Femtosecond Fabrication of Waveguides in Ion-Doped Laser Crystals

Abstract

Permanent refractive index change (PRICE) in dielectrics by means of the femtosecond laser pulses is a novel enabling technology in photonics. A wide range of photonic structures manufactured using this method has been demonstrated in glasses and crystals since the first observation of the underling phenomenon (Davis, 1996). While numerous waveguides, waveguide lasers and amplifiers, couplers and Bragg gratings were fabricated on the basis of this phenomenon, PRICE theory is far from accomplishment yet, although some basic principles are beyond any doubt. To date it is obvious that to understand femtosecond modification of a transparent dielectric the process should be separated in two stages. The first stage consists of the non-linear absorption of a femtosecond pulse and electron plasma generation. The second stage consists of energy transfer from the electron plasma to ions and structural changes in a dielectric. The first stage seems to be very analogous both in glasses and crystals, as it deals with electronic excitation and only material parameters required for its description are energy gap width and coefficients of multiphonon absorption (MPA). The second stage still rises many questions and should be considered as an incomplete chapter in PRICE theory. To date it is not clear whether PRICE proceeds in the same manner both in crystals and glasses. It is generally understood for glasses and associated with melting and densification (Glezer, 1997; Streltsov & Borelli, 2001). As a rule it gives positive refractive index change in the exposed region, and its magnitude can be as high as 10-2 (Allsop, 2010). Thus a straightforward way for waveguide inscription is open for glasses. Contrary to glasses PRICE is rather more complicated and intrigued in crystals. As a rule refractive index change is negative in the exposed region. For example, a widely accepted point of view relays on the assumption that a crystal undergoes amorphisation in the exposed region, and this causes stresses and positive refractive index change in the surrounding area (Gorelik, 2003; Apostolopoulos, 2004). Thus a waveguide is usually created in the area adjacent to tracks written by femtosecond beam. Since in this case waveguiding is due to an indirect effect accompanying femtosecond modification of crystal lattice, magnitude of refractive index change in the waveguide is not so high as in glass waveguides and does not exceed 1*10-3 (Nejadmalayeri, 2005; Torchia, 2008; Siebenmorgen, 2009; Silva, 2010; Bookey, 2007; Burghoff, 2007). This value is not enough to build compact waveguide lasers with diode pumping. Meantime in many cases crystals are more a attractive media for femtosecond fabrication of compact waveguide lasers in comparison to glasses, because they have better thermo-conductivity, high optical damage threshold and