Zhuoqi Tang

Progress in rare-earth-doped mid-infrared fiber lasers.

The progress, and current challenges, in fabricating rare-earth-doped chalcogenide-glass fibers for developing mid-infrared (IR) fiber lasers are reviewed. For the first time a coherent explanation is forwarded for the failure to date to develop a gallium-lanthanum-sulfide glass mid-IR fiber laser. For the more covalent chalcogenide glasses, the importance of optimizing the glass host and glass processing routes in order to minimize non-radiative decay and to avoid rare earth ion clustering and glass devitrification is discussed. For the first time a new idea is explored to explain an additional method of non-radiative depopulation of the excited state in the mid-IR that has not been properly recognized before: that of impurity multiphonon relaxation. Practical characterization of candidate selenide glasses is presented. Potential applications of mid-infrared fiber lasers are suggested.

Study of mid-infrared laser action in chalcogenide rare earth doped glass with Dy 3+ , Pr 3+ and Tb 3+

We present a study of chalcogenide glass fiber lasers doped with Dy3+, Pr3+ or Tb3+ that would operate in the mid-infrared wavelength range. A set of chalcogenide glass samples doped with different concentrations of rare earth ions is fabricated. The modeling parameters are directly extracted from FTIR absorption measurements of the fabricated bulk glass samples using Judd-Ofelt, Fuchtbauer–Ladenburg theory and McCumber theory. The modeling results show that, for all the dopants considered, an efficient mid-infrared laser action is possible if optical losses are kept at the level of 1dB/m or below.

Mid-infrared supercontinuum generation to 12.5μm in large NA chalcogenide step-index fibres pumped at 4.5μm

We present numerical modeling of mid-infrared (MIR) supercontinuum generation (SCG) in dispersion-optimized chalcogenide (CHALC) step-index fibres (SIFs) with exceptionally high numerical aperture (NA) around one, pumped with mode-locked praseodymium-doped (Pr(3+)) chalcogenide fibre lasers. The 4.5um laser is assumed to have a repetition rate of 4MHz with 50ps long pulses having a peak power of 4.7kW. A thorough fibre design optimisation was conducted using measured material dispersion (As-Se/Ge-As-Se) and measured fibre loss obtained in fabricated fibre of the same materials. The loss was below 2.5dB/m in the 3.3-9.4μm region. Fibres with 8 and 10μm core diameters generated an SC out to 12.5 and 10.7μm in less than 2m of fibre when pumped with 0.75 and 1kW, respectively. Larger core fibres with 20μm core diameters for potential higher power handling generated an SC out to 10.6μm for the highest NA considered but required pumping at 4.7kW as well as up to 3m of fibre to compensate for the lower nonlinearities. The amount of power converted into the 8-10μm band was 7.5 and 8.8mW for the 8 and 10μm fibres, respectively. For the 20μm core fibres up to 46mW was converted.

Low loss Ge-As-Se chalcogenide glass fiber, fabricated using extruded preform, for mid-infrared photonics

Chalcogenide glass fibers have attractive properties (e.g. wide transparent window, high optical non-linearity) and numerous potential applications in the mid-infrared (MIR) region. Low optical loss is desired and important in the development of these fibers. Ge-As-Se glass has a large glass-forming range to provide versatility of choice from continuously varying physical properties. Recently, broadband MIR supercontinuum generation has been achieved in chalcogenide fibers by using Ge-As-Se glass in the core/clad. structure. In the shaping of chalcogenide glass optical fiber preforms, extrusion is a useful technique. This work reports glass properties (viscosity-temperature curve and glass transition) and optical losses of Ge-As-Se fiber fabricated from an extruded preform. A robust cut-back method of fiber loss measurement is developed and the corresponding error calculation discussed. MIR light is propagated through 52 meters of a fiber, which has the lowest loss yet reported for Ge-As-Se fiber of 83 ± 2 dB/km at 6.60 μm wavelength. The fiber baseline loss is 83-90 dB/km across 5.6-6.8 μm, a Se-H impurity absorption band of 1.4 dB/m at 4.5 μm wavelength is superposed and other impurity bands (e.g. O-H, As-O, Ge-O) are ≤ 20 dB/km. Optical losses of fiber fabricated from different positions of the extruded preform are investigated.

Refractive index dispersion of chalcogenide glasses for ultra-high numerical-aperture fiber for mid-infrared supercontinuum generation

We select a chalcogenide core glass, AsSe, and cladding glass, GeAsSe, for their disparate refractive indices yet sufficient thermal-compatibility for fabricating step index fiber (SIF) for mid-infrared supercontinuum generation (MIR-SCG). The refractive index dispersion of both bulk glasses is measured over the 0.4 µm–33 µm wavelength-range, probing the electronic and vibrational behavior of these glasses. We verify that a two-term Sellmeier model is unique and sufficient to describe the refractive index dispersion over the wavelength range for which the experimentally determined extinction coefficient is insignificant. A SIF composed of the glasses is fabricated and calculated to exhibit an ultra-high numerical aperture >0.97 over the entire wavelength range 0.4-33 µm suggesting that the SIF glass pair is a promising candidate for MIR-SCG. Material dispersion characteristics and the zero dispersion wavelength, both critical design parameters for SIF for MIR-SCG, are derived.

Mid-infrared photoluminescence in small-core fiber of praseodymium-ion doped selenide-based chalcogenide glass

Rare earth (RE)-ion doped chalcogenide glasses are attractive for mid-infrared (MIR) fiber lasers for operation >4 μm. Our prior modeling suggests that praseodymium (Pr) is a suitable RE-ion dopant for realizing a selenide-based, chalcogenide-glass, step index fiber (SIF) MIR fiber laser operating at 4-5 μm wavelength. Fabrication of RE-ion doped chalcogenide glass fiber, especially with a small core, is a demanding process because crystallization must be avoided during the heat treatments required to effect shaping. Here, a 500 ppmw (parts per million parts, by weight) Pr3+-doped Ge-As-Ga-Se glass SIF with a 10 μm or 15 μm diameter core is reported; the cladding glass is Ge-As-Ga-Se-S. The multistage process to produce the fiber is outlined. Thermal and optical properties of the core/clad. glass pair, and the crystalline/amorphous nature and optical behavior of the small core fiber are reported. MIR photoluminescence and lifetime of a RE-ion doped chalcogenide glass small core fiber are reported for the first time.

Superior photoluminescence (PL) of Pr 3+ -In, compared to Pr 3+ -Ga, selenide-chalcogenide bulk glasses and PL of optically-clad fiber

The photoluminescent-(PL)-properties of Pr³⁺-ions in indium-containing selenide-chalcogenide bulk-glasses are found to be superior when compared with gallium-containing analogues. We observe circa doubling of mid-infrared (MIR) PL intensity from 3.5 to 6 μm for bulk glasses, pumped at 1.55 μm wavelength, and an increased excited state lifetime at 4.7 μm. PL is reported in optically-clad fiber. Ga addition is well known to enhance RE³⁺ solubility and PL behavior, and is believed to form ([RE³⁺]-Se-[Ga(III)]) in the glasses. Indium has the same outer electronic-structure as gallium for solvating the RE-ions. Moreover, indium is heavier and promotes lower phonon energy locally around the RE-ion, thereby enhancing the RE-ion PL behavior, as observed here.

Mid-infrared emission in Tb 3+ -doped selenide glass fiber

The mid-infrared (MIR) emission behavior of Tb3+-doped Ge–As–Ga–Se bulk glasses (500, 1000, and 1500 ppmw Tb3+) and unstructured fiber (500 ppmw Tb3+) is investigated when pumping at 2.013 μm. A broad emission band is observed at 4.3–6.0 μm corresponding to F57→F67, with an observed emission lifetime of 12.9 ms at 4.7 μm. The F47 level is depopulated nonradiatively and so it is proposed that Tb3+-doped Ge–As–Ga–Se fiber may operate as a quasi-three-level MIR fiber laser. Underlying glass-impurity vibrational absorption bands are numerically removed to give the true Tb3+ absorption cross section, as required for Judd–Ofelt (J–O) analysis. Radiative transition rates calculated from J–O theory are compared with measured lifetimes. A numerical model of the three-level Tb3+-doped fiber laser is developed for Tb3+ doping of 8.25×1024  ions m−3 (i.e., 500 ppmw) and dependence of laser performance on fiber length, output coupler reflectivity, pump wavelength, signal wavelength, and fiber background loss is calculated. Results indicate the feasibility of an efficient three-level MIR fiber laser operating within 4.5–5.3 μm, pumped at either 2.013 or 2.95 μm.

The local environment of Dy3+in selenium-rich chalcogenide glasses

The environment of Dy3+ is investigated when it is added as DyCl3 or Dy foil into two base glasses, Ge16.5As19−xGaxSe64.5, where x = 3 or 10 at%, at doping levels between 0 and 3000 parts per million by weight (ppmw) Dy3+. Extended X-ray Absorption Fine Structure demonstrates the glasses doped with Dy foil, or less than 1000 ppmw Dy3+ as DyCl3, contain Dy ions that are fully incorporated into the glass network and are coordinated by 7–8 Se atoms. However, when the DyCl3 dopant is present in concentrations ≥1000 ppmw Dy3+ the environment is dominated by Dy–Cl bonds. Furthermore, these Dy–Cl environments are nanocrystalline, retaining chemical order beyond the first coordination shell. By comparison with XRD and FTIR results, we report that the presence of α-Ga2Se3 crystallites in the glass, and the increased optical scattering in the fibres, are both related to the presence of DyCl3 crystallites.

The effect of the nature of the rare earth additive on chalcogenide glass stability

To realize the goal of mid-infrared lasing in rare-earth-ion-doped chalcogenide glass fibers, it is important to achieve as large a concentration as possible of the rare earth ion dopants in the host chalcogenide glass matrix, while minimizing optical loss. However, when a large amount of rare earth dopant is added to the chalcogenide glass, solubility problems can emerge which decrease glass stability and also impair optical properties. In this paper, the nature of the rare earth additive is demonstrated to be pivotal, affecting both the chalcogenide glass stability and the optical scattering loss. Dysprosium in the form of dysprosium metal foil, or as the salt dysprosium trichloride, is added to the Ge16.5As9Ga10Se64.5 batch and glasses are obtained using the melt-cool method. The nominal Dy3+ concentration is 2000 ppm. The glass melting results, and powder X-ray diffraction and Fourier transform infrared spectrometry characterization of the products, indicate major improvement in bulk glass stability, glass surface quality and optical loss when the dysprosium additive to the glass batch is in the form of Dy0 (metal foil) rather than DyIIICl3. Reasons for this improvement are suggested.