Electronically Tunable Wide Band Optical Delay Line Based on InGaAs Quantum Well Microresonators

A novel electronically tunable optical delay line based on InGaAs quantum well microresonators is proposed for high frequency RF transmission. The device utilizes the charge-controlled blue shift of the absorption edge in InGaAs quantum wells to change the effective refractive indices of the resonators and couplers, therefore, provides an efficient way to produce variable time delay. A theoretical model based on measurements is used to analyze the device performance. Simulation results for five 3 × 27？μm2 cascaded resonators with bias voltages <0.7？V show a continuous tuning range of 7~68？ps, a ripple delay <1.5？ps, and a useable bandwidth of 39.3？GHz. 1. Introduction Optical delay is a valuable concept for a multitude of microwave and millimeter wave applications. However to be useful, the value of this delay should be controlled electronically, and this has remained a challenging problem for the various implementations. This problem falls under the general classification of slow light which refers to the ability to decrease the group velocity of an optical medium in order to control the delay. Various approaches have been reported such as SOI ring resonators [1], SOI gratings [2], and LPCVD ring resonator [3]. But the most interest has centered on semiconductor waveguides because of the prospect of a high degree of device integration and room temperature operation as described recently [4]. The potential applications lie in the areas of information processing, communications, and the generalized RF area of Microwave Photonics (MWP). However, in all of these areas the intrinsic limitations of slow light have prevented its incorporation in high bit rate applications. All the proposed and reported tunable optical delay lines utilize the thermo-optic effect and, therefore, require high operating voltages and large footprints. For example, in the MWP area, silicon-on-insulator (SOI) microring resonators have been used to achieve full phase tuning over 40？GHz but the implementation employs temperature variations with microheaters to control the resonant frequency of the ring. The thermal response time in the ms range and the associated power dissipation are clearly limitations for the device applications. In this paper, we introduce a new concept for the microresonator which enables direct electrical control of slow light. This is implemented by the field effect control of charge in the input waveguide and the resonator separately, which shifts the absorption edge, resulting in index changes predicted by Kramers-Kronig relations, and provides coupling and