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
A high sensitive long period fiber grating（LPFG） sensor for the detection of nitrite is proposed, which is realized by coating multiple poly（sodium 4-styrensulfonate）（PSS） and poly（diallyldimethylammonium） chl...A high sensitive long period fiber grating（LPFG） sensor for the detection of nitrite is proposed, which is realized by coating multiple poly（sodium 4-styrensulfonate）（PSS） and poly（diallyldimethylammonium） chloride （PDDA） layers on the fiber grating surface. The sensitivity of this LPFG sensor is maximum when the number of assembled layers is 70. Under this condition, a nitrite concentration of 3×10^-3 mol/L, which is lower than the National Food Additive Standard, 4.2×10^-3 mol/L, can be distinguished. The sensitivity is further increased by 30% when nitrite was determined in the sucrose solution with a concentration of 65%, which provides a new solution for the best refraction index approaching matched index of the fiber cladding. Compared with chemical methods, this nitrite detection technology offers some advantages, such as high accuracy, non toxicity, high speed, low cost, without chemical reagent, and is suitable for foodstuff security detection.展开更多
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
The coupled-mode equations for fiber Bragg grating （FBG） and long period fiber grating （LPFG） undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temper...The coupled-mode equations for fiber Bragg grating （FBG） and long period fiber grating （LPFG） undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temperature change on the reflectivity spectrum of fiber Braggs grating and the transmission spectrum of long period fiber grating were investigated using the numerical simulation, and the dependence relationships of the central wavelength shift, the full-width-athalf-maximum, and the peak intensity upon temperature gradient were also obtained. These relationships may be used to design a novel fiber optical sensor which can simultaneously measure the temperature and temperature gradient.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
This paper focuses on the investigation of modal characteristics and sensing properties of long period grating photonic crystal fibers (LPG-PCFs). An improved effective index method is employed with an objective to st...This paper focuses on the investigation of modal characteristics and sensing properties of long period grating photonic crystal fibers (LPG-PCFs). An improved effective index method is employed with an objective to study its limitations for various designs of LPG-PCFs. Results so obtained with the above method are compared with the corresponding values of multiple multipole (MMP) method results which points the range of validity and applicability of the improved effective index method to LPG-PCFs. It is shown that this method is excellent when the surrounding media is assumed to be air. However, it becomes less accurate when the fiber is immersed into a liquid with a refractive index close to that of the cladding.展开更多
<div style="text-align:justify;"> In this paper, a hybrid optical fiber structure for solution concentration measurement with the temperature compensation is proposed. The structure consists of long pe...<div style="text-align:justify;"> In this paper, a hybrid optical fiber structure for solution concentration measurement with the temperature compensation is proposed. The structure consists of long period fiber grating (LPFG) and single mode-multimode-single mode (SMS) fiber structures. The sensing mechanism of the device is studied and verified by experiments. LPFG is sensitive to solution concentration and is affected by temperature crosstalk. SMS structure is not affected by solution concentration, but sensitive to ambient temperature. It can be used as a temperature compensation system. The sensitivity coefficients of LPFG and SMS on temperature and concentration were measured experimentally, and a dual-wavelength matrix was established to realize simultaneous measurement of solution temperature and concentration. </div>展开更多
A corrugated surface long period grating(LPG)was fabricated on a flat-shaped plastic optical fiber(POF)as a refractive index(RI)sensor by a simple pressing with the heat pressure and mechanical die press print method....A corrugated surface long period grating(LPG)was fabricated on a flat-shaped plastic optical fiber(POF)as a refractive index(RI)sensor by a simple pressing with the heat pressure and mechanical die press print method.The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics.Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance,and the sensor with a thinner flat thickness,a deeper groove depth of the corrugated surface LPG,and a longer LPG exhibited better RI sensing performance.When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600μm,an LPG with a period of 300μm,a groove depth of 200μm,and a length of 6 cm was fabricated on it,and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10^(−6) RIU.In addition,the influences of the POF cladding,tilting of LPG,and bending of the sensing structure were investigated.The results demonstrated that after removing the cladding and tilting or bending the LPG,the RI sensing performance was improved.When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/πcm,the highest sensitivity of 6563%/RIU was achieved with a resolution of 2.487×10^(−9) RIU in the RI range of 1.3330–1.4230.The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication,high sensitivity,and intensity modulation at the visible wavelengths.展开更多
The long period fiber grating （LPFG） is widely used as a sensor due to its high sensitivity and resolution. However, the broad bandwidth of the attenuation bands formed by the mode coupling between the fundamental c...The long period fiber grating （LPFG） is widely used as a sensor due to its high sensitivity and resolution. However, the broad bandwidth of the attenuation bands formed by the mode coupling between the fundamental core mode and the cladding modes constitutes a difficulty when the device is used as a conventional sensor. To overcome this limitation, a Michelson interferometer-type sensor configuration has been developed, using an LPFG grating pair formed by coating a mirror at the distal end of the LPFG. This sensor configuration is more convenient to use and is able to overcome the limitations of the single LPFG based sensor as the shifts in the attenuation bands being more easily detectable due to the formation of the sharp spectral fringe pattern in the LPFG based Michelson interferometer. In this work, I studied the LPFG based Michelson interferometer as the refractive index sensor and discussed the sensitivity enhancement of the LPFG based Michelson interferometer as a refractive index sensor by employing higher order cladding modes and by reducing the cladding radius. The results demonstrated the HE17 mode with a cladding radius of 62.5 μm, in the range of surrounding refractive index （SRI） of 1 - 1.45, and its resonant peak showed a wavelength shift of 26.99nm/RIU. When the cladding region was further reduced to 24μm, the resonant peak showed a wavelength shift of 569.88 nm/RIU, resulting in a sensitivity enhancement of nearly 21 times. However, as the cladding region was etched further, then the HE17 order cladding mode and higher mode would be cut off. Therefore, the implementation of high sensitivity for SRI sensing with the reduced cladding in the LPFG based Michelson interferometer is dependent on the proper combination of the cladding radius and cladding mode order.展开更多
采用F-P可调滤波器对长周期光纤光栅的某级透射谱进行波长扫描,借助探测器和示波器对信号光波在驱动信号周期内所形成的负脉冲间隔进行测量,实现信号解调。利用此系统对蔗糖溶液浓度从0到饱和进行测量,测得系统传感灵敏度的实验值为0.02...采用F-P可调滤波器对长周期光纤光栅的某级透射谱进行波长扫描,借助探测器和示波器对信号光波在驱动信号周期内所形成的负脉冲间隔进行测量,实现信号解调。利用此系统对蔗糖溶液浓度从0到饱和进行测量,测得系统传感灵敏度的实验值为0.02 ms Lg-1。展开更多
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 National Natural Science Foundation of China(Nos.60707016 and 60807030)
文摘A high sensitive long period fiber grating（LPFG） sensor for the detection of nitrite is proposed, which is realized by coating multiple poly（sodium 4-styrensulfonate）（PSS） and poly（diallyldimethylammonium） chloride （PDDA） layers on the fiber grating surface. The sensitivity of this LPFG sensor is maximum when the number of assembled layers is 70. Under this condition, a nitrite concentration of 3×10^-3 mol/L, which is lower than the National Food Additive Standard, 4.2×10^-3 mol/L, can be distinguished. The sensitivity is further increased by 30% when nitrite was determined in the sucrose solution with a concentration of 65%, which provides a new solution for the best refraction index approaching matched index of the fiber cladding. Compared with chemical methods, this nitrite detection technology offers some advantages, such as high accuracy, non toxicity, high speed, low cost, without chemical reagent, and is suitable for foodstuff security detection.
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Funded by the Natural Science Foundation of Wuhan University of Technology (471-38650024)the National Natural Science Foundation of China (No. 50802069)
文摘The coupled-mode equations for fiber Bragg grating （FBG） and long period fiber grating （LPFG） undergoing linear and quadratic temperature change were given. The effects of temperature gradient and quadratic temperature change on the reflectivity spectrum of fiber Braggs grating and the transmission spectrum of long period fiber grating were investigated using the numerical simulation, and the dependence relationships of the central wavelength shift, the full-width-athalf-maximum, and the peak intensity upon temperature gradient were also obtained. These relationships may be used to design a novel fiber optical sensor which can simultaneously measure the temperature and temperature gradient.
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.
文摘This paper focuses on the investigation of modal characteristics and sensing properties of long period grating photonic crystal fibers (LPG-PCFs). An improved effective index method is employed with an objective to study its limitations for various designs of LPG-PCFs. Results so obtained with the above method are compared with the corresponding values of multiple multipole (MMP) method results which points the range of validity and applicability of the improved effective index method to LPG-PCFs. It is shown that this method is excellent when the surrounding media is assumed to be air. However, it becomes less accurate when the fiber is immersed into a liquid with a refractive index close to that of the cladding.
文摘<div style="text-align:justify;"> In this paper, a hybrid optical fiber structure for solution concentration measurement with the temperature compensation is proposed. The structure consists of long period fiber grating (LPFG) and single mode-multimode-single mode (SMS) fiber structures. The sensing mechanism of the device is studied and verified by experiments. LPFG is sensitive to solution concentration and is affected by temperature crosstalk. SMS structure is not affected by solution concentration, but sensitive to ambient temperature. It can be used as a temperature compensation system. The sensitivity coefficients of LPFG and SMS on temperature and concentration were measured experimentally, and a dual-wavelength matrix was established to realize simultaneous measurement of solution temperature and concentration. </div>
文摘A corrugated surface long period grating(LPG)was fabricated on a flat-shaped plastic optical fiber(POF)as a refractive index(RI)sensor by a simple pressing with the heat pressure and mechanical die press print method.The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics.Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance,and the sensor with a thinner flat thickness,a deeper groove depth of the corrugated surface LPG,and a longer LPG exhibited better RI sensing performance.When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600μm,an LPG with a period of 300μm,a groove depth of 200μm,and a length of 6 cm was fabricated on it,and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10^(−6) RIU.In addition,the influences of the POF cladding,tilting of LPG,and bending of the sensing structure were investigated.The results demonstrated that after removing the cladding and tilting or bending the LPG,the RI sensing performance was improved.When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/πcm,the highest sensitivity of 6563%/RIU was achieved with a resolution of 2.487×10^(−9) RIU in the RI range of 1.3330–1.4230.The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication,high sensitivity,and intensity modulation at the visible wavelengths.
文摘The long period fiber grating （LPFG） is widely used as a sensor due to its high sensitivity and resolution. However, the broad bandwidth of the attenuation bands formed by the mode coupling between the fundamental core mode and the cladding modes constitutes a difficulty when the device is used as a conventional sensor. To overcome this limitation, a Michelson interferometer-type sensor configuration has been developed, using an LPFG grating pair formed by coating a mirror at the distal end of the LPFG. This sensor configuration is more convenient to use and is able to overcome the limitations of the single LPFG based sensor as the shifts in the attenuation bands being more easily detectable due to the formation of the sharp spectral fringe pattern in the LPFG based Michelson interferometer. In this work, I studied the LPFG based Michelson interferometer as the refractive index sensor and discussed the sensitivity enhancement of the LPFG based Michelson interferometer as a refractive index sensor by employing higher order cladding modes and by reducing the cladding radius. The results demonstrated the HE17 mode with a cladding radius of 62.5 μm, in the range of surrounding refractive index （SRI） of 1 - 1.45, and its resonant peak showed a wavelength shift of 26.99nm/RIU. When the cladding region was further reduced to 24μm, the resonant peak showed a wavelength shift of 569.88 nm/RIU, resulting in a sensitivity enhancement of nearly 21 times. However, as the cladding region was etched further, then the HE17 order cladding mode and higher mode would be cut off. Therefore, the implementation of high sensitivity for SRI sensing with the reduced cladding in the LPFG based Michelson interferometer is dependent on the proper combination of the cladding radius and cladding mode order.
