Research On Subwavelength Gratings Based Beam Splitters And A House-shaped Resonant Cavity For Optical Communication System

With the rapid development of IT technologies such as 5G,cloud computing,big data,and artificial intelligence,optical communication technology,as their main support,has also ushered in new changes and challenges.To achieve a higher data transmission rate,the performance of various optical modules and devices in optical communication systems has been greatly improved,miniaturization,high speed,and integration are their direction.Silicon-based sub wavelength grating,as an optical passive device,has been widely used in laser,photodetector,coupler,filter,sensor,and other optoelectronic devices due to its excellent optical diffraction characteristics,which can be used to realize more complex photonic integrated circuits.Besides,various functional devices based on microcavities with high-quality factors have greatly promoted the development of photon integration and photon chip.This article centers on the theoretical analysis and experimental research of subwavelength grating beam splitters and a kind of house-shaped optical resonator.The main innovations and research results are as follows:1.The wavefront phase modulation characteristics of nonperiodic subwavelength gratings are studied.A one-dimensional subwavelength grating power splitter with parallel transmitted beams is proposed.A 1×2 power beam splitter with 15 ° and 30 ° deflection angles and a 1:2 power ratio is designed.The simulation results show that the deflection angles of the two split beams are 14.4 ° and 29.5 ° respectively,and the power ratio is about 1:1.87,which is basically consistent with the design values.Besides,one-dimensional subwavelength grating combiner,one-dimensional subwavelength grating power splitter with focused transmitted beams,and one-dimensional subwavelength grating bifocus lens are also proposed,the performance of the devices is simulated and verified.2.A polarization splitter based on one-dimensional subwavelength gratings with a double-layer grating structure is proposed.A polarization splitter with a focal length of 40 ?m is designed,by which TM polarization could be reflected and converge at a certain point and TE polarization could be transmitted and converge at another point under the normal incidence of 1.55 ?m.The simulated focal length of TM reflected beam is 40 ?m,the full width at half maximum(FWHM)of the optical field at the focus is about 1.88 ?m,and the total reflectivity is about 90.8%;the focal length of TE transmitted beam is 38.3 ?m,the FWHM of the optical field at the focus is about 1.7 ?m,and the total transmissivity is about 82.4%.It can be seen that the device could separate the two orthogonal polarization effectively and make them converge.3.The 1×N power splitters based on the two-dimensional subwavelength gratings are proposed.In the theoretical analysis,the transmission type 1×3 and 1×4 power splitters with a focal length of 10?m are designed,the simulated focal lengths are 9.5?m and 9.7?m respectively,and the total transmissivities are 89%and 87.2%respectively,the FWHM of the light field intensity at each focus on the focal plane is less than 2 ?m.In practical use,a circular 1×3 power splitter with a focal length of 150 ?m and a radius of 216 ?m,and a square 1×4 power splitter with a focal length of 150 ?m and a side length of 370 ?m are fabricated on an SOI wafer.The images of the beam focuses can be clearly observed at about 170 ?m from the transmission surface.4.A convex cylindrical lens,a concave cylindrical mirror,and a cylindrical beam splitter based on two-dimensional subwavelength gratings are proposed.In the theoretical analysis,a grating convex cylindrical lens and a grating concave cylindrical mirror with a focal length of 6 ?m are designed,the simulated focal lengths are 5.85 ?m and 5.6 ?m respectively,the FWHMs of the light field intensity of the linear convergent spots are 0.82 ?m and 1.08 ?m,respectively.In practical use,a grating convex cylindrical lens with a period of 6?m and a focal length of 250 ?m,and an area of 400 ?m×400 ?m is fabricated on an SOI wafer.The far-field image of transmitted light at 600 ?m is measured,and a strip convergent spot is observed.The FWHMs of spot intensity in two orthogonal directions are 250 ?m and 680 ?m respectively.When the polarization of the incident light is changed,the normalized intensity of the strip spot remains the same,which indicates that the cylindrical lens based on the two-dimensional subwavelength grating has low polarization sensitivity.Meanwhile,a 1×2 cylindrical beam splitting lens is fabricated and its diffraction characteristics are tested.5.A beam deflector based on the two-dimensional subwavelength grating is proposed.In the theoretical analysis,a grating beam deflector with an area of 7.8 ?m×7.8 ?m,the two deflection angle components are?=30°(the angle between the projection of the transmitted beam in the grating plane and the x-direction)and ?=30°(the angle between the transmitted beam and the z-axis)respectively is designed,the simulated two deflection angle components are 31.4 ° and 29.5 ° respectively.In practical use,a grating beam deflector with an area of 400 ?m×400?m,both deflection angle components are 30 ° is fabricated,the measurement results show that the two deflection angle components are ?=29.5 ° and?=29.6 ° respectively,and the accurate deflection control of parallel beam is realized.6.In cooperation with others,the three elements and four elements symmetrical connected uni-traveling-carrier photodiode arrays integrated with subwavelength grating beam splitters are proposed and realized.The maximum RF output powers as high as 11.5 dBm@15 GHz and 13.1 dBm@15 GHz are achieved for them respectively,while the saturation photocurrents are 70 mA@15 GHz and 91 mA@15 GHz respectively at a-2 V bias voltage.Compared with the single uni-traveling-carrier photodiode with the same structure,the saturation characteristics of the integrated devices are greatly improved.7.A house-shaped optical resonant cavity composed of nonparallel reflectors is proposed.The resonance conditions of incident beams in different regions are analyzed.The simulation results show that the linewidth of the TE20,1 mode is less than 0.008 nm,and the quality factor is not less than 1.938×105,while the inclination angle of the top mirror is 1°,and the height and width of the cavity are 4.468 ?m and 14.976?m respectively.Compared with the parallel planar cavity with the same size parameters,light beams can be confined in a smaller region drastically in the house-shaped resonant cavity,which is characteristic of smaller linewidth,higher quality factor,and smaller mode volume.Besides,a conical optical resonator is proposed and its resonant characteristics are analyzed theoretically.