<title>Picosecond Optical Electronic Measurements</title>

Abstract

AbstractWe report the development of an electro -optic sampling system and its application to the characterizationof high speed GaAs Schottky photodiodes. This system achieves a temporal resolution of 2 ps and shot noiselimited sensitivity of 11 uV-Hz.IntroductionElectrical sampling oscilloscopes available today have the ability to measure pulses as short as 30 pico-seconds duration (FWHM) limited by sampling gate switching times and the difficulty of generating preciselytimed electrical trigger pulses. CW mode -locked lasers, on the other hand, provide a source of extremelystable, picosecond pulses, which, can be combined with the linear electro -optic effect12 to allow electronicand optoelectronic devices to be characterized with very high temporal resolution. An electro -optic sampling system3based on this concept has been constructed and used to evaluate the impulse response of GaAs Shottky photodiodes. Sampling System DescriptionThe key element of the electro -optic sampler is a Pockels cell light amplitude modulator consisting ofa 25 Q microstrip transmission line deposited on a Z face of a rectangular piece of LiTa03 (see Figure 1).The remaining X or Y faces are polished parallel for passage of a probe beam. To complete the Pockels cell,the crystal is placed between a pair of crossed polarizers. A crystal quartz plate is used as a compensatorto bias the net phase retardation to the linear portion of the modulator's response. For the 25Q trans-mission line, the switching voltage Vr of the modulator is 2.2 kV.A commercial synchronously pumped, mode -locked dye laser is used as the optical source and generates atrain of 5 ps pulses at a 246 MHz rate. This pulse train is split into three beams. The first beam is inci-dent upon a scanning autocorrelator to provide an independent measurement of the optical pulse. The secondbeam excites the high speed photodiode under test which in turn launches an electrical waveform onto themicrostrip transmission line. An electro -optic light modulator is included in the optical path before thephotodiode to translate the measurement from D.C. to 8.5 MHz, well above the low frequency noise of the dyelaser as shown in Figure 2. This high frequency chopping results in a 50 dB improvement in the signal tonoise ratio as compared to low frequency mechanical chopping. The third beam is focused through thesampling Pockels cell where its intensity is modulated by the electric field under the transmission line, thussampling the signal. The modulated probe beam is detected by the slow photodiode which feeds a spectrumanalyzer operating as a fixed tuned 8.5 MHz receiver with a 10 Hz video bandwidth. A stepper motor drivencube corner reflector introduces a variable delay between the excitation and probe beams and scans at a rate