Patent ID: 12241741

DETAILED DESCRIPTION

Embodiments of the present invention provide a laser sensor which is capable of performing self-mixing interference measurements without the need for additional systems, such that all position and/or velocity information needed can be obtained from the self-mixing interference measurement.

Embodiments of the present invention provide a system capable of detecting at least one of a position or velocity of an object with high accuracy without the need for additional systems like a camera.

According to a first aspect of the present invention, a laser sensor is provided. The laser sensor comprisinga laser source configured to emit a laser beam,optics configured to project the laser beam as a one- or two-dimensional patterned laser beam onto an object to be examined, such that a distance of the patterned laser beam from the laser source varies along the patterned laser beam projected on the object,a detector configured to determine a self-mixing interference signal generated by laser light of the patterned laser beam reflected by the object back into the laser source, andcircuitry configured to spectrally analyze the self-mixing interference signal and extract from the spectrum of the self-mixing interference signal multiple frequencies indicative of multiple distances along the patterned laser beam from the laser source and/or multiple velocities along the patterned laser beam with respect to the laser source.

The laser source according to embodiments of the present invention enables obtaining more information from the self-mixing interference signal as conventional SMI laser sensors. This is achieved by two differences with respect to conventional SMI techniques. A first difference is making use of a one- or two-dimensional patterned laser beam hitting the object to be examined. In contrast, in conventional SMI techniques, a laser beam is projected as a single point-like beam spot on the object so that there is only one distance of the laser beam spot on the object from the laser source at one point of time. By projecting the laser beam as a one- or two-dimensional patterned laser beam onto the object such that a distance of the patterned laser beam from the laser source varies along the patterned laser beam projected onto the object, there are multiple distances of the patterned laser beam from the laser source at one point of time. Thus, the back-reflected patterned laser beam generates an SMI signal which contains much more position and/or velocity information based on this multitude of distances than in case of a SMI signal based on a single beam spot projected on the object.

“One- or two-dimensional patterned beam” means that the projected laser beam has an extension in one or two dimensions. A one-dimensional patterned laser beam may be a straight or curved line or stripe, for example. The line may be a continuous line, a broken line, or a plurality of dots arranged in a line. A one-dimensional patterned beam may be characterized by a beam width in a first direction which is at least two times the beam width in a second direction perpendicular to the first direction. A two-dimensional patterned laser beam may have a beam profile extending over an area with at least one minimum of laser light intensity within the area, wherein the minimum may be zero. For example, a two-dimensional patterned beam may be a circle, ellipse, rectangle, a grid of a plurality of lines, a cross, etc.

The emitted laser beam may comprise a plurality of laser beams emitted by the laser source. A plurality of laser beams may be emitted in time-multiplexed manner. The laser source may comprise a single laser or a plurality of lasers. The laser source may comprise a single laser comprising a plurality of laser beam emission regions. For example, the laser may be a VCSEL array having two or more mesas, each mesa being a laser beam emission region.

A second difference between the laser sensor according to embodiments of the present invention and the conventional SMI laser sensors is that the laser sensor according to embodiments of the present invention spectrally analyses the self-mixing interference signal and extracts from the spectrum, e.g. the full spectrum or at least an essential part of the spectrum, multiple frequencies which are indicative of multiple distances along the patterned laser beam from the laser source and/or multiple velocities along the patterned laser beam with respect to the laser source. In conventional SMI laser sensors, only a peak frequency in the SMI signal is used to obtain position and/or velocity information from the SMI signal. By making use of spectral analysis of the SMI signal, much more and in particular more accurate information may be obtained from the SMI signal. The spectral analysis of the SMI signal may be performed in the frequency domain or in the time domain. For instance, after a Fast-Fourier Transformation (FFT) of the SMI signal, the information in a plurality of, or even in all bins, is used according to embodiments of the present invention.

Instead of getting one velocity and/or one distance as in case of conventional SMI sensors, a whole spectrum of information is available from which multiple distances and velocities can be analyzed simultaneously according to embodiments of the present invention.

The laser sensor according to embodiments of the present invention is particularly, but not exclusively, suitable for eye gaze angle detection. There is no need for an additional camera anymore. The laser sensor according to embodiments of the present invention is also particularly, but not exclusively, suitable for velocity profile detection in a flowing fluid, or for tilt detection of an object. Embodiments of the present invention provide a very cost effective and power effective solution to introduce spatial resolution over conventional systems using an array of a multitude of traditional SMI sensors.

The laser source of the laser sensor may be a laser diode, in particular a VCSEL. The detector may be configured as a photodiode integrated into the VCSEL, e.g. as an intra-cavity photodiode or an extra-cavity photodiode.

Exemplary embodiments of the present invention are defined in the dependent claims and as further indicated herein.

In an embodiment, the laser sensor comprises an electrical driver configured to provide a driving current to the laser source to cause the laser source to emit the laser beam, wherein the electrical driver may be configured to provide a modulated driving current to the laser source to cause the laser source to emit the laser beam with a periodically varying wavelength.

The periodically varying wavelength influences the self-mixing interference signal. If the object to be examined does not move, using a modulated laser beam with periodically varying wavelength enhances the accuracy of the detection of multiple positions. In case of a moving object, a triangular modulated laser beam results in different frequencies of the SMI signal for the rising and trailing (falling) edge, wherein the mean of both frequencies is an indicator of object distance, while the difference correlates with the double Doppler frequency of the SMI signal.

The pattern of the patterned laser beam projected onto the object may be selected from the group consisting of a continuous-line pattern, a dot-line pattern, a multiple-line pattern. For example, the patterned laser beam may be a straight or curved line or stripe, including a closed line like a circle, ellipse, rectangle, a grid, a cross, etc.

In a further embodiment, the optics is further configured to focus the patterned laser beam onto the object. Focusing the patterned laser beam onto the object advantageously increases the SMI signal strength, or in other words the signal-to-noise ratio (SNR). It is preferred to have the best focus position as best as possible matching the position of the surface of the object to be examined for optimum SMI signal. The optics may be adapted to project the laser beam such that the focus position is optimized at the surface of the object. This is of particular benefit, if the object has a non-planar surface.

The optics may be selected from the group consisting of a single cylinder lens, two or more crossed cylinder lenses, a cylinder lens having cylindrical surfaces with different orientations of cylinder axes of the cylindrical surfaces, free form optics, meta optics, diffractive elements, e.g. one or more optical gratings, holographical optics, mirrors, which may be spherical, cylindrical or free-form mirrors, wherein the mirrors may be partially reflecting.

Depending on the object to be examined, the optics may be chosen, accordingly, in particular to optimize the best focus position along the patterned laser beam.

According to a second aspect, a system for detecting at least one of a position or velocity of an object is provided, comprising a laser sensor according to the first aspect. As the system comprises a laser sensor according to embodiments of the present invention, the system can measure multiple positions and/or velocities with a single laser sensor.

The system may further comprise at least a second laser sensor comprising a second laser source configured to emit a second laser beam and second optics configured to project the second laser beam as a one- or two-dimensional patterned laser beam onto the object, wherein the patterned laser beam and the second patterned laser beam intersect on the object under a non-zero angle.

The non-zero angle may be 90° or an obtuse or acute angle. Using two patterned laser beams projected onto the object is advantageous, if the object can move in different directions.

The object to be examined may be a human eye, and the system may be configured to detect the gaze angle of the eye.

Detection of the gaze angle of the eye may be performed by analyzing the spectrum of the SMI signal for a gap in the spectrum which is indicative of the position of the pupil of the eye, as the patterned laser beam or a part thereof is, at the pupil position, dominantly reflected at the retina which has a larger distance from the laser source than the front surface (iris and sclera) of the eye. The system for eye gaze angle detection may be configured to project multiple patterned laser beams onto the eye in time-multiplexed manner.

The system according to embodiments of the present invention may also be configured to detect a tilt angle of an object. Detection of tilt of an object may be used in any applications, for example in quality control of products in a production line. Tilt detection may also be used to detect the gaze angle of the eye, as the tilt of the iris is a measure for the eye gaze angle.

The system may also be configured to detect a velocity profile of a flowing fluid. This is an embodiment of analyzing the spectrum of the SMI signal to extract multiple velocities along the patterned laser beam with respect to the laser source. For example, micro-particles in the fluid may reflect the patterned laser beam, generating a measurable SMI signal. In this case, it is preferred to have SMI signals from the focused position of the patterned laser beam only, so that it is preferred to have a relatively large numerical aperture at the fluid side for optimal position discrimination.

According to a third aspect, a method is provided, comprisingemitting a laser beam from a laser source,projecting the laser beam as a one- or two-dimensional patterned laser beam onto an object to be examined, such that a distance of the patterned laser beam from the laser source varies along the patterned laser beam projected on the object,determining a self-mixing interference signal generated by laser light of the patterned laser beam reflected from the object back into the laser source,spectrally analyzing the self-mixing interference signal and extracting from the spectrum of the self-mixing interference signal multiple frequencies indicative of multiple distances along the patterned laser beam from the laser source and/or multiple velocities along the patterned laser beam with respect to the laser source.

According to a fourth aspect, a computer program product comprising program code means for causing a laser sensor according to the first aspect or a system according to the second aspect to carry out the steps of the method according to the third aspect, when said computer program is carried out on a processor of the laser sensor or on a processor of the system.

Further advantageous embodiments are defined below.

Before referring to the drawings, the principles of self-mixing interference and laser sensor basics are explained.

The operating principle of a laser, e.g. a laser diode, e.g. a vertical cavity surface emitting laser, is based on optical resonators. Inside of the resonator, electrons are in exited state by external energy input. Radiation by spontaneous emission is reflected forth and back in the optical resonator and causes a stimulated emission, thus amplifying the resonant mode and producing coherent radiation. At one side of the lasing cavity, laser radiation can couple into free-space through a semi-transparent mirror. In case of a vertical cavity surface emitting Laser (VCSEL), the mirror structures are realized as distributed Bragg reflectors (DBR). A photodiode may be placed in the VCSEL, wherein the photodiode may be integrated into the lasing cavity, or may be placed outside the lasing cavity. Thus, a VCSEL with integrated photodiode, abbreviated as ViP, is formed.

The underlying physical effect of laser self-mixing is explained next. A beam laser emitted by the laser may be reflected at an object. “Reflected” as used herein is not only understood as specular reflection, but also as diffuse reflection, also referred to as scattering. If externally reflected laser radiation couples back into the laser cavity, stimulated emission in the laser cavity is modulated based on the phase of back-scattered photons. If twice the distance between laser cavity and external scattering surface (back and forth) equals an integer multiple of the wavelength of the laser light, scattered radiation and radiation inside of the laser resonator are in phase. This results in positive interference, whereby lasing threshold is reduced and laser output is slightly increased, which may be sensed by the photodiode integrated into the laser. At slightly increased distance, both radiation waves are out of phase and at some point negative interference occurs, reducing laser output power. If the distance to the scattering surface of the object is changed at constant speed, laser output power is oscillating between a maximum during constructive interference and a minimum during destructive interference. The resulting oscillation is a function of the speed of the scatterer (object) and laser wavelength.

The same effect, namely oscillating laser output power, can be observed if the distance between the laser cavity and the scattering surface is left constant, but the laser wavelength is changed. Laser wavelength change may be achieved by modulating the external energy, e.g. a driving current, used for driving the laser. Now, the phase of radiation between internal cavity inside the laser and external cavity between laser and scatterer is dependent on how many wavelengths “fit” inside the external cavity. However, the frequency of oscillation of output power is dependent on distance between laser and scattering surface, as the wavelength of the laser light typically is in the near infrared region, e.g. around 850 nm, and external cavity distances are in the region of several centimeters, so that slight change in laser wavelength can result in a full turn of external cavity laser phase. The higher the distance between laser and scattering surface, the less wavelength change results in a full turn of external cavity laser phase. Assessing the laser output power variation, the higher the distance to the scatterer, the higher the power variation frequency at constant laser wavelength change. Therefore, mapping the power monitoring photodiode into frequency domain, the peak frequency correlates with the distance between laser and scatterer. Laser wavelength change can be introduced by power modulation of the laser diode. For example, a driving current may be linearly modulated according to a triangular laser current.

If both effects superimpose, i.e. laser wavelength is modulated and scatterer moves, resulting beat frequencies as known from frequency modulated continuous wave radar systems occur. Due to the Doppler shift in frequency, the resulting beat frequency is lower for a target moving towards the laser sensor during ramp up of the frequency (according to the wavelength modulation) and higher during ramp down of the frequency. Thus, beat frequencies are to be calculated for rising and falling modulation segments individually. The mean of both frequencies is an indicator for object distance, while the difference correlates with the double Doppler frequency.

Next, with reference toFIG.1, a laser sensor will be described.FIG.1shows a sketch of a laser sensor10. The laser sensor10comprises a laser source12and a detector14. The detector14may be integrated with the laser source12. More specifically, the detector14may be a photodiode integrated into the layer structure of the laser source12, wherein the photodiode may be integrated as an intra-cavity photodiode or an extra-cavity photodiode. The laser sensor10may further comprise an electrical driver16and a controller18. The controller18is connected to the laser source12including the photodetector14. The electrical driver16supplies electrical power to the laser source12to cause the laser source12to emit a laser beam22, indicated by broken lines. The laser source12may be or comprise a vertical cavity surface emitting laser (VCSEL) with integrated photodiode, i.e. a ViP. The electrical driver16may be configured to provide a constant driving current or a modulated driving current to the laser source12. In case the electrical driver16provides modulated driving current to the laser source12, the modulated driving current may follow a triangular shape. The controller18is further configured to receive electrical signals provided by the detector14which are caused by self-mixing interference (SMI) of laser-light re-entering the laser cavity with laser light generated in the laser cavity.

The laser sensor10further comprises optics20. The optics20is configured to project the laser beam22as a one- or two-dimensional patterned laser beam onto an object24to be examined. The object24is illustrated inFIG.1by a broken line, wherein the broken line resembles the reflecting (scattering) surface of the object24.

Optics20is configured to project the laser beam as a one- or two-dimensional patterned laser beam26onto the object24to be examined such that a distance D of the patterned laser beam26from the laser source12varies along the patterned laser beam26projected onto the object24. InFIG.1, three distances D1, D2, D3are exemplarily shown, wherein D2and D3are larger than D1. An example of a patterned laser beam26is shown inFIG.2a). The patterned laser beam26inFIG.2a) is a line shaped beam, as an example for a one-dimensional projected patterned laser beam.FIG.2b) shows a crossed-line beam, as an example for a two-dimensional projected patterned laser beam. In general, the pattern of the projected patterned laser beam may be selected from the group consisting of a continuous-line pattern, as shown inFIG.2a), a dot-line pattern, which is a line comprising discrete dots, a grid of continuous lines or dot lines, A pattern as the crossed-line beam26′ shown inFIG.2b) may be achieved by using two laser sources12and corresponding optics20, wherein the latter may comprise two crossed cylinder lenses, one for each laser source, or an array of micro-lenses, wherein two subsets of lenses are orthogonal to each other.

The optics may be further configured to focus the patterned laser beam26onto the object25. The patterned laser beam26may be focused in direction of a first dimension only, e.g. the y-dimension inFIG.2a). The cross26′ inFIG.2b) may be focused in each of the short dimensions of the two crossing lines.

The optics20may be further configured to expand the laser beam22emitted by the laser source12in one or two dimensions. For example, the patterned laser beam26inFIG.1may have been expanded by the optics20to expand the long dimension of the patterned laser beam26. It is, however, also possible that the long dimension of the patterned laser beam26is achieved by the divergence of the laser beam22emitted by the laser source12without any additional expansion in the long dimension, for example if the object distance from the laser source is large enough.

Further, the optics22may be arranged to obliquely project the laser beam22onto the object24, as shown inFIGS.3and6. As shown inFIG.3, the object24may further have a non-planar surface. With an oblique projection of the laser beam22, the distance D of the patterned laser beam26from the laser source12stronger varies along the patterned laser beam26even for a planar object surface than in a symmetrical projection as shown inFIG.1.

The optics22may comprise one or more optical elements. An example of optics22is shown inFIG.4, wherein the optics comprises a cylinder lens28. In general, the optics22may be selected from the group consisting of a single cylinder lens, two or more crossed cylinder lenses, a cylinder lens having cylindrical surfaces with different orientations of cylinder axes of the cylindrical surfaces, free form optics, meta optics, one or more optical gratings, holographical optics, mirrors. Crossed cylinder lenses may be used, for example, to project and focus the laser beam22in form of a crossed line shape as shown inFIG.2b) when two laser sources are used, or to tune the line length independently from the line width.

With reference back toFIG.1, the detector14is configured to determine a self-mixing interference signal generated by laser light of the patterned laser beam26reflected by the object24back into the laser source14. The laser sensor10further comprises circuitry configured to spectrally analyze the self-mixing interference signal and extract from the spectrum of the self-mixing interference signal multiple frequencies indicative of multiple distances (e.g. distances D1, D2, D3) along the patterned laser beam26from the laser source12and/or multiple velocities along the patterned laser beam26with respect to the laser source12, as described herein.

The afore-mentioned function of the circuitry may be performed by the controller18of the laser sensor10.

With reference toFIGS.5to9, an embodiment of the laser sensor for eye gaze angle detection will be described.

FIG.5schematically shows a front view of a human eye E. P denotes the pupil of the eye E, I the iris of the eye, and S the sclera of the eye E. A first laser sensor101emits a laser beam221which is projected by optics like optics20inFIGS.1and4as a line shaped laser beam261onto the surface of the eye E. A second laser sensor102emits a second laser beam222which is projected by optics as a line shaped laser beam262onto the surface of the eye E. The patterned laser beam262may be orthogonal to the patterned laser beam261. The optics used for projecting the laser beam221and222onto the eye E may be a cylinder lens in each case, one cylinder lens of the laser sensor101and one cylinder lens of the laser sensor102.FIG.6shows a side view of the eye E, wherein only the laser beam221emitted by the laser sensor101and the patterned (line shaped) projected laser beam261are shown.

As shown inFIG.6, the laser beam221is obliquely projected onto the eye E in the region of the pupil E. The patterned laser beam261has an extension along its long dimension which preferably is larger than the pupil diameter.

The laser beams221and222are emitted as modulated laser beams, wherein the laser sources of laser sensor101and102are supplied with modulated driving current. In the present embodiment, a triangular modulation is used. Due to the wavelength variations, the phase of the laser light reflected from the eye E will vary, resulting for one fixed distance in one fixed frequency. However, having the laser beam261projected onto the eye E as shown inFIG.6, multiple different frequencies will be detected in the self-mixing interference signal, as the distance between the projected patterned laser beam261and the laser source101varies along the projected patterned laser beam261.

The SMI signal determined by the detector14(FIG.1) which is generated by laser light of the patterned laser beam261reflected by the eye E back into the laser source12of the laser sensor101thus comprises multiple frequencies corresponding to multiple distances along the patterned laser beam261from the laser source12of the laser sensor101.

The same distance variations and thus multiple frequencies are measured by the second laser sensor102along the patterned laser beam262on the surface of the eye E.

When part of the patterned laser beam261enters the pupil P, laser light travels further to the retina R. Since the distance of the retina R from the laser source12of the laser sensor101is larger than the distance of the iris I and sclera S from the laser source12of the laser sensor101, a gap is expected in the SMI spectrum. The position of this frequency gap is an absolute measure for the pupil P position. This will be explained with reference toFIGS.7and8.FIG.7shows a spectrum of the SMI signal resulting from the projected patterned laser beam261inFIG.6. The horizontal axis of the diagram inFIG.7shows the SMI signal frequency. SMI signal frequency may be translated into distance as explained above. The vertical axis inFIG.7shows the power or occurrence of the SMI signal in dependence on SMI signal frequency. SMI signals with low frequencies belong to low distances, corresponding to bottom part B (FIG.6) of the projected patterned laser beam261. At the pupil position, the patterned laser beam261is reflected at the retina, corresponding to large distances/frequencies. Medium frequencies correspond to the upper part U of the patterned laser beam261. There is a gap between the lower frequencies corresponding to the part B and the medium frequencies corresponding to the part U which gap corresponds to the position of the pupil P. Thus, the pupil position may be accurately measured.

If the eye E moves, i.e. rotates, and thus the gaze angle moves, the gap position (P inFIG.7) in the spectrum shifts due to the change in “missing” distance from the pupil location. The proper and exact pupil position may then be derived after averaging the gap positions in the spectrum on the rising and the falling edge of the triangle wavelength modulation of the laser beam221.

In order to verify the above, experiments have been performed using an artificial eye. The artificial eye is a 3D-printed sphere with a hole, representing the pupil. A laser beam is projected as a line shaped patterned laser beam on this artificial eye, using a f=20 mm cylinder lens with 2 f-2 f imaging. Triangle modulation from an external function generator with a modulation frequency of 8 kHz with a modulation laser current amplitude of 0.51 mApp is performed. The average laser power is 0.5 mW.

FIG.8shows the result of this experiment. The artificial eye is rotated clockwise in the sketch ofFIG.6. When the pupil P passes the projected patterned laser beam, low distances/frequencies disappear first, and upon further rotating the artificial eye, finally the high frequencies disappear. After the pupil P passed the patterned laser line beam, all frequencies are visible again.FIG.8shows an FFT spectrum as a function of relative rotation angle of the artificial eye. At around 30 degrees, the pupil P is passing the projected laser line beam, resulting in a dip in the power at these frequencies (bin20inFIG.8). The signal line at FFT bin14is an artefact caused by a reflection of some dirt on the cylinder lens front surface.

From this experiment, it can be understood that the orientation or gaze angle of the eye can be reconstructed from the measured SMI signal by spectral analysis of the SMI signal in the angle range where part of the laser light is falling in the pupil. Reconstruction may be done using a feed-forward neural network, wherein the result of this reconstruction is shown inFIG.9.FIG.9shows the reconstructed eye gaze angle for the experiment on the artificial eye, showing the eye angle as a function of time. The straight line41is the true value of the eye gaze angle, and the curve43is the SMI based measured value of the eye gaze angle.FIG.9shows that the eye gaze angle can be correctly reconstructed in the range with unique spectral signatures, which means up to two seconds. After two seconds, the pupil P of the artificial eye rotates away from the projected line shaped laser beam, and the pupil position cannot be reconstructed directly from the measurement spectra anymore. As can be seen inFIG.9, the practical eye tracking angles are limited to about +/−20 degrees. However, Doppler velocity signals will still be present. Integration of these velocity signals can be used to derive eye positions at even larger eye gaze angles.

In the experiment described above, the spectra were acquired with 8 kHz. For the reconstruction, 16 spectra have been analyzed. For the curve inFIG.9, another 9 point median filter was applied. The resulting update rate is 18 ms, so 56 Hz.

In a refinement of the embodiment described before, the optics20for projecting the laser beam22as a patterned laser beam onto the eye E may be optimized in terms of a better focus position of the patterned laser beam26on the eye E. In the experiment described above, this focus position is on a line at a distance of 40 mm from the laser source12of the laser sensor10. It would be preferred to have the best focus position as good as possible matching the position of the front surface of the eye for optimum SMI signals, i.e. the projected patterned laser beam26to be as best as possible focused along the curved line beam projected on the eye.

FIG.10shows the front position of the eye E as a distance towards the laser source12of the laser sensor10. α is the angle of the laser beam along the patterned line shaped beam26with respect to the horizontal axis H. At larger angles α, the distance of the front of the eye E to the laser source12is larger. A similar effect can be obtained for the focus position by placing an f=5 mm cylinder lens30(as optics20inFIG.1) at a 30° tilted position, wherein the distance of laser source to lens30for the central beam is 6.3 mm. Thus, using a tilted cylinder lens30enables to match the focus position better to the eye position as shown inFIG.10.FIG.11shows the distance of the focus position to the laser source as a function of angle α both for the focus position and the front position of the eye.

This is only an example how the SMI signals may be optimized by a better focus position. Further optimizations may be done by using e.g. free form optics. In that way, also the Gaussian intensity distribution of the projected patterned laser beam can be transformed into a more homogenous pattern.

Crossed cylinder lenses at different distances to the laser source and with different magnification factors (optionally different focal lengths) can also be used to create an elongated focus. This way, also the amount of laser light collected back into the laser source may be optimized. This can also be one optical element with the two crossed cylinder lenses distributed on the two surfaces of the optical element. Alternatives to cylindrical or free-form lenses made of optical material like glass or polymer to create an appropriate beam focus are optical grating structures, like imprinted surface gratings, holographical optics like photopolymer hologram layers, or meta lenses. These kinds of structures may also be used to create e.g. a line of spots instead of a continuous line. Other shapes as a line may also be considered.

FIG.12shows a system100which makes use of the teachings of the present disclosure according to an embodiment. System100is configured to detect the gaze of a human eye (not shown inFIG.12). The system100is configured in form of spectacles102comprising a frame with frame temple104and glasses106. The system100comprises laser sensors101,102,103, and104arranged on the frame104. It is to be noted here that the laser sensors101to104are arranged on the inner side of the frame104which faces the eye of the user wearing the spectacles. Each of the laser sensors101to104may be configured as described above with reference toFIG.1. The number of laser sensors may be less than four, wherein two laser sensors, e.g. laser sensors101and102may be sufficient for eye gaze angle detection. A further laser sensor105may be arranged on the frame temple104to measure extreme eye rotations. Some or all the laser sensors101to105could also be integrated in one or both of the spectacle's glasses106or in the nose pads. In one further possible embodiment, the laser sensors101to104may be arranged in the frame temple, but could emit the laser beams towards the glasses106, wherein a holographic optical element embedded in the glasses may deflect the laser beams towards the eye. In another embodiment, both the left and the right eye can be tracked simultaneously, using e.g. two sensors per eye.

The system100may be operated in different modalities. In a first modality, the laser sources of the laser sensors101to104may be operated at continuous waves (constant frequencies). In a second modality, the frequency of the laser sources may be modulated in time, e.g. following a triangular modulation pattern as described above.

With reference toFIGS.13to16, an embodiment of the system for eye gaze angle detection will be described which is modified with respect to the above embodiments described with respect toFIGS.5to9.

FIG.13shows, similar toFIG.5, a front view of a human eye E.FIG.13shows four projected patterned laser beams261,262,263,264, which are emitted by the laser sensors101,102,103, and104. The projected patterned laser beams261to264may be formed by optics like optics20, e.g. a cylinder lens or holographical optical element, to fit the desired shape of the sensing region on the eye E. Each of the projected patterned laser beams261,262,263,264forms a corresponding sensing region. In this example, four laser sensors, e.g. ViP sensors, are used with a cylinder lens to form the corresponding sensing region. The projected patterned laser beams261to264are stripes arranged in a two-dimensional pattern, e.g. forming a cross. It is, however, to be understood that other possible patterns of the projected laser beams are conceivable, e.g. circles, rectangles, grids, etc. In addition, adding more laser sensors increases the sensing surface size and therefore the coverable field of view. Less laser sensors can be used to reduce the overall power consumption. Due to the high temporal resolution of the sensor principle, time multiplexing can be used to decrease the power consumption of the necessary electronics and the optical power on the eye surface to fulfill eye safety considerations.

As described above, the laser sensors101to104measure distances towards the eye surface and velocities of the eye surface in the corresponding sensing region corresponding to the patterned laser beams261to264. In addition, the signal to noise ratio (SNR) may be determined from the measurement data to obtain information about the part of the eye (sclera S, iris I, pupil P) currently illuminated by the respective projected patterned laser beam261to264.

FIGS.14a) and14b) show in simplified manner frequency spectra measured by the two laser sensors104and102inFIG.12for an eye position as shown inFIG.13. The y-axis40shows the signal amplitude in relation to the frequency on the x-axis42.FIG.14a) shows the frequency spectrum in the sensing region of projected patterned laser beam264, andFIG.14b) shows the frequency spectrum in the sensing region of projected patterned laser beam262. As the eye inFIG.4is centered with respect to the four sensing regions261to264, the amplitude distribution in both SMI frequency spectra is similar, representing similar distance patterns.

If the eye E is rotated towards the sensing region corresponding to the projected patterned laser beam262, as shown inFIG.15, the Fourier spectrum of the SMI signal varies. The corresponding amplitude distribution (Fourier spectrum) changes accordingly due to the rotation of the eye E.FIGS.16a) and16b) show the changed amplitude distributions of the Fourier spectra in the two sensing regions264and262after eye rotation. The frequency spectra can be understood as a depth probability distribution. If the sensor region covers only a spot on a flat surface, the spectrum contains only a single peak at the frequency representing the optical path length between laser sensor and flat surface. If the laser spot is widened up as it is the case with the projected patterned laser beams261to264, and aims at a step-like surface with two distinct distances, the spectra will represent this by two peaks representing both distance levels. Depending on power distribution towards both surfaces, amplitude relation of both peaks will represent the integral power relation of both surface regions. The laser sensor which covers sensing region262measures a higher fraction of the iris and pupil region of the eye E. As a significant amount of photons enter through the pupil into the eye E to the retina, and hence produce SMI data correlating with an increased optical path length into the eye, this leads to an increase of the amplitudes for higher frequencies correlating with the retinal distance. On the other hand, the amplitude at high frequencies correlating with retinal distance measured by the laser sensor in the sensing region264decreases, as the photons are scattered by closer surface fractions of the eye like sclera and iris.

To calculate the pupil position and the gaze direction, the difference between the laser sensor signals obtained from different sensing regions can be used. Additional SMI information acquired by the laser sensors, like SNR or velocity, can be used to improve the system accuracy. Based on the measured time series, additional features like pupil size can be extracted from the laser sensor signals. With this information, an additional improvement of the accuracy of the pupil position estimation can be achieved. Other features that can be used to improve the system accuracy are full width half-maximum peak width or amplitude of spectral data or information from the time domain signal (e.g. thresholds, time differences, etc.).

In eye gaze angle detection, eye lashes may (partially) block the projected patterned laser beam, and consequently the signal quality may be reduced. Because the lashes provide signals from a closer distance, these false signals occurring at low frequencies in the frequency spectrum can be discarded from the real signals from the eye when looking at the frequency spectra.

An alternative to solve the eye lashes issue will be described with reference toFIG.19.

FIG.19shows a system for eye gaze angle detection which comprises laser sensors101and102. Laser sensor101comprises a laser source121which may be a VCSEL with integrated photodiode (ViP). The laser source121emits two (or more) laser beams2211and2212. In case the laser source121comprises a ViP, the ViP may comprise two (or more) laser emission regions, e.g. two (or multiple) mesas. The laser beams2211and2212are preferably emitted in a time-multiplexed manner. Optics201project the emitted laser beams2211and2212as patterned laser beams2411and2412in form of two line beams onto the eye E, as shown in the right-hand side diagram. The projected patterned laser beams2411and2412are projected at slightly shifted positions on the surface of the eye E. The optics201may be a single optical element, for example a cylinder lens, as described above. A controller18(e.g. an ASIC) including a pre-processing algorithm receives electrical signals provided by the detector integrated into the laser source121which are caused by self-mixing interference of laser-light re-entering the laser cavity of the laser source121. Pre-processing the electrical signals in controller18a1includes extracting from the frequency spectrum velocity distribution information Vel and distance distribution information Dis, wherein the velocity and distance distribution information is output to a controller18bwhich includes a post-processing algorithm (e.g. analytical/neural network) and outputs the eye gaze angle ‘ega’ measured in the horizontal direction. The laser sensor102may have the same configuration as the laser sensor101, except that laser beams2221and2222emitted by the laser source122are projected by optics202as patterned laser beams2421and2422onto the eye E to measure the eye gaze angle in the vertical direction. Again, velocity distribution information Vel and distance distribution information Dis is output from laser sensor102to the controller18bso that in combination with the velocity and distance distribution information from the laser source101, the 2D eye gaze angle of the eye E can be accurately measured.

The embodiment described with reference toFIG.19enhances the chance that one of the projected laser beams2411and2412hits the eye E, without being (partially) blocked by the lashes of the eye E. The same holds for the two projected laser beams2421and2422. Thereby, an improved accuracy for the eye gaze angle detection is obtained.

The laser sensors101and102may be arranged integrated in spectacles, as described above with reference toFIG.12.

A further modification of the previous embodiment may use multiple projected patterned laser beams, as shown inFIG.20.FIG.20shows projected patterned laser beams2411,2412,2413and2421,2422and2423projected onto the eye E. Using multiple projected laser beams, e.g. with a pattern in form of a grid of lines as shown not only is beneficial in case of lashes, but it can also be used to cover a larger area on the eye E so that there are more chances to see the pupil position in one of the laser beam lines. As already described above, other shapes of patterned laser beams than straight lines as shown inFIG.20may be used.

In the embodiment ofFIG.20, time-multiplexing can also be used to know from which laser beam line the signal comes in case of using a laser source with multiple laser emission regions and one photodiode only.

Another advantage of projecting patterned laser beams as shown inFIG.19andFIG.20is strongly reduced visibility of the laser light, because the irradiance (W/m2) on the retina reduces as compared to a system which projects a point-like spot on the eye. It is to be noted here that the human eye may see laser light even in a wavelength range from 750 to 950 nm. The laser beam projected onto the eye thus may disturb the user. Furthermore, it was observed in experiments that a laser beam line in horizontal direction is more visible to the user as compared to a laser beam line in vertical direction. In order to reduce visibility of the projected laser beam or laser beams, it is therefore beneficial to project the laser beam or laser beams onto the eye E in a diagonal line pattern, as shown inFIG.21for two laser beam lines241and242. In another example, the laser beam line pattern inFIG.20may be rotated by 45° as well.

A system comprising a laser sensor like laser sensor10inFIG.1making use of the principles of the present disclosure may also be configured for tilt detection, i.e. for detecting or measuring a tilt of an object, as shown inFIG.17for an object60which is tilted with respect to a reference, e.g. a vertical axis62. Tilt detection is already enabled by one laser sensor10according to the principles of the present teachings, by projecting a laser beam22as a one- or two-dimensional patterned laser beam26onto the object60such that a distance of the patterned laser beam26from the laser source of the laser sensor10varies along the patterned laser beam26projected on the object60, and spectrally analyzing the SMI signal and extracting from the spectrum of the SMI signal multiple frequencies indicative of multiple distances along the patterned laser beam from the laser source. The laser beam22may be wavelength modulated as described above, for example a triangle modulation of the laser beam22may be used. In case the object60does not have a tilt, the spectral width of the frequencies after FFT is minimal. When the object tilts, a broadening of the spectrum will occur, wherein the broadening increases with increasing tilt.

Tilt detection may also be used for eye gaze tracking. This is because the iris is a relatively flat part of the eye. Thus, when the eye is rotated away from the center position, the corresponding tilt of the iris can be observed in the SMI signal. Thus, tilt of the iris is a measure for the eye gaze angle. Sign information of the tilt may be derived by the observed Doppler frequencies.

Tilt detection and eye gaze tracking can be performed by extracting features from the frequency spectrum of the SMI signal, as its width, or by fitting a function to the spectrum (template matching) or by using neural networks.

One algorithm embodiment for tilt/eye gaze angle detection may consist of the following stages:1) Recording time domain SMI signal, preferably with modulation of the laser beam,2) transforming time-domain signal to frequency domain using FFT algorithm,3) extracting features from the spectrum (e.g. width at half peak maximum or transformation parameters of a template matching algorithm),4) mapping function from feature vector to tilt/eye angle.

A system comprising a laser sensor like laser sensor10inFIG.1may also be configured to detect a velocity profile of a flowing fluid.FIG.18shows a sketch of a system configured to detect a velocity profile of a flowing fluid80. The velocity profile is illustrated by arrows82, wherein the lengths of the arrows indicate the local velocity along the cross-section of the flowing fluid80. A laser sensor10emits a laser beam22which is projected into the fluid as a patterned one- or two-dimensional laser beam as described above. Micro-particles typically present in the fluid80scatter back the laser light from the position of the projected patterned laser beam26in the fluid80so that laser light from the position of the projected patterned laser beam26re-enters the laser source. Modulation of the emitted laser beam22is not necessarily required in this embodiment. It is preferred to have SMI signals from the focus position at the location of the projected patterned laser beam26only so that for this embodiment it is preferred to have a relatively large numerical aperture at the fluid side for optimal position discrimination. The strongest SMI signals will originate from the focus position, being at different positions in the fluid along the patterned laser beam26. Thereby, it is possible to determine the velocity profile in the fluid.

Generally, the principles of the present disclosure provide instead of one velocity and one distance as it is the case for normal SMI, a range of velocities and distances. Those skilled in the art will be able to make more applications based on the same SMI measurement principles of a patterned laser beam in combination with a spectral analysis.

As follows from the above description, the present disclosure also encompasses a method of detecting a plurality of velocities and/or distances, wherein the method comprises: emitting a laser beam22from a laser source12, projecting the laser beam22as a one- or two-dimensional patterned laser beam26onto an object to be examined, such that a distance of the patterned laser beam26from the laser source12varies along the patterned laser beam26projected on the object, determining a self-mixing interference signal generated by laser light of the patterned laser beam26reflected by the object back into the laser source12, and spectrally analyzing the self-mixing interference signal and extracting from the spectrum of the self-mixing interference signal multiple frequencies indicative of multiple distances of portions of the object from the laser source12, and/or multiple velocities of portions of the object with respect to the laser source12.

The teachings herein also encompass a computer program product, comprising program code means for causing a laser sensor like laser sensor10or a system like system100to carry out the steps of the method indicated before, when said computer program is carried out on a processor of the laser sensor or on a processor of the system.

A computer program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part with other hardware that may also be distributed in other forms, such as via the internet or other wired or wireless telecommunication systems.

While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.

The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.