A femtosecond laser-written monolithic waveguide laser (WGL) oscillator based on a distributed feedback (DFB) architecture and fabricated in ytterbium doped phosphate glass is reported. The device lased at 1033 nm with an output power of 102 mW and a bandwidth less than 2 pm when bidirectionally pumped at 976 nm. The WGL device was stable and operated for 50 hours without degradation. This demonstration of a high performance WGL opens the possibility for creating a variety of narrow-linewidth laser designs in bulk glasses.There is an ever-growing need for integrated optical devices for use in guided wave applications such as communications and sensing. International activity over the past 40 years has led to various technologies being used for the fabrication of such devices: ion exchange [1], silicon-on-insulator [2] and silica-on-silicon [3]. A relatively new fabrication method that also shows good promise in this field is that of ultrafast laser directwriting. It was shown in 1996 that focussed femtosecond laser pulses can induce a permanent refractive index change in dielectric media [4]. The index change is initiated by the electrons in the material absorbing energy from the radiation field via various nonlinear mechanisms. Because the absorption is nonlinear, material modification remains localised at the focal point where the laser intensity is highest. Hence, by translating the material through the focus of a femtosecond laser beam, a pathway of refractive index change can be produced using this technique. In many glasses femtosecond laser irradiation results in an increase in refractive index and hence waveguiding, while in crystalline hosts such as YAG and LiNbO 3 , waveguiding can only be achieved through suppressed cladding arrangements or induced stress fields as the index change is typically negative [5]. By varying the writing geometry, the material and femtosecond laser properties, 2D and 3D optical waveguide devices with different characteristics can be fabricated in the bulk of many transparent materials [6].Ultrafast laser writing has been successfully applied to active materials resulting in the creation of waveguide amplifiers and waveguide lasers (WGLs) [5,[7][8][9][10]. For example, a femtosecond laser encoded distributed feedback (DFB) colour center laser was reported in a LiF crystal operating in a pulsed mode at 704 nm using an optical parametric oscillator (OPO) as a pump [7]. However, the grating structure of this device was incoherent, the linewidth was significantly broader than that normally expected of DFB lasers and no output powers were reported. More recently, bulk glass WGL devices have been demonstrated using external fiber Bragg gratings (FBGs) to complete a cavity around a rare-earth doped waveguide amplifier [8,9]. This configuration is a bulk glass waveguide analogue to a fiber laser and allows for efficient pumping with 1