Nonlinear Optical Effects in High-Index Doped Silica Waveguides and Their Applications

高折射率摻雜石英波導的非線性光學效應及其應用

Student thesis: Doctoral Thesis

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Award date4 Jun 2020

Abstract

With the demand for increased bandwidth, we have witnessed a tremendous research effort in all-optical signal processing over the past two decades due to its ability to perform signal processing in the optical domain. This allows the elimination of the need for optical-to-electrical and electrical-to-optical conversion in the conventional electronic signal processing systems and has the potential to both satisfy the bandwidth demand while lowers the overall cost and power consumption at the same time. All-optical signal processing makes use of the various nonlinear optical phenomena to perform the processing function. Nonlinear optical processes such as self and cross phase modulations (SPM and XPM), four wave mixing (FWM), sum-frequency and high-harmonics generations (SFG and HHG) can all be utilized to perform the various processing functions. Among the various nonlinear optics technologies, integrated nonlinear photonics have the added advantages of field enhancement and increased interaction length, so to allow the observation of the nonlinear optics effects at a much lower power level.

The development of integrated nonlinear photonics circuits has benefited from the existing complementary metal-oxide semiconductor (CMOS) compatible foundry and its technology advancement from the microelectronics industry. There are a number of nonlinear optics phenomena demonstrated in silicon rich integrated optical platforms with the CMOS process over the past decade. Among these platforms, silicon (Si) nonlinear photonics circuits was one of the first investigated, although Si has very high nonlinearity with Kerr coefficient 100-fold higher than silica, its two-photon absorption (TPA) in the telecom band has greatly restricted its application in this popular band. One nonlinear optical platform that has gained attention in recent years is the high-index doped silica glass. The CMOS-compatible high-index doped silica has an adjustable refractive index between 1.45 to 1.9 in the C-band. Although its nonlinearity is an order of magnitude lower than Si, it has negligible linear and nonlinear loss in the telecom wavelengths leading to a very high nonlinear figure-of-merit (FOM). Moreover, its mature fabrication process and the availability of the vast array of supporting optical structures make this platform a prime candidate for the commercialization of these integrated nonlinear photonics circuits.

Although the properties and applications of the high-index doped silica nonlinear devices have been investigated since the late 2000’s, new and exciting applications continue to emerge as witnessed from the recent advances in quantum entanglement and processing with the high-index doped silica glass microring resonator (MRR) devices. It is clear that there are many potential applications in nonlinear optics with this platform waiting to be discovered. Therefore, the objective of this thesis is twofold: to seek ways to enhance the nonlinearity of the high-index doped silica glass waveguides and to investigate potential applications in nonlinear optics with these devices.

To enhance the nonlinearity of the conventional waveguide structure, the microstructured waveguide approach is adopted where silicon rich layers are embedded in the conventional waveguide to increase the nonlinear coefficient. This approach is highly flexible, as different materials can be embedded in the waveguide to obtain different nonlinear effects and the multilayered structure can be used to optimize the waveguide dispersion. By embedding a 20 nm thin film of silicon nanocrystal (Si-nc), we achieved a two to threefold enhancement of the nonlinear coefficient. Using a 50 cm long Si-nc embedded waveguide, we achieve the first demonstration of a 2-bit quantizer for analog-to-digital converter (ADC) among the CMOS-compatible nonlinear waveguide platforms and shows the proposed ADC as a promising platform for high-speed and ultra-broadband digitizing systems.

In the investigation of potential applications in the field of microwave photonics, we proposed and demonstrated a technique to monitor the radio-frequency (RF) spectrum of the optical signal based on the ultrafast response of χ(3) in these nonlinear waveguides. Our approach makes use of XPM process in the high-index doped silica slot waveguide with 50 cm length. Experimental results indicate that the potential photonic on-chip RF spectrum analyzer has a measurement bandwidth up to 1.7 terahertz. Key points to achieve this goal is to maintain high nonlinearity and flat dispersion in the measurement region. Through optimizing the parameters of the slot waveguide, not only the nonlinearity can be enhanced, but also the zero-dispersion wavelength can be tuned to be around 1550 nm, which is crucial for the operation. This enables various applications in monitoring ultrahigh speed telecommunication devices for the next generation photonic networks.

We have also demonstrated optical second- and third-harmonic generations (SHG and THG) using the high-index doped silica MRRs. A theoretical model is developed to describe the relationship between the thermal dynamics and Kerr effects inside the MRR. Using the extracted thermal nonlinear coefficients, the measured TH resonance shift is calibrated by subtracting the thermal nonlinear-induced phase mismatch to obtain the theoretical three-fold wavelength relationship along with the measured cubic power relationship. The SHG can be the result from symmetry breaking at the interface of high-index doped silica core and silica cladding layer for materials that lack the second order nonlinearity χ(2). It is worth noting that this is the first reported high-harmonics generation in this platform. This work enables new capability of frequency converter for tunable on-chip visible laser sources and provides additional insights into the mechanism of the harmonic generation emissions.

    Research areas

  • Nonlinear optics, High-index doped silica, CMOS-compatible, Micro-ring resonator, Four-wave mixing, Self-phase modulation, Cross-phase modulation, Third-harmonic generation, Second-harmonic generation