Zhongchao Duan

Mid-infrared supercontinuum generation in a suspended-core As2S3 chalcogenide microstructured optical fiber.

We demonstrate the supercontinuum (SC) generation in a suspended-core As(2)S(3) chalcogenide microstructured optical fiber (MOF). The variation of SC is investigated by changing the fiber length, pump peak power and pump wavelength. In the case of long fibers (20 and 40 cm), the SC ranges are discontinuous and stop at the wavelengths shorter than 3500 nm, due to the absorption of fiber. In the case of short fibers (1.3 and 2.4 cm), the SC ranges are continuous and can extend to the wavelengths longer than 4 μm. The SC broadening is observed when the pump peak power increases from 0.24 to 1.32 kW at 2500 nm. The SC range increases with the pump wavelength changing from 2200 to 2600 nm, corresponding to the dispersion of As(2)S(3) MOF from the normal to anomalous region. The SC generation is simulated by the generalized nonlinear Schrödinger equation. The simulation includes the SC difference between 1.3 and 2.4 cm long fiber by 2500 nm pumping, the variation of SC with pump peak power in 2.4 cm long fiber, and the variation of SC with pump wavelength in 1.3 cm long fiber. The simulation agrees well with the experiment.

Five-order SRSs and supercontinuum generation from a tapered tellurite microstructured fiber with longitudinally varying dispersion.

We try to obtain stable supercontinuum (SC) generation with broad bandwidth under relative simple pump conditions. We use a 1.3-m-long highly nonlinear tellurite microstructured fiber and pump it by a 15 ps 1064 nm fiber laser. One segment of the fiber is tapered from a core diameter of 3.4 μm to 1.3 μm. For the first time five-order stimulated Raman scatterings (SRSs) are observed for soft glass fibers. SC covering 730-1700 nm is demonstrated with the pump-pulse-energy of several nJ. The mechanisms of SC broadening are mainly SRS, self-phase modulation (SPM) and cross phase modulation (XPM). The tapered segment has two advantages. Firstly it increases the nonlinearity of fiber by several times. Secondly, it acts as a compensation for the dispersion of the untapered segment, and mitigates the walk-off between pump pulse and SRS peaks.

Size-dependent upconversion luminescence and quenching mechanism of LiYF 4 : Er 3+ /Yb 3+ nanocrystals with oleate ligand adsorbed

Er3+/Yb3+ co-doped LiYF4 nanocrystals were prepared by a facile solvothermal method. By adjusting the LiOH concentration, LiYF4 crystals with the size range from 16 nm to 2.0 μm were synthesized. Under the excitation by a 976 nm laser, upconversion quantum efficiency of the LiYF4: Er3+/Yb3+ samples were measured. It was observed that upconversion quantum efficiency tended to decrease with the reduction of particle size from microscale to nanoscale. A model was proposed to clarify the surface quenching mechanism influencing the size-dependent upconversion luminescence.

Supercontinuum generation in short tellurite microstructured fibers pumped by a quasi-cw laser

We investigate supercontinuum (SC) generation in highly nonlinear tellurite microstructured fibers pumped by a continuous wave (cw)/quasi-cw laser. We investigate two types of tellurite fibers. One type has the constant core diameter, and the other type has a longitudinally varying core diameter. For the fibers with a constant core diameter, when pumped in the anomalous dispersion region, the SC is symmetric in a fiber that has a zero dispersion wavelength close to the pump wavelength. For the fibers with a longitudinally varying diameter, the calculated phase-matching conditions show that they have a broad wavelength range of dispersive waves, and therefore the measured SC spectrum can be broader than one octave. In this work, the fiber lengths are as short as several tens of centimeters, and the pump power is in the watt level.

Ultrabroad supercontinuum generation through filamentation in tellurite glass

Although soft glass such as tellurite or chalcogenide glass is transparent in the range of 3–6 μm, fiber made of it is difficult to generate as a supercontinuum (SC) to that range because of the high loss, the wavelength limit of the pump source, and the challenges in light-coupling. To circumvent these problems, we developed an SC light source by using tellurite bulk glass through filamentation. For this scheme, the optical path length in the glass is very short due to the adopted high pump power, so the negative influence of material loss is reduced greatly. The light-coupling is straightforward, and the coupling efficiency is high. The bulk glass for SC generation is cheap, and can be fabricated easily. We have shown that under suitable pump conditions, the SC generation by filamentation can cover from visible to 6 μm. It is the broadest SC generation by tellurite (including glass and fiber). For suitable pump conditions, the glass was free of optical breakdown. If the interface reflections were deducted, the SC conversion efficiency was 87%; the SC conversion efficiency was stable. To the best of our knowledge, this is the first report on filamentation in tellurite glass which has a comparatively small bandgap.

Soliton self-frequency shift and third-harmonic generation in a four-hole As 2 S 5 microstructured optical fiber

Soliton self-frequency shift (SSFS) and third-harmonic generation (THG) are observed in a four-hole As2S5 chalcogenide microstructured optical fiber (MOF). The As2S5 MOF is tapered to offer an ideal environment for SSFS. After tapering, the zero-dispersion wavelength (ZDW) shifts from 2.02 to 1.61 μm, and the rate of SSFS can be enhanced by increasing the energy density of the pulse. By varying the average input power from 220 to 340 mW, SSFS of a soliton central wavelength from 2.206 to 2.600 μm in the mid-infrared is observed in the tapered segment, and THG at 632 nm is observed in the untapered segment.

Directly draw highly nonlinear tellurite microstructured fiber with diameter varying sharply in a short fiber length

We demonstrate theoretically and experimentally that it is feasible to draw the microstructured fiber with longitudinally varying diameter (FLVD) whose diameter varies sharply in a short fiber length. It is elucidated that during the fiber drawing process the tension is linearly proportional to the natural logarithm of the fiber drawing speed. As a result, the tension is not so sensitive to the fiber diameter. Moreover, this sensitivity can be decreased by using a large diameter ratio of preform to fiber. Owing to the low sensitivity the FLVD with diameter varying sharply in a short fiber length can be drawn directly from the preform. Additionally we show that the microstructural geometry of FLVD does not depend on the varying diameter. The deformation in microstructural geometry is determined by the fiber segment with the smallest diameter. We fabricate a FLVD of which the diameter decreases by 75% in a fiber length of 10 cm. By using this fiber we demonstrate the 600-1800 nm supercontinuum (SC) generation and the 532 nm second harmonic generation pumped by a picosecond fiber laser. The SC spectra by the conventional fibers with the largest and the smallest diameters of the FLVD are also shown, respectively. The comparisons show that the FLVD has the broadest SC spectrum due to its high nonlinearity, varying dispersion, and high damage threshold.

Tellurite Photonic Nanostructured Fiber

In this paper, we address the challenges faced in the fabrication process of nanostructured fiber. We show that a slight nonuniformity of holes of the preform results in a difference in the added pressure in the holes of the fiber during the fabrication process. It may not be a notable problem for the microstructured fiber, but it can result in serious deformation or even collapse for nanostructured fiber. By using a model, we propose a distortion factor that indicates the distortion degree of the geometry of fiber compared with the geometry of preform. The hole size of preform is the most important variable to the distortion factor. A large hole size in the preform is of great significance in decreasing the distortion. We also show that when the temperature is increased, the surface tension is decreased, but the viscosity is decreased much more quickly, so the distortion becomes severe. For minimum distortion in the nanostructured fibers we demonstrate, preforms with comparatively large and uniform inner holes are fabricated by inflating with inert gas. By using such preforms, we fabricate hexagonal core and triangular core nanostructured fibers with the smallest size recorded. Supercontinuum generation from the nanostructured fiber is demonstrated. In this paper, the glass we use for the demonstration is a soft glass. By using polymer or silica glass, which is more suitable for nanostructure fabrication, and by controlling the uniformity of holes in the original cane more accurately, various nanostructured fibers with even smaller size and more complex structure, or nanowire array, should be able to be fabricated by the inflation method.

A simple all-solid tellurite microstructured optical fiber

A simple all-solid tellurite microstructured optical fiber which has only one layer of high-index rods in the cladding is proposed and fabricated in the paper. The core and the cladding with the low index are made from the TeO(2)-ZnO-Na(2)O-La(2)O(3) glass, and the high-index rods are made from the TeO(2)-Li(2)O-WO(3)-MoO(3)-Nb(2)O(5) glass. The guiding regime in this fiber can be explained by ARROW model. The fiber can support the near- and mid-infrared light transmitting in the core within the transmission bands while the all-solid silica microstructured optical fiber cannot. When the pump light is outside the transmission bands, the light will transmit in six TLWMN rods.

Five-Octave-Spanning Supercontinuum Generation in Fluoride Glass

An ultra-broadband supercontinuum (SC) covering 0.2–8.0 µm which is more than 5 octaves, is demonstrated in a piece of fluoride glass. The 20 dB bandwidth spans from 0.39 to 7.4 µm. The filaments, bright conic visible emission, and interference fringe pattern are observed. The glass thickness, optical path, and pump conditions are optimized to enable the SC to cover the entire transmission range of the glass, which includes the whole functional group region of chemical bands and groups. The SC conversion efficiency is confirmed to be stable.