A method to optically probe flip chip packaged complementary metal-oxide-semiconductor (CMOS) microprocessors is described. The technique utilizes an infrared laser with a wavelength of 1064 nm to probe the electric field and the free carrier induced absorption modulation that occurs in a reverse biased P-N junction. This absorption modulation is related directly to the voltage across the junction. The mode-locked laser, which generates a train of 35 ps optical pulses at a repetition rate of 100 MHz, is focused through the heavily doped silicon onto the diffusion regions of the CMOS chip. The small modulations in laser power riding on the reflected optical beam are detected and recovered to measure the voltage across the junction. Time resolution is achieved by making the measurement stroboscopically, by phase locking the mode-locked laser to the tester driving the chip. Results from the flip chip packaged microprocessor have, been measured and are presented.