Abstract-In this paper we describe Ge-on-Si waveguides and Mach-Zehnder interferometers operating in the 5.2 -5.4 µm wavelength range. 3dB/cm waveguide losses and Mach-Zehnder interferometers with 20dB extinction ratio are presented. Index Terms-mid-infrared, photonic integrated circuits, germanium on silicon
I. INTRODUCTIONThe mid-infrared wavelength range is a region of interest for spectroscopic sensing applications, due to strong and specific absorption features of many molecules in that wavelength range [1]. This allows for a non-contact method to detect the presence and concentration of these molecules. Quantum cascade lasers and inter-band cascade lasers are used as coherent laser sources in this wavelength range. These are typically single-wavelength lasers with limited tuning range. Other options are the use of broadband incoherent thermal sources or coherent supercontinuum sources in combination with a spectrometer. Current instrumentation is bulky, expensive and power hungry, necessitating the need for a midinfrared photonic integration platform to mitigate these disadvantages. Several functions can be implemented on such a platform, such as wavelength multiplexers for arrays of single wavelength QCL lasers allowing to cover a broad wavelength range and the integration of spectrometers for spectral analysis of broadband sources after they passed through a gas or liquid under study. For telecom wavelengths, Silicon-on-Insulator (SOI) is now a standard waveguide platform. One of the main reasons for this is the compatibility with CMOS fabrication technology and the associated potential for large volume manufacturing at low cost. However, beyond 3.8 µm, the underlying buried oxide layer starts absorbing heavily [2]. There are several candidates for viable waveguide platforms for the mid-IR wavelength range beyond 3.8 µm as discussed primarily in [2]. There have been reports in recent literature on Ge-on-Si waveguides [3], free-standing Si waveguides [4] and silicon-on-sapphire waveguides [5]. In this paper we describe the fabrication of Ge-on-Si photonic integrated circuits, an attractive option for mid-IR photonics and present waveguide losses and a key photonic component in wavelength multiplexers and spectrometers: the Mach-Zehnder Interferometer (MZI).