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High-gain Tm

High-performance thermoelectric (TE) materials with great flexibility and stability are urgently needed to efficiently convert heat energy into electrical power. Recently, intrinsically crystalline, mechanically stable, and flexible inorganic TE fibers that show TE properties comparable to their bulk counterparts are of interest to researchers. Despite remarkable progress moving TE fibers towards room-temperature TE conversion, the figure-of-merit value (ZT) and bending stability still need enhancement. Herein, we report interfacial engineering enhanced thermoelectric properties of micro-nano polycrystalline TE fibers fabricated by thermally drawing Bi2Te3-based bulks in a glass-fiber template. The interfacial engineering effect comes from generating stress-induced oriented nanocrystals to increase electrical conductivity and producing strain-distorted interfaces to decrease thermal conductivity. The resulting fibers achieve a 40% higher ZT (~1.4 at 300 K) than their bulk counterparts and show a reversible bending radius of 50 µm, approaching the theoretical elastic limit. This fabrication strategy works for a wide range of inorganic TE materials and benefits the development of fiber-based micro-TE devices.

N-type bismuth telluride (Bi2Te3) based thermoelectric (TE) fibers were fabricated by thermal drawing and Bridgman annealing, and the influence of Bridgman annealing on the TE properties of n-type Bi2Te3-based TE fibers was studied. The Bridgman annealing enhanced the electrical conductivity and Seebeck coefficient because of increasing crystalline orientation and decreasing detrimental elemental enrichment. The TE performance of n-type Bi2Te3-based TE fibers was improved significantly by enhancing the power factor. Hence the power factor increased from 0.14 to 0.93 mW/mK2, and the figure-of-merit value is from 0.11 to 0.43 at ~300 K, respectively.

Nd3+ doped multi-component phosphate glass was prepared by the conventional melt-quenching method. The absorption spectrum, emission spectrum, and fluorescence lifetime of the prepared glass were measured. Based on the absorption spectrum, Judd-Ofelt (J-O) intensity parameters, spectroscopic quality factor, absorption and emission cross-sections, and figure of merit were calculated and analyzed. What is more, multi-material fibers with Nd3+ doped multi-component phosphate glass core and silicate glass cladding were successfully drawn by using the molten core method. The Nd3+ doping concentration reaches as high as 3.82 × 1020 ions/cm3 (3.86 wt.%). More importantly, 1.05 μm amplified spontaneous emission (ASE) was realized in a 4-cm-long as-drawn multi-material fiber when pumped by an 808 nm laser diode (LD). These results suggest that the Nd3+ doped multi-component phosphate glass multi-material fibers are promising gain material for 1.05 μm fiber laser applications.

Tin selenide (SnSe) exhibits high thermoelectric (TE) performance, which is a promising TE candidate for thermal-electrical energy conversion. In this work, Sn-Se alloy core borosilicate glass-clad fibers with ∼94 μm core and ∼220 μm cladding in diameter were fabricated by a molten core drawing method. X-ray diffraction (XRD) showed the core to be highly crystalline and consist of SnSe and SnSe2 poly-crystal grains. Based on EPMA measurement, there exists an inter-diffusion process between the core and cladding ions during the fiber drawing. The Seebeck coefficient of the Sn-Se core is −151 μV/K. The electrical conductivity of the Sn-Se core is as high as 6.2 × 103 S/m, which is close to that of SnSe single crystals. The demonstration of fibers with TE core materials and the integration with the conventional fiber fabrication technique represents the first step in providing the building blocks for fiber-integrated TE devices.

A new type of thermally sensitive fibers with an antimony selenide (Sb 2 Se 3 ) core and phosphate glass cladding is demonstrated. The fibers were fabricated by a molten core method and maintained overall diameters ranging from 250 to 800 μm and core diameters of 35–200 μm. A 2.8 cm long Sb 2 Se 3 core fiber, electrically contacted to external circuitry through fiber end facets, exhibited a four orders of magnitude change in conductivity after the whole fiber was heated from 25 to 195 °C. In addition, the fiber exhibited enhanced photoconductivity under illumination. These results indicate that Sb 2 Se 3 core multimaterial fibers have promising applications in temperature sensing, optical switches and photodetectors.

Rare-earth (RE) doped photonic glasses exhibit potential applications in the area of optical communication, laser lidar, remote sensing, infrared detection, and biomedical. Up to now, the widely used silica based photonic glasses encounted intrinsic limitations such as lower rare-earth doping concentration, narrower emission band, larger phonon energy, and lower gain coefficients. Therefore, it is necessary to study and develop novel photonic glasses and fibers to meet the needs of new wavelength bands, especially the near-and mid-infrared materials and devices. In this paper, the important applications of RE doped photonic glasses in optical fiber amplifiers and fiber lasers are briefly described. The basic requirements for realizing near-and mid-infrared luminescence and laser are summarized. The latest research progress in RE doped photonic glasses, fibers, and devices are reviewed with special emphasis on 1.0, 1.5, 2.0 and 3.0 μm wavelength bands. In addition, challenges, applications, and future advances of the near-and mid-infrared luminescence and laser in RE doped photonic glasses have also been dealt with.

For the first time to the best of our knowledge phosphate glass-clad optical fibers comprising tellurium (Te) semiconductor core have been fabricated using a molten core approach. The cores were found to be highly crystalline and phase-pure as evidenced by X-ray diffraction (XRD) and corroborated by Micro-Raman spectrum. Elemental analysis across the core/clad interface suggests that there is some diffusion of oxygen and phosphorus into the core region and, conversely, diffusion of Te into the cladding region. Unfortunately, the propagation loss of the Te core fibers was too high to measure due to the significant scattering from the grain boundaries, oxygen and phosphor precipitates. However, the larger Raman gain, infrared and terahertz transparency of tellurium over silicon and germanium should make these fibers of significant value for fiber-based mid- to long-wave infrared, terahertz waveguides and Raman-shifted infrared light sources.

Transparent Er3+-doped germanotellurite glass ceramics (GCs) with variable Te/Ge ratio were prepared by controllable heat-treated process. X-ray diffraction (XRD) and transmission electron microscope (TEM) confirmed the formation of nanocrystals in glass matrix. Raman spectra were used to investigate the evolution of glass structure and photon energy. Fourier transform infrared (FTIR) spectra were introduced to characterize the change of hydroxyl group (OH−) content. Enhanced 2.7 μm emission was achieved from Er3+-doped GCs upon excitation with a 980 nm laser diode (LD), and the influence of GeO2 concentration and heat-treated temperature on the spectroscopic properties were also discussed in detail. It is found that the present Er3+-doped GC possesses large stimulated emission cross section at around 2.7 μm (0.85 × 10−20 cm2). The advantageous spectroscopic characteristics suggest that the obtained GC may be a promising material for mid-infrared fiber lasers.

ZnO–TeO2–P2O5 glasses were prepared by melt-quenching method. The color of the glass samples changed from colorless to pale red and dark red with increasing TeO2 content. Coloration mechanism and nonlinear optical properties of ZnO–TeO2–P2O5 glasses have been investigated. Raman spectra and transmission electron microscope measurements indicated the precipitation of ZnTe quantum dots in the glasses and ZnTe quantum dots are the origin of coloration. Z-scan technique was used to examine the nonlinear optical properties of the glasses. The glass sample with 30 mol% TeO2 exhibits large third-order nonlinear optical susceptibility of 10−11 esu.

Transparent Er3+-doped germanotellurite glass ceramics (GCs) with variable Te/Ge ratio were prepared by controllable heat-treated process. X-ray diffraction (XRD) and transmission electron microscope (TEM) confirmed the formation of nanocrystals in glass matrix. Raman spectra were used to investigate the evolution of glass structure and photon energy. Fourier transform infrared (FTIR) spectra were introduced to characterize the change of hydroxyl group (OH−) content. Enhanced 2.7 μm emission was achieved from Er3+-doped GCs upon excitation with a 980 nm laser diode (LD), and the influence of GeO2 concentration and heat-treated temperature on the spectroscopic properties were also discussed in detail. It is found that the present Er3+-doped GC possesses large stimulated emission cross section at around 2.7 μm (0.85 × 10−20 cm2). The advantageous spectroscopic characteristics suggest that the obtained GC may be a promising material for mid-infrared fiber lasers.

The laser performance of Er3+-doped tellurite fiber lasers operating at 2.7 μm due to 4I11/2 → 4I13/2 transition has been theoretically studied by using rate equations and propagation equations. The effects of pumping configuration and fiber length on the output power, slope efficiency, threshold, and intracavity pump and laser power distributions have been systematically investigated to optimize the performance of fiber lasers. When the pump power is 20 W, the maximum slope efficiency (27.62%), maximum output power (5.219 W), and minimum threshold (278.90 mW) are predicted with different fiber lengths (0.05–5 m) under three pumping configurations. It is also found that reasonable output power is expected for fiber loss below 2 dB/ m. The numerical modeling on the two- and three-dimensional laser field distributions are further analyzed to reveal the characteristics of this multimode step-index tellurite fiber. Preliminary simulation results show that this Er3+-doped tellurite fiber is an excellent alternative to conventional fluoride fiber for developing efficient 2.7 μm fiber lasers.

The morphology of hexagonal phase NaYF4:Er3+ nanorods synthesized by hydrothermal method changed greatly after a continuing calcination, along with a phase transformation to cubic phase. Photoluminescence (PL) spectra indicated that mid-infrared (MIR) emission was obtained in both hexagonal and cubic phase NaYF4:Er3+ nanocrystals for the first time. And the MIR emission of NaYF4:Er3+ nanocrystals enhanced remarkably at higher calcination temperature. To prevent uncontrollable morphology from phase transformation, the cubic phase NaYF4:Er3+ nanospheres with an average size of ~100 nm were prepared via a co-precipitation method directly. In contrast, the results showed better morphology and size of cubic phase NaYF4:Er3+ nanocrystals have realized when calcined at different temperatures. And PL spectra demonstrated a more intense MIR emission in the cubic phase NaYF4:Er3+ nanocrystals with an increasing temperature. Besides, the MIR emission peak of Er3+ ions had an obvious splitting in cubic phase NaYF4. Therefore, cubic phase NaYF4:Er3+ nanospheres with more excellent MIR luminescent properties seems to provide a new material for nanocrystal-glass composites, which is expected to open a broad new field for the realization of MIR lasers gain medium.

Silicate glasses containing PbS quantum dots (QDs) with narrow size distribution were prepared through heat treatment. Transmission electron microscopy (TEM) results show that spherical PbS QDs are densely dispersed in the glassy matrix. Using ZnS–PbO to replace PbS as precursor of PbS QDs, the size distribution of PbS QDs in glasses becomes more uniform. Tunable infrared luminescence from 1100 to 2200 nm has been obtained by controlling the glassy matrix and preparation parameters. Obvious optical amplification at communication windows of 1.55 μm and 1.33 μm is probed, and the PbS QDs doped glasses using ZnS–PbO as precursor exhibit larger optical amplification. The PbS QDs doped glasses with intense optical amplification are considered to be promising candidate as gain medium for broadband fiber amplifier and tunable fiber laser.

Spectroscopic properties and energy transfer (ET) in Bi2O3(PbO)–GeO2–Ga2O3 (BPGG) glass doped with Tm3+ and/or Tb3+ have been investigated. It is noted that the Tm3+ single-doped BPGG glass exhibits broad 1.47-μm fluorescence peaked at 1465 nm with a full width at half-maximum (FWHM) of ∼134 nm. The incorporation of Tb3+ into Tm3+-doped BPGG glass could significantly decrease the 1.80 μm emission intensity and enhance the intensity ratio of 1.47-μm to 1.80-μm (I1.47/I1.80), which reveals that Tb3+ ion can be considered to be an effective sensitizer ion on improving the 1.47-μm emission. The products of FWHM × σ e p e a k and τ f × σ e p e a k for the 1.47-μm fluorescence are in the range of 5.66–6.63 × 10−26 cm3 and 8.76–10.02 × 10−25 cm2 s. Effects of Bi2O3 substitution for PbO on spectroscopic properties, such as 1.47-μm emission of Tm3+, Judd–Ofelt intensity parameters Ωt (t = 2, 4, 6), and the lifetime of the 3H4 level of Tm3+, have also been investigated.

Bi2Te3-based materials have been reported to be one of the best room-temperature thermoelectric materials, and it is a challenge to substantially improve their thermoelectric properties. Here novel Bi2Te3 core fibers with borosilicate glass cladding were fabricated utilizing a modified molten core drawing method. The Bi2Te3 core of the fiber was found to consist of hexagonal polycrystalline nanosheets, and polycrystalline nanosheets had a preferential orientation; in other words, the hexagonal Bi2Te3 lamellar cleavage more tended to be parallel to the symmetry axis of the fibers. Compared with a homemade 3-mm-diameter Bi2Te3 rod, the polycrystalline nanosheets’ preferential orientation in the 89-μm-diameter Bi2Te3 core increased its electrical conductivity, but deduced its Seebeck coefficient. The Bi2Te3 core exhibits an ultrahigh ZT of 0.73 at 300 K, which is 232% higher than that of the Bi2Te3 rod. The demonstration of fibers with oriented nano-polycrystalline core and the integration with an efficient fabrication technique will pave the way for the fabrication of high-performance thermoelectric fibers.

Amorphous selenium (a-Se) core fibers with glass cladding have immediately been fabricated using the molten core method. The high aspect ratio of the a-Se core fibers and the presence of the glass cladding surrounding the Se core make it convenient to convert the a-Se core into a polycrystalline structure. It is found that the two-step thermal annealing process allows for the increase of crystal grain size and decrease of the structure defects in the Se core. Therefore, a low propagation loss of 1.5 dB/cm at 1310 nm has been realized for the polycrystalline selenium (c-Se) core optical fibers obtained by the wo-step annealing a-Se core fibers, first at a 80 °C low temperature annealing followed by a 207 °C high temperature annealing, which is much lower than that reported for c-Se core optical fibers (2.6 dB/cm).

We demonstrated a kHz-linewidth single-frequency laser at 1.95 μm using the self-developed heavily Tm(3+)-doped single-mode germanate glass fiber with the net gain coefficient of 2.3 dB per centimeter. The maximum output power of the stable single longitudinal mode continuous wave laser is over 200 mW. The slope efficiency measured versus the absorbed pump power is 34.8%, the signal-to-noise ratio is higher than 68 dB and laser linewidth is less than 7 kHz. A wavelength-tuning from 1949.55 to 1951.23 nm was also demonstrated based on changing the tension on the fiber Bragg grating outside the cavity.

We present the demonstration of a compact linearly polarized low noise narrow-linewidth single-frequency fiber laser at 1014 nm. The compact fiber laser is based on a 5-mm-long homemade Yb(3+)-doped phosphate fiber. Over 164 mW stable continuous-wave single transverse and longitudinal mode lasing at 1014 nm has been achieved. The measured relative intensity noise is less than -135 dB/Hz at frequencies of over 2.5 MHz. The signal-to-noise ratio of the laser is larger than 70 dB, and the linewidth is less than 7 kHz, while the obtained linear polarization extinction ratio is higher than 30 dB.

An all-fiber 10.9 W single-frequency one-stage linearly-polarized master-oscillator power amplifier (MOPA) laser at 1560 nm has been demonstrated. The laser linewidth is less than 3.5 kHz and the polarization-extinction ratio (PER) is greater than 24 dB. The measured signal-to-noise ratio (SNR) is higher than 70 dB and the optical-to-optical conversion efficiency is 29.5%. No obvious stimulated Brillouin scattering and the devastating effects of unwanted coupling light were observed.

Tm3+ doped lead silicate glasses with good thermal stability were prepared by the melt-quenching method. Based on the absorption and emission spectra, Judd-Ofelt intensity parameters, absorption and emission cross sections, gain spectra, and σe × FWHM were calculated and analyzed. These results suggest that Tm3+ doped lead silicate glasses are promising as mid-infrared laser materials. Tm3+ doped lead silicate glass single mode (SM) fibers with cladding diameter of 125 μm and core diameter of 8.5 μm were then fabricated by the rod-in-tube technique. The Tm3+ doping concentration reached as high as 4.545 × 1020 ions/cm3. ~2.0 μm amplified spontaneous emission (ASE) was realized in a 3.5-cm-long as-drawn SM fiber when pumped by a homemade single mode 1560 nm fiber laser. The results indicate that these Tm3+ doped lead silicate glass single mode fibers are promising fiber material for 2.0 μm fiber laser applications.

To meet the growing demand for mid-infrared tunable fiber lasers, the spectroscopic and structural properties of Tm3+/Ho3+ co-doped barium tellurite glass fibers are systematically evaluated by absorption, Raman, and photoluminescence spectra measurements. The density, molar volume, refractive index, and glass transition temperature are assessed in detail to fully understand their basic physical and thermal properties. Benefitting from the multiple structural sites in a barium tellurite glass system, the maximum doping concentration of Tm2O3 reaches up to 6.0 wt.% without inducing any crystallization or phase separation. Such a high ion concentration is conducive to reducing the fiber length and obtaining an efficient laser output. Furthermore, an intense ~2.0 μm ultra-broad emission with a full width at half maximum (FWHM) of 382 nm is achieved in the Tm3+/Ho3+ co-doped sample upon excitation at 808 nm by properly adjusting Tm3+ concentration and fiber length. The larger emission cross-sections and higher gain coefficients along with excellent thermal stability indicate that this barium tellurite glass could be an attractive gain medium for mid-infrared ultra-broad tunable fiber lasers.

Er3+-doped transparent tellurite glass ceramics (GCs) containing PbTe3O7 nanocrystals were successfully prepared. The formation of PbTe3O7 nanocrystals was confirmed by X-ray diffraction (XRD) and transmission electron microscope (TEM). It can be deduced that Er3+ ions prefer to concentrate in the PbTe3O7 nanocrystals rather than in glass matrix. Intense 2.7 μm emission was achieved from Er3+-doped GCs upon excitation with a 980 nm LD. Besides, the present tellurite GC possesses large stimulated emission cross section (0.80 × 10−20 cm2) and a low pumping threshold around 2.7 μm indicating that the obtained GC might be an attractive candidate for mid-infrared laser or amplifier.

The purity of the synthesized orbital-angular-momentum (OAM) light in the fiber is inversely proportional to channel crosstalk level in the OAM optical fiber communication system. Here the relationship between the fiber structure and the purity is firstly demonstrated in theory. The graded-index optical fiber is proposed and designed for the OAM light propagation with the purity higher than 99.9%. 16 fiber modes (10 OAM modes) have been supported by a specific designed graded-index optical fiber with dispersion less than 35 ps/(km∙nm). Such fiber design has suppressed the intrinsic crosstalk to be lower than -30 dB, and can be potentially used for the long distance OAM optical communication system.

An experimental investigation on the effects of Niobium and Antimony (Nb and Sb) on the radiation response of the multicomponent phosphate glass used for higher optical gain single mode fiber has been carried out. The samples were irradiated with γ-rays up to 20 kGy. Optical absorption spectra and Electron Paramagnetic Resonance (EPR) were employed to investigate the radiation-induced-defects. We found that the radiation induced attenuation (RIA) decrease with increasing Nb5+ and Sb3+ doping concentration. Only the phosphorus oxygen hole center (POHC) related EPR signals were observed in the Nb5+-doped samples. Both POHC and phosphorus-oxygen electron centers (POEC) related EPR signals decrease obviously with 1 mol % Sb2O3 doping concentration. These experimental results were interpreted by a model that is based on competition between various defects and Nb- or Sb-ions for the holes and electrons generated by the absorption of γ-rays.

Enhanced broadband 1.8 μm emission has been demonstrated in Bi/Tm3+ co-doped fluorogermanate glasses under a selectable pumping scheme. Detailed studies based on structure analysis, absorption spectra, excitation and emission spectra, and dynamic luminescence decay curves indicate that the enhanced broadband 1.8 μm luminescence originates from extremely efficient energy transfer process from Bi to Tm3+. Furthermore, the detailed energy transfer mechanism between Bi and Tm3+ is rationally analyzed. This novel pumping scheme and sensitizing mechanism of Bi/Tm3+ co-doping could provide the experimental basis for ~2.0 μm laser flexibly pumped by visible lasers, optical parametric oscillators, commercial 808 and 980 nm laser diodes.

Cr3+/Tm3+ co-doped fluorogermanate glasses were prepared by melt-quenching technique, and their luminescent properties were investigated under excitation of Xenon lamp and 808 nm laser diode, respectively. Through strong sensitization of Cr3+, an enhanced 1.8 µm emission band of Tm3+: 3F4→3H6 can be achieved in an extremely extended excitation band of 380-900 nm. The possible energy transfer mechanism between Cr3+ and Tm3+ is rationally discussed. On the basis of experimental data, the highest energy transfer efficiency is up to 91.70%. Our results suggest this efficient Cr3+/Tm3+ co-doping system provides a selective pump mechanism for obtaining 1.8 µm laser and thus shows attractive applications in near-infrared fiber lasers and amplifiers.

The influence of fluoride and shielding gas (O2 or Ar) on the physical and spectroscopic properties of Er3+ doped TeO2-ZnO-ZnF2 glass system is investigated. The larger electronegativity of F than O accounts for the gradual decrease of refractive index, density, and J-O parameters with increasing ZnF2. An analysis on Fourier transform infrared transmission spectra reveals that the absorption coefficient of OH− around 3 μm as low as 0.247 cm−1 can be achieved when 30 mol% ZnF2 containing sample is treated with Ar gas during glass melting process. The reduction of OH− groups combined with the low multiphonon relaxation rate (207 s−1) contributes to the enhanced emissions at 1.5 and 2.7 μm, along with prolonged lifetimes of 4I11/2 and 4I13/2 levels. A high branching ratio (17.95%) corresponding to the Er3+: 4I11/2 → 4I13/2 transition, the large absorption and emission cross section (0.44 × 10−20 cm2 and 0.45 × 10−20 cm2), and good gain cross section demonstrate that oxyfluoride tellurite glass could be a promising material for a diode-pump 2.7 μm fiber laser.

Narrow linewidth microwave signals generated from a heterodyne detection configuration of a dual-wavelength (DW) single-frequency highly Er3+/Yb3+ co-doped phosphate fiber laser is presented. The oscillating cavity of the fiber laser consists of a dual-channel narrow-band fiber-Bragg-grating (DC-NB-FBG), a 0.7-cm-long Er3+/ Yb3+ co-doped phosphate fiber and a wideband FBG (WB-FBG). The wavelength selecting gratings are spatially separated to create partially separated resonant cavities. Highly Er3+/Yb3+ co-doped phosphate fiber ensures that the mode competition is relatively weak under low pump power. DW single-frequency lasing with laser linewidths of 3 kHz is achieved. A 12.014-GHz microwave signal with a 3-dB linewidth of 3 kHz is obtained from the heterodyne detection of the DW fiber laser.

Novel spinel hollow nanofibers were synthesized by a convenient single-nozzle electrospinning technique. Based on the composition and morphology results by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), a ‘gas-push’ model was proposed to elucidate the formation mechanism of hollow nanofibers during calcination. Photoluminescence (PL) properties of calcined ZnAl2O4: xCr3+ and MgAl2O4: xCr3+ nanofibers were investigated systematically. Under an excitation of 400 nm or 530–550 nm (4A2g→4T1g or 4A2g→4T2g transitions of Cr3+ ions), an emission band at 675–725 nm including a sharp emission line (R-line) at ∼687 nm with several vibrational sidebands was obtained. Compared with the spectroscopic properties of ZnAl2O4: xCr3+ nanofibers, the emission band was broadened and the decay lifetime was shortened for MgAl2O4: xCr3+ nanofibers. By calculation of crystal field strength, the value of Dq/B is estimated to be 3.25 and 2.72 for ZnAl2O4: xCr3+ and MgAl2O4: xCr3+ nanofibers, respectively. The difference of crystal field strength gives a good explanation of the spectroscopic and decay evolution between ZnAl2O4: xCr3+ and MgAl2O4: xCr3+ nanofibers.