DEVICE DESIGN AND CHARACTERIZATION FOR SILICON NITRIDE ON-CHIP OPTICAL FREQUENCY COMB APPLICATIONS
Kerr frequency comb, a sequence of equally spaced sharp lines in frequency domain generated via four-wave mixing process, has multiple applications such as spectroscopy, metrology, and atomic clocks. Conventional frequency combs generated from mode-locked laser have the limitations of low repetition rate and large volume. One novel platform, silicon nitride (SiN) microring resonator (MRR), can overcome such disadvantages. The SiN MRR is a low loss waveguide resonator and has good reliability and capacity for on-chip integration, which enables a portable solution for Kerr frequency comb.
This thesis focuses on the design and characterization of the SiN MRR to optimize the important performance characteristics for the applications.
In Kerr comb applications, phase coherence between the comb lines is required to eliminate unwanted signals in the systems. Therefore, the investigation of the coherent state in MRR based comb generation can benefit the development of comb generation techniques. In particular, dark pulses exhibit much higher comb conversion efficiency than the single soliton combs.
The tunability of Kerr comb is another important performance characteristic of the applications, which is useful for multiple applications, such as matching the comb line spacing to the wavelength multiplexing grid for coherent communication or aligning the on-chip laser wavelength and MRR resonance frequency during the integration. The theoretic analysis of thermal tuning and experimental characterization of resonance frequency tuning via an on-chip microheater are performed in this thesis to explore the thermal tuning efficiency and its limitation.
Another important performance characteristics of the frequency comb is the comb bandwidth. Large bandwidth comb will be beneficial for application like dual comb spectroscopy. In addition, octave-spanning Kerr comb is desired due to its capacity of f-2f self-referencing for comb line frequencies stabilization for the applications like atomic clocks. To demonstrate on-chip octave-spanning Kerr soliton, the dispersion engineering is utilized in the device design to optimize the pump dispersion and dispersive wave generation simultaneously. The octave-spanning solitons are achieved on SiN MRRs with around 900 GHz repetition rate.
Finally, two optical division approaches are demonstrated to read out the large repetition rate of the octave-spanning soliton on all-SiN platform with auxiliary combs to enable the locking of undetectable repetition rate with less complexity in the fabrication and integration. The first approach uses a 25 GHz soliton; whose repetition rate is directly detectable via a photodiode. The second approach employs a Vernier scheme with an 880 GHz soliton to provide an alternative optical division scheme with lower requirements in fabrication ultrahigh Q MRRs. The divided repetition rate can be locked to enable the fully stabilization of frequency comb to provide an on-chip high stability and low noise frequency comb source.