This paper presents the fabrication of a tunable Fabry-Perot (F-P) filter array using microelectromechanical systems (MEMS) technology for infrared multispectral imaging and infrared communication. The primary focus of the study is on the design and fabrication process of the F-P filter structure, resulting in the successful creation of a tunable 12×14 MEMS F-P filter array. The experimental findings demonstrate a tunable spectral range spanning from 3.78 μm to 4.89 μm, with a maximum wavelength tuning amplitude of 1.11 μm and a maximum spectral transmittance of 72%. The fabrication procedure demonstrates compatibility with the current CMOS process, hence enabling the incorporation of a tunable MEMS F-P filter module via MOEMS-CMOS heterogeneous integration technology. The MEMS F-P tunable filter, functioning as a dispersive component, exhibits promising potential for various applications such as wavelength division multiplexing, multispectral imaging, and gas sensing spectroscopy.
The tunability of optical transmittance spectra can be available by mounting one of the mirrors of the Fabry-Perrot cavity on a movable structure. The F-P filter prepared by adopting MEMS process can realize the advantages of miniaturization, array, and high output. The size of the MEMS F-P filter can be reduced to a few hundred micrometers. This feature introduces a new problem for the characterization of optical performance, that is, the incident light needs to be focused onto the mirror with a size of a few hundred micrometers. However, in the actual test, the incident light with a hundred-micron spot is usually a convergent beam with a certain cone angle. It is found that through theoretical analysis, compared to parallel incident light, the convergent light beam passed through the F-P cavity leads to the decrease at peak transmittance and the broadening of full width at half maximum. The reason for that was the converging light with a cone angle passing through the F-P cavity had different incident angles and caused diverse optical path difference. As a result, the light emitting from the cavity with various wavelength would appear in the transmission spectra. In summary, the test results under the converging light could not truly reflect the performance of the F-P cavity and the influence of the cone angle of incident light beam on the performance characterization of MEMS F-P filter was analyzed by theoretical arithmetic and simulation.
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.
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.
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.
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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