Hai-Zhong Weng

Narrow-linewidth microwave generation using AlGaInAs/InP microdisk lasers subject to optical injection and optoelectronic feedback

Narrow-linewidth and low phase noise photonic microwave generation under sideband-injection locking are demonstrated using an 8-μm-radius AlGaInAs/InP microdisk laser subject to optical injection and optoelectronic feedback. Microdisk laser subject to external optical injection at the period-one state provides the microwave subcarrier seed signal, and the optoelectronic feedback serves as direct current modulation to stabilize and lock the generated microwave signal without using the electrical filter. High-quality photonic microwave signals are realized with the 3-dB linewidth of less than 1 kHz and the frequency tunable range from 8.8 to 17 GHz. Single sideband phase noise of -101 dBc/Hz is obtained at a frequency offset of 10 kHz for the generated 14.7 GHz signal. Furthermore, the dependences of photonic microwave signal on the optical injection and optoelectronic feedback parameters are investigated.

Mode coupling in hybrid square-rectangular lasers for single mode operation

Mode coupling between a square microcavity and a Fabry-Perot (FP) cavity is proposed and demonstrated for realizing single mode lasers. The modulations of the mode Q factor as simulation results are observed and single mode operation is obtained with a side mode suppression ratio of 46 dB and a single mode fiber coupling loss of 3.2 dB for an AlGaInAs/InP hybrid laser as a 300-μm-length and 1.5-μm-wide FP cavity connected to a vertex of a 10-μm-side square microcavity. Furthermore, tunable single mode operation is demonstrated with a continuous wavelength tuning range over 10 nm. The simple hybrid structure may shed light on practical applications of whispering-gallery mode microcavities in large-scale photonic integrated circuits and optical communication and interconnection.

Mode Q factor and lasing spectrum controls for deformed square resonator microlasers with circular sides

Deformed square resonators with the flat sides replaced by circular sides are proposed and demonstrated to enhance mode Q factors and adjust transverse mode intervals using the regular ray dynamic analysis and numerical simulations. Dual-transverse-mode emissions due to the ultrahigh-Q factors with different wavelength intervals are realized experimentally for AlGaInAs/InP circular-side square microlasers, and the stationary condition of the dual-mode emission is satisfied because the high-Q confined modes have totally different mode numbers. Furthermore, optical frequency combs are generated using the dual-mode lasing microlaser as a seeding light source by cascaded four-wave mixing in a highly nonlinear optical fiber.

Localized-cavity-loss-induced external mode coupling in optical microresonators

We study the external mode coupling in optical microresonators with a loss-dependent coupling coefficient. In the case of dielectric optical microresonators with localized absorption or radiation loss, the mode coupling between whispering gallery modes (WGMs) can result in a destructive interference of the mode fields in the lossy area and consequently reduce the mode loss. Once the reduction overcomes the introduced loss, the actual mode loss is reduced by increasing the loss factor. An anomalous relation between the actual mode loss and the introduced loss factor is then found, while the mode frequencies show parallel, crossing, or anticrossing behavior under different coupling conditions; such phenomena can be well explained by the analysis based on the coupling theory. The results show that the localized cavity loss can suppress the original WGMs and generate high-Q coupled modes, which illustrate a possible way to dynamically control the optical modes inside the microresonator.

Locally deformed-ring hybrid microlasers exhibiting stable unidirectional emission from a Si waveguide

We propose and demonstrate a locally deformed-ring (LDR) hybrid microlaser to realize stable unidirectional emission from a silicon waveguide. The coupled modes eliminate the competition between clockwise (CW) and counter-clockwise (CCW) modes, and the highly unidirectional characteristics are achieved with an enhanced mode quality factor based on a locally deformed notch in the LDR resonator. Using a divinylsiloxane-benzocyclobutene bonding technique, a LDR hybrid microlaser is fabricated vertically coupled to a silicon waveguide with a radius of 20 μm, a ring width of 4 μm and a notch width of 500 nm. The threshold current is 7 mA, and single-mode lasing is achieved with the side-mode suppression ratio of 27 dB. Nearly total unidirectional output from the CCW direction of a Si waveguide is demonstrated with a unidirectional ratio of 0.053.

Mode engineering for circular-side square microcavity lasers

Whispering-gallery mode (WGM) microcavities with the merits of small mode volumes and high quality (Q) factors have attracted great research interests as potential low-power-consumption light sources for photonic integration. We propose and demonstrate deformed square microcavity lasers with the flat sidewalls replaced by circular arcs as converge mirrors to control the WGMs inside the laser cavity. The ray dynamic analysis results indicate that the circular-sides can confine the light rays with stable islands, although full chaotic dynamics are observed under certain deformation. With the numerical simulation of the circular-side square microcavities, ultrahigh-Q modes are obtained owing to the elimination of the scattering losses from the vertices, and a reduction of mode Q factors due to the chaotic ray dynamics is also observed. Different transverse modes have distinct light trajectories, which results in a difference of the effective roundtrip length and a controllable transverse mode interval. Low threshold lasing is achieved experimentally due to the high Q factors of the WGMs. The lasing spectra can be engineered by designing the cavity geometry for the waveguidecoupled circular-side square microcavity lasers. The robust structure and ultrahigh-Q of the waveguide-coupled microlasers provide a potential solution for the compact light sources in photonic integrated circuits.

Narrow-Linewidth microwave generation by optoelectronic oscillators with AlGalnAs/InP microcavity lasers

We present the 1.55μm AlGalnAs/InP microcavity laser based optoelectronic oscillators (OEOs) for narrow-linewidth photonics microwave signal generation. The microwave signals with 3-dB linewidths of about 300 Hz are obtained experimentally by the OEOs.

Ray dynamics and mode characteristics of square microcavities with circular sides

Square microcavities with the flat sides replaced by circular arcs are proposed. An enhancement of Q factor is realized due to the light converge effect, while a dip of Q factor is observed with the chaotic light rays. Ultrahigh Q can be obtained in the circular-side square microcavities with an output waveguide by properly designing the arc radius.

Optical frequency comb generation by four-wave mixing with a seeding source of dual-mode microlasers

Optical frequency comb (OFC) generation based on four-wave-mixing in a highly nonlinear fiber is investigated using a dual-mode microlaser as a seed source. The microlasers with lasing wavelength intervals from 1.11 to 0.36 nm are applied for OFC generation, and phase-locked comb is verified by frequency resolved optical gating analyses.

Ultrahigh-Q deformed square resonator with enhanced transverse mode interval

A mechanism of transverse mode control is demonstrated for square microresonators with the flat sides replaced by circular arcs, which results in ultrahigh-Q factors and large transverse mode interval. A numerical optimization shows that mode Q factors up to 1011 can be obtained for 2D deformed square resonators with a side length of 16 μm, a 1.5 μm vertex-output waveguide, and a refractive index of 3.2 confined by air. Dual-mode lasing with wavelength intervals in agreement with simulated results is realized for deformed square AlGaInAs/InP microlasers. Furthermore, the beating signals of 0.43, 0.31, and 0.16 THz are obtained by the autocorrelation traces, which verify the deformed microsquare lasers as a novel architecture for the further compact THz radiation source.