CN116819845A - Silicon-based optical phased array structure with high sideband suppression ratio and calculation method thereof - Google Patents
Silicon-based optical phased array structure with high sideband suppression ratio and calculation method thereof Download PDFInfo
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Abstract
The invention discloses a silicon-based optical phased array with a high sideband suppression ratio, which comprises a laser, a coupling beam splitter, a phase modulator, a transmission waveguide and a coupling grating, wherein the coupling beam splitter, the phase modulator, the transmission waveguide and the coupling grating are all integrated on a silicon dioxide substrate, an emergent beam of the laser enters the coupling beam splitter, the coupling beam splitter splits a single path of light into multiple paths of light, each path of split light enters the phase modulator for phase modulation, the transmission waveguide is connected with the phase modulator and the coupling grating, and the coupling grating radiates the multiple paths of light after phase modulation to a far field. The invention also discloses a calculation method of the silicon-based optical phased array structure with the high sideband suppression ratio. The advantages are that: according to the invention, the length of the coupling area between the array waveguide and the main waveguide is regulated, so that the near-field light intensity of the optical phased array meets Gaussian distribution, and the sideband suppression ratio of far-field light intensity is improved.
Description
Technical Field
The invention relates to the technical field of laser radars, in particular to a silicon-based optical phased array structure with a high sideband suppression ratio.
Background
With the vigorous development of new energy automobiles, the automatic driving technology also makes great progress. In such a development background, the laser radar can realize three-dimensional imaging with long distance and high precision, and the advantages thereof are increasingly prominent. The core component of the lidar is the beam scanning system therein. Compared with the traditional mechanical scanning structure, the optical phased array has the advantages of no internal moving parts, easy miniaturization, simple phase calibration, low cost and the like, and is more and more widely focused. The outgoing beam of the optical phased array appears in the far field as a bright spot with an angular width, which is essentially multi-beam interference. Therefore, the far-field light spot not only has a main lobe, but also contains various levels of side lobes, and the scanning accuracy of the optical phased array is greatly reduced due to the side lobes. Therefore, reducing the sidelobes of far-field light intensity and improving the sideband suppression ratio are one of the key works for designing and optimizing the optical phased array structure. The sideband suppression ratio of far field light intensity generally needs to reach more than 30 dB.
The traditional silicon-based optical phased array mostly adopts a multi-stage beam splitting structure, the optical power of the array waveguide is evenly distributed,
example Optica: publication number: 2334-2539, article name: non-redundant optical phased array, authors: T.Fukui, R.Tanomura, K.Komatsu, D.Yamashita, S.Takahashi, Y.Nakano and t. tantemura, the 3dB MMI cascaded spectral design described in this article implements an optical phased array of non-redundant construction, reducing the number of phase modulators without reducing the scan resolution, but with a sideband suppression ratio of only 7dB for far field measurements.
Example Optical Express: publication number: 1094-4089, article name: sub-wavelength-patch silicon-photonic optical phased array for large field-of-regard coherent optical beam steering, the authors were: Y.Zhang, Y.C.Ling, K.Zhang, C.Gentry, D.Sadighi, G.Whaley, J.Colosimo, P.Suni and s.j.ben Yoo, the 3dB MMI cascaded spectroscopic design described in this article achieves a large scan range in the horizontal and vertical directions. However, the disadvantage is also evident that the crosstalk of the waveguide array is large, and the sideband suppression ratio of the far field is only 7dB.
Silicon-based optical phased array structures generally increase the sideband suppression ratio of far-field light intensity by reducing crosstalk between array waveguides, or by increasing the accuracy of calibration of the initial phase. The suppression of the inter-waveguide array crosstalk is more realized by designing an array structure with a specific structure, for example: superlattice waveguides, plasmonic waveguides, metamaterial waveguides, etc. Although the symmetry break of the waveguide array structure can realize crosstalk suppression to a certain extent, the waveguide array structure also has the defects of difficult processing, insignificant crosstalk suppression (the best crosstalk suppression can be realized to be about-20 dB), high cost and the like. On the other hand, in practical application, the optical phased array keeps the initial phase of each waveguide consistent, and generates a phase gradient on the basis, thereby realizing light beam scanning. Meanwhile, in practical application, the phase lock of the waveguide is also important to keep facing environmental (temperature, humidity and the like) changes. At present, a deterministic random gradient descent method, a hierarchical random parallel gradient descent method and the like provide faster optimization convergence speed, realize higher phase calibration precision and can improve the sideband suppression ratio to more than 25 dB. However, the sideband suppression ratio improvement mode depending on the optimization algorithm is greatly influenced by environmental factors, and the instability is larger; furthermore, the increase in the sideband suppression ratio due to phase calibration does not fully satisfy the requirements of the radiating antenna.
Disclosure of Invention
One of the purposes of the invention is to provide a silicon-based optical phased array structure with a high sideband suppression ratio, which enables near-field light intensity of an optical phased array to meet Gaussian distribution and improves the sideband suppression ratio of far-field light intensity by adjusting the length of a coupling area between an array waveguide and a main waveguide.
The technical scheme adopted is as follows: the silicon-based optical phased array structure with the high sideband suppression ratio comprises a laser, a coupling beam splitter, a phase modulator, a transmission waveguide and a coupling grating, wherein the coupling beam splitter, the phase modulator, the transmission waveguide and the coupling grating are all integrated on a silicon dioxide substrate, an emergent beam of the laser enters the coupling beam splitter, the coupling beam splitter splits a single path of light into multiple paths of light, each path of split light enters the phase modulator for phase modulation, the transmission waveguide is connected with the phase modulator and the coupling grating, and the coupling grating irradiates the multiple paths of light after phase modulation to a far field.
Further preferably, the coupling beam splitter has a 1 XN beam splitting structure, and the width of the coupling beam splitter is W from 1 path 0 Is composed of main waveguide and beam-splitting waveguide array consisting of N sub-waveguides with width W 1 ，W 1 ＝W 0 N is a positive integer. The silicon-based optical phased array generally adopts a 1 XN beam splitting structure, wherein 1 represents a main waveguide and is used as a propagation channel for light source input; n is a beam splitting structure, dividing the output beam into N wavelets. According to the Huygens principle, the propagation phase of each sub-wave can be regulated, so that the regulation and control of the propagation direction of the output light can be realized.
Further preferably, the N sub-waveguides of the beam-splitting waveguide array have a length L i The (i=1, 2, …, N) straight waveguide and S-type Bezier waveguide are cascaded in turn on one side of the main waveguide, and the N sub-waveguides are unevenly divided into M groups, where:
wherein m is j For the number of sub-waveguides in the j (j=1, 2, …, M) th group, M is a positive integer.
The sub-waveguide consists of a straight waveguide and an S-shaped Bezier waveguide, the straight waveguide can realize transmission coupling with the main waveguide, and a part of light intensity is transferred into the sub-waveguide; the S-shaped Bezier waveguide guides the coupled light to be output, and the structure can reduce the loss of light transmission.
Further preferred in the technical scheme of the invention, the phase modulator provides additional phases for the beam splitting waveguide array, and the gradient of the phase modulator changesThe following are provided:
wherein lambda is the wavelength of the light beam of the light source, N eff For the effective refractive index of the light beam propagating in the waveguide, D is the spacing period of the output waveguide array, θ s Is the scan pointing angle of the optical phased array.
The angular scanning of the silicon-based optical phased array is achieved by providing additional phase to the beam splitting waveguide array through a phase modulator. And determining the requirement of the pointing angle thetas, and providing additional phases for the sub-waveguides to meet the above relation, so as to ensure that the emergent light of the silicon-based optical phased array is plane wave. The three-dimensional sensing function of lidar is just to require that the scanning beam be a plane wave.
Further preferably, the transmission waveguide is an evenly distributed array, the wavelet derivative is N, and the interval period of the transmission waveguide is d=αλ; where α is the array spacing factor. The invention constructs Gaussian near-field light intensity distribution mainly by adjusting the light power ratio transmitted by the neutron waveguide in the beam-splitting waveguide array. Under the design thought, only the transmission waveguide is ensured to be uniformly distributed, and the Gaussian distribution light intensity of the near field has a high sideband suppression ratio after being subjected to Fraunhofer diffraction.
Further preferably, the coupling grating is a sub-wavelength grating, and the grating period lambda meets the phase matching condition, specifically as follows:
wherein lambda is the wavelength of the light beam of the light source, n p For refractive index of light beam in exit space, N eff Is the effective refractive index of the light beam propagating in the waveguide.
The sub-wavelength grating can couple out the light beams transmitted in the waveguide, so that the scanning of far-field objects is realized. The phase matching condition ensures that the light source beam is coupled out from the end face of the sub-wavelength grating vertically.
Further preferably, the laser is an external light source, the laser is a monochromatic light source for one-dimensional scanning and a wide-spectrum light source for two-dimensional scanning.
The silicon-based optical phased array can realize two-dimensional scanning, and scanning in the horizontal direction is mainly realized by adding gradient phases to the waveguide array through the phase modulator, so that the scanning in the horizontal direction can be realized by a single-wavelength light source; in the vertical direction, light beams with different wavelengths are coupled out through the sub-wavelength grating to exhibit different exit angles, which is commonly referred to as wavelength tuning. Therefore, scanning in the vertical direction needs to be achieved by a broad spectrum light source.
The second purpose of the invention is to provide a calculation method of a silicon-based optical phased array structure with a high sideband suppression ratio, a mode coupling equation which is continuous in the propagation direction is constructed, coupling among waveguides is optimized in groups by adopting a genetic algorithm, and the sideband suppression ratio of the far-field light intensity of the silicon-based optical phased array is used as an optimized loss function.
The technical scheme adopted is as follows: the calculation method of the silicon-based optical phased array structure with the high sideband suppression ratio comprises the following steps:
s1, constructing an optical phased array beam splitting weight factor w by adopting Gaussian distribution n (n=1, 2, …, N) distribution model;
s2, according to the width W of the main waveguide 0 Width W of beam splitting waveguide 1 And waveguide spacing W g Calculating a coupling coefficient kappa; setting propagation mismatch constant delta beta, and calculating coupling length L c ；
S3, simultaneous main waveMode coupling equation set between the waveguide and N sub-waveguides, and obtaining main waveguide light field amplitude coefficient A (z) and sub-waveguide light field amplitude coefficient a n (z) (n=1, 2, …, N), where z is the propagation distance; with A (z) as boundary condition, wavelet light field amplitude coefficient a n (z) a distribution model w satisfying the beam-splitting weight factor n Conservation of total energy, and further obtaining the straight waveguide lengths L corresponding to the N sub-waveguides n (n＝1,2,…,N)；
S4, N sub-waveguides are unevenly divided into M groups, the number of sub-waveguides of each group being denoted S m (m=1, 2, …, M), the straight waveguides of the same group of neutron waveguides have the same length, and the common length is the average value L of the lengths of the corresponding straight waveguides in S3 m (S m )(m＝1,2,…,M)；
S5, constructing a loss function Fitness=f SMSR (S m θ), wherein SMSR represents the sideband suppression ratio of far field light intensity;
s6, searching the grouping mode of the optimal beam splitting waveguide array by adopting a genetic algorithm, namely S m The distribution function maximizes the sideband suppression ratio of the far field light intensity.
9. The computing method according to claim 8, wherein: after N sub-waveguides are grouped, the average length L of the straight waveguides of each group m (S m ) The difference is greater than 0.5 μm.
The silicon-based photonic device is processed with a certain size error, and the smaller average length difference is easily covered by the error in the processing process, so that the energy ratio in the corresponding sub-waveguide does not meet the numerical requirements proposed by the invention, and the far-field sideband suppression ratio of the actual device is also reduced.
The beneficial effects of the invention are as follows:
1. the silicon-based optical phased array structure with high sideband suppression ratio realizes the accurate control of the splitting ratio by designing and optimizing the lengths of the splitting waveguide and the main waveguide coupling area, and realizes the Gaussian near-field light intensity distribution, thereby improving the sideband suppression ratio of far-field light intensity and effectively suppressing the interference and noise from side lobes.
2. The invention has a silicon-based optical phased array structure with high sideband suppression ratio, the beam splitting waveguides are grouped unevenly, the size difference of the beam splitting waveguides is improved, and the practical application value is improved.
3. According to the calculation method of the silicon-based optical phased array structure with the high sideband suppression ratio, the general solution of the amplitude coefficients of the light fields in the main waveguide and the beam-splitting waveguide array is obtained according to the mode coupling theory, the light field amplitude attenuation of the main waveguide is used as a boundary condition, the beam-splitting weight factor distribution is used as a solving target, and the length of each beam-splitting waveguide coupling area can be rapidly obtained.
Drawings
FIG. 1 is a schematic diagram of a silicon-based optical phased array structure with high sideband suppression ratio in an embodiment of the invention;
FIG. 2 is a distribution of the number of sub-waveguides grouped in a sub-waveguide array of a beam splitting waveguide array according to an embodiment of the present invention;
FIG. 3 is a flow chart of a method for optimizing a silicon-based optical phased array design with a high sideband suppression ratio in an embodiment of the invention;
FIG. 4 is a length of a coupling region corresponding to a beam splitting waveguide array according to an embodiment of the present invention;
FIG. 5 is a graph showing the average adjacent lengths L of the grouped beam splitting waveguide arrays in accordance with an embodiment of the present invention m (S m ) A difference between;
FIG. 6 is a normalized distribution of the far field of light intensity at different scan angles (a) 0deg in an embodiment of the present invention; (b) 10deg; (c) 20deg; (d) 30deg; (e) 40deg; (f) 50deg.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be further described in detail with reference to the accompanying drawings 1 to 5 and examples. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
As shown in fig. 1, the present embodiment is a silicon-based optical phased array structure with a high sideband suppression ratio, which includes a laser 1, a coupling beam splitter 2, a phase modulator 3, a transmission waveguide 4, and a coupling grating 5, wherein the laser 1 is an off-chip light source, and the coupling beam splitter 2, the phase modulator 3, the transmission waveguide 4, and the coupling grating 5 are integrated on a silicon dioxide substrate. The laser 1 generates a light beam to be incident into a main waveguide 2-1 in the coupling beam splitter 2; the beam splitting waveguide array 2-2 is sequentially cascaded on one side of the main waveguide 2-1, and incident light is coupled into the beam splitting waveguide array 2-2 in a direct coupling mode; the beam splitting waveguide array 2-2 is characterized in that straight waveguides belonging to a coupling area are extended outwards through S-shaped Bezier waveguides and are connected with the phase modulator 3 through the straight waveguides; the phase modulator 3 realizes the phase modulation of the propagation light beam by changing the refractive index of the waveguide area, and the modulation phase distribution of the phase modulator 3 is in gradient descent or gradient ascent; one end of the transmission waveguide 4 is connected with the phase modulator 3, the other end is connected with the coupling grating 5, and the waveguide spacing is equal; the coupling grating 5 is a sub-wavelength grating, and realizes the output coupling of incident light.
In this embodiment, the laser 1 may be a monochromatic light source or a broad spectrum light source, where the monochromatic light source can only realize one-dimensional scanning of the optical phased array in the horizontal direction, and the broad spectrum light source can realize two-dimensional scanning of the optical phased array in the horizontal direction and the vertical direction by means of wavelength tuning.
In this embodiment, the beam splitting waveguide array 2-2 includes n=600 sub-waveguides, and is divided into m=20 groups, each group corresponding to the number S of waveguides m As shown in fig. 2.
In this embodiment the phase modulator 3 is connected to an external voltage, which applies an extra phase to the optical field passing through the region by thermo-optic effect, which varies the gradientThe following are provided:
wherein lambda is the wavelength of the light beam of the light source, N eff For the effective refractive index of the light beam propagating in the waveguide, D is the spacing period of the output waveguide array, θ s The corresponding relation between the modulation phase and the scanning pointing angle is expressed for the scanning pointing angle of the optical phased array.
The grating period Λ of the coupling grating 5 in this embodiment satisfies the phase matching condition of vertical emission:
wherein n is p Is the refractive index of the light beam in the exit space. In this embodiment, the coupling grating 5 applies a single radiation, so m=1.
As shown in fig. 3, the present embodiment is a method for calculating a silicon-based optical phased array structure with a high sideband suppression ratio, which includes the following steps:
s1, constructing an optical phased array beam splitting weight factor distribution model, namely Gaussian distribution. Beam splitting weight factor w n The concrete steps are as follows:
wherein μ is the mean and σ is the standard deviation.
S2, according to the main waveguide width W 0 Width W of beam splitting waveguide array 1 And waveguide spacing W g The coupling coefficient κ is calculated. Setting propagation mismatch constant delta beta=0, and calculating coupling length L c 。
S3, obtaining a main waveguide light field amplitude coefficient A (z) and a sub waveguide light field amplitude coefficient a by a mode coupling equation set of coupling between the simultaneous main waveguide 2-1 and N sub waveguides n (z) (n=1, 2, …, N), specifically as follows:
wherein z is the propagation distance,meanwhile, using A (z) as boundary condition, the amplitude coefficient a n (z) the Gaussian distribution satisfied by the splitting weight factor is satisfied, the total energy is conservedThe constant is as follows:
the propagation distance z meeting the requirement is the length L of the straight waveguide from which the beam-splitting waveguide array 2-2 is dependent n (n=1, 2, …, N), the distribution of which is shown by the dashed lines in fig. 4.
S4, the sub-waveguides of the beam-splitting waveguide array 2-2 are unevenly divided into M=20 groups, and the number of the sub-waveguides of each group is represented as S m (m=1, 2, …, M). Setting the lengths of the straight waveguides in the m group of neutron waveguides to be the same, wherein the common length is the average value L of the lengths of the corresponding straight waveguides in S3 m (S m )(m＝1,2,…,M)。
S5, constructing a loss function Fitness=f SMSR (S m θ), wherein SMSR represents the sideband suppression ratio of far-field light intensity, specifically as follows:
s51, according to the coupling mode equation (4), obtaining the optical field amplitude coefficient a of the beam splitting waveguide array 2-2 after grouping average n (L m (S m ) (m=1, 2, …, M), the light field distribution therein is:
wherein d=0.56 λ;
s52, obtaining the array factor AF of far-field amplitude according to the formula (6) (S m θ) as follows:
wherein θ is the far field angular distribution;
s53, approximation processing. Far-field SMSR is less affected by far-field light intensity envelope, so AF (S m θ) dominates. The sideband suppression ratio of AF may be approximated as the sideband suppression ratio of the far-field light intensity distribution. Finally, AF (S m θ) SMSR of far field distribution as a loss functionA number.
S6, searching the grouping mode of the optimal beam splitting waveguide by adopting a genetic algorithm, namely S m And a distribution function, so that the sideband suppression ratio of the light intensity far-field distribution is maximum. The structure of the coupling beam splitter is finally determined. Determining S m Distribution, then L after grouping n The distribution is shown as a solid line in fig. 4. As shown in fig. 5, the average length after grouping L m (S m ) The difference is larger than 0.5 μm, and the length can be effectively distinguished in actual processing. As shown in FIG. 6, the distribution of far field light intensity at different scan angles is shown with SMSR>39dB。
According to the mode coupling theory, a distribution model of the near-field output light intensity of the optical phased array can be regulated and controlled, and the sideband suppression ratio of the far-field light intensity is further improved to 39dB. The optical phased array optimized based on the grouping design has obvious size difference, which is easier to realize in actual processing.
The above embodiments are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereto, and any modification made on the basis of the technical scheme according to the technical idea of the present invention falls within the protection scope of the present invention.
Claims (9)
1. A silicon-based optical phased array structure with high sideband suppression ratio is characterized in that: comprises a laser (1), a coupling beam splitter (2), a phase modulator (3), a transmission waveguide (4) and a coupling grating (5), wherein the coupling beam splitter (2), the phase modulator (3), the transmission waveguide (4) and the coupling grating (5) are integrated on a silicon dioxide substrate,
the emergent beam of the laser (1) enters the coupling beam splitter (2), the coupling beam splitter (2) splits a single-path light into multiple paths of light, each path of split light simultaneously enters the phase modulator (3) for phase modulation, the transmission waveguide (4) is connected with the phase modulator (3) and the coupling grating (5), and the coupling grating (5) radiates the multiple paths of light after the phase modulation to a far field.
2. The silicon-based optical phased array structure with high sideband suppression ratio of claim 1, wherein: the coupling beam splitter (2) is 1 XNBeam splitting structure with width W of 1 path 0 Is composed of main waveguide and beam-splitting waveguide array consisting of N sub-waveguides with width W 1 ，W 1 ＝W 0 N is a positive integer.
3. The silicon-based optical phased array structure with high sideband suppression ratio of claim 2, wherein: the N sub-waveguides of the beam-splitting waveguide array have a length L i The straight waveguide and the S-shaped Bezier waveguide of (i=1, 2, …, N) are sequentially cascaded on the same side of the main waveguide, and the N sub-waveguides are unevenly divided into M groups, so that:
wherein m is j For the number of sub-waveguides in the j (j=1, 2, …, M) th group, M is a positive integer.
4. The silicon-based optical phased array structure with high sideband suppression ratio of claim 2, wherein: the phase modulator provides additional phase to the beam splitting waveguide array with a varying gradient Δφ as follows:
wherein lambda is the wavelength of the light beam of the light source, N eff For the effective refractive index of the light beam propagating in the waveguide, D is the spacing period of the output waveguide array, θ s Is the scan pointing angle of the optical phased array.
5. The silicon-based optical phased array structure with high sideband suppression ratio of claim 4, wherein: the transmission waveguide (4) is a uniformly distributed array, the wavelet derivative is N, and the interval period of the output waveguide array is D=αλ; wherein the alpha array spacing factor.
6. The silicon-based optical phased array structure with high sideband suppression ratio of claim 1, wherein: the coupling grating (5) is a sub-wavelength grating, and the grating period lambda meets the phase matching condition, and is specifically as follows:
wherein lambda is the wavelength of the light beam of the light source, n p For refractive index of light beam in exit space, N eff Is the effective refractive index of the light beam propagating in the waveguide.
7. The silicon-based optical phased array structure with high sideband suppression ratio of claim 1, wherein: the laser (1) is an external light source, the laser (1) is a monochromatic light source for one-dimensional scanning and a wide-spectrum light source for two-dimensional scanning.
8. The method of computing a silicon-based optical phased array structure with high sideband suppression ratio of any one of claims 1-7, wherein: the method comprises the following steps:
s1, constructing an optical phased array beam splitting weight factor w by adopting Gaussian distribution n (n=1, 2, …, N) distribution model;
s2, according to the width W of the main waveguide 0 Width W of beam splitting waveguide 1 And waveguide spacing W g Calculating a coupling coefficient kappa; setting propagation mismatch constant delta beta, and calculating coupling length L c ；
S3, obtaining a main waveguide light field amplitude coefficient A (z) and a sub waveguide light field amplitude coefficient a by a mode coupling equation set between the simultaneous main waveguide and N sub waveguides n (z) (n=1, 2, …, N), where z is the propagation distance; with A (z) as boundary condition, wavelet light field amplitude coefficient a n (z) a distribution model w satisfying the beam-splitting weight factor n Conservation of total energy, and further obtaining the straight waveguide lengths L corresponding to the N sub-waveguides n (n＝1,2,…,N)；
S4, N sub-waveguides are unevenly divided intoM groups, the number of sub-waveguides of each group being denoted S m (m=1, 2, …, M), the straight waveguides of the same group of neutron waveguides have the same length, and the common length is the average value L of the lengths of the corresponding straight waveguides in S3 m (S m )(m＝1,2,…,M)；
S5, constructing a loss function Fitness=f SMSR (S m θ), wherein SMSR represents the sideband suppression ratio of far field light intensity;
s6, searching the grouping mode of the optimal beam splitting waveguide array by adopting a genetic algorithm, namely S m The distribution function maximizes the sideband suppression ratio of the far field light intensity.
9. The computing method according to claim 8, wherein: after N sub-waveguides are grouped, the average length L of the straight waveguides of each group m (S m ) The difference is greater than 0.5 μm.
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