Dmitrii Kouznetsov

Short cladding-pumped Er/Yb phosphate fiber laser with 1.5 W output power

We report experimental results on a high-power, cladding-pumped, heavily Er∕Yb co-doped phosphate fiber laser of very short length. Up to 1.5W cw laser power was obtained from an11-cm-long multimode-core active fiber with optimized input and output couplers, when pumped by a 15W diode laser at 975nm. The fiber laser was demonstrated at 1535nm with a linewidth <1.2nm, and a good beam quality of M2<3.

Efficiency of pump absorption in double-clad fiber amplifiers. II. Broken circular symmetry

Absorption of an incoherent pump in a double-clad fiber amplifier is analyzed in the paraxial wave approximation. Different inner cladding shapes are considered. A small spiral distortion of an otherwise circular inner cladding shape is shown to enhance the coupling efficiency significantly relative to other geometries.

Role of undoped cap in the scaling of thin-disk lasers

The optimum design of a powerful thin-disk laser implies a compromise between amplified spontaneous emission (ASE), overheating, and the round-trip losses. The power enhancement of a composite thin-disk laser made of an undoped layer bonded over a thin active layer to reduce ASE losses is estimated analytically. Scaling laws for the parameters of a disk laser are suggested for cases both with and without an anti-ASE cap. Predictions of the maximal power achievable for a given laser material are compared to the published experimental data. The anti-ASE cap allows an increase of the maximal output power proportional to the square of the logarithm of the round-trip loss.

Surface loss limit of the power scaling of a thin-disk laser

We show that the general limit of power scaling of thin-disk lasers comes not only from the overheating and amplified spontaneous emission (ASE) but also from the surface loss. Overheating or thermal fracture increases the transverse size at the scaling, whereas ASE limits the gain-size product. The gain coefficient should decrease at the scaling. However, the round-trip gain should remain larger than the background loss; hence, the thickness should increase at the scaling. The limit of the output power per single active element occurs when the medium becomes too thick and cannot work efficiently without overheating. The maximum output power scales inversely with the cube of the surface loss coefficient. In the quasi-continuous regime, the average power scales inversely to the product of the duration of pulses to the repetition rate.

Switching of emissivity and photoconductivity in highly doped Yb3+:Y2O3 and Lu2O3 ceramics

Reversible jump of thermal emission accompanied with jump of photocurrent is observed in highly doped Yb3+:Y2O3 and Lu2O3 bulk ceramics pumped at 940nm wavelength. In contrast, when these materials are heated with a CO2 laser at 10.6μm wavelength, only a gradual increase of thermal emission and photoconductivity are observed up to the melting point. These results are interpreted as a ytterbium mediated, photoassisted avalanche of thermal emission.

Rough-surface capacitor: approximations of the capacitance with elementary functions

We consider the electrical capacitance of a parallel-plate capacitor with one rough electrode. The roughness is characterized by its amplitude and its wavenumber scale; the capacitance depends on both of these variables as well as the form of the roughness. We calculate the capacitance in the four limit cases of the two variables mentioned above. On the basis of these limit cases, we approximate the capacitance with elementary functions for the case of any surface roughness. We compare the capacitances calculated in this approach with precise numerical solutions for different forms of roughness.

High-power single-mode solid-state laser with a short, wide unstable cavity

A wide, short unstable cavity laser design is proposed for high-power, single-longitudinal, single-transverse-mode emission from a solid-state laser. Such a laser combines the single-mode master oscillator and the single-mode amplifier in a single piece. Design formulas are suggested; the efficiency and conditions of the single-longitudinal-mode operation are analyzed. Examples with Nd:YAG and Yb:YAG are considered. For a device of a few millimeters wide, the slope efficiency of approximately 50% and the threshold of a few watts are predicted.

Efficiency of pump absorption in double-clad fiber amplifiers. III. Calculation of modes

Using an analogy with quantum mechanics we employ a technique that enables us to calculate the efficiency of an incoherent pump in general-geometry double-clad fibers. This approach yields accurate estimates of the absorption rate of each mode of the pump in the first order of perturbation theory. Such estimates are compared with results of numerical simulations reported recently. The comparison confirms the high efficiency of a spiral-shaped cladding.

SOLUTION OF F(z + 1) = exp (F(z)) IN COMPLEX z-PLANE

Tetration F as the analytic solution of equations F(z ― 1) = ln(F(z)), F(0) = 1 is considered. The representation is suggested through the integral equation for values of F at the imaginary axis. Numerical analysis of this equation is described. The straightforward iteration converges within tens of cycles; with double precision arithmetics, the residual of order of I.e-14 is achieved. The numerical solution for F remains finite at the imaginary axis, approaching fixed points L, L * of logarithm (L = ln L). Robustness of the convergence and smallness of the residual indicate the existence of unique tetration F(z), that grows along the real axis and approaches L along the imaginary axis, being analytic in the whole complex z-plane except for singularities at integer the z < ―1 and the cut at z < ―2. Application of the same method for other cases of the Abel equation is discussed.

Perturbation theory for surface-profile imaging with a capacitive probe

We derive a perturbative series solution to the capacitance between two parallel electrodes with irregular profiles. The coefficients in the series are calculated using fast Fourier transform algorithms resulting in a very fast method. The applicability of the perturbative series solution is extended by introducing a spectral window function which can make the series converge in cases where the standard series does not converge. We show that the filtered perturbative solution is applicable to surface profiles with surprisingly large features. However, limitations on its applicability to surfaces with high spatial frequencies remain. Perturbation theory could be a powerful tool for simulating surface-profile images obtained by scanning a capacitive probe.