34 acts as an internal clock or clock generator of the TDC 28 , allowing for a time-resolved readout and analysis of the triggering signal applied to the stop signal input 32 of the TDC 28 . Therefore , similar to the operation of the detector unit 6 , a time-resolved assessment of a measurement signal characteristic for the photon emission at the light source 10 may be obtained . In a corresponding histogram with time increments in the , e . g . , picoseconds regime the ef fective emission time may be defined, for example , as the mean or median of the distribution . This way, the TDC' s 28 timestamp output 36 may provide a digital start timestamp TS-LS-START which corresponds to the start event SE , i . e . the initiation of the measurement cycle based on the main time reference MTR, and a digital stop timestamp TS-LS-STOP which corresponds to ef fective emission time of the light pulse . The time di f ference between the start timestamp TS-LS-START and the stop timestamp TS-LS-STOP of the light source unit 4 may be interpretated as delay time between triggering and actual light emission .

A corresponding evaluation ( and correction) unit 38 is configured to receive the timing information provided by both the light source unit 4 and the detector unit 6 and to derive from it a corrected time-of- f light value ToF' ( or a related quantity like a distance ) , i . e . a time-of- f light value in which the switch-on delay of the light source 10 is taken into account . Speci fically, the evaluation unit may receive a timestamps collection, comprising, for example , stop timestamps TS-D-STOP from the detector unit 6 and/or stop timestamps TS-LS-STOP from the light source unit 4 . Corresponding start timestamps TS-D-START and/or TS-LS-START may be transmitted as well . Additionally, a raw (uncorrected) time— of- f light value TOE derived in the detector unit 6 and/or a switch-on delay derived in the light source unit 4 and/or timing information from the main time reference MTR frame may be transmitted to the evaluation unit 38 and taken into account therein in a calculation to obtain time- corrected measurement parameters and thus a corrected time-of flight ToF' .

One should keep in mind that in practice a multitude of the described light source - detector units may be arranged side- by-side in an array configuration, wherein each of said units may represent one pixel of a multi-pixel screen and may comprise their own local oscillators .

Fig . 2 shows a time diagram with the various time references or frames involved in the measurement principle underlying the dToF sensor 2 of Fig . 1 .

The uppermost row shows an exemplary square wave signal (with a frequency in the MHz region) representing the main time reference MTR as discussed above . The hori zontal double arrow above indicates one period with period time PT .

The main time reference MTR may be mapped to a local time reference LSTR in the light source unit 4 , shown one row below, which is based on the high- frequency ( e . g . , GHz ) oscillations of the local oscillator 34 in the light source unit 4 , indicated one further row below and labelled local oscillations in light source , in short LSLO .

Similarly, the local oscillator 26 ( oscillating in the GHz region) establishes a local time reference DTR in the detector unit 6 , as visuali zed further below in the drawing . The according high- frequency oscillations are labelled local oscillations in detector, in short DLO .

Due to unavoidable dri ft and phase desynchroni zation of the local oscillators 26 , 34 and the main clock 8 , the time reference LSTR in the light source unit 4 and the time reference DTR in the detector unit 6 are slightly shi fted and/or distorted with respect to the main time reference MTR . In the LSTR and DTR parts of the diagram, phase/time j itter PTJ is indicated by respective hori zontal arrows for the light source regime and the detector regime . Furthermore , the respective moment of phase synchroni zation PS is indicted by a vertical arrow for each of these two regimes .

Further vertical arrows indicate the moment of ef fective light emission in the light source 10 , also called optical pulse emission OPE, as discussed above, and the related timestamping TS in the light source unit 4 by virtue of the TDC 28, only a few of the GHz-based time increments after the emission .

Similarly, vertical arrows indicate the moment of arrival of the reflected light pulse at the light detector 12, also called optical pulse detection OPD, and the related timestamping TS in the detector unit 6 by virtue of the TDC 16, only a few of the GHz-based time increments after the pulse arrival.

Finally, in the lowermost part of the diagram, horizontal arrows indicate the raw (uncorrected) time-of-f light ToF, based in principle on the time difference between the start event SE and the optical pulse detection OPD. By contrast, the corrected time-of-f light ToF' is based in principle on the time difference between the (true or effective) optical pulse emission OPE and the optical pulse detection OPD.

Summarizing the detailed description above, the light source 10 emits a light pulse triggered relative to the main time reference MTR with some delay and random jitter. After a while, if an object has been hit, the reflected pulse returns, and the arrival time is determined with reference to a time frame DTR established by the local oscillator 26 of the detector unit 6.

In the diagram of Fig. 2, the phase delay and random jitter are exaggerated compared with a real situation; however, they allow illustrating the benefit of the invention proposed.

What is key is to notice:

1. the delay between the main time reference MTR edge (i.e., start event SE) and the effective time the optical pulse is emitted (OPE) , and 2. the desynchronization between the local oscillator within respectively the light source and the detector.

The invention addresses these shortcomings, as described above .

On the implementation side, as schematically illustrated in Fig. 1, preferred key pieces or blocks are the time-to- digital converter (TDC) 28 and the local oscillator 34 integrated into the light source unit 4, using electronic blocks similar to those used within the detector unit 6. The surface penalty of these additional blocks in the light source unit 4 is negligible when compared with the overall surface of a VCSEL array driver, for example.

With these blocks, the important events (pulse emission and photon arrival time) can be timestamped according to the granularity of the local oscillators 26, 34 and the local start event trigger phase.

Both TDCs 16, 28 are started when the light pulse is triggered by the start event SE . On the detector side, the TDC 16 is stopped when a returning pulse is detected. On the light source side, the TDC 28 is stopped when the light pulse is truly emitted. As described above, this can be either done in the electrical domain using the light source driver 14 last stage signal applied to the emitter or in the optical domain using, for example, a photodiode.

The timestamps, e.g. TS-D-START, TS-D-STOP, TS-LS-START, TS- LS-STOP, shall then be exchanged or transferred to a calculator within the evaluation unit 38 that does the necessary corrections to generate the desired high precision time-of-f light ToF' .

This is schematically illustrated in Fig. 3, wherein the mapping between different time reference frames and the exchange of timestamps between them is shown. More specifically, the horizontal timeline represents the time (phase) local to the main time reference MTR, whereas the two slanted timelines represent the time (phase) local to the light source (upper line) and the time (phase) local to the detector (lower line) . Furthermore, various emission times for light pulse emission of the light source 10 and the corresponding detection times in the detector 12 are indicated, as well as their respective mappings to the main time reference MTR.

Timestamps exchange protocols and time synchronization algorithms are used in many technical domains such as satellites, networks, etc. and some are even standardized, e.g. in IEEE1588 or RFC 3161. They can also be employed in the scenario described herein.

The surface penalty to implement this functionality in an integrated circuit corresponds to the surface occupied by a three wires serial peripheral interface which can be considered to be negligible.

LIST OF REFERENCE SIGNS dToF sensor 2 light source unit 4 detector unit 6 main clock 8 light source 10 light detector 12 light source driver 14

TDC 16 start signal input 18 stop signal input 20 timestamp output 22 subtractor 24 local oscillator 26

TDC 28 start signal input 30 stop signal input 32 local oscillator 34 timestamp output 36 evaluation unit 38 photodetector 40 start event SE main time reference MTR detector time reference DTR light source time reference LSTR local oscillations in detector DLO local oscillations in light source LSLO time st amp TS start timestamp of detector TS-D-START stop timestamp of detector TS-D-STOP start timestamp of light source TS-LS-START stop timestamp of light source TS-LS-STOP period time PT phase/time jitter PT J phase synchronisation PS

(raw) time-of-f light ToF

(corrected) time-of-f light ToF' optical pulse emission OPE optical pulse detection OPD

Claims

1. A direct time-of-f light sensor (2) comprising a light source unit (4) with a light source (10) capable of emitting a light pulse and a detector unit (6) with a light detector (12) capable of detecting the light pulse after reflection at a distant object, the sensor (2) further comprising a main clock (8) defining a measurement cycle within a main time reference (MTR) , such that a start event (SE) specified in the main time reference (MTR) corresponds to triggering light pulse emission, wherein the light source unit (4) comprises a local oscillator (34) with a frequency higher, preferably considerably higher, than the frequency of the main clock (8) , defining a light source time reference (LSTR) , wherein the light source unit (4) further comprises circuitry for measuring a delay between light pulse triggering and actual light pulse emission and for generating according light source timestamps (TS-LS-START, TS-LS-STOP) relative to the light source time reference (LSTR) , and wherein an evaluation unit (38) is configured to receive the light source timestamps (TS-LS-START, TS-LS-STOP) and to use them for determining a time-of-f light (ToF' ) of the light pulse, taking into account said delay and/or local phase shifts .

2. A sensor (2) according to claim 1, wherein the detector unit (6) comprises a local oscillator (26) with a frequency higher, preferably considerably higher, than the frequency of the main clock (8) , defining a detector time reference (DTR) , wherein the detector unit (6) further comprises circuitry for measuring the arrival of a reflected light pulse and for providing according detector timestamps (TS-D-START, TS-D- STOP) relative to the detector time reference (DTR) , and wherein the evaluation unit (38) is configured to receive the detector timestamps (TS-D-START, TS-D-STOP) and to use them for determining the time-of-f light (ToF' ) .

3. A sensor (2) according to claim 2, wherein the local oscillator (26) of the detector unit (6) has a frequency which has the same order of magnitude as the frequency of the local oscillator (34) of the light source unit (4) .

4. A sensor (2) according to claim 2 or 3, wherein the detector unit (6) comprises a time-to-digital converter (26) clocked by the local oscillator (26) of the detector unit (6) and configured to generate the detector timestamps (TS-D- START, TS-D-STOP) .

5. A sensor (2) according to any one of the preceding claims, wherein the light source unit (4) comprises a time- to-digital converter (28) clocked by the local oscillator (34) of the light source unit (4) and configured to generate the light source timestamps (TS-LS-START, TS-LS-STOP) .

6. A sensor (2) according to claim 5, wherein the time to- digital-converter (28) of the light source unit (4) comprises a start signal input (30) which is triggered by the start event (SE) in the main time reference (MTR) .

7. A sensor (2) according to claim 5 or 6, wherein the time to-digital-converter (28) comprises a stop signal input (32) which is triggered by an electrical signal in an output stage of a light source driver (14) for the light source (10) .

8. A sensor (2) according to claim 5 or 6, wherein the time to-digital-converter (28) comprises a stop signal input (32) which is triggered by an electrical signal generated by a photodetector facing the light source (10) .

9. A sensor (2) according to any one of the preceding claims, wherein the local oscillator (34) of the light source unit (4) has a frequency in the regime of GHz, in particular from 1 to 10 GHz.

10. A sensor (2) according to any one of the preceding claims , wherein the main clock (8) has a frequency in the regime of MHz, in particular from 1 to 20 MHz.

11. A sensor (2) according to any one of the preceding claims, wherein the light source (10) is a laser diode, in particular a vertical-cavity surface-emitting laser.

12. A sensor (2) according to any one of the preceding claims, wherein the light detector (12) is a single-photonavalanche diode.

13. A sensor (2) according to any one of the preceding claims, wherein the start event (SE) is defined by an edge or flank of a square wave signal.

14. Method of measuring a time-of-f light of a light pulse with a direct time-of-f light sensor (2) comprising a light source unit (4) with a light source (10) capable of emitting a light pulse and a detector unit (6) with a light detector (12) capable of detecting the light pulse after reflection at a distant object, wherein a main clock (8) defines a measurement cycle within a main time reference (MTR) , such that a start event (SE) specified in the main time reference (MTR) corresponds to triggering light pulse emission, wherein a local oscillator (34) within the light source unit (4) with a frequency considerably higher than the frequency of the main clock (8) defines a light source time reference (LSTR) , wherein a delay between light pulse triggering and light pulse emission is measured and corresponding light source timestamps (TS-LS-START, TS-LS-STOP) relative to the light source time reference (LSTR) are generated, and the light source timestamps (TS-LS-START, TS-LS-STOP) are used for determining and/or correcting a time-of-f light (ToF' ) of the light pulse, taking into account said delay and/or local phase shifts.