Sensor device, appliance and method of detecting a displacement

Abstract

A sensor device (1) configured to detect a displacement (D) of a surface (15) with respect to the sensor device (1) is specified, wherein - the sensor device (1) comprises a laser (2) configured to illuminate the surface (15) with radiation (91); - the sensor device (1) is configured to detect a self-mixing interferometry (SMI) signal caused by a part of the radiation that is coupled back into a cavity (20) of the laser (2); - the sensor device (1) comprises a detector (3) configured to detect radiation reflected off the surface (15), wherein a detector signal of the detector (3) depends on the displacement of the surface (15); and - the sensor device (1) is configured to determine an absolute value and / or a frequency of the displacement (D) based on the SMI signal and to obtain a direction of the displacement (D) based on the detector signal. Further, an appliance (10) and a method of detecting a displacement are specified.

Description

[0001] Description

[0002] SENSOR DEVICE , APPLIANCE AND METHOD OF DETECTING A DISPLACEMENT

[0003] The present application relates to a sensor device , to an appliance , and to a method of detecting a displacement of a surface .

[0004] The measurement of a displacement of a reflecting surface towards an optical sensor can be measured using sel f-mixing interferometry, SMI . The displacement , for example for force sensor detection, may be determined using the number of fringes in the SMI signal . However, it is hard to detect whether the surface is moving towards or away from an SMI based sensor .

[0005] An obj ect to be achieved is to increase the reliability of a displacement measurement .

[0006] This obj ect is achieved inter alia by a sensor device , by an appliance and by a method according to the independent claims . Further developments and expediencies are subj ect of the dependent claims .

[0007] A sensor device configured to detect a displacement of a surface with respect to the sensor device is speci fied .

[0008] According to at least one embodiment of the sensor device , the sensor device comprises a laser configured to illuminate the surface with radiation . For example , the laser is configured to emit coherent radiation in the infrared spectral range , in the visible spectral range or in the ultraviolet spectral range . For example , the laser is a surface-emitting laser such as a vertical cavity surface emitting laser (VCSEL ) . Alternatively, the laser may be an edge emitting laser .

[0009] For example , the laser is a semiconductor laser chip . Thus , the laser itsel f does not comprise a housing .

[0010] According to at least one embodiment of the sensor device , the sensor device is configured to detect a sel f-mixing interferometry ( SMI ) signal caused by a part of the radiation that is coupled back into a cavity of the laser . Within the cavity the original laser radiation may interfere with the radiation coupled back into the cavity . This interference may af fect the laser emission characteristics such as the laser output power . For example , a photodiode can be used to capture a power read out signal . Alternatively or in addition, laser operation parameters such as the operation voltage or the operation current may be detected to obtain the SMI signal .

[0011] According to at least one embodiment of the sensor device , the sensor device comprises a detector configured to detect radiation reflected of f the surface . In particular, a detector signal depends on the displacement of the surface . For example , the detector is arranged laterally beside the laser when seen from the surface . For example , the detector may be configured such that the detector signal increases or decreases i f the surface is displaced towards the sensor device in response to an external force or pressure acting on the surface . For example , an average slope of the detector signal as a function of time is either positive or negative in a range from the beginning of the displacement action to the end of the displacement action .

[0012] For example , the external force is exerted by a user . For example , the surface is part of a user interface , for instance the user interface of an appliance . In other words , the user may interact with the appliance by pressing onto the surface . For example , di f ferent forces , di f ferent velocities and / or di f ferent durations of a press event may represent di f ferent user commands .

[0013] According to at least one embodiment of the sensor device , the sensor device is configured to determine an absolute value and / or a velocity of the displacement based on the SMI signal and to obtain a direction of the displacement based on the detector signal .

[0014] Thus , the detector signal may be used to determine whether the surface is moving towards the sensor device or moving away from the sensor device . For example , a displacement of the surface towards the sensor device may represent a button press event whereas a displacement away from the surface may represent a button release event of a user interface .

[0015] In at least one embodiment of the sensor device , the sensor device is configured to detect a displacement of a surface with respect to the sensor device , wherein the sensor device comprises a laser configured to illuminate the surface with a radiation . The sensor device is configured to detect a sel fmixing interferometry signal caused by a part of the radiation that is coupled back into a cavity of the laser .

[0016] The sensor device comprises a detector configured to detect radiation reflected of f the surface , wherein a detector signal depends on the displacement of the surface . The sensor device is configured to determine an absolute value and / or a velocity of the displacement based on the SMI signal and to obtain a direction of the displacement based on the detector signal .

[0017] As the detector may provide additional information in addition to the SMI signal , this additional information may be used to detect whether the surface is moving towards or moving away from the sensor device .

[0018] Thus , the SMI signal itsel f does not necessarily have to provide information regarding the direction of the displacement .

[0019] In particular, it is not required for the SMI signal to exhibit a slope asymmetry between the rising edge and the falling edge of an interference fringe . Such an asymmetry could be obtained using a beam shaping and / or beam steering optical element arranged between the sensor device and the surface , for example between the laser and the surface . However, integration of an optical component such as a lens increases costs and requires additional space in a direction perpendicular to the surface . The sensor device described above , in contrast , does not require additional optical components .

[0020] Compared to a sensor device that does not rely on sel f-mixing interferometry but only a conventional proximity signal , accuracy is increased as the accuracy is predominantly defined by the wavelength of the radiation emitted by the laser rather than by the signal-to-noise ratio . Furthermore , the sensor device can be operated at di f ferent working distances between the sensor device and the surface . For example , the working distance is in a range from 200 pm to 5 mm . For example , the driving current for the laser may be increased for higher working distances so that the higher working distance does not result in decreasing accuracy .

[0021] According to at least one embodiment of the sensor device , the SMI signal is obtained by monitoring an output of the laser . Thus , a displacement of the surface may be determined by monitoring the laser output , wherein the absolute level of displacement may be detected by counting the number of fringes occurring during the displacement , for example during a button press or button release event . Here , each fringe corresponds to a displacement of X / 2 wherein X is the wavelength of the radiation emitted by the laser . Alternatively or in addition, the velocity of the displacement can be derived from a frequency of the fringes in the SMI signal .

[0022] According to at least one embodiment of the sensor device , the laser output is monitored by the detector . Thus , the detector provides both the SMI signal and the detector signal used to obtain the direction of the displacement . For example , the direction of the displacement is obtained from an envelope function of the fringes occurring during the displacement . In other words , averaging over the fringes of the SMI signal allows to obtain information on the direction of the displacement .

[0023] According to at least one embodiment of the sensor device , the laser output is monitored by a further detector . Thus , the SMI signal used to determine the absolute value and / or the frequency of the displacement and the detector signal used to obtain the direction of the displacement may be obtained by two separate detectors .

[0024] According to at least one embodiment of the sensor device , the further detector is arranged on a side of the cavity that faces away from the surface . Radiation that exits from the cavity on a rear side of the laser facing away from the surface can be detected by the further detector . For example , the further detector is integrated into the laser . Thus , the laser and the further detector may be integrated in a common semiconductor chip . Alternatively, the laser may be arranged as a separate device between the further detector and the surface .

[0025] Such a further detector arranged underneath the laser when seen from the surface allows to obtain the interference fringes of the SMI signal during a displacement with high accuracy . However, the further detector cannot be used to obtain a reflected sense information in order to obtain the direction of the displacement as the laser is arranged in the beam path from the surface to the further detector . This directional information can be obtained from the detector provided in addition to the further detector .

[0026] According to at least one embodiment of the sensor device , the further detector is arranged laterally beside the laser when seen from the surface . For example , the further detector is arranged at a smaller distance from the laser than the detector . Thus , the further detector arranged closer to the laser may be used to obtain the SMI signal whereas the detector arranged further away from the laser is used to obtain the direction of the displacement . For example , the further detector is a photodiode .

[0027] According to at least one embodiment of the sensor device , the SMI signal is obtained by monitoring a laser operation parameter . For example , a laser current or a laser voltage may be monitored during operation of the sensor device . These laser operation parameters likewise allow to obtain an SMI signal providing information on the absolute value or the velocity of the displacement of the sensor device from the surface .

[0028] According to at least one embodiment of the sensor device , the sensor device is configured as a force or pressure sensor . I f the sensor device is used for a button, the sensor device may distinguish between a button press event and a button release event which is for example triggered by a user .

[0029] Further, an appliance is speci fied . According to at least one embodiment of the appliance , the appliance comprises a sensor device as described above .

[0030] According to at least one embodiment of the appliance , the surface is part of a user interface of the appliance . For example , the appliance is an electronic device , for instance a handheld electronic device such as a smartphone or a tablet or a wearable electronic device such as earbuds .

[0031] The sensor device is particularly suited for small electronic devices due to the low height of the sensor device ( extension perpendicular to the surface ) . Further, user interactions can be detected with particularly high accuracy . However, the sensor device is generally suited for appliances comprising a user interface , for example in a vehicle or in other appliances for consumer or industrial applications .

[0032] Further, a method of detecting a displacement of a surface is speci fied . The steps do not necessarily have to be performed in the indicated order .

[0033] According to at least one embodiment of the method, the method includes a step of illuminating the surface with a radiation from a laser . Thus , the radiation is coherent .

[0034] According to at least one embodiment of the method, the method includes a step of detecting a sel f-mixing interferometry signal caused by a part of the radiation that is coupled back into a cavity of the laser . The SMI signal may be obtained by monitoring a laser emission parameter or a laser operation parameter, for instance .

[0035] According to at least one embodiment of the method, the method includes a step of obtaining a detector signal resulting from a part of the radiation that is reflected of f the surface , wherein the detector signal depends on the displacement of the surface . Thus , the detector signal increases or decreases i f the surface is displaced with respect to its idle position .

[0036] According to at least one embodiment of the method, the method includes a step of determining an absolute value of the displacement based on the SMI signal and determining a direction of the displacement based on the detector signal . Consequently, the detector signal depending on the displacement of the surface may be used as additional information in addition to the number or frequency of interference fringes of the SMI signal , which is used to determine the absolute value or the velocity of the displacement with high accuracy .

[0037] In at least one embodiment of the method, the method includes the steps of illuminating the surface with a radiation from a laser, detecting a sel f-mixing interferometry signal caused by a part of the radiation that is coupled back into a cavity of the laser, obtaining a detector signal resulting from a part of the radiation that is reflected of f the surface , wherein the detector signal depends on the displacement of the surface , and determining an absolute value and / or a velocity of the displacement based on the SMI signal and determining a direction of the displacement based on the detector signal .

[0038] According to at least one embodiment of the method, the absolute value of the displacement is determined based on a number of interference fringes occurring during the displacement . Consequently, a displacement in the order of micrometers can be determined with high accuracy based on a displacement of X / 2 for each interference fringe detected during displacement . Alternatively or in addition the frequency of the interference fringes can be used to determine the velocity of the displacement with high accuracy .

[0039] According to at least one embodiment of the method, the SMI signal and the detector signal are obtained using a common detector . Thus , a single detector provided in addition to the laser may be suf ficient to determine the absolute value and / or the velocity of the displacement as well as the direction of the displacement .

[0040] According to at least one embodiment of the method, the direction of the displacement is obtained by using an envelope function of the interference fringes . In other words , averaging over the interference fringes provides a signal that increases or decreases with decreasing distance of the surface from the laser .

[0041] According to at least one embodiment of the method, the SMI signal is obtained separately from the detector signal . For example , the SMI signal can be obtained by monitoring the laser operation characteristics such as the operation current or operation terminal voltage or by monitoring the laser output using a further detector provided in addition to the detector that provides the detector signal .

[0042] The method can in particular be performed using the sensor device described above . Consequently, features described in connection with the sensor device may also apply to the method and vice versa .

[0043] Features described above in connection with at least one embodiment of the method or the sensor device or the appliance can be combined with other features described in connection with at least one embodiment of the method or the sensor device or the appliance unless they are contradictory .

[0044] In the exemplary embodiments and figures similar or similarly acting constituent parts are provided with the same reference signs . Generally, only the di f ferences with respect to the individual exemplary embodiments are described . Unless speci fied otherwise , the description of a part or feature in one exemplary embodiment applies to a corresponding part or feature in another exemplary embodiment as well .

[0045] In the Figures :

[0046] Figure 1 illustrates an exemplary embodiment of an appliance with a sensor device ;

[0047] Figures IB and 1C show examples of a signal S as a function of time in arbitrary units ;

[0048] Figures ID and IE show signals S for a first detector ( curves 85 ) and a second detector ( curves 86 ) as a function of a distance z between the sensor device and the surface ( Figure ID) and as a function of time during displacements ;

[0049] Figures 2A and 2B show an exemplary embodiment of a sensor device in top view ( Figure 2A) and in perspective view ( Figure 2B ) ;

[0050] Figure 3 shows an exemplary embodiment of an appliance with a sensor device in sectional view;

[0051] Figures 4A and 4B show exemplary embodiments of an appliance with a sensor device in sectional view; and

[0052] Figure 5 shows an exemplary embodiment of a method of detecting a displacement of a surface .

[0053] The elements illustrated in the figures and their si ze relationships among one another are not necessarily true to scale . Rather, individual elements or layer thicknesses may be represented with an exaggerated si ze for the sake of better representability and / or for the sake of better understanding .

[0054] Figure 1A illustrates an exemplary embodiment of a sensor device 1 configured to detect a displacement of a surface 15 of an appliance 10 with respect to the sensor device 1 . The sensor device 1 comprises a laser 2 configured to illuminate the surface 15 with a radiation 91 . The sensor device is configured to detect a sel f-mixing interferometry, SMI , signal cause by a part of the radiation that is coupled back into a cavity of the laser 1 ( reflected radiation 92 ) .

[0055] The sensor device 1 further comprises a detector 3 configured to detect radiation reflected of f the surface 15 ( reflected radiation 93 ) , wherein a detector signal depends on the displacement of the surface 15 . The sensor device 1 is configured to determine an absolute value of the displacement D based on the SMI signal and to obtain a direction of the displacement D based on the detector signal .

[0056] The detector 3 is designed such that the signal signi ficantly changes i f a displacement D of the surface 15 with respect to the detector 3 in the expected order of magnitude occurs . In other words , the radiation impinging onto the detector depends on the displacement D of the surface 15 . For example , the displacement D to be detected is at least 1 pm or at least 2 pm or at least 5 pm and / or at most 500 pm at most 200 pm or at most 100 pm .

[0057] The sensor device 1 further comprises a controller circuit 5 . For example , the control circuit 5 is an application-speci fic integrated circuit (AS IC ) . The Figure merely schematically illustrates the functional elements . The detector 3 and the control circuit board 5 may also be part of the same AS IC . Thus , the detector 3 may be integrated into the control circuit 5 .

[0058] In the exemplary embodiment shown in Figure 1A, the SMI signal is obtained by monitoring an output of the laser 2 using the detector 3 .

[0059] Figure IB illustrates an example of a detector signal S as a function of time measured during a displacement action .

[0060] The signal S can be the counts of an analog front end (AFE ) , for example .

[0061] During the displacement action interference fringes 89 occur due to alternating constructive and destructive interference between the original radiation generated in the laser 1 and the radiation 92 coupled back from the surface 15 into the cavity 20 of the laser .

[0062] As illustrated using arrow 81 , a modulation of the signal , i . e . the signal di f ference between the maximum and the minimum of the signal S in the interference fringes 89 , amounts to 0 . 3% in the example shown . By counting the number of interference fringes 89 , the absolute value of the displacement D caused by an external force F can be obtained with a high precision . Each interference fringe corresponds to a displacement of X / 2 , wherein X is the wavelength of the radiation 91 emitted by the laser 2 . The velocity of the displacement D can be derived from the frequency of the displacement . Arrow 82 further illustrates that the signal S during the displacement averaged over the interference fringes 89 decreases . Depending on the proximity function of the detector 3 used, this decrease may either be assigned to a displacement towards the sensor device 1 or to a displacement in a direction away from the sensor device 1 .

[0063] Figure 1C illustrates the signal S as a function of time during a press event 83 , where the displacement of the surface 15 with respect to the sensor device 1 decreases , and a subsequent release event 84 , where the displacement D increases . The direction of the displacement D can be derived from the slope of the envelope function during the displacements . During the press event 83 the slope of the envelope function is positive whereas the slope is negative during the release event 84 .

[0064] However, the slope of the envelope function depends on the proximity curve of the detector used . This is illustrated in Figures ID and IE for a first detector signal 85 and a second detector signal 86 .

[0065] As Figure ID shows , the signal S of the first detector 85 increases i f the distance z between the sensor device and the surface 15 is reduced, starting from an idle position 87 of the surface 15 . The signal S of the second detector 86 , in contrast decreases i f the distance z is reduced to values smaller than the idle position 87 .

[0066] Accordingly, the envelope function of the SMI signals of the first detector 85 and the second detector 86 have opposite signs as Figure IE illustrates . In both cases , however, the envelope function of the detector 3 can be used to determine the direction of the displacement .

[0067] This determination of the direction of the displacement neither requires an asymmetry of the interference fringes 89 nor complex computation algorithms .

[0068] By means of the sensor device 1 a high accuracy can be obtained for determining absolute values of the displacement D in the micrometer regime and / or a velocity of the displacement D . At the same time , the sensor device 1 has a comparably low complexity and can be produced at low cost . Further, a high degree of robustness can be obtained .

[0069] Furthermore , the sensor device 1 can be operated for di f ferent distances of the sensor device 1 from the surface 15 .

[0070] The sensor device 1 may be embodied as a surface mounted device ( SMD) so that all electrical contacts may be provided at a rear side of the sensor device 1 configured for being mounted to a substrate 6 of an appliance 10 , for instance .

[0071] The electrical contacts 17 of the sensor device 1 are depicted in Figure 2B .

[0072] For example , the appliance 10 may be a handheld or wearable electronic device , wherein the surface 15 is part of a user interface such as a button surface . The sensor device 1 may be used to detect user interactions such as button press or button release events and to distinguish between these events . Further, the force exerted by the user may be derived so that a soft press and a hard press may result in di f ferent sensor signals and represent di f ferent user commands . This may also apply for di f ferent velocities of the displacement

[0073] D .

[0074] Further, the sensor device 1 may include additional functions such as ambient light sensing, flicker light sensing or temperature sensing .

[0075] The sensor device 1 does not require any optical elements for beam shaping of the radiation 91 . In particular, there is no beam shaping element such as a lens between the laser 2 and the surface 15 . Thus , the height of the sensor device perpendicular to the surface 15 can be particularly small .

[0076] This facilitates its integration into tiny appliances 10 such as earbuds .

[0077] However, the sensor device 1 may also be used for other applications , for example in consumer electronics , automotive applications , or industrial applications , for example for a surface button and / or a force sensor .

[0078] Figures 2A and 2B illustrate an exemplary embodiment of a sensor device 1 in a top view and a perspective view .

[0079] The sensor device 1 according to this exemplary embodiment essentially corresponds to the sensor device described in connection with Figures 1A to IE .

[0080] As illustrated in Figure 2B, the electrical contacts 17 for an external electrical connection to the sensor device 1 may be provided on a side of the carrier 4 that faces away from the laser 2 . As illustrated in Figures 2A and 2B, the sensor device 1 may comprise one or more photosensitive areas in addition to the detector 3 but a single photosensitive area acting as the detector may be suf ficient . For example , the control circuit 5 and the detector 3 and / or further photosensitive areas are integrated in a common AS IC .

[0081] Figure 3 illustrates a further exemplary embodiment of a sensor device 1 . This exemplary embodiment substantially corresponds to the exemplary embodiment described in connection with Figures 1A to IE .

[0082] In contrast , the SMI signal is obtained by monitoring the laser operation characteristics , for example the laser terminal voltage , using a voltage meter 7 . The operation voltage at the terminals of the laser is also af fected by sel f-mixing interferometry ef fects , so that the signal of the voltage meter 7 may likewise comprise interference fringes as described in connection with Figures 1A to IE . Alternatively or in addition, the monitored laser operation characteristics may comprise or consist of a laser operation current . For example , an ampere meter is used to monitor the laser operation current .

[0083] The direction of the displacement D can be obtained from the signal of the detector 3 as described in connection with Figures 1A to IE . Thus , the detector 3 does not have to resolve any SMI interference fringes , but merely serves to provide additional proximity information as described in connection with Figures 1A to IE in order to determine the direction of the displacement D . The exemplary embodiments shown in Figures 4A and 4B substantially correspond to the exemplary embodiment described in connection with Figures 1A to IE .

[0084] In contrast , the sensor device 1 comprises a further detector 35 .

[0085] In the exemplary embodiment of Figure 4A, the further detector 35 is arranged on a side of the cavity 20 that faces away from the surface 15 . For example , the further detector 35 is integrated in the same semiconductor chip as the laser 2 . Alternatively, the further detector 35 may be a separate semiconductor chip arranged below the laser 2 . The further detector 35 may be used to obtain an SMI signal with high accuracy . As the laser 2 is arranged in the beam path between the further detector 35 and the surface 15 , the further detector 35 is not able to measure a proximity signal in parallel with an SMI signal .

[0086] In the exemplary embodiment of Figure 4B, the further detector 35 is arranged laterally beside the laser 2 . The further detector 35 is arranged closer to the laser 2 than the detector 3 . For example , the further detector 3 is a photodiode configured to obtain the SMI signal .

[0087] In the exemplary embodiments of Figures 4A and 4B, detector 3 can be used to determine the direction of the displacement as described in connection with Figures 1A to IE . Unlike in Figures 1A to IE , the signal of the detector 3 does not have to resolve the interference fringes caused by sel f-mixing interferometry as the SMI signal is obtained via the further detector 35 . Figure 5 illustrates an exemplary embodiment of a method of detecting a displacement of a surface . In the following, reference numerals regarding structural features refer to the previous figures for easier understanding .

[0088] In a step S I the surface 15 is illuminated with radiation 91 from a laser 2 .

[0089] In a step S2 a sel f-mixing interferometry signal caused by a part of the radiation that is coupled back into a cavity 20 of the laser 2 is detected .

[0090] In a step S3 a detector signal resulting from a part of the radiation that is reflected of f the surface 15 is obtained, wherein the detector signal depends on the displacement D of the surface 15 .

[0091] In a step S4 an absolute value or a velocity of the displacement is determined based on the SMI signal and a direction of the displacement is determined based on the detector signal .

[0092] The steps do not have to be performed in the described order . For example , steps S2 and S3 may be performed simultaneously or in an inverted order .

[0093] As described in connection with the previous exemplary embodiments , the absolute value or the velocity of the displacement D can be determined based on the number or the frequency of interference fringes 89 occurring during the displacement D . The SMI signal and the detector signal can be obtained using the same detector so that the sensor device 1 only requires a single detector . The direction of the displacement can be obtained by using an envelope function of the interference fringes as described above . The absolute value of the displacement can be obtained with high accuracy by determining the number of interference fringes during the displacement action .

[0094] The SMI signal may also be obtained separately from the detector signal , for example by means of a further detector or by monitoring the laser operation characteristics such as the operation current .

[0095] This patent application claims the priority of German patent application 10 2024 100 623 . 4 , the disclosure content of which is hereby incorporated by reference .

[0096] The invention described herein is not restricted by the description given with reference to the exemplary embodiments . Rather, the invention encompasses any novel feature and any combination of features , including in particular any combination of features in the claims , even i f this feature or this combination is not itsel f explicitly indicated in the claims or exemplary embodiments .

[0097] References

[0098] 1 sensor device

[0099] 10 appliance

[0100] 15 surface

[0101] 17 electrical contact

[0102] 2 laser

[0103] 20 cavity

[0104] 3 detector

[0105] 35 further detector

[0106] 4 carrier

[0107] 5 controller circuit

[0108] 6 substrate

[0109] 7 voltage meter

[0110] 81 modulation

[0111] 82 arrow

[0112] 83 press event

[0113] 84 release event

[0114] 85 signal of first detector

[0115] 86 signal of second detector

[0116] 87 idle position

[0117] 89 interference fringe

[0118] 91 radiation

[0119] 92 reflected radiation

[0120] 93 reflected radiation

[0121] D displacement

[0122] F external force

[0123] S I , S2 , S3 , S4 step

Claims

Claims1. A sensor device (1) configured to detect a displacement(D) of a surface (15) with respect to the sensor device (1) , wherein- the sensor device (1) comprises a laser (2) configured to illuminate the surface (15) with a radiation (91) ;- the sensor device (1) is configured to detect a self-mixing interferometry (SMI) signal caused by a part of the radiation that is coupled back into a cavity (20) of the laser (2) ;- the sensor device (1) comprises a detector (3) configured to detect radiation reflected off the surface (15) , wherein a detector signal of the detector (3) depends on the displacement of the surface (15) ; and- the sensor device (1) is configured to determine an absolute value and / or a velocity of the displacement (D) based on the SMI signal and to obtain a direction of the displacement (D) based on the detector signal.

2. The sensor device according to claim 1, wherein the SMI signal is obtained by monitoring an output of the laser ( 2 ) .

3. The sensor device according to claim 2, wherein the laser output is monitored by the detector (3) .

4. The sensor device according to claim 2, wherein the laser output is monitored by a further detector (35) .

5. The sensor device according to claim 4, wherein the further detector (35) is arranged on a side of the cavity (20) that faces away from the surface (15) .

6. The sensor device according to claim 4, wherein the further detector (35) is arranged laterally beside the laser (2) , wherein the further detector (35) is arranged at a smaller distance from the laser (2) than the detector (3) .

7. The sensor device according to claim 1, wherein the SMI signal is obtained by monitoring a laser operation parameter.

8. The sensor device according to any one of the preceding claims , wherein the sensor device (1) is configured as a force or pressure sensor.

9. An appliance (10) comprising a sensor device (1) according to any one of the preceding claims, wherein the surface (15) is part of a user interface.

10. The appliance according to claim 9, wherein the appliance (10) is a wearable or handheld electronic device.

11. A method of detecting a displacement (D) of a surface (15) , comprising the steps of: a) illuminating the surface (15) with a radiation (91) from a laser ( 2 ) ; b) detecting a self-mixing interferometry (SMI) signal caused by a part of the radiation (92) that is coupled back into a cavity of the laser (2) ; c) obtaining a detector signal resulting from a part of the radiation (93) that is reflected off the surface (15) ,wherein the detector signal depends on the displacement (D) of the surface (15) ; and d) determining an absolute value and / or a velocity of the displacement (D) based on the SMI signal and determining a direction of the displacement (D) based on the detector signal .

12. The method according to claim 11, wherein the absolute value of the displacement is determined based on a number of interference fringes (89) occurring during the displacement and / or the velocity of the displacement is determined based on a frequency of interference fringes (89) occurring during the displacement.

13. The method according to claim 11 or 12, wherein the SMI signal and the detector signal are obtained using a common detector (3) .

14. The method according to claims 12 and 13, wherein the direction of the displacement is obtained by using an envelope function of the interference fringes.

15. The method according to claim 11 or 12, wherein the SMI signal is obtained separately from the detector signal.

16. The method according to any one of claims 11 to 15, wherein the method is performed using a sensor device (1) according to any one of claims 1 to 8.

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