No abstract

Measuring strain without parasitic thermal influence is vital. A temperature compensated strain sensor is fabricated in fused silica using femtosecond laser micromachining. Utilizing femtosecond laser direct writing and femtosecond irradiation followed by chemical etching, two Bragg gratings are fabricated in the bulk of a fused silica substrate. By suspending one of the Bragg gratings in a cantilever, it is mechanically isolated from the rest of the substrate. Thermal and tensile characterization showed that both Bragg gratings are sensitive to thermal changes with a sensitivity around 10.5 pm/ • C, while only the non-isolated Brag grating is sensitive to strain with a sensitivity of 1.1 pm/µϵ. Hence, it is proven that the parasitic thermal influence on the strain sensor can be compensated by taking into account the response of the isolated Bragg grating.

With the increased interest in silicon photonics, smart integration and packaging technologies are essential to transform photonic integrated circuits (PICs) into functional photonic systems. Especially for sensing, the currently existing standard packaging technologies are too expensive and bulky. We developed a solution for integrating a 1 mm x 1 mm sensor PIC with a single mode fiber and packaging it in a 1.5 mm inner-diameter metal protective tube. The concept relies on interfacing a grating coupler with a fiber from the back side of the PIC employing a 300 µm ball lens mounted in a laser-fabricated fused silica precision holder. It is shown that the additional insertion loss caused by the ball lens interface is very limited. A packaged sensor was achieved by sequentially mounting the holder on a ceramic ferrule, then the PIC on the holder and finally gluing a metal tube surrounding the assembly, taking care that the PIC surface is flush with the end face of the tube. The back side fiber interface ensures that the PIC's surface remains accessible for sensing, while the tube protects the fiber-to-PIC interface. We demonstrated this concept by realizing a packaged phase shifted silicon Bragg grating temperature sensor operating around 1550 nm, which could be read out in reflection using a commercial interrogator. A temperature sensitivity of 73 pm/°C was found, and we demonstrated sensor functionality up to 180°C.

V-grooves as passive alignment structures allow for easy and cost-efficient packaging of planar optical sensors. Micronscale tolerances for alignment can be met during fabrication, eliminating the need for accurate alignment tools during assembly. Mechanical stability of the structure is verified with a temperature test.

We propose a simple optical interface for coupling between a single mode fiber and a grating on a Photonic Integrated Circuit (PIC), by coupling from the back side using a ball lens. This way, the top side of the PIC remains accessible for sensing.