Photonic Incremental Pressure Sensor Based on Optical Feedback in a Polymer Embedded VCSEL

A highly accurate integrated incremental pressure sensor is presented based on optical feedback in a vertical-cavity surface-emitting laser (VCSEL). This laser chip is embedded in a polymer host material and an external cavity, consisting of a compressible transducer material and a reflecting layer, is fabricated on top. The reflecting layer is coupling part of the emitted laser light back into the internal VCSEL cavity causing self-mixing interferometry. By applying pressure and consequently changing the external cavity length, this interference signal adopts a periodic shape corresponding to half the VCSEL wavelength. The use of unpackaged VCSELs limits the sensor dimensions and minimizes the distance between two adjacent sensing points. A proof-of-principle setup is developed and the integrated sensing principle has been demonstrated using a polydimethylsiloxane transducer layer. A 850-nm VCSEL is used and forces up to 300 mN are applied resulting in a 2-mV peak-to-peak variation of the electrical driving voltage.