Ytterbium-doped Silica Fiber Research Articles

Performance Enhancement of Ytterbium-Doped Fiber Amplifier Employing a Dual-Stage In-Band Asymmetrical Pumping

The performance of doped fiber amplifiers can be enhanced significantly with the help of multi-stage pumping technique provided that various critical parameters of pumps including their optical power and wavelength are optimized. We report the performance enhancement of a ytterbium doped fiber amplifier (YDFA) for a 1.02–1.08 m spectral region with an optimized design based on a novel dual-stage in-band asymmetrical pumping scheme. By accurately adjusting the optical power and wavelength of pumps in both the stages, a record peak gain of around 62.5 dB and output power of 4.5 W are achieved for a signal wavelength of 1.0329 m at an optimized length of Ytterbium-doped silica fiber and optimized doping concentration of Yb. Moreover, a minimum noise figure (NF) of 4 dB is observed for a signal wavelength of 1.0329 m at the optimized parameters. Similarly, the effect of using high and low pump powers at the first and the second stage, respectively, on NF of the amplifier is also investigated at different values of signal powers. It is observed that the value of NF increases significantly by using high pump power at the first stage and low pump power at the second stage.

Temperature dependence of the spectral properties of Yb3+/P5+/Al3+ co-doped silica fiber core glasses

Yb3+/P5+/Al3+ co-doped silica fiber core glasses with different P5+/Al3+ molar ratios were prepared by the sol-gel method and modified chemical vapor deposition (MCVD) method. The absorption and fluorescence spectra were measured using a temperature range from 25°C to 200°C. Results show that both the major absorption and emission peaks of the Yb3+ ions become weaker and wider with the temperature increasing. The absorption cross sections of HYPA1 (the molar compositions of 0.2Yb2O3-2P2O5-2Al2O3-95.8SiO2) preform slices made by MCVD with a composition similar to the commercial 20/400 ytterbium-doped silica fibers (YDFs) decrease by ∼31.8% (915 nm), ∼25.3% (940 nm) and ∼41.6% (975 nm), in addition, the emission cross sections decrease by ∼32.1% (1030 nm) and ∼22.7% (1080 nm), respectively, all which indicate the decline of the laser properties with the temperature increasing of Yb3+ doped glass samples. The highest absorption peak at 975 nm of HYPA1 is blue-shifted by 0.80 nm as temperature raised from 25°C to 200°C and the absorption coefficient of the peak wavelength is reduced by nearly 5%. Additionally, the results also demonstrate that the Rabs (the ratio of σabs@975 nm/σabs@915 nm) and Rem (the ratio of σem@1030 nm/σem@1080 nm) are affected not only by the P5+/Al3+ molar ratio but also by the temperature. This work is helpful for understanding the fiber laser performance variations with the increase of the fiber core temperature due to high power operation.

Enhanced radiation resistance of ytterbium-doped silica fiber by pretreating on a fiber preform

We propose a new pretreatment method on fiber preforms by loading deuterium (D2), pre-irradiation, combined with thermal annealing. The effect of the pretreatment condition on the optical loss at 1200 nm and a laser slope efficiency of ytterbium-doped silica fibers (YDFs) before and after γ-radiation was comparatively investigated. The related mechanism was revealed by combining the optical absorption, continuous wave electron paramagnetic resonance (CW-EPR), Raman, and Fourier transform infrared (FTIR) spectroscopies. A laser experiment shows that the radiation resistance of YDFs can be significantly improved by the pretreatment on a fiber preform. Also, this new pretreatment method has no obvious negative impact on the laser performance of non-irradiated YDFs. Furthermore, the vacuum experiment confirms that the YDF obtained by this method, named pretreated YDF, may have a long-term radiation stability when it is used in a vacuum environment (such as space). CW-EPR spectra show that the formation of color centers was effectively inhibited in pretreated YDF, which is correlated with the decrease of color center precursor and the existence of the deuterium radical as confirmed by Raman and FTIR spectra.

Yb3+ doped silica nanostructured core fiber laser.

We report development of ytterbium doped silica fiber with nanostructured core for laser applications. We study influence of non-continuous distributed Yb dopants on gain, beam quality, and fiber laser performance. The fiber core is composed of over 43 thousand nanorods with a central part doped with Yb. The diameter of each nanorod is 72 nm. With this method we obtained a flat refractive index profile with uniformity of 1.3 × 10-4 refractive index unit (RIU) despite the non-uniformity of 1.2 × 10-3 RIU in Yb doped preform rods used for the fiber development. We demonstrate a nanostructured core single-mode fiber laser with 61.8% of slope efficiency, and extremely low numerical aperture 0.027 of generated mode.

Suitability of FBG for Gain Flatness of 64 × 10 Gbps DWDM System Using Hybrid (EDFA+YDFA) Optical Amplifier in C + L Band up to 50 GHz (0.4 nm) Channel Spacing

Abstract Gain flatness of erbium-doped fiber amplifier (EDFA) is an important aspect to support high speed multimedia applications for smart devices, cloud computing, big data analysis and high definition television (HDTV) as it achieves equal power for all dense wavelength division multiplexing (DWDM) channels. In this paper, we compare 32 and 64 channels 10 Gbps DWDM system using hybrid EDFA+ Ytterbium-doped silica fiber amplifier (YDFA) in C + L band for 0.4, 0.8 and 1.6 nm wavelength spacing along with FBG for gain flattening. The system performance has been analyzed in terms of optical signal to noise ratio and spectral gain fluctuation observed from the optical power spectrums. It is observed that YDFA although extends the amplification window of EDFA from 1530 to 1610 nm (C to L band), but gain fluctuates over the operating wavelength range. Gain flatness is enormously improved (5–10 dB) by adding FBG as a gain flattening filter to the hybrid (EDFA+YDFA) amplifier. It is found that as the channel spacing decreases (from 1.6 to 0.4 nm) and the number of channels increases (from 32 to 64), the gain reduces due to inter-channel crosstalk and four wave-mixing effect.

用于高功率系统的掺镱石英光纤研究进展及发展趋势

用于高功率系统的掺镱石英光纤研究进展及发展趋势

Research on Photo-Radiation Darkening Performance of Ytterbium-Doped Silica Fibers for Space Applications

Yb3+/Al3+-doped and Yb3+/Al3+/P5+-doped silica fibers with almost identical Yb3+ doping concentrations were prepared using modified chemical vapor deposition methods. A harsh experimental environment for simultaneous photo-darkening (PD) and radio-darkening (RD) of an ytterbium-doped silica fiber (YDF) was established. The RD, PD, and photo-radio-darkening (PRD) of the YDF were characterized at two dose rates of 0.1 rad(Si)/s and 1 rad(Si)/s. The laser performances before and after the darkening process were tested under 974-nm pumping. The results demonstrated that the PD largely dominated the PRD kinetics of the YDF at a low dose rate (e.g., that in space) and that the PRD was not a superposition of RD and PD owing to the pump light bleaching if the induced loss was larger than the PD equilibrium level. The induced loss and defect types were investigated through absorption spectra and electron paramagnetic resonance. The results revealed that the P doping could enhance the PD, RD, and PRD resistances by inhibiting the formation of aluminum–oxygen hole centers.

Investigation of the photodarkening of Yb-Li co-doped silica fiber

In this work, we systematically studied the mitigation of photodarkening (PD) in ytterbium-doped silica fiber co-doped with lithium (Li). Adding a proper concentration of Li+ ions to the core glass composition, the PD-induced excess loss can be reduced by 25%. The results showed that the effect on numerical aperture and laser efficiency was very small. Compared with the sodium (Na) co-doped Yb/Al silica fiber at the same concentration, the mitigation of the PD effect through Li+ and Na+ ions co-doping was analyzed and discussed. Furthermore, a hypothesis was proposed for the mechanism of alkali metal Li and Na co-doping to mitigate the PD effect. More effective core composition and doping concentration were proposed to provide guidance of high output power gain fiber preparation.

An Adjustable EIT-Based FBG in Yb:silica Master-Oscillator Fiber Power-Amplifier System

An adjustable continuous-wave single-mode double-clad ytterbium-doped silica fiber laser is numerically investigated in the form of a master-oscillator fiber power-amplifier array. An optically-tunable coupler is suggested based on the nonlinear electromagnetically induced transparency (EIT) within fiber Bragg grating. Here, the Si nanocrystals is embedded in SiO2 over the selected regions of the output coupler. The transmittance is adjusted using a low power control field by modulating the refractive index in the EIT material.

978 nm Single Frequency Actively <inline-formula> <tex-math notation="TeX">$Q$ </tex-math></inline-formula>-Switched All Fiber Laser

A single-frequency actively Q-switched all-fiber laser at 978 nm was developed using a 2-cm-long commercial ytterbium-doped silica fiber and a pair of silica-fiber-based Bragg gratings. The Q-switching is enabled by the stress-induced polarization modulation in the short linear fiber laser cavity. The laser generates actively Q-switched single-frequency laser pulses with a pulse repetition rate ranging from tens of kilohertz to hundreds of kilohertz. The pulse duration and the average/peak power have been characterized. This is, to the best of our knowledge, the first demonstration of a single-frequency all-fiber Q-switched laser below 1 μm using commercial highly ytterbium-doped silica fiber.

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.

The three-electron bond =Si<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.

1018 nm掺镱光纤激光器

1018 nm掺镱光纤激光器

Improved setup and procedure for benchmarking of photodarkening in ytterbium doped silica fibers

This paper presents a new setup and a corresponding procedure for the accurate measurement of photodarkening in ytterbium doped fibers. The core-pumping scheme enables the use of short samples and a low pump power and the automation of the measurement permits long-term tests for an effective benchmarking of fibers with low photo-degradation. Exploiting temperature-controlled components and coherent detection, this arrangement allows the measurement of transmission drops below 2% over hundreds of hours. For the first time, a long-term photodarkening experiment (up to 450 h) has been carried out and correlated to the drop in output power experienced by a laser using the same type of fiber, demonstrating that the system can be employed in pre-compliance testing of active fibers prior to their use in a laser configuration.

Temporal evolution and correlation between cooperative luminescence and photodarkening in ytterbium doped silica fibers

The present work describes photodarkening from the viewpoint of cooperative luminescence. The temporal evolution of both effects was measured simultaneously by means of ytterbium doped aluminosilicate fibers for concentrations up to 1.8 wt% Yb3+. The quadratic dependence of photodarkening and cooperative luminescence versus dopant concentration was observed. The change in the photodarkening and cooperative luminescence mutual dynamics for highly and low doped fibers is ascribed to a different ion number which forms the cluster. Cooperative luminescence is proved to be a natural probe for photodarkening since it provides new pieces of information and contributes to the photodarkening mechanism description.

Analysis of temperature dependence of photodarkening in ytterbium-doped fibers

We examine the temperature dependence of photodarkening in ytterbium-doped silica fibers. A sequence of consecutive photodarkening experiments are performed over the same fiber sample, which shows good repeatability with no apparent changes in the glass structure. We find that during infrared irradiation, the level of saturation of the losses can be determined by the fiber core temperature, independent of the previous state of photodarkening losses and fiber temperature, and also at low temperatures where the thermal bleaching is not activated. We observe that variations in the fiber core temperature, induced by pump absorption due to photodarkening, affect the inversion level and photodarkening processes. These effects in turn cause a discrepancy in determining the ion dependence. We highlight the importance of performing the experiments under isothermal conditions and we propose a new approach to control the fiber temperature at room temperature and at elevated temperatures. The approach is based on an isothermal Galinstan bath. The appropriateness of this method is shown by comparing it to different cooling methods, and the results are supported by simulations.

Thermoluminescence characterization of traps involved in the photodarkening of ytterbium-doped silica fibers

The photodarkening (PD) mechanisms of ytterbium-doped silica optical fibers have still not been elucidated, although hardening routes have been proposed. Most basic questions are still under debate about the assignment of the darkening excitation bands into the UV range, the nature of absorbing centers (photoionized centers or trapped carriers?), or of traps accepting photo-released carriers (electron or hole traps?). We used thermoluminescence measurements to characterize traps populated by different radiation types. It is notably demonstrated that photodarkening involves silica hole traps. The popular idea that color centers are formed upon carrier trapping is not consistent with our observations.

Comparisons of multi-wavelength oscillations using Sagnac loop mirror and Mach-Zehnder interferometer for ytterbium doped fiber lasers

A multiwavelength Ytterbium-doped fiber ring laser operating at 1030 nm region is demonstrated using a Sagnac loop mirror and a Mach-Zehnder interferometer. We report the Performance comparisons of multi-wavelength oscillations in Yb3+ doped fiber lasers (YDFL) with typical commercial ytterbium doped silica fibers. By adjusting the polarization controller (PC), a widely tunable laser range of 22 nm from 1030 nm to 1050 nm is obtained. The Mach-Zehnder interferometer (MZI) design has exhibited simplicity in the operation for controlling the smallest wavelength spacing compared to Sagnac loop mirror method. In our observations, the smallest achieved stable wavelength spacing in Sagnac loop mirror setup and MZI setup were 2.1 nm and 0.7 nm, respectively. In case of nine-wavelength operation with a MZI setup, the stability, Full Width at Half Maximum (FWHM) and side mode suppression ratio (SMSR) of laser lines are not affected by increasing pump power, While for above four wavelength operation, the laser stability with Sagnac loop mirror becomes worse specially for higher input pump power and the power fluctuation among the wave-lengths would be also slightly larger.

Ytterbium fiber laser based on first-order fiber Bragg gratings written with 400nm femtosecond pulses and a phase-mask

A Fiber Bragg grating of 369 nm pitch was inscribed in a germanium-free double-clad ytterbium doped silica fiber using a femto-second pulse train at 400 nm wavelength and a phase mask. The photo-induced refractive index modulation of higher than 4 x 10(-3) was obtained and the accompanying photo-induced losses were subsequently removed by thermal annealing, resulting in a low loss (<0.1 dB), stable and high reflectivity (>40 dB) FBG. Based on this FBG, a monolithic Ytterbium fiber laser operating at 1073 nm with slope efficiency of 71% and output power of 13 W was demonstrated.

Highly ytterbium-doped silica fibers with low photo-darkening

Phosphorus co-doping is known to reduce clustering levels of rare earth ions in silica hosts. In this paper, ytterbium-doped silica fibers with approximately 8.9 wt% Yb(2)O(3), up to approximately 4700 dB/m peak core absorption at 976 nm, and low photo-darkening are demonstrated using high phosphorus co-doping. Measured gain as high as approximately 7 dB/cm is demonstrated in the fiber.