Silicon photonic on-chip spectral shaper for ultra-broadband radio frequency arbitrary waveform generation

Hao Shen, Purdue University

Abstract

Silicon photonics technology attracts increasing interest in lightwave communication systems, as well as in high speed interconnection within or in-between central processing units (CPUs). A novel application of silicon photonics is in the microwave area: high carrier frequency of lightwave and ultra-compact optical devices on a silicon platform enable an integrated solution for certain broadband Radio Frequency (RF) applications, such as RF Arbitrary waveform generation (RFAWG). RFAWG is widely used in both military (for example, electronic countermeasure, hostile detection) and commercial communication systems (radio over fiber, ultrawideband). Analog bandwidth of RFAWG realized by state-of-the-art electronic circuits is limited by 9.6 GHz. Using photonics technology, in particular, frequency-to-time mapping setup, we can easily demonstrate RFAWG with multi-tens of GHz bandwidth. The frequency-to-time mapping system can translate the spectral shape to the temporal profile of output waveform. Previously frequency-to-time mapping setups require bulk pulse shaper or arrayed waveguide grating as the spectral shaper, which limit the portability and consequently the wide deployment. In this thesis, an integrated 8-channel optical spectral shaper is demonstrated by cascading eight silicon microring add-drop filters on a silicon-on-insulator platform. Each microring is side coupled to a Mach-Zehnder arm, which is embedded in the through port waveguide. Compact metal heaters are fabricated on top of the microrings and the Mach-Zehnder arms for thermal control purpose. Thermal tuning changes the resonance wavelength and power spectrum extinction ratio of each microring add-drop filter. It consequently controls the spectral shaper in a programmable fashion. RF waveforms are demonstrated with fundamental frequencies from 10 GHz to 60 GHz. Various types of RF signals, including single frequency waveform, 180° phase shift RF burst, apodized single frequency waveform and chirped burst, have been generated through this chip. Our demonstration may be a starting point for the miniaturization of ultrabroad bandwidth microwave photonics and for system-level application of silicon photonics. In the setup introduced above, single mode fiber spool as the stretcher source is required to provide dispersion. This part is hard to be deployed on chip. An advanced design for RFAWG using tunable delay line is investigated. Here an optical pulse is separated into eight pulses by cascaded add-drop microring resonators. Each pulse is delayed by an all-pass microring train. Then eight pulses are recombined together to form the arbitrary waveform. In addition, simulation shows that the all pass delay line design is robust to fabrication errors. In this thesis, I first give an introduction to the silicon photonics technology and the microring resonators. In Chapter 2 I give the background of how to model the single and high order microring resonators, with two different methods. The simulation also covers some advanced microring resonator designs, like coupled optical resonator waveguide. In Chapter 3, I talk about the high quality factor microring resonator we have fabricated, the experimental characterization method and how to use this method to identify the loss of silicon waveguide. Chapter 4 focuses on the application of cascaded microring resonators as a multi-channel optical filter. In Chapter 5, I go through the first demonstration to use microring resonator as the on chip spectral shaper for radio frequency arbitrary waveguide generation, where frequency-to-time mapping scheme is utilized. In Chapter 6, I develop an advanced design with add-drop microring resonators and tunable on-chip delay line. In this design, RFAWG is achieved without the off chip stretcher source. It is a big step forward to the full scale integration of the whole system. In the end, the conclusion is given and future works are discussed.

Degree

Ph.D.

Advisors

Qi, Purdue University.

Subject Area

Electrical engineering

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