Abstract
We demonstrate a technique to continuously tune center frequency and repetition rate of optical frequency combs generated in silicon microring modulators and bandwidth scale them. We utilize a drive frequency dependent, microwave power induced shifting of the microring modulator resonance. In this work, we demonstrate center frequency tunability of frequency combs generated in silicon microring modulators over a wide range (∼8nm) with fixed number of lines. We also demonstrate continuously tunable repetition rates from 7.5GHz to 15GHz. Further, we use this effect to demonstrate a proof-of-principle experiment to bandwidth scale an 8-line (20dB band) comb generated from a single ring modulator driven at 10GHz to a comb with 12 and 15 lines by cascading two and three ring modulators, respectively. This is accomplished by merging widely spaced ring modulator resonances to a common location, thus coupling light simultaneously into multiple cascaded ring modulators.
Highlights
Optical frequency combs consist of uniformly spaced, coherent lines in wavelength space that have found extensive applications in areas of spectroscopy [1,2], sensing [3,4], distance ranging [5,6] etc
On-chip optical frequency comb generation has recently been investigated in CMOS compatible material platforms like silicon nitride [10,11], silicon oxide [12], hydex [13] aluminum nitride [14] etc. that typically use cascaded four-wave mixing and soliton processes in wavelength stabilized, microring resonator structures resulting in large bandwidth combs but with fixed repetition rate and limited center frequency tuneability
Frequency combs generated in such devices are attractive since they are integrated with pre-existing silicon photonics components without additional coupling losses, have continuously tunable center frequency and repetition rate and are CMOS fabrication compatible etc
Summary
Optical frequency combs consist of uniformly spaced, coherent lines in wavelength space that have found extensive applications in areas of spectroscopy [1,2], sensing [3,4], distance ranging [5,6] etc. We demonstrate simple mechanisms to overcome this constraint without the need of external compensation mechanisms In both linear and microring modulator systems, the number of comblines generated can be increased by varying device properties like doping concentration, or by increasing drive RF-power which requires cost ineffective use of higher power RF-amplifiers apart from requiring the addressing of critical modulator power handling issues [23]. We overcome this constraint as well by demonstrating a novel approach to achieve bandwidth scaling of optical frequency combs generated in microring modulators by cascading multiple microring modulators to a common bus waveguide We achieve both continuous center frequency and repetition rate tuning as well as bandwidth scaling by exploiting the effect of large-signal microwave power on silicon ring-modulator cavity resonances. We note that the microwave power induced thermo-optic shifting of the resonance described in this paper can be used to compensate for process variations in microring-modulator fabrication
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