Fiber Background Loss Research Articles

Realizing particle population inversion of 2.7 μm emission in heavy Er3+/Pr3+ co-doped low hydroxyl fluorotellurite glass for mid-infrared laser

In this work, the population bottleneck of Er3+: 4I11/2 → 4I13/2 was overcome for the first time in heavy Er3+/Pr3+ co-doped TeO2–BaF2–La2O3–LaF3 (TBLL) low hydroxyl fluorotellurite glasses. Infrared emission spectra and fluorescence lifetime decay curves reveal that Pr3+ ions could deplete the electrons from the Er3+: 4I13/2 level faster than those from the Er3+: 4I11/2 under 980 nm excitation. Specifically, the energy transfer (ET) efficiency of the Er3+: 4I13/2 → Pr3+: 3F3,4 process (ET1) reached 96.27%, while that of the Er3+: 4I11/2 → Pr3+: 1G4 process (ET2) is only 2.17% in the Er3+/Pr3+ co-doped glass. Additionally, the energy transfer mechanism of Er3+ and Pr3+ ions was investigated using the Dexter theory, where the energy transfer microscopic parameters CD-A are 13.21 × 10−40 cm6/s and 0.89 × 10−40 cm6/s for the ET1 and ET2 processes, respectively. Finally, a numerical simulations laser model was developed to discuss the laser properties of the Er3+/Pr3+ co-doped TBLL fibers. The simulation results indicate that a 2.7 μm laser with a maximum output power of 2.26 W and slope efficiency of 13.89% could be achieved when the fiber background loss is reduced to 0.5 dB/m. The above results suggest that the Er3+/Pr3+ co-doped TBLL glass has great potential applications in mid-infrared fiber lasers.

High content Er3+ doped ZBLAN glass: The spectral characteristics and high slope efficiency MIR laser investigation

The ZBLAN glass with high content of Er3+ was prepared and studied. The glass has a wide infrared transmittance range of 2.5–8.0 µm, which means a lower maximum phonon energy. By testing and analyzing the fluorescence spectrum and lifetime curves under different penetration depths, the intrinsic emission spectrum and lifetime of the sample are obtained without the influence of sub-absorption. Under 980 nm excitation, the maximum emission cross section and the lifetime of 2.7 µm are 6.10 × 10−21 cm2 and 6.49 ms, indicating that high-concentration doping has a larger laser quality factor. According to the rate equation and transmission equation, cascaded and non-cascaded fiber laser models are established to conduct research on 2.7 µm fiber lasers. The slope efficiency can reach 40.38% at a pump wavelength of 980 nm and a fiber background loss of 0.1 dB/m. The high emission cross section, long fluorescence lifetime and high slope efficiency output make ZBLAN glass heavily doped with Er3+ an ideal host material for mid-infrared lasers.

Experimental and numerical investigation of mid-infrared laser in Pr3+-doped chalcogenide fiber**Project supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 61605095), the Natural Science Foundation of Zhejiang Province, China (Grant No. LY19F050004), the Natural Science Foundation of Ningbo City (Grant No. 2015A610038), the Open Fund of the Guangdong Engineering Technology Research and Development

We report on a chalcogenide glass fiber doped with Pr3+ that can be used for commercialized 1.5- and 2- laser excitations by emitting broadband – fluorescence, which is extruded into a preform and then drawn into a step-index fiber. The spectroscopic properties of the fiber and glass are reported, and the mid-infrared fiber lasers are also numerically investigated. Cascade lasing is employed to increase the inversion population of the upper laser level. The particle swarm approach is applied to optimize the fiber laser parameters. The output power can reach 1.28 W at 4.89- wavelength, with a pump power of 5 W, excitation wavelength at , Pr3+ ion concentration at 4.22 ×1025 ions/m3, fiber length at 0.94 m, and fiber background loss at 3 dB/m.

Design of Highly Efficient Pr3+-Doped Chalcogenide Fiber Laser

This letter describes the design of a medium infrared laser based on a double clad structure of praseodymium (Pr3+)-doped chalcogenide glass. The overlap of the emission cross sections of Pr3+ in the transitions (3F2, 3H6 $\to $ 3H5, and 3H5 $\to $ 3H4) enables both transitions to simultaneously produce a single coherent wavelength in the range between 3.7 and $4.6~\mu \text{m}$ . A single pair fiber Bragg grating at the overlapping transition wavelengths is used to avoid fabrication complexity of a cascade pumping scheme. In addition, the proposed design takes advantages of the fact that one excited ion will radiate two photons in the overlapping transition wavelengths. In this letter, the laser performance is tested as a function of fiber length, pump power, signal wavelength, fiber background loss, and Pr3+-doped concentration. The modeling results reveal that 48% of slope efficiency could be produced when the fiber losses are about 1 dB/m.

E-band Nd3+ amplifier based on wavelength selection in an all-solid micro-structured fiber.

A Nd3+ fiber amplifier with gain from 1376 nm to 1466 nm is demonstrated. This is enabled by a wavelength selective waveguide that suppresses amplified spontaneous emission between 850 nm and 1150 nm. It is shown that while excited state absorption (ESA) precludes net gain below 1375 nm with the exception of a small band from 1333 nm to 1350 nm, ESA diminishes steadily beyond 1375 nm allowing for the construction of an efficient fiber amplifier with a gain peak at 1400 nm and the potential for gain from 1375 nm to 1500 nm. A peak small signal gain of 13.3 dB is measured at 1402 nm with a noise figure of 7.6 dB. Detailed measurements of the Nd3+ emission and excited state absorption cross sections suggest the potential for better performance in improved fibers. Specifically, reduction of the fiber mode field diameter from 10.5 µm to 5.25 µm and reduction of the fiber background loss to <10 dB/km at 1400 nm should enable construction of an E-band fiber amplifier with a noise figure < 5 dB and a small signal gain > 20 dB over 30 nm of bandwidth. Such an amplifier would have a form factor and optical properties similar to current erbium fiber amplifiers, enabling modern fiber optic communication systems to operate in the E-band with amplifier technology similar to that employed in the C and L bands.

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.

Mid-infrared nonlinear absorption in As2S3 chalcogenide glass.

We report mid-infrared (MIR) nonlinear absorption in As2S3 glasses which results from two-photon excitation of valence electron to the Urbach extension followed by strong linear absorption of excited states. The measured MIR nonlinear absorption can be 3 to 4 orders of magnitude stronger than the two-photon absorption in the near-infrared for similar laser intensities and does not result from contaminants, but it is intrinsic to As2S3 glasses. As2S3 fibers are widely used to generate supercontinuum by pumping them with high peak power laser pulses. For a 100 kilowatt peak power MIR soliton propagating in single mode As2S3 fiber, the nonlinear absorption can be of similar magnitude than the fiber background loss. Finally, for laser peak power around 1 MW, the MIR nonlinear absorption can be ~2 orders of magnitude larger than the fiber background loss in single mode As2S3 fiber.

Heavily Tm3+-Doped Silicate Fiber for High-Gain Fiber Amplifiers

We report on investigation the potential of a 7 wt% (8.35 × 1020 Tm3+/cm3) doped silicate fibers for high-gain fiber amplifiers. Such a high ion concentration significantly reduces the required fiber length of high-power 2 μm fiber laser systems and allows the high-repetition rate operation in 2 μm mode-locked fiber lasers. To evaluate the feasibility of extracting high gain-per-unit-length from this gain medium, we measure several key material properties of the silicate fiber, including the absorption/emission cross-sections, upper-state lifetime, fiber background loss, and photodarkening resistance. We show through numerical simulations that a signal gain-per-unit-length of 3.78 dB/cm at 1950 nm can be achieved in a watt-level core-pumped Tm3+-doped silicate fiber amplifier. In addition, an 18-dB 2013-nm amplifier is demonstrated in a 50-cm 7 wt% Tm3+-doped double-clad silicate fiber. Finally, we experimentally confirm that the reported silicate host exhibits no observable photodarkening.

Numerical Modeling and Optimization of Mid-Infrared Fluoride Glass Raman Fiber Lasers Pumped by $\hbox{Tm}^{3+}$-Doped Fiber Laser

Numerical modeling of multiorder cascaded fluoride glass Raman fiber laser (RFL) employing fiber Bragg gratings (FBGs) is presented. Calculations were compared with the recently reported results, and good agreement was achieved. The model was also used to optimize a fourth-order fluoride glass RFL pumped at 1.9 , which can generate a mid-infrared emission up to 3.36. The influences of launched pump power, fiber length, output coupling, fiber background loss, and insertion loss of FBG on laser performance were investigated. The fiber length of 53.1-85.5 m and reflectivity of output coupler of 0.34-0.51 were recommended to obtain a relative large output power. The calculated maximum output power of 2.96 W with slope efficiency of 19.2% and threshold of 24.7 W was obtained at launched pump power of 40 W. In addition, the strong effects of fiber background loss and insertion losses of FBGs on laser output performance emphasized the importance of development of high quality fiber and fluoride FBG writing technique.

Effect of the excited state absorption on the efficiency of erbium-doped DFB fiber lasers

In this paper, a model of erbium-doped distributed feedback fiber laser is proposed and discussed. The model is based on a fundamental matrix approach and accounts for all types of losses including the active fiber background loss and the loss originated from excited state absorption immanent to erbium ions. It is shown that the excited state absorption loss is a main limiting factor to the laser efficiency. The model permits to optimize the laser parameters including fiber Bragg grating strength and position of π-phase defect. It is shown that applying our optimization procedure the pump-to-signal conversion ratio can be increased by 60% for the same device length and pumping conditions.

Distributed Model for Actively Q-Switched Erbium-Doped Fiber Lasers

A distributed laser model is applied to explain the multi-peak shape of the pulses generated by the actively Q-switched erbium-doped fiber laser. In the model, all kinds of intra-cavity losses are taken into account, including the point losses produced by fiber splices and intra-cavity elements, and the distributed loss, such as the active fiber background loss and the loss caused by excited state absorption inherent in erbium-doped fibers. The model also takes into account the exact function that describes the switching dynamics of the Q-switch modulator. We show that the distributed model allows an accurate simulation of the laser pulse shape and energy, providing therefore a good agreement with the experimental results. The origin of a multi-peak pulse shape frequently observed in QS-EDFL and some other features that result from the model are discussed.

Theoretical Investigation of the Gain Profile of Erbium-Doped Fiber Amplifiers

An exact analytical expression for the gain profile of erbium-doped fiber amplifiers (EDFAs) is presented. It is shown theoretically that the shape of the gain profile of an EDFA is determined by the average gain per unit fiber length. It turns out that the optimum fiber length for minimum gain differences in an arbitrary bandwidth is exactly proportional to the desired gain and to the achieved gain differences. Furthermore it is shown that an erbium-doped fiber with a special wavelength-dependent fiber background loss produces a flat gain profile for any desired gain if the optimum fiber length is chosen. An analytical expression for the required background loss is given.

Noise performance evaluation of distributed erbium-doped fiber amplifiers with bidirectional pumping at 1.48 mu m

An accurate model for unsaturated fiber amplifiers with fiber background loss is used to determine optimal parameters for distributed amplification in erbium-doped fibers. Pump power at 1.48 mu m and erbium-doping levels required for transparency are studied for both unidirectional and bidirectional pumping schemes. An optimal erbium absorption coefficient which minimizes the required pump power is found. However, this case corresponds to the highest amplifier noise figure. The authors conclude that, in practice, distributed erbium-doped fiber amplifiers are not significantly advantageous compared to 980-nm-pumped lumped amplifier schemes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analysis of distributed erbium-doped fiber amplifiers with fiber background loss

An accurate model for erbium-doped fiber amplifiers with fiber background loss applicable to the unsaturated gain regime is presented. Exact analytical solutions are derived for the output pump power, gain, and amplified spontaneous noise as a function of input pump power in the cases of unidirectional or bidirectional pumping. An exact relation is also derived between the pump threshold and the pump required for fiber transparency. Such expressions are particularly useful to model distributed fiber amplifiers and to determine the optimal fiber parameters corresponding to a given pumping scheme. As an example, the analytical solutions are used to study the pump power requirement for distributed fiber amplifiers unidirectionally pumped at 1.48 mu m, and to determine an optimum Er/sup 3+/ absorption coefficient.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>