Non-ambiguity Range Research Articles

Precise Absolute Distance Measurement by Dual 70 MHz Mode-Locked Fiber Lasers

We employed a gold coated glass cover plate to fabricate the dual compact free running fiber laser combs. It had been used for a difference of repetition rates tunable LIDAR (light detection and ranging) system. Our study shows that the ranging precision is 0.4 μm and the non-ambiguity range is 2.1 m with averaging time of 0.4 s for the distance of 1.09 m. The system with 1 kHz-high update rate and the large non-ambiguity range is potentially useful for manufacturing application applied in absolute distance measurement.

Compact Dual-Comb Absolute Distance Ranging With an Electric Reference

To exploit the potential of a dual-comb absolute distance ranging system outside a well-controlled laboratory, a compact dual-comb structure is proposed. A beat frequency generated by the repetition rates of two frequency combs serves as an electric reference, which is easily built and maintains a long range and high resolution compared with traditional dual-comb systems. The performance of the proposed method is compared with that of a heterodyne interferometer. The residuals range within -116.6 to 117.2 nm, the standard deviations vary from 46.3 to 137.9 nm, and the non-ambiguity range extension remains reliable throughout a 10-m test. Compared with Michelson-type dual-comb interferometers, this compact dual-comb system omits the redundant optical reference arm, promising practical applications of distance ranging.

Large Non-Ambiguity Range and Precise Absolute Distance Measurement with Dual Mode-Locked Fiber Lasers

We employed a gold coated glass cover plate to fabricate dual compact free running fiber laser combs based LIDAR (light detection and ranging) system with tunable difference of repetition rates. Our study shows that the ranging precision is 0.4 μm and the non-ambiguity range is 2.1 m with averaging time of 0.4 s for the distance of 1.09 m by the time-of-flight method. The system with 1 kHz-high update rate and the large non-ambiguity range is potentially useful for manufacturing or machining application applied in absolute distance measurement.

Reliable non-ambiguity range extension with dual-comb simultaneous operation in absolute distance measurements

We report an absolute distance measurement scheme using two simultaneous optical frequency combs with different repetition rates for reliable non-ambiguity range extension. Since the non-ambiguity range extension is susceptible to distance drift during the repetition rate adjustment, pulse trains with two different repetition rates are coupled and directed simultaneously onto a target for coincident distance measurement. The simultaneous measurement avoids the process of adjusting the repetition rate and suppresses the influence of distance drift therein. The distances measured by the two repetition rates are distinguished by type II second harmonic generation. Target movement and atmospheric variation are made to imitate the distance drift, and experimental results show that the non-ambiguity range extension remains effective along the measurement.

Absolute distance measurement with extension of nonambiguity range using the frequency comb of a femtosecond laser

Abstract. We revisit the method of synthetic wavelength interferometry (SWI) for absolute measurement of longdistances using the radio-frequency harmonics of the pulse repetition rate of a mode-locked femtosecond laser.Our intention here is to extend the nonambiguity range (NAR) of the SWI method using a coarse virtual wave-length synthesized by shifting the pulse repetition rate. The proposed concept of NAR extension is experimen-tally verified by measuring a ∼13-m distance with repeatability of 9.5 μm (root-mean-square). The measurementprecision is estimated to be 31.2 μm in comparison with an incremental He – Ne laser interferometer. Thisextended SWI method is found to be well suited for long-distance measurements demanded in the fields oflarge-scale precision engineering, geodetic survey, and future space missions. © 2014 Society of Photo-OpticalInstrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.53.12.122403] Keywords: absolute distance measurement; femtosecond laser; synthetic wavelength interferometry.Paper140401SSreceivedMar.10,2014;revisedmanuscriptreceivedApr.8,2014;acceptedforpublicationApr.10,2014;publishedonline May 8, 2014.

Absolute distance measurement by intensity detection using a mode-locked femtosecond pulse laser.

We propose an interferometric method that enables to measure a distance by the intensity measurement using the scanning of the interferometer reference arm and the recording of the interference fringes including the brightest fringe. With the consideration of the dispersion and absorption of the pulse laser in a dispersive and absorptive medium, we investigate the cross-correlation function between two femtosecond laser pulses in the time domain. We also introduce the measurement principle. We study the relationship between the position of the brightest fringe and the distance measured, which can contribute to the distance measurement. In the experiments, we measure distances using the method of the intensity detection while the reference arm of Michelson interferometer is scanned and the fringes including the brightest fringe is recorded. Firstly we measure a distance in a range of 10 µm. The experimental results show that the maximum deviation is 45 nm with the method of light intensity detection. Secondly, an interference system using three Michelson interferometers is developed, which combines the methods of light intensity detection and time-of-flight. This system can extend the non-ambiguity range of the method of light intensity detection. We can determine a distance uniquely with a larger non-ambiguity range. It is shown that this method and system can realize absolute distance measurement, and the measurement range is a few micrometers in the vicinity of Nl(pp), where N is an integer, and lpp is the pulse-to-pulse length.

Absolute distance measurement by dual-comb nonlinear asynchronous optical sampling

A dual-comb nonlinear asynchronous optical sampling method is proposed to simplify determination of the time interval and extend the non-ambiguity range in absolute length measurements. Type II second harmonic generation facilitates curve fitting in determining the time interval between adjacent pulses. Meanwhile, the non-ambiguity range is extended by adjusting the repetition rate of the signal laser. The performance of the proposed method is compared with a heterodyne interferometer. Results show that the system achieves a maximum residual of 100.6 nm and an uncertainty of 1.48 μm in a 0.5 ms acquisition time. With longer acquisition time, the uncertainty can be reduced to 166.6 nm for 50 ms and 82.9 nm for 500 ms. Moreover, the extension of the non-ambiguity range is demonstrated by measuring an absolute distance beyond the inherent range determined by the fixed repetition rate.

光梳多波长绝对测距的波长选择及非模糊度量程分析

光梳多波长绝对测距的波长选择及非模糊度量程分析

Laser confocal feedback tomography and nano-step height measurement

A promising method for tomography and step height measurement is proposed, which combines the high sensitivity of the frequency-shifted feedback laser and the axial positioning ability of confocal microscopy. By demodulating the feedback-induced intensity modulation signals, the obtained amplitude and phase information are used to respectively determine the coarse and fine measurement of the samples. Imaging the micro devices and biological samples by the demodulated amplitude, this approach is proved to be able to achieve the cross-sectional image in highly scattered mediums. And then the successful height measurement of nano-step on a glass-substrate grating by combination of both amplitude and phase information indicates its axial high resolution (better than 2 nm) in a non-ambiguous range of about ten microns.

AUTOMATIC FEATURE-BASED POINT CLOUD REGISTRATION FOR A MOVING SENSOR PLATFORM

Abstract. The automatic and accurate alignment of multiple point clouds is a basic requirement for an adequate digitization, reconstruction and interpretation of large 3D environments. Due to the recent technological advancements, modern devices are available which allow for simultaneously capturing intensity and range images with high update rates. Hence, such devices can even be used for dynamic scene analysis and for rapid mapping which is particularly required for environmental applications and disaster management, but unfortunately, they also reveal severe restrictions. Facing challenges with respect to noisy range measurements, a limited non-ambiguous range, a limited field of view and the occurrence of scene dynamics, the adequate alignment of captured point clouds has to satisfy additional constraints compared to the classical registration of terrestrial laser scanning (TLS) point clouds for describing static scenes. In this paper, we propose a new methodology for point cloud registration which considers such constraints while maintaining the fundamental properties of high accuracy and low computational effort without relying on a good initial alignment or human interaction. Exploiting 2D image features and 2D/2D correspondences, sparse point clouds of physically almost identical 3D points are derived. Subsequently, these point clouds are aligned with a fast procedure directly taking into account the reliability of the detected correspondences with respect to geometric and radiometric information. The proposed methodology is evaluated and its performance is demonstrated for data captured with a moving sensor platform which has been designed for monitoring from low altitudes. Due to the provided reliability and a fast processing scheme, the proposed methodology offers a high potential for dynamic scene capture and analysis.

Frequency-comb-referenced multi-wavelength profilometry for largely stepped surfaces

3-D profiles of discontinuous surfaces patterned with high step structures are measured using four wavelengths generated by phase-locking to the frequency comb of an Er-doped fiber femtosecond laser stabilized to the Rb atomic clock. This frequency-comb-referenced method of multi-wavelength interferometry permits extending the phase non-ambiguity range by a factor of 64,500 while maintaining the sub-wavelength measurement precision of single-wavelength interferometry. Experimental results show a repeatability of 3.13 nm (one-sigma) in measuring step heights of 1800, 500, and 70 μm. The proposed method is accurate enough for the standard calibration of gauge blocks and also fast to be suited for the industrial inspection of microelectronics products.

Absolute distance measurement by dual-comb interferometry with adjustable synthetic wavelength

Absolute distances were measured using two femtosecond lasers of different pulse repetition rates by revisiting the dual-comb interferometric method proposed by Coddington et al (2009 Nature Photon. 3 351–6). The apparatus built for experiments was designed to eliminate the dead zones in the measurement range by separating the measurement pulses from the reference pulses using orthogonal polarization. In addition, the pulse repetition rate of the signal laser was made tunable in order to extend the non-ambiguity range (NAR) by adaptively adjusting the synthetic wavelength in consideration of the de facto measurement stability in the air. Actual tests performed in the open air proved that a target distance of 69.3 m is measured without interruptions at a 200 µs update rate in the presence of a ∼170 µm drift of the optical path length caused by the fluctuation of the refractive index of air. The proposed hardware system design for effective NAR extension will facilitate the use of dual-comb interferometry for various terrestrial applications.

Measurement of air refractive index fluctuation based on interferometry with two different reference cavity lengths

A measurement method based on interferometry with two different reference cavity lengths is presented and applied in air refractive index measurement in which the two cavity lengths and a laser wavelength are combined to generate two wavelength equivalents of cavity. Corresponding calculation equations are derived, and the optical path configuration is designed, which is inspired by the traditional synthetic wavelength method. Theoretical analyses indicate that the measurement uncertainty of the determined index of refraction is about 2.3×10(-8), which is mainly affected by the length precision of the long vacuum cavity and the ellipticity of polarization components of the dual-frequency laser, and the range of nonambiguity is 3.0×10(-5), which is decided by the length difference of the two cavities. Experiment results show that the accuracy of air refractive index measurement is better than 5.0×10(-8) when the laboratory conditions changes slowly. The merit of the presented method is that the classical refractometry can be also used without evacuation of the gas cavity during the experiment. Furthermore, the application of the traditional synthetic wavelength method may be extended by using the wavelength equivalents of cavity, any value of which can be easily acquired by changing cavity length rather than using actual wavelengths whose number is limited.

ACCURACY TEST OF MICROSOFT KINECT FOR HUMAN MORPHOLOGIC MEASUREMENTS

Abstract. The Microsoft Kinect sensor, a popular gaming console, is widely used in a large number of applications, including close-range 3D measurements. This low-end device is rather inexpensive compared to similar active imaging systems. The Kinect sensors include an RGB camera, an IR projector, an IR camera and an audio unit. The human morphologic measurements require high accuracy with fast data acquisition rate. To achieve the highest accuracy, the depth sensor and the RGB camera should be calibrated and co-registered to achieve high-quality 3D point cloud as well as optical imagery. Since this is a low-end sensor, developed for different purpose, the accuracy could be critical for 3D measurement-based applications. Therefore, two types of accuracy test are performed: (1) for describing the absolute accuracy, the ranging accuracy of the device in the range of 0.4 to 15 m should be estimated, and (2) the relative accuracy of points depending on the range should be characterized. For the accuracy investigation, a test field was created with two spheres, while the relative accuracy is described by sphere fitting performance and the distance estimation between the sphere center points. Some other factors can be also considered, such as the angle of incidence or the material used in these tests. The non-ambiguity range of the sensor is from 0.3 to 4 m, but, based on our experiences, it can be extended up to 20 m. Obviously, this methodology raises some accuracy issues which make accuracy testing really important.

Absolute distance measurement by spectrally resolved interferometry based on a femtosecond pulse laser

A spectrally resolved interferometry ranging method based on the optical frequency comb of a femtosecond pulse laser was presented. The approach utilized a frequency stabilized Ti:sapphire femtosecond pulse laser to provide a phase-locked summation of discrete quasi-monochromatic light modes of consecutive frequencies, which is seen as an optical comb in the frequency domain. In this method all modes interference signals of the optical frequency comb obtained by the femtosecond light pulses traveling along different optical path between the reference and measurement arms were utilized to measure absolute distance. A Fabry-Perot Etalon was used to filter smaller frequency modes from the broad-bandwidth femtosecond laser pulse produced. The actual measured light spectrum of the femtosecond laser source was used as the input data, the whole process of the spectrally resolved interferometry ranging method was simulated through MATLAB. The simulation results indicate that the spectrally resolved interferometry ranging method could satisfy the demand of the small scale measurement with a nonambiguity range of 5.75 mm and an associated resolution of nanometer level.

Many-Wavelength Interferometry with Thousands of Lasers for Absolute Distance Measurement

We demonstrate a new technique for absolute distance measurement with a femtosecond frequency comb laser, based on unraveling the output of an interferometer to distinct comb modes with 1GHz spacing. From the fringe patterns that are captured with a camera, a distance is derived by combining spectral and homodyne interferometry, exploiting about 9000 continuous wave lasers. This results in a measurement accuracy far within an optical fringe (λ/30), combined with a large range of nonambiguity (15cm). Our technique merges multiwavelength interferometry and spectral interferometry, within a single scheme.

Spectrally resolved laser ranging with frequency combs

The multiheterodyne beatnote between two frequency combs having pulses sliding one with respect to another is used to perform spectrally resolved ranging of diffuse reflectors at short distances. The sliding comb sources are generated using one mode-locked laser and a two-beam interferometer, but two properly controlled lasers could be used as well. A pseudo-random binary modulation of the pulses is used to increase the non-ambiguous range. Ranging with a spatial resolution of 21 cm and a spectral resolution of 10 cm(-1) over a 200 cm(-1) spectral range is demonstrated.

Analysis of Random Step Frequency Radar and Comparison With Experiments

Linear stepped frequency radar is used in wide-band radar applications, such as airborne synthetic aperture radar (SAR), turntable inverse SAR, and ground penetration radar. The frequency is stepped linearly with a constant frequency change, and range cells are formed by fast Fourier transform processing. The covered bandwidth defines the range resolution, and the length of the frequency step restricts the nonambiguous range interval. A random choice of the transmitted frequencies suppresses the range ambiguity, improves covert detection, and reduces the signal interference between adjacent sensors. As a result of the random modulation, however, a noise component is added to the range/Doppler sidelobes. In this paper, relationships of random step frequency radar are compared with frequency-modulated continuous wave noise radar and the statistical characteristics of the ambiguity function and the sidelobe noise floor are analyzed. Algorithms are investigated, which reduce the sidelobes and the noise-floor contribution from strong dominating reflectors in the scene. Theoretical predictions are compared with Monte Carlo simulations and experimental data

High-dynamic-range laser range finders based on a novel multimodulated frequency method

A continuous-wave phase-shift laser range finder employs a novel multimodulation frequency method associating an undersampling analog-digital converter (ADC) with digital synchronous detection. This presentation greatly improves measured phase accuracy and reduces prior art scheme complexities. The novel patented design includes one phase-lock-loop (PLL) chip to produce a multimodulation frequency, one analog-to-digital converter operating at a low sampling rate, and an effective algorithm to calculate the final distance, which has encoded computing codes, and is implemented into compact computing circuits but without mixers and redundant components. The experimental results prove that a nonambiguity range is easily achieved to 1.5 Km when the modulation frequency is operated at 0.1 MHz. The measured accuracy approaches 2.9 mm using the same apparatus when the modulation frequency is tuned to 14.5 MHz. Dynamic range can reach 5.2×105 without a very high modulation frequency below 15 MHz, as revealed by a detailed analysis.

Absolute distance measurement by dispersive interferometry using a femtosecond pulse laser

We describe an interferometric method that enables to measure the optical path delay between two consecutive femtosecond laser pulses by way of dispersive interferometry. This method allows a femtosecond laser to be utilized as a source of performing absolute distance measurements to unprecedented precision over extensive ranges. Our test result demonstrates a non-ambiguity range of ~1.46 mm with a resolution of 7 nm over a maximum distance reaching ~0.89 m.