The realization of a DNA biosensor based on Double Tilted Fiber Bragg Grating (DTFBG) for label-free detection has
been demonstrated. To our knowledge this is the first time that a biosensor has been realized with such kind of device.
The surface of the optical fiber has been functionalized with peptide nucleic acid (PNA) in order to capture DNA
strands. The changes of the interference fringes visibility of the grating, due to the PNA-DNA binding, proved the
occurred fiber hybridization. The re-use of the fiber for multiple measurements and the selectivity of the sensor have
been also investigated.
We present recent work and progress on Spectrally Coded Multiplexing (SCM). SCM as a generic multiplexing
technique adds new dimension of freedom and provides additional flexibility and channel capability needed for fibre and
fibre grating based sensor systems. We show a few examples of our newly developed SCM techniques based on
specially designed fibre gratings.
Various Moiré Long Period Gratings are fabricated and their thermal properties are measured. The occurrence of an anomalous switching effect between two thermal response curves is noted to occur at a certain wavelength. A deviation between them by as much as 1nm is observed. As the effect is stable and repeatable it offers the potential for various novel applications in the fields of sensing and optical logic.
A theoretical analysis of multiple scattering is undertaken. The effect it has upon systems that do not take multiple
scattering events or spectral shadowing into account through their signal processing routines is examined. An exact first
order compensation scheme and higher order non-exact compensation schemes are developed. A comparison is made
between the system performance that is obtained from the uncorrected, first-order corrected and higher-order corrected
systems. The comparison makes use of a worst-case time-independent scenario.
The composite cavity fibre laser (CCFL) is demonstrated for its capability in maintaining single longitudinal mode
operation, whilst having longer cavity length than typical distributed Bragg reflector or distributed feedback fibre lasers,
and hence also higher output power. These two attributes should enable the CCFL to be useful in sensing applications. A
long cavity length CCFL can be fabricated simply by writing three spectrally matched Bragg gratings directly into a
continuous length of doped fibre. This is analogous to the use of feedback cavities in semiconductor laser designs to
maintain single longitudinal mode. Results from in-house fabrications show that long cavity length CCFLs can be
fabricated to have single longitudinal mode and narrow linewidth characteristics similar to that of a distributed feedback
fibre laser, and also significantly higher output power.
An in-line fibre ring cavity is fabricated by writing two blazed gratings next to each other to form a Fabry Perot cavity. A
visibility of fringes as good as 0.032 in the reflection spectrum and 0.76 for transmission is obtained for the interference
between the forward propagating guided mode and the reverse propagating ghost mode of the blazed grating. The ability
to measure the external refractive index and the variability of this response with cavity length is demonstrated.
A multiplexing technique using amplitude-modulated chirped fibre Bragg gratings (AMCFBGs) is presented. This
technique realises the multiplexing of spectrally overlapped AMCFBGs with identical centre Bragg wavelength and
bandwidth. Since it is fully compatible with the wavelength division multiplexing scheme, the number of gratings that
can be multiplexed can be increased by several times. The discrete wavelet transform is used to demodulate such
multiplexed signal. A wavelet denoising technique is applied to the multiplexed signal in conjunction with the
demodulation. Strain measurements are performed to experimentally demonstrate the feasibility of this multiplexing
technique. The absolute error and crosstalk are measured. An application to simultaneous two-parameter sensing is also
demonstrated.
In order to increase the number of channels available to a grating based strain sensor system specialised gratings were designed that would allow the sensor system to be able to distinguish between a number of gratings located in the one WDM channel independently of the amount by which they overlap each other in the wavelength domain. Distinguishing between gratings is achieved by inscribing a carrier frequency in the grating spectrum, so that each grating can be addressed in the Fourier domain via the spectral information centred around the inscribed carrier frequency. Tests performed on the gratings successfully show the ability to distinguish between three spectrally overlapping gratings. The calculated value for Young's modulus, 72+/-3GPa, was found to be in keeping with the standard value of 70.3GPa for fused silica.
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