Abstract: 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.

Abstract: A method for analyzing gas concentration using doubly resonant photoacoustic spectroscopy, and a doubly resonant photaoacoustic gas detector comprising: i) a continuous wave light beam whose wavelength coincides with an absorption wavelength of a gaseous analyte; ii) a closed path optical cavity having at least two reflective surfaces; iii) an acoustic resonator chamber contained within said optical cavity, and comprising an acoustic sensor for detecting sound waves generated by a gaseous analyte present within said chamber, the light beam passing sequentially into, through and out of said chamber, and being repeatedly reflected back and forth through said chamber, and being modulated at a frequency which is equal to or equal to one-half of an acoustic resonance frequency of said acoustic resonator chamber.