Silicon Photonics for RF-to-optical link and high-precision metrology

<p dir="ltr">Silicon photonics provides a versatile and scalable platform for integrated photonics by enabling the seamless integration of photonic and electronic components on a single chip. As a CMOS (complementary metal-oxide-semiconductor)-compatible technology, it holds promises for a wide range of applications, including classical and quantum communication, spectroscopy, timekeeping, and radio or optical frequency synthesis, within compact, low-size, weight, and power (SWaP) architectures. However, realizing these capabilities requires careful design considerations. For example, achieving a fully stabilized, on-chip frequency comb demands both reduced mode volume and dispersion engineering to support octave-spanning bandwidths. This, in turn, pushes the repetition rate and carrier-envelope offset (CEO) frequency beyond the detection capabilities of conventional electronics, necessitating additional stabilization strategies. This thesis addresses a few challenges in integrated photonic design and demonstrates how the designed system can be used in quantum frequency processing, programmable frequency synthesis, and high-precision metrology.</p><p dir="ltr">The first chapter focuses on designing photonic components for quantum frequency processing on a CMOS-compatible platform. This includes the design of microring filters that enable a foundry-fabricated, on-chip Fourier transform pulse shaper with channel spacings narrower than those of commercially available devices. Additionally, we demonstrate the use of a back-end process to incorporate an organic electro-optic material on the same platform, inducing the Pockels effect, which is typically absent in silicon, to create an on-chip modulator.</p><p dir="ltr">The remainder of this thesis addresses the development and application of on-chip frequency combs as a bridge between the radio frequency (RF) and optical domains. Two major challenges associated with on-chip Kerr microcombs are addressed. First, fully stabilizing a Kerr comb requires detection and control of ~THz-range repetition rates and high CEO frequencies (~100 GHz for our case), both beyond the reach of standard photodetectors. Second, the large mode spacings of ~THz Kerr combs limit their utility in precision metrology. To address the first challenge, we employ the Vernier technique to achieve full stabilization of both microcombs using only three feedback loops. This enables programmable RF-to-optical frequency synthesis using the coherent dual-microcomb system. For the second challenge, overcoming the limitations imposed by the large THz mode spacing, we reduce the spacing to the GHz regime by modulating each microcomb with an electro-optic (EO) comb generator, driven by a divided version of its own repetition rate. This hybrid architecture enables static laser frequency calibration with kHz resolution and dynamic laser frequency calibration with very fast resolving capability, without requiring prior knowledge of the laser wavelength. Together, these advances demonstrate programmable optical frequency synthesis and high-precision metrology systems based on a phase-coherent dual-microcomb platform.</p>