Optical sensor module using polarized light

Abstract

An optical sensor module using polarized light such that, in some cases, crosstalk effects caused by light reflected from a cover glass or from a thin smudge layer on the cover glass can be eliminated or at least reduced by directing light of a first polarization toward a target through a cover glass and selectively detecting light of a second polarization orthogonal to the first polarization in the module.

Description

Technical Field

[0001] The present disclosure relates to an optical sensor module using polarized light. Background Art

[0002] Some handheld computing devices, such as smartphones, may provide a variety of different optical functions, such as one-dimensional (1D) or three-dimensional (3D) gesture detection, 3D imaging, time-of-flight or proximity detection, ambient light sensing, and / or front-facing two-dimensional (2D) camera imaging.

[0003] For example, optical proximity sensing systems are based on emitting light that is reflected by one or more objects in a scene. The reflected light is detected by a sensor, and the photo-generated electrons are analyzed to determine, for example, whether an object is present nearby. Summary of the invention

[0004] The present disclosure describes optical sensor modules that use polarized light. In some cases, crosstalk effects caused by light reflected from the cover glass or from a thin smudge layer on the cover glass can be eliminated or at least reduced by directing light of a first polarization through a cover glass toward a target and selectively detecting light of a second polarization orthogonal to the first polarization in the module.

[0005] For example, in one aspect, the present disclosure describes a device including an optical module. The optical module includes an emission channel and a detection channel. The emission channel is operable to emit light having a first polarization. The emission channel includes an optical transmitter operable to generate light of a certain wavelength. The detection channel is operable to selectively detect light of a second polarization orthogonal to the first polarization. The detection channel includes an optical receiver operable to detect light of the same wavelength as the emitter.

[0006] Some embodiments include one or more of the following features. For example, in some cases, the device includes a cover glass, and the optical module is operable to transmit at least some of the light generated by the light emitter through the cover glass, and to detect at least some of the light that is transmitted through the cover glass and reflected back through the cover glass by the object toward the module by the light receiver. In some cases, the emission channel is operable to emit light having a first linear polarization, and the detection channel is operable to selectively detect light having a second linear polarization orthogonal to the first linear polarization. In some embodiments, the emission channel is operable to emit light having a first circular polarization, and the detection channel is operable to selectively detect light having a second circular polarization orthogonal to the first circular polarization.

[0007] In some cases, the device includes a polarizer disposed in an emission channel so as to intersect light generated by the light emitter. In some cases, the light emitter includes an optical polarizer integrated as part of the light emitter. The light emitter may include, for example, a VCSEL structure having an asymmetric aperture, a reflection grating, or a sub-wavelength reflection grating. In some cases, the device includes a polarization analyzer operable to selectively allow only light having a second polarization to pass through for detection by the light receiver. In some embodiments, the light receiver includes an optical polarizer integrated as part of the light receiver. In some cases, the optical module is disposed behind a cover glass of the portable computing device.

[0008] The present disclosure also describes a method that includes transmitting light from an optical module toward an object through a cover glass of a portable computing device, wherein the light emitted through the cover glass has a first polarization. The method includes receiving, in the module, light that is reflected by the object and passes through the cover glass, wherein the light reflected by the object includes light having the first polarization and light having a second polarization orthogonal to the first polarization. The method also includes selectively detecting, in the module, light that is reflected by the object and has the second polarization.

[0009] Some embodiments include one or more of the following features. For example, in some cases, the method includes converting light emitted by a light emitting element into polarized light having a first polarization, wherein the conversion occurs before transmitting the light through a cover glass. In some cases, the method includes selectively blocking light that is reflected by an object and passes through a cover glass, wherein the blocked light has a first polarization, and wherein the blocking occurs before detecting light having a second polarization.

[0010] In some embodiments, the light transmitted through the cover glass has a first linear polarization, and the selectively detected light has a second linear polarization orthogonal to the first linear polarization. In some embodiments, the light transmitted through the cover glass has a first circular polarization, and the selectively detected light has a second circular polarization orthogonal to the first circular polarization.

[0011] In some cases, the cover glass reflects some light having the first polarization back into the module, and the method includes blocking the light reflected by the cover glass before detecting the light having the second polarization.

[0012] The method may include, for example, using light detected in the module for proximity sensing or for other applications.

[0013] Other aspects, features, and various advantages will become apparent from the following detailed description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 An example of a photovoltaic module is shown.

[0015] Figure 2 A first example of an optoelectronic module using polarized light is shown.

[0016] Figure 3 Shows Figure 2 Example of polarization directions of the emission and detection channels of an optoelectronic module.

[0017] Figure 4 A second example of an optoelectronic module using polarized light is shown.

[0018] Figure 5 A third example of an optoelectronic module using polarized light is shown. DETAILED DESCRIPTION

[0019] Figure 1 An example of an optoelectronic module 100 is shown, which includes a light emission channel and a light detection channel. The light emitter 106 and the light receiver 108 can be mounted on, for example, a printed circuit board (PCB) or other substrate. The light emitter 106 is operable to emit light of a specific wavelength or wavelength range. In some embodiments, the light emitter 106 is implemented as one or more laser diodes or vertical cavity surface emitting lasers (vertical cavity surface emitting laser, VCSEL).

[0020] In some cases, the inner wall 115 provides optical isolation between the two chambers of the module, i.e., the light emission chamber (channel) and the light detection chamber (channel). The optical components may include one or more corresponding passive optical elements (e.g., lenses) for each channel. Light from the emitter 106 is directed out of the module 100, and if it is reflected by the object 150 back into the detection channel of the module, it may be sensed by the light receiver 108.

[0021] The optical receiver 108 may include, for example, a photodetector (e.g., a photodiode or an array of spatially distributed photosensitive elements (e.g., pixels) and may also include logic and other electronics to read and process signals from the photodetector. The pixels and other circuits may be implemented, for example, in an integrated semiconductor chip.

[0022] The transmitter 106 and the optical receiver 108 can be electrically connected to the PCB, for example, by a conductive pad or wire bond. The PCB can be electrically connected to other components in the host device (e.g., a smart phone). The design of the smart phone and other portable computing devices cited in this disclosure may include one or more processors, one or more memories (e.g., RAM), a storage device (e.g., a disk or flash memory), a user interface (which may include, for example, a keyboard, a TFTLCD or OLED display, a touch or other gesture sensor, a camera or other optical sensor, a compass sensor, a 3D magnetometer, a three-axis accelerometer, a three-axis gyroscope, one or more microphones, etc., and software instructions for providing a graphical user interface), interconnections between these elements (e.g., a bus), and an interface for communicating with other devices (which may be wireless, such as GSM, 3G, 4G, CDMA, WiFi, WiMax, Zigbee or Bluetooth; and / or wired, such as through an Ethernet local area network, a T-1 Internet connection, etc.).

[0023] The control and processing circuitry (e.g., electronic control circuitry) 140 of the sensor may be implemented as, for example, one or more integrated circuits in one or more semiconductor chips with appropriate digital logic and / or other hardware components (e.g., readout registers; amplifiers; analog-to-digital converters; clock drivers; timing logic; signal processing circuitry; and / or a microprocessor). The control and processing circuitry 140 and associated memory 142 may reside in the same semiconductor chip as the optical receiver 108, or in one or more other semiconductor chips. In some cases, the control and processing circuitry 140 may be external to the module 100; for example, the control and processing circuitry may be integrated into a processor of a host device in which the module 100 is disposed.

[0024] In some cases, the host device (e.g., a smart phone or other portable computing device) in which the module 100 is integrated includes a transmissive cover (e.g., a cover glass) 132, under which the module 100 is disposed. The cover glass 132 can generate light reflections from both surfaces thereof, and in some cases, the reflected light can strike the light receiver 108, thereby generating background signals (i.e., cover glass crosstalk), which can deteriorate the signal of the optical sensor. In addition, stains (i.e., blurred or smeared marks, such as fingerprints or dirt or other contaminants) may appear on the cover glass 132. The stains can scatter the transmitter light, thereby generating additional background signals (smudge crosstalk). In addition, the stain crosstalk can change over time, for example, as the amount of stain and its distribution change.

[0025] The inventors of the present disclosure have recognized that, in some cases, when light from the emitter 106 is reflected by the polished surface of the cover glass or scattered by a thin layer of dirt, its polarization is substantially maintained; while when the emitter light strikes certain targets, such as skin or other body scattering targets (i.e., targets through which the emitter light penetrates), its polarization is scrambled into unpolarized light. Therefore, by directing light of a first polarization through the cover glass toward the target, and selectively detecting light of a second polarization orthogonal to the first polarization in the module, the crosstalk effect caused by light reflected from the cover glass or from dirt on the cover glass can be eliminated or at least reduced.

[0026] Figure 2 , which includes a first polarizer 202 disposed on the emitter 106 so as to intersect the path of the light from the emitter 106. The first polarizer 202 can operate as a filter that allows light waves of a specific polarization to pass through while blocking light waves of other polarizations. Thus, the first polarizer 202 can filter a light beam 206 emitted by the emitter 106 and having an undefined or mixed polarization into a light beam 208 having a well-defined polarization. In some embodiments, the first polarizer 202 is a linear polarizer, but in some cases it can be a circular polarizer.

[0027] The second polarizer 204 may be referred to as a polarization analyzer, which is operable to block all light except light having a polarization orthogonal to the light passing through the first polarizer 202 . Figure 3 An example of polarizers 202, 204 implemented as linear polarizers is shown, and the polarization directions (orientations) of the first and second polarizers 202, 204 are shown to be orthogonal to each other. If the polarizers 202, 204 are implemented as circular polarizers, one polarizer may have a right-handed orientation and the other polarizer may have a left-handed orientation.

[0028] In operation, unpolarized light 206 generated by emitter 106 is polarized into light 208 having a first polarization direction. Most of light 208 passes through cover glass 132, and some of this light 208 may be reflected by object 150 back to module 100. In many cases, object 150 disrupts incident light 208 into unpolarized reflected light 210. Figure 2 As shown, some reflected light 210 passes through cover glass 132 toward light receiver 108. Polarization analyzer 204 selectively allows only light 212 having a polarization direction orthogonal to that of light 208 to pass for detection by light receiver 108. The detected signal may be used for proximity sensing or other purposes.

[0029] Some of the polarized light 208 incident on the cover glass 132 may be reflected by one of the cover glass surfaces (or scattered by a thin layer of dirt on the cover glass surface). This reflected (or scattered) light 214, 216 substantially maintains the polarization of the light 208. Therefore, most or all of the reflected (or scattered) light 214, 216 will not pass through the polarization analyzer 204 and therefore will not be detected by the optical receiver 108. In this way, crosstalk effects caused by light reflected from the cover glass 132 or scattered by dirt on the cover glass can be eliminated or at least strongly reduced.

[0030] Preferably, the polarization of the light emitted from the module is aligned so that the light beam 208 generated for transmission out of the module 100 is p-polarized relative to the cover glass 132. The use of p-polarized light can better transmit the cover glass than using s-polarized light. In addition, at the Brewster angle, no p-polarized light is reflected from the surface of the cover glass. Therefore, by using p-polarized light, the amount of light reflected back to the receiver 108 by the cover glass 132 can be further reduced.

[0031] In some cases, each of polarizers 202, 204 is implemented as a grating.For a circular polarizer, a quarter wave plate can be placed after the linear polarizer so that the transmission axis of the linear polarizer is halfway between the fast and slow axes of the quarter wave plate (ie, 45°).

[0032] In some cases, optical polarizers can be integrated as part of an optical transmitter and an optical receiver, respectively, rather than being separate components from the transmitter and receiver. Figure 4 As shown, module 200 includes a polarized light transmitter 306 and a polarization sensitive receiver 308. In this case, the polarization direction of light 309 generated by transmitter 306 is orthogonal to the polarization direction of light detected by receiver 308.

[0033] In operation, most of the polarized light 309 generated by the emitter 306 passes through the cover glass 132, and some of this light 309 can be reflected by the object 150 back to the module 300. In many cases, the object 150 disrupts the incident light 309 into unpolarized reflected light 310. Figure 3 As shown, some reflected light 310 passes through cover glass 132 toward light receiver 308, which selectively detects only light having a polarization direction orthogonal to the polarization direction of light 309. The detected signal may be used for proximity sensing or other purposes.

[0034] Some of the polarized light 309 incident on the cover glass 132 may be reflected by one of the cover glass surfaces (or scattered by a thin layer of dirt on the cover glass surface). This reflected (or scattered) light 314, 316 substantially maintains the polarization of the light 208. Therefore, most or all of the reflected (or scattered) light 314, 316 will not be detected by the light receiver 308. In this way, crosstalk effects caused by light reflected from the cover glass 132 or light scattered by dirt on the cover glass can be eliminated or at least reduced.

[0035] For example, various VCSEL structures can be used to generate linearly polarized light. Therefore, in some cases, the VCSEL structure includes an asymmetric aperture. For example, the VCSEL structure can have an elliptical cross-section gain section so that the VCSEL device operates with an elliptical beam. The elliptical structure causes asymmetric thermal and electrical stresses applied to the VCSEL crystal structure, which causes a refractive index asymmetry. This optical asymmetry causes the VCSEL to emit light with linear polarization that is aligned with the refractive index asymmetry.

[0036] In other embodiments, a VCSEL structure operable to produce linearly polarized light includes one or more reflective gratings. In some cases, these structures can produce a substantially symmetrical circular output beam (i.e., a beam having a circular or substantially circular cross-section). The VCSEL can be top-emitting or bottom-emitting. In some embodiments, the reflective grating is functionally combined with a distributed Bragg reflector (DBR). In other cases, the reflective grating can be advantageously used even without an associated DBR.

[0037] In other embodiments, a VCSEL structure operable to generate linearly polarized light includes a sub-wavelength reflective grating.

[0038] Furthermore, in some cases, instead of providing only a single VCSEL as the emitter 106, 306, an array of VCSELs may be provided as the emitter.

[0039] In some embodiments, one of the optical transmitter or the optical receiver has a separate polarizer associated therewith, while the other of the optical transmitter or the optical receiver has an integrated optical polarizer. Figure 5 An example is shown in which the optical transmitter 406 is implemented as a VCSEL that produces polarized light, and the polarization analyzer 404 is disposed in front of the optical receiver 408. In operation, most of the polarized light 409 generated by the transmitter 406 passes through the cover glass 132, and some of this light 409 can be reflected by the object 150 back to the module 400. In many cases, the object 150 disturbs the incident light 409 into non-polarized reflected light 410. Figure 5 As shown, some reflected light 410 passes through cover glass 132 toward light receiver 408. Polarization analyzer 404 selectively allows only light 412 having a polarization direction orthogonal to that of light 409 to pass through for detection by light receiver 408. The detected signal may be used for proximity sensing or other purposes.

[0040] Some of the polarized light 409 incident on the cover glass 132 may be reflected by one of the cover glass surfaces (or scattered by a thin smudge layer on the cover glass surface). This reflected (or scattered) light 414 substantially maintains the polarization of the light 409. Therefore, most or all of the reflected (or scattered) light 414 will not be detected by the light receiver 408. In this way, crosstalk effects caused by light reflected from the cover glass 132 or scattered by a thin smudge layer on the cover glass can be eliminated or at least reduced.

[0041] In the foregoing example, the module is configured so that the optical receiver selectively detects only light having a polarization direction orthogonal to the polarization direction of the light generated for transmission out of the module. In some cases, it may be advantageous for the receiver to separately detect two polarizations of light (i.e., light having a polarization direction orthogonal to the polarization direction of the light generated for transmission out of the module, and light having a polarization direction parallel to the polarization direction of the light generated for transmission out of the module). For example, this may be achieved by providing two photodetectors with crossed polarizers, and generating a current signal on a corresponding sensor gate for each of the two orthogonal polarizations. Although the parallel polarized light may contain light reflected from the cover glass, it may still be useful for objects located at a greater distance.

[0042] In some applications, the cover glass can affect polarization, for example, by changing the polarization state of the light (e.g., from a linear state to a circular state). In this case, the system can be configured to compensate for this change. For example, the polarization axis of the transmitter can be aligned so that the transmission of light through the cover glass is maximized (e.g., parallel to the transmission axis in the case of a polarizer). In some cases, the polarization state of the transmitter can be selected so that reflections from the cover glass and the target object can be optimally distinguished. For example, if the cover glass acts as a quarter wave plate and the receiver employs a linear polarizer, the transmitter can be configured to emit circularly polarized light so that the reflection from the cover glass is polarized perpendicular to the linear polarizer of the receiver and is therefore absorbed by the polarizer.

[0043] The technology described in this disclosure can help improve optical proximity sensing. Generally speaking, a proximity sensor detects the presence of a nearby object without any physical contact. The proximity sensor emits a beam of electromagnetic radiation (e.g., infrared light) and looks for changes in the detected return signal. For example, in the context of a mobile device such as a smart phone, proximity sensing can be used so that the device recovers from a low power or sleep mode when the target is within a nominal range. Once the device has been awakened from sleep mode, if the target of the proximity sensor is stationary for a long period of time, the sensor will ignore it, and the device will eventually revert to sleep mode. Therefore, for example, during a phone call, when the mobile device is held to the user's ear, the proximity sensor can play a role in detecting and ignoring accidental touch screen taps. In some cases, the modules described in this disclosure can provide improved performance.

[0044] The technology described in this disclosure can also be used for other types of optical sensing, for example, for identifying mid-air gestures and hovering operations. In addition, the technology can be used for other types of optical sensors, such as spectral sensors, optical distance sensors, and imaging sensors.

[0045] Many embodiments have been described. However, various modifications may be made without departing from the spirit of the invention. For example, features described in connection with different embodiments may be combined into a single embodiment. Therefore, other embodiments are also within the scope of the claims.

Claims

1. An optical sensing device, comprising: An optical module comprising: an emission channel operable to emit light having a first polarization, the emission channel comprising a light emitter operable to generate light; and A detection channel, comprising a light receiver, wherein the light receiver comprises: a first photodetector selectively operable to detect light of a second polarization orthogonal to the first polarization; and a second light detector operable to detect light having said first polarization; and a cover glass that reflects some of the light having the first polarization back to the optical module; and wherein the optical module is operable to cause at least some of the light generated by the light emitter to be transmitted through the cover glass, and to cause at least some of the light that is transmitted through the cover glass and reflected back through the cover glass by an object toward the module to be detected by the light receiver.

2. The device of claim 1, wherein the emission channel is operable to emit light having a first linear polarization, and the first light detector is operable to selectively detect light having a second linear polarization orthogonal to the first linear polarization.

3. The device of claim 1, wherein the emission channel is operable to emit light having a first circular polarization, and the first light detector is operable to selectively detect light having a second circular polarization orthogonal to the first circular polarization.

4. The device of claim 1, comprising a polarizer disposed in an emission channel so as to intersect light generated by the light emitter.

5. The device of claim 1, wherein the light emitter comprises an optical polarizer integrated as part of the light emitter.

6. The device of claim 1, wherein the light emitter comprises a VCSEL structure having an asymmetric aperture.

7. The device of claim 1, wherein the light emitter comprises a VCSEL structure having a reflective grating.

8. The device of claim 1, wherein the light emitter comprises a VCSEL structure having a sub-wavelength reflection grating.

9. The device of claim 1, wherein the optical receiver comprises an optical polarizer integrated as part of the optical receiver.

10. The apparatus of claim 1, wherein the optical module is disposed behind a cover glass of a portable computing device.

11. An optical sensing method, comprising: transmitting light from an optical module through a cover glass of a portable computing device toward an object, wherein the light emitted through the cover glass has a first polarization; and wherein the cover glass reflects some of the light having the first polarization back to the optical module; receiving, in the module, light reflected by the object and passing through the cover glass, wherein the light reflected by the object includes light having the first polarization and light having a second polarization orthogonal to the first polarization; as well as selectively detecting, by a first light detector in the module, light reflected by the object and having the second polarization; as well as Light reflected by the object and having the first polarization is selectively detected by a second light detector in the module.

12. The method of claim 11, comprising converting light emitted by a light emitting element into polarized light having the first polarization, wherein the conversion occurs prior to transmitting the light through the cover glass.

13. The method of claim 11 or 12, wherein light transmitted through the cover glass has a first linear polarization.

14. The method of claim 13, wherein the selectively detected light has a second linear polarization orthogonal to the first linear polarization.

15. The method of claim 11 or 12, wherein light transmitted through the cover glass has a first circular polarization.

16. The method of claim 15, wherein the selectively detected light has a second circular polarization orthogonal to the first circular polarization.

17. A method according to claim 11 or 12, comprising using light detected in the module for proximity sensing.

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