Patent Description:
There have conventionally been proposed various types of device that are capable of scanning space with light.

PTL <NUM> discloses a configuration in which an optical scan can be performed with a mirror-rotating driving apparatus.

PTL <NUM> discloses an optical phased array having a plurality of two-dimensionally arrayed nanophotonic antenna elements. Each antenna element is optically coupled to a variable optical delay line (i.e. a phase shifter). In this optical phased array, a coherent light beam is guided to each antenna element by a waveguide, and the phase of the light beam is shifted by the phase shifter. This makes it possible to vary the amplitude distribution of a far-field radiating pattern.

PTL <NUM> discloses an optical deflection element including: a waveguide including an optical waveguide layer through the inside of which light is guided and first distributed Bragg reflectors formed on upper and lower surfaces, respectively, of the optical waveguide layer; a light entrance through which light enters the waveguide, and a light exit formed on a surface of the waveguide to let out light having entered through the light entrance and being guided through the inside of the waveguide.

One aspect of the present disclosure provides a novel optical device that is capable of achieving an optical scan through a comparatively simple configuration. Solution to Problem.

An optical device according to one aspect of the present disclosure includes a first mirror having a first reflecting surface and extending along a first direction, a second mirror having a second reflecting surface that faces the first reflecting surface and extending along the first direction, and an optical waveguide layer, located between the first mirror and the second mirror, that causes light to propagate along the first direction. A transmittance of the first mirror is higher than a transmittance of the second mirror. A reflection spectrum of at least either the first mirror or the second mirror with respect to light arriving from a direction normal to the reflecting surface includes, in a wavelength region in which a reflectance is higher than or equal to <NUM>%, a local maximum point and first and second points of inflection located closer to a long-wavelength side than the local maximum point.

It should be noted that general or specific embodiments may be implemented as a device, a system, a method, or any selective combination thereof.

One aspect of the present disclosure makes it possible to achieve an optical one-dimensional scan or two-dimensional scan through a comparatively simple configuration.

Prior to a description of embodiments of the present disclosure, underlying knowledge forming the basis of the present disclosure is described.

The inventor found that a conventional optical scan device has difficulty in scanning space with light without making a complex apparatus configuration.

For example, the technology disclosed in PTL <NUM> requires a mirror-rotating driving apparatus. This undesirably makes a complex apparatus configuration that is not robust against vibration.

In the optical phased array described in PTL <NUM>, it is necessary to divide light into lights, introduce the lights into a plurality of column waveguide and a plurality of row waveguides, and guide the lights to the plurality of two-dimensionally arrayed antenna elements. This results in very complex wiring of optical waveguides through which to guide the lights. This also makes it impossible to attain a great two-dimensional scanning range. Furthermore, to two-dimensionally vary the amplitude distribution of emitted light in a far field, it is necessary to connect phase shifters separately to each of the plurality of two-dimensionally arrayed antenna elements and attach phase-controlling wires to the phase shifters. This causes the phases of lights falling on the plurality of two-dimensionally arrayed antenna elements to vary by a different amount. This makes the elements very complex in configuration.

The inventor focused on the foregoing problems in the conventional technologies and studied configurations to solve these problems. The inventor found that the foregoing problems can be solved by using an optical waveguide element having a pair of mirrors facing each other and an optical waveguide layer sandwiched between the mirrors. One of the pair of mirrors of the optical waveguide element has a higher light transmittance than the other and lets out a portion of light propagating through the optical waveguide layer. As will be mentioned later, the direction of light emitted (or the angle of emission) can be changed by adjusting the refractive index or thickness of the optical waveguide layer or the wavelength of light that is inputted to the optical waveguide layer. More specifically, by changing the refractive index, the thickness, or the wavelength, a component constituting the wave number vector (wave vector) of the emitted light and acting in a direction along a lengthwise direction of the optical waveguide layer can be changed. This allows a one-dimensional scan to be achieved.

Furthermore, in a case where an array of a plurality of the optical waveguide elements is used, a two-dimensional scan can be achieved. More specifically, a direction in which lights going out from the plurality of optical waveguide elements reinforce each other can be changed by giving an appropriate phase difference to lights that are supplied to the plurality of optical waveguide elements and adjusting the phase difference. A change in phase difference brings about a change in a component constituting the wave number vector of the emitted light and acting in a direction that intersects the direction along the lengthwise direction of the optical waveguide layer. This makes it possible to achieve a two-dimensional scan. Even in a case where a two-dimensional scan is performed, it is not necessary to cause the refractive index or thickness of each of a plurality of the optical waveguide layers or the wavelength of light to vary by a different amount. That is, a two-dimensional scan can be performed by giving an appropriate phase difference to lights that are supplied to the plurality of optical waveguide layers and causing at least one of the refractive index of each of the plurality of optical waveguide layers, the thickness of each of the plurality of optical waveguide layers, or the wavelength to vary by the same amount in synchronization.

In this way, the present disclosure makes it possible to achieve an optical one-dimensional or two-dimensional scan through a comparatively simple configuration.

The phrase "at least one of the refractive index, the thickness, or the wavelength" herein means at least one selected from the group consisting of the refractive index of an optical waveguide layer, the thickness of an optical waveguide layer, and the wavelength of light that is inputted to an optical waveguide layer. For a change in direction of emission of light, any one of the refractive index, the thickness, and the wavelength may be controlled alone. Alternatively, the direction of emission of light may be changed by controlling any two or all of these three. The wavelength of light that is inputted to the optical waveguide layer may be controlled instead of or in addition to controlling the refractive index or the thickness.

The foregoing fundamental principles are similarly applicable to uses in which optical signals are received as well as uses in which light is emitted. The direction of light that can be received can be one-dimensionally changed by changing at least one of the refractive index, the thickness, or the wavelength. Furthermore, the direction of light that can be received can be two-dimensionally changed by changing a phase difference of light through a plurality of phase shifters connected separately to each of a plurality of unidirectionally-arrayed waveguide elements.

An optical scan device and an optical receiver device of the present disclosure may be used, for example, as an antenna in a photodetection system such as a LiDAR (light detection and ranging) system. The LiDAR system, which involves the use of short-wavelength electromagnetic waves (visible light, infrared radiation, or ultraviolet radiation), can detect a distance distribution of objects with higher resolution than a radar system that involves the use of radio waves such as millimeter waves. Such a LiDAR system is mounted, for example, on a movable body such as an automobile, a UAV (unmanned aerial vehicle, i.e. a drone), or an AGV (automated guided vehicle), and may be used as one of the crash avoidance technologies. The optical scan device and the optical receiver device are herein sometimes collectively referred to as "optical device". Further, a device that is used in the optical scan device or the optical receiver device is sometimes referred to as "optical device", too. The term "optical device" is also used to refer to an optical component that constitutes the optical scan device or the optical receiver device.

The following describes, as an example, a configuration of an optical scan device that performs a two-dimensional scan. Note, however, that an unnecessarily detailed description may be omitted. For example, a detailed description of a matter that is already well known and a repeated description of substantially the same configuration may be omitted. This is intended to facilitate understanding of persons skilled in the art by avoiding making the following description unnecessarily redundant. It should be noted that the inventors provide the accompanying drawings and the following description for persons skilled in the art to fully understand the present disclosure and do not intend to limit the subject matter recited in the claims. In the following description, identical or similar constituent elements are given the same reference numerals.

In the present disclosure, the term "light" means electromagnetic waves including ultraviolet radiation (ranging from approximately <NUM> to approximately <NUM> in wavelength) and infrared radiation (ranging from approximately <NUM> to approximately <NUM> in wavelength) as well as visible light (ranging approximately <NUM> to approximately <NUM> in wavelength). Ultraviolet radiation is herein sometimes referred to as "ultraviolet light", and infrared radiation is herein sometimes referred to as "infrared light".

In the present disclosure, an optical "scan" means changing the direction of light. A "one-dimensional scan" means changing the direction of light along a direction that intersects the direction. A "two-dimensional scan" means two-dimensionally changing the direction of light along a plane that intersects the direction.

<FIG> is a perspective view schematically showing an example of an optical scan device <NUM>. The optical scan device <NUM> includes a waveguide array including a plurality of waveguide elements <NUM>. Each of the plurality of waveguide elements <NUM> has a shape extending in a first direction (in <FIG>, an X direction). The plurality of waveguide elements <NUM> are regularly arrayed in a second direction (in <FIG>, a Y direction) that intersects the first direction. The plurality of waveguide elements <NUM>, while propagating light in the first direction, emit the light in a third direction D3 that intersects an imaginary plane parallel to the first and second directions. Although, in the present disclosure, the first direction (X direction) and the second direction (Y direction) are orthogonal to each other, they may not be orthogonal to each other. Although, in the present disclosure, the plurality of waveguide elements <NUM> are placed at equal spacings in the Y direction, they do not necessarily need to be placed at equal spacings.

It should be noted that the orientation of a structure shown in a drawing of the present disclosure is set in view of understandability of explanation and is in no way intended to restrict any actual orientation whatsoever. Further, the shape and size of the whole or a part of a structure shown in a drawing are not intended to restrict an actual shape and size.

Each of the plurality of waveguide elements <NUM> has first and second mirrors <NUM> and <NUM> (each hereinafter sometimes referred to simply as "mirror") facing each other and an optical waveguide layer <NUM> located between the mirror <NUM> and the mirror <NUM>. Each of the mirrors <NUM> and <NUM> has a reflecting surface, situated at the interface with the optical waveguide layer <NUM>, that intersects the third direction D3. The mirror <NUM>, the mirror <NUM>, and the optical waveguide layer <NUM> have shapes extending in the first direction (X direction).

The reflecting surface of the first mirror <NUM> and the reflecting surface of the second mirror <NUM> face each other substantially in a parallel fashion. Of the two mirrors <NUM> and <NUM>, at least the first mirror <NUM> has the property of transmitting a portion of light propagating through the optical waveguide layer <NUM>. In other words, the first mirror <NUM> has a higher light transmittance against the light than the second mirror <NUM>. For this reason, a portion of light propagating through the optical waveguide layer <NUM> is emitted outward from the first mirror <NUM>. Such mirrors <NUM> and <NUM> may for example be multilayer mirrors that are formed by multilayer films of dielectrics (sometimes referred to as "multilayer reflective films" or "distributed Bragg reflector (DBR)").

An optical two-dimensional scan can be achieved by controlling the phases of lights that are inputted to the respective waveguide elements <NUM> and, furthermore, causing the refractive indices or thicknesses of the optical waveguide layers <NUM> of these waveguide elements <NUM> or the wavelengths of lights that are inputted to the optical waveguide layers <NUM> to simultaneously change in synchronization.

In order to achieve such a two-dimensional scan, the inventor conducted an analysis on the principle of operation of a waveguide element <NUM>. As a result of their analysis, the inventor succeeded in achieving an optical two-dimensional scan by driving a plurality of waveguide elements <NUM> in synchronization.

As shown in <FIG>, inputting light to each waveguide element <NUM> causes light to exit the waveguide element <NUM> through an exit surface of the waveguide element <NUM>. The exit face is located on the side opposite to the reflecting surface of the first mirror <NUM>. The direction D3 of the emitted light depends on the refractive index and thickness of the optical waveguide layer and the wavelength of light. In the present disclosure, at least one of the refractive index of each optical waveguide layer, the thickness of each optical waveguide layer, or the wavelength is controlled in synchronization so that lights that are emitted separately from each waveguide element <NUM> are oriented in substantially the same direction. This makes it possible to change X-direction components of the wave number vectors of lights that are emitted from the plurality of waveguide elements <NUM>. In other words, this makes it possible to change the direction D3 of the emitted light along a direction <NUM> shown in <FIG>.

Furthermore, since the lights that are emitted from the plurality of waveguide elements <NUM> are oriented in the same direction, the emitted lights interfere with one another. By controlling the phases of the lights that are emitted from the respective waveguide elements <NUM>, a direction in which the lights reinforce one another by interference can be changed. For example, in a case where a plurality of waveguide elements <NUM> of the same size are placed at equal spacings in the Y direction, lights differing in phase by a constant amount from one another are inputted to the plurality of waveguide elements <NUM>. By changing the phase differences, Y-direction components of the wave number vectors of the emitted lights can be changed. In other words, by varying phase differences among lights that are introduced into the plurality of waveguide elements <NUM>, the direction D3, in which the emitted lights reinforce one another by interference, can be changed along a direction <NUM> shown in <FIG>. This makes it possible to achieve an optical two-dimensional scan.

The following describes the principle of operation of the optical scan device <NUM>.

<FIG> is a diagram schematically showing an example of a cross-section structure of one waveguide element <NUM> and an example of propagating light. With a Z direction being a direction perpendicular of the X and Y directions shown in <FIG> schematically shows a cross-section parallel to an X-Z plane of the waveguide element <NUM>. The waveguide element <NUM> is configured such that the pair of mirrors <NUM> and <NUM> are disposed so as to hold the optical waveguide layer <NUM> therebetween. Light <NUM> introduced into the optical waveguide layer <NUM> through one end of the optical waveguide layer <NUM> in the X direction propagates through the inside of the optical waveguide layer <NUM> while being repeatedly reflected by a first reflecting surface <NUM> of the first mirror <NUM> provided on an upper surface (in <FIG>, the upper side) of the optical waveguide layer <NUM> and a second reflecting surface <NUM> of the second mirror <NUM> provided on a lower surface (in <FIG>, the lower side) of the optical waveguide layer <NUM>. The light transmittance of the first mirror <NUM> is higher than the light transmittance of the second mirror <NUM>. For this reason, a portion of the light can be outputted mainly from the exit surface 30es of the first mirror <NUM>. In the following, the first reflecting surface <NUM> is simply referred to as "reflecting surface <NUM>", and the "second reflecting surface <NUM>" simply as "reflecting surface <NUM>".

In the case of a waveguide such as an ordinary optical fiber, light propagates along the waveguide while repeating total reflection. On the other hand, in the case of a waveguide element <NUM>, light propagates while being repeatedly reflected by the mirrors <NUM> and <NUM> disposed above and below, respectively, the optical waveguide layer <NUM>. For this reason, there are no restrictions on angles of propagation of light. The term "angle of propagation of light" here means an angle of incidence on the interface between the mirror <NUM> or <NUM> and the optical waveguide layer <NUM>. Light falling on the mirror <NUM> or <NUM> at an angle that is closer to the perpendicular can be propagated, too. That is, light falling on the interface at an angle that is smaller than a critical angle of total reflection can be propagated, too. This causes the group speed of light in the direction of propagation of light to be much lower than the speed of light in free space. For this reason, the waveguide element <NUM> has such a property that conditions for propagation of light vary greatly according to changes in the wavelength of light, the thickness of the optical waveguide layer <NUM>, and the refractive index of the optical waveguide layer <NUM>. The waveguide element <NUM> is referred to as "reflective waveguide" or "slow light waveguide".

The angle of emission θ of light that is emitted into the air from the waveguide element <NUM> is expressed by Formula (<NUM>) as follows:
[Math. <NUM>] <MAT>.

As can be seen from Formula (<NUM>), the direction of emission of light can be changed by changing any of the wavelength λ of light in the air, the refractive index nw of the optical waveguide layer <NUM>, and the thickness d of the optical waveguide layer <NUM>.

For example, in a case where nw = <NUM>, d = <NUM>, λ = <NUM>, and m = <NUM>, the angle of emission is <NUM> degree. Changing the refractive index from this state to nw = <NUM> changes the angle of emission to approximately <NUM> degrees. Meanwhile, changing the thickness to d = <NUM> without changing the refractive index changes the angle of emission to approximately <NUM> degrees. Changing the wavelength to λ = <NUM> without changing the refractive index or the thickness changes the angle of emission to approximately <NUM> degrees. In this way, the direction of emission of light can be greatly changed by changing any of the wavelength λ of light, the refractive index nw of the optical waveguide layer <NUM>, and the thickness d of the optical waveguide layer <NUM>.

Accordingly, the optical scan device <NUM> of the present disclosure controls the direction of emission of light by controlling at least one of the wavelength λ of light that is inputted to each of the optical waveguide layers <NUM>, the refractive index nw of each of the optical waveguide layers <NUM>, or the thickness d of each of the optical waveguide layers <NUM>. The wavelength λ of light may be kept constant without being changed during operation. In that case, an optical scan can be achieved through a simpler configuration. The wavelength λ is not limited to a particular wavelength. For example, the wavelength λ may be included in a wavelength range of <NUM> to <NUM> (from visible light to near-infrared light) within which high detection sensitivity is attained by a common photodetector or image sensor that detects light by absorbing light through silicon (Si). In another example, the wavelength λ may be included in a near-infrared wavelength range of <NUM> to <NUM> within which an optical fiber or a Si waveguide has a comparatively small transmission loss. It should be noted that these wavelength ranges are merely examples. A wavelength range of light that is used is not limited to a wavelength range of visible light or infrared light but may for example be a wavelength range of ultraviolet light.

In order to change the direction of emitted light, the optical scan device <NUM> may include a first adjusting element that changes at least one of the refractive index of the optical waveguide layer <NUM> of each waveguide element <NUM>, the thickness of the optical waveguide layer <NUM> of each waveguide element <NUM>, or the wavelength.

As stated above, using a waveguide element <NUM> makes it possible to greatly change the direction of emission of light by changing at least one of the refractive index nw of the optical waveguide layer <NUM>, the thickness d of the optical waveguide layer <NUM>, or the wavelength λ. This makes it possible to change, to a direction along the waveguide element <NUM>, the angle of emission of light that is emitted from the mirror <NUM>. By using at least one waveguide element <NUM>, such a one-dimensional scan can be achieved.

In order to adjust the refractive index of at least a part of the optical waveguide layer <NUM>, the optical waveguide layer <NUM> may contain a liquid crystal material or an electro-optical material. The optical waveguide layer <NUM> may be sandwiched between a pair of electrodes. By applying a voltage to the pair of electrodes, the refractive index of the optical waveguide layer <NUM> can be changed.

In order to adjust the thickness of the optical waveguide layer <NUM>, at least one actuator may be connected, for example, to at least either the first mirror <NUM> or the second mirror <NUM>. The thickness of the optical waveguide layer <NUM> can be changed by varying the distance between the first mirror <NUM> and the second mirror <NUM> through the at least one actuator. When the optical waveguide layer <NUM> is formed from liquid, the thickness of the optical waveguide layer <NUM> may easily change.

In a waveguide array in which a plurality of waveguide elements <NUM> are unidirectionally arrayed, the interference of lights that are emitted from the respective waveguide elements <NUM> brings about a change in direction of emission of light. By adjusting the phases of lights that are supplied separately to each waveguide element <NUM>, the direction of emission of light can be changed. The following describes the principles on which it is based.

<FIG> is a diagram showing a cross-section of a waveguide array that emits light in a direction perpendicular to an exit face of the waveguide array. <FIG> also describes the phase shift amounts of lights that propagate separately through each waveguide element <NUM>. Note here that the phase shift amounts are values based on the phase of the light that propagates through the leftmost waveguide element <NUM>. The waveguide array of the present disclosure includes a plurality of waveguide elements <NUM> arrayed at equal spacings. In <FIG>, the dashed circular arcs indicate the wave fronts of lights that are emitted separately from each waveguide element <NUM>. The straight line indicates a wave front that is formed by the interference of the lights. The arrow indicates the direction of light that is emitted from the waveguide array (i.e. the direction of a wave number vector). In the example shown in <FIG>, lights propagating through the optical waveguide layers <NUM> of each separate waveguide element <NUM> are identical in phase to one another. In this case, the light is emitted in a direction (Z direction) perpendicular to both an array direction (Y direction) of the waveguide elements <NUM> and a direction (X direction) in which the optical waveguide layers <NUM> extend.

<FIG> is a diagram showing a cross-section of a waveguide array that emits light in a direction different from a direction perpendicular to an exit face of the waveguide array. In the example shown in <FIG>, lights propagating through the optical waveguide layers <NUM> of the plurality of waveguide elements <NUM> differ in phase from one another by a constant amount (Δϕ) in the array direction. In this case, the light is emitted in a direction different from the Z direction. By varying Δϕ, a Y-direction component of the wave number vector of the light can be changed. Assuming that p is the center-to-center distance between two adjacent waveguide elements <NUM>, the angle of emission α<NUM> of light is expressed by Formula (<NUM>) as follows:
[Math. <NUM>] <MAT>.

In the example shown in <FIG>, the direction of emission of light is parallel to the X-Z plane. That is, α<NUM> = <NUM>°. In each of the examples shown in <FIG>, the direction of light that is emitted from the optical scan device <NUM> is parallel to a Y-Z plane. That is, θ = <NUM>°. However, in general, the direction of light that is emitted from the optical scan device <NUM> is not parallel to the X-Z plane or the Y-Z plane. That is, θ ≠ <NUM>° and α<NUM> ≠ <NUM>°.

<FIG> is a perspective view schematically showing an example of a waveguide array in a three-dimensional space. The bold arrow shown in <FIG> represents the direction of light that is emitted from the optical scan device <NUM>. θ is the angle formed by the direction of emission of light and the Y-Z plane. θ satisfies Formula (<NUM>). α<NUM> is the angle formed by the direction of emission of light and the X-Z plane. α<NUM> satisfies Formula (<NUM>).

In order to control the phases of lights that are emitted from the respective waveguide elements <NUM>, a phase shifter that changes the phase of light may be provided, for example, at a stage prior to the introduction of light into a waveguide element <NUM>. The optical scan device <NUM> of the present disclosure includes a plurality of phase shifters connected separately to each of the plurality of waveguide elements <NUM> and a second adjusting element that adjusts the phases of lights that propagate separately through each phase shifter. Each phase shifter includes a waveguide joined either directly or via another waveguide to the optical waveguide layer <NUM> of a corresponding one of the plurality of waveguide elements <NUM>. The second adjusting element varies differences in phase among lights propagating from the plurality of phase shifters to the plurality of waveguide elements <NUM> and thereby changes the direction (i.e. the third direction D3) of light that is emitted from the plurality of I waveguide elements <NUM>. As is the case with the waveguide array, a plurality of arrayed phase shifters are hereinafter sometimes referred to as "phase shifter array".

<FIG> is a schematic view of a waveguide array 10A and a phase shifter array 80A as seen from an angle parallel with a direction (Z direction) normal to a light exit face. In the example shown in <FIG>, all phase shifters <NUM> have the same propagation characteristics, and all waveguide elements <NUM> have the same propagation characteristics. The phase shifter <NUM> and the waveguide elements <NUM> may be the same in length or may be different in length. In a case where the phase shifters <NUM> are equal in length, the respective phase shift amounts can be adjusted, for example, by a driving voltage. Further, by making a structure in which the lengths of the phase shifters <NUM> vary in equal steps, phase shifts can be given in equal steps by the same driving voltage. Furthermore, this optical scan device <NUM> further includes an optical divider <NUM> that divides light into lights and supplies the lights to the plurality of phase shifters <NUM>, a first driving circuit <NUM> that drives each waveguide element <NUM>, and a second driving circuit <NUM> that drives each phase shifter <NUM>. The straight arrow shown in <FIG> indicates the inputting of light. A two-dimensional scan can be achieved by independently controlling the first driving circuit <NUM> and the second driving circuit <NUM>, which are separately provided. In this example, the first driving circuit <NUM> functions as one element of the first adjusting element, and the second driving circuit <NUM> functions as one element of the second adjusting element.

The first driving circuit <NUM> changes at least either the refractive index or thickness of the optical waveguide layer <NUM> of each waveguide element <NUM> and thereby changes the angle of light that is emitted from the optical waveguide layer <NUM>. The second driving circuit <NUM> changes the refractive index of the waveguide 20a of each phase shifter <NUM> and thereby changes the phase of light that propagates through the inside of the waveguide 20a. The optical divider <NUM> may be constituted by a waveguide through which light propagates by total reflection or may be constituted by a reflective waveguide that is similar to a waveguide element <NUM>.

The lights divided by the optical divider <NUM> may be introduced into the phase shifters <NUM> after the phases of the lights have been controlled, respectively. This phase control may involve the use of, for example, a passive phase control structure based on an adjustment of the lengths of waveguides leading to the phase shifters <NUM>. Alternatively, it is possible to use phase shifters that are similar in function to the phase shifters <NUM> and that can be controlled by electrical signals. The phases may be adjusted by such a method prior to introduction into the phase shifters <NUM>, for example, so that lights of equal phases are supplied to all phase shifters <NUM>. Such an adjustment makes it possible to simplify the control of each phase shifter <NUM> by the second driving circuit <NUM>.

An optical device that is similar in configuration to the aforementioned optical scan device <NUM> can also be utilized as an optical receiver device. Details of the principle of operation of the optical device, a method of operation of the optical device, and the like are disclosed in <CIT>, the disclosure of which is hereby incorporated by reference herein in its entirety.

The beam line width of light that is emitted from a slow light waveguide <NUM> determines the resolving power of a scan. A narrower beam line width leads to brings about improvement in resolving power of a scan, and a greater beam line width brings about a decrease in resolving power of a scan. The following describes a relationship between the beam line width and the angle of emission of light that is emitted from a conventional slow light waveguide <NUM>.

A distant pattern of light that is emitted from the slow light waveguide <NUM> is equivalent to the Fourier transform of the distribution of electric fields over the exit surface 30es shown in <FIG>. That is, an increase in length of propagation of light propagating through the optical waveguide layer <NUM> leads to a decrease in beam line width of the emitted light at a great distance. On the other hand, a decrease in length of propagation of light propagating through the optical waveguide layer <NUM> leads to an increase in beam line width of the emitted light at a great distance. The term "length of propagation" here means the distance that the intensity of light <NUM> that propagates through the optical waveguide layer <NUM> while attenuating decreases to <NUM>/e. e is the base of a natural logarithm. The term "beam line width" means the angle Δθ of spread to both sides with the angle of emission θ in the center. Specifically, the beam line width is described as a full width at half maximum of the emitted light in an angle spectrum.

<FIG> and <FIG> are diagrams schematically showing how light is emitted from the exit surface 30es in cases where an angle of propagation ϕ is relatively small and relatively large, respectively. For simplicity, assume that the reflectance of the mirror <NUM> is constant regardless of the angle of propagation ϕ. In the example shown in <FIG>, in which the angle of propagation ϕ is small, the number of times the reflecting surface <NUM> reflects the light <NUM> per unit length is large. Accordingly, a length of propagation Lp is short. In the example shown in <FIG>, in which the angle of propagation ϕ is large, the number of times the reflecting surface <NUM> reflects the light <NUM> per unit length is small. Accordingly, the length of propagation Lp is long. Since there is a positive correlation between the angle of propagation ϕ and the angle of emission θ, an increase in the angle of emission θ leads to an increase in the length of propagation Lp. It should be noted that the length of propagation Lp indicated by a double-headed arrow in each of <FIG> and <FIG> is a schematic representation and does not represent an actual length.

<FIG> is a diagram showing an example of a relationship between the length of propagation Lp and the beam line width Δθ of emitted light. The graph shown in <FIG> shows a result obtained by calculating, with varying lengths of propagation, the line width of a light beam that is emitted from one slow light waveguide <NUM> in which conditions such as the dimensions and dielectric constant of each constituent element are set as appropriate. As shown in <FIG>, an increase in the length of propagation Lp leads to a decrease in the beam line width Δθ of the emitted light. Since an increase in the angle of emission θ leads to an increase in the length of propagation Lp as mentioned above, an increase in the angle of emission θ leads to a decrease in the beam line width Δθ of the emitted light. Thus, since the beam line width Δθ of the emitted light depends on the angle of emission θ, a change in the angle of emission θ causes a change in resolving power of a scan.

The inventors found the foregoing problem and studied to configure an optical device to solve this problem. As a result, the inventors found that the foregoing problem can be solved by using, as one of two mirrors of a slow light waveguide, a non-conventional mirror having a special property. An embodiment of the present disclosure described below is based on these findings. The following describes an exemplary embodiment of the present disclosure.

For comparison, the reflection spectrum of a conventional DBR(s) that may be used as the mirror <NUM> and/or the mirror <NUM> in a slow light waveguide <NUM> is/are described here.

As shown in <FIG>, the light <NUM> propagates through the optical waveguide layer <NUM> while being reflected by the reflecting surface <NUM> of the mirror <NUM> and the reflecting surface <NUM> of the mirror <NUM>. At this point in time, the reflectances of the mirrors <NUM> and <NUM> are approximately <NUM>% even on the side from which light is emitted. In order to achieve such high reflectances, the mirror <NUM> and the mirror <NUM> may be formed from DBRs. The following describes an example of a reflection spectrum obtained in a case where light falls on a reflecting surface of a conventional DBR. The angle of incidence at which light falls on the reflecting surface is equivalent to the angle of propagation ϕ.

<FIG> is a diagram showing the reflection spectrum of the conventional DBR at an angle of incidence ϕ of <NUM> degree. An angle of incidence ϕ of <NUM> degree is equivalent to the angle at which light arrives from a direction normal to the reflecting surface of the DBR. The reflection spectrum was calculated by using a Synopsys' DiffractMOD. The refractive index of a medium on an entrance side of the DBR in this example is <NUM>. The DBR is equivalent to the mirror <NUM> of the slow light waveguide <NUM>, and the medium on the entrance side is equivalent to the optical waveguide layer <NUM> of the slow light waveguide <NUM>. This DBR has a structure in which nine high-refractive-index layers and eight low-refractive-index layers are alternately stacked. Each high-refractive-index layer has a refractive index of <NUM> and a thickness of <NUM>. Each low-refractive-index layer has a refractive index of <NUM> and a thickness of <NUM>. As shown in <FIG>, the reflection spectrum of the conventional DBR exhibits a design reflectance of approximately <NUM>% in a stopband and exhibits a low reflectance away from the stopband. The term "stopband" here means a wavelength region in which incident light is strongly reflected by a Bragg reflection attributed to a periodic structure.

<FIG> is a diagram showing the reflection spectrum of the conventional DBR at angles of incidence ϕ of <NUM> degree, <NUM> degrees, and <NUM> degrees. As shown in <FIG>, the reflection spectrum shifts toward a short-wavelength side as the angle of incidence ϕ increases. The following describes as an example how the reflectance of light at a wavelength λA of <NUM> and the reflectance of light at a wavelength λB of <NUM> vary according to the angle of incidence ϕ.

<FIG> are diagrams showing a relationship between the angle of incidence ϕ and the reflectance of light at a wavelength λA of <NUM> and a relationship between the angle of incidence ϕ and the reflectance of light at a wavelength λB of <NUM>, respectively. The range within which the angle of incidence ϕ is greater than or equal to <NUM> degree and less than or equal to <NUM> degrees is equivalent to the range within which the angle of emission θ is greater than or equal to <NUM> degree and less than or equal to <NUM> degrees. As shown in <FIG>, the reflectance of light at the wavelength λA is low in wavelength dependency. Accordingly, for the aforementioned reason, the beam line width Δθ of the emitted light becomes narrower as the angle of emission θ increases. Meanwhile, as shown in <FIG>, at the wavelength λB, which is close to an end of the stopband, the reflectance too sharply drops near an angle of incidence ϕ of <NUM> degrees. For this reason, when the angle of incidence ϕ falls within a range of <NUM> degree to approximately <NUM> degrees, the beam line width Δθ of the emitted light becomes narrower as the angle of emission θ increases, and when the angle of incidence ϕ falls within a range of approximately <NUM> degrees to <NUM> degrees, the beam line width Δθ of the emitted light increases as the angle of emission θ increases. In each of the examples shown in <FIG>, the beam line width Δθ of the emitted light varies greatly according to the angle of emission θ.

The inventors found from the above that an optical scan device in which the beam line width Δθ of emitted light does not vary greatly according to the angle of emission θ can be achieved by using a mirror whose reflectance slowly decreases as the angle of incidence ϕ increases. Specifically, by providing points of inflection from a local maximum value to a long-wavelength side in the reflection spectrum of a mirror, the inventors achieved a mirror whose reflectance slowly decreases as the angle of incidence ϕ increases. In the present embodiment, a chirp DBR in which the thicknesses of high-refractive-index and low-refractive-index layers are adjusted as appropriate was used as a mirror having points of inflection in the reflection spectrum. The term "chirp DBR" herein means a DBR in which the thicknesses of a plurality of high-refractive-index layers and/or the thicknesses of a plurality of low-refractive-index layers vary from layer to layer. The chirp DBR encompasses not only a DBR in which the thicknesses of a plurality of high-refractive-index layers and/or the thicknesses of a plurality of low-refractive-index layers gradually increase or decrease along a direction of stacking but also a DBR in which the thicknesses of a plurality of high-refractive-index layers and/or the thicknesses of a plurality of low-refractive-index layers irregularly or randomly vary along a direction of stacking.

<FIG> is a diagram showing the reflection spectrum of a chirp DBR of the present embodiment at an angle of incidence ϕ of <NUM> degree. In the example shown in <FIG>, the reflection spectrum includes, in a wavelength region in which the reflectance is higher than or equal to <NUM>%, one local maximum point PLM and points of inflection P<NUM> to P<NUM> located on a long-wavelength side of the local maximum point PLM. In the reflection spectrum, the reflectance monotonically decreases on the long-wavelength side of the local maximum point PLM. The term "point of inflection" here means a point at which the quadratic differential of the reflectance in relation to the wavelength reaches zero. At the points of inflection, the reflectance linearly varies with respect to the wavelength.

In the reflection spectrum of the chirp DBR of the present embodiment, the local maximum point PLM and the points of inflection P<NUM> to P<NUM> are present in a wavelength region that exhibits a reflectance higher than or equal to <NUM>%. The chirp DBR may be designed such that the local maximum point PLM and the points of inflection P<NUM> to P<NUM> are present in a wavelength region in which the reflectance is higher than or equal to <NUM>%.

<FIG> shows that a length of propagation greater than or equal to approximately <NUM> is required for the beam line width Δθ to be less than or equal to approximately <NUM>°. <FIG> is a diagram showing a relationship between the angle of incidence ϕ and reflectance at a length of propagation of <NUM>. As indicated by <FIG>, the reflectance needs to be higher than or equal to approximately <NUM>% in order for the length of propagation to be kept until an angle of incidence of <NUM> degrees.

<FIG> is a diagram showing an example of a relationship between the angle of incidence ϕ and reflectance at a wavelength λA of <NUM>. As shown in <FIG>, the reflectance slowly monotonically decreases in a very high wavelength region in which the reflectance ranges approximately from <NUM>% to <NUM>%. More specifically, in the wavelength region, the reflectance gradually decreases as the angle of incidence ϕ increases. The wavelength region is higher than or equal to approximately <NUM> and lower than or equal to approximately <NUM>. The reflectance does not sharply decrease as shown in <FIG>. The reflectance is high in a case where the angle of incidence ϕ is relatively small, and is low in a case where the angle of incidence ϕ is relatively large. A method for designing a chirp DBR for obtaining a desired reflection spectrum such as that shown in <FIG> is described, for example, in <NPL>).

As noted above, by providing points of inflection closer to a long-wavelength side than the local maximum point PLM of a reflection spectrum, changes in reflectance with respect to changes in the angle of incidence ϕ can be made slow. The local maximum point PLM and points of inflection of the reflection spectrum may be present in a wavelength region that exhibits a reflectance higher than or equal to <NUM>%. Such a configuration can make changes slow with the reflectance kept high. The local maximum point PLM and points of inflection of the reflection spectrum may be present in a wavelength region that exhibits a reflectance higher than or equal to <NUM>%. In the present embodiment, when there is one or more points of inflection, the reflectance slowly changes at least when the angle of incidence ϕ falls within a range of <NUM> degree to approximately <NUM> degrees. In particular, when there are two or more points of inflection, the reflectance slowly changes at least when the angle of incidence ϕ falls within a range of <NUM> degree to approximately <NUM> degrees. By thus providing two or more points of inflection, changes in reflectance with respect to changes in angle of incidence can be made slow in a wider angular range. Further, the slow changes in reflectance can be achieved while high reflectances are kept.

For comparison, the following describes relationships between the angle of emission θ and the length of propagation Lp in cases where a conventional DBR and a DBR of the present embodiment are used as the mirror <NUM>.

<FIG> is a diagram showing an example of a relationship between the angle of emission θ and the length of propagation Lp. The open circles represent a case where the mirror <NUM> of a slow light waveguide <NUM> is formed from the conventional DBR according to the aforementioned example. The filled circles represent a case where the mirror <NUM> of a slow light waveguide <NUM> is formed from the chirp DBR according to the aforementioned example. The mirror <NUM> of the slow light waveguide <NUM> is formed from a conventional DBR different from the aforementioned example. The DBR has a structure in which eleven high-refractive-index layers and ten low-refractive-index layers are alternately stacked. The high-refractive-index layers have a refractive index of <NUM> and a thickness of <NUM>. The low-refractive-index layers have a refractive index of <NUM> and a thickness of <NUM>. The optical waveguide layer <NUM> has a refractive index of <NUM>. Air, which is a medium on the side from which light is emitted, has a refractive index of <NUM>.

As represented by the open circles, in the conventional DBR, the length of propagation Lp increases as the angle of emission θ increases. On the other hand, as represented by the filled circles, it is found that in the chirp DBR of the present embodiment, the length of propagation Lp does not greatly change even when the angle of emission θ increases. Thus, the chirp DBR of the present embodiment makes it possible to reduce the dependence of the length of propagation Lp on the angle of emission θ. As long as the length of propagation Lp is substantially constant regardless of the angle of emission θ, the beam line width Δθ of the emitted light shown in <FIG> is substantially constant with respect to the angle of emission θ, too. In the example shown in <FIG>, the length of propagation Lp is approximately <NUM> on average. As shown in <FIG>, "Length of Propagation Lp ≈ <NUM>" is equivalent to "Beam Line Width Δθ of Emitted Light ≈ <NUM>°". Accordingly, even when the angle of emission θ changes, the beam line width Δθ of the emitted light can be kept approximately <NUM> degree. This makes it possible to inhibit the resolving power of a scan from varying according to the angle of emission θ. Further, since the beam line width Δθ of the emitted light is <NUM> degree, a high resolving power can be kept regardless of the angle of emission θ.

Although, in the aforementioned example, the reflectance monotonically decreases on the long-wavelength side of the local maximum point PLM, the reflectance does not necessarily need to monotonically decrease on the long-wavelength side of the local maximum point PLM. <FIG> is a diagram showing the reflection spectrum of another chirp DBR of the present embodiment at an angle of incidence ϕ of <NUM> degree. In the example shown in <FIG>, the reflection spectrum includes, in a wavelength region in which the reflectance is higher than or equal to <NUM>%, a local maximum point PLM1, a local maximum point PLM2 located on a long-wavelength side of the local maximum point PLM1, and points of inflection P<NUM> to P<NUM> located on the long-wavelength side of the local maximum point PLM1 and a short-wavelength side of the local maximum point PLM2. In the reflection spectrum, the reflectance decreases and then increases on the long-wavelength side of the local maximum point PLM1 and the short-wavelength side of the local maximum point PLM2 as the wavelength increases. That is, the reflectance does not monotonically decrease on the long-wavelength side of the local maximum point PLM1. The reflectance monotonically decreases on a long-wavelength side of the local maximum point PLM2. Even in this case, in a wavelength region in which the reflectance is extremely high ranging approximately from <NUM>% to <NUM>%, the reflectance slowly or, more specifically, gradually decreases as the angle of incidence ϕ increases. The wavelength region is higher than or equal to approximately <NUM> and lower than or equal to approximately <NUM>.

As noted above, the slow light waveguide <NUM> according to the present embodiment makes it possible to reduce the dependence of the beam line width Δθ of emitted light on the angle of emission θ. Furthermore, even when the angle of emission θ changes, the beam line width Δθ of the emitted light can be kept narrow. This effect is brought about in a case where at least either the mirror <NUM> or <NUM> of the slow light waveguide <NUM> has the following reflection spectrum. The reflection spectrum includes, in a wavelength region in which a reflectance with respect to an angle of incidence ϕ of <NUM> degrees is higher than or equal to <NUM>%, one local maximum point and first and second points of inflection located on a long-waveguide side of the local maximum point. A wavelength at the first point of inflection is shorter than a wavelength at the second point of inflection. The wavelength λ of light <NUM> that propagates through the optical waveguide layer <NUM> is a wavelength higher than or equal to the local maximum point and lower than or equal to the first point of inflection. The wavelength region may be included in a wavelength region higher than or equal to <NUM> and lower than or equal to <NUM> that may be used for the aforementioned LiDAR system. Either the mirror <NUM> or <NUM> may exhibit such a reflection spectrum, or both the mirrors <NUM> and <NUM> may exhibit such reflection spectra. Although, in the example shown in <FIG>, light is emitted from the mirror <NUM> and light is reflected by the mirror <NUM>, this example is not intended to impose any limitation. Light may be reflected by the mirror <NUM>, and light may be emitted from the mirror <NUM>. Alternatively, light may be emitted from both the mirrors <NUM> and <NUM>.

In the present embodiment, by providing points of inflection closer to a long-wavelength side than the local maximum point PLM of a reflection spectrum, changes in reflectance with respect to changes in the angle of incidence ϕ can be made slow, and a region in which the reflectance slowly changes is utilized. Therefore, the wavelength λ of light <NUM> that propagates through the optical waveguide layers <NUM> is a wavelength represented by the following formula using the local maximum point PLM and the first point of inflection P<NUM>:
[Math. <NUM>] <MAT>.

<FIG> is a diagram showing an example configuration of an optical scan device <NUM> in which elements such as an optical divider <NUM>, a waveguide array 10A, a phase shifter array 80A, and a light source <NUM> are integrated on a circuit board (e.g. a chip). The light source <NUM> may for example be a light-emitting element such as a semiconductor laser. In this example, the light source <NUM> emits single-wavelength light whose wavelength in free space is λ. The optical divider <NUM> divides the light from the light source <NUM> into lights and introduces the lights into waveguides of the plurality of phase shifters. In the example shown in <FIG>, there are provided an electrode 62A and a plurality of electrodes 62B on the chip. The waveguide array 10A is supplied with a control signal from the electrode 62A. To the plurality of phase shifters <NUM> in the phase shifter array 80A, control signals are sent from the plurality of electrodes 62B, respectively. The electrode 62A and the plurality of electrodes 62B may be connected to a control circuit (not illustrated) that generates the control signals. The control circuit may be provided on the chip shown in <FIG> or may be provided on another chip in the optical scan device <NUM>.

As shown in <FIG>, an optical scan over a wide range can be achieved through a small-sized device by integrating all components on the chip. For example, all of the components shown in <FIG> can be integrated on a chip measuring approximately <NUM> by <NUM>.

<FIG> is a schematic view showing how a two-dimensional scan is being executed by irradiating a distant place with a light beam such as a laser from the optical scan device <NUM>. A two-dimensional can is executed by moving a beam spot <NUM> in horizontal and vertical directions. For example, a two-dimensional ranging image can be acquired by a combination with a publicly-known TOF (time-of-flight) method. The TOF method is a method for, by observing light reflected from a physical object irradiated with a laser, calculating the time of flight of the light to figure out the distance.

<FIG> is a block diagram showing an example configuration of a LiDAR system <NUM> serving as an example of a photodetection system that is capable of generating such a ranging image. The LiDAR system <NUM> includes an optical scan device <NUM>, a photodetector <NUM>, a signal processing circuit <NUM>, and a control circuit <NUM>. The photodetector <NUM> detects light emitted from the optical scan device <NUM> and reflected from a physical object. The photodetector <NUM> may for example be an image sensor that has sensitivity to the wavelength λ of light that is emitted from the optical scan device <NUM> or a photodetector including a photosensitive element such as a photodiode. The photodetector <NUM> outputs an electrical signal corresponding to the amount of light received. The signal processing circuit <NUM> calculates the distance to the physical object on the basis of the electrical signal outputted from the photodetector <NUM> and generates distance distribution data. The distance distribution data is data that represents a two-dimensional distribution of distance (i.e. a ranging image). The control circuit <NUM> is a processor that controls the optical scan device <NUM>, the photodetector <NUM>, and the signal processing circuit <NUM>. The control circuit <NUM> controls the timing of irradiation with a light beam from the optical scan device <NUM> and the timing of exposure and signal readout of the photodetector <NUM> and instructs the signal processing circuit <NUM> to generate a ranging image.

The frame rate at which a ranging image is acquired by a two-dimensional scan can be selected, for example, from among <NUM> fps, <NUM> fps, <NUM> fps, <NUM> fps, <NUM> fps, or other frame rates, which are commonly used to acquire moving images. Further, in view of application to an onboard system, a higher frame rate leads to a higher frequency of acquisition of a ranging image, making it possible to accurately detect an obstacle. For example, in the case of a vehicle traveling at <NUM>/h, a frame rate of <NUM> fps makes it possible to acquire an image each time the vehicle moves approximately <NUM>. A frame rate of <NUM> fps makes it possible to acquire an image each time the vehicle moves approximately <NUM>. A frame rate of <NUM> fps makes it possible to acquire an image each time the vehicle moves approximately <NUM>.

The time required to acquire one ranging image depends on the speed of a beam scan. For example, in order for an image whose number of resolvable spots is <NUM> by <NUM> to be acquired at <NUM> fps, it is necessary to perform a beam scan at <NUM> per point. In this case, the control circuit <NUM> controls the emission of a light beam by the optical scan device <NUM> and the storage and readout of a signal by the photodetector <NUM> at an operating speed of <NUM>.

Each of the optical scan devices of the present disclosure can also be used as an optical receiver device of similar configuration. The optical receiver device includes a waveguide array 10A which is identical to that of the optical scan device and a first adjusting element that adjusts the direction of light that can be received. Each of the first mirrors <NUM> of the waveguide array 10A transmits light falling on a side thereof opposite to a first reflecting surface from the third direction. Each of the optical waveguide layers <NUM> of the waveguide array 10A causes the light transmitted through the first mirror <NUM> to propagate in the second direction. The direction of light that can be received can be changed by the first adjusting element changing at least one of the refractive index of the optical waveguide layer <NUM> of each waveguide element <NUM>, the thickness of the optical waveguide layer <NUM> of each waveguide element <NUM>, or the wavelength of light. Furthermore, in a case where the optical receiver device includes a plurality of phase shifters <NUM> or 80a and 80b which are identical to those of the optical scan device and a second adjusting element that varies differences in phase among lights that are outputted through the plurality of phase shifters <NUM> or 80a and 80b from the plurality of waveguide elements <NUM>, the direction of light that can be received can be two-dimensionally changed.

For example, an optical receiver device can be configured such that the light source <NUM> of the optical scan device <NUM> shown in <FIG> is substituted by a receiving circuit. When light of wavelength λ falls on the waveguide array 10A, the light is sent to the optical divider <NUM> through the phase shifter array 80A, is finally concentrated on one place, and is sent to the receiving circuit. The intensity of the light concentrated on that one place can be said to express the sensitivity of the optical receiver device. The sensitivity of the optical receiver device can be adjusted by adjusting elements incorporated separately into the waveguide array and the phase shifter array 80A. The optical receiver device is opposite in direction of the wave number vector (in the drawing, the bold arrow) shown, for example, in <FIG>. Incident light has a light component acting in the direction (in the drawing, the X direction) in which the waveguide elements <NUM> extend and a light component acting in the array direction (in the drawing, the Y direction) of the waveguide elements <NUM>. The sensitivity to the light component acting in the X direction can be adjusted by the adjusting element incorporated into the waveguide array 10A. Meanwhile, the sensitivity to the light component acting in the array direction of the waveguide elements <NUM> can be adjusted by the adjusting element incorporated into the phase shifter array 80A. θ and α<NUM> shown in <FIG> are found from the phase difference Δϕ of light and the refractive index nw and thickness d of the optical waveguide layer <NUM> at which the sensitivity of the optical receiver device reaches its maximum. This makes it possible to identify the direction of incidence of light.

The aforementioned embodiments can be combined as appropriate.

Claim 1:
An optical device comprising:
a first mirror (<NUM>) having a first reflecting surface (<NUM>) and extending along a first direction;
a second mirror (<NUM>) having a second reflecting surface (<NUM>) that faces the first reflecting surface (<NUM>) and extending along the first direction; and
an optical waveguide layer (<NUM>), located between the first mirror (<NUM>) and the second mirror (<NUM>), that causes light (<NUM>) to propagate along the first direction,
wherein a transmittance of the first mirror (<NUM>) is higher than a transmittance of the second mirror (<NUM>), and
characterized in that
a reflection spectrum of at least either the first mirror (<NUM>) or the second mirror (<NUM>) with respect to light (<NUM>) arriving from a direction normal to the reflecting surface (<NUM>, <NUM>) includes, in a wavelength region in which a reflectance is higher than or equal to <NUM>%, a local maximum point and first and second points of inflection located closer to a long-wavelength side than the local maximum point.