Micron-precision measurement using a combined frequency-modulated continuous wave ladar autofocusing system at 60 meters standoff distance

Continuous dynamic measurement of frequency scanning interferometry based on motion phase synchronization compensation and calibration

We present a continuous dynamic frequency scanning interferometry (DFSI) measurement method based on motion phase synchronization compensation and calibration. By introducing heterodyne interferometry (HI) synchronization measurement and frequency scanning interferometry (FSI) motion phase compensation, dynamic continuous measurement is achieved and effectively suppresses the distance error introduced by the Doppler effect (DE). Based on this, the influence of the initial optical frequency deviation (OFD) of the tunable laser and the OFD of the HI laser on the dynamic absolute distance measurement (DADM) is analyzed; the relationships between the error of DADM with the variation of the OFD and the target motion parameters are investigated; and the residual DE introduced by the OFD is shown as the fundamental cause of the degradation of the accuracy of DFSI. We propose an online optical frequency measurement method based on HI combined with H13C14N gas absorption cells to resolve this problem. High-precision motion phase compensation is achieved by calibrating the optical frequency (fixed frequency) of the measured HI laser and the initial frequency of the tunable laser online during measurement and then performing motion phase calibration. To verify the effectiveness of our method, an optical frequency calibration experiment, a continuous DADM experiment, and a precision evaluation experiment were conducted, and a highly accurate continuous DADM was achieved.

Temperature influence on optical fiber and temperature compensation method for a frequency modulation continuous wave absolute distance measurement system

A temperature compensation method is proposed to solve the ranging precision decrease problem of the frequency-modulated continuous wave distance measurement system. The set of phases spread frequency sampling method is used to correct the beat frequency signal non-linearity. The influence model of temperature on the optical fiber auxiliary interferometer is studied. The experimental results show that distance measurement error decreases from 0.3432 mm to 0.02260 mm, and the mean measurement standard deviation decreases from 0.1088 mm to 0.01733 mm on a maximum measurement range of 1.6 m after compensation.

FMCW LiDAR with an FM nonlinear kernel function for dynamic-distance measurement

Frequency-modulated continuous-wave (FMCW) LiDAR is an absolute-distance measurement technology with the advantages of high-precision, non-cooperative target measurement capabilities and the ability to measure distance and speed simultaneously. However, the existing range extraction method for FMCW LiDAR is associated with problems, such as requiring a high sample rate and dispersion mismatch. Here, we propose and demonstrate a dynamic range extraction method based on an FM nonlinear kernel function, which improves measurement accuracy without the use of a long auxiliary interferometer (as is required for the traditional method), reduces the influence of dispersion mismatch and the Doppler effect caused by target movement and can simultaneously measure the target motion information dynamically, with a lower measurement error than that of the existing range extraction method under the same conditions.

System Design and Simulation for Square Dance Movement Monitoring Based on Machine Learning

Since the reform and opening, China’s economy has grown rapidly, and people’s living standards have improved significantly. As one of the most effective ways to implement national fitness, square dance has gradually become the main lifestyle of urban communities, an important part of China’s sports construction, and an important indicator to reflect the fitness of the masses and the construction of a well-off society in an all-round way. On the other hand, with the rapid development of internet of things technology, many people can use intelligent bracelets based on machine learning technology to realize motion detection. This technology is also applicable in square dance, which is of great significance to exercise and protect health. This paper first reviews the research status of the internet of things communication protocol and cloud platform, then introduces and analyzes the MQTT communication protocol and Netty high-performance network framework, and studies the integration technology of the internet of things and machine learning. Then, according to the characteristics of the internet of things, a scheme to realize data preprocessing is proposed. The value to be completed is calculated based on the correlation of other attributes corresponding to the k-nearest neighbor model (KNN) and the regression model. Finally, the machine learning algorithm is used to train the results of the three models to obtain the final filling value. The whole scheme design allows the machine learning algorithm to obtain relatively high-quality data in the internal environment. This paper designs a sports monitoring data system for square dance by combining machine learning and internet of things technology, so as to promote national fitness.

Proof-of-concept study of the virtual optical scale bar by the pulse-to-pulse interferometry

The optical scale bar with calibrated or measured internal point-to-point length has many applications in coordinate measurements. In this paper, the virtual optical scale bar with two retroreflectors is constructed by the absolute distance measurement based on pulse-to-pulse interferometry. The temporal and dispersive coherence could be utilized to determine the adjustable internal length of multiple pulse-to-pulse intervals with high precision. The proposed scheme was combined with a pellicle beamsplitter to minimize systematic error. The influence of its thickness on precision is also discussed and calibrated in detail. Besides, a femtosecond mode-locked pulse laser with 100-MHz repetition rates was employed in our system to develop an optical scale bar and verify the feasibility of the proposed method. The sub-micron precision could be realized by temporal coherence with a piezo-driven stage or a simplified non-polarized scheme of dispersed coherence. It shows that this method could achieve a flexible and high-precision virtual optical scale bar for further practical applications.

Vibration compensation method based on instantaneous ranging model for triangular FMCW ladar signals

Triangular frequency-modulated continuous-wave (FMCW) laser radars (ladars) are extremely sensitive to vibration errors. An FMCW ladar 3D imaging system may suffer from severe vibrations and can use only one-period echoes for the ranging of each observation spot; consequently, it can provide only few measurement results. These vibrations may cause large errors because conventional vibration compensation methods are ineffective when applied to fast disturbances with limited measurement results. To solve this problem, we analyze the influence of vibrations on FMCW ladar ranging and propose a vibration compensation method based on an instantaneous ranging model for one-period triangular FMCW ladar signals. We first use a synchrosqueezing wavelet transform to extract time-frequency curves of the up- and down-dechirp signals and then build an instantaneous ranging model that can characterize local vibration errors. Based on the instantaneous ranges, we remove the disturbance vibration errors by taking the mean values of the instantaneous ranges and obtain the target range by using the triangular relations of the up and down observations. Experiments based on synthetic and real data verify the effectiveness of the proposed method and its superiority over the three-point method and Doppler shift method in compensating for vibrations with different frequencies and noise levels.

Frequency modulation nonlinear correction and range-extension method based on laser frequency scanning interference

A phase spread frequency sampling method is proposed. This method can be used to correct the nonlinearity in the beat frequency of a measurement signal. The proposed method expands the phase of the auxiliary interference beat signal, thereby satisfying the Nyquist sampling theorem, correcting the nonlinearity in the beat frequency of the measured signal, and solving the problem of limited range. The conditions over which the frequency sampling method can be applied are expanded. The measurement range is flexibly expanded by performing multiple phase expansions.

Precision improvement in frequency scanning interferometry based on suppression of the magnification effect

Frequency scanning interferometry (FSI) is a promising technique for absolute distance measurement and has been demonstrated in many industrial applications. However, in practice, the measurement precision is limited and sensitive to the variations of the measured distance while sweeping the optical frequency of the laser. The induced errors would be amplified by hundreds of times due to the magnification effect. In this paper, an incremental interferometer was established on the basic scheme of the FSI system for monitoring the variations of distance. The compensation could be achieved by multiplying the heterodyne signals from monitor and measurement interferometer without complex and time-costing data processing. The system performance has been verified by experiments for different kinds of vibrating targets. Finally, after compensation by suppression of the magnification effects, a measurement precision of 4.26 μm has been achieved in a range of 10 m.

Real-time dynamic absolute ranging with frequency scanning interferometry using a robust Monte-Carlo-based particle filter

A particle-filter-based data processing for a frequency-scanning interferometry system for dynamic long absolute ranging is presented in this paper. Because the particle filter is not restricted by linearity or noise Gaussianity requirements, the proposed system achieves high robustness and accuracy levels when tracking dynamic objects at long standoff distances. We experimentally demonstrate that the proposed system exhibits a standard deviation below 6 μm for a quasi-static target, and superior tracking performance for a vibrating target with vibration amplitude exceeding 10 μm and frequencies of 1-5 Hz at a 15 m standoff distance.

Dynamic cascade-model-based frequency-scanning interferometry for real-time and rapid absolute optical ranging

A new frequency-scanning interferometry (FSI) scheme using in-phase and quadrature (IQ) detection for real-time and rapid absolute optical ranging is presented. Dynamic measurement with FSI modulates interference signal frequency by both target movement and time-varying optical-frequency scanning rate; hence, a dynamic model is proposed to decouple dynamic absolute distance from the instantaneous frequency of interference signals. The unscented Kalman filter and particle filter algorithms are implemented for the nonlinear first-layer and non-Gaussian second-layer models, respectively. The proposed FSI scheme eliminates nonlinear optical-frequency scanning effects in dynamic measurements and realizes real-time measurement only current observed data are used. Experimental results verify high tracking performance for a vibrating target with approximately 10 μm amplitude and 50-500 Hz frequency.

Ultra-long range optical frequency domain reflectometry using a coherence-enhanced highly linear frequency-swept fiber laser source

We report on an ultra-long range optical frequency domain reflectometry (OFDR) using a coherence-enhanced highly linear frequency-swept fiber laser source based on an optoelectronic phase-locked loop (OPLL). The frequency-swept fiber laser is locked to an all-fiber-based Mach-Zehnder interferometer (MZI) to suppress sweep nonlinearity and enhance the laser coherence, leading to a high coherence linear frequency sweep of 1 GHz in the duration time of 25 ms. This enables the OFDR to realize an ultra-long range measurement with a high spatial resolution. As a result, we obtain a 10 cm transform-limited spatial resolution at a 20 km fiber within 25 ms measurement time, and a 72 cm spatial resolution over an entire 200 km fiber link within 5 ms measurement time. The proposed reflectometry provides a high-performance solution with both high spatial resolution and ultra-long measurement range for field real-time fiber network monitoring and sensing applications.

Vibration Compensation of the Frequency-Scanning-Interferometry-Based Absolute Ranging System

The frequency-scanning-interferometry-based (FSI-based) absolute ranging technology is a type of ranging technology possessing a high precision and no ranging blind area, so it can be used for non-cooperative targets. However, due to a tiny movement of a target, the Doppler shift and the phase modulation are introduced into the beat signal which results in ranging accuracy decrease. In order to solve this problem, first the model of vibration effect is established, and then the beat signals of two adjacent scanning periods are processed to produce a signal that is immune to vibration. The proposed method is verified by the experiments, and the experimental results show that the effect of vibration compensation is better for the target with a lower vibration velocity and at a lower vibration frequency (lower than 6 Hz). When the target is subjected to a sinusoidal vibration with an amplitude of 10 μm at a frequency of 1 Hz, by using the proposed method the standard deviation is reduced from 775 to 12 μm. Moreover, in the natural environment, by using vibration compensation the standard deviation is reduced from 289 to 11 μm.