Thermally induced dephasing in periodically poled KTiOPO4 nonlinear crystals Page: 3 of 5
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Thermally induced dephasing in periodically poled
KTiOPO4 nonlinear crystals
Z. M. Liao, S. A. Payne, J. W, Dawson, A. D. Drobshoff, C. A. Ebbers, D. M. Pennington, and I. Jovanovic
Lawrence Livermore National Laboratory, 7000 East Avenue, L-482, Livermore, CA 94550
L. R. Taylor
European Southern Observatory, Garching, Germany D85748
Abstract: Experimental data that exhibits a continuous-wave, second-harmonic intensity
threshold (15 kW/cm2) that causes two-photon nonlinear absorption which leads to time-
dependent photochromic damage in periodically poled KTiOPO4 is presented and verified through
a thermal dephasing model.
@ 2004 Optical Society of America
OCIS codes: (190.0190) Nonlinear Optics, (190.2620) Frequency Conversion, (190.4400) Nonlinear Optics, Materials
Periodically poled nonlinear materials have the potential to serve as versatile devices for laser developers because of
their ability to quasi-phase-match (QPM) over long lengths of crystal and allow noncritical phase matching for tight
focusing geometries, thereby enabling efficient frequency conversion of continuous-wave (CW) and low-peak-
power pulsed lasers. A number of crystals have been successfully poled for frequency conversion of which
periodically poled LiNbO3 (PPLN) and KTiOP04_PPKTP) have produced the most success. PPLN has
demonstrated high conversion efficiency (2.7W out of 6.5W) in CW single-pass operation before exhibiting
photorefractive damage . Recently, periodically poled stoichiometric LiTaO3 (PPSLT) has successfully
demonstrated 300 minutes of CW single-pass second harmonic generation at 1064_nm (1.6 W out of 18_W) without
observable damage . Unfortunately, there have been no reported CW high-power studies using PPKTP, although
no damage was observed in low power regime (-mW) . In pulsed systems however, PPKTP has been shown to
be capable of generating large amounts of 532_nm light (up to 6_W average power)  without observing
In bulk KTP crystals, laser intensity induced damage (gray tracking) is well documented . The transmission
degradation of the crystal across the visible wavelengths is generally attributed to the creation of color centers
through two-photon absorption. We describe our assessment of time-dependent conversion efficiency degradation in
PPKTP in a CW, single-pass configuration. This investigation has important consequences for pulsed operation as
well, particularly for longer pulses and for high-average-power systems.
2. Theory of nonlinear conversion with losses
The exact treatment of second harmonic generation (SHG) consisted of the solving the coupled wave equation of the
pump and the signal fields as it propagates through the crystal . Several works treated reduction of coupled wave
equations to a close-form analytical solution, notably Boyd and Kleinman (BK) .
The solution of SHG in the BK focusing regime (tight focusing) is given as
Pa = fgL P , (1)
where f is the focusing factor that accounts for non-ideal focusing, g is the nonlinear conversion coefficient, 7 is the
thermal dephasing factor averaged over the crystal lengthL, , and PIG, and P2, are the fundamental and the second-
harmonic powers, respectively. The thermal dephasing factor 7 is a function of the temperature detuning (AT) and
is directly related to the absorption of the crystal. The total absorption of the crystal is then the sum of the linear
absorption (aL) due to impurities and the time-dependent nonlinear absorption due to precursor defects, a=aL Es ~-
exp(-_ f2t)] where, Q is the defect-creation coefficient and a, is the saturated nonlinear absorption. Defect
generation is a function of 122, because the process is a two-photon absorption interaction. A time-dependent,
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Dawson, J W; Pennington, D M; Jovanovic, I; Liao, Z M; Payne, S A; Drobshoff, A D et al. Thermally induced dephasing in periodically poled KTiOPO4 nonlinear crystals, article, March 18, 2004; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1411831/m1/3/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.