Spatially integrated speckle intensity: maximum resistance to decorrelation caused by in-plane target displacement

Laser speckle produced from a diffuse object can be used in determining the angular position of a rotating object. When the object rotates the backscattered speckle pattern, which changes continuously but repeats exactly with every revolution, is sampled by a suitably positioned photodetector. The photodetector output signal is periodic, and one period is stored in the memory as a reference. Shaft position can then be determined by the comparison of this stored reference signal with the current photodetector output signal. When the shaft is axially displaced, for example, by vibration, the backscattered speckle pattern changes on the photodetector and the similarity between the reference signal and the current signal is reduced. We examine the cross correlation of the real-time photodetector output signal and the stored reference signal as a function of axial shaft position. Use of a rotating shaft when collecting data is shown to be an efficient means by which to make effectively several thousand independent estimates of the maximum axial displacement tolerable before decorrelation of the photodetector output. Theoretical results and experiments conducted show that the decorrelation displacement varies, according to optical configuration, to a maximum value of 0.7 of the beam diameter. This has important implications for a proposed laser torquemeter as well as additional applications in which changes to the sampled speckle pattern, including decorrelation, are either desirable or undesirable.     

Zonal wavefront sensor with reduced number of rows in the detector array

In this paper, we describe a zonal wavefront sensor in which the photodetector array can have a smaller number of rows. The test wavefront is incident on a two-dimensional array of diffraction gratings followed by a single focusing lens. The periodicity and the orientation of the grating rulings of each grating can be chosen such that the +1 order beam from the gratings forms an array of focal spots in the detector plane. We show that by using a square array of zones, it is possible to generate an array of +1 order focal spots having a smaller number of rows, thus reducing the height of the required detector array. The phase profile of the test wavefront can be estimated by measuring the displacements of the +1 order focal spots for the test wavefront relative to the +1 order focal spots for a plane reference wavefront. The narrower width of the photodetector array can offer several advantages, such as a faster frame rate of the wavefront sensor, a reduced amount of cross talk between the nearby detector zones, and a decrease in the maximum thermal noise. We also present experimental results of a proof-of-concept experimental arrangement using the proposed wavefront sensing scheme.     

Tailoring the Performances of Lead Halide Perovskite Devices with Electron‐Beam Irradiation

Lead halide perovskites are intensively studied in past few years due to their potential applications in optoelectronic devices such as solar cells, photodetectors, light‐emitting diodes (LED), and lasers. In addition to the rapid developments in material synthesis and device fabrication, it is also very interesting to postsynthetically control the optical properties with external irradiations. Here, the influences of very low energy (10–20 keV) electron beam of standard electron beam lithography are experimentally explored on the properties of lead halide perovskites. It is confirmed that the radiolysis process also happens and it can selectively change the photoluminescence, enabling the direct formation of nanolaser array, microsized light emitter array, and micropictures with an electron beam writer. Interestingly, it is found that discontinuous metallic lead layers are formed on the top and bottom surfaces of perovskite microplate during the radiolysis process, which can act as carrier conducting layers and significantly increase the photocurrent of perovskite photodetector by a factor of 217%. By using the electron beam with low energy to modify the perovskite, this method promises to shape the emission patterns for micro‐LED with well‐preserved optical properties and improves the photocurrent of photodetector.     

Multipoint laser Doppler vibrometry with single detector: principles, implementations, and signal analyses

A 20-point laser Doppler vibrometer with single photodetector is presented for noncontact dynamic measurement. A 5×4 beam array with various frequency shifts is generated by a 1.55 μm distributed feedback laser and four acousto-optic devices, and illuminating different points on vibrating objects. The reflected beams are coupled into a single-mode fiber by a pigtailed collimator and interfere with a reference beam. The signal output from a high-speed photodetector is amplified and then digitized by a high-speed analog-to-digital converter with a sampling rate of 1 gigasample per second (1 GS/s). Several methods are introduced to avoid the cross talk among different frequencies and extract the vibration information of 20 points from a one-dimensional signal. Two signal processing algorithms based on Fourier transform and windowed Fourier transform are illustrated to extract the vibration signals at different points. The experimental results are compared with that from a commercial single-point laser vibrometer. The results show simultaneous vibration measurement can be realized on multiple points using a single laser source and a single photodetector.     

Camera influence on the phase-measurement accuracy of a phase-shifting speckle interferometer

In a real phase-shifting interferometer the camera (i.e., a photodetector plus an analog-to-digital converter) cuts off intensities above some saturation level and provides a limited number of digitization steps. Owing to the intensity statistics of speckle fields, this might severely influence the accuracy of the calculated speckle phase. The optimum beam ratio and the modulation of the camera are computed. To calculate the standard deviation of the phase difference, first, we derive a relation that shows that the variances of the two measured phase frames are equal and that they must be added with the decorrelation-dependent variance. To obtain the minimum phase-measurement error of 25.1 mrad, it is found that the mean speckle intensity ought to be adjusted to be 0.058 times the saturation intensity of the camera and that the beam ratio is to be 5.7. The results are confirmed by computer simulation of a two-wavelength speckle interferometer.     

Measurement of aerosol-particle trajectories using a structured laser beam

What is believed to be a new concept for the measurement of micrometer-sized particle trajectories in an inlet air stream is introduced. The technique uses a light source and a mask to generate a spatial pattern of light within a volume in space. Particles traverse the illumination volume and elastically scatter light to a photodetector where the signal is recorded in time. The detected scattering waveform is decoded to find the particle trajectory. A design is presented for the structured laser beam, and the accuracy of the technique in determining particle position is demonstrated. It is also demonstrated that the structured laser beam can be used to measure and then correct for the spatially dependent instrument-response function of an optical-scattering-based particle-sizing system for aerosols.     

Optical Heterodyne Polarimeter for Studying Space-and Time-dependent State of Polarization of Light

Abstract

This paper describes an optical heterodyne polarimeter with a photodetector array by which the space- and time-dependent state of polarization (SOP) of light can be determined. Since no optical components for polarization control are used, the time response of the polarimeter is free from such components, but is basically limited to the frequency bandwidth of the photodetector array used. The signal and local oscillator beams are coherently photomixed to generate a beat photocurrent at every pixel of the photodetector array. The orthogonal linearly polarized two-frequency components of the local oscillator beam are superimposed with their respective counterpart orthogonally decomposed components of the elliptically polarized signal beam. The generated beat-photocurrent offers the significant physical parameters required for the determination of the space- and time-dependent SOP. The performance principle of the polarimeter is explained and confirmed experimentally.     

Turning angle sensor based on a periodic optical raster and photodetector matrix in full-frame mode

A new principle for the construction of an angle sensor based on a periodic optical raster and a photodetector matrix is proposed. An algorithm for determining the angular position of the axis is considered. Theoretical and experimental estimates of the error in measurements of the angle and a comparison of the estimates are presented.     

Liquid-level sensor with a high-birefringence-fiber loop mirror

A novel liquid-level sensor with a high-birefringence-fiber loop mirror (HBFLM) based on a uniform-strength cantilever beam (UCB) is proposed and demonstrated. Part of the high-birefringence fiber is pasted onto the central surface of the UCB. A hollow suspending pole is utilized to apply force at the end of the beam. The applied force varies with the change of the liquid level, leading to a change of transmission intensity. Thus the variation of liquid level can be determined via the laser wavelength within the quasi-linear transmission range of the HBFLM filter. Its sensitivity, resolution, and linear measurement range reach 0.047/cm, 10 mm, and 140 mm, respectively. The advantages of the sensor include simple structure, high sensitivity, low cost, and good repeatability, etc. The sensing signal can be directly detected by a photodetector and does not require complicated demodulation devices.     

毫米波大规模MIMO系统波束选择方案的设计

针对毫米波大规模MIMO系统采用全数字预编码时,所需射频链路数量过多而导致能量消耗高的问题,提出了一种基于透镜的波束选择方案。该方案首先通过分析用户受干扰的可能性,将所有的用户分为干扰用户组和非干扰用户组,然后对于非干扰用户,直接利用最大功率准则进行波束选取,而对于干扰用户,则通过低复杂度增量算法选择合适的波束使系统和速率最大化。仿真结果表明,在有效减少系统所需射频链路数量和降低计算复杂度的基础上,该方案的系统和速率能够达到接近全数字预编码方案的水平,并且能够获得更高的能量效率。     

Detection and tracking of the backreflection of potassium dihydrogen phosphate images in the presence or absence of a phase mask

The potassium dihydrogen phosphate (KDP) crystals present in the final optics assembly at the National Ignition Facility (NIF) are used for conversion of an infrared laser light beam into an ultraviolet beam. The conversion is highest for a certain incident angle, the alignment of which is determined from the position of the backreflection beam, which exhibits a distinct characteristic shape. When a phase-plate device is introduced before the final assembly to increase the uniformity of the beam, the backreflection pattern changes drastically. The algorithm that is best for tracking the special-shaped beam is no longer suitable for tracking the phase-modified beam. We discuss our detection schemes for both situations. In particular, we demonstrate how the algorithm senses the modified beam by using a newly proposed criterion of correlation peak pedestal area and executes an alternate algorithm in real time without operator intervention. This new algorithm continuously tracks the beam pattern to guarantee reliable and repeatable sensing. Results from simulation and real-world implementation of the algorithm at the NIF facility are presented.     

Information-theoretic analyses of a birefringent blur filter

We present the results of an information-theoretic analysis of an undersampled imaging system that contains a birefringent blur filter. We vary the spacing between replicas of the nonblurred point-spread function produced by the birefringent blur filter to find the value that yields the maximum information density. Comparison is made between this value and that yielded by a conventional qualitative design. We then analyze the effects of defocus aberration on the design of the birefringent blur filter. A wide search of blur-defocus space yields local peaks in the information density surface. The information density at these peaks is shown to exceed the maximum information density associated with the optimized birefringent blur filter or defocus alone, particularly when the fill factor of the photodetector array is low.     

Vector characterization of photodetectors, photoreceivers, andoptical pulse sources by time-domain pulse response measurements

Deconvolution of measurement system effects from pulse response measurements is demonstrated to yield reproducible, accurate characterization of the impulse response (and vector frequency response) of a photodetector or photoreceiver, as well as the intensity waveform of an optical pulse. Calibration is based on a <3-ps FWHM (full width at half maximum) optical pulse and a 50-GHz 3-dB bandwidth electrical sampling system. Vector characterization of a photodetector/photoreceiver to >40 GHz and an optical pulse source to >30 GHz are demonstrated. Calibration and effects of noise are discussed     

Spatial and spectral response of a Fabry-Perot interferometer illuminated by a Gaussian beam

A generalized study has been done of the transmission characteristics of a Fabry-Perot interferometer (FPI) illuminated by a Gaussian light beam impinging on it at normal and non-normal incidence. The theoretical approach is based on a plane-wave, angular-spectrum representation of both the incident Gaussian beam and the transmitted beam. Expressions are obtained for the FPI instrumental function and for the spatial distribution of the transmitted beam. Numerical results are presented for the FPI maximum transmission, effective finesse, and spectral displacement of the interference maximum.     

Rapid unbiased bipolar incoherent calculator cube

Presented in this paper is one of several possible electrooptical engagement array architectures for performing matrix-matrix multiplication using incoherent light. Essential components of this new signal-processing device include two dynamic light valves operating in a reflection mode, a 2-D photodetector array, and a single polarizing beam splitter.     

Radiant-flux integrating circuit for the discrimination of an infrared photodetector signal

A new principle is proposed for the design of an infrared photodetector signal discrimination circuit that operates by integrating the radiant flux and utilizes a pn junction fieldeffect transistor. A characteristic feature of the circuit is the capability of reducing the background signal to negligible levels. A calculation shows that a dopedsilicon IR photoresistor can be combined with the proposed signal discrimination circuit to obtain high photosensitivities. Experimental data confirm this conclusion and demonstrate the indicated feasibility of controlling the background signal. Moreover, the results of experiments show that the proposed IR photodetector model can also be used to achieve high detectivities, including situations where it operates in the integration mode and where it is irradiated by a beam of 4-MeV electrons at fluences up to 2.10^–15 cm^–.Translated from Izmeritel'naya Tekhnika, No. 10, pp. 23–26, October, 1994.     

Near-infrared photodetection with a diruthenium complex having redox-switchable wavelength response

We report on the electrochromic behavior of a dinuclear ruthenium complex, which provides high environmental stability together with an intense absorption maximum at 1237 nm in solution in its +3 oxidation state. To verify its optoelectronic properties, we used this complex as the photoactive material in a planar photodetector prototype for near-infrared light pulses, succeeding in the detection of a train of light pulses at a wavelength of 1280 nm.     

Accuracy of optical angle estimator operating through the turbulent atmosphere

Performance limitations of optical detector arrays (ODA) are evaluated taking into account background radiation, quantum noise, and internal noises of the photodetector as well as the impact of the atmosphere on the optical wave propagation. It is shown that the accuracy of the ODA-based angle estimator rapidly decreases when the range exceeds a certain limit. Thus, it is possible to define the maximum range of the ODA. Behavior of the maximum range as a function of wavelength and of target radiation energy is investigated. Large maximum ranges are obtained at important wavelengths of 0.63, 3.83, and 10.6 microm. It is shown that the ODA performance is limited by the quantum noise for the small-target radiation energy and by the atmospheric turbulence and attenuation for the large-target radiation energy.