Cladding mode coupling in highly localized fiber Bragg gratings II: complete vectorial analysis

Femtosecond laser direct writing large-area fiber Bragg grating based on diaphragm shaping

We propose and demonstrate a new method of direct writing large-area fiber Bragg grating by femtosecond laser through the coating. By adding an adjustable diaphragm before the focusing objective, we can precisely control the length of the refractive index modulation line along the femtosecond laser incident direction up to 29.1 µm. In combination with femtosecond laser scanning fabrication technology, a uniform refractive index modulation plane can be inscribed in the fiber in a single scanning. Based on the plane-by-plane inscription method, we have fabricated a high-quality high-reflectivity fiber Bragg grating and a chirped fiber Bragg grating on 20/400 double-clad fiber core. The reflectivity of both gratings is greater than 99%, and the insertion loss is as low as 0.165 dB and 0.162 dB, respectively. The thermal slope of chirped fiber Bragg grating without any refrigeration is 0.088 °C/W and there is no obvious temperature increase when using the water cooling. Therefore, the fabrication method of large-area fiber Bragg grating based on diaphragm shaping can efficiently fabricate high-quality fiber Bragg grating in the large core diameter fiber, which has an important application prospect in high-power all-fiber oscillators, especially all-fiber oscillators in special wavebands.

Trace detection of cadmium (II) ions based on an excessively tilted fiber grating

Cadmium (Cd2+) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd2+ sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd2+, and hence forming a complex. The grating excites high order cladding modes, and ensures a sufficient interaction between the light and hydrogel binding to Cd2+, providing highly sensitive monitoring. The results show that the sensor can detect 0-160 pM Cd2+ in aqueous solution. The maximum sensitivity is 10600 nm/µM, and the minimum detection concentration is 20 pM (about 0.004 ppb), which is much less than that of the international standard (3 ppb). The proposed sensor exhibits high sensitivity, ultra-low detection limit, specificity, and a compact structure, offering potential as a tool for Cd2+ detection in aqueous solution.

Femtosecond Laser Inscribed Excessively Tilted Fiber Grating for Humidity Sensing

We propose a humidity sensor using an excessively tilted fiber grating (Ex-TFG) coated with agarose fabricated using femtosecond laser processing. The processed grating showcases remarkable differentiation between TE and TM modes, achieving an exceptionally narrow bandwidth of approximately 1.5 nm and an impressive modulation depth of up to 15 dB for both modes. We exposed the agarose-coated TFG sensor to various relative humidity levels and monitored the resonance wavelength to test its humidity sensing capability. Our findings demonstrated that the sensor exhibited a rapid response time (2-4 s) and showed a high response sensitivity (18.5 pm/%RH) between the humidity changes and the resonant wavelength shifts. The high sensitivity, linearity, repeatability, low hysteresis, and excellent long-term stability of the TFG humidity sensor, as demonstrated in our experimental results, make it an attractive option for environmental monitoring or biomedical diagnosis.

High-temperature and stress response behavior of femtosecond laser pulses inscribed eccentric fiber Bragg gratings

Eccentric fiber Bragg grating (EFBG) is inscribed in standard communication single-mode fiber using femtosecond laser pulses, and the temperature and strain sensing characteristics are experimentally demonstrated and analyzed. The EFBG exhibits strong thermal stability and good robustness in high-temperature measurement up to 1000 °C, and undergoes different thermal sensitivities during Bragg peak and the strong resonance coupled cladding spectral comb. The temperature sensitivity linearly increases with respect to the effective index of the resonant modes. Such a situation also occurs in axial strain measurement. These characteristics are of high interest for multiparametric sensing at high temperatures.

Optimized femtosecond laser direct-written fiber Bragg gratings with high reflectivity and low loss

We propose and experimentally demonstrate a femtosecond laser plane-by-plane (Pl-b-Pl) technology for inscription of high-quality fiber Bragg gratings (FBGs). The spherical aberration (SA) was introduced to elongate the focal volume, and then combined with the scanning process, an expanded rectangular refractive index modification (RIM) region can be achieved. Such RIM regions exhibit a length of 15 µm and a width of 14 µm. Note that it consists of a negative region and a positive region. We have systematically studied the influence of the overlap between the RIM region and fiber core on the spectrum of FBG. After optimizing, the core of a conventional single-mode fiber (SMF) is covered completely by using the positive RIM region, resulting in a significant enhancement of the coupling strength coefficient (i.e., 3177.6 m^-1). A 500 µm long FBG assembled by using these RIM regions can achieve a high reflectivity of 95.83%. Moreover, the cladding mode resonances in transmission spectrum are suppressed thoroughly, since the localized effect in RIM region was avoided. In addition, this FBG exhibits a high birefringence of 2.13 × 10^-4. Therefore, the proposed fabrication method can be used to inscribe high-quality FBGs that could be used in many fields such as communication, fiber laser, polarization-selective filtering and multi-parameter sensing.

Strong cladding mode excitation in ultrathin fiber inscribed Bragg grating with ultraviolet photosensitivity

Strong UV-written Bragg gratings written in 50 µm-diameter cladding single mode fibers compatible with conventional fiber couple core guided light to dozens of cladding modes distributed across 140 nm in the 1400-1600 nm region, without the need for complex symmetry breaking mechanisms such as tilted, laterally offset, or localized gratings. The extent of the coupling to high order modes and the smaller cladding diameter both contribute to increasing the sensitivity to surrounding refractive index changes by more than one order of magnitude, and to an increased spacing between mode resonances to facilitate unambiguous measurements of larger index changes between 1.3 and 1.44. These improvements are confirmed by theoretical and experimental studies that also cover the temperature and strain differential sensitivities of the cladding mode resonances for complete multiparameter sensing capability.

Quantitative morphology of femtosecond laser-written point-by-point optical fiber Bragg gratings

We investigate the morphology of femtosecond laser, single pulse-inscribed, point-by-point (PbP) fiber Bragg gratings. Direct measurement of a PbP grating's refractive index profile was carried out with micro-reflectivity analysis. PbP gratings were imaged at sub-micrometer scale with scanning electron microscopy, Raman and photoluminescence studies were performed to probe the structural and electronic changes. Comparison of results from different characterisation techniques suggests that the creation of an increased refractive index region around the micro-void is due to contributions from both densification and the formation of highly polarizable non-bridging oxygen bonds.

Orientation-dependent fiber-optic accelerometer based on eccentric fiber Bragg grating

A highly localized eccentric fiber Bragg grating (EFBG) accelerometer was proposed, and its orientation-dependent measurement results were demonstrated experimentally. An EFBG was inscribed point-by-point (PbP) in a single-mode fiber (SMF) using a femtosecond laser, and the cladding mode was recoupled to excite the ghost mode through an abrupt taper. Owing to the asymmetry caused by the lateral offset of the EFBG, the ghost mode showed a significant directional response to acceleration. Furthermore, monitoring the fundamental core mode resonance can help calibrate accidental power perturbation or cross-sensitivity.

Localized tilted fiber Bragg gratings induced by femtosecond laser line-by-line inscription

Localized tilted fiber Bragg gratings (TFBGs) with low insertion loss are reported. A series of second-order TFBGs with tilt angles of 0°, 7°, 14°, and 21° was inscribed line by line directly in a single-mode fiber. For the 7° TFBG, the Bragg resonance was 2.4 dB at 1550 nm, and the maximum cladding-mode resonance reached 24.6 dB with an insertion loss of 0.8 dB, the same level as that for TFBGs fabricated by the phase-mask method. The range in cladding-mode resonance for the TFBGs obtained was wider than 170 nm with an intensity exceeding 20 dB. Combined with microscope images, the formation of these localized TFBGs and their spectral performance are discussed. The effect of an inscription offset along the $y$-axis direction was further investigated. With increasing offset, the intensity of the cladding-mode resonance dropped rapidly. The refractive index response sensitivity of the 7° TFBG was measured at 507.54 nm/RIU.

Ultrafast Laser Processing of Optical Fibers for Sensing Applications

A review of recent progress in the use of infrared femtosecond lasers to fabricate optical fiber sensors that incorporate fiber Bragg gratings (FBG) and random fiber gratings (RFG) is presented. The important advancements in femtosecond laser writing based on the phase mask technique now allow through-the-coating (TTC) fabrication of Bragg gratings in ultra-thin fiber filaments, tilted fiber Bragg gratings, and 1000 °C-resistant fiber Bragg gratings with very strong cladding modes. As an example, through-the-coating femtosecond laser writing is used to manufacture distributed fiber Bragg grating sensor arrays for oil pipeline leak detection. The plane-by-plane femtosecond laser writing technique used for the inscription of random fiber gratings is also reviewed and novel applications of the resultant devices in distributed temperature sensing, fiber lasers and fiber laser sensors are discussed.

Experimental investigation of point-by-point off-axis Bragg gratings inscribed by a femtosecond laser in few-mode fibers

Off-axis Bragg gratings with varied horizontal and vertical distances off the center in a step-index two-mode fiber were fabricated by 800 nm infrared-femtosecond laser pulses through a point-by-point technique. In this article, we experimentally investigate these gratings via measuring the transmitted power and the reflected intensity profiles under different input polarization, with multiple characteristics reported for the first time to the best of our knowledge. To highlight, we find that the birefringence induced to the LP₀₁ reaches its maximum magnitude at an intermediate offset, followed by the fast and slow axes switching at a further slightly increased offset. We also show that the peak reflectivity of the LP₁₁ exhibits strong polarization dependence, with the much stronger peak reflectivity constantly corresponding to the polarization perpendicular to the damage-point-to-center line, whereas the peak reflectivity of the LP₀₁ has almost no polarization dependence. Moreover, we report that the reflected mode patterns of the cross-coupling of the LP₀₁ and LP₁₁ are linked to the direction of linear polarization, through which one can selectively excite an arbitrarily oriented LP₁₁ by merely altering the polarization.

Polarization dependent cladding modes coupling and spectral analyses of excessively tilted fiber grating

We report on the detailed analyses of mode coupling from fiber core to cladding in excessively tilted fiber gratings (ETFGs). Cladding modes responsible for the typical dual peak pairs in the transmission spectrum of ETFGs are identified with phase matching condition, which suggests two set of dual peak pairs generated from coupling to cladding modes with even and odd azimuthal order. The polarization dependence of those dual peak pairs are also investigated by calculating the coupling coefficients of cladding modes for two orthogonal polarizations. With the calculated coupling coefficients, the measured polarization dependent spectra can be reproduced numerically.

Symmetry selective cladding modes coupling in ultrafast-written fiber Bragg gratings in two-mode fiber

The lower order cladding mode resonances of a fiber Bragg grating (FBG) are sensitive to fiber bending but their spectral density makes their response to bending very complex. In this work we present a simple method to reduce and control the number of low order cladding mode resonances via FBGs written in a two-mode fiber (TMF) with an ultrafast laser. Owing to the larger core size of the TMF, a slight break of the cylindrical asymmetry of the grating patterns can be induced when using femtosecond side-irradiation with a small change in the writing condition. This allows us to control the mode families coupled by the grating, and in particular to those modes that have positive or negative bending responses along certain bend directions. Experimental results demonstrate that several lower-order neighboring-cladding mode pairs coupled by the asymmetric TMFBG have antagonistic loss responses (by several dB) for different bending directions, thus allowing full 2D bending measurements with many applications in shape sensing. Finally, this device has similar advantages as tilted FBGs, i.e. temperature de-correlation and the possibility of increasing the signal to noise ratio by averaging simultaneous measurements on several pairs of resonances.

Multifunctional Smart Optical Fibers: Materials, Fabrication, and Sensing Applications

This paper presents a review of the development of optical fibers made of multiple materials, particularly including silica glass, soft glass, polymers, hydrogels, biomaterials, Polydimethylsiloxane (PDMS), and Polyperfluoro-Butenylvinyleth (CYTOP). The properties of the materials are discussed according to their various applications. Typical fabrication techniques for specialty optical fibers based on these materials are introduced, which are mainly focused on extrusion, drilling, and stacking methods depending on the materials’ thermal properties. Microstructures render multiple functions of optical fibers and bring more flexibility in fiber design and device fabrication. In particular, micro-structured optical fibers made from different types of materials are reviewed. The sensing capability of optical fibers enables smart monitoring. Widely used techniques to develop fiber sensors, i.e., fiber Bragg grating and interferometry, are discussed in terms of sensing principles and fabrication methods. Lastly, sensing applications in oil/gas, optofluidics, and particularly healthcare monitoring using specialty optical fibers are demonstrated. In comparison with conventional silica-glass single-mode fiber, state-of-the-art specialty optical fibers provide promising prospects in sensing applications due to flexible choices in materials and microstructures.

Control of higher-order cladding mode excitation with tailored femtosecond-written long period fiber gratings

We report on the detailed investigation of the core to cladding mode coupling in femtosecond-written long period fiber gratings (LPFG). It is shown that the excitation of higher-order cladding modes with strong selectivity and high precision is possible. The coupling behavior of several gratings, as well as its dependence on the modified core cross-section, is determined theoretically and confirmed experimentally by its spectral response. The presented tool paves the way for a completely new class of tailored LPFGs for different fiber integrated devices.

Refractive index sensor in eccentric fiber Bragg gratings using a point-by-point IR femtosecond laser

In this paper, we demonstrate point-by-point inscription of eccentric fiber Bragg grating (EFBG) in conventional single-mode fiber (Corning SMF-28) with second-order Bragg resonances within the C-band using an IR femtosecond laser. Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. EFBGs are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. The inscription conditions to optimize the grating fabrication have been explored to achieve second-order resonances of good spectral quality. Potential applications of EFBG in refractometry sensing are further illustrated. Two interrogation techniques for EFBG in refractometry are reported and experimentally tested.

Higher-order cladding mode excitation of femtosecond-laser-inscribed tilted FBGs

We study the modal behavior of plane-by-plane femtosecond laser fabricated tilted fiber Bragg gratings (FBGs). The focus is on the differential strain and temperature sensitivities between the cladding mode resonances of an nth grating order and those of the (n-i)th orders (with i=1-n), which are collocated in the same wavelength range. Whereas the Bragg mode exhibits an axial strain sensitivity of 1.2 pm/μϵ, we experimentally show that the strain sensitivity of ultrahigh-order cladding modes is negative and at -1.99  pm/μϵ in the same spectral window. Using a finite element mode solver, the modal refractive index value is computed to be well below 1, thus confirming that these modes, in reality, are leaky modes.

Plasmonic Optical Fiber-Grating Immunosensing: A Review

Plasmonic immunosensors are usually made of a noble metal (in the form of a film or nanoparticles) on which bioreceptors are grafted to sense analytes based on the antibody/antigen or other affinity mechanism. Optical fiber configurations are a miniaturized counterpart to the bulky Kretschmann prism and allow easy light injection and remote operation. To excite a surface plasmon (SP), the core-guided light is locally outcoupled. Unclad optical fibers were the first configurations reported to this end. Among the different architectures able to bring light in contact with the surrounding medium, a great quantity of research is today being conducted on metal-coated fiber gratings photo-imprinted in the fiber core, as they provide modal features that enable SP generation at any wavelength, especially in the telecommunication window. They are perfectly suited for use with cost-effective high-resolution interrogators, allowing both a high sensitivity and a low limit of detection to be reached in immunosensing. This paper will review recent progress made in this field with different kinds of gratings: uniform, tilted and eccentric short-period gratings as well as long-period fiber gratings. Practical cases will be reported, showing that such sensors can be used in very small volumes of analytes and even possibly applied to in vivo diagnosis.

Compact eccentric long period grating with improved sensitivity in low refractive index region

We demonstrate a compact eccentric long period grating with enhanced sensitivity in low refractive index region. With a period designed at 15 µm for coupling light to high order cladding modes, the grating is more sensitive to surrounding refractive index in low refractive index region. The intrinsically low coupling coefficients for those high order cladding modes are significantly improved with the eccentric localized inscription induced by the femtosecond laser. The fabricated grating is compact with a length of 4.05 mm, and exhibits an average sensitivity of ~505 nm/RIU in low refractive index region (1.3328-1.3544). The proposed principle can also work in other refractive index region with a proper choice of the resonant cladding modes.

Helical long-period grating manufactured with a CO2 laser on multicore fiber

We have demonstrated a helical long-period grating (HLPG) manufactured from multicore fiber (MCF) combined with twist and CO₂-laser heating technology. The MCF we used behaves as a kind of single mode fiber. The nineteen cores of MCF are distributed hexagonally which make the helical structure only exist in core area after heating and twisting. There are several strong couplings between the core and cladding modes at different wavelengths when the grating pitch was 900 µm. The extinction ratio of the main coupling reached 20 dB at 1550 nm. We have investigated the sensor characteristics of the HLPG. It is found that the grating presents clear sensitivity to refractive-index while exhibiting smaller cross-sensitivity to environmental temperature than conventional long-period gratings. The HLPG also shows great potential as a twist sensor, which has a sensitivity up to 0.198 nm/(rad/m) and can apply for measuring twist directions.

Fabrication of long period fiber gratings of subnanometric bandwidth

This Letter reports on the fabrication of long period fiber gratings having subnanometric bandwidths in the 1500 nm spectral region. Large gratings have been photo inscribed in a high NA fiber; the grating pitch and the order of the HE cladding mode are optimized to produce gratings with a large number of periods and prevent the coupling to TE, TM, or EH modes. Resonances with a FWHM of 0.83 and 0.68 nm have been achieved for gratings 15 and 20 cm long, respectively; the free spectral range between the transmission notches is 125 nm. The polarization effects and the sensitivity of the gratings to temperature and to strain variations are presented as well.

Behavior of femtosecond laser-induced eccentric fiber Bragg gratings at very high temperatures

In this work, eccentric Bragg gratings are photoinscribed in telecommunication-grade optical fibers. They are localized close to the core-cladding interface, yielding strong cladding mode resonance couplings and high photoinduced birefringence. Their transmitted amplitude spectrum is measured with polarized light while they are exposed to temperature changes up to 900°C. Despite the gratings' overall good thermal stability that confirms their robustness for high-temperature refractometry, we report an interesting polarization effect depending on both the cladding mode resonance family and mode order. While the core mode birefringence decreases with growing temperatures, certain cladding mode resonances show an increase in wavelength splitting between their orthogonally polarized components. This differential behavior is of high interest in developing high-resolution, multiparametric sensing platforms.

Polymer-clad silica fibers for tailoring modal area and dispersion

We demonstrate higher-order-mode (A_eff up to ∼2000  μm²) propagation in a 100 μm outer diameter pure-silica fiber with a low-index polymer jacket commonly used for fiber laser pump guidance. This simple structure obviates the need for complex designs deemed necessary for realizing large-mode-area fibers. Modes ranging from HE_1,12 to HE_1,22 were found to propagate stably over 15 m in this fiber. The index step is approximately 4 times larger than that obtained with fluorine down doping; thus the fiber supports even higher-order modes, which may have implications for building rare-earth-doped fiber lasers or achieving enhanced dispersion tunability for high-energy fiber nonlinear phenomena.

Off-axis ultraviolet-written fiber Bragg gratings for directional bending measurements

Off-axis fiber Bragg gratings are inscribed by ultraviolet irradiation limited to expose only a portion of the fiber core cross section. The coupling to cladding modes is significantly increased, and the amplitude of the cladding mode resonances becomes sensitive to bending in magnitude and direction. Sensitivities ranging from +1.17  dB/m(-1) to -1.25  dB/m(-1) were obtained for bending in different directions relative to the offset direction of the grating, for curvatures from 0 to 1.1  m(-1), a range ideal for the shape sensing of large structures. The bending sensor response is also shown to be independent of temperature and the surrounding refractive index.

Surface plasmon resonance in eccentric femtosecond-laser-induced fiber Bragg gratings

Highly localized refractive index modulations are photo-written in the core of pure silica fiber using point-by-point focused UV femtosecond pulses. These specific gratings exhibit a comb-like transmitted amplitude spectrum, with polarization-dependent narrowband cladding mode resonances. In this work, eccentric gratings are surrounded by a gold sheath, allowing the excitation of surface plasmon polaritons (SPP) for radially-polarized light modes. The spectral response is studied as a function of the surrounding refractive index and a maximum sensitivity of 50  nm/RIU (refractive index unit) is reported for a well-defined cladding-mode resonance among the spectral comb. This novel kind of plasmonic fiber grating sensor offers rapidity of production, design flexibility, and high temperature stability.

Detuning in apodized point-by-point fiber Bragg gratings: insights into the grating morphology

Point-by-point (PbP) inscription of fiber Bragg gratings using femtosecond laser pulses is a versatile technique that is currently experiencing significant research interest for fiber laser and sensing applications. The recent demonstration of apodized gratings using this technique provides a new avenue of investigation into the nature of the refractive index perturbation induced by the PbP modifications, as apodized gratings are sensitive to variation in the average background index along the grating. In this work we compare experimental results for Gaussian- and sinc-apodized PbP gratings to a coupled-mode theory model, demonstrating that the refractive index perturbation induced by the PbP modifications has a negative contribution to the average background index which is small, despite the presence of strong reflective coupling. By employing Fourier analysis to a simplified model of an individual modification, we show that the presence of a densified shell around a central void can produce strong reflective coupling with near-zero change in the average background index. This result has important implications for the experimental implementation of apodized PbP gratings, which are of interest for a range of fiber laser and fiber sensing technologies.

Femtosecond laser writing of optical edge filters in fused silica optical waveguides

The positional alignment of femtosecond laser written Bragg grating waveguides within standard and coreless optical fiber has been exploited to vary symmetry and open strong optical coupling to a high density of asymmetric cladding modes. This coupling was further intensified with tight focusing of the laser pulses through an oil-immersion lens to control mode size against an asymmetric refractive index profile. By extending this Bragg grating waveguide writing into bulk fused silica glass, strong coupling to a continuum of radiation-like modes facilitated a significant broadening to over hundreds of nanometers bandwidth that blended into the narrow Bragg resonance to form into a strongly isolating (43 dB) optical edge filter. This Bragg resonance defined exceptionally steep edge slopes of 136 dB/nm and 185 dB/nm for unpolarized and linearly polarized light, respectively, that were tunable through the 1450 nm to 1550 nm telecommunication band.

Polarization-maintaining fiber-optic-grating vector vibroscope

A fiber-optic vector vibroscope based on orthogonal polarization cladding-to-core recoupling is demonstrated. A compact structure in which a short section of polarization-maintained (PM) fiber stub containing a straight fiber Bragg grating (FBG) is spliced to another single-mode fiber. Two well-defined orthogonally polarized cladding modes reflected by the PM-FBG are recoupled at the junction and the coupling intensity shows an extremely high sensitivity to bending in the corresponding orthogonal directions. Both the orientation and amplitude of the vibrations can be determined unambiguously via dual-path power detection of these recoupled orthogonal-polarimetric cladding modes (LP(1,12) and LP(1,13)). Since spectral information is not required, temperature changes do not affect the sensor response, and power fluctuations can be referenced out by monitoring the power in the core mode (LP(0,1)) resonance.