A circularizated semiconductor laser diode (CSLD), such as for example a vertical cavity surface emitting laser (VCSEL) may be used for optical measurements. The CSLD may be used in a cell density probe to perform cell density determination and/or turbidity determination, such as in a biotech, fermentation, or other optical absorbance application. The cell density probe may comprise a probe tip section made from a polytetrafluoroethylene material, which provides sealability, ease of manufacture, durability, cleanability, optical semi-transparency at visible and near infrared wavelengths, and other advantages. The probe tip advantageously provides an optical gap that allows for in situ measurements of optical measurements including but not limited to absorbance, scattering, and fluorescence.