A compact, fiber-based spectrometer for biomedical application utilizing a tilted fiber Bragg grating (TFBG) as
integrated dispersive element is demonstrated. Based on a 45° UV-written PS750 TFBG a refractive spectrometer with
2.06 radiant/μm dispersion and a numerical aperture of 0.1 was set up and tested as integrated detector for an optical
coherence tomography (OCT) system. Featuring a 23 mm long active region at the fiber the spectrum is projected via a
cylindrical lens for vertical beam collimation and focused by an achromatic doublet onto the detector array. Covering
740 nm to 860 nm the spectrometer was optically connected to a broadband white light interferometer and a wide field
scan head and electronically to an acquisition and control computer. Tomograms of ophthalmic and dermal samples
obtained by the frequency domain OCT-system were obtained achieving 2.84 μm axial and 7.6 μm lateral resolution.
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.
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 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 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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