Published papers

We numerically study the chaotic synchronization of microresonator frequency combs. The chaotic state of microresonators could be a key factor in optical communications because the modulation instability state (chaotic) has a larger output than the soliton state (stable), which may enable us to realize a higher signal-to-noise ratio. In addition, it will allow secure communication. We show that two microresonator frequency combs in a leader-follower configuration can be synchronized in chaotic regimes. Interestingly, the follower comb synchronizes even when some longitudinal modes of the leader comb are absent. We also show that the Turing pattern comb in the follower ring becomes chaotic and synchronizes when we inject the leader's modulation instability comb.

Optical microcavity, which can localize light at a certain spot for a short period of time, has a wide range of applications, such as optical signal processing and optical frequency combs. Single crystal CaF2 has excellent optical properties and many research on the manufacturing process to create a microcavity has been done. However, due to its unstable thermal conditions, stable oscillation of Kerr comb could not be done. Single crystal MgF2, which also has excellent optical properties, has stable thermal conditions suitable to create a microcavity. Like CaF2, MgF2 microcavity can only be manufactured by ultra-precision cylindrical turning, yet its appropriate processing conditions is not known. In this study, a method to derive the critical depth of cut theoretically in ductile mode machining was proposed in consideration of specific cutting energy, mechanical properties and slip system of workpiece, and tool geometry. The critical depth of cut calculated by the proposed method roughly agreed with the experimental value on the MgF2 plane. Unlike CaF2, MgF2 has an optical anisotropy; therefore, in order to create a microcavity where the refractive angle is constant, the resonance part needs to be made on a cylinder with (001) plane as end face. Therefore, a microcavity was fabricated according to the critical depth of cut on a cylinder with (001) as end face derived from the proposed method, and the surface integrity was evaluated from the viewpoint of microcavity performance.

We utilized silica microtoroid resonators to achieve high repetition rate mode-locking and perfect soliton crystals. We then transfer graphene onto microtoroids to enable saturable absorption and study the impacts on its properties, including the Q value.

We generated terahertz waves at 300 GHz using a soliton comb, focusing on characteristics such as low phase noise, and successfully demonstrated an error-free 10 Gbit/s transmission with a bit-error rate below 10-9.

A coupled-cavity system with gain and loss enables mode-locking without natural saturable absorbers. Our theoretical and numerical findings demonstrate significant modulation in system Q near the exceptional point (EP), even with minor refractive index changes and minimal Kerr effect involvement.

We developed a systematic measurement method that simultaneously captures pump detuning, the optical spectrum, RF beat noise, and the linewidth of a microresonator frequency comb. This comprehensive measurement approach enables us to investigate and visualize the phase transition between two distinct states within the modulation instability (MI) comb: a low-noise state (MI comb phase 1) and a high-noise state (MI comb phase 2). While phase 2 is typically recognized as the MI comb, our findings suggest that phase 1 represents a transition from the Turing pattern to the MI comb, maintaining a low-noise profile despite exhibiting the spectral characteristics of an MI comb.

We demonstrated forward-error-correction (FEC)-free dense wavelength division multiplexing communication utilizing a Kerr soliton frequency comb generated from a high-Q crystalline microresonator with a 20-GHz free spectral range.

We demonstrated forward-error-correction (FEC)-free dense wavelength division multiplexing communication utilizing a Kerr soliton frequency comb generated from a high-Q crystalline microresonator with a 20-GHz free spectral range.

Data traffic has been increasing with the spread of the internet. Therefore, a network that connects data centers and metropolitan areas requires a high-capacity, low-latency, and low-power-consumption optical communication system [1]. Since microresonator-based optical frequency combs (i.e., microcombs) have many longitudinal modes, they are expected to be applied to wavelength division multiplexing (WDM) communications [2,3]. We describe a field demonstration of a multi-wavelength optical transmission with a soliton microcomb. We employed intensity modulation and direct detection (IM-DD) for a simple and low-latency communication system [3]. We also used a magnesium fluoride (MgF2) microresonator because it has a small free spectral range (FSR) for efficient bandwidth use. Although sophisticated experiments have already been reported in the laboratory, we believe that our demonstration using a commercially installed optical fiber in a metropolitan area is a significant step towards the practical use of a microresonator system.

We developed a method for coating CNT/PDMS on a high- Q silica microcavity that allows us to obtain saturable absorption that is needed for building a microresonator based Er-doped mode-locked laser.

We demonstrate a mechanically actuated soliton microcomb with a microwave repetition rate of 15 GHz in ultrahigh-Q crystalline microresonators. This enables direct initiation, subsequent long-term stabilization, and precise tuning of soliton combs.

Ultrahigh repetition rate lasers are vital light sources, and a mode-locked laser with whispering-gallery-mode (WGM) microresonators would provide a cost-effective on-chip solution. We have already reported numerically on the mode-locking in an erbium (Er) ion-doped resonator that was coated with polymer-dispersed carbon nanotubes (CNTs) [1]. Although mode-locking is possible, the required Q is still high, so the experiment remains challenging. In this work, we show that a coupled microresonator system with a gain resonator coupled to a nonlinear loss resonator will solve this problem. It is based on the idea of employing the Q revival of the gain cavity when the loss is increased at the cavity with additional loss, which is known to exist in the vicinity of the exceptional point [2].

We experimentally demonstrate the reconstruction of multiple wavelength light with an ultra-compact spectrometer. The spectrometer is based on a random photonic crystal and operates with a spectral resolution beyond the fabrication error.

We experimentally demonstrate the reconstruction of multiple wavelength light with an ultra-compact spectrometer. The spectrometer is based on a random photonic crystal and operates with a spectral resolution beyond the fabrication error.

Mode-locked fiber lasers are gaining a lot of attention in the industry because of their excellent performance while being relatively inexpensive. Although fiber ring is robust and has been widely used, it only achieves a repetition rate of several hundreds of MHz due to the long fiber needed to build the resonator. On the other hand, Whispering-Gallery-Mode (WGM) microresonators can achieve an extremely high repetition rate due to the smallness of the device. We explore the possibility of a passively mode-locked laser with CNTs as a saturable absorber and erbium-doped microresonator.

Using a soliton microcomb, we generated 300 GHz waves and demonstrated a transmission experiment at 10 Gbit/s with an ultra-low bit-error rate of 10-9. We also demonstrated a photonic filter in the 300 GHz region, which adds another functionality in the terahertz regime.

We developed a compact prism coupling module conforming to the 1U CubeSat standard and demonstrated soliton microcomb generation with an ultrahigh-Q MgF resonator to explore the robustness of the miniaturized multi-wavelength light source.

By numerically investigating the synchronization of cascaded microresonator frequency combs, we find the optimal transmittance parameters and discover that partial injection from the leader is sufficient. We also present preliminary experimental results.

We present first-time demonstration of short-reach and low-latency optical communication within a real network, employing a microresonator frequency comb as a light source. The modulated signal is transmitted through a 9-km single-mode optical fiber installed in a metropolitan network. This demonstration paves the way for realizing low-latency massively parallel optical communication, which is the key to beyond-5G and 6G network. For a proof-of-concept experiment, we employ an MgF2 crystalline microresonator with a 20-GHz free-spectral range that could be used for dense wavelength division multiplexing communication. We generated a stable soliton comb and modulated it with simple 10-Gbps intensity modulation and direct detection to achieve a small excess delay of 3.1 µs.