Planar coupling to high-Q lithium niobate disk resonators

Whispering gallery mode microsphere resonator based on cylindrical air cavity coupling

In this Letter, an optical fiber whispering gallery mode microsphere resonator based on cylindrical air cavity coupling is proposed and demonstrated. The cylindrical air cavity is vertical to the axis of a single-mode fiber and in touch with the fiber core, fabricated by using femtosecond laser micromachining together with hydrofluoric acid etching. A microsphere is inserted into the cylindrical air cavity and in tangential contact with the inner cavity wall, which is in touch with or inside the fiber core. The light traveling in the fiber core is coupled into the microsphere via an evanescent wave when the light path is tangential to the contacting point of the microsphere and the inner cavity wall, resulting in whispering gallery mode resonance when the phase-matching condition is satisfied. Such a device is highly integrated, robust in structure, low in cost, stable in operation, and has a good quality factor (Q) of 1.44 × 10⁴.

Modified genetic algorithm for high-efficiency dispersive waves emission at 3 µm

Mid-infrared dispersive waves generated from supercontinuum generation are of great significance for gas sensing, environmental monitoring, and molecular spectroscopy. But the conversion efficiency of mid-infrared dispersive waves is degraded at longer wavelengths, which limits the application of mid-infrared dispersive waves. Here, we present a genetic algorithm (GA) which is modified by using a simulated binary crossover method and non-uniform mutation process. The modified genetic algorithm (MGA) optimizes the central wavelength, peak power and time duration of the pump to generate high-efficiency dispersive waves at around 3 µm. The conversion efficiency of mid-infrared dispersive waves is increased from 1.5% to 2.29%. These results are useful for gas sensing and environmental monitoring.

THz Pyro-Optical Detector Based on LiNbO₃ Whispering Gallery Mode Microdisc Resonator

This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a potential bolometer detector in the THz range. The resonator is theoretically characterized in the stationary regime by its thermo-optic and thermal coefficients. Considering a Q-factor of 10⁷, a minimum detectable power of 20 μW was evaluated, three orders of magnitude above its noise equivalent power. This value opens up the feasibility of exploiting LiNbO₃ disc resonators as sensitive room-temperature detectors in the THz range.

Tapered nanoscale chalcogenide fibers directly drawn from bulk glasses as optical couplers for high-index resonators

We report production of air-clad tapered chalcogenide fibers by directly drawing bulk glasses between cleaved tips of tapered silica fibers. Exploiting these tapered fibers with nanoscale waists as evanescent optical couplers, we demonstrate phase-matched coupling of light into on-chip whispering gallery mode chalcogenide microresonators with coupling efficiencies as high as 95%. To the best of our knowledge, this is the first-time demonstration of critical coupling into high-index microresonators by using high-index tapered fibers. The tapered chalcogenide fibers can also be utilized as optical couplers for microresonators made of various high-index materials, as well as for nonlinear optical applications.

Ultrahigh Q whispering gallery mode electro-optic resonators on a silicon photonic chip

Crystalline whispering gallery mode (WGM) electro-optic resonators made of LiNbO₃ and LiTaO₃ are critical for a wide range of applications in nonlinear and quantum optics, as well as RF photonics, due to their remarkably ultrahigh Q(>10⁸) and large electro-optic coefficient. Achieving efficient coupling of these resonators to planar on-chip optical waveguides is essential for any high-yield and robust practical applications. However, it has been very challenging to demonstrate such coupling while preserving the ultrahigh Q properties of the resonators. Here, we show how the silicon photonic platform can overcome this long-standing challenge. Silicon waveguides with appropriate designs enable efficient and strong coupling to these WGM electro-optic resonators. We discuss various integration architectures of these resonators onto a silicon chip and experimentally demonstrate critical coupling of a planar Si waveguide and an ultrahigh QLiTaO₃ resonator (Q∼10⁸). Our results show a promising path for widespread and practical applications of these resonators on a silicon photonic platform.

Lithium-Niobate-Silica Hybrid Whispering-Gallery-Mode Resonators

Lithium-niobate-silica hybrid resonators with quality factors higher than 10(5) are fabricated by depositing a layer of polycrystalline lithium niobate on the flat top surfaces of inverted-wedge silica microdisk resonators. All-optical modulation with improved performance over silica-only resonators and electro-optic modulation not achievable in silica-only resonators are realized in the hybrid resonators.

Modeling the whispering gallery microresonator-based optical modulator

We present a theoretical analysis and numerical simulations of an electro-optic double resonant modulator based on interaction of fundamental whispering gallery modes with a radio frequency field in a dielectric microdisk made from electro-optical material. Models of the modulator in two dimensions and three dimensions are developed and compared. Both optical and RF fields are simulated using the finite element method. The magnitude of the effect in such a system may be maximized with an optimum configuration of a microstrip resonator used for radio-frequency coupling.

Waveguides in single-crystal lithium niobate thin film by proton exchange

The proton exchanged (PE) planar and channel waveguides in a 500 nm thick single-crystal lithium niobate thin film (lithium niobate on insulator, LNOI) were studied. The mature PE technique and strong confinement of light in the LN single-crystal thin film were used. The single mode and cut-off conditions of the channel waveguides were obtained by finite difference simulation. The results showed that the single mode channel waveguide would form if the width of the PE region was between 0.75 μm and 2.1 μm in the β(4) phase. The channel waveguide in LNOI had a much smaller mode size than that in the bulk material due to the high-refractive-index contrast. The mode size reached as small as 0.6 μm(2). in simulation. In the experiment, the refractive index and phase transition after PE in LNOI were analyzed using the prism coupling method and X-ray diffraction. Three different width waveguides (5 μm, 7 μm and 11 μm) were optically characterized. Near-field intensity distribution showed that their mode sizes were 3.3 μm(2).,5 μm(2). and 7 μm(2). The propagation losses were evaluated to be about 16 dB/cm, 12 dB/cm and 11 dB/cm, respectively. The results indicate that PE is a promising method for building more complicated photonic integrated circuits in single-crystal LN thin film.

Long period grating-based fiber coupler to whispering gallery mode resonators

We present a new method for coupling light to high-Q silica whispering gallery mode resonators (WGMs) that is based on long period fiber gratings (LPGs) written in silica fibers. An LPG allows selective excitation of high-order azimuthally symmetric cladding modes in a fiber. Coupling of these cladding modes to WGMs in silica resonators is possible when partial tapering of the fiber is also implemented in order to reduce the optical field size and increase its external evanescent portion. Importantly, the taper size is about one order of magnitude larger than that of a standard fiber taper coupler. The suggested approach is therefore much more robust and useful especially for practical applications. We demonstrate coupling to high-Q silica microspheres and microbubbles detecting the transmission dip at the fiber output when crossing a resonance. An additional feature of this approach is that by cascading LPGs with different periods, a wavelength selective addressing of different resonators along the same fiber is also possible.

High-quality LiNbO3 microdisk resonators by undercut etching and surface tension reshaping

We report ultra-smooth LiNbO(3) microdisk resonators fabricated by selective ion implantation, chemical etching, and thermal treatment. The undercut microdisk structure is produced by chemically etching the buried lattice damage layer formed by selective ion implantation. By thermal treatment, surface tension smoothes and reshapes microdisk surface topography. The resonant characteristics of microdisk resonators are simulated by finite element method and are well consistent with the experimental results. The 20μm-diameter microdisk resonator has the FSR of 16.43nm and the Q factor of 2.60 × 10(4). The produced LiNbO(3) microdisk resonators can be utilized in new microdisk applications with electro-optic and nonlinear-optic effects.