Based on a novel method for overcoming DC drift in RF subcarrier phase detection scheme for fibre optic sensors
in this paper we propose an improved method for the measurement of small displacements and vibrations. The method
works in open loop and is characterized by low distortions in the modulation process, good signal-to-noise ratio and rather
low cost. Considering the receiver ideal, we obtained for the measurements of small distances a minimum of 0.74
a 6.6 × 10<sup>-7</sup> dB dynamic range.
Also, we evaluated the probability density and modelled it vs the phase error for different values of the average
number of photoelectrons generated by the signal and the phase noise parameter. The presented system can be exploited in
the measurement of small distances, vibrations and seismic detection.
We analyze the crosstalk of compliant cylinder type fiber optical accelerometers using 3D finite element analysis. For a standard design we obtain vibrational modes (transversal and torsional) having their eigenfrequencies below the eigenfrequency of the principal mode, i.e. the longitudinal mode vibrating parallel to the axis of axial symmetry. Through the inclusion of two diaphragms we show that for this new topology the principal mode is the fundamental mode. We have constructed such topology and tested it. A responsivity of 2000 rad/g at an optical wavelength of 1550 nm and a crosstalk of -26 dB was achieved, ignoring one resonance.
Optical Coherence Tomography (OCT) is used to perform in vivo high-resolution imaging of biological tissue microstructures. In the present study, we evaluate the optimum conditions to use Avalanche photodiodes (APDs) in OCT to achieve maximum signal-to-noise (S/N) ratios. The optical sources employed in OCT have a large bandwidth.
Due to beating within the source line-width, excess photon noise (EPN) is generated in addition to shot noise (SN). Usually, high speed OCT requires large optical power, which makes the EPN to dominate over the SN. Therefore, balanced detection is used to reduce the EPN. We analyse the optimisation of the OCT configuration with respect to the APD noise performance. When using APDs, another parameter has to be considered in the S/N analysis: that of the
voltage across the APDs, which determines, both, the gain and the noise.
Inhomogeneous and anisotropic materials offer a rich variety of possibilities for constructing novel and efficient
electro-optic and acousto-optic devices. Here we illustrate this by discussing some aspects of the acousto-optics
of TeO<sub>2</sub>, a material having a high degree of acoustic anisotropy, giving a design example. We also present
some recent results concerning the electro-optics and acousto-optics of inhomogeneous structures, in the form of
periodically poled LiNbO<sub>3</sub>.
We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter which combines a piezoelectric transducer array and the acoustic interaction medium in a single lithium niobate wafer
piece. By applying an rf-E<sub>y</sub>-field, a chirped acoustic grating, formed by domain-inversion, excites an X-propagating longitudinal wave which is converted at a mechanically free boundary into a Y-propagating X-polarized slow-shear wave which itself couples collinearly propagating e- and o-polarized optical waves. A relative (absolute) conversion efficiency of 80%/W was measured at 980 nm. Regarding the filter properties the obtained FWHM in the frequency domain reflects the use of the overall length of the device as the acousto-optic interaction medium, whereas the sidelobe suppression is with approximately -3dB higher than the theoretical value of approximately -9 dB. Possible causes of such behavior are discussed and an improved design is suggested.