CN110187438B - A Tunable Harmonic Guided Grating Surface Emitter - Google Patents
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
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Abstract
本发明公开了一种可调谐波导光栅面发射器。波导光栅结构连接在输入光波导的输出末端，波导光栅结构设有光栅槽，光栅槽内充满填充材料；方形石墨烯层铺设在波导光栅结构顶面，光栅衍射区域侧方设有金属电极，两侧的金属电极均经条带石墨烯连接臂接到方形石墨烯层，金属电极接到外部电源电路的两端，外部电源电路施加电压通过金属电极、条带石墨烯连接臂和方形石墨烯层形成通路，通过方形石墨烯层作为主要加热区域对波导光栅结构加热调控光栅衍射角，通过光栅衍射区域将波导中传输的模式光向波导上方衍射出去。本发明通过调节石墨烯两端电压来调控光栅衍射角，结构简单，有望实现小型化、低成本、调控速度快的光波导相控阵激光雷达。
The invention discloses an adjustable harmonic guide grating surface emitter. The waveguide grating structure is connected to the output end of the input optical waveguide. The waveguide grating structure is provided with grating grooves, and the grating grooves are filled with filling materials; the square graphene layer is laid on the top surface of the waveguide grating structure, and metal electrodes are arranged on the sides of the grating diffraction area. The metal electrodes on the side are connected to the square graphene layer through the strip graphene connecting arms, the metal electrodes are connected to both ends of the external power circuit, and the external power circuit applies voltage through the metal electrodes, the strip graphene connecting arms and the square graphene layer. A passage is formed, and the waveguide grating structure is heated and controlled by the square graphene layer as the main heating area to control the diffraction angle of the grating, and the mode light transmitted in the waveguide is diffracted to the upper side of the waveguide through the grating diffraction area. The invention adjusts the diffraction angle of the grating by adjusting the voltage at both ends of the graphene, has a simple structure, and is expected to realize a miniaturized, low-cost, and fast-adjusting optical waveguide phased array laser radar.
Description
Technical Field
The invention relates to a tunable waveguide grating surface emitter, in particular to a hybrid integrated tunable waveguide grating surface emitter taking graphene as a heater.
Background
The laser radar scans and samples the detection target by adopting a moving laser beam to acquire characteristic quantities such as the position, the speed and the like of the detection target. Traditional mechanical type laser radar leans on mechanical rotation to scan the space, and this kind of mode response speed is slower, and machinery is ageing easily moreover, and reuse nature is relatively poor, and whole device is bulky simultaneously, inconvenient transportation and carry. Compared with the prior art, the phased array laser radar for researching fire heat in recent years can change the emission direction of the laser beam by adjusting the relative phase of multiple paths of signals, and has the advantages of high response speed, small size and the like.
With the development of photonic integration technology, optical waveguide phased array laser radars have been widely studied. The optical waveguide phased array laser radar can simultaneously integrate a light beam scanning device and a control logic circuit on a single chip, and is favorable for further meeting the requirements of low energy consumption, small size, high response speed, low cost and the like.
Most of optical waveguide phased array laser radars control the emergent direction of light beams by regulating and controlling phase difference in the transverse direction, a plurality of waveguides are required to be designed, and the phase difference between adjacent waveguides is fixed. Mode light can be emitted along one direction after passing through the array waveguide with fixed phase difference, the emitting angle of the light beam in the transverse direction can be changed by changing the phase difference, and the light beam is emitted in a near-plane wave mode by etching the grating at the emitting end of the waveguide. According to the diffraction principle of the grating, the diffraction angles of the gratings with the same size and different refractive indexes are different, and the effective refractive indexes of the same material for light waves with different wavelengths are different. Therefore, a general optical waveguide phased array controls the light beam emission direction by controlling the wavelength of the light source in the longitudinal direction. In order to obtain a large-angle regulation range, the used laser light source can provide enough wavelength selection, however, the tunable laser with multiple wavelengths is generally expensive, and the designed grating has high dependency on the tunable laser, which is not beneficial to the use of the optical waveguide phased array laser radar.
On the other hand, photonic integrated devices that use the thermo-optic effect of waveguides to achieve the modulation effect are also a great subject of research. The conventional integrated thermo-optically controllable device usually employs a metal heating electrode, and in order to prevent the influence of a metal material on light transmission in the waveguide, a thicker low-refractive-index isolation layer is usually required to be added between the waveguide core layer and the metal heating electrode. The thermal conductivity of the isolation layer is generally low, so that heat is not easily conducted to the optical waveguide after the electrodes are electrified, the heating efficiency of the thermo-optical device is reduced, and the thermo-optical regulation speed is limited. Therefore, there is a need for a transparent heater with a modulating grating structure that has high heating efficiency and does not significantly affect the light propagating in the optical waveguide.
Disclosure of Invention
The invention aims to provide a tunable waveguide grating surface emitter, which is characterized in that single-layer graphene is directly paved on a grating as a micro heater, so that the grating can be effectively heated, the refractive index of an optical waveguide is increased, too large loss is not introduced into the waveguide, and meanwhile, the tunable waveguide grating surface emitter is convenient to integrate, low in cost and high in reliability.
The invention adopts the specific technical scheme that:
the structure mainly comprises an input optical waveguide, a grating diffraction region and a miniature graphene heater which are arranged between an upper cladding layer and a lower cladding layer, wherein the grating diffraction region comprises a waveguide grating structure and a filling material; the miniature graphene heater comprises a square graphene layer laid on a grating diffraction region, strip graphene connecting arms, metal electrodes and an external power supply circuit; square graphite alkene layer is laid at waveguide grating structure top surface, grating diffraction zone's both sides side all is equipped with metal electrode, the metal electrode of both sides all is connected to square graphite alkene layer through respective strip graphite alkene linking arm, the metal electrode of both sides is connected to external power supply circuit's both ends, external power supply circuit applys voltage and passes through metal electrode, strip graphite alkene linking arm and square graphite alkene layer form the route, heat waveguide grating structure as main heating region through square graphite alkene layer, thereby regulation and control grating diffraction angle, diffract the mode light of transmission in with the waveguide through grating diffraction zone and go out to the waveguide top.
The input optical waveguide and the grating diffraction region form a core layer strip optical waveguide, the material of the filling material is different from the material of the core layer strip optical waveguide and the graphene material, and the refractive indexes of the filling material, the upper cladding material and the lower cladding material of the waveguide are lower than that of the core layer strip optical waveguide.
The metal electrodes are positioned on two sides of the waveguide grating structure and have a spacing distance from the waveguide grating structure, so that influence on waveguide light is avoided.
The filling material is only filled in the grating grooves and does not need to be filled above the waveguide or other areas.
The filling material is a low-refractive-index material, and the low refractive index is specifically set to be 1.45.
The input optical waveguide, the grating diffraction area and the miniature graphene heater are all manufactured by adopting single-chip integration.
The lower cladding layer is made of low-refractive-index materials, and the low refractive index is specifically set to be about 1.45.
The lower cladding can be partially removed, the mainly removed area is located right below the grating diffraction area, an air gap is formed after the partial lower cladding is removed, the refractive index is 1, and the heating efficiency of the graphene can be improved through the treatment.
The upper cladding is air.
The input optical waveguide and the grating diffraction region form a core layer strip optical waveguide, the core layer strip optical waveguide comprises a plurality of core layer strip optical waveguides and only one miniature graphene heater, the core layer strip optical waveguides are arranged on the lower cladding layer in parallel, and the miniature graphene heater is laid on the grating diffraction region of the common plurality of core layer strip optical waveguides, so that the plurality of grating diffraction regions are cascaded.
The optical waveguide is positioned between the upper cladding material and the lower cladding material, is etched into a strip structure, and can allow the internal transmission of the fundamental mode light.
All materials except metal electrodes have transparent characteristic in the C wave band of optical communication, and the material absorption loss of the whole structure only accounts for a very small part of the total loss.
The grating is etched at the tail end of the optical waveguide, the etched area is filled with other materials, the surface of the optical waveguide is smooth, and then the single-layer graphene is paved. Due to the fact that the graphene is high in heat conductivity coefficient, after voltage is applied to two ends of the graphene, the grating can be heated quickly, the design process is simple, and good heating effect can be achieved. When the optical waveguide phased array is used as an emergent end of the optical waveguide phased array, the refractive index of the optical waveguide can be quickly and effectively changed, namely the diffraction angle of an emergent grating is changed, so that the effect of beam scanning is achieved.
The invention has the beneficial effects that:
1. the structure is simple, the design is convenient, the manufacture is simple and convenient, and the manufacture cost of the device can be obviously reduced.
2. The graphene has high heat conductivity coefficient, and can rapidly heat the grating after being powered on.
3. The graphene is only one atomic layer thick, the absorption rate of the graphene to vertical incident light is only 2.3%, and the graphene does not introduce too large loss when added above the optical waveguide.
4. The graphene material has good flexibility and structural strength, can be easily processed into a micro-nano structure, can realize a thermo-optic regulation and control photonic integrated device with a micro-nano size by using the graphene transparent nano heating electrode, can also be used for heating the photonic integrated device with an uneven surface, and makes up the limitation of the traditional metal heating electrode on application.
5. And the grating area is fully paved with the square graphene, so that the operation is simple. If a plurality of gratings are arranged in parallel, the area of the single-layer graphene is directly increased.
In conclusion, the grating diffraction angle is regulated and controlled by regulating the voltage at two ends of the graphene, the structure is simple, and the optical waveguide phased array laser radar is expected to realize miniaturization, low cost, low loss, good material performance and high regulation and control speed.
Drawings
FIG. 1 is a top view of the structure of the present invention.
3 fig. 3 2 3 is 3 a 3 sectional 3 view 3 a 3- 3 a 3' 3 of 3 fig. 3 1 3. 3
Fig. 3 is a sectional view B-B' of fig. 1.
Fig. 4 is a cross-sectional view of C-C' of fig. 1.
Fig. 5 is a cross-sectional view D-D' of fig. 1.
Fig. 6 is a cross-sectional view E-E' of fig. 1.
3 fig. 3 7 3 is 3 a 3 cross 3- 3 sectional 3 view 3 a 3- 3 a 3' 3 with 3 the 3 lower 3 cladding 3 layer 3 removed 3 from 3 the 3 portion 3 just 3 below 3 the 3 grating 3 region 3. 3
Fig. 8 is a top view of the waveguide grating array after filling.
Fig. 9 is a structural diagram of the design of the light guide phased array as the exit end of the invention.
In the figure: 1. the optical waveguide structure comprises a lower cladding layer, 2, an input optical waveguide, 3, a waveguide grating structure, 4, a filling layer, 5, a square graphene heater, 6, strip graphene connecting arms, 7, a metal electrode and 8, and an air gap formed after part of the lower cladding layer is removed.
Detailed Description
The invention is further illustrated by the following figures and examples.
As shown in fig. 1, the structure of the implementation mainly includes three parts, namely an input optical waveguide 2, a grating diffraction region and a micro graphene heater, which are arranged between an upper cladding and a lower cladding, wherein the lower cladding 1 is made of silicon dioxide, and the upper cladding is air.
As shown in fig. 2-6, the grating diffraction region includes a waveguide grating structure 3 and a filling material 4, the waveguide grating structure 3 is connected to the output end of the input optical waveguide 2, a plurality of parallel grating grooves are formed on the top surface of the waveguide grating structure, each grating groove is perpendicular to the extending direction of the waveguide, two sides of the grating groove penetrate through the waveguide grating structure, the grating grooves are filled with the filling material 4, the filling material only fills the etched part of the optical waveguide, and the surface of the filling material 4 is flush with the top surface of the waveguide grating structure after filling; the input optical waveguide 2 and the grating diffraction region 3 form a core layer strip-shaped optical waveguide and adopt silicon materials.
As shown in fig. 1, the micro graphene heater includes a square graphene layer 5 laid on a grating diffraction region, a strip graphene connecting arm 6, a metal electrode 7, and an external power circuit; the square graphene layer 5 is laid on the top surface of the waveguide grating structure, the size of the square graphene layer 5 is equal to or equal to that of the grating diffraction region, the area of the graphene does not exceed that of the grating diffraction region, and covers all grating grooves and the filling material 4 therein, metal electrodes 7 are arranged on two side sides of the grating diffraction region, the metal electrodes 7 on the two sides are connected to the square graphene layer 5 through respective strip graphene 6, the metal electrodes 7 on the two sides are connected to two ends of an external power circuit, the external power circuit applies voltage to form a passage through the metal electrodes 7, the strip graphene connecting arms 6 and the square graphene layer 5, the square graphene layer 5 is used as a main heating region to heat the waveguide grating structure, and further regulating and controlling the diffraction angle of the grating, and diffracting the mode light transmitted in the waveguide out above the waveguide through the grating diffraction region.
In specific implementation, the grating diffraction region comprises a grating structure and a filling material, a grating groove is formed on the upper surface of the strip waveguide through etching, and then the material with low refractive index is filled in the groove, so that the surface of the optical waveguide is flattened again. The square graphene layer is a main heating area, and one side of the strip graphene connecting arm is connected with the square graphene and is paved on the metal electrode. The metal electrodes 7 are located on both sides of the waveguide grating structure and spaced apart from the waveguide grating structure.
Graphene has certain resistance and heat conduction capability, and a closed loop can be formed by connecting the graphene layer with a metal electrode and a power supply. When the power supply is switched on, the graphene generates heat and rapidly conducts the heat to the grating layer below. The material of the grating layer has a certain thermo-optic effect, and the refractive index of the material is changed along with the temperature rise.
The working process of the invention used as the emergent end of the optical waveguide phased array is as follows:
according to bragg condition of grating diffraction: n iseff-nc·sinθ＝λ/Λ(neff: effective refractive index of optical waveguide, nc: upper cladding refractive index, θ: grating diffraction angle, λ: wavelength of incident light, Λ: grating period), the effective refractive index n of the optical waveguide with the same structure is heated by the single-layer graphene transparent heating electrodeeffChanges occur and the grating diffraction angle theta changes accordingly. The heating temperature of the graphene can be regulated and controlled by only regulating the voltage applied to the two ends of the graphene electrode, so that the diffraction angle of the grating is regulated and controlled, and the effect of beam scanning is achieved.
To increase the heating efficiency of the graphene, so that the same refractive index changes, the lower cladding layer portion under the grating diffraction region can be etched away, forming a suspended grating structure, as shown in fig. 7. Because only the portion of the lower cladding layer below the diffraction region of the grating is removed, the upper structure does not collapse. Since the thermal conductivity of air is low, it can be seen as a poor conductor of heat, and graphene heats the suspended grating region, the heat will not diffuse downward rapidly, so the power required to achieve the same heating effect is less than that of fig. 1-6.
When the tunable waveguide grating surface emitter is used in an optical waveguide phased array lidar, the same size grating is etched at the end of each aligned waveguide as shown in fig. 8. Mode light propagating by two adjacent waveguides has a fixed phase difference, and when the mode light propagates to the grating region, due to the phase difference, diffracted light deviates from the vertical direction of the upper surface of the waveguide in the transverse direction. Adjusting the phase difference between each waveguide allows the scan angle in the lateral direction to be adjusted. As shown in fig. 9, after the parallel waveguide grating is filled and leveled, a layer of graphene is directly laid on the parallel waveguide grating, the whole graphene is heated, the temperature of the grating is increased, the temperature of each grating is basically consistent, and therefore the diffraction angle of each grating is basically consistent. The scanning angle in the longitudinal direction can be adjusted by adjusting the voltage at the two ends of the graphene, so that the optical waveguide phased array laser radar can realize two-dimensional laser beam scanning in a certain range.
Specific embodiments of tunable waveguide grating surface emitters are given below.
Example 1
Only a single core layer strip optical waveguide and one micro graphene heater are arranged as shown in fig. 1.
Silicon nanowire optical waveguides based on silicon-on-insulator (SOI) materials are selected: the core layer is made of silicon material, the thickness is 220nm, and the refractive index is 3.4744; the lower cladding material is silicon dioxide, the thickness is 2 μm, and the refractive index is 1.4404; the upper cladding material is air and has a refractive index of approximately 1. The TE polarization mode is used considering the center wavelength of the incident light as 1550 nm.
The core layer is etched into a strip waveguide with a certain width by adopting the processes of photoetching and the like, the coupling grating is etched at the front end, the light in the optical fiber is coupled into the silicon waveguide, and the tail end is also etched into a grating structure, but the size of the grating structure is different from that of the coupling grating. The etching depth is 0.065 μm, the period is 0.95 μm, the duty ratio is 0.7, and the whole grating diffraction region is 25 μm long. At normal temperature, the diffraction angle of the grating is about 65 degrees, and the diffraction angle is much larger than that of the common coupling grating.
And after the grating etching is finished, filling HSQ photoresist in the grating groove to ensure that the upper surface of the whole silicon waveguide is flat. And then transferring the graphene material to the upper part of the grating, and forming a square and strip structure by processes such as graphical processing and the like. Then used for etching SiO2Sputtering gold in the window to form a gold electrode, attaching one end of a strip graphene connecting arm on the gold electrode, and connecting a circuit, thereby obtaining the silicon-graphene mixed tunable waveguide grating surface emitter.
And voltage is applied to two ends of the graphene, and the temperature of the miniature graphene heating electrode is continuously increased along with the increase of the voltage in a certain range. When the graphene temperature reaches 740K, the temperature of the silicon grating is about 700K, which is equivalent to being higher than the room temperature400K. The thermo-optic coefficient of silicon is about + 1.84X 10-4K-1The increase of 400K is an increase of 0.0736 in the refractive index. The diffraction angle of the grating deflects by nearly 10 degrees before and after the temperature of the grating rises, and the heating efficiency of the graphene is about 0.20 degrees/mW. The heating efficiency is higher than that of a common metal heating electrode, the graphene has no great influence on an optical field, and the peak diffraction efficiency of the grating is not greatly changed in the heating process.
And etching the silica lower cladding layer below the diffraction region of the grating by using a reagent, and controlling the thickness of the removed silica according to the etching rate. The power required to heat the suspended waveguide grating to 700K is lower than that required for the non-suspended waveguide grating, in which case the graphene heating efficiency is approximately 0.33 °/mW, while the diffraction efficiency of the waveguide grating is not too much.
Example 2
As shown in fig. 8 and 9, the specific implementation includes a plurality of core layer strip optical waveguides and only one micro graphene heater, the plurality of core layer strip optical waveguides are arranged in parallel on the lower cladding layer, and the micro graphene heater is laid on the grating diffraction regions of the plurality of common core layer strip optical waveguides, so that the plurality of grating diffraction regions are formed in cascade. In order to make the graphene laying smoother, after the input waveguide and waveguide grating etching are completed, HSQ photoresist is used for filling a grating etching area and a spacing area between adjacent waveguides. Due to the increased graphene area, the heating efficiency is reduced compared to example 1 by providing higher power to increase the waveguide to the same temperature. The same mode light is input into each waveguide, but the adjacent waveguides maintain a fixed phase difference (the phase difference can be achieved by growing the waveguides or heating the waveguides, etc.). Four waveguide arrays with the width of 1 mu m are designed, the distance between adjacent waveguides is 2 mu m (the mutual crosstalk between the waveguides is prevented), and the fixed phase difference is pi. The fixed phase difference enables a light beam which can be approximately 30 degrees in the transverse direction to deflect, and the diffraction angle of the grating is changed by about 10 degrees in the longitudinal direction through heating of the miniature graphene heater. The whole structure can thus achieve a beam angle scan of 30 deg. × 10 deg..
The above-described embodiments are intended to illustrate rather than to limit the invention, and any modifications and variations of the present invention are within the spirit of the invention and the scope of the appended claims.
Claims (10)
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| Application Number | Priority Date | Filing Date | Title |
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| CN201910400466.3A CN110187438B (en) | 2019-05-15 | 2019-05-15 | A Tunable Harmonic Guided Grating Surface Emitter |
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| Application Number | Priority Date | Filing Date | Title |
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| CN201910400466.3A CN110187438B (en) | 2019-05-15 | 2019-05-15 | A Tunable Harmonic Guided Grating Surface Emitter |
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