Pulsed frequency-modulation spectroscopy as a means for fast absorption measurements

Absorption measurements are accomplished by utilizing short pulses of frequency-modulated (FM) light. The absorption is measured by detecting the heterodyne beat signal that occurs when the FM spectrum is distorted by the absorption feature of interest. By using a single short laser pulse it is demonstrated that the beat signal can build up far above the noise level within 1 microsec. The entire absorption structure can be probed by a few laser pulses. Thus pulsed FM spectroscopy permits ultrafast absorption measurements to be made by using an overall light exposure several orders of magnitude smaller than is necessary for traditional absorption techniques.