Er3+/Yb3+ Co-Doped Fluorotellurite Glass Fiber with Broadband Luminescence.

In order to address the 'capacity crisis' caused by the narrow bandwidth of the current C band and the demand for wide-spectrum sensing sources and tunable fiber lasers, a broadband luminescence covering the C + L bands using Er3+/Yb3+ co-doped fluorotellurite glass fiber is investigated in this paper. The optimal doping concentrations in the glass host were determined based on the intensity, lifetime, and full width at half maximum (FWHM) of the fluorescence centered at 1.5 µm, which were found to be 1.5 mol% Er2O3 and 3 mol% Yb2O3. We also systematically investigated this in terms of optical absorption spectra, absorption and emission cross-sections, gain coefficients, Judd-Ofelt parameters, and up-conversion fluorescence. The energy transfer (ET) mechanism between the high concentrations of Er3+ and Yb3+ was summarized. In addition, a step-indexed fiber was prepared based on these fluorotellurite glasses, and a wide bandwidth of ~112.5 nm (covering the C + L bands from 1505.1 to 1617.6 nm) at 3 dB for the amplified spontaneous emission (ASE) spectra has been observed at a fiber length of 0.57 m, which is the widest bandwidth among all the reports based on tellurite glass. Therefore, this kind of Er3+/Yb3+ co-doped fluorotellurite glass fiber has great potential for developing broadband C + L band amplifiers, ultra-wide fiber sources for sensing, and tunable fiber lasers.

Supercontinuum generation in As2S3 chalcogenide waveguide pumped by all-fiber structured dual-femtosecond solitons.

Supercontinuum sources with high compactness are essential for applications such as optical sensing, airborne detection and communication systems. In the past decades, the adoption of bulky optical parametric amplifier to pump various chalcogenide glass waveguides are widely reported for on-chip mid-infrared supercontinuum generation, but this usually leads to a large volume of the whole system, and is not practical. Therefore, integrating advanced femtosecond fiber lasers with optical waveguides using nano-fabrication technology are highly desired. However, the scarcity of compact pump sources and the dispersion-matched high-nonlinearity waveguide in short wavelength regions have hindered the advancement of integrated supercontinuum source performances in the near and mid-infrared region. In this study, we demonstrate a broadband supercontinuum source from As2S3 waveguide pumped by a compact dual-femtosecond solitons pulse source. The laser is completely fiber structured, and its wavelength can be readily tuned from 2 to 2.3 µm using Raman soliton self-frequency shift technology in a Tm3+-doped fiber amplifier. Furthermore, the As2S3 waveguide is designed with controllable dispersion and high nonlinearity for a broadband supercontinuum generation. These results will advance the development of on-chip supercontinuum sources based on chalcogenide waveguides.

Design and fabrication of large-mode-area multicore chalcogenide fiber with low bending loss.

Multicore fiber (MCF) has a larger mode-area (LMA) compared to traditional single-core fiber, making it easy to get a mode area of more than 3000 µm2 with an optimized MCF structure. Here, a fine-structured 19-core fiber based on chalcogenide glass was fabricated using a combined method involving extrusion, drilling, and rod-in-tube for the first time. The fiber has a minimum transmission loss of 1.8 dB/m at 6.7 µm. When the bending radius exceeds 6 cm, a low bending loss of about 0.6 dB appears, and the experimental data are in good agreement with the simulation results. In addition, the supermode characteristics of the 19-core fiber are analyzed from both perspectives of simulation and experiment, and these results are perfectly in good agreement. We believe it opens a new way to develop high-power and bend-resisting fiber with such kind of multicore structure.

Origin of thermally activated Er3+ emission in GeGaSe films and waveguides.

The origin of the dead or active emission from Er in various Er-doped films has been unclear. Here we took Er-doped GeGaSe as examples and investigated the correlation between the intensity of the photoluminescence (PL) spectra, the content of the activated Er ions, and the intensity of the absorption spectra in the waveguides. We found the linear correlation between the content of Er ions, photoluminescence, and absorption intensity. This provides clear evidence that thermal annealing can promote the conversion of Er metals into Er ions, and such a conversion is essential for practical applications, in which the number of the activated Er ions rather than the nominal Er contents in the materials plays an important role in achieving emission and thus effective optical amplification and lasing.

On-chip Er-doped Ta2O5 waveguide amplifiers with a high internal net gain.

We designed and fabricated a double-layered structure Er3+:Ta2O5 waveguide and investigated its optical amplification performance in C band. The pump laser threshold for zero gain at 1533 nm was 2.5 mW, and the internal net gain was ∼4.63 dB/cm for a lunched pump power of 36.1 mW at 980 nm and signal input power of -30.0 dBm (1 µW). The relationship between the internal gain and the signal input power was also investigated, and a large internal net gain of 10.58 dB/cm was achieved at a signal input power of ∼-47.1 dBm. The results confirm the potentials of the use of Ta2O5 as a host material for optical waveguide amplification.

A simplified mid-infrared anti-resonant chalcogenide fiber with fewest resonant peaks.

High-power laser delivery in the mid-infrared via hollow-core fibers is attractive, but it is too difficult to be fabricated using chalcogenide glasses. Here, we designed a mid-infrared hollow-core anti-resonant chalcogenide fiber (HC-ARCF) with a simplified Kagome cladding micro-structure for the first time. Then, the fiber was firstly fabricated through a precision mechanical drilling and pressured fiber drawing method. Ultra-thin walls of 2µm in the fiber lead to the fewest resonance peaks in the 2-5µm among all reported HC-ARCFs. All the fundamental mode, the second-order mode, tube mode and node mode in the fiber were excited and observed at 1550 nm. The power and spectral properties of the core and cladding of HC-ARCF are studied for the first time. The fiber can deliver high-power of 4.84 W without damage with core-coupling, while the threshold of the node in the cladding is only 3.5 W. A broadening of the output spectrum from 1.96 to 2.41µm due to the high nonlinearity at the node was successfully observed under short-pulse laser pumping at 2µm. The potentials of the fiber used for mid-infrared high-power laser delivery via core, or nonlinear laser generation via node, were thus demonstrated.

Disrupted topological organization of functional brain networks is associated with cognitive impairment in hypertension patients: a resting-state fMRI study.

PURPOSE: To investigate the alterations of topological organization of the whole brain functional networks in hypertension patients with cognitive impairment (HTN-CI) and characterize its relationship with cognitive scores. METHODS: Fifty-seven hypertension patients with cognitive impairment and 59 hypertension patients with normal cognition (HTN-NC), and 49 healthy controls (HCs) underwent resting-state functional magnetic resonance imaging. Graph theoretical analysis was used to investigate the altered topological organization of the functional brain networks. The global topological properties and nodal metrics were compared among the three groups. Network-based statistic (NBS) analysis was used to determine the connected subnetwork. The relationships between network metrics and cognitive scores were also characterized. RESULTS: HTN-CI patients exhibited significantly decreased global efficiency, lambda, and increased shortest path length when compared with HCs. In addition, both HTN-CI and HTN-NC groups exhibited altered nodal degree centrality and nodal efficiency in the right precentral gyrus. The disruptions of global network metrics (lambda, Lp) and the nodal metrics (degree centrality and nodal efficiency) in the right precentral gyrus were positively correlated with the MoCA scores in HTN-CI. NBS analysis demonstrated that decreased subnetwork connectivity was present both in the HTN-CI and HTN-NC groups, which were mainly involved in the default mode network, frontoparietal network, and cingulo-opercular network. CONCLUSION: This study demonstrated the alterations of topographical organization and subnetwork connectivity of functional brain networks in HTN-CI. In addition, the global and nodal network properties were correlated with cognitive scores, which may provide useful insights for the understanding of neuropsychological mechanisms underlying HTN-CI.

High-Q chalcogenide racetrack resonators based on the multimode waveguide.

We propose and demonstrate a high quality (Q) factor racetrack resonator based on uniform multimode waveguides in high-index contrast chalcogenide glass film. Our design features two carefully designed multimode waveguide bends based on modified Euler curves, which enable a compact 180° bend and reduce the chip footprint. A multimode straight waveguide directional coupler is utilized to couple the fundamental mode without exciting higher-order modes in the racetrack. The fabricated micro-racetrack resonator shows a record-high intrinsic Q of 1.31×106 for selenide-based devices, with a relatively low waveguide propagation loss of only 0.38 dB/cm. Our proposed design has potential applications in power-efficient nonlinear photonics.

Single-mode suspended large-core chalcohalide fiber with a low zero-dispersion wavelength for supercontinuum generation.

Chalcogenide glass possesses outstanding advantages, such as supercontinuum generation, but its nonlinear applications were limited by large zero-dispersion wavelength (ZDW). Traditional suspended-core fibers can shift the ZDW to near IR with a tiny core size of less than 5 µm but a large evanescent wave loss exists in these fibers. In this paper, we prepared a novel suspended-core fiber (SCF) based on chalcohalide glasses for the first time via the extrusion method, in which the ZDW of the fundamental mode in the fiber with a core size of larger than 30 µm was successfully shifted to 2.6 µm. We also calculated confinement loss (CL) of propagation modes and fundamental mode energy ratio in the fiber. We found that the minimum CL ratio of the high order modes (LP11) to the CL of the fundamental mode is 124, indicating that the single-mode operation condition is satisfied when the wavelength is more than 4.6 µm. The lowest transmission loss is 1.2 dB/m at 6.5 µm. An ultra-broad supercontinuum spectrum, covering from 1.6 to 12 µm was generated in this suspended-core fiber pumped by a 5 µm femtosecond laser. Such a wide SC in the chalcogenide SCF is due to the large core size. All these results demonstrate the potential to use the large core SCF in the application of a mid-IR laser.

Se-H-free As2Se3 fiber and its spectral applications in the mid-infrared.

The complete removal of the impurities like Se-H in Se-based chalcogenide glasses has been challenging in the development of highly transparent chalcogenide glass fiber. In this paper, several purification methods, including dynamic distillation, static distillation, and combined distillation method, were adopted with an aim of purifying arsenic selenide glass with ultra-low content of the impurities. The experimental results demonstrated that the Se-H can be completely eliminated in the arsenic selenide glass host and fiber without the introduction of any chloride. We further explored the applications of such low loss and Se-H-free chalcogenide glass fiber in the mid-infrared. It was found that, using such a Se-H free fiber, a flattened supercontinuum spectrum above the -30 dB level from 1.2 to 13 µm was generated from the Se-H free fiber with a 5.5 µm laser pumping. The sensitivity was found to be improved 5.1 times for CO2 gas in the 3 to 6 µm wavelength range.

Real-time change of optical losses in chalcogenide waveguides induced by light illumination.

We prepared several GeGaSe waveguides with different chemical compositions and measured the change of optical losses induced by light illumination. Together with some experimental data in As2S3 and GeAsSe waveguides, the results showed that maximum change of the optical loss can be observed in the waveguides under bandgap light illumination. The chalcogenide waveguides with close to stoichiometric compositions have less homopolar bonds and less sub-bandgap states, and thus are preferential to have less photoinduced losses.

High-Q, submicron-confined chalcogenide microring resonators.

We demonstrate high quality (Q) factor microring resonators in high index-contrast GeSbSe chalcogenide glass waveguides using electron-beam lithography followed by plasma dry etching. A microring resonator with a radius of 90 µm shows an intrinsic Q factor of 4.1 × 105 in the telecom band. Thanks to the submicron waveguide dimension, the effective nonlinear coefficient was determined to be up to ∼110 W-1m-1 at 1550 nm, yielding a larger figure-of-merit compared with previously reported submicron chalcogenide waveguides. Such a high Q factor, combined with the large nonlinear coefficient and high confinement, shows the great potential of the GeSbSe microring resonator as a competitive platform in integrated nonlinear photonics.

High quality, high index-contrast chalcogenide microdisk resonators.

We demonstrate the high quality (Q) factor microdisk resonators in high index-contrast chalcogenide glass (ChG) film GeSbSe using electron-beam lithography followed by plasma dry etching. High confinement, low-loss, and single-point-coupled microdisk resonators with a loaded Q factor of 5×105 are measured. We also present pulley-coupled microdisk resonators for relaxing the requirements on the coupling gap. While adjusting the wrap-around coupling waveguides to be phase-matched to the resonator mode, a single specific microdisk radial mode can be excited. Moreover, the thermal characterization of microdisk resonators is carried out to estimate the thermo-optic coefficient of 6.7×10-5/K for bulk ChG.

Design and fabrication of As2Se3 chalcogenide waveguides with low optical losses.

In this paper, we report the fabrication and characterization of chalcogenide-based planar waveguides for possible applications in broadband light sources and/or biochemical sensing. ${{\rm Ge}_{11.5}}{{\rm As}_{24}}{{\rm Se}_{64.5}}$Ge11.5As24Se64.5 film as bottom cladding followed by another layer of ${{\rm As}_2}{{\rm Se}_3}$As2Se3 was deposited on a thermally oxidized silicon wafer using thermal evaporation, and the waveguides were patterned directly on the ${{\rm As}_2}{{\rm Se}_3}$As2Se3 layer by UV exposure followed by inductively coupled plasma dry etching. The device structure was optimized by using commercial software (COMSOL Multiphysics) based on complete vector finite components, and the fundamental mode of the waveguide was calculated. By optimizing the geometry of the waveguide, the zero dispersion wavelength was shifted to a short wavelength (at $\sim{2}.{3}\;\unicode{x00B5} {\rm m}$∼2.3µm), which facilitates supercontinuum generation with shorter wavelength pump source. The insertion loss of the rib waveguides with different widths was measured using the cut-back method, and the best propagation loss at 1550 nm was 1.4 dB/cm.

Mid-infrared flattened supercontinuum generation in all-normal dispersion tellurium chalcogenide fiber.

We have prepared a well-structured tellurium chalcogenide (ChG) fiber with a specialized double cladding structure by an improved extrusion method, and experimentally demonstrated an ultra-flat mid-infrared (MIR) supercontinuum (SC) generation in such a fiber. The step-index fiber had an optical loss of <1 dB/m in a range from 7.4 to 9.7 µm with a minimum loss of 0.69 dB/m at 7.87 µm. Simulation showed that an all-normal dispersion profile can be realized in this double cladding tellurium fiber. An ultra-flat MIR SC spectrum (~3.2-12.1µm at -10 dB, ~2-14 µm at -30 dB) was generated from a 22-cm long fiber pumped with a femtosecond laser at 5 µm (~150 fs, 1 kHz). Then the degree of coherence was calculated out based on a simulation, showing that a high coherent MIR SC (from 2.9 to 13.1 µm) can be generated in this double-cladding tellurium fiber.

1.2-15.2 µm supercontinuum generation in a low-loss chalcohalide fiber pumped at a deep anomalous-dispersion region.

A novel low-loss selenium-based chalcohalide fiber, with a low zero-dispersion wavelength, was prepared by an innovative preparation process. The composition optimized fiber has a wide transmission range of up to 11.5 µm, a lowest fundamental mode zero-dispersion wavelength of 4.03 µm, and a minimum optical loss of 1.12 dB/m at 6.4 µm, which provides a possibility to replace As2S3 and As2Se3 in a cascade of ZrF4-BaF2-LaF3-AlF3-NaF(ZBLAN)-As2S3-As2Se3 fiber in the practical all-fiberized supercontinuum (SC) source. Meanwhile, the broadest SC spectrum, ∼1.2 to 15.2 µm, was achieved by pumping a 12-cm-long fiber with a femtosecond laser at a deep anomalous-dispersion region. Furthermore, simulations are adopted to interpret the results as well as to demonstrate spectral evolution along the fiber. To the best of our knowledge, this is the broadest SC spectrum reported in any selenium-based chalcogenide fiber.

Shortening Nucleation Time to Enable Ultrafast Phase Transition in Zn1Sb7Te12 Pseudo-Binary Alloy.

Zn1Sb7Te12 thin films have been deposited by magnetron co-sputtering of ZnTe and Sb2Te3 targets. The microstructure, phase-change speed, optical cycling stability, and crystallization kinetics have been investigated during thermal annealing and laser irradiation. The thermal-annealed and laser-irradiated films give a clear evidence of the coexistence of trigonal Sb2Te3 and cubic ZnTe phases, which are homogeneously distributed in a single alloy as confirmed by advanced scanning transmission electron microscopy. The formation of both phases increases the initial nucleation sites, leading to the rapid phase-change speed in the Zn1Sb7Te12 film. The film has a minimum crystallization time of ∼3 ns at 70 mW with almost no incubation period for the formation of critical nuclei compared to Ge2Sb2Te5 and other Zn-based films. Moreover, the complete crystallization of Zn1Sb7Te12 thin films is achieved within 10 ns. The ultrafast two-dimensional nucleation and crystal growth speed in Zn1Sb7Te12 obtained from the laser-irradiated system is almost 7 times faster compared to that in Ge2Sb2Te5 film. Controlling the crystallization process through doping ZnTe into Sb2Te3 is thus promising for the development of high-speed optical switching technology.

Greater than 50% inversion in Erbium doped Chalcogenide waveguides.

We report Er-doped Ge-Ga-Se films and waveguides deposited using co-thermal evaporation and patterned with plasma etching. Strong photoluminescence at 1.54 µm with intrinsic lifetime of 1 ms was obtained from deposited films with 1490 nm excitation. Erbium population inversion up to 50% was achieved, with a maximum of ~55% possible at saturation for the first time to the author's knowledge, approaching the theoretical maximum of 65%. Whilst gain was not achieved due to the presence of upconversion pumped photoinduced absorption, this nonetheless represents a further important step towards the realization of future chalcogenide Erbium doped waveguide amplifiers at 1550 nm and in the Mid-infrared.

Broadband mid-infrared supercontinuum generation in 1-meter-long As2S3-based fiber with ultra-large core diameter.

In this study, the supercontinuum (SC) generation in a 1-m-long As2S3 fiber with a 200 µm core diameter was demonstrated experimentally. The high-purity As2S3 fiber we used exhibited very low optical loss with a background loss of approximately 0.1 dB/m at a wavelength of 2-5 µm. SC generation was studied by pumping the fiber at different wavelengths and different peak powers. A strong spectral broadening with a 30 dB spectral flatness spanning from 1.4 to 7.0 µm was obtained when the fiber was pumped with 150 fs short pulses at 5.0 µm. The SC generation in bent fiber was also studied. The result showed that the bending radius of the fiber will significantly affect the SC spectra bandwidth and the output power. The SC spectra in the used fiber could still be maintained when it was bent to a radius of 5 cm.

Experimental demonstration of linearly polarized 2-10 µm supercontinuum generation in a chalcogenide rib waveguide.

This Letter reports the production of a supercontinuum extending from ≈2 µm to >10 µm generated using a chalcogenide buried rib waveguide pumped with 330 femtosecond pulses at 4.184 µm. This is, to the best of our knowledge, the broadest mid-infrared supercontinuum generated in any planar waveguide platform. Because the waveguide is birefringent, quasi-single-mode, and uses an optimized dispersion design, the supercontinuum is linearly polarized with an extinction ratio >100. Dual beam spectrophotometry is performed easily using this source.