Yuhang Li

Demonstration of microfiber knot laser

The authors demonstrate a 1.5 mu m wavelength microfiber laser formed by tightening a doped microfiber into a knot in air. The 2-mm-diameter knot, assembled using a 3.8-mu m-diameter microfiber that is directly drawn from Er:Yb-doped phosphate glass, serves as both active medium and resonating cavity for lasing. Single-longitudinal-mode laser with threshold of about 5 mW and output power higher than 8 mu W is obtained. Their initial results suggest a simple approach to highly compact lasers based on doped microscale optical fibers. (c) 2006 American Institute of Physics.

Observation of a nonlinear microfiber resonator

Measurements of the intensity transfer function of a silica microfiber resonator are shown to follow a wide variety of hysteresis cycles, depending on the cavity detuning and the scanning frequency of the range of input powers. We attribute these observations to a nonlinear phase shift of thermal origin and provide a simple model that reproduces well our measurements. The response time is found to be around 0.6 ms.

Effect of Host Polymer on Microfiber Resonator

We observe that changing the surrounding material from air to a low refractive index polymer can considerably alter the transmission spectrum of a microfiber knot around a given wavelength. We report on a silica microfiber knot resonator about 180 m in diameter. Using a supercontinuum source, we study its transmission when air and when a low index polymer are used as cladding. The resonator shows similar extinction ratio and Q-factor for both cladding materials. However, embedding the resonator in the polymer down-shifts the optimal operating wavelength by about 20%.

Demonstration of critical coupling in microfiber loops wrapped around a copper rod

The authors demonstrate critical coupling condition in microfiber loops wrapped around a copper rod. The critical coupling condition is achieved by tuning the coupling coefficient to balance the circulation loss. A maximum extinction of 30dB has been obtained around the critical coupling point with a quality factor of about 4000. The resonance wavelength can be tuned by applying an electric current through the copper rod. The copper rod, usually deemed as a high-loss medium for handling light at optical frequency, serves as a low-index robust support for achieving critical coupling in the microfiber loops with acceptable loss.

All-fiber Fabry–Perot resonators based on microfiber Sagnac loop mirrors

We demonstrate all fiber Fabry-Perot resonators (F-P resonators) based on Sagnac loop mirrors assembled with tellurite microfibers. As-assembled F-P resonators, with dimensions of hundreds of micrometers, show clear resonant responses with typical quality factor of about 5700, free spectral range of about 1 nm, and a maximum extinction ratio of 18 dB. The reflectivity of the loop mirror and the effective cavity length of the F-P resonator can be tuned by micromanipulation under an optical microscope. F-P resonators demonstrated here show advantages of small size, good resonance, easy fabrication, tunability, and easy integration with fiber systems.

Theoretical study of microfiber resonator devices exploiting a phase shift

Phase shifts within microfiber resonators can be exploited to demonstrate compact and fast-responding devices. Two examples, a sensor and a bistable device, where the origins of the phase shift are fundamentally different, are investigated. In the sensor the phase change originates from the change of refractive index of the medium surrounding the microfiber ring. This is a linear mechanism which translates into a change of resonance wavelength. Calculations of a silica microfiber ring immersed in an aqueous solution and operating at a wavelength of 1550 nm show that with a fiber 550 nm in diameter the sensitivity approaches a maximal value of about 1137 nm/RIU. In contrast to the sensitivity, the detection limit is critically dependent on the Q factor of the microfiber resonator, and with state of the art microfiber resonators we predict a detection limit of the order of 10 −7 RIU. In the bistable device the phase shift is assumed to originate from the nonlinear optical Kerr effect. In contrast to the sensor, the nonlinearity affects the shapes of the resonances, a phenomenon responsible for bistability. Analytical formulae are derived to evaluate the main parameters at play. We investigate the suitability of several glass materials to realize a microfiber bistable device in air operating at a wavelength of 1550 nm. While the threshold for bistability is predicted to be of the order of tens of watts for silica, it drops to less than 30 mW for G2S2, an easily processed chalcogenide glass.

Modeling bending losses of optical nanofibers or nanowires

Bending losses of nanofibers or nanowires with circular 90 degrees bends are simulated using a three-dimensional finite-difference time-domain (3D-FDTD) method. Dependences of bending losses on wavelength and polarization of guided light are investigated, as well as the diameters, refractive indices, and bending radii of nanowires. The acceptable bending losses (approximately 1 dB/90 degrees) predicted in glass, polymer, and semiconductor nanowires with bending radii down to micrometer level may offer valuable references for assembling highly compact photonic integrated circuits or devices with optical nanowires.

Modeling rare-earth doped microfiber ring lasers

We propose a compact laser configuration based on resonating both the pump and signal light along a microfiber ring doped with active ions. We estimate the minimum Q-factor to obtain lasing and find that values already demonstrated in passive microfiber rings will be sufficient. We model the performance of this device in steady state using rate equations and show that pump resonance can significantly reduce the threshold and increase the quantum efficiency, especially for rings made of materials with weak active ion absorption. Numerical examples for erbium and ytterbium doped devices are presented. Taking into account scattering and coupling losses the optimum pump coupling factor is calculated. The dependences of the quantum efficiency and threshold power on the coupling losses are also investigated. We predict that efficient ytterbium-doped lasers can be obtained with a ring diameter down to a few tens of micrometers.

Injectable nanohydroxyapatite-chitosan-gelatin micro-scaffolds induce regeneration of knee subchondral bone lesions

Subchondral bone has been identified as an attractive target for KOA. To determine whether a minimally invasive micro-scaffolds could be used to induce regeneration of knee subchondral bone lesions, and to examine the protective effect of subchondral bone regeneration on upper cartilage, a ready-to-use injectable treatment with nanohydroxyapatite-chitosan-gelatin micro-scaffolds (HaCGMs) is proposed. Human-infrapatellar-fat-pad-derived adipose stem cells (IPFP-ASCs) were used as a cellular model to examine the osteo-inductivity and biocompatibility of HaCGMs, which were feasibly obtained with potency for multi-potential differentiations. Furthermore, a subchondral bone lesion model was developed to mimic the necrotic region removing performed by surgeons before sequestrectomy. HaCGMs were injected into the model to induce regeneration of subchondral bone. HaCGMs exhibited desirable swelling ratios, porosity, stiffness, and bioactivity and allowed cellular infiltration. Eight weeks after treatment, assessment via X-ray imaging, micro-CT imaging, and histological analysis revealed that rabbits treated with HaCGMs had better subchondral bone regeneration than those not treated. Interestingly, rabbits in the HaCGM treatment group also exhibited improved reservation of upper cartilage compared to those in other groups, as shown by safranin O-fast green staining. Present study provides an in-depth demonstration of injectable HaCGM-based regenerative therapy, which may provide an attractive alternative strategy for treating KOA.

Self-collimation and superlensing in wavy-structured two-dimensional photonic crystals

Wavy-structured two-dimensional (2D) photonic crystal (PC) is possessed of isofrequency contours of rectangular form and is therefore capable of self-collimation. The self-guiding phenomenon is studied by calculating the intensity of the electromagnetic waves through slabs of the PC. Superlensing based on self-collimation of wavy-structured 2DPC is also investigated by analyzing the field pattern where point source is used. The full width at half maximum of image point is calculated to be 0.28λ. The position of the image is always located in the near-field domain of the slab. Image quality degrades gradually as the slab thickness or the source-slab distance increases.