Transmission Spectrum Of Long-period Fiber Grating Research Articles

Generation of all-fiber third-order orbital angular momentum modes based on femtosecond laser processing of long-period grating

The generation of orbital angular momentum (OAM) modes is very important, for they have a variety of applications such as in optical tweezers, quantum optics, and optical communication systems. Particularly, how can high-order OAM modes be generated efficiently in fibers with the advantage of low cost and compatible with fiber system? The Traditional method for first order to third order OAM is based on long period fiber grating (LPFG) fabricated by carbon dioxide laser. However, high power and large focused spot of carbon dioxide laser are unfavorable for stable and repeatable generation of higher-order OAM, which needs the LPFG with small grating pitch. In order to solve this problem, a third-order OAM mode converter based on femtosecond microfabrication is proposed and fabricated for the first time. With the advantage of 4.4 μm focused spot size near the core, lower power and lower heat absorption efficiency, this method can be more stable and promising. Therefore, we first carry out the mode filed analysis and simulate the intensity and phase profiles of the superposed mode field in LP odd-even mode on different scales and phases patterns to obtain OAM mode. Second, we use the coupled-mode theory to analyze and simulate the transmission spectrum of LPFG, which guides the setting of the grating parameters such as the grating pitch, the depth of modulation and the length of the grating. By experimental verification, an asymmetric modulated long-period fiber grating with a pitch setting to 194 μm is fabricated on a six-mode fiber. The fundamental mode can be converted into the third-order angular linear polarization mode LP<sub>31</sub> mode with 98% mode conversion efficiency near 1550 nm, and further converted into the OAM<sub>±3</sub> modes by superposition of the odd and even LP<sub>31</sub> mode with ±π/2 phase difference. At the same time, this fiber grating can also generate LP<sub>12</sub> mode with 90% mode conversion efficiency near 1325 nm. Then we can take the same approach to transform LP<sub>12</sub> mode into OAM modes with angular first-order as well as radial second-order. The experimental result is consistent with the simulation result. Thus, this scheme provides an idea for generating the high-order OAM modes in all-fiber systems by using only one grating with high repeatability.

Rapid and sensitive detection of Staphylococcus aureus by using a long-period fiber grating immunosensor coated with egg yolk antibody

The rapid and reliable detection of bacteria plays an important role in clinical and veterinary practice. A stable, label free, compact, and sensitive long-period fiber grating (LPFG) sensor based on egg yolk antibody (IgY) was proposed for the detection of Staphylococcus aureus (S. aureus). LPFG was fabricated with laser writing technology, and specific IgY was further immobilized on the grating region and then combined with the corresponding bacteria. S. aureus was detected by tracking the change of resonance wavelength in the LPFG transmission spectrum caused by bacteria-antibody interaction induced by the increase of biological cover thickness and density after the immune reaction. The testing results showed that the proposed sensor was selective and sensitive to S. aureus measurement, and the detection limit is approximately 33CFU/ml. The proposed antibody immobilization method is very simple, and the optical fiber can be manufactured in batch to reduce the cost. The detection time of the sensor is around 20min, which is fast and suitable for detection. The assay was successfully applied for the quantitative analysis of S. aureus in natural waters and met the needs of on-site screening trace pathogenic bacteria in food safety control.

Graphene oxide-functionalized long period fiber grating for ultrafast label-free glucose biosensor.

A label-free glucose biosensor is constructed successfully based on the long period fiber grating (LPFG) functionalized with graphene oxide (GO)-glucose oxidase (GOD) via the chemical crosslink method. GO coated on the surface of LPFG can immobilize GOD by the plentiful binding sites because of its favorable combination of exceptionally high surface-to-volume ratio. The structure and characterization of GOD-GO-modified LPFG are studied by the optical microscope, Fourier transformation infrared spectrometer (FTIR), Raman spectroscopy, scanning electron microscope (SEM) and atomic force microscopy (AFM), respectively. The reaction between GOD and glucose create gluconic acid and H2O2, which will lead to an evident shift of LPFG transmission spectrum due to the greater change of the surrounding refractive index (SRI). The GOD-GO-modified LPFG sensor shows a linear response with a response coefficient of 0.77 nm/(mg/mL). This biosensor has good selectivity and can be used for the detection of practical sample. The GOD-GO-modified LPFG biosensor has great prospect in the pharmaceutical research and medical diagnosis fields.

Long Period Fiber Grating Fabrication by Two-Step Infrared Femtosecond Fiber Laser Exposure

We develop a two-step infrared (IR) femtosecond fiber laser exposure technique to flexibly fabricate long period fiber gratings (LPFGs) with a high peak band-rejection efficiency of 35.4 dB, insertion loss of 4.36 dB, and 3 dB bandwidth of 13 nm. First, we etch periodic corrugations via IR femtosecond laser in the cladding of standard single mode fiber, leading to efficient laser beam-focusing on the cladding as well as stress-optic refractive index (RI) modulation. Then, we carry out femtosecond fiber laser irradiation under the corrugations, to further introduce RI modulation within fiber core. We experimentally investigate the impacts of fabrication parameters including laser focus location and individual exposure time with respect to the transmission spectra of LPFGs. The fabricated LPFG has a temperature coefficient of 114 pm/°C and a polarization dependent loss of 1.78 dB, which may find useful applications in sensors.

Long-period fiber grating sensor for detection of viruses

Fast and reliable detection of viruses, including those specific to bacteria – bacteriophages – is crucial in clinical and veterinary practice, during biotechnological processes, and in basic research. In this paper, a highly sensitive long-period fiber grating (LPFG) label-free immunosensor is described for the detection of T7 bacteriophages. The LPFGs were tuned up to their highest refractive index sensitivity at the dispersion turning point of higher order cladding modes. The simple and reliable fiber surface functionalization was performed using 3-(triethoxysilyl)propylsuccinic anhydride to covalently bind anti-T7 antibodies. The T7 phage detection was possible by tracing the shift of the resonance wavelength in the LPFG transmission spectrum caused by the increase of the thickness and density of the bio-overlay, in particular induced by antigen-antibody interactions. The obtained sensor was selective and the limit of detection was lower than 5×103PFU/mL. Moreover, regeneration of the sensor surface by removing all organic layers was demonstrated. The regeneration procedure can reduce the costs of production and give the possibility for performing other measurements on the same LPFG structure. The applied procedure enabled real-time measurements and can be easily extended for the detection of other viruses.

Long period fiber grating based sensor for the detection of triacylglycerides

In this paper, stable, label free enzyme based sensor using long period fiber grating (LPG) is described for the detection of triacylglycerides. A stable covalent binding technique for lipase enzyme immobilization on an optical fiber is reported. An active and stable attachment of the functional group of the enzyme on the fiber surface is achieved using this method. Enzyme immobilization is confirmed by Scanning Electron Microscopy (SEM) and Raman Spectroscopy. The stability is confirmed by lipase p-nitrophenyl palmitate (PNP) assay. In contrast to widely used amperometric based biosensor, where a number of enzymes are required, only one enzyme, namely, lipase is required in our sensor. The sensor shows optimum response within one minute at a temperature of 37°C and pH of 7.4. The sensor is based on the shift in resonance wavelength of the LPG transmission spectrum due to the interaction of triacylglycerides with the enzyme. The biosensor is highly specific towards triacylglycerides and is unaffected by the presence of many other interfering substances in serum. Interaction between the bio-molecules and the long period grating surface is also modeled theoretically using a four layer model for the LPG fiber with the bio-recognition layer and the results obtained are consistent with experimentally obtained results. The sensor shows a high sensitivity of 0.5nm/mM and a low detection limit of 17.71mg/dl for the physiological range of triacylglycerides in human blood.

Various characteristics of long-period fiber grating-based refractive index sensor

In this paper, the response of long-period fiber grating (LPFG) as a refractive index sensor is theoretically investigated. The response has been checked for the surrounding refractive index (SRI) variation from 1 to 1.45. The sensitivity enhancement of LPFG as a refractive index sensor by employing higher-order cladding modes and by reducing the cladding radius is discussed. In the range of SRI from 1 to 1.45, HE13 mode has shown an overall wavelength shift of 0.0015μm and HE17 mode shows an overall wavelength shift of 0.0042μm, thus increasing the sensitivity of HE17 mode 2.8 times to that of HE13 mode. It was also found that reducing cladding radius by 10μm in case of HE17 mode results in an overall wavelength shift of 0.0066μm for SRI range from 1 to 1.45 and results in increased sensitivity up to 1.6 times. Also discussed is the variation of physical parameters such as grating length and peak-induced index change (PIIC) which influences the transmission spectrum of LPFG for its efficient use as a refractive index sensor.

Theory of Fourier mode coupling for long-period fiber gratings

A novel theory, namely, Fourier mode coupling (FMC) theory for long-period fiber gratings (LPFGs) is proposed in this paper. During analyzing the co-propagating coupling between the core mode and cladding modes in LPFGs, the Fourier transform relations among the amplitude coefficients of co-propagating coupled-modes are found for the first time, to the best of authors’ knowledge. The general expressions of the coupling and transmission spectra of LPFGs are also deduced from the combination of Fourier transform with the well-known coupled-mode theory. In the proposed FMC theory, the spectral characteristics of the LPFGs without over-coupling are derived from the calculation of co-propagating mode coupling in the spatial domain spectrum, which is the Fourier transform result of refractive index perturbation in the LPFG. According to the FMC theory, the spectra of the LPFGs in different perturbation amplitudes and lengths are numerically simulated here. A measured transmission spectrum is also compared with the calculated transmission spectra based on the FMC theory and the coupled mode theory, respectively. The comparison shows that the FMC theory and the derived spectra for LPFGs are in consistance with the coupled-mode theory and the practical spectra of LPFGs respectively. The FMC has many features, these being simple, fast and accurate, which could be employed for spectrum analysis of any LPFG with an arbitrary distribution of refractive index perturbation along the fiber axis.

Characterization with the Simulation of Transmission Spectra in Long-Period Fiber Grating Fabricated by the Point Exposure Method

Long-Period Fiber Gratings (LPFGs) which have a periodic structure in the refractive index in the longitudinal direction of the fiber with a period of about 0.5 mm, convert incident light in the fundamental LP01 mode into higher-order LP0m modes through mode coupling. Because the higher-order modes cannot propagate in a single-mode fiber but are lost outside the fiber, an LPFG works as an attenuator. LPFGs with different Bragg wavelengths were fabricated by using the point exposure method in which the period was freely changed. Just after excimer laser light exposure, transmission spectra of LPFGs were measured. Furthermore, compared to the simulation, the higher-order modes of peaks in the transmission spectra of experimentally fabricated LPFGs were identified.

薄膜消光系数对镀膜长周期光纤光栅传输谱的影响

薄膜消光系数对镀膜长周期光纤光栅传输谱的影响