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Opt Express. 2013 Aug 26;21(17):19538-43.
Saeed Khan1, 2 and Sasan Fathpour1,2
1CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL 32816 USA.
2Department of Electrical Engineering and Computer Science, University of Central Florida, Orlando, FL 32816 USA.
Abstract
High-speed and tunable integrated optical delay lines are demonstrated based on silicon grating waveguides apodized by the super-Gaussian function. The submicron channel waveguides with inward-apodized gratings are fabricated by deep-ultraviolet optical lithography. Characterization of the compact delay lines shows that they offer true-time delays as long as 132 ps, tuning range of ~86 ps, and a minimum bit rate of ~13 Gb/s. For lower bit rates, delays as high as 220 ps and tuning range of 174 ps are feasible.
Additional Information
Optical variable true-time delay lines are key components in a wide range of photonic and electronic-photonic systems such as optically-steered phased-arrayed antennas (PAA), light detection and ranging (LIDAR), optical storage, sampling, communications and signal processing, optical interconnects for supercomputers and data centers, and optical coherence tomography. Integrated chips that can buffer optical signals have been harder to achieve since there is always trade-offs between size, loss and bandwidth.
To address these challenges and to offer a compromised solution, a series of papers by Prof. Sasan Fathpour at CREOL, the College of Optics and Photonics at the University of Central Florida and his graduate student, Saeed Khan, proposes and demonstrates apodized grating waveguides for tunable optical delay lines based on the versatile silicon photonics technology. The latest papers published in Optics Express1 and Optics Letters2 and report demonstration of such devices and compares their performances with delay lines based on other technologies such as ring-resonators and photonic crystal waveguides.
The principle operation of the novel devices is based on delaying light in multiple reflections in grating waveguides, similar to the well-known mirror-effect in distributed Bragg reflectors. Tuning of the delay can be achieved by either the thermo-optic effect or electronic tuning via carrier modulation in the waveguides. The problem with uniform gratings, however, is that group delay ripples, from the optical cavity formed by the sharp ends of the grating, prevents operation in the transmission mode. Apodizing the grating profile solves this problem1 but, because of the high dispersion in the wavelength range of interest, the devices have small bandwidth. This problem can be solved, in turn, by cascading two of the waveguides with opposite (inward and outward) apodized grating profiles2. The complementary waveguides cancel each other’s dispersions and much higher bandwidth line is achieved. Measurements on the cascaded devices shows that they have true time delays of 82 ps and a tuning range of 32 ps, and can operate at bit rates as high as 107 Gb∕s. That is an enhancement of around 3 times in tunability–bit rate product compared with a single-grating device.
1. S. Khan and S. Fathpour “Demonstration of Tunable Optical Delay Lines Based on Apodized Grating Waveguides” Optics Express, vol. 21, pp. 19538-19543, August 2013.
2. S. Khan and S. Fathpour “Demonstration of Complementary Apodized Cascaded Grating Waveguides for Tunable Optical Delay Lines” Optics Letters, vol. 38, pp. 3914-3917, October 2013.
