A high performance fast-Fourier-transform spectrum analyzer for measuring spin noise spectrums

A high performance fast-Fourier-transform (FFT) spectrum analyzer, which is developed for measure spin noise spectrums, is presented in this paper. The analyzer is implemented with a field-programmable-gate-arrays (FPGA) chip for data and command management. An analog-to-digital-convertor chip is integrated for analog signal acquisition. In order to meet the various requirements of measuring different types of spin noise spectrums, multiple operating modes are designed and realized using the reprogrammable FPGA logic resources. The FFT function is fully managed by the programmable resource inside the FPGA chip. A 1 GSa/s sampling rate and a 100 percent data coverage ratio with non-dead-time are obtained. 30534 FFT spectrums can be acquired per second, and the spectrums can be on-board accumulated and averaged. Digital filters, multi-stage reconfigurable data reconstruction modules, and frequency down conversion modules are also implemented in the FPGA to provide flexible real-time data processing capacity, thus the noise floor and signals aliasing can be suppressed effectively. An efficiency comparison between the FPGA-based FFT spectrum analyzer and the software-based FFT is demonstrated, and the high performance FFT spectrum analyzer has a significant advantage in obtaining high resolution spin noise spectrums with enhanced efficiency.

Review on Speckle-Based Spectrum Analyzer

Accurate spectral measurement and wavelength determination are fundamental and vital for many fields.A compact spectrum analyzer with high performance is expected to meet the growing requirements,and speckle-based spectrum analyzer is a potential solution.The basic principle is based on using the random medium to establish a speckle-to-wavelength mapping relationship for spectrum reconstruction.This article introduces current speckle-based spectrum analyzers with different schemes and reviews recent advances in this field.Besides,some applications by using speckle-based spectrum analyzers are also introduced.Finally,the existing challenges and the future prospects of using speckle for spectrum recovery are discussed.

Time-resolved energy spectrum measurement of a linear induction accelerator with the magnetic analyzer

We recently set up a time-resolved optical beam diagnostic system. Using this system, we measured the high current electron beam energy in the accelerator under construction. This paper introduces the principle of the diagnostic system, describes the setup, and shows the results. A bending beam line was designed using an existing magnetic analyzer with a 300 mm-bending radius and a 60° bending angle at hard-edge approximation. Calculations show that the magnitude of the beam energy is about 18 MeV, and the energy spread is within 2%. Our results agree well with the initial estimates deduced from the diode voltage approach.

FPGA implementation of 500-MHz high-count-rate high-time-resolution real-time digital neutron-gamma discrimination for fast liquid detectors

Fast neutron flux measurements with high count rates and high time resolution have important applications in equipment such as tokamaks.In this study,real-time neutron and gamma discrimination was implemented on a self-developed 500-Msps,12-bit digitizer,and the neutron and gamma spectra were calculated directly on an FPGA.A fast neutron flux measurement system with BC-501A and EJ-309 liquid scintillator detectors was developed and a fast neutron measurement experiment was successfully performed on the HL-2 M tokamak at the Southwestern Institute of Physics,China.The experimental results demonstrated that the system obtained the neutron and gamma spectra with a time accuracy of 1 ms.At count rates of up to 1 Mcps,the figure of merit was greater than 1.05 for energies between 50 keV and 2.8 MeV.

Fabrication of fiber-embedded multi-core photonic crystal fibers

A novel fabrication method of multi-core photonic crystal fibers is proposed on the basis of a fiber-embedded technique. A taper tower is used to modify the structures of the fiber preform, and four steps of fiber fabrication and different structures of fiber samples are given. The mode structures and beating characteristics of a photonic crystal fiber sample with two successive cores are investigated in detail with the help of a supercontinuum light source, a charge-coupled device （CCD） camera, and an optical spectrum analyzer. The test results show a clear beating phenomenon between two orthotropic polarization modes with a 2.8-nm peak interval in wavelength.

Novel spectral characterization method for color printer based on the cellular Neugebauer model

The cellular Yule-Nielson spectral Neugebauer （CYNSN） model to characterize a typical CMYK （i.e.,cyan,magenta,yellow,and black） 4-ink color printer is presented.By reconstructing spectral reflectance,models with high accuracy of spectral and colorimetric predictions are built.A novel cell-searching algorithm is proposed and used together with the iteration method to invert the cellular Neugebauer model efficiently.Large numbers of high quality hardcopy samples are produced to evaluate model performance and prove the feasibility of the algorithm.The spectral-based model performs better compared with the usual model based on CIE1931 XYZ tristimulus values.

A cost-effective 100-Gb/s transmitter with low-speed optoelectronic devices and high spectral efficiency

A 100-Gb/s high-speed optical transmitter is proposed and experimentally demonstrated. Based on frequency-quadrupling technique, two sub-channels with a fixed 50-GHz spacing are obtained from one laser source. Using return-to-zero differential quadrature phase-shift keying （RZ-DQPSK） modulation format and polarization multiplexing （PolMux）, only low-speed electronic devices of 12.5 GHz are needed for the 100-Gb/s transmitter. This eliminates the need of ultrahigh-speed optoelectronic devices and thus greatly reduces the cost. The experimental results show that this transmitter can achieve good performance in dispersion tolerance of a 25-km single mode fiber （SMF）.

Progress of Optical Fiber Sensors and Its Application in Harsh Environment

Fiber sensors have been developed for industry application with significant advantages.In this paper,Fiber sensors for oil field service and harsh environment monitoring which have been investigated in Tsinghua University are demonstrated.By discussing the requirements of practical applications,the key technologies of long-period fiber grating(LPFG)based fiber sensor,optical spectrum analyzer for oil detection,laser induced breakdown spectroscopy(LIBS)system for soil contamination monitoring,and seismic sensor arrays are described.

Coherent optical OFDM scheme with inter-carrier interference self-cancellation and common phase error compensation

Schemes integrating inter-carrier interference （ICI） self-cancellation and common phase error （CPE） com- pensation for coherent optical orthogonal frequency division multiplexing （CO-OFDM） systems are investi- gated. The purpose of our research is to counteract the impacts of laser phase noise and fiber nonlinearity. We propose two ICI self-cancellation-based CO-OFDM schemes, and adopt a pilot-aided decision feedback （DFB） loop for CPE compensation. The proposed schemes are compared with conventional CO-OFDM schemes at the same spectral efficiency. Simulations show that our schemes can not only enhance laser linewidth tolerance of the CO-OFDM system, but also present strong robustness against fiber nonlinearity.

Fabrication of 3D colloidal photonic crystals in cavity of optical fiber end face

The natural sedimentation method combining with the isothermal heating evaporation induced selfassembly is presented to fabricate three-dimensional（3D） colloidal crystals in the curved cavity of the fiber end face.A～50-μm-deep cavity is etched by hydrofluoric acid.The colloidal crystals are characterized by optical and scanning electron microscopy.A 1380-nm stop band is observed by measuring their transmission spectra at normal incidence by an optical spectrum analyzer（OSA）.

High-sensitivity sensor based on mode number-encoded multi-longitudinal mode fiber laser

We propose a high sensitivity sensor based on a mode number-encoded multi-longitudinal mode fiber laser. The fiber laser incorporates a uniform fiber Bragg grating （FBG） and a fiber Fabry-Perot interferometer （FFPI） as sensitive components in the cavity. The sensor counts the number of longitudinal modes （NLM） of fiber laser, which is caused by the mismatch between the reflection band of FBG and the transmission band of FFPI resulting from the application of external perturbation to the FBG. An electrical spectrum analyzer is adopted to analyze the NLM. The strain sensor is experimentally demonstrated to have sensi- tivity of as high as 0.02 με/mode.

Novel schemes to generate multi-level modulation formats for ultra-capacity coherent detection transmission systems

We summarize several novel schemes to generate multi-level modulation formats for high capacity transmission system with high spectral efficiency. We show that multi-level 8QAM, 16QAM, 32QAM, and 64QAM （QAM： quatlrature amplitude modulation） optical signals can be generated by commercial optical and electrical devices. Employing these multilevel modulation formats, we have realized PDM-8QAM （PDM： polarization division multiplexing） 32 Tb/s and PDM-36QAM 64-Tb/s signals transmission over 580-and 320-km fibers, respectively.

Nonlinear spectral imaging microscopy of rabbit aortic wall

Employing nonlinear spectral imaging technique based on two-photon-excited fluorescence and second- harmonic generation （SHG） of biological tissue, we combine the image-guided spectral analysis method and multi-channel subsequent detection imaging to map and visualize the intrinsic species in a native rabbit aortic wall. A series of recorded nonlinear spectral images excited by a broad range of laser wavelengths （730- 910 nm） are used to identify five components in the native rabbit aortic wall, including nicotinamide adenine dinucleotide （NADH）, elastic fiber, flavin, porphyrin derivatives, and collagen. Integrating multichannel subsequent detection imaging technique, the high-resolution, high contrast images of collagen and elastic fiber in the aortic wall are obtained. Our results demonstrate that this method can yield complementary biochemical and morphological information about aortic tissues, which have the potential to determine the tissue pathology associated with mechanical properties of aortic wall and to evaluate the phaxmacodynamical studies of vessels.

All-optical amplitude-shift keying and differential phase-shift keying to differential phase amplitude-shift keying format combination in highly nonlinear fiber

We propose an all-optical modulation formats combination scheme that merges an amplitude-shift keying （ASK） signal and a differential phase-shift keying （DPSK） signal into a single differential phase amplitude- shift keying （DPASK） signal based on parametric amplification in a highly nonlinear fiber. By optimizing the power of the ASK channel, formats combination of ASK and DPSK to DPASK signal is successfully demonstrated by computational simulation. The demodulation process of the generated DPASK pulses is investigated and the relationship between optical signal-to-noise ratio （OSNR） penalty and the input ASK power is presented. The proposed scheme may be used for increasing spectral efficiency and all-optical logic device.

Novel method for measurement of effective cavity length of DBR fiber

A novel method to measure the effective cavity length of distributed Bragg reflector（DBR） fiber laser with fiber Bragg grating（FBG） Fabry-Perot cavity as the resonator is proposed.The effective cavity length of DBR fiber laser is accurately measured by measuring the frequency spacing of two adjacent longitudinal modes in the cavity without the precise physical length of FBGs.The measuring accuracy as high as 10^（-5） mm can be obtained by setting the resolution bandwidth of the spectrum analyzer.

Prediction and Measurement Techniques for Radiated EMI of Power Converters with Cables

Recently,radiated electromagnetic interference(EMI)has become a research hotspot in power electronics systems,as the switching frequencies of power electronics systems have increased significantly with the adoption of wide-bandgap devices.In this article,a generalized radiated EMI model for power electronics converters with power cables is first reviewed.The radiated EMI model is then developed for a flyback power converter with critical ground impedance included.Based on the developed model,accurate high-frequency parameter extraction techniques and a radiated EMI prediction technique are developed and experimentally validated.Finally,essential measurement techniques are identified and developed to accurately extract parameters for accurate EMI prediction.The effects of the resolution bandwidth of the spectrum analyzer and critical PCB ground impedance on the radiated EMI are experimentally validated.PCB’s impact on the common-mode(CM)choke’s impedance and the radiated EMI is further validated.Techniques for minimizing the undesired near-field couplings in parameter extraction are discussed.The predicted EMI properly agreed with the measured EMI in the range of 30-230 MHz based on the EN550323 m class B standard.