Experimental Study Of Femtosecond Laser Writing Of Optical Waveguides

As a kind of novel waveguide writing technology, femtosecond laser writing technology has been developed dramatically in the past ten years, and is characterized by simple, flexible, efficient, and low cost, which shows obvious advantage over traditional writing technology such as ion exchange, ion implantation, and chemical vapor deposition. Femtosecond laser writing technology is capable of fabricating photonic chips such as photonic lanterns, 3D filter, 3D coupler, etc. Additionally, it has a wide application prospect in the aspects of making integrated photonic devices of high density, due to the interaction between femtosecond pulse laser and materials to make three-dimensional photonic devices in bulk materials. Thus, this technology creates a new field, i.e., astronomy photonics. The main work of this paper includes the following aspects:1. Waveguides consisting in double line structure are written in Ti:Sapphire crystal in transverse scheme by a femtosecond laser with a repetition rate of 1 k Hz, pulse duration of 120 fs, and wavelength of 800 nm. The dependence of the processing conditions, i.e., inscribing depth, writing speed and the track distance on the waveguide formation are systematically studied in experiment. The waveguide with a good guiding mode is achieved with pulse energy of 2 μJ, inscribing depth of 175 μm, writing speed of 90 μm/s and track distance of 26 μm. And the double line waveguide shows polarization dependent optical guiding, benefiting from the production of nano-level stripe during waveguide writing process. Based on the near-field mode intensity, the refractive index profile of the optical waveguide is reconstructed, and the maximum positive refractive index change is about 1.9×10-4. In addition, the propagation loss of the waveguide is 1.82 d B/cm, which is measured by the scattering technique,2. By near-infrared femtosecond laser with repetition rate of 1 k Hz, central wavelength of 800 nm and pulse width of 120 fs, sing line(Type I) and depressed circular cladding waveguides are transversely written for the first time in Er3+ doped fluorotellurite glass with slit shaping technology. Based on the near-field mode intensity, the refractive index profile of the single line and depressed circular cladding waveguides are reconstructed, respectively. The largest positive refractive index change for Type I waveguide is about 1×10-4, slightly smaller than that(1.9×10-4) of depressed cladding waveguide. The propagation loss of the single line waveguide is about 1.04 d B/cm. The loss of the depressed circular cladding waveguide is less than 1.88 d B/cm. By experimentally contrasting fluorescence spectra between host material and guiding area of two kinds of waveguides, we can conclude that the action of femtosecond laser almost has no an impact on fluorescence properties.