The optimized structure design in improving the bandwidth of fast steering mirror

In this paper the mechanical structure and the operating principle of a fast steering mirror (FSM) are introduced, and a novel FSM based on magnetostrictive actuators is designed. The modal analysis of the FSM is carried out with the help of the finite element method, and relative experiments are also conducted. The experimental results and that of simulation are given and in good qualitative agreement with each other. This paper pays attention to ameliorating the FSM structure for higher mechanical natural frequency and perfect control performance. The experimental results would be helpful for the construction design of the FSM.

Free-space optics (FSO) communication enjoys desirable modulation rates at unexploited frequency bands, however, its application is hindered by atmospheric turbulence which causes phase shifting in laser links. Although a single deformable mirror (DM) adaptive optics (AO) system is a good solution, its performance remains unsatisfactory as the proportion of tilts aberrations becomes relatively high. This condition happens when the incident angle of the laser beam for the optical receiver dynamically shifts. To tackle this problem, we introduce a fast steering mirror (FSM), DM cascaded AO architecture, based upon which we also propose an atmospheric turbulence compensation algorithm. In this paper, we compare the compensation ability of FSM and DM towards tilts aberrations. Furthermore, we gain model matrices for FSM and DM from testbed and simulatively verify the effectiveness of our work. For a Kolmogorov theory-based atmospheric turbulence disturbed incident laser beam where the tilt components take up 80% of the total proportion of wavefront aberrations, our proposed architecture compensates the input wavefront to a residual wavefront root mean square (RMS) of 116 wavelength, compared to 16 wavelength for single DM architecture. The study intends to overcome atmospheric turbulence and has the potential to guide the development of future FSO communications.

Fast steering mirror (FSM) is one of the most important components in electro-optical tracking system and access to FSM model is the basis for controlling and fault diagnosis. This paper presented a correlation identification method based on Invert-Repeated m-sequence which can be used in the electro-optical tracking system to achieve the model of FSM under low sampling rate. Firstly, this article discussed the properties of the Invert-Repeated m-sequence and program implemented in matlab language, then analyzed the principle of correlation identification method based on Invert-Repeated m-sequence by utilizing Wiener-Hopf equation which is simple to achieve with strong anti-jamming capability and small perturbations on the system. Finally, a FSM model with the experiment data got by Dynamic Signal Analyzer was built in Matlab/Simulink and identified by the method mentioned in the paper. The experiment showed that correlation identification method which has certain actual application value, based on Invert-Repeated m-sequence can obtain more accurate recognition results even if the FSM system’s output signal contained large variance noise.

Purpose– Since the use of advanced finite element analysis (FEA) in the design of steel structures has been increasing its popularity in order to avoid unsafe or highly conservative designs, a solid know-how in computer-aided design (CAD) and engineering (CAE) codes is necessary. Therefore the purpose of this paper is to provide an extensive review of useful guidelines concerning modelling, simulation and result validation for the accurate performance of those analyses.Design/methodology/approach– Such guidelines are obtained from international steel design codes like Eurocode 3 and DNV, publications from experienced CAE engineers and renowned FE software companies like Ansys and Altair. Topics like mesh independence, the effect of the load sequence on the load bearing capacity and steel fracture criteria are underlined.Findings– Since the use of advanced FEA in the design of steel structures is becoming more and more traditional due to the increase of its competitiveness when compared to the use of (very) conservative design rules, a solid know-how in CAD and CAE codes is necessary.Practical implications– This work will be quite useful for structural steel stress engineers, contributing for a safer use of FEA in research and design.Originality/value– This work will be quite useful for structural steel stress engineers, contributing for a safer use of FEA in research and design.

The objective of the research is to investigate an effective method of using servo control through the use of a fast steering mirror and/or acoustic actuators and sensors for optical jitter suppression. The experimental testbed is built in an anechoic chamber at Duke University. Two optical benches are used to isolate metrology components from vibrating optical elements. A Helium-Neon laser and a position sensor are mounted on one optical bench and a turning flat mirror and a 3-axis fast steering mirror are on the other optical bench. External acoustic disturbances from the speaker are used to generate vibration in the optical components. A speaker housed within an enclosure and a fast steering mirror are the two allowable control actuators and external microphones are used for reference measurement, microphones/accelerometers mounted on or nearby the optical elements and the optical sensor are the three allowable control sensors in the experiment. The generalized predictive controller is used to conjugate the process of the system identification algorithm and the process of the controller design, and can be updated adaptively in the changes of the operating environment. The generalized predictive controllers are designed for each actuator and each sensor: Case 1) Fast steering mirror and position detector, Case 2) Control speaker and position detector, Case 3) Fast steering mirror and external microphones. The generalized predictive controller for all control actuators and sensors is also designed.

Optical wireless communication (OWC) can provide the last mile high data rate and broadband access to indoor users. Infrared-based communication (IRC) can also achieve high data rate OWC transmission using a directional and collimated line-of-sight (LOS) optical beam. However, a collimated optical beam with beam steering ability is required to provide the LOS communication channel to different mobile users. Different optical beam steering approaches have their limitations. Although the mechanical-based approach using fast steering mirror (FSM) usually has a large footprint and limited optical steering speed; it can provide a large field-of-view (FOV) and support a wide wavelength window with polarization, optical mode, and modulation format independence operations. These are crucial for the OWC systems as wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) are employed to boost the transmission capacity. In this work, we propose and reveal a proof-of-concept experimental demonstration of an adaptive fast steering mirror (FSM)-based optical beam tracking and alignment system. To enhance the FOV and the stability of the OWC transmission, high-precision adaptive optical tracking via a two-FSM system with a light spot approaching method is proposed and demonstrated. The operation mechanism of optical alignment using two FSMs to establish the reference optical axis path is discussed in detail. The first FSM is utilized to correct the beam displacement issue from the reference optical axis, while the second FSM is utilized to correct the incident beam angle issue from the reference optical axis. Experimental result shows that the FOV can be enhanced from 0.05° (i.e., using only a single fiber collimator) to 51.85° using the proposed FSM-based system. A high-speed and stable orthogonal frequency division multiplexing (OFDM) OWC transmission at 89.3 Gbit/s, 87.03 Gbit/s, and 84.47 Gbit/s after free space propagation distances of 3 m, 10 m, and 20 m, respectively, can be achieved, fulfilling the hard-decision forward-error-correction (HD-FEC) requirement (i.e., bit-error-rate, BER = 3.8 × 10-3).

The fast steering mirror is a commonly used image motion compensation device in space cameras. The mechanical properties of the support structure will directly affect the response speed and closed loop bandwidth of the fast steering mirror system. A fast steering mirror driven by voice coil motor in a low earth orbit space camera was taken as the research object, and a two-axis flexible support structure based on cross-shaped flexible hinges was designed, which had the advantages of compact structure and small center drift. According to the approximate differential equation of cantilever beam deflection, the mathematical model of the rotational stiffness of the two-axis flexible support was established. The main structural parameters of the flexible hinge were selected for the resonant frequency requirements of the fast steering mirror mechanism, and a modal analysis of the mechanism was performed by using the finite element software MSC.Patran. The analysis results show that the resonant frequency of the fast steering mirror mechanism in the two working directions is 18.6 Hz and 18.7 Hz, while the resonant frequencies in other non-working directions are above 292.2 Hz, which proves that the choice of flexible hinge structural parameters is reasonable. In order to test the correctness of the theoretical model, a prototype of the fast steering mirror was processed and the experimental platform was built. The rotational stiffness and resonant frequency of the mechanism were tested. The results of the test and the theoretical analysis are consistent within the allowable error range.

This paper presents a compact and high-performance piezoelectric micro-electro-mechanical system (MEMS) fast steering mirror (FSM) designed for use in laser inter-satellite links (ISLs). The FSM features a large optical aperture of 10 mm and is batch fabricated using an 8-inch wafer-level eutectic bonding process, packaged into a volume of 26 × 22 × 3 mm3. Notably, the piezoresistive (PZR) sensor is integrated on the spring of the FSM to facilitate precise beam control. Furthermore, an intermediate directional defect structure is novelly designed to create a Stress Concentration Region (SCR), effectively improving PZR sensitivity from 3.3 mV/(V∙mrad) to 5.4 mV/(V∙mrad). In this article, various performance metrics of the FSM are tested, including the mechanical characteristics, PZR sensor properties, and mirror optical quality, which all meet the requirements for laser ISLs. Results indicate that the FSM achieves a high resonant frequency (>1 kHz) and a low nonlinearity of 0.05%@ ± 2.1 mrad. A remarkable minimum angular resolution of 0.3 μrad and a repeated positioning accuracy of 1.11 μrad ensure exceptional pointing precision. The open-loop control is driven by the double-step algorithm, resulting in a step response time of 0.41 ms and achieving a control bandwidth over 2 kHz. Additionally, the integrated angular sensor demonstrates a nonlinearity of 0.09%@ ± 1.05 mrad, a sensitivity of 5.1 mV/(V∙mrad), and a minimum angular resolution of 0.3 μrad. Under quasi-static driven conditions (500 Hz @ ± 2 mrad), the maximum dynamic deformation of the mirror surface is merely 2 nm.

This paper develops a tip-tilt motion controller of fast steering mirror (FSM) in the Giant Magellan telescope (GMT). A mathematical model of tip-tilt motion system of FSM is derived, and then based on this model, stability analysis is carried out. A heuristic adaptive controller is designed for the tip-tilt motion control with modeling error. The heuristic controller consists of two different adaptations such as initial adaptive control and adaptation at steady state errors. Through numerical simulations, the validity of the controller is illustrated. After that this paper addresses several practical issues like disturbance from wind, actuator saturation and resonance frequency from mechanical structure in implementing the controller to the actual system.

Experimental study on adaptive optical wavefront correction with dual mirrors in free space optical communication

The main function of the dynamic scan and stare imaging system is to quickly and continuously search a large area and perform high-resolution imaging. To eliminate image motion during the scanning process, this paper proposes an image motion compensation control method with dual-channel control. First, an improved model-assisted active disturbance rejection control is proposed, in which an auxiliary model is integrated into the algorithm to improve the control accuracy and response speed of the rotation rate of the platform. Second, a fast steering mirror (FSM) is introduced into the control system to compensate for the scanning speed and multiple disturbances. By adopting the amplitude frequency characteristics of the tracking differentiator, a parallel tracking differentiator filter is designed to suppress the interference of gyroscope noise on the FSM. When the system is disturbed by multiple frequency disturbances, the residual error of the image motion compensation is less than the spatial angular resolution of one pixel through the dual-channel stable control of the platform and the FSM. In the scan imaging experiment results, the average value of the grayscale variance function for the compensated images is close to 90% that of the static reference images.

In the precise pointing system of satellite laser communication, the piezoelectric fast steering mirror (FSM) is used in the system. In the free state, the hysteresis between the input voltage and the output displacement of FSM is obvious, which has a serious impact on the precision positioning accuracy. Therefore, aiming at the hysteresis problem of the fast steering mirror in satellite laser communication, a phenomenal hysteresis mathematical model based on Bouc-Wen operator is established by analyzing the hysteresis mechanism of the piezoelectric fast steering mirror. An improved nonlinear particle swarm optimization (PSO) parameter identification method is proposed, which greatly improves the optimization efficiency of particle swarm optimization. On this basis, a feedforward inverse compensation linearization method is proposed. In order to verify the effectiveness of the proposed model, a fast steering mirror system experiment is established. The experimental results show that the improved PSO parameter identification method proposed in this paper can accurately identify the parameters in the model, and the proposed feedforward linearization method can improve the linearity of piezoelectric ceramic actuator by more than 90%, which can meet the actual index requirements of the fast steering mirror.KeywordsFast steering mirrorHysteresisBouc-WenFeedforward linearization

Many engineering projects require automated control of analog voltages over a specified range. We have developed a computer interface comprising custom hardware and MATLAB code to provide real-time control of a Thorlabs adaptive optics (AO) kit. The hardware interface includes an op amp cascade to linearly shift and scale a voltage range. With easy modifications, any linear transformation can be accommodated. In AO applications, the design is suitable to drive a range of different types of deformable and fast steering mirrors (FSM’s). Our original motivation and application was to control an Optics in Motion (OIM) FSM which requires the customer to devise a unique interface to supply voltages to the mirror controller to set the mirror’s angular deflection. The FSM is in an optical servo loop with a wave front sensor (WFS), which controls the dynamic behavior of the mirror’s deflection. The code acquires wavefront data from the WFS and fits a plane, which is subsequently converted into its corresponding angular deflection. The FSM provides ±3° optical angular deflection for a ±10 V voltage swing. Voltages are applied to the mirror via a National Instruments digital-to-analog converter (DAC) followed by an op amp cascade circuit. This system has been integrated into our Thorlabs AO testbed which currently runs at 11 Hz, but with planned software upgrades, the system update rate is expected to improve to 500 Hz. To show that the FSM subsystem is ready for this speed, we conducted two different PID tuning runs at different step commands. Once 500 Hz is achieved, we plan to make the code and method for our interface solution freely available to the community.

To meet the requirements of the adaptive optics systems with high bandwidths and large excursion angles, we propose a fast steering mirror (FSM) with an ultrahigh natural frequency and a large angular range. The proposed FSM is driven by a preloaded piezoelectric ceramic stack actuator (PCSA), which has a higher shear stress limit in the working direction. We describe the structure of the preloading device and analyze the stiffness improvement of the preloaded PCSA. Then we introduce the structure of the proposed FSM and perform theoretical analysis based on the established static model and dynamical model. We also build an experimental setup of the proposed FSM. The experimental results show that the angular range of the proposed FSM is up to 8.4 mrad, and its first natural frequency is 6660 Hz, which surpass the performances of current FSMs.

A 37 element adaptive optics system has been built in the Institute of Optics and Electronics for experimental research of atmospheric effect compensation. In this system a 37 subaperture Shack-Hartmann sensor with frame rate 380 Hz is used as wavefront sensor, a 37/55 element deformable mirror and a fast steering mirror as wavefront correctors, a digital signal processor with peak operation speed 100 Mops as wavefront processor and controller, and 39 channels of high voltage amplifier for controlling the wavefront correctors. This system has been integrated with a turbulence cell developed by the Anhui Institute of Optics and Fine Mechanics and experiments for compensating for wavefront errors induced by the turbulence of the cell with different strength have been conducted. Another Shack- Hartmann wavefront sensor is used for measuring the strength of turbulence. The integrated probability density functions of both the coherence length r<SUB>0</SUB> and the Strehl ratio of S of the focused spot are used to describe the strength of turbulence and the sharpness of focused spots with and without correction. In this paper the adaptive optics system and the experimental results are briefly reported.