JP7134441B2 - optical deflection device - Google Patents

Description

The present invention relates to an optical deflection device that controls the traveling direction of light.

The technical field of laser radar or lidar equipment (LiDAR (Light Detection and Ranging, Laser Imaging Detection and Ranging)) using laser measurement that acquires the distance to surrounding objects as a two-dimensional image is used in automatic driving of cars and three-dimensional It is used for map production, etc., and its basic technology can also be applied to laser printers and laser displays.

In this technical field, a light beam is applied to an object, the light reflected by the object is detected, distance information is obtained from the time difference and frequency difference, and the light beam is scanned two-dimensionally. Acquire wide-angle three-dimensional information by .

An optical deflection device is essential for optical beam scanning. In the past, mechanical mechanisms were used for rotating the entire device, mechanical mirrors such as polygonal mirrors and galvanometer mirrors, and compact integrated mirrors using micromachine technology (MEMS technology). , high cost, and instability in a vibrating moving body.

As a non-mechanical optical deflection device, a phased array type and a diffraction grating type have been proposed, which realize optical deflection by changing the wavelength of light and the refractive index of the device. However, the phased array type optical deflection device has a problem that it is very difficult to adjust the phases of a large number of optical radiators arranged in an array, and a high-quality sharp optical beam cannot be formed. On the other hand, a diffraction grating type optical deflection device can easily form a sharp beam, but has a problem that the optical deflection angle is small.

To solve the problem of a small light deflection angle, the inventor of the present invention has proposed a technique for increasing the light deflection angle by coupling a slow light waveguide to a diffraction mechanism such as a diffraction grating (Patent Document 1). . Slow-light light is generated in a photonic nanostructure such as a photonic crystal waveguide, has a low group velocity, and is characterized by a large change in the propagation constant due to a slight change in the wavelength or refractive index of the waveguide. have If a diffraction mechanism is installed inside or close to this slow-light waveguide, the slow-light waveguide is coupled to the diffraction mechanism and becomes a leaky waveguide, radiating light into free space. At this time, a large change in the propagation constant is reflected in the deflection angle of the radiated light, resulting in a large deflection angle.

FIG. 9A shows a schematic of the device structure and the emitted light beam, in which a diffractive mechanism is introduced into a photonic crystal waveguide propagating light with low group velocity (slow light). An optical deflection device 101 includes a photonic crystal waveguide 102 having a double periodic structure in which two types of circular holes with different diameters are repeated along the waveguide in the plane of the photonic crystal. The double periodic structure constitutes a diffractive mechanism, which converts the slow-light propagating light into radiation conditions and radiates it into space.

An optical deflection device 101 has a photonic crystal waveguide 102 formed by a lattice arrangement 103 in which low refractive index portions 111 are arranged in a high refractive index member 110 on a clad 113 made of a low refractive index material such as SiO ₂ . The grating arrangement 103 of the low refractive index portion 111 is, for example, a double periodic structure of a periodic structure of repeating large-diameter circular holes and a periodic structure of repeating small-diameter circular holes. A portion of the lattice arrangement 103 of the photonic crystal waveguide 102 where the circular holes 111 are not provided forms a waveguide core 112 through which incident light propagates.

9B and 9C are diagrams for explaining the beam intensity distribution of the radiation light beam, FIG. 9B showing the beam intensity distribution in the vertical direction, and FIG. 9C showing the beam intensity angular distribution in the horizontal direction.

In FIG. 9B, the radiated light beam gradually leaks out along the waveguide core to form a sharp beam with uniform longitudinal beam intensity distribution. In FIG. 9C, the lateral beam intensity angular distribution has a wide angular distribution.

A light beam formed by an optical deflection device using a photonic crystal waveguide is longitudinally polarized by changes in the wavelength of light incident on the optical deflection device, or the refractive index of the waveguide or the equivalent refractive index of the waveguide mode. radiation angle changes. Thus, the optical deflection device changes the emission angle by rapidly and continuously changing the wavelength of the incident light, or the refractive index of the waveguide or the equivalent refractive index of the guided mode, thereby changing the emitted light beam to can be scanned.

An optical deflection device functions as an optical deflector only by providing the function of scanning the emitted light beam.

In a practical optical deflection device, when changing the wavelength of incident light, the wavelength spectrum required for the laser light source has the following requirements.
(a) The wavelength linewidth is as narrow as, for example, 1 MHz or less.
(b) The amplitude of the wavelength is a wide range, for example, 30 nm or more.
(c) the wavelength can be continuously scanned;
(e) The output is a high output close to 100 mW, for example.
(f) it is small;

However, at present, there is no laser light source that satisfies such conditions. Currently available practical semiconductor lasers cannot continuously scan the wavelength for requirement (c), but the wavelength can be set to a specific value over a period of milliseconds to seconds, and other requirements can be satisfied. What satisfies is known.

Therefore, at present, as an optical deflection device that can be realized, a configuration in which the above-described semiconductor laser is used as a light source and a light beam is scanned by sequentially changing the refractive index of a waveguide using a specific wavelength is assumed. be.

When such an optical deflection device is applied to a lidar apparatus (LiDAR), typical specifications of the lidar apparatus include, for example, 320×32≈10,000 two-dimensional pixels for obtaining a range image, and a frame of the image. Assuming a rate of 10 frames per second, a total of 100,000 points/second of data must be scanned. This amount of data corresponds to a scanning rate of 100 kHz. In order to obtain image data by scanning a light beam with an optical deflection device, the change speed of the refractive index to be changed for scanning the radiation light beam must be high enough to correspond to this scanning speed.

In the optical deflection device, as a means to change the refractive index of the waveguide or the equivalent refractive index of the waveguide mode, at present, micromachines such as micromachines, nonlinear effects (optical Kerr effects), carrier plasma effects, and thermo-optic effects etc. can be considered.

Among these means, an optical deflecting device using a micromachine such as a micromachine is difficult to operate at a high speed like the micromachine as the optical deflecting mirror described above, and it is difficult to obtain the scanning speed described above. There is also the problem of instability and low reliability.

An optical deflection device using a nonlinear effect (optical Kerr effect) requires high optical power of the order of 10 W, for example, in a photonic crystal waveguide using silicon (Si) as a waveguide material. However, when an optical power of 100 mW or more is continuously introduced, nonlinear absorption (two-photon absorption) occurs, not only attenuating the light but also damaging the waveguide.

The carrier plasma effect is the effect of changing the refractive index of the semiconductor by forming a pn junction diode in a semiconductor such as Si and applying a forward or reverse bias to it to change the amount of carriers in the waveguide. be. An optical deflection device using the carrier plasma effect can provide a high-speed response of 10 GHz or more, but the amount of change in refractive index is as small as 10 ⁻³ or less, making it difficult to obtain a sufficient scanning range. In addition, light absorption occurs at the same time, and the absorption loss when propagating slow light through an optical deflection device with a length of 1 mm or longer, which is assumed for an optical deflection device, is as large as 10 dB or more. difficult.

The thermo-optic effect is the effect in which heating of a material causes a change in refractive index. An optical deflection device using the thermo-optic effect depends on the heating temperature. For example, when Si is used, the refractive index change per 1 K temperature change is 0.000186. When the temperature is changed, a large refractive index change of 0.0744 occurs. This refractive index change corresponds to a large deflection angle of 31°. FIG. 9D shows the change in the deflection angle θ when the temperature change ΔT is applied to the Si layer of the Si photonic crystal slow light deflector. The characteristics of FIG. 9D are obtained by setting the refractive index of the Si photonic crystal slab to 3.5, the thickness to 210 nm, the refractive index of the upper and lower claddings to 1.45, and the lattice constant a of the photonic crystal to 400 nm. This is an example in which the diameter of the circular hole is 2r=210±5 nm in the double periodic structure in which the diameter of the circular hole is repeated.

10A, 10B, and 10C show structural examples of a heating mechanism for generating a thermo-optic effect in the slow light waveguide. In the heating mechanism 201A shown in FIG. 10A, a heater 203 is formed on the side of the photonic crystal waveguide 202 in the horizontal direction, and the heater 203 is energized by a power source 205 to perform heating. The heater 203 can be formed of, for example, a TiN heater that can be formed by a SiCMOS process (Non-Patent Document 1).

A heating mechanism 201B shown in FIGS. 10B and 10C forms a heater 203 directly above (or directly below) a photonic crystal waveguide 202 (Non-Patent Document 2). In the SiO ₂ cladding (not shown) on top of the photonic crystal waveguide 202, by placing the heater 203 at a distance of 1 μm or more from the waveguide, light absorption can be kept sufficiently low. In addition, by providing it at a position close to the waveguide, it is possible to achieve a high heating efficiency and a small heat capacity, and obtain a faster response than the heating mechanism 201A of FIG. 10A. 10A and 10B, reference numeral 204 indicates a heating area by the heater 203. FIG.

Non-Patent Document 3 discloses a technique of using a Si layer that guides light as a heater in a Si photonic crystal. This heating technique controls the phase by heating the delay line that guides light to each antenna cell in a phased array, bending the middle of the waveguide and passing an electric current across the curved portion. Thus, the structure is such that absorption of light in the waveguide is suppressed (Non-Patent Document 3).

Patent application 2016-10844

Applied Physics Letters," Photonic crystal tunable slow light device integrated with multi-heaters" Norihiro Ishikura, Ryo Hosoi, Ryo Hayakawa, Takemasa Tamanuki, Mizuki Shinkawa, Toshihiko Baba 100,221110(2012) 221110-1~221110-3 published online 31 May 2012 (See Fig.1.) Frontiers In Physics Volume2 Article61 p1~p9 "Theoretical and experimental investigation of low-volgage and low-loss 25-Gbps Si photonic crystal slow light Mach-Zehnder modulators with interleaved p/n junction" Yosuke Terada, Hiroyuki Ito, Hong C. Nguyen and Toshihiko Baba November 2014 (See Fig.2., Fig.7.) Nature LETTER "Large-scale nanophotonic phased array" Jie Sun, Erman Timurdogan, Ami Yaacobi, Ehsan Shah Hosseini Michael R. Watts vol493 p195-199, 10 JANUARY 2013 (see Fig4a.)

In the heating mechanism 201A shown in FIG. 10A, since the heater 203 itself is generally made of an opaque material, the light is absorbed when the heater 203 is installed near the guided light of the photonic crystal waveguide 202. FIG. Therefore, the heater 203 is formed so as to be in contact with Si at a position sufficiently distant from the center of the photonic crystal waveguide 202, and the heat reaches the center through Si, which has excellent thermal conductivity. In either configuration, since the center of the waveguide is away from the heater, the temperature at the center of the waveguide is lower than that around the heater, and the heating efficiency is low. In addition, since the heating area increases and the overall heat capacity increases, it is difficult to achieve high-speed response.

In the heating mechanism 201B shown in FIGS. 10B and 10C, the heater is provided directly above the waveguide center of the photonic crystal waveguide. prevent the upward radiation of Therefore, it is necessary to radiate the light to the back side of the substrate, which restricts the overall configuration of the device.

In order to avoid this, a configuration in which the heater is formed in the _SiO2 cladding layer immediately below the waveguide is considered.

Conventionally, however, photonic crystal waveguides are formed by processing the uppermost Si layer of an SOI substrate, which is composed of a _SiO2 layer and a Si layer on a Si substrate. In order to form the heater in the _SiO2 layer immediately below the Si layer, instead of using the SOI substrate, a _SiO2 layer is formed on the Si substrate by chemical vapor deposition or the like, and then the heater is formed. It is necessary to form a layer structure by forming a _SiO2 layer and then forming a Si layer.

In this layered structure, the Si layer that serves as the waveguide layer is an amorphous layer instead of the high-quality single crystal layer of the SOI substrate. As a result, in addition to an increase in waveguide loss, there is the problem of difficulty in integrating optical modulators and photodetectors required for LiDAR.

In the technique of using the Si layer as a heater in Non-Patent Document 3, the distribution of guided light is biased around the outer periphery of the waveguide by bending the waveguide, and current is passed through the inner periphery of the waveguide. Light absorption is suppressed by installing a mechanism. On the other hand, since the waveguide of the photonic crystal optical deflection device that changes the radiation angle of the light beam emitted linearly is linear, it is difficult to apply a waveguide heater that requires a curved portion.

Therefore, in the optical deflection device, it is difficult to satisfy the heating efficiency, the high-speed response, and the low loss of the waveguide with the conventionally proposed heating mechanism.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a heating mechanism that improves heating efficiency, reduces waveguide loss, and achieves high-speed response in an optical deflection device that thermally changes the refractive index.

The present invention relates to a heating mechanism for generating a thermo-optic effect in a slow light waveguide, and the heating mechanism is formed on a semiconductor forming the slow light waveguide.

By doping silicon, which is the material of the photonic crystal waveguide, to make it possible to conduct electricity, and only near the center of the waveguide as an undoped intrinsic region, voltage is concentrated there, resulting in conduction. To enable concentrated heating in the center of the wave path. Heating the center of the waveguide through which light propagates can improve the heating efficiency and speed up the heating response. In addition, since the center of the waveguide is an intrinsic region without doping, it is possible to suppress the absorption of propagating light accompanying carriers and reduce the loss of the waveguide.

In the optical deflection device of the present invention, in a photonic crystal waveguide having a lattice arrangement in which low refractive index portions are periodically arranged in the plane of a high refractive index member, the region in which the lattice arrangement is formed is an intrinsic region, and an impurity region and a high-concentration diffusion layer region sandwiching the intrinsic region on both sides. The heating mechanism forms a heating mechanism having a current path by the impurity regions and the high-concentration diffusion layer regions arranged on both sides of the intrinsic region. In this heating mechanism, the high-concentration diffusion layer region is arranged outside the impurity region, has an electrode thereon, and makes ohmic contact with the electrode.

By applying a current from a power supply to the impurity region--intrinsic region--impurity region through the electrode and the high-concentration diffusion layer region, heat is generated in the intrinsic region having a high electric resistance to perform heating. Further, since the intrinsic region corresponds to the waveguide core, heating the intrinsic region can change the refractive index of the waveguide or the equivalent refractive index of the waveguide mode.

The impurity region and the high-concentration diffusion layer region constituting the heating mechanism may be the p-type impurity region and the p-type high-concentration diffusion layer region, or may be the n-type impurity region and the n-type high-concentration diffusion layer region. A heating mechanism for forming the p-type impurity region and the p-type high-concentration diffusion layer region has a pi-p structure, and a heating mechanism for forming the n-type impurity region and the n-type high-concentration diffusion layer region has an nin structure. be. Here, i represents the intrinsic region.

(Comb-like impurity region)
The heating mechanism may have a configuration in which a part of the impurity region is provided in the intrinsic region, and in this configuration, a comb-like impurity region is formed in the intrinsic region. The comb-like impurity region can be provided along the arrangement of the low refractive index regions provided in the intrinsic region. According to the configuration in which the impurity region is provided in the intrinsic region, light absorption can be reduced even with the same electrical resistance, and further, by the configuration in which the heating portion is provided at a position closer to the waveguide core, heat generation is concentrated in the central portion of the waveguide, Heating efficiency and heating response speed can be increased. A comb-like impurity region is not provided in a portion where light propagates and radiates in the waveguide core.

(Asymmetric arrangement of impurity regions)
When the heating mechanism is, for example, a pi-p structure, there is a difference in the electric field strength between when the current direction is from the p-type impurity region to the intrinsic region and when the current direction is from the intrinsic region to the p-type impurity region. This difference in electric field strength causes the temperature distribution to be biased, and the peak of the temperature distribution to shift. The same temperature distribution is obtained for the heating mechanism of the nin structure.

In the heating mechanism of the present invention, in order to eliminate the peak deviation of the temperature distribution, in the impurity region of the pip structure, the impurity region on the side to which the electrode on the negative side is connected is replaced with the electrode on the positive side. The asymmetrical arrangement is such that the center of the intrinsic region is closer than the impurity region on the side to be connected. This asymmetrical arrangement peaks the temperature distribution in the waveguide section in the intrinsic region, which results in higher heating efficiency and faster response. In the impurity region of the nin structure, the asymmetric arrangement is reversed with respect to the impurity region of the pi-p structure.

(Divided configuration of heating area)
The heating mechanism is provided with a plurality of heating units divided in the length direction of the waveguide core, and each of these heating units can individually control heating. By individually controlling the heating of each heating unit, the heating condition along the length of the waveguide core can be adjusted, thereby adjusting the emission angle of the light radiation beam along the length of the waveguide core.

Adjustment of the emission angle can be used to adjust the size of the target area of the beam scan or to compensate for non-uniformities in the light beam associated with device fabrication.

As described above, the heating mechanism of the optical deflection device of the present invention can improve heating efficiency, reduce waveguide loss, and achieve high-speed response.

It is a figure for demonstrating the schematic structure of the optical deflection device of this invention. It is a figure for demonstrating the schematic structure of the optical deflection device of this invention. It is a figure for demonstrating the structure of the heating mechanism of this invention, and distribution of an electric field when a voltage is applied between two electrodes. FIG. 4 is a diagram for explaining deviation of peaks of temperature distribution; FIG. 4 is a diagram showing a configuration in which impurity regions are asymmetrically arranged according to the present invention; FIG. 4 is a diagram showing an example of temperature distribution in a configuration in which impurity regions are asymmetrically arranged according to the present invention; FIG. 3 is a diagram for explaining a heating mechanism A having comb-shaped impurity regions according to the present invention; FIG. 4 is a diagram for explaining characteristics due to the configuration of comb-like impurity regions of the heating mechanism of the present invention, and is a diagram showing an example of a temperature distribution obtained by finite element analysis; FIG. 4 shows absorption loss versus comb width; FIG. 4 is a diagram showing frequency characteristics of a heating response; FIG. 2 is a diagram for explaining the configuration of a heating mechanism provided with a plurality of divided heating units B of the present invention; FIG. 4 is a diagram for explaining an example of a heating mode of each heating unit, and is a diagram showing a state in which heating is not performed by the heating mechanism. FIG. It is the figure which showed the state evenly heated by each heating unit of a heating mechanism. FIG. 5 is a diagram showing a state in which the temperature of a heating unit located far from the incident light side is increased to increase the refractive index and increase the radiation angle. FIG. 10 is a diagram showing a state in which the temperature of a heating unit located near the incident light side is increased to increase the refractive index and increase the radiation angle. FIG. 3 illustrates the state of a beam of light radiation in an element with non-uniformity during fabrication of an optical deflection device; FIG. 10 is a diagram showing a state in which the direction of radiation of an optical radiation beam emitted from a waveguide core whose temperature is controlled by each heating unit is corrected; FIG. 2 is a diagram for showing an overview of a device structure in which a diffraction mechanism is introduced into a slow light waveguide and a radiated light beam; FIG. 4 is a diagram showing a beam intensity distribution in the vertical direction; FIG. 4 is a diagram showing a beam intensity distribution in the lateral direction; FIG. 4 is a diagram showing changes in deflection angle when a temperature change is applied to the Si layer of the Si photonic crystal slow light deflector. FIG. 4 is a diagram for showing a structural example of a heating mechanism for generating a thermo-optical effect in a slow light waveguide; It is a figure which shows the example which formed the heater just above the photonic crystal waveguide. It is a figure which shows the example which formed the heater directly under the photonic crystal waveguide.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 1A and 1B, schematic configuration examples of the optical deflection device and the heating mechanism of the present invention will be described, and FIG. 2 will be used to describe the configuration of the heating mechanism of the present invention. 4B will be used to explain the configuration for adjusting the temperature distribution peak of the heating mechanism of the present invention, and FIGS. , 7, and 8A to 8F, description will be given of a configuration example and an operation example of the split heating unit of the heating mechanism of the present invention.

(Outline of optical deflection device)
1A and 1B are diagrams for explaining the outline of the optical deflection device and heating mechanism of the present invention. In FIG. 1A, an optical deflection device 1 includes a photonic crystal waveguide 2 in which low refractive index regions 11 are periodically arranged in a lattice in the plane of a high refractive index member 10 .

The photonic crystal waveguide 2 is formed by a lattice arrangement 3 in which low refractive index portions 11 are periodically arranged in a high refractive index member 10 made of a semiconductor such as Si. The low refractive index portion 11 can be, for example, a circular hole provided in the high refractive index member 10 . A photonic crystal waveguide 2 is provided on a clad 13 made of a semiconductor material such as Si.

In the photonic crystal waveguide 2, a waveguide core 12 for propagating light is formed by providing a portion where the low refractive index portion 11 is not provided in a portion of the lattice arrangement 3. FIG. In the configuration in which the low refractive index portion 11 is a circular hole, the waveguide core 12 is formed by providing a part of the grating arrangement 3 where the circular hole is not arranged. The incident light that enters the waveguide core 12 is radiated to the outside from the waveguide core 12 while propagating through the waveguide core 12 in the longitudinal direction. Note that arrows in FIGS. 1A and 1B schematically indicate incident light and emitted light beams.

(Overview of heating mechanism)
The optical deflection device 1 has a heating mechanism A for heating the waveguide core 12 . In FIG. 1B, the heating mechanism A includes an intrinsic region 21, and a plurality of semiconductor regions including an impurity region 22 and a high-concentration diffusion layer region 23 sandwiching the intrinsic region 21 in the lattice arrangement 3 forming the photonic crystal waveguide 2. Consists of The high-concentration diffusion layer region 23 is provided with an electrode 24 on the outer side opposite to the side where it is joined to the impurity region 22 and makes ohmic contact with the electrode 24 . In the heating mechanism A, a current path is formed from one electrode 24 to the other electrode 24 via the high-concentration diffusion layer region 23, the impurity region 22, the intrinsic region 21, the impurity region 22, and the high-concentration diffusion layer region 23. , heat is generated with the intrinsic region 21 having a high electric resistance as a peak region of the temperature distribution by passing an electric current, and the waveguide core 12 is heated to change the refractive index.

The impurity region 22 and the high-concentration diffusion layer region 23 constituting the heating mechanism A may be a p-type impurity region and a p-type high-concentration diffusion layer region, or may be an n-type impurity region and an n-type high-concentration diffusion layer region. A heating mechanism for forming the p-type impurity region and the p-type high-concentration diffusion layer region has a pi-p structure, and a heating mechanism for forming the n-type impurity region and the n-type high-concentration diffusion layer region has an nin structure. be. Note that i represents the intrinsic region.

The p-type impurity region 22p is provided at a sufficient distance from the center of the waveguide core 12 to avoid light absorption associated with doping. Since the center of the waveguide core 12 is an intrinsic region (i region) with no doping, the heating mechanism A has a pip structure or nini structure with the waveguide core 12 as the intrinsic region 21. Become. Since the intrinsic region 21 has a large electrical resistance, the intrinsic region 21 is effectively heated when a voltage is applied between the two electrodes 24 to cause current to flow.

The heating mechanism A can have either a pi-p structure or an ni-n structure. also has a large light absorption coefficient. Therefore, when the heating mechanism A is configured with the nin structure, the doping concentration of the n-type impurity region 22n should be set to that of the p-type impurity region in order to obtain the same light intensity as that of the pi-p structure. It should be lower than the doping concentration of 22p.

In addition to the pi-p structure and ni-n structure described above, a pin-type structure in which one side of the intrinsic region 21 is p-type and the other side is n-type is also conceivable. However, when the pin structure is forward biased and current flows, the central intrinsic region Since the carrier density of 21 is increased, light absorption is increased. For example, in the pi-p structure and n-i-n structure, the intrinsic region (i region) has a carrier density of 1×10 ¹⁷ cm ⁻³ or less and hardly contributes to light absorption. On the other hand, the intrinsic region (i region) in the pin structure has a carrier density of 1×10 ¹⁸ cm ⁻³ or more. This carrier density of 1×10 ¹⁸ cm ⁻³ or more corresponds to a large loss of 100 dB/cm or more in the slow light waveguide. This loss is 10 dB or more even if the length of the optical deflection device is set to be as short as 1 mm. Therefore, the pin structure heating mechanism is not suitable for optical deflection devices due to the loss.

FIG. 2(a) shows the pi-p structure of the heating mechanism, and FIG. 2(b) shows the electric field strength distribution when a voltage is applied to this heating mechanism. In the heating mechanism A shown in FIG. 2(a), p-Si p-type impurity regions 22p are arranged on both sides of an intrinsic Si region 21, and p ⁺ -Si p-type high-concentration diffusion layer regions 23 are formed on the outside thereof. A circular hole 11 is provided in a portion of the p-type impurity region 22p and the intrinsic region 21 excluding the central portion thereof. In FIG. 2A, the diameter of the circular hole 11 is 220 nm, and the lateral width of the intrinsic region 21 is Li.

In the lattice arrangement 3, when the low refractive index portions are formed by filling the insides of the circular holes 11 with an insulator SiO ₂ , the electric field inside the circular holes 11 is high, and the electric field between the circular holes 11 and 11 is high. The electric field is high because the current path is narrowed in between.

In the electric field intensity distribution shown in FIG. 2(b), parts of capital letters A, B, C, and D, parts of H, and parts denoted by I, J, and K correspond to p-type impurity regions. Parts of D, parts of E, F, G, and H correspond to intrinsic regions, and lower case letters a to j correspond to holes in doped SiO ₂ .

The electric field intensity in the central part of the electric field intensity distribution corresponds to the high electric field intensity range indicated by (i) in the index shown on the right side of FIG. 2(b), corresponding to the range of low field strength indicated by (ii) in the index shown on the right hand side, where the width of the intrinsic region Li is 4.0 μm and the doping of the p-type impurity region has an acceptor concentration of N _A =1.05×10 ¹⁸ cm ⁻³ .

(Asymmetric arrangement of impurity regions)
The heating mechanism A has a symmetric pip structure in which p-type impurity regions are arranged on both sides of an intrinsic region (i-region). There is a difference in the electric field intensity between the direction from the intrinsic region (i region) to the intrinsic region (i region) and the direction from the intrinsic region (i region) to the p-type impurity region (p region). The electric field intensity toward (p region) becomes larger.

This difference in electric field strength causes a bias in the temperature distribution, and heat generation is greater in the i-region→p-region. As a result, the maximum value of the temperature distribution is not at the center of the waveguide, and the peak of the temperature distribution is deviated.

FIG. 3 is a diagram for explaining the deviation of the peak of the temperature distribution. FIG. 3(a) shows a configuration in which the impurity region 22 and the high-concentration diffusion layer region 23 are arranged symmetrically with respect to the intrinsic region 21, and FIG. 3(b) schematically shows the temperature distribution of this symmetrically arranged heating mechanism. showing. Here, the temperature distribution is shown when current is applied to the heating mechanism A from left to right. The peak point P of the temperature distribution shifts from the central point of the intrinsic region 21 to the negative side.

It is expected that an asymmetrical temperature distribution will cause left-right asymmetry in the formation of a light beam, making it impossible to form a high-quality beam. In order to make the temperature distribution bilaterally symmetrical with respect to the center of the waveguide, the heating mechanism A of the present invention places the p-type impurity region 22p (impurity region on the right side in the figure) connecting the negative electrodes by Ls. It has an asymmetric configuration that is centered. This asymmetric arrangement of the impurity regions is not limited to the pi-p structure, but can be applied to the nin structure to make the temperature distribution symmetrical. However, in the case of the nin structure, the n-type impurity region connecting the positive electrode is brought to the center of the waveguide to form an asymmetric structure.

FIG. 3(c) shows a configuration example in which the impurity region 22 is shifted toward the center of the waveguide, and FIG. 3(d) shows the temperature distribution in this configuration. The temperature distribution peaks at the center of the waveguide, and the temperature distribution is bilaterally symmetrical with respect to the waveguide.

4A and 4B show an example of temperature distribution in a structure in which impurity regions are arranged asymmetrically. FIG. 4A shows a configuration example in which the position of the impurity region on the negative electrode side is shifted toward the center side of the waveguide by Ls as in FIG. 3C, and FIG. Here, the applied voltage is 30 V and the lateral length Li of the intrinsic region is 2.5 μm.

According to the temperature distribution in FIG. 4B, the peak of the temperature distribution shifts to the right where the electric field strength increases in a completely symmetrical situation (Ls=0 nm). When the shift amount Ls is 300 nm, the center of the waveguide becomes the peak of the temperature distribution, and a symmetrical temperature distribution with respect to the waveguide is obtained.

Therefore, the heating mechanism of the present invention is an asymmetric impurity region in which the impurity region on the side to which the electrode on the negative side is connected is closer to the center of the intrinsic region than the impurity region on the side to which the electrode on the positive side is connected. This asymmetrical arrangement aligns the peak of the temperature distribution with the waveguide portion in the intrinsic region. Heating efficiency and high-speed responsiveness are thereby further enhanced.

(Comb-like impurity region)
In the heating mechanism, the impurity region can be provided in the intrinsic region, and a comb-shaped impurity region is formed in the intrinsic region.

FIG. 5 shows a heating mechanism A with comb-like impurity regions. In the heating mechanism A, the comb-like portion 6 of the impurity region is provided within the intrinsic region 21 . Comb-like regions 6 are formed along the array of circular holes 11 in intrinsic region 21 . A comb-like impurity region is not provided in a portion where light propagates and radiates in the waveguide core.

The comb-like portion 6 can be formed by performing oblique comb-like doping along the rows of circular holes arranged in the triangular lattice of the photonic crystal. This oblique comb-like shape can reduce the electrical resistance if the light absorption is the same, and can suppress the light absorption if the electrical resistance is the same. Furthermore, since the heat-generating portion can be brought closer to the vicinity of the center of the waveguide, the heat generation is concentrated near the center of the waveguide, and the heating efficiency and the responsiveness of heating are enhanced. In addition, the comb shape has the effect of reducing the current limitation and limiting the doping region to reduce the attenuation of light absorption.

FIG. 6A is an example of a temperature distribution obtained by finite element analysis. Here, the applied voltage is 15 V, and the temperature changes when the widths Wc of the comb-shaped portions are 110 nm, 130 nm, and 150 nm when the gaps lc between the comb-shaped portions on both sides in the central portion of the waveguide are 200 nm and 400 nm. ΔT[K] is shown. Note that the p-doping acceptor concentration is N _A =1.50×10 ¹⁸ cm ⁻³ .

When the gap lc is 200 nm, a temperature change of 400 K, which is a realistic temperature change, can be obtained, and even when the gap lc is 400 nm, a temperature change close to about 400 K can be obtained.

FIG. 6B shows absorption loss versus comb width Wc. When ΔT at the center of the waveguide reaches 400K, the loss is 3.5 dB/cm (=0.35 dB/mm) when the width Wc of the comb-shaped portion is 130 nm and the gap lc is 400 nm. Assuming that the length of the optical deflection device 1 is 3 mm, which forms a high-quality longitudinal beam, the total loss is about 1 dB, which is well acceptable. The parameters such as gap lc and comb width Wc and doping acceptor concentration N _A in FIG. 6B are the same as in FIG. 6A.

FIG. 6C shows the frequency characteristics of the heating response. The voltage between the p-type impurity regions is 15 V, the comb width Wc is 130 nm, and the gap lc is 400 nm. The 3 dB cutoff frequency of this heating frequency characteristic is 110 kHz. For example, when the number of pixels is 10,000, the frequency response satisfies a scanning speed of 100 kHz per pixel corresponding to a frame rate of 10 frames per second.

(Divided configuration of heating area)
The heating mechanism A can be configured to include a plurality of heating units B divided along the length of the waveguide core.

FIG. 7 shows a configuration in which the heating mechanism A is provided with a plurality of divided heating units B. As shown in FIG. Each heating unit B has the same structure as the heating mechanism A described above. The heating of each heating unit B can be individually controlled. FIG. 7 shows a configuration example in which the temperature controller 27 performs heating control on A1, A2, and A3, each of which is a unit of a plurality of heating units B. As shown in FIG. The number of heating units B for which heating control is performed may be set to a plurality such as three as shown in FIG. 7, or heating may be controlled for each unit.

Each heating unit B is individually controllable so that the heating state in the lengthwise direction of the waveguide core can be adjusted, thereby changing the radiation angle of the light radiation beam in the lengthwise direction of the waveguide core. can be adjusted. Also, by adjusting the emission angle, it is possible to adjust the width of the target area in the beam scanning and to correct the non-uniformity of the light beam due to the manufacturing of the device.

An example of a heating mode of each heating unit will be described with reference to FIGS. 8A to 8F. FIG. 8A shows a state in which heating is not performed by the heating mechanism, and FIG. 8B shows a state in which uniform heating is performed by each heating unit of the heating mechanism. The light beam is deflected by uniform heating by the heating mechanism. This uniform heating state is similar to the heating state by a single heater.

FIG. 8C and FIG. 8D show the state when uneven heating is performed by adjusting the heating state of the heating unit. FIG. 8C shows the state in which the temperature of the heating unit located far from the incident light side is raised to increase the refractive index and increase the radiation angle, and FIG. 8D shows the state in which the heating unit located close to the incident light side It shows a state in which the temperature is increased to increase the refractive index and increase the radiation angle. This control state allows the beam to be intentionally divergent or convergent. This heating aspect gives LiDAR the ability to adaptively switch between zooming in and out of a wide target area or a narrow target area.

FIG. 8E shows the state of a beam of optical radiation in an element with non-uniformity during fabrication of an optical deflection device. A beam of light emitted from a non-uniform element will have non-uniform radiation directions due to diffusion and convergence. On the other hand, FIG. 8F shows the radiation from the waveguide core temperature-controlled by each heating unit by controlling the heating state of each heating unit in the element having non-uniformity during fabrication of the optical deflection device. The emitted direction of the beam of optical radiation is corrected and adjusted to the desired angle of emission to improve the quality of the beam of optical radiation.

It should be noted that the present invention is not limited to the above embodiments. Various modifications are possible based on the gist of the present invention, and these are not excluded from the scope of the present invention.

The optical deflection device of the present invention can be mounted on automobiles, drones, robots, etc., and can be mounted on personal computers and smartphones to easily capture the surrounding environment. It can be applied to switches and the like.

In the above-described embodiments, light in the near-infrared wavelength region is used on the assumption that Si is used as the high refractive index member constituting the photonic crystal waveguide of the optical deflection device. By applying it to a visible light material as a refractive index member, further application to projectors, laser displays, retinal displays, 2D/3D printers, POS, card readers, etc. is expected.

This application claims priority based on Japanese Patent Application No. 2017-076112 filed on April 6, 2017, and the entire disclosure thereof is incorporated herein.

REFERENCE SIGNS LIST 1 optical deflection device 2 photonic crystal waveguide 3 lattice arrangement 6 comb-like portion 10 high refractive index member 11 circular hole (low refractive index portion)
12 waveguide core 13 cladding 21 intrinsic region 22 impurity region 22n n-type impurity region 22p p-type impurity region 23 high-concentration diffusion layer region 23n n-type high-concentration diffusion layer region 23p p high-concentration diffusion layer region 24 electrode 27 temperature control unit 101 Optical deflection device 102 Photonic crystal waveguide 103 Lattice arrangement 110 High refractive index member 111 Circular hole (low refractive index portion)
112 waveguide core 113 clad 201A heating mechanism 201B heating mechanism 202 photonic crystal waveguide 203 heater 204 heating region 205 power supply A heating mechanism B heating unit

Claims (8)

In a photonic crystal waveguide having a lattice arrangement in which low refractive index portions are periodically arranged in the plane of a high refractive index member,
The lattice array is
comprising an intrinsic region, and an impurity region and a high-concentration diffusion layer region sandwiching the intrinsic region on both sides;
In the impurity region,
An optical deflection device constituting a heating mechanism , wherein the impurity region on the side to which the electrode on the negative side is connected is asymmetrically arranged closer to the center of the intrinsic region than the impurity region on the side to which the electrode on the positive side is connected. . In a photonic crystal waveguide having a lattice arrangement in which low refractive index portions are periodically arranged in the plane of a high refractive index member,
The lattice array is
comprising an intrinsic region, and an impurity region and a high-concentration diffusion layer region sandwiching the intrinsic region on both sides;
The optical deflection device, wherein the high-concentration diffusion layer region constitutes a heating mechanism having an electrode on the side opposite to the side that is joined to the impurity region . In a photonic crystal waveguide having a lattice arrangement in which low refractive index portions are periodically arranged in the plane of a high refractive index member,
The lattice array is
comprising an intrinsic region, and an impurity region and a high-concentration diffusion layer region sandwiching the intrinsic region on both sides;
In the impurity region,
the impurity region on the side to which the negative electrode is connected is asymmetrically arranged closer to the center of the intrinsic region than the impurity region on the side to which the positive electrode is connected;
The optical deflection device, wherein the high-concentration diffusion layer region constitutes a heating mechanism having an electrode on the side opposite to the side that is joined to the impurity region . the impurity region is a p-type impurity region,
the high-concentration diffusion layer region is a p-type high-concentration diffusion layer region;
The optical deflection device according to any one of claims 1 to 3, wherein said heating mechanism constitutes a pi-p structure. the impurity region is an n-type impurity region,
the high-concentration diffusion layer region is an n-type high-concentration diffusion layer region,
The optical deflection device according to any one of claims 1 to 3, wherein said heating mechanism constitutes an nin structure. In the heating mechanism,
6. An optical deflection device according to any one of claims 1 to 5 , wherein the intrinsic region comprises comb-like impurity regions. 7. The optical deflection device according to claim 6 , wherein said comb-shaped impurity region is provided along the arrangement of said low refractive index portions provided in said intrinsic region. The heating mechanism includes a plurality of heating units divided in the length direction of a waveguide core, which is a region of the intrinsic region in which the impurity region is not provided, for propagating light, and the heating unit 8. The optical deflection device according to any one of claims 1 to 7, wherein the heating control of is individually controllable.

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