Ultrafast Photomodulation Spectroscopy: a device-level tool for   characterizing the flow of light in integrated photonic circuits
  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. 
Roman Bruck, Ben Mills, David J. Thomson, Frederic Y. Gardes, Youfang   Hu, Graham T. Reed and Otto L. Muskens
physics.optics cond-mat.mes-hall physics.ins-det