Analysis of the refractive index change of optical waveguide in LiNbO_3 using a femtosecond laser

摘要

We used a commercially available 75 MHz regeneratively amplified laser system emitting 50 femtosecond pulses of energies up to 3nJ at a wavelength of 800 run. All waveguides were fabricated by focussing the femotsecond pulse train polarised parallel to the x-axis to a distance of approximately 125 μm below the sample surface using a 0.65 NA, ×40 microscope objective and translating the sample along the y axis. To find the optimum waveguide fabrication parameters the translation speed was varied from 2 to 100 μm/s. We introduces a method of measuring the refractive index of optical waveguide in ten micrometer. Useing CCD to measure the two-dimensional near-field light intensity distribution of the output cross-section of the waveguide, by measuring the two-dimensional near-field light intensity distribution of the output cross-section of the waveguide can be calculated the two-dimensional distribution of refractive index of waveguides .The context detailedly gives measurement results about femtosecond laser inducing the near-field intensity of lithium niobate optical waveguide cross-section and calculations of refractive index of optical waveguide. The results show that the refractive index of waveguides showed a large central, gradually reduce and the change of refractive index in the range of 0.001. This method is of great significance to measure the optical waveguide refractive index distribution.