Journal Club FY 2020 | Tanabe Photonic Structure Group

Journal Club

By fiscal year (April-December)

FY2020

Presentation Details:

The WGM resonator has chiral symmetry, and it is not easy to control the optical output in the CW and CCW directions. In this paper, we develop a microlaser source that takes advantage of the symmetry breaking by varying the intensity of the pump light. Here, a selection ratio of nearly 160:1 in the CW direction was experimentally achieved, and this robust and reconfigurable system may be used as a new platform in the future.

Presentation Details:

With optical spectra equally spaced by frequencies in the microwave or radio frequency domain, optical frequency combs have been used not only to synthesize optical frequencies from a microwave reference but also to generate ultra-low noise microwaves via optical frequency division. In this study, two frequency combs, a soliton microcomb and a semiconductor gain-switched comb, are combined to demonstrate low-noise microwave generation based on a novel frequency division technique. A semiconductor laser driven by a sinusoidal current and injection-locked to a microcavity soliton is used to lock the spectral purity of the dissipating soliton to an integer multiple of the microcomb repetition rate. The gain-switched comb outputs a dense optical spectrum that divides the line spacing of the soliton microcomb. With the potential for integration, the integration of the two chip-scale devices opens the door to a wide range of applications for frequency comb technology.

Presentation Details:

State-of-the-art microcomb-based RF filters require pulse shapers, increasing system cost, footprint, and complexity. In this study, we demonstrate a comb-based RF filter that utilizes soliton combs and does not require an additional pulse shaper. In addition, many microcomb states (single soliton comb, two soliton comb, and perfect soliton crystal) can be applied to easily reconfigure the RF filter.

Presentation Details:

A system with saturable nonlinearity brings a variety of phenomena not found in linear systems. In this study, we theoretically and experimentally demonstrate the saturable nonlinearity (saturable absorption and gain) when Er ions are added to a silica microcavity. The results show that the isolated model agrees well with the experimental results for low addition concentrations and can be used as a basic description for the analysis of saturable nonlinearities in the case of high addition concentrations. However, for the high concentration case, it was found that the clustering effect of Er ions must also be taken into account to obtain reasonable agreement with the experimental data. Therefore, the saturable nonlinearity was experimentally evaluated by varying the ion concentration. The experimental results are highly consistent with theory, including optical bistability and nonlinear parity-time symmetry. The results are consistent with the theory and suggest the possibility of other applications of the theory based on saturable nonlinearity, such as optical bistability and nonlinear parity-time symmetry.

Presentation Details:

Long-term control of the carrier envelope phase (CEP) of mode-locked lasers is an essential technique for applications such as ultrafast optics and precision measurement. In this paper, we have successfully stabilized the CEP of an Er:Yb mode-locked laser with an accuracy of less than 14mrad for more than 75 hours by using a control system that combines short-term control by a feed-forward method and long-term control by a feedback method. The performance of the control system was also evaluated against changes in the external environment.

Presentation Details:

Superconducting nanowire single-photon detectors (SNSPDs) are currently the primary single-photon counting technology at near-infrared wavelengths, with efficiencies exceeding 90%, jitter of less than 3 ps, reset times of a few ns, and dark count rates below Hz. However, unlike Superconducting Transition Edge Sensors (TES) and Microwave Mechanics Inductance Detectors (MKID), SNSPD lacks photon number resolution. In this paper, impedance matching with a tapered transmission line is used to provide a kΩ load impedance to the SNSPD without latching while connecting the readout electronics at 50Ω, making the SNSPD output amplitude sensitive to the number of photon-induced hot spots and enabling more practical photon number resolution is possible.

Presentation Details:

Machine learning techniques such as neural networks have attracted attention as powerful tools for nanostructure inverse problems. However, due to the complexity of the relationship between nanostructures and their optical properties, the optimal solution is often not unique, which may cause convergence problems for the algorithm. In this paper, we analyze this problem using an algorithm called the Mixture Density Network, and clarify its usefulness.

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Optical car coms have the potential to be an important component of wavelength division multiplexing (WDM) communication systems, and recent experiments have demonstrated their potential for high-speed communication at tens of Tbit/s. However, the output power of chip-scale comb sources is typically lower than that of conventional outputs, requiring additional amplifiers and compromising optical signal-to-noise ratio (OSNR). This paper examines the effects of comb power and optical carrier-to-noise ratio (OCNR) on WDM performance. In addition, we use soliton combs as a particularly interesting example to investigate the impact of line-to-line power variations on achievable OSNR and transmission capacity. These may help to compare different types of comb sources and to benchmark them with respect to achievable transmission performance.

Presentation Details:

Nonlinear optical parametric oscillation is possible in micro optical resonators due to their low mode volume and high Q-value. The key to these studies is the control of the relative resonant frequency. Conventional methods for silicon nitride resonators have tuned the global resonant modes by varying the cross-sectional area of the ring resonator. This method may not be suitable when competing nonlinear processes are to be introduced. To solve this problem, we present a method of multiple selective mode splitting (MSMS) to tune targeted resonance frequencies.

Presentation Details:

Wavelength-tunable microlasers in the visible light range play an important role in lighting technology, displays, and sensing. However, most wavelength-tunable microlasers are known to operate in multimode mode. In this study, RGB emission was obtained by adding a material with gain in the visible region to a WGM resonator. Furthermore, we developed a wavelength-tunable single-mode laser by coupling these resonators and extracting only specific wavelengths.

Presentation Details:

Dissipative cur solitons provide broadband coherent, low-noise frequency combs and stable time pulse trains that show great potential applications in spectroscopy, communications, and metrology. Breathing solitons are a specific class of dissipative cur solitons that exhibit periodic oscillations in pulse width and peak intensity. Here, we have investigated breathing dissipative cur solitons in silicon nitride (Si3N4) microrings, and both simulations and experiments show that the breathing period has an uncertainty on the order of about megahertz (MHz). This instability is a major obstacle for future applications. By applying the modulation signal to a pump laser, the breathing frequency can be injection-locked to the modulation frequency and tuned to tens of MHz or better with significant suppression of frequency noise. Our results provide a secondary entry point to control soliton dynamics in microcavities and open new avenues for practical applications of breathing solitons.

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Among nonlinear optical effects, induced Brillouin scattering shows the highest gain in solid materials and has demonstrated advanced photonics capabilities in waveguides. On the other hand, the efficiency of induced Brillouin scattering could be further improved by utilizing the large compressibility of gas. In this paper, a hollow-core fiber filled with gas at high pressure is used to achieve Brillouin amplification more than six times greater than that observed with solid silica fiber. This technique can be applied to any wavelength band, suggesting that various types of light can be amplified in hollow-core waveguides. In addition, a low-threshold Brillouin fiber laser and a high-performance dispersive temperature sensor have been implemented using this mechanism, demonstrating the potential of hollow-core fiber for new applications.

Presentation Details:

In recent years, microcombs have become capable of high repetition rates up to 1 THz, which has advantages in many areas such as wavelength multiplexing, coherent sampling, and self-referencing. However, detection at such repetition rates is difficult due to the limited bandwidth of photodiodes and electronics. In this paper, we report a dual-comb Vernier frequency division method ("Dual-comb Vernier frequency division method") to solve this problem. It utilizes a free-running 216 GHz vernier soliton and samples and divides the repetition frequency of the main soliton from 197 GHz to 995 MHz. This demonstration relaxes the equipment requirements for microcomputer repetition frequency detection and has potential applications in various fields such as optical clocks and microwave photonics.

Presentation Details:

Although microcombs are expected to be used in a variety of applications, their low energy efficiency and difficulty in control have hindered their real-world diversion. Here, we experimentally realized a chiral soliton, an agile on-off switch and tunable dual comb, by introducing a laser gain medium (erbium ions) into a soliton microcomb. By tuning the erbium gain, multi-soliton states can be generated and the soliton capable region can be extended.

Presentation Details:

Continuous fine tuning of nonlinear processes in whispering gallery mode (WGM) resonators is still in its infancy. Fine tuning is achieved by extending a piezoelectric device with an embedded WGM resonator composed of lithium niobate crystals. The advantages of this method include smaller dispersion and a faster fine tuning speed than thermal fine tuning. In this paper, we demonstrate continuous mode-hopping-free tuning of a pump light near 1 µm with second harmonic generation in the range of 28 GHz and parametric optical oscillation in the range of 4.5 GHz using this technique.

Presentation Details:

We proposed and experimentally demonstrated that PT symmetry can be implemented in wavelength space, a non-spatial parameter. A wavelength-space PT-symmetric OEO (Optical Microwave Oscillator) was designed to generate high-quality microwave signals. We showed that eliminating spatial overlap reduces system complexity and significantly improves stability compared to spatially PT-symmetric systems.

Presentation Details:

Circularly polarized coherent vacuum ultraviolet light makes it possible to observe instantaneous phenomena such as the spin state of electrons and the molecular structure of living organisms, and is expected to lead to the discovery of new phenomena and characteristics never seen before. However, it is difficult to control light in the vacuum ultraviolet region, and it has been difficult to generate coherent circularly polarized pulsed light. In this paper, we report the development of a simple method for converting circularly polarized light in the vacuum ultraviolet region by irradiating circularly polarized femtosecond laser light in the visible light region onto a four-fold rotationally symmetric photonic crystal.

Presentation Details:

100 GBd data transmission experiments with modulation schemes OOK and PAM4 were performed for a silicon-organic hybrid Mach-Zehnder modulator with 0.7 dB low phase-shift insertion loss, with line rates up to 200 Gbit/s. The bit error rate is below the hard-decision forward error correction (HD-FEC) threshold of 7%, resulting in a net data rate of 187 Gbit/s. This is the highest PAM4 data rate ever achieved with a sub-1 mm silicon photonic Mach-Zehnder modulator.

Presentation Details:

High-speed operation of the laser frequency is a prerequisite for phase-locking, frequency stabilization, and stable transmission in the optical carrier. Although soliton microcombs have been demonstrated at the system level as chip-scale frequency comb light sources, phase-locking of MHz actuation bandwidth has not been achieved on-chip until now. In this study, high-speed soliton microcomb actuation is performed using a monolithically integrated AlN actuator.

Presentation Details:

Electromagnetic wave induced transparency (EIT), a quantum interference effect that inhibits light absorption in an opaque medium, has found a wide range of applications such as slow light generation, optical storage, frequency conversion, and optical quantum memory. In this study, we have achieved EIT in a new system that does not require external control of temperature or optical power by controlling the direction of light propagation in the resonator.

Presentation Details:

With both strong Pockels and Kerr nonlinear optical effects and a very large band gap, single-crystal aaluminium nitride (AlN) is an attractive platform in nonlinear optics. In this paper, well etched, fully integrated AlN microcavities on sapphire with a high Q-value (2.1×10^6) in the TE00 mode are fabricated by photolithography for the first time. mode at a power of 406 mW. Due to its high confinement performance, a higher-order mode, the TE10 mode, also excites car combs spanning 1270 to 1850 nm at 316 mW of power. Furthermore, frequency conversion to visible light due to harmonic generation was also observed during the generation of the Kercomb. This study is expected to lead to the construction of a large-scale, low-cost integrated nonlinear platform based on AlN.

Presentation Details:

In recent years, research has made it possible to generate frequency combs using CMOS-compatible photonic integrated circuits. However, the soliton microcombs currently being developed have been difficult to use in microwave photonics because they operate at repetition rates far beyond what conventional electronics can detect. In this paper, we demonstrate a soliton microcomb that operates in two widely used microwave bands, X-band (~10 GHz) and K-band (~20 GHz). The combs generated by the low-noise fiber laser produce microwave signals with phase noise levels comparable to modern electronic microwave oscillators. Furthermore, such a low soliton repetition rate is important for the generation of future high-density wavelength-division multiplexed channels, which could have a variety of applications.

Presentation Details:

Raman lasers based on integrated WGM resonators have enabled many applications ranging from communications to bio-detection. These devices take advantage of the high Q-value of the WGM resonator to compensate for the inherently low silica Raman gain, enabling induced Raman scattering laser oscillation at sub-mW thresholds. However, to realize an anti-Stokes Raman scattering laser with the same nonlinear optical effect, the low Raman gain of silica results in a low oscillation efficiency. In this study, we improved the performance of the device by adding metal to the resonator. As a result, the SRS laser oscillation efficiency was increased by a factor of 10 or more, and the sub-mW threshold and efficiency improvement were confirmed for the SARS as well.

Presentation Details:

A luminescent solar concentrator (LSC) is a photon managing device that can harvest, direct and concentrate solar light to small areas, enabling However, the intrinsic photon loss through the so-called escape cone of the LSCs significantly limits their light harvesting and concentrating performance. In this work, we introduce a facile and low-cost In this work, we introduce a facile and low-cost approach for the fabrication of a three-dimensional (3D) macroporous photonic crystal (PC) filter as an efficient photon reflector, which can be coated We demonstrate that by controlling the PC reflection band to match the emission profile of the QD emitters, the light trapping efficiency of the PC coated LSC (PC-LSC) can be significantly improved from 73.3% to 95.1% as compared to the conventional In addition, we have developed a simulation model that considers the PC reflector effect. experimental and simulation results show that the enhancement in LSC device performance induced by the PC reflector increases with increasing In fact, simulation data predicts a maximum of 13.3-fold enhancement in external quantum efficiency (EQE) and concentration factor (C Moreover, the simulation result offers insight into the relationship between photon output Our study sheds light on future design and fabrication of LSC devices with enhanced photon collection and concentrating efficiencies through novel and wavelength-selective photon reflectors.

Presentation Details:

Interactions and multiplications between dissipative cur solitons have been studied extensively in recent years. Soliton molecules, in which different solitons are bound to each other, are an effective way to study the interactions between solitons. The formation of soliton molecules inside micro optical resonators has been observed only in homonuclear soliton molecules, in which solitons are bound to each other with a relatively wide spacing (at least wider than the width of the soliton) due to dispersive waves. For example, multisolitons and soliton crystals fall into this category.
In this paper, we confirm the formation of heteronuclear soliton molecules, in which solitons are bound to each other at a narrower distance (about the width of a soliton or less) due to mutual phase modulation between solitons with different group velocities. The spectra of heteronuclear soliton molecules show different structures from those of ordinary solitons, and are expected to be applied to various fields.

Presentation Details:

Valley photonic crystals (VPhC) are an attractive platform for implementing topologically protected optical waveguides in photonic integrated circuits (PICs). The realization of slow light modes in topology waveguides could lead to further miniaturization and functionality of PICs. In this paper, we report a method for realizing topological slow light waveguides in semiconductor slab-based VPhCs.

Presentation Details:

There have been many studies on all-optical switches using photonic crystals. Silicon and III-V semiconductors (InGaAsP) are often used as photonic crystals, but in this study, switching using Si photonic crystals and InAsP/InP nanowires was performed to take advantage of the advantages of both.

Presentation Details:

We show that a single dark pulse carcom can produce a high enough noise ratio to transmit 1.84 Pbit / s of data. This was achieved by 223 WDM modulated with 32 Gbaud, DP-QAM over 37 core fiber.

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In recent years, frequency microcombs in micro-optical resonators have achieved an accuracy approaching that of laser frequency combs through different physical phenomena. However, real-time investigation of the origin of the dynamical phenomena and the high power stability of microcombs has not been fully achieved so far. In this study, we will elucidate the transition dynamics of microcombs from a chaotic state to mode-locking. Furthermore, we propose a dispersion-controlled resonator as a new platform for understanding fast resonator dynamics and achieving high-power microcombs.

Presentation Details:

Saturable absorbers have been used to generate high-speed pulses, and semiconductor saturable absorbing mirrors (SESAMs), graphene, and carbon nanotubes have been used in the past, but new saturable absorbers have been developed recently. In this study, mode-locked pulses using ferric tetroxide as a saturable absorber were demonstrated for the first time, and its characteristics were measured.

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What is Journal Club?
This is an open lecture series held in Tanabe Photonic Structures Laboratory. Students who are graduate students or above will survey papers related to optics and related technologies such as photonics, materials, bioscience, etc., and explain them in an easy-to-understand manner.

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