Retro Reflector Research Articles

Optical interferometry implementation considerations for sinusoidal low frequency accelerometer measurements and calibrations

Vibration metrology utilizing sine excitation stimulus, based either on fringe counting or minimum point methods, employs Michelson interferometers for the calibration of accelerometers. The former method combines the optical reference and measurements arms from a static mirror and reflected by the accelerometer respectively which are then directed to a suitable photodiode for processing. The stroking motion due to the oscillation of the accelerometer will contribute to the misalignment of the two optical paths and there are three methods to compensate for this. The first requires the mounting of a mirror on the accelerometer itself and the effect of the added mass from the mirror can be accounted for and corrected. The second requires the mounting of three mirrors on the mounting surface of the accelerometer but this requires the alignment of the Michelson interferometer for each mirror. The third is to use two retro reflectors so that the returning measurement beams are always along the same path with the reference; however, this requires the alignment of the interferometer for each retro-reflector. This paper will discuss such considerations on the optical systems, their components and associated instrumentation and discuss the design of an alternative approach that can overcome certain limitations.

Design and manufacturing of an optimized retro reflective marker for photogrammetric pose estimation in ITER

Retro reflective markers can remarkably aid photogrammetry tasks in challenging visual environments. They have been demonstrated to be key enablers of pose estimation for remote handling in ITER. However, the strict requirements of the ITER environment have previously markedly constrained the design of such elements and limited their performance. In this work, we identify several retro reflector designs based on the cat’s eye principle that are applicable to the ITER usecase and propose a methodology for optimizing their performance. We circumvent some of the environmental constraints by changing the curvature radius and distance to the reflective surface. We model, manufacture and test a marker that fulfils all the application requirements while achieving a performance increase of at least 50% over the previous solution in the targeted working range.

Open-Path Atmospheric Transmission of Diode-Pumped Alkali Lasers in Maritime and Desert Environments.

A tunable diode laser absorption spectroscopy (TDLAS) device has been developed to study long-path atmospheric transmission near diode pumped alkali laser (DPAL) emission wavelengths. By employing a single aperture and retro reflector in a mono-static configuration, the noise associated with atmospheric and platform jitter were reduced by a factor of ∼30 and the open-air path length was extended to 4.4 km and over a very broad spectral range, up to 120 cm-1. Water vapor absorption lines near the rubidium (Rb) and cesium (Cs) variants of the DPAL near 795 and 894 nm, oxygen lines near the potassium (K) DPAL near 770 nm, and water vapor absorption in the vicinity of the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser 1.064 μm and chemical oxygen iodine laser (COIL) 1.3 μm lines were studied. The detection limit for path absorbance increases from ΔA = 0.0017 at 100 m path length to 0.085 for the 4.4 km path. Comparison with meteorological instruments for maritime and desert environments yields agreement for the 2.032 km path to within 1.5% for temperature, 4.5% for pressure, and 5.1% for concentration, while agreements for the 4.4 km path are within 1.4% for temperature, 7.7% for pressure, and 23.5% for concentration. An intra cavity output spectroscopy (ICOS) device was also used as a spectral reference to verify location of atmospheric lines. Implications of TDLAS collection system design on signal-to-noise (S/N) are discussed as well as the effect of path turbulence on baseline noise and inform the selection of the DPAL variant least affected by molecular absorption.

Position estimation using neural networks in semi-monolithic PET detectors

Objective. The goal of this work is to experimentally compare the 3D spatial and energy resolution of a semi-monolithic detector suitable for total-body positron emission tomography (TB-PET) scanners using different surface crystal treatments and silicon photomultiplier (SiPM) models. Approach. An array of 1 × 8 lutetium yttrium oxyorthosilicate (LYSO) slabs of 25.8 × 3.1 × 20 mm3 separated with Enhanced Specular Reflector (ESR) was coupled to an array of 8 × 8 SiPMs. Three different treatments for the crystal were evaluated: ESR + RR + B, with lateral faces black (B) painted and a retroreflector (RR) layer added to the top face; ESR + RR, with lateral faces covered with ESR and a RR layer on the top face and; All ESR, with lateral and top sides with ESR. Additionally, two SiPM array models from Hamamatsu Photonics belonging to the series S13361-3050AE-08 (S13) and S14161-3050AS-08 (S14) have been compared. Coincidence data was experimentally acquired using a 22Na point source, a pinhole collimator, a reference detector and moving the detector under study in 1 mm steps in the x- and DOI- directions. The spatial performance was evaluated by implementing a neural network (NN) technique for the impact position estimation in the x- (monolithic) and DOI directions. Results. Energy resolution values of 16 ± 1%, 11 ± 1%, 16 ± 1%, 15 ± 1%, and 13 ± 1% were obtained for the S1 3-ESR + B + RR, S1 3-All ESR, S14-ESR + B + RR, S14-ESR + RR, and S14-All ESR, respectively. Regarding positioning accuracy, mean average error of 1.1 ± 0.5, 1.3 ± 0.5 and 1.3 ± 0.5 were estimated for the x- direction and 1.7 ± 0.8, 2.0 ± 0.9 and 2.2 ± 1.0 for the DOI- direction, for the ESR + B + RR, ESR + RR and All ESR cases, respectively, regardless of the SiPM model. Significance. Overall, the obtained results show that the proposed semi-monolithic detectors are good candidates for building TB-PET scanners.

Effects of source spatial partial coherence on intensity statistics of optical beams in mono-static turbulent channels.

We investigate, via both experimental measurements and wave-optics computer simulations, the statistical characteristics of the fluctuating intensity, such as the average intensity, the beam wander and the scintillation index, of the Gaussian Schell-model (GSM) beams on passing through double-pass, monostatic turbulence channels with either a retro-reflector (RR) or a flat mirror (FM). Our experimental results reveal that the enhanced backscatter (EBS) gradually weakens as the spatial coherence of the GSM source decreases, and eventually disappears for the sufficiently low source spatial coherence states. The r.m.s beam wander remains practically invariant with the variation of the source coherence width in the range from 0.2 to 6.0 mm both in the case of the RR and the FM, which formed the RR case being much smaller. In addition, it is found that the long-term scintillation index of the untracked beam with the RR is smaller than that with the FM, while the situation is reversed for the short-term scintillation index of the tracked beam. In both cases, the scintillation index decreases as the spatial coherence of the GSM source decreases. The obtained computer simulation results agree reasonably well with the experimental results. In addition, the effects of spatial coherence on statistical characteristics of the GSM beams along a 1 km propagation distance through the double-pass monostatic turbulence are also investigated using wave-optics simulation. We also carry out evaluation and comparison of the intensity probability density functions for the RR and FM cases and for various source coherence states that are of utmost importance for free-space optical communications in retro-reflection modulation regime. In addition, our findings will be beneficial for the development of remote sensing and directed energy laser applications in the presence of air turbulence.

The satellite distance radiometer with a passive retroreflection of a request radio signal

In prospect projects of the lunar information and navigation support system, the using of a satellite range finder for solving the problem of ballistic support of near-moon spacecrafts is considered. In this paper we consider a mathematical model for calculating the radio line of a satellite rangefinder with passive retro reflection of the requested radio signal from selenodetic points. The model allows the calculation of the signal-to-noise ratio in the reflected radio signal and the standard deviation of the determination of the distance from the spacecraft to the passive radio retroreflector under various conditions. The calculations take into account both the reflection of the signal from the radio retroreflector and from the surrounding surface lunar soil. Various types of radio retroreflectors in the form of combinations of corner reflectors and in the form of a Luneburg lens are considered. The calculations of the radio line using the indicated types of retro radio reflectors for different values of the orbit height, signal frequency and other parameters are performed. It is shown that, at certain values of the parameters of the radio line, the standard deviation of the determination of the range of the order of units of meters and less is achieved both for the Luneburg lens and the combined angular reflector. The final choice should be made taking into account their mass, as well as the possibilities of transforming the radio retroreflector from transportation to working condition.

Calibration of Gamma Ray Impacts in Monolithic-Based Detectors Using Voronoi Diagrams

Molecular imaging systems, such as positron emission tomography (PET), use detectors providing energy and a 3-D interaction position of a gamma ray within a scintillation block. Monolithic crystals are becoming an alternative to crystal arrays in PET. However, calibration processes are required to correct for nonuniformities, mainly produced by the truncation of the scintillation light distribution at the edges. We propose a calibration method based on the Voronoi diagrams. We have used $50 \times 50 \times 15$ mm3 LYSO blocks coupled to a $12\times 12$ SiPMs array. We have first studied two different interpolation algorithms: 1) weighted average method (WAM) and 2) natural neighbor (NN). We have compared them with an existing calibration based on 1-D monomials. Here, the crystal was laterally black painted and a retroreflector (RR) layer added to the entrance face. The NN exhibited the best results in terms of XY impact position, depth of Interaction, and energy, allowing us to calibrate the whole scintillation volume. Later, the NN interpolation has been tested against different crystal surface treatments, allowing always to correct edge effects. Best energy resolutions were observed when using the reflective layers (12%–14%). However, better linearity was observed with the treatments using black paint. In particular, we obtained the best overall performance when lateral black paint is combined with the RR.

Optimization of FSO quantum backscatter communication.

In this paper, the free-space optics (FSO) quantum backscatter communication (QBC), inspired by the quantum illumination concept, is described. In this method, the transmitter generates entangled photon pairs. The signal photon is transmitted in the direction of the modulated retro reflector (MRR) or the tag antenna, and the idler photon is moved to the receiver. The MRR communicates by modulating the received photon and retro-reflecting it back to the receiver. QBC could provide performance improvement in comparison to conventional backscatter communications. In this work, a mathematical model of the FSO QBC systems is derived. In an FSO system, the pointing direction jitter is a stochastic process that reduces the communication performance. In this paper, optimization of FSO QBC performance is proposed to minimize the error probability. This could be done by adapting the telescope gain to jitter a standard deviation amplitude.

Comparison of slant open-path flux gradient and static closed chamber techniques to measure soil N<sub>2</sub>O emissions

Abstract. Improving direct field measurement techniques to quantify gas emissions from cropped agricultural fields is challenging. We compared nitrous oxide (N2O) emissions measured with static closed chambers to those from a newly developed aerodynamic flux gradient (FG) approach. Measurements were made at a vegetable farm following chicken manure application. The FG calculations were made with a single open-path Fourier transform infrared (OP-FTIR) spectrometer (height of 1.45 m) deployed in a slant-path configuration, sequentially aimed at retro reflectors at heights of 0.8 and 1.8 m above ground. Hourly emissions were measured with the FG technique, but once a day between 10:00 and 13:00 with chambers. We compared the concurrent emission ratios (FG∕chamber) of these two techniques and found N2O emission rates from a celery crop farm measured at midday by FG were statistically higher (1.22–1.40 times) than those from the chambers measured at the same time. Our results suggest the OP-FTIR slant-path FG configuration worked well in this study: it was sufficiently sensitive to detect the N2O gradients over our site, giving high temporal resolution N2O emissions corresponding to a large measurement footprint.

A Six-Axis Heterodyne Interferometer System for the Joule Balance

A six-axis heterodyne interferometer system has been designed for the joule balance to measure the relative displacement between the suspended coil and the exciting coil. The essential optical components in this system are three dual-axis heterodyne interferometers. Each interferometer can simultaneously measure the displacements of one retro-reflector (RR) on the suspended coil and one RR on the exciting coil. Because of the common path structure, this interferometer is susceptible to periodic nonlinearity caused by double-frequency mixing. However, a method can be used to eliminate the periodic errors. The measured displacements can be strictly locked to integral multiples of half the wavelength. This method was theoretically analyzed and finally verified by the experiment.

Performance Study of a Large Monolithic LYSO PET Detector With Accurate Photon DOI Using Retroreflector Layers

Clinical and organ-dedicated PET systems typically require a high efficiency imposing the use of thick scintillators, normally through crystal arrays. To provide depth of interaction (DOI) information, two or more layers are sometimes mounted in the staggered or phoswich approach. In this paper, we are proposing an alternative using thick and large monolithic crystals. We have tested two surface treatments for a 50 mm $\times50$ mm $\times20$ mm LYSO block. We provide data in this paper as close as 5 mm to the lateral walls. We left those walls black painted and the exit face coupled to the photosensor ( $12\times12$ SiPM array) polished. The entrance face was: 1) black painted or 2) coupled to a retroreflector (RR) layer. These configurations keep a good DOI linearity and, on average, reached 4 mm DOI resolution, measured as the full width at half of the maximum. Approaches using RR layers return constant and good energy resolutions nearing 12%, compared to a range of 15%–16% in the case of totally black painted blocks. The best result concerning the detector spatial resolution was obtained when one of the smallest RR was used (120 $\mu {\mathrm{ m}}$ corner cube size), being 1.7 mm at the entrance crystal layer and 0.7 mm in the layer closest to the photosensor. These values worsen at least 30% for the black treatment case.

Deep turbulence effects mitigation with coherent combining of 21 laser beams over 7 km.

We demonstrate coherent beam combining and adaptive mitigation of atmospheric turbulence effects over 7km under strong scintillation conditions using a coherent fiber array laser transmitter operating in a target-in-the-loop setting. The transmitter system is composed of a densely packed array of 21 fiber collimators with integrated capabilities for piston, tip, and tilt control of the outgoing beams wavefront phases. A small cat's-eye retro reflector was used for evaluation of beam combining and turbulence compensation performance at the target plane, and to provide the feedback signal for control of piston and tip/tilt phases of the transmitted beams using the stochastic parallel gradient descent maximization of the power-in-the-bucket metric.

Electromagnetic Embossing of Optical Microstructures

Electromagnetic forming (EMF) is a high-speed forming process that is already established in the macroworld. Due to its advantages like high deformation rate and cheaper tools, it is introduced to microforming. In this research, the replication of prismatic optical microstructures is investigated. EN AW-1050A (Al99.5) micrometal sheets with a thickness of 50 μm and 300 μm are electromagnetically micro-embossed. With this technique, it is possible to successfully replicate triangular cross section micro V-grooves of 86.6 μm in width and 24.1 μm in depth with an average surface roughness of Sa = 44 nm. The microstructures of the embossing tool are generated by diamond micro chiseling (DMC), a novel machining process to produce microstructures with discontinuous geometry, like miniature cube corner retro reflectors and V-grooves with well-defined endings.

Study of retro reflector array for the polarimeter-interferometer system on EAST Tokamak

In this paper, we experimentally verify the feasibility of replacing individual retro reflectors (RRs) with retro reflector array (RRA) in EAST POlarimeter/INTerferometer (POINT) system, by considering mode transformation and power wastage. Being exposed to plasma environment directly, RRs have risks of deformation, erosion and deposition. RRA is preferable because it can be installed within a smaller space and provide a gap of several centimeters for the shutter design. This protective structure can reduce the cost of device maintenance and bring down system errors. According to Helmholtz-Kirchhoff integral theorem, the optimized incident diameter for the RRA, constituted by seven hexagonal RR cells, is 40 mm in POINT system. The corresponding bench tests are carried out by measuring the propagation properties of reflected beams by plane RRA for perpendicular incidence and reflected beams by terrace RRA for oblique incidence. The experimental results illustrate that RRA can be satisfactorily applied in POINT system at the optimized incident diameter. In view of the energy wastage caused by plasma film coating, it is found that RRA has more advantages for diagnostics using shorter wavelengths, such as the case in ITER.

Positioning and precision validation of Chang'E-3 Lander based on multiple LRO NAC images

High precision positioning of Chang'E-3 lunar lander is of great importance for the mission operations and relevant scientific investigations. In this research, we calculate the location of Chang'E-3 lander using Lunar Reconnaissance Orbiter(LRO) narrow angle camera(NAC) images and validate the precision with the Lunar Ranging Retro Reflectors(LRRRs). First, we pinpoint the Chang'E-3 lander and other landmarks(LRRR etc.) on multiple LRO NAC images by manual interpretation and high-precision image matching. Then the geodetic coordinates are derived from the image coordinates with the rigorous sensor models established with the refined orbiter positions and attitudes from NAIF SPICE kernels. Finally, the average geodetic coordinates are calculated and considered as the locations of Chang'E-3 and other landmarks. As a result, the location of Chang'E-3 is determined to be 44.1219?N, 19.5113?W using 14 LRO NAC images. The five LRRRs with high-precision absolute locations available are used as check points to evaluate the poisoning errors. The results show that the location accuracy can reach 20 m with a single LRO NAC image, and positioning the lander using multiple images reduces random errors and provides better accuracy than that using a single image.

GIOVE-B solar radiation pressure modeling for precise orbit determination

Previous studies have identified systematic errors in the orbit and clock estimates of the GIOVE and Galileo IOV satellites in the order of ±20cm. These errors are visible as periodic variations in the Satellite Laser Ranging (SLR) and clock residuals. For IOV, these variations could be attributed to the contribution of a stretched satellite body and it was shown that a simple a priori box model for the solar radiation pressure can significantly reduce these errors. GIOVE-B has similar dimensions as the IOV satellites but its orientation is different: for GIOVE-B the narrow side of the satellite points to the Earth rather than the longitudinal side. In addition, an extra plate carrying, amongst others, the laser retro reflector array is mounted on the spacecraft introducing shadowing effects. These features are considered with a simple box-plate model. This model reduces the periodic clock errors and the SLR residual RMS of GIOVE-B by a factor of two. Most importantly, the box-plate model reduces the SLR offset from 11cm to less than 1cm. The largest part of this reduction comes from considering the plate and its shadowing effects.

1차원적 단순배열구조 재귀반사체의 반사출력광 특성 및 응용

재귀반사체(Retro Reflector)는 그 특별한 기능을 살려 도로 교통안전, 특수 블라인드, 광소자 resonator 등 다양한 분야에서 활용 영역이 넓어져 가고 있다. 특히 근래에는 '재귀반사구조체'를 창문의 차양 및 채광 구조물로서 도입하여 조명을 스마트화하는 경우도 제시되고 있다. 이에 본 연구는, 태양광 조절용 구조물로서, 띠구조를 1차원적으로 단순배열한 '주름판형-재귀반사체', St-RR(Strap type-Retro Reflector) 구조를 정의하고, 입사각도에 따라 그 반사광의 특성을 분석하였다. 또한, 이 특성 결과를 용이하게 파악할 수 있도록 시각화한 특성 도표를 고안하였고, 이를 이용하여 각종 태양광 조절 구조물을 보다 정량적으로 설계할 수 있음을 보였다. Retro-reflectors have been used for a wide range of applications such as traffic safety, special blinds, optical devices, etc. We analyzed the characteristics of the reflected light of a strap retro-reflector as a function of incidence angle. It is expected that various solar control structures may be designed more quantitatively using the characteristic chart we have prepared.

Study of the propagation properties of 432 [formula omitted]m laser reflected by retro reflector array

Some recent researches showed that retro reflector array (RRA) has installation superiorities that single retro reflector (RR) does not possess in the far-infrared (FIR) polarimeter/interferometer (PI) system for Tokamak. As a key precondition for replacing RR with RRA, the reflected light from RRA must change to Gaussian profile to guarantee the detectors get stronger signals and the threshold distance to Gaussian profile should be less than the optical path between the first wall and detectors. In this paper, the propagation properties of the reflected beams from RR and RRAs of different combinations have been measured by a beam profiler. The experimental results show that the reflected beam profiles from RRA of two retro reflectors (RRs) evolve into Gaussian distributions beyond a certain reflected optical distance and the reflected beam profiles from RRA of three RRs have a similar tendency to be Gaussian distributions with increasing reflected optical distance. The theory predictions, deriving from the integral theorem of Helmholtz and Kirchhoff, provide the same evolutions with experiments, but the threshold distances to Gaussian profile existing distinctions. These results suggest that RRA could be applied in the FIR PI system for the future ITER.

Retroreflective imaging system for optical labeling and detection of microorganisms.

A retroreflective imaging system for imaging microscopic targets over macroscopic sampling areas is introduced. Detection of microorganism-bound retroreflector (RR) targets across millimeter-scale samples is implemented according to retroreflection directionality, collimation, and contrast design characteristics. Retroreflection directionality is considered for corner-cube (CC) and spherical geometries. Spherical-RRs improve directionality and reliability. Retroreflection collimation is considered for spherical-RRs. Retroreflective images for micro-CC-RRs and micro-spherical-RRs with varying refractive indices show optimal results for high refractive index BaTiO3 micro-spherical-RRs. A differential imaging technique improves retroreflection contrast by 35 dB. High refractive index micro-spherical-RRs and differential imaging, together, can detect microscopic RR targets across macroscopic areas.

Results of a combined monolithic crystal and an array of ASICs controlled SiPMs

In this work we present the energy and spatial resolutions we have obtained for a γ-ray detector based on a monolithic LYSO crystal coupled to an array of 256 SiPMs. Two crystal configurations of the same trapezoidal shape have been tried. In one approach all surfaces were black painted but the exit one facing the photosensor array which was polished. The other approach included a retroreflector (RR) layer coupled to the entrance face of the crystal powering the amount of transmitted light to the photosensors. Two coupling media between the scintillator and the SiPM array were used, namely direct coupling by means of optical grease and coupling through an array of light guides. Since the same operational voltage was supplied to the entire array, it was needed to equalize their gains before feeding their signals to the Data Acquisition system. Such a job was performed by means of 4 scalable Application Specific Circuits (ASICs). An energy resolution of about 24.4% has been achieved for the direct coupling with the RR layer together with a spatial resolution of approximately 2.9mm at the detector center. With the light guides coupling the effects of image compression at the edges are significantly minimized, but worsening the energy resolution to about 33.1% with a spatial resolution nearing 4mm at the detector center.