The interaction of a fiber Bragg grating and longitudinal acoustic waves in a highly birefringent suspended-core fiber is investigated for the realization of a multi-wavelength reflection property. The modulated grating couples power from the fast and slow polarization modes to shifted superposed modes supported by the grating. The grating reflectivity of the superposed modes are tuned by the voltage of an electrical signal. Up to five different wavelength reflection peaks have been generated indicating new possibilities for compact and fast fiber-integrated multi-wavelength dynamic filters.
An ytterbium-doped fiber laser mode-locked by the interaction of a fiber Bragg grating and longitudinal acoustic waves in a suspended core fiber is experimentally investigated. An optimized design of an acousto-optic modulator is also proposed. The results indicate output pulses with a width of less than 550 ps at a repetition rate of 10 MHz. The reduction of the power consumed by the transducer and the grating length points out to more efficient, compact and fast acousto-optic modulators for mode-locked all-fiber lasers.
Longitudinal acoustical modes propagating in a fiber are characterized using an extrinsic Fabry-Perot interferometer. The
method allows the detection of the fiber axial deflection peaks which correspond to the acoustic resonances. The method
can be further used for estimating properties of acoustic excited fibers, such as acoustic frequency and wavelength,
which are essential in the design of acousto-optic devices employing Bragg gratings.
Experimental and numerical demonstration of the acousto-optic effect applied in long period grating by means of
flexural waves is presented. The interaction between acoustic and optical waves is modeled with help of the method of
assumed modes, which delivers the strain field inside the grating and the transfer matrix method, which, given the strain
field as input, calculate the resultant grating spectrum. The experimental and theoretical results are found to be in good
agreement. The main effect of the bends in the grating is the break of degeneracy of the circular cladding modes, leading
the attenuation band to be changed. Among all the applications of this methodology, it is important to mention the
possibility of use as a tunable filter, laser cavity gain controller, switching device and transducer in sensing systems.
The development of a fiber extrinsic Fabry-Perot interferometer for measuring vibration amplitude and resonances of
piezoelectric elements is reported. The signal demodulation method based on the use of an optical spectrum analyzer
allows the measurement of displacements and resonances with high resolution. The technique consists basically in
monitoring changes in the intensity or the wavelength of a single interferometric fringe at a point of high sensitivity in
the sensor response curve. For sensor calibration, three signal processing techniques were employed. Vibration
amplitude measurement with 0.84 nm/V sensitivity and the characterization of the piezo resonance is demonstrated.
Acoustic waves, generated by exploiting the acousto-optic effect within silica optical fibres, were used to enhance the
sensitivity of a long period grating. Most of the physical parameters measured by using an LPG involve simple linear and
passive transformations of spectral shifts. However, by adding a temporal element using acoustic waves, parameters that
require dynamic assessment become accessible. In this work we demonstrate the measurement of fluid viscosity by
measuring the rise time and acousto-optic efficiency. We show results of sensor characterization and suggest the
possibility of monitoring a chemical reaction in real time.
A stable, tunable, CW, narrow-linewdith ring distributed feedback (DFB) fibre laser fabricated in erbium-doped photonic
crystal fibre is demonstrated. Wavelength tuning of this device using a piezoelectric transducer was utilized to probe the
P11 molecular absorption band of acetylene (C2H2) with a resolution of 0.45pm. Furthermore, by applying an AC
voltage this absorption band could be continually-swept, allowing square-wave modulation of the DFB output at
repetition rate of 500Hz.
We report some of our recent progress in the area of Bragg grating writing in photonic crystal fibres (PCFs). The various
challenges that PCFs present are discussed and the methods used to overcome these challenges are presented. The
fabrication of highly-durable type-IIa gratings in highly nonlinear photonic crystal fibre is demonstrated, the rotational
variance of grating inscription is also investigated through both experiments and numerical modeling. In other
experiments we fabricate a narrow-linewidth distributed feedback (DFB) laser in erbium-doped PCF, achieving stable,
single-mode and CW operation. The potential of such a DFB PCF in sensing applications is assessed by accurately
measuring an absorption line of acetylene gas.
The measurement of the tuning speed in fiber Bragg gratings is an increasingly important parameter for characterizing the dynamics of
tunable devices. Optical spectrum analyzers and wavelength meters are not suitable due to their slow response for measuring the
wavelength shift in time. In this paper we report on a technique for estimating the tuning speed of a fiber Bragg grating which is based
on accessing the wavelength shift through a calibration curve and measuring the time in which the shift occurs with the help of a fast
photo-detector.
We succeeded in recording broad FBG in standard single-mode fibre optic, using saturated hydrogen loading and over-exposure to the UV beam. The reflection spectra of the obtained gratings presents a FWHM line-width between 3 nm and 10 nm, and peak reflectivity in the range from 20 dB to 25 dB. The dispersion of the gratings is also measured and the dispersion coefficient results values about 2.5 ps/nm, at least 10 times smaller than that of conventional broad gratings recorded with chirped refractive index profiles.
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