Denis S. Lipatov

Effect of the AlPO4 join on the pump-to-signal conversion efficiency in heavily Er-doped fibers.

Heavily Er-doped fibers (EDFs) based on P(2)O(5)-Al(2)O(3)-SiO(2) (PAS) ternary glass have been studied. A unique feature of this glass is the formation of a AlPO(4) join having a structure similar to that of SiO(2) glass and a refractive index below it. It is found that the Er(3+) absorption and emission spectra in the PAS EDFs are defined by the dopant (Al(2)O(3) or P(2)O(5)) present in excess and are close to those of the corresponding binary glass (Al(2)O(3)-SiO(2) or P(2)O(5)-SiO(2)). The presence of the AlPO(4) join results in the enhancement of the pump-to-signal conversion efficiency in the PAS EDFs as compared with the EDFs based on the P(2)O(5)-SiO(2) and Al(2)O(3)-SiO(2) (with 1.5 mol. %Al(2)O(3) and less) binary glasses. The PAS host glass is advantageous in the case of large-mode-area active fibers.

Millijoule pulse energy 100-nanosecond Er-doped fiber laser

We report, for the first time to our knowledge, on a single-mode millijoule-level 100-nanosecond Er-doped fiber laser operating near 1550 nm. The system features a newly developed 35-μm-core Yb-free double-clad Er-doped fiber based on P(2)O(5)-Al(2)O(3)-SiO(2) glass matrix and produces pulses with energy as high as 1 mJ at repetition rates of 1-10 kHz.

Record-peak-power all-fiber single-frequency 1550 nm laser

In this paper, we present results on the amplification of 2 MHz linewidth nanosecond pulses in a recently developed ytterbium-free erbium-doped large mode area fiber cladding pumped at 980 nm. A record peak power of 4 kW for single-frequency single-mode silica-based fiber sources near 1550 nm is demonstrated. Pulse instabilities arising in an all-fiber laser scheme from stimulated Brillouin scattering are described.

High power all-fibered femtosecond master oscillator power amplifier at 1.56 μm

Direct amplification of output from chirped pulse oscillator (CPO) to 3.3 W of average power (pulse energy of 118 nJ in 20 ps pulse duration before compression) was achieved in a properly designed cladding pumped large mode area Er-doped fiber. Various configurations of CPO cavity with different FWHM of output spectrum and pulse duration were investigated. Fourier limit compression with 480 fs pulse duration and 32 kW peak power has been obtained for pulses with 14.8 nm FWHM spectrum. Subsequent nonlinear compression in a standard SMF-28 fiber yielded pulses as short as 145 fs.

Radiation Resistance of Er-Doped Silica Fibers: Effect of Host Glass Composition

With the aim to find the best-suited host glass for radiation-resistant erbium-doped fibers (EDF), radiation-induced absorption (RIA) is measured and analyzed after γ -irradiation to 3.0-4.5 kGy in 18 silica optical fibers with various concentrations of dopants: Al₂O₃ , P₂O₅ , GeO₂ and Er₂O₃ . The RIA dependence on the content of Al₂O₃, P₂O₅ and Er₂O₃ in singly doped fibers is investigated. RIA of fibers codoped simultaneously with Al₂O₃ and P₂O₅ are found to be significantly lower than those of singly Al₂O₃ -doped fibers. This is explained as the result of the formation of AlPO₄-joins in the silica network. GeO₂ codoping of AlPO₄ -doped silica is shown to further reduce RIA. Nevertheless, the application of H₂ -loading and photobleaching of RIA by 980-nm laser radiation shows that the AlPO₄ - and GeO₂ -codoped silica is outperformed by P-free Al₂O₃- and GeO₂-codoped silica, which is, therefore, concluded to be the best host glass composition for radiation resistant EDFs.

Large-mode-area highly Yb-doped photodarkening-free Al 2 O 3 -P 2 O 5 -SiO 2 -Based fiber

Large-mode area (LMA) Yb-doped fibers are routinely used in high-power CW and pulsed fiber lasers and amplifiers. Solubility of Yb³⁺ ions in pure silica glass is rather small; therefore co-doping with Al<inf>2</inf>O<inf>3</inf> is required to suppress clustering. Single-mode operation in LMA fibers (core diameter ∼ 20 µm) is possible, when the refractive index difference between the core and the cladding (Δn) is about 0.002 or less. Both Al<inf>2</inf>O<inf>3</inf> and Yb<inf>2</inf>O<inf>3</inf> have a relatively high molar refractivity (∼0.0025/mol.% and 0.001/wt.%, respectively), and therefore only a small Al<inf>2</inf>O<inf>3</inf> concentration can be incorporated into a highly Yb-doped LMA fiber. Clustering of Yb³⁺ ions results in photodarkening (PD) - degradation of the laser properties with time due to a grey loss increase. Utilization of the phosphorosilicate glass matrix allows one to suppress PD almost completely (the grey loss due to PD is 10–100 times smaller in comparison with that in the aluminosilicate fiber) [1], but a typical Δn in such fibers is about 0.005–0.015.

Fabrication of heavily Er2O3 doped aluminophosphosilicate glass fibers

This work examines vapor-phase doping of aluminophosphosilicate (APS) glasses with erbium oxide using Er(dpm)3 (erbium dipivaloylmethanate) as a metalorganic precursor. We describe two vapor-phase processes for Er2O3 doping of APS glasses for fiber cores. In one of them, a standard MCVD procedure, all the oxides of the glass core are deposited simultaneously. With this procedure, the erbium content of the APS glasses does not exceed 1 wt % because of the reaction of AlCl3 with POCl3 and Er(dpm)3. In the other procedure, a porous Al2O3-P2O5-SiO2 layer is infiltrated with Er2O3 from the vapor phase. Owing to the separation of the reactant flows, this procedure ensures considerably higher rare-earth doping levels in the APS glasses, up to several weight percent.

MW peak power diffraction limited monolithic Yb-doped tapered fiber amplifier

Chirped pulse monolithic fiber amplifier based on a newly developed tapered polarization maintaining Yb-doped fiber has been developed and optimized. A novel amplification regime in a relatively long (220 cm) tapered fiber of improved design, which has been theoretically predicted, allowed us to achieve an ultimate high peak power. In this regime, the signal propagates most of the fiber without amplification and growths very rapidly only in the last 80 cm of the tapered fiber, which has a mode field area of approximately 1000 μm2 near the output. We have demonstrated amplification of 20 ps chirped pulses centered at 1056-nm with spectral width of 20 nm to 0.7 MW peak power directly from the tapered fiber amplifier. The pulses had a diffraction limited quality (M2 ~ 1.124) and could be compressed down to 350 fs with 50% efficiency. In addition, amplification of narrow-band 9 ps pulses centered at 1064 nm to a peak power of 1.8 MW directly from the tapered fiber amplifier was demonstrated.

Monolithic high peak-power coherent Doppler lidar system

In this work we present a monolithic lidar system, based on a newly-developed double-clad large mode area (LMA) polarization-maintaining Er-doped fiber and specially designed LMA passive components. Optimization of the fiber designs resulted in as high as 100 W of SBS limited peak power. The amplifier and its passive components (circulator and collimator) were integrated in an existing lidar system. The enhanced lidar system provides three times increase of scanning range compared to one based on standard telecom-grade amplifiers.

Monolithic sub-MW peak power tapered ytterbium-doped fiber amplifier

ABSTRACT In this study, we present a novel monolithic ytterbium-doped fiber amplifier with more than 0.5 MW peak power output power. The amplifier is based on a 2.1 m long tapered fiber with core/cladding diameters changing from 10/80 µm (at the signal input end) to 50/430 µm (signal output, pump input). The fiber has all-glass polarization-maintaining design, that make possible utilization of conventional FC adapter and standard angle polishing to 7° for thick end. Pump absorption was measured to be 8 dB at 915 nm for the whole fiber length. Despite a very large mode area (~30 µm) the tapered fiber demonstrates low bend sensitivity (it is possible to coil tapered fiber with 9 cm radius) and a diffraction limited beam quality. In the amplifier the pump power was coupled through the thick end by means of collimating and focusing lenses. Dichroic mirror employed to separate output signal and counter-propagating pump power. We obtained 3.5 W of average output power for a 5 mW seed signal (coupled by usual fusion splicing through a thin end) corresponding to a 28 dB gain. The amplified pulses have duration of about 5 ps and energy of about 3.3 µJ that corresponding to over 0.5 MW peak power. The spectral width was 28 nm operating with center wavelength of 1057 nm.To the best of our knowledge it is the highest peak power obtained directly from the all-fiber amplifier.. Keywords: Yb-doped tapered fiber, all-fiber megawatt system, low bend sensitive tapered fiber, ultrashort pulses amplification 1. INTRODUCTION The very rapid power scaling of both pulsed and cw Yb-doped fiber lasers dictates stringent requirements on the utilized fibers, mainly in terms of nonlinear effects threshold. Conventional single-mode Yb-doped fibers exhibit a relatively low threshold for nonlinear effects since the light is confined in a small core (6-10 µm in diameter) and propagates along long distances (above 10 m in some cases). The increasing of nonlinearity threshold is the urgent problem in the case of pulsed fiber systems, which have been demonstrated to reach power levels of several hundreds of kW and even approach MW power level. The most simple and obvious way to solve this problem is the enlargement of core diameter resulting in the enlarged mode filed diameter of the fundamental mode. In the same time, this approach has so me drawbacks: the fiber becomes a multimode, leading to degradation of output beam quality, and extremely bend-sensitive, resulting in requirement to keep the fiber straight, i.e. the system becomes cumbersome. The main goal of this work was to develop a novel approach based on utilizing of so-called tapered fibers that allows one to obtain large mode field areas and maintain an acceptable bend-loss sensitivity at the same time. The fiber parameters optimization and its using in the final amplification stage of picosecond chirped-pulse amplification system have been demonstrated. The obtained output peak power was record for all-fiber amplifying systems and approached by power level of microstructured fibers based systems.