Defects in LiNbO3—I. experimental aspects

Comparative study of defect structures in lithium niobate with different compositions

Both absolute and relative nonlinear optical coefficients of six nonlinear materials measured by second-harmonic generation are discussed. A single-mode, injection-seeded, Q-switched Nd:YAG laser with spatially filtered output was used to generate the 1.064- mu m fundamental radiation. The following results were obtained: d/sub 36/(KD*P)=0.38 pm/V, d/sub 36/(KD*P)=0.37 pm/V, mod d/sub 22/(BaB/sub 2/O/sub 4/) mod =2.2 pm/V, d/sub 31/(LiIO/sub 3/)=-4.1 pm/V, d/sub 31/(5%MgO:MgO LiNbO/sub 3/)=-4.7 pm/V, and d/sub eff/(KTP)=3.2 pm/V. The accuracy of these measurements is estimated to be better than 10%. >

Absolute and relative nonlinear optical coefficients of KDP, KD*P, BaB/sub 2/O/sub 4/, LiIO/sub 3/, MgO:LiNbO/sub 3/, and KTP measured by phase-matched second-harmonic generation

Zinc doping is shown to reduce the photorefraction in LiNbO3:Zn. The damage-resistant crystal LiNbO3:Zn has demonstrated a conversion efficiency of approximately 50% for frequency doubling of 1.06-μm radiation. The dependence of optical characteristics on the ZnO concentration in the melt reveals a sharp change of the optical properties at the threshold concentration of 4–6 mol % Zn. The optical and nonlinear-optical data of LiNbO3:Zn are similar to those of LiNbO3:Mg, but the former shows better optical performance.

Optical-damage-resistant LiNbO(3):Zn crystal.

Laser oscillation in Nd:MgO:LiNbO3 has been demonstrated. Thresholds as low as 3.6 mW and slope efficiencies up to 39% were achieved in a resonantly pumped miniature device. The electro-optical and nonlinear-optical properties of the host were also used to make active internal Q-switched and self-frequency-doubled lasers. Photorefractive damage is shown to be greatly suppressed compared with that for non-MgO-doped material. Absorption spectra, fluorescence spectra, and lifetime measurements are also reported.

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Nd:MgO:LiNbO 3 spectroscopy and laser devices

We have confirmed greatly improved resistance to photorefractive damage in compositions of lithium niobate containing 4.5 at. % MgO or more. Holographic diffraction measurements of photorefraction demonstrated that the improved performance is due to a hundredfold increase in the photoconductivity, rather than a decrease in the Glass current. The diffraction efficiency shows an Arrhenius dependence on temperature, with an activation energy of 0.1 eV for the damage‐resistant compositions, compared with 0.5 eV for undoped or low‐magnesium compositions. The damage‐resistant compositions are distinguished by a 2.83‐μm absorption line instead of the usual 2.87‐μm line due to the OH‐stretch vibration.

Increased optical damage resistance in lithium niobate

Optical absorption, electron spin resonance (ESR), thermoluminescence, and x‐ray‐induced luminescence techniques have been used to characterize point defects in LiNbO3. A series of crystals with different magnesium‐doping levels and Li/Nb ratios were investigated, and the defects were introduced either by x‐ray irradiation at low temperatures (10–85 K) or by reduction at high temperature in a vacuum. The samples may be classified into two groups, according to their response to radiation and reduction. A threshold (i.e., a sharp change in behavior due to a small change in composition) marks the change from one type of response to the other. The concentrations of both magnesium and lithium affect the threshold level. An electron trap, represented by an optical absorption band peaking at 1200 nm and a broad ESR spectrum centered at gc=1.82, is observed in several of the heavily‐doped crystals after irradiation. A corresponding hole trap produced during the same irradiation has an optical absorption peak near...

Point defects in Mg‐doped lithium niobate

Compositions of lithium niobate containing 4.5 at.% or more MgO have the ability to transmit, without distortion, light 100 times as intense as undopecl compositions. Holographic diffraction measurements of photorefraction have demonstrated that the improved performance is due to a hundredfold increase in the photoconductivity, rather than to a decrease in the Glass current. The damage-resistant compositions are also distinguished by a thermal activation energy of 0.1 eV for the diffraction efficiency, an OH-stretch vibration at 2.83 Am, a lattice phonon absorption at 21.2 Am, an electron spin resonance signal for Fe impurities at 1500 G, and, after reduction by heating in a vacuum, an optical absorption band at 1.2 um. (The corresponding values for undopedl LiNbO3 are 0.5 eV, 2.87 um, 21.8 um, 790 G, and 0.5 um, respectively.) The high photoconductivity is thus related to a distinctive electronic environment for impurities in the damage-resistant crystals. The photoconductivity strongly affects the impedance and time constants of signal processing devices made of LiNbO3.

Magnesium-doped lithium niobate for higher optical power applications

Optical absorption spectra were obtained after reducing (i.e., vacuum annealing) a series of LiNbO3 crystals grown from melts having various Mg concentrations and Li/Nb ratios. A band peaking at 500 nm, and assigned to oxygen vacancies containing two electrons, was the only absorption present in one set of crystals following reduction. In contrast, two overlapping bands peaking near 1200 and 760 nm were present in the other set of crystals immediately after the reduction. The 1200‐nm band is assigned to a previously unreported electron trap and the 760‐nm band to oxygen vacancies containing only one electron. These data are interpreted in terms of a threshold level for Mg doping; however, the threshold Mg doping level is not a constant but depends on the ratio of Mg ions to Li vacancies.

H. E. Tomaschke

Papers

Increased optical damage resistance in lithium niobate

Point defects in Mg‐doped lithium niobate

Magnesium-doped lithium niobate for higher optical power applications

Threshold effect in Mg‐doped lithium niobate