The measurable range of FMCW LiDAR is limited by receiver bandwidth and laser coherence length. By combining a swept optical frequency comb with phase noise compensation, we achieved 14.7 mm precision over 8 km.

We propose and demonstrate a novel four-port evanescently coupled Ge-on-Si photodetector (PD) designed to overcome performance limitations under high optical power. This study aims to mitigate the space-charge effect that causes photocurrent saturation and bandwidth degradation in conventional Ge-on-Si PDs. In our device, the input optical signal is divided into four waveguides that evanescently couple light into a central Ge absorption region from four sides. This configuration effectively distributes the optical intensity, thereby reducing the space-charge effect. The fabricated PD exhibits a linear responsivity of 1.08 A W-1, a large maximum photocurrent of 54 mA, and a 3 dB bandwidth of approximately 15 GHz at an output current of 15 mA. These results indicate that the proposed PD offers a promising solution for high-power and high-speed applications, demonstrating strong potential for advanced radio-frequency photonics.

We demonstrate novel high-saturation-current waveguide-coupled vertical Ge-on-Si photodetectors, achieving a saturation current of 21.4 mA, a dark current of 30.1 nA at −3 V, and a 3 dB bandwidth of 15.2 GHz at −3 V.

We propose a novel method for non-contact measurement of the refractive index of samples using FMCW LiDAR. Based on the proposed measurement principle, we measured the refractive index of cutting oil. The measurement results followed a Gaussian distribution, with a standard deviation of 2.41x10(-4).

In the field of structural health monitoring with optical fiber sensing, installing instrumentation close to the structure is often challenging. By connecting the structure to the measuring device with a transmission optical fiber (this section not subject to measurement), advantages such as indoor installation of the measuring device and shared monitoring of multiple structures can be achieved. However, in such cases, part of the measurement range is consumed by the length of the transmission fiber, significantly limiting the effective measurement range. In this paper, we propose a novel optical frequency domain reflectometry (OFDR) configuration that bypasses the transmission fiber. We demonstrate remote strain measurements on a 1-km section of the fiber under test, located 25 km away from the OFDR system. Strain measurements of up to 1000 mu e with a spatial resolution of 10 cm were successfully achieved.

The slowing down of Moore's law has driven the development of application-specific processors for deep learning. Analog photonic processors offer a promising solution for accelerating matrix-vector multiplications (MVMs) in deep learning by leveraging parallel computations in the optical domain. Intensity-based photonic MVM processors, which do not utilize the phase information of light, are appealing due to their simplified operations. However, existing intensity-based schemes for such processors often employ wavelength multiplexing or mode multiplexing, both of which have limited scalability due to high insertion loss or wavelength crosstalk. In this work, we present a scalable intensity-based photonic MVM processor based on the concept of waveguide multiplexing. This scheme employs multiport photodetectors (PDs) to sum the intensities of multiple optical signals, eliminating the need for multiple wavelengths or modes. A 16-port Ge PD with a 3 dB bandwidth of 11.8 GHz at a bias voltage of-3 V is demonstrated, and it can be further scaled up to handle 250 ports while maintaining a 6.1 GHz operation bandwidth. A 4 x 4 circuit fabricated on a Si-on-insulator platform is used to perform MVMs in a three-layer neural network designed for classifying Iris flowers, achieving a classification accuracy of 93.3%. Furthermore, the performance of large-scale circuits in a convolutional neural network for Fashion-MNIST is simulated, resulting in a classification accuracy of 90.53%. This work provides a simplified and scalable approach to photonic MVM, laying a foundation for large-scale and multi-dimensional photonic matrix-matrix multiplication in optical neural networks. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

This study demonstrates the ability of the Rayleigh-based phase-noise compensated optical frequency-domain reflectometry (PNC-OFDR) sensing method to monitor the distributed temperature field with an ultra-short data acquisition period of 2 ms, a spatial resolution of 2 cm, and a temperature resolution of 0.1 degrees C. A heating cable (H-cable) was embedded within a cylindrical concrete mortar specimen and subjected to various heating powers. A sensing optical cable (temperature measurement cable) was placed adjacent to the H-cable to monitor the temperature distribution continuously. Two water-holding boxes were installed along the specimen at two positions to retain water. The study's results indicated that the PNC-OFDR technique demonstrated a high sensitivity to even small temperature changes, enabling it to pinpoint water locations at two distinct points accurately. The research determined the minimal heating power required to successfully locate the water positions. The magnitude of the heating power exerted a significant impact on the temperature change. Three distinct phases of temperature increment were observed for a given heating period: rapid, fast, and gentle increase. The insights gained from this study have the potential to be applied in natural fields, allowing for the detection of groundwater and seepage phenomena in vulnerable slopes.

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.

The complex impulse responses of two types of coupled 2-core fibers, with homogeneous and heterogeneous cores, were measured by using a measurement system based on linear optical sampling. The measurements were performed over a bandwidth of 20 nm (∼2.5 THz), and the spatial-mode dispersion (SMD) was analyzed. Several lengths of both homogeneous and heterogeneous core fibers were prepared to investigate the length dependence of SMD. The SMDs for divided frequency bands were examined by numerically analyzing the measured broadband impulse response. Consequently, the power impulse response at each subband was obtained, and the variation in the SMDs was examined. In addition, the statistical distribution of the SMD was revealed to be different for each type of the 2-core fiber. The SMD observed at a specific band is handled by the receiver in space-division multiplexing transmission systems. Finally, we present a measurement of mode dependent loss (MDL) of a 2-core fiber, and discuss the impact of time axis inaccuracy to the MDL measurement. These analyses performed are expected to be beneficial for designing such systems.

Civil structures e.g. bridges, tunnels, and dams are essential to human societies. Currently, these complex engineered structures are challenged by aging issues. It is crucial to monitor the conditions of such structures in realtime to ensure their protection and conduct sufficient maintenance and rehabilitation when they begin to show omens of degradation or damage. Observation of Rayleigh scattering spectra from optical fibers using fiber Rayleigh reflectometry enables distributed sensing of static and dynamic strain in structural health monitoring for civil structures. Its key performance indices are the spatial resolution, the strain dynamic range, the measurement range, and the refresh rate. This article reviews tunable-wavelength optical time-domain reflectometry and coherent optical frequency-domain reflectometry and discusses the performance indices of each method in terms of the performance indices listed above. After analytical derivation, we have found that signal-to-noise ratios of both schemes are the same, which is a valuable discovery. In addition, we enumerate and review recent major industrial developments of both schemes.

To the best of the authors' knowledge, we present the first -ever demonstration of the two-dimensional linear optical sampling of fiber spatial modes. The images of the fiber cross sections excited by the LP01 or LP11 modes are directly projected onto a two-dimensional photodetector array and coherently sampled by local pulses with a uniform spatial distribution. Consequently, the spatiotemporal complex amplitude of the fiber mode is observed with a time resolution of a few picoseconds by using electronics with a bandwidth of only a few MHz. Such ultrafast and direct observation of vector spatial modes enables the characterization of the space-division multiplexing fiber itself with high time accuracy and wide bandwidth.

We propose a FMCW LiDAR using a wavelength-swept optical frequency comb, which overcomes the ranging limitation caused by the receiver bandwidth. We have established the fundamental part of our proposal and conducted proof-of-concept ranging experiments. With optical fiber transmission for emulating long distance free-space propagation, we have succeeded in ranging corresponding to 1605-m free-space propagation with a 10-MHz FMCW receiver.

Long range strain measurement is presented by OFDR employing wide and rapid wavelength swept light source. The measurement range of ~1000 µε with µε-level accuracy is achieved over 140 m, potentially at 150 Hz.

Transmission length dependency of complex impulse responses of coupled 2-core fibers are investigated using coherent sampling with picosecond time resolution over 20-nm bandwidth. Spectrally decomposed analysis is accomplished to observe the statistical nature.

We propose and demonstrate an overall non-mechanical spectrally steered laser rangefinder using the dispersion-tuned swept laser (DTSL) and a passive diffractive element. The DTSL has no mechanical moving parts, making it possible to achieve an inertial-free high wavelength sweeping speed. The inherent intensity-modulation characteristic of the DTSL allows the modulation phase-shift method to be applied, similar to that used for an amplitude-modulated continuous-wave (AMCW) rangefinder. Since the pulse repetition rate of the DTSL is chirped, standard signal processing techniques for AMCW are not applicable. In this paper, we propose a novel chirped amplitude-modulated phase-shift (CAMPS) method with a signal processing technique to obtain the phase-shift information from a chirped amplitude-modulated signal. As a proof of concept, we demonstrated the CAMPS LiDAR with an axial ranging resolution of ∼50 μm at a scanning speed of 10 kHz.

This paper describes a method to evaluate the modulated waveforms output by a high-speed external phase modulator over a wide wavelength range by using linear optical sampling (LOS) and a wavelength-swept light source. The phase-modulated waveform is sampled by LOS together with the reference signal before modulation, and the modulation waveform is observed by removing the phase noise of the light source extracted from the reference signal. In this process, the frequency offset caused by the optical-path length difference between the measurement and reference interferometers is removed by digital signal processing. A pseudo-random binary-sequence modulated signal is observed with a temporal resolution of 10 ps. We obtained a dynamic range of similar to 40 dB for the measurement bandwidth of 10 nm. When the measurement bandwidth is expanded to entire C-Band (similar to 35 nm), the dynamic ranges of 37 similar to 46 dB were observed, depending on the wavelengths. The measurement time was sub-seconds throughout the experiment.

We demonstrate a novel signal-processing method for spectrally-steered LiDAR using DTSL for 3D imaging at >10 kHz. The proposed method gives ranging-resolution of <100 μm, which is 3-4 times better than conventional AMCW methods.

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.

微孔作为一种常见结构,被广泛应用于生物医疗、航空航天、三维封装等领域.飞秒激光具有的超短脉冲持续时间和超高峰值功率特性使其在高质量微孔加工方面具有独特优势.本文综述了近年来飞秒激光时空整形微孔加工方法及其应用,包括飞秒激光时空整形方法、时域/空域整形的电子动态调控微孔加工以及微孔在增透减反、切割以及油水分离、雾气收集、气体收集等方面的应用,并讨论了时空整形飞秒激光微孔加工目前所面临的挑战和未来研究方向.

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.