Fabrication of a real time ultrafast optical pulse monitor using a two-photon GaN photodetector

点击翻译按钮获取中文摘要

A compact ultrafast optical pulse autocorrelator contained a two-photon semiconductor photodetector was fabricated aiming at an estimation of pico- or femto-seconds optical pulse duration. We started with making examination of two-photon absorption properties of commercially available semiconductor elements (light-emitting diodes or photodiodes). As a result, it was found that all of InGaN, AlGaAs light emitting diodes and GaAsP, GaP photodiodes prepared for the experiment had a good sensitivity as two-photon photodetectors. In the next year, a novel structure of autocorrelator was proposed and tested for picosecond optical pulse measurements. An interferometer inside the auocorrelator consists of close placed two pieces of silica glass ; one is fixed and the other is rotated. The pulse beam component passing through the rotating silica glass is yielded a relative time delay corresponding to the variation of path-length due to the rotation. Then both of pulse beam components passing through the glasses overlap each other only by focusing them on a photodetector with a condensing lens. The intensity autocorrelation trace was thus obtained in real time, without great difficulty in alignment, by sensing the two-photon signal from the above photodetector as a function of the time delay. In addition, a phase fringe-resolved autocorrelation trace was also obtained by utilizing a diffraction of the pulse beam at the boundary of the glasses. We confirmed experimentally that this autocorrelation traces could be monitored, at least, in a wide range from 740 nm to 840 nm and at input pulse average power as low as 100 mW, respectively. We also treated these autocorrelation measurements numerically using Fraunhofer-diffraction formula, taking into account the influence of the dispersion of silica glasses. As a result, we estimated that the proposed autocorrelator was available to measure ultrafast optical pulse as short as tens of femtoseconds.