We have developed and used an infrared diode laser absorption spectroscopy diagnostic to measure concentrations of fluorocarbon radicals, including CF, CF2, and CF3, during selective oxide etching with a low-pressure, high-density plasma reactor. The fluorocarbon radicals are the precursors for the etch process, as well as for the protective polymer that provides selectivity to the nonoxidized materials. The ability to measure the radical concentrations is important for developing a reproducible oxide etch process. We utilize wavelength modulation to increase the detection sensitivity for operation at the low pressures used in high-density plasma reactors. We use a N2O reference cell to provide wavelength calibration, facilitating identification of the fluorocarbon absorption lines. The N2O cell also provides actinometric calibration, which allows us to compare measurements under different experimental conditions and at different times. We present measurements in an inductively coupled plasma (ICP) etch tool comparing CFx=1-3 concentrations in CFH3:Ar to those in a C2F6:H-2:Ar chemistry. We also present measurements of the rotational temperature of CF2, which is an essential parameter for extracting concentration measurements from the diode laser measurements.