Amplitude-modulated continuous-wave laser scanner with focusing optics is expected to realize high-precision 3D measurement which requires accuracy of mm or less. Since such modulation scheme employs periodical intensity modulation, the longitudinal resolution and the unambiguous range are in a trade-off. Another problematic situation is the case that the target object is large so that the scanning range exceeds the unambiguous range. The acquired 3D point clouds contain phase jumps at the boundary of the unambiguous range imposed by the modulation frequency. In this paper, we propose multiple solutions to cope with these problems. First of all, our system utilizes dual-frequency modulation to overcome trade-off between the resolution and the unambiguous range. With appropriate electronics for demodulation, the hardware imposed longitudinal resolution of 19.2 μm +-38 μm and the unambiguous range of 48 cm +-614.3 μm were realized. The unambiguous range can be shifted within 5 m range using a mechanical focusing optics. However, such an attractive laser scanner still suffers from ranging ambiguity. The noise on the laser light modulated by the lower modulation frequency results in phase deviation which can incur ranging errors at the integer times of the half cycle of the higher modulation frequency. We have coped with such ranging ambiguity by synthesizing ranging errors with the data in the correct range. Especially, defocused 3D point clouds contaminated by severe ranging errors were analysed and restored. The measurement range can thus be elongated by >20 times of the depth-of-focus using such data processing without manipulating the mechanical focusing optics. Next, we developed an algorithm as a remedy to prevent phase jumps at the unambiguous range. With exploitation of the relationship between the intensity and spatial information, the phase unwrapping was performed to recover the spatial continuity. Thereafter, the unambiguous range can be elongated to be >48 cm. With the above-all mentioned configuration and data processing, we have overcome the ranging ambiguity inherent in the amplitude-modulated continuous-wave laser scanner.

We present long-range strain measurements using phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) over 1.5 km with a strain range of 1000 μϵ. In conventional OFDR, the observed Rayleigh scattering spectral correlation cannot be maintained over the coherence length of the laser. The use of PNC-OFDR extends this correlation distance by a factor of 100 or more, confirming high Rayleigh correlation over the entire 1.5 km section. As a result, we achieved strain measurements of up to 1000 μϵ at the end of the fiber under test (FUT), with a spatial resolution of 10 cm. In addition, we develop a scheme to observe fast dynamic strain by decomposing the Rayleigh scattering spectrum into sub-bands. Thanks to a newly proposed statistical processing technique, such a scheme eliminates the need for prior measurement of the reference spectrum in a stationary state. With our scheme, we observed dynamic strain up to 200 Hz at a measurement distance over 200 m. Such a result greatly surpasses the conventional dynamic strain measurement by OFDR.

Forward Brillouin scattering (FBS) has been regarded as a key tool for distributed sensing of strain, temperature, acoustic impedance, etc. To date, FBS-based distributed sensing has been experimentally reported only by time-domain techniques. In this paper, we demonstrate correlation-domain distributed temperature sensing using cascaded FBS seeded by backward stimulated Brillouin scattering (SBS). By localizing the backward SBS using synthesized optical coherence function, cascaded FBS can also be localized. In the proof-of-concept experiment, we detect a 104 m long heated section in a 397 m long highly nonlinear fiber.

The laser scanner is a powerful tool for 3D geometry measurement. The amplitude-modulated continuous-wave laser scanner, which benefits from high accuracy measurement operating in the intermediate range, is suitable for high-precision 3D industrial inspection. However, due to the limited dynamic range, laser scanners suffer from data loss and deterioration. This poses challenges for 3D modelling, conversion of 3D point cloud data to 3D solid data etc. We chose the operation wavelength at 1530-nm region. The dynamic range of an amplitude-modulated continuous-wave laser scanner was enhanced by implementation of a combination of a low-noise and high-gain optical preamplifier and PIN photodiodes, a pellicle beamsplitter, and an automatic gain control using a polarization-independent high-speed variable optical attenuator based on electro-optic ceramics in the receiver. In these manners, receiver sensitivity of -72 dBm was achieved and receiver dynamic range of around 37 dB was realized. We expect that our results contribute to high-definition industrial inspection for Industry 4.0.