Phase-shifted Long-period Fiber Grating Research Articles

Simultaneous Detection of Temperature, Strain, Refractive Index, and pH Based on a Phase-Shifted Long-Period Fiber Grating

A novel π-phase-shifted (PS) long-period fiber-grating (LPFG) sensor operating near the phase-matching turning point (PMTP) was proposed. The PS-LPFG sensor can simultaneously detect temperature, strain, refractive index (RI), and pH. Due to the introduction of π phase shift, the double resonance peak of the selected cladding mode was divided into four loss peaks, which is convenient for multiparameter measurement. By monitoring the resonant-wavelength shifts of four dips of PS-LPFG, simultaneous detection of temperature, strain, RI, and pH can be achieved. Moreover, using a simple LPFG design on one fiber and a multiparameter matrix algorithm, the crosstalk between multiple signals was eliminated. Graphene oxide/polyvinyl alcohol (GO/PVA) hydrogel was coated onto the grating sensing area to measure the change in pH, and the sensitivity was higher than that of the same type of fiber-grating sensor. Results showed that the maximum sensitivity of temperature, strain, RI, and pH of the fabricated phase-shifted grating sensing structure were 0.2128 nm/°C, 0.1225 nm/μϵ, 718 nm/RIU, and -1.0166 nm/pH, respectively. Overall, the proposed sensor had great potential applications in monitoring marine environments and biochemistry sensing.

Miniature Temperature-Independent Curvature Sensor Based on a Phase-Shifted Long-Period Fiber Grating Using Deep Tapering

A miniature temperature-independent curvature sensor was proposed and prepared by using a phase-shifted long-period fiber grating (LPG) fabricated from deeply tapered conventional single-mode fibers (SMFs). As a typical example, a sensor made from a phase-shifted deeply tapered long-period grating (PS-DTLPG) with a length of 3.4 mm and a maximum curvature sensitivity of 12.05 nm/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was successfully demonstrated. The proposed curvature sensor is of the temperature-independence and has features of high sensitivity, high reliability, miniature size and low cost, which could have potential applications in structural health monitoring, especially for the medical industry and shape reconstruction.

Fiber-optic label-free biosensor for SARS‐CoV‐2 spike protein detection using biofunctionalized long-period fiber grating

With COVID-19 widespread worldwide, people are still struggling to develop faster and more accurate diagnostic methods. Here we demonstrated the label-free detection of SARS-CoV-2 spike protein by employing a SARS-CoV-2 spike antibody-conjugated phase-shifted long-period fiber grating (PS-LPFG) inscribed with a CO2 laser. At a specific cladding mode, the wavelength separation (λD) between the two split dips of a PS-LPFG varies with the external refractive index, although it is virtually insensitive to ambient temperature variations. To detect SARS-CoV-2 spike protein, SARS-CoV-2 spike antibodies were immobilized on the fiber surface of the fabricated PS-LPFG functionalized through chemical modification. When exposed to SARS-CoV-2 spike protein with different concentrations, the antibody-immobilized PS-LPFG exhibited the variation of λD according to the protein concentration, which was caused by bioaffinity binding-induced local changes in the refractive index at its surface. In particular, we also confirmed the potential of our sensor for clinical application by detecting SARS-CoV-2 spike protein in virus transport medium. Moreover, our sensor could distinguish SARS-CoV-2 spike protein from those of MERS-CoV and offer efficient properties such as reusability and storage stability. Hence, we have successfully fabricated a promising optical transducer for the detection of SARS-CoV-2 spike protein, which can be unperturbed by external temperature disturbances.

Simultaneous Measurement of pH and Temperature with Phase-Shifted Long-Period Fiber Grating Inscribed on High-Birefringence Fiber by CO₂ Laser Pulses.

We propose an optical fiber grating sensor capable of simultaneously measuring pH and temperature based on a phase-shifted long-period fiber grating (PS-LPFG) inscribed on high-birefringence fiber (HBF). The PS-LPFG was fabricated on HBF with CO₂ laser pulses, and a phase shift π was induced by inserting a grating-free fiber region (GFFR) between two identical LPFGs with a grating period of ˜510 μm. The length of the GFFR was set as half of the grating period to induce a π phase shift. With the spectral characteristics of a π-PS-LPFG exhibiting two split attenuation bands, the PS-LPFG written on HBF, which is referred to as the HB-PS-LPFG, can create two polarization-dependent transmission spectra with dual-resonance dips at different wavelengths according to two orthogonal input polarization states, e.g., linear horizontal polarization (LHP) and linear vertical polarization (LVP). For simultaneous measurement of pH and temperature with the fabricated HB-PS-LPFG as a sensor head, the inter-resonance wavelength separation of the dual-resonance dips in each transmission spectrum obtained for an LHP or LVP input signal was exploited as a sensor indicator. By investigating the wavelength changes of the two sensor indicators, which were induced by pH and temperature variations, linear and independent spectral responses to both pH and temperature variations were experimentally confirmed in a pH range from 1 to 11 and a temperature range from 25 to 65 °C. Owing to the unique pH and temperature responses of the fabricated HB-PS-LPFG, ambient variations in pH and temperature could be simultaneously estimated from the measured wavelength changes and sensitivities of the two sensor indicators.

Optimizing output spectral linewidth of fiber laser utilizing phase-shifted long-period fiber grating

Suppressing nonlinear effects in fiber laser by special fiber gratings has attracted growing attentions. Currently, most researches mainly focus on suppressing stimulated Raman scattering. However, the mitigation of the nonlinear effects, such as self-phase modulation or four wave mixing, which broaden the main lobe of the output laser spectrum has not been paid attention. We report a novel method for universally and conveniently optimizing output spectral linewidth of fiber laser by applying phase-shifted long-period fiber grating (PS-LPFG). A detailed design philosophy and method of PS-LPFG was proposed. Further, a symmetrical PS-LPFG was fabricated to verify the feasibility of the method. The central wavelength of the grating’s passband was designed to match the signal light of a 1075 nm fiber laser. A 200 W continuous-wave oscillator was also established to evaluate the PS-LPFG’s performance. Experimental results demonstrate that the spectral broadening of the main lobe is clearly alleviated. The 30 dB linewidth of the evaluation system is narrowed approximately 32.9% by the PS-LPFG. The 0.03 dB insertion loss of the fabricated grating avoids the deterioration of the signal laser’s power. Furthermore, the 0.023 °C/W thermal slope of the grating allows for potential applications in higher power fiber lasers.

직교 입력 편광에 따른 위상 천이 장주기 격자의 pH 및 온도 응답에 관한 연구

In this paper, we demonstrated an optical fiber sensor based on a phase-shifted long-period fiber gratin g (PS-LPFG) that could realize simultaneous measurement of surrounding pH and temperature variations. The PS-LPFG composed of a grating-free fiber region (GFFR) in the middle of two identical LPFGs was fabricated on high-birefringence fiber (HBF) with CO₂ laser pulses. In addition, a π phase shift formed by setting the length of the GFFR as half of the grating period of an LPFG allows a transmission dip to split into two attenuation bands at two different resonance wavelengths. Since HBF can preserve a linear state of polarization (SOP) if the input SOP is aligned with one of its two principal axes (fast or slow axis), a PS-LPFG inscribed on HBF (HB-PS-LPFG) can create two different transmission spectra for two orthogonal input SOPs, i.e., linear horizontal polarization (LHP) and linear vertical polarization (LVP). To investigate pH and temperature responses of the fabricated HB-PS-LPFG, the dip wavelength separation bet ween two different attenuation bands was used as a sensor indicator for an LHP or LVP input SOP. Consequently, pH- and temperature-induced wavelength variations of the two sensor indicators showed linear and independent spectral responses to both pH and temperature variations in a pH range from 3 to 11 and a temperature range from 25 to 65 °C. With these unique pH or temperature responses of the two sensor indicators, the fabricated sensor could realize simultaneous measurement of pH and temperature.

Utilizing phase-shifted long-period fiber grating to suppress spectral broadening of a high-power fiber MOPA laser system

Abstract 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 kilowatt-level 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 high-power fiber MOPA laser systems.

Multiple orbital angular momentum mode switching at multi-wavelength in few-mode fibers.

Mode division multiplexing has attracted great attention because it can potentially overcome the limitation of single-mode fiber traffic capacity. However, it has been challenging to realize multiple modes controlling and switching due to the intrinsic overlap of the modes in the transmission waveguide. As a solution, we propose a cascaded phase-shifted long-period fiber grating (PS-LPFG) based multiple mode switching scheme. Using the PS-LPFGs, the multiple guided orbital angular momentum (OAM) modes selective controlling and switching at multi-wavelength can be achieved in few-mode fibers by regulating the grating resonance condition. In principle, a N × N mode switch matrix can be realized by cascading CN2 gratings, where each grating acts as a mode switch element to achieve a couple selected OAM mode switching and meanwhile the other modes are under nonblocking status. As a proof of the concept, a 2 × 2 mode switching between two OAM modes at different wavelengths based on one PS-LPFG element is demonstrated in our experiments. The switching efficiency of the two modes at two wavelengths 1537nm and 1558nm are ∼98.4% and ∼98.7%, respectively. The proposed multiple OAM mode switch has potential applications in the future hybrid multi-dimensional multiplexing optical fiber communication systems.

A new phase-shifted long-period fiber grating for simultaneous measurement of torsion and temperature

A novel phase-shifted long-period fiber grating (PS-LPFG) for the simultaneous measurement of torsion and temperature is described and experimentally demonstrated. The PS-LPFG is fabricated by inserting a pre-twisted structure into the long-period fiber grating (LPFG) written in single-mode fiber (SMF). Experimental results show that the torsion sensitivities of the two dips are −0.114 nm/(rad/m) and −0.069 nm/(rad/m) in the clockwise direction, and −0.087 nm/(rad/m) and −0.048 nm/(rad/m) in the counterclockwise direction, respectively. The temperature sensitivities of the two dips are 0.057 nm/°C and 0.051 nm/°C, respectively. The two dips of the PS-LPFG exhibit different responses to torsion and temperature. Simultaneous measurement of torsion and temperature can be implemented using a sensor. The feasibility and stabilization of simultaneous torsion and temperature measurement have been confirmed, and hence this novel PS-LPFG demonstrates potential for fiber sensing and engineering applications.

Analysis for the phase-diffusion effect in a phase-shifted helical long-period fiber grating and its pre-compensation.

Through analyzing the structure of the phase-shifted helical long-period fiber grating (HLPG), which is fabricated based on the thermally twisting method, we have shown that there exists a phase-diffusion effect when the thermal region is larger than the grating period itself, i.e., the inserted phase preset at particular period will be diffused to several neighboring periods, which causes a large distortion in the transmission spectrum. We have analytically proved that this kind of phase-diffusion effect can be quantified by doing the convolution between the preset phase function and the phase-diffusion function in spatial domain. According to the analytical results, we have proposed and successfully demonstrated a pre-compensation method to solve the phase diffusion effect. As an example, a phase-shifted HLPG with π phase-shift precisely inserted at middle position of the grating has been presented.

Recent progress in design and fabrication of novel long-period fiber grating

Long-period fiber grating (LPFG) is a kind of wide-range transmission passive photonic device with extensive applications in the field of fiber communication and fiber sensing. In this review, from the angle of refractive index spatial modulation, we extract three characteristic parameters of LPFG:grating period length, index modulated depth and normal orientation of grating plane, and classify LPFG as two types:uniform LPFG (none of these three parameters changes) and nonuniform LPFG (at least one of them changes), and analyze the deficiency of LPFG, including larger size than fiber Bragg grating, no reflection peak, too large bandwidth, polarization loss from single-side exposure, etc. We define the concept of novel LPFG (NLPFG) as the LPFGs based on general LPFG but having new structures and new characters by importing new factors from different aspects, like grating formed mechanism, grating structure, making material, processing technique, application performance, etc. Then we point out that the research significance of NLPFG lies in improving and exploring its real usable property, and making it practical by overcoming the defects of general LPFG in structure, property and application. We expound new techniques of LPFG fabrication, such as multi-exposure, apodized exposure, outfield action, coating and filling, fiber incised and welded, multi-dimensional modulation, and show some NLPFG examples written with these techniques. We build the spatial model of NLPFG to expand the refraction index modulation region from only fiber core to both core and cladding, and to correctly mark the direction of grating plane with tilted angle and azimuth angle. On this basis, we propose the design theory of NLPFG by adding those two angles into the coupling mode coefficient and solving the coupling mode equation. We also expound three different NLPFG design processes, as the direct design to start from given factors of grating, the reserve design to calculate the factors back from expected function or spectrum, and the direct-reserve design combined by them. Meanwhile, we introduce some typical design methods of NLPFG, like geometrical structure changed method, materials changed method, medium coated and embedded method, etc. In addition, we review the recent fabrication and typical application of NLPFG, then introduce different LPFG devices based on excentric core LPFG, multi-core LPFG, few-mode LPFG, stagger LPFG, mismatched LPFG, over-melted LPFG, phase-shift LPFG, tuning LPFG, coupled LPFG and cascaded LPFG, and show their sensing applications in strain, twisting, bending, temperature, displacement, gas concentration and biology. Finally, we provide a developing prospect of the research on NLPFG and give three possible means to improve the research, as innovating new gating structures, exploring new design methods and developing new fabrication techniques.

Cryogenic Temperature Response of Reflection-Based Phase-Shifted Long-Period Fiber Gratings

In this study, we investigated the temperature behavior of phase-shifted long-period fiber gratings (PS-LPFGs) inscribed in two types of optical fiber: B/Ge and SMF28. The experiments were carried out from 5 to 305 K using a superconducting quantum interference device magnetometer. The average temperature sensitivity obtained of −0.43 nm/K for PS-LPFGs inscribed in the B/Ge fiber is one order of magnitude larger than for PS-LPFGs inscribed in the SMF28 fiber, in the 60–240 K range. Values ranging from −0.08 nm/K up to 0.2 nm/K were obtained in the 5–35 K temperature range, which are considerably better than previous results achieved for metal-coated FBGs and also for LPFGs inscribed in a similar B/Ge codoped fiber. Nevertheless, further work is required in order to correctly address sensor reliability.

Fabrication of Phase-Shifted Long-Period Fiber Grating and Its Application to Strain Measurement

Several kinds of phase-shifted long-period gratings (LPGs) have been fabricated using a high-repetition-rate CO2 laser, where both the grating and the inserted phase shift are simultaneously produced in a commonly used single-mode fiber. Furthermore, using one of the fabricated phase-shifted LPGs, a temperature-insensitive sensor that allows for large axial strain measurement with a value up to 7600 $\mu \varepsilon $ , is proposed and first demonstrated. Measurement accuracy for the strain ranging from 340 to 7600 $\mu \varepsilon $ is estimated to be ±30 $\mu \varepsilon $ , while the temperature is changed in a range of 20 °C–140 °C.

The Evolution of Dual-Wavelength Dissipative Soliton in Yb-Doped Fiber Lasers

Dual-wavelength dissipation soliton (DS) in the all-normal-dispersion ytterbium-doped fiber laser (YDFL) is reported. Using a phase-shift long period fiber grating (PS-LPFG) as an all-fiber format spectral filter in the laser cavity, self-starting dual-wavelength pulses are obtained by nonlinear polarization evolution (NPE). The mode-locking mechanism of the ANDi cavity with different central wavelengths has analyzed. Experimental results show that the NPE together with the PS-LPFG play key roles in the generation of the dual-wavelength, meanwhile, a switchable dual-wavelength mode-locked fiber laser can achieve through change the polarization of the cavity. DOI: http://dx.doi.org/10.11591/telkomnika.v11i10.2961

Phase-shift formed in a long period fiber grating and its application to the measurements of temperature and refractive index.

A novel approach to calibrate a phase-shift formed in a long-period fiber grating (LPG) is firstly proposed and numerically demonstrated, which is based on the use of either intensity- or wavelength-interrogation technique to the main loss-peak of the phase-shift LPG in the spectrum. Moreover, by using a CO2 laser with high-repetition-rate pulses emission, an equivalent phase-shift is successfully created at middle of the LPG. As an application of the proposed calibration scheme, measurement for the temperature and the refractive index of the ambient solution has been proposed and successfully demonstrated by using a phase-shifted LPG.

Reflection-based phase-shifted long period fiber grating for simultaneous measurement of temperature and refractive index

We report a reflection-based phase-shifted long period fiber grating (PS-LPFG) and demonstrate its capability for simultaneous measurement of temperature and external reflective index (RI). The sensor device comprises a grating directly written by CO2 laser and silver-coated end face. A π-shifted LPFG is presented with two attenuation bands through its reflection spectrum. These two bands have different sensitivity towards temperature and external RI that can be used for simultaneous measurement of the two variables. The experimental results show that this probe-type PS-LPFG performs well in terms of linearity and sensitivity.

Arbitrarily switchable multi-wavelength Yb-doped fiber lasers with phase-shifted long-period fiber grating

An arbitrarily switchable multi-wavelength Yb-doped fiber laser (YDFL) based on a phase-shifted long-period fiber grating (PS-LPFG) is proposed and demonstrated. The PS-LPFG, which was fabricated by a CO2 laser under asymmetric exposure, acts as a spectral filter with anisotropic transmission spectrum at different states of polarization. The capability of arbitrary wavelength switching can be attributed to the combination of the polarization-dependent loss (PDL) of the PS-LPFG and the nonlinear polarization rotation (NPR) effect. The method suggests a new possibility to achieve even more wavelengths output with an all-fiber format, arbitrarily switchable and tunable capability in the Yb-doped fiber laser system in a simple, stable and well controlled manner.

镀膜相移长周期光纤光栅大间隔双峰滤波特性

镀膜相移长周期光纤光栅大间隔双峰滤波特性

Fabrication and characteristics of phase-shifted beat gratings induced by CO2 laser

We propose and investigate a phase-shifted long-period fiber grating (LPFG) written by CO2 laser in twisted single-mode fibers (SMFs).For the beating effect, two peaks of LPFG derived from phase-shift further split into two peaks, respectively. The evolution and characterization of the transmission spectra were investigated. The channel spacing of the phase-shifted LPFGs written in twisted Er-doped fiber could be much narrower than that of which written in twisted normal SMF. Polarization dependent loss of the phase-shifted beat-gratings is less than 0.6 dB. This type of gratings provides potential applications in optical communication systems, such as course wavelength division multiplexing devices. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2526–2530, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26340

Tunable high energy giant chirped passively mode-locked Yb-doped fiber laser

A tunable, low-repetition rate, all-normal-dispersion Yb-doped fiber laser (YDFL) that is passively mode locked based on a phase shifted long period fiber grating (PS-LPFG) is demonstrated and proposed. The mode-locking mechanism of the laser is based on nonlinear polarization evolution (NPE). Using a PS-LPFG as the spectral filter in the laser cavity, the mode-locked output wavelength can be tuned continuously and smoothly over a spectral range of 10 nm, which is the first time implementation of a tunable giant chirped pulse with all-fiber format bandpass filter in YDFL. The maximum output pulse energy is 38.9 nJ at the repetition rate of 2.499 MHz.