Kaijun Che

Widely-tunable, passively Q-switched erbium-doped fiber laser with few-layer MoS2 saturable absorber

We propose and demonstrate a MoS2-based passively Q-switched Er-doped fiber laser with a wide tuning range of 1519.6-1567.7 nm. The few-layer MoS2 nano-platelets are prepared by the liquid-phase exfoliation method, and are then made into polymer-composite film to construct the fiber-compatible MoS2 saturable absorber (SA). It is measured at 1560 nm wavelength, that such MoS2 SA has the modulation depth of ∼ 2% and the saturable optical intensity of ∼ 10 MW/cm(2). By further inserting the filmy MoS2-SA into an Er-doped fiber laser, stable Q-switching operation with a 48.1 nm continuous tuning from S- to C-waveband is successfully achieved. The shortest pulse duration and the maximum pulse energy are 3.3 μs and 160 nJ, respectively. The repetition rate and the pulse duration under different operation conditions have been also characterized. To the best of our knowledge, it is the first demonstration of MoS2 Q-switched, widely-tunable fiber laser.

Directional emission InP/GaInAsP square-resonator microlasers

InP/GaInAsP square-resonator microlasers with an output waveguide connected to the midpoint of one side of the square are fabricated by standard photolithography and inductively-coupled-plasma etching technique. For a 20-mum-side square microlaser with a 2-mum-wide output waveguide, cw threshold current is 11 mA at room temperature, and the highest mode Q factor is 1.0x10(4) measured from the mode linewidth at the injection current of 10 mA. Multimode oscillation is observed with the lasing mode wavelength 1546 nm and the side-mode suppression ratio of 20 dB at the injection current of 15 mA.

Multiple-Port InP/InGaAsP Square-Resonator Microlasers

Multiple-port semiconductor microlasers can be used as the light sources in photonic integrated circuits through on-chip waveguide connection. By directly jointing two and four coupling waveguides at vertices, two- and four-port electrically pumped InP/InGaAsP square-resonator microlasers are proposed and fabricated by standard photolithography and inductively coupled plasma etching techniques. The mode characteristics of square resonators with two and four coupling waveguides are investigated by 2-D finite-differences time-domain (FDTD) simulation. The influences of the thickness of insulating layer SiO2 and the width of coupling waveguides on the mode Q factors are studied for the square resonators laterally confined by an insulating layer SiO2 and p-electrode layers Ti/Au. Room-temperature continuous-wave operations are achieved for a square microlaser with a side length of 15 μm and two 1-μm-wide coupling waveguides at threshold current of 26 mA. In addition, a 20-μm-side microlaser connected with four 1-μm-wide bus waveguides is realized at 270 K. At last, the four-port square resonator with the same size as the fabricated lasers is studied by FDTD simulation, which yields the mode Q factors at the same magnitude as that measured result of 1.34 × 104.

Ultralow-threshold cascaded Brillouin microlaser for tunable microwave generation.

We experimentally demonstrate an ultralow-threshold cascaded Brillouin microlaser for tunable microwave generation in a high-Q silica microsphere resonator. The threshold of the Brillouin microlaser is as low as 8 μW, which is close to the theoretical prediction. Moreover, the fifth-order Stokes line with a frequency shift up to 55 GHz is achieved with a coupled pump power of less than 0.6 mW. Benefiting from resonant wavelength shifts driven by thermal dynamics in the microsphere, we further realized tunable microwave signals with tuning ranges of 40 MHz at an 11 GHz band and 20 MHz at a 22 GHz band. To the best of our knowledge, it was the first attempt for tunable microwave source based on the whispering-gallery-mode Brillouin microlaser. Such a tunable microwave source from a cascaded Brillouin microlaser could find significant applications in aerospace, communication engineering, and metrology.

Low-threshold stimulated Brillouin scattering in high-Q whispering gallery mode tellurite microspheres.

We demonstrate the first observation of stimulated Brillouin scattering (SBS) in a high-Q whispering gallery mode tellurite microsphere. Tellurite glass with composition of 70TeO₂-20ZnO-5Na₂O-5La₂O₃ (molar ratio) was prepared in-house using a melt-quenching technique. Moreover, tellurite microspheres with Q in excess of 13 millions at 1550 nm were fabricated by melting tellurite microwires using a CO₂ laser. By pumping the tellurite microspheres with a tunable single frequency laser, SBS is further realized with a threshold as low as 0.58 mW. At last, the beat notes between the pump and the Stokes signals were measured, which indicated the Brillouin frequency shift is at the 8.2 GHz band for our tellurite glass. Our results could propel significant applications utilizing SBS by employing tellurite microspheres.

0.1–1-THz High-Repetition-Rate Femtosecond Pulse Generation From Quasi-CW Dual-Pumped All-Fiber Phase-Locked Kerr Combs

We report the generation of tunable high-repetition-rate (0.1-1 THz) femtosecond pulses based on a quasi-continuous wave (CW) dual-pumped all-fiber cascaded four-wave-mixing (FWM) scheme without requiring a cavity resonator. By synchronously injecting two pulse-modulated lasers into an 85-m-long dispersion-flat highly nonlinear fiber, cascaded FWM processes under only ~30-mW average pump power are efficiently initiated to form the Kerr frequency combs near 1.56 μm. Phase locking of the Kerr combs is confirmed by real-time measurements using an autocorrelator, and stable ultrashort pulse trains are observed. The pulse repetition rate can be tuned continuously in the range of 0.1~1 THz by controlling the wavelength spacing between the two pump lasers. The pulse duration can be correspondingly adjusted from 1.66 ps to 262 fs. Such quasi-CW dual-pumped all-fiber cascaded-FWM frequency combs with the advantages of compactness and flexibility could provide a practical solution for generating ultrahigh-repetition-rate femtosecond pulses.

Optical coupling and emission of metal -insulator confined circular resonators

We numerically investigate the direct and indirect optical interactions of pair circular resonators laterally confined by metal-insulator waveguide. The direct optical interaction shows the split of quality (Q) factors of bonding and antibonding states only happens for thick insulator. The indirect optical interaction through a waveguide is proposed to control the modes resonance and collect the output emissions. The Q factors of resonant modes versus the coupling distance (width of waveguide) are studied. The results show whispering gallery modes(WGMs) engaged into interaction are strongly coupled with the guided waves of waveguide once its width is close to the cut-off width of guided waves, while the coupled mode of two WGMs is not limited by this condition. High Q factor mode, combined with a robust wide emission waveguide(close to the cut-off width of second-order guided waves), can be realized from the bonding states of WGM and coupled WGM with an added wave envelope in waveguide. In addition to the pair resonators, the studies on four resonators interacted with each other through waveguide are also addressed and wide waveguide output is anticipated.

Optical processing between two metallically hybrid microdisks

A waveguide is bridged between two metallically hybrid microdisks for achieving nonlocal optical processing. The whispering gallery mode (WGM) is split into two modes, one is intrinsic WGM and the other is oscillating mode (OM) whose photons oscillate between two microdisks through the waveguide. The characteristics of OM on the waveguides width and length are studied. The results show the fundamental WGM oscillation only happens at certain lengths and widths of the waveguide, and strong optical interaction between two microdisks is accompanied by a high-loss waveguide. The optical processing through a waveguide between two individual resonators or photon producers may be applied to dynamic control on the photon states.

External cavity lasing pumped stimulated Brillouin scattering in a high Q microcavity

Stimulated Brillouin scattering (SBS) in a microcavity is usually realized by employing a wavelength tunable external cavity diode laser (TECDL) as the pump source. In this Letter, we report the observation of SBS in a high Q microcavity based on a TECDL-free scheme. The microcavity is employed as a mode-reflecting mirror for constructing a fiber-ring laser and, simultaneously, pumped by the fiber-ring lasing with intrinsic resonance latching. Several regimes are observed in a microcavity with a diameter of ∼215  μm, such as single lasing pumped SBS and multiple regular lasing pumped SBSs (single or cascaded). The microwave signals from the beat notes of the composite output lasing are measured with full-width at half-maximum on the scale of kilohertz at ∼11 and ∼22  GHz, indicating the high coherence between the pump and the Brillouin lasing.

Competitive excitation and osmotic-pressure-mediated control of lasing modes in cholesteric liquid crystal microshells

We examined the end-pumped lasing behaviors of dye doped cholesteric liquid crystal (DDCLC) microshells which were fabricated by glass capillary microfluidics. Several kinds of mode resonances, including distributed feedback, Fabry–Perot (FP), and whispering gallery (WG) modes, can be robustly constructed in each individual DDCLC microshell by varying the beam diameter, namely, tuning the DDCLC gain area. The FP and WG modes were further confirmed experimentally, and the corresponding lasing mechanisms are clearly revealed from the unique material characteristics of DDCLC and the geometrical structure of the microshell. Additionally, we demonstrated that the osmotic pressure can be used to shrink/expand the microshell, productively tuning the excitation of lasing modes in a controlled manner. We wish our findings can provide a new insight into the design of DDCLC microlasers with tunable optical properties.