Gratings In Photonic Crystal Fiber Research Articles

Cascaded Bragg gratings in photonic crystal fiber for plasmonic cladding mode-based biosensing of HER2 protein

Spectral multiplexing of biosensors in a single optical fiber has been a long-standing challenge, which we address here for the first time by combining photonic crystal fibers (PCF) with fiber Bragg grating technology. We exploit the features of the optical transmission spectrum of a straight fiber Bragg grating written in a PCF that allows exciting cladding mode resonances within a spectral span of about 60 nm, which is significantly narrower than the width of the transmission spectra of tilted gratings in standard single-mode step-index fibers. More specifically, we consider the cladding mode resonances that feature effective index values close to the refractive index of phosphate buffered saline, and we demonstrate plasmonic label-free biodetection of HER2 (human epidermal growth factor receptor 2) protein. We report on the simultaneous monitoring of the wavelength shifts of said cladding mode resonances from two spatially separated biofunctionalized Bragg gratings and we find that the PCF sensor is able to detect the protein concentration of 8.62 nM with high reproducibility.

The Design of Bending Long Period of Photonic Crystal Fiber Grating Sensors

The bending photonic crystal fiber grating sensor is an important role in underwater monitoring and fire alarm systems. It is studied that the resonant wavelength expression of bending long period photonic crystal fiber gratings is deduced, it is designed that a bending long period photonic crystal fiber grating sensor system, it is calculated in theory that between the bending long period photonic crystal fiber gratings sensor resonance wavelength and the grating period and the bending strain. The result is shown by calculating and analysing in theory, the grating curvature is increased by the increase of the bending strain of the grating, and the resonance wavelength of the grating sensor is drifted, the drift amount is increased, one in this grating, the drifted amount of the resonant wavelength is 0.014 nm.

Theoretical model and spectrum characteristics of superimposed photonic crystal fiber grating

The superimposed gratings have attracted considerable interest because they can extend the potential applications of gratings. Superimposed gratings are fabricated by inscribing multiple gratings at the same section of the fiber, and they can demonstrate various features simultaneously. A number of optical devices based on superimposed gratings have been reported, such as multi-wavelength filters, beam shapers, ultrahigh repetition rate optical pulse generators, etc. Photonic crystal fiber (PCF) can bring new optical characteristics by changing the sizes, spacings and arrangements of the air holes in the fiber. In this paper, we present the spectra of the superimposed gratings inscribed in a photonic crystal fiber. A numerical mode is proposed based on the V-I transmission matrices. The traditional cosinoidal variation of refractive index is replaced with a square-type refractive index variation, and the scattering occurs at a localized discrete location. According to the simulations, the reflection spectra and time delays of a superimposed Bragg grating and superimposed chirped Bragg grating are analyzed. A superimposed Bragg grating and a superimposed chirped Bragg grating are fabricated in the single mode photosensitive PCFs under the irradiation of a 193 nm ultraviolet laser. The superimposed Bragg grating is composed of four subgratings with resonance wavelengths at set spacings. And under a phase mask displacement of 1.03 mm, the superimposed chirped Bragg grating has a periodic resonance with a period of 0.82 nm. The results show that the spectrum of superimposed Bragg grating can be flexibly customized by the parameters of each subgrating. Superimposed chirped Bragg gratings have good linear group delays and flat periodic resonance amplitudes, and the resonance period can be adjusted by displacing the phase mask. The grating spectra obtained from experiments are in good agreement with the theoretical analyses. The research results in this paper provide an important basis for designing, fabricating, and applying the superimposed PCF gratings.

Numerical modeling of femtosecond laser inscribed IR gratings in photonic crystal fibers.

During grating inscription in photonic crystal fibers (PCFs) the intensity of the inscribing laser beam is non-uniformly distributed over the core region due to the interaction with the air holes in the fiber's microstructure. In this paper we model and study the non-uniformity of the index modification and its influence on the grating reflection spectra, taking into account the non-linear nature of the index change. For femtosecond laser inscription pulses at 800 nm, we show that the intensity redistribution in the PCF core region can result in Type II index changes even if the peak intensity of the incident beam is well below the corresponding threshold. Our coupled mode analysis reveals that the non-uniform nature of the index change can seriously affect the reflectivity of the grating due to a limited overlap of the guided mode with the transverse index modulation profile for almost all angular orientations of the PCFs with respect to the inscription beam. We also evaluate the influence of PCF tapering and we found that for the considered PCF a significant increase in the induced index change and reflectivity is observed only for taper diameters below 40 μm.

Measurement and Analysis of Loss for Long-Period Photonic Crystal Fiber Gratings

In this work, we used a fiber splicer to fabricate a long-period grating (LPG) in a photonic crystal fiber (PCF) for power loss measurement. Photonic crystal fiber grating (PCFG) is long-period grating which is produced by using electric arc to heat the PCF. For the measurement of power loss, we will use beam profile and power meter to analyze in detail the phenomenon of loss factor and parameter of the measurement. We will also do experiment on both polished and unpolished long-period photonic crystal fiber grating and compare the results. Utilizing two long-period grating of photonic crystal fiber (polished and unpolished) to analyze and compare the difference of optical field distribution and power loss. The unpolished PCFG shows the equally filed distribution with wavelength by beam profile measurement. But, the polished PCFG is sensitive with wavelength that the field distribution of wavelengths 1520 and 1530nm are concentrated on the upper half of field distribution and the wavelength 1530nm is more concentrated on the side. The wavelength of polished long-period grating of photonic crystal fiber has sensitive response and loss with wavelength increase and increment. Another measurement is probing photonic crystal fiber grating of power loss measurement using power meter. The PCFG can be evaluated by field distribution and power for operating wavelength and power loss. The measurement of operating wavelength of the long-period fiber grating is 1550nm.

Numerical and experimental investigation of long-period gratings in photonic crystal fiber for refractive index sensing of gas media

We have used the finite-difference frequency-domain (FDFD) method to simulate the core mode to cladding mode couplings in long-period gratings (LPGs) in photonic crystal fiber (PCF). Four sets of LPG-PCF have been fabricated with respective periodicities of 590, 540, 515, and 490 μm, resulting in corresponding resonance wavelengths (RWs) of 1241, 1399, 1494, and 1579 nm. We show both theoretically and experimentally that the longer the RW, the more sensitive the LPG-PCF is to the index change in Ar. We demonstrate a robust sensitivity of 517 nm per refractive index unit using the LPG-PCF at 1579 nm RW.

光子晶体光纤光栅及其在激光器中的应用

光子晶体光纤光栅及其在激光器中的应用

Study on transmission characteristics of photonic crystal fiber gratings with periodic air hole deformation

The paper studies a kind of improved photonic crystal fiber gratings fabricated by CO2 laser heating method. The effective refractive index of cladding induced by periodic air hole deformation is computed using multipole method, and the relationship between the effective refractive index and the collapse of air-holes is discussed, thereby the modulation expression of effective refractive index is obtained. The grating transmission characteristics are simulated. The results indicate that with the diameter of air-holes increasing from 3.3 μm to 3.7 μm, the resonance wavelength shows blue-shift, the resonance peak intensifies, and the bandwidth becomes narrow. As the collapse degree of cladding enhances, the resonance wavelength shows red-shift, the transmission increases, and the bandwidth tends to narrow.

Spectral characteristics and thermal evolution of long-period gratings in photonic crystal fibers fabricated with a near-IR radiation femtosecond laser using point-by-point inscription

The spectral properties of long-period gratings (LPGs) fabricated in photonic crystal fibers using femtosecond laser pulses by the point-by-point technique, without oil-immersion of the fiber, are investigated in detail. Postfabrication spectral monitoring at room temperature showed significant long-term instability of the gratings and stable spectra only after 600 h. The stabilized spectral properties of the gratings improved with increasing annealing temperature. The observed changes in resonant wavelength, optical strength, and grating birefringence were correlated to the laser inscription energy and were further used to study the mechanism of femtosecond inscription. Furthermore, the femtosecond-laser inscribed LPGs were compared to electric-arc fabricated LPGs. Comparison of experimental results with theoretical models of LPGs and laser propagation during inscription indicate that the major processes responsible for the index change are permanent compaction and thermally induced strain, the latter can be significantly changed through annealing.

Long period gratings in photonic crystal fibers

The authors review the recent advances in fabricating long-period gratings (LPGs) in photonic crystal fibers (PCFs). The novel light-guiding properties of the PCFs allow the demonstration of novel sensors and devices based on such LPGs. The sensitivity of these PCF LPGs to temperature, strain and refractive index is discussed and compared with LPGs made on conventional single-mode fibers. In-fiber devices such as tunable band rejection filters, Mach-Zehnder interferometers are discussed.

Long-period gratings in photonic crystal fiber as an optofluidic label-free biosensor

Using long-period gratings (LPG) inscribed in photonic crystal fiber (PCF) and coupling this structure with an optically aligned flow cell, we have developed an optofluidic refractive index transduction platform for label-free biosensing. The LPG–PCF scheme possesses extremely high sensitivity to the change in refractive index induced by localized binding event in different solution media. A model immunoassay experiment was carried out inside the air channels of PCF by a series of surface modification steps in sequence that include adsorption of poly(allylamine hydrochloride) monolayer, immobilization of anti-rat bone sialoprotein monoclonal primary antibody, and binding interactions with non-specific goat anti-rabbit IgG (H + L) and specific secondary goat anti-mouse IgG (H + L) antibodies. These adsorption and binding events were monitored in situ using the LPG–PCF by measuring the shift of the core-to-cladding mode coupling resonance wavelength. Steady and significant resonance changes, about 0.75 nm per nanometer-thick adsorbed/bound bio-molecules, have been observed following the sequence of the surface events with monolayer sensitivity, suggesting the promising potential of LPG–PCF for biological sensing and evaluation.

Sensing properties of femtosecond laser-inscribed long period gratings in photonic crystal fiber

The use of near infrared, high intensity femtosecond laser pulses for the inscription of long period fiber gratings in photonic crystal fiber is reported. The formation of grating structures in photonic crystal fiber is complicated by the fiber structure that allows wave-guidance but that impairs and scatters the femtosecond inscription beam. The effects of symmetric and asymmetric femtosecond laser inscriptions are compared and the polarization characteristics of long period gratings and their responses to external perturbations are reported.

热激法PCFG制备工艺中热传导特性研究

The fiber gratings fabrication technology with the heating method in a photonic crystal fiber (PCF) based on structural change is examined. The principle of photonic crystal fiber gratings (PCFGs) is analyzed in theory. The heat transfer theory and finite element method are used to examine the thermal field distribution in the fiber and the influence of the air hole structure in the cladding, and the parameters of the laser beam in the process of grating fabrication are discussed. The results show that gratings can be formed by the periodic collapse of air holes in the cladding of PCFs. Under double-point heating condition, the energy is uniformly distributed in the radial direction and is approximate to Gaussian distribution in the axial direction. With the same size of the luminous spot, as the layers and radius of the air holes increase, the laser power needed to make the air holes collapse decreases. With the same laser power, as the luminous spot radius increases, the needed heating time increases. Moreover, the relationship between the laser power needed and the air filling rate is obtained as the number of layers of the air holes changes from 1 to 7. This kind of PCFG can overcome the long-term thermal instability of conventional gratings in substance and thus has great potential applications in the related field of optical fiber sensors.

Heat transfer characteristics in fabrication of heat method in photonic crystal fiber grating

The grating fabrication technology with heat method in photonic crystal fiber based on its structural change is researched. The principle of photonic crystal fiber grating is analyzed theor etically. Heat transfer theory and finite element method are both used to analyze the thermal field distribution in the fiber, as well as the influence of air hole structure in the cladding, and the parameters of laser beam in the process of grating fabrication are discussed. The research results show that the grating can be formed by the periodic air hole collapse in the cladding of photonic crystal fiber. Under double-point heating, the energy will be uniformly distributed in the radial direction and approximated by the Gaussian distribution in the axial direction. The collapsed air holes in the cladding accelerate the process of forming grating. In the same size of luminous spot, as the layers and the radii of air holes increase, the laser power for collapsing fiber decreases. Moreover, the relationship between laser power and air filling rate is obtained by stimulating the grating fabrication process in photonic crystal fiber with 1 to 7 layers of air holes. This kind of photonic crystal fiber grating can improve the thermal and long-term stability of conventional grating, and so it will have great potential applications in the relevant fields of optical fiber sensors.

结构性改变长周期光子晶体光纤光栅成栅机理研究

结构性改变长周期光子晶体光纤光栅成栅机理研究

Spectral characterization of helicoidal long-period fiber gratings in photonic crystal fibers

We report a new helicoidal long-period grating by twisting a photonic crystal fiber under CO 2 laser irradiation and experimentally investigated its novel transmission characteristics. The fabricated helicoidal PCF-LPG has a relatively short length of ∼1.4 cm with a low polarization dependent loss less than 1 dB and thermal shift of ∼3.7 pm/°C. The proposed device endows the unique resonant peaks tuning capability more than ∼18 nm by applying torsion stress, which has not been achieved in prior PCF-LPGs. From these novel thermo-optic and photo-mechanical properties, the proposed device can be applied for various components, including optical filters, attenuators, and sensors.

CO_2 laser writing of long-period fiber grating in photonic crystal fiber under tension

We demonstrate that the efficiency of CO(2) laser writing of long-period fiber gratings in a solid-core photonic crystal fiber (PCF) can be enhanced greatly by applying tension to the fiber during the writing process through the mechanism of frozen-in viscoelasticity. Using this mechanism, we are able to write strong gratings in PCFs with a dosage of CO(2) laser radiation low enough not to cause any significant fiber structure deformation.

Finite element analysis of a novel weak-pressure sensor based on fiber Bragg gratings in photonic crystal fibers

The fiber Bragg grating has been widely used in sensors. We have studied the spectral properties of uniform Bragg gratings in photonic crystal fibers under transverse pressure. By the finite element method, the relation between the birefringence and the pressure was simulated in bare photonic crystal fibers. The results show that the birefringence is changed even under weak pressure. A new method based on polarization-dependent loss is presented for measuring pressure. The maximal amplitude of the polarization-dependent loss varies linearly in the pressure range of 0 to 4 MPa with a slope of 0.75 MPa−1.

Compact In-Fiber Interferometer Formed by Long-Period Gratings in Photonic Crystal Fiber

We reported the design and implementation of an in-fiber Mach-Zehnder interferometer (MZI) based on a pair of long-period gratings (LPGs) written on a photonic crystal fiber (PCF). The LPG was fabricated by using a pulsed CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser to carve grooves periodically along the PCF. The MZI relies on the interference between the fundamental core mode and a cladding mode of the PCF. The MZI was further demonstrated as a temperature sensor and a strain sensor. The temperature and strain sensitivities were measured to be 42.4 pm/degC and - 2.6 pm/muepsiv, respectively. We also fabricated an MZI on a single-mode fiber, which has a temperature sensitivity of 1215.56 pm/( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">deg</sup> Cmiddotm) and a strain sensitivity of + 0.445 pm/muepsiv.

Stress-Induced Versatile Tunable Long-Period Gratings in Photonic Crystal Fibers

We report a method for generating versatile tunable long-period gratings (LPGs) in photonic crystal fibers (PCFs). The central wavelength and rejection bandwidth of LPGs can be tuned over a broad spectral range with adjustable transmission loss. Spectral fringes with uneven spacing are observed in chirped LPGs, depending on the interaction length and the amount of chirping between the two ends of the LPG, and can be removed by reducing the chirping and increasing the grating length. By utilizing a stress-induced LPG, gain flattening of an erbium-doped fiber amplifier has also been demonstrated.