Absolute Distance Measurement System Research Articles

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.

All-fiber beat-frequency laser heterodyne absolute distance measurement system

To realize precise absolute distance measurement, an all-fiber beat-frequency laser heterodyne distance measurement system based on a fiber-optic interferometric structure was proposed and demonstrated. An acousto-optic frequency shifter is introduced into one arm of a fiber-optic Mach–Zenhder interferormeter to generate the beat-frequency laser beams in two fiber-optic paths. By a ternary sinusoidal curve fitting method to extract the initial phases of the two beat-frequency laser signals and their phase difference, a distance measurement precision of several tens micrometers can be realized in a distance range of several meters. Experiments showed a maximum relative error of 0.0548% and a resolution of 83.333 μm in a distance range of 0 to 600 mm.

Analysis on coupling efficiency of the fiber probe used in frequency scanning interference distance measurement

Fiber probe is a key component in absolute distance measurement system. It has dual tasks of delivery and collection of light. However, little information is given about how light coupling efficiency behaves as the target distance and tilt angle change, although it is of great importance to improve signal-to-noise ratio of the measurement system. In this paper, a theoretical model of fiber probe is proposed. Assuming that the target surface is a mirror, we deduce the coupling efficiency of the light reflected from target surface which is deviated from the ideal image plane with distance Δl. When Δl=-5 mm, the difference of the light coupling efficiency between Zemax simulation result and the theoretical calculation is 0.0017. In addition, Zemax is utilized to simulate the variation of coupling efficiency when target distance and tilt angle change both for reflective and diffuse target surface. It is concluded that the rate of descent for the coupling efficiency is greater when target surface is placed in front of the ideal image plane, and the working range of the fiber probe increases with the increase of ideal image distance. The variation of coupling efficiency with respect to target distance basically follows the same rule for both cases. Nevertheless, the coupling efficiency of diffuse target surface is less sensitive to the tilt angle. The findings will be helpful to guide us to make full use of fiber probe and provide a simple and economical approach to improve the measurement precision.

Comb-calibrated frequency sweeping interferometry for absolute distance and vibration measurement.

We present here an absolute distance measurement system based on frequency sweeping interferometry calibrated with a frequency comb. The instantaneous frequency of the sweeping laser is calibrated at some discrete frequencies corresponding to the comb lines using a narrow bandpass filter. A large number of frequency calibration peaks have been derived in sub-measurements based on fringe counting, leading to a precision improvement by a factor of 10 in a full sweep. A precision of 75nm for distances around 0.84m (corresponding to 89×10-9 in relative term) has been achieved. Also, 1.7nm of vibration measurement in terms of RMS noise has been obtained benefiting from the dense lines of a frequency comb and the sub-measurement principle.

基于光纤三波长激光自混合干涉的绝对距离测量系统

基于光纤三波长激光自混合干涉的绝对距离测量系统

Error Correction for FSI-Based System without Cooperative Target Using an Adaptive Filtering Method and a Phase-Matching Mosaic Algorithm

In our frequency scanning interferometry-based (FSI-based) absolute distance measurement system, a frequency sampling method is used to eliminate the influence of laser tuning nonlinearity. However, because the external cavity laser (ECL) has been used for five years, factors such as the mode hopping of the ECL and the low signal-to-noise ratio (SNR) in a non-cooperative target measurement bring new problems, including erroneous sampling points, phase jumps, and interfering signals. This article analyzes the impacts of the erroneous sampling points and interfering signals on the accuracy of measurement, and then proposes an adaptive filtering method to eliminate the influence. In addition, a phase-matching mosaic algorithm is used to eliminate the phase jump, and a segmentation mosaic algorithm is used to improve the data processing speed. The result of the simulation proves the efficiency of our method. In experiments, the measured target was located at eight different positions on a precise guide rail, and the incident angle was 12 degrees. The maximum deviation of the measured results between the FSI-based system and the He-Ne interferometer was 9.6 μm, and the maximum mean square error of our method was 2.4 μm, which approached the Cramer-Rao lower bound (CRLB) of 0.8 μm.

Synthetic-wavelength-based dual-comb interferometry for fast and precise absolute distance measurement.

We present an absolute distance measurement system using a phase-stable dual-comb system with 56.09 MHz repetition rate and 2 kHz repetition rate difference. A relative phase stability of 0.1 rad in 0.5 ms between two combs is achieved using a mutual locking scheme. The dual-comb ranging system combines the time-of-flight (TOF) method, synthetic-wavelength interferometry (SWI), and carrier wave interferometry (CWI). Each method provides a particular ambiguity range and resolution, and they can be applied simultaneously and linked to enhance the precision and measurement rate of the ranging system. The experimental results demonstrate that a precision of 1.2 μm is obtained without time averaging, and the precision can be improved to 3 nm with only 10 ms averaging time using the SWI method described in this study. The precision reaches a sub-nanometer when the averaging time exceeds 0.1 s. A system with high accuracy and short averaging time would enhance fast measurement performance in various industrial applications. The ambiguity range is about 2.67 m in our system, we test the performance of the system with 1.5 mm range at 1.5 m distance.

Research on the influence of laser-tuning characteristics on all-fiber distributed Bragg reflector self-mixing rangefinder

We proposed an all-fiber tunable distributed Bragg reflector (DBR) laser rangefinder, which realized high-accuracy measurement of the absolute distance in the range of 1.88 to 3.33 m based on the self-mixing effect. The theoretical analysis of the all-fiber tunable DBR laser shows that wavelength tuning range and frequency of the laser can influence the measurement range and resolution. In addition, the experimental results indicate that the influence factors of measurement accuracy and sensitivity in absolute distance measurement are modulation response linearity, experimental equipment, and circuit bandwidth, which is in good agreement with theory analysis. Moreover, increasing the linear wavelength tuning range of the tunable DBR laser is an essential method to improve the measurement accuracy and sensitivity of the absolute distance measurement system.

Dual-Comb Absolute Distance Measurement Based on a Dual-Wavelength Passively Mode-Locked Laser

A new absolute distance measurement system is demonstrated in this paper, which is based on a dual-wavelength passive mode-locked fiber laser. By controlling the intracavity loss and spectral filtering, the cavity mode-locked by a single-wall carbon nanotube saturable absorber is able to generate two pulse trains at slightly different repetition rates simultaneously with high mutual coherence. Comparisons with the commercial interferometers located on the 80-m large-scale dimensional standard show a combined measurement uncertainty of 6 $\mu$ $m+1\times {10}^{-7}L\,(k=2)$ , in an averaging time of 1 s. Our compact dual-comb system provides a low-cost way for ranging, with continuous range, rapid measurement, and high resolution.

Absolute distance measurement system with micron-grade measurement uncertainty and 24 m range using frequency scanning interferometry with compensation of environmental vibration.

We establish a theoretical model of the Doppler effect in absolute distance measurements using frequency scanning interferometry (FSI) and propose a novel FSI absolute distance measurement system. This system incorporates a basic FSI system and a laser Doppler velocimeter (LDV). The LDV results are used to correct for the Doppler effect in the absolute distance measurement signal obtained by the basic FSI system. In the measurement of a target located at 16 m, a measurement resolution of 65.5 μm is obtained, which is close to the theoretical resolution, and a standard deviation of 3.15 μm is obtained. The theoretical measurement uncertainty is 8.6 μm + 0.16 μm/m Rm (k = 2) within a distance range of 1 m to 24 m neglecting the influence of air refractive index, which has been verified with experiments.

Large-scale absolute distance measurement using inter-mode beat of a femtosecond laser

Large-scale absolute distance measurement system with high accuracy plays a significant role in science and engineering applications. In many fields such as aerospace technology, large-scale manufacture, geodetic survey and civil engineering, absolute distance measurement systems with a range of up to kilometers and accuracy of better than several micrometers are generally required. Traditional laser ranging methods such as the time-of-flight method and the interferometry method are difficult to achieve both large scale and high accuracy. With the development of femtosecond optical frequency comb technology, several ranging methods with larger range and higher accuracy are developed. In the frequency domain, the optical frequency comb has a large number of stable mode lines, or the longitudinal modes, at regular intervals, which generates the inter-mode beat signal. In this study, based on the inter-mode beat of a femtosecond laser, an absolute distance measurement system using multi-wavelength interferometric method is demonstrated. It has a simple experimental setup with high accuracy but in a limited range of 2.5 m due to the 2-period of phase detection. To achieve a large-scale measurement system, the measurement range of the experimental system is extended by using the synthetic wavelength generated by tuning the repetition frequency of the laser. With a repetition frequency change of 0.2 MHz, a synthetic wavelength of up to 1.5 km is realized, thus the measurement range of the experimental setup can be extended to 0.75 km. Besides the reference and measurement path beams, a monitor path beam and two alternately opened mechanical shutters are used to measure and compensate for the phase drift due to the unbalanced drift of the electronic circuit. By using this method, the standard deviation of the phase measurement results in 30 min is 0.022 in the experiment, and the phase drift can be compensated for very well. The measurement results from the experimental system are compared with the results from a commercial heterodyne interferometer, and the comparison between results shows a precision of better than 50 m in a displacement of 1125 mm. In the experiment, the repeatability of absolute distance measurement using the range extending method is better than 3 m, thus the range of the distance measurement system can be theoretically extended up to 7.5 km. In conclusion, we demonstrate that a large-scale absolute distance measurement system using inter-mode beat of a femtosecond laser, has a range of up to 7.5 km, an accuracy of better than 50 m and a repeatability of better than 3 m. The accuracy of the experimental system can be further improved by using photodetectors with higher bandwidth so that a higher inter-mode beat and a shorter wavelength can be used.

Absolute distance ranging by means of chirped pulse interferometry

Large-range and high-accuracy absolute distance measurement plays an important role in many practical applications, such as industrial production, aerospace and scientific research, etc. In this paper, a method is proposed for absolute distance measurement by chirped pulse interferometry based on the femtosecond optical frequency comb. The interference spectra obtained in experiments are analyzed by means of the principle of the dispersive interference, and the distance can be determined by the shift of the widest fringe in the interference spectra. An absolute distance measurement system can be set up based on the modified Michelson interferometer with a pair of gratings to chirp the reference pulses in the reference arm. Experimental results are in agreement well within 33 m in a range up to 65 m, i.e. a relative precision of 5.110-7. In addition, the optimization of the measurement uncertainty is theoretically and experimentally performed.

高精度双干涉光路调频连续波激光绝对测距系统

高精度双干涉光路调频连续波激光绝对测距系统

Multi-channel absolute distance measurement system with sub ppm-accuracy and 20 m range using frequency scanning interferometry and gas absorption cells.

We present an implementation of an absolute distance measurement system which uses frequency scanning interferometry (FSI). The technique, referred to as dynamic FSI, uses two frequency scanning lasers, a gas absorption cell and a reference interferometer to determine the unknown optical path length difference (OPD) of one or many measurement interferometers. The gas absorption cell is the length reference for the measurement system and is traceable to international standards through knowledge of the frequencies of its absorption features. The OPD of the measurement interferometers can vary during the measurement and the variation is measured at the sampling rate of the system (2.77 MHz in the system described here). The system is shown to measure distances from 0.2 m to 20 m with a combined relative uncertainty of 0.41 × 10⁻⁶ at the two sigma level (k = 2). It will be shown that within a scan the change in OPD of the measurement interferometer can be determined to a resolution of 40 nm.

Thermally independent fibre optic absolute distance measurement system based on white light interferometry

A fully automated system is described that utilises white light interferometry to accurately measure the spacing between two plates. The system measures spacing in the range from 0 to 2 mm, with 5 µm accuracy, at operating temperatures spanning at least 0–70°C. Results of measurements verifying this performance are presented. The system is immune from both fibre chromatic dispersion and spurious optical path differences introduced by environmental changes.

Absolute distance measurement system using a femtosecond laser as a modulator

The generation of broadband microwave frequency comb from a femtosecond pulse train by direct photodetection opens the possibility for high-accuracy length measurements of long distances. We demonstrate a relatively simple realization of this measurement principle: an electronic distance measurement system based on a time-of-flight approach, driven by a femtosecond fibre laser source as a modulator. By the evaluation of the phase shifts of two distinct comb frequencies, a coarse and a fine measurement of the absolute distance can be performed. The range of the measurement system is demonstrated up to a length of 100 m. The experimental comparison of the femtosecond laser system with a conventional reference counting interferometer shows a precision better than ±10 µm at 100 m, corresponding to a relative measurement uncertainty of 1 × 10−7 L. The limiting factors for the measurement uncertainty of the system are theoretically investigated and shown to be of the same order of magnitude.

Origin and Control of Sinusoidal Nonlinearities in Wavelength-Tuned Littman External Cavity Laser Diodes

In a recent paper, we presented the influence of sinusoidal nonlinearities in the wavelength sweeping speed on the performance of an absolute distance interferometry setup. Nonlinearities in an optical frequency modulation system are generally the source of degradation in performance. The interest of this study comes from the experimental observation of the good behavior of an absolute distance measurement system with the presence of sinusoidal nonlinearities in the wavelength sweeping speed obtained from an external cavity laser diode (ECLD). This paper presents experimental results obtained with a custom ECLD based on a Littman-Metcalf setup. We link the sinusoidal nonlinear sweep to some mechanical properties of the cavity. The main source of these nonlinearities comes from the digitization of the piezoelectric transducer excitation. Different types of excitation like super-Gaussian and ramp, including or not sinusoidal modulations, were implemented and analyzed. By this way, we are able to reduce the spectral width by a factor 3 without adding linearization electronics such as a servo-loop control on injection current or nonuniform samplers, and our system achieves a 3 times 10- 6 relative uncertainty on distance measurement with a number of samples reduced by a factor 8.

Design and implementation of a vacuum-compatible laser-based subnanometer-resolution absolute distance measurement system

We describe the design and implementation of a vacuum compatible laser-based absolute distance measurement gauge with sub-nm resolution. The present system is compatible with operation in the 10{sup -8} Torr range and with some minor modifications could be used in the 10{sup -9} Torr range. The system is based on glancing incidence reflection and dual segmented diode detection. The system has been implemented as a focus sensor for extreme ultraviolet interferometry and microlithography experiments at Lawrence Berkeley National Laboratory's Advanced Light Source synchrotron radiation facility and 1{sigma} operational measurement noise floor of 0.26 nm has been demonstrated.