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• A novel photonic skin flexible sensor based on partially neutralized sodium polyacrylate is reported for the first time in this paper. • This photon skin flexible sensor has achieved relative humidity sensing for the first time. • The photonic skin flexible sensor has a relative humidity sensitivity of up to 0.1411 dB/%, with a fast response time of 170 ms. • The temperature response exhibits outstanding sensitivity (0.4 dB/°C) in the human skin surface temperature range (30 ∼ 48 °C). Biomimetic skin sensing devices play an important role in next-generation healthcare, robotics, and bioelectronics. In recent years, biomimetic skin-sensing technologies have gained significant attention due to their broad-ranging utility in the fields of healthcare and automation, and future wearable devices designed for health monitoring, robotic applications, and ultra-precise industrial positioning will necessitate the incorporation of flexible optical systems. A novel photonic skin flexible sensor (PSFS) based on partially neutralized sodium polyacrylate (PAS-50) is reported for the first time in this paper. In this study, a novel wearable sensor was designed for real-time detection of temperature and humidity. A single-mode tapered fiber (SMTF) structure based on the principle of evanescent wave is proposed for fast response and real-time monitoring of humidity and temperature. The sensor incorporates an optical micro/nanofiber (MNF) as the core sensing node, while a hydrophilic and ductile Sodium Polyacrylate (PAS) is chosen as the humidity-sensitive material that overcomes the limitation of commonly reported electrical, optical, and material-based flexible devices. Due to the strain amplification effect of the MNF, the sensitivity of the material to relative humidity (RH) is significantly improved, reaching 0.1411 dB/%. In addition, the temperature response exhibits outstanding sensitivity (0.4 dB/°C) in the human skin surface temperature range (30 ∼ 48 °C). Notably, the sensor exhibits a rapid response with a remarkable response time of 170 ms. The proposed PSFS presents novel opportunities for the advancement of future biomimetic skin devices, thus playing a pivotal role in the advancement of next-generation robotics and medical monitoring technologies. [ABSTRACT FROM AUTHOR]

Fiber optic curvature sensors face several challenges that limit their application in structural health monitoring. These include high fabrication and demodulation costs, poor stability and robustness. In this paper, we propose a novel Mach–Zehnder interferometer (MZI) sensor that overcomes these limitations.The sensor was fabricated through the splicing of a seven-core fiber(SCF) and two single-mode fibers (SMF) with a common fusion splicer. The proposed sensor exhibited a good linear response, with a sensitivity of 22 nm/m − 1 in the range of 0–0.313 m − 1. The MZI sensor was demodulated by a neural network-based system and a data augmentation algorithm, reducing the need for large data samples and improving demodulation accuracy on sparse datasets. Overall, the sensor is characterized by fabrication simplicity, high mechanical strength, and compatibility with existing optical devices, making it applicable to various fields, such as aerospace, civil engineering, and mechanical manufacturing. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a simple structure, ultra-narrow linewidth, extremely high stability and high optical signal to noise ratio single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL). Nested Fiber Ring (NFR), nested in the ring cavity of EDFL, is employed to serve as a high quality mode filter to eliminate the dense longitudinal mode of the EDFL. The mode-selection capability of the RIR is investigated and discussed theoretically and experimentally. Saturable absorber, which composes of a section of unpumped polarization maintaining erbium-doped fiber and a Fabry–Perot fiber Bragg grating (FP-FBG), is employed to produce a dynamic narrowband grating to ensure the SLM laser output. Saturable absorber, consisted of a section of unpumped polarization maintaining erbium-doped fiber, produces a dynamic narrowband grating to ensure the SLM laser output. We achieve a ultra-narrow linewidth SLM laser output of 550 Hz. The laser has an optical signal-to-noise ratio (OSNR) of more than 59 dB. In addition, the stability of the wavelength and power fluctuations below 0.03 nm and 0.02 dB, respectively. The single-longitudinal-mode fiber laser has a compact structure, ultra-narrow linewidth, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on. • We propose a new structure of ultra-narrow linewidth single-longitudinal-mode erbium-doped fiber laser based on Nested Fiber Ring structure and Saturable Absorber. • The new structure has the advantages of simple structure, easy preparation and high stability. • The output performance of the proposed fiber laser is excellent, the 3-dB linewidth is 550 Hz and the optical signal-to-noise ratio is 59 dB. • The filtering function of the Nested Fiber Ring was theoretically derived and its filtering effect was analyzed. [ABSTRACT FROM AUTHOR]

• The effect of gamma rays on two experimental fiber-optic cables was studied. • The cables were irradiated using the dose of 80.5 kGy and rate of 5.5 Gy/h. • The cables were irradiated using the dose of 65.5 kGy and rate of 0.94 Gy/h. • MM fibers after irradiation have lower optical losses at 1300 nm than 850 nm. • SM fiber with the lowest influence of gamma rays had losses of 3.5 dB at 1310 nm. This paper reports on our study of the effect of gamma-ray irradiation on two types of experimental fiber-optic cables. The first type of cable, CTFS, had a jelly-filling compound, whereas the second type, CTDS, was without the jelly. The cables contained three multi-mode (A, B, C) and five single-mode (D, E, F, G, H) optical fibers. The study was performed using two gamma-irradiation facilities with 60Co sources for slow long-term irradiation with the dose of 80.5 kGy at the low dose rate of 5.5 Gy/h and with the dose of 65.5 kGy at the higher dose irradiation rate of 0.94 kGy/h. After the irradiation with the dose of 80.5 kGy, the C fiber in the CTDS cable had the optical loss of 5.91 dB; 140 days after the gamma treatment at the wavelength of 1300 nm, it was 4.96 dB. The lowest optical losses for a single-mode fiber after irradiation with the dose of 65.5 kGy were found for the D fiber in the cable CTFS, namely 3.26 dB at 1490 nm and 3.49 dB at 1310 nm. In the fiber E in the CTDS cable, the optical loss was 3.62 dB at 1550 nm. In the fiber F in the CTDS cable, it was 3.86 dB at 1310 nm. [ABSTRACT FROM AUTHOR]

• A wavelength injection locking actively Q-switched wide locking range random fiber laser (RFL) based on single-mode fiber (SMF) is proposed. • A narrow linewidth laser with tunable wavelength and power is injected into the cavity through the wavelength tunable laser (TLS), and the laser mode of RFL is selected by mode resonance. • In the C-band of 1530 to 1565 nm, different laser modes with stable center wavelength and peak power can be selected by changing the wavelength of the injected light. In this paper, a wavelength injection locking actively Q-switched wide locking range random fiber laser (RFL) based on single-mode fiber (SMF) is proposed. Within the C-band range from 1530 nm to 1565 nm, different laser modes can be selected by changing the wavelength of the injected light. Under injection locking state, the power fluctuation within one hour is less than 0.1dBm. When high nonlinear optical fiber (HNLF) is used in the RFL, the pump power is increased from 107 mW to 300 mW, the fixed sine wave modulation signal frequency is 20 kHz, the modulation voltage is 5 V, and the injection power is 15dBm. The RFL is in injection locking state and the output pulse is stable 4 pulses. When HNLF in RFL is replaced by SMF with the same length and the injection locking state is maintained, the output pulse is single pulse. Furthermore, the pump power increases from 107mW to 300mW, the pulse width increase from 1.91 μs to 3.24 μs, and the pulse repetition frequency is 20 kHz. In addition, the laser has wavelength tunable properties in the C-band range and serves as an ideal light source in the fields of laser imaging and pulse coding. [ABSTRACT FROM AUTHOR]

• PDMS is introduced to fiber-optic Fabry-Perot interferometer for sensitization. • The sensor has a wide range of temperature measurement from −30 °C to 85 °C and has high temperature sensitivity. • The sensor's specifications are tested to prove its excellent performance. • The sensor is simple to make and has the potential for mass production. In this paper, a highly sensitive temperature sensor based on polymer-coated Fabry-Perot interferometer (FPI) is proposed and demonstrated. The FPI temperature sensor consists of the end face of single-mode fiber (SMF) and the Polydimethylsiloxane (PDMS) cap on the end face. By varying the number of coatings, the thickness of the PDMS cap can be flexibly controlled to obtain FPI sensors with different cavity lengths. The temperature sensitivity is studied experimentally from −30 °C to 85 °C, and it is found that the sensor shows different sensitivity in different temperature intervals, up to 0.698 nm/°C. Compared to similarly constructed sensors, the sensor in this paper has a large temperature range while maintaining a high sensitivity. In addition, the performance of the sensor is tested by experiments and the results show that the sensor possesses the advantages of good stability, fast response time and good repeatability. [ABSTRACT FROM AUTHOR]

• The coupler is simply made of SMF-NCF-SMF offset splicing. • The coupled structure can realize the coexistence of MMI with a large FSR and WGM with small FWHM. • The sensor can simultaneously achieve temperature detection with a large measuring range and low detection limit. • Asymmetric Fano lines can be obtained under the modulation of interference. In this paper, a coupler based on a single-mode fiber (SMF)- no-core fiber (NCF)- SMF offset structure was proposed and demonstrated, which can generate the evanescent field to excite whispering gallery mode (WGM) of a microsphere cavity. By reasonably controlling the length and offset distance of NCF, the multimode interference (MMI) and WGMs can be generated at the same time. The experimental results show that the temperature measurement with a large measurement range and low detection limit can be realized simultaneously with the assistance of the interference envelope, which solves the problem of large range measurement difficulty caused by the dense dips of WGM to some extent. In the range of 22 ℃-80 ℃, the sensitivity of WGM resonant dips is ∼10.5 pm/℃. On the other hand, symmetric Lorenz lines with a quality factor of 8.2 × 103 and asymmetric Fano lines with a slope of -21.94 dB/nm were obtained under the modulation of interference. The coupler proposed in this paper has the advantages of simple preparation and strong structural robustness, and shows good potential in applications such as sensors and filters. [ABSTRACT FROM AUTHOR]

• An optical fiber strain sensor based on metal welding has been presented. • The proposed fiber sensor has the strain sensitivity of 9.11 pm/με. • The fiber sensor has the advantages of easy fabrication and low cost. • The relationship between capillary length and strain sensitivity is investigated. This paper presents a strain sensor based on Fabry-Perot (F-P) interference. The F-P strain sensor is composed of a single-mode fiber (SMF) and a silica capillary, which are encapsulated by metal welding. The relationship between the cavity length and sensitivity is analyzed theoretically, then the F-P strain sensor is calibrated and tested. The experiment results show that the strain sensitivity of the fabricated sensor reaches 4.4 pm/με when the length of the silica capillary is 12 mm and the F-P cavity length is 436 μm. In addition, the temperature sensitivity of the F-P sensor is 11.7 pm/°C. With the increase of the capillary, the strain sensitivity increases. When the capillary length is 20 mm and the cavity length is 341 μm, the sensitivity of the sensor reaches 9.11 pm/με. The sensor proposed fabricated by metal welding, has the advantages of high sensitivity, simple fabrication, simple structure, and low cost. [ABSTRACT FROM AUTHOR]

Pressure sensing holds immense significance in the realm of fiber optic sensors, particularly in the context of industrial and environmental safety monitoring. In this paper, we propose and experimentally validate an optical fiber structural sensor based on Mach-Zehnder interferometer (MZI) for pressure measurement. The sensor adopts a cascaded spliced single-mode-multimode-tapered single-mode fiber (SMF-MMFTapered SMF, SMTS) structure, taking advantage of the mode mismatch effect between different fiber types and the high sensitivity of the evanescent field surrounding the tapered single-mode fiber. A segment of multimode fiber (MMF) is incorporated as the mode coupler to enhance the multimode interference (MMI). Experimental results demonstrated the sensor's robustness to pressure and temperature changes, with measurement sensitivities of the interference wavelengths Dip A and Dip B at 5.751 nm/KPa and 1.394 nm/KPa, respectively, within the pressure range of 0 ∼ 4410 Pa. The sensor also exhibited a response of 1.113 nm/°C and 0.857 nm/°C in the temperature range of 27.5 °C ∼ 41 °C. The sensor indicates a low temperature crosstalk of only 0.194 KPa/°C. By applying temperature compensation to correct pressure measurements, the accuracy of pressure sensing can be significantly improved. The sensor offers distinct advantages, including high sensitivity, low temperature crosstalk, compact structure, and simple fabrication, making it a promising candidate for applications in biochemistry and industrial production. [ABSTRACT FROM AUTHOR]

• A compact concentration sensor based on the modified thin-diameter polarization-maintaining fiber is demonstrated. • The relationship between the measurement sensitivity and the length and corroded time of the TDPMF is discussed. • The SC concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%, and the temperature sensitivity is 0.08179 nm/°C. • The MMF-MTDPMF-MMF structure sensor has excellent repeatability and stability in SC concentration sensing. Sodium citrate, as strong alkali and weak acid salt, play an essential role in the food and pharmaceutical industries. However, lack of detection method selectivity, limited biocompatibility and complexity of the manufacturing process are challenges that hinder the practical application of the sensors. In this paper, a improved-sensitivity sodium citrate concentration sensor with simple and compact interrogation setup is proposed. The sensor is fabricated by fusion splicing the sandwich structure of modified thin-diameter polarization-maintaining fiber, two pieces of multimode fiber, and single-mode fiber. In theoretical modeling, the properties of the structure are investigated. In the experiment, the evanescent field generated by the modified thin-diameter polarization-maintaining fiber plays a vital role in the detection of different sodium citrate concentrations. The sodium citrate concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%. The sensor provided refractive index sensitivity of 3173.2 nm/RIU. In addition, the response, repeatability, and stability are also evaluated by the experiments. Our results show that the sodium citrate concentration sensor holds great promise for serving as a soft and biocompatible probe for monitoring sodium citrate in situ. [ABSTRACT FROM AUTHOR]

The polarization characteristics of the optical fiber have spurred interest in a plethora of smart fiber optic sensors with extremely compact structures due to the capability of in-line configuration, except that the sensing sensitivity remains too low to meet the practical requirements. In this paper, fiber Lyot filter (FLF) is leveraged to generate Vernier effect for sensitivity enhancement of birefringence-based and versatile fiber optic sensor. The proposed sensing structure is composed of two sections of polarization maintaining fibers (PMF) with the reflective scheme, which can be applied to birefringence sensing scenarios such as strain and temperature measurement. The working principle analysis derived from Jones matrix indicates that the combination of Vernier effect and the reflective structure catalyzes a significant improvement in sensitivity. The length of the PMF used in this sensor, the fusing angle between two PMFs, and the birefringence properties of the PMF are all numerically investigated to generate better Vernier performance. Also, experiments are carried out for validation, and sensitivity as high as −12.96 nm/rad is achieved. The compact and in-line structure of the proposed sensor can enable itself to perform excellent and versatile sensing on various occasions. • Fiber birefringence sensors are widely used in many applications. Low sensitivity due to the slight birefringence change of the polarization maintaining fiber hinder their development, and the practicality and stability of existing structures (core-offset fiber, tapered fiber, etc.) used to generate Vernier effect are unsatisfactory. Therefore, we investigate the Lyot filter-induced Vernier effect for sensitivity improvement of fiber birefringence sensor. • The Lyot filter is used to generate Vernier effect for sensitivity improvement of fiber birefringence sensor. • This approach provides a practical and stable structure with high sensitivity and easy packaging. • Simulations and experiments show that the sensor is stable with high sensitivity, providing more opportunities for versatile sensing in various occasions. [ABSTRACT FROM AUTHOR]

• A transverse pressure sensor based on Sagnac cascaded FBG is proposed. • Based on PMF birefringence effect. • FBG structure is introduced to improve sensitivity. • Finite element method combined with experiment. In the field of engineering, the deformation and lateral stress in structural health detection have attracted extensive attention to obtain the structural health information of building objects in time. Based on the birefringence effect of polarization maintaining fiber (PMF), a transverse pressure sensor based on Sagnac cascade fiber bragg grating (FBG) was proposed and prepared. The performance of the sensor is analyzed by the combination of the finite element method and experiment. The simulation results show that the cross section of PMF is deformed under the pressure along the fast axis and the slow axis and then modulates the refractive index. The experimental results show that by adding FBG structure, the lateral pressure sensitivity of the sensor is 408.7 pm/N, and the linear fit is 98.3%. The lateral pressure sensitivity of the structure without FBG is 281.7 pm/N, and the linear fit is 99.3%. That is to say, the FBG structure increases the sensitivity of the sensor by 45.1%. The experimental results are in full agreement with the theoretical derivation. The optical fiber lateral pressure sensor proposed in this paper has the characteristics of simple fabrication, low cost, and high repeatability. It plays an important role in the field of structural stress monitoring. [ABSTRACT FROM AUTHOR]

• An all-fiber electro-optic modulator derived from the large-transverse-offset MZI coated with ITO is first proposed. • The modulator spectral shift efficiency is about 0.406 nm/V with an excellent linear correlation coefficient of 0.995. • The rise time of the modulator is about 0.65 s and the fall time is 0.5 s, respectively. In this paper, we first propose a fiber electrothermal modulator derived from the large-transverse-offset Mach-Zehnder interferometer (MZI) coated with transparent and conductive indium-tin-oxide (ITO). The fiber MZI was fabricated by welding single-mode fiber (SMF) with large-transverse-offset fusion. The fiber modulator is that the thermo-optic effect of the ITO film on the surface of the fiber leads to a change in the refractive index, which in turn causes the shift of the interference spectrum. Specifically, it is based on the thermo-optic effect of a large-transverse-offset MZI coated with ITO. Through the external electric potential, the ITO film generates Joule heat and then changes the effective refractive index of the cladding to realize fiber phase modulation. In the experiment, the spectrum is significantly shifted by 5.55 nm at the wavelength of 1650 nm and can obtain an optical modulator with a rise (fall) time of 0.65 (0.5) s. The proposed fiber modulator shows great performance in the optical switches and sensing. [ABSTRACT FROM AUTHOR]

• This paper proposes a double-ring cavity erbium-doped random fiber laser based on bi-directional rayleigh scattering in a single-mode fiber. • The tunable laser output is realized by using the Mach-Zehnder interferometer composed of two 2 × 2 couplers as the dressing filter. • The stable and wavelength tunable random fiber laser could be realized by Mach-Zehnder interferometer filter. A wavelength-tunable double-ring cavity erbium-doped random fiber laser (RFL) based on a Mach–Zehnder interferometer (MZI) was proposed in this study. The MZI structure, which was based on two 2 × 2 optical fiber couplers, exhibits a comb-filtering effect. Rayleigh scattering (RS) was generated by a 10-km-long single-mode fiber (SMF) to provide randomly distributed feedback. A double-ring cavity structure based on forward and backward RS was built using two 2 × 2 fiber couplers. The threshold of the laser was 104 mW, and the slope efficiency of the total power output to the pump power was approximately 1.2%. When the pump power was 158 mW, a laser beam with a wavelength of 1555.3 nm was formed. By adjusting the polarization controller (PC), a single-wavelength switched RFL at 1547.9 nm and 1559.6 nm was obtained. The maximum peak power difference was less than 0.67 dB, the signal-to-noise ratio (SNR) was greater than 17.27 dB, and the peak power fluctuation was less than 0.92 dB during a scan time of 30 min at 25 °C. Regarding dual-wavelength switchable laser emission, two groups of different wavelengths with an SNR greater than 16.88 dB, power fluctuation less than 0.81 dB, and wavelength fluctuation less than 0.26 nm could be achieved under the same monitoring conditions. By further adjusting the PC, three- and four-wavelength RFLs were achieved, and the SNRs exceeded 16.31 dB and 14.64 dB, respectively. The proposed RFL has promising application prospects for distributed sensing systems and speckle imaging. [ABSTRACT FROM AUTHOR]

• The influence of PAPR of baseband OFDM signal and the MB-OFDM UWB signal is investigated. • The DFT-spread scheme with CF scheme is applied to reduce the PAPR of the MB-OFDM UWB signal. • A combination of DFT-spread and pre-compensation scheme is introduced to improve system performance. This paper explores the peak-to-average power ratio (PAPR) influence of the base band orthogonal-frequency-division-multiplexing (OFDM) signal and the multiband orthogonal-frequency-division-multiplexing ultra-wideband (MB-OFDM UWB) signal, and compares the performance of discrete fourier transform spread (DFT-spread) scheme with clipping filter (CF) scheme to reducing PAPR in MB-OFDM UWB signal. On this basis, this paper also puts forward a scheme for restraining the influence of high PAPR and frequency selective fading (FSF) in 64 quadrature amplitude modulation (QAM) MB-OFDM UWBoF system, which is based on the combination of DFT-spread and pre-compensation. The results of simulation show that this scheme can improve the receiver sensitivity by 3.30 dB after the transmission of 70 km standard single-mode fiber (SSMF) with the bit error rate (BER) of 1.0 × 10-3, which compared with the situation without DFT-spread and pre-compensation. [ABSTRACT FROM AUTHOR]

• LEAF optical fiber Brillouin signature allows temperature and strain discrimination. • Irradiation exposure of LEAF improves temperature and strain discrimination. • Measurement only requires one fiber and one Brillouin-based interrogation technique. • Temperature and strain sensitivities of LEAF are affected by the irradiation. • Suitable technique under irradiation depending on the application. In this paper, we investigate the accessible performances in terms of strain and temperature discrimination using Brillouin Optical Time Domain Analysis (BOTDA) combined with the LEAF fiber (from Corning) when exposed to γ-rays up to doses of 1 MGy(SiO 2). This optical fiber offers a multipeak Brillouin Gain Spectrum (BGS) with unique dependencies of each of its peaks over temperature and strain. The discrimination capability evolution of LEAF is investigated at high γ doses and dose rates to evaluate the radiation effects on the sensing performances. Gamma irradiation leads to modifications of the LEAF BGS signature through two main effects. First, the Radiation Induced Attenuation (RIA) of this germanosilicate optical fiber limits the BGS amplitude, sensing range and capability for discrimination over BGS amplitude. Second, radiations modify the Brillouin scattering properties by slightly changing the refractive indices and the acoustic velocities of the fiber core and cladding leading to a small change in temperature and strain dependences as well as a limited Radiation Induced Brillouin Frequency Shift (RI-BFS) causing a direct error on the measured parameters. Results exhibit an overall decrease of temperature and strain uncertainties through discrimination process after 1 MGy irradiation reaching 0. 9 ° C and 29 μ ε for 0.1 MHz frequency uncertainty. [ABSTRACT FROM AUTHOR]

The all-fiber voltage sensor is difficult for application because of its complex structure, difficult signal demodulation, and sensitivity to the environment. In this paper, a wavelength modulated all-fiber voltage sensor with a simple structure is designed and proposed using straight tapered single-mode fiber and stacked piezoelectric crystal as a voltage sensing unit. The simulation analysis of its performance shows that it can realize the voltage measurement of 0 kV ∼ 10.41 kV, and the accuracy can reach 0.2%. Additionally, the tapered fiber has good strain sensitivity and temperature insensitivity, and its sensitivity is up to 0.03206 nm / μ ε with low bending loss. [ABSTRACT FROM AUTHOR]

Multidimensional optical modulation (MOM) is one of the promising techniques for spectrally efficient optical access network transmissions. However, inter-dimensional interference (IDI) in MOM limits the signal-to-interference-plus-noise ratio (SINR) giving trade-off between each dimension. In this paper, we propose IDI-tolerant four-dimensional (4-D) set-partitioning quadrature amplitude modulation with M−ary differential phase shift keying (SP-QAM-MDPSK) based on direct-detection (DD) systems. We compared the proposed scheme with QAM-MDPSK without set-partitioning and experimentally demonstrate SP-16QAM-16DPSK over 20-km single-mode fiber, results in nearly 3 dB signal-to-noise ratio (SNR) gain at 3 × 10 - 2 symbol error rate (SER) compared to 16QAM-4DPSK, which shows that SP-QAM-MDPSK is tolerant to the possible IDI. [ABSTRACT FROM AUTHOR]

• An actual DML-based MB-OFDM UWBoF system was constructed and investigated. • The ISFA algorithm is applied to improve the SNR and channel estimation accuracy of the DML-based MB-OFDM UWBoF system. • OCT precoding is employed to average the SNR of DML-based MB-OFDM UWBoF systems. In this paper, an intensity-modulated direct-detection (IM/DD) multiband orthogonal-frequency-division-multiplexing ultra-wideband over fiber (MB-OFDM UWBoF) transmission system using a low-cost directly modulated laser (DML) is analyzed and experimentally verified. Precoding techniques and inter-symbol frequency-domain averaging (ISFA) algorithm are used in this experiment. Precoding techniques are used to suppress the noise introduced by DML and the power fading due to the imperfect frequency response of optoelectronic devices and frequency selective fading. Orthogonal Circular Matrix Transform (OCT) precoding and Discrete Fourier Transform (DFT) precoding were compared when the ISFA algorithm was used. In this scheme, for a single-mode fiber (SMF) link at a distance of 20 km, the 64 Quadrature amplitude modulation (QAM) mapped MB-OFDM UWBoF system using OCT precoding with ISFA and DFT precoding with ISFA scheme can improve the sensitivity of the optical receiver by 0.4 dBm and 0.284 dBm, respectively, compared to no precoding with ISFA at a BER of 3.8e-3. The minimum BER of the 64QAM mapped MB-OFDM UWBoF system scheme with OCT precoding and ISFA scheme is 2.56e-5, which achieves two orders of magnitude improvement compared to the BER threshold of the 7% hard-decision forward error correction (HD-FEC, 3.8e-3). [ABSTRACT FROM AUTHOR]

• In this paper, a new core modulation method is proposed for the first time. A novel long period fiber grating with arched fiber core is fabricated and the sensing characteristics are investigated. • The fiber etched by the CO 2 laser is put into the fiber fusion splicer for heating and melting. Under the action of liquid surface tension, the fiber core is deflected to the etching side, forming an arched core. • The experimental results show that the arched core long period fiber grating has excellent bending and strain characteristics. In this paper, the fabrication of arched fiber core based on single-mode fiber is first proposed and experimentally studied about the relevant sensing characteristics. A novel kind of long-period fiber grating (LPFG) with good sensing performance is formed by increasing the number of arched cores with a fixed cycle. The arched core long-period fiber grating (AC-LPFG) has excellent bending sensitivities with −25.06 nm/m−1, −4.26 nm/m−1, 16.84 nm/m−1, −4.92 nm/m−1 in four different directions, and it shows that the AC-LPFG can distinguish the bending direction very well. Meanwhile, the strain sensitivity and temperature sensitivity of the AC-LPFG are also experimentally tested to be −5.9 pm/μm and 84.5 pm/℃ respectively. The excellent bending and strain characteristics of the AC-LPFG indicate that the arched fiber core has similar effects as some special fibers, which will surely get closer attention and more research applications in the future. [ABSTRACT FROM AUTHOR]

• A way of multipoint displacement measurement using fiber-loop ring-down (FLRD) method is proposed. • Single-mode optical fiber wrapped around the cylinder airbag surface (CA-SMF) is used as the displacement sensor. • The proposed FLRD system has low intracavity loss, so there is no need to supplement the intensity of pulse light in the system. • OTDR is developed by ourselves, which makes the sensing system simple and efficient. • We have demonstrated that the length and intrinsic losses of FLRD do not affect the sensing performance of displacement sensors in a reasonable range. In this paper, we propose a method of multipoint displacement measurement using fiber-loop ring-down (FLRD) technology. We demonstrated the feasibility of multipoint displacement measurement using three cascaded FLRDs. Single-mode optical fiber wrapped around the cylinder airbag surface (CA-SMF) is used as the displacement sensor. Due to the low insertion loss of CA-SMF and the high-power modulated signal output of OTDR, it is not necessary to add any device for compensating the pulse signal power in FLRD. Finally, we also consider the errors that arise in the engineering manufacturing process. We have experimentally demonstrated that the length and intrinsic losses of FLRD do not affect the sensing performance of displacement sensors in a reasonable range, which is extremely helpful for practical engineering applications. [ABSTRACT FROM AUTHOR]

• An amplitude temperature sensor based on chirped fiber Bragg gratings is presented. • An accuracy is 0.48 °C and a sensitivity is 0.20 µW/°C in the range of 30–70 °C. • The power of signal can be scaled by increasing the number of pairs of CFBGs. In this paper we study an original technique for creating an amplitude temperature sensor based on n -pairs of chirped fiber Bragg gratings (CFBGs). The scalable design of the sensor allows to increase the range of the measured signal. The use of the amplitude interrogation technique ensures high speed, elementary nature of the scheme and simple operating conditions. The application of superimposed CFBGs as a sensitive element of an amplitude temperature sensor is investigated. As a result of the work, a method for measuring temperature with a fiber-optic sensitive element based on n -pairs of CFBGs is tested. Two superimposed CFBGs are inscribed in a standard telecommunication single-mode optical fiber using a KrF excimer laser system and Talbot interferometer. One of the superimposed structures is used as a reference element of the sensor and the second – as a sensitive element and is placed in a brass tube, where it is fixed on one side with an adhesive joint. The sensor has an accuracy of 0.48 °C and a sensitivity of 0.20 µW/°C for temperature measurement in the range of 30–70 °C. The results obtained are of great importance in the development of fiber-optic temperature control systems. Implementation of the sensor based on n -pairs of CFBGs allows scaling the power of signal, increasing the ability to detect a slight change in ambient temperature. The created sensor works according to the amplitude polling technique, which is characterized by high accuracy and speed of measurement, minimized scheme, simplified use and easy processing of the detected signal. In addition, the implemented interrogation method corresponds to the trend of developing low-cost fiber-optic systems without losing their main advantages. [ABSTRACT FROM AUTHOR]

• An all-fiber half waveplate is placed at the center of the whole spun highly birefringent fiber to compensate for the depolarization in the fiber. • The all-fiber half waveplate is made of the helically wound spun highly birefringent fiber without breaking the whole fiber. • The advantages of the proposed method are simple, effective and without breaking the fiber. • Simulations and experiments have demonstrated the excellent performance of the proposed method. Nowadays, the spun highly birefringent single-mode optical fiber is increasingly being considered for current-sensing applications. However, when a broadband polarized light is launched into the fiber, apparent depolarization will occur with its propagation. This leads to a decrease in the polarization sensibility. In this paper, we propose a kind of method to compensate for the depolarization in spun highly birefringent fibers. An all-fiber half waveplate made of the helically wound spun highly birefringent fiber is placed at the center of the whole fiber. To obtain this half waveplate, the torsion of the helix should be equal to the inherent polarization rotation of the spun highly birefringent fiber. The advantages of the proposed method are simple, effective and without breaking the fiber. Simulations and experiments have demonstrated the excellent performance of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrated a novel seed source for the continuous wave Yb-Raman fiber amplifier, which adopted an 1140 nm single mode laser diode (LD) as Raman signal seed and a conventional 1085 nm fiber oscillator as the Raman pump seed. By splicing a section of long passive fiber in the Raman pump seed cavity, optimizing the Raman pump seed power and the cooling condition, the higher order Raman threshold was doubled and over 500 W output Yb-Raman fiber amplifier was achieved. In addition, our experiment also proved that, for the first time, splicing long passive fiber in the seed oscillator cavity improved the temporal performance of the first order Raman in the conventional fiber master oscillator power amplifier, which could increase the higher order Raman threshold as a result. The seed structure based on a single mode LD and a long cavity length fiber oscillator, could provide a highly compact and cost effective seed source for the high power Yb-Raman fiber amplifier with enhanced higher order Raman suppression. [ABSTRACT FROM AUTHOR]

• Ring based Hybrid FSO- Fiber Optic Architecture. • Same wavelength transmission for downstream and upstream Transmission. • A reliable hybrid 4 × 10 Gbps fiber optic-FSO based ring architecture. • Backup protection at CO side as well as at user side. This paper proposes a reliable hybrid 4 × 10 Gbps fiber optic-FSO based ring architecture. The proposed architecture aims to provide reliable and bandwidth-efficient transmission. The proposed architecture has a fault protection capability in presence of the link failure occurring at the feeder network (FN) and distribution network (DN) towards the central office (CO) and optical networking unit (ONU) respectively. To avail the benefits of free space optics (FSO), it is integrated with the optical fiber at the user end. The architecture transmits the data on the ring in only a clockwise direction for both downstream and upstream transmission and therefore enables the utilization of the same wavelengths without any interference. i.e. the proposed architecture improves the bandwidth utilization and needs only half of the spectrum. In the proposed architecture, four 10 Gbps signals are propagated through a 20 km single-mode fiber (SMF) and a 200 m length of free-space/optical fiber link for the downstream and upstream transmissions. We also compared the signal transmission performance of architecture in terms of bit error rate (BER) for different weather conditions of the FSO link at the user side. The architecture works well for the different weather conditions and can transmit data through the FSO link for upto 700 m and 200 m for clear and heavy rain weather conditions respectively. Protection against faults in the fiber links; improves the survivability and reliability of the network. The proposed architecture is useful in rural and urban hilly areas where signal transmission interrupts frequently due to various geographical challenges. [ABSTRACT FROM AUTHOR]

• A novel compound-ring resonator of an optical fiber laser configurated with two interleaved sub-ring resonators has been developed. • Single-longitudinal-mode laser has been emitted from the optical fiber laser. • A Fabry-Perot etalon has been designed as a criterion to which the frequency of the laser has been locked. • The frequency of the laser has been stabilized by the Fabry-Perot etalon and an active feedback controlling circuit. In this paper we report the development of a robust highly stable frequency single-longitudinal-mode optical fiber laser based on a Fabry-Perot etalon and an active feedback controlling system. The laser is fundamentally structured on an all-fiber compound-ring resonator which is configurated with two interleaved and optical-path-overlapped sub-ring resonators. By employing a length of erbium-doped fiber as the gain medium and a FBG as a reflector, the effective linewidth of the gain spectrum of the compound-ring resonator is restricted to be as narrow as the reflected spectrum of the FBG, which is about 0.043 nm. Because of the vernier effect between the two interleaved sub-ring resonators, the longitudinal mode spacing of the compound-ring resonator can be expanded to be as wide as the effective linewidth of the gain spectrum of it, which permits only one longitudinal mode oscillating. By employing an external Fabry-Perot etalon as a criterion and using an active feedback controlling system to tune the length of the cavity of the compound-ring resonator, the ambiance disturbances to the cavity can be compensated and the frequency of the emitted laser is locked to the offset frequency of the Fabry-Perot etalon. Therefore the frequency of the emitted laser has been stabilized to be 10−8. [ABSTRACT FROM AUTHOR]

• Enhanced Mach-Zehnder interferometer multimode–single-mode–multimode fiber optic refractive index sensor based on surface plasmon resonance is proposed and experimentally demonstrated. • Simultaneous using of SPR effect and chemical etching provides a high-performance Mach-Zehnder RI sensor. • Reducing the waist of the sensing head of the SPR-MZI sensor results in the improvement of sensitivity (about 50%) in a wide range of RI near to 1.333. • High sensitivity, simple fabrication, and reproducibility make these sensors promising devices for chemical and biosensing applications. In this paper, an all-fiber Mach-Zehnder interferometer (MZI) sensor for refractive index (RI) measuring is presented, which is based on Multimode–Single-mode–Multimode (MSM) fiber. The effects of both reducing the radius of the sensing part and the surface plasmon resonance (SPR) on its efficiency are investigated. Increasing the interaction of high-order modes with external media, caused by etching the cladding layer of the single-mode fiber part, significantly improves the sensitivity. Both wavelength and intensity interrogation approaches are employed to study the Multimode–etched Single-mode–Multimode (MESM) fiber sensor. The intensity and the wavelength sensitivities for the RI measurement in the range of 1.428–1.458 are obtained as −2306.754 %/RIU and 1315.74 nm/RIU, respectively. Finally, the MESM-SPR sensor is proposed and characterized. Results exhibit high performance in the RI range of 1.333 to 1.357, in which the sensitivity of 1437.056 nm/RIU is achieved. The advantages like low cost, high sensitivity, and simple fabrication methods make these sensors promising devices for chemical, food industry, and biosensing applications. [ABSTRACT FROM AUTHOR]

• Minimalistic single-mode fiber biosensors are reviewed; • The fabrication methods for express, minimalistic biosensors are outlined; • A comparison between traditional and minimalistic biosensors paves the road for novel applications; • Minimalistic biosensors require the handling of quasi-random optical spectra. Optical fiber biosensors are receiving significant interest, as they allow real-time, low-limit, and high precision detection of biological analytes such as biomarkers, proteins, and small cells. Classical biosensor designs include gratings, interferometers, and plasmonic structures; however, these systems require a complex design, hard to manufacture on high volumes. Recently, several biosensors having a minimalistic design, rapid and suitable to be automated, have been proposed; the key asset is the scalability potential, in view of fabricating numerous devices for disposable use or for large parallel immunoassays. This new sensor class features reflector-less sensors, shallow tapers, and ball resonators; a pseudo-random spectrum characterizes all these biosensors. In this paper, we provide a bird-eye view of these minimalistic designs and a perspective of their use in advanced biosensing applications. [ABSTRACT FROM AUTHOR]

• All fiber-optic sensor based on a Balloon-like interferometer for pH measurement was demonstrated. • The Mach-Zehnder interferometer was fabricated by bending standard single-mode fiber into a Balloon shape. • In addition to the high sensitivity, the proposed sensor exhibits a fast response time with excellent stability. In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability. [ABSTRACT FROM AUTHOR]

Recently there has been an interest in upgrading legacy multimode fibers such as 50 μm OM2 fiber to higher data rates through single mode transmission. A fiber modal adapter hosting a modal conditioning single-mode fiber (MCSMF) has been demonstrated as one robust solution to convert OM2 links into single mode transmission links for higher data rate transmission. Legacy multimode fibers also include OM1 fiber which has higher index contrast between the core and cladding and a larger core of 62.5 μm diameter. In this paper, we conduct a study to understand the MCSMF mode field diameter and connector offset tolerance requirements for OM1 fiber, which affect the transmission performance of single mode transmission. We find that despite very different refractive index profiles between OM1 and OM2 fibers, the fiber modal adapter concept is still applicable to OM1 and only one modal conditioning single-mode fiber design is needed to accommodate the needs for both fiber types. The detailed tolerances to the mode field diameter mismatch and the connector offset are numerically studied. The transfer functions for 1000-m of OM1 and OM2 with the adapters are measured, which show very high modal bandwidths at tens of GHz in contrast to very low modal bandwidths without the adapters. Transmission at 25 Gb/s using a 25G LR transceiver, 100 Gb/s using a 100G CWDM transceiver, and 400 Gb/s using a 400G LR transceiver are demonstrated with error free performance and open eye diagrams over 1000-m of OM1 and OM2. We find that the approach of using MCSMF can support fundamental mode transmission up to 100 Gb/s over this distance without obvious limitations. [ABSTRACT FROM AUTHOR]

• We propose a low threshold multi-wavelength erbium-doped fiber ring laser with NOLM and Lyot filter. • The threshold pump power of the laser is 45 mW, and can get 12 stable lasing lines, the side mode suppression ratio is greater than 25 dB. • This laser's channel spacing can be changed among of 0.08 nm, 0.34 nm and 0.89 nm by changing the PMF length. • This laser has simple structure and good stability. In this paper, the performance of a switchable multi-wavelength erbium-doped fiber laser (MWEDFL) incorporating Lyot filter and nonlinear optical loop mirror (NOLM) is described. The NOLM consisting of a polarization controller (PC2) and a single-mode fiber is effective to suppress the mode competition caused by the homogeneous broaden line in the erbium-doped fiber amplifier (EDFA). The comb filter is developed by using PC1 and polarization-maintained fiber (PMF). The wavelength interval can be switched among 0.08 nm, 0.34 nm, and 0.89 nm using different lengths of PMF via careful adjusting of the PCs. When the pump power is 45mW, the laser can emit 12 stable lasing lines within the 3 dB bandwidth, and the side mode suppression ratio (SMSR) is greater than 25 dB. The spectral power and wavelength fluctuations of 1.9 dB and 0.16 nm, respectively. Experimental results reveal that the demonstrated laser has good stability and low threshold power. [ABSTRACT FROM AUTHOR]

• Recycling fused optical fiber to develop a miniature self-referred Fabry-Perot microcavity sensor. • The physical theory and Finite Element Method (FEM) of the Fabry-Perot microcavity were considered to fabricate an efficient sensor. • This sensor simultaneously measurements the refractive index and temperature. • This sensor has the advantages of low cost, high repeatability, and small dimension, making it a desirable sensor for chemical and biomedical applications. In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source. [ABSTRACT FROM AUTHOR]

• The high oxygen activity in the germanosilicate fiber reduces its attenuation. • Oxygen deficit in GeO 2 - doped fibers reduces their radiation resistance. • The small h-parameter in an elliptical core is created by its shape and stresses. • Doping silica fibers with GeO 2 does not reduce their radiation resistance. This paper aims to consider fabrication of single-mode polarization-maintaining (PM) optical fibers with elliptical core, doped with 20 mol. % GeO 2 and to study their optical properties and radiation resistance. The optical fiber preform was fabricated by chemical vapor deposition method. The low-oxygen central piece of the core in the preform was removed by high-temperature chemical gas-phase etching. The single-mode optical fibers 125 µm in diameter, with 65 µm thick polymer protection coating were drawn. The results of measurements of the optical properties at a wavelength of 1.3–1.55 μm confirmed that the highly-doped GeO 2 optical fiber waveguides have a low optical loss and high radiation resistance owing to gas phase etching. Polarization-maintaining factor (h-parameter) of ≈ 8∙10-6 m−1 was achieved. As to the radiation resistance, our single-mode PM optical fiber with elliptical core appears to be quite competitive to the known PANDA PM optical fibers with a core made of pure silica glass. [ABSTRACT FROM AUTHOR]

• A simple balloon-like fiber sensing system for angle and micro-displacement is proposed. • A new way for angle and micro-displacement measurement. • The proposed fiber sensing system has the advantages of low manufacturing cost, easy manufacture. In this paper, a simple balloon-like fiber sensing system based on a Mach-Zehnder modal interferometer by bending single-mode fiber (SMF) into a balloon-like structure is proposed, and the experiment proves that the system can be used for angle and micro-displacement measurement. The sensor system is only composed of SMF. Firstly, the transmission spectrum curves under different sensitive lengths and bending diameters are analyzed. It is found that the maximum extinction ratio of 24 dB can be achieved when the bending diameter of SMF is 8 mm. Then, by comparing the angle and micro-displacement sensitivity under different sensitivity lengths, it is found that the angle and micro-displacement sensitivity of balloon-like structures increase with the increase of sensitivity lengths. The angle measurement sensitivity is 253.1 pm/°, the detection range is −90° ∼ 90°, and the micro-displacement measurement sensitivity is 327.4 pm/μm. This sensor system has a great potential in the field of angle and micro-displacement measurement, and provides a new idea for angle and micro-displacement measurement. [ABSTRACT FROM AUTHOR]

• Hybrid ARoF Super-PON architecture for broadband service and 5G mobile communication is presented. • Generation of the mm-wave signal by heterodyne photonic upconversion is implemented. • Several IF frequencies in Ka-band are addressed to investigate the performance of the ARoF Super-PON communication system. • The proposed communication system includes commercially available SFP+ modules in RoF transmitters to maintain cost-effectiveness and simplify the network deployment. Fifth-generation mobile communications are delivering higher data rates and low latency services for costumer by utilizing the benefits of optical fiber. In this paper, we propose the architecture of a hybrid millimeter-wave (mm-wave) analog radio-over-fiber (ARoF) Super-PON optical network for future broadband service and the latest fifth-generation 5G/5G + cellular mobile communications. Our proposed infrastructure shows the trade-off between technology and costs where the optical access network architecture is technologically transformed to transport intermediate frequencies (IF) through the existing fiber optical distribution network (ODN) by integrating baseband units (BBUs) into optical line terminals (OLTs) and the remote radio units (RU) into optical network terminals (ONTs), leaving the traditional broadband architecture unchanged. Performance of investigated mm-wave ARoF Super-PON transmission systems in the C-band downlink scenario for both broadband and mobile service is shown. With the proposed hybrid architecture, it is possible to provide cost-effective deployment of future 5G/5G+ networks as 5G key performance indicators (KPIs) are met and increase PON's utilization for multiple services by sharing frequency bands and infrastructure. Several intermediate frequencies in Ka-band (26.5 – 40 GHz) are addressed to evaluate the performance of our investigated ARoF Super-PON transmission system for each corresponding frequency in different scenarios of up to 50 km over standard single-mode optical fiber (SMF) link. [ABSTRACT FROM AUTHOR]

• A more practical receiving underwater acoustic signals model of DASF system is established. It is of great significance to carry out the basic research related to DASF system on the bottom of shallow water. • Single-mode optical fiber model is established, and takes this as a prototype, the receiving acoustic signal model of DASF in shallow water is established. The rationality of this model is verified by lake test and simulation. • In this paper, taking the application of land-based DAS system to perceive underwater acoustic signals as a breakthrough, theoretical modeling and experiments are mutually verified. To better apply the distributed acoustic sensing optic fiber (DASF) based on phase-sensitive optical time domain reflection (Ф-OTDR) to underwater target detection, on the basis of the two-layer optical fiber model, the single-mode optical fiber model composed of core, one-layer cladding and multi-layer coatings is established, which is closer to the reality. Based on this model, the receiving signal model of DASF is set up. The relationship between the phase change of backward Rayleigh scattering (BRS) signal in optical fiber before and after the disturbance caused by acoustic field is derived. Experiment in the lake is carried out, which results show that the data received by the underwater distributed optical fiber system can better reconstruct as the acoustic source information in shallow water, and the feasibility of DASF sensing underwater acoustic signals in shallow water is verified. The rationality of receiving signal model of DASF is verified by comparing the simulation modeling with the experimental results. [ABSTRACT FROM AUTHOR]

• Simultaneous temperature and strain measurement using TCF based MZI is proposed. • A segment of TCF is spliced between two SMFs with offset to excite cladding modes. • The impact of fiber offset value on the fringe visibility and the sensitivity is explored. • Simultaneous measurement is validated and the maximum measurable strain is 7000 με. The spectral response of multi-mode interference in optical fibers shows promising application prospects in simultaneous measurement. This paper presents a thin core fiber (TCF) based Mach-Zehnder interferometer (MZI) for simultaneous measurement of temperature and strain. High order cladding modes are excited by splicing a segment of TCF between two single mode fibers (SMFs) with lateral offset. Due to the different spectral responses of the high order cladding modes, the resonant dips in the transmission spectrum are utilized to realize the simultaneous measurement. The sensor is experimentally characterized and the simultaneous measurement is validated. The temperature and strain resolutions of the fabricated sensors are 0.64 °C and 5.61 με, 0.23 °C and 9.14 με, 0.21 °C and 13.29 με, respectively. The maximum strain can be measured is 7000 με. The proposed sensor possesses appealing characteristics of easy fabrication, high performance and low cost, and is anticipated to be applied in the structural health monitoring and damage identification. [ABSTRACT FROM AUTHOR]

• The nonlinear effects are investigated in 5G fronthaul system based on WDM-PON architecture. • In the 12-channel case, the FWM effect is studied in O-band whilst the XPM effect is discussed in C-band by simulation. • The results show that all channels cannot achieve the BER threshold of 1.0 × 10 - 3 except for the first and last channels in O-band, and the sensitivity penalty of probe channel caused by the XPM effect is 3.6 dB in C-band. Wavelength-division multiplexing passive optical network (WDM-PON) is considered as a mainstream solution for 5G fronthaul transmission system to cope with the rapidly increasing bandwidth demand. In such a multi-wavelength system, nonlinear effects limit the achievable transmission distance and channel capacity. Therefore, in this paper, the impact of nonlinear effects on a multi-wavelength transmission system with 25 Gb/s/λ non-return-to-zero (NRZ) and a 200-GHz channel spacing over 25 km standard single-mode fiber (SSMF) is investigated through experiment and simulation. The effects of four-wavelength mixing (FWM) in O-band as well as cross-phase modulation (XPM) in C-band are both studied and discussed. Experiments are performed for 3- and 4-channel transmission cases, while 12-channel transmission is performed by simulation. The results show that in the O-band scenario, the FWM effect severely affects the performance of the channels near the zero-dispersion wavelength. Besides, the XPM products overlap the signal in the C-band, which also degrades the sensitivity. Based on these analyses, the nonlinear effects should be fully considered when planning wavelengths for 5G fronthaul transmission, including wavelength distribution and channel spacing selection. [ABSTRACT FROM AUTHOR]

• The best transmission deep happened in specific range of diameters of bent structure of fiber optics. • Multimode, two mode and single mode of BSBSs under simultaneous change of diameters and length monitored. • A specific ratio of cladding to core is necessary to achieve effective coupling core to cladding. In this paper, a simple, novel design of bent fiber optics with changeable length and diameter (CLD) under displacement while attached to Faraday mirror was analyzed. The work aimed to achieve the largest transmission dip for an optimal range of bending diameters of CLD fibers. A comparison of multi-mode, two-mode, and single-mode bent fibers under simultaneous changes of bending diameters and lengths was first performed. The experimental results revealed an optimal ratio of cladding to core to achieve effective coupling core to cladding. Secondly, this design was shown to be sensitive to changes in humidity and temperature. Our calculations and experimental results of single-mode CLD bent fibers as a relative humidity (RH) and temperature sensor in ranges of 60–90 RH% and 30–120 °C yield sensitivities 275 pm/RH% and 44.9 pm/°C, respectively. Therefore, this design can be applied for the practical use of environmental sensing. [ABSTRACT FROM AUTHOR]

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF's misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses. [ABSTRACT FROM AUTHOR]

• The non-linearly decreasing inertia weight is more suitable for three DOF stage alignment. • A new relation between the inertia weight and iteration is established. • The proposed PSO algorithm presents the good performance via simulation and experiment. • The increase of search coefficient can change the global and local search ability in improved PSO algorithm. In this paper, we investigate a novel improved Particle Swarm Optimization (IPSO) algorithm to enhance the coupling precision between a laser diode and a single-mode fiber. It mainly relies on the nonlinear adjustment of inertia weight. Several variations of PSO are used as references. The simulated and experimental results show that the proposed strategy has an excellent search success rate with few iterations in multi-degree of freedom alignment. This is largely due to the improvement of exploration via the adjustment of inertia weight. These results confirm that the improved PSO algorithm can be applied reliably to improve both the efficiency and accuracy in laser-fiber coupling. [ABSTRACT FROM AUTHOR]