Gain competition induced mode evolution and resonance control in erbium-doped whispering-gallery microresonators

Magnon-induced chaos in an optical PT-symmetric resonator

Optomagnonics supports optical modes with high-quality optical factors and strong photon-magnon interaction on the scale of micrometers. These novel features provide an effective way to modulate the electromagnetic field in optical microcavities. Here in this work, we studied the magnon-induced chaos in an optomagnonical cavity under the condition of parity-time symmetry, and the chaotic behaviors of electromagnetic field could be observed under ultralow thresholds. Even more, the existence optomagnetic interaction makes this chaotic phenomenon controllable through modulating the external field. This research will enrich the study of light matter interaction in the microcavity and provide a theoretical guidance for random number state generation and the realization of the chaotic encryption of information on chips.

Observation of parity-time symmetry in microwave photonics

Symmetry plays a crucial role in explorations of the laws of nature. Parity-time (PT) symmetry phenomena can lead to entirely real spectra in non-Hermitian systems, which attracts considerable attention in the fields of optics and electronics because these phenomena provide a new tool for the manipulation of oscillation modes and non-reciprocal signal transmission. A potential new field of application is microwave photonics, an interdisciplinary field in which the interaction between microwaves and optical signals is exploited. In this article, we report the experimental use of PT symmetry in an optoelectronic oscillator (OEO), a key microwave photonics system that can generate single-frequency sinusoidal signals with high spectral purity. PT symmetry is theoretically analyzed and experimentally observed in an OEO with two mutually coupled active oscillation cavities via a precise manipulation of the interplay between gain and loss in the two oscillation cavities. Stable single-frequency microwave oscillation is achieved without using any optical/electrical filters for oscillation mode selection, which is an indispensable requirement in traditional OEOs. This observation opens new avenues for signal generation and processing based on the PT symmetry principle in microwave photonics.

Rapid and high precision measurement of opto-thermal relaxation with pump-probe method

Opto-thermal relaxation is one of the most important properties of nonlinear optical materials. Rapid and high precision measurement of this parameter is vital in both fundamental research and applications. Current measurement uses either complicated structure with poor precision or high power heating source with low efficiency. Here, we propose a pump-probe method (PPM) to optically measure the thermal relaxation using whispering gallery mode (WGM) microcavities. When the pump laser shines on a microcavity, the materials absorb the input power resonantly and heat up. Then the heat dissipates from the cavities to the surroundings. The opto-thermal effect induces a refractive index change reflected in the signal light transmission spectra. By analyzing the curve character of the transmission spectra of the signal response in the spontaneous relaxation process, the thermal relaxation time can be rapidly measured with high precision. Additionally, we systematically verify the PPM using microtoroids under various pump powers and at various locking points of the signal laser mode. The small rate of refractive index changes (∼10^-8) can be discerned with an input pump power as low as 11.816 μW. Hence, the PPM can be used to detect refractive index perturbation, like gas or liquid sensing, temperature fluctuations with ultra-high sensitivity and be applied to optical materials analysis efficiently.

Optothermal control of gains in erbium-doped whispering-gallery microresonators

Erbium-doped whispering-gallery-mode (WGM) microcavities have great potential in many important applications, such as the precision detection and the micro/nano laser. However, they are sensitive to the fluctuations from the pump laser and the environment. Here we demonstrate the precise controlling of transmission spectra and optical gains using optothermal scanning methods in erbium-doped WGM microcavities. The transmission spectrum of the probe signal exhibits the transition between asymmetric Fano-like resonance and the Lorentz peak (or dip) through tuning the input frequency and the scanning speed of the pump laser. In particular, the analytical calculations can fit well with our experimental results through adiabatically eliminating the anticlockwise optical mode. This Letter shows that the optothermal control of gains is more robust to external noises, which paves a crucial step toward the application in the ultra-sensitive detection.

Optomechanically engineered phononic mode resonance

Optomechanics describes the interaction between the optical field and mechanics, and the optomechanical system provides an ideal interface between photons and phonons. The role of the electromagnetic field during optomechanical interaction is studied in this paper as it is regarded as a phonon transmission medium. An analytical model is built to study the phononic mode resonance and reveals the transmission properties of the phonons, which are related to the variance of the frequency of the electromagnetic field. Moreover, when one mechanical mode is driven, different mode resonant properties could be achieved on the transmission spectrum of phonons between the two mechanical modes. We believe that the current work provides significant results for the research of phononic devices.

Pump induced lasing suppression in Yb:Er-doped microlasers

A pump source is one of the essential prerequisites in order to achieve lasing in a system, and, in most cases, a stronger pump leads to higher laser power at the output. However, this behavior may be suppressed if two pump beams are used. In this work, we show that lasing around the 1600 nm band can be suppressed completely if two pumps, at wavelengths of 980 nm and 1550 nm, are applied simultaneously to an Yb:Er-doped microlaser, whereas it can be revived by switching one of them off. This phenomenon can be explained by assuming that the presence of one pump (980 nm) changes the role of the other pump (1550 nm); more specifically, the 1550 nm pump starts to consume the population inversion instead of increasing it when the 980 nm pump power exceeds a certain value. As a result, the two pump fields lead to a closed-loop transition within the gain medium (i.e., the erbium ions). This study unveils an interplay similar to coherence effects between different pump pathways, thereby providing a reference for designing the laser pump, and may have applications in lasing control.

Enhanced Fano resonance in a non-adiabatic tapered fiber coupled with a microresonator

We achieved enhanced Fano resonance by coupling a bottle resonator with a special non-adiabatic tapered fiber, where there is a high intensity distribution ratio between high-order and fundamental modes in the tapered region, as well as single mode propagation in the waist region. The resonance line shape is theoretically proved to be related to the intensity distribution ratio of the two fiber modes and their phase shift. An enhanced Fano line shape with an extinction ratio over 15 dB is experimentally reached by improving the intensity distribution ratio and tuning the phase shift. The results can remarkably improve the sensitivity of whispering-gallery mode microresonators in the field of optical sensing.

Tunable Raman laser in a hollow bottle-like microresonator

A tunable Raman laser in the hollow bottle-like microresonator is demonstrated. By controlling the pump laser frequency, we have demonstrated continuous Raman laser frequency tuning. We also have studied the interesting transient mode evolution with Raman gain by sweeping the pump and probe laser, and verified the thermal tuning mechanism by theoretical simulations. By mechanically stretching the resonator, we have achieved the large range frequency tuning of the Raman laser, with the tuning range of 132 GHz with the resolution about 85 MHz. The demonstrated tunable Raman laser can be used as a source for future optical applications.

Fabrication of a microtoroidal resonator with picometer precise resonant wavelength

Fabricating an optical microresonator with precise resonant wavelength is of significant importance for fundamental research and practical applications. Here, we develop an effective method to fabricate ultra-high Q microtoroid with picometer-precise resonant wavelength. Our method adds a tuning reflow process, using low-power CO₂ laser pulses, to the traditional fabrication process. It can tailor resonant wavelength to a red or blue direction by choosing a proper laser power. Also, this shift can be controlled by the exposure time. Meanwhile, quality factor remains nearly unchanged during this tailoring process. Our method can greatly reduce the difficulties of experiments where precise resonances are required.