Compact packaged silicon photonic Bragg grating sensor based on a ball lens interface

Abstract

We demonstrate a ball-lens based optical interface for coupling between a single mode fiber and a silicon grating coupler from the back side of a photonic integrated circuit (PIC). This allows compact packaging solutions and keeps the PIC top side accessible, e.g. for interacting with the environment in case of sensing applications. Different configurations employing 300 mu m diameter ball lenses made of both LASF-35 glass and sapphire were investigated. Without any modifications to the grating coupler, the peak coupling efficiency (for TE polarization and operating wavelength around 1550 nm) was experimentally found to be only between 1.4 dB and 2.2 dB lower when interfacing with a ball lens from the back side compared to coupling from the top side using a cleaved fiber. Furthermore, no noticeable change in the bandwidth was found. The potential of the ball lens interface was proven by realizing a compact (diameter less than 1.5 mm) optical temperature sensor probe based on a phased shifted silicon photonic Bragg grating structure on a PIC. The probe consisted of a single mode fiber which was terminated with a 1.25 mm ceramic ferrule onto which a ball lens holder and a PIC was glued. The holder was realized in fused silica glass with a femtosecond laser direct-write technology and served to accurately position the ball lens with respect to the fiber and PIC. The assembly allowed to cleanly read out the Bragg grating spectrum in reflection, using a single fiber interface and was compatible with commercial interrogator systems. A Bragg wavelength shift of 73 pm/degrees C was found in a tested temperature range between 15 degrees C and 60 degrees C.