Optical Calibration Research Articles

Beat frequency measurement of the stabilized He-Ne laser 633 nm calibration in SNSU-BSN

In the metrology area, typical stabilized lasers are used as length primary standard. KIM-1, i.e. the iodine stabilized He-Ne laser in National Measurement Standards Laboratory-National Standardization Agency of Indonesia (SNSU-BSN) has been traceable to SI through CCL-K11 inter-laboratory comparison. The result was suitable for KIM-1 to be used as the length primary standard in SNSU-BSN. Beat frequency measurement has been applied in the optical frequency and wavelength calibration system for stabilized He-Ne laser 633 nm using KIM-1 as a reference standard. In the calibration replica, a dual-frequency mode (Agilent 5519B) took a role as DUT laser, which emits a pair of beams with a central wavelength of 632.991 354 nm in the vacuum with frequency difference 3.4 to 4.0 MHz and ±0.02 ppm stability for a typical lifetime. As a measurement result, the beat frequencies of the 1st and 2nd polarized beam of Agilent 5519B against KIM-1 are (121.33±0.06) MHz and (118.59±0.06) MHz.

Phase-locked two-color visible frequency comb system based on 1.5-μm all-polarization-maintaining fiber laser

A two-color visible frequency comb system based on a 1.5-μm all-polarization-maintaining (PM) fiber femtosecond laser was developed. This configuration relies on the implementation of three amplifiers, seeded by a single master oscillator. With the repetition rate (fr) of the oscillator locked to a reference frequency of 200 MHz, the output of the first amplifier was used to generate the feedback signal and achieve simultaneous phase lock of the carrier envelop offset frequency (fceo). The remaining two independently configurable amplifiers followed by highly nonlinear fibers and MgO-doped periodically poled lithium niobite crystals were used to produce visible comb lights at 543 and 633 nm (in air), respectively. By referencing to a hydrogen maser, the Allan deviations of fr and fceo at a gate time of 1 s are 438 μHz and 63 mHz, respectively. The spectral bandwidths of the 543- and 633-nm comb lights are 0.157 and 0.174 nm, respectively, and the single-mode powers of these comb lights are higher than 1 μW. The multiple-branch all-PM fiber-based visible frequency comb system exhibiting a narrow spectrum and a high single-mode power will facilitate the development of optical clocks and wavelength standard calibrations.

Dynamic Crosslinking of the Actin Cytoskeleton Governs Cell Mechanics

Cells precisely control their mechanical properties to organize and differentiate into tissues. The architecture and connectivity of cytoskeletal filaments changes in response to mechanical and biochemical cues, allowing the cell to rapidly tune its mechanics from highly-crosslinked, elastic networks to weakly-crosslinked viscous networks. While the role of actin crosslinking in controlling actin network mechanics is clear in purified actin networks, its mechanical role in the cytoplasm of living cells remains unknown. Here, we probe the frequency- dependent viscoelastic properties of the cytoplasm in living cells using multiharmonic excitation and in situ optical trap calibration. At previously unexplored long timescales, we observe that the cytoskeleton fluidizes. The mechanics are well-captured by a model in which actin filaments are dynamically connected by a single dominant crosslinker. A disease-causing point mutation (K255E) of the actin crosslinker α-actinin 4 (ACTN4) causes its binding kinetics to be insensitive to tension. Under normal conditions, the viscoelastic properties of wild type (WT) and K255E+/- cells are similar. However, when tension is reduced through myosin II inhibition, WT cells relax 3x faster to the fluid-like regime while K255E+/- cells are not affected. These results indicate that dynamic actin crosslinking enables the cytoplasm to flow at long timescales.

Optical test procedure for characterization and calibration of robotic fiber positioners for multiobject spectrographs

The recent burgeoning interest in massive multiobject spectroscopy has pushed the development of massive optical fiber positioning systems. These systems rely on precise fiber placement to detect the light spectra of many stars and galaxies. One successful approach is the use of robotic fiber positioners, which allow one to automate and scale up observations. However, due to the need for high precision and accuracy, each positioner must be calibrated and verified to comply with the requirements. The calibration measurements are nontrivial, and the large number of the robotic positioners up to thousands can lead to a prohibitively long time for calibration. We describe and validate an optical calibration setup and procedure for robotic fiber positioning systems. Based on the measurements results, we have developed models describing the behavior of the positioners and we introduce new performance metrics that allow one to verify the stringent positioner specifications and furthermore help to identify and analyze design and manufacturing flaws.

Underwater Photometry System of the SNO+ Experiment

The SNO+ experiment is a large-scale liquid scintillator-based experiment, adapting the Sudbury Neutrino Observatory (SNO) detector located at SNOLAB, Canada. The main physics goal is to investigate the Majorana nature of neutrinos through the search for the neutrinoless double-beta decay of 130Te. The camera system of SNO+ is designed to photograph calibration sources and triangulate their locations with an accuracy of a couple of centimeters. This will lead to better calibrations and more accurate physics measurements in SNO+. The camera system, when operated in a special mode with underwater lights turned on, also allows monitoring of the physical state of the detector. The optical calibration source was deployed in the water filled SNO+ detector in the summer of 2017. Pictures of the deployed source were taken using the camera system while the underwater lights were turned on. The triangulation analysis of the pictures gave us an opportunity to test the position accuracy of the deployed source in SNO+ using the camera system.

Estimation of parameters from time traces originating from an Ornstein-Uhlenbeck process.

In this article, we develop a Bayesian approach to estimate parameters from time traces that originate from an overdamped Brownian particle in a harmonic potential, or Ornstein-Uhlenbeck process (OU). We show that least-square fitting the autocorrelation function, which is often the standard way of analyzing such data, is significantly underestimating the confidence intervals of the fitted parameters. Here, we develop a rigorous maximum likelihood theory that properly captures the underlying statistics. From the analytic solution, we found that there exists an optimal measurement spacing (Δt=0.7968τ) that maximizes the statistical accuracy of the estimate for the decay-time τ of the process for a fixed number of samples N, which plays a similar role than the Nyquist-Shannon theorem for the OU process. To support our claims, we simulated time series with subsequent application of least-square and our maximum likelihood method. Our results suggest that it is quite dangerous to apply least-squares to autocorrelation functions both in terms of systematic deviations from the true parameter values and an order-of-magnitude underestimation of confidence intervals. To see whether our findings apply to other methods where autocorrelation functions are typically fitted by least-squares, we explored the analysis of membrane fluctuations and fluorescence correlation spectroscopy. In both cases, least-square fits exhibit systematic deviations from the true parameter values and significantly underestimate their confidence intervals. This fact emphasizes the need for the development of proper maximum likelihood approaches for such methods. In summary, our results have strong implications for parameter estimation for processes that result in a single exponential decay in the autocorrelation function. Our analysis can directly be applied to single-component dynamic light scattering experiments or optical trap calibration experiments.

Evaluation of Different Calibration Approaches for Merchantable Volume Predictions of Norway Spruce Using Nonlinear Mixed Effects Model

Research Highlights: Determination of merchantable wood volume is one of the key preconditions for sustainable forest management. This study explores accuracy of calibrated predictions of merchantable wood volume of Norway spruce (Picea abies (L.) H. Karst.) using stem taper curves (STC) in a form of a mixed model. Background and Objectives: The study is devoted to the determination of merchantable wood volume (over bark) of individual standing stems based on the integration of an STC model calibrated using upper diameter measurements. Various options of upper diameter measurement were tested and their impact on the accuracy of merchantable wood volume prediction was evaluated. Materials and Methods: To model stem taper curves, a Kozak 02 function was applied in a form of a nonlinear, mixed effects model. Accuracies of calibrated merchantable wood volume predictions obtained through remote (optical) upper diameter measurements were compared to accuracies corresponding to contact measurements by a caliper. The performance of two alternative methods used in the Czech National Forest Inventory (NFI) and forestry practice, involving diameter at breast height and total tree height as the only predictors, were also tested. The contact measurements were performed at identical stem positions after felling the respective sample tree. The calibration was done in order to account for factors inherent in particular location, and, optionally, also in a particular sample stem (within the respective location). Input data was sourced as part of a dedicated survey involving the entire territory of the Czech Republic. In total, 716 individual spruce trees were measured, felled and analysed at 169 locations. Results: In general, the best merchantable volume predictions were obtained by integrating the STC fitted (and calibrated) by minimising errors of stem cross-sectional areas instead of diameters. In terms of calibrated predictions, using single-directional, caliper measurement of upper diameter at 7 m (after felling) led to the best accuracy. In this case, the observed mean bias of merchantable volume prediction was only 0.63%, indicating underestimation. The best optical calibration strategy involved upper diameter measurements at two heights (5 and 7 m) simultaneously. Bias of this volume prediction approach was estimated at 2.1%, indicating underestimation. Conclusions: Concerning the prediction of merchantable stem volume of standing Norway spruce trees, STC calibration using two optical upper diameter measurements (at 5 and 7 m) was found to be practically applicable, provided a bias up to 3.7% can be accepted. This method was found to be more accurate than the existing national alternatives using diameter at breast height and the total tree height as the only predictors.

A Simply Equipped Fourier Ptychography Platform Based on an Industrial Camera and Telecentric Objective.

Fourier ptychography microscopy (FPM) is a recently emerged computational imaging method, which combines the advantages of synthetic aperture and phase retrieval to achieve super-resolution microscopic imaging. FPM can bypass the diffraction limit of the numerical aperture (NA) system and achieve complex images with wide field of view and high resolution (HR) on the basis of the existing microscopic platform, which has low resolution and wide field of view. Conventional FPM platforms are constructed based on basic microscopic platform and a scientific complementary metal–oxide–semiconductor (sCMOS) camera, which has ultrahigh dynamic range. However, sCMOS, or even the microscopic platform, is too expensive to afford for some researchers. Furthermore, the fixed microscopic platform limits the space for function expansion and system modification. In this work, we present a simply equipped FPM platform based on an industrial camera and telecentric objective, which is much cheaper than sCMOS camera and microscopic platform and has accurate optical calibration. A corresponding algorithm was embedded into a conventional FP framework to overcome the low dynamic range of industrial cameras. Simulation and experimental results showed the feasibility and good performance of the designed FPM platform and algorithms.

RadCalNet: A Radiometric Calibration Network for Earth Observing Imagers Operating in the Visible to Shortwave Infrared Spectral Range

Vicarious calibration approaches using in situ measurements saw first use in the early 1980s and have since improved to keep pace with the evolution of the radiometric requirements of the sensors that are being calibrated. The advantage of in situ measurements for vicarious calibration is that they can be carried out with traceable and quantifiable accuracy, making them ideal for interconsistency studies of on-orbit sensors. The recent development of automated sites to collect the in situ data has led to an increase in the available number of datasets for sensor calibration. The current work describes the Radiometric Calibration Network (RadCalNet) that is an effort to provide automated surface and atmosphere in situ data as part of a network including multiple sites for the purpose of optical imager radiometric calibration in the visible to shortwave infrared spectral range. The key goals of RadCalNet are to standardize protocols for collecting data, process to top-of-atmosphere reflectance, and provide uncertainty budgets for automated sites traceable to the international system of units. RadCalNet is the result of efforts by the RadCalNet Working Group under the umbrella of the Committee on Earth Observation Satellites (CEOS) Working Group on Calibration and Validation (WGCV) and the Infrared Visible Optical Sensors (IVOS). Four radiometric calibration instrumented sites located in the USA, France, China, and Namibia are presented here that were used as initial sites for prototyping and demonstrating RadCalNet. All four sites rely on collection of data for assessing the surface reflectance as well as atmospheric data over that site. The data are converted to top-of-atmosphere reflectance within RadCalNet and provided through a web portal to allow users to either radiometrically calibrate or verify the calibration of their sensors of interest. Top-of-atmosphere reflectance data with associated uncertainties are available at 10 nm intervals over the 400 nm to 1000 nm spectral range at 30 min intervals for a nadir-viewing geometry. An example is shown demonstrating how top-of-atmosphere data from RadCalNet can be used to determine the interconsistency between two sensors.

Signal Processing Considerations on the Optical Measurement of Acoustic Particle Velocities in Free-Field Conditions

To overcome the limitations of existing acoustic standards based on microphones, the optical method based on photon correlation has been considered as a potentially new primary standard. In this article, the experimental measurement and signal processing procedures to measure acoustic pressures optically, thus realizing the acoustic pascal in a direct manner, from captured photon sequences are described. The assessment of specific major contributing parameters associated with the signal processing of the photon correlation method is investigated and discussed in detail for free-field measurements. To extract the information relating to the acoustic particle velocities resulting from captured photon sequences, specific signal processing procedures were investigated. First, the captured electrical pulse sequences are converted to a series of binarized photon event sequences. Next, the sequences are divided into time sets related to the time length of individual acoustic cycles at each frequency step; a gating window equal to a short part of the acoustic cycle itself is applied on the aforementioned time sets. The gating process is performed by shifting the time window through the acoustic cycle, with the autocorrelation function calculated for each phase offset step, which yields the acoustic particle velocity and, hence, pressure directly. Single and double gating are applied and their effects are investigated. In addition, the effect of specific signal processing parameters is studied and selection criteria are proposed. Comparison between optical and reciprocity calibrations in terms of sound pressure and device sensitivity in the frequency range of 0.5–16-kHz range revealed discrepancies within 0.01–1.3 dB.

Calibration of Displacement Laser Interferometer Systems for Industrial Metrology.

Displacement laser interferometer systems are widely used for the calibration of machine tools and CMMs (Coordinate Measuring Machines). Additionally, they are often the workhorse in dimensional calibration laboratories, where they act as the basic metrological traceability link for many calibrations. This paper gives a review of the calibration of such systems, where several approaches, such as the calibrations of separate components or the system as a whole, are reviewed. The calibrations discussed are: the laser frequency, the counting system, software evaluation of the environmental conditions, environmental and material temperature sensor calibration and the calibration of optics that is part of the system. For these calibrations considerations are given about the ways these can be carried out and about establishing the re-calibration intervals.

An Approach to Modeling Closed-Loop Kinematic Chain Mechanisms, Applied to Simulations of the da Vinci Surgical System

Open-sourced kinematic models of the da Vinci Surgical System have previously been developed using serial chains for forward and inverse kinematics. However, these models do not describe the motion of every link in the closed-loop mechanism of the da Vinci manipulators; knowing the kinematics of all components in motion is essential for the foundation of modeling the system dynamics and implementing representative simulations. This paper proposes a modeling method of the closed-loop kinematics, using the existing da Vinci kinematics and an optical motion capture link length calibration. Resulting link lengths and DH parameters are presented and used as the basis for ROS-based simulation models. The models were simulated in RViz visualization simulation and Gazebo dynamics simulation. Additionally, the closed-loop kinematic chain was verified by comparing the remote center of motion location of simulation with the hardware. Furthermore, the dynamic simulation resulted in satisfactory joint stability and performance. All models and simulations are provided as an open-source package.

A calibration method for spatial pose of a laser beam

An articulated laser sensor, which is a new kind of trans-scale, non-contact metrological instrument, is principally made up of two articulated laser sensing modules. Each module is composed of two rotary tables and one collimated laser. Calibrating the spatial pose of a laser beam is a key aspect of the measurement system. In this paper we propose a one-dimensional linear displacement optical calibration device for high stability and easily performed calibration. Through analysis of space vectors and image processing, a calibration method for spatial pose of the laser beam is introduced in detail. The calibration method is divided into two parts: calibration of the direction vector of the laser beam and a fixed point on the laser beam. Firstly, the direction vector of the laser beam is calibrated using the proposed calibration device. Then a set of points on the laser beam is obtained and the linear fitting is performed. By comparing the direction vector of the fitted line and the direction vector of the above laser beam, the optimal fixed point on the fitted line is obtained. The simulation and experimental results show that the proposed calibration method is accurate and effective.

Corrections to “FengYun-3 B Satellite Medium Resolution Spectral Imager Visible On-Board Calibrator Radiometric Output Degradation Analysis” [Jun 19 3240-3251

In the above paper [1] , the name of the institution of the authors Dandan Zhi, Tanqi Yu, and Yan Pan were incorrect, they are with the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Key Laboratory of Optical Calibration and Characterization, Hefei 230031, China, and also with the University of Science and Technology of China, Hefei 230026, China (e-mail: 1416652331@qq.com).

Improving branching fraction calibration methods: The optical to ultraviolet bridge

Herein we report new measurements of key Ar ii branching ratios (BRs) that span the optical to ultraviolet (UV) wavelength region. Our new measurements are the first to use a standard detector, a NIST calibrated Si photodiode, instead of standard sources. The use of a NIST calibrated Si photodiode requires a stable source with sufficient output to yield power measurements with a good signal-to-noise ratio from the NIST calibrated Si photodiode after filters are used to isolate wavelength regions from the source. In this work we used a Hg pen lamp which has good stability, but is a line source instead of a continuum source. The Hg i lines fell near the Ar ii lines of interest and were easy to isolate using filters. A complete calibration of the filter transmission as a function of wavelength is not necessary when a line source is used. The selected Ar ii BRs are the best for testing and/or improving a bridge between an optical calibration based on a tungsten standard lamp and an UV calibration based on a deuterium lamp.

Using Muon Rings for the Calibration of the Cherenkov Telescope Array: A Systematic Review of the Method and Its Potential Accuracy

The analysis of ring images produced by muons in an Imaging Atmospheric Cherenkov Telescope (IACT) provides a powerful and precise method to calibrate the IACT optical throughput and monitor its optical point-spread function (PSF). First proposed by the Whipple collaboration in the early 90's, this method has been refined by the so-called second generation of IACT experiments: H.E.S.S., MAGIC and VERITAS. We review here the progress made with these instruments and investigate the applicability of the method as the primary throughput calibration method for the different telescope types forming the future Cherenkov Telescope Array (CTA). We find several additional systematic effects not yet taken into account by previous authors and propose several new analytical methods to include these in the analysis. Slight modifications in hardware and analysis need to be made to ensure that such a calibration works as accurately as required for the CTA. We derive analytic estimates for the expected muon data rates for optical throughput calibration, camera pixel flat-fielding and monitoring of the optical PSF. The achievable statistical and systematic uncertainties of the method are also assessed.

Lambert Compensation, Nonlinear Estimation, and Control Architecture of a Novel Optical Calibration Instrument

In this paper, an optical calibration instrument is proposed for mass production of irradiation angle sensors. A Lambert-based current compensation principle with nonlinear parameter identification tools are given to improve the precision of sensor-specific characteristic functions through eliminating the adversary effects of indoor light attenuation and manufacturing defects. The newly designed instrument, endowed with the proposed tools, is a smart device that is automatic, fast, efficient, and accurate. Outdoor experimental results show that the instrument calibrates the sensor in a good accuracy up to 0.23° at the presence of 16° manufacture errors of inclines. Therefore, the instrument is promising in mass production in terms of intelligent manufacture process.

Micro-optical vibrometer/accelerometer using dielectric microspheres.

In this paper, an optical vibrometer/accelerometer is designed using micro-optical dielectric sensors based on whispering gallery modes (WGM) phenomena. The proposed design is created as a sphere made from polydimethylsiloxane (PDMS). The dielectric sensor would be laid between a piezo stack and a proof mass of 10-4 kg (implemented as body force to the sensor) in the vertical position. Once the piezo stack starts to impose a vibration in the vertical direction, the proof mass starts to vibrate, leading to a change in the morphology of the sensor. In turn, the WGM shifts would be seen on the transmission spectrum and would be related to the vibration/acceleration of the piezo stack. An analysis based on the force transmissibility from the piezo stack to the optical sensor and calibrations are carried out along with preliminary designs and experiments. Results showed that the proposed sensor could reach a very high resolution up to 8nano-g with sensitivity dλ/da≈2.33 pm/μg.

Improved colour-modelled CH* and C2* measurement using a digital colour camera

The present paper proposes an improved colour-modelled CH* and C2* measurement using a digital colour camera. A complete measurement description has been given. As an important part of the method, the camera sensor spectral response has been obtained through the optical calibration process. With this spectral response, an improved model that pivots on the relationship between image colour and flame chemiluminescence has been established, and its accuracy has been evaluated by determining CH*/C2* ratios of the premixed propane (C3H8) flames at different equivalence ratios (Φ = 0.93 − 1.53). The two sets of results rendered by the improved and the conventional methods have been compared rigorously.

Improving characterization capabilities in new single-photon avalanche diode research.

Many novel and promising single-photon avalanche diodes (SPADs) emerged in recent years. However, some of them may demonstrate a very high dark count rate, even tens of megahertz, especially during the development phase or at room temperature, posing new challenges to device characterization. Gating operation with a width of 10 ns can be used to suppress the dark counts not coincident with the photon arriving time. However, as a side effect of the fast-gating operation, the gating response could be much higher than the avalanche signal and is usually removed by various circuit-based cancellation techniques. Here, we present an alternative method. A high-speed digital storage oscilloscope (DSO) is used to extract the weak avalanche signals from the large gating response background by waveform subtraction in software. Consequently, no complex circuit and precise tuning for each SPAD are needed. The avalanche detection threshold can be reduced to 5% of the full vertical scale of the DSO or 5 mV, whichever is greater. The timing resolution can be better than 2 ps for typical avalanche signals. Optical alignment and calibration are easy. The feasibility of on-wafer test with an RF probe station is discussed. All the advantages and features listed above make this method very useful in new SPAD research.