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EP-4235234-B1 - OPTICAL FILTER AND SENSOR SYSTEM

EP4235234B1EP 4235234 B1EP4235234 B1EP 4235234B1EP-4235234-B1

Inventors

  • HENDRIX, KAREN DENISE
  • BRADLEY, RICHARD, A., JR.
  • GRIGONIS, MARIUS
  • OCKENFUSS, GEORG

Dates

Publication Date
20260513
Application Date
20130716

Claims (15)

  1. An optical filter (600) comprising: a filter stack (610) including alternating layers of hydrogenated silicon (611) and layers of a lower-refractive index material (612) including an oxide; wherein the optical filter (600) is designed for substantially allowing light in a wavelength range that includes a wavelength between 800 to 1100 nm to pass through it and exhibits a blocking level outside the wavelength range of the optical filter (600) of greater than OD2 over a wavelength range of 400 nm to 1100 nm.
  2. The optical filter (600) of claim 1, wherein the filter (600) has a blocking level of greater than OD3 over a wavelength range of 300 nm to 1100 nm that are outside of the wavelength range of the optical filter (600).
  3. The optical filter (600) of claim 1 or claim 2, wherein the filter (600) has a center wavelength that shifts by less than 20 nm in magnitude with a change in incidence angle from 0° to 30°.
  4. The optical filter (600) of any of claims 1 to 3, wherein the filter stack (610) is on a first side of a substrate (620) and an antireflective coating (630) is on a second side of the substrate (620).
  5. The optical filter (600) of claim 5, wherein the antireflective coating (630) is a Ta 2 O 5 and SiO 2 coating.
  6. The optical filter (600) of any of the preceding claims, wherein the layers of hydrogenated silicon (611) have a refractive index of greater than 3 over the wavelength range of 800 nm to 1100nm.
  7. The optical filter (600) of any of the preceding claims, wherein the plurality of hydrogenated silicon layers (611) have a refractive index of greater than 3.5 over a wavelength range of 800 nm to 1100 nm.
  8. The optical filter (600) of any of the preceding claims, wherein the plurality of hydrogenated silicon layers (611) have a refractive index of greater than 3.6 at 830 nm.
  9. The optical filter (600) of claim 5 and optionally any of claims 6 to 8, wherein the layers of lower-refractive index layers (612) are composed of silicon dioxide (SiO 2 ) and wherein at least one of the layers of hydrogenated silicon layers (611) is thicker than at least one of the layers of lower-refractive index layers (612).
  10. The optical filter (600) of any of the preceding claims, wherein the layers of the lower-refractive index material (612) including an oxide have a refractive index of less than 3 over the wavelength range of 800 nm to 1100nm.
  11. The optical filter (600) of any of the preceding claims, wherein the layers of the lower-refractive index material (612) including an oxide have a refractive index of less than 2.5 over the wavelength range of 800 nm to 1100nm.
  12. The optical filter (600) of any of the preceding claims, wherein the layers of the lower-refractive index material (612) including an oxide are composed of at least one of silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), titanium dioxide (TiO 2 ), niobium pentoxide (Nb 2 O 5 ), tantalum pentoxide (Ta 2 O 5 ), or mixtures thereof.
  13. The optical filter (600) of any of the preceding claims, wherein the optical filter (600) has a transmittance level, within the passband, of greater than 90%.
  14. An optical system (1000), comprising: a light source (1010) for emitting light having a wavelength between 800 to 1100 nm; a sensor (1030); and an optical filter (600, 1020) of claim 1..
  15. The optical system (1000) of claim 14, wherein the optical filter (600, 1020) is according to any one or more of claims 2 to 13.

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to optical filters and to sensor systems comprising optical filters. More particularly, the present invention relates to optical filters including hydrogenated silicon layers and to sensor systems comprising such optical filters. BACKGROUND OF THE INVENTION In a typical gesture-recognition system, a light source emits near-infrared light towards a user. A three-dimensional (3D) image sensor detects the emitted light that is reflected by the user to provide a 3D image of the user. A processing system then analyzes the 3D image to recognize a gesture made by the user. An optical filter, more specifically, a bandpass filter, is used to transmit the emitted light to the 3D image sensor, while substantially blocking ambient light. In other words, the optical filter serves to screen out ambient light. Therefore, an optical filter having a narrow passband in the near-infrared wavelength range, i.e., 800 nm to 1100 nm, is required. Furthermore, the optical filter must have a high transmittance level within the passband and a high blocking level outside of the passband. Conventionally, the optical filter includes a filter stack and a blocking stack, coated on opposite surfaces of a substrate. Each of the stacks is formed of high-refractive-index layers and low-refractive-index layers stacked in alternation. Different oxides are, generally, used for the high-refractive-index layers and the low-refractive-index layers, such as TiO2, Nb2O5, Ta2O5, SiO2, and mixtures thereof. For example, some conventional optical filters include a TiO2/SiO2 filter stack and a Ta2O5/SiO2 blocking stack, in which the high-refractive index layers are composed of TiO2 or Ta2O5, respectively, and the low-refractive-index layers are composed of SiO2. In a first conventional optical filter designed to transmit light in a wavelength range of 829 nm to 859 nm over an incidence angle range of 0° to 30°, the filter stack includes 71 layers, the blocking stack includes 140 layers, and the total coating thickness is about 24 µm. Transmission spectra 100 and 101 at incidence angles of 0° and 30°, respectively, for this optical filter are plotted in FIG. 1. In a second conventional optical filter designed to transmit light at a wavelength of 825 nm over an incidence angle range of 0° to 20°, the filter stack includes 43 layers, the blocking stack includes 82 layers, and the total coating thickness is about 14 µm. Transmission spectra 200 and 201 at incidence angles of 0° and 20°, respectively, for this optical filter are plotted in FIG. 2. In a third conventional optical filter designed to transmit light in a wavelength range of 845 nm to 865 nm over an incidence angle range of 0° to 24°, the filter stack includes 77 layers, the blocking stack includes 148 layers, and the total coating thickness is about 26 µm. Transmission spectra 300 and 301 at incidence angles of 0° and 24°, respectively, for this optical filter are plotted in FIG. 3. With reference to FIGS. 1-3, the first, second, and third conventional optical filters, generally, have a high transmittance level within the passband and a high blocking level outside of the passband. However, the center wavelength of the passband undergoes a relatively large shift with change in incidence angle. Consequently, the passband must be relatively wide to accept light over the required incidence angle range, increasing the amount of ambient light that is transmitted and reducing the signal-to-noise ratio of systems incorporating these conventional optical filters. Furthermore, the large number of layers in the filter stacks and blocking stacks increases the expense and coating time involved in fabricating these conventional optical filters. The large total coating thickness also makes these conventional optical filters difficult to pattern, e.g., by photolithography. To enhance the performance of the optical filter in the gesture-recognition system, it would be desirable to reduce the number of layers, the total coating thickness, and the center-wavelength shift with change in incidence angle. One approach is to use a material having a higher refractive index than conventional oxides over the wavelength range of 800 nm to 1100 nm for the high-refractive-index layers. In addition to a higher refractive index, the material must have also have a low extinction coefficient over the wavelength range of 800 nm to 1100 nm in order to provide a high transmittance level within the passband. The use of hydrogenated silicon (Si:H) for high-refractive-index layers in optical filters is disclosed by Lairson, et al. in an article entitled "Reduced Angle-Shift Infrared Bandpass Filter Coatings" (Proceedings of the SPIE, 2007, Vol. 6545, pp. 65451C-1-65451C-5), and by Gibbons, et al. in an article entitled "Development and Implementation of a Hydrogenated a-Si Reactive Sputter Deposition Process" (Proceedings of the Annual Technical Conference, Society of Vac