Device-level characterization of the flow of light in integrated photonic circuits using ultrafast photomodulation spectroscopy

Advances in silicon photonics have resulted in rapidly increasing complexity in integrated circuits. New methods that allow the direct characterization of individual optical components in situ, without the need for additional fabrication steps or test structures, are desirable. Here, we present a device-level method for the 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 the quantitative characterization of a range of properties of silicon photonic devices, from group indices in waveguides, to quality factors of a ring resonator, and 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 at the device level.