Two-dimensional Photodetector Array Research Articles

Ultrafast measurement of vector spatial modes by using two-dimensional linear optical sampling.

To the best of the authors' knowledge, we present the first-ever demonstration of the two-dimensional linear optical sampling of fiber spatial modes. The images of the fiber cross sections excited by the LP01 or LP11 modes are directly projected onto a two-dimensional photodetector array and coherently sampled by local pulses with a uniform spatial distribution. Consequently, the spatiotemporal complex amplitude of the fiber mode is observed with a time resolution of a few picoseconds by using electronics with a bandwidth of only a few MHz. Such ultrafast and direct observation of vector spatial modes enables the characterization of the space-division multiplexing fiber itself with high time accuracy and wide bandwidth.

Ultra-Low Noise and Self-Powered β-Ga2O3 Deep Ultraviolet Photodetector Array with Large Linear Dynamic Range

Self-powered ultraviolet (UV) photodetectors are the need of the hour for the current technology to harvest the charge carriers without power consumption. β-Ga2O3 deep UV photodetectors working at zero bias have been investigated here for potential applications in high-definition imaging. The photodetector exhibits an ultralow dark current of 8.6 ± 3.4 fA at zero bias. The initial results of low frequency noise measurements are reported for β-Ga2O3 deep UV photodetectors. An ultralow noise current of 9.1 × 10–16 A/Hz1/2 at 1 Hz is obtained in the self-powered condition. Such an ultralow noise current suggests the potential of this device in detecting very weak optical signals. The thermal noise current limited transport mechanism is found to dominate in comparison to shot noise and flicker noise in the self-powered mode. The linear dynamic range (LDR) of 88.5 dB is achieved which is very useful for high-resolution imaging. The dark current, noise floor, and LDR of the photodetector are the benchmark for β-Ga2O3 self-powered deep UV photodetectors. A 3 × 4 two-dimensional photodetector array with uniform dark and photocurrent across all of the pixels is also demonstrated. The outcomes of the present work are encouraging for imaging applications of β-Ga2O3 based energy-efficient deep UV photodetectors.

A Phase-Retrieving Coherent Receiver Based on Two-Dimensional Photodetector Array

To achieve an ultra-compact and flexible coherent transceiver solution, a carrier-less coherent detection technique using a two-dimensional (2-D) photodetector array (PDA) is proposed based on the idea of phase retrieval (PR). In the proposed PR-based coherent receiver, the optical phase information is digitally reconstructed from multiple intensity-only measurements acquired by arrayed photodetectors; neither local light sources, optical hybrids, nor spatial demultiplexers are needed. We develop a robust and low-complexity PR algorithm, which exploits prior knowledge on the optical modulation format, for the proposed PR receiver. As a proof-of-concept, the coherent detection of 10-Gbaud optical phase-modulated signals, including QPSK, 16-QAM, coherent optical OFDM, and polarization-multiplexed QPSK signals, using only a 2-D PDA is demonstrated experimentally.

25-Gbaud 4-WDM Free-Space Optical Communication Using High-Speed 2-D Photodetector Array

We present a newly developed two-dimensional photodetector array (2D-PDA) device and its application for short-range free-space optical (FSO) communication using a wavelength-division multiplexed beam. The 2D-PDA enables direct optical coupling to the FSO beam, simplifying the optical alignment process needed for 10-μm single-mode fiber coupling and reducing the need for beam-tracking function. In the wavelength-division multiplexing (WDM) FSO communication demonstration, a four-wavelength 25-Gbaud parallel beam filtered by a 4-WDM filter through free space was successfully received by 4-pixels in the 2D-PDA. This paper discusses the design and fabrication in the 2D-PDA device, as well as the FSO communication demonstration using four combined WDM filters.

High Speed 2-D Photodetector Array for Space and Mode-Division Multiplexing Fiber Communications

We report a high-data-rate two-dimensional photodetector array (2D-PDA) operated over 10 GHz for compact receiver systems in multicore fiber (MCF), few-mode fiber (FMF), and FMF-MCF, which can be directly coupled to multiparallel beams from the MCF and FMF. The 2D-PDA (pixel size of 20–30 μm, pixel number of 32) were designed and characterized for the frequency response and crosstalk. In the on-wafer probing test, the averaged 3-dB bandwidth of 11.2 GHz and 19.8 GHz were measured for 30-μm and 20-μm pixels, respectively. In the MCF direct-coupling demonstration with the 2D-PDA, the selected 16 pixels could work well at 25 Gb/s (nonreturn-to-zero: NRZ). By using triangular-shape-aligned seven-core fiber and the square-shape-aligned seven cores in the 2D-PDA, the coupling efficiency and alignment tolerance were investigated for the universal MCF photoreceiver. In the demonstration of the mode-division multiplexing signal detection with 2D-PDA, the multiplexed three-mode signal with 20-Gbps quadrature phase-shift keying was directly coupled to the 2D-PDA. Through the electrical phase matching system with the multiple-input multiple-output function, the three-mode signal (20 Gbps in each mode) could be successfully detected and separated error-free.

Image Sensors Meet LEDs

A new class of visible light communication (VLC) systems, namely image sensor (IS) based VLC systems, has emerged. An IS consists of a two-dimensional (2D) array of photodetectors (PDs), and then VLC systems with an IS receiver are capable of exploiting the spatial dimensions invoked for transmitting information. This paper aims for providing a brief survey of topics related to the IS-based VLC, and then provides a matrix representation of how to map a series of one dimensional (1D) symbols onto a set of 2D symbols for efficiently exploit the associate grade of freedom offered by 2D VLC systems. As an example, the matrix representation is applied to the symbol mapping of layered space-time coding (L-STC), which is presented to enlarge the coverage of IS-based VLC that is limited by pixel resolution of ISs.

Analysis on 3D object measurement based on fringe projection

A coordinate measuring method is presented, which is specially devised to perform the measurement of coordinates with projected fringe techniques of projectors in three dimensions. The system is composed of two parts: one is a target which can move freely in three dimensions, and the other is a stationary two-dimensional array of photodetectors. The mini-projector is tied to the target, and the projected fringe is monitored by the photodetectors. The phase of the photodetectors can be precisely measured with the phase-shifting algorithm, so that, the xyz location of the target can be determined with the geometric model of multilateration using the method of optimization. In this paper, the measuring principle, iterative method, computer simulation and preliminary results are given. The phase-shifting technique has the advantages of high accuracy and noise endurance. The method will provide the basis for follow-up iterative optimization calculation. The experimental results prove that the proposed coordinate measuring method is of high precision.

Faraday current sensor using space-variant analyzers

A Faraday current sensor using spatially inhomogeneous analyzers is presented. In the proposed optical architecture, the light waves coming from the sensor head travel in free space reaching a space-variant analyzer, and then they are acquired by a (one-or two-dimensional) photodetector array. This allows a full determination of the Faraday rotation induced by the electrical current, even in the case of a nonideal optical system (e.g., analyzer with poor polarization ratio). The proposed method resembles well-known techniques of phase-shifting interferometry. Validation experiments using linearly variant and azimuthal (radial) analyzers are presented and different detection schemes are compared.

Near-infrared camera in polycrystalline germanium integrated on complementary-metal-oxide semiconductor electronics

The authors demonstrate a two-dimensional array of Si-integrated photodetectors equipped with readout electronics and operating in the near infrared. The chip is realized in polycrystalline Ge on a silicon complementary metal oxide semiconductor circuitry and includes 512pixels, 64 analog to digital converters, dark current cancellation, and test/calibration facilities. They describe its design, fabrication, characterization, and operation as a near-infrared image sensor.

멀티레터레이션법을 이용한 절대간섭계의 점광원 위치 결정법

A CCD camera is used to determine the position of the point sources that influence the measurement accuracy in the absolute interferometer. The principle of determination method is based on the GPS in which the position is determined by more than 3 distance information from the known positions. Two-dimensional array of photo-detectors in the CCD camera is used as known positions. Performing optimization of the cost function constructed with phase values measured at each pixel on the CCD camera, the position coordinates of each source is precisely determined.

거친 표면 형상측정을 위한 점광원 절대간섭계의 오차해석과 시스템 변수의 보

An absolute interferometer system with multiple point-sources is devised for tile 3-D measurement of rough surface profiles. The positions of the point sources are determined to be the system parameters that influence the measurement accuracy, so they are calibrated precisely prior to performing actual measurements. For the calibration, a CCD camera composed of a two-dimensional array of photo-detectors was used. Performing optimization of the cost function constructed with phase values measured at each pixel on the CCD camera, the position coordinates of each point source is precisely determined. Measurement results after calibration performed for the warpage inspection of chip scale packages (CSPs) demonstrate that the maximum discrepancy is 9.8 mm with a standard deviation o( 1.5 mm in comparison with the test results obtained by using a Form Taly Surf instrument.

High-performance nonscanning Fourier-transform spectrometer that uses a Wollaston prism array.

A high-performance nonscanning Fourier-transform spectrometer is reported that is composed mainly of a Wollaston prism array and a two-dimensional photodetector array. It is a substantial improvement over existing Wollaston prism based nonscanning Fourier-transform spectrometers because it offers finer spectral resolution and smaller size. Such spectrometers will find important applications in remote chemical and biological sensing, environmental monitoring, medical diagnosis, etc. Experimental results are consistent with theoretical analyses.

Volumetric phase-measuring interferometer for three-dimensional coordinate metrology

We present a volumetric interferometer that has been specially designed to improve the measurement accuracy of the three-dimensional coordinate metrology by reducing excessive Abbe offsets. The interferometer is composed of two main parts; one is a three-dimensionally movable target and the other is a stationary two-dimensional array of photo-detectors. The target made of point diffraction sources emanates two spherical wavefronts, of which interference intensity is monitored by the photo-detectors. Applying phase shifting allows the phases of the photo-detectors to be precisely obtained, and the measured phases are fitted to a geometric model of multilateration so as to determine the xyz-location of the target by minimizing least square errors. A prototype built on a CMM demonstrates that the proposed interferometer is capable of measuring the xyz-coordinates of the probe with a volumetric uncertainty of less than 1.0 μm.

Absolute distance measurement by two-point-diffraction interferometry.

We present a point-diffraction interferometer that has been specially devised to perform absolute distance measurements in three dimensions. It is composed of two main parts: One is a target that moves in three dimensions, and the other is a stationary two-dimensional array of photodetectors. The target is made of point-diffraction sources that emit two spherical wave fronts, whose interference is monitored by the photodetectors. Application of a phase-shifting technique allows the phase values of the photodetectors to be precisely measured, which are then fitted to a geometric model of multilateration so as to determine the xyz location of the target by minimization of least-squares errors. Experimental results show that the proposed diffraction interferometer is capable of measuring the xyz coordinates of the target with a volumetric uncertainty of less than 1.0 microm over a working volume of a 100-mm side.

A light-transmitting two-dimensional photodetector array using a-Si pin photodiodes and poly-Si TFT's integrated on a transparent substrate

A light-transmitting two-dimensional photodetector array (32/spl times/32 cells) using amorphous silicon (a-Si) pin photodiodes and polysilicon (poly-Si) thin film transistors (TFT's) integrated on a transparent substrate has been developed for use in a free-space optical switching network. The fabrication and the characteristics of the photodetector array are discussed. With driving circuits and sensing amplifiers, this photodetector array shows a minimum detectable power of /spl minus/25 dBm and an insertion loss of 0.4 dB for an incident optical beam with a diameter of 550 /spl mu/m. By monitoring the positions and the states of input optical beams, this photodetector array can be used to control the optical paths in photonic switching systems, such as a 1024-input-port optical concentrator. >

Patterned wafer inspection using laser holography and spatial frequency filtering

An automatic patterned wafer inspection system for use in integrated circuit fabrication is described. The system is based on the techniques of optical spatial frequency filtering and laser holography. The system can locate defects as small as 0.5 μm over a 150 mm patterned silicon wafer in ∼30 min. The wafer under inspection is placed in front of a lens and is flood illuminated through beam splitting optics with a collimated laser beam. The light reflected and diffracted from the wafer is collected by the lens through the beam splitter. In the back focal plane of the lens, the Fourier transform (spatial frequency) pattern of the wafer circuit is formed. A spatial filter placed in this plane effectively blocks the transmission of the light from the repetitive circuit features of all dies of the wafer. The wave front transmitted through the filter is then recorded in a hologram. The conjugate to the recorded wave front is later reconstructed from the hologram. The reconstructed wave front thus reverse ray traces the filter and the lens, forming a real image of the wafer. In this image, the defects are greatly intensified relative to the filtered circuit features. An analysis of the optical wave fronts in the system is provided. The defect information is obtained by scanning a two-dimensional photodetector array through the holographic image and recording the size and location of each point of light above a predetermined threshold. Test wafers containing electron beam programmed defects in a trilevel resist structure were prepared and inspected. Performance results are described.

The retina as a two-dimensional detector array in the context of color vision theories and signal detection theory

A comprehensive discussion on the human retina and its color-mechanism topography as a two-dimensional photodetector array being the first phase of the human visual sensory processor is presented. The treatment is a logical deduction from optimum statistical detection theory and its application to biological sensory systems. The evolution of color vision and of the application of statistical communications theory to sensory systems and the issues common to both that merge them into a unified research thrust in vision are briefly reviewed. The basic problems of retinal topography and available data are surveyed in detail to provide the constraints for the model. The model performance is confronted with actual visual performance data, particularly threshold and discrimination data. It is concluded that the theory of statistical communications and associated techniques of signal detectability, combined with the existing empirical findings provide a most powerful means to investigate sensory communication. As argued in earlier researches of sensory communications based on the same approach, a different new concept is defined for the sensory system and the various issues raised in the problem formulation.