Opt. Express 19, 11780–11785 (2011)

Tunable optical true-time delay lines are essential components in optical communication networks and optical coherence tomography set-ups. However, these devices are often bulky, slow or have low time resolution. Silicon is an attractive material for such applications, but previous studies based on relatively slow thermo-optical tuning have shown what seems to be an inherent trade-off between loss level and device size. Saeed Khan and co-workers from the University of Central Florida in the USA have proposed an electronically tunable optical true-time delay line to avoid this trade-off. Their approach is based on a silicon-on-insulator rib waveguide sandwiched in a p–n junction. Apodized gratings are used to increase the time delay and the free-carrier plasma effect to tune the delay of the waveguide at a fixed wavelength. Three designs of apodized gratings were studied: Gaussian profile in mesa and grooved structures, and chirped raised-cosine type. They found that the grooved-type device has the highest tuning range (660 ps) with the lowest loss (<2.2 dB), which corresponds to a loss per unit time delay of around 3.3 dB ns−1 — significantly lower than current state-of-the-art technologies. It is also said to be able to operate at bit rates of >20 Gbit s−1 with a tunable delay of around 40 ps and a loss of <10 dB.