Due to the short working wavelength of light detection and ranging (LIDAR), the information of the distance and angular position of the target can be detected more accurately. Therefore, LIDAR has high research significance and wide application prospects in both military and civilian fields. The main components of this technology include the laser emitting module, receiving optical system, detection module and digital information processing system. The receiving optical system is the key factor for the miniaturization of LIDAR. Therefore, we optimized the design and prepared an optical system with a micro-nano structure according to the requirements of the field of view (FOV), focal length and modulation transfer function (MTF). The quality of the micro-nano optical lenses design and preparation directly affects the overall LIDAR system performance. In order to measure and analyze the optical characteristics of the micro-nano optical lenses, a multi-functional optical characteristic testing system is designed and built. The testing system is used to measure and calculate the optical characteristic parameter in the assembled micro-nano optical lenses. Compare the measured value of the optical characteristic parameter with the theoretical value, the measured result meets the design requirements of the micro-nano optical lens. Our experimental data demonstrated the testing system has practical significance for the design, preparation and image quality evaluation of micro-nano optical lenses.
In the silicon photonics field, coupling occupies an important position of propagating the light from the space to the waveguide. There are two normal coupling way. The one is end-face coupling and the other one is surface coupling. And the more popular way is to use the surface coupling, which can be put on anywhere of the chip and is much easier to measure. The specific surface coupling format is grating coupler. Grating coupler can be both input and output coupler and match the fiber to propagate the light from and to the space. However, the one-dimensional grating coupler, used in the most of silicon photonic chips, has polarization selectivity and can only transfer one single mode (TE mode) in the waveguide. That means the half of the light would be wasted during coupling. In order to improve the efficiency of the coupler, two-dimensional grating coupler is a better solution. It has two orthogonal waveguides and propagate the transverse-electric (TE) mode with opposite directions. And the transverse-magnetic (TM) mode is transferred to the TE mode when the light changes the propagating direction. In this paper, the two-dimensional grating coupler is designed to match the light whose wavelength is from 1260 nm to 1290 nm. The calculation and simulation method is finitedifference time domain (FDTD). After modeling and optimizing the structure, the coupling efficiency is 26.8%.
This paper presents a readout integrated circuit (ROIC) for 32×32 single photon avalanche diode (SPAD) array. The ROIC integrates 32×32 active quenching circuit and time-to-digital converter (TDC) circuit. Each ROIC unit has a novel active quenching circuit (AQC) and an in-pixel TDC. The ROIC and the detectors are integrated by Flip-Chip .The novel quenching circuit with active reset function is proposed to reduce the dead time. A dual-counter-based TDC is designed to prevent the metastability of the counter. The sensor is fabricated in 180-nm CMOS BCD technology. The simulation results show the novel active quenching circuit effectively reduces the dead time down to 10 ns. The 13bit-TDC helps the system achieve centimeter-accuracy detection.
Space-chip coupling using silicon photonic grating coupler is of great significance for OPA-based LIDAR (Optical Phased Array, OPA), free-space data communication, and so on. However, Silicon-based grating couplers are commonly used for fiber-chip coupling and space-chip coupling is rarely mentioned. In order to obtain the optimal coupling effect, commercial three-dimensional Finite Difference Time-Domain (3D FDTD) software is employed to simulate the coupling process and analyze the characteristics of spatial light coupling. Because the spot size is in the order of micrometer, we first build a vector beam with three variables of numerical aperture, lens diameter and beam diameter for simulation. Afterwards, the incident location of the spatial light beam, the incident angle and the grating width are scanned to explore the influence of these parameters on coupling efficiency. We have found that the total coupling efficiency changes with grating width exponentially. That is, the total coupling efficiency firstly increases with the grating width, and does not change after reaching the maximum value. However, the coupling efficiency of the fundamental mode decreases gradually after reaching the maximum value. This indicates that higher-order modes are more likely to be excited when the width is greater than the optimized grating width. Besides, the coupling efficiency varies parabolically with the incident angle and location of the spatial light beam. There exists optimal incident angle and location on the parabola symmetry axis to get the maximum coupling efficiency. Furthermore, the best incident position is half of the beam diameter from the beginning of grating coupler.
Transmission characteristics of Fabry-Perot (F-P) filter based on silicon substrate with different transmissivity of high reflection (HR) coatings, incident angle, and interference orders are investigated. The results show that the transmissivity of HR coatings has great effect on full width at half maximum (FWHM) of transmission spectrum, the FWHM of F-P filter reduced from 209 nm to 3.4 nm with the reflectivity of HR coatings increased from 84.7% to 99.6%. The peak wavelength shifts from 1546.3 nm at 0° to 1542.6 nm at 5°, indicating that the FWHM of transmission spectrum broadens as the incident angle increases. The 1st, 2nd, 3rd, and 4th order interference are 3.4 nm, 2.3 nm, 1.8 nm, and 1.5 nm, respectively. Thus, in the applications tuning in a narrow wavelength range, F-P filter can be designed to operate in high-order mode to achieve a narrow transmission spectrum.
A mesa-type normal incidence separate-absorption-charge-multiplication (SACM) Ge0.95Sn0.05/Si avalanche photodiode (APD) was fabricated. The 60-μm-diameter avalanche photodiode achieved a responsivity of ~5A/W (gain=24) and ~3.1A/W (gain=20) at 98% breakdown voltage (-14.2V) under 1310nm and 1550nm illumination respectively with a low dark current of 10μA. The −3 dB bandwidth for a 60-μm-diameter APD is about 1-1.25GHz for gains from 5 to 20, resulting in a gain-bandwidth product of 25GHz for a C-band communication wavelength of 1550nm.
The photonic crystal structure can be utilized for improving the transmission within a broadband, and suppressing the dark current of detector efficiently as well. Considering such an advantage, the study on the multi-level profile photontrapping structure is performed; meanwhile, the enhancement of HgCdTe mid-wavelength infrared detector based on such a structure is analyzed. With the help of FDTD model and FEM model, via optimizing the structure, a multi-level profile photon-trapping detector scheme with a quantum efficiency enhancement of 20% is established. The proposed simulation results and structure are crucial for further acquiring HgCdTe detector with enhanced SNR.
The reflected or radiated electromagnetic wave from natural objects exhibits different polarization characteristics. By detecting the polarization properties of light waves, more information from the target can be obtained, as well as the target recognition capability can be strengthened. In this paper, the research progress of the image processing method for infrared polarization detection is introduced. The traditional extraction methods of polarization components and the theory of polarization information processing are initially demonstrated. The fabricated 128×128 integrated polarization infrared detector, whose extinction ratio is 10:1, is also proposed. By using of the image reconstruction method, polarization images captured from this detector, which can acquire four polarization components simultaneously, are processed. Further, the degree and angle of polarization of natural scene images are obtained as well.