EP4453604A1

EP4453604A1 - Improved wide-field-of-view lidar

Info

Publication number: EP4453604A1
Application number: EP22835826.3A
Authority: EP
Inventors: Jean-Paul CROMIERES
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Paris Saclay
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Paris Saclay
Priority date: 2021-12-21
Filing date: 2022-12-16
Publication date: 2024-10-30
Also published as: FR3131004A1; FR3131004B1; WO2023117816A1

Abstract

The invention relates to a lidar system (10) based on measuring a time-of-flight, comprising: an emission device (DE) configured to emit light pulses towards a scene at an angle greater than or equal to 5°; a reception device (DR) comprising: o a photodetector (PD) configured to receive pulses (Ir) reflected or backscattered by at least one element (Ei) of the scene and to convert said pulses into an electrical signal, o an amplification circuit (CA) configured to amplify said electrical signal, a processing unit (UT) for processing said amplified electrical signal, configured to digitize the amplified electrical signal and determine a distance (di) from said at least one element (Ei) based on said digitized amplified electrical signal, the amplification circuit (CA) comprising a transimpedance amplifier (TIA), a transformer (T) comprising a primary (P) and a secondary (S), a capacitor (C) and an inductor (L) arranged in series with said capacitor, the primary of the transformer being connected to an anode of the photodetector, the secondary being connected to said capacitor, said capacitor being connected to an input of said transimpedance amplifier.

Description

DESCRIPTION

TITLE: Enhanced Wide Field of View Lidar

FIELD OF THE INVENTION

The present invention relates to the field of lidars performing a measurement by time of flight (“time of flight” in English or TOF), and more particularly lidars having a field of view greater than 5°. We are interested here in static lidars (without moving mechanical parts) with low consumption, small dimensions and low cost. This type of lidar finds, for example, applications in the detection of obstacles. STATE OF THE ART

[0002] A lidar (Light Detection And Ranging) is a device used to measure distance by measuring the time of flight of a light pulse.

[0003] For this, it emits a light pulse of high power and of short duration (typically a few ns), and recovers a time later a pulse reflected/backscattered on an obstacle. Knowing the speed of propagation of light, the distance is deducted from this delay, called time of flight. It is calculated according to the formula: in which c = 3. 10 ⁸ m/s is the speed of light, R the delay linked to the distance d between the emitter and the obstacle. Thus, for an obstacle 1m away, it is assumed that the delay is 6.67 ns.

[0004] The lidar consists of:

-an optical pulse transmitter (laser diode or light-emitting diode), controlled by a logic component of the microprocessor, microcontroller or FPGA type -an optoelectronic receiver whose role is to convert the reflected pulse into an electrical signal in compliance with measurement quality, in particular by maximizing the measurement signal-to-noise ratio, denoted S/N

-a unit for processing the electrical signal supplied by the receiver to deduce the distance from the obstacle. This device can be a time-distance converter, or “Time to Distance Convert” (TDC), or a digital signal processing system based on a microprocessor, microcontroller or FPGA.

[0005] Typically in the case of the use of a laser diode transmitter, the optical power emitted can be up to a few tens of watts over a period of a few nanoseconds. The orders of magnitude of powers received from the echoes are typically from a few nanowatts to a few hundred milliwatts. The luminous background can vary from zero power for complete darkness up to 1kW/m ² (120klux) in case of full sun. Thus, an illumination of 10W/m ² corresponding to average artificial lighting illuminating a 5x5mm ² silicon photodiode will generate a photo generation current of approximately 500μA, full sun a current of approximately 20mA (taking into account the entire spectrum solar).

[0006] Lidars are used to measure distances, map surfaces, detect objects. There are several types of lidars:

[0007] The narrow-field 1D lidar presents an emitted pulsed light beam of small aperture (lidar low field of view - Field Of View - FOV). It measures the distance from a specific point, where the object is located. The optoelectronic components (beam emission diode, reception photodiode) are then associated with optical components (lenses, filters, etc.) for collimating and focusing the beams. [0008] FIG. 1 illustrates a lidar 1 D L0 according to the state of the art. It comprises an emission device DE0 for emitting light pulses in the direction of a scene at a low angle (FOV), typically less than 3°. The emissive element ELO is for example a laser diode or a light-emitting diode. Typically collimating optics (not shown) is coupled to the emitter to obtain low divergence of the illumination beam.

[0009] The lidar L0 also comprises a reception device DR0 (or receiver) comprising a photo-detector PD0 which can receive pulses reflected or backscattered by at least one element (Ei i index of the element) of the scene and for converting the reflected pulses into an electrical signal and a CAD amplification circuit that amplifies the electrical signal. Conventionally, the receiver (photodetector and amplification circuit) has an impulse response hr(t) which can be measured and/or determined by calculation. Typically an optic of reception (not shown) is coupled to the photodetector to obtain a low reception solid angle, and thus to recover the useful light as a priority

[0010] A processing unit UTO controls the transmission, typically via a logic component of the microprocessor, microcontroller or FPGA type, digitizes the amplified electrical signal, and processes it so as to extract the useful information, i.e. say the presence of the elements in the detection field and their respective distance.

[0011] The optoelectronic components (beam emission diode, reception photodiode) are then often associated with optical components (lenses, filters, etc.) for collimating and focusing the beams. These narrow field (typically less than 3°) and short range 1D lidars are marketed for obstacle detection applications in light applications of autonomous or semi-autonomous moving objects such as drones or robots in the broad sense (vacuum cleaners and autonomous mowers, radio-controlled vehicles with obstacle detection, etc.). They have supplanted the traditional ultrasonic rangefinders whose field is much wider and of which we do not know exactly which object will constitute its first detected obstacle.

[0012] 1D lidars have the advantage of being relatively compact and low cost but have a major drawback which is a narrow field of view, less than 3° (Safran LRF3013, Benewake TF02). FIG. 2 illustrates a drone D equipped with a 1D lidar L0 according to the state of the art in descent action on uneven terrain: obstacle detection is uncertain due to the narrow field.

[0013] The need for obstacle detection requires a wide field of observation, as illustrated in FIG. 3 in which the drone D is equipped with a 3D lidar enabling it to detect obstacles during its landing.

[0014]To obtain a wide field, 3D lidars with scan technology are used today. The pulsed beam is also of low aperture, but coupled with a mechanical scanning system to irradiate an entire portion of space: several shots directed towards several places are necessary to produce a map. 3D lidars make it possible to establish a precise cartography of the environment by the use of more or less complex mechanical systems or even by the use of MEMS (Micro Electro Mechanic Systems). They are effective but have the disadvantage of being oversized with regard to light applications, in terms of:

-Volume of data: precise mapping requires heavy processing of data, of which it is possible to imagine that not all of them need to be processed -Significant physical size

- High consumption (higher than W in most cases)

- Low measurement refresh rate (a few tens of Hz)

[0015] This is why in the field of reversing radars requiring the detection of obstacles in a large field of view, it is camera devices and not lidars which have supplanted or supplemented ultrasonic sensors. Indeed for anti-collision protection the wide field of an ultrasonic sensor is an advantage over the traditional 3D lidar, which to scan a wide field needs to be mounted on a rotating plate. In addition, ultrasonic sensors are a priori easier to use due to the low propagation speed of acoustic waves compared to that of electromagnetic waves, with the consequence of echo times that are much longer and easier to measure. The latter are notably found in numerous applications such as vehicle anti-collision, robotics, detection of the presence of objects or living beings.

[0016] However, the characteristics of wavelength and propagation speeds of acoustic waves mean that ultrasonic sensors have fundamental defects:

- the high sensitivity to atmospheric and environmental conditions: humidity, rain, temperature, noise, and it is difficult to obtain ranges beyond one meter with reasonable sensitivity in these climatic conditions, - the presence of parasitic secondary lobes in the measurement field with risk of false detections, - insensitivity to low roughness surfaces (less than mm) seen under strong incidence (phenomenon of total reflection preventing their detection).

- the difficulty of detecting thin, small or surface objects - the slowness of measurement is a limit to the detection of obstacles moving relative to the sensor

[0017] A lidar having the wide field properties of ultrasonic sensors, which would illuminate a scene according to a cone greater than 5°, see 10° or 20°, would make it possible to scan a large portion of space and detect echoes from different elements/obstacles present in the scene or observation field, with the possibility of a single pulse (no need for scanning anymore). In addition, a wide FOV lidar, almost insensitive to high humidity and/or rain and having the possibility of detecting a smooth painted surface under high incidence, would have a competitive advantage in the fields of anti-collision still reserved for ultrasound. It could also be used for autonomous movement of robots or drones.

[0018] But the wide field is not the natural domain of the lidar. Indeed, the backscattered optical flux decreases with the distance (d) in d ^-4 for small objects instead of decreasing in d ^-2 for a narrow field, which limits its range. Indeed the difference is made on the density of incident light intensity. In a so-called "narrow" field, it is considered that all the incident energy is included on the surface of the obstacle (in other words, the surface of the obstacle is greater than the illuminated surface according to the solid angle): obstacle receives all the energy from the transmitter. Each point of the obstacle then backscattered energy towards the receiver according to a law in d ^-2 . In the case of a wide field, the obstacle is completely included in the cone of illumination: the obstacle receives only part of the energy of the transmitter according to a law in d ^-2 , and backscattered this energy towards the receiver also according to a law in d ^-2 , with overall an energy on receiver in d ^-4 of the energy of the transmitter. In addition, large objects in the background obscure small ones in the foreground. The centimeter resolution is more difficult to obtain, the costs are a priori higher than for an ultrasonic sensor.

[0019] In addition, a wide field of view poses problems at the level of the reception device.

The most suitable and most commonly used receiver assembly is called a transimpedance amplifier (“Trans Impedance Amplifier”, or TIA) associated with a photodiode PD0. These elements are known to professionals. The TIA circuit, based on operational amplifiers, can be complex. It is configured to transform the current from the photodetector into electrical voltage.

[0021] In its most basic version, the TIA-type CAD amplification circuit consists of a resistor R0 and an operational amplifier Amp0. Its structure is shown in Figure 4. [0022] Considering the ideal components:

-The PD0 photodiode transforms the luminous flux into photo generation current i _ph0 .

-The TIA transforms the current i _ph0 into voltage according to the relationship: [0023] V _s0 = — R ₀ i _ph0 .

The value of resistor R ₀ sets the gain of the amplifier. The sensitivity of

TIA is tied to this value.

The most widely used photodetector is a PIN type photodiode, which is more reliable and simpler to implement than an avalanche photodiode. The current from the photodiode can be described by the relationship: in which :

S is the sensitivity of the photodiode, of the order of 0.6A/W, Ø _e (t) is the light power received,

I ₀ is the sum of the reverse static currents of the photodiode (saturation current, black current), represents the sum of the intrinsic noise of the photodiode (mainly shot noise).

The captured light power Ø _e (t) can also consist of a dynamic part φ _e (t) such as the reflected pulse and a static part caused by a light background Ø _e0 , caused by the sun For example. The relationship is then written:

For example, a silicon PIN photodiode generates approximately 600mA/W. The photodiode is usually followed by a transimpedance amplifier whose gain is generally a compromise between the desired bandwidth and the sensitivity (detection capacity) of the sensor. It can also be limited by the intensity of the reverse static currents of the photodiode.

In reality, the operational amplifier AmpO and the resistor R ₀ are sources of noise. FET technology is most often used for the operational amplifier because of its attractive characteristics of very low noise current, in the order of femtoA/√Hz. Under these conditions, it is known that the current noise added by the TIA mainly originates from the resistance R ₀ , whose noise spectral density is: where k = 1.38.10 ^-23 JK ^-1 is the Boltzmann constant, and T the temperature in

Kelvins. This leads to a noise current source i _n (t) parallel to the signal source i _ph0 . The signal to noise ratio S/N is then proportional to √R ₀ (see for example Wikipedia: “Transimpedance noise considerations”).

It is therefore appropriate, in terms of sensor sensitivity and S/N measurement signal-to-noise ratio, to use a high resistance R ₀ , leading to a high gain. During detection, the ambient light is amplified in the same way as the optical echo signals. It was mentioned previously that there are very different orders of magnitude between the currents generated by the echo pulses (a few μA for an echo of a few μW) and that induced by the ambient luminosity (a few tens of mA for a 5x5mm ² silicon photodiode detecting any the solar spectrum).

In order to limit the photogeneration current typically induced by the sun, which can lead to saturation of the TIA, it is possible to place an optical filter just on the surface of the photodiode. It can be the color filter integrated into the photodiode proposed by the manufacturers (broad spectrum of the order of 300nm), or even an interference filter (narrow spectrum of the order of 10nm and angular tolerance of less than 3°, which presents a major directivity drawback).

By typically considering an average ssoollaaiirree power of 1 Wm ^-2 ..nm ^-1 around a working wavelength of 900 nm illuminating a PIN photodiode of dimensions 5mmx5mm, a quick calculation gives the values of the voltage of output of the TIA (see table I below) for two values of R ₀ :

- R ₀ = 10kΩ corresponding to a standard value in TIAs used for state-of-the-art Lidars, achieving a sensitivity/noise/bandwidth compromise.

-R ₀ = 1 MΩ

Table I

[0036] A typical TIA supply voltage is 3 to 5V. Any higher theoretical output voltage VsO will have the effect of saturating it. It follows that:

- Resistance to direct sunlight can only be envisaged as it is with the use of the low-sensitivity TIA fitted with a resistor R ₀ = 10kΩ and a very fine interference filter positioned upstream of the photodiode.

- Resistance to direct sunlight is not possible as it is when using the high-sensitivity low-noise TIA fitted with a resistance R ₀ = 1 MΩ. [0037] Thus holding the sensor in direct sunlight is antagonistic to the use of a highly sensitive TIA. This is a major limitation for lidars used outdoors (“outdoor” lidar). Furthermore, for a lidar with a wide field of view, the addition of an interference filter cannot be considered because of its implementation difficulties, in particular its low opening angle. Photodiode average current compensation solutions are proposed in the literature and by component manufacturers specializing in time-of-flight detection.

[0039] One of them often consists in subtracting the average current resulting from the ambient illumination. An example component, the MAX40658 produced by Maxim Integrated, illustrates this method. Its effectiveness is limited by the saturation of the compensation circuit and leads at least to the use of a narrow band optical filter (interference filter) to minimize the power of the sun on the photodiode, without guarantee of operation for the strongest luminosity. . But the interference filter is not compatible with wide field detection, because it only works on an angular zone close to the normal. Another solution was proposed in 2020 by Juha Kostamovaara & Al in the publication "A wide dynamic range laser radar receiver based on Input Pulse-shaping technique", IEEE transactions on circuits and systems-l: regular papers, Vol 67 , n°8, 2020. The authors use an inductance which short-circuits the average current generated by ambient lighting. However, this method has the drawback of drastically limiting the gain capacities of the amplifier by the diversion of the photogeneration current.

[0041] An object of the present invention is to overcome some of the aforementioned drawbacks by proposing a Lidar with a large field of view having improved sensitivity and rendered insensitive to ambient lighting, this being obtained by the addition of a component in the receiving device.

DESCRIPTION OF THE INVENTION

The subject of the present invention is a lidar system by measuring a time of flight comprising:

-an emission device configured to emit light pulses in the direction of a scene at an angle greater than or equal to 5°

-a reception device comprising:

*a photo-detector configured to receive pulses reflected or backscattered by at least one element (Ei) of the scene and to convert said pulses into an electrical signal,

*an amplification circuit configured to amplify said electrical signal,

a unit for processing said amplified electrical signal configured to digitize the amplified electrical signal and determine a distance (di) of said at least one element from said digitized amplified electrical signal. The amplification circuit comprises a transimpedance amplifier, a transformer comprising a primary and a secondary, a capacitor and an impedance arranged in series with the capacitor, the primary of the transformer being connected to an anode of the photo-detector, the secondary being connected to said capacitor, said capacitor being connected to an input of said transimpedance amplifier.

According to one embodiment, the frequency operating range of the transformer includes the band [10 MHz; 350MHz], According to one embodiment, a transformation ratio equal to the ratio of the number of turns of the secondary to the number of turns of the primary is strictly greater than 1.

According to one embodiment, the capacitance C and the inductance L satisfy the relationship:

[0047]

(L + n ² L _p )C > L _p C _ph with L _p transformer primary inductance, C _ph . the transition capacity of the photodiode, n transformation ratio defined by: with n _p and n _s number of turns respectively of the primary and the secondary.

According to one embodiment, the inductor verifies the relationship: with: n _p and n _s number of primary and secondary turns respectively,

L _p inductance of the transformer primary, C _ph . transition capacitance of the photodiode, f _{l_b} minimum frequency of interest.

According to one embodiment, said inductor verifies the relationship:

0.5 Ls <L< 2 Ls

L inductance and Ls inductance of the transformer secondary

[0050] According to one embodiment, the capacitor C verifies the relation [0051] with Cph transition capacity of the photodiode.

According to one embodiment, the reception device further comprises a so-called damping resistor between the photodetector and the primary of the transformer.

The following description presents several embodiments of the device of the invention: these examples do not limit the scope of the invention. These embodiments present both the essential characteristics of the invention as well as additional characteristics related to the embodiments considered.

The invention will be better understood and other characteristics, objects and advantages thereof will appear during the detailed description which follows and with reference to the accompanying drawings given by way of non-limiting examples and in which:

FIG. 1 already cited illustrates a low field of view lidar according to the state of the art.

[0056] Figure 2 already cited illustrates a drone equipped with a lidar 1 D according to the state of the art in descent action on rough terrain.

[0057] FIG. 3 already cited illustrates the advantage of a wide-field lidar for the detection of obstacles during the landing of a drone.

FIG. 4 already cited illustrates a simplified amplification circuit for lidar according to the state of the art. FIG. 5 illustrates a wide field of view TOF lidar according to the invention.

Figure 6 illustrates an amplification circuit according to the invention.

FIG. 7 illustrates an embodiment of the lidar reception device according to the invention with a basic transimpedance amplifier circuit.

Figure 8 illustrates the equivalent diagram of a photodiode. FIG. 9 illustrates a circuit illustrating the notion of noise gain.

FIG. 10 illustrates the equivalent diagram of the reception device according to the invention comprising a photodiode, a transformer, an additional impedance Z and a transimpedance amplifier.

Figure 11 shows the equivalent diagram of Figure 10 with the elements connected to the primary reported to the secondary of the transformer.

Figure 12 illustrates an embodiment of the amplification circuit according to the invention in which an inductance L placed in series with the capacitor C is added.

[0067] Figure 13 illustrates an overall diagram with an impedance Z comprising a capacitor C and inductance L.

Figure 14 illustrates the asymptotic diagram of signal gain (A) and noise gain (B) of the circuit of Figure 13. FIG. 15 illustrates the asymptotic diagram of signal gain (A) and noise gain (B) for three values of f _M (f _M1 , f _M2 , f _M3 ).

FIG. 16 illustrates the equivalent diagram of the circuit of FIG. 13 in which the noise voltage source has been deliberately omitted. FIG. 17 illustrates an embodiment of the reception device according to the invention comprising a resistor in series with the photodiode.

Figure 18 illustrates various simulated signals.

DETAILED DESCRIPTION OF THE INVENTION

A wide FOV TOF lidar 10 according to the invention is illustrated in FIG. 5. It comprises an emission device DE configured to emit light pulses in the direction of a scene at an angle (FOV) greater than or equal to 5° , preferably 10°. The emissive element is for example a laser diode or a light-emitting diode. The choice of wavelength for a lidar according to the invention is wider than for a lidar with a narrow field, because for eye safety the fact of using a wide field greatly limits the risks. Preferably, the illumination wavelength close to the maximum sensitivity of the detector is chosen for the purpose of optimizing reception. According to one embodiment, the desired large field of view is obtained by using the natural divergence of the transmitter (approximately ten degrees to a few tens of degrees depending on the components and the emission axes) which has the advantage of eliminate an optic and therefore gain in size and simplicity. According to another embodiment, an optic coupled to the transmitter makes it possible to obtain the desired FOV.

The lidar 10 also comprises a reception device DR (or receiver) comprising a photo-detector PD configured to receive pulses reflected or backscattered by at least one element (Ei i index of the element) of the scene and to converting the reflected pulses into an electrical signal and an AC amplifier circuit configured to amplify the electrical signal. The detector can be used without optics or coupled with detection field adaptation optics.

A processing unit UT controls the transmission, typically via a logic component of the microprocessor, microcontroller or FPGA type, digitizes the amplified electrical signal, and processes it so as to extract the useful information, i.e. the presence of the elements (Ei i index d of the element) in the detection field and their respective distance (distance di) .

In the example of Figure 5 there are two elements E1 and E2 in the detection field, respectively a pole P and a vehicle V. The initial pulse li is emitted at time t=0, and the photo-detector receives, at time t1, a first pulse Ir1 from the backscatter of the pole P and, at time t2, a second pulse Ir2 from the backscatter of the vehicle V. We call R ₁ the delay related to the distance d ₁ between the transmitter and the post P and R ₂ the delay linked to the distance d ₂ between the transmitter and the vehicle V. The detector also receives ambient light such as sunlight. For the example of a reversing lidar according to the invention on board a vehicle V, photo 1 illustrates the scene illuminated by the lidar of the vehicle V. In general, the lidar according to the invention can be on board any object in movement: automobile, drone, robot, cane for the visually impaired... For proper operation, the lidar must detect the presence of the pole P and determine its distance d1 from it without being disturbed by the backscatter of the vehicle V.

According to another example, the Lidar is static and detects the presence of static or moving objects. A first consequence of the opening of the field is the spatial spreading of the energy emitted leading to less illumination of the obstacles which in return provide lesser echoes, more difficult to measure than in the case of an emission of focused or collimated beam.

A second consequence of opening the detection field is the greater probability of finding a strong emissive source there, such as the sun.

[0081] Finally, the proximity of obstacles is synonymous with the measurement of short echo times, a priori requiring rapid detection electronics.

We are interested here in the detection of lidar signals having a frequency of interest included in the band [10 MHz, 350 MHz]. This frequency band is linked, among other things, to the shape of the pulse and to the separation of the return pulses that one wishes to measure (for example distinguishing 1 m of spacing).

The object of the invention is to overcome the parasitic signal generated by the presence of ambient illumination, that is to say the cancellation of the component continuous photogeneration current while maintaining high sensitivity and low noise. For this, the CA amplification circuit of the lidar 10 according to the invention comprises a transimpedance amplifier circuit TIA, a transformer T comprising a primary P and a secondary S and a capacitor C as shown in FIG. 6. The photodetector has in a conventional manner an anode An and a cathode Cath. The TIA transimpedance amplifier circuit is a circuit suitable for lidar applications known from the state of the art. The primary P of the transformer being connected to the anode An of the photo-detector, the secondary S is connected to the capacitor C, itself connected to an input of the transimpedance amplifier. It is recalled that the primary P and secondary S circuits of a transformer are, as a first approximation, linked by the relationship:

[0085] with Up and Us respective voltages at the terminals of the primary and the secondary, Ip and Is respective intensities at the primary and at the secondary as illustrated in FIG. 6, n _p and n _s number of turns respectively of the primary and of the secondary, and n the transformation ratio.

An example of an amplification circuit according to the invention, with a basic transimpedance amplifier comprising an amplifier Amp and a resistor R, is illustrated in FIG. 7. Of course, the invention is compatible with any more complex TIA used for detection. lidar, including several amplification components (operational amplifiers, transistors).

The principle of operation of the transformer lies in the conversion of a current in its primary, which is here the photodiode current i _ph (t) into a magnetic field B(t), itself reconverted into an electric field. E(t), therefore in a voltage Vs(t) at the secondary of the transformer. We have previously described the photodiode current:

[0088] I ₀ sum of the reverse static currents of the photodiode (saturation current, black current), sum of the intrinsic noise of the photodiode (mainly shot noise).

Preferably, the photodetector is fitted with a colored filter, having for example a transmission between 750 nm and 1100 nm (emission at 900 nm), or a Δλ = 350 nm. It should be recalled that an interference filter cannot be used because its low angular field (3° maximum) is not compatible with the wide detection field of the lidar according to the invention.

By way of example, the photogeneration current i _phs generated by a face sun with a power density or irradiance 1rs of 0.89 W/m ² /nm is determined, the photodetector (PIN diode with detecting surface A=25μmx25μm) coupled to a color filter with a bandwidth of 350 nm: i _phs (t) = SAI _rs .Δλ = 0.6 x 25.10 ^-6 x 0.89 x 350 = 0.52mA

It is known that the electric field is a function of the derivative of the magnetic field induced by i _ph (t). Consequently, the voltage and the current from the transformer only depend on the variations of i _ph (t). The voltage from the TIA will be of the form:

Without characterizing the coefficient k, it is established here that all the static components of the photodiode current i _ph (t) are eliminated at the secondary of the transformer:

- the parasitic inverse intrinsic currents of the photodiode lo (saturation current, black current),

- the photo generation currents due to any light sources, weak or intense (sun).

The addition of a capacitor C is linked to the problem of noise reduction as explained below.

It is known that the transimpedance amplifier ideally fulfills its role only if the current source connected to its input is ideal and sees its infinite output impedance. A real current source has a finite output impedance. This can be resistive, inductive or capacitive, or a mix of several elements. In particular, a photodiode can be likened to a current source with a capacitive output impedance Z _s =C _ph . The equivalent diagram of a photodiode is recalled in figure 8.

Like any electronic system, the TIA intrinsically generates noise symbolized at its inputs as a single source of voltage noise and a single source of current noise, the spectral densities of which can be considered constants for the frequencies of interest. The amplification technology with FET (Field Effect Transistor) elements adding only little current noise, the high value of the resistance against feedback R of the TIA also adding little noise, we are interested here in the voltage noise source e _n , referred to the input. The TIA, whatever its electronic structure, then behaves like an amplifier with respect to its source of noise voltage, of a value conventionally called "noise gain", illustrated in Figure 9.

The noise gain G is defined as:

Gn = Vs _n /e _n = 20.log[1+ R/Zs] With Vs _n output voltage with i _ph =0.

[0097] All the spectral components of the noise must be considered here, from DC (offset voltage) to infinite frequencies. The impedance of a transformer being a low value resistance in DC, the use of a high value TIA gain resistor leads to a high noise gain with which an offset voltage even of low value (typically a few tens of microvolts) leads to saturation of the TIA. It is therefore appropriate to add an impedance Z in series with the secondary of the transformer to increase the branch impedance of the circuit and then limit the noise gain. FIG. 10 illustrates the equivalent diagram photodiode PD (i _ph , Cph)/transformer T (primary P of inductance Lp, secondary S of inductance Ls)/additional impedance Z/transimpedance amplifier TIA. The TIA is represented here according to its basic version but it is understood that the reasoning applies for a more complex TIA circuit.

FIG. 11 represents the photodiode/transformer/Z/TIA equivalent diagram with the elements connected to the primary connected to the secondary of the transformer. The impedance of the transformer loaded by the photodiode is called Zeq. It should be noted that the source i _ph does not appear there for the sake of simplification. The noise gain formula G for the circuit according to the invention of FIG. 11 is written:

Gn = 20. Iog[ 1 + R/(Zeq+Z)] (2) [0099] In order not to add additional thermal noise (Johnson noise), the impedance Z must be produced by absolutely favoring the use of purely reactive components (inductor, capacitor), or to limit the use of resistive elements. [0100] Thus an essential element constituting Z is the capacitor C placed between the TIA and the transformer T. The role of this is to minimize the low-frequency gain (in particular the DC) that the system composed solely of the together AOP (Amp) / resistance R / transformer T (for the simplified case with an amplification component of the operational amplifier type, but this remains valid for a more complex amplification circuit, for example with several operational amplifiers). Indeed without this capacity this assembly would behave as a non-inverting amplifier with infinite gain with respect to the offset voltage of the TIA, with the effect of its saturation. [01O1] However, an impedance Z consisting of a single capacitor is not optimal because the overall impedance achieved Zeq + Z inevitably tends towards 0 at high frequencies, leading to an increase in the noise gain for these frequencies.

Thus, in the amplification circuit according to the invention CA, another constituent element of Z is added, which is an inductance L placed in series with the capacitance C, as shown in FIG. 12. This is a means of raising the impedance Z at high frequencies and therefore of lowering the noise gain for these frequencies. The impedance L is an additional impedance different from the impedance Ls of the secondary of the transformer. The respective position of the impedance and the capacitance, arranged in series between the input of the amplifier and the secondary of the transformer, is irrelevant.

The structure of the amplification circuit as claimed (T+C+L+TIA) therefore solves two problems. The lifting of the gain resistance limit, as the various DC stray currents are filtered out. It is then possible to envisage a very high gain sensor (high resistance R) and to improve the detection capacity. The sensor is virtually insensitive to ambient light. It is then possible to detect obstacles with the sun in front. It is thus perfectly adapted to the use of a large F.O. lidar. V.

A fundamental characteristic of the transformer for its use in the circuit according to the invention is the non-saturation of its magnetic circuit by the direct current generated, in particular by the sun. Saturation of the magnetic core would have the effect of deforming the useful signal or even its absence of transmission, the coefficient k then evolving towards 0. The deformation of the useful signal leads to non Linearities leading to limitations in signal processing With the aim of minimizing non-linearities, it is advisable to choose a transformer capable of transporting currents ten times higher than the photogeneration current due to the sun in front. Considering the full sun situation resulting in a photogeneration current of 0.52mA, the nominal operating current of the transformer must be greater than or equal to 5mA.

[0105] The information-carrying phenomena involved in the time-of-flight measurement (echo rise time, echo pulse width) are at the level of ten nanoseconds. This leads to an equivalent frequency of This frequency very roughly constitutes the limit of low cut-off frequency that the transformer T must have. The low cut-off frequency of the transformer is chosen to be lower, of the order of ten MHz or one MHz.

In a similar way, it is possible to roughly characterize the high cut-off frequency of the transformer. It is a priori necessary to measure and separate events on time scales of the order of a nanosecond.

This leads to an equivalent frequency of The frequency of transformer high cutoff is preferably chosen to be greater than this value. Consequently, a so-called radiofrequency transformer is chosen, the bandwidth of which preferably includes the range 10 MHz to 350 MHz. These transformers also have the advantage of being of reduced volume (the volume occupied by a transformer is inversely linked to its working frequencies).

Conventionally, the transformation ratio is designated as voltage: a voltage step-up transformer sees a transformation ratio n greater than

1. In the lidar application of the invention, it is sought, in addition to the suppression of the DC component, to obtain an amplification of the useful current. If a current I _p is injected into the primary of the step-up transformer, the current in the secondary will have the value . The notion of noise gain was explained above. This is limited by adding an impedance Z comprising a capacitor C in series with an inductor L, and possibly a resistor Rad also in series (not shown on the diagrams). A global diagram with an impedance Z comprising a capacitance C and inductance L is represented in figure 13. The current leaving the secondary is called ls=l _tia .

From formula (1) and the equivalent diagram of FIG. 13, the inventors have determined the following characteristics of the noise gain Gn.

The noise gain reveals three resonance frequencies:

• : series resonance frequency L, n ² L _p , C leading to a maximum noise gain

• parallel resonance frequency L _p , C _ph leading to a minimum of noise gain

• : parallel resonance frequency of an equivalent inductance dependent on L, L _p and n, and C _ph resulting in maximum noise gain

• f _m , is an elbow frequency dependent on R, L, L _p , of an order of magnitude greater than GHz, well beyond the frequencies of interest. [0112] Because of the formulas, we necessarily have f _m <f _M .

In order to optimize the measurement signal-to-noise ratio, the noise gain should be minimized at the frequencies of interest. Cph being imposed (which we want to be as low as possible) we determine L and Lp so that fM is lower than the frequencies of interest. The asymptotic noise gain diagram Gn corresponding to the circuit of Figure 13 is shown in Figure 14, B.

[0114] As a reminder, we have:

[0115] Ls = n ² Lp

[0116] The slopes are:

• -20db/dec (-1 ) for f _LC << f < f _m

• +20db/dec (+1) for f _m < f < f _M

• -20db/dec (-1) for f _M < f < f _m ,

A first element for minimizing Gn is to shift the resonance f _LC towards low frequencies, i.e. f _LC << f _m , whence:

[0118] (L + n ² L _p )OL _p C _ph (3) L _p is the primary inductance of the transformer, and C _ph the transition capacitance of the photodiode, n the transformation ratio.

A second noise gain reduction factor at the frequencies of interest is the bringing of the frequency f _M , partly controlled by L, closer to the frequency determined by L _s and C _ph . Indeed, the more the frequency f _M moves away from f _m , the more the noise gain increases in the frequency range of interest, as illustrated in figure 15, B, which schematizes the asymptotic diagram of noise gain for three different values of f _M (f _M1 , f _M2 , f _M3 ).

A third noise gain reduction factor at the frequencies of interest is the positioning of the maximum noise gain at a frequency lower than the minimum frequency of interest.

From the circuit of Figure 13, considering that the noise source e _n is of a random nature, generating a white noise of constant spectrum, it is established that the noise intensity I _{TIA_noise} (i _ph =0) at the frequencies of interest is approximately:

[0123] (4)

[0124] with

The Z dipole comprising L and C decreases the noise gain but also impacts the signal gain. From the diagram of figure 13 can be deduced that of figure 16 called small signals making it possible to describe the circuit by disentangling the primary and secondary of the transformer. We deliberately omitted the source of noise voltage, which makes it possible to calculate the signal gain, and the capacitance C is omitted from the reasoning (resonance frequency generated with L and Ls very low compared to the frequencies of interest).

The effects of capacitor C are not taken into account, this acting mainly for frequencies much lower than f _m , outside the frequency band of interest. From the diagram of FIG. 16, the asymptotic diagram of signal gain Gs illustrated in FIG. 14, A and FIG. 15, A is determined for three different values of f _M (f _M1 , f _M2 , f _M3 ). [0127] The study of the signal gain reveals a cutoff frequency

The formula for the simplified signal intensity I _{TIA_signal} is established by:

(5) The calculation of the signal-to-noise ratio in the frequency range of interest leads to:

(6)

The signal-to-noise ratio is even better than the photodiode capacitance C _ph . is low and the transformation ratio is high, i.e. n>1. This n>1 result is counter-intuitive because one would have thought that a value of n<1 would have resulted in a better signal-to-noise ratio based on the formula: ls = 1/n.lp

The frequency f _M corresponding to the maximum noise gain or to the maximum noise spectral density must preferably be located at a value lower than the lowest frequency of interest f _{i_b} . The transformation of the formula expressing f _M makes it possible to fix:

(7)

It is also appropriate to minimize the value of L for the preservation of a high signal gain, according to the established expression (5). As such, a good compromise is:

Values outside this range will lead to non-optimal operation of the assembly: reduction in the signal-to-noise ratio or reduction in signal gain.

The values of the primary and secondary inductances L _p and L _s respectively are linked by the transformation ratio n by: For optimum operation, it is advisable to take a value of L approaching Ls, so as not to lower the signal gain too much. Thus preferably 0.5 Ls <L< 2 Ls.

With the condition L = L _s = n ² L _p the condition (3) implies:

(8)

A too low value of f _LC would lead to an increase in the noise gain at low frequencies (long-term noise) which could also lead to saturation of the TIA. In practice, it has been established by the inventors that a preferential range of choice of C is:

(9)

The AC amplification circuit according to the invention illustrated in the figure has several resonance modes with the effect of a pseudo-oscillating impulse response. These pseudo oscillations do not pose a problem when the lidar processing unit has a module carrying out an appropriate processing, transforming these oscillations into suitable signals. For other lidar applications that do not include a TF module, these oscillations can be troublesome. According to an embodiment illustrated in FIG. 17, the reception device DR according to the invention further comprises a resistor, called a damping resistor Rph, placed in series with the photodiode PD, which damps the oscillations. The resistance Rph adding noise and causing a voltage drop through the static currents crossing it, it is advisable to choose the minimum value to achieve the desired effect while minimizing it. Typically this resistance Rph is from a few ohms to a few hundred ohms.

FIG. 18 illustrates various simulated signals. For the simulation we have: L=Ls, n = 4, C= 62pF, Ls = 80μH, Cph= 10pF, R=500Ω Optical pulse: duration 6ns - 5 μW - delay 20ns

The transimpedance amplifier TIA is considered with an infinite band gain product, and the noise source is e _n =4 nV/√hz.

In FIG. 18 A: signal 10 is equal to Itia with an amplification circuit comprising the transformer, a capacitance but no inductance. The signal is still noisy. In FIG. 18B: signal 20 is equal to Itia with an amplification circuit comprising a capacitor and an inductor. The signal is now very quiet and usable for lidar detection.

In FIG. 18 C: signal 30 is equal to Itia with an amplification circuit comprising a capacitor, an inductor and a damping resistor Rth of 500 Q. The oscillation has almost disappeared.

The determination of the moment of reception of the echo pulse (20 ns) is carried out by a series of processing operations (not detailed here) carried out on the signals 20 or 30.

Claims

1. lidar system (10) by measuring a time of flight comprising: an emission device (DE) configured to emit light pulses in the direction of a scene at an angle greater than or equal to 5° a reception device (DR) comprising: o a photo-detector (PD) configured to receive pulses reflected (Ir) or backscattered by at least one element (Ei) of the scene and to convert said pulses into an electrical signal, o an amplification circuit (CA) configured to amplify said electrical signal, a processing unit (UT) of said amplified electrical signal configured to digitize the amplified electrical signal and determine a distance (di) of said at least one element (Ei) from said digitized amplified electrical signal . the amplification circuit (CA) comprising a transimpedance amplifier (TIA), a transformer (T) comprising a primary (P) and a secondary (S), a capacitor (C) and an inductor (L) arranged in series with said capacitor (C), the primary of the transformer being connected to an anode of the photodetector, the secondary being connected to said capacitor, said capacitor being connected to an input of said transimpedance amplifier.

2. A lidar system according to the preceding claim wherein a frequency operating range of the transformer includes the band [10 MHz; 350MHz].

3. Lidar system according to one of the preceding claims, in which a transformation ratio (n) equal to the ratio of the number of turns of the secondary (ns) to the number of turns of the primary (np) is strictly greater than 1.

4. Lidar system according to one of the preceding claims, in which the capacitance C and the inductance L satisfy the relationship:

(L + n ² L _p )C > L _p C _ph with L _p inductance of the primary of the transformer, C _ph the transition capacitance of the photodiode, n transformation ratio defined by: with n _p and n _s number of turns of the primary and the secondary.

5. lidar system according to one of the preceding claims wherein said inductor (L) verifies the relationship: with: n _p and n _s number of primary and secondary turns respectively,

L _p transformer primary inductance, C _ph photodiode transition capacitance, f _{i_b} minimum frequency of interest.

6. lidar system according to one of the preceding claims wherein said inductance (L) verifies the relationship:

0.5 Ls <L< 2 Ls

L inductance and Ls inductance of the transformer secondary

7. Lidar system according to one of the preceding claims, in which the capacitor C verifies the relationship with Cph transition capacitance of the photodiode.

8. lidar system according to one of the preceding claims wherein the receiving device further comprises a resistance called damping resistance (Rph) between the photodetector and the primary of the transformer.