This paper demonstrates a highly effective method for fibre Bragg grating (FBG) inscription into a seven core fibre (7CF) and its application as an effective vector bending sensor. The development of multicore fibres (MCFs) and enabling the fabrication of FBGs into them result in a solution for multi-parameter measurements such as temperature/strain/bending/twist. The FBG in the central core of 7CF is on the neutral axis and therefore it is sensitive only to thermal and insensitive to deformational change, whereas the FBGs in the none-central cores that are evenly distributed over the 7CF cross-section can facilitate the measurement of structure deformation, such as bending, loading and twist. Furthermore, the FBGs in different cores respond to the physical perturbations differently in various orientations, offering vector sensing to measure both amplitude and direction of the structure change. The 7CF-FBG sensors are highly applicable for mechanical structures and flexible medical instruments.
In this paper, we report a highly effective relative humidity (RH) sensor implemented on graphene oxide (GO) coated long period grating (LPG). The GO nanocolloides bonded onto a cylindrical fibre cladding enables the LPG with strong evanescent waves to absorb more water molecules increasing its RH sensitivity. In an LPG, the phase matching condition occurs when a forward propagating core mode is coupled with the co-propagating lower order cladding modes generating evanescent waves to interact with the surrounding medium. This unique effect of LPGs can be more enhanced with multilayer GO deposition. There is an expansion of GO film with the absorption of more water molecules as RH increases. The absorption of water molecules on GO coating increases the conducting carrier (holes) density on it, thus decreasing the refractive index of GO film. The combined effect of increasing evanescent waves and modulated refractive index makes the GO coated LPGs as effective RH sensors. Our recently achieved results have shown the RH sensitivity of the GO coated LPG is about 0.01 dB/%RH. We have also investigated the effect on GO layer thickness, showing thicker layer increases the RH response of the LPG cladding mode resonances in lower wavelength region.
In this paper, we have theoretically analyzed and experimentally demonstrated the spatial distribution of the radiation mode of 45° tilted fiber grating (TFG). The simulation results have shown the intensity distribution of the radiation mode along the fiber axis exhibited an exponential reduction. In experiment, we have observed the radiation mode pattern of a 5-mm long 45°TFG. The captured profiles along the radial direction and axial direction were in good agreement with the simulated results of the model.
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.
The fabrication of submicron-height sine-like relief of a trifocal diffractive zone plate using a nanoimprinting technique is studied. The zone plate is intended for use in combined trifocal diffractive–refractive lenses and provides the possibility to form trifocal intraocular lenses with predetermined light intensity distribution between foci. The optical properties of the designed zone plate having the optical powers 3 D, 0, −3 D in the three main diffraction orders are theoretically and experimentally investigated. The results of the theoretical investigations are in good agreement with experimental measurements. The effects of the pupil size (lens diameter) as well as the wavelength-dependent behavior of the zone plate are also discussed.
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.
In-fiber microchannels were fabricated directly in standard single mode fiber using the femtosecond laser inscribe and etch technique. This method of creating in-fiber microchannels offers great versatility since it allows complex three dimensional structures to be inscribed and then etched with hydrofluoric acid. Four in-fiber microchannel designs were experimentally investigated using this technique. Device characteristics were evaluated through monitoring the spectral change while inserting index matching oils into each microchannel - a R.I. sensitivity up to 1.55 dB/RIU was achieved. Furthermore, a simple Fabry-Pérot based refractometer with a R.I. sensitivity of 2.75 nm/RIU was also demonstrated.
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.
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 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.
In this paper, we report a simple fibre laser torsion sensor system using an intracavity tilted fibre grating as a torsion
encoded loss filter. When the grating is subjected to twist, it induces loss to the cavity, thus affecting the laser oscillation
build-up time. By measuring the build-up time, both twist direction and angle on the grating can be monitored. Using a
low-cost photodiode and a two-channel digital oscilloscope, we have characterised the torsion sensing capability of this
fibre laser system and obtained a torsion sensitivity of ~412μs/(rad/m) in the dynamic range from -150° to +150°.
We have proposed and demonstrated a fibre laser system using a microchannel as a cavity loss tuning element for
surrounding medium refractive index (SRI) sensing. A ~6μm width microchannel was created by femtosecond (fs) laser
inscription assisted chemical etching in the cavity fibre, which offers a direct access to the external liquids. When the SRI
changes, the microchannel behaves as a loss tuning element, hence modulating the laser cavity loss and output power. The
results indicate that the presented laser sensing system has a linear response to the SRI with a sensitivity in the order of 10-5.
Using higher pump power and more sensitive photodetector, the SRI sensitivity could be further enhanced.
μA microchannel was inscribed in the fibre of a ring cavity which was constructed from two 0.1%:99.9% couplers and a
10m fibre loop. Cavity ring down spectroscopy (CRDS) was used to measure the refractive index (RI) of gels infused
into the microchannel with high resolution. The ring down time discloses a nonlinear increase with respect to the RI of
the gel and sensitivity up to 300μs/RI unit (RIU) and resolution of 5×10-4 were obtained.
The properties of etched large angle tilted gratings (81o) are investigated. The attenuation peaks of the modes are
found to shift to shorter wavelengths at a rate of ~5nm/min in a solution of 10% HF acid. The most sensitive
modes are examined for different etching times creating the relationship that longer etching times results in an
increase in overall sensitivity. Comparing the sensitivity of the tilted fibre grating, etched for 6 hours,
506.9nm/unri and the most sensitive LPG, period 164μm, 389.2nm/unri in the RI range 1.30-1.40 shows that the
produced tilted grating is more sensitive than the LPG and in this range would ideally be suited for the used in
Bio-sensing applications.
We report here the fabrication, charaterisation and refractive index sensing of two microchanneled chirped fiber Bragg
gratings (MCFBGs) with different channel sizes (~550μm and ~1000μm). The chirped grating structures were UV-inscribed
in optical fibre and the microchannels were created in the middle of the CFBGs by femtosecond (fs) laser
assisted chemical etching method. The creation of microchannels in the CFBG structures gives an access to the external
index liquid, thus inducing refractive index (RI) sensitivity to the structure. In comparison with previously reported FBG
based RI sensors, for which the cladding layers usually were removed, the MCFBGs represent a more ideal solution for
robust devices as the microchannel will not degrade the structure strength. The two MCFBGs were spectrally
charaterised for their RI and temperature responses and both gratings exhibited unique thermal and RI sensitivities,
which may be utilised for implementation of bio-chemical sensors with capability to eliminate temperature crosssensitivity.
The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The
principal advantage of high-energy pulses is their ability for grating inscription in any material type without
preprocessing or special core doping. In the field of fibre optical sensing LPGs written in photonic crystal fibre have a
distinct advantage of low temperature sensitivity over gratings written in conventional fibre and thus minimal
temperature cross-sensitivity. Previous studies have indicated that LPGs written by a point-by-point inscription scheme
using a low repetition femtosecond laser exhibit post-fabrication evolution leading to temporal instabilities at room
temperatures with respect to spectral location, strength and birefringence of the attenuation bands. These spectral
instabilities of LPGs are studied in photonic crystal fibres (endlessly single mode microstructure fibre) to moderately
high temperatures 100°C to 200°C and their performance compared to fusion-arc fabricated LPG. Initial results suggest
that the fusion-arc fabricated LPG demonstrate less spectral instability for a given constant and moderate temperature,
and are similar to the results obtained when inscribed in a standard single mode fibre.
Fibre Bragg Grating (FBG) array sensors have been successfully embedded in aluminium alloy matrix by ultrasonic
consolidation (UC) technique. The temperature and loading responses of the embedded FBG arrays have been
systematically characterised. The embedded grating sensors exhibit an average temperature sensitivity of ~36pm/°C,
which is three times higher than that of normal FBGs, and a loading responsivity of ~0.1nm/kg within the dynamic range
from 0kg to 3kg. This initial experiment clearly demonstrates that FBG array sensors can be embedded in metal matrix
together with other passive and active fibres to fabricate smart materials to monitor the operation and health of
engineering structures.
A series of LPGs was inscribed in photonic crystal fibre by a low repetition femtosecond laser system. When subjected
to bending they were found to be spectrally sensitive to bend orientation and displayed a strong polarisation dependence.
A series of LPGs with the same period was inscribed by femtosecond laser into photonic crystal fibre with various
powers. All suffered post-fabrication spectral evolution at low temperatures, apparently related to inscription power.
We report a distinctive polarisation mode coupling behaviour of tilted fibre Bragg gratings (TFBGs) with tilted angle
exceeding 45°. The ex-45° TFBGs exhibit pronounced polarisation mode splitting resulted from grating structure
asymmetry induced birefringence. We have studied and analysed the property of ex-45° TFBGs under transverse load
applied to their equivalent fast- and slow-axis. The results show that the coupling between the orthogonally polarised
modes takes place only when the load is applied to its fast-axis, giving a prominent directional loading response. This
transverse load related polarisation property may be exploitable for implementation of optical fibre vector sensors
capable of measuring the magnitude and orientation of the applied transverse load.
The use of high intensity femtosecond laser sources for inscribing fibre gratings has attained significant interest. The
principal advantage of high-energy pulses is their ability for grating inscription in any material type without pre-processing
or special core doping - the inscription process is controlled multi-photon absorption, void generation and
subsequent local refractive index changes. The formation of grating structures in photonics crystal fibre has proven
difficult, as the presence of holes within the fibre that allow wave-guidance impair and scatter the femtosecond
inscription beam. Here we report on the consistent manufacture of long period gratings in endlessly single mode
microstructure fibre and on their characterisation to external perturbations. Long period gratings are currently the subject
of considerable research interest due to their potential applications as filters and as sensing devices, responsive to strain,
temperature, bending and refractive index. Compared to the more mature fibre Bragg grating sensors, LPGs have more
complex spectra, usually with broader spectral features. On the other hand they are intrinsically sensitive to bending and
refractive index. Perhaps more importantly, the fibre design and choice of grating period can have a considerable
influence over the sensitivity to the various parameters, for example allowing the creation of a bend sensor with minimal
temperature cross-sensitivity. This control is not possible with FBG sensors. Here we compare the effects of symmetric
and asymmetric femtosecond laser inscription.
We report the implementation of a low-cost high-resolution WDM interrogation system operating around 800nm region
with bandwidth up to 60 nm and resolution of 13 pm by utilising a tilted fibre Bragg grating as an out-coupling device
and a CCD-array detector. The system has been evaluated for interrogating fibre Bragg grating based strain, temperature
sensors, giving sensitivities of 0.6 pm/&mgr;&Vegr; and 5.6 pm/°C which are in good agreement with previously reported values.
Furthermore, the system has been utilised to detect refractive index change of sample liquids, demonstrating a capability
of measuring index change as small as 10-5.
An 800nm band fiber Bragg grating sensing interrogation system using TFBG as the core wavelength division
component is presented. A charge coupled device (CCD) linear array is put on the focal plane of the lens to
detect the light. TFBG is used to tap light out of the fiber core to fiber cladding. The sensing wavelength is
795 to 830nm, with accuracy of 20pm and scan speed 100Hz. Using FBG sensor, we achieve the temperature
sensitivity as 1.8°C and strain sensitivity as 18με.
We report the first experimental measurements on the spectral modification of Type IA fibre Bragg gratings, incorporated in an optical network, which result from the use of high-power, near infrared lasers. The fibre grating properties are modified in a controlled manner by exploiting the characteristics of the inherent 1400nm absorption band of the optical fibre, which grows in strength during the Type IA grating inscription. If the fibre network is illuminated with a high power laser, having an emission wavelength coincident with the absorption band, the Type IA centre wavelength and chirp can be modified. Furthermore, partial grating erasure is demonstrated. This has serious implications when using Type IA gratings in an optical network, as their spectrum can be modified using purely optical methods (no external heating source acts on the fibre), and to their long-term stability as the grating is shown to decay. Conversely, suitably stabilised gratings can be spectrally tailored, for tuning fibre lasers or edge filter modification in sensing applications, by purely optical means.
We report experimental findings for tailoring the temperature and strain coefficients of Type I and Type IA fibre Bragg
gratings by influencing the photosensitivity presensitisation of the host optical fibre. It is shown that by controlling the
level of hydrogen saturation, via hot and cold hydrogenation, it is possible to produce gratings with lower thermal
coefficients. Furthermore, there is a larger difference between the Type I and Type IA thermal coefficients and a
significant improvement in the matrix condition number, which impacts the ability to recover accurate temperature and
strain data using the Type1-1A dual grating sensor.
We present an implementation of high-sensitivity optical chemsensors based on FBGs UV-inscribed in D-shape and multimode fibres and sensitized by HF-etching treatment, demonstrating a capability of detecting chemical concentration changes as small as < 0.5%.
We demonstrate a high sensitivity biosensor by fine tailoring mode dispersion and sensitivity of dual-peak LPGs using light-cladding-etching method. The etched device has been used to detect concentration of Hemoglobin protein in sugar solution, showing a sensitivity as high as 20nm/1%.
We report experimental findings for tailoring the temperature and strain coefficients of Type 1 and Type 1A fibre Bragg gratings by influencing the photosensitivity presensitisation of the host optical fibre. It is shown that by controlling the level of hydrogen saturation, via hot and cold hydrogenation, it is possible to produce gratings with lower thermal coefficients. Furthermore, there is a larger difference between the Type 1 and Type 1A thermal coefficients and a significant improvement in the matrix condition number, which impacts the ability to recover accurate temperature and strain invariant data using the Type1-1A dual grating sensor.
We report experimental measurements of reversible wavelength tuning, chirping and permanent thermally induced decay of type1A fibre Bragg gratings using purely optical means. The grating properties are modified in a controlled manner by utilising the characteristics of the intrinsic 1400nm absorption band of the optical fibre and two tuneable lasers coincident with that band. It is shown that the type 1A grating can be selectively tuned in a grating network and that selective tuning can be attained in a common section of suitably prepared optical fibre. Applications of this approach for tuning fibre lasers or edge filter modification in sensing applications are discussed.
A dual-parameter optical sensor has been realized by UV-writing a long-period and a Bragg grating structure in D-fiber. The hybrid configuration permits the detection of the temperature from the latter and measuring the external refractive index from the former responses, respectively. The employment of the D-fiber allows as effective modification and enhancement of the device sensitivity by cladding etching. The grating sensor has been used to measure the concentrations of aqueous sugar solutions, demonstrating the potential capability to detect concentration changes as small as 0.01%.
Type 1A fibre Bragg gratings (FBG) form only after the erasure of a standard grating in hydrogenated germanosilicate fibre, under prolonged UV exposure. They are distinct from other grating types as they exhibit a uniquely large increase in the mean index of the core, readily identifiable by a large red shift in the Bragg wavelength. Type 1A gratings can surpass the conventional mean index change by a factor of six with a typical red shift of up to 20nm, interpreted as a mean index increase of up to 1.9x10-2. Importantly, 1A gratings have been shown to exhibit the lowest temperature coefficient of all FBG, which makes them ideal for use as temperature compensated, dual grating sensors. We report on the formation of Type 1A gratings and the correlation between the mean index change of the grating and the growth of a loss band at close to 1400nm that is associated with the formation of OH centres within the fibre. We present annealing data comparing the decay of Type 1 and Type 1A gratings. Finally, we demonstrate a dual temperature compensated strain sensor system, based on two adjoining Type 1 and Type 1A gratings, which have been formed using a common phase mask, yet with central wavelengths many nm apart.
Point-probe optical fiber chem-sensors have been implemented using cladding etched fiber Bragg gratings. The sensors possess refractive index sensing capability that can be utilized to measure chemical concentrations. The Bragg wavelength shift reaches 8 nm when the index of surrounding medium changes from 1.33 to 1.44, giving maximum sensitivity more than 10 times higher than that of previously reported devices. More importantly, the dual-grating configuration of the point-probe sensors offers a temperature reference function, permitting accurate measurement of refractive index encoded chemical concentrations.
We report a new concept of biochemical sensor device based on long-period grating structures UV-inscribed in D-fiber. The surrounding-medium refractive index sensitivity of the devices has been enhanced significantly by a hydrofluoric acid etching process. The devices have been used to measure the sugar concentrations showing clearly an encoding relation between the chemical concentration and the grating spectral response, demonstrating their capability for potential biochemical sensing applications.
The curvature- or bend-sensing response of long-period gratings (LPG) UV-inscribed in D-shaped fiber has been investigated experimentally. Strong fiber orientation dependence of the spectral response when such LPGs are subjected to dynamic bending has been observed and is shown to form the basis for new vector sensors.
Optical fibre strain sensors using Fibre Bragg Gratings (FBGs) are poised to play a major role in structural health monitoring in a variety of application from aerospace to civil engineering. At the heart of technology is the optoelectronic instrumentation required to convert optical signals into measurands. Users are demanding compact, lightweight, rugged and low cost solutions. This paper describes development of a new device based on a blazed FBG and CCD array that can potentially meet the above demands. We have shown that this very low cost technique may be used to interrogate a WDM array of sensor gratings with highly accurate and highly repeatable results unaffected by the polarisation state of the radiation.
In this paper, we present results showing that sensors may be interrogated with an RMS error of 1.7pm, drift below 0.12pm and dynamic range of up to 65nm.
In this paper dispersion compensation of two WDM channels using two cascaded Chirped fiber gratings (CFGs) fabricated by double exposure method as reported. The power penalty of dispersion compensation of two channels are both less than 1 dB.
We fabricated a bandpass filter based on Moire Bragg grating in fiber with a uniform phase mask. We employed a stretch and two-exposure technique, in which the fiber was exposed to UV light from a KrF excimer through a phase mask and then the fiber is stretched and given another exposure at the same region. Due to the stretch, the periods of these two grating are slightly different, and there is a transmission between two reflection peaks at the Bragg wavelength of these two gratings. Applying different stretch can control the bandpass width of the filter. We measured the stretch characterization of a uniform Bragg grating and found the Bragg wavelength of the grating shifts linearly with the stretched length. We theoretically analyzed the grating structure and its reflection spectrum. The filter’s characteristics can be optimized by choosing appropriate parameters. We will give a theoretical discussion concerning which parameters and how they affect the filter's operation.
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