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A fiber-optic acoustic pressure sensor based on aluminum-polyimide diaphragm is proposed in this paper. The sensing diaphragm exhibits excellent properties including heat resistance and long-term stability (4 months). Together with hybrid configuration of Mach-Zehnder and Sagnac interferometer with triple detection passive demodulation algorithm, the sensor can measure the absolute amplitude of acoustic pressure. The sensor has a high sensitivity of 110 nm/Pa with the acoustic pressure amplitude of 2.05 mPa–7.15 Pa and gives a minimum detectable pressure of 37.7 μPa/Hz 1/2 at 4 kHz. The sensor also demonstrates a flat frequency response in the range of 600 Hz–6 kHz and is expected to be used for weak acoustic pressure sensing and photo-acoustic spectroscopy. [ABSTRACT FROM AUTHOR]

This paper presents a triple detection fiber differential interferometer and its passive demodulation scheme. The interferometer is based on an all fiber Mach–Zehnder and Sagnac hybrid configuration, which is composed of a fiber ASE source, a section of delay fiber, a 3×3 fiber coupler and several other fiber components. In the interferometer, the signal beam and reference beam travel along the same path but in opposite directions. The received signal is demodulated by a triple detection passive demodulation scheme. The interferometer can measure the absolute amplitudes and frequencies of phase sensitive signals with large dynamic range, and the low frequency environmental disturbance is removed simultaneously due to the phase compression mechanism. The experimental results demonstrate that the phase demodulation resolution is 6×10 −5 rad and the maximum measuring amplitude is up to 90 rad. This method can be used to measure many kinds of parameters such as vibration and refractive index. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the dispersive and absorptive properties in a three-level ladder-type SQW system that both cascade transitions are driven by the same strong coherent field and are probed by the same weak probe field. It is found that the positive or negative refractive index with vanishing absorption can be also easily obtained in this solid-state system. Our scheme shows many intrinsic characteristics that other schemes do not have, which may provide some possibilities for the technological applications in solid-state optoelectronics. [Copyright &y& Elsevier]

Abstract: We investigated the optical bistability of a quantum dot molecule under coherent excitation and considering the spontaneous exciton decay and pure dephasing as two decoherence channels. Because of the efficient destructive quantum interference established by the tunneling coupling, it is found that the appearance and disappearance of OB can easily be controlled by adjusting the position of the tunneling induced transparency window. Our scheme opens the possibility to control OB with electric gates. [Copyright &y& Elsevier]

Abstract: In-phase and quadrature-phase (I/Q) signals often need to be formed in the laser interferometric vibration measurement technique. To avoid the disadvantages of traditional I/Q signals forming methods such as effect of piezoelectric ceramic (PZT) for generating high frequency carrier, or optical configuration with complicated structure, a novel interferometric vibration measurement sensor with quadrature detection is proposed. The sensor utilizes simple optical configuration which contains 1/8 wave plate to generate two I/Q signals, then the signals were processed by arctangent algorithm which is compiled by Labview software through data acquisition card. Theoretical analysis and experimental Lissajous figures synthesis prove the phase orthogonality of the two signals. The experimental results indicate that the system can measure the vibration displacement accurately. [Copyright &y& Elsevier]

Nanoscale optical sensing plays a crucial role in achieving high‐precision non‐contact distance and displacement measurements. As one of the promising alternatives, structured light can greatly simplify optical sensing systems. In this study, a novel optical phenomenon of a structured beam called the “Dumpling‐shaped Structured Light Field” (DSLF) is revealed, which forms during the focusing of cylindrical lens beams. The 3D DSLF comprises two mutually perpendicular primary and secondary foci, offering unique possibilities in micro/nano processing and sensing applications. A thorough investigation of the DSLF is conducted, revealing a correlation between the secondary foci and the phase status at the objective lens's entrance pupil. The propagation of DSLF under tight focus conditions has been verified and discussed through methods of femtosecond laser two‐photon polymerization and light field analysis. Finally, thanks to the unique 3D intensity distribution of 3D DSLF, the applicability of DSLF in high‐precision position and vibration sensing has been demonstrated. By detecting the horizontal and vertical dimensions of the reflected DSLF, position and vibration sensing is achieved with a Z‐direction accuracy of 10 nm (λ/80). This research contributes to understanding structured light, and offers practical applications in various fields, including micro/nano laser processing, optical imaging, sensing, etc. [ABSTRACT FROM AUTHOR]

Abstract: Based on Michelson interferometer and phase generated carrier (PGC) homodyne demodulation technique, an optical interferometer system is built, and a novel method using the central angles of Lissajous figures to measure micro-vibration displacement is proposed. The Lissajous figures are obtained by synthesizing two interferometric signals that their vibration directions are orthogonal and their intensities are equal. Through theoretical analysis, software simulation and experimental test, it is concluded that the central angles of Lissajous figures have a linear relationship with the phase modulation coefficients and the micro-vibration displacements when the phase modulation coefficients are small. So the micro-vibration displacements can be directly obtained by measuring the central angles of Lissajous figures. The method is simple and convenient. The experimental results indicate that the micro-vibration displacements measuring range can reach 5–775nm, with a resolution of about 5nm. [Copyright &y& Elsevier]

Laser Doppler velocimetry (LDV) based on a differential laser Doppler system has been widely used in fluid mechanics to measure particle velocity. However, the two outgoing lights must intersect strictly at the measurement position. In cross-interface applications, due to interface effects, two beams of light become easily disjointed. To address the issue, we present a laser velocimeter in a coaxial arrangement consisting of the following components: a single-frequency laser (wavelength λ = 532 nm) and a Twyman–Green interferometer. In contrast to previous LDV systems, a laser velocimeter based on the Twyman–Green interferometer has the advantage of realizing cross-interface measurement. At the same time, the sensitive direction of the instrument can be changed according to the direction of the measured speed. We have developed a 4000 m level laser hydrothermal flow velocity measurement prototype suitable for deep-sea in situ measurement. The system underwent a withstand voltage test at the Qingdao Deep Sea Base, and the signal obtained was normal under a high pressure of 40 MPa. The velocity contrast measurement was carried out at the China Institute of Water Resources and Hydropower Research. The maximum relative error of the measurement was 8.82% when compared with the acoustic Doppler velocimeter at the low-speed range of 0.1–1 m/s. The maximum relative error of the measurement was 1.98% when compared with the nozzle standard velocity system at the high-speed range of 1–7 m/s. Finally, the prototype system was successfully evaluated in the shallow sea in Lingshui, Hainan, with it demonstrating great potential for the in situ measurement of fluid velocity at marine hydrothermal vents. [ABSTRACT FROM AUTHOR]

In this paper, a low coherence fiber differentiating interferometer and its passive demodulation schemes are presented and experimentally demonstrated. The interferometer which adopts a broadband light resource and balanced detection is a hybrid configuration of Mach–Zehnder and Sagnac interferometer. The two passive demodulation schemes employ an approximate method and a differential-and-cross-multiply (DCM) method to obtain the vibration signal. The phase measurement resolution (PMR) of both methods can reach 6 × 10 −5 rad. Comparing with the approximate method which is usually adopted to restore weak signal, the experiment results demonstrated the dynamic range of the DCM demodulation method has increased 30 dB, which extends the measurement range and can be used widely in various physical fields. [ABSTRACT FROM AUTHOR]

Microseismic monitoring technology is widely used in tunnel and coal mine safety production. For signals generated by ultra-weak microseismic events, traditional sensors encounter limitations in terms of detection sensitivity. Given the complex engineering environment, automatic multi-classification of microseismic data is highly required. In this study, we use acceleration sensors to collect signals and combine the improved Visual Geometry Group with a convolutional block attention module to obtain a new network structure, termed CNN_BAM, for automatic classification and identification of microseismic events. We use the dataset collected from the Hanjiang-to-Weihe River Diversion Project to train and validate the network model. Results show that the CNN_BAM model exhibits good feature extraction ability, achieving a recognition accuracy of 99.29%, surpassing all its counterparts. The stability and accuracy of the classification algorithm improve remarkably. In addition, through fine-tuning and migration to the Pan II Mine Project, the network demonstrates reliable generalization performance. This outcome reflects its adaptability across different projects and promising application prospects. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the effect of quantum interference on the refractive property in a three-level semiconductor quantum well system. It is found that the refractive properties can be significantly improved via the coupling field, the pumping field and the relative phase. Otherwise, the positive or negative refractive index with vanishing absorption can be also easily obtained in this solid-state system, and the switch between positive refractive index and negative refractive index just by tuning the relative phase. [Copyright &y& Elsevier]

Abstract: We investigated the optical bistability and multistability in an Er3+-doped ZrF4–BaF2–LaF3–AlF3–NaF optical fiber inside an optical ring cavity. It is found that the optical bistability and multistability can be easily controlled via adjusting properly the parameters of the corresponding system. Our scheme may provide some new possibilities for technological applications in optoelectronics and optical-fiber communication. [Copyright &y& Elsevier]

We demonstrate a new scheme for realizing the Optical Bistability (OB) in an erbium-doped-fiber laser with an external control laser. It is found that the OB can be significantly modified by changing the power and the wavelength of the control laser. We give an explanation of the bistability phenomenon based on numerical simulations, which are agreed very well with our experimental results. Our scheme provides a guideline for optimizing and controlling the OB in an erbium-doped-fiber laser, which might be useful for optical communications. [ABSTRACT FROM AUTHOR]

Highlights: [•] A strain-insensitive high temperature fiber sensor is proposed. [•] The sensor is based on a fiber modal interferometer. [•] A piece of small-core photosensitive fiber is used in the sensor. [•] The sensor owns the highest temperature sensitivity. [•] The temperature to strain cross sensitivity of the sensor is low. [ABSTRACT FROM AUTHOR]

We investigate the two-dimensional (2D) probe absorption spectrum in a four-subband three-coupled semiconductor quantum-well system driven by two orthogonal standing-wave lasers. It is found that, due to the quantum interference effect, the 2D spatial distribution of probe absorption spectrum can be easily controlled via adjusting the system parameters. Thus, our scheme shows the underlying probability for the formation of the 2D electron localization in a solid. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a study of the reconstruction of target motion waveform based on the self-mixing interference (SMI) from the sampling rate of data acquiring card. It has been found that the ratio of sampling rate to motion frequency of target (RSF) is of importance in the sampled SMI waveform. Based on the experimental SMI waveforms and the corresponding sampling rate, a quantitative relationship between RSF and the number of peaks of SMI waveform are given, showing that the lowest RSF is 10 points for the normal waveform in every monotony interval of SMI waveform. [Copyright &y& Elsevier]

Abstract: We theoretically investigate the absorption–amplification response of the probe field in a four-level cold 87Rb atomic system. It is found that the incoherent pumping fields play very important roles in realizing the amplification of the probe field. The transmission property of the probe field is also studied. [Copyright &y& Elsevier]

Abstract: We investigate the one- and two-dimensional atom localization behaviors via spontaneous emission in a coherently driven five-level atomic system by means of a radio-frequency field driving a hyperfine transition. It is found that the detecting probability and precision of atom localization behaviors can be significantly improved via adjusting the system parameters. More importantly, the two-dimensional atom localization patterns reveal that the maximal probability of finding an atom within the sub-wavelength domain of the standing waves can reach unity when the corresponding conditions are satisfied. As a result, our scheme may be helpful in laser cooling or the atom nano-lithography via atom localization. [Copyright &y& Elsevier]

Abstract: We theoretically investigate the features of two-photon absorption in a four-level inverted-Y quantum well system. We find that not only the strengths of applied laser fields but also the total closed interaction phase can affect the two-photon absorption dramatically, which can be used to suppress or enhance two-photon absorption. More importantly, the double electromagnetically induced two-photon transparency can be achieved in this system. Thus, it may provide some new possibilities for technological applications in the two-photon lasing, the pulse propagation, and the two-photon entanglement in solid-state quantum information processing. [Copyright &y& Elsevier]

Abstract: We investigate the transient behaviors of the dispersion and the absorption in a three-level GaAs/AlGaAs semiconductor quantum well system. It is found that the Fano interference and the energy splitting affect the transient behaviors dramatically, which can be used to manipulate efficiently the gain-absorption coefficient and group velocity of the probe field. The dependence of transient electron population on the Fano interference and the energy splitting is also discussed. [Copyright &y& Elsevier]

Abstract: We propose and demonstrate a novel switchable quadruple-wavelength erbium-doped fiber ring laser (EDFL) based on two-segment Lyot–Sagnac filter which is used as the wavelength-selective filter. Due to the deeply saturated spectral hole-burning (SHB) effect in an ordinary erbium-doped fiber, the laser realizes stable single-wavelength, dual-wavelength, triple-wavelength, and quadruple-wavelength output by controlling two polarization controllers (PCs) appropriately. The optical signal-to-noise ratio (OSNR) is over 30dB. The peak fluctuation is less than ~2dB over 1.5h at room temperature. [Copyright &y& Elsevier]

Abstract: A novel configuration of fiber laser with frequency modulation is presented. Frequency modulation, stable polarization state and narrow linewidth are realized by using the waveguide phase modulator, polarization maintaining devices and saturable absorber. It is shown that the laser output reaches 23mW, linewidth is less than 1kHz, polarization extinction ratio is higher than 20dB and maximum value of frequency deviation can reach 7.5MHz. [Copyright &y& Elsevier]

Abstract: Laser interferometry based on self-mixing effect widely used to measure displacement, velocity, vibration and distance due to compactness, non-contact, as well as low cost. An important use of the self-mixing effect inside a laser diode is in optical self-mixing vibrometer. We have experimentally investigated self-mixing interference produced by the feedback of light from a vibrated target into a vertical-cavity surface-emitting laser for sensing applications. In this paper, the self-mixing vibrometer employing different feedback regime is discussed and a theoretical analysis is also proposed. Numerical analysis focused on the distortion self-mixing waveform. Numerical analysis and experimental results show that mode-hop occurs when the laser self-mixing vibrometer employing at high feedback level. [Copyright &y& Elsevier]

In order to control the divergence angle of the optical fiber and reduce the focal spot size of the beam, we have proposed and experimentally proved a simple and low-cost method for fabricating microlens on the end facet of the fiber, which involves only two steps: dripping and curing. We successfully fabricated hemispherical microlenses with different sizes on the fiber end facets and proved that the shape of microlens can be adjusted through multiple titration and curing processes. The measured characteristics of the beam output from a lensed fiber are in agreement with numerical simulation results. [ABSTRACT FROM AUTHOR]

In order to control the divergence angle of the optical fiber and reduce the focal spot size of the beam, we have proposed and experimentally proved a simple and low-cost method for fabricating microlens on the end facet of the fiber, which involves only two steps: dripping and curing. We successfully fabricated hemispherical microlenses with different sizes on the fiber end facets and proved that the shape of microlens can be adjusted through multiple titration and curing processes. The measured characteristics of the beam output from a lensed fiber are in agreement with numerical simulation results. [ABSTRACT FROM AUTHOR]

We observed that the sensitivity of a fiber sensor is changed by introducing fast and slow light. In this fiber sensor configuration, fast and slow light are generated by a double Lorenz fiber Bragg grating (DL-FBG) and the probe pulse through DL-FBG will have different delays by varying the input laser wavelength. The maximum delay is 180 ps (slow light) and the maximum advance is 90 ps (fast light). Experimental result shows that the fiber sensor sensitivity obtains enhancement by introducing slow light, while the sensitivity of fiber sensor decreases when combining with the fast light. [ABSTRACT FROM AUTHOR]