We propose a simple distributed sensor with millimeter-order spatial resolution using a multichannel fiber Bragg grating (FBG). The multichannel FBG was designed by using the layer peeling (LP) algorithm with a tailored group delay characteristic, in which each channel is spatially separated. The multichannel FBG can be easily used in a low cost WDM sensing system for distributed sensing with millimeter-order spatial resolution.
We demonstrated a compact stable room-temperature multiwavelength erbium doped fiber laser by employing a 45° tilted fiber gratings (TFGs) based all-fiber polarization interference filter. Benefiting from the filter, the channel number, the linewidth, the uniformity and stabilization of the multiwavelength laser were greatly improved. The filter also worked as a polarizing functional device in nonlinear polarization rotation leading to multiwavelength operation. More than 60 wavelengths (within 3dB bandwidth) lasing with a linewidth of 0.03nm and a signal-to-noise ratio of 31dB were obtained. The wavelength spacing was 0.164nm agreeing with the value of the filter and it can be flexibly controlled by adjusting the length of the filter.
Enzyme-functionalized dual-peak long-period fiber grating (LPFG) inscribed in 80-μm-cladding B/Ge codoped single-mode fiber is presented for sugar-level and specific glucose detection. Before enzyme functionalization, the dual-peak LPFG was employed for refractive index sensing and sugar-level detection and high sensitivities of ∼4298.20 nm/RIU and 4.6696 nm/% were obtained, respectively. Glucose detection probe was attained by surface functionalization of the dual-peak LPFG via covalent binding with aminopropyl triethoxysilane used as a binding site. Optical micrographs confirmed the presence of enzyme. The surface-functionalized dual-peak LPFG was tested with D-(+)-glucose solution of different concentrations. While the peak 2 at the longer wavelength was suitable only to measure lower glucose concentration (0.1 to 1.6 mg/ml) recording a high sensitivity of 12.21±0.19 nm/(mg/ml), the peak 1 at the shorter wavelength was able to measure a wider range of glucose concentrations (0.1 to 3.2 mg/ml) exhibiting a maximum resonance wavelength shift of 7.12±0.12 nm/mg/ml. The enzyme-functionalized dual-peak LPFG has the advantage of direct inscription of highly sensitive grating structures in thin-cladding fibre without etching, and most significantly, its sensitivity improvement of approximately one order of magnitude higher than previously reported LPFG and excessively tilted fibre grating (Ex-TFG) for glucose detection.
We propose and demonstrate, for the first time to our best knowledge, the use of a 45° tilted fiber grating (TFG) as an infiber lateral diffraction element in an efficient and fiber-compatible spectrally encoded imaging (SEI) system. Under proper polarization control, the TFG has significantly enhanced diffraction efficiency (93.5%) due to strong tilted reflection. Our conceptually new fiber-topics-based design eliminates the need for bulky and lossy free-space diffraction gratings, significantly reduces the volume and cost of the imaging system, improves energy efficiency, and increases system stability. As a proof-of-principle experiment, we use the proposed system to perform an one dimensional (1D) line scan imaging of a customer-designed three-slot sample and the results show that the constructed image matches well with the actual sample. The angular dispersion of the 45° TFG is measured to be 0.054°/nm and the lateral resolution of the SEI system is measured to be 28 μm in our experiment.
This paper presents a highly sensitive ambient refractive index (RI) sensor based on 81° tilted fiber grating (81°-TFG) structure UV-inscribed in standard telecom fiber (62.5μm cladding radius) with carbon nanotube (CNT) overlay deposition. The sensing mechanism is based on the ability of CNT to induce change in transmitted optical power and the high sensitivity of 81°-TFG to ambient refractive index. The thin CNT film with high refractive index enhances the cladding modes of the TFG, resulting in the significant interaction between the propagating light and the surrounding medium. Consequently, the surrounding RI change will induce not only the resonant wavelength shift but also the power intensity change of the attenuation band in the transmission spectrum. Result shows that the change in transmitted optical power produces a corresponding linear reduction in intensity with increment in RI values. The sample shows high sensitivities of ~207.38nm/RIU, ~241.79nm/RIU at RI range 1.344–1.374 and ~113.09nm/RIU, ~144.40nm/RIU at RI range 1.374–1.392 (for X-pol and Y-pol respectively). It also shows power intensity sensitivity of ~ 65.728dBm/RIU and ~ 45.898 (for X-pol and Y-pol respectively). The low thermal sensitivity property of the 81°-TFG offers reduction in thermal cross-sensitivity and enhances specificity of the sensor.
Due to the limitation of the lens effect of the optical fibre and the inhomogeneity of the laser fluence on different cores, it is still challenging to controllably inscribe different fibre Bragg gratings (FBGs) in multicore fibres. In this article, we reported the FBG inscription in four core fibres (FCFs), whose cores are arranged in the corners of a square lattice. By investigating the influence of different inscription conditions during inscription, different results, such as simultaneous inscription of all cores, selectively inscription of individual or two cores, and even double scanning in perpendicular core couples by diagonal, are achieved. The phase mask scanning method, consisting of a 244nm Argon-ion frequencydoubled laser, air-bearing linear transfer stage and cylindrical lens and mirror setup, is used to precisely control the grating inscription in FCFs. The influence of three factors is systematically investigated to overcome the limitations, and they are the defocusing length between the cylindrical lens and the bare fibre, the rotation geometry of the fibre to the irritation beam, and the relative position of the fibre in the vertical direction of the laser beam.
A polymer-core/silica-cladding hybrid optical fiber is implemented by filling a capillary with UV-curable epoxy and a following UV-laser scanning exposure. A fiber Bragg grating is successfully inscribed in parallel using a phase mask. The experimental results show a reduced thermal response for the FBG and a theoretical analysis for such a hybrid optical fiber is performed which corroborates existing of a turning temperature for minimized thermal response.
Cardiovascular health of the human population is a major concern for medical clinicians, with cardiovascular diseases responsible for 48% of all deaths worldwide, according to the World Health Organisation. Therefore the development of new practicable and economical diagnostic tools to scrutinise the cardiovascular health of humans is a major driver for clinicians. We offer a new technique to obtain seismocardiographic signals covering both ballistocardiography (below 20Hz) and audible heart sounds (20Hz upwards). The detection scheme is based upon an array of curvature/displacement sensors using fibre optic long period gratings interrogated using a variation of the derivative spectroscopy interrogation technique
We have experimentally demonstrated an active loading sensor system based on a fiber ring laser with single-polarization output using an intra-cavity 45°-tilted fiber grating (45°-TFG). When the laser cavity fiber subjected to loading, the laser output is encoded with the load and can be measured and monitored by a power metre. A loading sensitivity as high as 0.033/ (kg•m-1) has been achieved using this laser. The experiment results clearly show that single polarization fiber laser may be developed to a low-cost high-sensitivity loading sensor system.
Multiwavelength lasing in the random distributed feedback fiber laser is demonstrated by employing an all fiber Lyot filter. Stable multiwavelength generation is obtained, with each line exhibiting sub-nanometer line-widths. A flat power distribution over multiple lines is also obtained, which indicates the contribution of nonlinear wave mixing towards power redistribution and equalization in the system. The multiwavelength generation is observed simultaneously in first and second Stokes waves.
We propose and demonstrate a technique for monitoring the recovery deformation of the shape-memory polymers (SMP) using a surface-attached fiber Bragg grating (FBG) as a vector-bending sensor. The proposed sensing scheme could monitor the pure bending deformation for the SMP sample. When the SMP sample undergoes concave or convex bending, the resonance wavelength of the FBG will have red-shift or blue-shift according to the tensile or compressive stress gradient along the FBG. As the results show, the bending sensitivity is around 4.07 nm/cm −1 . The experimental results clearly indicate that the deformation of such an SMP sample can be effectively monitored by the attached FBG not just for the bending curvature but also the bending direction.
Recently, we have extended fibre grating devices in to mid-IR range. Fibre Bragg gratings (FBGs) and long-period
gratings (LPGs) with spectral responses from near-IR (800nm) to mid-IR ( ~ 2μm) have been demonstrated with
transmission loss as strong as 10-20dB. 2μm FBG and LPG showed temperature and refractive index (RI) sensitivities
of ~ 91pm/°C and 357nm/RIU respectively. Finally, we have performed a bio sensing experiment by monitoring the
degradation of foetal bovine serum at room temperature. The results encouragingly show that the mid-IR LPGs can be an
ideal biosensor platform as they have high RI sensitivity and can be used to detect concentration change of bio- samples.
We present femtosecond laser inscribed phase masks for the inscription of Bragg gratings in optical fibres. The principal
advantage is the flexibility afforded by the femtosecond laser inscription, where sub-surface structures define the phase
mask period and mask properties. The masks are used to produce fibre Bragg gratings having different orders according
to the phase mask period. The work demonstrates the incredible flexibility of femtosecond lasers for the rapid
prototyping of complex and reproducible mask structures. We also consider three-beam interference effects, a
consequence of the zeroth-order component present in addition to higher-order diffraction components.
We demonstrate highly sensitive temperature and strain sensors based on an all-fiber Lyot filter structure, which is
formed by concatenating two 45°-TFGs (tilted fiber gratings) with a PM fiber cavity. The experiment results show
the all-fiber 45°-TFG Lyot filter has very high sensitivity to strain and temperature. The 45°-TFG Lyot filters of two
different cavity lengths (18cm and 40 cm) have been evaluated for temperature sensing by heating a section of the
cavity from 10°C to 50°C. The experiment results have shown remarkably high temperature sensitivities of
0.616nm/°C for 18cm and 0.31nm/°C for 40cm long cavity filter, respectively. The 18cm long cavity filter has been
subjected to strain variations up to around 550με and the filter has exhibited strain sensitivities of 0.02499nm/με and
0.012nm/με for two straining situations, where its cavity middle section of 18cm and 9cm were stretched,
respectively.
We report a linear response optical refractive index (RI) sensor, which is fabricated based on a
micro-channel created within a Fabry Perot (F-P) cavity by chemical etching assisted by femtosecond
laser inscription. The experimental results show the F-P resonance peak has a linear response with the
RI of medium and the measuring sensitivity is proportion to the length of micro-channel. The sensor
with 5 μm -long micro-channel exhibited an RI sensitivity of 1.15nm/RIU and this sensitivity increased
to 9.08nm/RIU when widening the micro-channel to 35μm. Furthermore, such micro-channel FP
sensors show a much broader RI sensing dynamic range (from 1.3 to 1.7) than other reported optical
fiber sensors.
We present a compact, portable and low cost generic interrogation strain sensor system using a fibre Bragg grating
configured in transmission mode with a vertical-cavity surface-emitting laser (VCSEL) light source and a GaAs
photodetector embedded in a polymer skin. The photocurrent value is read and stored by a microcontroller. In addition,
the photocurrent data is sent via Bluetooth to a computer or tablet device that can present the live data in a real time
graph. With a matched grating and VCSEL, the system is able to automatically scan and lock the VCSEL to the most
sensitive edge of the grating. Commercially available VCSEL and photodetector chips are thinned down to 20 μm and
integrated in an ultra-thin flexible optical foil using several thin film deposition steps. A dedicated micro mirror plug is
fabricated to couple the driving optoelectronics to the fibre sensors. The resulting optoelectronic package can be
embedded in a thin, planar sensing sheet and the host material for this sheet is a flexible and stretchable polymer. The
result is a fully embedded fibre sensing system - a photonic skin. Further investigations are currently being carried out to
determine the stability and robustness of the embedded optoelectronic components.
Fiber Bragg gratings can be used for monitoring different parameters in a wide variety of materials and constructions.
The interrogation of fiber Bragg gratings traditionally consists of an expensive and spacious peak tracking or spectrum
analyzing unit which needs to be deployed outside the monitored structure. We present a dynamic low-cost interrogation
system for fiber Bragg gratings which can be integrated with the fiber itself, limiting the fragile optical in- and outcoupling
interfaces and providing a compact, unobtrusive driving and read-out unit. The reported system is based on an
embedded Vertical Cavity Surface Emitting Laser (VCSEL) which is tuned dynamically at 1 kHz and an embedded
photodiode. Fiber coupling is provided through a dedicated 45° micromirror yielding a 90° in-the-plane coupling and
limiting the total thickness of the fiber coupled optoelectronic package to 550 μm. The red-shift of the VCSEL
wavelength is providing a full reconstruction of the spectrum with a range of 2.5 nm. A few-mode fiber with fiber Bragg
gratings at 850 nm is used to prove the feasibility of this low-cost and ultra-compact interrogation approach.
We report the fabrication of a refractive index (RI) sensor based on a liquid core fibre Bragg grating (FBG). A micro-slot
FBG was created in standard telecom optical fibre employing the tightly focused femtosecond laser inscription aided
chemical etching. A micro-slot with dimensions of 5.74(h) × 125(w) × 1388.72(l) μm was engraved across the whole fibre
and along 1mm long FBG which gives advantage of a relatively robust liquid core waveguide. The device performed the
refractive index sensitivity up to about 742.72 nm/RIU.
We experimentally demonstrated a highly sensitive twist sensor system based on a 45° and an 81° tilted fibre grating
(TFG). The 81°-TFG has a set of dual-peaks that are due to the birefringence induced by its extremely tilted structure.
When the 81°-TFG subjected to twist, the coupling to the two peaks would interchange from each other, providing a
mechanism to measure and monitor the twist. We have investigated the performance of the sensor system by three
interrogation methods (spectral, power-measurement and voltage-measurement). The experimental results clearly show
that the 81°-TFG and the 45°-TFG could be combined forming a full fibre twist sensor system capable of not just
measuring the magnitude but also recognising the direction of the applied twist.
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