Waveguide fabrication using femtosecond laser pulses

Abstract

Tightly focused femtosecond (fs) laser pulses can be used to modify the refractive index of glass within the focal region of the laser beam. By scanning the glass with respect to the laser focus waveguide structures can he fabricated inside the glass. This technique holds tremendous potential as a fabrication method for threedimensional all-optical integrated components with applications in the photonics industry. The ability to use this technique with different glass compositions-specifically tailored for a specific photonics application- is critical to its successful exploitation. For example, optical amplification will require a glass with high concentrations of rare-earth ions, such as apbosphate glass, whereas for optical switching a glass with a high nonlinear coefficient is needed. Consequently. it is important to understand bow glass composition effects waveguide fabrication with fs laser pulses and how different glasses are structurally modified after exposure to the fs laser pulses. We have used confocal laser spectroscopy to monitor the changes in glass structure that are associated with waveguide fabrication. Using a low power continuous wave (cw) Ar laser as excitation source we can measure both Raman and fluorescence spectra of the modified regions. Raman spectroscopy provides us with information on the network structure. whereas fluorescence measurements reveal the presence of optically active point defects in the glass. We can also capture microscope images of the modified region using regular white light microscopy. In this presentation we give an overview of OUT work on the fabrication of waveguides inside two typcs of glasses: fused silica and Schon IOG-1, a phosphate glass. Fused silica (Coming 7940) is a common optical material ideal for passive device fabrication. Phosphate glasses, such as Schott IOG-I, are potentially well suited for fabricating activc devices because of the high doping concentrations of rare earth ions allowed in the material. Waveguides were fabricated in these materials by focusing fs laser pulses from an amplified Tisapphire laser system (800 nm wavelength, 130 fs pulse duration, l KHz repetition rate) into the glass through a lox microscope objective. Pulse energies typically ranged from 0.5 to 5 pJ. For fused silica good quality (e IdB/cm loss), single mode (at 633 om) waveguides can be fabricated with a An on the order of IOd. Confocal fluorescence spectroscopy of the waveguide regions shows that nonbridging oxygen hole centers (NBOHC) are formed in the waveguide core. Confocal Raman spectroscopy shows that additional structural changes in the glass network are occuning as a result of waveguide fabrication with fs laser pulses [I .2]. As the fs pulse energy used to modify the glass is increased, the 495 cm and the 605 cm-' peaks, which are due to breathing modes from 4- and 3-membered ring structures in the silica network, increase in intensity. The increase in these small ring structures is associated with densification of the material and this is responsible for the index increase of the glass. For IOG-I phosphate glass the observed waveguide behavior is qualitatively different from that in fused silica. Whereas in fused silica waveguide formation takes place in the focal region of the fs laser beam, in IOG-1 waveguides are not formed in the focal region of the fs laser beam, but rather in regions surrounding the exposed region. Fluorescence imaging shows that non-bridging oxygen hole centers are formed in the central exposed region but not in the waveguide regions. The differences in behavior between fused silica (referred to as Type I behavior) and IOG-l phosphate glass (Type Ill can be understood in terms of the differences in thermo-mechanical behavior and in particular the differences in dependence of density and index on the fictive temperature of the glass.