Applications and Integration of Optical Frequency Combs

Optical frequency combs have a wide range of applications in science and technology, including but not limited to timekeeping, optical frequency synthesis, spectroscopy, searching for exoplanets, ranging, and microwave generation. The integration of microresonator with other photonic components enables the high-volume production of wafer-scale optical frequency combs, soliton microcombs. However, it faces two considerable obstacles: optical isolation, which is challenging to integrate on-chip at acceptable performance levels, and power-hungry electronic control circuits, which are required for the generation and stabilization of soliton microcombs. In this thesis, we describe the design and early commissioning of the laser frequency comb for astronomical calibration using electro-optic modulation. We also focus on the realization of a novel and compact chip-scale optical frequency comb, soliton microcomb, including the progress made towards the visible soliton microcomb generation and the demonstration of low power operation of a soliton microcomb along contours of constant power in the phase space. We introduce a soliton spectrometer using dual-locked counter-propagating soliton microcombs to provide high-resolution frequency measurement. Finally, we look into the integration of lasers and high-Q microresonators. The self-injection locking process has been shown to create a new turnkey soliton operating point that eliminates difficult-to-integrate optical isolation as well as complex startup and feedback loops. Moreover, this technique also simplifies the access to high-efficiency dark soliton states without special dispersion engineering of microresonators

Vernier spectrometer using counter-propagating soliton microcombs

Acquisition of laser frequency with high resolution under continuous and abrupt tuning conditions is important for sensing, spectroscopy and communications. Here, a single microresonator provides rapid and broad-band measurement of frequencies across the optical C-band with a relative frequency precision comparable to conventional dual frequency comb systems. Dual-locked counter-propagating solitons having slightly different repetition rates are used to implement a Vernier spectrometer. Laser tuning rates as high as 10 THz/s, broadly step-tuned lasers, multi-line laser spectra and also molecular absorption lines are characterized using the device. Besides providing a considerable technical simplification through the dual-locked solitons and enhanced capability for measurement of arbitrarily tuned sources, this work reveals possibilities for chip-scale spectrometers that greatly exceed the performance of table-top grating and interferometer-based devices

Quantum diffusion of microcavity solitons

Coherently pumped (Kerr) solitons in an ideal optical microcavity are expected to undergo random quantum motion that determines fundamental performance limits in applications of the soliton microcombs. Here this random walk and its impact on Kerr soliton timing jitter are studied experimentally. The quantum limit is discerned by measuring the relative position of counter-propagating solitons. Their relative motion features weak interactions and also presents common-mode suppression of technical noise, which typically hides the quantum fluctuations. This is in contrast to co-propagating solitons, which are found to have relative timing jitter well below the quantum limit of a single soliton on account of strong correlation of their mutual motion. Good agreement is found between theory and experiment. The results establish the fundamental limits to timing jitter in soliton microcombs and provide new insights on multisoliton physics

Dirac solitons in optical microresonators

Mode-coupling-induced dispersion has been used to engineer microresonators for soliton generation at the edge of the visible band. Here, we show that the optical soliton formed in this way is analogous to optical Bragg solitons and, more generally, to the Dirac soliton in quantum field theory. This optical Dirac soliton is studied theoretically, and a closed-form solution is derived in the corresponding conservative system. Both analytical and numerical solutions show unusual properties, such as polarization twisting and asymmetrical optical spectra. The closed-form solution is also used to study the repetition rate shift in the soliton. An observation of the asymmetrical spectrum is analysed using theory. The properties of Dirac optical solitons in microresonators are important at a fundamental level and provide a road map for soliton microcomb generation in the visible band

Application of evolution-based uncertainty design on gear

The evolution of mechanical parameters, a factor affecting the mechanical reliability, has gathered more attention nowadays. However, studies on time varying uncertainty can hardly be found. A new method based on evolution-based uncertainty design (EBUD) is applied to the design of gear in this paper. Considering the wear evolution over the lifetime, a tooth wear’s time-varying uncertainty model based on the continuous-time model and Ito lemma is established. Drift and volatility functions dependent on the drift rate and volatility rate of rotational speed and torque are used to express the time-varying uncertainty of tooth thickness. The method can predict the reliability and provide an instruction in reliability improving, maintenance and repair of the gear system

Interleaved difference-frequency-generation for mid-infrared microcomb spectral densification

Generation of mid-infrared combs (3.3 micron band) with GigaHertz line spacing is demonstrated by interleaved difference-frequency-generation. The method, applied to a 22 GHz repetition-rate microcomb, is useful for spectral densification of sparse microcomb spectra