Research On Grating Beam Shaper For Photodetector In Optical Communication System

With the continuous expansion of optical communication system scale,the continuous improvement of transmission rate and the continuous increase of communication capacity,photodetector(PD)with high quantum efficiency and high response bandwidth has become the focus of research.Researchers have proposed many schemes to improve the performance of photodetector.For example,the bandwidth of photodetector can be improved by reducing the area of the absorption region and extending the length of depletion layer to reduce the capacitance or reducing the thickness of the absorption layer to shorten the carrier transport time.The results show that the distribution of incident light field will affect the distribution of internal electric field and carrier of the photodetector,thus affecting the performance of the photodetector.In order to improve the performance of PD,the distribution of incident light field will be changed by using beam shaping technology,and the expected distribution of light field will be obtained.In this dissertation,a novel beam shaper based on sub-wavelength grating is studied,and the main innovations and research results are as follows:1.The related theories of photodetector and beam shaping are studied,and the basic theories and characteristics of sub-wavelength grating,such as wave-front phase modulation and broadband high transmittance,are analyzed emphatically.The design method of grating beam shaper is summarized.2.A beam shaper structure of the bar periodic diffraction grating with beam flattening function is proposed.At 1550 nm wavelength,a flat top spot with a diameter of 5.1?m can be obtained by using Gaussian light with a waist radius of 4.2?m as the light source.The transmittance is over 98%,and it is insensitive to polarized light.The simulation results show that the diameter of flat top spot on the output plane is proportional to the period of the diffraction grating.3.The beam shaper structure of the bar aperiodic sub-wavelength grating with focusing function is designed.At 1550nm wavelength,TM polarized light is used as the light source and incident vertically,the transmittance of the structure is 92%,the focal length is 20?m,the FWHM of the beam in the focal plane is 0.89?m.In the wavelength range of 1500nm to 1700nm,the transmittance is higher than 80%and the spot size in the focal plane is almost unchanged.4.Based on the corresponding relationship between ring sub-wavelength grating and bar sub-wavelength grating,a concentric ring sub-wavelength grating beam shaper with converging function is designed.At 1550nm wavelength,with radially polarized light as light source and incident vertically,the transmittance of the structure is 74.5%,the focal length is 20?m,the FWHM of the beam in the focal plane is 2.15?m.5.A two-dimensional block aperiodic sub-wavelength grating beam shaper with converging function is designed to solve the problem of polarization sensitivity.At 1550nm wavelength,the transmittance of the structure is 89.35%,the focal length is 5.8?m,the FWHM of the beam in the focal plane is 1.23 ?m.In the wavelength range of 1500nm to 1900nm,the structure can achieve better beam convergence and beam shaping effect.6.The experimental scheme of PD performance under different incident light distribution is designed,and the experimental test system of PD performance under different incident light distribution is built together with others.When the reverse bias voltage is 3V and the incident power is 0.61mW,the 3dB bandwidth of PD is about 11.29GHz and 10GHz at vertical and oblique incidence,respectively,which is 12.9%higher than that at oblique incidence.