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An internal-and-external-cavity Fabry–Perot interferometer (IECFPI) model was mathematically constructed. Based on the IECFPI configuration, a crosstalk-free fiber optic sensor for simultaneous measurement of refractive index (RI) and temperature was proposed and experimentally demonstrated. The internal cavity was a silica cavity, while the external one consisted of an open cavity and silica cavity. The open cavity was formed by an offset single-mode fiber (SMF) sandwiched between two sections of SMF, and the silica cavity was formed by a well-cleaved SMF. A sensitivity coefficient (SC) matrix of the proposed sensor was used to determine the RI and temperature by monitoring the wavelength shifts of the envelope and fine fringes. The proposed sensor can be used for simultaneous measurement of RI and temperature based on the inverse of the SC matrix. Experimental results demonstrated that the RI sensitivities for the envelope and fine fringes were 1084.9 and 124.2 nm/RIU, respectively. Temperature sensitivities of 3.5 and 9.8 pm/°C were also achieved for the envelope and fine fringes, respectively. The crosstalk between the RI and temperature was thus eliminated. In addition, the proposed sensor has a compact size, simple fabrication, and an all-fiber structure, making it a potential candidate for many applications, such as the food industry and environment monitoring.

We propose and demonstrate a novel and simple wavelength switchable multi-wavelength erbium-doped fiber (EDF) laser based on polarization-dependent loss (PDL) modulation with cascaded fiber Bragg gratings (FBGs) as the wavelength comb filter. PDL is generated and controlled by different polarization angles between the polarization state of each lasing line and the optical fiber polarizer in the laser cavity. The PDL modulation balances the difference between the total loss and the wavelength-dependent lasing gain. The relations among gain, loss, and laser output power under different PDL conditions are numerically analyzed on the basis of a simplified Giles model in an EDF laser system. Results show that multi-wavelength lasing lines can be built up by properly tuning the PDLs in different wavelengths. The amount of lasing lines can be switched by changing the PDL intensity in a certain wavelength. In an experiment, the PDL in that certain wavelength is modulated by utilizing four polarization controllable elements, each of which includes an FBG and a corresponding polarization controller (PC). One PC is used to control the polarization state of the reflected lasing line on the center wavelength of the FBG. A four-wavelength individually switchable EDF laser system is demonstrated by utilizing this wavelength switchable method, in which the switching operation for the single-, dual-, triple-, and four-wavelength laser output is verified. The experimental results are consistent with the theoretical results. All wavelength switching operations are conveniently controlled by merely adjusting the PCs. This simple structure and flexible wavelength switchable operations show that this multi-wavelength EDF laser has great potential in practical applications.

Laser-ultrasonic excitation holds promising prospects in nondestructive testing and structural health monitoring. This paper proposes a multipoint, energy-equal laser-ultrasonic actuator based on multiple peanut-shape (PNS) structures in a single mode fiber link. The PNS structures were fabricated by fiber micro-matching operations using a fiber splicer. The PNSs effectively couple the laser energy from the core mode into the cladding modes in the fiber. Moreover, the coupling ratio can be adjusted by controlling the taper diameter of the PNSs. For energy control, multiple PNS structures are fabricated and connected in order from small to large coupling ratios. Experiments were performed on an actuator with four PNSs of different taper diameters (138.71 $\mu \text{m}$ , 143.42 $\mu \text{m}$ , 158.23 $\mu \text{m}$ , and 180.19 $\mu \text{m}$ ) distributed along a single fiber. With its simple structure, energy control, and multipoint functionality, this all-fiber laser-ultrasonic actuator shows great potential in future multipoint ultrasound applications.

A femtosecond soliton erbium-doped fiber ring laser with the symmetrical saturable absorber (SA) of graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) is proposed and proved. Based on the nonlinear multimode interference (NL-MMI) effect of SA, the fiber laser achieved femtosecond soliton mode-locking operation and repetition rate tuning. The duration of soliton was 364 fs at 1562.5 nm and repetition rate was tunable from 10.29 MHz to 763.36 MHz. In the structure of SA, a short step index multimode fiber (SIMF) was inserted between two graded index multimode fibers (GIMFs) to generate more high-order modes, adjust the self-focusing length, and improve operation stability of mode-locked fiber laser. The SA also has the advantages of immunity to the external environment variation, high damage threshold and inoxidizability, which make the laser operate stably for a long time.

A high-sensitivity fiber-optic strain sensor, based on the Vernier effect and separated Fabry–Perot interferometers (FPIs), is proposed and experimentally demonstrated. One air-cavity FPI is used as a sensing FPI (SFPI) and another is used as a matched FPI (MFPI) to generate the Vernier effect. The two FPIs are connected by a fiber link but separated by a long section of single-mode fiber (SMF). The SFPI is fabricated by splicing a section of microfiber between two SMFs with a large lateral offset, and the MFPI is formed by a section of hollow-core fiber sandwiched between two SMFs. By using the Vernier effect, the strain sensitivity of the proposed sensor reaches $\text{1.15 nm/}\mu \varepsilon $ , which is the highest strain sensitivity of an FPI-based sensor reported so far. Owing to the separated structure of the proposed sensor, the MFPI can be isolated from the SFPI and the detection environment. Therefore, the MFPI is not affected by external physical quantities (such as strain and temperature) and thus has a very low temperature cross-sensitivity. The experimental results show that a low-temperature cross-sensitivity of $\text{0.056 } \mu \varepsilon /^ {\circ }{\text{C}}$ can be obtained with the proposed sensor. With its advantages of simple fabrication, high strain sensitivity, and low-temperature cross-sensitivity, the proposed sensor has great application prospects in several fields.

We theoretically and experimentally demonstrate an all-fiber multi-channel ultrasonic sensor using a switchable fiber Bragg gratings (FBGs) filter in an erbium-doped fiber laser (EDFL). The FBGs are not only used as the filter to determine the laser wavelength, but also as the sensing element to induce the intensity response of the lasing line via cavity loss modulation from the ultrasound-induced relative spectral shift of the matched FBGs. The switchable FBGs filter consists of a tuning FBG and several sensing FBGs. Multi-channel ultrasound detection is achieved via this filter by tuning an FBG to scan the spectra of multiple sensing FBGs. The ultrasonic-detection channel can be flexibly and efficiently switched via tuning the tunable FBG to match with any one of the sensing FBGs. A novel dynamic response model of the EDFL based on wavelength-dependent gain is proposed and developed to study the effects of different spectrally matched FBG positions and the consistency of the ultrasound response of each detection channel. The theoretical results show that the optimal sensing position is at the overlapping regions of spectra when the slope of the FBGs is greatest and the laser power is relatively high. Meanwhile, the consistency of the ultrasound response for each detection channel is influenced by the laser-resonance condition of the EDFL and can be optimized by adjusting the laser-cavity loss. Such theoretical results are well-verified experimentally. This sensor-laser-integrated system achieves high ultrasonic detection response since cavity-loss modulation is related to the amplifying effect of the laser. Using this method, an eight-channel switchable ultrasonic-sensing system has been demonstrated, covering nearly a 16-nm spectral range. By further controlling the cavity loss in the laser system, the consistency of ultrasonic response of each detection channel was optimized. The proposed all-fiber system has multiple- detection channels in a single laser system and consistent ultrasonic-detection response at each channel; it shows great potential for fiber-optic-ultrasonic-detection applications.

We demonstrate the absorption-based fiber humidity sensors working near 1950 nm wavelength which is a strong water absorption peak. No additional coating of humidity-sensitive material is needed for the proposed relative humidity (RH) fiber sensors. Two sensor structures including a taper fiber and a microfiber knot resonator are fabricated. For the taper fiber sensor, upto 0.18 mW/% RH sensitivity has been achieved via direct power measurement. For the fiber knot sensor, the RH variations induce both extinction ratio change and wavelength shift. The optical spectra of the fiber knot resonator under different RH are measured, and the extinction ratio and wavelength shift sensitivity are measured to be 0.034 dB/% RH and 10 pm/% RH, respectively. The response time for the tapered fiber is 1.13 s for RH increasing from 28.6% to 53.4% and 2.27 s for the reverse operation. The microfiber knot sensor can respond in 0.8 and 1.55 s accordingly. The sensor performance for the two structures is also investigated under different temperatures from 25 °C to 45 °C and the results indicate negligible impact from the temperature variations. We believe the simple fabrication and low cost of the sensor will make it potential and practical for the RH measurement.

This study experimentally demonstrates a high-sensitivity fiber-optic relative humidity (RH) sensor based on sensitivity amplification and a reduction mechanism, employing an internal–external Fabry–Perot cavity (IEFPC) Vernier effect and a chitosan film as a Fabry–Perot (FP)-sensing cavity. The proposed sensor is constructed using cascaded FP interferometers comprised of an air cavity formed by a hollow-core fiber (HCF), a chitosan cavity, and an air–chitosan hybrid cavity. The chitosan cavity is fabricated by dipping the HCF into a chitosan solution to form a thin chitosan film. Thus, the thickness of the chitosan film could be controlled precisely based on dipping time and capillary effect. As the optical path lengths of an air–chitosan hybrid cavity and an air cavity are similar, the IEFPC Vernier effect is generated, amplifying the air–chitosan hybrid cavity’s low sensitivity to the chitosan cavity’s high sensitivity. The experimental results agree with the theoretical analysis, supporting the fact that the sensor’s sensitivity is related only to the thickness of the chitosan film. The sensitivity of the sensor reaches up to 7.15 nm/% RH, ranging 40%–92% RH at 25°C. Fabrication of the proposed sensor is cost-effective. The proposed sensor also exhibits superior stability performance, a low-temperature cross-sensitivity of 0.0068% RH/°C, and repeatable fabrication. The proposed IEFPC Vernier effect model functions well for cascaded cavities, which plays a guiding role in the sensitivity improvement of such a structure within a fiber-optic sensing context.

A Q -switched mode-locked square noise-like pulse (QMLSNLP) is generated in a nonlinear polarization rotation passively mode-locked fiber laser. When the pump power changes from 154 mW to 414 mW, the frequency of the Q -switched envelope varies from 21.7 kHz to 38.9 kHz, while the duration of the Q -switched envelope decreases from 5.1 µ s to 3.2 µ s , correspondingly. In the meantime, QMLSNLP keeps the fundamental repetition rate constant, and the pulse duration increases from 3.4 ns to 6.7 ns. By inserting different lengths of single-mode fiber into the ring cavity, the evolutions of QMLSNLP are measured and analyzed. According to the cavity parameters and experimental results, impacts of the cavity length on QMLSNLP are investigated in detail.

We designed an erbium-doped fiber laser based on a semiconductor saturable absorber mirror. The switch of Q-switched mode-locking and continuous-wave mode-locking was obtained by adjusting the pump power.

Tian, J.J.; Liu, Y.J., and Yin, Z.D., 2020. Relationship between coastal foreign direct investment & industrial upgrading of China under the condition of the Belt and Road initiative. In: Guido Aldana, P.A. and Kantamaneni, K. (eds.), Advances in Water Resources, Coastal Management, and Marine Science Technology. Journal of Coastal Research, Special Issue No. 104, pp. 695–699. Coconut Creek (Florida), ISSN 0749-0208.At present, the issuance of the relationship between foreign direct investment & the transformation and upgrading of domestic industrial structures has been controversial under the condition of the construction of the “Belt and Road” of China. Based on the data from 2008 to 2018, the VAR model is applied for studying the interrelationship between foreign direct investment in sixty four countries along the “Belt and Road” and the transformation and upgrading of domestic industrial structures according to the experience since China has joined in Word Trade Organization (“WTO”); and it is found that in this kind of countries, the direct investment is without any statistical significance on the industrial upgrading of China so far; nevertheless, the industrial restructuring of China still in turn accelerates the going out intention of companies and promotes direct investment in this kind of countries.

The orbital angular moment (OAM) of light has been proved to be useful in plenty of applications. By transmitting the OAM of the focused light field to a particle, it will be orbited around the optical axis. Therefore, it is necessary to study the OAM distribution of the focused light field used to manipulate the particles. In this application, the widely used paraxial approximation is no longer sufficient due to the tightly focused beam. We employ the higher-order Poincaré sphere to represent the Laguerre–Gaussian (LG) beams with arbitrary polarization. Then the Rayleigh–Sommerfeld integral method and the q -parameter method are used to derive the analytical expression of the light field on the focal plane. Based on this, the OAM density expression of the tightly focused LG beam is derived. In the numerical simulation, we study and analyze the unique intensity distributions and OAM distributions of tightly focused linear polarized, radial polarized, and circular polarized LG beams. The results could be leveraged to further explore the applications of the polarized vortex beam.

We propose and demonstrate a multi-point, self-adaptive fiber ultrasonic sensor based on fiber Bragg gratings (FBGs). The sensor is constructed from matched FBG pairs (FBGPs) and a tunable optical filter in a fiber ring laser. FBGPs with different Bragg wavelengths are used as both the comb filter to produce potential lasing lines and the ultrasonic sensing elements. The ultrasonic signal is directly detected by monitoring the intensity response at each laser channel to the cold-cavity loss modulation from the spectral shift of matched FBGPs. The laser channel between the FBGPs is actively switched using the tunable optical filter, thereby achieving multi-point and switchable detection yet avoiding mode competition in the lasers. Meanwhile, to achieve self-adaptability, the two FBGs of the FBGP are fixed side by side at the same location of the structure such that both FBGs respond similarly to static strain and temperature variation. The multi-point system is promising for the robustness in the practical applications because it is insensitive to static strain ranging over 1400 μ ϵ and temperature variation from 20 °C to 50 °C, respectively.

We propose and demonstrate a fiber-optic cascaded-cavity Fabry-Perot interferometer (FPI) for simultaneous measurement of air pressure and temperature. The open-cavity hybrid FPI consists of an air-cavity and a silica-cavity formed by a section of silica tube and photonic crystal fiber, respectively. The air-cavity and silica-cavity exhibit different sensitivities to air pressure and temperature. Thus, the proposed sensor can be used to implement air pressure and temperature sensing simultaneously. The spectra of the air-cavity and silica-cavity are extracted from the total reflection spectrum with designed bandpass filters. The air pressure and temperature sensitivities of the air-cavity are 4.04 pm/kPa and 0.87 pm/°C, respectively, and those of the silica-cavity are 3.36 pm/kPa and 14.36 pm/°C, respectively. Moreover, the temperature and air pressure cross-sensitivity can be reduced by using a sensitivity coefficients matrix of two cavities. With the advantages of compact size, easy fabrication, and all-fiber structure, the proposed sensor is a potential candidate for practical applications.

A rapid and precise phase-retrieval method based on Lissajous ellipse fitting and ellipse standardization is demonstrated. It only requires two interferograms without pre-filtering, which reduces its complexity and shortens the processing time. The elliptic coefficients obtained by ellipse fitting are used for ellipse standardization. After compensating phase-shift errors by ellipse standardization, the phase distribution is extracted with high precision. It is suitable for fluctuation, noise, tilt-shift, simple and complex fringes. This method is effective for the number of fringes less than 1. The reliability of the method is verified by simulations and experiments, indicating high accuracy and less time consumption.

In this paper, we employ a symmetrical graded index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) structure as artificial saturable absorber to achieve mode-locked operation in an Erbium-doped fiber laser. After optimizing the cavity length, pump power and orientation of polarization controller, noise-like pulse (NLP) is built in anomalous dispersion region experimentally. It is worth noting that except for the traditional NLP, a special NLP with unique features is obtained whose temporal and spectral properties are different from the known case on NLP reported before. According to the experimental results, we also acquire and compare properties of subpulse within NLP envelope with the change of pump power. Last but not least, harmonic of the peculiar NLP is found under appropriate cavity conditions.

In this study, we propose and demonstrate a multipoint fiber-optic laser-ultrasound transducer based on fiber waist-enlarged tapers (WETs), which are fabricated by taper fusing two single-mode fibers. The WET structure can effectively extract some of the light energy from the fiber core and transfer it to the cladding to form a series of higher-order modes that can be used to excite ultrasound waves. The light energy in the cladding can be absorbed and converted into ultrasound waves by the photoacoustic composite coating on the fiber near WET. The coupling efficiency of the WET is dependent upon its taper length and, therefore, can be easily controlled by the splicer during fabrication. Thus, by controlling coupling efficiencies of the WETs, a multipoint laser-ultrasound actuator with a balanced signal strength can be obtained. We constructed a prototype of the actuator and successfully generated ultrasound waves with balanced strengths at four points by connecting four WETs with coupling ratios of 25.24%, 32.25%, 49.52%, and 90.62% in a fiber link. This multipoint laser-ultrasound actuator is simple to fabricate and provides a balanced ultrasonic signal strength, and thus, has various potential applications in fiber-optic active ultrasound testing technologies.

A wavelength tunable, dark pulse, and bright pulse-switched dual-wavelength fiber ring laser is reported. Through adding high nonlinear optical fiber as a nonlinear medium in the fiber ring cavity, the double-wavelength dark-bright pulse pair can be formed. The separation of dark-bright pulse pair can be achieved by polarization beam splitter. The dark pulse and bright pulse with orthogonal polarization correspond to different wavelengths, respectively. Then, via tunable optical filter and polarization controller to filter out the wavelength we need and adjust the polarization, we achieved the interconversion of dark pulse and bright pulse in the same wavelength and the tunableness of wavelengths of dark pulse and bright pulse in our laser.

A high sensitivity humidity fiber sensor based on all-agar Fabry–Perot (FP) interferometer is proposed and experimentally demonstrated. The sensor is fabricated by filming a thin layer of moisture-sensitive agar to single mode fiber with end dipping method. Due to the all-agar FP cavity structure, both the size and the RI changes of agar film in the swelling effect are fully utilized to induce the frequency shift, the sensor exhibits a high sensitivity of 4.20 nm/%RH, with relative humidity (RH) ranging from 25%RH to 95%RH at 25 °C. Moreover, owing to the short FP cavity, $7.10~\mu \text{m}$ , the proposed sensor provides a short response time less than 340 ms with the humidity level changed from 64.0%RH to 74.2%RH. The proposed sensor also exhibits other superior performances in terms of good stability, low cost, and easy fabrication. Such good characteristics make the proposed sensor a good candidate for practical applications.

A high-sensitivity Fabry–Perot interferometer (FPI) strain sensor based on a concave-core photonic crystal fiber (CPCF) was proposed and demonstrated. The FPI was formed by directly splicing a CPCF to a standard single-mode fiber. The CPCF was fabricated by cleaving a high-numerical-aperture solid-core photonic crystal fiber under axial tension using a traditional fiber cleaver. An FPI strain sensor with a cavity length of 4.85 μ m and a strain sensitivity of 31.58 pm/ μ ϵ was successfully produced. The sensor also demonstrated a low temperature sensitivity of 0.653 pm/°C, reducing the cross-sensitivity between tensile strain and temperature. The benefits of the proposed sensor include simple fabrication, high strain sensitivity, and low temperature cross-sensitivity, making it attractive for practical applications.

We propose and demonstrate a fiber sensor for simultaneous temperature and strain measurements. The proposed sensor is implemented by a cascaded-cavity Fabry–Perot (FP) fiber interferometer. The two cascaded FP cavities comprise a micro-air-cavity in a hollow-core tube fiber and a micro-silica-cavity in a standard single-mode fiber. To separate the interference spectrum of each FP cavity, the total spectrum is filtered in the frequency domain through band-pass filters, whose central frequencies were predesigned based on the relationship between the spatial frequency and free spectral range of each FP cavity. The different cross-sectional areas and thermal-optic coefficients of the two FP cavities confer different sensitivities to temperature and strain. Both parameters were measured simultaneously by tracking the wavelength shifts in the filtered interference spectra of the FP cavities. Moreover, the temperature–strain cross-sensitivity was compensated by solving a sensitivity-coefficient matrix equation for the two cavities, using the calibrated temperatures and strains. Other advantages of the proposed sensor are simple fabrication and an all-fiber structure. Owing to these properties, the proposed sensor is potentially applicable to real sensing applications.

In this paper, we investigated the mode splitting in subwavelength grating metamaterial ring resonators caused by the break of rotational symmetry, which can be leveraged to detect 25 nm diameter single nano particle.

We propose to place subwavelength segments in the evanescent field region of strip waveguide to reduce grating strength. Millimeter-long grating is designed which is essential to the implementation of large aperture optical phased arrays.

We report a high sensitivity ultrasonic/acoustic sensor based on all-fiber Fabry-Perot interferometer. The proposed sensor is made by splicing a single mode fiber onto a fiber tube with an ultra-thin silica diaphragm, the thickness of which is less 0.9 μm. The diaphragm is fabricated by an electrical arc discharge technique with a fiber tube. The experimental results show that the proposed sensor has a high sensitivity to ultrasound detection as high as 3.68 mV/Pa at 60 kHz. Also it has a wide frequency response, which can reach 30-860 kHz. Due to the sensor fully composed of silica, it is anti-electromagnetic interference and anti-erosion. Moreover, it has a low temperature response, only 8.6 pm/°C.

A fiber-optic sensor for simultaneous measurement of temperature and pressure is proposed and experimentally demonstrated. The sensor is composed of a hollow core fiber (HCF) sandwiched by two segments of single mode fiber (SMF), in which Fabry-Perot interference (FPI) combined with antiresonant (AR) mechanism are applied to the simultaneous measurement of temperature and pressure. The reflection spectrum of the sensor is composed of a sinusoid caused by axial FPI and several resonant wavelengths induced by AR mechanism occurred in cladding of the HCF. Two kinds of mechanisms have different responses to temperature and pressure, so the proposed sensor can simultaneously measure the pressure and temperature without pressure-temperature cross-talk. The advantages of the proposed sensor including small size, compact structure, and easy fabrication.

We develop and investigate a novel multi-channel switchable and self-adaptive ultrasonic sensor by integrating fiber Bragg gratings (FBGs) in an erbium-doped fiber (EDF) laser (EDFL). The sensor system consists of a sensing FBGs string, a matched FBGs string and a tunable optical filter in an EDFL. The reflection spectra of the two FBGs strings are matched, where every sensing FBG has a corresponding matched FBG with the similar central wavelength and the overlapped position is at the linear slope of the two FBGs. The two FBGs strings are used comb filter to produce the potential lasing line in the laser system. When impinging the ultrasonic waves onto the FBGs, two FBGs strings also act as the sensing element. As the ultrasonic waves induce different wavelength shifts between sensing FBGs and matched FBGs, the ultrasonic signal is detected via the laser intensity variation from the ultrasound-induced relative spectra shift of FBGs. However, the EDF is a homogeneous broadening medium at room temperature, which will bring the mode competition in the laser, leading to an instable lasing oscillation, if multiple FBGs are included directly into an EDFL to increase the laser wavelength (the detection channel) and achieve a multi-channel detection. To solve this problem, in the proposed system the laser channel and the mode competition effect in the EDFL is avoided. A numerical model is used to analyze the laser establishment and dynamic ultrasonic response in this channel switchable ultrasonic sensor. Experimentally, eight channels switchable ultrasonic sensor is achieved. In addition, the side-by-side fixed structure of two FBGs strings contributes to the system can operate in the environmental interference that ranges nearly 2500μe static strain and 40°C temperature variation. Due to the multi-channel, good embeddability and high robustness, the multi-channel ultrasonic sensor has great potential in the future applications.

A dual-wavelength soliton fiber ring laser with adjustable repetition rate was reported. Through adding highly nonlinear fiber (HNLF) as nonlinear medium in the fiber ring cavity, the dual-wavelength bright-dark soliton pair can be formed. The 1st to 4th, 5th to 8th and 9th to 11th harmonics bright-dark solitons pairs were obtained by rotating the Polarization controller (PC) when the power are 160 mW, 230 mW and 340 mW, respectively. In order to investigate the influence of loss on the laser system, a variable optical attenuator (VOA) was embedded in the ring laser cavity. Then the dual-wavelength with Kelly sidebands was obtained by properly adjusting the PC and VOA. The center wavelengths of the dual-wavelength laser were located at 1557.8 nm and 1568.5 nm and the 3 dB bandwidths of two wavelengths are 2.2 nm and 2.1 nm, respectively. The spacing between the two wavelengths was 10.7 nm. Due to the existence of nonlinearity, the 1st to 22th harmonics bright solitons sequence (corresponding to the repetition rate tunable from 196.8 kHz to 4.33 MHz) was obtained by increasing the pump power. The intervals of bright-dark solitons pair and bright solitons all were $5.08\ \mu\mathrm{s}$ , which corresponds to fundamental repetition rates are 196.8 kHz. The signal-to-noise ratio of the bright solitons was approximately 58 dB, which means the operation of laser was stable.

Photoacoustic ultrasound excitation has great potential in structural nondestructive testing and applications for medical treatments as a promising alternative to electrical ultrasound. This study proposes and demonstrates a multipoint optical fiber laser-ultrasonic transducer system, wherein the fiber-optic ultrasonic transducer is fabricated by a coreless fiber segment's fusion with single-mode fibers at each end. Simulation and experiment results show that the transducer coupling ratio is dependent on the coreless fiber's length. The structure of such an ultrasonic transducer is easily manufactured. Thus, the structures of these optical fiber ultrasonic transducers with different coupling ratios are connected in the order of small to large coupling ratios. In this manner, multipoint ultrasonic excitation with equal intensities at each excitation point can be obtained using this simple and low-cost method. Using laser guidance through the optical fiber to generate ultrasound can efficiently solve some shortcomings of traditional ultrasonic transducers, such as large volume, small bandwidth, and electromagnetic interference. Moreover, this type of fiber-optic ultrasound transducer has higher mechanical strength than other fiber-optic ultrasound transducers and is expected to be useful in structural health-monitoring of buildings.

We report on the influence of the average nonlinear coefficient on noise-like pulse (NLP) in a figure-of-eight erbium-doped fiber laser. Since the nonlinear switching point of nonlinear amplifying loop mirror (NALM) shifts with distinct laser configurations, through changing the length of highly nonlinear fiber (HNLF) in NALM, properties of output pulse have been compared and analyzed. The spectral evolutions occur and the peak power of NLP presents a decline when the HNLF is lengthened in NALM. Also, the tuning range of the duration of NLP shows a difference based on the average nonlinear coefficient selected.

Because of the high index contrast, current silicon photonics based optical phased arrays cannot achieve small beam divergence and large field-of-view simultaneously without increasing fabrication complexity. To resolve the dilemma, we propose an ultra-long waveguide grating antenna formed by placing subwavelength segments within the evanescent field of a conventional strip waveguide. Bound state in the continuum effect is leveraged to suppress the sidewall emission. As a proof of concept, we theoretically demonstrated a millimeter-long through-etched waveguide grating antenna with a divergence angle of 0.081° and a feature size compatible with current silicon photonics foundries.

A switchable and tunable multiwavelength erbium-doped fiber laser is proposed and demonstrated based on a two-stage Lyot filter and intensity-dependent loss (IDL) effect induced by a nonlinear optical loop mirror (NOLM). The proposed laser can achieve multi-parameter individually tuning functions, such as adjusting the channel-spacing, number, and spectral-range of lasing line. The channel spacing can be switched via adjusting the beam's polarization in the Lyot filter among 0.53, 0.78, and 1.56 nm. Meanwhile, the number and the spectral range of the multiwavelength lasing line can be independently controlled by IDL modulation through the combination of adjusting pump power and the birefringence in the loop of the NOLM. The individual tuning method of the proposed laser on each specific output characteristic provides more convenience and practicality to the applications.

We report on the generation of high-order solitons with order continuously adjustable in an erbium-doped fiber laser mode-locked by a graded index multimode fiber–step index multimode fiber–graded index multimode fiber (GIMF–SIMF–GIMF) saturable absorber (SA). The SA has advantages of high damage threshold, easy fabrication and low cost. The fundamental soliton to eleventh-order soliton were obtained by adjusting the pump power and PC. It was found that both increasing pump power and adjusting PC can affect the solitons splitting during experiment. The generation of high-order solitons is due to the peak power clamping effect of laser cavity and the pulse shaping effect of GIMF–SIMF–GIMF SA. Last but not least, this is the first time to adjust the order of solitons continuously from the first to eleventh in a fiber laser mode-locked by GIMF–SIMF–GIMF SA.

A fiber-optic sensor for the simultaneous measurement of strain and temperature is proposed and experimentally demonstrated based on Fabry–Perot (FP) interference and the antiresonance (AR) mechanism. The sensor was implemented using a single-mode fiber (SMF)–hollow-core fiber–SMF structure. A temperature sensitivity of 21.11 pm/°C was achieved by tracing the troughs of the envelope caused by the AR mechanism, and a strain sensitivity of 2 pm/µe was achieved by detecting the fine fringes caused by the FP cavity. The results indicate that the dual-parameter sensor is stable and reliable.

In this article, the peak-power expression of dissipative-soliton-resonance (DSR) pulse is derived, in a normal-dispersion fiber laser, by using the equation for the resonance curve and the relationship between the parameters of cubic–quintic complex Ginzburg–Landau (CGLQ) equation and the cavity parameters. According to the peak-power expression, the peak power of DSR pulses is inversely proportional to the average dispersion and the square of the gain bandwidth, is proportional to the square of the saturation power. That is to say, we can improve the peak power of DSR pulse in three ways: Firstly, increase the saturation power in the case of sinusoidal-saturable-absorber transmission curve; secondly, decrease the gain bandwidth properly; thirdly, decrease the average dispersion suitably. In addition, the further results of numerical simulations are obtained by using pulse-tracing technique, which are in good agreement with the theoretical derivation of peak power of DSR pulses. This means that our work can serve as a guideline to design high-peak-power DSR fiber laser.

Circular polarization shift-keying (CPolSK) modulation is crucial for optical communication in free space optical communication. In this paper, we propose a new optical communication system with multiple orbital angular momentum (OAM) modes for division multiplexing CPolSK modulation signals. A well-designed reference beam is employed to interfere with the CPolSK-OAM-division multiplexing (CPolSK-OAM-DM) signals for coherent detection. Then, the complex modulation signal is converted into intensity change directly detected by photo-detector. The communication performance can be further improved by subtracting the two interference beams detected. In numerical simulation, the demodulated bit error rate (BER) is 2.25 × 10−4 with the signal noise ratio (SNR) of 12-dB, and it can be reduced to 1.25 × 10−4 after difference operation. Through mapping the signals to both the initial phases and circular polarization states of the multiplexing channels, we construct the phase-CPolSK-OAM-DM system, the demodulated BER is 1.07 × 10−4 with the SNR of 15-dB. The results demonstrate that the CPolSK-OAM-DM modulation can effectively expand the communication capacity of the CPolSK system. The CPolSK-OAM-DM system has great potential in the field of high-speed broadband optical communications.

The batch verification methods can relieve the bottleneck problem of vehicular authentication efficiency to accelerate the authentication speed during the group construction phase. However, in current batch verification methods, there exist some deficiencies such as the authentication methods applied in the actual scenario are sensitive to the group initialization and dynamic construction process and the intra-group communication efficiency have not been improved due to lack of effective dynamic group management. This paper proposes a fast and secure batch verification scheme based on dynamic group management and certificateless public-key cryptography. First, the two-way authentication between the proxy vehicle and the roadside unit (RSU) is established, and then the batch verification method relying on RSU-assisted are proposed based on dynamic management of the intra-group vehicles. Our method fully considers the security requirements of the group while solving certification efficiency. Theoretical analysis and simulation experiments show that compared with other existing solutions, the verification scheme in this paper reduces the computational delay and transmission overhead whilst increasing robustness in dynamic environments.

Ultrasonic nondestructive testing plays a more and more important role in modern production and construction. In this paper, a novel ultrasonic energy transfer technique based on peanut shape structure is proposed and demonstrated. The ultrasonic device is able to effectively couple the laser energy in the fiber core to the cladding of the single-mode fiber (SMF) by using the micro machined fiber structure. The laser energy coupled to the cladding can be absorbed by the ultrasonic conversion material coated on the surface of the SMF. Due to the ultrasonic conversion material has high thermal elastic coefficient, it can produce mechanical expansion and contraction process after absorbing the laser, resulting in the generation of the ultrasonic signal. The ultrasonic transducer based on peanut shape taper only needs to be fused by SMF and fiber splicer. The proposed laser-ultrasonic transducer has many advantages, such as simple preparation, cost-effective, and high energy conversion efficiency. Such characteristics makes the proposed laser-ultrasonic transducer attractive for practical applications.

We investigate the conversion of mode locking of bright pulses, dark pulses and dark-bright pulses in an Erbium-doped fiber laser (EDFL) based on a Molybdenum disulfide (MoS2) films saturable absorber (SA) experimentally. When the pump power is 150 mW, by changing the polarization controller (PC) to change the polarization state in the laser cavity， we observed the bright pulse trains, the dark pulse trains and the dark-bright pulses trains in turn. All pulse interval are about 58 ns, and the repetition frequency of pulse is 1.7 MHz. The study provides a method for the conversion among bright pulses, dark pulses and dark-bright pulses.

An all-fiber Fabry-Perot interferometer (FPI) strain sensor based on offset splicing fibers and Vernier effect is proposed and experimentally demonstrated. The proposed sensor consists of two separated air-cavity FPIs connected by a long section of single mode fiber (SMF) in a fiber link. The two separated FPIs have approximately equal optical paths, so that the Vernier effect can be generated. One FPI is used as the sensing FPI (SFPI) for strain measurement, which is formed by splicing a section of microfiber between two SMFs with large lateral offset. The other FPI is used as the reference FPI (RFPI) to employ the Vernier effect and amplify the sensitivity, which is formed by splicing a section of silica tube between two SMFs. Compared to a single FPI based sensor, the strain sensitivity of the proposed sensor with Vernier effect can be improved by tens of times. The strain sensitivity of our proposed sensor reaches 1.3 nm/μe, which is the highest strain sensitivity of fiber sensor based on FPI and wavelength demodulation mechanism. Since the aircavity SFPI is insensitive to temperature, the proposed sensor also exhibits low temperature sensitivity of 50.2 pm/℃. With the advantages of high strain sensitivity, low temperature cross-sensitivity, compact size and easy fabrication, the proposed sensor has great applications in many fields.

Functional soft materials exhibiting distinct functionalities in response to a specific stimulus are highly desirable towards the fabrication of advanced devices with superior dynamic performances. Herein, two novel light-driven chiral fluorescent molecular switches have been designed and synthesized that are able to exhibit unprecedented reversible Z/E photoisomerization behavior along with tunable fluorescence intensity in both isotropic and anisotropic media. Cholesteric liquid crystals fabricated using these new fluorescent molecular switches as chiral dopants exhibit reversible reflection color tuning spanning the visible and infrared region of the spectrum. Transparent display devices have been fabricated using both low chirality and high chirality cholesteric films that operate either exclusively in fluorescent mode or in both fluorescent and reflection mode, respectively. The dual mode display device employing short pitch cholesteric film is able to function on demand under all ambient light conditions including daylight and darkness with fast response and high resolution. Moreover, the proof-of-concept for a "remote-writing board" using cholesteric films containing one of the light-driven chiral fluorescent molecular switches with ease of fabrication and operation is disclosed herein. Such optically rewritable transparent display devices enabled by light-driven chiral fluorescent molecular switches pave a new way for developing novel display technology under different lighting conditions.

In this paper, we reported on the experimental generation of dark pulses from a passively mode-locked fiber laser based on the interaction between the evanescent light and two dimensional material molybdenum disulfide (MoS 2 ) on the microfiber. The MoS 2 nanosheets are prepared through the liquid-phase exfoliation (LPE) method and transferred onto a tapered microfiber by the optical deposition method. The MoS 2 -deposited microfiber is the key mode-locker for dark pulse generation. In experiment, by properly adjusting the polarization controller and pump power, we obtained dark pulses output. The center wavelength of the laser is 1533.4 nm, the interval of the pulse train is 33.3 ns and repetition rate is measured to be 30 MHz.

A multiwavelength erbium-doped fiber laser based on tunable comb spectral filter and intensity-dependent loss modulation is proposed and experimentally demonstrated. The laser allows fine and multifunctional tunable operations of channel-spacing, peak-location, spectral-range, and wavelength-number. More specifically, channel-spacing switch from 0.4 nm to 0.2 nm and peak-location adjustment within half of free spectrum range are obtained via controlling the tunable comb filter. The wavelength-number and the spectral-range of the lasing lines can be accurately controlled by intensity-dependent loss modulation in the laser cavity, enabled by a power-symmetric nonlinear optical loop mirror. In addition, fine control over the wavelength-number at fixed spectral-range is realized by simply adjusting the pump power. More important, the tunable operation process for every type of specific parameter is individual, without influences for other output parameters. Such features of this fiber laser make it useful and convenient for the practical application.

The bright-dark soliton pair in a passively mode-locked fiber laser based on the topological insulator: Bi2Se3/polyvinyl alcohol (PVA) film in the proper polarization state and the pump power is experimentally demonstrated. By carefully adjusting the pump power and the polarization state, the bright-dark soliton pair could be easily observed. Furthermore, it is found experimentally that the formation of the bright-dark soliton pair is mainly due to the polarization effect based on the high nonlinear effect of topological insulator: Bi2Se3/PVA film. We also demonstrate that the pulse shape of the bright-dark soliton pair can be affected by the total net cavity dispersion in the fiber laser.

A tunable multiwavelength erbium-doped fiber laser (MWEDFL) based on nonlinear optical loop mirror (NOLM) and tunable birefringence fiber filter (BFF) is proposed and demonstrated. By combination of intensity-dependent loss modulation induced by NOLM and pump power adjustment, the proposed laser can achieve independent control over the number of lasing lines, without affecting other important characteristics such as channel spacing and peak location. In addition, the laser allows wavelength tuning with both the peak location and the spectral range of lasing lines controllable. Specifically, the peak location of lasing lines can be controlled to scan the whole spectral range between adjacent channels of comb filter by adjusting the BFF. Moreover, the spectral range of lasing lines can be controlled by adjusting NOLM. This tunable MWEDFL may be useful for fiber-optic communication and fiber-optic sensing.

An all-fiber Fabry-Perot (FP) interferometer (FPI) sensor which can simultaneously measure strain and temperature is proposed and experimentally demonstrated. The FPI sensor is composed of a cascaded-cavity structure with a hollow-core tube fiber cavity and a standard single-mode fiber (SMF) cavity. Different cross section areas and thermal-optic coefficients of the two cavities make them differently sensitive to temperature and strain. The individual interference spectrum based on the air cavity or silica cavity can be extracted by band-pass filtering the spectrum of the sensor in frequency domain. The temperature and strain sensitivity of each cavity are obtained by tracing the wavelength shifts of the interference spectra. Furthermore, the temperature-strain cross-talk is eliminated by solving a cofficient matrix equation composed by the temperature and strain sensitivities of the two FP cavities. The strain sensitivities of two FP cavities achieved are 1.173 nm/μe and 1.86 nm/μe. The temperature sensitivities of two FP cavities are 0.00574 nm/°C and 0.00549 nm/°C. The advantages of the proposed sensor including simultaneous measurement of temperature and strain, no temperature-strain cross-talk, simple fabrication, and all-fiber structure make it appropriate for the real sensing applications.

A novel side-excitation fiber-optic laser-ultrasound transducer based on fused taper fiber is proposed and demonstrated. The transducer achieves ultrasound generation at optical fiber sidewall and performs a relative high conversion efficiency. In the experiments, 514 mV peak-to-peak power ultrasound wave has been obtained at the fiber sidewall with input 0.04 mJ laser pulse. Theoretical analysis and experimental results show that the strength of generated ultrasound wave can be controlled by adjusting the taper length of the fused fiber taper. The proposed method is promising candidate to achieve the multi-point laser-ultrasonic generation along a fiber.

A novel multi-wavelength switchable laser system is proposed and demonstrated, which is based on polarization hole burning (PHB) effect in an erbium-doped fiber (EDF) laser (EDFL). In this EDFL, multiple fiber Bragg gratings with different wavelengths are used as comb filter. The PHB effect is enhanced by polarization separating of the multi-wavelength lasing lines through adjusting the polarization controllers (PCs). So the lasing lines with different wavelength, different polarization state can consume the erbium-doped particles with different polarization, leading to a suppression of the mode competition effect in the EDF and a stable multi-wavelength lasing operation. In the experiment, we obtain four-wavelength stable lasing oscillation.

A novel fiber-optic laser-ultrasound transducer based on core-offset splicing fiber is proposed. The experimental operation and working mechanism of the transducer based on ultrasonic generation at sidewall of a core-offset splicing fiber are demonstrated. A 20% coupling ratio core-offset structure was used to fabricate the transducer. Around 480 mV peak-to-peak power ultrasound waves were obtained on the fiber sidewall at the core-offset splicing point with 0.04 mJ laser pulse in the system, which shows a high photoacoustic conversion efficiency. The strength of generated ultrasonic signal can be controlled by adjusting the misalignment size of the core-offset structure. Moreover, the efficient ultrasonic generation at optical fiber sidewall based on core-offset splicing fiber has potential in achievement of multipoint ultrasound generation.

A novel method to realize two point fiber-optic ultrasonic sensor system has been demonstrated, which is based on fiber Bragg grating (FBG) in an erbium-doped fiber (EDF) ring laser (EFRL). There are two pairs of FBGs with different Bragg wavelength in this system. Each pair of FBGs has a long FBG (LFBG) and a short FBG (SFBG), which have an appropriate matching operating point and determine one potential lasing line. Two pairs of FBGs are used as both the comb filter and the ultrasonic sensors. Due to the different spectral response of the LFBG and SFBG to ultrasound, the cavity loss will change regularly by the ultrasound, and the intensity of the laser output will be modulated by the cavity loss. Meanwhile, an adjustable attenuator is added to the fiber ring cavity. By adjusting the attenuator, the gain and loss of the two specific wavelengths can be balanced, and the laser oscillation at two specific wavelengths is stabled. With the modulation of the adjustable attenuator, multiplexing ultrasonic EFRL sensors can be achieved within a single cavity fiber laser. The proposed method has been experimentally demonstrated by using a dual-channel laser system with a single section of gain medium, EDF.