Ultra-broadband integrated asymmetric power splitters for the mid-infrared based on refractive-index-engineered femtosecond laser written Y-junction

As the mid-infrared range finds increasing interest in numerous applications, the demand for integrated optical components operating at these wavelengths is becoming ever more critical. In this work, we present a novel method for fabricating 1×2 beam splitters with precisely controlled power distribution among the two output arms. The devices are realized using femtosecond laser writing in chalcogenide glass and the output power balance is tuned by inducing refractive index variations (∆n) of different magnitudes in the splitting region of each arm. The power ratio between the two outputs can be continuously adjusted from 50:50 to 95:5 without introducing additional losses. The components are evaluated at a wavelength of 4.5 µm, but measurements using a supercontinuum source show that the coupling ratio remains constant over an unprecedented spectral range exceeding 1 µm. Finally, these results are extended to three-dimensional components with the fabrication of a 1×6 splitter featuring controlled power distribution across the output arms.