By applying both low-frequency wavelength modulation and high-frequency phase modulation to a laser diode, we develop a sensitive, high-bandwidth chemical diagnostic tool that is applicable to a variety of gas-phase processing environments. Specific chemical species are identified and monitored through their infrared absorption spectra, and the modulation methods allow for sensitive detection that is free of window and other reflection-driven interference fringes. Absorbance limits of 5.3 x 10(-8) and 1.9 x 10(-7) are obtained for an AlGaAs diode laser and a lead-salt diode laser, respectively. We discuss applications to plasma etching and chemical vapor deposition.