Patent ID: 12189905

DETAILED DESCRIPTION

FIG.1shows a schematic diagram of an embodiment of an optical device1. The device has a display2(e.g. a LED, OLED or μLED display) comprising display circuitry for controlling the display in order display an image. A proximity sensor3is located behind the display2and is configured the detect objects in front of the display2. The device further comprises a cover glass4for protecting the display2. In use, the proximity sensor3emits light that is transmitted through the display2and through the cover glass4. The light is reflected from an object5in front of the cover glass. The reflected light is transmitted back through the cover glass4, through the display2and back to the sensor3. The received light is used to detect the object5(e.g. to measure the distance and/or velocity to the object).

The display2is typically made from silicon and comprises metal circuitry (e.g. copper metal lines and vias). The metal circuitry causes a large portion of light to be reflected from the display2, thereby decreasing the overall transmission. In addition, the silicon material will absorb a portion of the light, depending on the wavelength of the light. Overall, the transmissivity of the display2may be around 5%, i.e. only about 5% of the light emitted from the sensor3is transmitted through the display2. This loss of light occurs both as the light is transmitted out of the device1and as it is reflected back through the display2to the sensor3.

Hence, a high light intensity may be required for sufficient light to be reflected back to the sensor3. However, the display2can be disturbed by the high light intensity. For example, the intensity of pixels in the display2change and black spots may form upon irradiation with high intensity. To mitigate this, the sensor3is configured to use a long wavelength sufficiently far away from the visible spectrum. With a suitably long wavelength, the display2is not disturbed or only marginally disturbed. For a silicon based display2, the sensor3may be configured to emit light having a wavelength above 1100 nm.

Due to the absorption spectrum of the display material (e.g. silicon) (seeFIG.5a), the display2will have a wavelength dependent transmissivity (seeFIG.5b). Typically, the display will have a low transmissivity for wavelength shorter than 1000 nm (depending on the display material and thickness), and the sensor3is configured to operate at a longer wavelength. Hence, the long wavelength used by the sensor3both increases the transmission (and thereby SNR) and reduces the risk of display distortion.

The minimum (or reduced) transmissivity of the display generally corresponds to the maximum absorption of the display, since the reflectivity from the display circuitry is largely wavelength-independent. The sensor3may be configured to operate at a wavelength where the absorption of the display is at most 5% in absolute value. This may be, for example, at a wavelength at least 100 nm or at least 200 nm above the wavelength where the absorption transition step occurs (FIG.5a). For example, if the absorption step of the display occurs around 950 nm, then the sensor3can be configured to operate at 1050 nm or more.

The proximity sensor3comprises an emitter being a laser, such as a VCSEL, EEL, or QDLs. A GaAs-based laser (e.g., VCSEL) works well up to about 1150 nm or 1200 nm.

GaInNAs-based laser (e.g. VCSEL) technologies can be fabricated on GaAs substrates for longer wavelengths above 1200 nm or 1400 nm or even higher, but typical power conversion efficiencies are lower than in traditional GaAs based VCSELs with and InGaAs quantum well.

GaInNAsSb-based laser (e.g. VCSEL) technologies can also be fabricated on GaAs substrates for longer wavelengths above 1200 nm or 1400 nm or higher, but this decreases its power conversion efficiency.

The sensor3may be a SMI sensor, wherein the emitter and receiver of the sensor3is the same, and the sensor3is arranged such that light is reflected back into the emitter. The emitter comprises a resonator (e.g. the laser cavity in a VCSEL). In the resonator, coherent mixing occurs with light generated by the emitter with light reflected back from the object5into the emitter (i.e. there is interference between the light directly generated in the emitter and the light reflected back into the emitter). This affects the emitter output and the emitted light depends on the coherent mixing. Hence, the emitter output (or a related parameter) can be used to determine the distance and/or velocity to the object5. For example, the supply signal (i.e. the input voltage or current) to the emitter may be used as a parameter to detect the object5. Alternatively, optical detection may be used, wherein the intensity of the emitted light is directly measured with a light detector for determining the distance/velocity.

For SMI detection, light is emitted from an emitter being a resonant light source (i.e. having an optical resonator in which the light circulates), e.g. a laser, and a portion of the light leaving the resonator is fed back into the resonator a, e.g., after the light has interacted with an object by, e.g., reflection or scattering. The feed-back light interacts with the light in the resonator and introduces a disturbance in the light source by interference. This effect can be sensed and can be related to the interaction with the object, such as the distance to the object or a velocity of the object (relative to the light source/resonator exit mirror). Sensing can be accomplished optically, wherein the emitted light intensity is monitored, e.g., using a photodiode. For example, a beam splitter can be positioned close to the exit mirror to let most of the light exiting the exit mirror pass and reflect a small portion to a photo detector. Alternatively, the other mirror of the resonator can be made partially transmissive (e.g., 99% instead of 100% reflective), and the light detector is positioned close that mirror. This can be a more compact solution than using a beam splitter. Alternatively, sensing can be accomplished electrically, wherein a feed signal for the light source is monitored. For example, if the light source is driven with constant current, and the change in voltage is determined. If the light source is driven with a constant voltage, then change in current is determined instead. The electrical signal may be noisier than the optically obtained signal.

Using SMI-based sensing can eliminate the need for a detector that is sensitive at the optical wavelength, since the light is received back by the laser. This also avoids the loss of SNR by using a detector at the edge of its wavelength range, or adding a non-silicon detector for detection of the long wavelength.

In addition, the SMI-based sensors can be very small, which is important for application in mobile devices such as smartphones and smartwatches. SMI-based detection is also very insensitive to background light, and enables absolute distance and/or velocity measurements.

While optical detection for SMI may provide accurate results, for long wavelengths from above about 1100 nm or 1150 nm, light detectors which are based on a different semiconductor material may be required (e.g., InGaAs, GaInNAs, GaInNAsSb or Ge), which can increase the complexity and cost of the device1.

Hence, electrical detection for SMI may be advantageous. It enables a smaller form factor (as no additional light detector is required) and avoids the problems which can arise from having to detect light having relatively long wavelengths, such as above 1100 nm (e.g. problems of increased costs and selection of semiconductor materials which are more difficult to handle than, e.g., Si)

Reflections from the display2and/or the cover glass4back into the emitter may be problematic for SMI detection. However, due to the relative short distance between the display/cover glass and the sensor3, compared to the distance to the object5, the signal reflected from the display/cover glass can be filtered out from the signal reflected from the object5. To further reduce the amount of reflections from the display/cover glass, a lens, which focuses the light outside the cover glass4(closer to the object5) may be used. Alternative or in addition, the emitter may be tilted (e.g. by 1 to 3 degrees) off the vertical axis to avoid mirror reflections back from the display/cover glass. Light reflected from the object5will tend to be more diffuse, and is therefore less impacted by the tilted emitter.

The proximity sensor3may be used to simply indicate the presence of an object within a certain distance threshold, rather than being used to measure the absolute distance to that object.

FIG.2shows a schematic diagram of an embodiment of an optical device1, which may be the optical device1ofFIG.1. Similar or equivalent features of embodiments in different figures have been given the same reference numerals to aid understanding and are not intended to limit the illustrated embodiments. The optical device1comprises a display2having display circuitry6(e.g. metal lines for connecting to display pixels), a cover glass4, and an optical sensor3comprising an emitter7and a receiver8. The emitter7emits light9that is transmitted through the display2and reflects off an object5. The reflected light10is transmitted through the display2and into the receiver8. Time of flight (TOF) or intensity measurements may be used to detect the object5.

FIG.3shows another embodiment of an optical device1which may be the optical device1ofFIG.1when using an SMI sensor3. The optical device1comprises a display2having display circuitry6, a cover glass4, and an optical sensor3comprising an emitter7being a laser (e.g. a VCSEL) and also being the receiver8of the sensor3. The emitter7emits light9that is transmitted through the display2and reflects off an object5. The reflected light10is transmitted through the display2and into the receiver8. The sensor3further comprises an electrical detection module11, which is used to measure the input voltage and/or input current to the emitter7. The measured current or voltage is indicative of the interference of light inside the laser, and can be used to detect the object. For example, the absolute distance and/or velocity of the object can be determined from the measured voltage or current.

This may be a particularly useful embodiment, wherein the long wavelength (e.g. λ>1100 nm) of the emitter7for higher transmission and less display distortion/disturbance is combined with SMI, which does not require a separate detector that is capable of detecting the long wavelengths.

FIG.4shows another embodiment of an optical device1which may be the optical device1ofFIG.1when using an SMI sensor3. The optical device1is similar to that ofFIG.3, but instead of using an electrical detection module, a separate light detector12is used to measure the light intensity from the emitter7of the sensor3. The object5can be detected by the measured intensity.

FIG.5ashows the absorption coefficient of Silicon in the range of 300 nm to 1200 nm. The absorption is strongly increasing towards shorter wavelengths (logarithmic scale). From the graph, any Silicon thickness transmission T can be computed with the following expression: T=exp(−α*t), where a is the absorption coefficient and t is the Silicon thickness (this expression is not taking into account the refractive index).

FIG.5bshows the transmission spectra of Silicon computed from the absorption coefficient (FIG.5a) for various thicknesses (10 μm, 100 μm and 1000 μm). One can see that for a Silicon thickness of 100 μm, wavelengths above 1100 nm may be suitable for use in the disclosed device.

FIG.6shows the sensitivity of different semiconductor materials to light as a function of wavelength. The sensitivity relates to the absorption of light, since light that is transmitted through the material cannot be sensed. As can be seen from the graph, GaAs has a maximum sensitivity at a lower wavelength (around 800 nm) compared to silicon. Hence, if a device with a display based on GaAs is used, the proximity sensor may be operated at a lower wavelength. The wavelength should be chosen such that it is longer than the wavelength at which the maximum sensitivity of the display material occurs.

FIG.7is a flow diagram showing the steps of a method proximity detection in order to determine the distance to or velocity of an object in front of a display. The method may be carried out using an optical device as described above (e.g. the optical device1ofFIG.1). The method comprises emitting light though the display (step S1), and receiving in the emitter light reflected back from the object and through the display (step S2). The method further comprises measuring a change in the emitter output (step S3) and, using the measured change, determining the distance to or velocity of the object (step S4). The change may be inferred or determined from the electrical (voltage or current) input to the emitter.

Although the aforementioned description uses a proximity sensor for the behind display sensing, it will be understood by the skilled person that any other optical sensing device can be equally used instead of the proximity sensor and the alternative arrangement would still be within the scope of the claimed disclosure.

Although specific embodiment have been described above, the claims are not limited to those embodiments. Each feature disclosed may be incorporated in any of the described embodiments, alone or in an appropriate combination with other features disclosed herein.

Reference Numerals1 Optical Device2 Display3 Sensor4 Cover glass5 Object6 Display circuitry7 Emitter8 Receiver9 Emitted light10 Reflected light11 Electrical detection module12 Light detector