High- and ultrahigh-Q whispering-gallery mode resonators have the capability to trap photons by total internal reflection for a duration ranging from nanoseconds to milliseconds. These exceptionally long photon lifetimes enhance the light–matter interactions at all scales, namely at the electronic, molecular, and lattice levels. As a consequence, nonlinear photon scattering can be triggered with very low threshold powers, down to a few microwatts. The possibility to efficiently harness photon–photon interactions with a system optimizing size, weight, power, and cost constraints has created a new, quickly thriving research area in photonics science and technology. This topic is inherently cross-disciplinary, as it stands at the intersection of nonlinear and quantum optics, crystallography, optoelectronics, and microwave photonics. From a fundamental perspective, high-Q whispering-gallery mode resonators have emerged as an ideal platform to investigate light–matter interactions in nonlinear bulk materials. From an applied viewpoint, technological applications include time-metrology, aerospace engineering, coherent optical fiber communications, or nonclassical light generation, among others. The aim of this paper is to provide an overview of the most recent advances in this area, which is increasingly gaining importance in contemporary photonics.

Nonlinear photonics with high-Q whispering-gallery-mode resonators

Narrow-linewidth lasers and optical frequency combs generated with mode-locked lasers have revolutionized optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators has opened new horizons in academic research and industrial applications. These combs, as was naturally assumed, however, require narrow-linewidth, single-frequency pump lasers. We demonstrate that an ordinary cost-effective broadband Fabry–Perot laser diode at the hundreds of milliwatts level, self-injection-locked to a microresonator, can be efficiently transformed to a powerful single-frequency, ultra-narrow-linewidth light source with further transformation to a coherent soliton comb oscillator. Our findings pave the way to the most compact and inexpensive highly coherent lasers, frequency comb sources, and comb-based devices for mass production. A broadband multi-frequency Fabry–Perot laser diode, when coupled to a high-Q microresonator, can be efficiently transformed to an ~100 mW narrow-linewidth single-frequency light source, and subsequently, to a coherent soliton Kerr comb oscillator.

Narrow-linewidth lasing and soliton Kerr microcombs with ordinary laser diodes

Deformation characteristics and surface generation modelling of crack-free grinding of GGG single crystals

Narrow linewidth lasers and optical frequency combs generated with mode-locked lasers revolutionized optical frequency metrology. The advent of soliton Kerr frequency combs in compact crystalline or integrated ring optical microresonators opens new horizons for applications. These combs, as was naturally assumed, however, require narrow-linewidth single-frequency pump lasers. We demonstrate that a regular multi-frequency Fabry-Perot laser diode self-injection locked to an optical whispering gallery mode (WGM) microresonator can be first efficiently transformed to a single-frequency ultra-narrow-linewidth source and then to coherent soliton comb oscillator with low power consumption and possibility of further integration.

Narrow linewidth lasing and soliton Kerr-microcombs with ordinary laser diodes

We present a novel compact dual-comb source based on a monolithic optical crystalline MgF2 multi-resonator stack. The coherent soliton combs generated in the two microresonators of the stack with the repetition rate of 12.1 GHz and difference of 1.62 MHz provided after heterodyning a 300 MHz wide radio frequency comb. An analogous system can be used for dual-comb spectroscopy, coherent LIDAR applications, and massively parallel optical communications.

Soliton dual frequency combs in crystalline microresonators.

To realize ultimately efficient signal processing, it is necessary to replace electrical signal processing circuits with optical ones. The optical micro-resonator, which localizes light at a certain spot, is an essential component in optical signal processing. Single-crystal calcium fluoride (CaF2) is the most suitable material for a highly efficient optical micro-resonator. The CaF2 resonator can only be manufactured by ultra-precision machining processes, because its crystal anisotropy does not allow the application of chemical etching. However, the optical micro-resonator's performance depends definitely on the surface integrity. This study investigated the relationship between surface quality after ultra-precision machining and crystal anisotropy. Firstly, crack initiation was investigated on the (1 0 0), (1 1 0), and (1 1 1) planes using the micro-Vickers hardness test. Secondly, brittle-ductile transition was investigated by orthogonal cutting tests. Finally, cutting performance of cylindrical turning was evaluated, which could be a suitable method for manufacturing the CaF2 resonator. The most difficult point in cylindrical turning of CaF2 is that the crystalline plane and cutting direction vary continuously. In order to manufacture the CaF2 optical micro-resonator more efficiently, analysis was conducted on crack initiation and surface quality of all crystallographic orientations from the perspective of slip system and cleavage.

Experimental study of crystal anisotropy based on ultra-precision cylindrical turning of single-crystal calcium fluoride

Investigation of the cutting mechanisms and the anisotropic ductility of monocrystalline sapphire

We fabricated a crystalline whispering gallery mode microcavity by using a computer-controlled ultraprecision cutting process to control the cavity cross section. We used a numerical simulation to show that a wide-spanning optical Kerr frequency comb is generated by tailoring the dispersion of a crystalline whispering gallery mode microcavity. To control the dispersion, we designed the cross-sectional shape of the device and fabricated it by using ultraprecision cutting. Both the measured value and the numerical result show that the microcavity has an anomalous dispersion over one octave.

Dispersion tailoring of a crystalline whispering gallery mode microcavity for a wide-spanning optical Kerr frequency comb

A resolved stress model is presented to re-discuss the anisotropic deformation mechanism of single-crystal calcium fluoride in plunge-cut tests. The authors have investigated the machinability of CaF2 in ultra-precision cutting. However, the influence of crystal anisotropy on the brittle–ductile transition was discussed in a qualitative manner using optical microscopy and white light interferometer. Moreover, only the primary {100} slip system and { 111 } cleavage were considered as deformation mechanisms. In this study, the ductility and brittleness of CaF2 is semi-quantitatively re-discussed on the basis of computation of the resolved stresses, by considering a secondary slip system and cleavage. The surface morphologies of the machined surface were characterized using field-emission scanning electron microscopy. The primary {100} slip system is assumed to be dominant for the ductility. The brittle fracture initiated by the { 111 } cleavages was observed in a different manner in dependency rwith cutting directions.

Yuta Mizumoto

Papers

Experimental study of crystal anisotropy based on ultra-precision cylindrical turning of single-crystal calcium fluoride

Investigation of the cutting mechanisms and the anisotropic ductility of monocrystalline sapphire

Dispersion tailoring of a crystalline whispering gallery mode microcavity for a wide-spanning optical Kerr frequency comb

Revisit of the anisotropic deformation behavior of single-crystal CaF2 in orthogonal cutting

Basic study on Ultraprecision machining of Single-crystal Calcium Fluoride