Frequency-doubling of a CW fiber laser using PPKTP, PPMgSLT, and PPMgLN

Abstract

Second-harmonic generation (SHG) in PPKTP, PPMgSLT, and PPMgLN crystals is analyzed by frequency-doubling CW light from a 1064 nm fiber laser over a range of powers up to 10 W. Data for fundamental powers less than 3 W is used to determine the effects of the fundamental laser linewidth on SHG and to identify imperfections in the periodicity and boundary sharpness of the crystals' poled domains which can reduce SHG. Data for fundamental powers greater than 3 W is used to diagnose and model limiting effects on SHG such as pump depletion and thermal dephasing. Thermal dephasing was found to reduce second-harmonic power by 25% or more for input fundamental powers approaching 10 W.

Concept for power scaling second harmonic generation using a cascade of nonlinear crystals

“…Although 3.6 W of SH light could be generated with two PPMgCLN crystals, strong phase matching instabilities in these crystals lead to a distorted SH beam profile, lowered overall efficiency and erratic fluctuations in output power. These are probably all manifestations of strong thermal dephasing, as other authors have also observed [5]. For any practical purpose, such deteriorate behavior renders this configuration of limited use.…”

“…However, not all nonlinear media are suited for high powers of the second harmonic (SH) because of adverse effects such as photochromism [2,3], photorefractivity [4] or thermal dephasing due to absorption [3,5,6].…”

“…It is well documented in the literature [5,6] that one of the most efficient crystals in current widespread use, periodically poled MgO-doped congruent lithium niobate (PPMgCLN), is in general not suitable for generating more than 2-3 W of light in the green spectral region due to thermal dephasing and thermal lensing caused by SH absorption. Alternative crystal materials with better power handling ability, such as periodically poled MgO-doped stoichiometric lithium tantalate (PPMgSLT) [7,8,9], have a lower nonlinear coefficient and are less efficient at generating second harmonic light as a consequence.…”

Frequency doubling of incoherent light from a superluminescent diode in a periodically poled lithium niobate waveguide crystal

“…We assume that the conversion efficiency is mainly limited by the acceptance band width of the crystal as investigated in [31]. In our case, the acceptance bandwidth of the crystal is 140 pm and the bandwidth of the ASE source is about 10 nm giving a ratio of ∆λ ASE /∆λ P P LN ≈ 70.…”

Threshold for strong thermal dephasing in periodically poled KTP in external cavity frequency doubling

“…The latter was determined in our crystal by both a direct transmission experiment and from finesse measurements of the external resonator. Several authors [1][2][3][4] have reported on thermal issues with high input powers in periodically poled crystals, due to absorption, but to our knowledge not for wavelengths lower than 423 nm.…”