Optical sensor system

Abstract

The present invention addresses the problem of providing an optical sensor system in which a single aperture is used, an optical element for splitting wavelengths of incident light is used to guide only a light beam having a specific wavelength, and the guided light beam is sensed by a corresponding sensor, thereby enabling the optical sensor system to be made smaller, thinner, and more attractive in design. The problem is solved by at least having: a light-guiding substrate; a diffraction element 1 for splitting incident light into at least two light beams having different wavelengths; a diffraction element 2 for diffracting the light beams split by the diffraction element 1; and a plurality of sensors each detecting a corresponding light beam of a plurality of light beams split by the diffraction element 1.

Description

Optical Sensor System

[0001] The present invention relates to an optical sensor system, and more particularly to providing a sensor configuration that can be made smaller, thinner, and more aesthetically pleasing.

[0002] In recent years, sensing technologies that utilize the wavelength characteristics of transmittance, reflectance, and absorptance of various substances have begun to be used.

[0003] For example, as exemplified by facial recognition on smartphones, the accuracy of facial recognition is improved by collecting three-dimensional information of the face using near-infrared light in addition to camera images using normal visible light.

[0004] Another example of vital sensing is a sensor that measures blood saturated oxygen concentration (SpO2) using wavelengths in the visible light and near-infrared light regions.

[0005] These sensor systems are equipped with sensors (including two-dimensional sensors) corresponding to each wavelength, and apertures with lenses that allow light to enter each sensor. Therefore, the device requires multiple apertures and space to accommodate these components. In addition, because light of other wavelengths can become noise in these sensor systems, bandpass filters or the like are sometimes placed in front of the sensors to measure only specific wavelengths.

[0006] In contrast, Patent Document 1 describes a sensor system (imaging device) that individually detects multiple light beams in different wavelength ranges (wavelength bands), including a separation unit that separates incident light into light beams in two or more wavelength ranges and a detection unit that detects each of the separated light beams in the two or more wavelength ranges and outputs a signal that allows tunable wavelength extraction in post-processing. In this sensor system, the separation unit is exemplified by an optical element that includes a separation surface that separates incident light into light beams in two or more wavelength ranges, and that is positioned at a predetermined angle with respect to the optical axis of the incident light. Specifically, the separation unit is exemplified by a dichroic filter that separates incident light into reflected light and transmitted light using the separation surface.

[0007] According to the sensor system described in Patent Document 1, each sensor requires only one aperture having a lens for introducing light. However, this device requires a separation unit such as a dichroic filter to be positioned at an angle relative to the optical axis of the incident light, which requires space for this. Furthermore, the light separated by the dichroic filter travels in different directions, for example at an angle of 90°, before entering the corresponding sensor. Therefore, it is necessary to secure an optical path for each light traveling in a different direction, which requires space for this.

[0008] Japanese Patent Application Laid-Open No. 2020-22121

[0009] In view of these considerations, the object of the present invention is to provide an optical sensor system that can be made smaller, thinner, and more stylish by using a single opening, guiding only specific wavelengths using an optical element that separates the wavelengths of incident light, and having each sensor sense the guided light.

[0010] [1] An optical sensor system comprising at least a light-guiding substrate, a diffraction element 1 that separates incident light into light of at least two wavelengths, a diffraction element 2 that diffracts the light separated by the diffraction element 1, and a plurality of sensors that respectively sense the plurality of light beams separated by the diffraction element 1. [2] The optical sensor system according to [1], wherein the diffraction element 1 separates the incident light into light with a wavelength in the visible light region and light with a wavelength in the short-wave infrared region. [3] The optical sensor system according to [1], wherein the diffraction element 1 separates the incident light into light with a wavelength in the visible light region, light with a wavelength in the near-infrared region, and light with a wavelength in the short-wave infrared region. [4] The optical sensor system according to any one of [1] to [3], wherein either the diffraction element 1 or the diffraction element 2, or both the diffraction element 1 and the diffraction element 2, are liquid crystal diffraction elements.

[0011] The present invention can provide a sensor system that is smaller, thinner, and has improved design.

[0012] FIG. 1 is a diagram schematically showing an example of an optical sensor system of the present invention. FIG. 2 is a diagram schematically showing an example of the light separation function in the optical sensor system of FIG. 1. FIG. 3 is a diagram schematically showing another example of an optical sensor system of the present invention. FIG. 4 is a diagram schematically showing another example of an optical sensor system of the present invention. FIG. 5 is a diagram schematically showing another example of an optical sensor system of the present invention. FIG. 6 is a schematic diagram for explaining an example of a reflective liquid crystal diffraction element. FIG. 7 is a schematic diagram for explaining an example of a reflective liquid crystal diffraction element. FIG. 8 is a schematic diagram for explaining the function of an example of a reflective liquid crystal diffraction element. FIG. 9 is a schematic diagram for explaining an example of a transmissive liquid crystal diffraction element. FIG. 10 is a schematic diagram for explaining an example of a transmissive liquid crystal diffraction element. FIG. 11 is a schematic diagram for explaining the function of an example of a transmissive liquid crystal diffraction element. FIG. 12 is a schematic diagram for explaining the function of an example of a transmissive liquid crystal diffraction element.

[0013] Hereinafter, the optical sensor system of the present invention will be described in detail based on preferred embodiments shown in the drawings.

[0014] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0015] In addition, in this specification, visible light (light with a wavelength in the visible light region) refers to light with a wavelength (wavelength region) of 380 to 800 nm, near-infrared light (NIR (Near InfraRed)) refers to light with a wavelength of more than 800 nm and not more than 1000 nm, and short-wave infrared light (light with a wavelength in the short-wave infrared region, SWIR (Short Wave InfraRed)) refers to light with a wavelength of more than 1000 nm and not more than 2500 nm.

[0016] Fig. 1 schematically shows an example of an optical sensor system of the present invention. The optical sensor system shown in Fig. 1 includes a light-guiding substrate 101, a first diffraction element 201, a second diffraction element 202, and a first sensor 301 and a second sensor 302. In the illustrated example, the first diffraction element 201 is bonded to one surface (the upper surface in the figure) of the light-guiding substrate 101, and the second diffraction element 202 is bonded to the other surface (the lower surface in the figure) of the light-guiding substrate 101. In this example, the surface on which the second diffraction element 202 is disposed is the light incident surface of the light-guiding substrate 101.

[0017] The light guiding substrate 101 is a known light guiding element (light guiding plate) used as a light guiding plate in a backlight unit of a liquid crystal display device.

[0018] The first diffraction element 201 is the diffraction element 1 of the present invention. Therefore, the first diffraction element 201 is a diffraction element that separates incident light into light of at least two wavelengths. In the illustrated example, the first diffraction element 201 is, as an example, a reflective diffraction element that selectively reflects short-wave infrared light (SWIR light) and transmits other light. The first diffraction element 201 thereby separates the incident light into SWIR light and light other than SWIR light. Furthermore, in the illustrated example, the first diffraction element 201 diffracts and reflects SWIR light in a rightward direction toward the light incident side. That is, in the illustrated example, the first diffraction element 201 diffracts and reflects SWIR light in a rightward direction in the figure.

[0019] The second diffraction element 202 corresponds to the diffraction element 2 of the present invention. Therefore, the second diffraction element 202 is a diffraction element that diffracts the light separated by the first diffraction element 201, which is the diffraction element 1. Therefore, the second diffraction element 202 is provided at a position where the SWIR light diffracted and reflected by the first diffraction element 201 is incident. In the illustrated example, the second diffraction element 202 is, for example, the same diffraction element as the first diffraction element 201. Therefore, the second diffraction element 202 also diffracts and reflects the SWIR light to the right toward the light incident side. In FIG. 1 , the first diffraction element 201 and the second diffraction element 202 are upside down. Therefore, the second diffraction element 202 diffracts and reflects the right-handed circularly polarized SWIR light separated by selective reflection by the first diffraction element 201 to the left in the figure.

[0020] The second diffraction element 202 is not limited to a diffraction element that selectively diffracts and reflects light in a specific wavelength range, such as SWIR light. For example, the second diffraction element 202 may diffract and reflect light in a wide wavelength range, from visible light to SWIR light. However, considering the efficiency of light diffraction and reflection, like the first diffraction element 201, the second diffraction element 202 is preferably a diffraction element that selectively diffracts and reflects light of a specific wavelength (wavelength range), such as SWIR light, NIR light, red light, green light, and blue light. This also applies to the diffraction element 204 described below.

[0021] In the optical sensor system of the present invention, the diffraction element 1 does not limit the separation between SWIR light and light of other wavelengths. In other words, in the present invention, there are no limitations on the light separated by the diffraction element 1. For example, the diffraction element 1 may separate incident light into near-infrared light (NIR light), infrared light in a predetermined wavelength range exceeding SWIR light, red light, green light, blue light, and light of any wavelength (wavelength range) in the entire visible light range, and other light. Alternatively, the diffraction element 1 may separate incident light into light of two predetermined wavelengths, such as NIR light and SWIR light. Furthermore, in the optical sensor system of the present invention, the diffraction element 1 may separate incident light into light of three or more wavelengths.

[0022] Furthermore, in the optical sensor system illustrated in the figure, the diffraction element 1 (first diffraction element 201) selectively reflects light of a predetermined wavelength, such as SWIR light, and transmits other light, but the present invention is not limited to this. That is, in the optical sensor system of the present invention, the diffraction element 1 may selectively transmit light of a predetermined wavelength and reflect other light. Alternatively, in the optical sensor system of the present invention, the diffraction element 1 may selectively reflect light in a predetermined wavelength range, selectively transmit light in a predetermined wavelength range, and absorb light in other wavelength ranges.

[0023] In the optical sensor system of the present invention, the sensor measures (senses) each of the multiple light beams separated by the diffraction element 1. In the optical sensor system shown in FIG. 1 , the first sensor 301 measures the light transmitted through the first diffraction element 201. On the other hand, the second sensor 302 measures the SWIR light selectively reflected by the first diffraction element 201. Therefore, the first sensor 301 is disposed at a position where the light transmitted through the light guiding substrate 101 and the first diffraction element 201 can be incident. Furthermore, the second sensor 302 is disposed at a position where the SWIR light reflected by the second diffraction element 202 and emitted from the light guiding substrate 101 can be incident.

[0024] FIG. 2 schematically illustrates an example of the light separation effect in the optical sensor system shown in FIG. 1 . In this example, incident light 401, which is a mixture of visible light 402 and SWIR light 403, enters the optical sensor system. For example, the incident light 401 enters the optical sensor system from the normal direction of the light guiding substrate 101 toward a position corresponding to the first diffraction element 201. The incident light 401 that has entered the optical sensor system passes through the light guiding substrate 101 and enters the first diffraction element 201. As described above, the first diffraction element 201 selectively reflects SWIR light and transmits other light. Therefore, the incident light 401 is separated by the first diffraction element 201 into visible light 402, which is transmitted light, and SWIR light 403, which is reflected light.

[0025] Visible light 402 transmitted through the first diffraction element 201 is incident on the first sensor 301 and sensed (photometrically measured). As described above, the first diffraction element 201 is a reflective diffraction element that diffracts and reflects SWIR light in the right direction in the figure. Therefore, SWIR light 403 diffracted and reflected by the first diffraction element 201 is guided within the light-guiding substrate 101 and enters the second diffraction element 202. The SWIR light 403 incident on the second diffraction element 202 is diffracted in the left direction in the figure by the second diffraction element 202, thereby changing the direction of the light and being reflected toward the second sensor 302, exiting the light-guiding substrate 101 and being sensed by the second sensor 302.

[0026] Another example of the optical sensor system of the present invention is shown schematically in Figure 3. Note that the optical sensor system of the present invention shown hereinafter has a number of the same components as the optical sensor system shown in Figure 1 (Figure 2) described above, so the same components are given the same reference numerals, and the explanation will mainly focus on the different parts.

[0027] In the optical sensor system shown in FIG. 3, a first diffraction element 201A, which is the diffraction element 1 of the present invention, is attached to the center of the longitudinal direction of one surface (the bottom surface in the figure) of the light guiding substrate 101. In addition, a second diffraction element 202 is arranged at both ends of the longitudinal direction of the other surface (the top surface in the figure) of the light guiding substrate 101. As described above, the second diffraction element 202 is the optical element 2 of the present invention. In this example, too, the surface on which the second diffraction element 202 is arranged becomes the light incident surface of the light guiding substrate 101. As in the example shown in FIG. 1 (FIG. 2), the first diffraction element 201A, which is the diffraction element 1 of the present invention, is a reflective diffraction element that selectively reflects SWIR light and transmits light of other wavelengths. Here, in the example shown in FIG. 3, the first diffraction element 201A separates and reflects SWIR light in two different directions.

[0028] As in the previous example, both of the two second diffraction elements 202 are provided at positions where SWIR light diffracted and reflected by the first diffraction element 201A is incident. In this example, as an example, the second diffraction element 202 is the same as the first diffraction element 201A. However, the present invention is not limited to this, and both of the two second diffraction elements 202 may diffract and reflect the incident SWIR light only in one direction toward the corresponding second sensor 302. Furthermore, as in the previous example, the first sensor 301 is disposed at a position where light transmitted through the first diffraction element 201A and emitted from the light guiding substrate 101 can be incident. Furthermore, the two second sensors 302 are each disposed at a position where SWIR light reflected by the corresponding second diffraction element 202 and emitted from the light guiding substrate 101 can be incident.

[0029] As an example, in the optical sensor system shown in FIG. 3 , incident light 401, which is a mixture of visible light 402 and SWIR light 403, enters the optical sensor system from the normal direction of the light guiding substrate 101 at a position corresponding to the first diffraction element 201A. The incident light 401 passes through the light guiding substrate 101 and enters the first diffraction element 201A. As described above, the first diffraction element 201A selectively diffracts and reflects SWIR light and transmits other light. Therefore, the incident light 401 is separated by the first diffraction element 201A into visible light 402, which is transmitted light, and SWIR light 403, which is reflected light. The visible light 402 that has passed through the first diffraction element 201A enters the first sensor 301 and is sensed.

[0030] 3, the first diffraction element 201A separates and reflects the SWIR light 403 in two different directions. In this example, the first diffraction element 201A diffracts and reflects the SWIR light to the right and left in the figure. Therefore, the SWIR light diffracted to the right in the figure is guided within the light-guiding substrate 101 and enters the second diffraction element 202 on the right in the figure, and the SWIR light diffracted to the left in the figure is guided within the light-guiding substrate 101 and enters the second diffraction element 202 on the left in the figure.

[0031] The SWIR light incident on the second diffraction element 202 on the right side in the figure is diffracted and reflected by the second diffraction element 202 in the left direction in the figure (right direction with respect to the incident light), thereby changing the direction of the light toward the second sensor 302, exiting the light guiding substrate 101 and being sensed by the corresponding second sensor 302. On the other hand, the SWIR light incident on the second diffraction element 202 on the left side in the figure is diffracted and reflected by the second diffraction element 202 in the right direction in the figure (left direction with respect to the incident light), thereby changing the direction of the light toward the second sensor 302, exiting the light guiding substrate 101 and being sensed by the corresponding second sensor 302.

[0032] Another example of the optical sensor system of the present invention is shown in Fig. 4. The optical sensor system of the present invention shown in Fig. 4 is an example of a configuration that separates near-infrared light (NIR light) in addition to visible light and SWIR light.

[0033] The optical sensor system shown in Fig. 4 has the basic configuration of the optical sensor system shown in Fig. 1 (Fig. 2). Specifically, the optical sensor system shown in Fig. 4 further includes a diffraction element 203 laminated on the first diffraction element 201 in the optical sensor system shown in Fig. 1 (Fig. 2), and the diffraction element 203 is attached to one surface (the bottom surface in the figure) of the light guiding substrate 101. Furthermore, a diffraction element 204 is attached to the other surface (the top surface in the figure) of the light guiding substrate 101, between the second diffraction element 202 and the first diffraction element 201 in the longitudinal direction of the light guiding substrate 101. Furthermore, a third sensor 303 is arranged at a position corresponding to the light reflected by the diffraction element 204. In this example, the surface on which the second diffraction element 202 and the diffraction element 204 are arranged is the light incident surface of the light guiding substrate 101.

[0034] As described above, the first diffraction element 201 is a reflective diffraction element that selectively diffracts and reflects SWIR light. On the other hand, the diffraction element 203 stacked on this first diffraction element 201 is a reflective diffraction element that selectively diffracts and reflects NIR light and transmits other light. Like the first diffraction element 201, the diffraction element 203 diffracts and reflects NIR light in the right direction toward the light incident side, i.e., the right direction in the figure. In this example, the diffraction element 203 has a smaller diffraction angle than the first diffraction element 201. On the other hand, the diffraction element 204 diffracts and reflects the NIR light reflected by the diffraction element 203 in the right direction toward the incident side, i.e., the left direction in the figure, thereby causing the NIR light to be incident on the third sensor 303. Therefore, the diffraction element 204 is positioned so that the NIR light reflected by the diffraction element 203 can be incident. The third sensor 303 is disposed at a position where the NIR light reflected by the diffraction element 203 and emitted from the light guiding substrate 101 can be incident thereon.

[0035] In the optical sensor system shown in Fig. 4, the diffraction element 203 is basically the same as the first diffraction element 201, i.e., the diffraction element 1 of the present invention, except that the wavelength of light that is selectively reflected is different. Also, in the optical sensor system shown in Fig. 4, the diffraction element 204 is basically the same as the second diffraction element 202, i.e., the diffraction element 2 of the present invention.

[0036] In the optical sensor system shown in FIG. 4 , as an example, incident light 401, which is a mixture of visible light, NIR light, and SWIR light, enters the optical sensor system from the normal direction of the light guiding substrate 101 at a position corresponding to the first diffraction element 201, as in the above example. The incident light 401 passes through the light guiding substrate 101 and enters the laminate of the first diffraction element 201 and the diffraction element 203. The incident light 401 first enters the diffraction element 203. As described above, the diffraction element 203 is a reflective diffraction element that selectively diffracts and reflects NIR light and transmits other light. Therefore, the incident light 401 entering the diffraction element 203 is separated by the diffraction element 203 into NIR light 404, which is reflected light, and visible light 402 and SWIR light 403, which are transmitted light. The visible light 402 and SWIR light 403 that have passed through the diffraction element 203 then enter the first diffraction element 201. The first diffraction element 201 selectively diffracts and reflects the SWIR light and transmits other light. Therefore, the visible light 402 and SWIR light 403 that have entered the first diffraction element 201 are separated by the first diffraction element 201 into the SWIR light 403, which is reflected light, and the visible light 402, which is transmitted light.

[0037] As a result, incident light 401, which is a mixture of visible light, NIR light, and SWIR light, is separated into visible light 402, NIR light 405, and SWIR light 403 by the laminate of the first diffraction element 201 and the diffraction element 203. Note that in this example, the stacking order of the first diffraction element 201 and the diffraction element 203 is not limited to that shown in the figure and may be reversed. Even if this stacking order is reversed, the incident light 401 can be separated into visible light 402, NIR light 405, and SWIR light 403 by exactly the same effect.

[0038] Visible light 402 transmitted through the first diffraction element 201 (laminate) is incident on the first sensor 301 and sensed. NIR light 404 diffracted and reflected by the diffraction element 203 in the right direction in the figure is guided within the light guiding substrate 101 and is incident on the diffraction element 204, which is arranged closer than the second diffraction element 202. As described above, the diffraction angle of the diffraction element 203 is smaller than that of the first diffraction element 201. The NIR light 404 incident on the diffraction element 204 is diffracted and reflected in the left direction in the figure by the diffraction element 204, thereby changing the direction of the light to the direction of the third sensor 303, exiting the light guiding substrate 101 and being sensed by the third sensor 303. On the other hand, SWIR light 403 diffracted and reflected by the first diffraction element 201 in the right direction in the figure is guided within the light guiding substrate 101 and is incident on the second diffraction element 202. The SWIR light 403 incident on the second diffraction element 202 is diffracted and reflected by the second diffraction element 202 to the left in the figure, thereby changing the direction of the light toward the second sensor 302, and is emitted from the light-guiding substrate 101 and sensed by the second sensor 302.

[0039] 4, the diffraction angle of the diffraction element 203 that selectively diffracts and reflects NIR light is set smaller than that of the first diffraction element 201 that selectively diffracts and reflects SWIR light. However, the present invention is not limited to this, and the diffraction angle of the diffraction element 203 that selectively diffracts and reflects NIR light may be set larger than that of the first diffraction element 201 that selectively diffracts and reflects SWIR light.

[0040] In all of the examples shown in FIGS. 1 to 4 , light reflected by the diffraction element 1 (first diffraction element 201 (diffraction element 203)) is not totally reflected at the interface between the light guiding substrate 101 and air, but directly enters the diffraction element 2 (second diffraction element 202 (diffraction element 204)). However, the optical sensor system of the present invention is not limited to this. That is, in the present invention, an embodiment using a reflective diffraction element 1 may be configured such that light diffracted and reflected by the diffraction element 1 is totally reflected two or more times at the interface between the light guiding substrate 101 and air, is guided, and then enters the diffraction element 2.

[0041] Here, when the diffraction element is a type that diffracts and reflects light according to the polarization of incident light, such as a reflective liquid crystal diffraction element described below, repeated total reflection of the light within the light guiding substrate 101 may result in a loss of polarization, resulting in a decrease in the diffraction and reflection efficiency of the diffraction element 2. In consideration of this point, particularly when the diffraction element is a type that diffracts and reflects light according to the polarization of incident light, such as a reflective liquid crystal diffraction element, it is preferable that the light reflected by the diffraction element 1 directly enters the diffraction element 2 without being totally reflected at the interface between the light guiding substrate 101 and the air, as shown in Figures 1 to 4.

[0042] The optical sensor systems of the present invention shown in FIGS. 1 to 4 all use a reflective diffraction element as the diffraction element 1 (first diffraction element 201). However, the present invention is not limited thereto, and the diffraction element 1 may also be a transmissive diffraction element. Generally, in a transmissive diffraction element, the longer the wavelength of incident light, the larger the diffraction angle of transmitted light. For example, among visible light, NIR light, and SWIR light, the diffraction angle of transmitted light is smallest for visible light and largest for SWIR light. Therefore, when a transmissive diffraction element is used for the diffraction element 1, this can be used to separate incident light into light of at least two wavelengths. For example, in the case of a transmissive liquid crystal diffraction element, unlike a reflective liquid crystal diffraction element using cholesteric liquid crystal, it is not possible to separate only light of a specific wavelength, but the diffraction angle of light depending on the wavelength varies, just like in the case of a reflective liquid crystal diffraction element. Therefore, by dividing the diffraction angle of the light to be separated into light with an angle larger than the Brewster angle of the light-guiding substrate and light with an angle smaller than the Brewster angle of the light-guiding substrate, it becomes possible to separate the light into one light that is emitted from the light-guiding substrate and the other light that travels through repeated total reflection within the light-guiding substrate.

[0043] 5 is a schematic diagram of an example of an optical sensor system of the present invention that uses a transmission type diffraction element as the diffraction element 1. In the optical sensor system shown in FIG. 5, the first diffraction element 201B, which is the diffraction element 1, is a transmission type diffraction element. The second diffraction element 202 is the same as in the example described above. As in the example described above, incident light 401, which is a mixture of visible light and SWIR light, enters the first diffraction element 201B from the normal direction of the light guiding substrate 101. As described above, the diffraction angle of visible light by the first diffraction element 201B is small, and the diffraction angle of SWIR light is large. Therefore, the visible light 402 separated by the first diffraction element 201B is incident on the interface between the light-guiding substrate 101 and air at an angle smaller than the Brewster angle of the light-guiding substrate 101, is emitted from the light-guiding substrate 101, and is incident on the first sensor 301 for sensing.

[0044] On the other hand, SWIR light 403 separated at a large diffraction angle by first diffraction element 201B is incident on the interface between light guiding substrate 101 and air at an angle larger than the Brewster angle of light guiding substrate 101 and is totally reflected. The totally reflected SWIR light 403 propagates within light guiding substrate 101 while repeatedly undergoing total reflection, and is incident on second diffraction element 202. As in the above example, SWIR light 403 incident on second diffraction element 202 is diffracted by second diffraction element 202 in the left direction in the figure, and the direction of the light is changed toward second sensor 302. The SWIR light exits light guiding substrate 101 and is sensed by second sensor 302.

[0045] As described above, the optical sensor system of the present invention, which uses a light-guiding substrate, a diffraction element 1 that separates incident light into at least two wavelengths, and a diffraction element 2 that diffracts the light separated by the diffraction element 1, does not require a component that is arranged at an angle with respect to the incident light, and the light path for the light separated according to wavelength can be formed by the light-guiding substrate, as shown in FIGS. 1 to 5 . As a result, the optical sensor system can be made smaller and thinner. Furthermore, by thinning the optical sensor system, the degree of freedom in system design can be improved, which in turn can enhance the designability of the system.

[0046] Although the optical sensor system of the present invention described above separates and detects visible light and SWIR light, or even NIR light, the present invention is not limited to this. That is, in the present invention, the light to be detected, i.e., the light separated by the diffraction element 1, is not limited to light of the wavelengths (wavelength ranges) described above, and may be, for example, infrared light, red light, green light, blue light, etc., in a predetermined wavelength range beyond SWIR light. Among these, SWIR light and NIR light are suitable as detection targets.

[0047] In the optical sensor system of the present invention, there are no limitations on the type of diffraction element, and various known diffraction elements can be used. Examples of diffraction elements include surface relief diffraction elements, volume hologram diffraction elements, and liquid crystal diffraction elements. Among these, liquid crystal diffraction elements are preferred because they can maintain high diffraction efficiency regardless of the diffraction angle.

[0048] The liquid crystal diffraction element may be either a transmission type or a reflection type, and may be appropriately selected depending on the constraints on the configuration and arrangement.

[0049] The diffraction angle, diffraction efficiency, and diffraction wavelength of the liquid crystal diffraction element can be adjusted appropriately to suit the application. The diffraction angle depends on the in-plane pitch (diffraction period (1 period Λ)), and the diffraction efficiency depends on the film thickness. The diffraction wavelength depends on the film thickness in the case of a transmission type and on the chiral pitch length in the case of a reflection type, so it can be adjusted appropriately to suit the application.

[0050] The liquid crystal diffraction element may be used by stacking or adhering elements that diffract light of different wavelengths.

[0051] An example of a reflective liquid crystal diffraction element is shown in FIGS. 6 and 7. As shown in FIG. 6, the reflective liquid crystal diffraction element includes a support 50, an alignment film 52, and a cholesteric liquid crystal layer 54 that functions as a reflective liquid crystal diffraction element. FIG. 7 is a diagram showing the orientation of the liquid crystal compound within the principal plane of the cholesteric liquid crystal layer 54. In the following description, the principal plane of the cholesteric liquid crystal layer 54 is defined as the X-Y plane, and the cross section perpendicular to this X-Y plane is defined as the X-Z plane. As shown in FIG. 6, the cholesteric liquid crystal layer 54 is a layer in which the liquid crystal compound is cholesterically aligned. FIGS. 6 to 8 show an example in which the liquid crystal compound that constitutes the cholesteric liquid crystal layer 54 is a rod-shaped liquid crystal compound.

[0052] 6 includes a support 50, an alignment film 52, and a cholesteric liquid crystal layer 54, but the present invention is not limited to this. The reflective liquid crystal diffraction element may include, for example, only the alignment film 52 and the cholesteric liquid crystal layer 54, or may include only the cholesteric liquid crystal layer 54.

[0053] The support 50 supports the alignment film 52 and the cholesteric liquid crystal layer 54 and may be made of, for example, glass or various resin films. The alignment film 52 is an alignment film for orienting the liquid crystal compound constituting the cholesteric liquid crystal layer 54 into a predetermined liquid crystal alignment pattern. The alignment film 52 may be a known alignment film used for orienting liquid crystal compounds, such as a photo-alignment film using a photo-alignment material. As will be described later, in the liquid crystal diffraction element, the cholesteric liquid crystal layer 54 has a liquid crystal alignment pattern in which the orientation of the optic axis 40A (see FIG. 7 ) derived from the liquid crystal compound 40 changes while continuously rotating along one in-plane direction. Therefore, the alignment film 52 is formed so that the cholesteric liquid crystal layer 54 can form this liquid crystal alignment pattern. In the following description, "the orientation of the optic axis 40A rotates" may also be simply referred to as "the optic axis 40A rotates."

[0054] In the reflective liquid crystal diffraction element, a cholesteric liquid crystal layer 54 is formed on the surface of the alignment film 52. The cholesteric liquid crystal layer 54 is a cholesteric liquid crystal layer formed by fixing a cholesteric liquid crystal phase, and has a liquid crystal alignment pattern in which the direction of the optical axis derived from the liquid crystal compound changes while continuously rotating along at least one direction in the plane.

[0055] As conceptually shown in Figure 6, the cholesteric liquid crystal layer 54 has a helical structure in which liquid crystal compounds 40 are stacked in a spiral, similar to a normal cholesteric liquid crystal layer, and the helically spiraling liquid crystal compounds 40 are stacked at multiple pitches, with one helical pitch (helical pitch P) being the configuration in which the liquid crystal compounds 40 are stacked in a spiral through one rotation (360° rotation).

[0056] As is well known, cholesteric liquid crystal phases exhibit selective reflection for either left- or right-handed circularly polarized light at a specific wavelength. Whether the reflected light is right-handed or left-handed circularly polarized light depends on the twist direction (sense) of the helix of the cholesteric liquid crystal phase. When the twist direction of the helix of the cholesteric liquid crystal phase is right-handed, the cholesteric liquid crystal phase selectively reflects right-handed circularly polarized light, and when the twist direction of the helix is ​​left-handed, the cholesteric liquid crystal phase selectively reflects left-handed circularly polarized light. For example, in the case of a cholesteric liquid crystal layer 54 that selectively reflects right-handed circularly polarized light, the twist direction of the helix of the cholesteric liquid crystal phase is right-handed.

[0057] As is well known, the selective reflection wavelength (selective reflection center wavelength) exhibiting selective reflection depends on the helical pitch P of the cholesteric liquid crystal phase. The longer the helical pitch P of the cholesteric liquid crystal layer 54, the more selectively light of longer wavelengths is reflected. Therefore, the wavelength of light reflected (diffracted) by the liquid crystal diffraction element can be set by, for example, appropriately adjusting the helical pitch P of the cholesteric liquid crystal layer 54. Furthermore, the half-width Δλ (nm) of the selective reflection wavelength range (circularly polarized light reflection wavelength range) exhibiting selective reflection depends on the Δn and helical pitch P of the cholesteric liquid crystal phase, and follows the relationship Δλ = Δn × P. Therefore, the width of the selective reflection wavelength range can be controlled by adjusting Δn.

[0058] 7, in the X-Y plane of the cholesteric liquid crystal layer 54, the liquid crystal compounds 40 are aligned along a plurality of alignment axes D (X direction) that are parallel to each other in the X-Y plane, and on each alignment axis D, the orientation of the optical axis 40A of the liquid crystal compounds 40 changes while continuously rotating in one direction in the plane along the alignment axis D. In the illustrated example, the optical axis 40A rotates clockwise toward the alignment axis D. In a liquid crystal diffraction element having the cholesteric liquid crystal layer 54, the direction of this alignment axis D (X direction), i.e., the direction in which the optical axis 40A of the liquid crystal compounds 40 rotates in the plane of the cholesteric liquid crystal layer 54, is the periodic direction in the diffraction structure of the diffraction element.

[0059] In a reflective liquid crystal diffraction element having a cholesteric liquid crystal layer 54, in the liquid crystal orientation pattern of such a liquid crystal compound 40, the length (distance) over which the optical axis 40A rotates 180° in the direction of the arrangement axis D, along which the optical axis 40A continuously rotates and changes in the plane, is one period Λ (diffraction period (in-plane pitch)) in the diffraction structure of the diffraction element.

[0060] On the other hand, the liquid crystal compounds 40 forming the cholesteric liquid crystal layer 54 have optical axes 40A aligned in the direction of the alignment axis D, i.e., the direction perpendicular to the periodic direction (Y direction in FIG. 4 ). In other words, the liquid crystal compounds 40 forming the cholesteric liquid crystal layer 54 have optical axes 40A of the liquid crystal compounds 40 aligned in the Y direction, such that the angle formed between the optical axes 40A of the liquid crystal compounds 40 and the alignment axis D (X direction) is equal.

[0061] As is well known, a normal cholesteric liquid crystal layer without a liquid crystal orientation pattern specularly reflects incident light. Therefore, when light is incident on a normal cholesteric liquid crystal layer from the normal direction, the normal light is reflected back in the normal direction. In contrast, the cholesteric liquid crystal layer 54 with the above-described liquid crystal orientation pattern reflects incident light at an angle inclined toward the direction of the arrangement axis D (periodic direction) relative to the specular reflection. In other words, the cholesteric liquid crystal layer 54 with a liquid crystal orientation pattern is a reflective liquid crystal diffraction element. The diffraction effect of the cholesteric liquid crystal layer 54 will be described below with reference to the schematic diagram of FIG. 8 .

[0062] As an example, the cholesteric liquid crystal layer 54 is configured to polarize right-handed circularly polarized SWIR light RR Therefore, when light is incident on the cholesteric liquid crystal layer 54, the cholesteric liquid crystal layer 54 selectively reflects right-handed circularly polarized light R of SWIR light. R It reflects only light and transmits all other light.

[0063] The cholesteric liquid crystal layer 54 has a liquid crystal orientation pattern in which the optical axis 40A of the liquid crystal compound 40 changes while rotating along the direction of the alignment axis D, i.e., the periodic direction. The liquid crystal orientation pattern in the cholesteric liquid crystal layer 54 is a periodic pattern along the direction of the alignment axis D. Therefore, right-handed circularly polarized light R of SWIR light incident on the cholesteric liquid crystal layer 54 is polarized. R As conceptually shown in FIG. 8, the right-handed circularly polarized light R of the SWIR light is reflected (diffracted) in a direction according to the period of the liquid crystal orientation pattern. R is reflected (diffracted) in a direction tilted toward the direction of the arrangement axis D with respect to the XY plane (the main surface of the cholesteric liquid crystal layer). As a result, when the cholesteric liquid crystal layer 54 is applied to the first diffraction element 201 and the first diffraction element 201A, SWIR light included in the incident light that is incident on the light guiding substrate 101 from the normal direction is reflected (diffracted) at an angle toward the second diffraction element 202, and other light can be transmitted.

[0064] In the cholesteric liquid crystal layer 54, the direction of the alignment axis D, which is one direction in which the optical axis 40A rotates, can be appropriately set to adjust the reflection direction of light. For example, when reflecting circularly polarized light of the same wavelength and rotation direction, the reflection direction of the circularly polarized light can be reversed by reversing the direction of the alignment axis D, i.e., the periodic direction. In other words, the reflection direction of the same circularly polarized light can be reversed by reversing the rotation direction of the optical axis 40A of the liquid crystal compound 40, which is oriented in the alignment axis D, i.e., the periodic direction, to the counterclockwise direction.

[0065] Furthermore, in liquid crystal layers having the same liquid crystal orientation pattern, the reflection direction is reversed depending on the helical gyration direction of the liquid crystal compound 40, i.e., the gyration direction of the reflected circularly polarized light. For example, if the helical gyration direction is right-handed, right-handed circularly polarized light is selectively reflected. By having a liquid crystal orientation pattern in which the optical axis 40A rotates clockwise along the alignment axis D, right-handed circularly polarized light is reflected with a tilt toward the alignment axis D. Also, for example, if the helical gyration direction is left-handed, left-handed circularly polarized light is selectively reflected. A liquid crystal layer having a liquid crystal orientation pattern in which the optical axis 40A rotates clockwise along the alignment axis D reflects left-handed circularly polarized light with a tilt toward the direction opposite to the alignment axis D. In other words, a cholesteric liquid crystal layer with an opposite helical gyration direction can diffract (reflect) light with a tilt toward the same direction by reversing the alignment axis D direction, i.e., by 180°.

[0066] Therefore, for example, by stacking a cholesteric liquid crystal layer 54 that selectively reflects right-handed circularly polarized SWIR light and a cholesteric liquid crystal layer 54 that selectively reflects left-handed circularly polarized SWIR light, both of which have the same liquid crystal orientation pattern, a first diffraction element 201A is obtained that separates the SWIR light contained in incident light 401 into right-handed and left-handed circularly polarized light and reflects it in two different directions, as shown in Figure 3. In this case, in the optical sensor system shown in Figure 3, the SWIR light 403 diffracted and reflected by the first diffraction element 201A to the right in the figure is right-handed circularly polarized light, and the SWIR light 403 diffracted and reflected to the left in the figure is left-handed circularly polarized light. Alternatively, in this case, in the optical sensor system shown in Figure 3, the SWIR light 403 diffracted and reflected by the first diffraction element 201A to the right in the figure is left-handed circularly polarized light, and the SWIR light 403 diffracted and reflected to the left in the figure is right-handed circularly polarized light. Furthermore, by stacking a cholesteric liquid crystal layer 54 that selectively reflects right-handed circularly polarized SWIR light and a cholesteric liquid crystal layer 54 that selectively reflects left-handed circularly polarized SWIR light, which have the same liquid crystal orientation pattern except for the opposite rotation directions of the liquid crystal compounds toward the alignment axis D, a first diffraction element 201 (diffraction element 203) and a second diffraction element 202 (diffraction element 204) that diffract and reflect incident light in one direction regardless of polarization can be obtained, as shown in Figures 1 (2) and 4. Note that if the first diffraction element 201 (diffraction element 203) and the second diffraction element 202 (diffraction element 204) are formed from a single cholesteric liquid crystal layer, the light selectively reflected by each diffraction element will be either right-handed circularly polarized light or left-handed circularly polarized light.

[0067] As described above, in a reflective liquid crystal diffraction element, the length over which the optical axis of the liquid crystal compound in the liquid crystal orientation pattern in the liquid crystal layer rotates 180° is one period Λ (diffraction period) in the periodic structure of the diffraction element. In a cholesteric liquid crystal layer having a liquid crystal orientation pattern, the shorter the period Λ, the greater the diffraction. That is, the shorter the period Λ, the greater the inclination of the reflected light relative to the specular reflection direction of the incident light. In other words, the shorter the diffraction period Λ, the greater the angle between the specular reflection direction and the reflected light. Therefore, by adjusting the period Λ of the liquid crystal orientation pattern according to the diffraction angle required for the diffraction element, light can be diffracted and reflected in the desired direction. Furthermore, in a cholesteric liquid crystal layer having this liquid crystal orientation pattern, the reflection angle (diffraction angle) of light varies depending on the wavelength of the reflected light. Specifically, the longer the wavelength of light, the greater the angle of the reflected light relative to the incident light.

[0068] 9 and 10 are schematic diagrams showing a transmissive liquid crystal diffraction element. The liquid crystal diffraction element shown in FIGS. 9 and 10 has a support 50, an alignment film 52, and a liquid crystal layer 56. The support 50 and the alignment film 52 are the same as those described above. As shown in FIG. 9, in the transmissive liquid crystal diffraction element, the liquid crystal compound 40 forming the liquid crystal layer 56 does not twist and rotate helically in the thickness direction, and the optical axis 40A is located at the same position in the plane direction.

[0069] Like the cholesteric liquid crystal layer 54, the liquid crystal layer 56 also has a liquid crystal orientation pattern in which the direction of the optical axis 40A derived from the liquid crystal compound 40 changes while continuously rotating along the arrangement axis D direction, i.e., the X direction, in the plane. That is, in this liquid crystal layer 56 (transmissive liquid crystal diffraction element), the periodic direction is also the arrangement axis D direction (X direction). Also in this liquid crystal layer 56, as shown in FIG. 10 , one period Λ (diffraction period) of the diffractive structure is the length over which the optical axis 40A rotates 180° along the arrangement axis D direction, i.e., the periodic direction. Meanwhile, the liquid crystal compounds 40 forming the liquid crystal layer 56 are arranged at equal intervals in the Y direction perpendicular to the X direction, i.e., the Y direction perpendicular to the arrangement axis D (periodic direction), which is the direction in which the optical axis 40A continuously rotates.

[0070] In the liquid crystal layer 56, the liquid crystal compounds aligned in the Y direction have the same angle between their optical axes 40A and the X direction. A region where the liquid crystal compounds 40, whose optical axes 40A and alignment axis D form the same angle, are arranged in the Y direction is referred to as a region R. In this case, the in-plane retardation (Re) value in each region R is preferably half the wavelength, i.e., λ / 2.

[0071] When circularly polarized light enters such a liquid crystal layer 56, the transmitted light is diffracted (refracted) and the direction of the circular polarization is changed. This action is conceptually shown in Figures 11 and 12. Assume that the liquid crystal layer 56 has a thickness multiplied by the refractive index difference of the liquid crystal compound 40. As shown in Figure 11, when left-handed circularly polarized incident light L1 enters the liquid crystal layer 56 when the refractive index difference of the liquid crystal compound 40 in the liquid crystal layer 56 multiplied by the thickness is λ / 2, the incident light L1 is given a phase difference of 180° as it passes through the liquid crystal layer 56, and the transmitted light L2 is converted to right-handed circularly polarized light. Furthermore, because the liquid crystal orientation pattern formed in the liquid crystal layer 56 is a periodic pattern in the direction of the alignment axis D, the transmitted light L2 travels in a direction different from the traveling direction of the incident light L1. In this way, the left-handed circularly polarized incident light L1 is converted to right-handed circularly polarized transmitted light L2, which is tilted at a certain angle toward the alignment axis D with respect to the incident direction.

[0072] On the other hand, as shown in FIG. 12 , when the product of the refractive index difference and thickness of the liquid crystal compound 40 in the liquid crystal layer 56 is λ / 2, when right-handed circularly polarized incident light L4 enters the liquid crystal layer 56, the incident light L4 is given a phase difference of 180° as it passes through the liquid crystal layer 56 and is converted into left-handed circularly polarized transmitted light L5. Furthermore, because the liquid crystal orientation pattern formed in the liquid crystal layer 56 is a periodic pattern in the direction of the alignment axis D, the transmitted light L5 travels in a direction different from the direction of propagation of the incident light L4. At this time, the transmitted light L5 travels in a different direction from the transmitted light L2, i.e., in the opposite direction from the alignment axis D relative to the incident direction. In this way, the incident light L4 is converted into left-handed circularly polarized transmitted light L5 tilted at a certain angle in the opposite direction from the alignment axis D relative to the incident direction. In other words, this liquid crystal layer 56 is a transmissive liquid crystal diffraction element.

[0073] Similar to the cholesteric liquid crystal layer 54, the liquid crystal layer 56 can adjust the diffraction angles of the transmitted light L2 and the transmitted light L5 by changing the period Λ (diffraction period) of the formed liquid crystal orientation pattern. Specifically, the shorter the period Λ of the liquid crystal orientation pattern, the greater the diffraction angle of the transmitted light L2 and L5. Therefore, by adjusting the period Λ according to the desired refraction angle of the transmitted light, the diffraction element can diffract and transmit light of each wavelength range in the desired direction. Furthermore, by reversing the rotation direction of the optical axis 40A of the liquid crystal compound 40, which rotates along the direction of the array axis D, i.e., the periodic direction, the diffraction direction of the transmitted light can be reversed. That is, in the examples shown in Figures 9 to 12, the rotation direction of the optical axis 40A facing the direction of the array axis D, i.e., the periodic direction, is clockwise. However, by changing this rotation direction counterclockwise, the diffraction direction of the transmitted light can be reversed.

[0074] Furthermore, in the liquid crystal layer 56 functioning as a transmissive liquid crystal diffraction element, the longer the wavelength of light, the greater the diffraction (refraction). Therefore, as shown in FIG. 5, by using a transmissive liquid crystal diffraction element as the first diffraction element 201B, for example, when incident light containing a mixture of visible light and SWIR light is incident, the visible light and SWIR light can be separated due to the difference in the magnitude of diffraction. Furthermore, as described above, visible light with a small diffraction angle by the first diffraction element 201B can be incident on the interface between the light guiding substrate 101 and air at an angle smaller than the Brewster angle and emitted from the light guiding substrate 101. On the other hand, SWIR light with a large diffraction angle by the first diffraction element 201B can be incident on the interface between the light guiding substrate 101 and air at an angle larger than the Brewster angle, and can be guided within the light guiding substrate 101 by repeated total reflection.

[0075] 1 to 5 show the diffraction element attached to the light-guiding substrate, but it is not necessarily required to be attached. Note that, for attaching the diffraction element, a known adhesive such as OCA (Optically Clear Adhesive) can be used as long as it can transmit the light to be measured.

[0076] The light guiding substrate travels through the interior of the light guiding substrate while undergoing total reflection due to the difference in refractive index between the material of the light guiding substrate and air on both the front and back main surfaces. It is also desirable to suppress unintended interfacial reflections due to the difference in refractive index between the substrate and the diffraction element. Therefore, it is desirable for the light guiding substrate to have a refractive index in the range of 1.45 to 2.0.

[0077] As described above, the light-guiding substrate can be made of various known light-guiding members, such as light-guiding plates used in backlight units of liquid crystal display devices. Suitable materials for the light-guiding substrate include, but are not limited to, glass, acrylic, and polycarbonate. Furthermore, a thickness of 0.5 to 5.0 mm is preferable for the light-guiding substrate from the standpoints of weight reduction and durability (crack prevention). Here, in cases where the diffraction element is capable of diffracting incident light at a larger angle, such as a liquid crystal diffraction element, even if the thickness of the light-guiding substrate is thin, at 0.5 to 2.0 mm, the angle between the separated light beams is large, allowing multiple sensors to be installed horizontally in parallel.

[0078] As the sensor, various known sensors (optical sensors) can be used as long as they are capable of measuring light of the wavelength to be measured. Examples of sensors include a CCD (Charge Coupled Device) sensor, a CMOS (Complementary Metal-Oxide-Semiconductor) sensor, an InGaAs (Indium gallium arsenide) sensor, and a photodiode array. If necessary, the sensor may have a filter such as a band-pass filter, low-pass filter, or high-pass filter that blocks light of wavelengths other than the light to be measured.

[0079] Although typical embodiments of the present invention have been described above, the present invention is not limited to these, and various embodiments that can be understood by a person skilled in the art can be applied as long as they fall within the scope of the claims of the present invention.

[0080] 40 Liquid crystal compound 40A Optical axis 50 Support 52 Alignment film 54 Cholesteric liquid crystal layer 56 Liquid crystal layer 101 Light guiding substrate 201, 201A, 201B First diffraction element 202 Second diffraction element 203, 204 Diffraction elements 301 First sensor 302 Second sensor 303 Third sensor 401 Incident light 402 Visible light 403 SWIR light 404 NIR light

Claims

1. An optical sensor system having at least a light-guiding substrate; a diffraction element 1 that separates incident light into light of at least two wavelengths; a diffraction element 2 that diffracts the light separated by said diffraction element 1; and a plurality of sensors that respectively detect the plurality of light beams separated by said diffraction element 1.

2. The optical sensor system according to claim 1, wherein the diffraction element 1 separates the incident light into light having a wavelength in the visible light region and light having a wavelength in the short wave infrared region.

3. The optical sensor system of claim 1, wherein the diffraction element 1 separates the incident light into light having a wavelength in the visible light region, light having a wavelength in the near infrared region, and light having a wavelength in the short wave infrared region.

4. The optical sensor system of claim 1, wherein either the diffractive element 1 or the diffractive element 2, or both the diffractive element 1 and the diffractive element 2, are liquid crystal diffractive elements.

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