Carbon nanotubes dispersed in ethanol are aligned by an electric field. Due to the orientation of these elongated particles, the dispersion exhibits anisotropic behaviour.Transmission experiments show rotation of the linear polarisation of an incident laser beam. Alignment and relaxation times and the influence of the magnitude of the electric field have been measured.
Broadband monolithic acousto-optic tunable filters that combine a piezoelectric transducer array and an acousto-optic interaction medium in a single crystal have been investigated. A linearly chirped acoustic superlattice with an optical tuning range of lambda = 1.3-1.6 mum was formed by domain inversion in LiNbO(3) . X-propagating longitudinal acoustic waves are excited in a crossed-field scheme by a rf E(y) field applied to the superlattice and couple collinearly propagating e- and o-polarized optical modes. At mu = 1.319 mum and mu = 1.55 mum the spectral bandwidths (FWHM) were 1.54 and 2.3 nm, respectively. A relative conversion efficiency of 43%/W and a maximum conversion efficiency of 51% were measured at 1.319 mum.
In integrated-optical directional couplers formed by two parallel waveguides we measure the difference Deltabeta = beta(even) - ss(odd) between the propagation constants of the supermodes. We couple them locally by heating a fine spot on one of the guides. When the spot is scanned along the coupler, the output power from one of the guides is found to vary periodically. The period of variation is the modal beat length Lambda = 2pi/Deltabeta. We demonstrate this technique with directional couplers fabricated by K-ion exchange in glass. Beat lengths in the range 0.6 -2.2 mm are measured with an accuracy of +/-0.3%.
We report on an extensometer based on fibre Bragg gratings (FBG) which offers a relatively large and high dynamic range for the monitoring of the relative displacement between two adjacent buildings with individual foundations located in an earthquake-prone area. The sensor consists of a sensing FBG subjected to strain and a reference FBG. The fibre containing the strain-sensing FBG is directly surface mounted onto the walls of both buildings, with the fixations ∼1 m apart. A large range of ±20 mm is achieved by desensitization through a tilt of the fibre axis with respect to the displacement vector (∼6° away from the normal). The sensor is illuminated by a broadband source (SLD with λ0 = 815 nm) and the reflected signal is monitored directly by a CCD-based optical spectrum analyser. The recorded spectra are curve-fitted to two Gaussian distribution functions on a PC. A precision of 36 µm displacement with an integration time of 3 ms has been achieved.
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