RECONSTRUCTION OF LOW CONTRAST WAVEGUIDE INDEX PROFILE CONSIDERING THE EVANESCENT FIELD

BIASETTI, DEMIAN A.; TORCHIA, GUSTAVO A.; TEJERINA, MATÍAS

Sevilla

Conferencia; EuroMat; 2013

In last years, a substantial amount of works have been published about thereconstruction of waveguide index profile, many of them are based on the inversion ofthe scalar wave equation [1] for TE propagation modes and measuring the near field. Inour work, we also use this equation but we consider the evanescent field as a keyexperimental data to be acquired and analyzed. From this, it is possible to obtain abetter fitting of the real low contrast waveguides index profile, and get more importantinformation allowing us to determine at the same time the effective index and the shapeof the cross guidance section. In particular we investigated waveguides fabricated bydirect writing [2,3] technique using femtosecond laser pulses on fused silica andLiNbO3 crystals.In order to achieve a good fitting of the waveguide?s shape, it is necessary toimprove either the spatial and intensity resolution of the near field. In this sense, westudy the way under which experimental conditions can minimize the error in thenumerical determination of the refractive index profile after processing experimentaldata. So to do that, it is required a CCD camera of ten bit at least to register the nearfield especially due the low SNR in the evanescent field region. Additionally, anadequate optical zoom to register the guided mode in the CCD is also considered in thiswork.Finally, we test a fit algorithm of reconstruction with the simulated data of TEpropagation modes. From this and comparing with real data it seems to be possible, in aself-consistent way, enhance the accuracy of the retrieval information corresponding tothe transversal shape of these kinds of waveguides. This procedure allows us obtainingthe effective and absolute refractive index profile, instead of a relative determination ofthese values, usually reported in the literature.