Since their inception nearly 25 years ago, optical microresonators have had a major impact on many fields related to linear and non-linear optics. Optical microresonators are used in photonics technologies for laser stabilization, optical filtering and wavelength division multiplexing, as well as in nonlinear optics for Raman lasers, frequency comb generators, and Brillouin lasers. Optical microresonators are useful tools in cavity quantum electrodynamics and cavity optomechanics, where they couple mechanical motion with optical fields.
The capability for optical microresonators to achieve this result is due to the phenomena of the whispering gallery mode present in all microresonators. This phenomenon occurs when laser light is coupled into a circular waveguide, such as a glass ring or disk. When the light strikes the boundary of the waveguide at a grazing angle it is reflected back into the waveguide. The light wave can make many trips around the waveguide before it is absorbed, but only at frequencies of light that fit perfectly into the circumference of the waveguide. If the circumference is a whole number of wavelengths, the light waves superimpose perfectly each trip around.
The so-called “whispering gallery” microresonators can even detect and measure individual nanoparticles. This creates a more reliable and accurate detector for nanoparticles, and may help establish better safety standards for industrial manufacturing of products containing nanoparticles. Medical applications of whispering gallery microresonators include nanoparticle detection within the body, ensuring that nanoparticles are present at treatment or diagnostic sites and absent from areas where they could cause health complications.
Some prior art microresonators are fabricated using advanced clean room techniques and require a large number of fabrication steps including optical and electron beam lithography and several etching steps. Another prior art technique for fabricating microresonators from crystalline materials requires cumbersome manual polishing of the microresonator material, which can take several days. These techniques significantly increase the complexity of fabrication, end cost, and overall time required for fabrication of microresonators, creating obstacles to more wide-scale use.
Control and stabilization of optical frequency combs enables a range of scientific and technological applications, including frequency metrology at high precision, spectroscopy of quantum gases and of molecules from visible wavelengths to the far infrared, searches for exoplanets, and photonic waveform synthesis. Recently, a new class of frequency combs based on monolithic microresonators has emerged, which offer significantly reduced bulk, cost, and complexity beyond what is possible with conventional femtosecond-laser technology. Such factors stand in the way of next generation applications that will require high-performance optical clocks for experiments outside the lab.
In microcomb systems, the comb generation relies on parametric conversion provided by nonlinear optical effects and is enabled by high-quality factors (Q) and small mode volumes of microresonators. To date, microcombs have been explored with a number of microresonator technologies, including microtoroids, crystalline microresonators, microrings, fiber cavities, machined disks, and disk microresonators. Unique comb spectra have been demonstrated, featuring octave spans and a wide range of line spacings.
Microcombs present a challenge for frequency stabilization. Specifically, the center frequency of a microcomb spectrum is matched to a pump laser, and line spacing must be controlled by changing the microresonator's physical properties. Future metrology applications of microcombs will require stabilization of the line spacing with respect to fixed-optical and microwave frequency standards. Hence, the key factors for stabilization are line spacing in the measurable 10's of GHz range, low intrinsic fluctuations, and the capability for fast modulation. Additionally, a threshold power for comb generation in the milliwatt range and the potential for integration with chip-based photonic circuits would enable portable applications.
In prior art, only thermal control via the power of a pump laser has been used in microresonators. However, this technique is not applicable in microresonators with small thermal effect or slow thermal response time.
It is desirable to create optical microresonators quickly and with a minimal number of processing steps.
It is also desirable to stabilize microcomb frequencies in optical microresonators without resorting to thermal control.