Rapid-aging civil infrastructures have become social issues worldwide. Developed countries have faced difficulties e.g. cost increase in infrastructure maintenance, shortage of highly-skilled human resources. Thereafter, infrastructures have been inspected infrequently e.g. every 1~10 year. 3D mapping powered by light detection and ranging (LiDAR) platforms has recently been brought to public attention. In infrastructure maintenance, LiDAR having high-precision and long-measurement range is expected to enhance inspection frequencies and lower the maintenance cost. Time-of-flight (ToF) approach is generally utilized for long-range measurement, however it suffers from low depth resolution. Although development of dual-comb interferometry enabled high-precision ToF rangefinders, sophisticated stabilization is required for two lasers. On the other hand, frequency-modulated continuous-wave (FMCW) LiDAR can perform ranging with a single laser. However, its measurement range is limited by its receiver bandwidth. Although FMCW LiDAR with low-speed analog-to-digital converters can conduct long-distance ranging by reducing its wavelength-sweeping rate, the measurement refresh rate is sacrificed. In this article, we propose a long-range FMCW LiDAR employing wavelength-swept optical frequency comb which overcomes ranging limitation caused by its receiver bandwidth. In addition, the spectrum of such optical frequency comb is apodised to circumvent its inherent ranging ambiguity. We have performed proof-of-concept experiments using the proposed LiDAR. With optical fibers for emulating beam propagation, we have succeeded in ranging corresponding to 1,605-m free-space propagation with a 10-MHz FMCW receiver. Our proposal will make an important contribution to infrastructure maintenance.
In this article, we review various fiber diagnostic methods based on distributed sensing such as optical timedomain reflectometry, optical frequency-domain reflectometry etc., fiber characterization schemes employing digital coherent receivers and various interferometric approaches. We also go through multiple industrialized fiber diagnostic instruments provided by leading manufactures in the installation and maintenance market.
Free-space optical communication is a line-of-sight wireless communication scheme, which is preferred for its number of prime advantages over radio frequency wireless communication, such as no spectrum licensing, large bandwidth, inherent security, electromagnetic compatibility/electromagnetic interference immunity etc. Moreover, free-space optical communication also benefits from low-cost installation and maintenance. It has been studied for the next generation access networks, inter-building connections, ground-to-unmanned aerial vehicle links, underwater communication applications, inter-satellite links, deep space links etc. Among various detection approaches utilized in free-space optical communication, coherent detection can achieve the best sensitivity in a bandwidth-limited condition, effectively demodulate optical multilevel coded signals to attain high spectral efficiency, offer excellent background noise rejection. However, such an attractive free-space optical communication suffer from waveform distortion, scintillation, phase fluctuations etc. after transmission in atmospheric channels. Its link losses are almost dependent on atmospheric effects and climatic conditions. In this article, we present an up-to-date survey on coherent free-space optical communication, the atmospheric turbulent effects especially the impacts of turbulence in free-space optical links, and countermeasures against such impairments.
Amplitude-modulated continuous-wave laser scanner with focusing optics can realize extremely high-precision 3D measurement. Since amplitude-modulated continuous-wave scheme employs periodical modulation, the longitudinal resolution and the maximum unambiguous range are in a trade-off. Our system utilizes dual-frequency modulation compromise such trade-off. However, such an attractive laser scanner suffers from ranging ambiguity due to aliasing, which is the systematic error inherent in amplitude-modulated continuous-wave scheme. We have removed the ranging ambiguity by aliasing synthesis. Secondly, the acquired 3D point clouds contain phase jumping at the maximum unambiguous range. With leveraging the relationship between the intensity and spatial information, the phase jumping was unwrapped to recover the spatial continuity. Thirdly, the 3D point clouds in the defocused region of the amplitude-modulated continuous-wave laser scanner distort since the depth-of-focus of focusing optics is generally cm order. The 3D point clouds in the defocused area are contaminated by aliasing which can also be regarded as a ranging ambiguity problem. We have experimentally restored the 3D point clouds by aliasing synthesis with the assistance of intensity information. The ranging area can be elongated by at least ten times of the depth-of-focus with such data processing. With the above-all mentioned configuration and data processing, we have compromised the ranging ambiguity inherent in the amplitude-modulated continuous-wave laser scanner comprehensively. We expect that our results contribute to high-precision industrial inspection for Industry 4.0.
With the rise of Industry 4.0, smart factory is fast becoming a key concept in infrastructure. To realize the autonomous production system, it is necessary to ensure the parts are properly manufactured. 3D scanners are expected to play a vital role in quality assessment in smart factories. Especially, amplitude-modulated continuous-wave laser scanners benefit from high accuracy and high sensitivity which are suitable for industrial inspection. However, due to the limited dynamic range of receiver electronics, such laser scanners fail to obtain the data points in 3D measurement of highly reflective objects. This impairment deteriorates the performance of conversion of 3D point clouds to solid data for shape inspection, 3D modeling, reverse engineering etc. We coped with receiver saturation by adopting a high-speed polarizationindependent variable optical attenuator in our laser scanner. With such a lase scanner, we have succeeded in prevention of data loss due to receiver saturation.
Amplitude-modulated continuous-wave laser scanners can realize 3D measurement with high longitudinal resolution. With installation of focusing optics in the laser scanning system, the high lateral resolution can be realized, which is enhanced by the focused beam spot size <100 um. However, the depth-of-focus of the focusing optics is generally several cm. The 3D data of the defocused objects are contaminated by aliases distributed by integer times of the half cycle of the periodical modulation. Aliasing is an impairment inherent in the amplitude-modulated continuous-wave scheme. We experimentally recovered the defocused data drastically by synthesizing those aliases. The ranging area can be elongated by at least ten times with such data processing compared with the depth of focus. Our results will contribute to highprecision industrial inspection for Industry 4.0.
Laser scanners have emerged as powerful instrument for high-precision 3D geometry measurement and high-resolution reverse engineering in combination with 3D printers etc. However, such attractive laser scanners have failed to address to fading phenomenon, which was a critical issue of laser scanning systems. Although extensive research has been carried out on the fading issue, no single study exists which effectively coped with such an impairment inherent in 3D geometry measurement using laser scanners. In this article, we propose a 3D laser scanner having a tunable high-speed polarization scrambling scheme and cope with the fading phenomenon. To our knowledge, this is the first report of versatile removal of the fading phenomenon inherent in laser scanning systems
We demonstrated distributed strain measurement using the slope-assisted Brillouin optical correlation-domain analysis (SA-BOCDA) with polarization maintaining fiber (PMF), which has no polarization scrambler. The 0.35- m-long strained section in 47-m-long PMF was clearly measured with 1-kHz sampling ratio and no averaging. Next, we proposed the new concept of the human interactive sound effector using SA-BOCDA with PMF, which means that the distributed measurement data was used as sound waveform directly. Then we demonstrated the sound wave controlling with SA-BOCDA with PMF. This result will be useful for not only the sensing application but also the musical instrument application based on nonlinear optical phenomenon.
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