A technique called ultrafast photomodulation spectroscopy (UPMS) could help not only with testing and characterization of photonic chips, but in making them more reliable and robust. Made from silicon, photonic chips are a key component in the future advancement of worldwide optical networks. As the complexity of integrated circuits in silicon photonics advances, simpler methods for differentiating individual optical components in situ, and ultimately fabricating better photonic chips, are needed. The technique is fast and robust, and could be used for industrial testing. Images courtesy of University of Southampton. The “device-level method for the characterization of photonic chips” is based on localized modulation that uses pulsed laser excitation, wrote University of Southampton researchers in a recent study. “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.” Light traveling in the chip is confined to the silicon and cannot be measured from the outside. The noncontact, all-optical UPMS technique helps solve this problem, as it can determine when the light inside the chip is at specific positions. It uses UV femtosecond laser pulses to change the silicon’s refractive index in those exact areas of the photonic chip. “Monitoring the transmission of the chip while the refractive index is locally changed gives a precise picture of how the light flows through it,” said Dr. Roman Bruck of the university’s physics and astronomy research group. “This allows testing of individual optical elements on the chip, a crucial step in the design optimization to ensure its flawless operation.” Ultrafast photomodulation spectroscopy uses UV femtosecond laser pulses to characterize photonic chips. The researchers have demonstrated UPMS’ versatility in imaging, as well as the quantitative characterization of the properties of silicon photonic devices — group indices in waveguides, quality factors of a ring resonator and the mode structure of a multimode interference device. UPMS provides important information on devices of complex design, the researchers said, and is easily applicable for testing at the device level. The method is also quick and robust, with the potential to be used for industrial testing in the photonics industry. “Because the changes induced by the technique are fully reversible, this testing method is nondestructive and, after testing, the chip can be used for its intended application,” Bruck said. Next, the researchers hope to establish UPMS as a standard characterization tool and make photonic chips overall more reliable. The work was funded by the Engineering and Physical Sciences Research Council, and was published in Nature Photonics (doi: 10.1038/NPHOTON.2014.274). For more information, visit: www.southampton.ac.uk.