IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 27, NO. 13, JULY 1, 2015 1379 Single-Pass Multi-Watt Second-Harmonic-Generation in Congruent and Stoichiometric LiTaO 3 Mukesh Kumar Shukla, Samir Kumar, and Ritwick Das Abstract—We present a comparison of an efficient, single- pass second-harmonic-generation (SHG) of a continuous-wave Yb-fiber laser using a 1) congruently grown LiTaO 3 crystal and 2) stoichiometrically grown LiTaO 3 crystal. Using a Yb-fiber laser pump source delivering a single-frequency output of 40 W at 1064 nm, we have generated ∼8.5 and 14.5 W of single- frequency green-radiation at 532 nm from 8 mol% MgO-doped periodically poled, congruently grown LiTaO 3 (MgO:cPPLT) crystal and 0.5 mol% MgO-doped periodically poled, stoichio- metrically grown LiTaO 3 (MgO:sPPLT) crystal, respectively. We obtained SHG conversion efficiencies in excess of 24% for 5-cm long MgO:cPPLT and ≥36% for 3-cm long MgO:sPPLT. The impact of thermal dephasing was evident in variation of SH power in case of MgO:cPPLT crystal. However, the mani- festations due to thermal effect were found out be significantly weaker in case of MgO:sPPLT. This lead to saturation of SHG efficiency at pump powers ≥20 W for MgO:cPPLT. The long-term peak-to-peak power-fluctuation in case of MgO:sPPLT is recorded to be ≈4% for the green beam over a 4-h duration which shows improved performance over MgO:cPPLT output (peak-to-peak power-fluctuation ≈11%). In addition, the 532-nm radiation exhibited single-frequency characteristics with linewidth of 12 MHz (MgO:cPPLT) and 5 MHz (MgO:sPPLT). Index Terms—Nonlinear optics, laser. H IGH-POWER, continuous wave (cw), spectrally-pure light sources in the red-green-blue are always in great demand for various applications spanning the areas of medicine to material-technology and most importantly basic sciences [1], [2]. By riding on the advantage of highly devel- oped nano-micro fabrication technology of semiconductors, the high-power diode lasers with suitable spectral and spatial characteristics are widely available for the red and blue spectral bands. However, the Indium (In) doped Gallium Nitride (GaN) multi-quantum well (MQW) structures which form the backbone of green - diode lasers, exhibit intolerably low thermal stability. High volatility of In as well as high equilibrium vapor pressure of nitrogen are primary reasons behind such degradation and therefore, experimentally realizing a compact high-power green diode lasers with desirable characteristics is a challenge [3]. In addition to green-diode lasers, argon ion (Ar + - ion) gas laser emitting multi-watt optical power at λ ≈ 514 nm is most widely available. However, poor wall-plug efficiency, Manuscript received January 27, 2015; revised March 20, 2015; accepted April 6, 2015. Date of publication April 15, 2015; date of current version June 3, 2015. This work was supported by the Science and Engineer- ing Research Board through the Department of Science and Technology, Government of India under Project SR/S2/LOP-09/2012. (Corresponding author: Mukesh Kumar Shukla.) The authors are with School of Physical Sciences, National Institute of Science Education and Research, Bhubaneswar 751005, India (e-mail: mukesh.s@niser.ac.in; samir.kumar@niser.ac.in; ritwick.das@niser.ac.in). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2015.2421643 along with complex architecture, limits the employability of such systems to sophisticated laboratory environment [4]. The advent of high-power near-infrared (1 μm) laser sources have opened a plausible route for realizing high-power green source by efficiently doubling the frequency of such sources using appropriate nonlinear crystals. Primarily, such laser sources operate on the principle of internal second-harmonic- generation (SHG) of near-infrared (IR) lasers using nonlinear crystals such as LBO (LiB 3 O 5 ) and KTP (KTiOPO 4 ) [5], [6]. However, the intra-cavity configuration requires the cavity to be actively stabilized for optimum spectral features which inevitably increases complexity and cost of the system. Addi- tionally, these systems require efficient cooling mechanisms for optimally regulating the nonlinear crystal temperature which leads to further escalation of architectural complexity. Such issues could be addressed by adopting a single-pass frequency-doubling of present generation rare-earth doped, air- cooled high-power fiber laser systems operating near 1 μm wavelength in a quasi-phase-matched (QPM) nonlinear crys- tals. Due to optical fiber based active-medium, fiber-laser output beam exhibits extremely good spatial characteris- tics ( M 2 ≤ 1.05) as well as desirable spectral qualities (ν ≤ 1 MHz ) at cw powers in excess of 50 W. The efficiency of frequency conversion in such cases crucially depends on pos- sibility to optimum thermal management in nonlinear crystal. In the context of frequency conversion, congruently-grown periodically-poled LiNbO 3 (PPLN) exhibits maximum nonlin- ear coefficient (d eff ≈ 16 - 17 pm/ V ) amongst the nonlinear crystals with broad transparency window (≈0.35-5.0 μm) and long lengths (≥80 mm). However, low photorefractive damage threshold in addition to infrared-induced absorption at visible frequencies limits its usage at high fundamental powers. On the other hand, periodically-poled KTiOPO 4 , commonly abbreviated as PPKTP, exhibits moderate nonlinear coefficient (d ef f ≈ 10 pm/ V ) and high resistance to photorefractive damage. Unfortunately, PPKTP has low thermal conductivity (≥3.4 W/ m - K ) which renders it unsuitable at high-power applications. Interestingly, another QPM nonlinear crystal i.e. periodically-poled LiTaO 3 (PPLT) exhibits improved features such as high thermal conductivity (≥8 W/ m - K ), extremely high photorefractive damage threshold (by MgO doping) and negligible green-induced-infrared absorption (GRIIRA). In addition, PPLT crystals could be grown into long lengths (≥30 mm) which are favorable for high-power frequency conversion [7], [8]. In this letter, we investigate the generation of single-frequency green radiation by frequency-doubling of Ytterbium (Yb)-fiber laser using two variants of PPLT crystal in a single-pass configuration. The QPM crystals are (i) congruently-grown, (8-mol%) MgO-doped PPLT (MgO : cPPLT) and (ii) stoichiometrically-grown, (0.5-mol %) MgO-doped PPLT (MgO : sPPLT). The MgO : cPPLT is l = 50 mm long and 0.5 mm thick (t ) with 1041-1135 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.