Patent ID: 12216271

The accompanying drawings are intended to depict embodiments of the present invention and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. Also, identical or similar reference numerals designate identical or similar components throughout the several views.

DETAILED DESCRIPTION

In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result.

Referring now to the drawings, embodiments of the present disclosure are described below. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

As used herein, the term “connected/coupled” includes both direct connections and connections in which there are one or more intermediate connecting elements.

Embodiments of the present disclosure enables changes in a wavelength of a laser beam over a wide wavelength range and at a high speed.

Embodiments of the present disclosure are described referring to the drawings. Like reference signs are applied to identical or corresponding components throughout the drawings and redundant description thereof may be omitted.

The embodiments to be described below exemplify an optical device, which implements the technical concepts of the present disclosure. However, no limitation is intended therein. Unless otherwise specified, shapes of components, relative arrangements thereof, and values of parameters described below are not intended to limit the scope of the present invention but are intended to exemplify the scope of the present invention. The relative positions of the elements illustrated in the drawings are exaggerated for purpose of clear illustration.

An optical device according to an embodiment includes a light emitter, a first reflector, a second reflector, a base, and a piezoelectric member as a piezoelectric body. The second reflector faces the first reflector with the light emitter between the first reflector and the second reflector. With a space between the light emitter and the second reflector, the base supports the second reflector. The piezoelectric body deforms in response to application of drive voltage. The base includes a first region and a second region having a lower stiffness than the first region. The second reflector and the piezoelectric body are in the second region.

The distance between the first reflector and the second reflectors, or the inter-reflector distance refers to a resonator length. In the optical device, the piezoelectric body deforms the second region to drive the second reflector, in response to application of drive voltage. Further, the optical device emits a laser beam whose wavelength changes with the distance between the first reflector and the second reflector.

In the electrostatic actuation method that electrostatically actuates a movable portion to change the resonator length, for example, the actuation of the movable portion at a high speed and with a larger amount of displacement is restricted due to limitations of a resonance frequency or a spring constant of the movable portion. There is room for improvement in changing the wavelength of a laser beam at a high speed and in a wide wavelength range.

In at least one embodiment, the resonator length is changed by deforming a thin second region using the piezoelectric body and driving the second reflector at a high speed and with a large amount of displacement. This enables the wavelength of a laser beam to change over a wide range at a high speed.

The following describes a light source device that emits multiple laser beams in parallel, according to an embodiment. In the drawings to be described below, an X-direction, or the X-axis is a given direction within a plane parallel to a reflecting surface of the first reflector, and a Y-direction, or the Y-axis is a direction orthogonal to the X-axis within the plane parallel to the reflecting surface of the first reflector. Further, a Z-direction, or the Z-axis is a direction orthogonal to each of the X-axis and the Y-axis. The light source device emits a laser beam along the Z-axis.

However, these axes are used for convenience of description, and no particular limitation is imposed on the orientation of the optical device. The optical device is used in any orientation.

First Embodiment

A light source device according to a first embodiment will be described.

The overall configuration of a light source device1is described with reference toFIGS.1A and1B.

FIGS.1A and1Bare illustrations of an exemplary configuration of the light source device1.FIG.1Ais a plan view of the light source device1.FIG.1Bis a cross-sectional view taken along line A-A′ inFIG.1A. InFIGS.1A and1B, the light source device1includes a base10, a joint13, and a vertical-cavity surface-emitting lasers (VCSEL) device20.

The base10is a plate having a rectangular shape in plane view and includes a first region11and a second region12. The second region12is substantially circular in plan view at substantially the center of the base10, and serves as a movable portion. The second region12is formed to have a thickness (i.e., the length along the Z-axis) smaller than the thickness of the first region11(i.e., the second region12is thinner than the first region11). The first region11is another region other than the second region in the base10, and serves as a support.

The second region12includes a second reflector121and a piezoelectric element122on the −Z-surface of the second region12. The −Z-surface of the second region12faces a light emitter211to be described below, meaning that the −Z-surface of the second region12refers to a surface of the second region12, which faces the light emitter211.

The second reflector121is formed at substantially the center of the second region12and is substantially rectangular in planar view. The piezoelectric element122is an example of a piezoelectric body surrounding the second reflector121. The piezoelectric element122is in a ring shape surrounding the second reflector121. The piezoelectric element122may be in another shape other than a ring shape as long as the second reflector121is surrounded by the piezoelectric element122.

The piezoelectric element122deforms (for example, expands and contracts) in accordance with a drive voltage applied through an electrode. The deformation of the piezoelectric element122causes the second region12to elastically deform, thus allowing the second reflector121to displace in the Z-direction.

In the light source device1, the VCSEL device20is disposed upstream from the base10in the Z-direction (i.e., downstream from the base10in the −Z-direction), with the joint13between the base10and the VCSEL device20. The base10and the VCSEL device20are joined together to be attached firmly to each other by the joint13using, for example, an atomic diffusion bonding method.

The VCSEL device20includes a mesa21and a first reflector22. The mesa21is an island-shaped structure including a light emitter211. The VCSEL device20includes the first reflector22downstream from second reflector121in the −Z-direction with the light emitter211between the first reflector22and the second reflector121. In other words, the first reflector22faces the second reflector121with the light emitter211therebetween.

The light emitter211emits light in accordance with electric current injected through the electrode. The first reflector22, the second reflector121, and the light emitter211between the first reflector22and the second reflector121constitute a resonator. In the resonator, light emitted by each light emitter211is reflected alternately by the first reflector22and the second reflector121, and travels back and forth between the first reflector22and the second reflector121so as to be amplified.

More specifically, light emitted by the light emitter211is reflected alternately by a reflecting surface22A of the first reflector22and a reflecting surface121A of the second reflector121, meaning that light emitted by the light emitter211travels back and forth between the reflecting surface22A and the reflecting surface121A, thus to be amplified.

The amplification of light causes the occurrence of lasing when gain and loss are balanced. Thus, the light emitter211emits a laser beam. The light source device1emits a laser beam through the lower-reflectivity surface between the second reflector121and the first reflector22. When the reflectivity is lower for the second reflector121than the first reflector22, the light source device1emits a laser beam through the second reflector121in an emission direction31, or a first direction as illustrated inFIG.1B. When the reflectivity is lower for the first reflector22than the second reflector121, the light source device1emits a laser beam through the first reflector22in an emission direction32, or a second direction as illustrated inFIG.1B.

In response to drive voltage applied to the piezoelectric element122, the second region12elastically deforms and causes the second reflector121to displace along the Z-axis. The displacement of the second reflector121changes the distance between the first reflector22and the second reflector121, thus causing the resonator length to vary. The variation in resonator length enables the light emitter211to emit a laser beam having a wavelength changeable with the resonator length.

Next, a configuration of the base10and its surroundings will be described with reference toFIG.2.FIG.2is an enlarged cross-sectional view of the configuration of the base10and its surroundings according to an embodiment.

The base10includes the second reflector121and the piezoelectric element122on one silicon on insulator (SOI) substrate that has undergone, for example, an etching process.

InFIG.2, the first region11includes a support layer111, an oxide insulating layer112, and a silicon active layer113. In contrast, a component constituting the second region12may be formed by removing the support layer111and the oxide insulating layer112from the SOI substrate through etching, and is thus composed of the silicon active layer113alone. In some embodiments, a component constituting the second region12may include a supporting layer and a supporting layer, in addition to the silicon active layer113.

The second region12has a thickness in the Z-direction smaller than the length in the X-direction or the Y-direction, to have an elasticity and a low stiffness in the Z-direction. The second region12is movable by deformation of the piezoelectric element122. The first region11, which has a higher stiffness than the second region12does, deforms by a small amount due to deformation of the piezoelectric element122. Such a first region11supports the second region12.

The second reflector121is on the −Z-surface of the second region12, or on the −Z-surface of the first region11. The second reflector121has a structure having a reflectivity sufficient to enable the occurrence of lasing, including: a multilayer-film mirror or a metal thin film in which two or more thin films having different refractive indexes are alternately laminated on the component constituting the second region12; and a high contrast grating (HCG) having a periodic structure in which a material with a thickness equivalent to a wavelength is periodically arranged on the component constituting the second region12.

The piezoelectric element122includes a lower electrode122A, a piezoelectric portion122B, and an upper electrode122C, which are stacked on the second region12. On the upper electrode122C, an overcoat122D is provided to protect the piezoelectric element122.

The upper electrode122C and the lower electrode122A contain gold (Au) or platinum (Pt). The piezoelectric portion122B contains a piezoelectric material such as lead zirconate titanate (PZT) or aluminum nitride (AlN). In some embodiments, an insulating layer is between the piezoelectric elements122to prevent shorting between the piezoelectric elements122. The piezoelectric portion122B, when a positive or negative voltage in the polarization direction is applied thereto, is deformed (for example, expanded or contracted) in proportion to the potential of the applied voltage and exhibits inverse piezoelectric effect.

The joint13include an adhesive layer, a diffusion preventing layer, and a joining layer, which are sequentially stacked in the −Z-direction. The joining layer uses Titanium (Ti), the diffusion preventing layer uses platinum (Pt), and the adhesive layer uses gold (Au) or aluminum oxide (Al2O).

The shape of the second region12in plan view is not limited to a circular shape, and the second region12may have a quadrangular shape, a triangular shape, an elliptical shape, or an asymmetric shape. A part of the second reflector121and the piezoelectric element122may be included in the first region11, instead of being entirely including in the second region12.

In the present embodiment, the piezoelectric element122is between two joints13. In some embodiments, one or more joints13are provided in the light source device1. For example, one joint13, which is arranged to surround the piezoelectric element122, increases the symmetry of the geometry of the surface to be joined with the VCSEL device20and prevents tilting of the base10joined with the VCSEL device20. In this arrangement, a part of the joint13is open to lead out the wiring for driving the piezoelectric element122, to the outside of the joint13.

For another example, two or more joints13, which are arranged symmetrically about the center of the second reflector121in the plane of the second reflector121, prevents tilting of the base10jointed together with the VCSEL device20. For still another example, three joints13are arranged at an interval of 120° with respect to the center of the second reflector121. Further, four joints13are arranged at the four sides of the second reflector121.

In some embodiments, the joint13serves as an electrode by electrically connecting the piezoelectric element122to the joint13.

Further, the second reflector121is arranged at substantially the center of the ring-shaped piezoelectric element122to prevent overlapping of the piezoelectric element122with the second reflector121along the Z-axis.

Next, a configuration of the VCSEL device20will be described with reference toFIG.3.FIG.3is a cross-sectional view of an exemplary configuration of the VCSEL device20.

In the light source device1, the first reflector22included in the VCSEL device20and the second reflector121separate from the VCSEL device20constitute a resonator. In this configuration, the VCSEL device20, which is a surface-emitting semiconductor laser including one reflector constituting the resonator, is referred to as a half-VCSEL.

InFIG.3, the VCSEL device20includes a mesa21, a first reflector22, a semiconductor substrate23, an antireflection film24, and a groove25. On the +Z-surface of the semiconductor substrate23, the first reflector22, a spacer layer221, and the mesa21are stacked. On the −Z-surface of the semiconductor substrate23, the antireflection film24is formed. The groove25is formed by removing the spacer layer221and the first reflector22around the mesa21by etching.

The first reflector22is a semiconducting multilayer reflector formed on the semiconductor substrate23such as an n-GaAs substrate. The first reflector22includes, for example, a low-refractive-index layer made of n-Al0.9Ga0.1As and a high-refractive-index layer made of n-Al0.2Ga0.8As.

Between the refractive-index layers of the first reflector22, a composition-graded layer having a thickness of 20 nm in which the composition gradually changes from one composition to the other composition is provided to reduce the electrical resistance. Each of the refractive-index layers includes ½ of the adjacent composition-graded layer, and has an optical thickness of λ/4 where λ denotes the oscillation wavelength. Note that when the optical thickness is λ/4, the actual thickness D of the layer is D=λ/4 n (where n denotes a refractive index of a medium of that layer).

The spacer layer221on the first reflector22is, for example, a non-doped AlGaInP layer.

The mesa21includes the light emitter211, the spacer layer212, a selective oxide layer213, a pair of semiconducting multilayer reflectors214, a contact layer215, an insulating layer216, and an electrode217.

The spacer layer212is formed on the light emitter211. The spacer layer212is, for example, a non-doped AlGaInP layer.

A region including the spacer layer212and the light emitter211is also referred to as a resonator structure or a resonator region, includes ½ of the adjacent composition-graded layer, and has an optical thickness of one wavelength (λ).

In the mesa21, the light emitter211is provided between the spacer layer221and the spacer layer212. The light emitter211emits light in response to injection of electric current, and amplifies light traveling back and forth between the first reflector22and the second reflector121, which constitute the resonator. The light emitter211may be referred to as an active layer. The light emitter211is an active layer having a three quantum well structure including three quantum well layers and four barrier layers. Each quantum well layer is, for example, an InGaAs layer, and each barrier layer is, for example, an AlGaAs layer. The light emitter211is disposed in the center of the resonator structure, which is a position corresponding to the antinode in the standing wave distribution of the electric field, so as to achieve a high stimulated-emission rate.

The selective oxide layer213includes an oxidized region213aand a non-oxidized region213b. The selective oxide layer213contains p-AlAs and has a thickness of 30 nm. The selective oxide layer213is between the pair of semiconducting multilayer reflectors214. For example, the selective oxide layer213is within the second pair of a high refractive index layer and a low refractive index layer when counted from the spacer layer212. The selective oxide layer213may include layers such as the composition-graded layer and an intermediate layer. In the present embodiment, the selective oxide layer213includes layers actually oxidized.

The contact layer215is formed on the semiconducting multilayer reflector214. The contact layer215is, for example, a p-GaAs layer.

The mesa21and the groove25is formed by removing a part of the lamination of the contact layer215, the semiconducting multilayer film reflector214, the spacer layer212, and the light emitter211through etching.

The mesa21includes the insulating layer216on its surface. Examples of material insulating layer216include SiN, SiON, and SiO2. The insulating layer216includes an opening218that exposes a part of the contact layer215of the mesa21. The insulating layer216includes the opening218at a position overlapping with the non-oxidized region213bin plan view.

The electrode217, which is electrically connected to the contact layer215through the opening218, is formed on the insulating layer216of the mesa21. The electrode217is, for example, a lamination in which a layer of Ti, a layer of Pt, and a layer of Au are stacked in this order in the +Z-direction inFIG.3.

The groove25includes the insulating layer216on its surface. A part of the insulating layer216forms an opening252that exposes a part of the semiconductor substrate23.

The insulating layer216on the groove25includes an electrode251electrically connected to the contact layer215through the opening252. The electrode251is, for example, a lamination in which a layer of germanium alloy (AuGe), a layer of nickel (Ni), and a layer of gold (Au) are stacked in this order in the +Z-direction inFIG.3.

The wiring219is electrically connected to each of the electrodes217and251. The wiring219is, for example, a lamination in which a of Ti, a layer of Pt, and a layer of Au are stacked in this order in the +Z-direction inFIG.3.

FIGS.4A and4Bare illustrations of an exemplary operation of the second region12.

FIG.4Ais an illustration of generation of stress.FIG.4Bis an illustration of deformation of the second region.

In response to a drive voltage applied to the piezoelectric element122in the second region12to cause contraction of the piezoelectric element122, the piezoelectric element122is contracted. Since the end of the second region12is connected to the first region11, the end of the second region12is not displaced in the Z-direction because of the deformation of the piezoelectric element122.

Since the piezoelectric element122is bonded to the second region12, the contraction stress S of the piezoelectric element122is transmitted to the second region12. A neutral axis N is present at a given position along the Z-axis within the second region12. A compressive stress Ta is generated in a first area of the second region12, which is between the neutral axis N and the piezoelectric element122, and a tensile stress Tb is generated in a second area of the second region12, which is on the other side of the neutral axis N. The compressive stress Ta and the tensile stress Tb elastically deform and warp the second region12and thus displace its surface in the −Z-direction as illustrated inFIG.4B.

In response to a drive voltage applied to piezoelectric element122to cause expansion of the piezoelectric element122, a tensile stress is generated in the first area of the second region12, which is between the neutral axis N and the piezoelectric element122, and a compressive stress is generated in the second area of the second region12, which is on the other side of the neutral axis N. These stresses elastically deform the second region12and thus displace its surface in the +Z-direction.

FIGS.5A and5Bare illustrations of another exemplary operation of the second region12.FIG.5Ais an illustration of generation of stress.FIG.5Bis an illustration of deformation of the second region12. InFIGS.5A and5B, two piezoelectric elements122E and122F are on the +Z-surface of the second region12.

In response to a drive voltage applied to the piezoelectric elements122E and122F to cause contraction of the piezoelectric elements122E and122F, the second region12deforms in the same manner as inFIG.4A. In a portion of the second region12on which the piezoelectric elements122E and122F are not formed, stresses are generated in directions opposite to those of the stresses generated immediately below the piezoelectric elements122E and122F. Such stresses cause the portion of the second region12to protrude in the +Z-direction. The amount of displacement of that portion varies depending on the dimension in the in-plane direction. In other words, the portion of the second region12on which none of the piezoelectric elements122E and122F are provided deforms by an amount of deformation that differs with the area of the portion.

FIG.6is an illustration of an exemplary oscillation of the second region12. The second region12oscillates while satisfying an equation of motion of the second region12when caused to oscillate with a resonance frequency determined by the dimension and geometry of the second region12, in response to a drive voltage applied to the piezoelectric element122.

Assuming that the second region12is circular having a radius of a and its periphery rendered entirely stationary, an amount of displacement u (r, φ, t) along the Z-axis at a position r relative to the center of the second region12, an angle φ, and a time t is given by the following Formulae 1 to 3:

umn⁡(r,ϕ,t)=(A⁢⁢cos⁡(c⁢⁢λmn⁢t)+B⁢⁢sin⁡(c⁢⁢λmn⁢t))⁢Jm⁡(λmn⁢r)⁢(C⁢⁢cos⁡(m⁢⁢ϕ)+D⁢⁢sin⁡(m⁢⁢ϕ))Formula⁢⁢1c=Tρ⁢⁢hFormula⁢⁢2Jx⁡(2.405a⁢r)Formula⁢⁢3

In Formula 1, A, B, C, and D represent constants, Jmn represents a Bessel function, and m (=0, 1, 2, . . . ) represents an order of the Bessel function. Further, n (n=1, 2, 3, . . . ) represents the n-th r at which the value of the Bessel function becomes zero (0). In Formula 2, T represents internal stress, p represents density, and h represents the thickness of the second region12.

The amount of displacement in along the Z-axis is zero over the entire time t because the periphery of the second region12is fixed. For the primary resonance, the amount of displacement is not zero at any point on the second region12except the periphery. In the primary resonance, the diameter of a corresponds to the first zero point in the Bessel function.

The value of the zero point in the Bessel function is determined in advance. The first point is 2.405, and the second point is 5.520. The value of the Bessel function at any position r (r≤a) in the first resonance is obtained by substituting the time t determined by the dimensions, stress information, resonance frequency of the second reflector121into the above Formula 1: u (r, φ, t) to estimate the amount of displacement of the second reflector121per unit time during resonance.

FIG.6is an illustration of resonant oscillation of the second region12. The resonator length changes with the position of the second reflector121along the Z-axis in the second reflector121. With a change in resonator length, the wavelength of a laser beam emitted from the light emitter211changes.

To linearly change the wavelength of a laser beam emitted from the light emitter211with drive voltage applied to the piezoelectric element122, the drive voltage is determined based on the amount of displacement per unit time, which is determined by the dimension and frequency of the second region12.

In some examples, the wavelengths of the laser beams are changed by oscillating the second region12at frequencies sufficiently far from the resonance frequency. This is advantageous in that the wavelengths of the laser beams are changed at various frequencies: using the frequencies significantly different from the resonance frequency allows the amount of displacement to remain significantly unchanged with changes in frequency although a great amount of displacement like the resonant actuation is not obtained.

For the linear relation between the drive voltage and the amount of displacement of the second reflector121, the drive voltage linearly changes with time, and the amount of displacement of the second region12also linearly changes with time. Using that linear relation between the drive voltage and the amount of displacement facilitates adjustment of the wavelengths of the laser beams unlike using the resonant actuation.

FIG.7is a graph of the relation between drive voltage of the piezoelectric element122and the amount of displacement of the second reflector121, according to an embodiment. InFIG.7, a linear relation exists between the drive voltage of the piezoelectric element122and the amount of displacement of the second reflector121.

FIG.8is a graph of the relation between the wavelength of a laser beam emitted from the light emitter211and the distance between the first reflector22and the second reflector121, according to an embodiment. InFIG.8, the wavelength of a laser beam emitted from the light emitter211increases as the distance between the first reflector22and second reflectors121(i.e., the resonator length) increases. The wavelength of a laser beam emitted from the light emitter211decreases as the resonator length decreases.

The linearity of the wavelength of the laser beam over time is determined by, for example, the linearity of the amount of displacement of the second reflector121along the Z-axis with respect to drive voltage applied to the piezoelectric element122and the linearity of the wavelength of the laser beam with respect to the resonator length. With a linear relation between the drive voltage and the amount of displacement of the second reflector121along the Z-axis, the resonator length linearly changes with the drive voltage.

InFIG.8, changes in the distance between the reflectors within the range, in which the wavelength of the laser beam relatively linearly changes with the distance between the reflectors, enables a linear change in the wavelength of the laser beam with the drive voltage.

Further, using frequencies significantly different from the resonance frequency to actuate the second region12produces a substantially linear relation between the drive voltage and the amount of displacement of the second reflector121. In view of the above description, selecting the range in which the wavelength of the laser beam linearly changes with the distance between the reflectors enables a linear change in the wavelength of the laser beam over time.

The following describes the operational effects of the light source device1.

Conventionally, a wavelength variable laser used as a light source of a frequency modulated continuous wave light detection and ranging (FMCW LiDAR) device is known. Such a wavelength variable laser includes: an optical resonator including a first mirror and a second mirror; a gain region between the first mirror and the second mirror; and an electrostatically-actuated micro-electromechanical systems (MEMS) actuator. The MEMS actuator regulates air gap between the first mirror and the second mirror.

However, the electrostatic actuation enables determination of the amount of displacement and the speed of actuation (i.e., the resonance frequency) based on a balance between an electrostatic attractive force between parallel flat plates sandwiching the air gap and a restoring force of the movable portion connected to the parallel flat plates.

For example, assuming that one of the parallel flat plates is fixed to the fixed portion and the other is connected to the support through the movable portion, the electrostatic attractive force depends on the area W of the parallel flat plate, the vacuum dielectric constant ε, the drive voltage V, and the displacement amount x, and the spring restoring force depends on the spring constant k and the displacement x. In response to application of drive voltage, a displacement x is obtained to balance the electrostatic attractive force against the spring restoring force.

The resonance frequency f of the parallel flat plate is given by using a spring constant k, a constant c, and a mass m of the parallel flat plate. Under the relation between the area W of the parallel flat plate, the resonance frequency f, the vacuum dielectric constant ε, the drive voltage V, the spring constant c, and the displacement amount x, the displacement amount x decreases and the resonance frequency increases with increasing spring constant k. Further, the mass m is given by the product of the area W, the thickness h, and the density p.

It can be found from the above-described relations that a smaller density, a smaller thickness, and a higher drive voltage are to be obtained to shift a curve indicating the relation between the resonance frequency and the displacement amount, which are limited by the spring constant, to increase the speed (i.e., a higher frequency) and the amount of displacement.

For example, if other variables are fixed, the density, the thickness, and the voltage are to be 1/100, 1/100, and 10 times as great, respectively, to decuple the resonance frequency.

Further, new material is to be developed to reduce the density to 1/100, and new material is alto to be developed to reduce the thickness of the movable portion. Development of such materials is not easy. Further, the increase in the drive voltage may be limited by specifications such as the size, operation reliability, and power consumption of an optical device, including a light source device.

As described above, the electrostatic actuation has difficulties in actuating the parallel plates at high speeds with greater amount of displacement to change the resonator length. Its challenge is in changing the wavelength of the laser beam over a wide range of wavelength at high speeds.

In view of such circumstances, the light source device according to an embodiment includes the light emitter211, the first reflector22, the second reflector121, the base10, and the piezoelectric element122(or a piezoelectric body). The first reflector22and the second reflector121face each other with the light emitter between the first reflector22and the second reflector121. With a space between the light emitter211and the second reflector121, the base10supports the second reflector121. The piezoelectric element122deforms in response to application of drive voltage. The base10includes a first region11and a second region12having a lower stiffness than the first region11. The second reflector121and the piezoelectric element122are in the second region12. The piezoelectric element122deforms the second region12to drive the second reflector121to displace. The displacement of the second reflector121changes the distance between the first reflector22and the second reflector121. The light source device1emits a laser beam whose wavelength is changeable with the distance between the first reflector22and the second reflector121.

The stiffness changes with factors including dimensions (lengths and thicknesses) of objects and elastic moduli of materials constituting the objects. In the present embodiment, the second region12has a lower thickness than the first region11to allow the first region11to have a relatively high stiffness and the second region12to have a relatively low stiffness. The resonant frequency of the movable portion is determined by its dimensions and the mechanical properties of the material of the movable portion. If the constituent material is constant, the spring constant decreases as the size of the movable portion increases, and the resonance frequency that is proportional to the spring constant decreases. However, when the support having a higher stiffness than the movable portion is provided around the movable portion, the substantial fixed end of the movable portion approximates to an area at and around the boundary between the low-stiffness region and the high-stiffness region. In other words, the area at and around the boundary between the first region11and the second region12becomes a substantially fixed end of the second region12. Further, using the first region11made of material having a higher stiffness than the second region12eliminates the need to increase the thickness of the first region11, and enables a relatively high-stiffness first region11and a relatively low-stiffness second region12.

In the above-described embodiment, the base10includes a low-stiffness area and a high-stiffness area. In some examples, the base10includes no high-stiffness area (e.g., the silicon active layer113), and an area where a low-stiffness layer (e.g., the silicon active layer113) is connected to the joint13provides a high-stiffness area serving as the fixed end of the movable portion. In this case, the size of the base10is to be reduced sufficiently to prevent a decrease in spring constant and maintain the mechanical strength of the movable portion having a low stiffness.

The piezoelectric element122drives the second reflector121to change the resonator length, which corresponds to the distance between the reflectors22and121. The light emitter211emits a laser beam whose wavelength is changeable with the resonator length.

The piezoelectric element122deforms the thin second reflector121, the deformation of the thin second reflector121drives the second reflector121. In this configuration, driving the second reflector121at high speeds and with a great amount of displacement changes the resonator length. For example, the second reflector121can be driven at a driving speed of 1 MHz or higher and with an amount of displacement of 200 nm or greater. This enables changes in the wavelengths of the laser beams over a wide wavelength range at high speeds.

FIGS.9A,9B, and9Care graphs of the relation between drive voltage applied to the piezoelectric element122and the wavelength of a laser beam emitted from the light source device1. For example, a tunable laser beam emitted from the light source1has a wavelength band ranging from 920 nm to 950 nm, and a drive voltage applied to the piezoelectric element122is below 5 V. Since the amount of deformation of the piezoelectric element122(i.e., the amount of change in the volume of the piezoelectric element122) linearly changes with the drive voltage, the amounts of displacement of the second region12and the second reflector121along the Z-axis substantially linearly change with the deformation of the piezoelectric element122. To allow a substantially linear change in the wavelength of a laser beam emitted from the light source device1with the drive voltage applied to the piezoelectric element122, the second reflector121is to be displaced with wavelengths in and around the range in which the oscillation wavelength substantially linearly changes with the distance between the reflectors, or the resonator length (e.g., with oscillation wavelengths close to or of 940 nm inFIG.8).

The volume of a voltage-signal source is to be reduced to reduce the sizes of the light source device1and other devices mounted on the light source device1. The volume of a voltage-signal source usually increases as voltage and its modulation speed increase. Further, the volume of a voltage-signal source, which additionally serves to correct non-linearity of wavelengths, increases more. The light source device1allows an approximately linear change in wavelength with drive voltage and enables changes in wavelength with a low drive voltage over a wide wavelength range, which is sufficient to achieve intended performance. Such a light source device1achieves a compact tunable-laser light source that emits a light beam whose wavelength is adjustable at a speed of MHz order.

In the present embodiment, the piezoelectric element122is provided in an annular shape around the second reflector121in the second region12. This arrangement allows a uniform distribution of driving force over the second reflector121in an inward direction from outside the second reflector121, and facilitates adjustment of displacement of the second reflector121.

In the above-described embodiments, the piezoelectric element122is on the surface included in the second region12and facing the light emitter211. However, this is only one example.

FIGS.10A and10Bare enlarged cross-sectional views of configurations of a second region12and its surroundings according to variations of an embodiment.FIG.10Ais an illustration of a configuration of a second region12and its surroundings according to a first variation of an embodiment.FIG.10Bis an illustration of a configuration of a second region12and its surroundings, according to a second variation of an embodiment.

InFIG.10A, a light source device1aaccording to the first variation includes the second region12between the second reflector121and the piezoelectric element122(i.e., the piezoelectric element122is at the opposite side of the second region12from the second reflector121), with the second reflector121overlapping with the piezoelectric element122. This configuration eliminates a restriction on the area of the piezoelectric element122due to the second reflector121, and thus allows an increase in the area of the piezoelectric element122. Increasing the area of the piezoelectric element122increases stress generated by the piezoelectric element122, and allows the second reflector121to displace more significantly.

InFIG.10B, a light source device1baccording to the second variation includes a piezoelectric element122on the −Z-surface of the second region12and a second reflector121overlapped on the −Z-surface of the piezoelectric element122. This configuration, in which the piezoelectric element122and the second reflector121are on one side of the second region12, also eliminates a restriction on the area piezoelectric element122due to the second reflector121and thus allows an increase in the area of piezoelectric element122. This configuration further eliminates the need for forming a film on each side of the second region12.

UnlikeFIG.10Bin which the piezoelectric element122and the second reflector121are on the −Z-surface of the second region12(i.e., downstream of the second region12in the −Z direction), the piezoelectric element122and the second reflector121may be on the +Z-surface of the second region12(i.e., downstream of the second region12in the +Z-direction).

FIGS.11and12are illustrations of a base10according to a first variation of an embodiment.

FIG.11is a plan view of the base10according to the first variation.FIG.12is a cross-sectional view taken along line A-A′ inFIG.11.

As illustrated inFIGS.11and12, the second region12of the base10includes multiple movable beams314connecting the second reflector121and the first region11. The movable beam314may not have a linear shape, and may be curved or partly bent. Further, the multiple movable beams314include two or more movable beams314. The two or more movable beams314are preferably disposed so as to be rotationally symmetrical. This facilitates driving of the second reflector121to displace along the Z-axis while maintaining parallelism between the second reflector121and the first reflector22.

Each movable beam314is formed by removing the support layer111, the oxide insulating layer112, and the silicon active layer113in a region316through the semiconductor processing, including dry etching. The region316is surrounded by the first region11. Similarly to the movable beams314, through the semiconductor processing, the piezoelectric element322is formed on the movable beams314of the second region12and on a portion of the first region11, which is a peripheral area of the region316. The spring constant of the movable beams314is changed by adjusting the dimension of the movable beam314to set the relation between the resonance frequency of the second reflector121and the amount of displacement of the second reflector121along the Z-axis during resonance.

The base10according to the present variation includes the piezoelectric element322on the movable beams314and on the peripheral area of the region316. Simultaneously applying voltage to the entirety of the piezoelectric element322on both the movable beams314and the peripheral area of the region161aincreases the amount of displacement of the second reflector121along the Z-axis during resonance. With a change in the volume of the piezoelectric element322on the movable beams314in response to application of voltage, the movable beams314on which the piezoelectric element322is provided deform, and the deformation of the movable beams314moves, or displaces the second reflector121(i.e., the deformation of the movable beams314changes the position of the second reflector121). For example, applying a sinusoidal voltage to the piezoelectric element322on the movable beams314sinusoidally displaces the second reflector121along the Z-axis over time. For another example, applying a voltage signal with a frequency equal to or close to the resonance frequency of the movable beams314to the piezoelectric element322allows the movable beams314to undergo excitation to thus exhibit a resonance phenomenon. The occurrence of the resonance phenomenon allows a greater amount of displacement of the second reflector121than a non-resonance status.

Further, applying, to the piezoelectric element322in the peripheral area of the region316, the same sinusoidal voltage signal as that of the piezoelectric element322on the movable beams314deforms the silicon active layer113near the peripheral area of the region316with a smaller amount of displacement than the movable beams314. When the oscillation caused by the deformation of the silicon active layer113is transferred to the movable beams314mechanically connected to the peripheral area of the region316, the movable beams314are excited to exhibit the resonance phenomenon. Using contraction and expansion of the piezoelectric element322, the movable beams314, which is to receive a greater amount of displacement at a higher speed, and the peripheral area mechanically connected to and surrounding the movable beams314are deformed to move, or displace the second reflector121on the movable beams314at high speeds and with a greater amount of displacement.

FIGS.13and14are illustrations of a base10according to a second variation of an embodiment.FIG.13is a plan view of the base10according to the second variation.FIG.14is a cross-sectional view taken along line B-B′ inFIG.13.

In the second variation as illustrated inFIGS.13and14, the second variation include, as the second reflector121, a reflector311having a high contrast grating (HCG) that servers to ensure its reflectivity. The reflector311has holes periodically formed on a portion of the silicon active layer113. The resonance frequency of the movable beams314is commonly inversely proportional to its mass. The reflector composed of a multilayer reflector or a metal thin film, in which multiple thin films are overlaid on top of another, has a large mass. In contrast, the reflector311having the HCG of a single silicon layer according to the present variation achieves the reflectivity equivalent to that of the second reflector121according to the first variation. Thus, the use of the reflector311as the second reflector121achieves a higher resonance frequency. Notably, the material of the HCG reflector is not limited to silicon, and may be any material having a refractive index different from the refractive index of the space through which light propagates. In the second variation, the second reflector121according to the first variation is replaced by a reflector311having an HCG. In some examples, the second reflector121according to the first embodiment may be replaced by a reflector311having have the HCG.

FIGS.15and16are illustrations of a base10according to a third variation of an embodiment.FIG.15is a plan view of the base10according to the third variation.FIG.16is a cross-sectional view taken along line C-C′ inFIG.15.

As illustrated inFIGS.15and16, the third variation differs from the first variation in that the movable beams314is made of a multilayer-film reflector315instead of the silicon active layer113. In the second variation, the reflector311includes an HCG in the movable beams314of the silicon active layer113. In contrast, the second reflector121is composed of the multilayer-film reflector315substituted for the movable beams314. The reflector composed of the multilayer film mirror or the metal thin film involves forming a reflector on the silicon active layer313, whereas the HCG reflector involves forming an HGC periodic structure on the silicon active layer313. However, in the present variation, a single structure enables both movement of the multilayer-film reflector315and reflection of light, which eliminates the needs for processing the element.

Further, the emission direction of the tunable laser beam emitted from the light source device1can be selected by adjusting the number of layers of the multilayer film to control its reflectivity. For example, the light source device with the base10according to any one of the first variation and the second variation is not to emit a laser beam having a wavelength in an absorption band of the silicon active layer113through the silicon active layer113. However, the light source device with the base10according to the present variation, which includes no silicon active layer113, allows the second reflector121to emit a laser beam even having a wavelength in the absorption band of the silicon active layer113in the direction31inFIG.1B. Notably, in the third variation, the silicon active layer113constituting the movable beams314according to the first variation is the multilayer-film reflector315. Alternatively, the silicon active layer113according to the first embodiment may be a multilayer-film reflector.

In the first variation to the third variation, at least two movable beams314are included as a part of the second region12. For example, the movable beam314has a width of 1 μm to 100 μm in the shorter-side direction, a length of 10 μm to 1000 μm in the longer-side direction, and a thickness of 50 μm to 100 μm in the thickness direction. The movable beam314having such dimensions enables a higher resonance frequency (e.g., above 1 MHz) and a greater wavelength sweep width while achieving a reduction in the size of the device.

Second Embodiment

Next, a light source device1caccording to a second embodiment will be described. Like reference signs denote like elements as in the first embodiment, and overlapping description is omitted. The same applies to the following embodiments and variations described below.

FIG.17is a plan view of an exemplary configuration of a light source device1c. InFIG.17, the light source device1cincludes a second base110supporting the base10, which differs from the first embodiment. The second base110includes a second support100and second movable portions13a, and13b. The second support100is an example of a third region, and the second movable portions13aand13bare examples of a fourth region.

The second movable portion13ahas a meandering structure, in which adjacent ends162aof the movable beam131aand the movable beam132aare couple to each other, and adjacent ends161aof the movable beam133aand the movable beam134aare coupled to each other. One end of the second movable portion13ais coupled to the second support100through a first coupling portion151a, and the other end of the second movable portion13ais coupled to the base10through a second coupling portion152a. Each of the movable beams131a,132a,133a, and134ais an example of a beam.

The movable beam131a,132a,133a, and134aincludes piezoelectric elements141a,142a,143a, and144aon their −Z-surfaces, respectively. Each of the piezoelectric elements141a,142a,143a, and144ais an example of a base driver.

Each of the piezoelectric elements141a,142a,143a, and144adeforms (e.g., contracts) with a drive voltage applied thereto through electrodes on the second support100. The movable beams133aand134aare elastically deformed by the deformation of the piezoelectric elements141aand142a, and the movable beams131aand132aare elastically deformed by the deformation of the piezoelectric elements143aand144a.

Further, the second movable portion13bhas a meandering structure, in which the movable beam131band the movable beam132bare coupled to each other at their ends161b, and the movable beam133band the movable beam134bare coupled to each other at their ends162b. One end of the second movable13bis coupled to the second support100through a first coupling portion151b, and the other end of the second movable portion13bis coupled to the base10through a second coupling portion152b. Each of the movable beams131b,132b,133b, and134bis an example of a beam.

The movable beams131b,132b,133b, and134binclude piezoelectric elements141b,142b,143b, and144bon the −Z-surfaces of the movable beams131b,132b,133b, and134balong the Z-axis. Each of the piezoelectric elements141b,142b,143b, and144bis an example of a base driver.

Each of the piezoelectric elements141b,142b,143b, and144bdeforms (e.g., contracts) with a drive voltage applied thereto through electrodes on the second support100. The movable beams131band132bare elastically deformed by the deformation of the piezoelectric elements141band142b, and the movable beams133band134bare elastically deformed by the deformation of the piezoelectric elements143band144b.

The movable beams131a,132a,133a,134a,131b,132b,133b, and134bare thinner and stiffer than the first region of the base10and the second support100. This configuration allows an area in and around each of the first coupling portion151aand the second coupling portion152ato be substantially a fixed end of the second movable portion13a, and also allows an area in and around each of the first coupling portion151band the second coupling portion152bto be substantially a fixed end of the second movable portion13b.

When the amount of displacement of the second movable portion13ais equal to the amount of displacement of the second movable portion13b, the base10is translationally moved by that amount of displacement along the Z-axis. When the amount of displacement of the second movable portion13ais different from the amount of displacement of the second movable portion13b, the base10is caused to tilt along the Y-axis.

For example, when the amount of displacement of the second movable portion13ais smaller than the amount of displacement of the second movable portion13b, the −Y-end portion of the base10is displaced by a relatively small amount in the −Z-direction, and the +Y-end portion of the base10is displaced by a relatively large amount in the −Z-direction. This causes the base10to tilt along the Y-axis.

FIG.17is a side view of the base10, on which the second reflector121is provided. The VCSEL device20is assumed to be disposed downstream of the second support100in the −Z-direction and connected to the second support100through the joint13.

In the light source device1according to the first embodiment, the distance along the Z-axis between the VCSEL device20and the base10might vary in each manufactured light source device1. If the distance along the Z-axis between the VCSEL device20and the base10deviates from a desired distance between the reflectors, a laser beam having a desired wavelength may not be obtained.

The correction of the distance between the reflectors by adjusting the distance between the VCSEL device20and the base10during the joining operation takes time and effort. For example, applying an offset drive voltage to the piezoelectric element122to translationally move the second reflector121along the Z-axis so as to correct the distance between the reflectors restricts the amount of the translational movement of the second reflector121along the Z-axis, and may fail to completely correct the distance between the reflectors, which significantly deviates from a desired value.

To change the distance between the VCSEL device20and the base10to correct the distance between the reflectors, the present embodiment applies bias voltage to the piezoelectric elements141a,142a,143a,144a,141b,142b,143b, and144bof the second movable portions13aand13bto deform the movable portions13aand13bto drive the base10along the Z-axis. This configuration eliminates the needs for adjusting (correcting) the distance between the VCSEL device20and the base10and facilitates correction of the distance between the reflectors.

The second movable portion13aand the13beach having a meander structure enable a more significant displacement of the base10than the driving of the piezoelectric element122to move the second reflector121. The configuration of the present embodiment enables correction of the distance between the reflectors, which even significantly displaces from a desired distance between the reflectors.

For the base10tilted relative to the VCSEL device20, the base10is caused to tilt by the deformation of the second movable portions13aand13bto change the tilt angle of the base10relative to the VCSEL device20so as to correct the distance between the reflectors. In some examples, the distance between the reflectors is corrected by changing both the distance between the VCSEL device20and the base10and the tilt angle of the base10relative to the VCSEL device20.

In the light source device1c, the second reflector121and the base10may be driven separately. In this case, a drive voltage whose waveform continuously and periodically changes with time (e.g., a sinusoidal waveform and a triangular waveform) is applied to the piezoelectric element122on the second region of the base while maintaining a bias voltage applied to the piezoelectric elements122of the second movable portions13a, and13b. This allows continuous modulation of the wavelength of a laser beam at high speeds. The central wavelength of the laser beam is adjusted while maintaining the range in which the wavelength continuously changes, by changing a bias voltage applied to the piezoelectric elements in the second movable portions13a, and13b. This is due to synergy between the second movable portions13aand13bthat allows a significant change in the distances between the reflectors and the second region that allows sweep of wavelengths at high speeds. A typical electrostatic tunable laser has difficulties in simultaneously modulating the wavelength of the laser beam at high speeds in a continuous manner and modulating the central wavelength of the laser beam. This is because, the electrostatic driving unit drives the reflectors using an electrostatic attractive force uniformly acting between two surfaces facing each other with an air gap therebetween, and a portion to be driven or driving characteristics cannot be divided within a plane.

To separately drive the second reflector121and the base10, the resonance frequency of the base10coupled to the second movable portions13aand13bthrough the first coupling portion151aand the second coupling portion152a, respectively is to be different from the resonance frequency of the second reflector121in the second region12by 50 Hz or greater. If the second reflector121is caused to oscillate with the resonance frequency of the second region12, which is equal to the resonance frequency of the base10or with an absolute difference of below 50 Hz between the resonance frequency of the second region12and the resonance frequency of the base10, the entirety of the base10including the second reflector121is excited by the oscillation of the second reflector121, and the base10whose displacement is fixed by the bias voltage might be displaced. With a difference between the resonance frequency of the base10and the second movable portions13a, and13band the resonance frequency of the second reflector121and the second region12, noticeably of 50 Hz or greater, the wavelength of the laser beam is stably swept at high speeds while changing the central wavelength of the laser beam by changing the distance between the reflectors.

With the drive frequency greater for the second reflector121than for the base10, the second region12is to have a higher stiffness than the movable beams131a,132a,133a,134a,131b,132b,133b, and134bof the second movable portions13a, and13b. Further, the weight is to be smaller for the second region12than for the second movable portions13a, and13b.

In the present embodiment, two second movable portions13a, and13bare incorporated in the light source device1c. Alternatively, the light source device1cmay include three or more movable portions. Still alternatively, the second movable portions13aand13bmay have a meander structure composed of three or more movable beams131, instead of two movable beams131aand131b.

Third Embodiment

Next, a light source device1caccording to a second embodiment will be described.

FIG.18is a plan view of an exemplary configuration of a light source device1d. As illustrated inFIG.18, the light source device1dincludes a base10d, which is substituted for the base10of the light source device1caccording to the second embodiment. The base10dincludes four second regions12a,12b,12c, and12d. The four second regions12a,12b,12c, and12dare two-dimensionally arranged within the plane of the base10d.

The second region12aincludes a second reflector121aand a ring-shaped piezoelectric element122asurrounding the second reflector121a. The second region12bincludes a second reflector121band a ring-shaped piezoelectric element122bsurrounding the second reflector121b. The second region12cincludes a second reflector121cand a ring-shaped piezoelectric element122csurrounding the second reflector121c. The second region12dincludes a second reflector121dand a ring-shaped piezoelectric element122dsurrounding the second reflector121d.

The piezoelectric elements122a,122b,122c, and122ddrive the second reflectors121a,121b,121c, and121d, independently of each other.

The second regions12a,12b,12c, and12dserve the same function as the second region12, and the second reflectors121a,121b,121c, and121dserve the same function as the second reflector121. The piezoelectric elements122a,122b,122c, and122dhas the same configuration and serve the same function as those of the piezoelectric element122.

FIG.18illustrates one side of the base10d, on which the second reflectors121a,121b,121c, and121dare provided. The VCSEL device20is assumed to be disposed downstream of the second support100in the −Z-direction and connected to the second support100through the joint13.

The VCSEL device20includes multiple light emitters211for the second reflectors121a,121b,121c, and121d, respectively. Each of the second reflectors121a,121b,121c, and121dand its corresponding first reflector22forms a resonator with a corresponding light emitter therebetween. Each light emitter211emits a laser beam having a wavelength according to the resonator length.

The light source device1din which multiple light emitters211emits light beams achieves an increase in the intensity of the laser beam to be emitted. In the light source device1d, the multiple light emitters211are driven to emit light beams in a staggered manner. This configuration allows a shorter duration of light emission of each light emitter211, and enables a reduction in heat generation due to the emission of laser beams, thus achieving a longer duration of light emission of the light source device1das a whole.

Fourth Embodiment

Next, a LiDAR device200according to a fourth embodiment will be described.FIG.19is a block diagram of a configuration of a LiDAR device200as an example of a distance measurement apparatus. The LiDAR device200is, for example, an FMCW LiDAR device that measures a distance to an object.

As illustrated inFIG.19, the LiDAR device200includes a light source device1, a photo-coupler202, a light mixer203, a photosensor204, an analog-to-digital (AD) converter205, and a frequency analytical processor206.

The light source device1emits a tunable laser beam whose wavelength changes with a drive voltage. The photo-coupler202splits the tunable laser beam emitted from the light source device1into two light beams: a first light beam as an irradiation wave230and a second light beam as a reference wave231, with a predetermined energy ratio. The photo-coupler202irradiates an object300with the first light beam as the irradiation wave230, and directs the second light beam as the reference wave231to the light mixer203.

The first light beam hitting the object300is reflected or scattered from the object300back to the light mixer203, turning a return wave232. The light mixer203superimposes the return wave232on the reference wave231to generate an interference wave.

The return wave232is delayed because of the distance to the object300. For the object300moving relative to the LiDAR device200, a frequency shift of a wave from such object300occurs because of the Doppler effect.

The photosensor204receives the interference wave generated by the light mixer203and outputs a voltage signal according to a light intensity of the interference wave. A voltage signal (beat signal) obtained from the interference wave generated by the light mixer203includes a frequency difference between the reference wave231and the return wave232and a frequency shift due to the Doppler effect.

The AD converter205A/D converts an analog voltage signal output from the photosensor204, into a digital signal and outputs the digital signal to the frequency analytical processor206. The frequency analytical processor206analyzes the input digital signal through Fourier transform, and calculates a frequency difference between the reference wave231and the return wave232from frequency peak data detected by the analysis. Using the frequency difference, the LiDAR device200acquires and outputs at least one of the distance to the object300and the relative speed of the object300.

For typical FMCW-LiDAR devices using tunable laser beams, linearity of wavelength changes with time is a challenge. The accuracy of measurement might significantly decrease if the level of linearity is low.

Incorporating the light source device1enables the LiDAR device200to change wavelengths over a wide range at high speeds and perform measurement with a tunable laser beam having a high linearity of wavelength changes with time, thus achieving a higher accuracy of measurement.

For two-dimensional scanning a ranging space with continuous tunable laser beams to acquire a three-dimensional point group, one wavelength sweep is performed for one point group. Notably, the term “wavelength sweep” refers to changing a wavelength with time. The ranging space refers to a space in which a distance to an object is measured.

The wavelength sweep rate is obtained by the expression: F×N (Hz), which is F×N times for at least one second where N denotes the number of ranging points per frame of the LiDAR device200, and F denotes a frame rate.

For example, assuming that the number of ranging points is the product of the number of points in the horizontal direction and the number of points in the vertical direction, and that the number of ranging points in each direction is 102, the total number is 104. The minimum wavelength sweep rate is 104×102=106(Hz) where the frame rate is 102.

That is, 1 (MHz) is obtained.

To increase the frame rate and resolution of the LiDAR device200, the wavelength sweep rate is to be increased to about 1 MHz.

Further, the accuracy of measurement of the LiDAR device200depends not only on the linearity of the wavelength sweep but also on the width of the variable wavelength range. Specifically, as the wavelength range is wider, the accuracy of measurement increases, and distance measurement with a submillimeter wave becomes possible. To expand the wavelength range, the amount of displacement of the second reflector121is to be increased to increase the distance between the reflectors.

In the light source device according to a comparative example in which the movable portion is driven by the electrostatic actuation, the amount of displacement of the reflector by the driving of the movable portion is not proportional to the first power of the voltage.FIG.20is a graph of the relation between drive voltage and the amount of displacement of the reflector in a light source device according to a comparative example. As illustrated inFIG.20, the slope of the amount of displacement with respect to the drive voltage is not constant.

This phenomenon is caused by the followings: the position of the reflector is determined by the conditions for balancing the electrostatic attractive force acting between the reflector and the flat plate facing the reflector with the spring restoring force of the movable portion; and the electrostatic attractive force is proportional to the square of the voltage.

Incorporating such a light source device that drives the movable portion with the electrostatically actuation into the LiDAR device takes effort to distort drive voltage in advance to obtain a higher linearity of wavelength sweep, resulting in complicated control.

The resonance frequency of the movable portion is proportional to the square root of the reciprocal of the density of the movable portion, the square root of the reciprocal of the thickness of the movable portion, and the drive voltage. In view of such relations, to increase the resonance frequency tenfold, the density or thickness is to be reduced to one-hundredth, or the drive voltage is to be increased up to tenfold.

It is a challenge to reduce the density or thickness of the movable portion while maintaining its mechanical strength sufficient to prevent breakage of the movable portion. Further, increasing the drive voltage is restricted by the specification such as the size, operational reliability, and power consumption of the optical device, including a light source device.

The resonance frequency of the electrostatically-actuated movable portion is inversely proportional to the amount of displacement thereof. With such an inverse relation, the amount of displacement sufficient to increase the wave range cannot be obtained while achieving actuation with approximately one MHz. As described above, incorporating the electrostatically-actuated light source device, which actuates the movable portion with the electrostatic actuation, into the FMCW-LiDAR device fails to achieve a higher accuracy of measurement.

However, using the light source device1according to an embodiment of the present disclosure allows a tunable laser beam whose wavelength is changeable over a wide wavelength range at high speeds. This increases the frame rate of the LiDAR device200to increase the spatial resolution of the object300within a plane (i.e., an XY plane) and improve the accuracy of measurement of at least one of the distance to the object300and the speed of the object300relative to the LiDAR device200.

Fifth Embodiment

Next, a fifth embodiment will be described. The fifth embodiment relates to a mobile object.FIG.21is an illustration of a vehicle as a mobile object according to a fifth embodiment. A vehicle500as an example of a mobile object according to the fifth embodiment is mounted with a LiDAR device200according to the fourth embodiment at an upper portion of the windshield. The LiDAR device200is an example of a distance measurement apparatus that measures at least one of a distance to an object and the speed of the object. The light source device1emits a light beam to an object502, and the photosensor204receives the light beam reflected and returned from the object502. The LiDAR device200according to the present embodiment calculates a distance to the object502around the vehicle500based on the laser beam reflected from the object502. The measurement result of the LiDAR device200is input to a controller as processing circuitry included in the vehicle500, and the controller controls the operation of the mobile object based on the measurement result of the LiDAR device200. Alternatively, the controller displays a warning on a display provided in the vehicle500for the driver501of the vehicle500based on the measurement result of the LiDAR device200.

The fifth embodiment in which the LiDAR device200is mounted on the vehicle500enables recognition of the location of an object502around the vehicle500at a high precision. The installation position of the LiDAR device200is not limited to an upper and front portion of the vehicle500, and the LiDAR device200may be mounted at a side surface or a rear portion of the vehicle500. In this example, the LiDAR device200is provided in the vehicle500. Alternatively, the LiDAR device200may be provided in an aircraft or a ship. Still alternatively, the LiDAR device200may be provided in mobile objects such as drones and robots that autonomously move without a driver.

Numerous additional modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the disclosure of the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims.

In addition, the numbers such as ordinal numbers and quantities used above are all examples for specifically describing the technology of the present invention, and the present invention is not limited to the exemplified numbers. In addition, the relation of connection between the components are illustrated for specifically describing the technology of the present invention, and the relation of connection for implementing the function of the present disclosure is not limited thereto.

The above-described embodiments are illustrative and do not limit the present invention. Thus, numerous additional modifications and variations are possible in light of the above teachings. For example, elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of the present invention.

Any one of the above-described operations may be performed in various other ways, for example, in an order different from the one described above.

Each of the functions of the described embodiments may be implemented by one or more processing circuits or circuitry. Processing circuitry includes a programmed processor, as a processor includes circuitry. A processing circuit also includes devices such as an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), and conventional circuit components arranged to perform the recited functions.