We demonstrate experimentally fabrication of CO2 laser written long-period fiber gratings （LPFGs） in single-mode fiber （SMF） under tension. New transmission dips can be generated due to the frozen-in stress in the...We demonstrate experimentally fabrication of CO2 laser written long-period fiber gratings （LPFGs） in single-mode fiber （SMF） under tension. New transmission dips can be generated due to the frozen-in stress in the fiber under tension. Dynamics analysis of laser writing process is performed to study the mechanism of the grating inscription. Experimental results show that the wavelength shift of new generated resonance dips can shift towards the longer or shorter wavelength with the increase of laser scanning cycles for gratings with different pitches.展开更多
Based on coupled-mode theo ry , the eigenvalue equation of five-layered long-period fiber grating(LPFG) sens or with Ag film and gas-sensitive film overlays are firstly studied. The probl em of resolving complex eigen...Based on coupled-mode theo ry , the eigenvalue equation of five-layered long-period fiber grating(LPFG) sens or with Ag film and gas-sensitive film overlays are firstly studied. The probl em of resolving complex eigenvalue equation on five-layered LPFG is analyzed, a nd the method of resolution is also given. Then the eigenvalue equation of three -layered metal cladding LPFG is analyzed, and the complex transcendental equati on is also discussed. The computing result shows that the coupling between the l ow-order EH modes and the core mode is much stronger than that between the low -order HE modes and the core mode.展开更多
An effective method to fabricate two-helix long-period fiber gratings （TH-LFGs） is presented. Based on the coupling mode theory, the conversion of optical vortices （OVs） in TH-LFGs are analyzed in detail. The conv...An effective method to fabricate two-helix long-period fiber gratings （TH-LFGs） is presented. Based on the coupling mode theory, the conversion of optical vortices （OVs） in TH-LFGs are analyzed in detail. The conversions of OVs with different topological charges： 0 → ±2 and 1 → 3 are simulated as three examples and the conversion efficiency higher than 98% can be realized.展开更多
A refractive index （RI） sensor based on hybrid long-period fiber grating （LPFG） with multimode fiber core （MMFC） is proposed and demonstrated. The surrounding RI can be determined by monitoring the separation be...A refractive index （RI） sensor based on hybrid long-period fiber grating （LPFG） with multimode fiber core （MMFC） is proposed and demonstrated. The surrounding RI can be determined by monitoring the separation between the resonant wavelengths of the LPFG and MMFC since the resonant wavelengths of the LPFG and MMFC will shift in opposite directions when the surrounding RI changes. Experimental results show that the sensor possesses an enhanced sensitivity of 526.92nm/RIU in the RI range of 1.387-1.394 RIU. The response to the temperature is also discussed.展开更多
To obtain the influence rules of the coating parameters of a long-period fiber grating(LPFG)with respect to temperature,strain and refractive index sensing properties,based on the mode coupling theory,a strict four-la...To obtain the influence rules of the coating parameters of a long-period fiber grating(LPFG)with respect to temperature,strain and refractive index sensing properties,based on the mode coupling theory,a strict four-layer theorietical model of a metal film coated LPFG is established,and these parameters that affect the spectral characteristics of the metal film coated LPFG are studied.The simulation results show that there is an optimal metal film thickness on the surface of the LPFG that will induce the surface-plasmon resonance(SP R)effect,which results in higher sensitivity to the environmental temperature and refractive index but has little influence on the strain There is theoretical evidence that when the silver thickness is between0.8and1.2nm,the refractive index sensitivity will reach the peak point of42.4026,at which the refractive index sensor sensitivity is increased by4.S%.The theoretical results of coating a long-period fiber grating provide a good theoretical basis and guidance for LPFG design and parameters optimization展开更多
Long period fiber gratings are emerging as a potential candidate in the list of surrounding refractive index optical fiber sensors. Their sensitivity can be enhanced greatly if the grating period, fiber dimensions and...Long period fiber gratings are emerging as a potential candidate in the list of surrounding refractive index optical fiber sensors. Their sensitivity can be enhanced greatly if the grating period, fiber dimensions and surrounding refractive index are optimized in a way to operate at a point called turn around point on phase matching curves of these gratings. Turn around point LPFGs are well known for their ultrahigh sensitivity to external parameters. Potential of operating LPFG at or near turn around point has been investigated by many researchers in various applications including physical parameter sensing, adulteration detection, radiation dose, etc. Since TAP LPFGs are in investigation phase therefore a lot of rigorous & efficient work in finding techniques for optimizing their potential as sensor in chemical, biochemical, structural health monitoring is still to be carried out. A brief review of work carried out in this domain till now is presented here and key findings from literature review are highlighted.展开更多
Vector bending sensing has been consistently growing in many fields.A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating[LPFG]with an off-axis micro helix taper is propose...Vector bending sensing has been consistently growing in many fields.A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating[LPFG]with an off-axis micro helix taper is proposed and experimentally demonstrated.The grating is composed of several sections of single-mode fiber with gradually larger lengths,and the off-axis micro helix tapers with fixed lengths when they are fabricated by using the arc discharge technology.The large refractive index modulation in the micro-helix taper greatly reduces the sensor size.The total length of the sensor is only 4.67 mm.The micro-helix taper-based LPFG can identify the bending direction due to the asymmetric structure introduced by the micro helix.The experimental results show that the transmission spectra of the sensor have distinct responses for different bending directions,and the maximum bending sensitivity is 14.08 nm/m^(-1)in the range from 0.128 m^(-1)to 1.28 m^(-1).The proposed bending sensor possesses pronounced advantages,such as high sensitivity,small size,low cost,and orientation identification,and offers a very promising method for bend measurement.展开更多
Long period fiber gratings （LPFGs） with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of...Long period fiber gratings （LPFGs） with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of axial stress along a fiber during laser writing or post- processing with lower energy density laser irradiation. Variations of focal condition, pulse energy of laser irradiation and the number of grating periods lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus larger out-of-band loss. The out-of-band loss is primarily caused by Mie scattering from the laser processed cites, and it can be reduced by decreasing the duty cycle of grating pitch instead of lowing down the actual power of laser irradiation.展开更多
We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity.This sensor is configured by a triangular-prism-shaped long-period fiber gr...We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity.This sensor is configured by a triangular-prism-shaped long-period fiber grating,which is fabricated by the high frequency CO_(2) laser polished method.The unique design of the triangular-shaped structure breaks the rotational symmetry of the optical fiber and provides high sensitivity for torsion measurement.In preliminary experiments,the torsion response of the sensor achieves a good stability and linearity.The torsion sensitivity is 0.54 nm/(rad/m),which renders the proposed structure a highly sensitive torsion sensor.展开更多
Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot.At present,research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser.Howeve...Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot.At present,research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser.However,the suppression of spectral broadening,caused by self-phase modulation or four-wave mixing,is still a challenging attribute to the close distance between the broadened laser and signal laser.If using a traditional fiber grating with only one stopband to suppress the spectral broadening,the signal power will be stripped simultaneously.Confronting this challenge,we propose a novel method based on phase-shifted long-period fiber grating(PS-LPFG)to suppress spectral broadening in a high-power fiber master oscillator power amplifier(MOPA)laser system in this paper.A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique.The resonant wavelength of the fabricated PS-LPFG is 1080 nm,the full width at half maximum of the passband is 5.48 nm,and stopband extinction exceeds 90%.To evaluate the performance of the PS-LPFG,the grating is inserted into the seed of a kilowattlevel continuous-wave MOPA system.Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%,providing an effective and flexible way for optimizing the output linewidth of highpower fiber MOPA laser systems.展开更多
Long-period gratings （LPGs） are fabricated in a photonic crystal fiber （PCF） using the symmetric point- by-point CO2 laser irradiation method to explore the sensitivity characterization of PCF-LPG. Numerical sim- ...Long-period gratings （LPGs） are fabricated in a photonic crystal fiber （PCF） using the symmetric point- by-point CO2 laser irradiation method to explore the sensitivity characterization of PCF-LPG. Numerical sim- ulation is used to guide the investigation. It is found that the refractive index （RI） sensitivity of PCF-LPG depends on the coupled cladding modes as well as the coupled resonance wavelength （RW） of the LPG. Experimental studies show that the longer the RW, the higher the RI sensitivity for the same cladding mode. At similar RWs, the lower the cladding mode, the higher the RI sensitivity of PCF-LPG.展开更多
A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber(STS) structure is proposed and optimized theoretically.The sensor is composed of two sing...A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber(STS) structure is proposed and optimized theoretically.The sensor is composed of two single-mode fibers connected by a section of long-period fiber grating fabricated on thin-core fiber.After optimization and benefitting from the phase matching point, the loss peak of the structure can reach 62.8dB theoretically.The wavelength of the characteristic peak is fixed at the phase matching point, so intensity detection can be achieved.The sensitivity can reach 272.5dB/RIU.The structural optimization in this Letter provides a reference for the fabrication of an easy-made all-fiber sensor without extra cladding.展开更多
基金Project supported by the Shanghai Pujiang Program (Grant No.09PJ1404600)the New Century Excellent Talents in University, Ministry of Education of China (Grant No.NCET-10-0082)+1 种基金the Shanghai Shuguang Program (Grant No.08SG40)the Shanghai Leading Academic Discipline Project (Grant No.S30108)
文摘We demonstrate experimentally fabrication of CO2 laser written long-period fiber gratings （LPFGs） in single-mode fiber （SMF） under tension. New transmission dips can be generated due to the frozen-in stress in the fiber under tension. Dynamics analysis of laser writing process is performed to study the mechanism of the grating inscription. Experimental results show that the wavelength shift of new generated resonance dips can shift towards the longer or shorter wavelength with the increase of laser scanning cycles for gratings with different pitches.
基金"Shu Guang"Plan of Education Committee of Shanghai (02SG32) Natural Science Foundation of ScienceCommittee of Shanghai(03ZR14071)
文摘Based on coupled-mode theo ry , the eigenvalue equation of five-layered long-period fiber grating(LPFG) sens or with Ag film and gas-sensitive film overlays are firstly studied. The probl em of resolving complex eigenvalue equation on five-layered LPFG is analyzed, a nd the method of resolution is also given. Then the eigenvalue equation of three -layered metal cladding LPFG is analyzed, and the complex transcendental equati on is also discussed. The computing result shows that the coupling between the l ow-order EH modes and the core mode is much stronger than that between the low -order HE modes and the core mode.
基金Supported by the National Natural Science Foundation of China under Grant No 61275049
文摘An effective method to fabricate two-helix long-period fiber gratings （TH-LFGs） is presented. Based on the coupling mode theory, the conversion of optical vortices （OVs） in TH-LFGs are analyzed in detail. The conversions of OVs with different topological charges： 0 → ±2 and 1 → 3 are simulated as three examples and the conversion efficiency higher than 98% can be realized.
基金Supported by the National Natural Science Foundation of China under Grant Nos 61327012 and 61505160the Natural Science Foundation of Shaanxi Province under Grant No 2016JQ6021the Shaanxi Key Laboratory of Optical Information Technology under Grant No OIT201601
文摘A refractive index （RI） sensor based on hybrid long-period fiber grating （LPFG） with multimode fiber core （MMFC） is proposed and demonstrated. The surrounding RI can be determined by monitoring the separation between the resonant wavelengths of the LPFG and MMFC since the resonant wavelengths of the LPFG and MMFC will shift in opposite directions when the surrounding RI changes. Experimental results show that the sensor possesses an enhanced sensitivity of 526.92nm/RIU in the RI range of 1.387-1.394 RIU. The response to the temperature is also discussed.
基金National Natural Science Foundation of China(No.51309001)Scientific Research Key Project of Anhui Province(No.KJ2017A041)
文摘To obtain the influence rules of the coating parameters of a long-period fiber grating(LPFG)with respect to temperature,strain and refractive index sensing properties,based on the mode coupling theory,a strict four-layer theorietical model of a metal film coated LPFG is established,and these parameters that affect the spectral characteristics of the metal film coated LPFG are studied.The simulation results show that there is an optimal metal film thickness on the surface of the LPFG that will induce the surface-plasmon resonance(SP R)effect,which results in higher sensitivity to the environmental temperature and refractive index but has little influence on the strain There is theoretical evidence that when the silver thickness is between0.8and1.2nm,the refractive index sensitivity will reach the peak point of42.4026,at which the refractive index sensor sensitivity is increased by4.S%.The theoretical results of coating a long-period fiber grating provide a good theoretical basis and guidance for LPFG design and parameters optimization
文摘Long period fiber gratings are emerging as a potential candidate in the list of surrounding refractive index optical fiber sensors. Their sensitivity can be enhanced greatly if the grating period, fiber dimensions and surrounding refractive index are optimized in a way to operate at a point called turn around point on phase matching curves of these gratings. Turn around point LPFGs are well known for their ultrahigh sensitivity to external parameters. Potential of operating LPFG at or near turn around point has been investigated by many researchers in various applications including physical parameter sensing, adulteration detection, radiation dose, etc. Since TAP LPFGs are in investigation phase therefore a lot of rigorous & efficient work in finding techniques for optimizing their potential as sensor in chemical, biochemical, structural health monitoring is still to be carried out. A brief review of work carried out in this domain till now is presented here and key findings from literature review are highlighted.
基金supported in part by the National Natural Science Foundation of China(NSFC)(Nos.62205087,62175049,and 62105077)Natural Science Foundation of Heilongjiang Province in China(No.YQ2021F002)+1 种基金Fundamental Research Funds for the Central Universities(Nos.3072022TS2501 and 3072021CFJ2504)China Postdoctoral Science Foundation(No.2022M710933)。
文摘Vector bending sensing has been consistently growing in many fields.A low-cost and high sensitivity vector bending sensor based on a chirped long-period fiber grating[LPFG]with an off-axis micro helix taper is proposed and experimentally demonstrated.The grating is composed of several sections of single-mode fiber with gradually larger lengths,and the off-axis micro helix tapers with fixed lengths when they are fabricated by using the arc discharge technology.The large refractive index modulation in the micro-helix taper greatly reduces the sensor size.The total length of the sensor is only 4.67 mm.The micro-helix taper-based LPFG can identify the bending direction due to the asymmetric structure introduced by the micro helix.The experimental results show that the transmission spectra of the sensor have distinct responses for different bending directions,and the maximum bending sensitivity is 14.08 nm/m^(-1)in the range from 0.128 m^(-1)to 1.28 m^(-1).The proposed bending sensor possesses pronounced advantages,such as high sensitivity,small size,low cost,and orientation identification,and offers a very promising method for bend measurement.
文摘Long period fiber gratings （LPFGs） with different spectral characteristics were fabricated with 1 kHz, 50 fs laser pulses. The contrast of resonant rejection band can be significantly increased by a proper amount of axial stress along a fiber during laser writing or post- processing with lower energy density laser irradiation. Variations of focal condition, pulse energy of laser irradiation and the number of grating periods lead to the generation of resonance rejection band of LPFGs from single-peak to multi-peak plus larger out-of-band loss. The out-of-band loss is primarily caused by Mie scattering from the laser processed cites, and it can be reduced by decreasing the duty cycle of grating pitch instead of lowing down the actual power of laser irradiation.
基金supported by the Joint Research Fund in Astronomy under cooperative agreement between the National Natural Science Foundation of China(NSFC)and Chinese Academy of Sciences(CAS)(Nos.U1831115,U1631239,and U1931206)the Dean Project of Guangxi Key Laboratory of Wireless Broadband Communication and Signal Processing(No.GXKL06190106)the Key Projects of Natural Science Foundation of Heilongjiang Province(No.ZD2019H003)。
文摘We propose and investigate a compact optical fiber sensor that aims to measure the torsion in both amount and direction with high sensitivity.This sensor is configured by a triangular-prism-shaped long-period fiber grating,which is fabricated by the high frequency CO_(2) laser polished method.The unique design of the triangular-shaped structure breaks the rotational symmetry of the optical fiber and provides high sensitivity for torsion measurement.In preliminary experiments,the torsion response of the sensor achieves a good stability and linearity.The torsion sensitivity is 0.54 nm/(rad/m),which renders the proposed structure a highly sensitive torsion sensor.
基金supported by the National Key Research and Development Program of China(No.2017YFB1104402)the Pre-research Foundation of Equipment Development Department(No.61404140105)+3 种基金the Key Laboratory of Optical System Advanced Manufacturing Technology of the Chinese Academy of Sciences(No.KLOMT190101)the Jiangsu Provincial Key Research and Development Program(No.BE2019114)the Basic Research Program of Jiangsu Province(No.BK20190456)the National Natural Science Foundation of China(No.62005120)。
文摘Suppressing nonlinear effects in high-power fiber lasers based on fiber gratings has become a hotspot.At present,research is mainly focused on suppressing stimulated Raman scattering in a high-power fiber laser.However,the suppression of spectral broadening,caused by self-phase modulation or four-wave mixing,is still a challenging attribute to the close distance between the broadened laser and signal laser.If using a traditional fiber grating with only one stopband to suppress the spectral broadening,the signal power will be stripped simultaneously.Confronting this challenge,we propose a novel method based on phase-shifted long-period fiber grating(PS-LPFG)to suppress spectral broadening in a high-power fiber master oscillator power amplifier(MOPA)laser system in this paper.A PS-LPFG is designed and fabricated on 10/130 passive fiber utilizing a point-by-point scanning technique.The resonant wavelength of the fabricated PS-LPFG is 1080 nm,the full width at half maximum of the passband is 5.48 nm,and stopband extinction exceeds 90%.To evaluate the performance of the PS-LPFG,the grating is inserted into the seed of a kilowattlevel continuous-wave MOPA system.Experiment results show that the 30 dB linewidth of the output spectrum is narrowed by approximately 37.97%,providing an effective and flexible way for optimizing the output linewidth of highpower fiber MOPA laser systems.
基金supported by the United States National Science Foundation under Grant No.DMR-1206669the Ministry of Education,Youth,and Sport of the Czech Republic under Grant No.LH 11038 within the U.S.–Czech S&T Cooperation agreement
文摘Long-period gratings （LPGs） are fabricated in a photonic crystal fiber （PCF） using the symmetric point- by-point CO2 laser irradiation method to explore the sensitivity characterization of PCF-LPG. Numerical sim- ulation is used to guide the investigation. It is found that the refractive index （RI） sensitivity of PCF-LPG depends on the coupled cladding modes as well as the coupled resonance wavelength （RW） of the LPG. Experimental studies show that the longer the RW, the higher the RI sensitivity for the same cladding mode. At similar RWs, the lower the cladding mode, the higher the RI sensitivity of PCF-LPG.
基金supported by the National Natural Science Foundation of China (Nos. 61827817 and 61525501)
文摘A refractive index intensity detecting sensor with long-period grating written in the single-mode–thin-core–single-mode fiber(STS) structure is proposed and optimized theoretically.The sensor is composed of two single-mode fibers connected by a section of long-period fiber grating fabricated on thin-core fiber.After optimization and benefitting from the phase matching point, the loss peak of the structure can reach 62.8dB theoretically.The wavelength of the characteristic peak is fixed at the phase matching point, so intensity detection can be achieved.The sensitivity can reach 272.5dB/RIU.The structural optimization in this Letter provides a reference for the fabrication of an easy-made all-fiber sensor without extra cladding.
