Optical frequency combs (OFCs) play a key role in a variety of applications and have been implemented primarily using mode-locked lasers, Kerr resonators, and electro-optic (EO) modulation. EO modulation-based OFC generation may not yield as many comb lines nor span as broad a bandwidth as modelocked lasers or Kerr resonators; however, it offers a high degree of tunability in central frequency and comb spacing. Integrated solutions are of interest as they significantly reduce device footprint and enable large scale system integration. In this paper, we demonstrate flexible on-chip OFC generation using two cascaded EO Mach-Zehnder modulators in silicon photonics. We demonstrate quasi-rectangular OFCs with 9 lines and a comb spacing of up to 10 GHz with an amplitude variation (comb flatness) within 6.5 dB. The corresponding time-domain waveforms have a good fit with sinc-shaped Nyquist pulses having a full-width at half maximum duration as short as 11.4 ps.

An optical true time delay line (OTTDL) is a fundamental building block for signal processing applications in microwave photonics and optical communications. Here, we experimentally demonstrate an index-variable OTTDL based on an array of 40 subwavelength grating (SWG) waveguides in silicon-on-insulator. Each SWG waveguide in the array is 34 mm long and arranged in a serpentine manner; the average incremental delay between waveguides is about 4.7 ps, and the total delay between the first and last waveguides is approximately 181.9 ps. The waveguide array occupies a chip area of ∼ 6.5 m m × 8.7 m m = 56.55 m m 2 . The proposed OTTDLs bring potential advantages in terms of compactness as well as operation versatility to a variety of microwave signal processing applications.

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We use cascaded microring modulators in silicon photonics for electro-optic comb generation and achieve 7 comb lines with a spacing of 5 GHz, and 5 comb lines with a spacing of up to 10 GHz.