Femtosecond laser fabrication of directional couplers and Mach-Zehnder interferometers

Abstract

The use of femtosecond lasers for photonic device fabrication in glass has become an active area of research in recent years. Since the first demonstration of laser modification of refractive index in glass, a variety of devices such as couplers, interferometers, gratings, 3D structures, active waveguides, and void structures have been successfully demonstrated. In contrast to conventional semiconductor-based fabrication processes, the femtosecond laser fabrication process utilizes a single step, enabling rapid prototyping of a variety of devices, including three-dimensional structures. In order to make progress toward the building of more complex photonic structures, it is important to fabricate reliable couplers and Mach-Zehnder interferometers. To enable telecommunications applications, it is useful to demonstrate their functionality around 1550 nm. We demonstrate the femtosecond laser fabrication of symmetric directional couplers and unbalanced Mach-Zehnder interferometers and the measurement of their wavelength characteristics. We demonstrate an in-depth characterization of the spectral characteristics of symmetric directional couplers and show that they can be tailored by controlling the physical parameters of the device. A wavelength-independent 3dB directional coupler is designed for the wavelength range of 1500 to 1600 nm. We demonstrate high extinction Mach-Zehnder devices over the same wavelength range. The spectral data from a series of unbalanced Mach-Zehnders is used to find the waveguide propagation constant. In addition, the change in spectral behavior of Mach-Zehnders device is used to estimate the dependence of waveguide propagation constant on writing speed. The ability to fabricate couplers and Mach-Zehnder devices with good repeatability and flexibility is an important step toward the design of complex femtosecond laser written integrated devices