This PDF file contains the front matter associated with SPIE Proceedings Volume 11902, including the Title Page, Copyright information and Table of Contents

Spectroscopy is an essential tool to explore the interaction between light and matter. With the extensive study of ultrafast phenomena, ultrafast spectrum analysis is in great demand. In view of the limited acquisition frame rate of the conventional spectroscopy, the ultrafast temporal focusing mechanism was proposed and demonstrated, and it is capable of capturing arbitrary waveform signal, with the acquisition frame rate up to 100 MHz. Moreover, several approaches have been proposed to further improve its accuracy and bandwidth, it has achieved 2-pm spectral resolution and 58-nm observation bandwidth, and the observation range has been further extended to microwave and terahertz span. Furthermore, this spectroscopy has successfully characterized ultrafast phenomena and mechanisms of devices, and has been applied to several ultrafast spectral imaging systems.

In practical engineering applications, the real-time monitoring of structural physical field is very important, which requires overall performance improvements on the Brillouin optical fiber sensing, including fast measurement, quick data demodulation and small amount of data storage. A novel Brillouin optical time-domain analysis (BOTDA) scheme has been proposed based on compressed sensing and pattern recognition algorithms, which paves the way for the real time Brillouin optical fiber sensing. In order to test the sensing performance, the Brillouin gain spectrum is measured by the traditional fast BOTDA, where the frequency step and span are 4 MHz and 500 MHz, respectively. By employing the proposed algorithms, the strain information is directly obtained with only 35% of the full data, verifying the feasibility of real-time measurements.

Brillouin-based optical fiber sensors have been developed over the past few years and played a significant role for distributed temperature and strain measurements, which include many properties such as high measurement accuracy, large measurement range and environmental suitability. Among these sensors, Brillouin optical fiber sensors via optical chirp chain (OCC) become an ideal choice for ultrafast distributed measurement to distinguish quick-changing events in practical applications. This paper begins with the introduction of the concept and the generation schemes of OCC, the spectra distortion characteristic of OCC Brillouin signal are analyzed. The efforts towards such OCC based Brillouin optical fiber sensing for ultrafast measurement are reviewed here as well, with the OCC based Brillouin optical time domain analysis and the OCC based Brillouin optical time domain reflectometry, which give distinguished performance for dynamic measurement, long distance measurement and one end access measurement. Meanwhile, the consequent future challenges of OCC based distributed sensing are discussed and presented, such as as high spatial resolution and high accuracy measurement.

Distributed acoustic sensing technology has unique advantage in diverse applications, such as seismology, mineral exploration, and so on. We explored the methods to increase measurement range, distance and speed, which are the most important requirement for field deployment. Heterogeneous sideband linear frequency modulated optical pulse with different frequency modulated bandwidth was used to realize simultaneous measurement of acoustic events in different dynamic ranges. Weak fiber Bragg gratings and phase of constructed single frequency were proposed for long distance sensing. Fast processing algorithm based on a graphics processing unit for a linear frequency modulation pulse demodulation was realized. The experiment results verified our methods well.

High-resolution 3D optical imaging is important for unmanned underwater vehicles (UUVs) in the applications of target detection and recognition, underwater engineering, automatic navigation, scientific research, and natural resources exploration. Compared with stereo camera and 3D laser scanning imaging, 3D range-gated imaging (3D RGI) has longer detection range and higher spatial resolution at the same time. This paper presents a survey on 3D range-gated imaging methods for underwater detection. Up to now, there are two main methods developed, including time slicing method and gated range intensity correlation method. The literature about 3D RGI has been reviewed in different detection applications for UUV. We also introduce our works in 3D gated range-intensity correlation imaging for fishing net detection and marine life in-situ detection. This paper is beneficial for the 3D RGI technique in underwater detection applications for UUV.

Publisher’s Note: This paper, originally published on 9 October 2021, was withdrawn on 13 October 2021 per author request.

In this work, we demonstrate an ultra-narrow linewidth SOA fiber laser with single-longitudinal-mode (SLM) operation, wherein the side mode is suppressed by the Rayleigh backscattering (RBS) in a distributed self-injection feedback structure (DSIFS). Herein, the high scattering fiber (HSF) with a large scattering coefficient is employed as the DSIFS to soar the amount of RBS signal. In this experiment, a hybrid laser is constructed by the HSF serving as the feedback medium of ring main cavity. The SLM operation and linewidth narrowing of the proposed laser at different pump currents under the premise of stable laser output are discussed. Our experimental results show that the side mode suppression ratio of the SOA fiber laser is 70 dB with an output linewidth is less than 540 Hz. The 50 m HSF is employed to achieve laser side mode suppression and deep compression of linewidth, which greatly simplifies the system and improves performance of the laser

An all-fiber flat-top orbital angular momentum (OAM) mode convertor with ultra-wideband is numerically demonstrated, which is based on a helical long period fiber grating (HLPG) with phase modulation. The proposed phase-modulated HLPG is formed in a single mode fiber (SMF) and works at dispersion turning point. A flat-top OAM mode convertor with a bandwidth of ∼371 nm and a conversion efficiency of ∼100% has been successfully obtained. The temperature and strain effects on the proposed OAM mode convertor have been also theoretically investigated, respectively. The results show that the proposed phase-modulated HLPG based flat-top OAM mode convertor has a certain resistance to external temperature and strain changes, which will be beneficial to reduce its packaging cost and difficulty. The proposed device may find potential applications in the OAM mode division multiplexing (MDM) system combined with the wavelength division multiplexing (WDM) technique.

Microwave photonic filter (MPF) based interrogation solutions have attracted considerable research interest, through the sensing information conversion from the optical domain to the microwave domain and high-resolution electrical spectrum analyzing processing techniques. In order to overcome the trade-off between measurement accuracy and interrogation speed existing in the traditional direct-detection method, an efficient machine learning algorithm is introduced into the MPF-based interrogation system. Compared with the traditional direct-detection method, the proposed method can achieve better measurement accuracy under the sparsely sampled frequency response, whilst the interrogation speed is greatly improved. In addition, the well-trained model has strong adaptability to the amplitude variation of the microwave photonic filtering response.

An approach to measuring ultra-wideband frequency responses of high-speed photodetectors (PDs) is proposed based on low-speed photonic sampling. The optical frequency comb lines of a mode-locked laser diode (MLLD) can be used as the ultra-wideband and scalable optical stimulus of PD. The relative frequency response of PD can be extracted by analyzing the frequency components at comb lines of the MLLD. Thereinto, the uneven response of the MLLD can be eliminated through the specific frequency photonic sampling, thereby realizing the self-referenced and ultra-wideband measurement of PD. Moreover, the measuring frequency range is 2M-fold expanded with respect to the operating range of the microwave modulation frequency.

High-accuracy and high-speed optoelectronic oscillator (OEO)-based magnetic field and electric current fiber measurement is presented in this paper. Thanks to the Giant-Magnetoresistance Effect and Faraday Effect, the magnetic field and electric current can be transformed to the changes in characteristics of light propagating in optical fiber. Then, the use of OEO can translate the magnetic field and electric current dependent changes in optical domain to the oscillating frequency changes in microwave domain in order to increase the interrogation speed and resolution. The theoretical analysis and principle are investigated systematically. Experimental results are then provided to validate the OEO-enabled magnetic field and electric current measurement techniques.

A photonic-assisted transceiver for microwave Doppler frequency shift (DFS) measurement, which possesses capabilities of generating a frequency-quadruplicated transmitted signal and evaluating the speed and the orientation of the moving object, is proposed and demonstrated by simulating. In the transmitter, the frequency-quadruplicated transmitted signal is obtained by beating the carrier-suppressed ±2nd sidebands. In the receiver, a low-frequency reference signal is introduced to estimate the sign of the DFS. Different from the other DFS measurement solutions based on microwave photonics, the proposed approach takes the advantages of generating a high transmitted signal, employing a low reference signal, and having no use of an optical filter.

Simultaneous measurement and visualisation of multi-parameters are crucial in understanding and characterising complex combustion. A measurement technique coupled with deflection tomography and particle image velocimetry was proposed to obtain three-dimensional temperature and velocity fields of swirling combustion. A combustion system was developed to generate a non-premixed swirling flame. Moiré patterns in four view angles and tracer particle images illuminated by volume light source were captured simultaneously using a hybrid measurement system. Three-dimensional temperature and velocity fields were quantitatively reconstructed. The characteristics of swirling combustion were analysed on the bases of visualisations of temperature distribution, velocity distribution, streamlines. In each cross-section with varying heights from the nozzle, the high velocity and high temperature areas showed similar closed or semi-closed annular structure. In the central longitudinal section, the V-shaped high temperature and high velocity regions showed the swirling structure of combustion flow field. The high temperature area did not coincide with the high velocity area but was located relatively downstream. Additionally, the high velocity area was in the periphery of the high temperature area.

With the evolution of communication system toward higher capacity and higher speed, optoelectronic and microwave devices, such as photodiodes and electro-optic modulators, have been developed to operate at tens of or even beyond one hundred gigahertz. Terahertz bands and the corresponding devices are also playing increasingly important roles for future information systems. Traditional measurement methods and instruments, such as vector network analyzers and terahertz time-domain spectroscopy, are expensive, and bulky as well as other performance limitations. On the other hand, optical frequency comb (OFC) is one of the ultrafast optics technologies that has become a powerful and essential tool for numerous high-resolution metrology applications. Recently, dual-comb metrology techniques based on two sets of combs with slightly different comb-tooth spacing have attracted more attention, which have been applied in optical spectroscopy, THz domain metrology, absolute distance measurement and microwave signal characterization. However, the traditional dual-comb systems need two complicated and costly frequency-stabilized ultrafast lasers, and it prevents the application of these attractive schemes to field-deployed applications. In the past few years, there had been many efforts to explore solutions to realize simplified dual-comb systems. One approach is based on the generation of dual combs with high mutual coherence from one mode locked laser. Here, we illustrate the real-time, signal characterization and identification techniques enabled by the simplified dual-comb lasers.

A frequency-shifted dual-carrier method is proposed for microwave characterization of Mach-Zehnder modulators based on low frequency detection. The proposed method utilizes the heterodyne products between the beats of two modulated sidebands, and achieves calibration-free microwave measurement of Mach-Zehnder modulators with the help of electrical spectrum analysis. Our method features low-frequency detection with only one microwave source and avoids the responsivity correction introduced by the photodetector. In the experiment, the frequency response of a Mach-Zehnder electro-optic intensity modulator is measured by using the proposed method, where the measurement results fit in with those obtained by using the conventional optical spectrum analysis method.

Soliton molecules, exhibiting similarities to molecular-like behavior, have attracted tremendous attention. With the dramatic recent advances in real time spectroscopic techniques as well as time-domain probes, the detailed nature of various soliton molecular has been revealed. Particularly, time-stretch dispersive Fourier transform (DFT) technique has been applied to unveil the complex internal dynamics of bound states. Nevertheless, the observation time frame of such technique is limited to hundreds of micro seconds mainly confined by the storage capability of the real time oscilloscopes, as well as by the number and the distribution of soliton pulses. Here, we experimentally demonstrate a new probing concept, orbital angular momentum (OAM)-resolved method, for visual extraction of long-term internal phase motions within soliton molecules produced by a mode locked laser.

Photonic integrated circuits (PICs) play an important role in transporting and processing analog information. Reconfigurable optical computing chip can meet the needs of implementing miniaturized and multi-functional signal processors which has become a research hotspot in recent years. Here, we designed and fabricated a reconfigurable photonic integrated computing chip based on a quadrilateral MZI topology network. Several analog signal processing functions including temporal differentiation, integration and Hilbert transformation are experimental demonstrated with a processing bandwidth of 40 GHz. By reconfiguring optical path and changing the spectral ratio of the MZI structure in the network, the functions can be switched and the operation order can be tuned. The quadrilateral network is universal and can be extended to a larger network structure in the future.

Based on frequency entangled sources and arrival time measurements, a best time synchronization stability result of fewer than 100 fs has been reported over 20km fiber links, verifying the superiority of quantum time synchronization. In this paper, a two-way quantum time transfer over 50 km fiber links with the same frequency standard was implemented, with a short-term stability of 2.63 ps at an averaging time of 18 s and a long-term stability of 79.2 fs at 73700 s. The system accuracy in terms of the 50km fiber transfer length is measured as 524.75 ps, the agreement of which with the theoretical simulation illustrates that improving the spectral consistency of the two entangled sources will significantly improve the accuracy. This result shows that the fiber-optic two-way quantum time transfer can be successfully extended to a metropolitan fiber link distance of around 50 km or longer, not only maintaining the transfer stability well below picosecond but also promising further improvements in synchronization accuracy.

In this study, a modified OAM Mach-Zehnder (M-Z) interferometer for displacement measurement has been proposed and experimentally proved, where two conjugate OAM beams propagate on two arms respectively. The traditional M-Z OAM interferometer was modified by introducing a reflection module in the reference arm. As a result, the optical path shift mismatched reflections on two arms is eliminated, which improves the quality of the petal-like pattern produced by the interference between two conjugate OAM beams. And a significant rotation angle of the petal-like pattern vs. the tiny displacement of the tested object can be clearly observed. By accurately measuring the rotation angle of the petal-like pattern, a tiny displacement ranging from 50 to 800 nm with resolution of ~50 pm has been measured successfully.

A linear frequency modulation (LFM)-Costas waveform generation scheme is proposed and experimentally demonstrated based on a Fourier domain mode locking (FDML) optoelectronic oscillator (OEO). The FDML OEO is established based on stimulated Brillouin scattering (SBS) effect, where the Costas-coded probe light is realized by employing an open-loop voltage controlled oscillator (VCO) via electro-optic frequency shift. In the experiment, an LFM-Costas waveform with a 12-bit Costas sequence, a chirp rate of 39.234 MHz/μs and a period of 20.390 μs is generated in the frequency range of 8.020 GHz to 8.820 GHz. The generated LFM-Costas waveform has good pulse-to-pulse coherence according to the correlation results, and has been used to detect two targets with a distance difference of 0.30 m and 0.20 m

Basic principle, modeling method, error analysis and process design of products assembly and adjustment are analyzed. In order to improve the resolution of the imaging system and reach the theoretical limit, the off-axis system optimization method is promoted from the perspective of theory to engineering. The simulation results show that the system can meet the application requirements of MTF and other related criteria. And an engineering method is explored for real-time construction and control of dynamic systems.

Real-time identification of frequency-hopping millimeter-wave (mm-wave) signals is a real challenge, due to the high demand for detection bandwidth and processing speed. In this paper, we propose and demonstrate a novel microwave photonic approach to identifying frequency hopping of mm-wave signals based on the concepts of photonic time stretch (PTS) and Reservoir Computing (RC). The PTS scheme allows the modulated signal to be slowed down hence reducing the required detection bandwidth. The developed RC model offers unique features such as being more efficient and time saving in temporal data pattern classification than traditional methods. According to the simulation, RC can recognize the hopping instants and give a precise hop timing estimation in real-time.

Vortex beams have attracted attention due to their unique properties and wide applications. In this paper, a novel scheme for generating the first-order orbital angular momentum (OAM) modes based on anti-resonant reflecting guidance mechanism in ring-core fibers (RCFs) is proposed and experimentally demonstrated. By directly fusion splicing with the standard single-mode fiber (SMF), anti-resonant reflecting guidance of guided-core modes in RCF can be basically motivated. Consequently, the excited modes can well transmit inside the low-refractive-index central core of the RCFs with exhibiting a Gaussian field distribution (defined as the anti-resonant mode, AR mode). Because of different mode coupling coefficients under external pressure on the RCF, these AR modes can be coupled to the HE11 and HE21 modes, respectively. Results show that, by adjusting the polarization state of injected light beams as well as the pressure on the RCF, OAM modes with topological charge numbers of both the positive (L = +1) and negative (L = -1) can be separately generated at efficiencies of 63.2% and 61.7%, respectively

Nebulization therapy is a common treatment for respiratory diseases. The particle size distribution and physicochemical properties of the atomized droplets of different nebulizers have a great impact on the treatment. Droplets larger than 5 microns and less than 10 microns generally can only reach the upper airway, those with 5-10 microns can reach the nasopharynx, those below 5 microns can reach the bronchi, and those smaller than one micron can enter the alveoli. This study provides a polarization measurement system for atomized droplets. In the experimental device, we used a 532 nm laser as the light source. In the incident module, we used a polarizer and a quarter wave plate to form a polarization generator to generate a specific incident polarization state. In the detection module, we measured the polarization scattering signals at four different angles, namely 30°, 60°, 85° and 115°. When the droplet passes through the scattering region, the trigger channel set in the forward direction of 10° first detects the signal, and then triggers the other four channels to complete the detection of the polarization signal. The liquid medicines we used in the experiment were sterile water, acetylcysteine, budesonide and sodium chloride solution. For different liquid medicines, we compared the nebulization effects of two nebulizers, oxygen-driven nebulizer and compressed air nebulizer. The suspended droplets produced by the nebulizer first pass through a pressure-stabilizing box with a built-in fan to realize the uniform dispersion of the atomized droplets, and then use the sheath flow device to realize single droplet detection according to the principle of hydrodynamic focusing. Experiments show that this method can obtain multidimensional polarization information for each measurement and enables fast real-time detection of the polarization scattering signal of liquid droplets.

Dual-comb spectroscopy has become a powerful tool for spectral analysis because of its ultra-high resolution and fast frame rate. To capture the stable spectrum in the frequency domain, it is essential to lock the repetition frequency and the carrier envelope offset frequency of the mode-lock laser. By introducing a phase-lock loop to a piezoelectric ceramic based cavity, the repetition frequency can be easily synchronized to an external clock. To lock the carrier envelope offset frequency, a phase-shifted fiber Bragg grating with 132-MHz narrow pass band is introduced, and it is capable of filtering out single comb line of each comb. Therefore, the post-calibration signal for carrier envelope offset frequency can be obtained from the beating signal of the filtered adjacent comb lines. Based on this post-calibration scheme, a stabilized dual-comb spectroscopy is proposed and demonstrated with 120-nm observation bandwidth, 1-pm spectral resolution, and 100-Hz acquisition frame rate.

As a key component of all-optical switching network terminal technology, ultra-fast wavelength-tunable laser arrays are critical to the high integration and performance improvement of the entire optical switching system. In this article, the array of 2×8 matrix grating DFB laser arrays that we used are based on reconstruction equivalent chirp technology. The wavelength range is in the C-band. Based on this, we designed an excellent control circuit and a high-speed drive circuit for the needs of the laser array to provide the laser array with a drive current that can control the ultra-high-speed switching of channels. The final experimental results show that the channel spacing of the 16 channels of the laser array is 100G, In addition, the switching time of any two channels is less than 10 nanoseconds, and the wavelength selection time has nothing to do with the wavelength range. During the switching process, the centre wavelength drift of each channel is always within the channel wavelength error specified by WDM under the ITU-T standard.

The signal filtering of the grating encoder is of great significance to the measurement accuracy, aiming at eliminating the background noise potentially from the temperature changes, airflow fluctuations, and mechanical vibrations. Compared with the traditional time-frequency analysis methods, including wavelet transform, fast Fourier transform (FFT), and time Fourier transforms (TFT), the empirical mode decomposition (EMD) algorithm owing to no basis functions and high adaptability, is widely applied for signal decomposition. Here, we extended the EMD algorithm for the background-noise-based signal filtering in a grating encoder, with the experimental parameters of 5 µm/s moving speed and ~19 mm stroke. Simultaneously, a laser interferometer, as a reference, was additionally assembled to calibrate the measurement results of the grating encoder. The measurement signal was collected by NI acquisition card with a 1000 Hz sample rate and processed by EMD algorithm. Here, EMD decomposed the signal into multiple intrinsic mode functions (IMFs), which were reconstructed by removing the noise and DC components according to the correlation coefficients. Compared with the measurement results of the laser interferometer, the measurement displacement with a 6.2 µm error was solved by the phase correction and arctangent calculation from the reconstructed signals. Finally, our proposed signal-filtering approach based on the EMD algorithm exhibits a stable, accurate, and real-time calculation performance applicable for the grating encoder with ultra-high precision positioning.

Wavelength-swept laser is one of the most critical common components of fiber Bragg grating (FBG) sensors. However, a fast, stable, integrated and low-cost multi-channel wavelength-swept laser array is still unavailable. In this article, a multi-channel simultaneous wavelength-swept DFB laser array based on the reconstruction-equivalent-chirp (REC) technique is proposed and manufactured. The REC technology simplifies the fabrication process and greatly reduces the cost of the laser. The laser array contains 4 lasers in parallel with integrated heating resistance for thermal tuning to broaden the wavelength-tuning range. Each single-wavelength DFB laser introduces a π-phase shift structure, which is used to improve the single-mode performance of the laser. Meanwhile, the active multiplexer responsible for coupling and an optical amplifier (SOA) responsible for compensating the coupling loss of the laser are also integrated on the same chip. The driving circuits of the laser use FPGA to control the DAC chip to obtain precise current output and realizes continuous linear output of wavelength by changing the injection current size. The packaged laser module can realize a continuous wavelength-swept range of 2.4 nm per channel with the SMSR over 40 dB, achieving a simultaneous sweep of four channels with good scanning linearity and scanning speed. The work of this paper realizes the integration of linear wavelength-swept light sources, which creates the conditions for a low-cost, small-volume multi-channel sensing system in the future.

Autocorrelator, as a pulse measuring equipment, can effectively characterize pulse width and pulse waveform with high resolution. Limited by the refresh rate, conventional autocorrelator are incapable of capture the ultrafast processes of pulses, such as soliton transitions. In this paper, we proposed an ultrafast autocorrelator based on broadband radio frequency (RF) spectrum analyzer. The RF spectrum under test is firstly transformed to the continuous wave via the cross-phase modulation. Then the RF spectrum is mapped to the temporal domain through time stretch and finally measured by a real-time oscilloscope. The ultrafast autocorrelation trace is obtained from the inverse Fourier transform of the mapped RF spectrum. With the 60-GHz detected bandwidth of the photodetector adopted, the simulated ultrafast autocorrelator achieves the measurement rate of 50 MHz, the resolution of 300 fs, and the measuring window of 600 ps.