  Advances in silicon photonics have resulted in rapidly increasing complexity
of integrated circuits. New methods are desirable that allow direct
characterization of individual optical components in-situ, without the need for
additional fabrication steps or test structures. Here, we present a new
device-level method for characterization of photonic chips based on a highly
localized modulation in the device using pulsed laser excitation. Optical
pumping perturbs the refractive index of silicon, providing a spatially and
temporally localized modulation in the transmitted light enabling time- and
frequency-resolved imaging. We demonstrate the versatility of this all-optical
modulation technique in imaging and in quantitative characterization of a
variety of properties of silicon photonic devices, ranging from group indices
in waveguides, quality factors of a ring resonator to the mode structure of a
multimode interference device. Ultrafast photomodulation spectroscopy provides
important information on devices of complex design, and is easily applicable
for testing on the device-level.
