ELIMINATION OF PHASE NOISE OF PULSED-LASER DIODES IN ELECTROOPTIC SAMPLING SYSTEMS

The multimode operation of pulsed laser diode sources is converted int o excess noise in electrooptic probing systems for electrical waveform s and prevents measurements with shot-noise limited sensitivity. The t ransfer function of the sampling system is determined and analyzed in terms of wavelength-and electric field dependence. It is shown that by proper orientation of the polarizing components within the optical be am path points of operation can be found which both have a vanishing w avelength dependence of the transfer function and maximum sensitivity to the electric fields to be measured. Excess noise sources due to wav elength fluctuations of the laser are thus eliminated and shot-noise l imited voltage resolution is obtained. Our system has been optimized t o yield a high voltage sensitivity of 2.3 mV/square-root Hz for a long itudinal LiNbO3 probe crystal. At 28 muW of detected optical power thi s is only 3 dB above the shot noise limit.