LIDAR, measurements

TECHNICAL FIELD

Various examples of the invention generally relate to light detection and ranging, lidar. Various examples of the invention specifically relate to a respective scan system using frictionless motion of a steering mirror and having a coaxial optical system.

BACKGROUND

Light detection and ranging (LIDAR; sometimes also referred to as laser ranging or LADAR) allows to provide a 3-D point cloud of a scene. Objects can be accurately detected. Ranging is possible. Pulsed or continuous-wave laser light is transmitted along a transmit beam and, after reflection at an object, detected along a receive beam. This allows to determine the distance to the object (z-position).

LIDAR can be employed in crowded areas. Here, eye-safety concerns are required to be met. Thus, the laser power needs to be limited to avoid damage to the human eye. Examples include application of LIDAR for automotive use cases.

To determine the x-y-position (lateral position) of the object, the laser light can be steered. There are various options available for steering laser light. Examples include non- mechanical beam steering using optical-phased arrays (see, e.g., US20160161600A1 ) or flash lidar (see, e.g., US20170343653A1 ). Such techniques often face restrictions in range; typical ranges can be between 30 m to 100 m for eye-safety compliant laser powers. Another option available for steering laser light includes mechanical beam steering. Here, a scanner including a deflection unit such as a mirror is used to steer the laser light. Macroscopic approaches are known in which a bearing - e.g., a ball bearing - is used to move the mirror, see, e.g., US201 10216304A1 or EP2541273B1. Such approaches implementing bearings often face restrictions in durability. When moving the mirror, friction at the bearing results in wear out; this, in turn, limits the time between failure. Also, bearing-based scanners are often bulky. In automotive use cases, sometimes, respective scan units are mounted to the top of the vehicle, because it is difficult to integrate them with the chassis. On the other hand, macroscopic approaches typically allow for large mirrors which helps to increase the range. Typical ranges can be up to 250 m for eye- safety compliant laser powers.

Another sub-class of mechanical beam steering employs frictionless scanning using micro-mirrors and elastic mounts that can elastically deform to move the mirror. See, e.g., US20100296146A1 or US20150062677A1. Sometimes, frictionless scanning is also referred to as solid-state scanning.

SUMMARY

Therefore, a need exists for advanced techniques of frictionless, mechanical steering laser light for lidar. A need exists for techniques which overcome or mitigate at least some of the above-identified restrictions and drawbacks.

This need is met by the features of the independent claims. The features of the dependent claims define embodiments.

A scan system includes a steering mirror having a reflective surface. The scan system also includes an elastic mount of the steering mirror. The scan system also includes an actuator configured to actuate, by elastic deformation of the elastic mount, resonant motion of a mass-spring system formed by the steering mirror and the elastic mount. The scan system also includes a laser configured to emit primary light along a transmit beam. The scan system also includes a detector configured to detect secondary light along a receive beam, the transmit beam and the receive beam being aligned along an overlap section. The overlap section includes the reflective surface of the steering mirror. A size of the reflective surface of the steering mirror is in the range of 120 mm ² to 230 mm ² , optionally in the range of 180 mm ² to 220 mm ² . A divergence of at least one axis of the primary light along the overlap section is in the range of 0.09° to 0.15°, at the steering mirror.

A scan system includes a steering mirror having a reflective surface and an elastic mount of the steering mirror. The scan system also includes an actuator configured to actuate, by elastic deformation of the elastic mount, resonant motion of a mass-spring system. The mass-spring system is formed by the steering mirror and the elastic mount. The scan system also includes a laser configured to emit primary light along a transmit beam. The scan system also includes a detector configured to detect secondary light along a receive beam, the transmit beam and the receive beam being aligned along an overlap section, the overlap section comprising the reflective surface of the steering mirror. The primary light has a fast axis and a slow axis, the fast axis having a larger divergence than the slow axis. A slow-axis field width of the primary light along the slow axis is not smaller than 90 % of a respective width of the steering mirror perpendicular to an optical axis of the transmit beam.

A scan system includes a steering mirror having a reflective surface and an elastic mount of the steering mirror. The scan system also includes an actuator configured to actuate, by elastic deformation of the elastic mount, resonant motion of a mass-spring system formed by the steering mirror and the elastic mount. The scan system also includes a laser configured to emit primary light along a transmit beam. The scan system also includes a detector configured to detect secondary light along a receive beam: The transmit beam and the receive beam are aligned along an overlap section, the overlap section comprising the reflective surface of the steering mirror. The scan system also includes a collimator lens configured to collimate at least one axis of the primary light. The collimator lens is arranged in the overlap section. A scan system includes a steering mirror having a reflective surface. The scan system also includes an elastic mount of the steering mirror. The scan system also includes an actuator configured to actuate, by elastic deformation of the elastic mount, resonant motion of a mass-spring system formed by the steering mirror and the elastic mount. The scan system also includes a laser configured to emit primary light along a transmit beam. The scan system also includes a detector lens and a detector arranged in a focal plane of the detector lens and configured to detect secondary light along a receive beam, the transmit beam and the receive beam being aligned along an overlap section, the overlap section comprising the reflective surface of the steering mirror: The scan system also includes a beam splitter comprising a coupling mirror having a reflectivity of at least 86 % and configured to split the transmit beam and the receive beam at an end of the overlap section, wherein the coupling mirror is arranged in-between the detector lens and the detector.

The various examples can be combined with each other to form further examples. For example, it would be possible to arrange the beam splitter in between the detector lens and the detector even if the slow axis and fast axis collimation is performed in between the laser and the beam splitter. Further, relative dimensioning of the transmittance and the divergence as described above can also be employed for scenarios where, e.g., the beam splitter is arranged in between the detector lens and the detector and/or where the collimator lens is arranged in the overlap section of the transmit beam and the receive beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a scan system including a coaxial optical system and a frictionless steering mirror according to various examples.

FIG. 2 schematically illustrates an example implementation of the coaxial optical system according to various examples. FIG. 3 schematically illustrates a field width of primary light transmitted along a transmit beam of the coaxial optical system according to various examples.

FIG. 4 schematically illustrates a field width of primary light transmitted along a transmit beam of the coaxial optical system with respect to a size of the steering mirror according to various examples.

FIG. 5 schematically illustrates a field width of primary light transmitted along a transmit beam of the coaxial optical system with respect to a size of the steering mirror according to various examples.

FIG. 6 schematically illustrates a dependency of a transmittance of the transmit beam with respect to the receive beam of the coaxial optical system on a divergence of a slow axis of the primary light according to various examples.

FIG. 7 schematically illustrates an example implementation of the coaxial optical system according to various examples.

FIG. 8 schematically illustrates a multifocal lens body according to various examples.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following, embodiments of the invention will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the invention is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only.

The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof.

Hereinafter, techniques of using a steering mirror to steer light are described. The steering mirror has a reflective surface. For example, a reflectivity of a reflective surface the steering mirror can be larger than 86 %, optionally larger than 90 %. A light beam of the light can be deflected by reflection at the reflective surface. For example, the reflective surface can be coated with gold or silver or another reflective metal coating or another coating.

The steering mirror may be deflected by reversible deformation at least one spring of an elastic mount of the steering mirror. Tailored deflection of the steering mirror facilitates steering of the light. The at least one spring can deform reversibly, i.e. , without structural damage to the material - e.g., silicon such as crystalline silicon. As a general rule, in the various examples described herein, one or more springs may be used to implement the elastic mount.

Thus, in other words, techniques are described which employ a solid-state, frictionless scanner to steer light.

For example, the techniques described herein may facilitate 1 -D or 2-D steering of light. Specifically, the light can be scanned. Scanning of light can correspond to repetitively redirecting light using different transmission angles. A scan rate or repetition rate of the scanning can be defined by steering cycles. For this, the steering mirror can be deflected accordingly. Larger scanning areas correspond to larger changes in the transmission angles during scanning; larger changes in the transmission angles can be achieved by larger deflection of the steering mirror. Thereby, a FOV of the scanning can be increased. It is possible to implement transmission angles by deflecting the steering mirror in accordance with one or more degrees of freedom of motion of a mass-spring system formed by the elastic mount and the steering mirror. For example, the steering mirror may be rotated, tilted, shifted, etc.. Examples of the degrees of freedom of motion that can be used to steer light include flexure and torsion of the at least one spring of the mass-spring system.

According to some examples, resonant motion of the mass-spring system is possible. Specifically, the respective freedom of motion may exhibit a respective resonance characteristic - sometimes also referred to as frequency response, i.e., deflection as a function of frequency. The resonance characteristic may have a peak of certain width in frequency domain. It is possible to select a driving force to exhibit a frequency within this resonance peak. For this, an actuator can be appropriately controlled. By resonant motion, large changes in the transmission angles can be achieved. Large scanning areas can be implemented.

Example actuators that can be controlled to drive the elastic scan unit resonantly include piezoelectric comb drives, magnetic drives, piezoelectric actuators, etc..

As a general rule, the techniques described herein may find application in various use cases. Example use cases include, but are not limited to: LIDAR with lateral resolution, spectrometers, projectors, endoscopes, etc. hereinafter, for sake of brevity, reference is primarily made to LIDAR use cases; similar techniques may be readily employed for other use cases.

The steering mirror and the elastic mount can be part of a scan unit. A scan system (or, simply, scanner) may include the scan unit, a light source configured to emit light to be scanned, and/or a detector configured to receive secondary light. The scan system can also include one or more actuators to actuate the elastic mount, to thereby deflect the steering mirror. According to various examples, it would be possible to steer laser light. For example, coherent or incoherent laser light can be used. Polarized or non-polarized laser light may be used. Pulsed laser or continuous-wave laser light may be used. For example, short laser pulses having a width in the range of picoseconds or nanoseconds may be used. For example, a pulse duration in the range of 0.5 - 3 nanoseconds may be used. The laser light can have a wavelength in the range of 700 - 1800 nanometers, specifically of 1550 nanometers or 950 nanometers. A laser-light beam may be formed by multiple spatial modes (multi-mode laser light).

As a general rule, the light source may be implemented using a laser diode. For example, a broad-area laser diode may be used. The laser diode may be spatially multi-mode providing multi-mode laser light. For example, an edge-emitting broad-area laser diode may be used.

As a general rule, an emitting area (sometimes also called active area) of the laser diode may not be shaped as a circle or square, but rather asymmetric. The active area may have a short side defining a so-called fast axis (FA) and may have a long side defining a so-called slow axis (SA). The FA and the SA may be defined perpendicular by the direction of propagation of the laser light and perpendicular to the optical axis (OA). Specifically, a divergence of the laser light may be large along the FA and may be small along the SA. The divergence defines the opening angle of the laser light along the FA or the SA, respectively. In other words, the width of the laser light quickly broadens along the FA and slowly broadens along the SA.

As a general rule, the beam profile of the laser light in the FA and the SA is defined by the respective side length w _SA , w _FA of the active area, as well as a emission profile defining the divergence W _5A , W _{RA \}

^W SA ^X ^SA ' ^{= C} SA (1 ) w _FA x W _ra · = c _FA (2) Eq. (1 ) and (2) result in divergence of the laser light to be asymmetric as well. As a general rule, laser diodes may be used that have the following properties: ^W SA = 50 mt ^h — 250 mt ^h , w _FA = 1 mth— 15 mth,

D. _SA = 10 deg — 30 deg,

W _RA = 25 deg — 45 deg,

Detected light may be scattered at an object of the environment and, hence, may be non- coherent.

According to various examples, it is possible that laser light that is complying with eye- safety regulations is scanned. Such eye safety regulations can specify an average laser power, a peak laser power, e.g., dependent on a pulse length, etc.. An example eye safety regulation that can be used is described in British Standards Institute, BS EN 60825- 1 :2014, Safety of laser products. Equipment classification and requirements, published 31 August 2014.

Various techniques are based on the finding that for such elastic scan units employed in eye-safety regime of laser light, it can be difficult to achieve ranges above 100 m, e.g., given an object reflectivity of 50%. Various techniques help to increase the range to above 100 m, e.g., to achieve a range of 100 m to 200 m.

To increase the range, spatial filtering can be employed. Spatial filtering relates to a scenario where the primary laser light is transmitted along the transmit beam and via the steering mirror towards the object; and secondary light reflected by the object is collected along a receive beam and via the same steering mirror. In other words, the transmit beam is at least partly overlapping with the receive beam. Specifically, the transmit beam and the receive beam can be overlapping in an overlap section which includes the reflective surface of the steering mirror.

Sometimes, such a technique of spatial filtering is referred to as a coaxial optical system, because the OA of the transmit beam and the OA of the receive beam are aligned. As a general rule, there may be an offset between the transmit beam and the receive beam for a coaxial optical system, as long as there is an overlap between the beam profiles of the transmit beam and the receive beam. In a coaxial optical system, the transmit beam and the receive beam may share at least one common aperture, e.g., defined by the steering mirror.

Coaxial optical systems generally allow to restrict the solid angle from which light is collected and focused onto the detector. Thereby, background noise is reduced, by reducing the number of background photons, e.g., due to sunshine. This helps to increase the signal-to-noise ratio; thereby, the range is increased.

As a general rule, when implementing a coaxial optical system, a beam splitter is used to separate the transmit beam and the receive beam at an end of the overlap section. There are various options available to implement the beam splitter. In one example, two prisms can be used that are glued together and have a distance which facilitates total reflectivity at this transition of the refractive index. Another option relies on a semi-reflective mirror, e.g., using a silver coating having a reflectivity of less than, e.g., 80 % across the width of the beam profile. A third option employs a fully reflective mirror (coupling mirror) having a reflectivity of, e.g., larger than 90%, optionally larger than 86%, further optionally larger than 90%. The lateral dimensions of this coupling mirror - perpendicular to the OA - are smaller than the width of the receive beam; thereby, some of the secondary light can pass by the coupling mirror without being deflected by the latter, and hence can reach the detector. The beam splitter, the steering mirror, etc. define a coaxial optical system. The optical system is generally associated with a transmittance. The transmittance can be defined as

T ^P TX

(3)

PRX’ i.e., as the ratio between (/) power of the primary laser light in the transmit beam just upstream of an overlap section between the transmit beam and the receive beam and (//) power of the secondary laser light in the receive beam just downstream of the overlap section.

In Eq. (3), losses in the environment are not considered, e.g., it is (artificially) assumed that all light leaving the scan system along the transmit beam also re-enters the scan system along the receive beam.

The transmittance of the transmit beam with respect to the receive beam can be reduced below 1 by various factors:

A first factor is the steering mirror: For example, steering mirrors used in the optical system can have a size which is smaller than the field width of the primary light; then, primary light is lost and the transmittance is reduced ( Tmirmr ).

A second factor are optical losses due to the use of non-perfect optical elements, e.g., lenses, sub-100% reflectivity of mirror surfaces, etc. ( Tosses ). Typically, Tosses ^a 0.7 to 0.9. For example, if gold is used to define a reflective surface of the coupling mirror and/or a reflective surface of the steering mirror, then Tmatenai = 0.86 - 0.96 of that particular optical element.

A third factor is the use of the beam splitter which can result in reduced transmittance

( T splitter ) The overall transmittance T of the coaxial optical system - i.e., of the transmit beam 121 just upstream of the overlap section 125 and of the receive beam 122 just downstream of the overlap section 125 - is provided by the various influencing factors identified above:

T ⁼ Tlosses X T mirror X T splitter- (4)

A lower transmittance T limits the range. Various techniques are based on the finding that in Eq. (4) specifically the beam splitter transmittance T _sp ntter can significantly limit the overall transmittance T and, thereby, the range. For example, in reference implementations in which a prism-beam splitter is used, T _sp ntter can be as low as 0.25.

To mitigate this drawback, according to various examples, a beam splitter including a coupling mirror is used. The size of the coupling mirror is tailored so as to provide for a large beam-splitter transmittance, and, hence, a large overall transmittance. This helps to increase the range of LIDAR measurements.

In further detail, in such a scenario, the transmittance T _sp atter of the beam splitter depends on the geometric configuration of the coaxial optical system formed by the coupling mirror and the steering mirror, and optionally by lenses, etc.. In particular, a lateral size of the coupling mirror - perpendicular to the OA of the transmit beam and the receive beam - determines the transmittance of the beam splitter.

According to various examples described herein, the coaxial optical system is configured to provide a transmittance not less than 0.40, e.g., in the range of 0.50 to 0.85. For example, the transmittance without losses (i.e., Ti ₀ sses- = 1 ) can be in the range of 0.70 to 0.85. Specifically, lateral dimensions of the coupling mirror and/or of the steering mirror can be set accordingly. It has been found that such a transmittance provides for a optimized trade-off situation between (/) range of LIDAR measurements due to large transmittance and (/ ^' / ^' ) lateral resolution of LIDAR measurements due to low divergence. According to various examples described herein, the beam splitter can be arranged in- between a detector lens configured to focus secondary light onto the detector, and the detector. Thereby, it is possible to provide for a compact footprint of the optical system.

FIG. 1 schematically illustrates a scan system 100 according to various examples. The scan system 100 includes a control device 90, a laser diode 101 , and a detector 102. The scan system 100 defines a coaxial optical system.

The laser diode 101 is controlled by the control device 90 to emit primary laser light 1 1 1 , along a transmit beam 121. The transmit beam 121 passes a beam splitter 130, then passes a steering mirror 150, then passes a housing window 151 of the scan unit and then reaches a surrounding 190.

For example, a broad-area laser diode as described above may be employed.

As a general rule, a reflective surface of the steering mirror 150 can have an elliptical shape. The reflective surface may be arranged in a plane that has a normal vector enclosing an angle with respect to the OA of the transmit beam 121 , e.g., a 45° angle. A projection of the reflective surface into a plane perpendicular to the OA can have a circular cross section.

As a general rule, according to the various examples described herein, a steering mirror 150 can be used that has a reflective surface of a size that (/) is small enough to facilitate resonant motion of a mass-spring system formed by an elastic mount of the steering mirror and the steering mirror and that (/ ^' / ^' ) is large enough to facilitate low-divergence primary laser light and (///) is large enough to facilitate spatial filtering using a coaxial optical system.

As a general rule, the size of the reflective surface of the steering mirror may be in the range of 120 mm ² to 230 mm ² , optionally in the range of 180 mm ² to 220 mm ² . For example, an elliptical cross section of the reflective surface having radial lengths along the semi-minor and semi-major axes of 12 mm and 16.97 mm = 203 mm ² . This provides for a circular shape of a projection of the reflective surface into a plane perpendicular to the OA of the transmit beam 121 , if the OA is tilted by 45° to a normal vector of the reflective surface.

The primary laser light may be scattered at an object (not illustrated in FIG. 1 ), to generate non-coherent secondary light 1 12. The secondary light 1 12 - as well as background light and/or interfering light - travels along a receive beam 122 through the window 151 , passes the mirror 150 and then reaches the beam splitter 130. The beam splitter deflects the secondary laser light 1 12 towards a detector 102.

The OA of the transmit beam 121 and the OA of the receive beam 1 12 are aligned; hence, a coaxial optical system is implemented. Both, the transmitter aperture of the transmit beam 121 , as well as the detector aperture of the receive beam 122 are defined by the steering mirror 150.

As a general rule, the detector 102 may be an avalanche photo diode (APD) array or a single-photon APD array.

FIG. 1 illustrates an overlap section 125 of the transmit beam 121 and the receive beam 122. The overlap section 125 has and end at the beam splitter 130.

The control unit 90 is configured to actuate the steering mirror 150 to implement different scanning angles. For this, an actuator 901 is coupled with the control unit 90. Example actuators 901 include electrostatic drives, magnetic drives, or piezoelectric drives. The actuator 901 is configured to exert a force onto a first end of an elastic mount. Thereby, reversible deformation of a spring of the elastic mount 902 is caused, deflecting the steering mirror 150 that is coupled to a second end of the elastic mount 902 which is opposite from the first end. A spring element can extend between the first and second end (not illustrated in FIG. 1 ). Example implementations of the elastic mount 902 are described in US20180143322A1 : Fig. 62 and in DE 10 2016 014 001 A1 and in DE102009058762A1 and in US20100290142A1.

The control unit 90 can read out the detector 102. The control unit 90 can be implemented by an application-specific integrated circuit (ASIC) and/or a field-programmable array (FPGA) and/or a general purpose processor. The control unit 90 may include an analog- to-digital converter and/or a time-to-digital converter.

In the example of FIG. 1 , the optical system as a so-called pre-scanner optical system. This is because there are no lenses etc. having a significant optical impact on the transmit beam 121 arranged downstream of the steering mirror 150 along the transmit beam 121 (as would be the case for a post-scanner optical system, not illustrated herein). Thus, the divergence of the transmit beam 121 present at the steering mirror 150 corresponds to the divergence of the transmit beam 121 leaving the scan system 100 towards the surrounding / environment 190. Additional beam shaping behind the steering mirror 150 is not provided for in the pre-scanner optical system.

Details with respect to the optical system of the scan system 100 of FIG. 1 are also illustrated in FIG. 2.

FIG. 2 illustrates aspects with respect to the scan system 100. FIG. 2 is a ray tracing schematic of the optical system of the scan system 100, specifically of the transmit beam 121 and the receive beam 122. FIG. 2 illustrates a FA 716 of the primary laser light 121 (the SA is not illustrated).

In FIG. 2, a collimator lens 211 is arranged in the transmit beam 121 , in-between the laser diode 101 and the beam splitter 130 implemented by a coupling mirror. A further collimator lens 201 is also arranged in the transmit beam 121 , in between the laser diode 101 and the beam splitter 130. The collimator lens 21 1 is configured to collimate the FA 716 of the primary light 1 1 1. The collimator lens 201 is configured to collimate the SA of the primary light 1 1 1. A fixed mirror 202 deflects the primary light 1 1 1 towards the coupling mirror 131 of the beam splitter 130. FIG. 2 illustrates the FA field width 321 -1 , W _T X,FA of the primary light 1 1 1 at point A.

The coupling mirror 131 is arranged within the receive beam 122 along which the secondary light 1 12 travels. The primary light 1 1 1 and the secondary light 1 12 travel in opposite directions. An OA 199 of the transmit beam 121 and of the receive beam 122 in the overlap section 125 is illustrated in FIG. 2.

FIG. 2 illustrates the reflective surface 151 of the steering mirror 150. The steering mirror 150 encloses an angle of 45° with the OA 199. Thereby, periscope-type scanning by rotation around a rotational axis 152 can be implemented. To provide for the rotation around the rotation axis 152 as illustrated in FIG. 2, techniques as described in DE 10 2016 014 001 A1 may be employed. As a general rule, other types of motion of the steering mirror 150 can be used, e.g., a tilt, etc.. The particular type of elastic mount employed for moving the steering mirror 150 is not germane for the functioning of the various techniques described herein.

As a general rule, the size of the reflective surface 151 of the steering mirror 150 can be in the range of 120 mm ² to 230 mm ² , optionally in the range of 180 mm ² to 220 mm ² . This provides for resonance frequencies - and thereby scanning frequencies of the LIDAR measurements - in the range of 60 Flz to 500 Hz. Such scanning frequencies are helpful to provide for acceptable refresh rates of LIDAR images. Also, resilience against unintended motion of the mirror due to low-frequency shock is provided.

FIG. 2 also illustrates the field width 322, w _RX of the secondary light 1 12. The field width 322 is defined by a detector aperture of the receive beam 122. The size of the detector aperture is defined by the size of the steering mirror 150. The field width 322 is larger than the size of the coupling mirror 130. Hence, the transmittance of the beam splitter 130 T _sp mer is larger than 0. Some of the light reaches the detector 102, through a detector lens 252.

As a general rule, the geometrical size of the coupling mirror 131 defines the transmittance of the beam splitter 130, T sputte _r . For example, if the size of the coupling mirror 131 along the FA 716 perpendicular to the OA 199 is larger (at a fixed field width 322), the transmittance of the beam splitter 130 T _sp mer reduces, because less secondary light 1 12 reaches the detector 130.

The detector lens 252 has a focal length 102A such that the secondary light 1 12 is focused onto the detector 130. A field stop 253 is provided close to the focal plane of the detector lens 252.

The transmittance T of the transmit beam 121 with respect to the receive beam 122 in the coaxial optical system illustrated in FIG. 1 is defined as the ratio between the power of the primary light 1 11 at point A of the transmit beam 121 and the power of the secondary light 1 12 at point B of the receive beam 122, under the assumption that the environment of the scan system 100 fully reflects the light and no light is added in the environment. I.e., all primary light 1 1 1 that travels to the environment re-enters as secondary light 1 12.

It is hereinafter assumed that the transmittance is set by the geometric configuration of the coaxial optical system. Hence, the transmittance is set by the geometrical configuration of the coupling mirror 131 and the steering mirror 150 ( T _mimr ) and by the geometrical configuration of the beam splitter 130 ( Tsp e _r ), see Eq. (4). Other losses are neglected, e.g., losses due to sub-100% reflectivity of the material of the reflective surfaces of the mirrors 131 , 150. Hence, it is assumed that Ti _0S ses. = 1 in Eq. (4).

Next, details with respect to the design of the coaxial optical system to tailor the transmittance T are described in connection with FIGs. 3 - 6. FIG. 3 schematically illustrates a beam profile 701 of the primary light 1 1 1. FIG. 3 illustrates the rectangular-shaped beam profile 701 along the SA 715 - having a small divergence - and along the FA 716 - having a large divergence.

The field width 321 -1 along the FA 716 (labeled WTX,FA) and the field width 321 -2 along the SA 715 (labeled WTX,SA) are illustrated. The field widths 321 -1 , 321 -2 are defined by one or more collimator lenses - e.g., the collimator lens 201 of FIG. 1 - under the constraints imposed by the laser diode 101 by Eqs. (1 ) and (2). For example, if the field width 321 -1 or 321 -2 is reduced by the collimator lens 201 , then the divergence will increase, according to Eq. (1 ).

FIG. 4 schematically illustrates the beam profile 701 of the primary light 1 1 1 with respect to the projection 151 a of the reflective surface 151 into the plane perpendicular to the OA 199. FIG. 4 also illustrates the beam profile 701 of the primary light 1 1 1 with respect to the coupling mirror 131 of the beam splitter 130.

As a general rule, as illustrated in FIG. 4, it is possible that the size of the coupling mirror 131 is not larger than the size of the beam profile 701 of the primary light 1 1 1. Specifically, the size of the coupling mirror 131 can be matched to the size of the beam profile 701 of the primary light 1 1 1 . For example, the field width 321 -1 may be in the range of 80 % to 120 % of the respective size 131 -1 of the coupling mirror 130. For example, the field width 321 -2 may be in the range of 80 % to 120 % of the respective size 131 -2 of the coupling mirror 130. Thereby, reduced transmittance of the coupling mirror 131 of the beam splitter 130 for the primary light 1 1 1 is avoided.

Further, as illustrated in FIG. 4, the size of the reflective surface 151 of the steering mirror 150 limits the aperture of the transmit beam 121 , for the SA 715. Specifically, the respective size 151 a-1 of the projection 151 a of the reflective surface 151 is smaller than the field width 321 -2 of the primary light 1 1 1 along the SA 715. Not all primary light 1 1 1 is reflected by the steering mirror 150 - which reduces the mirror transmittance Tmimr < 1 (non-reflected light is highlighted by the arrows in FIG. 4). On the other hand, the divergence is comparably small, because the field width 321 -2 along the SA 715 is large, see Eq. (1 ). Spatial modes of the primary light 1 1 1 that have a comparably low divergence are selected due to the alignment of the center of the projection 151 a of the reflective surface 151 of the steering mirror 150 with the OA 199; spatial modes of the primary light 1 1 1 that have a comparably high divergence are cut-off at the arrows.

As a general rule, it may not be easily possible to compensate for a loss of primary light 1 1 1 at the mirrors 131 , 150. This is because the maximum transmit power of the laser diode 101 may be limited.

As a general rule, it would be possible that the laser diode 101 is operated at transmit powers for emitting the primary light 1 1 1 that are within 20 % of the thermal breakdown point. The thermal breakdown point may be associated with transmit powers than result in irreversible damage to the integrity of the laser diode 101.

In FIG. 4, the size of the coupling mirror 131 is matched to the size of the beam profile 701 of the primary light 1 1 1. Hence, the transmittance is not reduced by losses of the primary light 1 1 1 at the coupling mirror 131.

In FIG. 4, the size of the projection 151 a , Asmirror defines the detector aperture of the receive beam 122. Hence, the size of the projection 151 a defines the field width 322 of the secondary light 1 12.

As illustrated in FIG. 4, the size 158 of the projection 151 a along the SA 751 is smaller than the size 131 -2 of the coupling mirror 131 along the SA 751. Hence, primary light 1 1 1 is lost at the steering mirror 150. A ^* is the area of the beam profile 701 that falls within the projection 151 a, i.e., that is reflected by the steering mirror 150 (dashed lines in FIG. 4, 701 ^* ) .

Thus, the transmittance of the primary light 1 12 along the transmit beam 121 in such a scenario is given by: where A cmirror is the area of the coupling mirror 131 , l.e., Acmirror ~ WTX.SA X WTX,FA-

Secondary light 122 is collected across the entire area of the projection 151 a. Hence, the size of the projection 151 a , Asmirror with respect to the size of the coupling mirror, Acmimr defines the beam splitter transmittance T _sp ntter, and the transmittance of the secondary light 1 12 along the receive beam 122 is given by:

Finally, the overall transmittance is given by:

Again, Eq. (7) is under the assumption that the size of the reflective area 131 of the coupling mirror 130 matches the size of the beam profile 701.

FIG. 5 schematically illustrates the beam profile 701 of the primary light 1 1 1 with respect to the projection 151 a of the reflective surface 151 into the plane perpendicular to the OA 199. FIG. 5 also illustrates the beam profile 701 of the primary light 1 1 1 with respect to the coupling mirror 131 of the beam splitter 130.

The scenario of FIG. 5 generally corresponds to the scenario of FIG. 4. However, in the scenario FIG. 5, the collimator lens 201 is configured to reduce the field width 321 -2 of the primary light 1 1 1 along the SA 715. Then, the field width 321 -2 is smaller than the respective size 151 a-1 of the projection 151 a of the reflective surface 151 of the steering mirror 150. Thus, the mirror transmittance Tmimr = 1 . Primary laser light 1 1 1 is not lost at the mirror 150. On the other hand, the divergence of the primary light 1 1 1 along the SA 715 is large, because the field width 321 -2 is small, see Eq. (1 ). In FIG. 4 and FIG. 5, a mount of the coupling mirror 131 is not illustrated for sake of simplicity.

Further, from a comparison of FIGs. 4 and 5, it follows that the coupling mirror 131 can be dimensioned smaller in the scenario FIG. 5 than in the scenario FIG. 4, due to the reduced field width 321 -2. Specifically, the length 131 -2 of the respective side of the coupling mirror 131 can be reduced. Thus, in the scenario of FIG. 5, the transmittance T _sp mer of the beam splitter 130 is larger than for the scenario of FIG. 4, see Eq. (4).

As will be appreciated from the discussion of FIGs. 4 and 5 above, there is a correlation between (/) transmittance of the transmit beam 121 with respect to the receive beam 122 and (/ ^' /) divergence of the primary light 1 1 1 : There is generally a tendency that larger transmittance results in larger divergence of the primary light 1 1 1. This is illustrated in FIG. 6.

FIG. 6 illustrates a dependency of the transmittance 601 -604 T of the optical system as a function of the divergence 610. In FIG. 6, this dependency is plotted for various sizes of the reflective surface 151 of the steering mirror 150. The dependency of FIG. 6 is obtained from Eq. (7). FIG. 6 is under the assumption of Tiosses = 1 , i.e., no losses due to imperfections of the optical elements as such.

As a reference, FIG. 6 illustrates the transmittance T 609 for a conventional prism-based beam splitter. A prism-based beam splitter typically has T _sp ntter,rei=0.25. Flence, the horizontal dashed line of the transmittance 609 provides a best-case reference for such a scenario using a prism-based beam splitter. The reference transmittance is not dependent on the divergence of the primary light 1 1 1.

Differently, using a coaxial optical system according to various examples, larger sizes of the reflective surface 151 , Asmirror result in higher transmittances 601 -604, because the transmittance of the beam splitter increases, of. Eq. (7). This dependency is highlighted by the vertical dotted arrow in FIG. 6. As general rules:

(A) The lateral resolution of the LIDAR image can be better for smaller divergence. This is because the spot of the primary laser light illuminates a smaller target area of the object for smaller divergence. Thus, the spatial averaging increases for increasing divergence. Thus, it is generally desired to design the coaxial optical system be towards the left of the plot of FIG. 6.

(B) A smaller transmittance results in a smaller signal at the detector 102. A smaller signal limits the range, because less and less photons of the secondary laser light 1 12 reach the detector 102. Thus, a better range of LIDAR measurements is achieved for larger transmittance. Thus, it is generally desired to design the coaxial optical system to be towards the top of the plot of FIG. 6.

From FIG. 6, it is apparent that there is a trade-off situation between (A) lateral resolution/small divergence and (B) range/large transmittance.

The regime 671 in FIG. 6 corresponds to the scenario of FIG. 4: if the field width 321 -2, WTX.SA increases, more and more primary light 121 is lost at the steering mirror 150 (arrows in FIG. 4) - hence, the transmittance 601 -604 decreases, of. Eq. (5) and Eq. (1 ). One the other hand, if the field width 321 -2, WTX,SA increases, the divergence 610 decreases.

The regime 673 in FIG. 7 corresponds to the scenario of FIG. 5: if the field width 321 -2 increases, the divergence 610 decreases; on the other hand, if the field width 321 2 increases, the coupling mirror 131 becomes larger and larger, and, in turn, the Turner decreases - see Eq. (7) - and along with T _S pntter the transmittance decreases.

The transition regime 672 between the regimes 671 and 672 corresponds to a scenario where the field width 321 -2 of the primary light 1 1 1 along the SA 715 has approximately the size of the respective width 131 -2. In the model of FIG. 6, the regime 671 is characterized by a linear change of transmittance 601 -604 with divergence 610. The regime 673 is characterized by a sub-linear change of transmittance 601 -604 with divergence 610.

This transition regime 672 is a sweet spot of the trade-off situation between (A) lateral resolution/small divergence and (B) range/large transmittance. This is because further increase in (B) range/large transmittance comes at the cost of overly increased divergence, i.e., overly decreased lateral resolution. At the same time, further (A) reduction of the divergence, i.e., further increase of the spatial resolution strongly reduces (B) transmittance and, hence, the range.

Thus, as a general rule, the collimation of the primary light 1 11 and the size of the steering mirror 150 can be set to the transition regime 672 between a linear and a sub-linear dependency of the transmittance 601 -604 on the divergence 610.

As a further general rule, the SA field width 321 -2, WTX,SA of the primary light 1 1 1 just ahead / upstream of the mirror 150 may not be smaller than 90 % of the respective width 158 of the reflective surface 151 perpendicular to the OA 199, i.e., of the projection 151 a. Also, the SA field width 321 -2 of the primary light 1 1 1 just ahead / upstream of the mirror

150 may not not larger than 120 % of the respective width 158 of the reflective surface

151 perpendicular to the OA 199, to avoid excessive loss of primary light 1 1 1 (reduced transmittance according to Eq. (5)) that cannot be easily compensated by the transmit- power-limited laser diode 101.

Beyond such general rules for the relative dimensioning of the various parts of the optical system of the scan system 100, it is also possible to derive quantitative teachings from FIG. 6. Specifically, as has been mentioned above, for resonantly-driven steering mirrors 150, it is often helpful to limit the mass of the steering mirror 150. In detail, if the mass of the steering mirror 150 increases, a resonance frequency of the respective motion reduces. On the other hand, it has been observed that resonance frequencies below, e.g., 60 Hz or 80 Hz cannot meet the requirements of various use cases, including automotive use cases. This is due to a reduced refresh rate of LIDAR images, as well as increased susceptibility to shock. A typical implementation of the steering mirror 150 is based on micro-electromechanical (MEMS) fabrication techniques. For example, a material of the steering mirror 150 can be silicon, e.g., single crystalline silicon. For example, lithography and wet etching and/or dry etching, e.g., reactive ion beam etching can be employed to fabricate the steering mirror 150. On a mirror back-side, sometimes, fins and cavities can be provided to provide for reduced mass at high structural rigidity; this is sometimes referred to as back-side reinforcement. Then, within such an approach of MEMS fabrication of the steering mirror 150, it has been observed that reflective surfaces 151 of the steering mirror 150 that are larger than, e.g., 220 mm ² result in a resonance frequency that falls below, e.g., 60 Flz to 80 Hz. On the other hand, from FIG. 6 it follows that areas of the reflective surface 151 of the steering mirror 150 that are, e.g., significantly smaller than 180 mm ² will necessarily result in a divergence above 0.15°. At specified ranges of the LIDAR measurements of 150 m to 250 m, divergences above 0.15° are typically unacceptable in terms of reduced lateral resolution at these large ranges. Thus, from FIG. 6 it follows that in a coaxial optical system using a frictionless, resonantly driven steering mirror 150, it is desirable to implement a reflective surface 151 having a size in the range of 180 mm ² to 220 mm ² (it is noted that the projection 151 a of the reflective surface 151 into the plane perpendicular to the OA of the optical system can be smaller). Also, the divergence can be in the range of 0.09° to 0.15°. Smaller divergences can be undesirable, due to reduced transmittance. Also, smaller divergences may not be required, because of the inherently range-limited design of the LIDAR measurements at approximately 150 m to 250 m, corresponding to spot width of roughly 43 cm at 250 m for a divergence of 0.10° along the SA 715. The transmittance can be in the range of 0.7 to 0.85, at Ti ₀ sses- = 1 , as illustrated in FIG. 6. If losses due to imperfections of the optical elements are considered to amount to Tosses ^a 0.57, then it can be assumed that the transmittance is not smaller than =0.4. This is still significantly larger than the reference transmittance 609 of 7 sputter, ref=0.25.

As will be appreciated, above, with respect to FIGs. 4 - 6, primarily limitations of the coaxial optical system with respect to the SA 715 have been discussed. For example, the field width 321 -1 of the primary light 1 1 1 along the FA 716 can be much smaller than the field width 321 -2 of the primary light 1 12 along the SA 716, at the coupling mirror 131 and at the steering mirror 150. Specifically, as illustrated in Figs. 4 and 5, the field width 321 - 1 of the primary light 1 1 1 along the FA 716 can be smaller than 50% of the respective width 159 of the projection 151 a of the reflective surface 151 of the steering mirror 150 into the plane perpendicular to the OA 199. Typically, the quality of the laser light emitted by the laser diode 101 along the FA 716 is much better than the quality of the laser light emitted by the laser diode 101 along the SA 715. c _FA « c _SA , see Eqs. (1 ) and (2). Flence, it is possible to restrict the field width 321 -1 of the primary light 1 1 1 along the FA 716, while maintaining a small divergence of the primary light 1 1 1 along the FA 716. A collimator lens for collimating the FA 716 may be integrated within the housing of the laser diode 1 1 1.

The coaxial optical system of the scan system 100 according to the example of FIG. 2 is only one example implementation. Other configurations of the coaxial optical system are conceivable. Another example implementation of the optical coaxial system of the scan system 100 is illustrated in connection with FIG. 7. Also for the optical coaxial system illustrated in FIG. 7 or other implementations of the coaxial optical system, the techniques generally described with respect to FIGs. 3 - 6 may be readily applied.

FIG. 7 illustrates aspects with respect to the scan system 100. FIG. 7 is a ray tracing schematic of the optical system of the scan system 100, specifically of the transmit beam 121 and the receive beam 122. FIG. 7 illustrates a FA 716 of the primary laser light 121 (the SA is not illustrated).

The scenario of FIG. 7 generally corresponds to the scenario of FIG. 2. For example, also the scenario FIG. 7 illustrates a coaxial optical system.

In the example of FIG. 7, the beam splitter 130 is arranged in between the detector lens 252 and the detector 102 (while in the example of FIG. 2, the detector lens 252 is arranged in between the beam splitter 130 and the detector 102).

Such an arrangement of the beam splitter 130 as illustrated in FIG. 7 has various effects. For example, it can be possible to integrate the laser diode 101 and the detector 102 into a common housing. Thereby, robustness against environmental influences such as humidity, etc. can be increased. Also, packaging of a reduced footprint can be achieved.

In the scenario FIG. 7, a lens 702 is illustrated. The lens 702 acts upon the SA 715 of the primary light 1 1 1 and does not act or does not act significantly on the FA 716 of the primary light 1 1 1 . The lens 702 does not collimate the SA 715 of the primary light 1 1 1. Rather, in a first implementation, the lens 702 is configured to narrow a beam width of the primary light 1 1 1 along the SA 715 towards the beam splitter 130; while, in a second implementation, the lens 702 is configured to broaden a beam width of the primary light 1 1 1 along the SA 715 towards the beam splitter.

The primary light 1 1 1 entering the lens 702 does not need to be collimated, it may be divergent, e.g., at least along the SA 715.

Considering a scenario where the field width 321 -2 of the primary light 1 1 1 reduces towards the coupling mirror 131 of the beam splitter 130. For example, it would be possible that the coupling mirror 131 is arranged in the image plane of the lens 702 - while the laser diode 101 is arranged in an object plane of the lens 702. Then, the respective width 131 -2 of the coupling mirror 131 along the SA 715 can be reduced, along with the reduced field width 321 -2 of the primary light 1 1 1. This, in turn, increases the transmittance of the optical system T of the scan unit 100, because the beam splitter transmittance T _sp nuer is increased, cf. Eq. (6). Specifically, the transmittance 601 -604 can be increased beyond the dependencies as illustrated in FIG. 6.

To avoid highly divergent primary light 1 1 1 along the SA 715 leaving the scan unit 100 towards the environment 190 at the steering mirror 150, the collimator lens 201 for the SA 715 is arranged in the transmit beam 121 behind the beam splitter 130, i.e., the collimator lens 201 is arranged in the overlap section 125.

Generally, it would possible to separately implement the collimator lens 201 and the detector lens 252. Flowever, in the example of FIG. 7, the collimator lens 201 and the detector lens 252 are integrated in a common, multifocal lens body 800 (cf. FIG.9) providing, in a first region 801 , a focal length to focus the secondary light 1 12 onto the detector 102 and providing, in a second region 802 collimation of the primary light 1 11 (of. FIG. 8). The multifocal lens body 800 can be segmented into the regions 801 and 802. The region 801 encloses the region 802. As will be appreciated, secondary light 1 12 incident on the region 802 will be lost, anyway, due to reflection at the coupling mirror 131. Hence, provisioning the collimator lens 201 in the overlap section 125, per se, does not significantly affect the transmittance of the transmit beam 121 with respect to the receive beam 122. By integrating the lenses 201 , 252 into the common lens body 800, a particular compact design of the optical system can be achieved.

Further, by provisioning the lens 702, it is possible to increase the divergence of the primary light 1 1 1 along the SA 715. Thereby, the distance in between the laser diode 101 and the collimator lens 201 can be reduced. A reduced distance between the laser diode 101 and the collimator lens 201 , in turn, facilitates a shorter focal length of the detector lens 252. This, in turn facilitates a smaller footprint of the coaxial optical system. For example, the coaxial optical system may be integrated into a smaller housing.

A similar effect can also be achieved if the lens 702 directly broadens the primary light 1 1 1 , i.e., if the SA field width 321 -2 of the primary light is directly widened. This may correspond to a virtual image plane. Here, the distance between the lens 702 and the virtual image plane may be at least 20 % of the distance between the lens 702 and the collimator lens 201 , to achieve sufficient broadening of the SA field width 321 -2.

Further, as a general rule, above scenarios have been described in which the lens 702 selectively acts on the primary light 1 1 1 along the SA 715. However, in other examples, it would also be possible that the lens 702 acts on the FA 716, in addition to or alternatively to the SA 715.

Summarizing, above, techniques have been described which facilitate implementation of an eye-safety compliant LIDAR sensor unit including a scan system implementing a coaxial optical system. A resonantly driven steering mirror is employed.