Using finite element analysis (FEA), a model has been constructed to predict the thermo-fluidic and optical properties of a microstructure optical fibre (MOF). The properties under study include external temperature, input water velocity and optical fibre geometry. Under laminar flow the steady-state temperature is dependent on the water channel radius while independent of the input velocity. A critical channel radius is observed below which the steady-state temperature of the water channel is constant, while above, the temperature decreases. The MOF has been found capable of supporting multiple modes whose response to temperature was dominated by the thermo-optic coefficient of glass, despite the larger thermo-optic coefficient of water. This is attributed to the majority of the light being confined within the glass, which increased with increasing external temperature due to a larger difference in the refractive index between the glass core and the water channel.
We have developed a silicon MEMS optical accelerometer in which the motion of the proof mass is mechanically
amplified using a V-beam mechanism prior to transduction. The output motion of the V-beam is detected using a Fabry-Pérot interferometer (FPI) which is interrogated in reflection mode via a single-mode optical fibre. Mechanical
amplification allows the sensitivity of the accelerometer to be increased without compromising the resonant frequency or
measurement bandwidth. We have also devised an all-optical method for calibrating the return signal from the FPI, based
on photothermal actuation of the V-beam structure using fibre-delivered light of a different wavelength. A finite-element
model has been used to predict the relationship between the incident optical power and the cavity length at steady state,
as well as the step response which determines the minimum time for calibration. Prototype devices have been fabricated
with resonant frequencies above 10 kHz and approximately linear response for accelerations in the range 0.01 to 15 g.
We present data on the development a new type of optical fibre polariser and the characterisation of its wavelength
properties. The device is fashioned using a two step process. Firstly, a standard UV long period grating (LPG) with a
period of 330μm is inscribed into hydrogenated SMF-28, followed by femtosecond laser ablation of a groove parallel to
the fibre axis. The UV inscribed LPGs have inherently low birefringence. However, the removal of the cladding layer
parallel to the location of the LPG within the fibre core (as a result the ablation) modifies the cladding modes that couple
with the LPG. Furthermore, the groove breaks the fibre symmetry introducing a non-uniform stress profile across the
fibre cross section leading to significant birefringence. We show that increasing the depth of the groove increases the
birefringence, and this behaviour coupled with the ability to control the wavelength location of the LPGs attenuations
peaks results in a polariser able to operate at almost any wavelength and birefringence. The maximum birefringence
reported here as polarisation mode splitting was approximately 39±0.1nm with a polarisation loss of 10dB.
The distinct behaviour of femtosecond laser inscribed long period gratings, with a non-uniform index perturbation within
the optical fibre core, has been studied experimentally. The non-uniform laser-induced perturbation results in light
coupling from the core mode to a greater number of cladding modes than is the case with their UV laser inscribed
counterparts, and this is made evident from the surrounding refractive index (SRI) grating response. Femtosecond
inscribed long period gratings are shown to simultaneously couple to multiple sets of cladding modes. A 400μm LPG is
shown to result in attenuation peaks that have both blue and red wavelength shifts over a 1250nm to 1700nm wavelength
range. This gives rise to SRI sensitivities far greater than anything achievable by monitoring a single attenuation peak.
The maximum sensitivity produced by monitoring a single attenuation peak was 1106nm/RIU, whereas monitoring
opposing wavelength shifts resulted in a significantly improved sensitivity of 1680nm/RIU.
We demonstrate the development of femtosecond laser inscribed superstructure fiber gratings (fsSFG) in silica optical
fibre. We utilise a single step process, to inscribe low loss and polarisation independent, sampled gratings in optical
fibres using the point by point femtosecond laser inscription method. Our approach results in a controlled modulated
index change with complete suppression of any overlapping LPG structure leading to highly symmetric superstructure
spectra, with the grating reflection well within the Fourier design limit. We also solve Maxwell's equations and calculate
the back reflection spectrum using the bidirectional beam propagation method (BiBPM). Experimental results validate
our numerical analysis and the estimation of inscription parameters such as ac index modulation, wavelength and the
relative peak strength. We also explore how changes in the grating's period influence the reflection spectrum.
A series of surface plasmonic fibre devices were fabricated using multiple coatings deposited on a lapped section of a
single mode fibre and post-fabrication UV laser irradiation processing with a phase mask, producing a surface relief
grating structure. These devices showed high spectral sensitivity in the aqueous index regime ranging up to 4000
nm/RIU for wavelength and 800 dB/RIU for intensity. The devices were then coated with human thrombin binding
aptamer. Several concentrations of thrombin in buffer solution were made, ranging from 1nM to 1μM. All the
concentrations were detectable by the devices demonstrating that sub-nM concentrations may be monitored.
A series of surface plasmonic fibre devices were fabricated using multiple coatings deposited on a lapped section of a
single mode fibre. Guided mode coupling to surface plasmons was promoted following UV laser irradiation of the coated
region through a phase mask, which generated a surface relief grating structure. The devices showed high spectral
sensitivities and strong coupling for low refractive indices as compared to other grating fibre devices. The plasmonic
devices were used to detect the variation in the refractive indices of alkane gases with measured wavelength and
coupling sensitivity to index of 1900 nm/RIU and 4000 dB/RIU, respectively.
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.
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 on the use of thin film coatings, both single and multi-layered, deposited on the flat side of a
lapped, D-shaped fibre to enhance the sensitivity of two kinds of surface plasmon resonance based optical
fibre sensors. The first kind involves the use of a tilted Bragg grating inscribed within the fibre core, prior to
fibre coating, while the second relies on a surface relief grating photoinscribed after the fibre has been coated.
Some of the devices operate in air with high coupling efficiency in excess of 40dB and an estimated index
sensitivity of Δλ/Δn = 90nm from 1 to 1.15 index range showing potential for gas sensing. Other sensors
produced index sensitivities (Δλ/Δn) ranging from 6790nm to 12500nm in the aqueous index regime. The
materials used for these fibre optical devices are germanium, silica, silver, gold and palladium.
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