Valentin A. Tyrtyshnyy

Low-threshold mode instability in Yb 3+ -doped few-mode fiber amplifiers

Spatio-temporal instability of the fundamental mode in Yb(3+)-doped few-mode PM fiber amplifiers with a core diameter of 8.5 μm was registered at 2-30 Watts pump power. Both experimental and theoretical analysis revealed the nonlinear power transformation of the LP(01) fundamental mode into high-order modes. Numerical simulation revealed self-consistent growth of the higher-order mode and traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb(3+) ions). Experimental results and numerical calculations showed the increase of the instability threshold along with an increase of the signal frequency bandwidth.

Third harmonic frequency generation by Type-I critically phase-matched LiB 3 O 5 crystal by means of optically active quartz crystal

We present a method of third harmonic generation at 355 nm by frequency mixing of fundamental and second harmonic radiation of an ytterbium nanosecond pulsed all-fiber laser in a type-I phase-matched LiB(3)O(5) (LBO) crystal where originally orthogonal polarization planes of the fundamental and second harmonic beams are aligned by an optically active quartz crystal. 8 W of ultraviolet light at 355 nm were achieved with 40% conversion efficiency from 1064 nm radiation. The conversion efficiency obtained in a type-I phase-matched LBO THG crystal was 1.6 times higher than the one achieved in a type-II LBO crystal at similar experimental conditions. In comparison to half-wave plates traditionally used for polarization alignment the optically active quartz crystal has much lower temperature dependence and requires simpler optical alignment.

Generating high-quality beam in a multimode LD-pumped all-fiber Raman laser

We report on the first demonstration of an all-fiber CW Raman laser based on a multimode graded-index fiber directly pumped by multimode fiber-coupled laser diodes. A joint action of Raman clean-up effect and mode-selection properties of special fiber Bragg gratings inscribed in the central part of the graded-index fiber core, results in high-efficiency conversion of a multimode (M2~26) pump at 915 nm into a high-quality (M2~2.6) output beam at 954 nm. About 50 W output power has been obtained with slope efficiency of 67%. The proposed development and integration of key multimode fiber technologies opens the door to new type of LD-pumped high-power high-beam-quality fiber lasers that may operate at almost any wavelength defined by available LDs.

Resonant acoustic calorimetry of the interaction of laser radiation with nonlinear-optical crystals

Resonant acoustic calorimetry technique gives the opportunity to measure the crystal temperature during linear and nonlinear interaction of the laser radiation with crystals. It is based on the registration of the piezoelectric resonance frequency change caused by the interaction of the laser radiation with crystals. Piezoelectric resonance is observed by measuring the dependence of the sample electrical impedance on the external electric field frequency. It is shown that inhomogeneous crystal heating can be characterized by the equivalent temperature depending on the laser power. Crystal equivalent temperature can be directly determined from the measured piezoelectric resonance frequency.

Influence of a backward reflection on low-threshold mode instability in Yb 3+ -doped few-mode fiber amplifiers.

An influence of backward reflection on spatio-temporal instability of the fundamental mode in Yb3+-doped few-mode polarization maintaining fiber amplifiers with a core diameter of 10 μm was studied experimentally and theoretically. The mode instability threshold was registered to decrease dramatically in the presence of a backward reflection of the signal from the output fiber end; an increase of the signal bandwidth or input power resulted in the increase of the threshold. Numerical simulation revealed a self-consistent growth of the higher-order mode LP11 and a traveling index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb3+ ions). The presence of the backward-propagating wave resulted in four-wave mixing on the common index grating induced by the interference field of pairs of the fundamental LP01 and LP11 modes.

Low-threshold mode instability in Yb3+-doped few-mode fiber amplifiers: influence of a backward reflection

Spatio-temporal instability of the fundamental mode in Yb3+-doped few-mode PM fiber amplifiers with a core diameter of 8-10 μm was studied experimentally and theoretically. The mode instability threshold was registered at 1-100 Watts pump power. The threshold was found to decrease dramatically in the presence of a backward reflection of the signal from the output fiber end or an external counter-propagating beam; an increase of the signal bandwidth or input power resulted in the increase of the threshold. Numerical simulation revealed a self-consistent growth of the higher-order mode and a traveling electronic index grating accompanying the population grating induced by the mode interference field (due to different polarizability of the excited and unexcited Yb3+ ions).

Piezoelectric resonance calorimetry of nonlinear-optical crystals under laser irradiation

Novel method is proposed for determination of nonlinear-optical crystal both heat transfer and optical absorption coefficients by measuring kinetics of the laser-irradiated crystal temperature-dependent piezoelectric resonance frequency. When laser radiation propagates through the crystal its temperature evaluation with time is directly determined from crystal piezoelectric resonance frequency shift, which is precisely measured by analyzing crystal response to the applied ac electric voltage. Heat transfer and optical absorption coefficients are obtained using measured characteristic time of crystal laser heating kinetics by solving nonstationary heat conduction equation. Experiments were performed with nonlinear-optical α-quartz, lithium triborate (LBO) and periodically poled lithium niobate (PPLN) crystals.

Resonant acoustic calorimetry of the interaction of high-power laser radiation with crystals

Influence of the high-power laser radiation on the piezoelectric resonance in the nonlinear-optical KTiOPO4 crystal was observed. Equivalent crystal temperature can be directly determined from the piezoelectric resonance frequency during interaction of the laser radiation with crystals. Frequency kinetics of the different piezoelectric resonances of the KTiOPO4 crystal were observed for the different pump laser powers up to 20 W. The KTiOPO4 optical absorption coefficient and the amplitude of the crystal inhomogeneous temperature distribution were obtained from the kinetics data. A theoretical model of the piezoelectric resonances on the basis of the Rayleigh-Ritz method is proposed.

Anisotropy of Bulk Degradation of LBO Crystal Induced by DUV Laser Radiation at 266 nm

Dependence of degradation time of LBO crystal on UV propagation direction, polarization and power was investigated at different crystal temperatures. Anisotropy of bulk degradation induced by DUV radiation at 266 nm was observed.

Equivalent temperature of nonuniformly heated nonlinear-optical crystals in course of laser radiation frequency conversion

Novel method of piezoelectric resonance spectroscopy is introduced for nonlinear-optical crystals equivalent temperature measurement during frequency conversion of laser radiation. Equivalent temperature of the crystal heated by laser radiation is directly determined from its strongly temperature sensitive piezoelectric resonance frequencies. Method was applied for PPLN crystal temperature measurement during second harmonic generation of CW ytterbium fiber laser radiation. Temperature tuning curves of PPLN and its phase matching temperature dependence on pump power were precisely measured using concept of the equivalent temperature. Hysteresis of PPLN temperature and optical bistability of second harmonic power in respect to pump power were observed.