Micromechanical structure for optical sensor

Abstract

A micromechanical structure for an optical sensor, comprising a microbeam (100) flexing about a first direction, an actuator (230) controlled to modify the orientation of the microbeam (100) about said direction (x), and at a free end of said microbeam (100), phased optical array antennas (223) (120) arranged to emit a light beam steerable about a second direction (y) transverse to the first direction. The micromechanical structure further comprises a sensor embedded in the microbeam transmitting a signal representative of said current orientation of the microbeam about the first direction. Figure for the abstract: 6

Description

Title of the invention: Micromechanical structure for optical sensor. Technical field and background.

[0001] The invention relates to the field of integrated optics, with applications in the field of LiDAR (Light Detection and Ranging) systems. It uses phased optical array (OPA) technology, possibly on-chip. It is envisioned, for example, for the automotive sector – a LiDAR in a vehicle could detect a pedestrian or other obstacle. The development of LiDARs based on integrated photonics equipment is likely to significantly reduce the cost of sensors while improving performance.

[0002] The operation of a phased optical array (POA) is based on a power divider that distributes the beam emitted by a coherent light source, typically a laser, among a series of optical antennas whose emitting ends are placed along a straight line or row (hence the term 1D POA) and which each emit a fraction of the optical power of the source. These antennas are spaced from each other along the line by a distance that is often constant and on the order of the wavelength of the emitted signal, typically a few millimeters or less. The system also includes phase modulators, generally one per antenna, which control the phase differences between the optical signals emitted from one antenna to another. A linear phase gradient is applied between the signals emitted by each antenna along the line, and interference is produced. This interference takes the form of a beam directed in a given direction.By changing the slope of the linear phase gradient, it is possible to change the emission direction to the left or right of the antenna array and thus perform a sweep, or scan.

[0003] This type of circuit therefore makes it possible to direct an optical beam in a chosen direction without any moving mechanical parts. This technique is called "solid-state beam steering" or solid-state beam steering, or semiconductor beam steering. It is discussed in the article Doylend et al. Two-dimensional free-space beam steering with an opticalphased array on silicon-on-insulator, Opt. Express, 2011, vol. 19, no. 22, p. 21595. However, without another beam orientation strategy, the orientation, and therefore the scanning, is performed along a single direction (denoted q>).

[0004] One method for directing the beam around a second direction consists of modifying the wavelength of the source and taking advantage of the fact that optical antennas are diffraction gratings, which implies that their emission angle 0 depends on the wavelength of the light.

[0005] This method therefore allows for a 2D scan (q > x 0) of the beam using a 1D phased optical array (PAO). However, to obtain a large scan angle at 0 (typically more than 10°), it is necessary to significantly modify the laser wavelength (often by more than 100 nm since a typical sensitivity is A0 ~ 0.1° for AX ~ 1 nm). This is a limitation, as most readily available laser sources do not offer such wavelength variability, in combination with the other performance requirements of a LIDAR system, such as high power, a narrow linewidth, and the possibility of frequency modulation.

[0006] The realization of 2D phased optical arrays (PAAs) containing an emitter array has also been proposed, notably by the article Sun et al, Large-scale nanophotonic phased array, Nature, vol. 493, no. 7431, 2013. Such an array makes it possible, by individually controlling the phase of each emitter, to direct the emitted beam along two complementary directions (q> x 0).

[0007] However, the realization of an optical emitter matrix implies strong constraints on the circuit and consequently such an architecture leads to low-performance systems, either because the emitted power is low, or because the scanning amplitude is limited.

[0008] We also know from FR3098606A1, FR3112216A1, FR3112217A1, FR3112218A1 and from the article Guerber S. et al. Active opticalphased array integrated within a micro-cantilever, Communications Engineering volume 3, 76, 2024, an implementation based on MEMS technology -Microelectromechanical Systems - combined with an integrated photonics approach.

[0009] As illustrated in Figures 1 and 2, which are microscope images, we are interested in a silicon support 1 (a wafer) carrying an ordered series of control terminals 4, for example 8 in number, an optical input 5 of the optical fiber type for the light from a laser and electrical terminals 6 to apply a voltage to a piezoelectric actuator.

[0010] A microbeam 10, constituting a MEMS technology object, is routed from the support 1. Its upper surface is equipped with a piezoelectric actuator 11 – PZT – connected to the electrical terminals 6. The microbeam thus comprises an active element that allows it to be tilted in a controllable manner, in this case a lead zirconate titanate (PZT) piezoelectric element. This element is activated on demand by applying a voltage between the two electrical terminals 6.

[0011] The microbeam 10 is also equipped on its upper surface with a phased optical array OPA. The phased optical array consists of a splitter 12 connected to the input optics 5, separating the light into different paths, for example the stated number of 8 paths in waveguides, phase modulators 13 - one for each waveguide from the separator 12 - and antennas 14 constituting the ends of the waveguides emitting in free space the phase waves modulated by the phase modulators 13. The phase modulators 13 are controlled individually via the control terminals 4.

[0012] Thus, the presented system comprises a 1D optical pickup array (OPA) on a microbeam-type MEMS object, which, as is known, can potentially enter into mechanical resonance and therefore sweep significant angular amplitudes. The sweep directions of the OPA and the microbeam are orthogonal, so as to allow 2D scanning.

[0013] [Fig. 1] When no voltage is applied across the terminals of the piezoelectric element 11, the microbeam 10 hangs slightly due to its weight, the bending being limited by the rigidity of the material. The angle 0 takes a first value, as can be seen in [Fig. 1].

[0014] [Fig.2] If a voltage of a few volts is applied to the piezoelectric element 11, through the electrical terminals 6, and as can be seen in [Fig.2], the microbeam is straightened, which has the effect of changing the beam emission angle 0. By combining this system with a phased optical grating OPA capable of scanning in the direction q> perpendicular to 0, it is possible to scan the emitted beam in two dimensions without changing the wavelength of the beam.

[0015] Furthermore, by applying a chosen sinusoidal signal to the microbeam via the PZT actuator, it is possible to obtain a large variation of angle 0, on the order of several tens of degrees, due to a resonance phenomenon of the microbeam.

[0016] However, to enable widespread deployment of this device in consumer applications such as the automotive industry, it is desirable to be able to track the position of the microbeam in real time, as it represents one of the LiDAR emission angles. Such tracking ensures the proper functioning of the system and the safety of people – an unintentional and undetected immobilization of the microbeam would lead to a concentration of power that is dangerous to the eyes in particular, and must therefore, if it occurs, be detected in order, for example, to cut off the laser emission.

[0017] The integration of a real-time tracking device for the position of the microbeam is therefore the object of this invention.

[0018] The integration of strain gauges on MEMS microbeams has been addressed in the context of the development of piezoresistive gauges, for example in the document Behrens et al, Piezoresistive cantilever as portable micro force calibration standard J. Micromechanics Microengineering, vol. 13, no. 4, pp. S171-S177, 2003. Such a system, composed of a microbeam and a strain gauge, is used to create pressure sensors, gas flow velocity sensors, or to measure the deflection of the tip of an atomic force microscope, as described in the document Dukic et al, Piezoresistive AFM cantilevers surpassing standard optical beam deflection in low noise topography imaging Sci. Rep., vol. 5, no. 1, p. 16393, 2015. These systems do not incorporate a phased optical grating. Presentation of the invention

[0019] To overcome the difficulties and shortcomings encountered with the prior art, a micromechanical structure for an optical sensor is therefore proposed, comprising a microbeam in bending around a first direction, an actuator controlled to modify the orientation of the microbeam around said first direction and, at a free end of said microbeam, phased optical array antennas arranged to emit a light beam orientable around a second direction transverse to the first direction.

[0020] The micromechanical structure is remarkable because it also includes a sensor embedded in the microbeam transmitting a signal representative of a current orientation of the microbeam around the first direction.

[0021] The signal can be used by a modulation means based on said signal from a command transmitted to the actuator or from a command transmitted to the light source. The system's behavior can then be corrected to prevent an accumulation of excessive light power in a given direction.

[0022] According to optional and advantageous features:

[0023] - the microelectronic structure may include an optical reading means for measure the signal transmitted by the sensor.

[0024] - the on-board sensor includes a waveguide comprising a material with an index of refraction depending on a mechanical stress of said material varying with the orientation of the microbeam around said first direction.

[0025] - the micromechanical structure may include a Mach-Zender interferometer said onboard sensor comprising a sensor arm of said Mach-Zender interferometer.

[0026] - the microelectronic structure may include a light splitter for transmitting light from the same source to the phased optical array antennas and to the on-board sensor, which is then an optical sensor.

[0027] - the microelectronic structure may include a controlled phase modulator to adapt a phase of a wave, for example a light, transmitted to said onboard sensor, and more specifically to modify an amplitude of an interference signal used by said onboard sensor.

[0028] - the on-board sensor may include a waveguide making at least one forward and a return along the microbeam.

[0029] - the on-board sensor may include a piezoelectric sensor.

[0030] The invention also relates to a light detection and telemetry device comprising a micromechanical structure for an optical sensor according to the principles mentioned, and a coherent light source for emitting through phased optical array antennas and a controller for controlling the actuator and the phased optical array to orient in two dimensions a light beam emitted for detection and telemetry.

[0031] The invention also relates to a motor vehicle comprising such a light-based detection and telemetry device for detecting obstacles to the movement of the vehicle. List of figures

[0032] Fig. 1 presents, as already mentioned, a microscopic image of a MEMS microbeam equipped with a PZT actuator and an OPA phased optical array, at rest, according to the prior art.

[0033] Figure 2 presents, as already mentioned, a similar image of the same object, but the actuator being activated, still according to the prior art.

[0034] Figure 3 presents an embodiment of the invention.

[0035] Figure 4 presents a technical means used in one embodiment of the invention.

[0036] Fig. 5 presents a physical principle used in an embodiment.

[0037] Figure 6 presents a second embodiment of the invention.

[0038] Fig. 7 presents a top view of the system of Fig. 6.

[0039] Figure [8] shows the exploitation of the measurements obtained with the system of Figure [6].

[0040] Figure [Fig.9] presents a variant of the system of Figure [Fig.6].

[0041] Fig. 10 presents another variant of the system of Fig. 6. Description related to the figures

[0042] [Fig. 3] In [Fig. 3], a LiDAR constructed using a planar support 1 comprises a microbeam 100 cut from the support 1, which is equipped with a piezoelectric actuator (not shown) for bending around a direction X parallel to the plane of the support 1 and perpendicular to the direction of the microbeam. Thus, the bending allows the free end of the microbeam to be oriented at an angle θ.

[0043] The microbeam is equipped with an optical input 110 and a phased optical array OPA 120 implanted on the microbeam, which allows a scan to be carried out at an angle q>, which combined with the previous scan forms a two-dimensional scan (q> x 0).

[0044] The principles presented in relation to figures 1 and 2 are taken up again, possibly modified, in this embodiment.

[0045] The integration of a position sensor allows real-time tracking of the direction of the emitted beam according to the angle 0.

[0046] In the LIDAR comprising the microbeam 100 and on it the phased optical array 120, a position sensor 150 is integrated on the microbeam 100, for example a piezoelectric gauge, or for example an optical gauge, and this sensor allows to follow in real time the position of the microbeam 100 according to the angle 0, and therefore the direction of the beam emitted by the LiDAR according to 0.

[0047] A controller 190 takes into account this position information, and uses it for a modulation of a command transmitted to the piezoelectric actuator or of a command transmitted to the light source applied to the optical input 110, which provides guarantees in terms of safety and reliability.

[0048] The tracking of the beam position according to q> is ensured elsewhere within the photonic circuit.

[0049] Another embodiment will be presented. It uses an optical gauge, manufactured using an MZI Mach-Zenhder interferometer.

[0050] [Fig. 4] In [Fig. 4], the principle of the Mach-Zenhder MZI interferometer is recalled. A Mach-Zenhder interferometer uses, on a waveguide fed by an optical input 60, a splitter 61 which distributes the injected light towards two independent arms: the reference arm 62 and the sensing arm 64, including a specific path 65. The light beams having traveled through the two arms are then mixed via a combiner 68 to produce a single output beam which is used at the optical output 69. In the case of coherent light, the transmission of the MZI interferometer depends on the phase difference between the two arms. Thus, the transmittance of an MZI interferometer has a sinusoidal evolution as a function of the wavelength of the light applied at the input with maxima which correspond to constructive interference (signals from the two arms in phase) and minima to destructive interference (signals from the two arms in antiphase).A Mach-Zenher interferometer (MZI) therefore allows a phase variation Aq> to be translated into an intensity variation AI, which is simpler to detect.

[0051] [Fig. 5] As illustrated in [Fig. 5], the refractive indices of certain solid materials, such as silicon, depend on the stress to which the material is subjected, due to a so-called photoelastic effect. Thus, the phase of light propagating in a waveguide subjected to stress is modified. The photoelastic effect is on the order of An = 10⁶ for a stress of 40 MPa, which can be applied with a PZT actuator as presented in the article by Tang et al., Hybrid integrated ultralow- linewidth and fast-chirped laser for FMCW LiDAR, Opt. Express, vol. 30, no. 17, p. 30420, 2022.

[0052] Thus, in the figure, we have represented, seen from the side, the support 1 and the microbeam 100 carrying a silicon waveguide 80 on its upper face with an upward bend (a) which brings the silicon of the waveguide 80 into compression hence the change in refractive index + An, without bending (b), and with a downward bend (c) equivalent to the upward bend shown in (a), which brings the silicon of the waveguide 80 into stretching hence the change in refractive index - An.

[0053] [Fig.6] It is proposed to use the principle of the Mach-Zenhder MZI interferometer to make an optical position sensor or gauge, one of the arms of the MZI interferometer serving as an area exposed to deformations, which vary over time, the other as a fixed reference and for example unconstrained.

[0054] Fig. 6 again shows a support 1, a microbeam 100 in the z direction, bending about the X direction, an optical input 110 on the support 1 near the microbeam and a phased optical array 120 placed between the optical input 110 and the free end of the microbeam. This includes a light splitter 221 separating the light from the optical input 110 into N beams conducted by parallel waveguides arranged side by side on the support 1 and then on the microbeam 100, phase modulators 222, one modulator for each of the waveguides (the control of the modulators is not shown) and antennas 223 placed at the end of the microbeam 100 to emit the light guided by the different parallel waveguides, the active parts of the antennas 223 forming a straight row on the distal part of the microbeam 100.Phase modulation allows the light beam to be oriented at an angle q around the y direction (in the xy plane).

[0055] The principles presented in relation to figures 1 and 2 are again taken up, possibly modified, in this embodiment.

[0056] An MZI 250 interferometer is further constructed comprising an optical input 251 and an optical output 252 on the support 1, the reference arm 255 of which, outside the microbeam, is also on the support 1, and the sensing arm 256 of which is placed, in whole or in part, on the microbeam 100. For this purpose, the sensing arm 256 is, for example, essentially composed of two straight segments parallel to each other and a bend forming a half-turn connecting the two segments. The two segments are placed parallel to the waveguides, over a significant length of the microbeam, the bend being optimally aligned with the antennas 223, such that the two segments of the sensing arm 256 traverse the entire length of the microbeam.

[0057] The microbeam 100 is equipped with a piezoelectric actuator 230, controlled by a voltage applied to it (the terminals are not visible in the figure), this which causes variations in the bending of the microbeam and therefore allows the light beam to be oriented around the x direction (in the yz plane), according to the angle 0.

[0058] When the microbeam 100 is actuated by the actuator 230, a stress is created within the sensor arm 256. This stress creates a difference in the refractive index of the waveguide over a length that is greater the further the sensor arm 256 is installed along the entire length of the microbeam 100, up to its near end. The reference arm 255, on the other hand, is not subjected to any stress. Measuring the light intensity at the output of the MZI then allows the current position of the microbeam 100 to be determined according to the principle explained in relation to [Fig. 4]. The optical output 252 reads this intensity, for example, using a photodiode.

[0059] Again a controller 190 takes into account the position information, and uses it for a modulation of a command transmitted to the piezoelectric actuator or of a command transmitted to the light source applied to the optical input 110, which provides guarantees in terms of safety and reliability.

[0060] [Fig.7] A top view of the system of [Fig.6] is shown [Fig.7], with The position sensor is integrated onto a microbeam on which a phased optical array (PAA) photonic circuit, comprising a beam splitter, modulators, and antennas, has been previously or concurrently mounted. The microbeam again has an actuator, which in this embodiment is a PZT actuator. The optical position sensor is mounted on the microbeam with one arm of the MZI interferometer—this is the sensor arm—and the other arm is outside the microbeam—this is the reference arm. The reference arm can also consist of two parallel segments connected by a U-turn bend. It is proposed that the length of the sensor arm be greater than the length of the reference arm.

[0061] Consider an MZI interferometer with the following characteristics: - length of the reference arm Lref = 8 mm, difference in length between the two arms AL = 50 µm, hence the relationship 1 = l - + A /

[0062] - refractive indices at rest in the waveguides of the reference arm and the arm of equal sensor nref= ncapt = 3. The sensor arm also undergoes a maximum modification (addition or subtraction) of the refractive index A n.

[0063] Wave vectors are further introduced in the sensing arm and in the reference arm: " and

[0064] An analytical simulation can be performed using the equation describing the output intensity of an MZI interferometer:

[0065] î 4 = - 1 + cos(^ ref L wf - < ■%• •■.'.< < V.' ? ? f ■' Vxi'fc / v v.vj^.' k. ''I'

[0066] The light intensity at the output of the MZI Io is therefore a function of the wavelength.

[0067] [Fig. 8] As can be seen in [Fig. 8], the peak of destructive interference at the output of the MZI interferometer shifts in wavelength by a value AX when a constraint is applied by the PZT actuator (active PZT actuator An = 106) relative to the unconstrained state (inactive PZT actuator An = 0). By adjusting the laser wavelength close to a peak value of interference from the MZI interferometer, it is therefore possible to obtain a sensitive intensity variation AI0, dependent on the position of the microbeam and thus allowing its determination.

[0068] [Fig.9] A variant is shown with reference to [Fig.9].

[0069] Fig. 9 again shows a support 1, a microbeam 100 in bending, an optical inlet 110 on the support 1 near the microbeam and a phased optical array 120 placed between the optical inlet 110 and the free end of the microbeam.

[0070] An MZI 250 interferometer is further constructed, comprising an optical input 251 and an optical output 252 on the support 1, whose reference arm 255, outside the microbeam, is also on the support 1, and whose sensing arm 356 is placed, in whole or in part, on the microbeam 100. The sensing arm 356 is composed of several segments in series forming multiple round trips on the microbeam in order to increase the length of the sensing arm and thus the sensitivity of the gauge. Performing multiple passes in this way allows for the accumulation of a greater phase delay in the sensing arm relative to the reference arm. This solution, which increases sensitivity, requires a waveguide surface forming the upper sensing arm, which has been accommodated by increasing the width of the microbeam compared to the embodiment of [Fig. 7].

[0071] [Fig. 10] A variant shown in [Fig. 10] incorporates a phase modulator on the reference arm, which allows the wavelength of the interference peaks of the interferometer to be modified.

[0072] Figure 10 again shows a support 1, a microbeam 100 in bending, an optical input 110 on the support 1 near the microbeam, and a phased optical grating 120 placed between the optical input 110 and the free end of the microbeam. An MZI interferometer 250 is further provided, comprising an optical output 252 on the support 1 and whose sensing arm 256 is placed, in whole or in part, on the microbeam 100.

[0073] But the reference arm 455, which is placed as before on the support 1 outside the constraint zone, includes an electrically controlled phase modulator 420, which allows, by seeking the maximum interference of the interferometer of Mach-Zehnder, to increase the amplitude of the signal without having to change the wavelength of the laser, and therefore to easily increase the sensitivity of the device.

[0074] Furthermore, the variant uses a portion of the laser source 110 used for the phased optical grating OPA to power the interferometer MZI, by means of a two-way optical splitter 410. The optical input of the interferometer is thus coupled to the second channel of the optical splitter 410, while the phased optical grating 120 is coupled to the first channel of this optical splitter 410. This avoids having to use a specific laser for the interferometer MZI, whereas a laser is used for the phased optical grating OPA.

[0075] The micromechanical structure for optical sensor presented is used for scanning a scene or environment, by a fixed or moving object, and typically in the context of a LIDAR, for example implemented in an automobile for the purpose of preventing collisions with pedestrians or vehicles.

Claims

Demands

1. Micromechanical structure for optical sensor, comprising a microbeam (100) in bending about a first direction (x), an actuator (230) controlled to modify the orientation of the microbeam (100) about said first direction (x) and at a free end of said microbeam (100), phased optical array antennas (223) (120) arranged to emit a light beam orientable about a second direction (y) transverse to the first direction (x), characterized in that the micromechanical structure further comprises a sensor embedded (150) in the microbeam transmitting a signal representative of a current orientation of the microbeam about the first direction (x).

2. Micromechanical structure for optical sensor according to claim 1, characterized in that the on-board sensor (150) comprises a waveguide (256) comprising a material with a refractive index dependent on a mechanical stress of said material varying with the orientation of the microbeam (100) around said first direction (x).

3. Micromechanical structure for optical sensor according to claim 1 or claim 2, characterized in that the micromechanical structure comprises a Mach-Zender interferometer (250), said on-board sensor (150) comprising a sensing arm of said Mach-Zender interferometer (250).

4. Micromechanical structure for optical sensor according to any one of claims 1 to 3, characterized in that the microelectronic structure includes a light splitter (410) for transmitting light from the same source to the phased optical array antennas (223) and to the on-board sensor (150) which is an optical sensor.

5. Micromechanical structure for optical sensor according to any one of claims 1 to 4, characterized in that the microelectronic structure comprises a controlled phase modulator (420) for modifying an amplitude of an interference signal exploited by said on-board sensor (150).

6. Micromechanical structure for an optical sensor according to any one of claims 1 to 5, characterized in that the embedded sensor (150) includes a waveguide (256; 356) making at least one round trip along the microbeam (100).

7. Light detection and rangefinding device comprising a micromechanical structure for optical sensor according to any one of claims 1 to 6, a coherent light source for emitting by phased optical array antennas (120) and a controller for controlling the actuator (230) and the phased optical array (120) to orient along two dimensions (q>, 0) a light beam emitted for detection and rangefinding.

8. Motor vehicle comprising a light detection and telemetry device according to claim 7 for detecting obstacles to the movement of the vehicle.

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