Co-doped Silica Fiber Research Articles

γ irradiation induced effects on bismuth active centres and related photoluminescence properties of Bi/Er co-doped optical fibres.

We investigate the effects of γ irradiation on bismuth active centres (BACs) and related photoluminescence properties of bismuth/erbium co-doped silica fibre (BEDF), [Si] ~28, [Ge] ~1.60, [Al] ~0.10, [Er] ~ <0.10 and [Bi] ~0.10 atom%, fabricated by in-situ solution doping and Modified Chemical Vapor Deposition (MCVD). The samples were irradiated at 1 kGy, 5 kGy, 15 kGy, 30 kGy and 50 kGy doses, and dose rate of 5.5 kGy/h, at room temperature. The optical properties of BEDF samples are tested before and after γ irradiation. We found that high dose γ irradiation could significantly influence the formation and composition of BACs and their photoluminescence performance, as important changes in absorption and emission properties associated with the 830 nm pump produces the direct evidence of γ irradiation effects on BAC-Si. We notice that the saturable to unsaturable absorption ratio at pump wavelength could be increased with high dose γ irradiation, indicating that emission and pump efficiency could be increased by γ irradiation. Our experimental results also reveal good radiation survivability of the BEDF under low and moderate γ irradiation. Our investigation suggests the existence of irradiation related processing available for tailoring the photoluminescence properties and performance of bismuth doped/co-doped fibres.

50 μm core diameter Yb³⁺Al³⁺/F⁻ codoped silica fiber with M²

This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a Yb(3+)/Al(3+)/F(-) codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter >6 mm was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean M2<1.1 in this 50 μm core diameter fiber was achieved at a bend diameter of 0.35 m. The core glass with the refractive index nearly equal to pure silica glass is suitable for the fiber design, such as large-mode-area photonic crystal fiber.

Effects of macro-bending on 1500-nm amplified spontaneous emission, gain, and noise figure of erbium-gallium co-doped fiber

The relationships among macro-bending loss, power of the 1500-nm amplified spontaneous emission (ASE), gain, and noise figure of an erbium-gallium co-doped silica fiber amplifier are investigated and explained. The dependence of macro-bending loss on different bending radii is examined. Using different fiber lengths and bending radii, the effects of macro-bending on ASE, gain, and noise figure are analyzed in comparison to an unbent fiber. The ASE power changes because macro-bending alters the number of Er3+ ions in the I13/24 level that decay to the I15/24 level emitting photons of shorter and longer wavelengths. The trade-off relationship that exists between the change in the ASE power and signal loss, where both result from macro-bending, explains the gain change. Fiber length also affects the changes in the ASE power and gain. Noise figure in the longer-wavelength region increases. In the shorter-wavelength region, for a long fiber, the noise figure improves only slightly. For a short fiber, it worsens due to gain decrement. The findings from this study explain the reason for gain improvement upon suppressing either a competing or a noncompeting ASE via filters or macro-bending in other rare-earth-doped fibers.

Enhanced Erbium–Zirconia–Yttria–Aluminum Co-Doped Fiber Amplifier

An efficient optical amplifier is demonstrated using an improved erbium–zirconia–yttria–aluminum co-doped fiber (Zr-EDF) as the gain medium. With a combination of both Zr and Al, we could achieve a high erbium doping concentration with an absorption loss of around 80.0 dB/m at 980 nm. The Zr-EDF is obtained from a fiber preform, which is fabricated in a ternary glass host, i.e., a zirconia–yttria–aluminum co-doped silica fiber, using a modified chemical vapor deposition, in conjunction with a solution doping process. At the optimum length of 1 m, the Zr-EDF amplifier produces a flat gain of 38 dB within a wavelength region between 1530 and 1565 nm with a gain variation of less than 3 dB when the input signal power and 980 nm pump power are fixed at $-$ 30 dBm and 130 mW, respectively. The highest gain of 40.3 dB is obtained at 1560 nm wavelength. Compared with the previous Zr-EDF amplifier, the proposed Zr-EDFA shows improved gain, particularly at longer wavelengths. The gain is enhanced by about 15.8 dB at a wavelength of 1560 nm for an input signal of $-$ 30 dBm.

Enhanced green upconversion luminescence in Yb3+/Tb3+-codoped silica fiber based on glass phase-separated method

We reported on an Yb3+/Tb3+-codoped silica fiber with a large fiber core prepared from nanoporous silica glass based on glass phase-separated method. The measured refractive index profile indicated an excellent homogeneity of the doped active fiber core. Intense green upconversion emission from Tb3+ centered at 543 nm was obtained in the Yb3+/Tb3+-codoped silica fiber under 976-nm excitation. It is suggested that the green upconversion emission is dominated by a two-photon absorption process. It is found that the Al3+ ions as a modifier can facilitate the energy transfer from Yb3+ to Tb3+ in the porous glass fiber. The energy transfer efficiency from Yb3+ to Tb3+ was calculated.

Spectroscopy and 2 μm laser behaviors of Al3+/Ho3+ CO-doped silica fiber

A Ho3+-doped silica glass, 0.08Ho2O3–1.6Al2O3–98.32SiO2 (mol%), was prepared by the sol–gel method combined with high temperature sintering. The 2μm luminescence properties of the Al3+/Ho3+ co-doped silica core glass were investigated. A single-mode double cladding Al3+/Ho3+ co-doped silica fiber with a 15μm core diameter and 0.08 core numerical apertures was prepared. Pumped with a 1.95μm Tm3+ fiber laser, 320mw laser output centered at 2040nm with M2 factor of 1.14 was obtained from a 150-cm-long double-cladding fiber. The laser has a threshold of 850mw launched power with a slope efficiency of 48.46%. For the first time a robust single-mode 2μm fiber laser was realized in a fiber with Al3+/Ho3+ co-doped silica core glass prepared by the sol–gel method.

Photodarkening-Induced Absorption and Fluorescence Changes in Yb Fibers

We study the photodarkening effect in a double clad ytterbium-doped silica fiber fabricated by modified chemical vapor deposition and solution doping. A common measurement technology for photodarkening is used to study the photodarkening-induced absorption spectra. We present a fast and simple method to observe the PD effect by measurement of the fluorescence spectra. The method proposed here can be used to observe the influence for a short time, and thus reduces the requirements of system stability and heat management. It is obtained that under the condition of 45.5% Yb ion inversion level, photodarkening-induced excessive loss at an equilibrium state is above 4.5dB/m and florescence intensity degradation is above 10% after 500min pumping at 1041 nm for the home-made normal Yb/Al co-doped silica fiber.

Characterization of Nb/Al codoped silica fiber by writing long-period gratings with CO2 laser

In this paper, we report a long-period grating (LPG) in Nb/Al codoped silica fiber with CO2 laser pulse writing technology. The temperature and strain characteristics of this Nb/Al-LPG were studied experimentally. Based on the experimental results, the thermooptic coefficient and elastooptic coefficient of the core of the Nb/Al codoped fiber were evaluated to be 7.99×10−6/°C and −0.222.

The three-electron bond =Si&lt;O_2:∙Yb absorption center of pre-darkened ytterbium-doped silica

The formation and bleaching of color centers during annealing of pre-darkened ytterbium-doped silica fibers is modeled by three-electron bond (TEB) = Si<O(2):∙Yb absorption centers. The nature of a center and underlying mechanism for its annealing in formation, shift and dissociation of chemical bonds is described in terms of a Markov statistical model with state change set by Bose-Einstein phonon statistics. The center hold one terminal and four active states with activation energies for transitions among these found to match bond energies of molecular oxygen in ionic character bonds of 1 and 1½ bond order. Experimentally observed in- and decrease in absorption during ramp and isothermal annealing of pre-darkened ytterbium co-doped silica fibers are hereby matched by a set of = Si<O(2):∙Yb centers.

Suppression of photo‐darkening effect by Ca additive in Yb‐doped silica glass fibre

It is found that Ca additive effectively suppresses the photo-darkening effect in Yb-doped silica fibre even at 6.0 wt% of high Yb2O3 concentration. Ca ion works as a stabiliser to maintain the Yb3+ valence state. It is hypothesised that this phenomenon may reflect the local charge balance effect in silica glass. This Yb-Ca co-doped silica fibre can be useful for high-power fibre lasers.

All-fiber passively mode-locked thulium/holmium laser with two center wavelengths

We have demonstrated a self-starting, passively mode-locked Tm/Ho codoped fiber laser that lases at one of two center wavelengths. An amplified 1.56 μm distributed feedback laser pumps a ring laser cavity which contains 1 m of Tm/Ho codoped silica fiber. Mode locking is obtained via nonlinear polarization rotation using a c-band polarization sensitive isolator with two polarization controllers. The laser is able to pulse separately at either 1.97 or 2.04 μm by altering the intracavity polarization during the initiation of mode locking. The codoped fiber permits pulsing at one of two wavelengths, where the shorter is due to the Tm(3+) emission and the longer due to the Ho(3+) emission. The laser produces a stable pulse train at 28.4 MHz with 25 mW average power, and a pulse duration of 966 fs with 9 nm bandwidth.

Theoretical analysis of characteristics for 2 μm Tm3+:Ho3+ co-doped silica fiber laser pumped by a 1550 nm fiber laser

We established a theoretical model of 2μm Tm3+:Ho3+ co-doped silica fiber laser pumped by a 1550nm fiber laser based on the rate-equation theory and performed the numerical simulation using Runge–Kutta algorithm and Newton–Raphson algorithm. The intracavity power distributions of both pump and laser of the Tm3+:Ho3+ co-doped silica fiber laser based on the Tm3+:Ho3+ co-doped silica fiber supplied by the National Optics Institute in Canada (NOIC) were obtained. The effects of the output reflectivity R4(λs) at the output laser wavelength λs and the concentrations of Tm3+ and Ho3+ in the fiber on laser output performance were analyzed. In order to achieve a high laser output power, the optimal R4(λs) of 0.13 was verified and the optimal Tm:Ho ratio of 1:2.4 was proposed. Finally, better output performance for the fiber laser based on the optimized Tm3+:Ho3+ co-doped silica fiber was obtained than the laser using the fiber supplied by the NOIC. This theoretical model and numerical simulation results will guide the fabrication of 2μm Tm3+:Ho3+ co-doped all-fiber lasers pumped by 1600-nm-band (1500–1750nm) Er3+:Yb3+ co-doped silica fiber lasers.

Upconversion luminescence in Tm3+/Yb3+ co-doped double-clad silica fibers under 980 nm cladding pumping

An investigation is reported of the visible and near-infrared upconversions in Tm3+/Yb3+ co-doped double-clad silica fibers (TYDFs) under excitation at 980 nm. The TYDFs used were fabricated using the modified chemical vapor deposition (MCVD) and solution doping techniques. Three distinct upconversion luminescences were observed at wavelengths of 482, 649 and 816 nm and their intensities found to increase with Yb3+ concentration. The intensity of blue and red fluorescence bands at 482 and 649 nm were found to be the highest with LTY-8 fiber, which had Tm3+ and Yb3+ concentrations of 5.6 × 1019 and 15.5 × 1019 ions/cc, respectively. The upconversion luminescence intensity was also observed to decrease with an increase in temperature. The main emission switched from 482 nm to 816 nm as the temperature increased above 200°C.

Four-Wave-Mixing in Zirconia-Yttria-Aluminum Erbium Codoped Silica Fiber

The generation and characterization of the Four-Wave-Mixing (FWM) effect in an Erbium Doped Zirconia-Yttria-Alumino Silicate Fiber (EDZF) is described. The EZDF is fabricated from a conventional silica preform by Modified Chemical Vapour Deposition (MCVD) and also solution doping to add glass modifiers and nucleating agents, with the resulting preform annealed and drawn into a fiber strand with a 125 ± 0.5 µm diameter. A 4 m long EZDF, ZEr-B with a propagation loss of 0.68 dB/m and an erbium concentration of 3000 ppm is used to investigate the FWM effect. The FWM power levels are measured to be approximately - 45 dBm at a region of 1565 nm and show good agreement with the theoretical predicted values. A non-linear coefficient of 14 W−1km−1 is also measured, along with chromatic and slope dispersion values of 28.45 ps/nm.km and 3.63 ps/nm2.km, which agree with the predicted values. The fabricated ZEr-B fibre has many potential applications utilizing the FWM effect, including the generation of multi-wavelength outputs.

Photo darkening of rare earth doped silica

The photo darkening (PD) absorption spectra from unseeded amplifier operation (by 915 nm pumping) of ytterbium / aluminum and co-doped silica fibers is after prolonged operation observed to develop a characteristic line at 2.6 eV (477 nm). This line is proposed to be due to inter center excitation transfer from type II oxygen deficiency centers ODC(II) to Tm3+ trace impurities. The ODC(II) is proposed to be the result of a displacive transition of a 4-fold silica ring hosting two 3-fold silicon units that presents two non-bridging oxygen to Yb3+ (as part of its 6-fold coordination by oxygen). The displacive transition is initiated by a charge disproportionation process which leads to NBO transfer in forming dioxasilirane (2-fold coordinated silicon with two NBO attached) next to silylene (2-fold coordinated silicon with a lone electron pair). In collaboration with a valence electron of Yb3+ a new ½ / 1½ chemical bond is formed on dioxasilirane which comprises the PD color center for the visible and near-infrared. Difference in solid acidity of the silica material co-doped with Yb/Al and Yb/P may explain the observed difference in spectral shapes by change of bond order to the formed chemical bond.

Double-pass erbium-doped zirconia fiber amplifier for wide-band and flat-gain operations

The double-pass erbium-doped zirconia fiber amplifier (EDZFA) is proposed and demonstrated to provide a wide-band amplification as well as flat-gain operation in both the C- and L-band regions using only a single-gain medium. The proposed amplifier utilizes an erbium-doped zirconia fiber (EDZF) with erbium ion concentration of 2800 ppm as a gain medium. The medium is fabricated in a ternary glass host, zirconia–yttria–aluminum codoped silica fiber through solution doping technique along with modified chemical vapor deposition (MCVD). Compared to a single-pass operation, the double-pass EDZFA shows a better gain performance. At input signal power of 0 dBm and the optimum EDZF length of 2 m, a flat gain of around 16 dB is achieved by the proposed double-pass amplifier with gain variation of approximately 2.5 dB throughout the wavelength range from 1530 to 1590 nm. However, the noise figure of the double-pass amplifier is slightly higher than that of the single-pass due to inefficient population inversion at the input part of the amplifier.

Wideband EDFA Based on Erbium Doped Crystalline Zirconia Yttria Alumino Silicate Fiber

A wideband erbium-doped fiber amplifier (EDFA) is demonstrated using an Erbium-doped zirconia fiber as the gain medium. With a combination of both Zr and Al, we could achieve a high erbium doping concentration of 4320 ppm in the glass host without any phase separations of rare-earths. The Erbium doped fiber (EDF) is obtained from a fiber preform, which is fabricated in a ternary glass host, zirconia-yttria-aluminum codoped silica fiber using a MCVD process. Doping of Er2O3 into Zirconia yttria-aluminosilicate based glass is done through solution doping process. The maximum gain of 21.8 dB is obtained at 1560 nm with 2 m long of EDF and co-pumped with 1480 nm laser diode. At high input signal of -4 dBm, a flat-gain at average value of 8.6 dB is obtained with a gain variation of less than 4.4 dB within the wavelength region of 1535-1605 nm and using 3 m of EDF and 100 mW pump power. The corresponding noise figure is maintained below 9.6 dB at this wavelength region.

Theoretical modelling of dual-wavelength pumpedYb3 + –Tm3 + co-doped silica fibre laser

Numerical simulations have, for the first time to our knowledge, been carried out to characterize theYb3 + –Tm3 + co-doped silica fibre laser (YTFL), defined by a fibre grating and an end mirror, by usingthe rate equations, which take into consideration both the energy transfer processes fromYb3 + toTm3 + ions and the cross-relaxation processes among differentTm3 + ions. A dual-wavelength pumping scheme with one at 805 nm and the other at 975 nm is usedto pump the YTFL. We have investigated the wavelength-dependent output power ofthe YTFL, from 1750 to 2200 nm, which takes its maximum output power at ∼ 1800 nm. The effect of the cross-relaxation processes in theTm3 + -doped silica fibre laser has been studied. The results indicate that these processes are beneficialto the laser and should be considered in the theoretical modelling. The influence of theYb3 + concentration on the characteristics of the YTFL has also been analysed and the results show thatYb3 + dopants can improve the output power and slope efficiency of the laser.

Performance comparison of Zr-based and Bi-based erbium-doped fiber amplifiers

In this Letter, we present a comprehensive comparison of the performance of a zirconia-based erbium-doped fiber amplifier (Zr-EDFA) and a bismuth-based erbium-doped fiber amplifier (Bi-EDFA). The experimental results reveal that a Zr-EDFA can achieve comparable performance to the conventional Bi-EDFA for C-band and L-band operations. With a combination of both Zr and Al, we could achieve a high erbium-doping concentration of about 2800 ppm (parts per million) in the glass host without any phase separations of rare earths. The Zr-based erbium-doped fiber (Zr-EDF) was fabricated using in a ternary glass host, zirconia-yttria-aluminum codoped silica fiber through a solution-doping technique along with modified chemical vapor deposition. At a high input signal of 0 dBm, a flat gain at average value of 13 dB is obtained with a gain variation of less than 2 dB within the wavelength region of 1530-1575 nm and using 2 m of Zr-EDF and 120 mW pump power. The noise figures are less than 9.2 at this wavelength region. It was found that a Zr-EDFA can achieve even better flat-gain value and bandwidth as well as lower noise figure than the conventional Bi-EDFA.

Spectral characteristics of the erbium-bismuth co-doped silica fibers and its application in single frequency fiber laser

We developed two kinds of erbium (Er)-bismuth (Bi) co-doped silica fibers with different erbium doping concentrations. After investigation of its spectral characteristics, it was found that the co-doping of the bismuth could alleviate the clustering of the erbium ions in the silica fiber and thus improve the fluorescence efficiency. However, the near-infrared (NIR) fluorescence spectrum attributed to the bismuth ions was different from that reported before and needs further comprehensive investigation. For the heavy Er doped erbium-bismuth (Er-Bi) co-doped fiber (about 8000 ppm Er concentration), the optical absorption coefficients at the wavelengths of 976 and 1530 nm are more than 40 and 90 dB/m, respectively. A couple of fiber Bragg gratings with proper reflectivity were chosen to be spliced to 2 cm long of this fiber forming a compact linear cavity fiber laser. Upon the excitation of the 976 nm laser diode, stable single frequency fiber laser operation was realized with a maximum side mode suppression ratio more than 60 dB and a line-width less than 1 kHz. By splicing a segment of the Er fiber to the output end of the fiber laser, the fiber laser worked in a master oscillator pumping amplifier (MOPA) structure and emitted an increased laser power of more than 100 mW.