Abstract
A cascaded-ring-resonator-loaded Mach–Zehnder modulator (CRR-MZM) is presented in which a number of cascaded ring resonators (RRs) are loaded in the interferometer as phase modulators. The ability of the design to provide enhanced modulation efficiency at a wide optical bandwidth is demonstrated in comparison with a conventional single-RR-type modulator without an interferometer. The optimization of RRs for the CRR-MZM is investigated experimentally by measuring the transmission spectra in both intensity and group delay of RRs having various structural parameters. Using the optimized parameters, we fabricated a CRR-MZM with 10 cascaded RRs loaded on each arm of the interferometer on a silicon-on-insulate substrate. The RRs had pin-diodes along the waveguides, which were operated with forward bias voltage. Its modulation efficiency was enhanced by a factor of 4.4 at the expense of additional loss of less than 3.5 dB compared with a standard non-resonant MZM. 10 Gb/s-operations of CRR-MZM were successfully demonstrated using pre-emphasized RF signals with amplitude of 1.5 Vpp in a wavelength range of 2 nm.
Highlights
Optical modulators play a key role in optical communication systems by converting highspeed electrical signals, typically 10 Gb/s or even higher, to optical signals
The ability of the design to provide enhanced modulation efficiency at a wide optical bandwidth is demonstrated in comparison with a conventional single-RRtype modulator
small footprints have become highly desirable since silicon photonics was recognized as an enabling technology
Summary
Optical modulators play a key role in optical communication systems by converting highspeed electrical signals, typically 10 Gb/s or even higher, to optical signals. A tiny change in the refractive index according to the electrical input signals causes sufficient intensity variation by shifting the resonant wavelengths; we call this type of modulator a single-ring-resonator-based spectral shifting modulator (SRR-SSM). An optical bandwidth as small as 0.1 nm was commonly reported for these modulators [7–13] This narrow operating window made those modulators difficult to use in practical systems without precise dynamic tuning of the resonant wavelengths of the RRs [12,13]. This need for a tuning system could restrict the application of those modulators. For a given refractive index change, it is essential to increase the Q value of the resonators, to obtain large modulation efficiency This inevitably causes a narrow optical bandwidth. 2 nm, which is about 20 times wider than those reported for SRR-SSMs, while maintaining an enhancement of 4.4 times in the modulation efficiency due to the optical resonance
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