Amplification Properties of Femtosecond Laser-Written Er3+/Yb3+ Doped Waveguides in a Tellurium-Zinc Glass

We report on the fabrication and characterization of active waveguides in a TeO2-ZnO glass sample doped with Er3+/Yb3+ fabricated by direct laser writing with a femtosecond laser delivering 150？fs pulses at 1？kHz repetition rate. The waveguides exhibit an internal gain of 0.6？dB/cm at 1535？nm, thus demonstrating the feasibility of active photonics lightwave circuits and lossless components in such a glass composition. 1. Introduction In the recent years, direct laser writing of waveguides and photonic lightwave circuits in glass and crystals has become a potential alternative to conventional fabrication methods such as ion exchange and lithography [1, 2]. The most prominent demonstrations of integrated optical devices for telecom applications have been realized by means of direct ultraviolet (UV) writing, exploiting UV coherent radiation to induce a permanent refractive index change into a glass substrate [3, 4]. This method has proved to be reliable and capable of competing with clean room processing in terms of waveguide loss, with the inherent advantage that laser writing enables fast prototyping and requires much lower ownership costs of fabrication. However, its widespread application has been hampered by the complexity of the writing procedure that requires plasma enhanced chemical vapor deposition (PECVD) glass substrates and hydrogen loading to enhance the photosensitivity. Furthermore, UV writing in bulk glass has been cumbersome and limited to two-dimensional structures. An extensive study of femtosecond (fs) laser pulses to locally modify the structure and refractive properties of optical glasses and other dielectrics via nonlinear absorption has been conducted in the recent years [5]. The mechanism undergoing this optically induced change is still a hot research topic, though it is believed that the process is triggered by a rapid absorption of the pulse energy through nonlinear excitation mechanisms [6]. Direct laser writing of optical waveguides and photonic lightwave circuits is currently one of the most widely studied applications of femtosecond (fs) laser micromachining in transparent dielectrics. Moreover, direct laser writing opens up new routes in fabrication of three-dimensional (3D) waveguides inside transparent glass substrates, which is otherwise impossible by conventional ion exchange and photolithographic processes [7]. Channel waveguides written using ultrafast lasers in erbium (Er) doped glasses and ytterbium/erbium (Yb/Er)-codoped glasses for integrated amplifiers and lasers operating in the C-band (1530–1565？nm) have been