A novel device for the detection and characterisation of static magnetic fields is presented. It consists of a femtosecond
laser inscribed fibre Bragg grating (FBG) that is incorporated into an optical fibre with a femtosecond laser micromachined
slot. The symmetry of the fibre is broken by the micro-slot, producing non-uniform strain across the fibre cross
section. The sensing region is coated with Terfenol-D making the device sensitive to static magnetic fields, whereas the
symmetry breaking results in a vectorial sensor for the detection of magnetic fields as low as 0.046 mT with a resolution
of ±0.3mT in transmission and ±0.7mT in reflection. The sensor output is directly wavelength encoded from the FBG
filtering, leading to simple demodulation through the monitoring of wavelength shifts that result as the fibre structure
changes shape in response to the external magnetic field. The use of a femtosecond laser to both inscribe the FBG and
micro-machine the slot in a single stage, prior to coating the device, significantly simplifies the sensor fabrication.
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.
We present to the best of our knowledge the first example of femtosecond laser inscription/ablation of phase/amplitude
masks for the demonstrated purpose of inscribing Bragg gratings in optical fibers. We show that the utilization of a
femtosecond laser for the mask production allows for great flexibility in controlling the mask period. The masks are used
to produce 1st, 2nd and 3rd order fiber Bragg gratings (FBGs) in SMF-28. The work demonstrates the proof of concept and
flexibility for the use of femtosecond lasers for the rapid prototyping of complex and reproducible mask structures. Our
inscription studies are augmented by considerations of three-beam interference effects that occur as a result of the strong
zeroth-order component that is present in addition to higher-order diffraction components.
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.
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.
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