7.5-μm wavelength fiber-chip grating couplers for Ge-rich SiGe photonics integrated circuits

The mid infrared (MIR) region, which ranges from 2 μm to 20 μm, has attracted a lot of interest, particularly for novel applications in medical diagnosis, astronomy, chemical and biological sensing or security, to name a few. Most recently, Germanium-rich Silicon Germanium (Ge-rich SiGe) has emerged as a promising waveguide platform to realize complex mid-IR photonic integrated circuits. The Ge-rich SiGe graded buffer benefits from a wide transparency window, strong 3rd order nonlinearity, and the compatibility with mature large-scale fabrication processes, which in turn, paves the way for the development of mid-IR photonic devices that afford improved on-chip functionalities, altogether with compact footprints and cost-effective production. Albeit, low-loss waveguides and wideband Mach-Zehnder interferometers (MZIs) have been recently successfully demonstrated at mid-IR wavelengths, the coupling of light between external access ports, typically optical fibers, and integrated circuits remains challenging. Surface grating couplers provide technologically attractive scenario for light coupling, since they allow flexible placement on the chip, thereby enabling automatic testing of fabricated devices on a wafer-scale, preferred for large-volume developments. In this work, we report two designs for surface grating couplers implemented on the Ge-rich SiGe graded buffer. The grating couplers are designed for transverse electric (TE) and transverse magnetic (TM) polarizations, respectively, both operating at 7.5 μm wavelength. In particular, the TE-designed grating coupler with an inverse taper excitation arrangement yields a coupling efficiency of 6.3% (-12 dB), a 1-dB bandwidth of 300 nm, and reduced back-reflection less than 1%. Furthermore, the TM-designed grating coupler with a conventional taper injection stage predicts an improved coupling performance up to 11% (-9.6 dB), with a 1-dB bandwidth of 310 nm, and only 1% back-reflection. These results open up the way for the realization of complex and multifunctional photonics integrated circuits on Ge-rich SiGe platform with operation at midIR wavelengths.