With the development of laser technology, microwave photonic technology and optical communication technology, the frequency modulated continuous wave (FMCW) Light Detection and Ranging (LIDAR) has received more and more attention from scientific researchers. The main components of this technology include a laser emitting module, receiving optical system, detection module and digital information processing system. Here, we report the miniaturized graded-index (GRIN) lens fiber array used in FMCW LIDAR. The GRIN lens is a radial gradient index lens with the advantages of short focal length and large numerical aperture. Therefore, we used the Zemax software to design a GRIN lens with a large field of view (FOV) and high transmittance, and its FOV is 2°. In order to improve the FOV of the optical receiving system, the 2×8 GRIN lenses fiber array is fixed based on the compound eye arrangement, and the FOV can be increased to 4°×16°. The GRIN lenses fiber array and the chip of FMCW LIDAR are combined through the optical packaging. The experimental data demonstrated the distance measurement function of the device has realized.
We present the demonstration of an integrated Frequency Modulated Continuous Wave (FMCW) coherent solid-state LIDAR (Light detection and ranging) on a silicon platform. The grating coupler array, the multimode interferometer (MMI) and the balanced detector array are implemented on one chip. The silicon-based grating coupler array receives the signal light and couples it into the silicon-based waveguide. Then the signal light is coherently beaten against the local light in the MMI, whose two outputs with 180° phase difference are detected by the balanced germanium (Ge) photodetector array. An external readout circuit composed of transimpedance amplifiers (TIAs) and bandpass filters is used to convert the photocurrent to voltage, from which the measured distance can be obtained through fast Fourier transform (FFT) and spectrum analysis. Here, on-chip space distance measurement was performed within the eye-safe 1550 nm band. Our prototype, fabricated entirely in a 300 mm wafer facility, has the advantages of low-cost, high integration and performance, which may enable extensive application of LIDARs in consumer products, such as selfdriving cars, drones, and robots.
KEYWORDS: LIDAR, Signal processing, Photodetectors, Silicon photonics, Signal to noise ratio, Waveguides, Signal detection, Sensors, Solid state electronics, Silicon
We present a high SNR signal processing system for coherent solid-state LIDAR. A receiving frequency processing system is initially developed. In this LIDAR system, a frequency-modulated continuous wave (FMCW) laser is used as the transmitter, and balanced detectors array based on silicon photonic technology is used as the laser echo receiver. The receiving processing system includes multi-channel low-noise transimpedance amplifier, band-pass filter, high resolution ADC and output buffers. Based on the signal processing system, frequency signal processing with a high signal-to-noise ratio is realized, and the distance detection is realized to confirm on-chip balanced-photodetector-based coherent ranging. The system can be integrated by CMOS technology in the future and realizing three-dimensional integration through through-silicon-via (TSV) with the silicon photonic chip to get low integration complexity, low power consumption, low optical loss, and large array integration.
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.
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.
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%.
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.
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