We report a study on the thermo optical properties of some nonlinear materials using the z scan technique. Both open aperture and closed aperture z scan transmittance signals were recorded to study the refractive and absorptive nonlinearities. The samples chosen were metal phthalocyanines viz CoPc, NiPc, ZnPc and CuPc. Dimethyl Formamide (DMF) and Dimethyl Sulphoxide (DMSO) were used as solvents. The laser source was the Q switched Nd: YAG with 10 Hz repetition rate and a pulse width of 7 ns. The Q switched envelope of the pulses give negative nonlinearity in all these samples which can be due to either thermal lens effect or electronic transitions among triplet states. Usually the thermal lens effect is neglected in z scan signals recorded using high repetition rate pulses. In our work, we have also carried out dual beam thermal lens measurements inside the sample, using a low power He-Ne as probe. We used the theory of the thermal lens formalism and also that of the Kerr type nonlinearities to interpret the obtained data. These studies yield the nonlinear optical constants and also the thermo optic constants of the samples under investigation.
In this work, we report the influence of ambient refractive index changes on the output power of a multimode fiber with long period grating (LPG) written on it. It is seen experimentally that the cladding mode profiles of such a LPG sensor is very sensitive to the change in refractive index of the surrounding medium. In order to study the influence of ambient refractive index, we have utilized LPG with different periods (100 micrometer, 250 micrometer and 500 micrometer) and multimode fibers with different core diameter (200 micrometer and 400 micrometer). The variation in the output power as a function of different period and fiber dimensions is also investigated. In addition to the many advantages of optical sensors, the unique features such as low insertion loss, low back reflection and easy fabrication of LPG sensors make the present measurement system simple, reliable and accurate for measuring refractive index of the medium surrounding the cladding.