The infrared digital Fourier transform spectrometer has several advantages, including small size, light weight, high stability, increased throughput, and enhanced spectral resolution, making it a valuable tool in the biomedical field. The data acquired by this instrument directly is interference data, and the required spectral data is obtained through a spectral recovery algorithm. The reconstruct spectral is determined by the acquired data quality and spectral recovery algorithm. However, the type of silicon photonics-based Fourier transform infrared spectrometers often encounter non-uniform optical path differences in collected interference data, due to the limitations in hardware design and manufacturing processes, leading to the spectral obtained by commonly used spectrum reconstruction methods inaccuracies. In this paper, a spectral recovery algorithm based on deep neural networks is proposed for the reconstruction of spectra from non-uniformly sampled interference data, Compared to other spectral recovery methods, the proposed method achieves better spectral angle (SA) and relative quality error (RQE) between the reconstructed spectra and ideal spectra.
In order to obtain a high-resolution image and realize the all-day imaging simultaneously, a visible and long-wave infrared dual band imaging detector is proposed and designed, by a meta surface structure through which the infrared light has been absorbed and the visible light passed. With a monolithic chip, the visible light of 400 nm to 900 nm and the long-wave infrared light of 8 μm to 14 μm have been detected, which have the apparent size and weight advantages. This validates the improved integration of structure and results in considerable reduction of the complexity of the imaging system, which is advantageous to target recognition in complex environment.
Polarization detection in the Medium Wave Infrared (MWIR) region presents broad applications in environmental monitoring, aerial reconnaissance and military target spying. Here, we have fabricated aluminum gratings with 1000 nm and 400 nm pitch by Electron Beam Lithography (EBL) on Si substrate with double sides antireflection coating. The polarization property is characterized by the transmittance and extinction ratio in medium wave infrared measured by the FTIR-based test system. Both Transverse Electric (TE) wave transmittances of Al wire-grid polarizers remain at low value indicating that the TE wave is effectively blocked by the designed grating. The Transverse Magnetic (TM) wave transmittance of Al grating with 400 nm pitch in medium wave infrared augments significantly leading to the increase of extinction ratio and the better polarization performance compared with that of the Al grating with 1000 nm pitch. The average TM transmittance of Al polarization grating with 400 nm pitch exceeds 80% in medium wave infrared which can meet the requirements of polarization detection integrated into medium wave infrared detectors. As the polarization angle of incident light rotates, the transmittances of both Al gratings are of remarkably periodic, which follows Malus’ law. The extinction ratios of Al grating with 400 nm pitch and 1000 nm pitch realize 219:1 and 90:1 at 4.5 μm, respectively. Al wire-grid polarizers manufactured in this paper can be a promising candidate for optical system applications, especially in the field of medium wave infrared detectors.
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.
The graphene-based silicon modulator can benefit from the outstanding optical and electrical properties of graphene which makes this kind of modulator a potential candidate for applications such as optical communication and optical interconnect which have strict performance requirements, like broadband operation, low optical loss, low-cost and high speed. In this work, a graphene-based high speed electro-absorption modulator is constructed. Firstly, a typical graphene-oxide-silicon (GOS) structure modulator with one-layer graphene is designed and simulated. The mode field distribution and single mode loss is simulated. Secondly, in order to improve the performance of the graphene-based modulator, the structure of the modulator is optimized. Instead of one-layer graphene, two layers are used. By adjusting the position of graphene in the structure of modulator, a graphene-oxide-graphene (GOG) structure modulator is designed. The structures of graphene layers on top of the silicon-based waveguide and graphene layers sandwiched between two silicon-based waveguides are designed, and the latter one is chosen because it has the advantage of stronger interaction between graphene and light field. By simulation, the extinction ratio of this proposed modulator is as high as 15 dB, and the modulation depth is about 93.3%. The operation bandwidth is calculated to be 52 GHz with low insertion loss of 1.55 dB.
A new on-chip Fourier transform spectrometer has been developed for spectrum analysis application. This spectrometer, based on a thermally tuned Mach-Zehnder Interferometer(MZI) with silicon photonics technology, is small size, light weight and low power consumption. Experimental data have acquired in the O-band domain and the processing chain to convert raw data to spectral data is described. These experimental results show that the spectral resolution is close to the one expected, but also that the signal to noise ratio is limited by various factors. We discuss the origin of those limitations and suggest solutions to circumvent them.
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.
The optical on-chip integration technology can be used to realize spectrum analysis. Compared with the traditional bulky high performance optical spectrum analyzer which is always used in laboratory, on-chip spectrometer has the advantages of chip-scaled, low-cost and suitable for detection in complicated environment. For example, it can be utilized for detection of toxic gases such as carbon monoxide and hydrogen fluoride. In the past decade, several kinds of on-chip spectrometer, dispersive spectrometer and Fourier transform spectrometers, have been demonstrated as promising candidates for wide range of spectral application. In this paper, a digital Fourier transform spectrometer based on an interferometer whose arms consist of several optical switches which are connected by Mach-Zechnder Interferomter (MZI) with different arm lengths is demonstrated. The optical switch based on thermo-optic effect is used to select the different arms of an MZI, so the combination of optical switches and MZIs can lead to a series of different optical path difference. The proposed on-chip spectrometer is designed for Original-band (O-band) and the prospective spectral resolution is 0.3 nm. The future effort will focus on the test of the proposed spectrometer and the followed data processing including dispersive compensation and spectral reconstruction.
A sensitive structure with built-in T-shaped beams was studied in the paper to achieve high fill-factor and high sensitivity for Micro-Optical-Electro-Mechanical-Systems (MOEMS) application. The silicon proof mass in the structure was supported by four identical T-shaped beams, which were distributed symmetrically and orthogonally in the plane to suppress the in-plane cross coupling. In particular, the T-shaped beam was composed of three parts: stress releasing structure, cantilever and flexible linking structure. The stress releasing structure was used to avoid torsion or warpage caused by residual stress and improve the sensitivity at the same time. The mechanical properties were studied systematically by finite element simulation. The stiffness in z-axis direction was much lower than the in-plane stiffness of the structure, indicating high z-axis sensitivity and small cross coupling error. The reason for high sensitivity of the sensitive structures was fully illustrated by comparing the displacement responses for different beams. The simulation results indicated that the sensitivity was improved more than twice because the stress releasing structure and flexible linking structure reduced the axial stress caused by deflection. Finally, the optical performances were also evaluated in terms of bandwidth and tuning range when used for MEMS Fabry Perot Optical Tunable Filter. The wavelength tuning range achieved about 1.8μm in long-wave infrared waveband by controlling the applied voltage.
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%.
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.
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.
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