LIQUID DETECTION WITH INGAASP SEMICONDUCTOR-LASERS HAVING MULTIPLE SHORT EXTERNAL CAVITIES

A liquid detection system consisting of a diode laser with multiple sh ort external cavities (MSXC's) is reported. The MSXC diode laser opera tes single mode on one of 18 distinct modes that span a range of 72 nm . We selected the modes by setting the length of one of the external c avities using a piezoelectric positioner. One can measure the transmis sion through cells by modulating the injection current at audio freque ncies and using phase-sensitive detection to reject the ambient light and reduce 1/f noise. A method to determine regions of single-mode ope ration by the rms of the output of the laser is described. The transmi ssion data were processed by multivariate calibration techniques, i.e. , partial least squares and principal component regression. Water conc entration in acetone was used to demonstrate the performance of the sy stem. A correlation coefficient of R(2) = 0.997 and 0.29% root-mean-sq uare error of prediction are found for water concentration over the ra nge of 2-19%. (C) 1996 Optical Society of America