Abstract

Visible or near-infrared (hereafter called simply visible) femtosecond pulses make possible the generation of far-infrared (THz) [1] and even mid-infrared [2] femtosecond pulses. A powerful technique used to characterize these pulses is Fourier-transform spectroscopy [3], yielding the power spectrum through a Fourier-transform of the recorded linear autocorrelation. Although such a measurement can be performed using an infrared Michelson interferometer, a sequence of two collinear infrared pulses separated by a variable time delay can more simply be generated using other techniques [4]. Here, we propose and demonstrate a new Fourier-transform spectrometer in which two time-delayed visible pulses are focused on a single infrared generator, as shown in fig. 1, thus generating a sequence of two infrared pulses. The signal recorded as a function of time delay yields the linear autocorrelation of the infrared pulse. However, when the two incident visible beams are collinear, their interaction in the generator results in an additional emission. As a consequence, the signal then recorded as a function of time delay, shown in fig. 2(a), resembles the interferometric second-order autocorrelation of the visible pulse, with a contrast larger than 2 and high-frequency fringes. Fortunately, a non-collinear geometry eliminates this crossed emission, while preserving a nearly collinear geometry in the infrared. Indeed, since the natural angle of diffraction is larger in the infrared than in the visible, we can choose an angle θ between the two incident beams such that two non-collinear incident visible beams yield two strongly overlapping infrared beams, as shown in the inset of fig. 1. Therefore, the two infrared beams can interfere on the detector, nearly as much as if they were truly collinear. In this case, the detected signal is the linear autocorrelation of the infrared pulse, with a constrast close to 2, as expected (fig. 2(b)). A Fourier transform provides the power spectrum of the infrared radiation (inset). The broad spectral width results from the use of 15-fs optical pulses on GaAs as the infrared generator [2], and the sharp cut-off above 14 µm is due to our detector spectral response.

© 1996 Optical Society of America