JP7430906B2 - Beam scanning wide angle system - Google Patents

Description

The present invention relates to a beam-scanning widening system from a beam steering element.

Various methods have been reported for beam steering devices that enable one-dimensional or two-dimensional scanning of the propagation direction of a laser beam, such as autonomous driving LiDAR (Light Detection and Ranging) and projectors. .

A deflection element that bends the beam propagation direction is essential to a beam steering device, and a deflection element that can operate at higher speeds is required for LiDAR and projector applications. One of the deflection elements that meets this requirement is a MEMS mirror. The MEMS mirror uses the principle of electromagnetic induction to swing a minute mirror at high speed, and the propagation direction of the beam can be controlled at high speed. Taking advantage of these properties, MEMS mirrors are actively used as components of beam steering devices installed in LiDAR.

However, on the other hand, the optical deflection angle of the MEMS mirror is smaller than that of other methods, and is generally limited to less than 10 degrees, so it is not possible to illuminate a wide area at a short distance.

Several methods have been proposed as techniques for widening the optical deflection angle of a MEMS mirror. One is a method using a bulk lens (see Non-Patent Documents 1, 2, and 3). According to this conventional technique, for example, the traveling direction of the beam is refracted by a concave lens, and the deflection angle of the beam can be expanded. The magnitude of the deflection angle can be designed according to the concave curvature of the bulk lens.

There are also other methods that use flat lenses such as Fresnel lenses and DOEs (Differtactive Optical Elements), which make it possible to construct optical systems that are lighter and more compact than bulk lenses.

Other methods have been proposed that utilize an isotropic volume hologram or a polarization diffraction grating to realize beam steering without mechanically moving parts (see Patent Document 1 and Non-Patent Document 4). This method combines the beam deflection function of a polarization diffraction grating in multiple stages to achieve a large deflection angle.

Patent Document 2 describes a technique of combining a polarization diffraction grating and a MEMS mirror on a MEMS array. In this technique, a polarization diffraction element in which polarization diffraction gratings having different grating vector directions and periods are distributed in an array or continuously is used as a wavefront control element. With this technology, by changing the incident area to the polarization diffraction element using a MEMS mirror, the propagation direction of the light beam can be controlled in parallel or randomly by combining the propagation direction control by the MEMS mirror and the propagation direction control by the polarization diffraction element. Since the system configuration is complicated because a plurality of MEMS mirrors and their array devices are combined in multiple stages, a system that can widen the deflection angle using existing beam steering elements is desired.

Furthermore, since the MEMS mirror utilizes a resonance phenomenon, the driving of the mirror is limited to a sine waveform. For this reason, the scanning speed of the beam is not constant; it becomes slow near the maximum optical deflection angle and fast near the minimum optical deflection angle, resulting in uneven brightness within the projected two-dimensional area. Become. Improvement of this brightness unevenness is desired in any application method such as processing, display, measurement, etc., and a technology that equalizes the scanning speed of the beam within the projected two-dimensional area (irradiation surface) is desired. There is.

JP2011-257764A JP 2019-45574 Publication

K. Iida, T. Morikawa, T. Hano, S. Shimizu, and K. Tezuka, “Development of 3D range sensor with super-wide angle detection to observe vehicle surrounding,” 19th ITS World Congress, no. AP-00079, Austiam Oct. 2012. X. Lee, and C. Wang, “Optical design for uniform scanning in MEMS-based 3D imaging lidar,” Appl. Opt. 54, 2219-2223 (2015). J. Zhou, and K. Qian, “Low-voltage wide-field-of-view lidar scanning system based on a MEMS mirror,” Appl. Opt. 58, A283-A290 (2019). J. Kim, C. Oh, S. Serati, and M. J. Escuti, “Wide-angle, nonmechanical beam steering with high throughput utilizing polarization gratings,” Appl. Opt. 2636-2639 (2011).

Several examples of conventional techniques for widening the deflection angle of MEMS mirrors have been listed above, but there is a technology that uses existing beam steering elements to widen the deflection angle and to equalize the scanning speed of the light beam on the irradiation surface. desired.

In view of the above-mentioned problems, an object of the present invention is to provide a beam scanning wide-angle system having the functions of widening the deflection angle of beam scanning and uniformizing the scanning speed on the beam irradiation surface.

A light beam widening system according to one aspect of the present invention includes a light source, a beam steering element, a polarization control element disposed between the light source and the beam steering element, and a light beam emitted from the light source passing through the beam steering element. and a polarization diffraction element arranged so as to be incident on the beam, the polarization diffraction element having a local grating period and having a function of increasing the deflection angle of the light beam from the deflection angle given by the beam steering element. , and has a function of keeping the scanning speed of the light beam constant in a spatial plane parallel to and separated from the plane of the polarization diffraction element.

In this aspect, it is preferable that the polarization diffraction element is arranged at a position where a light beam can directly enter from the beam steering element.

Furthermore, in this aspect, the local grating period of the polarization diffraction element is a distribution of optical axis orientations formed with one-dimensional or two-dimensional regularity within the plane of the polarization diffraction element. preferable.

Moreover, in this aspect, it is preferable that the polarization diffraction element is configured to include an optically anisotropic material. In this case, the polarization diffraction element includes a liquid crystal polymer film having a photoreactive side chain, and the photoreactive side chain causes at least one of photocrosslinking and photoisomerization reactions. Preferably.

Moreover, in this aspect, it is preferable to provide a plurality of polarization diffraction elements.

As described above, according to the present invention, it is possible to provide a beam scanning wide angle system having the functions of widening the deflection angle of beam scanning from a beam steering element and uniformizing the scanning speed on the beam irradiation surface.

1 is a schematic diagram of a beam scanning wide-angle system according to a first embodiment; FIG. FIG. 2 is a diagram schematically illustrating light beam scanning by a beam steering element in a configuration that does not include a polarization diffraction element. FIG. 2 is a diagram showing a polarization micrograph of a polarization diffraction element having a uniform grating period within the element plane and the distribution of the optical axis orientation φ within the element plane (optic axis distribution). 4 is a diagram showing an example of the state of diffracted light when the light passes through the polarization diffraction element of FIG. 3. FIG. FIG. 7 is a diagram showing the dependence of the diffraction efficiency of ±1st-order light on the ellipticity of the incident light when the retardation Γ is π/2. FIG. 3 is a diagram showing an example of the optical axis distribution of a polarization diffraction element in which local grating periods are distributed so that the deflection angle of light differs depending on the incident position of the light ray. (a) When distributed in one-dimensional (x-axis) direction. (b) When distributed in two dimensions (x-axis and y-axis). FIG. 7 is a diagram schematically showing a beam scanning wide angle system including a polarization diffraction element having a function of widening the beam scanning angle and uniformizing the speed of beam scanning according to a second embodiment. In the beam scanning wide-angle system (one-dimensional) according to Embodiment 2, (a) temporal change in beam exposure position, (b) spatial distribution of local grating period of the polarization diffraction element, and (c) optics of the polarization diffraction element FIG. 7 is a diagram showing an optical axis distribution shown by contour lines in which the amount of rotation of the axial direction φ is plotted every 300π. In the beam scanning wide-angle system (two-dimensional) according to Embodiment 2, (a) temporal change in the beam exposure position, (b) spatial distribution of local grating period of the polarization diffraction element, and (c) optics of the polarization diffraction element FIG. 7 is a diagram showing an optical axis distribution that reproduces the sum of phase distributions in the x direction and the y direction of the axial direction φ. It is a polarization micrograph of the polarization diffraction element of an Example. FIG. 2 is a diagram illustrating a beam scanning wide-angle system according to an embodiment. FIG. 7 is a diagram showing an example in which the deflection angle θ ₁ of one-dimensional scanning of the MEMS mirror is widened by 1.61 times. It is a figure explaining deflection angle (theta).

Hereinafter, embodiments of the present invention will be described in detail using the drawings. However, the present invention can be implemented in many different forms and is not limited to the specific examples described in the embodiments and examples below.

[Embodiment 1]
FIG. 1 schematically shows a beam scanning wide-angle system (hereinafter referred to as "this system") 1 according to the first embodiment. As shown in this figure, the system 1 includes a light source 2, a beam steering element 4, a polarization diffraction element 5, and a polarization control element 3 disposed between the light source 2 and the beam steering element 4.

<Light source 2>
In this system 1, the light source 2 literally refers to something that can emit light. Although the light emitted by the light source 2 of the system 1 is not particularly limited, it is desirable that the light be a laser beam having a single wavelength λ. Examples of light sources that emit laser light include semiconductor lasers, all-solid-state lasers, and DPSS (Diode Pumped Solid State) lasers.

The wavelength λ of the light emitted by the light source 2 is not limited as long as it satisfies formula (4) described below, but if it is, for example, a wavelength of 0.1 mm or more and 1 mm or less, the system can be used in a millimeter wave laser etc. mounted on a car etc. be able to. Further, if the thickness is 1 μm or more and 10 μm or less, it can be used for LiDAR etc. used in automatic driving etc. Furthermore, if it is 300 nm or more and 900 nm or less, it can be used for a laser projector or the like. Therefore, the wavelength of the light emitted by the light source 2 is preferably 300 nm or more and 1 mm or less, and more preferably 300 nm or more and 10 μm or less.

<Polarization control element 3>
In the present system 1, the polarization control element 3 is an element that controls the polarization state of the light having the wavelength λ emitted by the light source 2. Specifically, as illustrated in FIG. 1, the polarization control element 3 preferably includes a λ/4 plate 32, and more preferably includes a polarizer 31 at the front stage thereof.

The polarizer 31 makes it possible to convert unpolarized light into linearly polarized light, and the λ/4 plate 32 makes it possible to convert linearly polarized light into right-handed or left-handed circularly polarized light. Moreover, if the λ/4 plate 32 is directed at an angle other than 0°, 45°, 90°, or 135° from the vibration direction of the linearly polarized light emitted from the laser, it is possible to convert the light into elliptically polarized light. Further, as a method for producing circularly polarized light, a retardation having a variable retardation Γ such as an electro-optic modulator or a liquid crystal retarder may be used.

The polarizer 31, the λ/4 plate 32, and other elements can control the ellipticity of the circularly polarized light before it is incident on the beam steering element 4 by the polarization control element 3. Furthermore, by controlling the rotation direction of the circularly polarized light before it enters the polarization diffraction element 5 using the polarization control element 3, the optical path direction of the beam emitted from the polarization diffraction element 5 can be changed.

<Beam steering element 4>
In this system 1, the beam steering element 4 is capable of deflecting the incident position and propagation direction of circularly polarized light incident on the polarization diffraction element 5.

The beam steering element 4 can take various forms, and is preferably capable of externally controlling a mirror reflection surface such as a MEMS mirror, and controlling the propagation direction of the incident light into a Lissajous wave shape. . However, the beam steering element is not limited to a beam steering element having a reflective structure, and may be a transmission type beam steering element such as a wedge prism or a diffraction grating having a rotation mechanism.

FIG. 2 is a diagram schematically showing the beam scanning by the beam steering element 4 when the polarization diffraction element 5 is not arranged. The light deflected by a beam steering element 4 such as a MEMS mirror is scanned on a coordinate axis x ₂ that is a distance d ₀ away from the reference coordinate axis z. At this time, if the deflection angle θ ₁ optically given by the MEMS mirror does not respond linearly to time, the distance that the light beam moves on x ₂ per unit time (scanning speed) is no longer constant. That is, the positional intervals at which the light beam exposes x ₂ are irregular. Luminance unevenness occurs because the light density is not made uniform.

The polarization diffraction element 5, which will be described next, has the function of widening the deflection angle θ ₁ given by the beam steering element 4 during beam scanning while correcting this brightness unevenness.

<Polarization diffraction element 5>
The polarization diffraction element 5 has a structure in which the optical axis direction φ is periodically modulated, more specifically, it has a structure in which the optical axis direction φ is periodically distributed. It is preferable to have a characteristic of having a grating period and deflecting the circularly polarized light at a deflection angle (diffraction angle) corresponding to the local grating period Λ(x) at the position where the circularly polarized light is incident on the plane of the polarization diffraction element 5.

Here, the "local grating period" of the polarization diffraction element is obtained by dividing π by the rotation angle per unit length at a certain position, and is expressed by the following formula.

The polarization diffraction element 5 included in this system is preferably one that has an efficiency of deflecting light in a desired direction at 90% or more, preferably 95% or more, more preferably 98% or more, and most preferably 100%. The polarization diffraction element 5 preferably has an ellipticity of 95% or more, preferably 97% or more, more preferably 99% or more, and most preferably 100% of the light to be polarized in a desired direction.

The spatial distribution structure of the optical axis azimuth φ in the polarization diffraction element 5 is not limited, and can be selected as appropriate depending on the characteristics of the beam steering element 4 (driving frequency and Lissajous figure drawn by the scanning locus). The functions of the polarization diffraction element will be described based on a polarization diffraction element having an anisotropic spatial distribution in which the optical axis direction periodically rotates linearly and continuously with respect to a one-dimensional direction as shown in FIG.

Here, "one-dimensionally regular optical axis direction" refers to a plane perpendicular to the reference axis z, where the optical axis of the light beam perpendicularly entering the polarization diffraction element 5 is the reference axis z. A state in which the optical axis azimuth φ is formed according to an arbitrary rule along an arbitrary uniaxial direction within the optical axis. The state in which the optical axis direction periodically rotates linearly and continuously with respect to the one-dimensional direction shown in FIG. 3 is an example of the most typical regularity.

When light is incident on the polarization diffraction element, a Pancharatnam Berry phase (PB phase) corresponding to the optical axis direction φ is periodically imparted to the light, causing diffraction (FIG. 4). Using the Jones vector |L> of left-handed circularly polarized light in the following equation (2) and the Jones vector |R> of right-handed circularly polarized light in the following equation (3), the Jones matrix of a polarization diffraction element with a uniform grating period distribution is , is defined by the following formula (4).

Here, T ^OC is the Jones matrix of the polarization diffraction element, Γ = πΔnd/λ is the retardation of the polarization diffraction element (Δn: birefringence of the polarization diffraction element, d: thickness of the polarization diffraction element), Λ is the polarization diffraction element , where t is the time, x is the spatial coordinate of the horizontal axis, y is the spatial coordinate of the vertical axis, φ is the orientation of the grating vector from the x-axis, i is the imaginary unit, and <L| and <R| represent adjoint matrices of |L> and |R>, respectively.

Further, by calculating the complex amplitude of transmitted light for left-handed circularly polarized light and right-handed circularly polarized light from formula (4), the following formulas (5) and (6) can be obtained. Here, the second term in each of equations (5) and (6) corresponds to the diffracted light components of the +1st order light and -1st order light, and the diffracted light intensity I _L ⁺¹ of the +1st order light with respect to the left circularly polarized light and the right The diffracted light intensity I _R ^-1 of −1st-order light with respect to circularly polarized light is given by the following equations (7) and (8), respectively. From these equations, the polarization diffraction element can obtain 100% diffraction efficiency for incident circularly polarized light when the retardation Γ is π/2.

The propagation direction of the diffracted light is expressed by the following equation (9) when the angle of incidence on the polarization diffraction element 5 is θ _in as shown in FIG. From this equation, it can be seen that the light incident on the polarization diffraction element 5 is deflected by an angle θ _out depending on the grating period Λ of the polarization diffraction element and the wavelength λ of the incident light (θ _out is the deflection angle or the diffraction angle ). The propagation direction of the diffracted light is reversed in the ±1st order direction according to the rotation direction of the circularly polarized light. Note that the Jones vector |E _arb > when arbitrary elliptically polarized light is incident is expressed as in the following equation (10), and the diffracted light intensity I _arb ^± of the ±1st-order light is determined by the following equation (11). Here, ε and Ψ represent the ellipticity angle and azimuth angle of the incident polarized light. From equation (11), when the retardation Γ of the polarization diffraction element is π/2, the diffraction efficiency of the ±1st order light has the characteristic that the intensity ratio between the ±1st order lights changes depending on the ellipticity of the incident light. , its dependence on the ellipticity angle is shown in FIG.

As described above, when the retardation Γ satisfies the condition of π/2, the polarization diffraction element exhibits the function of diffracting the incident circularly polarized light into either the ±1st order light with 100% diffraction efficiency. Therefore, it is possible to deflect the beam in a specific direction without generating unnecessary diffracted light.

When using a polarization diffraction element to widen the angle of a beam steering element, it is preferable that the polarization diffraction element has high diffraction efficiency. In the polarization diffraction element, the ideal phase difference with the highest diffraction efficiency can be found from the above equations (7) and (8).

Specifically, the polarization diffraction element used in this system has a diffraction efficiency of 5% or more, that is, a phase difference (δ=2πΔnd/λ) in the range of 0.448+2πm to 5.82+2πm (m: natural number), preferably 50 % or more, that is, the phase difference (δ = 2πΔnd/λ) is in the range of 1.57 + 2πm to 4.71 + 2πm (m: natural number), ideally the diffraction efficiency is 100%, that is, the phase difference (δ = 2πΔnd/λ). λ) is preferably 3.14+2πm (m: natural number).

In addition, in order to concentrate energy on a desired diffraction order, the absolute value of the ellipticity angle of the circularly polarized light incident on the polarization diffraction element 5 is 26 degrees or more, preferably 32 degrees or more, more preferably 37 degrees or more, and most preferably, as shown in FIG. is preferably 45deg.

Note that, as described above, the deflection angle θ _out of the light beam transmitted through the polarization diffraction element depends on the grating period Λ of the polarization diffraction element. Therefore, by giving a distribution to the grating period of the polarization diffraction element, it is possible to control the deflection angle for each incident position of the light beam to the polarization diffraction element.

[Embodiment 2]
Based on this idea, the state in which the scanning speed on the screen 6 in a spatial plane parallel to the plane of the polarization diffraction element 5 at a distance d ₂ is non-uniform due to beam scanning using only the beam steering element 4 can be explained by polarization diffraction. By correcting the distance that the light beam moves on the screen per unit time by controlling the deflection angle θ _out by the element 5, the scanning speed of the light beam on the screen can be made constant, and the deflection angle θ of the light beam can be adjusted to a wide angle. can be converted into

Here, "the deflection angle of the light beam" θ means that the light beam scanned by the beam steering element 4 is polarized by polarization diffraction, with the reference axis z being the light axis that is perpendicular to the polarization diffraction element surface and the spatial plane xy. This is the angle that a line passing through the emission point P _PG in the element 5 and the point P _ex exposed on the spatial plane xy makes with respect to the reference axis z (FIG. 13). When the polarization diffraction element 5 is not arranged, the deflection angle θ ₁ given by the beam steering element 4 with respect to the reference axis z is equal to θ.

In the case of a MEMS mirror, etc., the light beam scanning speed becomes slower toward the outer periphery of the exposure area. For example, for one-dimensional light beam scanning, as shown in FIG. By shortening the local grating period of the polarization diffraction element (both ends of the x-axis) continuously or discretely from the center of the element (from the y-axis) toward the outside, changes in scanning speed can be corrected. It becomes possible.

In addition, FIG. 6(b) is an example of an optical axis direction formed with two-dimensional regularity, and by distributing the grating vector direction of the polarization diffraction element on the xy plane, two-dimensional light beam scanning is possible. It is also possible to widen the angle of view and to make the scanning speed uniform, that is, to make the scanning speed uniform.

<Wide angle and uniform speed of beam scanning of beam steering element using polarization diffraction element>
Based on the function of the polarization diffraction element described above, the principle of widening the deflection angle θ of the light beam scanned by the beam steering element and making the scanning speed constant will be specifically described.

FIG. 7 is a diagram schematically showing the present system 1' according to the second embodiment. This system 1' has a configuration in which the system 1 scans a light beam in space and is equipped with a screen 6 that irradiates the light beam, and is an example of a configuration using one polarization diffraction element 5. This system 1' is arranged in a spatial plane parallel to the plane of a light source 2 (laser light source), a polarization control element 3, a beam steering element 4 (MEMS mirror), a polarization diffraction element 5, and a distance d ₂ away from the polarization diffraction element 5. It consists of a screen 6.

As the screen 6, for example, when the present system is used as a projector, a screen that projects an image can be used, and when the system is used as an exposure device, an object to be exposed can be used. Other objects to be measured can also be used when used in sensing applications.

The beam emitted from the laser light source is first converted into left-handed circularly polarized light or right-handed circularly polarized light by the polarization control element 3, and then the polarization angle θ ₁ (t ) [deg] can bend the propagation direction. Here, t represents time, and θ ₁ (t) represents the time response of the deflection angle.

The high-speed axis time response of a MEMS mirror generally has a sinusoidal waveform, and is expressed by the following equation (12). Here, θ ₀ [deg] is the maximum deflection angle with respect to the z-axis at which the MEMS can perform light beam scanning, and f is the drive frequency [Hz].

Using θ ₁ (t), the incident position x ₁ of the light beam with respect to the x-axis in the xy plane, which is a distance d ₁ away from the center point of the mechanical operation axis of the MEMS mirror along the reference axis z, is expressed by the following equation (13). be done. If the local grating period at the position x ₁ of the polarization diffraction element 5 placed at a distance d ₁ is Λ(x ₁ ), then the light beam passing through the corresponding position is further diffracted from θ ₁ (t), and the diffraction is The angle θ ₂ (t) is expressed by the following equation (14). At this time, the polarization angle (deflection angle) θ of the light beam propagation direction from the axis parallel to the z-axis passing through the position x ₁ of the polarization diffraction element 5 is the deflection angle θ ₁ of the MEMS and the diffraction angle θ ₂ of the polarization diffraction element 5. The sum is θ ₁ +θ ₂ . Therefore, the exposure position x ₂ with respect to the x-axis on the plane of the screen 6, which is arranged on the xy plane parallel to the plane of the polarization diffraction element 5 and a distance d ₂ apart along the coordinate axis z, is expressed by the following equation (15). .

Consider widening the deflection angle θ ₁ due to MEMS light beam scanning by a factor of β. At this time, if the maximum value of the deflection angle of the MEMS is θ ₁ ^max (in the case of formula (12), θ ₁ ^max =θ ₀ ), the maximum diffraction angle θ ₂ ^max required for the polarization diffraction element is calculated using the following formula (16). ).

The local grating period at the incident position x ₁ ^max on the polarization diffraction element at the time when the MEMS reaches its maximum deflection angle is expressed by the following equation (17) using equation (16). That is, the local grating period at a specific position of the polarization diffraction element is determined, which is necessary to widen the deflection angle θ ₁ given by the MEMS mirror by a factor of β.

In order to widen the deflection angle θ ₁ given by the MEMS mirror and at the same time make the beam scanning speed uniform, the amount of displacement between the exposure positions x ₂ (t) of the light beam at fixed time intervals, expressed by the following equation (18), must be It is sufficient if Δx becomes constant.

In other words, it is sufficient to determine the one-dimensional spatial distribution of the grating period Λ(x ₁ ) of the polarization diffraction element along the x-axis so that the amount of displacement Δx is a constant value, and in this case, the boundary condition of Λ(x ₁ ^max ) By imposing this, it is possible to obtain a spatial distribution of the local grating period of the polarization diffraction element that simultaneously satisfies the widening of the angle by a factor of β and the uniformity of the beam scanning speed. Note that the following equation (19) holds true between the local grating period Λ and the optical axis azimuth φ of the polarization diffraction element, and as a result, the optical axis azimuth φ (x ₁ ) along the x-axis of the polarization diffraction element One-dimensional spatial distribution can be designed.

FIG. 8 is an example of a polarization diffraction element 51 that satisfies both the functions of widening the angle and uniformizing the scanning speed in one-dimensional scanning of the MEMS mirror calculated based on equations (12) to (19).

The polarization diffraction element 51 has a one-dimensional grating period so that the scanning range is widened and the scanning speed is made uniform, corresponding to the scanning locus drawn by the MEMS mirror, which is the beam steering element 4, and its time response. It has a specific spatial distribution.

Here, the basic characteristics of the light beam scanning of the MEMS mirror are that the maximum deflection angle θ ₀ =10 [deg] and the drive frequency f = 100 [Hz]. Further, d ₁ =0.1 [m], d ₂ =1.0 [m], λ=532 [nm], and β=5. The retardation Γ of the polarization diffraction element 51 is π/2, and the diffraction efficiency is 100%.

FIG. 8(a) shows the temporal change in the exposure position _x2 on the screen 6 when the light beam is scanned along the x-axis by the MEMS mirror, and FIG. 8(b) shows the position of the x-axis in the plane of the polarization diffraction element 5. Each represents the distribution of the local lattice period Λ at x ₁ . FIG. 8(c) is a plot of contour lines of the optical axis distribution of the polarization diffraction element from the local grating period Λ and the amount of rotation of the optical axis azimuth φ every 300π. Since the scanning waveform of the MEMS mirror has symmetry between the positive and negative regions of x ₂ , only 1/4 period was plotted.

In FIG. 8A, the broken line plots the time change of the exposure position x 2 on the screen when the polarization diffraction element 51 is not arranged, and the solid line plots the time change of the exposure position x ₂ on the screen when the polarization diffraction element 51 is arranged. It can be seen that when the polarization diffraction element 51 is not arranged, the change in the exposure position x ₂ on the screen with respect to the time t-axis is nonlinear, and the scanning speed is not constant.

On the other hand, when the polarization diffraction element 51 is arranged, the change in the exposure position x ₂ on the screen with respect to the time t-axis is linear, and it can be seen that the scanning speed is made uniform. Furthermore, from the graph of FIG. 8(a), by arranging the polarization diffraction element 51, the exposure position x ₂ is increased compared to when the polarization diffraction element 51 is not arranged, that is, the exposure range is expanded, and the deflection angle θ is wide-angle. It can be seen that it has changed.

The grating period of the polarization diffraction element and the distribution (optic axis distribution) of the optical axis azimuth φ in the element plane (optic axis distribution) that achieve wide angle and uniform speed of beam scanning are as shown in FIGS. 8(b) and 8(c). , the period becomes shorter as one goes from the center of the element to the outside. In the case of a MEMS mirror whose beam scanning characteristics follow equation (12), the scanning speed becomes slower as it goes outside the exposure area, so this can be corrected by accelerating the scanning speed by increasing the diffraction angle toward the outside of the exposure area. It is for the sake of being there.

FIG. 9 is an example of a polarization diffraction element 52 that satisfies both the functions of widening the angle and uniformizing the scanning speed in two-dimensional scanning of a MEMS mirror. Since a general MEMS mirror is driven in two axes, a slow axis and a fast axis, the deflection angle θ ₁ of MEMS light beam scanning changes linearly with respect to the y-axis direction (low-speed axis). ), the maximum deflection angle was θ ₀ =10 [deg], and the driving frequency f was 10 [Hz]. The maximum deflection angle θ ₀ and driving frequency f of MEMS light beam scanning in the x-axis direction (fast axis) are the same as those of the polarization diffraction element 51 that satisfies both the functions of widening the angle in one-dimensional scanning and uniformizing the scanning speed. The deflection angle θ of the beam scan was expanded and the velocity was made constant for each displacement of the x-axis and y-axis.

FIG. 9A shows the results of comparing the time dependence of the exposure position with respect to the y ₂ axis with and without the polarization diffraction element 52. Since linear scanning is performed in the y-axis direction, the degree of relaxation of the nonlinearity of the beam scanning speed is not noticeable, but it can be clearly seen that the deflection angle θ is widened.

FIG. 9B is a plot of the two-dimensional spatial distribution of the local grating period Λ of the polarization diffraction element 52 in the x-axis (fast axis) direction and the y-axis (slow axis) direction, respectively. FIG. 9(c) is a plot of the optical axis azimuth φ so as to reproduce the sum of the phase distributions in the x direction and the y direction according to equation (19). It can be seen that a parabolic distribution of the optical axis orientation φ in the element plane (optic axis distribution) is obtained.

In the embodiment shown in FIG. 7, an example is shown in which a single polarization diffraction element is used, but a polarization diffraction element is also used in which a plurality of polarization diffraction elements having the same or different optical axis distributions are arranged in multiple stages in the direction of travel of the light beam. It's okay.

<Method for manufacturing polarized light diffraction element 5>
Preferably, the polarization diffraction element 5 includes an optically anisotropic material. The optically anisotropic material is not limited as long as it can ensure high optical anisotropy, but the optically anisotropic material has a photoreactive side chain that causes at least one of photocrosslinking and photoisomerization reactions. It is preferable to use a photoreactive polymer having the following properties, and a photoreactive polymer liquid crystal exhibiting liquid crystallinity is more preferable.

The polarization diffraction element 5 including an optically anisotropic material can be manufactured by a known method, for example, WO2016/072436, Japanese Patent Application No. 2019-164405, J. Appl. Phys. 94, 1298 (2003), etc., it is possible to produce a polarization diffraction element 5 having a structure in which the optical axis direction φ is periodically modulated.

Alternatively, the polarization diffraction element 5 may be made of an optically anisotropic material that utilizes structural birefringence based on a sub-wavelength structure having a periodic structure shorter than the wavelength λ of the light emitted by the light source 2.

<Features of this system>
This system has a simple configuration in which a polarization diffraction element fabricated by the above method and having a structure in which the optical axis azimuth φ is periodically modulated within the element plane is placed after the existing beam steering element. Therefore, it is easy to downsize the entire device. Conventionally, a method using a refractive bulk lens has been used to widen the angle of beam scanning, but the present system has advantages over this conventional method in terms of weight and system size. Additionally, in this system, it is possible to widen the beam deflection angle by simply preparing a single layer of anisotropic film with a thickness of several μm as a polarization diffraction element, which is advantageous in terms of making the system lighter and smaller. . Furthermore, since it can be manufactured using a process with a high degree of manufacturing freedom, such as a liquid crystal photo-alignment method, it is advantageous in terms of manufacturing cost.

Here, the effects of the present system 1' according to the second embodiment were confirmed. The present invention will be specifically explained below using examples, but is not limited only by these examples.

<Preparation of polarized light diffraction element>
A polarization diffraction element was fabricated by recording a polarization hologram on a polarization-sensitive liquid crystal polymer. As a recording material, a polymer liquid crystal film having a thickness of 3.5 μm and having a photocrosslinkable side chain was formed on a TAC (Tri-Acetyle Cellulose) film. In order to induce optical anisotropy, polarization holograms were recorded by coaxially superimposing 360 nm laser beams with circularly polarized light in opposite directions, giving spherical phases of different curvatures coherently on the recording material. After recording, a heat treatment was performed at 130° C. for 5 minutes, followed by cooling to induce optical anisotropy in the polarization diffraction element. In this example, a polarization diffraction element formed by recording a polarization hologram while giving a curvature phase difference corresponding to a focal length of 155 mm was used.

<Characteristics of polarization diffraction element>
A polarized light micrograph of the produced polarized light diffraction element is shown in FIG. This photo was taken with a polarized diffraction element sandwiched between orthogonal polarizers, showing that the optical axis is distributed in correspondence to brightness and darkness. According to this figure, the fabricated polarization diffraction element has an optical axis distribution in which the optical axis orientation φ is distributed such that the local grating period becomes continuously shorter from the element center to the outer periphery. It can be seen that the outer peripheral portion has a function of deflecting incident light to a greater extent.

<Wide-angle scanning of MEMS mirror reflected light>
The produced polarization diffraction element was arranged in the form of the optical system shown in FIG. 11, and the locus of the light beam scan projected onto the screen 6 was imaged by long exposure using a separately arranged imaging camera (not shown). As the light source 2, a YAG laser (Torus532-100s, manufactured by Quantum) was used. A beam with a wavelength of 532 nm emitted from a YAG laser was transmitted through a polarization control element 3 consisting of a polarizer and a quarter-wave plate, and converted into circularly polarized light. Thereafter, it was reflected by a two-axis MEMS mirror (manufactured by Mirror Technology) used as the beam steering element 4, and one-dimensional scanning was performed in the x-axis (high-speed axis) direction with a sine signal of frequency f = 100 [Hz]. The above polarized light diffraction element is placed at a distance d ₁ =0.098 [m] from the MEMS mirror, and the light rays are directed to the screen placed at a distance d ₂ =0.559 [m] from the polarized light diffraction element. Projected. We observed the trajectory of the light beam scan projected onto the screen using long-time exposure with an imaging camera.

FIG. 12 is an example of the trajectory of the imaged light beam scan. The upper part of the figure is the trajectory of a light beam whose deflection angle θ is widened due to the deflection action of the polarization diffraction element 5, and the lower part is the light beam with a deflection angle θ ₁ caused only by the original MEMS mirror that does not pass through the polarization diffraction element 5. This is the scanning trajectory.

The width of the original beam scan projected onto the screen is 25 [mm], which is ±1.09 [deg] when converted to the maximum deflection angle θ ₁ (=θ ₀ ). On the other hand, the width of the beam scan after being subjected to the deflection action of the polarization diffraction element is 38 [mm], which is ±1.76 [deg] when converted to the maximum deflection angle θ.

Therefore, the maximum deflection angle θ ₀ of the light beam given by MEMS can be widened by 1.61 times, proving that this system can widen the deflection angle θ ₁ of the light beam scanned by the existing beam steering element. did it. In addition, since we were able to widen the deflection angle θ of light beam scanning using a polarization diffraction element made of a thin film with a thickness of 3.5 μm, we also demonstrated that it is possible to downsize the system compared to bulk lenses using conventional technology. did it.

As described above, according to this example, one of the effects of the present invention was to widen the angle of light beam scanning. In addition to this embodiment, various effects such as equalization of the speed of beam scanning can be obtained depending on the design of the spatial distribution of the optical axis azimuth φ of the polarization diffraction element.

The present invention makes it possible to widen the deflection angle by using a beam steering element typified by an existing MEMS mirror. Furthermore, it becomes possible to improve non-uniformity in beam scanning speed. In view of these characteristics, there is a possibility of industrial use as a beam scanning system. A more specific explanation is as follows.

In order to popularize autonomous driving, efforts are being made to develop lidar (a system with a light source that scans laser light two-dimensionally) to sequentially acquire environmental information outside the vehicle. Since the present invention enables beam steering over a wide range with high speed and uniform brightness with a small system, it is expected that it has a high possibility of being applied to lidar.

It is also expected to be used in projectors. By combining rotational drive at a frequency exceeding about 60 Hz, which is a typical video display frequency, and output time control of the semiconductor laser light source, any image can be projected onto the subject.

Although the invention has been given as examples in the fields of automatic driving technology and display, the application of the beam steering method can also be expected to be expanded to the medical field, security field, processing field, etc., and is not limited to these fields.

1,1'... Ray scanning wide angle system 2... Light source 3... Polarization control element 4... Beam Steering element 5, 51, 52...Polarization diffraction element 6...Screen

Claims (7)

a light source and
a beam steering element;
a polarization control element disposed between the light source and the beam steering element; and a polarization diffraction element disposed so that the light beam emitted from the light source can enter through the beam steering element;
The polarization diffraction element has a local grating period, and
having a function of increasing the deflection angle of the light beam from the deflection angle given by the beam steering element, and
A beam scanning wide-angle system characterized by having a function of making the scanning speed of the beam constant in a spatial plane parallel to the plane of the polarization diffraction element. The beam scanning wide angle system according to claim 1, comprising the polarization diffraction element arranged so that the beam can directly enter from the beam steering element. The beam scanning wide angle system according to claim 1, wherein the local grating period is a distribution of optical axis orientations formed with regularity one-dimensionally or two-dimensionally within the plane of the polarization diffraction element. . The beam scanning wide angle system according to claim 1, comprising a plurality of the polarization diffraction elements. The beam scanning wide angle system according to claim 1, wherein the polarization diffraction element includes an optically anisotropic material. The beam scanning wide angle system according to claim 5, wherein the polarization diffraction element includes a liquid crystal polymer film having a photoreactive side chain. The beam scanning wide angle system according to claim 6, wherein the photoreactive side chain causes at least one of photocrosslinking and photoisomerization.