Optical Misalignment Research Articles

High-precision optical modeling method for galvanometer-driven dual-camera systems.

To build a galvanometer-driven dual-camera sensing system, it is important to accurately correlate the wide-view image coordinates with the pan-tilt mirror angles for adjusting the incident light path of the zoom-in camera. Existing optical modeling methods assume sufficient target distance and simplify dual-camera optical centers as coincident. However, this simplification is not valid in many practical cases and might cause severe system malfunctions, such as complete loss tracking of important targets. To address this problem, we propose a novel approach, to the best of our knowledge, to facilitate high-precision optical modeling and calibration of galvanometer-driven dual-camera systems. The proposed method takes into consideration the dual-camera optical center misalignment issue and builds a model for accurate estimation and rectification of target localization errors under various optical configurations. Qualitative and quantitative experimental results demonstrate the superiority of our method, improving the performance of galvanometer-driven dual-camera systems for high-precision optical sensing applications.

Desensitization design method of an off-axis three-mirror anastigmatic optical system based on the nodal aberration theory

A design method for an off-axis three-mirror anastigmatic optical system with low misalignment sensitivity is proposed. Based on the transformed pupil coordinate and nodal aberration theory, the analytical expressions between the optical parameters and misalignments are derived and an as-built performance evaluation model is established. The effects of mirror spacings and the off-axis magnitude value on the misalignment sensitivity are analyzed. With the desensitization design method proposed in this paper, a low-sensitivity off-axis TMA optical system can be designed. The simulation experiments show that, the misalignment sensitivity of the off-axis TMA optical system obtained by our method is approximately 60% that of the system obtained by nominal performance optimization method, and the as-built performance is about 1.49∼1.54 times that of the nominal performance optimization method.

The Active Optics System on the Vera C. Rubin Observatory: Optimal Control of Degeneracy among the Large Number of Degrees of Freedom

The Vera C. Rubin Observatory is a unique facility for survey astronomy that will soon be commissioned and begin operations. Crucial to many of its scientific goals is the achievement of sustained high image quality, limited only by the seeing at the site. This will be maintained through an active optics system that controls optical element misalignments and corrects mirror figure error to minimize aberrations caused by both thermal and gravitational effects. However, the large number of adjustment degrees of freedom available on the Rubin Observatory introduces a range of degeneracies, including many that are induced by noise due to imperfect measurement of the wave-front errors. We present a structured methodology for identifying these degeneracies through an analysis of image noise level. We also present a novel scaling strategy based on truncated singular value decomposition that mitigates the degeneracy and optimally distributes the adjustment over the available degrees of freedom. Our approach ensures the attainment of optimal image quality, while avoiding excursions around the noise-induced subspace of degeneracies, marking a significant improvement over the previous techniques adopted for Rubin, which were based on an optimal integral controller. This new approach is likely to also yield significant benefits for all telescopes that incorporate large numbers of degrees of freedom of adjustment.

A Framework for Iterative Phase Retrieval Technique Integration into Atmospheric Adaptive Optics—Part I: Wavefront Sensing in Strong Scintillations

The objective of this study, which is divided into two parts, is twofold: to address long-standing challenges in the sensing of atmospheric turbulence-induced wavefront aberrations under strong scintillation conditions via a comparative analysis of several basic scintillation-resistant wavefront sensing (SR-WFS) architectures and iterative phase retrieval (IPR) techniques (Part I, this paper), and to develop a framework for the potential integration of SR-WFS techniques into practical closed-loop non-astronomical atmospheric adaptive optics (AO) systems (Part II). In this paper, we consider basic SR-WFS mathematical models and phase retrieval algorithms, tradeoffs in sensor design and phase retrieval technique implementation, and methodologies for WFS parameter optimization and performance assessment. The analysis is based on wave-optics numerical simulations imitating realistic turbulence-induced phase aberrations and intensity scintillations, as well as optical field propagation inside the SR-WFSs. Several potential issues important for the practical implementation of SR-WFS and IPR techniques, such as the requirements for phase retrieval computational grid resolution, tolerance with respect to optical element misalignments, and the impact of camera noise and input light non-monochromaticity, are also considered. The results demonstrate that major wavefront sensing requirements desirable for AO operation under strong intensity scintillations can potentially be achieved by transitioning to novel SR-WFS architectures, based on iterative phase retrieval techniques.

Normal-Incidence Germanium Photodetectors Integrated with Polymer Microlenses for Optical Fiber Communication Applications.

We present a novel photon-acid diffusion method to integrate polymer microlenses (MLs) on a four-channel, high-speed photo-receiver consisting of normal-incidence germanium (Ge) p-i-n photodiodes (PDs) fabricated on a 200 mm Si substrate. For a 29 µm diameter PD capped with a 54 µm diameter ML, its dark current, responsivity, 3 dB bandwidth (BW), and effective aperture size at -3 V bias and 850 nm wavelength are measured to be 138 nA, 0.6 A/W, 21.4 GHz, and 54 µm, respectively. The enlarged aperture size significantly decouples the tradeoff between aperture size and BW and enhances the optical fiber misalignment tolerance from ±5 µm to ±15 µm to ease the module packaging precision. The sensitivity of the photo-receiver is measured to be -9.2 dBm at 25.78 Gb/s with a bit error rate of 10-12 using non-return-to-zero (NRZ) transmission. Reliability tests are performed, and the results show that the fabricated Ge PDs integrated with polymer MLs pass the GR-468 reliability assurance standard. The demonstrated photo-receiver, a first of its kind to the best of our knowledge, features decent performance, high yield, high throughput, low cost, and compatibility with complementary metal-oxide-semiconductor (CMOS) fabrication processes, and may be further applied to 400 Gb/s pulse-amplitude modulation four-level (PAM4) communication.

Effects of primary aberration on the spatiotemporal optical vortex focus

A spatiotemporal optical vortex (STOV) with transverse orbital angular momentum has several potential applications. However, refractive index mismatch, beam tilt and optical path misalignment are often inevitable during the application of an optical system. To investigate the focusing field of light pulse, the effects of primary aberrations such as the spherical aberration, coma, and astigmatism were analyzed. The simulation results indicate that three-dimension spatiotemporal distributions of the focusing light pulse are affected by primary aberration. On the principal coordinate planes, coma can distort the intensity structure and shift the STOV focus perpendicular to the propagation direction, while spherical aberration and astigmatism induce the actual STOV focus to shift along the propagation direction. Astigmatism do not affect the intensity structure of spatiotemporal plane without spiral phase, but stretch the 3D STOV focus along one spatial axis. Coma and astigmatism are necessary to be avoided to obtain a perfect STOV focus. It is helpful to improve the applications of STOV focus, such as optical tweezers, microscopy, and communications.

Multi-bounce self-mixing in terahertz metasurface external-cavity lasers

The effects of optical feedback on a terahertz (THz) quantum-cascade metasurface vertical-external-cavity surface-emitting laser (QC-VECSEL) are investigated via self-mixing. A single-mode 2.80 THz QC-VECSEL operating in continuous-wave is subjected to various optical feedback conditions (i.e., feedback strength, round-trip time, and angular misalignment) while variations in its terminal voltage associated with self-mixing are monitored. Due to its large radiating aperture and near-Gaussian beam shape, we find that the QC-VECSEL is strongly susceptible to optical feedback, which is robust against misalignment of external optics. This, in addition to the use of a high-reflectance flat output coupler, results in high feedback levels associated with multiple round-trips within the external cavity-a phenomenon not typically observed for ridge-waveguide QC-lasers. Thus, a new theoretical model is established to describe self-mixing in the QC-VECSEL. The stability of the device under variable optical feedback conditions is also studied. Any mechanical instabilities of the external cavity (such as vibrations of the output coupler), are enhanced due to feedback and result in low-frequency oscillations of the terminal voltage. The work reveals how the self-mixing response differs for the QC-VECSEL architecture, informs other systems in which optical feedback is unavoidable, and paves the way for QC-VECSEL self-mixing applications.

Off-axis telescope misalignment correction based on defocus spot moment features

Optical mirror misalignments, which are caused by assembly mistakes and changes in the surrounding environment (such as gravity, temperature, and atmosphere), degrade the system’s imaging performance. Therefore, active misalignment correction is essential for ensuring the image quality of the off-axis telescope. In this paper, a novel misalignment correction method without wavefront sensors is proposed. The point spread functions (PSFs) of the system are analytically related to the optical mirror misalignments. On this basis, a fully connected neural network (FCNN) is used to establish the mapping relationship between the misalignments and the discrete orthogonal unbiased finite impulse response (UFIR) moment features, which can effectively characterize changes of intensity and geometric of the spot image. The simulation and experimental results in this paper justify the effectiveness and practicality of the proposed method. This approach offers a low-cost and straightforward technical method for achieving high imaging quality throughout the alignment and observation phases. This approach can prevent the accumulation of errors caused by wavefront detection and the high delay of multiple iterations.

Accurate ray-tracing optical model for coded aperture spectral snapshot imagers.

The image formation in coded aperture spectral imagers is key information for processing the acquired compress data, and the optical system design and calibration of these instruments require great care. We propose an analytical model for CASSI systems that builds upon ray-tracing equations of each optical component. The model takes into account optical distortions, sampling effects, and optical misalignments, and allows accurate modeling and fast calibration. Numerical comparisons with a simpler model usually exploited in the literature are provided, and an experimental validation is presented.

Performance analysis of AUV-carried RISs-aided multihop UWOC convergent with RF MRC systems over WGG oceanic turbulence

To establish reliable connection between Internet of Underwater Things (IoUT) devices and terrestrial data centers, this work first proposes a reconfigurable intelligent surface (RIS) aided multihop underwater wireless optical communication (UWOC) convergent with radio frequency (RF) uplink system. Specifically, the RIS carried by an autonomous underwater vehicle (AUV) is introduced into UWOC link to relax the line-of-sight (LOS) requirement and the maximal-ratio combining (MRC) receiver is adopted at the terrestrial data center to mitigate the RF link fading. It is assumed that the underwater thermocline channel is subject to the newly proposed Weibull-generalized gamma (WGG) turbulence distribution and the RF link composite fading follows Fisher-Snedecor F distribution. Additionally, the optical link misalignment is characterized by the zero-boresight pointing errors model. With the decode-and-forward (DF) relaying scheme, the analytical closed-form expressions of the outage probability (OP) and average bit error rate (ABER) of this system are mathematically achieved, and then the impacts of air bubbles, thermohaline gradient, the number of RIS elements, pointing errors, system structure, and the number of receive antennas are further investigated. Meanwhile, the analytical results are verified by Monte Carlo (MC) simulations. Results reveal that this hybrid system performance would degrade with the increased air bubble levels and thermohaline gradients. Notably, RIS can effectively alleviate the impact of underwater turbulence and this effect would be more pronounced as the number of RIS elements increases. This work will benefit the design and research of hybrid UWOC-RF system.

A Generic Error Simulation in Digital Photoelasticity By Jones Calculus

Photoelasticity is a whole field experimental technique to obtain stress fields in both 2-D and 3-D elasticity problems. With advancements in Digital Image Processing (DIP), a separate branch of photoelasticity namely digital photoelasticity came into existence wherein the intensity of the images is used for photoelastic parameter estimation. The accuracy of parameter evaluation by digital photoelastic techniques depends on the choice of the optical arrangements for parameter estimation, the inherent error in optical elements and any misalignments introduced during experimentation. A generic approach to simulation of error in digital photoelasticity by Jones calculus is presented in this paper. The simulation accounts for inherent error in quarter-wave plates and also accommodates the error due to the optical misalignment of the various optical elements. The applicability of the new methodology is shown for the most popular six-step phase shifting technique. The methodology proposed is generic and is applicable to study the influence of error sources on parameter estimation for various techniques proposed in digital photoelasticity.

Measurement of linear shear through optical vortices in digital shearing speckle pattern interferometry.

Digital shearing speckle pattern interferometry (DSSPI) is a powerful interferometric technique used to visualize the slope contours undergoing static and dynamic deformations. Precise determination of the shear amount is crucial for quantitative analysis in DSSPI. However, accurately measuring the shear amount is often challenging due to factors such as optical device dimensions, deflections, aberrations, and misalignments. In this paper, we propose a novel method utilizing optical vortices deflection in pseudo-phase for shear measurement. This method eliminates the need for attaching calibration objects and replacing the light source, making it applicable to inaccessible or fragile samples. Experimental results demonstrate the effectiveness and accuracy of the proposed method in determining shear amounts in DSSPI. The method can be easily automated and integrated into existing setups, offering broader application prospects.

Calibration of the Geometric Error Parameters of Laser Trackers in Site Environments

Laser trackers (LTs) are indispensable instruments in large-scale dimensional metrology. Nevertheless, geometric errors are prone to increase in site environments due to the occurrence of mechanical and optical misalignments, which deteriorates the measurement accuracy of the LT. The geometric error parameters are difficult to determine by traditional measuring or calibrating approaches. In this paper, a classification calibration method is proposed without any other etalons or instruments that accurately estimate the geometric error parameters in a site environment. First, to determine the environmental sensitivity error parameters, a refractive index correction model in a variable temperature environment is constructed, which performs well in reducing the interference of environmental temperature fluctuations. Subsequently, a spatial measurement network is established to determine two-face sensitivity error parameters using the difference between the frontsight and backsight measurements of the targets. Finally, the two-face non-sensitivity error parameters are solved by minimizing the residuals of the coordinate transformation from each LT station to the reference frame. Indeed, the effects of temperature fluctuation and geometrical misalignments on the measurement accuracy are deeply analyzed, which provide meaningful guidance for calibration and compensation of the geometric errors of the LT. The effectiveness of the proposed method is validated by a series of site experiments. The results show that the length error is reduced to 0.030mm on average, which is 44% lower than that of the network method.

Artificial Neural Network-Based Transmission Power Control for Underwater Wireless Optical Communication System

Several scientists have proposed using an underwater wireless optical communication system (UWOC) to deliver high-speed data services using the abundant optical spectrum. However, wireless optical signal propagation faces an antagonistic environment when using undersea channels due to various factors like scattering, absorption, turbulence, and optical link misalignment between the transmitter and the receiver. These factors will attenuate the optical signal and lead to degrading system performance. To reduce these factors impact on the communication system's performance, transmitted optical power (OTP) should be increased. Since the UWOC system is battery-powered, increasing OTP will consume more electrical power. Therefore, it is necessary to adjust OTP to a value commensurate with the underwater channel changes. So, an ANN model is proposed in this article for link adaptation, which can adjust the OTP level in tandem with the underwater channel conditions. Data for training, testing, and validation of the proposed system reliability was collected experimentally, and tap water was used as a transmission medium. Evaluation of the proposed model outcome demonstrates that reliable performance is achieved in predicting OTP needed in multiple scenarios. The MSE of the predicted OTP is(9.5×10-3,1.5×10-2, and 1.7×10-2) dBm in the training, testing, and validation stages, respectively. The regression values of the training, testing and validation sets are (0.9997,0.9990, and 0.9996). The results achieved by the proposed model prove it is reliable to be applied in UWOC systems.

Automatic Alignment of an Orbital Angular Momentum Sorter in a Transmission Electron Microscope Using a Convolutional Neural Network.

We report on the automatic alignment of a transmission electron microscope equipped with an orbital angular momentum sorter using a convolutional neural network. The neural network is able to control all relevant parameters of both the electron-optical setup of the microscope and the external voltage source of the sorter without input from the user. It can compensate for mechanical and optical misalignments of the sorter, in order to optimize its spectral resolution. The alignment is completed over a few frames and can be kept stable by making use of the fast fitting time of the neural network.

Wavelength calibration of birefringent interferometers for 2-D measurement of plasma flow.

Imaging birefringent interferometers are used to measure plasma flow in 2-D via the Doppler shift of a spectral emission line. Applications include plasma physics study in fusion energy experiments and in the Earth's upper atmosphere. We present a new, to the best of our knowledge, method for wavelength calibration that does not require measurement at the rest wavelength of the targeted spectral line, nor measurement using a tuneable laser source. This is useful when such light sources are not available. Fringes measured at known wavelengths from the emission lines of gas-discharge lamps are used to constrain an instrument model which can generate the required calibration data. In the process, optical path difference, dispersion and misalignments are characterized. The "2π ambiguity" of interferogram phase data is handled using circular statistics, allowing the wavelength span of the calibration lines to far exceed the unambiguous measurement range of the instrument. The technique is demonstrated to an accuracy of ±1 pm (±0.7 km/s flow-equivalent) over a 40 nm visible wavelength range.

Alignment and use of microbeam with full-field x-ray microscopes.

Demonstration tests of the alignment of Fresnel zone plate focusing optics using a full-field x-ray microscope and microbeam x-ray diffraction measurements combined with the full-field x-ray microscope were performed. It was confirmed that the full-field x-ray microscope enables direct two-dimensional observation of a microbeam with sub-micrometer spatial resolution. This allowed visualization of the misalignment of the focusing optics, resulting in accurate alignment of the optics within a short time. In addition, the microscope could be used to observe the sample as well as the microbeam, which enabled clarification of the position and two-dimensional shape of the microbeam on the sample. This realized a measurement procedure that a 100-μm-size sample was imaged with sub-micrometer spatial resolution, and then, microbeam-use measurements were performed for only the region of interest determined by the microscope, which has been difficult with conventional microbeam applications. The combination of observations by a full-field x-ray microscope and measurements using a microbeam is expected to open a new style of measurement.

Research on the influence mechanism and modification technology of UV-curing adhesive on the HPLD thermal-induced misalignment

The demand for output performance and reliability of the HPLD (high-power laser diode) has become increasingly stringent with the development of application requirements. The thermal effect of the HPLD will lead to optical components' thermal-induced misalignment, which will significantly affect the output quality and power of the laser. In this paper, the influence of the thickness and uniformity of UV-curing adhesive on the thermal-induced misalignment of the bonding module is revealed. Then the thermal stability of UV-curing adhesive is optimized by the microparticles composite modification method. Finally, the single emitter array laser is packaged with the optimized modified UV-curing adhesive, the thermal-induced misalignment of beam-shaping components can be reduced by 8 times, and the power reduction ratio of the laser is reduced from 3.8% to 0.72%. This research supports efficient control of the thermal-induced misalignment which to ensures high-performance packaging and manufacturing for HPLD.

Interference and noise analysis for hybrid FSO/RF‐based 6G mobile backhaul

Optical wireless communication, or free space optics, is a promising solution for backhauls in sixth-generation mobile systems. However, the susceptibility of optical links to weather conditions has led to FSO links being furnished with radio frequency (RF) backups. These Hybrid FSO/RF systems provide enhanced link availability but lead to RF resource wastage. Cognitive radio technology, in contrast, is well known for its optimal use of RF resources and may be combined with an FSO link to create a Cognitive Hybrid FSO/RF system. This work uses such a system to analyze a configuration for a mobile backhaul in sixth-generation mobile systems. This configuration can seamlessly coexist with established large scale RF cellular networks. The performance of this configuration is analyzed with respect to outage probability and average bit error by considering the impact of optical channel turbulence, misalignment loss, RF interference, and noise. Mathematical closed-form expressions are verified by simulations.

Minimizing the influence of higher-order transverse modes on the precision of cavity ring-down measurements

In cavity ring-down (CRD) measurements, the excitation of higher-order transverse modes and their random occurrence together with the fundamental mode inside the ring-down cavity (RDC) inevitably induce measurement errors of the decay time constant, which lead to a reduction in the measurement precision of the CRD instrument. To analyze and minimize the influence of higher-order transverse mode excitations on the CRD measurement precision, in this paper we report on the experimental observations of the impact of mode matching lens, aperture inside the cavity and trigger threshold on the excitation probability of fundamental and higher-order transverse mode excitations. The theoretical analysis and experimental results showed that when the optical misalignment is negligibly small, the occurrence probability of higher-order transverse mode excitations can be effectively suppressed by using a single lens between the laser and RDC to match preferably the fundamental model of the laser beam into the RDC, or inserting an aperture of an appropriate diameter inside the RDC, or using a high trigger threshold to record only CRD events with high amplitudes, or the combination of these approaches. For example, when the mode-matching lens is located at the optimal experimental position, and combined with the 2.2 mm diameter aperture, a 100% suppression of the excitation of high-order transverse modes can be achieved. Our results demonstrated that these methods effectively eliminated the decay time constant determination errors caused by the higher-order transverse mode excitations and therefore improved the precision of CRD experiments for high reflectance measurements. In practice the combination of mode-matching and appropriate trigger threshold setting is preferable for highly precise and yet fast CRD measurements.