VCSELs with two-sided beam emission for pressure sensor applications

A novel type of all-optical pressure sensor has been developed. In this context, a vertical-cavity surface-emitting laser (VCSEL) has been modified in its design to provide simultaneous light emission from both facets. One beam serves as measuring signal while the other establishes a reference; and both paths lie on the same optical axis. The VCSELs are based on active InGaAs quantum wells for laser output close to 960 nm wavelength where the GaAs substrate is transparent. From both top and bottom facet, single-polarization and single-mode beams are observed, having a power ratio of 1:2 to 1:4. In this paper we give insight into this new sensing application for VCSELs, describe the laser fabrication and the static operation characteristics as well as the noise properties which have paramount importance for high performance of the sensor. With regard to the sensor application in acoustics, the focus of the noise measurements is put on the low-frequency, i.e. kHz, regime. While laser diode noise performance is readily available for the MHz to GHz frequency range, only very limited data exists in the Hz to kHz domain. The relative intensity noise of both beams is measured and compared and the mutual correlation properties are investigated. The frequency noise is quantified.