WO2004027357A1

WO2004027357A1 - Photodetector array and method for compensating stray light

Info

Publication number: WO2004027357A1
Application number: PCT/DE2003/002764
Authority: WO
Inventors: Helmut Riedel; Andreas Dahl Von; Christian Lang; Friedrich Zywitza
Original assignee: Conti Temic Microelectronic Gmbh
Priority date: 2002-09-13
Filing date: 2003-08-19
Publication date: 2004-04-01
Also published as: DE10393757D2; AU2003266130A1

Abstract

Disclosed is a photodetector array for compensating stray light, comprising a photodetector unit (2) for detecting and determining a photocurrent (IPh), and an integrated unit (6) which compensates undesired signal portions (SL) underlying in the photocurrent (IPh). A compensation signal (IHL) which represents the undesired signal portions (SL) is connected counter to the photocurrent (IPh).

Description

Photodetector arrangement and method for Störlichtkompensation

Scenes are often actively illuminated in optical metrology. The active lighting, z. As infrared light, modulated or non-modulated light, is added to the existing stray light, z. B. background light of the sun, other light sources such as floodlights, fluorescent tubes. This results in a limitation of the usable dynamic range in a photodetector arrangement with which the scene is observed. In many cases, the intensity of the active illumination is below the intensity of the disturbance light with respect to its intensity. Therefore, the detector signal is dominated by the disturbing light, so that the desired useful signal from the active illumination forms only a small fraction of the total signal.

In particular, applications in which photodetectors are used for distance measurements using the pulse transit time method or the phase correlation method are limited in their performance by stray light. Examples of this from vehicle technology are laser radar or SD range cameras.

In order to be able to increase the performance of such systems, ways have to be found with which signals resulting from stray light can be suppressed. The aim is to have the majority of the dynamic range of the detector for the detection of the useful signal available. Although this problem can be alleviated somewhat by using photodetectors with extremely large dynamic range, the need for a sufficiently good signal-to-noise ratio remains when using such detectors. Even with sensors with a large dynamic range, the signals generated by active illumination remain small compared to the signals caused by stray light.

At present, various concepts for highly dynamic photodetectors are described in the literature. The concepts described therein use components with a logarithmic characteristic curve for signal compression (Höfflinger et al.: Cameras for Robust Vehicle Vision, International Conference on Active and Passive Automobile Safety, Capri, 1996) or control the integration time in accordance with the illumination intensity occurring at the photodetector. M. Böhm et al .: "High Dynamic Range Image Sensors in Thin Film on ASIC Technology for Automotive Applications", Advanced Microsystems for Automotive Applications, Springer-Verlag, Berlin, pp. 157-172, 1998).

A separation of photosignals, which are caused by an interaction of active illumination and stray light, can be achieved with current methods and systems only by means of several temporally successive measurements. In this case, the photosignal or the optical signal is determined by the cumulative effect of stray light and active illumination in a first measurement. In a subsequent second measurement, the photosignal of the disturbing light is determined when the active illumination is switched off. This order of measurements can also be reversed. The useful signal can then be determined by subtracting the Störlichtsignals from the total signal. This procedure corresponds to the procedure for Correlated Double Sampling (CDS for short). Proceeding from this, the object of the invention is to specify a particularly simple photodetector arrangement for interfering light compensation and a particularly simple method for compensating for interfering light for a photodetector arrangement.

The first object is achieved by a photodetector arrangement for Störlichtkompensation with a photodetector unit for detecting and determining a photocurrent and with an integrated compensation unit for compensation of the photocurrent underlying Störsignalanteilen such that the photocurrent, a Störsignalanteile representing compensation signal is countered.

Advantageous developments of the invention are part of the dependent claims.

The invention is based on the consideration that for a particularly simple, fast and safe Störlichtkompensation a photodetector arrangement should be specified, with the aid of a compensation of the Störlichtsignals directly on the photosensitive component, z. B. the photodetector or photoelement, and thus directly at the point of origin of the photoelectric or optical signal caused by the disturbance light is possible. In particular, expensive time-sequential measurements should be reliably avoided. For this purpose, a compensation unit in the manner of a circular structure or negative feedback is coupled directly to the photodetector unit. In this case, by means of the compensation unit of the size to be measured, d. H. the measurement signal or the optical signal and the resulting photocurrent, the output signal, d. H. the compensation signal, the counteracted in the feedback compensation unit.

The main advantages of the invention consist in the fact that by such a photodetector arrangement with a compensation unit arranged in a feedback branch and a resulting feedback signal processing method compared to a conventional so-called "Correlated Double Sampling method" for the Störlichtkompensation already after an initialization process, the detected measurement signal is processed on the basis of the feedback compensation signal and thus the Störlichtkompensation already during a first Thus, the third step, which is necessary for the so-called "correlated double sampling method", eliminates the difference between the two signals, which considerably shortens the signal processing time and enables higher refresh rates in imaging systems ,

The compensation unit expediently comprises at least one storage element, in particular a storage capacity, for determining the compensation signal. For determining the usage signal on which the photocurrent is based, a further memory element is provided. As a result, a separation of the useful signal is made possible by the interference signal, so that the dynamic range of photodetectors is extended so that even with measurement signals with high noise or background level and low Nutzsignalanteil the useful signal component can be determined.

The compensation unit preferably comprises a reset circuit for initializing the photodetector arrangement. The reset circuit puts the arrangement in the initial or basic state. This ensures that subsequently the compensation signal representing the interference signal component can be determined and fed to a feedback branch. Advantageously, the compensation unit comprises a switching element, which is provided state-dependent for determining the compensation signal. In a preferred embodiment, the compensation signal is detected as a voltage signal. For conversion of the voltage signal in a proportional current signal is preferably a voltage-current converter is provided.

Depending on the type and construction of the photodetector arrangement, this comprises, for example as photomix detector, a plurality of photodetector units arranged parallel to one another, followed by a compensation unit having a number of memory elements corresponding to the number of photodetector units for determining the respectively associated useful signal and another common memory element for determining the compensation signal comprises. For splitting the compensation signal common to all measuring and photosignals, a current mirror arrangement is connected downstream of the common memory element by means of which the compensating signal is switched counter to each photosignal. As an alternative to determining a compensation signal common to all measuring or photosignals, the photodetector arrangement can comprise a compensation unit for a separate and thus signal-related compensation, which has a number of memory elements corresponding to the number of photodetector units for determining the respectively associated useful signal and a corresponding number of has further memory elements for determining the respectively associated compensation signal.

The photodetector arrangement is preferably constructed with the aid of integrated electronic components, whereby the compensation unit is assigned directly to the photodetector unit and is thus integrated. As a result, the photodetector arrangement can be embodied as a so-called "active pixel sensor" (APS for short), which is constructed in a simple manner in CMOS technology, for example - formed detector units as an image sensor of a line or matrix camera. The second object is achieved according to the invention in a method for interfering light compensation for a photodetector arrangement, wherein a photocurrent is detected by means of a photodetector unit whose Störsignalanteile be compensated such that the photocurrent representing the Störsignalanteile and counter-formed compensating signal formed by an integrated compensation unit , It is essential that the method is not limited exclusively to photodetectors, but can in principle be applied to all signals which are composed of interfering and useful signal. The method can be implemented in the form of integrated components in the semiconductor detector. Thus, photodetectors are possible as so-called "Active Pixel Sensors" (APS) whose dynamic range can be used as far as possible for the detection of an active scene lighting.

The advantages achieved by the invention are in particular that by the negative feedback of a compensation signal to be processed photodetector signal to Störsignal- shares, z. B. of interfering light sources, is largely reduced by the useful signal components, ie the useful light is separated from the stray light. The counter- or feedback signal (= compensation signal) and therefore the degree of compensation is preferably adjustable via a control terminal ("control") .Such immediate compensation of interference signal components of the detected photodetector signals is unaffected by subsequent signal processing In addition, the photodetector arrangement is suitable for single detectors or for row or array arrangements, eg for an Active Pixel Sensor (called APS for short) and Photonic mixer detectors (PMD for short) in semiconductor technology Such a photodetector arrangement eliminates the need for expensive analog-digital conversion with subsequent value storage and subtraction. real-time capability, allowing high frame rates and short measurement times in image-capturing systems.

Advantageous embodiments and further developments of the inventive concept are the further description with reference to the drawings.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

1 shows a schematic representation of the photodetector arrangement;

2 shows a timing diagram for the control of the photodetector arrangement according to FIG. 1;

3 shows a schematic representation of the photodetector arrangement according to the invention with simplified circuitry for a photonic mixer detector (PMD).

The same or functionally identical elements have been provided in all figures - unless otherwise indicated - with the same reference numerals.

1 shows a schematic representation of a photodetector arrangement 1 for Störlichtkompensation. The photodetector arrangement 1 comprises a photodetector unit 2 with a photodetector circuit 3 and a photovoltaic cell 4 for the detection of a photocurrent Ip. The measuring signal or the photocurrent Ip is determined by means of a photovoltaic cell 4, e.g. B. a photodiode detected optical signal 0, which is in an electrical signal, the photocurrent Ip _h (also called photodetector) is converted. The optical signal 0 is composed of a noise signal component SL, z. B. stray light, sunlight, and a Nutzsignalanteil NL, z. B. infrared light. In order to compensate for the To current Ip _h underlying interference signal components SL is the photocurrent I _P a the Störsignalanteile SL largely corresponding compensation signal I _HL countered according to:

with Iph_κomp = compensated photocurrent.

To determine the compensation signal I _HL , a compensation unit 6 is provided, which is connected downstream of the photodetector unit 2. The compensation unit 6 includes a storage element C _HL - In this case the memory element C _HL is used to determine a signal proportional to the compensation signal I _HL voltage value V _C _ _HL - An additional storage element C _S i _g is used for determining a the photocurrent I _P underlying wanted signal NL with reference to a the compensated photoelectric current Ip _h _κ _o mp representing voltage signal V_si _g - Depending on the type of signal and function is assigned ig a voltage-current converter 8 and a comparison amplifier unit 10 of each storage element C _HL and C _S. To initialize the photodetector array 1 is associated beyond the respective storage element C and C _HL _S ig, as well as the photodetector unit 2, a reset circuit 12th

All elements or components of the photodetector arrangement 1 are preferably arranged directly on the photodetector or photoelement 4 on a semiconductor. The photodetector arrangement 1 with the photoelement 4 and the photodetector circuit 3 and the compensation unit 6 thus represents a possible embodiment of a so-called Active Pixel Sensor (abbreviated to APS).

During operation of the photodetector arrangement 1, the photodetector or the photoelement 4, for example a photodiode, a photogate detector, supplies the photocurrent I _Ph , which is processed by means of the photodetector circuit 3 and the compensation unit 6 to the compensated photocurrent Ip _h _κ _om . The photocurrent I _Ph is the use of active scene lighting caused by stray light of the scene, the Störsignalanteilen SL, and by the additional active scene lighting, the Nutzsignalanteilen NL. If the active scene illumination is switched off and a switch Si associated with the memory unit C _{HL is closed} at a time Ti, the compensated photocurrent Iph_κomp in this case is generated largely exclusively by the stray light which is present at the memory unit C _HL Z-B of a capacitance a voltage signal is determined on the basis of the voltage values V _C _ _HL , in particular integrated.

If the switch Si is opened after a time T ₂ , the result of the signal integration is kept as the voltage value V _C _ _HL at the memory _element C _HL .

By means of the downstream voltage-current converter 8, a proportional current value is generated from the voltage value V _C _ _HL , which represents the compensation signal I _L. Depending on the specification, a conversion or compensation factor k representing the degree of compensation can be set with "Control" at the voltage / current converter 8. Depending on the type and structure of the photodetector arrangement 1, the compensation signal I _HL can be adjusted by means of the conversion factor k adjustable or fixed.

If one switches the time T _2, the active scene illumination and closes when the switch Si has a storage element C _S ig associated second switch S, so is the voltage-current converter 8 due to the noise light power by means of the compensation signal I _HL directly to the photo detector circuit 3, so that the corresponding interference-relevant current component is compensated directly at its formation, namely immediately after the photoelement 4. By means of a suitable adjustment of the conversion factor k of the voltage-current converter 8, a predetermined degree of compensation is set, ie by means of the compensation factor k the compensation signal I _H χ, a complete compensation or a partial compensation of the interference signal components SL can take place. Thus, with appropriate adjustment of the voltage-current converter 8 to the memory element C _S i _g only the compensated photocurrent Ip _h _κ _om p generated by the active scene lighting is determined, in particular integrated. That is, the memory element C _S i _g , in particular its dynamic range, is used to determine the voltage signal of the active illumination, ie the useful signal components NL.

At a time T ₃ , the signal integration is aborted by opening the switch S ₂ . The voltage signal V _c _sig of the memory element C _S i _g (also called integration capacity) is held and guided with the switch S ₃ closed via the amplifier unit 10 to the output line 16 (also called "signal line").

The timing diagram representing the photodetector arrangement 1 is shown in FIG. After expiry of the timing scheme or according to specification, the photodetector arrangement 1 can be restored to the initial state by means of a reset pulse (reset) by means of the reset circuit 12. That is, the resetting of the memory elements C _HL and C _S i _g is in each case via the associated reset circuit 12, by which a Initialisierungspegel is defined at the capacitors. the arranged on the photovoltaic element 4 photodetector circuit 3 comparable also evaluates the reset pulse for initialization of the photosensitive member 4. in addition, during the signal integration, ie during signal processing, keep the potential at the photoelement 4 constant.

The control of the photodetector arrangement 1 can be described in more detail with reference to FIG 2. In this case, for example, the time profile of an active scene _lighting ΔE _Mθ d dar- posed. The active scene illumination ΔE _Mo a is added to the disturbance light E _DC , which in the simplest case is shown here as a direct signal, for example a signal obtained from the sunlight.

The modulation type and signal shape of the active scene _lighting ΔE _Mθd can be arbitrary. In the limiting case, the active scene lighting ΔE _o a may even represent a DC signal which is connected within the time period (ΔT = T ₃ -T ₂ ). In principle, all signal forms, for example rectangular signals, sine signals, triangular signals, pseudo noise signals, pulse group signals, etc., are suitable for the method according to the invention. It should only be noted that the time average values of the respective signal forms are formed by the integrating functionality of the described method. This applies both to the signal of the active scene _illumination ΔE _Mθd and to the interference light signal ΔE _D c (corresponds to the interference signal _components SL).

The method and the photodetector arrangement 1 can be used both for a single photodetector unit 2 with a single photoelement 4 and for a row or array arrangement of photovoltaic cells 4. An example of such an arrangement is shown in FIG. The application in a photodetector arrangement 1 in a two-channel system is particularly advantageous as a so-called photonic mixer device (PMD) .The photonic mixer device 18 is used as a mixer component for mixing electrical signals E and optical signals 0. The photonic mixer 18 comprises at least two paired photodetector units 2, each with associated photoelement 4 and a signal source V _Mo a for generating the electrical signal E. The electric signal E generated by the signal source V _mod is in the photonic mixer 18 with the optical signal 0. The result of the mixture is determined by one of the number of photodetector inputs. 2 corresponding number of signal paths Signal_A and Signal_B simultaneously provided.

In detail, during operation of the photodetector arrangement 1 charge carriers, which are generated by an active scene lighting ΔE _Mod in the respective semiconductor or photoelement 4, are distributed in the mixture with the electrical signal E according to a specific scheme to the two photodetector units 2 of a detector pair. The number of charge carriers resulting from the active scene illumination ΔE _Mo a (= useful signal components NL) can be very small in relation to the number of charge carriers generated by stray light (= interference signal components SL). Analogous to the photodetector arrangement 1 with a single photodetector unit 2 and a single photovoltaic cell 4, in photomix detector 18 a number of memory elements C _S i _g _A and C _S i _g _B corresponding to the number of photodetector units 2 is determined by means of a respective useful signal component NL relevant voltage signal V _c _si _g _A and V _c _si_B provided.

To determine the compensation signal I _L , for example, a common further memory _element C _{HL is} provided in addition to the memory elements C _S i _g _ _A and C _S ι _g _ _B for the voltage signals V _c _si _g _A and V _c _ _S ig_B of Nutzsignalanteils NL. Alternatively, in a manner not shown, the compensation signal I _HL signal-related, ie per voltage signal V _c _si _g _A and V _c _si _g _B / be determined. For this purpose, the compensation unit 6 a number of further memory elements C _HL corresponding to the number of photodetector units 2 for determining a respectively associated compensation signal, z. B. I _HL _si _g _ _A to I _H L_si _g _z include. To output the voltage signals V _c _sig_A and V_si _g _B, an associated output line 16 with integrated amplifier unit 10 and associated switch S _{3 is} provided in each case. In other words, the suppression or compensation of the disturbing light signal or the interfering signal components SL can be assigned to each detector unit 2 of a detector pair in the case of a photonic mixer detector 18. Since a detector pair is usually constructed symmetrically in a photonic mixer 18, it is sufficient to determine the interference signal components SL only at a detector unit 2 of the detector pair. For symmetry reasons, the same signal is then switched on as the compensation signal I _{HL of} the second detector unit 2. This results in a simplified wiring, as shown in FIG. That to

As a result of the symmetry properties, feedback in the two detector units 2 of the detector pair required compensation signal I _HL can be generated in a simple manner by forming a copy of this current, for example by means of a simple current mirror arrangement 20. The current mirror arrangement 20 is connected in a feedback or feedback line 22 of the compensation unit 6.

The initialization of the photodetector arrangement 1 according to FIG. 3 is analogous to the initialization of the photodetector arrangement 1 according to FIG. 1. In the photodetector arrangement 1 with the photonic mixer 18, a significantly higher dynamic range is made possible, which considerably increases the performance of such components in technical applications.

The present invention has been described with reference to the foregoing description for two alternative embodiments to best explain the principle of the invention and its practical application, but it will be understood that the invention, if suitably modified, can be embodied in various other embodiments.

For example, the proposed photodetector arrangement 1 moreover comprises a plurality of parallel arranged photoelements 4 with associated photodetector unit 2. sen, which are used for example in a row arrangement as image sensor in line scan cameras application. Furthermore, row arrangements are possible as multichannel optical systems for the separation of different modulation channels. The control and signal readout of the individual pixels of such line arrangements is usually carried out with so-called multiplexer components. The same applies to a two-dimensional matrix arrangement, as used in area sensors for video cameras. Multiplexer modules are used for control and readout of the detector elements in each case for the rows and the columns of the matrix arrangement.

LIST OF REFERENCE NUMBERS

1 photodetector arrangement

2 photodetector unit

3 photodetector circuit 4 photoelement

6 compensation unit

8 voltage-current converter

10 amplifier unit

12 Reset circuit 14 Connection

16 output line

18 photonic mixer

20 current mirror arrangement

22 feedback or feedback line

A, B signal paths

Csi _g ; Csig A r Csi B / Csi _g 1 /

C _S i _{g 2 /} C _H storage elements (= integration capacity) E electrical signals

Ec stain light

IH sig_A - IHL sig_z compensation signals

Ip _h photocurrent IptKo _m p compensated photocurrent k Conversion or compensation factor

NL useful signal components

SL interference signal components

0 optical signals Sig_A, Sig_B useful signals

Si, S ₂ , S ₃ switches

Ti, T ₂ , T ₃ times

Tsi, T _s2 , T _s3 times, in particular switching points of

counter

c sig_ _B voltage signals of the useful signals Sig_A,

Sig_B V _C _ _HL Voltage value, voltage signal

V _Mθ d signal source ΔE _MOD scene _lighting

Claims

claims

1. photodetector arrangement (1) for interfering light compensation with a photodetector unit (2) for detecting and determining a photocurrent (Ip _h ) and with an integrated compensation unit (6) for compensation of the photocurrent (I _P h) underlying interference signal components (SL) in such a way that the photocurrent (I _Ph ) is counteracted by a compensation signal (I _HL ) representing the interference signal components (SL).

2. Photodetector arrangement according to claim 1, wherein the compensation unit (6) comprises at least one storage element (C _HL ), in particular a storage capacity, for determining the compensation signal (I _HL ).

3. photodetector arrangement according to claim 1 or 2, wherein the compensation unit (6) comprises a reset circuit (12).

4. photodetector arrangement according to one of claims 1 to 3, wherein at least one switching element (Si) for determining the compensation signal (I _HL ) is provided.

5. Photodetector arrangement according to one of claims 1 to 4, wherein a voltage-current converter (8) for converting the as

Voltage value (V _C _ _HL ) detected compensating signal (I _H L) is provided in a proportional current value.

6. photodetector arrangement according to one of claims 1 to 5, wherein a plurality of mutually parallel photodetector units (2) are provided, which is followed by a compensation unit (6) having a number of photodetector units (2) corresponding number of memory elements ( C _S ig_ _a / C _S ig_ _B) for determining the respective associated desired signal (NL, Sig_A, SIG_B) and another common memory element (C _HL) for determining the compensation signal (IHL).

7. The photodetector arrangement according to claim 6, wherein the common memory element (C _HL ) is a current mirror arrangement

(20) is connected downstream.

8. Photodetector arrangement according to one of claims 1 to 5, wherein a plurality of mutually parallel photodetector units (2) are provided, which is followed by a compensation unit (6), one of the number of photodetector units (2) corresponding number of memory elements (C _s i _g _ _A / C _si g_ _B ) for determining the respectively associated useful signal (NL, Sig_A, Sig_B) and a corresponding number of further memory elements (C _HL ) for determining the respective associated compensation signal (I _HL ).

9. photodetector arrangement according to one of claims 1 to 8, wherein a plurality of mutually parallel arranged photodetector units (2) as a photonic mixer (18) are formed.

10. photodetector arrangement according to one of claims 1 to 9, wherein the photodetector unit (2) and / or the compensation unit (6) are formed as an integrated circuit.

11. Photodetector arrangement according to one of claims 1 to

10, wherein a plurality of a row or matrix arrangement forming photodetector units (2) are designed as image sensor of a line or matrix camera.

12. A method for Störlichtkompensation for a photodetector arrangement (1), wherein by means of a photodetector unit (2) a photocurrent (I _P ) is detected whose Störsignalanteile (SL) are compensated such that the photocurrent (Ip _h ) a the Störsignalanteile (SL) representing and by means of an integrated compensation unit (6) educated compensating signal (I _H L) is countered.

13. The method of claim 12, wherein the compensation signal (IH _L ) is detected as a voltage signal (V _C _HL).

14. The method of claim 12 or 13, wherein the compensation signal (I _HL ) is determined after an initialization process.

15. The method according to any one of claims 12 to 14, wherein the compensation signal (I _HL ) is determined event-controlled.

16. The method according to any one of claims 12 to 15, wherein for the compensation signal (I _HL ) a degree of compensation is set is.