Abstract
We investigate the third-order nonlinearity of DANS and DAN2 side-chain polymers by tunable third-harmonic generation and nonlinear distributed coupling. The large third-order susceptibility can be interpreted in terms of microscopic cascading of second-order hyperpolarizabilities.

Abstract
A scheme to frequency multiplex a group of sensors based on all-fiber Michelson interferometers is presented. Lead insensitivity is obtained by using the two fiber leads of the configuration as an extra Michelson interferometer whose differential phase is kept constant by active compensation. Topics concerning the system design, sensor sensitivity, and crosstalk between sensors are investigated. Experimental and numerical computational results are presented.

Abstract
We describe a scheme in which a multimode laser diode, with effective coherence length less than 250 µm, is used to illuminate sensors multiplexed in time and with their status read by coherence sensing. Data is presented which shows that the spectral characteristics of the laser light remain essentially unchanged upon direct modulation of the laser injection current or whilst subject to laser feedback. This has allowed the multiplexing scheme network to be implemented without recourse to either a Bragg cell modular or an optical isolator. The technique was applied to measure displacement, strain and temperature. © 1991.

Abstract
A reflective array of optical-fiber sensors multiplexed in time and with their status read using coherence sensing associated with directly modulated multimode laser-diode illumination is investigated. Sensor sensitivity as determined by primary noise sources is evaluated and numerical results are presented. The concept is demonstrated with two all-fiber Michelson interferometers and applied to the measurement of periodic and quasi-static parameters. It is shown that the effect of feedback light into the laser cavity on the level of the system noise floor is negligible, making unnecessary the use of source optical isolation.

Abstract
Distributed grating coupling of light pulses into planar waveguides, exhibiting third-order nonlinear-optical effects, is studied theoretically. The nonlinear interaction taking place between a beam that is Gaussian in time and space and one or more waveguide media with intensity-dependent refractive indexes or absorptions is analyzed, with emphasis on the different effects that result from the relative importance of two-photon absorption, dispersive index changes, and their saturation. Criteria for the evaluation of real and imaginary parts of chi-(3) by means of grating coupling are outlined.

Abstract
We report the experimental evaluation of the intensity-dependent index of refraction of a new class of third order nonlinear materials, side chain polymers containing 4-dialkylamino-4'-nitro-stilbene and 4-dialkylamino-4'-nitro-diphenylbutadiene as side groups. The measurements, based on nonlinear grating coupling into planar waveguides with 30 ps pulses at 1.064-mu-m, showed large electronic nonresonant n2 Kerr coefficients.