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.
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.
Terahertz imaging technology has been widely used in various fields. In continuous-wave terahertz imaging system, when the large size object is located at the unfocused position, Bessel beam with non-diffractive properties show its large depth of focus advantage over Gaussian beam. Bessel beam can be generated by the axicon, which has high conversion efficiency. The non-diffraction distance and the main lobe size of the Bessel beam depend on the parameters of the axicon and incident light wavelength. We analyzed that the influence on the axial two-dimensional intensity distribution of a zero-order Bessel beam by changing the axicon parameters and the incident Gaussian beam size. Experimentally, the axicon with different parameters were fabricated using different materials. Then the two-dimensional intensity distribution of the Bessel beam in the axial and transverse direction were recorded and analyzed. The experimental results is basically consistent with the theoretical ones.
Terahertz (THz) radiation is able to penetrate many different types of nonpolar and nonmetallic materials without the damaging effects of x-rays. THz technology can be combined with computed tomography (CT) to form THz CT, which is an effective imaging method that is used to visualize the internal structure of a three-dimensional sample as cross-sectional images. Here, we reported an application of THz as the radiation source in CT imaging by replacing the x-rays. In this method, the sample cross section is scanned in all translation and rotation directions. Then, the projection data are reconstructed using a tomographic reconstruction algorithm. Two-dimensional (2-D) cross-sectional images of the chicken ulna were obtained through the continuous-wave (CW) THz CT system. Given by the difference of the THz absorption of different substances, the compact bone and spongy bone inside the chicken ulna are structurally distinguishable in the 2-D cross-sectional images. Using the filtered back projection algorithm, we reconstructed the projection data of the chicken ulna at different projection angle intervals and found that the artifacts and noise in the images are strikingly increased when the projection angle intervals become larger, reflected by the blurred boundary of the compact bone. The quality and fidelity of the 2-D cross-sectional images could be substantially improved by reducing the projection angle intervals. Our experimental data demonstrated a feasible application of the CW THz CT system in biological imaging.
Terahertz frequency range spans from 0.1 to 10 THz. Terahertz radiation can penetrate nonpolar materials and nonmetallic materials, such as plastics, wood, and clothes. Then the feature makes the terahertz imaging have important research value. Terahertz computed tomography makes use of the penetrability of terahertz radiation and obtains three-dimensional object projection data. In the paper, continuous-wave terahertz computed tomography with a pyroelectric array detectoris presented. Compared with scanning terahertz computed tomography, a pyroelectric array detector can obtain a large number of projection data in a short time, as the acquisition mode of the array pyroelectric detector omit the projection process on the vertical and horizontal direction. With the two-dimensional cross-sectional images of the object are obtained by the filtered back projection algorithm. The two side distance of the straw wall account for 80 pixels, so it multiplied by the pixel size is equal to the diameter of the straw about 6.4 mm. Compared with the actual diameter of the straw, the relative error is 6%. In order to reconstruct the three-dimensional internal structure image of the straw, the y direction range from 70 to 150 are selected on the array pyroelectric detector and are reconstructed by the filtered back projection algorithm. As the pixel size is 80 μm, the height of three-dimensional internal structure image of the straw is 6.48 mm. The presented system can rapidly reconstruct the three-dimensional object by using a pyroelectric array detector and explores the feasibility of on non-destructive evaluation and security testing.
Terahertz (THz) continuous-wave digital holography is an advanced interference imaging technique that can reconstruct quantitative distributions of amplitude and phase of the sample in real time with high resolution. In this paper, a reflective off-axis holographic system is presented. A Gaussian fitting method is applied to enhance the hologram contrast and Laplacian of Gaussian filter is used to obtain the reconstructed distance automatically. Furthermore, spectrum filtering method and angular spectrum algorithm are used to obtain the complex amplitude of the one-yuan chinese metal coin. The results confirm the prospective application of terahertz digital holography in the surface morphology for reflective samples.
Terahertz computed tomography makes use of the penetrability of terahertz radiation and obtains three-dimensional (3-D) object projection data. Continuous-wave terahertz digital holographic tomography with a pyroelectric array detector is presented. Compared with scanning terahertz computed tomography, a pyroelectric array detector can obtain a large quantity of projection data in a short time. To obtain a 3-D image, in-line digital holograms of the object are recorded from various directions and reconstructed to obtain two-dimensional (2-D) projection data; then 2-D cross-sectional images and 3-D images of the internal structure of the object are obtained by the filtered back projection algorithm. The presented system can rapidly reconstruct the 3-D object and reveals the internal 3-D structure of the object. A 3-D reconstruction of a polyethylene straw is presented with a 6% error in retrieved diameter.
Lightwave, as a kind of electromagnetic wave, possesses a frequency much higher than the response frequency of the current photodetectors, so the complex amplitude of the lightwave cannot be measured directly due to the loss of the phase information. Some invisible optical concepts such as phase, wavefront etc. are difficult to understand for the undergraduate students. To make the wavefront of the light wave visible, a digital holographic teaching apparatus, as a kind of comprehensive experimental platform, is developed by our teachers and students under the support of Daheng Science & Technology company. Digital holography (DH) can retrieve the quantitative amplitude and phase information of the object wavefront, which makes the wavefront display possible. In the teaching apparatus, the digital hologram is recorded by the off-axis lensless Fourier transform (LFT) digital holographic system, and the reconstruction of the hologram is achieved by the inverse Fourier Transform. The system is controlled by the Labview software. Except the quantitative amplitude and phase information, the spectrum of the hologram in the spatial-frequency domain etc. can also be visualized in the user interface of the apparatus. This can help students intuitively comprehend many optical content including phase, optical interference, diffraction, optical Fourier transform and diffraction propagation calculation et al., which has the characteristics of being open, flexible and systematic. Furthermore, the analytical and manipulative ability of students can be effectively advanced, and students can also generally understand the actual demand, work pattern and product sales of the company.
Digital holography is the product of the optical holography, computer technology and photoelectric detection technology, and has the advantage of high-speed, real-time, full field of view, non-contact and quantitative phase contrast imaging. However, the numerical aperture of the hologram is limited due to the smaller sensitive area of the photoelectric sensor and the larger pixel size, and it is uneasy to meet the practical requirement on the imaging resolution. An approach is presented to achieve the high-resolution digital holographic imaging based on a spatial light modulator(SLM). An amplitude spatial light modulator is placed between the object and the CCD in the lensless Fourier transform digital holographic imaging system. The distribution of a diffraction grating is loaded into the SLM. In this way, more light including the high-frequency content, diffracted from the object, can be collected by the CCD. The standard resolution target is used as the object. The reconstructed image is obtained by the Fresnel diffraction propagation algorithm, which exhibits three diffraction orders. The results show that the resolution is improved from 62.5 μm to 31.3 μm.
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