Abstract:Laser phase modulation spectrum with 25 frequency bands is generated by modulating a single frequency laser with two cascaded phase modulators (PMs) with driving voltage amplitudes at 3.2 V and 7.8 V, respectively. And the time delay self-heterodyne method is adopted to measure and analyze the coherent characteristics of the original single frequency laser light and the generated multi-frequency light from two phase modulation schemes. By comparison of laser linewidth, the experimental results show that the laser phase modulation does not change the coherent characteristics of each frequency band, and the laser phase modulation spectra benefit the performance optimization for the Rayleigh scattering based optical fiber sensing system.

Abstract:In this paper, the shape of the object is reconstructed based on the fiber Bragg grating (FBG) flexible sensor encapsulated by polydimethylsiloxane (PDMS) material, and the influence of the number and layout of the sensors on the reconstruction accuracy of the object is studied. The COMSOL simulation software is used to verify the accuracy of the algorithm, and four sensors are prepared to measure and reconstruct the bent aluminum plate under set conditions. The maximum relative error of the shape reconstruction span is 1.203%, and the maximum relative error of the height is 2.802%. The research results provide the application basis for the shape detection of soft robots.

Abstract:A system of fiber Bragg grating (FBG) temperature sensing demodulation based on light power detection is proposed in this paper. Compared with the traditional demodulation method based on wavelength scanning, light power detection is more direct and avoids the use of spectrometer. Moreover, the light power in the system is converted into the electrical signal by the receiver optical subassembly (ROSA) and converted to the digital signal. The micro controller unit (MCU) processes the digital signal to realize the real-time temperature monitoring, which avoids the use of optical power meter (OPM). With the advantages of simple structure and low cost, the system is portable and practical. The experimental results show that the linearity coefficients R-square between light power and the sampling voltage are 0.999 08 and 0.998 93 in the temperature range from 10 °C to 85 °C, respectively. According to the results, the proved sensor has a repeatability error of 1%, a linearity error of 1.35%, and a hysteresis error of 0.7%, which indicates that the system is of high stability and high precision. The experimental results are consistent with the theory, which verifies the system's feasibility.

Abstract:In this letter, a titanium aluminum carbide (Ti3AlC2) coated D-shaped fiber is proposed and demonstrated as a new saturable absorber (SA) for Q-switched laser pulse generation. In preparing the SA, the Ti3AlC2 powder is dispersed in liquid polyvinyl alcohol (PVA) before the solution is dropped and left to dry onto a polished surface of D-shape fiber. The SA is added to an erbium-doped fiber laser (EDFL) cavity to modulate the cavity loss for Q-switching. The Q-switched laser is obtained at 1 561 nm. The pulse width of the pulses can be varied between 7.4 µs and 5.1 µs with a corresponding repetition rate range from 41.26 kHz to 54.35 kHz, when the pump power is increased from 42.2 mW to 71.5 mW. At 71.5 mW pump, the pulse energy is obtained at 70.3 nJ. The signal-to-noise ratio (SNR) of the fundamental frequency is recorded at 67 dB, which indicates the stability of the laser.

Abstract:Aiming at the problem of high requirement for the signal generator in the Brillouin optical time-domain analysis (BOTDA) system, a quarter of the Brillouin frequency shift (BFS) of modulation is proposed to reduce the required bandwidth of the sensing system. A functional model for solving the intensity of each-order sideband of the output light of electro-optic modulator (EOM) is proposed and applied, so the spectrum with suppressed the carrier and the first-order sidebands while maximizing the second-order sidebands is obtained. Compared with the latest scheme, the intensity of the second-order sidebands is increased by 21.1% based on this functional model. In the experiment, the second-order upper sideband and the second-order lower sideband are used as continuous wave (CW) probe light and pump pulse light, respectively, which ultimately reduces the required bandwidth of radio frequency (RF) signal sources to a quarter of the BFS (reduced from ~11 GHz to ~2.75 GHz), and the frequency sweep range is also reduced to a quarter of the original.

Abstract:The different compositions of the ternary alloyed CdSexS1-x quantum dots (QDs) and CdSexS1-x/ZnS core/shell quantum dots (CSQDs) have been synthesized by the chemical routes. The radii of these QDs were determined by transmission electron microscope (TEM). The optical properties of these QDs were investigated by the absorption and fluorescent measurement. It was found that the absorption and fluorescent emissions were tuned by the component ratio, and the Commission Internationale de l´Eclairage (CIE) coordinates of the fluorescent spectra also depended on the composition. Compared with the CdSexS1-x QDs, the CdSexS1-x/ZnS CSQDs exhibit the fluorescence enhancement due to the surface passivation by shell coating. The composition-tuned optical properties may allow them to be used as fluorescent markers in biological imaging and to fabricate multicolor light emitting diode (LED).

Abstract:Simultaneously improving the communication speed and equalizing the nonlinear frequency response are still challenging for tunnel visible light communication (TVLC) system. Here, we propose and numerically investigate a frequency domain pre-equalization scheme for discrete multitone (DMT) modulation TVLC system. The amplitude of each subscriber is appropriately pre-equalized by optimized nonlinear compensation parameters. Simulation results demonstrate that our proposed equalization technique can resist the channel attenuation of the signal high-frequency part and further flatten the nonlinear channel response. Without forward error correction technique, the bit error ratio (BER) performance can reach 7.66×10-6 in a 2.05 Gbit/s DMT-TVLC system.

Abstract:Accurate measurement of the thin liquid film thickness in pipes is the foundation for studying the characteristics of the film. In this paper, an interferometry-based measurement of liquid film thickness in transparent pipes is developed, which can greatly improve the accuracy, extend the lower limit of measurement and provide a new technical approach for the calibration and traceability. The light intensity distribution is established based on the optical path analysis and a mathematical model. A new algorithm to solve the direction ambiguity is developed to reconstruct the phase distribution. Besides, the effect of the pipe wall is taken into account, which can be suppressed by image subtraction and enhancement technology. The proposed method is of high accuracy and robustness, whose reconstruction errors are 0.064% and 0.25% for the smooth and slight fluctuating liquid films, respectively.

Abstract:The identification of rice seeds is crucial for agriculture production. An inverse Fourier transform (IFT) method based on laser-induced breakdown spectroscopy (LIBS) is proposed to identify five kinds of rice seeds. The LIBS data of the samples were preprocessed by inverse fast Fourier transform (IFFT), and the time-domain signals of rice seeds were obtained. The back propagation (BP) neural network was used to establish full spectrum, segmented spectrum, time-domain full spectrum and time-domain segmented spectrum discrimination models. Compared with the original spectrum, the time-domain spectrum can significantly improve the identification accuracy. The time-domain full-spectrum identification accuracy reached 95.28%, and the time-domain segmented spectrum identification accuracy reached 94.36%, whose identification time was only a few seconds. The results demonstrate that LIBS detection technology combined IFFT and BP neural network is fast and accurate, which provides a new idea for batch detection of rice seeds.

Abstract:Lithium-ion battery has attracted more and more attention under the background of energy crisis and environmental pollution, and safety issues have become the focus of attention. A new detection method is proposed in this paper to overcome the problems of poor versatility, low accuracy and poor sensitivity in aerosol and smoke detection in the early warning of lithium-ion battery failure. It is a photodetector based on Mie scattering theory, which can assist temperature, voltage and current data to realize online detection. The experiments have shown that a lithium-ion battery with a battery capacity of only 300 mA was overcharged under low power conditions. The aerosol and smoke generated by the battery failure and rupture were easily detected by the detector. A small amount of aerosol and smoke could be detected under overdischarge conditions, and the feasibility of the method was fully verified. The results show that on-line detection of lithium-ion battery failure can be achieved under extreme conditions with few nuisance alarms, and it is characterized by low cost, high speed, high efficiency and good universality.

Abstract:In the vehicle to vehicle (V2V) communication based on optical camera communication (OCC) system, how to achieve high reliability and low latency communication is still a problem. In this paper, we propose a lightweight light-emitting diode (LED) detection algorithm based on deep learning to detect the vehicle LED position at different communication distances, which can improve LED detection accuracy and inference speed. In addition, we design an LED segmentation recognition algorithm to reduce the bit error rate (BER) of the vehicle OCC system. The experimental results demonstrate the effectiveness of the proposed algorithms in real traffic scenes.