Design and fabrication of a sub-3†dB grating coupler on an X-cut thin-film lithium niobate platform.

Thin-film lithium niobate (TFLN) based integrated photonic devices have been intensively investigated due to their promising properties, enabling various on-chip applications. Grating couplers (GCs) are wildly used for their flexibility and high alignment tolerance for fiber-to-chip coupling. However, achieving high coupling efficiency (CE) in TFLN GCs often requires the use of reflectors, hybrid materials, or extremely narrow linewidths of the grating arrays, which significantly increases the fabrication difficulty. Therefore, there is a demand for high-CE GCs on TFLN with simple structure and easy fabrication processes. In this paper, combining process capabilities, we demonstrate a highly efficient apodized GC by linearly optimizing the period length and the fill factor on a 600-nm-thick TFLN platform. Without any reflector or hybrid material, we achieve a remarkable coupling loss of -2.97 dB at 1555 nm on the 600-nm-thick X-cut TFLN platform with only a single lithography and etching step. Our work sets a new benchmark for CE among GCs on the 600-nm-thick TFLN platform.

Hybrid integrated Si3N4 external cavity laser with high power and narrow linewidth.

We have designed and fabricated a hybrid integrated laser source with full C-band wavelength tunability and high-power output. The external cavity laser is composed of a gain chip and a dual micro-ring narrowband filter integrated on the silicon nitride photonic chip to achieve a wavelength tuning range of 55 nm and a SMSR higher than 50 dB. Through the integration of the semiconductor optical amplifier in the miniaturized package, the laser exhibits an output power of 220 mW and linewidth narrower than 8 kHz over the full C-band. Such a high-power, narrow-linewidth laser diode with a compact and low-cost design could be applied whenever coherence and interferometric resolutions are needed, such as silicon optical coherent transceiver module for space laser communication, light detection and ranging (LiDAR).

Fiber Bragg gratings inscribed in nanobore fibers.

The nanobore fiber (NBF) is a promising nanoscale optofluidic platform due to its long nanochannel and unique optical properties. However, so far, the applications of NBF have been based only on its original fiber geometry without any extra functionalities, in contrast with various telecom fiber devices, which may limit its wide applications. Here, we provide the first, to the best of our knowledge, demonstration of NBF-based fiber Bragg gratings (FBGs) introduced by either the femtosecond (fs) laser direct writing technique or the ultraviolet (UV) laser phase mask technique. Moreover, the FBG fabricated via the UV laser was optimized, achieving a high reflectivity of 96.89% and simultaneously preserving the open nanochannel. The NBF-based FBGs were characterized in terms of temperature variation and the infiltration of different liquids, and they showed high potential for nanofluidic applications.

Distributed fiber mountain seismic monitoring and steady-state analysis under natural earthquakes.

Mountain dynamic response monitoring plays important roles in geological disaster evolution monitoring and warning. A distributed mountain seismic monitoring and steady-state analysis method is demonstrated with distributed acoustic sensing (DAS) and a natural earthquake stimulus. In the field test, the seismic detection capability is first verified by comparing the recorded seismic waveforms from DAS and existing seismic stations. The vibration signal difference between steady-state and unsteady-state mountain parts is apparent; the operational modal analysis method is utilized to extract the response difference and to monitor the disaster evolution process. The proposed method has many advantages, including being easy to deploy, all-weather online monitoring, etc. It is believed that the proposed method will broaden the DAS application scope and promote the development of geological disaster early warning such as landslides and collapses.

Line shape of a delayed self-heterodyne varied with noise types and delays.

The delayed self-heterodyne and self-homodyne (DSH) method is widely used for measuring the line shapes of high coherent lasers. This method results in an autocorrelation of a laser line under the condition of a delay that is much larger than its coherent time. In practice, the delay is often not so long, especially for very narrow linewidth lasers, resulting in errors in rebuilding the laser's line shape from the DSH line. Many papers were devoted to the topic, but most of them are based on the formula for white noise. Analytical formulas of phase variance for 1/f noises are presented in this paper; the DSH line shapes for different noise types and different delay lengths are simulated based on the formulas. Some experimental data of the DSH line, combined with the power spectral density of frequency noise, are processed, showing good agreement with the theoretical analysis. It is indicated that the DSH line shape shows complicated behaviors varied with the delay, with noise types, and with the measurement duration. Such effects are to be compensated for in retrieving the laser's linewidth from the DSH data.

A Low Temperature Coefficient Time-to-Digital Converter with 1.3 ps Resolution Implemented in a 28 nm FPGA.

Time-to-digital converter (TDC) is the key technology to realize accurate time delay measurement in high-precision optical fiber time-frequency transmission and synchronization, optical sensing and many scientific applications. The performance of FPGA-TDC based on the carry chain is sensitive to the operating temperature. This paper presents a parallel multichain cross segmentation method, without multitime measurements, which merges multichain into an equivalent chain, achieving low temperature coefficient and maintaining high precision. The equivalent chain breaks the limit of the intrinsic cell delay of a single carry chain, improves the precision and reduces the impact of temperature variation significantly. A two-channel TDC based on parallel multichain cross segmentation method is implemented in a 28 nm fabrication process Kintex-7 FPGA. The results show that the performance of TDC is improved with the increase of the number of chains. The 10-chain TDC with 1.3 ps resolution, 4.6 ps single-shot precision performs much better than the plain TDC with 11.4 ps resolution, 8.7 ps single-shot precision. The resolution is stable with 0.0002 ps/°C temperature coefficient under an operating temperature range from 25 °C to 70 °C. Moreover, the proposed method reduces the complexity of the circuit and the resource usage.

Fiber-optic time-frequency transfer in gigabit ethernet networks over urban fiber links.

We demonstrate a new optical pulse amplitude modulation (PAM) scheme where joint ultrastable time-frequency and gigabit ethernet data transfer with the same laser wavelength is realized. Time transmission is compatible with the White Rabbit (WR) based on gigabit ethernet networks, and frequency transmission is achieved by using 100MHz radio frequency (RF) modulation and the round-trip compensation methods. The laser is on-off keying (OOK) modulated by the WR signal, the RF and WR signal are modulated by optical PAM in a Mach-Zehnder interferometer modulator (MZM), and the local and remote site are connected by 96km urban fiber in Shanghai. The experimental results demonstrate that the frequency instabilities are 5.7E-14/1 s and 5.9E-17/104s, and the time interval transfer of 1 pulse per second (PPS) signal with less than 300fs stability after 104 s are obtained. This novel scheme can transmit frequency signals at hydrogen-maser-level stability in the gigabit ethernet network.

All-fiber narrow-bandwidth rectangular Optical filter with reconfigurable bandwidth and tunable center wavelength.

In this manuscript, a novel narrow-bandwidth rectangular optical filter based on multi-phase-shifted fiber Bragg grating (MPSFBG) is proposed. Using the local temperature control technology, the precise controllable phase shifts are introduced at different positions of the fiber Bragg grating (FBG). Therefore, the bandwidth of the MPSFBG-based filter with good shape factor can be reconfigured from 70 MHz to 1050 MHz by flexibly controlling the numbers and the positions of the phase shifts introduced in the MPSFBG. In addition, the center wavelength of the MPSFBG-based filter can be tuned through controlling the MPSFBG's environment temperature, and the tuning range of 22 GHz is realized. This is one of the best results for the narrow-bandwidth rectangular optical tunable filter with reconfigurable bandwidth. It can be widely used in the processing of reconfigurable signals in the optical communication networks and microwave photonics.

Absolute phase marking technology and fiber-optic remote coherent phase transmission.

Fiber-optic time and frequency synchronization technology demonstrates ultra-high synchronization performance and has been gradually applied in various fields. Based on frequency synchronization, this study addressed the problems of period ambiguity and initial phase uncertainty of the phase signal to realize the coherent transmission of the phase. An absolute phase marking technology was developed based on high-speed digital logic with zero-crossing detection and an optimized control strategy. It can realize picosecond-level absolute phase marking and provide a picosecond-level ultra-low peak-to-peak jitter pulse marking signal to eliminate phase period ambiguity and determine initial phase and transmission delay. Thus, by combining the high-precision phase measurement capability of the synchronized frequency signal and long-distance ambiguity elimination capability of the pulse-per-second signal, a high-precision remote coherent phase transmission over an optical fiber is realized. After frequency synchronization, the peak-to-peak jitter between the local and remote phase-marking signals can be only 3.3 ps within 10,000 s measurement time. The uncertainty of the coherent phase transmission is 2.577 ps. This technology can significantly improve the phase coherence of fiber-optic time and frequency transmission and provide a new approach to achieve peak-to-peak picosecond-level reference phase marking and high-precision fiber-optic remote coherent phase transmission. This demonstrates broad application prospects in coherence fields such as radar networking.

Distributed optical fiber hydrophone based on Φ-OTDR and its field test.

In this letter, a distributed optical fiber hydrophone (DOFH) based on Φ-OTDR is demonstrated and tested in the field. The specially designed sensitized optical cable with sensitivity up to -146 dB rad/µPa/m is introduced, and an array signal processing model for DOFH is constructed to analyze the equivalence and specificity of the distributed array of acoustic sensors. In the field test, a 104-meter-long optical cable and a Φ-OTDR system based on heterodyne coherent detection (Het Φ-OTDR) is utilized, and underwater acoustic signal spatial spectrum estimation, beamforming and motion trajectory tracking with high accuracy can be realized. As far as we know, this is the first report on the field trial of DOFH based on Φ-OTDR. The DOFH has the potential to achieve an array range of tens of kilometers, with elements spaced up to the meter level and flexible configuration, which has a broad application prospect for marine acoustic detection.

High SNR Φ-OTDR with Multi-Transverse Modes Heterodyne Matched-Filtering Technology.

Phase-sensitive optical time domain reflectometer (Φ-OTDR) has attracted attention in scientific research and industry because of its distributed dynamic linear response to external disturbances. However, the signal-to-noise ratio (SNR) of Φ-OTDR is still a limited factor by the weak Rayleigh Backscattering coefficient. Here, the multi-transverse modes heterodyne matched-filtering technology is proposed to improve the system SNR. The capture efficiency and nonlinear threshold are increased with multiple transverse modes in few-mode fibers; the incident light energy is permitted to be enlarged by a wider probe pulse by using heterodyne matched-filtering without spatial resolution being deteriorated. As far as we know, this is the first time that both multi-transverse modes integration method and digital heterodyne matched filtering method have been used to improve the SNR of Φ-OTDR simultaneously. Experimental results show that the noise floor is reduced by 11.4 dB, while the target signal is kept. We believe that this proposed method will help DAS find important applications in marine acoustic detection and seismic detection.

Ultralow noise DFB fiber laser with self-feedback mechanics utilizing the inherent photothermal effect.

Single frequency laser sources with low frequency noise are now at the heart of precision high-end science, from the most precise optical atomic clocks to gravitational-wave detection, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference cavity. Despite the tremendous progress, these laser systems are relatively high in terms of complexity and cost, essentially suitable for the laboratory environment. Nevertheless, more and more commercial applications also demand laser sources with low noise to upgrade their performance, such as fiber optic sensing and LiDAR, which require reduced complexity and good robustness to environmental perturbations. Here, we describe an ultralow noise DFB fiber laser with self-feedback mechanics that utilizes the inherent photothermal effect through the regulation of the thermal expansion coefficient of laser cavity. Over 20 dB of frequency noise reduction below several tens of kilohertz Fourier frequency is achieved, limited by the fundamental thermal noise, which is, to date, one of the best results for a free-running DFB fiber laser. The outcome of this work offers promising prospects for versatile applications due to its ultralow frequency noise, simplicity, low cost, and environmental robustness.

Effect of linewidth on intensity noise induced by stimulated Brillouin scattering in single-mode fibers.

Non-linear effects and intensity noise characteristics are critical factors that influence the performance of fiber communication systems as well as fiber-based sensors. It is observed in our experiment that relative intensity noise (RIN) subsequent to fiber transmission has a strong dependence on laser linewidth. Over a short transmission distance, RIN decreases with a narrowing laser linewidth. For longer distances, a narrower laser linewidth will result in a smaller RIN in a frequency range higher than 1 MHz and a larger RIN in a low-frequency range. In this study, the Brillouin linewidth parameter is introduced into a stimulated Brillouin scattering (SBS) three-wave coupling equation to simulate RIN variation phenomenon. Excellent agreement between the theoretical and experimental RIN spectra was obtained. We initially prove that the phenomenon is primarily owing to SBS. It is believed that the experimentally observed phenomena and theoretical justification presented in this study is significant in improving the performance of communication systems and fiber-based sensors.

Artificial neural network for the reduction of birefringence-induced errors in fiber shape sensors based on cladding waveguides gratings.

Cladding waveguide fiber Bragg gratings (FBGs) provide a compact and simple solution for fiber shape sensing. The shape sensing accuracy is limited by birefringence, which is induced by bending and the non-isotropic FBG structure (written by femtosecond laser point-by-point technique). An algorithm based on an artificial neural network for fiber shape sensing is demonstrated, which enables increased accuracy, better robustness, and less time latency. This algorithm shows great potential in the application of high-accuracy real-time fiber shape measurements.

Multi-source aliasing suppression for distributed fiber acoustic sensing with directionally coherent enhancement technology.

Distributed fiber acoustic sensing (DAS) can detect almost all disturbances along the sensing fiber and is widely applied. However, the signals from multiple adjacent disturbance sources are superimposed, according to the sensing principle. A directionally coherent enhancement technology is demonstrated for DAS to suppress multi-source aliasing in air. In preliminary works, two situations are considered for feasibility verification. The submerged weak target signal is effectively extracted from strong broadband noise, and two different same-frequency signals from two sources are separately rebuilt with the same detected signal. As far as we know, this is the first time that the directionally coherent enhancement is proposed for DAS and the multi-source aliasing is suppressed. This technique will help DAS find new important foreground in acoustic detection of large-scale plants with many similar noisy devices, including discharge detection in high voltage substations and acoustic emission flaw detection in mechanical factories.

Narrow linewidth swept laser source based on cascaded multi-wavelength injection of DFB lasers.

A linearly swept laser source over broadband with a fast sweep rate and narrow linewidth is realized using a novel optoelectronic scheme based on a multi-wavelengths (mutually coherent) injected distributed feedback (DFB) laser. Under the condition of multi-wavelengths injection, the injection-locking and four-wave mixing (FWM) process can occur simultaneously in the DFB laser, inducing a swept laser source with a sweep range of 100 GHz and sweep rate of 10 THz/s. Furthermore, with the phase noise character analyzation of the swept laser source, the phase noise deterioration due to the radio frequency (RF) signal is studied quantitatively. Besides the influence of the RF signal noise, the phase noise deterioration in the FWM process can be suppressed completely with the phase-locked pump beam and signal beam based on the injection-locking principle. This low phase noise swept laser source with sub-kilohertz linewidth could have wide applications in lidar.

Recent Progress in Distributed Fiber Acoustic Sensing with Φ-OTDR.

Distributed fiber acoustic sensing (DAS) technology can continuously spatially detect disturbances along the sensing fiber over long distance in real time. It has many unique advantages, including, large coverage, high time-and-space resolution, convenient implementation, strong environment adaptability, etc. Nowadays, DAS becomes a versatile technology in many fields, such as, intrusion detection, railway transportation, seismology, structure health monitoring, etc. In this paper, the sensing principle and some common performance indexes are introduced, and a brief overview of recent DAS researches in Shanghai Institute of Optics and Fine Mechanics (SIOM) is presented. Some representative research advances are explained, including, quantitative demodulation, interference fading suppression, frequency response boost, high spatial resolution, and distributed multi-dimension localization. The engineering applications of DAS, carried out by SIOM and other groups, are summarized and reviewed. Finally, possible future directions are discussed and concluded. It is believed that, DAS has great development potential and application prospect.

Photonic high-fidelity storage and Doppler frequency shift of broadband RF pulse signals.

A microwave photonic system which can simultaneously realize the functions of rapidly tunable Doppler frequency shift (DFS) and high fidelity storage of broadband RF signals is proposed and verified. Single-sideband carrier-suppression modulation combined with dual-AOM frequency shifting ensures large-range and fast-responding DFS. And time-gated semiconductor amplifier (SOA) based fiber delay loop can realize high-fidelity RF pulse storage with high extinction ratio switching and amplification characteristics of time-gated SOA. A spurious rejection ratio greater than 40 dB, tuning range of DFS greater than ± 3 MHz, response speed of DFS less than 30 ns, and high fidelity storage of 4 GHz-12 GHz RF signals with greater than 381 circulations (corresponding 80 us delay time) are realized by the proposed structure. The maximum signal-to-noise ratio (SNR) is 13.6 dB within 381 circulations. Based on the experimental data, the simulation results show that the delay time also could be extended to 10 times more.

Practical multi-class event classification approach for distributed vibration sensing using deep dual path network.

Influenced by severe ambient noises and nonstationary disturbance signals, multi-class event classification is an enormous challenge in several long-haul application fields of distributed vibration sensing technology (DVS), including perimeter security, railway safety monitoring, pipeline surveillance, etc. In this paper, a deep dual path network is introduced into solving this problem with high learning capacity. The spatial time-frequency spectrum datasets are built by utilizing the multidimensional information of DVS signal, especially the spatial domain information. With the novel datasets and a high-parameter-efficiency network, the proposed scheme presents good reliability and robustness. The feasibility is verified in an actual railway safety monitoring field test, as a proof-of-concept. Seven types of real-life disturbances were implemented and their f1-scores all reached up to 97% in the test. The performance of this proposed approach is fully evaluated and discussed. The presented approach can be employed to improve the performance of DVS in actual applications.

High-stability and multithreading phase-coherent receiver for simultaneous transfer of stabilized optical and radio frequencies.

We demonstrate a high-stability and multithreading coherent receiver for simultaneous distribution of stabilized optical and radio frequencies (RFs). The technique is based on a monolithic electroabsorption modulator integrated with a distributed feedback laser, which can purify and amplify the optical carrier while recovering the RF signal as a high-speed photodetector. The large-dynamic-range and high-bandwidth phase-locking system preserves the stability of the receiver for optical and RF signals to 3.5×10-20 and 6.4×10-18 at 1000 s, respectively. Furthermore, a dual-stabilization system using this novel receiver is proposed for simultaneous transfer of ultrastable optical carriers and RF signals over a 263 km fiber link. The transferred frequency stabilities of the optical carrier and the 9.1 GHz signal are 6.5×10-20 and 1.6×10-17, respectively, for an averaging time of 10,000 s.